INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The auditory cortex (ACx) integrates and processes information carried along thalamocortical pathways to perform its major functions (for reviews, see de Ribaupierre 1997; Ehret 1997; Phillips 1995). Much is known about auditory thalamocortical pathways from anatomical and physiological studies. The main input to ACx is the primary thalamocortical pathway (often referred to as the lemniscal pathway) that projects from the ventral division of the medial geniculate (MGv) to the middle layers of primary ACx (Caviness and Frost 1980; Romanski and LeDoux 1993; Willard and Ryugo 1983; reviewed in Winer 1992). Physiological recordings from MGv and ACx indicate that this pathway mediates short-latency responses to acoustic stimuli and carries precise information about stimulus frequency, intensity, and timing (thalamus: Bordi and LeDoux 1994; Calford 1983; Edeline et al. 1999; Lennartz and Weinberger 1992; ACx reviewed in: de Ribaupierre 1997; Eggermont 1998; Ehret 1997). There are also nonprimary thalamocortical pathways (referred to as nonlemniscal or adjunct), including projections from the dorsal and medial divisions of the MG (MGd, MGm), the peripeduncular nucleus, and other nonprimary thalamic nuclei (Arnault and Roger 1990; Clerici and Coleman 1990; Herkenham 1980; LeDoux et al. 1985; Linke and Schwegler 2000; Ryugo and Killackey 1974). Typically these projections target superficial layers of temporal cortex. Responses of nonprimary thalamic cells tend to be weaker, have longer latencies, and more variability than MGv (Allon et al. 1981; Bordi and LeDoux 1994; Calford 1983; Edeline et al. 1999; Lennartz and Weinberger 1992), so their influence on cortex is expected to be more subtle (discussed in McGee et al. 1992; Sukov and Barth 2001; Weinberger 1995). Thus ACx function depends on the integration of both primary and nonprimary inputs.

Despite a large number of anatomical and physiological studies, very little is known about the cellular and synaptic mechanisms by which thalamic inputs are transmitted to, and processed in, ACx. Some progress has been made using in vivo intracellular recordings (e.g., Metherate and Ashe 1993; Mitani and Shimokouchi 1985; Sukov and Barth 2001), but such studies are hampered by the difficulty of making precise electrophysiological and pharmacological measurements in vivo. Yet detailed intracellular information must be obtained to address even seemingly simple questions such as how synaptic integration produces neuronal receptive fields. Brain-slice preparations offer an ideal environment to make precise intracellular and pharmacological manipulations, and considerable information has been obtained from ACx slices (Buonomano and Merzenich 1998; Hefti and Smith 2000; Kudoh and Shibuki 1997; Metherate and Ashe 1994). Nonetheless, the issue of thalamocortical processing has not been addressed directly because thalamocortical axons are severed during the preparation of conventional (coronal) slices and thus cannot be stimulated selectively.

To address similar problems in the somatosensory system, Agmon and Connors (1991) developed a slice preparation that maintained connections from the ventrobasal complex to barrel cortex. Since its development, that preparation has been applied to numerous issues fundamental to the understanding of thalamocortical processing in the somatosensory system. These include determining which cortical cells, as defined by position, morphology, and intrinsic properties, receive direct thalamic input (Agmon and Connors 1992; Gibson et al. 1999; Porter et al. 2001); what transmitters mediate thalamocortical transmission (Gil and Amitai 1996a); functional differences between intracortical and thalamocortical synapses (Gil and Amitai 1996b; Gil et al. 1997, 1999); and mechanisms of synaptic plasticity (reviewed in Castro-Alamancos and Connors 1997; Feldman et al. 1999).

To facilitate similar progress in the understanding of auditory forebrain mechanisms, we have developed a brain-slice preparation that maintains an intact auditory thalamocortical pathway. In initial studies using slices from rat and mouse, we demonstrated the likely feasibility of such a preparation by showing that stimulation within the thalamus or thalamocortical pathway could elicit cortical responses (Metherate and Cruikshank 1999). More recently, we have focused on the mouse brain because its smaller size permits more intact connections within a slice and for future transgenic studies. We have substantially refined the preparation to the point that we can reliably identify and study the primary auditory thalamocortical pathway in vitro. The present study represents an extensive anatomical and physiological characterization of this more refined preparation. We also introduce a second slice preparation, obtained from the area immediately ventral to the primary preparation, that may contain a nonprimary pathway. Several features of this second preparation are characterized and contrasted with those of the primary preparation. Portions of this study have appeared in abstract form (Cruikshank et al. 1999).

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Preparation of slices

All procedures followed the University of California, Irvine, animal-use regulations. Slices were taken from postnatal day (P)13-P19 FVB mice and maintained in vitro. Slice planes were nearly horizontal, but with the lateral end typically raised 15°, as illustrated in Fig. 1A. This plane was chosen so that thalamocortical axons, and parts of the MGv and ACx, could all be obtained within a single slice (Fig. 1).

View larger version (103K): [in this window] [in a new window]   Fig. 1. Anatomical positions of the primary and shell slices. A: drawing on right shows schematic of coronal mouse brain section at level of medial geniculate (MG) and auditory cortex (adapted from Franklin and Paxinos 1997). The ventral MG (MGv, black) is surrounded by a shell of nonprimary auditory nuclei (gray). Likewise, primary auditory cortex (ACx, blackalso indicated by asterisk) is bordered dorsally and ventrally by nonprimary auditory cortical areas (gray). The dotted line shows the approximate 15° angle that allows both MGv and ACx to be obtained within a slice. A, left: the left hemisphere of an actual mouse brain, viewed from the front, illustrating the planes of the primary and shell slices; they are 600 µM thick, and cut at about the angle indicated in the adjacent drawing. Orientation: lateral toward right, dorsal toward top. B: same hemisphere as in A but viewed from lateral side. Orientation: anterior toward left, dorsal toward top. C: same as B except the cortex was dissected away, revealing midbrain and thalamic structures (SC, superior colliculus; IC, inferior colliculus). MG is visible, and its bottom 30-50% appears to be within the primary slice. Shell slice is below MG. The structure immediately anterior to MG is the lateral geniculate (LG). D and E: the primary and shell slices are laid down flat and viewed from the dorsal side (as in recording chamber). Orientation: anterior is toward left, lateral is toward bottom. Typical recording sites in the middle layers of temporal cortex are indicated by asterisks. The MGv (in primary slice) and a typical position for shell stimulation (in shell slice) are indicated by arrows. Dashed rectangles indicate approximate areas of the fluorescent images in Figs. 8, A1 and B1 (primary), and 10, A1 and B1 (shell).

Following decapitation under halothane anesthesia, brains were rapidly removed and placed in 0-4°C artificial cerebrospinal fluid (ACSF; in mM: 125 NaCl, 2.5 KCl, 1.25 KH₂PO₄, 25 NaHCO₃, 1.2 MgSO₄, 2 CaCl₂, and 10 dextrose; bubbled with 95% O₂-5% CO₂). Before slicing, the brains were blocked by making three cuts with a handheld razor blade while submerged in cold ACSF. First the anterior 25% of the brain was removed with a coronal cut. The remaining brain block was then propped forward to rest on its cut surface, and a second cut was made in a nearly horizontal plane (but 15° medial/lateral slant), which split the brain into dorsal and ventral portions. The dorsal portion was discarded, then the ventral portion was lifted from the ACSF and glued to the stage of a Vibroslice with its freshly cut surface facing down (i.e., dorsal surface down; Vibroslice from Campden, WPI; glue was Superglue, WPI). The glued brain block was immediately re-immersed in cold ACSF, and a third blocking cut was made, this time midsagitally, thus separating the left and right hemispheres.

The 15° slant on the second cut caused a greater portion of one of the hemispheres to remain intact (usually the left); it was this more intact hemisphere that was set to face the Vibroslice blade and from which slices were taken for recording. However, the "unused" hemisphere was also kept glued in place for support during slicing. Slices were cut in 400-600 µm sections starting near the ventral surface of the brain. As the region below the MG and ACx became visible, slices were retained, and placed either in the recording chamber or a holding chamber filled with ACSF at room temperature. Generally two to four slices were examined under a dissecting microscope (in recording chamber) to determine if they were in appropriate planes. If correctly blocked and sectioned, there would ultimately be two useful slices per brain (the "primary slice" and the "shell slice:" Fig. 1), each of which had distinct physiology and anatomy as presented in RESULTS.

In initial experiments (n = 29 slices), slices were cut at 600 µM, to ensure a high probability of intact thalamocortical connections. However, in later experiments, after learning more about the pathway trajectory, slice thickness was successfully reduced to 500 µM (n = 38; also n = 2 @ 400 µM), thus improving chances of maintaining healthy preparations.

Electrophysiology

Recordings took place in an interface chamber (Haas model, Med Systems) maintained at 34°C, with a liberal flow of warmed humidified gas (95% O₂-5% CO₂) and ACSF passing over the slices. An initial incubation period of 1-2 h preceded data acquisition.

Sharp intracellular recording microelectrodes were pulled from filamented glass (1.0 mm OD; A-M Systems) on a horizontal puller (p97, Sutter Instruments) and had resistances of 60-140 M when filled with 3 M K-acetate (+10 mM HEPES buffer, pH 7.3). Extracellular recording microelectrodes were also pulled from filamented glass (1.5 mm OD; A-M Systems), on the same puller, and had resistances of 0.5-5.0 M when filled with ACSF. Neural signals were amplified (CyberAmp AT-401 and Axoclamp 2B, Axon Instruments), monitored on oscilloscope (Tektronix), then digitized (5-20 kHz) and stored on computer (PowerMac, Apple Computer). Extracellular electrical stimuli (0.1-0.2 ms, 1-325 µA) were delivered via concentric bipolar electrodes (Ultrasmall, 25 µM inner core, 200 µM overall diameter, F. Haer) and a constant current isolation unit (Axon Instruments). Control of experiments and off-line analysis were done with computers (AxoData and AxoGraph, Axon Instruments; PowerMac, Apple Computer).

Stimulation and recording sites were visualized with a dissecting microscope (Fig. 1, D and E). Stimuli were delivered to the MG (primary slice, Fig. 1D) or the region ventral to the MG (shell slice, Fig. 1E, indicated by arrow). Responses were recorded in temporal cortex, from the cortical column with the largest extracellular response in layer 4 (i.e., the "focus"see Laminar response profile of primary slice is dominated by middle layer CSD sink). Intracellular recordings were mainly in layers 3-4 (30-50% of the distance from the pia to the white matter; asterisks in Fig. 1, D and E), and simultaneous extracellular recordings were usually in layers 3-4 for the primary slice, and layer 1 for the shell slice (<250 µM lateral distance between intra- and extracellular sites in layers 3-4). To determine the laminar locations of current sinks and sources, one-dimensional current source density (CSD) analysis was performed (Agmon and Connors 1991; Johnston and Wu 1995; Mitzdorf 1985). First, extracellular responses were recorded at evenly spaced locations (either 125 or 150 µM spacing) beginning two positions above the pia and ending either in the deep cortical layers or below layer 6. The CSD value for a given location (at a given time point) was then calculated by subtracting twice the voltage at that location from the sum of the voltages at the two nearest locations and dividing the result by the square of the distance separating recording sites.

To test for the roles of glutamate receptors in evoked responses, the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonopentanoic acid (APV; 50 µM) and the -amino-3-hydroxy-5-methylisoxazole-4-proprionic acid/kainate (AMPA/KA) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, 20 µM) were applied to the bath (both drugs from Research Biochemicals International). CNQX stock solution included dimethyl sulfoxide (DMSO; final concentration 0.4%). In experiments designed to block synaptic transmission, Ca²⁺ concentration in the ACSF was reduced and replaced with Mg²⁺ ("low calcium," in mM: 0.2 CaCl₂ and 3.0 MgSO₄, all other salts the same as normal ACSF). In experiments designed to selectively suppress polysynaptic responses (see INTRACELLULAR RECORDINGS IN LAYERS 3-4 OF THE PRIMARY SLICE REVEAL CONSISTENT EARLY RESPONSE AND MORE IRREGULAR LATE POLYSYNAPTIC RESPONSE), ACSF with high concentrations of divalent cations was used ("high divalent ACSF"; in mM: 115 NaCl, 2.5 KCl, 25 NaHCO₃, 4.2 MgCl₂, 7 CaCl₂, and 10 dextrose). Besides the adjustments in Ca²⁺ and Mg²⁺ concentrations (which were 3.5 times normal values), sulfates and phosphates were also left out to prevent Ca²⁺ precipitation (Crepel and Ben-Ari 1996; Luhmann and Prince 1990), and NaCl was reduced to maintain osmolarity (see preceding text).

Anatomy

FIXATION AND SECTIONING OF RECORDED SLICES. To examine the exact planes of section, some slices were processed for either Nissl staining or parvalbumin (PV) immunohistochemistry after electrophysiological recording (described in RESULTS). For both types of labeling, whole slices were first fixed by immersion in 4% paraformaldehyde (in 0.1 M Na-phosphate buffer: PB, pH 7.2) for 12 h, then sectioned on a vibratome at 50 µM.

PV IMMUNOHISTOCHEMISTRY. PV procedures were carried out with free-floating sections at room temperature with agitation, except where specified. Sections were pretreated for 30 min in 0.5% H₂O₂, rinsed in phosphate-buffered saline (PBS; pH 7.15; 0.1 M PB, 0.5 M NaCl) containing 0.1% Tween 20, then incubated for 2 h in PBS containing 0.3% Triton X-100. Next, nonspecific sites were blocked using an Avidin/Biotin Blocking Kit (Vector Labs), followed by immersion in 10% horse normal serum for 2 h (HNS, Vector Labs). Sections were then incubated at 4°C for 12-48 h in a combined antibody solution (to reduce background staining) (Hierck et al. 1994) containing a 1:500 dilution of monoclonal PV antibodies (pa-235, Sigma), a 1:500 dilution of biotinylated anti-mouse IgG (Vector Labs), 0.1 M PB, 0.5 M NaCl, 0.3% Triton X-100, 2% HNS, and 0.1% mouse normal serum (MNS, Sigma). Standard peroxidase visualization followed. Briefly, sections were rinsed in PBS containing 0.1% Tween-20, incubated in Avidin/Biotin complex for 2 h (Vectastain Elite ABC-Peroxidase Kit, Vector Labs), rinsed again in PBS, then in 0.05 M Tris buffer (TB; pH 8.0). Next they were preincubated in nickel/DAB solution (0.3% nickel ammonium sulfate, 0.033% diaminobenzidine, TB), then reacted for 2-5 min by adding H₂O₂ (0.01%). Following rinses in TB and 0.1 M PB, sections were mounted on gelatin-coated slides, air dried, dehydrated with increasing concentrations of alcohol, cleared with Histo-Clear (National Diagnostics), and coverslipped using Permount (Fisher).

NISSL STAINING. In the Nissl material, after sectioning at 50 µM (see preceding text), tissue was rinsed in 0.05 M PB, mounted on gelatin-coated slides, then air dried for 12-36 h. Next, the slide-bound sections were immersed for 2-24 h in a solution of 50% chloroform and 50% alcohol, rehydrated in descending concentrations of alcohol in H₂O, then immersed in 0.06% cresyl violet stain (0.6 mg/ml cresyl violet acetate, 0.72 mg/ml Na-acetate, 4.9 µl/ml 1 N acetic acid, H₂Ousually 300 ml) for ~5 min, followed by 30 s of rinsing in H₂O. After that, the tissue was dehydrated, cleared, and coverslipped as described for the immunoreacted tissue (see preceding text), except that acetic acid was added to the 95% alcohol dehydration step to accelerate differentiation of the cresyl violet stain (0.5 ml glacial acetic per 200 ml alcohol solution).

FIXATION AND SECTIONING OF WHOLE BRAINS. To compare Nissl and PV patterns in the coronal plane, sections were cut from whole fixed brains. Under deep anesthesia [50-100 mg/kg pentobarbital sodium (Nembutal)], mice were transcardially perfused with 0.9% saline until most visible blood was flushed from the animal, then with 4% paraformaldehyde (see preceding text) for ~10 min. Solutions were 4-8°C and perfused at 10-15 ml/minute using a peristaltic pump (Fisher). The brains were postfixed overnight at 4°C (same fixative), embedded in 3% agarose (to increase stability during sectioning), then sectioned on a vibratome at 50 µM in the coronal plane. Every third section was Nissl stained (see preceding text), and adjacent sections were immunoreacted for PV (see preceding text) and calbindin (not shown).

TRACT TRACING WITH Di-I. The fluorescent lipophilic tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di-I; D-282, Molecular Probes) was used to examine fiber pathways in previously recorded primary and shell slices and in some whole brains. First the slices and brains were fixed by immersion and perfusion respectively (described in the preceding text), then allowed to postfix 2 days. Next, 25- to 100-µM-diam particles of Di-I were inserted into targets in the still intact slices and brains under a dissecting microscope, using a broken micropipette to manipulate the Di-I (targets indicated in RESULTS). The slices or brains were then placed in small plastic cups and covered in warm liquid agarose (3%) that was made with PB containing 0.1% sodium azide. After the agarose hardened, the cups were filled to the top with PB (including 0.1% sodium azide), sealed with parafilm, and placed in the dark at 30-38°C for 1-4 mo to permit Di-I diffusion. The slices or brains were subsequently sectioned on a vibratome at 50 µM (for whole brains, sectioning was in horizontal plane), rinsed in PB, then mounted on gelatin-coated slides, and coverslipped with Vectashield mounting medium (Vector Laboratories).

SLICE PLANE EXAMINATION IN UNSTAINED WHOLE BRAINS. To compare the slice planes to the positions of the MG and other macroscopic features, six brains were fixed by perfusion (see preceding text) and cut on a vibratome in the planes normally used for physiological experiments. For clarity, no blocking was done so that the slice planes could be seen in the context of the whole brains. For the same reason, only three cuts were made on the vibratome: one cut separated the dorsal part of the brain from the primary slice, a second separated the primary and shell slices, and a third separated the shell slice and the ventral part of the brain (Fig. 1). The slice thicknesses were 600 µM and were in the approximate planes of physiologically recorded primary and shell slices, based on positions and appearances of structures contained within them (Fig. 1, D and E).

MICROSCOPY AND IMAGE ACQUISITION. The Nissl and PV labeling patterns were examined using standard bright-field microscopy at a variety of magnifications (×25-1,000), and images were captured with a digital camera (SPOT, Diagnostic Instruments) attached to an Olympus microscope. Di-I labeling was examined using epifluorescence microscopes, again at a variety of magnifications, and images were captured with digital cameras (Zeiss and Olympus). Photographs of the unstained tissue (Fig. 1) were taken using a Polaroid camera (MicroCam) fitted to a dissecting microscope (WPI), then the photo prints were digitized with a scanner (Color OneScanner, Apple Computer).

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

As indicated in METHODS, when the mouse brain was cut along the near horizontal plane illustrated in Fig. 1, A and B, two distinct slice preparations could be obtained, each with unique physiological properties. The dorsal-most of these will be referred to as the "primary slice," and the more ventral will be called the "shell slice" (Fig. 1). Initially, the two preparations were distinguished based on gross anatomical features and by relating these features to the effective stimulation loci and cortical responses profiles. For example, in the unstained primary slice, the MG can be seen directly, and the lateral geniculate (LG), hippocampus, striatum, and other structures have characteristic shapes (Fig. 1D). When the MG is stimulated in a living primary slice, it results in a strong middle layer response in auditory cortex (described in the following text). The shell slice, and the structures contained within it, also have characteristic shapes. For example, the hippocampus is wider in the medial-lateral direction, giving it a more rounded appearance (Fig. 1E). More importantly, the shell slice is located in a sufficiently ventral plane that it contains no obvious MG nucleus. Thus in the shell slice, the region just medial to the hippocampus (indicated by an arrow in Fig. 1E) corresponds to the area ventral to the MG nucleus. Stimulation of this region consistently results in an upper layer current sink in temporal cortex (described in the following text), contrasting with the middle layer sink produced by MG stimulation in the primary slice.

Primary slice

ANATOMICAL FEATURES: PRIMARY SLICE CONTAINS MGV AND ACX. Figure 1C provides a lateral view of the mouse brain with part of the neocortex and hippocampus dissected away to facilitate comparison between the positions of the slices and the MG as a whole. The MG appears as a rounded protuberance posterior to the LG and ventral to the superior colliculus (SC). Notice that the primary slice contains the bottom 30-50% of the visible portion of the MG, which mostly consists of the core ventral division of the MG (MGv). Given that the slicing angle was chosen to obtain both the MGv and ACx within the same plane (Fig. 1A, right; see METHODS), it would seem logical that primary slices that contain the MGv would also contain primary ACx. These observed and inferred features of the primary slice, combined with its distinct middle layer response profile, are consistent with the known projection of the MGv to middle layers of primary auditory cortex (Caviness and Frost 1980; Romanski and LeDoux 1993; Willard and Ryugo 1983; reviewed in Winer 1992).

View larger version (90K): [in this window] [in a new window]   Fig. 2. Parvalbumin (PV) immunohistochemistry facilitates localization of auditory forebrain structures in thalamocortical slices. A: Nissl-stained coronal section (50 µM thick). Many structures within and surrounding the auditory thalamus can be differentiated in coronal plane using cytoarchitecture. The label "auditory cortex" is centered on approximate position of primary ACx. It is bordered dorsally and ventrally by nonprimary ("belt") areas (as indicated in Fig. 1A) (Franklin and Paxinos 1997). Besides position, primary ACx was identified by its more laminated cytoarchitecture (dense cell packing in layers 2-4 and 6, sparse in layer 5) than adjacent nonprimary areas, and the PV pattern (following text) (Cruikshank et al. 2001). Orientation: lateral toward left, dorsal toward top. B: PV pattern in section adjacent to A. Notice the dense PV labeling in MGv and sharp boundary along its ventral border (e.g., PP is virtually white). There is also dense and laminated PV labeling at the apparent position of primary ACx. The adjacent cortical areas have weaker PV labeling (e.g., region ventral to primary ACx but dorsal to rf has only lightly labeled somata). More ventrally, the perirhinal cortex (surrounding rf) has virtually no PV labeling. White lines in B (labeled 1-3) indicate the approximate planes of sections in C, 1-3. C: a primary thalamocortical slice was sectioned into 50-µM segments and PV immunolabeled. Orientation: anterior is toward left, lateral is toward bottom. The section in 1 was 100 µM from dorsal surface of slice, section 3 was the most ventral section of slice, section 2 was 50 µM above section 3. PV-immunopositive MGv is present in sections 1 and 2 but not 3. ACx expressed dense and laminated PV labeling in all 3 sections. Further described in RESULTS. APT, anterior pretectal nucleus; cp, cerebral peduncle; LG, lateral geniculate; MGd, dorsal division of MG; MGm, medial division of MG; PIN, posterior intralaminar nucleus; PP peripeduncular nucleus; rf, rhinal fissure; RT, reticular thalamic nucleus; SN, substantia nigra; str, superior thalamic radiation; VB, ventrobasal complex.

LAMINAR RESPONSE PROFILE OF PRIMARY SLICE IS DOMINATED BY MIDDLE LAYER CSD SINK. The dominant feature of the cortical response to MG stimulation in the primary slice is a robust negative field potential and associated CSD sink in layers 3-4. The example in Fig. 3A illustrates this and other typical features of the laminar response. In an initial procedure, the cortex was "mapped," by moving the electrode horizontally within layer 4, until finding the anterior-posterior position with the largest fast negative field potential evoked by MG stimulation (i.e., the "focus" of the response). The laminar profile was then determined for the cortical column corresponding to that focus by recording extracellular responses in evenly spaced intervals. Figure 3A1 shows the field potentials and CSD traces resulting from that procedure, conducted in normal ACSF. Following a 100-µA MG stimulus, the largest negative field potential and CSD sink (upward deflection) were recorded at 450 µM from the pia surface (total cortical thickness was 1,275 µM; Fig. 3A). Besides the large middle layer sink, the slice also displayed clear supragranular and infragranular CSD sources (downward deflections) at 150-300 and 750 µM, respectively. In addition, there was a small sink on the cortical surface.

View larger version (56K): [in this window] [in a new window]   Fig. 3. Laminar profiles and low-Ca2+ sensitivity of thalamocortical responses in the primary slice. A1: laminar field potentials and associated current source densities (CSDs) for example slice in normal artificial cerebrospinal fluid (ACSF). One-hundred-fifty-micrometer separation between recordings; numbers on left indicate depth of recording sites within ACx (gray background). "Pia" indicates surface of layer 1, "WM" indicates border between white matter and layer 6. MG stimulus intensity = 100 µA. Traces represent averages of 8-10 responses (gray-scale plot described in the following text). Notice large fast field potentials and CSD sinks around layers 3-4. Associated sources are immediately above and below. Also note small sink on surface; this was the smallest of the CSD responses classified as "surface sinks" (n = 6; see main text). A2: responses of same slice during perfusion of low-Ca2+ ACSF. Note the near absence of any evoked field potentials or CSDs. Responses recovered after return to normal ACSF (not shown). B: gray-scale CSD profiles of all primary slices, arranged by recording date. Areas under the raw traces were first measured (initial 20 ms after artifact), then gray-scale plots were generated using DeltaGraph (DeltaPoint). Values were normalized to the maximum sink for a given slice (max sink = +1), and plotted from +1 (black) to 1 (white), with 20 linear gray steps between (scale in B). Plot heights normalized to total cortical thickness. Some slices were not recorded to the white matter, so they don't reach 100%. Note generally consistent sink positions (dark bands) in layers 3-4. For further explanation, see RESULTS.

PRIMARY SLICE RESPONSES REQUIRE SYNAPTIC TRANSMISSION: CALCIUM SENSITIVITY. Blocking synaptic transmission, by lowering Ca²⁺ concentrations in the ACSF (replaced with Mg²⁺; see METHODS), profoundly suppressed the MG-evoked responses across all cortical laminae. Figure 3A2 illustrates this effect. After confirming the typical middle layer profile in normal ACSF (Fig. 3A1), low calcium ACSF was perfused over the slice for ~1 h (until reductions in layer 4 field potentials had become asymptotic), then the laminar profile was re-determined. Figure 3A2 shows that the MG-evoked field potential, and CSD traces are almost flat under low calcium conditions, with only extremely weak responses in the lower layers remaining (600-1,200 µM deep). Four other primary slices were tested in this way. All had a virtually complete (and reversiblein normal ACSF) blockade of middle layer responses during low calcium perfusion. For two of those slices, a short latency (3-4.5 ms) spike-like field potential remained (i.e., small and narrow) but only in the lower layers. These results suggest that the majority of the MG-evoked extracellular responses in the primary slice are synaptically mediated. The residual responses that sometimes remain in the lower layers likely result from direct activation of either afferent fibers or antidromically activated cells.

INTRACELLULAR RECORDINGS IN LAYERS 3-4 OF THE PRIMARY SLICE REVEAL CONSISTENT EARLY RESPONSE AND MORE IRREGULAR LATE POLYSYNAPTIC RESPONSE. Given the predominant middle layer CSD sinks and the known projection to this same region from the MGv, it seemed likely that major monosynaptic thalamocortical responses in the primary slice occur at the foci of the fast sinks in the middle cortical layers. Thus as a first step in the intracellular investigation of the thalamocortical responses, it seemed reasonable to direct recordings near those sinks. To do this, the auditory cortex was first mapped with an extracellular electrode to find the "focus" of the MG-evoked response in layers 3-4 (as described for the laminar recordings), then the intracellular recordings were conducted at or near that focus (within ~250 µM). Sixty-six cells, located between 30 and 50% of the distance from the pia to white matter, were recorded from 24 primary slices. Mean values of passive membrane properties (±SE) were as follows: resting potential = 69.3 ± 0.9 mV, input resistance = 46.8 ± 2.1 M, spike threshold (current) = 0.39 ± 0.03 nA, spike threshold (membrane potential) = 48.7 ± 0.5 mV, spike height = 64.1 ± 0.7 mV, spike width (at half-amplitude) = 0.87 ± 0.03 ms. Three cells had categorically narrower spikes (range: 0.28-0.44 ms) than the other 63 (range 0.64-2.21 ms). They also fired at high rates (100 Hz for hundreds of milliseconds during depolarizing current injection) and had deep/fast afterhyperpolarizations (AHPs), consistent with the fast spiking cell type (Connors and Gutnick 1990; McCormick et al. 1985; Porter et al. 2001). The majority of cells with wider spikes displayed strong spike frequency adaptation and had more shallow/gradually developing AHPs, consistent with the regular spiking cell type (Connors and Gutnick 1990; McCormick et al. 1985; Porter et al. 2001). We observed no obvious differences in synaptic responses between cell types and will report the data together.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Early and late responses of cells in layers 3-4 of the primary slice. A: responses of example cell to increasing intensities (in µA: 12, 25, 50, 100, 200). Each trace is average of 8-10 responses. Interstimulus interval (ISI) = 10 s. Note increases in slopes and amplitudes with increasing intensities. The apparent differences in spike thresholds are artifacts of averaging. Inset: 4 individual responses for 200-µA stimulus (2 with early and late responses, 2 with early responses only). Note that failure of late response results in smooth decay phase. Spikes are truncated. B: average early response amplitudes as a function of intensity. The "n" represent the number of cells for each intensity. Note the graded increases in peak amplitudes with increasing intensities. C: effects of "high divalent ACSF" (see METHODS) on late responses. Top: intracellular (same cell as in A), bottom: simultaneous middle layer field potential. Stimulus intensity = 200 µA, ISI = 30 s. Average of 10 trials. Note that average control late response at this slower stimulus rate is larger than average in A. Intracellular and extracellular late responses are strongly suppressed by high divalent ACSF, but the early response peaks are similar to control amplitudes. Explained further in RESULTS.

EARLY MG-EVOKED RESPONSE IN ACX CONTAINS FAST AMPA AND SLOWER NMDA EPSPS. Within the early response, there were multiple components that could be dissociated physiologically and pharmacologically. Figure 5A illustrates this for a cell in layer 4. The MG was stimulated at 0.1 Hz, which largely exhausted the late response discussed in the preceding text (not shown: note the difference in time scale relative to Fig. 4), and the remaining early response was recorded while the steady-state membrane potential of the neuron was varied with intracellular current injection. Because of unique voltage dependencies, the different subcomponents of the early response were most distinct at depolarized steady-state potentials (Fig. 5A, top). Following an MG stimulus in control solution (Fig. 5A1), there was an initial depolarization that peaked quickly (labeled fast EPSP), then a sharp negative deflection to below the baseline membrane potential (IPSP; discussed in the next 2 sections), and finally a second depolarizing peak (slow EPSP). Both the fast and slow EPSPs could trigger action potentials when the cell was "held" a few millivolts below threshold (shown for slow EPSP). The slow EPSP had clear nonconventional voltage dependence, being more prominent at depolarized steady-state potentials, suggesting the involvement of NMDA receptors.

View larger version (28K): [in this window] [in a new window]   Fig. 5. Differential effects of glutamate receptor antagonists on the 3 components of the primary slice early response: fast excitatory postsynaptic potential (EPSP), fast inhibitory PSP (IPSP), slow EPSP. A1: example middle layer cell showing 3 components of early response. Resting potential = 68 mV. Steady-state potentials adjusted by injecting intracellular current (in nA, from bottom trace to top: 0.2, 0, 0.2, 0.4). MG stimulus time indicated by arrow. Stimulus intensity = 150 µA. Response components are labeled at most depolarized steady-state level. Traces at 3 most hyperpolarized steady-state potentials are averages of 7-10 responses. Individual responses presented for depolarized potential to illustrate trials with and without action potentials (AP) initiated by slow EPSP. For other trials, the fast EPSP could induce a spike. Scale bars apply to all intracellular traces (A, 1-3). A2: 20 µM 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX) blocked the fast EPSP and fast IPSP, leaving a slow EPSP with nonconventional voltage dependence. A3: the combination of 20 µM CNQX + 50 µM DL-2-amino-5-phosphonopentanoic acid (APV) blocked all the MG evoked responses for the cell (the slow EPSP recovered after removal of APV, not shown). B: field potential from same slice as in A (simultaneous, layer 3-4). CNQX blocked the large fast potential, leaving a smaller slow potential (B2). Addition of APV (CNQX + APV) blocked the remaining slow potential (B3), which recovered on subsequent removal of APV (B4, CNQX). A large portion of the fast potential recovered during 2.5-h rinse in normal ACSF (B4, ACSF). C: group effects of glutamate antagonists on fast EPSP (1), slow EPSP (2), and fast and slow field potentials (3; 9 cells, 9 field potentials). Fast EPSPs and field potentials were measured at latency corresponding to peak (initial 20 ms) in control conditions. Slow EPSPs and field potentials were measured at latency corresponding to peak of response during CNQX (at depolarized steady-state potentials for intracellular: the most depolarized value that was still sub-threshold for spiking was used). While the fast EPSP was nearly abolished by CNQX, the slow EPSP was only moderately suppressed; the latter effect was weaker for depolarized steady-state potentials than at resting potentials. Addition of APV reversibly blocked the slow EPSP (completely) even at depolarized potentials. The fast and slow group field potentials had similar effects as the corresponding EPSPs plus partial recovery in normal ACSF.

NATURE OF THE FAST IPSP. The cell in Fig. 5 expressed a clear IPSP in control solution. For example, while both the fast and slow EPSPs could trigger spikes, there were never action potentials during the IPSP, even when strong depolarizing current was injected into the cell to produce "spontaneous" action potentials (not shown). The IPSP had distinct voltage dependence, being more prominent at depolarized steady-state membrane potentials, and reversing at approximately 62 mV (Fig. 5A1). This data, together with the fast time-course, suggest a GABA_A receptor-mediated mechanism (Avoli 1986; Connors et al. 1988; Cox et al. 1992; Hefti and Smith 2000). Note that the IPSP was blocked during CNQX application, consistent with the interpretation that glutamatergic synapses drive the interneurons responsible for the IPSP (see DISCUSSION).

View larger version (28K): [in this window] [in a new window]   Fig. 6. Differences in responses evoked by MG vs. on-beam stimuli: IPSPs and laminar profiles. A: intracellular responses evoked by MG stimuli (100 µA) and on-beam stimuli (60 µA) for resting potentials (MG, 73 mV; on beam, 71 mV) and depolarized steady-state potentials (48 mV for both, using 1.0 nA of intracellular current). Each trace represents average of 7-10 responses (response with spikes excluded). Note stronger IPSP for on-beam stimulation, despite similar initial EPSP slopes for on-beam and MG stimulation. B: example of on-beam-evoked laminar field potential and CSD profile (gray scale derived from traces). The CSD profile is compared with an MG-evoked CSD profile from same slice. Conventions and procedures as in Fig. 3. Note the large on-beam-evoked CSD sinks in the supragranular and infragranular layers, whereas the large MG evoked sink is in the middle layers as usual. See RESULTS for detailed explanation of IPSP and laminar comparison.

MG-EVOKED RESPONSE DIFFERS FROM RESPONSE PRODUCED BY CONVENTIONAL COLUMNAR (ON BEAM) STIMULATION. Aside from recent studies with intact thalamocortical slices (see INTRODUCTION), in vitro studies have generally employed stimulation of layer 6, or the white matter just below layer 6, in an attempt to activate thalamocortical axons projecting to cells recorded in the cortical column above (discussed in Agmon and Connors 1991; Kenan-Vaknin and Teyler 1994; Kirkwood and Bear 1994). This conventional white matter/layer 6 stimulation will be referred to as "on-beam" stimulation. Here we compared responses in primary slices evoked by MG and on-beam stimulation and found clear differences between them relating to IPSP strengths and CSD distributions. These two results will each be described in turn.

TRACT TRACING IN THE PRIMARY SLICE: ANATOMICAL CONNECTIONS BETWEEN THE MGV AND ACX. To anatomically investigate the thalamocortical pathway in the primary slice, we performed tract tracing studies using the lipophilic dye Di-I. Following physiological recording, slices were fixed, then a particle of Di-I (50 µM diam) was placed either in the MG near the stimulation site (n = 8 slices) or in the ACx near the focus of the middle layer response (n = 5 slices). The dye was allowed to diffuse along the pathway(s) for 1-2 mo (30-38°C), then the slices were re-sectioned into 50-µM-thick segments and imaged using conventional epifluorescence microscopy (see METHODS).

View larger version (98K): [in this window] [in a new window]   Fig. 7. Anterograde and retrograde 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di-I) tracing along the thalamocortical pathway in the primary slice. Orientation for all panels: anterior toward left, lateral toward bottom. The approximate positions of the images in A1 and B1 are indicated by the rectangle in Fig. 1D. A1: low-magnification image of MG, ACx, and surrounding structures, following Di-I application to the MG (*). - - -, a region that includes ACx and parts of striatum and is expanded in A2. The white arrow indicates a region of increased labeling density, approximately where the thalamocortical pathway crosses the reticular thalamic nucleus. A2: medium-magnification image showing the pattern of cortical labeling. In this section, labeling mostly involves anterograde-filled processes with many branches in layers 3-4. Other sections from same slice also had retrograde labeling of deep layer pyramidal cells. A3: high-magnification image from same section showing plexus of processes in layers 3-4. B1: low-magnification image following Di-I application to ACx (*). Orientation similar to A1. B2: part of MG outlined by dashed box in B1 is shown at higher magnification, illustrating 2 retrograde-filled MG somata. B3: image from adjacent section of same slice showing more MG somata. Details of the primary slice tracing presented in Tract tracing in the primary slice: anatomical connections between the MGv and ACx.

Shell slice

ANATOMICAL FEATURES OF THE SHELL SLICE. The shell slice was located ventral to the primary slice (Fig. 1). Gross anatomical observations (e.g., Fig. 1, C and E) and knowledge of the ventral boundaries of the primary slice (see preceding text) indicate that the thalamic portions of the shell slice are located below the MG proper. This region contains nonprimary or "shell" auditory thalamic subdivisions (e.g., the PPD and PIN; Figs. 1A and 2A) and more ventral nonauditory structures (e.g., substantia nigra, cerebral peduncle; Figs. 1A and 2A). Similar observations and reasoning suggest that the shell slice may contain portions of the "belt" auditory cortical area, ventral to primary ACx.

LAMINAR RESPONSE PROFILE OF SHELL SLICE IS DOMINATED BY A SURFACE CSD SINK. The dominant evoked response in the shell slice, to stimulation of the region ventral to the MG (indicated by arrow in Fig. 1E; hereafter referred to as the "shell region"), was a strong CSD sink on the surface of cortical layer 1 (Fig. 8). Generally, the field potential at the position of the sink was predominantly negative in polarity and had a similar time course as the sink. In contrast, the field potentials in the middle and lower layers were nearly always biphasic with a fast negative phase that preceded the surface sink and a slower positive phase having a similar latency as the surface sink. The example in Fig. 8A illustrates these and other characteristics of the shell slice laminar response (conventions and methodology as in primary slice experiments: Fig. 3). In addition to the surface sink, there was also a CSD source immediately below the sink (150 µM depth; Fig. 8A1). The gray-scale representation of this CSD profile is drawn to the right of traces and in the group plot (Fig. 8B).

View larger version (56K): [in this window] [in a new window]   Fig. 8. Laminar pattern and low-Ca2+ sensitivity of cortical responses in the shell slice. A1: laminar field potentials and associated CSDs for example shell slice in normal ACSF. Orientation, conventions, and procedures similar to Fig. 3. Stimulus intensity = 150 µA. Notice large CSD sink at the cortical surface, and the source immediately below. Also note predominantly negative surface field potential and fast biphasic middle and lower layer potentials. The onsets of late field potential responses are also visible at the end of the traces (negative polarity in lower and middle layers, positive polarity on the surface). A2: responses of same slice during perfusion of low-Ca2+ ACSF. Note the near absence of any evoked CSD components; arrow indicates a small surface source that sometimes remained. Also note the clear negative field potentials in the middle and lower layers. Responses recovered after return to normal ACSF (not shown). B: fast CSD profiles of all shell slices, arranged by recording date. The surface sinks (dark bands) were largest in all cases except the slice on the far right, for which the infragranular sink had greater area. For further explanation, see RESULTS.

SURFACE CSD SINK IN THE SHELL SLICE REQUIRES SYNAPTIC TRANSMISSION. Blocking synaptic transmission, by lowering extracellular Ca²⁺ concentrations, strongly suppressed responses evoked by shell region stimulation (Fig. 8A2). Across slices (n = 5), all major sinks and sources were essentially eliminated. The only consistent CSD feature still present in low-Ca²⁺ ACSF was a small source on the cortical surface (Fig. 8A2, ); while this source could sometimes be seen even in control conditions, its duration became longer in low-Ca⁺² ACSF, probably due to the removal of the large overlapping sink (Fig. 8A).

INTRACELLULAR RECORDINGS IN SHELL SLICE REVEAL EARLY AND LATE RESPONSES: EARLY RESPONSE TIME COURSE SIMILAR TO SURFACE CSD SINK. Intracellular recordings were made from nine cells in three shell slices; eight of these cells were located in layers 3-4, and one cell was recorded from approximately layer 6. Mean values of passive membrane properties were as follows: resting potential = 65.0 ± 2.3 (SE) mV, input resistance = 44.1 ± 5.7 M, spike threshold (current) = 0.31 ± 0.04 nA, spike threshold (membrane potential) = 47.3 ± 0.9 mV, spike height = 60.3 ± 1.1 mV, spike width (at half-amplitude) = 1.04 ± 0.05 ms. The range of spike widths was 0.82-1.36 ms, and they all exhibited strong spike frequency adaptation, consistent with the regular spiking cell type (Connors and Gutnick 1990).

View larger version (20K): [in this window] [in a new window]   Fig. 9. Early and late responses in the shell slice: intracellular recordings from cells in layers 3-4 and field potentials in layer 1. A: example responses from middle layer intracellular recording (top) and simultaneous layer 1 extracellular recording (bottom). Stimulus intensity = 100 µA. Stimulus times indicated by arrows. Traces are averages of 6 responses. The intracellular trace on left (800-ms sweep duration) illustrates early depolarization, followed by negative inflection (to slightly below baseline), then a large-amplitude, long-duration, late response. The layer 1 field response has negative polarity during early response, and positive polarity during late response. Right: traces show early responses at expanded time scale (70-ms duration; outlined by box in long sweeps on left). Note the similar onsets of early intracellular depolarization and layer 1 field negativity (- - -). Interstimulus interval (ISI) = 10 s. B: another set of example responses from middle layer intracellular and layer 1 extracellular recordings. Stimulus intensity = 150 µA. Again, there were similar onset latencies between the intracellular and extracellular recordings. Note that cells in both A and B show robust late responses in average traces despite 0.1-Hz stimulus rates and despite differences in early response amplitudes. C: peak amplitudes of the early responses across cells (n = 8, middle layers). Note the small mean sizes compared with primary slice (Fig. 4B).

TRACT TRACING IN THE SHELL SLICE: TRANSPORT OF Di-I FROM STIMULATION SITE LABELS FIBERS IN LAYER 1 AND PYRAMIDAL CELLS IN LAYER 5. Tracing studies were conducted in shell slices using the same methods as in the primary slices (see TRACT TRACING IN THE PRIMARY SLICE: ANATOMICAL CONNECTIONS BETWEEN THE MGV AND ACX). Di-I was placed either near the site of stimulation in the shell region (n = 5) or near the focus of the recordings in the temporal cortex (n = 3). Figure 10A illustrates the results for a slice with application near the stimulation site (asterisk). Panel 1 shows that the main route of dye movement was anterior-lateral, initially along the edge adjacent to the hippocampus, then across the striatum to the temporal cortex (Fig. 10A1). There was also clear posterior-going Di-I flow that eventually curved medially before leaving the slice plane (Fig. 10A1). Panel 2 focuses on the pattern of cortical labeling (same slice, different section). The most obvious feature was a dense band of pyramidal cells and processes in layer 5 (shown at higher magnification in 3). A more subtle aspect of the cortical labeling was the dense network of processes in layer 1. They are most readily seen at high magnification (4-6). Panel 4 shows a concentration of these fine processes in the most superficial aspects of layer 1. Panels 5 and 6 focus on individual superficial axons; notice that many other large and small caliber processes can also be seen, mostly behind the focal plane of the images. These included both spiny dendrites and axons with beady varicosities (not shown).

View larger version (115K): [in this window] [in a new window]   Fig. 10. Anterograde and retrograde Di-I tracing in the shell slice. Orientation for all panels: anterior toward left and lateral (i.e., temporal cortical layer 1) is toward bottom, similar to Fig. 7. The approximate positions of the images in A1 and B1 are indicated by the rectangle in Fig. 1E. A1: low-magnification image of shell pathway. Di-I travels in 2 directions from site of application near stimulation area (*): anterior-lateral to temporal cortex and posterior then out of slice plane. A2: medium magnification image of a nearby section from same shell slice showing pattern of cortical labeling. Retrograde labeling of cells in layer 5 is apparent. Only sparse labeling exists in layers 3-4. Area outlined by dashed box in infragranular layers is shown at expanded magnification in A3. Area outlined by box in supragranular layers is expanded in A4. A3: numerous pyramidal cells and processes in layer 5. A4: dense concentrations of processes in layer 1. A5 and A6: high-magnification images from adjacent section illustrating processes in upper layer 1. Black arrows point to probable axons containing beady varicosities. White arrows point to larger caliber processes which are probably branches of apical dendrites of retrograde filled pyramidal cells. In other focal planes, dendritic branches displayed visible spines. B: low-magnification image of shell pathway following Di-I application to temporal cortex (*). Orientation similar to A1. Arrow points to labeling in shell stimulation area. Note that pathway labeling continues past arrow (in posterior direction) toward target outside slice plane. Details of the shell slice tracing presented in RESULTS.

LAYER 5 CELLS PROJECT THROUGH THE SHELL STIMULATION REGION TO THE INFERIOR COLLICULUS AND APPEAR TO HAVE COLLATERAL PROJECTIONS TO LAYER 1. One possible source of input to layers 1 and 5 of the shell slice that does not involve direct projections from the thalamus could be axon collaterals emanating from pyramidal cells in layer 5. Given the dense retrograde labeling of layer 5 cells following Di-I application in the shell area, it is clear their axons pass through the effective stimulation sites. Thus it is possible that these cells could be antidromically activated by the shell stimuli. If they also have collaterals projecting to layers 1 and 5, such antidromic activation could subsequently produce the observed synaptic sinks in those layers.

View larger version (122K): [in this window] [in a new window]   Fig. 11. Di-I application in the inferior colliculus of whole fixed brains: labeling of layer 5 pyramidal cells with axons going through shell stimulation region, and axons in layer 1. Orientation for all panels: anterior toward left, and lateral (i.e., temporal cortical layer 1) toward bottom, similar to Fig. 10. Di-I was placed in the inferior colliculus, outside planes of images. A: low-magnification image at approximately same plane as Fig. 10A1, showing labeled tract through position typically effective for shell stimulation (arrow), and continuing toward temporal cortex. B: medium-magnification image of a nearby section from same brain showing pattern of temporal cortical labeling. Retrograde labeling of cells in layer 5 is apparent. Dashed box outlines a region of layers 1-5 that is expanded in C. C: expanded image illustrating layer 5 pyramidal cells and ascending processes, 1 of which clearly reaches superficial layer 1 (bottom). Dashed box in C outlines a region of layers 1-2 that is expanded in F. D-F: high-magnification images from same section (F) and two adjacent sections (D and E) illustrating processes in layers 1-2. Ascending processes can be observed crossing the layer 1-2 border in D and F. Many fine horizontally oriented processes, some with varicosities, can be seen close to the surface of layer 1. Thick white lines just beyond cortical surface in E and F are bubble artifacts. See RESULTS for more explanation.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The major finding of this study is that an in vitro preparation can be obtained from the mouse brain that contains functionally intact thalamocortical connections from the MGv to the ACx. Stimulation of the MGv in this preparation (i.e., the primary slice) activates not only thalamocortical synapses but also appears capable of initiating polysynaptic activity, indicating the presence of at least partially intact intracortical connectivity. A second major finding is that auditory thalamocortical transmission involves both AMPA and NMDA types of glutamate receptors. Third, this paper presents (for the first time, to our knowledge) a detailed description of laminar CSD profiles in ACx evoked by MG stimulation. Fourth, we observed that these MG-evoked profiles, and the intracellular responses in layer 4, differ from those produced by conventional columnar stimulation, indicating that columnar stimulation may not be a reliable means of selectively activating thalamocortical inputs. Finally, a second in vitro preparation (the shell slice) is presented in which subcortical stimulation appears to selectively activate synapses in superficial layer 1 of temporal cortex. This observation is consistent with known projections of nonprimary auditory thalamic nuclei near the stimulation site, suggesting that the preparation may provide a tool for investigating mechanisms of a nonprimary system; however, alternative interpretations are also presented. Each of these findings will be elaborated in the following text.

Primary slice includes functionally intact thalamocortical connections from the MGv to ACx: histology, tract tracing, and laminar physiology

MAJOR CONNECTION IS FROM MGV TO MIDDLE LAYERS OF ACX. Gross anatomical observations, Nissl stains, and PV-immunohistochemistry all indicated that parts of the MGv and primary ACx were contained in the primary slice. Consistent with this, the Di-I tracing experiments in the primary slice showed major projections from the MG to cortical layers 3-4, the principal target zone of MGv axons (Caviness and Frost 1980; Romanski and LeDoux 1993; Willard and Ryugo 1983; reviewed in Winer 1992). Furthermore, stimulation of the MG-evoked responses in auditory cortex that were dominated by fast CSD sinks in layers 3-4. Together these data strongly support the hypothesis that the primary slice contains functional projections from MGv to middle layers of primary ACx, and that activation of these projections was the predominant cause of the observed responses. Additional support comes from the observation that similar middle layer responses could also be obtained from stimulation of the superior thalamic radiation (Fig. 2C) or "downstream" parts of the thalamocortical pathway (e.g., in the striatum; Fig. 7A1), but could not be evoked a mere 100-200 µM outside of the pathway (e.g., in the hippocampus, LG, or ventral-basal nucleus) even when these ineffective regions were closer to the cortex than the effective sites. Thus nonspecific activation, or spread of stimulation current to the cortex itself, is highly unlikely (Agmon and Connors 1991). It should be made clear that stimulation outside the MG (in primary slice experiments) was limited to preliminary characterization of the slices before formal data collection; responses described in RESULTS were exclusively evoked by stimulation of the MG proper or the border between the MG and superior thalamic radiation but never further downstream (see Metherate and Cruikshank 1999 for effective loci along the pathway).

SECONDARY INPUTS TO LAYERS 1 AND 5-6. In addition to the middle layer CSD sinks, the primary slices also sometimes had fast secondary sinks in the deep layers and/or at the surface of layer 1. Based on onset latency comparisons with the main sink, it was suggested in RESULTS that these secondary sinks might represent monosynaptic responses to direct thalamic inputs (LAMINAR RESPONSE PROFILE OF PRIMARY SLICE IS DOMINATED BY MIDDLE LAYER CSD SINK). Consistent with such direct inputs, Di-I application in the MG often resulted in labeled axons in layers 5-6 and occasionally in layer 1 (in addition to the middle layersdiscussed in the preceding text). Cells in the MGv could be the origin of some of these secondary inputs. For example, MGv projections are known to branch in the infragranular cortical layers around the junction between layers 5 and 6 (Romanski and LeDoux 1993; Willard and Ryugo 1983), and some reports also indicate a sparse projection to layer 1 (Cetas et al. 1999; Romanski and LeDoux 1993). Another possible source of the secondary inputs is the medial division of the MG (MGm), which was often part of the primary slice. The major targets of MGm are layers 1 and 5-6 (Herkenham 1980; Linke and Schwegler 2000; Ryugo and Killackey 1974; Willard and Ryugo 1983). Finally, the PPD might have contributed to the superficial input; it reportedly has robust layer 1 projections (Linke and Schwegler 2000) and was generally part of the bottom of the primary slice.

CORTICOTHALAMIC CELLS AND POSSIBLE NONSYNAPTIC ACTIVITY. Many infragranular pyramidal cells underwent retrograde labeling after Di-I application in the MG, indicating that there were corticothalamic projections within the slice. This is an interesting finding in its own right, suggesting that the preparation might provide a means to investigate corticothalamic transmission (Bartlett and Smith 1999; Castro-Alamancos and Calcagnotto 1999; Golshani et al. 1998; Kao and Coulter 1997). It also reveals the possibility that corticothalamic pyramidal cells could be antidromically activated by MG stimulation and thereby contribute to MG-evoked responses either directly or synaptically (via intracortical axon collaterals). However, Rose and Metherate (2001), using this same preparation, showed that the probability of antidromically activating infragranular cells with MG stimulation was <3% (2/69 cells). Rose and Metherate tested with 500-µA stimulus intensities, whereas intensities in the laminar experiments of the present study were limited 200 µA, presumably decreasing chances of antidromic activation even further. Along these lines, blockade of synaptic transmission in the present study (with low-calcium ACSF) profoundly suppressed evoked responses in all layers, indicating little antidromic or other nonsynaptic contributions. In fact, virtually no responses remained in the middle and superficial layers of any tested slices, providing a potentially interesting contrast with the somatosensory thalamocortical system (Agmon and Connors 1991). However, in two slices, the deep infragranular layers did maintain small narrow field potentials in low calcium; these could represent antidromic spikes, and if the cells that produced them were to have local collateral synapses, they might contribute further to the sinks in normal ACSF.

Intracellular recordings in layers 3-4 reveal that auditory thalamocortical transmission is mediated by both AMPA and NMDA receptors and that thalamic stimuli can initiate intracortical activity in vitro

LAYER 3-4 CELLS HAVE EARLY AND LATE INTRACELLULAR RESPONSES. Intracellular recordings were made from neurons located near the major sink and zone of thalamocortical termination in layers 3-4. This location was chosen to optimize chances of recording from cells that received direct thalamic input. There were two major temporal components to the MG-evoked responses in these cells that were referred to as the early and late responses. The early responses had short latencies and durations that generally matched the local field potentials and CSD sinks, suggesting they were part of the underlying basis of the sinks. They also had consistent shapes from trial to trial. In contrast, the late responses had long durations and were far less consistent from trial to trial, sometimes failing entirely. Early and late responses were further differentiated by their sensitivities to a number of manipulations designed to suppress polysynaptic activity; all of them depressed the late responses much more than the early responses, consistent with the late responses being predominantly polysynaptic intracortical phenomenon, and the early responses depending on fewer synaptic links. Exactly how many links are involved in the early response remains to be determined. The correlation between the onsets of the early PSP and the fast sink (and field potential) combined with the observed and known projections of the MGv to layers 3-4, suggests that the fastest parts of the early EPSPs are monosynaptic. However, it is clear that the IPSPs within the early response must be di- or polysynaptic (discussed in the following text). Experiments are underway to address the issue of synaptic order. As alluded to at the beginning of the DISCUSSION, the presence of long-lasting polysynaptic activity indicates that some functionally intact intracortical networks are preserved in the preparation, and that they can be activated by thalamic inputs. This broadens the utility of the primary slice preparation. For a more extensive discussion of these late responses, see Metherate and Cruikshank (1999).

EARLY RESPONSES WERE COMPOSED OF FAST AND SLOW EPSPS AND FAST IPSPS. The relatively "secure" early responses were themselves composed of three subcomponents. First, there was a fast AMPA/KA receptor-mediated EPSP, which typically had a weak conventional voltage dependence (Fig. 5C1) (Cox et al. 1992; Jones and Baughman 1988). A second component, present in most cells, was a fast IPSP. In cases where this IPSP was robust, it usually had a reversal potential between the resting membrane potential and spike threshold (Fig. 5A), consistent with a GABA_A receptor mechanism (Avoli 1986; Connors et al. 1988; Cox et al. 1992; Hefti and Smith 2000). On average, the IPSPs followed the onset of the fast EPSP by ~3 ms, enough time for at least one synaptic delay. This is supported by the observation that IPSPs were blocked by CNQX (see also Hablitz and Sutor 1990; Metherate and Ashe 1994); if the IPSPs had been the result of direct inhibitory projections from the thalamus, then antagonism of AMPA/KA receptors could not have blocked them. Instead, it is likely that excitatory thalamic projections drove cortical inhibitory interneurons to fire and those interneurons synapsed on the recorded cells, producing the IPSPs (discussed in Castro-Alamancos and Connors 1997; Douglas and Martin 1991; Porter et al. 2001). This provides additional confidence that the preparation behaves in the expected way for a thalamocortical system. The third major component of the early response was a slow NMDA receptor-mediated EPSP. The pharmacologically isolated slow EPSPs typically had nonconventional voltage dependencies, gradual rising and decay phases, and relatively late peaks, all of which distinguished them from the fast EPSPs. In some cases, the isolated slow EPSP also appeared to have a longer onset latency than the fast EPSP (e.g., Fig. 5A), suggesting the possibility that the recorded slow EPSPs may not have been monosynaptic.

TRANSMISSION AT THALAMOCORTICAL SYNAPSES APPEARS TO BE MEDIATED BY BOTH NMDA AND AMPA/KA RECEPTORS. The latency issue just mentioned appears to complicate the pharmacological data, and so some additional explanation will be presented regarding what these data might say about transmission at thalamocortical synapses. The basic protocol after recording control responses was to first apply CNQX alone, then CNQX + APV, followed by CNQX alone again, and finally control ACSF (order illustrated in Fig. 5C3). Importantly, it was found that combined application of CNQX + APV completely blocked the middle layer responses in both intracellular and extracellular recordings. This indicates, with near certainty, that transmission at the middle layer thalamocortical synapses is mediated by NMDA receptors or AMPA/KA receptors or both. Because CNQX by itself left a clear slow component that was reversibly blocked by APV, it is highly probable that part of the monosynaptic thalamocortical response is mediated by NMDA receptors, even if the recorded responses were not definitively monosynaptic (i.e., hence the sometimes longer latency in CNQX). This subtractive logic cannot be used to implicate AMPA/KA receptors because APV was never applied by itself. However, if one assumes that the fast EPSP in the middle layers is a monosynaptic event, then the virtually complete suppression of the fast EPSP by CNQX (RESULTS; Fig. 5) does implicate AMPA/KA receptors in transmission at thalamocortical synapses. Although it is difficult to prove that a response is monosynaptic, the fast EPSPs and the fast field potentials were recorded at the cortical focus, which in the pharmacological experiments was confirmed to be the position with the shortest latency middle layer field potential. Finally, other studies point to the involvement of AMPA/KA receptors in thalamocortical transmission (Armstrong-James et al. 1993; Gil and Amitai 1996a; Hagihara et al. 1988; Salt et al. 1995; reviewed in Castro-Alamancos and Connors 1997).

Differences in IPSPs and laminar profiles produced by MG vs. conventional stimulation; relationship to acoustic responses in vivo

Generally, the IPSPs produced by MG stimulation were weaker than expected based on previous ACx slice studies in which stimuli were delivered in a conventional fashion, to white matter/layer 6 below the recording site (on-beam) (Buonomano and Merzenich 1998; Metherate and Ashe 1994). The natural question that emerged was whether this was due to differences between the inputs activated by the two types of stimuli or if some other variable(s) produced the lower than expected inhibitory drive in the present study. For example, it is known that some aspects of cortical inhibitory systems are not fully developed in juvenile rodents (e.g., Dunning et al. 1999; Luhmann and Prince 1991; reviewed in Sutor and Luhmann 1995). In fact, this was indicated by the relatively modest cortical PV labeling observed here; PV is a marker for a class of cortical inhibitory neurons and is expressed at much higher levels in the adult mouse ACx (Cruikshank et al. 2001; del Rio et al. 1994). Another possibility was that the slicing procedure or some other aspect of the preparation compromised inhibition. To control for these potential preparation-dependent effects, we directly compared on-beam- and MG-evoked IPSPs within the same slices. On-beam stimulation generally produced robust IPSPs. Thus although the inhibition might not be fully developed in these juvenile slices, this did not preclude clear expression of inhibitory responses using conventional stimulation. In contrast, for the same neuron, MG-evoked IPSPs were weaker even when EPSP strengths were matched. These findings indicated that MG and on-beam stimuli do not activate the same sets of synapses in cortex. This conclusion was further supported by the observation that MG and on-beam stimuli evoked different laminar profiles. While the MG stimuli produced expected profiles with dominant middle layer CSD sinks, on-beam stimuli evoked profiles that were highly variable between slices and whose largest sinks rarely appeared in the middle layers. The laminar profiles undoubtedly depend on the origins and targets of the cells and axons near the stimulating electrodes; it is possible that these factors differ between slices more for the on-beam stimuli than the MG stimuli, contributing to the greater response variability.

It remains to be determined whether or not the cortical activation (e.g., IPSP magnitude and laminar profile) produced by MG stimulation in the primary slice represents a "natural" pattern, similar to that produced by acoustic stimulation in vivo. Some basis for optimism emerges by comparing the present results with the laminar patterns of click-evoked responses in vivo. Recordings from ACx of rodents and primates indicate that the fastest evoked CSD sinks occur most consistently in layers 3-4, and in some cases, about equally fast smaller sinks occur in layers 5-6, similar to the MG-evoked pattern here (Barth and Di 1990; Muller-Preuss and Mitzdorf 1984; Steinschneider et al. 1992). On the other hand, intracellular sound-evoked responses of ACx cells in vivo have clear IPSPs (de Ribaupierre et al. 1972; Metherate and Ashe 1996; Volkov and Galazyuk 1992), which may differ from the present results. However, it is difficult to compare IPSPs across studies because the degree to which they can be observed, and their amplitudes, depends heavily on methodology and criteria. These issues might best be resolved by combining in vivo and in vitro approaches within a single study e.g., in vivo recordings of responses evoked by both sound and MG stimulation, combined with in vitro thalamocortical experiments from the same brains.

Shell slice

SHELL NUCLEI OF THE AUDITORY THALAMUS ARE DISTINCT FROM MGV. The rodent auditory thalamus includes a constellation of nonprimary nuclei that surround the primary MGv in a shell-like fashion (Cruikshank et al. 2001). As indicated in the INTRODUCTION, these nonprimary nuclei are distinguished from the MGv by their distinct physiological and anatomical properties, including strong projections to layer 1 (Herkenham 1980; Linke and Schwegler 2000; Ryugo and Killackey 1974; Willard and Ryugo 1983; reviewed in Winer 1992). Few groups have investigated the functional effects of the nonprimary inputs to ACx (Weinberger et al. 1995b), but discoveries by Barth and colleagues are intriguing. They found that stimulation of one of the subdivisions (the posterior intralaminar nucleusthey refer to it as PIL, but we will use the more conventional PIN) induces high-frequency (gamma band) oscillations in ACx (Barth and MacDonald 1996; Sukov and Barth 2001). In addition, prolonged stimulation of PIN produced steady increases in cortical cell excitability, depolarization, and spiking. Based on laminar analysis, the inputs responsible for these effects were hypothesized to be on layer 1 dendrites (Sukov and Barth 1998). To investigate cellular mechanisms by which nonlemniscal systems regulate cortical activity, we began to look at the feasibility of obtaining a slice preparation with intact nonlemniscal projections, and so the shell slice was developed.

MANY FEATURES OF THE SHELL SLICE SUGGEST AN INTACT PROJECTION FROM PPD TO LAYER 1. The shell slice was taken from the region of the brain immediately below the primary slice and, in the majority of cases, appeared to contain the PPD and nonprimary auditory cortex. These two regions have been shown to be interconnected via projections from PPD to layer 1 (Linke and Schwegler 2000). Stimulation of the region in or near the PPD (referred to as the shell region) produced a dominant sink at the surface of layer 1 in nearly every case. That sink was mediated by synaptic transmission as indicated by the effects of low calcium and glutamate antagonists. The latency of the surface sink matched the fast intracellular PSPs recorded from middle layer cells, suggesting that the PSPs might be produced by synapses in layer 1 on apical dendrites of the recorded cells. Consistent with this scenario, application of tracer in the shell stimulation region produced relatively dense fiber labeling in superficial layer 1 and very little in layers 3-4. Shell stimulation also produced long-duration late responses with large fluctuations. The late responses appeared to be induced more easily with shell stimulation than with MG stimulation (in the primary slice). In a previous study, we showed that virtually identical late potentials had fluctuations with significant power in the gamma band (Metherate and Cruikshank 1999). These observations are broadly consistent with the finding by Barth and colleagues that stimuli presumed to activate nonlemniscal pathways evoke cortical gamma band activity whereas lemniscal stimulation does not (Barth and MacDonald 1996). In summary, the data from the shell slice reviewed thus far support the hypothesis that the preparation contains a nonlemniscal pathway from PPD to layer 1 of temporal cortex, and that stimulation of this pathway caused the observed responses.

SOME OR ALL OF THE SHELL SLICE RESPONSES MIGHT BE MEDIATED BY AXON COLLATERALS OF ANTIDROMICALLY ACTIVATED CORTICOTECTAL CELLS. Although about two-thirds of the shell slices contained at least part of the PPD, the other one-third appeared to be located more ventrally, entirely below any of the auditory thalamic nuclei (ANATOMICAL FEATURES OF THE SHELL SLICE), indicating that the presence of these nuclei were not required for shell-type responses. In addition, the effective stimulation sites, and the subcortical components labeled after cortical Di-I applications, mostly involved a narrow white matter tract rather than auditory thalamic nuclei (see preceding text). Finally, there was a low incidence of retrograde filling of thalamic somata after cortical Di-I application. These results hinted that the nonprimary auditory thalamic nuclei may not be a major source of cortical input within the shell slice, leading to a search for an alternative explanation. The high incidence of retrograde-filled layer 5 pyramidal cells seen after dye application at the shell stimulation site provided a clue: because they were labeled, their axons must have passed through the stimulation site (Clerici and Coleman 1990; Rouiller and Welker 1991). Given this, it seemed likely that shell stimuli could activate these cells antidromically. If they also had axon collaterals to layer 1, they might contribute to the surface sink. Such a mechanism would be consistent with the conspicuous nonsynaptic field potentials in the lower and middle layers of the shell slice; the lower layer component could result from somatic spikes and the middle layers potentials could be produced by depolarization and/or spiking in proximal apical dendrites (Stuart and Sakmann 1994). Along these lines, the single infragranular cell recorded in this study appeared to display antidromic spiking. The laminar position of the back-labeled cells suggested that they might project to the tectum, so control tracing experiments were conducted in which Di-I was placed in the inferior colliculus to determine whether or not axons of auditory corticotectal cells passed through the shell stimulation area and if these cells had collateral branches to layer 1. Both questions were answered positively; therefore it is possible that this indirect mode of activation contributed to the layer 1 sink.

Conclusions

In summary, the primary slice is an in vitro preparation containing functionally intact portions of the primary auditory thalamocortical system. Just as the analogous somatosensory thalamocortical slice has proven an invaluable tool for elucidating fundamental mechanisms in that system (see INTRODUCTION), so too should the primary slice for the auditory thalamocortical system. In fact, this has already begun with the present work (cf., the pharmacological and laminar results) and with previously published studies conducted in parallel (Hsieh et al. 2000; Metherate and Cruikshank 1999; Rose and Metherate 2001). In contrast, the shell slice may constitute a "nonprimary" in vitro system. This possibility was supported by a reasonable body of evidence, although a plausible alternative interpretation was presented. Thus at present it should be used with caution. At any rate, the shell slice does appear to provide a novel way of selectively activating layer 1 synapses with extrinsic stimulation (Cauller and Connors 1994; Hess and Donoghue 1999). In closing, it should be mentioned that there may be an alternative to the shell slice for activating a nonprimary pathway in vitro. This would involve stimulation of the MGm within the primary slice. The MGm was often included within the primary slice. Assuming its projections to the ACx are intact, this preparation would have the added asset of containing both primary and nonprimary systems within the same slice, allowing for the study of their interaction.