INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

The perirhinal cortex (PC) is thought to be a crucial part of the medial temporal lobe declarative memory system, not only as a relay point for the information stream from the sensory cortices to the hippocampal formation, but also as an associative area for many kinds of information. In the hippocampal memory system (Squire and Zola-Morgan 1991), the pathway from the PC to the entorhinal cortex (EC) is regarded as one of the main paths into the entorhinal-hippocampal network, which is crucially involved in memory formation (Burwell et al. 1995; Suzuki 1996; Witter 1993). The significance of the PC in memory processing has been tested by assessing the behavioral effects of specific lesions (Corodimas and LeDoux 1995; Meunier et al. 1993; Wiig and Bilkey 1995). In the rat, the PC receives inputs from somatosensory, auditory, visual, and olfactory association areas (Burwell 2001; Burwell et al. 1995; Suzuki 1996). The PC projects robustly to the lateral EC (LEC), although these afferents provide 16% of its cortical input and are thus relatively less strong in the rat than they are in the monkey (Burwell and Amaral 1998a,b). Notwithstanding these rather elaborate studies of the structure and significance of the PC, only a few studies have addressed the connections of the PC with the entorhinal-hippocampal system, including one using electrophysiological methods in the rat in vivo (Naber et al. 1999) and two in guinea pig isolated whole brain (Biella et al. 2000, 2001).

The PC is also thought to be involved in emotional memory, in view of the prominent interconnections of the PC with the amygdala, which is considered to be an emotion-related area (Cahill et al. 1995; LeDoux 1987; Ono et al. 1995). In monkeys, the polar portion of the PC has powerful and reciprocal connections with a number of amygdaloid nuclei, including the lateral, basal, and accessory basal nuclei (Suzuki 1996). In rats, the PC has its strongest interconnections with the lateral nucleus, although minor reciprocal connections with the accessory basal nucleus have also been described (Burwell et al. 1995; Krettek and Price 1977a,b; Suzuki 1996). Herzog and Otto (1998) found that lesions limited to the PC attenuated the expression of fear related to an olfactory conditioning stimulus, but not to the training context, and suggested that the PC is a critically important component of the neural system, mediating the acquisition or expression of associations between olfactory and foot-shock inputs. Likewise, lesions of the PC interfere with the expression of conditioned fear responses to visual (Rosen et al. 1992), auditory (Campeau and Davis 1995), and contextual stimuli (Corodimas and LeDoux 1995). Moreover, in electrophysiological experiments in anesthetized cats, Collins et al. (2001) recorded spontaneous activity dominated by a slow focal oscillation at 1 Hz onto which a faster rhythm (approximately 30 Hz) was superimposed in both the PC and the amygdala; they suggested that the PC and the amygdala are functionally coupled. Despite the many efforts to study the relationship between the PC and amygdala, the functional organization between the two is still poorly understood. To investigate functional organization of the brain, optical imaging of neural activity with a voltage-sensitive dye is a powerful technique. By using voltage-sensitive dyes, neural activation and pathways in brain slices were successfully monitored (Barish et al. 1996; Grinvald et al. 1982; Iijima et al. 1996; Jackson and Scharfman 1996; Tanifuji et al. 1994; Tominaga et al. 2000; Yuste et al. 1997). Here, we describe a functional connection between the PC and the entorhinal-hippocampal circuitry, more specifically, the influence of amygdala stimulation on perirhinal-entorhinal neural propagation. For this study, we obtained rat brain slices, including the PC, EC, hippocampal formation, and lateral amygdaloid nucleus (AMG). These sections were used for in vitro experiments, in which we used an optical imaging technique with a voltage-sensitive dye to analyze the spatiotemporal distribution of excitatory activity elicited by separate and combined stimulation to the PC and amygdala.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Slice preparation and solutions

Male Wistar rats (100-160g) were deeply anesthetized by ether and decapitated. Slices (400-µm thickness) were prepared using a vibratome (DTK-3000W, Dosaka, Japan) in ice-cold sucrose artificial cerebrospinal fluid (s-ACSF) (Moyer and Brown 1998). The s-ACSF was composed of (in mM) 248 sucrose, 5 KCl, 1.25 NaH₂PO₄, 2 MgSO₄, 1 CaCl₂, 1 MgCl₂, 22 NaHCO₃, and 10 D-glucose, and oxygenated with a mixture of 95% O₂-5% CO₂, pH 7.4, chilled to 4°C. The slices including the PC, EC, hippocampal formation, and AMG were cut at an angle indicated by line h shown in Fig. 1A. For this, the hemispheres were separated and their dorsal part removed by razor cuts parallel to line h, and each hemisphere was glued with its dorsal cut surface to a vibratome stage. Figure 1C shows an example of a Nissl-stained slice cut at the level of line h. With the use of such slices, we investigated propagation of neural activity following stimulation of perirhinal layers II/III and of amygdala. The known anatomical neural connectivity in such slices is illustrated as a simplified block diagram in Fig. 1B. In some experiments we used slices cut at approximately 45°, as indicated by line a in Fig. 1A. To prepare such slices, first the rostral and caudal parts of the hemisphere were removed by razor cuts parallel to line a, and the remaining portion of the hemisphere was glued with its rostral cut surface to a vibratome stage. An example of a Nissl-stained slice cut at this angle is shown in Fig. 1D. Such angled slices have the benefit that the perirhinal-entorhinal border can be easily identified. We used also this type of slice for investigating the perirhinal-entorhinal pattern of spreading activity and to verify whether the presence of a physiological connection between perirhinal and entorhinal cortices depended on the angulation of the slices. With the use of the lipophylic tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) (Gelperin and Flores 1997; Godement et al. 1987), we verified the existence of connectivity between the perirhinal and entorhinal cortices in slices cut at angles h and a, and between amygdala and PC in slices cut at angle h.

View larger version (71K): [in this window] [in a new window]   Fig. 1. Preparation of rat brain slices and relationship between electrical and optical signals. A: lateral view of the rat brain indicating the position of the perirhinal cortex (areas 35 and 36) to either side of the rhinal sulcus. Approximate orientations of the in vitro slices are illustrated by h and a lines, respectively. B: simplified block diagram of the known neural connectivity of the rat perirhinal cortex (PC). C: Nissl-stained slice cut at angle h in A: this type of slice includes the hippocampus, entorhinal cortex (EC), PC, and lateral amygdaloid nucleus (AMG). MEC, medial entorhinal cortex; LEC, lateral entorhinal cortex. The rhinal sulcus (rs) indicates the border between the entorhinal and perirhinal cortices. The optical recording was performed in the area indicated by the rectangle. D: Nissl-stained slice cut at angle a in A: the boundary of the PC and LEC can be distinguished easily. By use of these types of slices, spread of excitatory neural activity from the PC to the EC was investigated. E: relationship between electrical and optical signals evoked by perirhinal stimulation in a slice. Traces elicited by various stimulus intensities were measured in layers II/III of the PC near the rs.

Before the start of recording, slices were submerged in the incubation chamber with standard ACSF for more than 1 h. The standard ACSF used for incubation and recording consists of (in mM) 124 NaCl, 5 KCl, 1.25 NaH₂PO₄, 2 MgSO₄, 2 CaCl₂, 22 NaHCO₃, and 10 D-glucose, oxygenated with a mixture of 95% O₂-5% CO₂, pH 7.4. The ACSF was maintained at room temperature (26-28°C).

Optical recording system and staining with voltage-sensitive dye

The optical recording methods were similar to those reported elsewhere (Barish et al. 1996; Iijima et al. 1996). For recording, a slice was submerged in a recording chamber under the tandem type microscope, which was constructed in our laboratory (Iijima et al. 1996; Takashima et al. 1999). We used a 50-mm Nikon camera lens (F1.25) as the first objective and a 105-mm camera lens (F1.85) for the second objective, which altogether results in an approximately twofold magnification. This optical apparatus allowed us to record from approximately a 5 × 5-mm area of the brain slice. The slice was stained in the recording chamber for 3 min with the voltage-sensitive dye RH-795 (0.5 mg/ml ACSF). The voltage-sensitive dye has excitation and emission maxima at 530 and 712 nm, respectively. Changes in fluorescence of the dye decrease proportionally to the changes of membrane depolarization. After the staining, extra dye was washed out with oxygenated superfusion solution and the preparation was incubated in the recording chamber for another 10-15 min before the optical measurements were performed. The stimulus-evoked responses were recorded as fractional changes of fluorescence by the image processor with the use of a metal-oxide semiconductor (MOS)-based solid-state camera developed in our laboratory (Ichikawa et al. 1993). Illumination from a tungsten-halogen lamp was passed through a heat filter (<10% transmittance for 700-1,600 nm light). The excitation light filtered at 535 nm (±20 nm band-pass) was reflected down onto the preparations by a dichroic mirror (half-reflectance wave length of 580 nm). The fluorescence from the preparation was projected to the image sensor through a long-wavelength pass filter (50% transmittance at 600 nm). The fractional changes were detected and stored in the frame memory of the imaging system connected to a PC/AT computer. The resolution of the MOS-image sensor was 128 × 128 pixels. To improve the signal-to-noise ratio, we binned 2 × 2 pixels into 1 pixel, and thus images are represented as 64 × 64 pixels. By using this optical imaging system, we acquired the 500 or 1,000 frames by the rate of 0.6, 1.2, or 2.4 ms/frame. To represent the spread of neural activity, we superimposed color-coded optical signals on the bright-field image of the slice. In this procedure, we applied a color code to the fraction of the optical signal, which exceeded the baseline noise. That is, optical signals whose sizes were close to the baseline noise were ignored. For reducing the baseline noise, we sometimes averaged four identical recordings acquired with a 5-s interval. The data set was averaged in the frame memory directly. The optical signal was analyzed off-line using custom-made software or Origin software (OriginLab Corp.). Recordings were made in a total of 58 slices.

Field potential recording

A glass recording electrode filled with normal medium (5-10 M) was positioned in the part of perirhinal layers II/III under the rhinal sulcus. The field potential was filtered at 1 kHz low-pass filter, amplified (MEZ-8300, Nihonkoden, Japan), and digitized with the use of the optical imaging system through its external input terminal. The field potential was continuously monitored throughout the experiment, and no photodynamic effect on its amplitude was observed by the optical recording.

Electrical stimulation to the PC and the amygdala

The stimulating electrode was a tungsten bipolar electrode with a tip separation of 150 µm. Since anatomical studies indicate that layers II/III of the PC receive projections from sensory areas (Burwell and Amaral 1998b; Faulkner and Brown 1999), we placed the stimulus electrode for the PC on layers II/III. Initially, a single-pulse electrical stimulation was applied. To mimic input from the sensory cortices to the PC in a brain slice, we stimulated an extensive area of layers II/III of PC using a stimulus intensity that evokes maximum response in PC. In Fig. 1C, electrical and optical signals extracted from layers II/III of PC are shown following different stimulus intensities ranging from 40 to 100 µA. The stimulus intensity and duration that evoked the maximum response in layers II/III of the PC was 80-100 µA and 300 µs, respectively. This set of parameters was chosen for all subsequent experiments. Similarly, we established the optimal protocol for amygdala stimulation. We needed to stimulate the amygdala repeatedly (40 Hz) with 100-150 µA for 300 µs.

Drugs

The entorhinal cortex has been reported to harbor many more cells containing the calcium-binding protein parvalbumin than is the case for the PC. Since it has been well established in these areas that parvalbumine colocalize for 100% with GABA, it is likely that, in the EC, more GABA-containing cells will be present compared with the PC (Burwell et al. 1995; Miettinen et al. 1996). To investigate the spatiotemporal distribution of excitatory activities between the PC and the entorhinal-hippocampal circuit, synaptic inhibition was partly suppressed by applying a low concentration of the GABA_A antagonist bicuculline (1-2 µM) (()-bicuculline methiodide, Sigma-Aldrich Co.) in the recording solution. Under such conditions, our optical imaging system has already measured the entorhinal-hippocampal excitatory neural spread elicited by the electrical stimulation to the LEC in rat brain slices without inducing spontaneous paroxysmal activities (Iijima et al. 1996). Under similar conditions, it is expected that the neural spread of excitatory activities from the PC to the EC might be also observed following electrical stimulation to the PC. In some experiments, the neural spreads of excitatory activities from the perirhinal to the entorhinal cortices were investigated by increasing the concentration of bicuculline (5 and 10 µM). Under 10 µM concentration, spontaneous synchronous excitatory activities were recorded by glass electrode, whereas, with 5 µM of bicuculline, no spontaneous activity was recorded. We measured the optical signals related to evoked response only in those conditions during which no spontaneous activity was recorded by the glass electrode.

Histology

The brain slices were fixed with 4% paraformaldehyde for more than 1 wk. Subsequently, the slices were dipped in PBS with 30% sucrose for more than 10 h and cut at 70-µm thickness with the use of a freezing microtome (SM-2000R, Leicamicrosystems). Sections were Nissl stained by 0.25% thionin solution. Figure 1, C and D shows the histological result. The areas of the perirhinal and entorhinal cortices were delineated on the bases of cytoarchitectural criteria (Burwell et al. 1995). By comparing these histological data with the optical imaging data, we could identify the area in which neural propagation was recorded.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Associative stimuli to the PC and the amygdala

Initial data were collected from 23 slices kept in ACSF with 1-2 µM bicuculline. A representative experiment is illustrated in Fig. 2. The traces in Fig. 2A represent changes in the optical signal over time, recorded from selected points indicated in Fig. 2B (AMG, PC, MEC; medial entorhinal cortex, DG; dentate gyrus). Figure 2C shows enlarged traces of perirhinal layers II/III and area 35 in Fig. 2A. In the case of a single stimulus in the perirhinal cortex (indicated in Fig. 2A, left column), a response in part of the perirhinal cortex was evoked 5 ms after the stimulation, and the amplitude of the response became maximal at 40 ms. In this case, no changes in the optical signal were observed in the medial EC or DG, but a small response was observed in the amygdala. Figure 2A (middle column) shows the responses evoked by repetitive stimulation of the amygdala (5 pulses at 40 Hz; indicated in the lowest time chart). The activity in the amygdala increased during the stimulation period. Activity in the deep layers of the PC was evoked 10 ms after the stimulation was applied and increased slowly until 65 ms. As with perirhinal stimulation, amygdala stimulation did not evoke responses in the medial EC or DG. When the PC and the amygdala were stimulated simultaneously, however, responses corresponding to a 0.02% fractional change in the medial EC and DG were evoked (Fig. 2A, right column). Note that, in this case, the trace for the deep layers of area 35 in the PC indicates enhanced neural activity (red asterisk in the red trace), which was caused by the association of both the amygdala and PC stimuli, whereas the responses in layers II/III of area 36 were suppressed (red arrow in the red trace) compared with the result of the amygdala stimulation (black arrow). The respective latencies of the changes in activity in the medial EC and DG were 100.8 and 104.8 ms after the stimulation was applied.

View larger version (63K): [in this window] [in a new window]   Fig. 2. Optical recording of associative inputs in the PC. A: matrix of optical signals indicating the responses elicited by stimulation of the PC, AMG, or both areas. For the amygdala stimulation, repeated electrical stimulation (5 pulses at 40 Hz) was applied as indicated below. Associative stimulation of both the PC and the amygdala resulted in a (asterisk) response in the deep layers of the perirhinal cortex (area 35), while there was no similar response in layers II/III (parallel between black and red arrows). B: schematic drawing of a brain slice showing the AMG, perirhinal layers II/III (PC II/III), area 35 of perirhinal cortex, MEC, and dentate gyrus (DG). C: initial phase of the depolarization on an expanded time scale. Left and right traces were extracted from perirhinal layers II/III and perirhinal area 35 shown in B, respectively. Red traces indicate the responses elicited by associative stimulation of both the PC and the AMG. Blue and green traces correspond to the responses elicited by the stimulation of PC or AMG alone, respectively. D: series of selected images taken at different time intervals after the electrical stimulation (time indicated below each image). Depolarization is measured as the fractional changes in fluorescence in each pixel, this value is encoded in "pseudocolor " as indicated in the scale and is superimposed on a bright-field image of the slice. a: neural propagation pattern elicited by a single-pulse stimulation to layers II/III in the PC. b: result of AMG stimulation (5 pulses at 40 Hz). c: result of associative stimulation to both the PC and AMG. Single pulse stimulation of the PC with 5-pulse stimulation at 40 Hz evoked entorhinal-hippocampal neural activation.

Real-time imaging of these results is shown in Fig. 2D. The evoked responses in perirhinal layers II/III elicited by the perirhinal stimulation increase until the image acquired at 48 ms. Subsequently, the neural responses decrease (Fig. 2Da). In the middle row (Fig. 2Db), the activity in the perirhinal cortex, evoked by the amygdala stimulation, is maximal in the 60-ms frame. The neural responses decrease after 60 ms, despite applying the stimulus at 40 Hz continuously. In the results shown in (Fig. 2D, a and b), there are fractional changes of about 0.01% in the deep layers part on the border of the perirhinal-entorhinal cortices, but no responses are observed in the medial EC or hippocampal formation. In Fig. 2Dc, we illustrate the effect on the perirhinal-entorhinal-hippocampal network of combined stimulation of both the amygdala and layers II/III of the PC. Combined stimulation evokes much greater signal changes in the deep layers than when only one site is stimulated, especially in area 35 at 48 and 60 ms, although the responses in layers II/III of the PC are less than in the images at 60 and 96 ms following amygdala stimulation. The activity in the deep layers of area 35 is followed by responses in all layers of the LEC at 60 ms. This evoked activity appears to propagate medially (to 96 ms), and responses are observed in the DG of the hippocampal formation at 132 ms. Similar observations were obtained in 13 of 23 slices. Similar experiments were carried out in 6 slices without bicuculline. Although the extent of the spread of activity in the horizontal and vertical directions in the PC was not strongly affected, evoked potentials were shorter and of smaller amplitude compared with the 1-2 µM bicuculline condition. Without bicuculline, combined stimulation produced similar effects in area 35 and adjacent parts of the LEC, although extensive medial spread of excitation followed by evoked hippocampal activities was not observed (not illustrated).

Responses elicited by repetitive stimulus

Since it is generally accepted that the PC is one of the major sources of multimodal cortical input for the hippocampal system, the finding that single stimulation of the PC does not evoke activity in either the EC or the hippocampal formation merits further investigation. In 21 slices bathed in ACSF with 1-2 µM bicuculline, we tested whether repetitive stimulation of the PC leads to activation of the EC. This was combined with repetitive amygdala stimulation. A representative experiment is illustrated in Fig. 3, which shows that no entorhinal-hippocampal neural activity was evoked under all experimental conditions despite repeated stimulation of the PC or the amygdala. In Fig. 3A, the responses elicited by a stimulus electrode in the superficial layers of the PC are mainly propagated in the rostrocaudal direction within the stimulated superficial layers until 19.2 ms. The activity can be seen to spread to the deep layers of the PC in the images taken from 19.2 to 28.8 ms. Responses encoded in red, which represents the peak value of the fractional change in this acquisition, are prominent in layers II/III at 43.2 ms. Irrespective of the stimulation protocol used, the overall features of the observed pattern of perirhinal activation were similar (Fig. 3A, b and c). As seen from Fig. 3C, top, repetitive stimulation did not lead to marked increases in the amplitude of the neural response in layers II (no increase) or V (approximately 10% increase), but the onset of recorded activation was delayed slightly. The evoked neural responses reached the steady state during acquisition, and no seizure activity developed in any case. In the three experimental situations, we never observed activity in the EC or hippocampal formation, even when we applied repetitive stimulation at 40 Hz (Fig. 3C, top, LEC trace).

View larger version (80K): [in this window] [in a new window]   Fig. 3. Optical recording of perirhinal neural activity elicited by stimulation of the PC or AMG alone. These results were obtained from the same slice shown in Fig. 2. The recording area and stimulation site are shown in Fig. 2B. A: time-series of images illustrating the distribution of activity evoked by electrical stimulation of the PC with 1, 5, and 10 pulses at 40 Hz. The time after the electrical stimulation is indicated below each image. B: time-series of images illustrating the distribution of activity evoked by electrical stimulation to the AMG with 1 and 5 pulses at 40 Hz. C: time course of the optical signals extracted from part of perirhinal layers II/III (PC II/III), perirhinal layers V/VI (PC V/VI), the LEC, and the AMG. The top and bottom traces show the results of PC and AMG stimulus experiments, respectively. The stimulus timing is shown above each set of traces. In every case, there is no neural activity in the EC or hippocampus. We applied a low concentration of bicuculline (2 µM); we observed seizure activities in none of the conditions.

Figure 3B shows the propagation pattern after the amygdala stimulation. Figure 3B, a and b shows that both single and repetitive stimulation resulted in a restricted activation increase in a single spot in the deep layers of the PC, with subsequent activation of the superficial layers. This pattern is fairly stable up until 24 ms. After this time, neural activation propagates to an extensive area of the PC, from the deep layers to the superficial layers. Comparing the repetitive stimulation image at 38.4 ms with the single-pulse result, the neural responses are increased, especially in the deep layer and middle layer parts of the PC, which show much higher responses. The depolarization of layers II/III reached a peak at 67.2 ms. With amygdala stimulation, we recorded the same or higher amplitudes of neural activity in the PC than following perirhinal stimulation. Figure 3C (bottom) shows examples of optical signal traces extracted from layers II/III, layers V/VI of PC, the lateral entorhinal cortex (LEC), and the amygdala (AMG). The neural activity in the amygdala increases during periods of repetitive stimulation. In the PC, although repetitive stimuli yield larger amplitude responses compared with single- pulse stimulation, the profiles of traces following 5- and 10-pulse stimulation are very similar. Despite this increased activity, we never observed lasting depolarization (total of 21 slices). Based on these results, the 5-pulse stimulus at 40 Hz was considered the optimal stimulus for perirhinal activation.

Stimulation of the deep layers of the PC near the perirhinal-entorhinal border

The results of single stimulation of the PC with or without costimulation of the amygdala (Fig. 2) indicate that sufficient neural activation in the deep layers near the perirhinal-entorhinal border is essential for subsequent activation of the LEC. In a subsequent series of experiments, we analyzed the relevance of activation in the deep layers of the PC in more detail. Figure 4 illustrates that activation of the deep layers of area 35 in the PC is important for the neural spread from the perirhinal to the entorhinal cortices. The slice was cut at the angle shown in Fig. 1A, parallel to a. To elucidate the physiological connections from the perirhinal to the entorhinal cortices, we initially cut slices at various angles. By cutting at angle a, we obtain brain slices in which the connectivity of interest is maintained and in which the boundary of the PC and the LEC can be distinguished easily (Burwell et al. 1995). A Nissl-stained section is shown in Fig. 4B. The perirhinal area was divided into areas 35 and 36 based on differences in cytoarchitecture. Optical measurements were performed in the rectangular area shown in this figure. We applied bicuculline at a concentration of 2 µM. The neural responses elicited by stimulation of the superficial, middle, or deep layer parts of area 36, at the border with area 35, were measured. Repetitive stimulation at 40 Hz to the superficial or middle layers evoked neural activity that propagated to an extensive area of the PC and temporal cortex, but neural activity was not initiated in the LEC (Fig. 4A, a and b). A single-pulse stimulation to the deep layers evoked neural activity in area 35 and a small response reached the area-35/LEC border (24-ms image in Fig. 4Ac). Repetitive stimulation of the deep layers caused much higher responses in area 35 of the PC and in the LEC near the perirhinal-entorhinal border (72-ms image in Fig. 4Ad), spreading in the direction of the medial entorhinal cortex (Fig. 4Ad, 72-216 ms). Figure 4B illustrates the time course of optical signals reflecting the imaging results shown in the two lower rows in Fig. 4A, i.e., single and repetitive stimulation of the deep layers of PC . The traces extracted from area 35 in the PC indicate that the neural response evoked by repetitive stimulation in this area increased slowly (approximately 150 ms), until the amplitude was approximately doubled (shown as the dotted area in Fig. 4B2). This increasing depolarization of area 35 propagated medially into the entorhinal cortex. This set of experiments was performed on 28 slices. The results obtained in slices taken at different rostral to caudal levels all had similar features (we did not use slices including the postrhinal cortex). Moreover, we obtained similar results when slices were recorded in the presence of 5 µM bicuculline (n = 20).

View larger version (82K): [in this window] [in a new window]   Fig. 4. Optical recordings showing that stimulation of the perirhinal deep layers is effective for neural propagation to the EC from the PC. A: time series of images illustrating the distribution of activity in the different stimulation conditions. The brain slice cut along the a line in Fig. 1A was used. The optical recording was made in the area enclosed by the dotted rectangle on the Nissl-stained slice in Fig. 4B. The stimulus electrode was placed at the superficial, middle, and deep layers of the PC. The stimulus site and stimulus frequency are indicated above each series. Blue dots on the first images (0 ms image) mark the stimulation sites of the bipolar electrode. On some images, the morphological features were superimposed. Perirhinal-entorhinal neural spread was only observed when deep layers were stimulated repetitively; single-pulse stimulation was not effective. B: time course of optical signals elicited by stimulation of the deep layers of the PC. In the traces extracted from area 35, the depolarization was increased slowly by repetitive stimulation (shown by the dotted circle on the red trace). These data were obtained with the low concentration of bicuculline (2 µM). TE, temporal cortex.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

This study examined the propagation of perirhinal-evoked activity into the entorhinal cortex and the associative effect of amygdala input. The main findings can be summarized as follows. First, propagation of activity from the perirhinal to the entorhinal cortices requires sufficient neural activation in the deep layers near the perirhinal-entorhinal border (area 35). Second, stimulation of the superficial part of the PC, which receives many projections from other cortical areas, alone does not evoke sufficient activity to initiate the entorhinal response from area 35. Third, the perirhinal deep layers can be activated by repeated stimulus of the AMG and combined stimulation of both the PC and the amygdala evoked much higher depolarization in the deep layers of the area 35, leading to initiation of propagation from the PC into the entorhinal-hippocampal circuit.

Rat perirhinal and entorhinal connections have been reported in detail (Burwell and Amaral 1998a; Burwell et al. 1995; Naber et al. 1997). On the other hand, the physiological correlates of such connectivity have not been investigated adequately. This study used optical recordings to obtain data about the physiological characteristics of the neural connectivity between the PC and the EC. It has been suggested that area 35 of the PC projects strongly to the lateral part of the EC and that these projections to the EC originate in layers III and V and terminate preferentially in layers II and III (Burwell and Amaral 1998a; Burwell et al. 1995). Naber et al. (1997) reported that anterogradely labeled fibers from the PC are widely distributed in the LEC, in both the superficial and deep layers. In addition, they indicated that injections that selectively involved the deep layers of the PC produced more labeled fibers in the LEC than injections in superficial layers. Our results indicate that the perirhinal deep layers are essential for the propagation of excitatory activity from the PC to the LEC. Moreover, our finding that only stimulation to the deep layers evoked sufficient neural activity in area 35 to initiate perirhinal-entorhinal excitation propagation supports the anatomical findings.

In our experiment, inhibition was partly suppressed with the application of a low concentration of bicuculline. However, even under such conditions, neural activity in the LEC was not evoked by either perirhinal superficial layers stimulation or amygdala stimulation alone. If the anatomical organization of the perirhinal-to-entorhinal connections is taken into consideration, the following questions arise. First, was the angle of the slice that we used suitable? Our results were obtained using several differently angled slices [horizontal and 45° slices (this study), and coronal slices (Iijima et al. 1994)]. Irrespective of the angle of the slice, comparable observations were collected. Therefore the angle of the slice does not appear to be critical. Second, are sufficient projection fibers to the EC included in a slice 400-µm thick? Although it cannot be denied that some projection fibers to the LEC will be severed or removed in such a slice preparation, anatomical data indicate that such connections will be sufficiently preserved (Burwell and Amaral 1998a; Naber et al. 1999). In fact, the preservation of the relevant circuitry has been confirmed by us using DiI-crystal placement in relevant parts of the slice. Moreover, recent experiments using preparations of the guinea pig whole brain (Biella et al. 2000, 2001) also demonstrated very weak direct perirhinal-to-entorhinal connectivity with physiological methods, and the connections that were observed are mediated by polysynaptic circuits. In that study, stimulation of area 36 induced activation within area 36, but not in the EC. In addition, we obtained similar data in a preliminary study using an optical imaging technique with the guinea pig whole brain. These observations strongly suggest that our results concerning the difficulty of activating the EC by perirhinal stimulation is not specific to brain slices. These considerations suggest that the neural architecture in the perirhinal-entorhinal border area blocks propagation from the PC to the EC. This blockade could involve a strong inhibitory system in area 35, because a high concentration of bicuculline appeared to induce neural spread from the PC to the LEC (data not shown). In addition, the results reported here (Fig. 4B, dotted circle in the red trace) seem to indicate that neurons in area 35 are not easily depolarized to the level that causes propagation of activity from the perirhinal to the entorhinal cortices. This feature might be the result of a polysynaptic structural feature or a synaptic delay resulting from the membrane electrical properties of neurons in this area. Perirhinal layers II/III (Beggs et al. 2000; Faulkner and Brown 1999), VI (Faulkner and Brown 1999), and V (Moyer et al. 2000) contain many late-spiking neurons, which characteristically possess slowly inactivating potassium channels. Such a neuron might have delayed and sustained firing properties in response to long synaptic trains. It has been suggested that these cells show prolonged synaptic integration and are involved in tonic neural activation measured in rat perirhinal neurons during the delay period in an odor-guided, delayed nonmatching-to-sample task (Young et al. 1997).

Another explanation for this apparent perirhinal-entorhinal gating mechanism is that neurons in the PC have a structural feature particularly suitable to associate incoming inputs from different origins. Indeed, the PC is thought to be a zone of convergence from higher-order sensory association areas and a number of different subcortical structures (Suzuki 1996). That is, perirhinal cells that project to EC and/or their presynaptic cells in PC should receive input from many areas in the brain. In other words, stimulation of a restricted perirhinal area might leave many nondepolarized fibers. In fact, we showed that stimulation of an extensive area of perirhinal layers II/III at maximum stimulus intensity (Fig. 3A) or stimulation of two sites in perirhinal layers II/III (data not shown) did not induce sufficient neural activity in the deep layers for propagation to the EC to occur. In contrast, perirhinal stimulation associated with repetitive stimulation of the amygdala evoked a much stronger response in the deep layer part of the perirhinal-entorhinal border than perirhinal stimulation alone. These results suggest that some neurons in the deep layers that were evoked by perirhinal-amygdala associative stimulation were not evoked by perirhinal stimulation alone.

Stimulation of amygdala by itself undoubtedly evoked neural responses in the PC (Fig. 3B). This result confirms the anatomical observation that fibers from the amygdala terminate in perirhinal layers V and III (Krettek and Price 1977b). The responses observed in layers II/III should result from complex monosynaptic responses of the amygdala and polysynaptic responses in the PC. Our results also showed that repetitive stimulation evoked much higher depolarization (approximately 40% increase) in perirhinal neurons in the deep and superficial layers. Lisman (1997) suggested that the best stimulus for exciting a cell (that is, a neural code) consists of brief bursts of high-frequency firing. However, excess repetitive stimuli were not effective to result in neural depolarization in perirhinal neurons. Consequently, as the maximum stimulus for our experiments, we used 5 pulses at 40 Hz to stimulate the amygdala. In our experiments, continuous stimuli to the PC or the amygdala were not effective in producing long-lasting perirhinal depolarization. In addition, we showed that associative stimuli to both areas suppressed the neural responses delivered from the amygdala in layers II/III of the PC (Fig. 2A, black to red arrows) and enhanced the responses in the deep layers (Fig. 2A, red asterisk). This postsynaptic response in the PC suggests that the excitability of perirhinal neurons was increased by associative or repetitive inputs, which arrive during the early phase (approximately 75 ms) but not during the decay phase. Moreover, the enhanced neural activity in the perirhinal deep layers resulting from associative stimuli to the PC and amygdala was only observed when both stimuli were applied during an approximately 100-ms time window (data not shown).

We postulate that one of the mechanisms initiating propagation from the PC to the EC is through emotion-related inputs from the amygdala that become associated with higher-order sensory inputs to the PC, triggering neural propagation from the perirhinal to the entorhinal cortices. It might be also possible that direct thalamoperirhinal inputs to the PC (Guldin and Markowitsch 1983; Wouterlood et al. 1990) play an additional role. It has been suggested that the dichotomy between the roles of the amygdala and the PC is not absolute. For example, perirhinal lesions performed after classical fear conditioning reduce or abolish conditioned fear responses (Corodimas and LeDoux 1995; Rosen et al. 1992). In addition, many data suggest that the AMG modulates the neural processes involved in the formation of declarative memory (Cahill 2000; Cahill and McGaugh 1998). For instance, long-term explicit memory of emotionally arousing stories is enhanced compared with memories of neutral stories, and lesions of the amygdala abolish this effect (Adolphs et al. 1997; Cahill et al. 1995). Moreover, brain-imaging studies have found a high correlation between long-term recall of emotionally arousing or neutral material and the amount of amygdala activation observed when these stimuli were first presented (Cahill 1996; Hamann et al. 1999). Currently, it is unclear how the amygdala modulates declarative memory. The reciprocal amygdala-perirhinal connections, in addition to the amygdala-hippocampal connections (Krettek and Price 1977a, 1977b; Russchen 1982) constitute an obvious possibility, but little is known of their physiology. The present results are in line with the idea that amygdala-perirhinal connections are critically involved, showing that stimulation of the PC associated with amygdala stimulation evoked neural activity in the EC, and this activity subsequently propagated into the DG, whereas stimulation of either PC or amygdala alone did not evoke responses in the EC or in the DG.

In conclusion, the optical signal traces extracted from the deep layers of PC indicate that perirhinal deep layer cells are activated by simultaneous associative stimuli to both the amygdala and PC. Needless to say, we cannot conclude that inputs from sensory cortices and inputs from the amygdala terminate on a single perirhinal neuron associatively, because our results are not obtained from single-cell-level recordings. However, the fact that the neural activation evoked in the PC propagated to the entorhinalhippocampal circuit only in case of associative stimulation to the PC and the amygdala suggests that the association of higher-order sensory inputs and emotion-related inputs could take place in the PC. Moreover, it appears likely that information transfer from the PC to the entorhinalhippocampal circuit is strongly dependent on the association of that information with concurrent activation of the amygdala.