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Department of Molecular and Integrative Physiology, University of Illinois; Department of Pharmacology, University of Illinois College of Medicine; and Beckman Institute, University of Illinois, Urbana, Illinois
Submitted 6 June 2005; accepted in final form 9 August 2005
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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; Sherman and Guillery 1996, 2001; Steriade et al. 1997). Input from the retina in the visual system accounts for <10% of the total synaptic inputs onto individual dLGN relay neurons (Van Horn et al. 2000). The majority of synaptic inputs (>90%) onto thalamic relay neurons originates from inhibitory neurons (local interneurons and thalamic reticular neurons), deep layer neocortex, and various brain stem nuclei (Sherman and Guillery 2001). Similar proportions of inputs have also been described in the somatosensory system (Liu et al. 1995). The extensive complement of nonretinal inputs to the dLGN combine to modulate the responsiveness of relay cells to their retinal inputs and thereby influences the transfer of visual information to cortex.
The innervation of thalamic nuclei by various brain stem nuclei involves several different neuromodulators: cholinergic innervation from the pedunculopontine region, noradrenergic innervation from the locus coeruleus, serotonergic innervation from the dorsal raphe nucleus, and dopaminergic innervation from the mesencephalic reticular formation (De Lima and Singer 1987; De Lima et al. 1985; Hallanger et al. 1987; Hughes and Mullikin 1984; Jones 1985; McCormick 1992b; Morrison and Foote 1986; Papadopoulos and Parnavelas 1990a,b). One proposed function of these brain stem afferents is the regulation of action potential discharge mode, which is related to levels of arousal as well as sleep/wake cycles (McCormick 1992a,b; Sherman and Guillery 2001; Steriade et al. 1993). Another possible functional role of these neuromodulators is to alter the transfer properties of thalamic relay in the attentive state (e.g., Albrecht et al. 1996; Kayama 1985; Zhao et al. 2002). Characterizing how these neuromodulators affect the properties of thalamic neurons is therefore critical in understanding the ascending modulation of thalamocortical function.
Various in vivo studies suggest the potentially important role of the neuromodulators in modifying the activity of thalamic relay neurons. Dopamine has been found to modify visual responses; however, the mechanisms underlying these actions remain unknown. In general, dopamine inhibits visually evoked unit activity in the majority of dLGN neurons in rats in vivo (Albrecht et al. 1996), suggesting that dopamine may modify contrast gain in dLGN. In addition, recent studies suggest that dopamine produces a concentration-dependent, biphasic response: suppression of visually evoked responses at high concentrations and facilitation of visually evoked responses at lower concentrations (Zhao et al. 2002). To help resolve these issues, we used intracellular recording techniques to investigate the actions of dopamine on individual dLGN neurons.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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; Govindaiah and Cox 2004). The animals were deeply anesthetized with pentobarbital sodium (50 mg/kg) and decapitated. The brains were quickly removed and placed into chilled (4°C), oxygenated (5% CO2-95% O2) slicing medium containing (in mM) 2.5 KCl, 1.25 NaH2PO4, 10.0 MgCl2, 2.0 CaCl2, 26.0 NaHCO3, 11.0 glucose, and 234.0 sucrose. Parasagittal or coronal slices (300- to 350-µm thick) containing the dLGN were cut using a vibrating tissue slicer, transferred to a holding chamber containing oxygenated, warm physiological saline (32°C), and incubated in physiological saline for 
1 h prior to recording. Individual slices were transferred to a submersion type recording chamber that was maintained at 30 ± 1°C and continuously perfused (3 ml/min) with oxygenated physiological saline. The physiological saline contained (in mM) 126.0 NaCl, 2.5 KCl, 1.25 NaH2PO4, 2.0 MgCl2, 2.0 CaCl2, 26.0 NaHCO3, and 10.0 glucose. The solution was continuously gassed with 95% O2-5% CO2 to a final pH of 7.4. Recording procedures
Intracellular recordings, using the whole cell configuration, were obtained with the visual aid of a modified microscope equipped with differential interference contrast optics (Axioskop 2FS, Carl Zeiss). A low-power objective (5x) was used to identify specific thalamic nuclei, and a high-power (63x) water-immersion objective was used to identify individual neurons. Recording pipettes were pulled from 1.5 mm OD capillary tubing (Garner Glass, Claremont, CA) and had tip resistances of 3–6 M
when filled with an intracellular solution containing (in mM) 117.0 K-gluconate, 13.0 KCl, 1.0 MgCl2, 0.07 CaCl2, 0.1 EGTA, 10.0 HEPES, 2.0 Na-ATP, 0.4 Na-GTP, and 0.3% biocytin. The pH and osmolarity of internal solution were adjusted to 7.3 and 290 mosM, respectively. The internal solution resulted in a 10 mV junction potential that has been corrected in the voltage measures. All recordings were obtained using either an Axoclamp 2B or a Multiclamp 700A amplifier (Axon Instruments, Foster City, CA). For current-clamp recordings, an active bridge circuit was continuously monitored and if necessary, adjusted to balance the drop in potential produced by passing current through the recording electrode. Recordings included in this study had initial access resistances ranging from 7 to 15 M and typically remained stable throughout the recording.
Drug application
Concentrated stock solutions of various pharmacological agents were originally prepared in appropriate diluents and diluted in physiological saline to a final concentration prior to use. Agonists were typically applied via a short bolus using a syringe pump (Cox and Sherman 2000; Govindaiah and Cox 2004). Dopamine and SKF38393 were prepared fresh just before application. Dopamine was prepared with 0.08% ascorbic acid to prevent oxidation. Stock solutions of SKF38393, forskolin, and KT5720 were initially made with dimethyl sulfoxide. All antagonists were bath applied 7–10 min prior to application of agonists. Dopamine and phenylephrine were purchased from Sigma (St. Louis, MO), whereas all remaining compounds were purchased from Tocris (Ellisville, MO).
Data analysis
The acquisition and analyses of data were accomplished using pClamp software (Axon Instruments). The apparent input resistance of neurons was calculated from the linear slope of the voltage-current relationship obtained by applying constant current pulses ranging from –100 to +40 pA (800-ms duration). A change in the apparent input resistance of the neuron during agonist application was determined by alterations in the membrane response to single-intensity hyperpolarizing current pulses (10–20 pA, 500 ms, 0.2 Hz). To reduce voltage-dependent changes in apparent input resistance, we would manually adjust the membrane potential using current injection at the peak of the agonist-induced depolarization to the preagonist level and then obtain our measures of the response to the hyperpolarizing current pulses. Quantitative analyses are expressed as means ± SD, and statistical significance was tested using Mann Whitney U test, Wilcoxon paired, paired Student's t-test, or one-way ANOVA unless otherwise noted. When an ANOVA test was conducted, Tukey-Kramer multiple comparisons tests were used to test differences between specific groups. P values <0.05 were considered statistically significant.
Histology
Neuronal subtypes, relay or interneurons, were confirmed by morphological reconstruction. The recording pipettes contained 0.3% biocytin, and after the recordings, individual slices were fixed overnight in 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4). Slices were incubated overnight in an avidin-biotin-horseradish peroxidase complex (ABC Elite, Vector Labs, CA) and reacted with diaminobenzidine using established procedures to visualize recorded neurons (Cox and Sherman 2000; Horikawa and Armstrong 1988). Slices were then mounted with permount and the neurons were examined using bright-field microscopy.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Whole cell current- and voltage-clamp recordings were obtained from a total of 167 relay neurons and 17 interneurons in dLGN. The relay and interneurons were identified by their morphological and electrophysiological properties (Fig. 2, A, ii and iii, and C, ii and iii) and were similar to those described previously (Govindaiah and Cox 2004; Pape and McCormick 1995; Williams et al. 1996; Zhu et al. 1999). All recorded cells were filled with biocytin for post hoc reconstruction, and among these, the morphology of 113 relay neurons and 9 interneurons were recovered. The membrane potential and input resistances of relay neurons were –65.2 ± 3.2 mV and 334 ± 106 M, respectively. Interneurons had a lower average resting membrane potential of –56.6 ± 6.9 mV (n = 17; P < 0.01, Mann-Whitney test) and a greater apparent input resistance (653 ± 224 M; n = 8; P < 0.01, Mann-Whitney test).
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, n = 34). The dLGN relay neurons responded to dopamine in a dose-dependent manner over the range of concentrations tested (2–200 µM; Fig. 1, B–D). The peak amplitude of the depolarizations produced by increasing concentrations of dopamine averaged 1.8 ± 1.3 (n = 10), 4 ± 1.5 (n = 15), 6.3 ± 2.2 (n = 13), 7.5 ± 3.1 (n = 24), and 8.9 ± 3.3 mV (n = 10) at 2, 10, 50, 100, and 200 µM, respectively (Fig. 1C). Overall, the dopamine-mediated depolarization significantly differed across concentrations (P < 0.001, ANOVA with Tukey-Kramer multiple comparisons). The amplitude of the depolarizations obtained with lower dopamine concentrations (2 and 10 µM) significantly differed from each of the depolarizations produced by higher dopamine concentrations (50, 100, and 200 µM, P < 0.05). However, these higher concentrations (50, 100, and 200 µM) did not significantly differ from each other. In a subset of cells (n = 12), multiple doses (2–200 µM) of dopamine were applied in the same cells at 10-min intervals. The peak amplitude of the dopamine depolarizations typically showed dose dependence (Fig. 1D).
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Depolarizing actions of dopamine are mediated through dopamine D1-like receptors
To evaluate the receptor subtype underlying the dopamine-mediated depolarization in relay neurons, we tested the actions of specific D1- and D2-like receptor agonists on the relay neurons. The D1-like receptor agonist, SKF38393 (2–50 µM), depolarized the majority of neurons tested (84%: 27/32; Fig. 2Ai). In contrast, the D2-like agonists, (±)-2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin hydrochloride (PPHT, 10–50 µM; n = 10) and bromocriptine (25–50 µM; n = 6), did not alter the membrane potential or apparent input resistance of the relay neurons (Fig. 2Ai). Similar to dopamine, the depolarizations produced by SKF38393 persisted in TTX (1 µM), and the peak amplitude of the depolarization was dose dependent: 1.3 ± 0.5 (n = 3), 3.8 ± 1.0 (n = 5), 3.8 ± 1.5 (n = 10), 4.5 ± 1.8 (n = 10), and 4.5 ± 1.9 mV (n = 4) at 2, 5, 10, 25, and 50 µM, respectively (Fig. 2B). Associated with the depolarizing action of SKF38393 (10–50 µM) was a significant decrease in the apparent input resistance of the relay neurons (pre-SKF38393: 364 ± 112 M vs. post-SKF38393: 329 ± 106 M; P < 0.05, paired t-test, n = 12).
To further determine the specific subtype of dopamine receptor underlying the postsynaptic depolarization in relay neurons, we tested the sensitivity of the dopamine agonist-mediated responses to the D1-like dopamine receptor antagonist SCH23390. The depolarization produced by dopamine (50–100 µM) was significantly attenuated by SCH23390 (5–10 µM; P < 0.01, n = 9; Fig. 3, A and B). The average attenuation of the dopamine-mediated depolarization was 59 ± 6% with 5 µM (n = 4) and 70 ± 11% with 10 µM SCH23390 (n = 5); however, there was no significant difference between the different SCH23390 concentrations (P > 0.05, Fig. 3B). Considering reports that SCH23390 may also act as a serotonergic receptor agonist in certain brain regions (Bourne 2001; Millan et al. 2001), it is important to note that application of SCH23390 did not significantly alter the membrane potential or input resistance of these neurons. Prior to SCH23390 application, the resting membrane potential and apparent input resistance averaged –70.4 ± 3.8 mV and 356.2 ± 72.3 M (n = 9), respectively. After wash in of SCH23390, the membrane potential averaged –70.5 ± 3.9 and input resistance averaged 349.3 ± 57.7 M (n = 9), neither of these were significantly different from the pre-SCH23390 condition (P > 0.5; Wilcoxon).
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Our data using SCH23390 clearly demonstrated that the SKF38393-mediated depolarization was nearly eliminated in the presence of the antagonist; however, the blockade of the dopamine-mediated depolarization was not as complete. To test the potential cross-talk of dopamine at the concentrations tested (50–100 µM) with other catecholamine receptors, we tested the 
1-adrenergic antagonist prazosin and SCH23390 on catecholamine-mediated depolarizations in relay neurons. The 1-adrenergic agonist phenylephrine (100 µM) depolarized dLGN relay neurons with average peak amplitude of 6.5 ± 2.7 mV (n = 11; Fig. 4Ai). In contrast to the actions of phenylephrine, the 2 receptor agonist clonidine (20–100 µM, n = 5; Fig. 4Aii) and the 
-adrenergic receptor agonist cimaterol (50–100 µM, n = 9; Fig. 4Aiii) failed to alter the membrane potential of dLGN relay neurons. The phenylephrine-mediated depolarization was significantly reduced by 91 ± 9% in the presence of the 1-adrenergic antagonist prazosin (5–25 µM; n = 5, P < 0.01; Fig. 4, Ai and B). In contrast, the phenylephrine-mediated depolarization was not significantly altered by SCH23390 (91 ± 18% of control, n = 6, P > 0.1; Fig. 4B). The adrenergic antagonist, prazosin, produced a smaller, but statistically significant reduction of the dopamine (50–100 µM)-mediated depolarization by 25 ± 8% (n = 5, P < 0.05; Fig. 4B).
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1-adrenergic (prazosin) antagonists. Dopamine (200 µM) produced a depolarization that was significantly attenuated to 44 ± 19% of control by 10 µM SCH23390 (Fig. 4, C and D; P < 0.05, n = 6). Subsequent addition of prazosin (5 µM) further reduced the dopamine-mediated depolarization to 10 ± 5% of control (Fig. 4, C and D; P < 0.05, n = 6). In contrast, the depolarization produced by a lower dopamine concentration (10 µM), was reduced by 93 ± 4% (n = 4) in the presence of SCH23390 alone. The actions of these antagonists were partially reversible after a 15- to 25-min wash. Our data suggest that at lower concentrations, dopamine selectively activates D1-like receptors to depolarize thalamic relay neurons; however, at higher concentrations, dopamine also activates 1-adrenergic receptors to further depolarize the thalamic neurons. Activation of D1-like receptors increases Ih in dLGN relay neurons
The ionic nature of the dopamine-mediated depolarization has not yet been identified in thalamic neurons. Given that the depolarization is associated with a weak increase in apparent input resistance, we initially tested the effect of the K+ channel blocker, Cs+. Using voltage-clamp recording techniques with a holding potential of –60 mV, dopamine (50 µM) produced an inward current (40.4 ± 21.2 pA, n = 15) consistent with the membrane depolarization observed using current-clamp recording techniques. Bath application of Cs+ (3 mM) significantly reduced the dopamine depolarization to 28.0 ± 9.0% of control (data not shown, n = 6, P < 0.05, Wilcoxon). However, extracellular Cs+ has been shown to not only block K+ currents, but also the hyperpolarization activated mixed cation current, Ih (Halliwell and Adams 1982; McCormick and Pape 1990b). We next used voltage-clamp recording techniques in conjunction with ramped voltage commands (–60 to –120 mV) to quantify alterations in input conductance. In TTX (1 µM), dopamine produced an inward current with an average peak of 47.6 ± 23.0 pA (n = 9). During the dopamine-mediated inward current, the slope of the current response was reduced in most cells (6/9, Fig. 5, A and B). Given the significant contribution of Ih to hyperpolarizing regions of the ramped voltage commands, we used the selective Ih blocker, ZD7288 (Fig. 5, A and B: +ZD7288). In ZD7288 (100 µM), the dopamine mediated inward current was significantly reduced to 27.8 ± 18.1% control (n = 9, P < 0.01, Wilcoxon) indicating that dopamine produces an increase in Ih that likely contributes to the membrane depolarization. In the presence of ZD7288, the remaining dopamine-sensitive current is linear (Fig. 5B: +ZD7288) and has an average reversal potential of –106 ± 31 mV (n = 9), indicative of Kleak. Thus our data suggest that the dopamine-mediated depolarization is composed of two components: an increase in Ih and a reduction in Kleak.
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1-adrenergic receptors, we carried out the voltage-clamp experiments with lower dopamine concentrations. At 10 µM, dopamine produced a smaller but consistent inward current (12.7 ± 6.5 pA, n = 5) compared with 50 µM dopamine (Fig. 5C). In ZD7288 (100 µM), the dopamine mediated inward current was completely blocked (pre-ZD7288: 13.6 ± 7.3 pA, ZD7288: 0.5 ± 0.6 pA; n = 4, P < 0.05, paired t-test) indicating at low concentrations, the dopamine-mediated inward current results only from an increase in Ih (Fig. 5, C and D). To specifically address the mechanism underlying the depolarizing response following activation of D1-like dopaminergic receptors, we repeated the voltage-clamp experiments using the selective agonist, SKF38393. With a holding potential of –60 mV in TTX (1 µM), SKF38393 produced an inward current that averaged 13.8 ± 8.2 pA (n = 7) associated with an apparent increase in conductance (Fig. 6A). Using slow ramped voltage commands, we analyzed the conductance change produced by SKF38393 by calculating the "resting" conductance of the cell prior to and after SKF38393 application in the linear response range of the neurons (Vhold: –60 to –80 mV, Fig. 6B). SKF38393 produced a significant increase in the neurons' conductance from 3.1 ± 0.9 nS to 3.6 ± 1.0 (n = 7; P < 0.01, paired t-test). We next tested the affect of the Ih blocker, ZD7288, on the SKF38393-mediated excitation. In ZD7288 (50–100 µM), the inward current produced by SKF38393 was strongly attenuated (Fig. 6A). In all four cells tested, the response to SKF38393 in the presence of ZD7288 averaged 0.1 ± 1.4 pA, a significant reduction of the inward current (P < 0.01, paired t-test). In addition, SKF38393 did not alter the conductance of the neurons in ZD7288 (2.02 ± 0.5 vs. 2.1 ± 0.5 nS, P > 0.3, n = 4). These data suggest that activation of D1-like dopaminergic receptors leads to an increase in Ih, which in turn accounts for the excitatory actions (inward current, membrane depolarization) of dopamine in thalamic relay neurons.
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The D1-like dopamine receptors are positively coupled to the activation of adenylyl cyclase/cAMP/protein kinase A signal transduction pathway (Stoof and Kebabian 1984). We next tested whether the excitatory actions of dopamine on thalamic neurons involved the cAMP pathway. Bath application of the adenylyl cyclase (AC) activator, forskolin (0.5–20 µM, n = 21), produced a depolarization similar to SKF38393 (Fig. 7A). The peak amplitude of the depolarization averaged 2.5 ± 1.1 (n = 6), 3 ± 1 (n = 7), 3 ± 1 (n = 6), and 4.3 ± 1 mV (n = 10) at 0.5, 2, 5, and 20 µM forskolin (Fig. 7B). In contrast, when the phospholipase C inhibitor, U-73122 (200 µM), was included in the recording pipette, the dopamine-mediated depolarizations were not significantly altered (Fig. 7F; n = 8). These data indicate that activation of AC can depolarize thalamic relay neurons.
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To test whether the dopamine-mediated depolarization involves cAMP-dependent PKA activation, we tested the sensitivity of the dopamine response to the selective PKA inhibitor, KT5720 (Huang and Hsu 2003; Kase et al. 1987). In control conditions, dopamine (10 µM) produced an average depolarization of 3.4 ± 1.1 mV (n = 5). The antagonist, KT5720 (1–5 µM), was applied for 8–15 min, and the subsequent dopamine application produced a similar depolarization (3.3 ± 1 mV) that did not significantly differ from the control response (Fig. 7, Ei and G; P > 0.1, paired t-test, n = 5). In a different series of experiments, KT5720 (10 µM) was included in recording pipettes. Using a similar experimental protocol as with Rp-cAMP, the initial dopamine-mediated depolarization (within 2 min after whole cell recording configuration) averaged 5.1 ± 0.9 mV (n = 4). The second dopamine application (13 min later) averaged 5.0 ± 0.5 mV and did not differ significantly from the first response (Fig. 7Eii, P > 0.1, paired t-test, n = 4). In same neurons, continuous bath application of cAMP agonist forskolin (25 µM, 5–10 min) produced a membrane depolarization that plateaued after a few minutes (Fig. 7F). The membrane potential was manually adjusted to the preforskolin level, and the subsequent dopamine application produced a significantly reduced depolarization (0.9 ± 0.2 mV; n = 4, Fig. 7Eii). The population data clearly indicate that the dopamine-mediated depolarization was significantly occluded following activation of cAMP by forskolin (Fig. 7I, P < 0.01, paired t-test, n = 4). These data suggest that the depolarizing action of dopamine via Ih is mediated through a cAMP-dependent, PKA-independent mechanism.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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; McCormick and Prince 1988). Overall, our study indicates a potentially important role of D1-like receptors in influencing the excitability of thalamic relay neurons and ultimately modulating the gating properties of thalamic information transfer to the neocortex.
The h current (Ih) is a hyperpolarization-activated mixed cation current that was originally described in cardiac cells as If but has since been identified as Ih in a widespread variety of neural as well as nonneural cells (Brown and DiFrancesco 1980; Halliwell and Adams 1982; Pape 1996). In the thalamus, Ih has been well characterized in relay neurons and plays an integral role in the rhythmic activity of these neurons as well as an important contributor to the resting membrane potential (Lüthi and McCormick 1998; McCormick and Pape 1990a,b; Pape 1996). An interesting aspect of Ih is that this current can be directly modulated by alterations in cAMP activity (Frere and Luthi 2004; Kaupp and Seifert 2001; Pape 1996; Santoro and Tibbs 1999). Our data further support the modulatory role of Ih by cAMP in a PKA-independent manner. Considering activation of dopamine receptors is associated with changes in cAMP levels, it is not entirely surprising that dopamine may alter Ih activity. In other tissues, dopamine, via D2 receptors, has been found to decrease Ih (Akopian and Witkovsky 1996; Vargas and Lucero 1999). The novelty of our findings is the increase in Ih that we have observed in thalamic neurons is associated with activation of D1-like receptors.
Within the thalamus, a variety of neuromodulators, that arise from brain stem nuclei, have been found to alter Ih activity in relay neurons, including serotonin, norepinephrine, and histamine (McCormick and Pape 1990a; McCormick and Williamson 1991). The increase in Ih has been associated with increases in cAMP activity, and this may serve as a final common pathway for these neuromodulators. Our data add a newcomer to this list: dopamine. The activity of dopaminergic neurons in the mesencephalic reticular formation have been associated with a variety of arousing events such as novel stimuli and changes in the conditions of a salient environment (Horvitz 2000; Horvitz et al. 1997). The activity of noradrenergic neurons in the locus coeruleus are correlated with an organisms state of arousal (Aston-Jones et al. 1991; Hirsch et al. 1983; Livingstone and Hubel 1981; McCarley et al. 1983; Nakai and Takaori 1974; Rasmussen et al. 1986). Considering the release of these neuromodulators are associated with arousal levels, attention, and stimulus novelty, one hypothesis regarding a common function may be that these modulators tend to produce a heightened level of sensitivity, although further investigation regarding this idea is required.
Visual information is transferred from the retina to primary visual cortex via dLGN relay neurons. A variety of neurotransmitters/modulators that are released by intrathalamic neurons, corticothalamic neurons, as well as brain-stem-projecting neurons can modify neuronal excitability and thereby influence visual information that passes through the dLGN (Steriade et al. 1997). The predominant source of synaptic innervation of dLGN relay neurons is from nonretinal sources with only a small minority of synapses (<10%) that arise from retinal ganglion cells (Van Horn et al. 2000). These nonretinal, modulatory inputs can influence the excitability of thalamic circuits thereby altering the input/output function of the thalamus. In addition, these modulatory inputs may alter the action potential discharge mode of these neurons, which is related to the arousal state of the organism or novelty of the stimulus (Sherman and Guillery 1996, 2002; Steriade et al. 1993). For example, many neurotransmitters including acetylcholine, histamine, glutamate, serotonin, and norepinephrine clearly alter the excitability of relay neurons and transition the firing mode from a burst to tonic discharge mode (Cox and Sherman 2000; McCormick 1992b; Monckton and McCormick 2002; Turner and Salt 2000).
Dopaminergic innervation of the thalamus is relatively sparse compared with other catecholamines such as norepinephrine or serotonin, but nonetheless dopamine receptors are also present within the dLGN (Camps et al. 1990; Khan et al. 1998; Mijnster et al. 1999; Papadopoulos and Parnavelas 1990a). However, the functional role of dopamine in thalamic functioning is less established than that of the other catecholamines. Of the limited number of in vivo studies, dopamine tends to produce primarily inhibitory actions, that is, activation of dopaminergic receptors decreases the light-evoked as well as baseline activity of presumed relay neurons (Albrecht et al. 1996; Zhao et al. 2002). Dopamine has been shown to inhibit flash-evoked activity in the majority of rat dLGN neurons but can also facilitate the activity of other dLGN neurons (Albrecht et al. 1996). Dopamine has also been shown to produce concentration-dependent effects on visually evoked discharge of dLGN relay neurons: at high concentrations, dopamine suppresses the visual response but may facilitate the relay neuron activity at lower concentrations (Zhao et al. 2002). These previous in vivo studies have also suggested that dopamine may alter the activity of dLGN interneurons thereby decreasing relay neuron activity. In addition, the excitatory actions of dopamine have been associated with the activation of D2-like receptors, whereas the inhibitory actions may be due to activation of D1-like receptors (Albrecht et al. 1996; Zhao et al. 2002). Despite these generalizations of dopamine-mediated activity within the dLGN, it is clear that there are as many exceptions to these generalizations regarding the inhibitory and excitatory responses produced by dopamine in in vivo experiments.
Our study provides the first data demonstrating dopamine-mediated actions in dLGN thalamic relay neurons using intracellular recording techniques. We found that both dopamine and the D1 receptor agonist, SKF38393, produced robust membrane depolarizations in relay neurons only, whereas D2 receptor agonists, PPHT and bromocryptine, did not alter the membrane potential of relay neurons. Furthermore, we found no evidence that dopaminergic agonists alter the membrane potential of dLGN interneurons. Clearly, the excitatory actions of dopamine that we have observed using the in vitro slice preparation cannot easily account for the general findings reported in vivo. The inhibitory actions observed in vivo could arise from excitation of adjacent thalamic reticular nucleus neurons but do not appear to involve dLGN interneurons as originally suggested. Future studies should also focus on the possible actions of dopamine on synaptic activity. It is clear that further investigation at both the reduced level (in vitro preparation) and intact level (in vivo) are required to resolve these differences and ultimately understand the cellular mechanisms and functional significance regarding the role of dopamine in visual processing.
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FOOTNOTES |
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Address for reprint requests and other correspondence: C. L. Cox, Dept. of Molecular and Integrative Physiology, University of Illinois, 2357 Beckman Institute, 405 N. Mathews Ave., Urbana, IL 61801 (E-mail: clcox{at}life.uiuc.edu)
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). B: population data illustrating the peak depolarization to different SKF38393 concentrations. The numbers in the graph represent the number of cells tested at each concentration. Ci: current-clamp recording from a dLGN interneuron. Dopamine SKF38393, and bromocriptine did not alter the membrane potential or apparent input resistance of this cell. Cii: micrograph is the interneuron from which the recordings in Ci were obtained. Note the relatively long and simple branch pattern of the dendrites. Calibration, 50 µm Ciii: membrane response to hyperpolarizing and depolarizing current steps indicate obvious depolarizing sag to hyperpolarizing current steps and the apparent lack of burst discharge.
