Department of Neurology and Neurological Sciences, Stanford University Medical Center, Stanford, California 94305
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ABSTRACT |
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Zhang, Shuanglin J., John R. Huguenard, and David A. Prince. GABAA receptor-mediated Cl currents in rat thalamic reticular and relay neurons. J. Neurophysiol. 78: 2280-2286, 1997. Spontaneous and evoked inhibitory postsynaptic currents (sIPSCs and eIPSCs) and responses to exogenously applied
-aminobutyric acid (GABA), mediated by GABA type A (GABAA) receptors, were recorded in inhibitory neurons of nucleus reticularis thalami (nRt) and their target relay cells in ventrobasal (VB) nuclei by using patch clamp techniques in rat thalamic slices. The decay of sIPSCs in both nRt and VB neurons was best fitted with two exponential components. The decay time constants of sIPSCs in nRt neurons were much slower (
1 = 38 ms;
2 = 186 ms) than those previously reported in a variety of preparations and two to three times slower than those in VB neurons (
1 = 17 ms;
2 = 39 ms). GABAA receptor-mediated Cl
currents directly evoked by local GABA application also had a much slower decay time constant in nRt (225 ms) than in VB neurons (115 ms). Slow decay of GABA responses enhances the efficacy of recurrent intranuclear inhibition in nRt. The results suggest a functional diversity of GABAA receptors that may relate to the known heterogeneity of GABAA receptor subunits in these two thalamic nuclei.
Inhibitory circuits in the thalamus play a critical role in the generation of normal and pathophysiological thalamocortical rhythms (Steriade et al. 1993 Thalamic slices
Animal preparation was the same as described in detail in Huguenard and Prince (1994b) Solutions and drugs
The aCSF was composed of (in mM) 126 NaCl, 26 NaHCO3, 2.5 KCl, 1.25 NaH2PO4, 2 MgCl2, 2 CaCl2, and 10 glucose and had a pH of 7.3 when equilibrated with the mixture of 95% O2-5% CO2. aCSF containing both 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 µM) and 2-amino-5-phosphonovalerianic acid (APV; 200 µM) was used to block ionotropic glutamate receptor-mediated excitatory postsynaptic currents. 100 µM GABA and 20 µM bicuculline methiodide (BMI) solutions were made by adding GABA and BMI stock solutions to the CNQX/APV-containing aCSF. The slice was completely submerged in a total volume of 300 µl and continuously perfused at a rate of 4 ml/min. We estimated that 3-4 min were required to completely change solutions. The pipette solution contained (in mM) 135 CsCl, 5 QX-314, 2 MgCl2, 10 ethylene glycol-bis( Electrophysiology
All voltage-clamp recordings were obtained at room temperature (22-24°C) with the continuous recording mode of an Axopatch-200 amplifier (Axon Instruments, Foster City, CA) and the whole cell recording configuration of the patch-clamp technique (Hamill et al. 1981
Data analysis
Signals were filtered at 5 kHz ( Individual nRt and VB neurons within rat horizontal thalamic slices were visually identified by their location and some clearly discernable morphological characteristics, such as the shape of their cell bodies and distribution of proximal processes. The borders of nRt could be easily recognized in live slices, and neurons were selected that were well within its lateral and medial aspects, marked by the internal capsule and external medullary lamina, respectively. VB neurons were recorded from a region Spontaneous IPSCs in nRt versus VB neurons
After ionotropic glutamate receptors were blocked with CNQX (20 µM) and APV (200 µM), spontaneous IPSCs (sIPSCs) were recorded in both nRt and VB neurons under whole cell voltage clamp at a holding potential of
Electrically evoked IPSCs in nRt and VB neurons
We next compared the properties of evoked IPSPs in the two nuclei. Immediately after obtaining a whole cell recording, a few test stimuli were delivered via a bipolar tungsten electrode placed within nRt to check the synaptic responsiveness of the neuron. Once responses were clearly seen, monosynaptic eIPSCs were isolated by switching the perfusion solution from normal aCSF to one containing CNQX and APV. After ~4 min, when excitatory ionotropic synaptic responses were blocked, a series of test stimuli of increasing intensity (10-250 µA and 10-200 µs) was applied to obtain the threshold for evoking monosynaptic IPSCs. Stimuli were adjusted to be 1.5 times threshold for eliciting a detectable monosynaptic IPSC in each neuron (100-300 µA; 60-300 µs). The average peak eIPSC amplitude in nRt cells (130.8 ± 25.0 pA, n = 10) was significantly smaller than that in VB neurons (465.7 ± 100.6 pA, n = 11, P = 0.006; Fig. 5D).
GABA-evoked responses in nRt and VB neurons
To further assess the properties of IPSCs in VB and nRt neurons we tested responses to pressure pulses of GABA (100 µM; 7-27 kPa and 10-200 ms) delivered onto the visualized recorded neuron via a micropipette. The peak amplitude of the GABA response, which varied depending on the pressure and duration of pressure ejections and the tip size of ejection pipettes, was adjusted to be approximately the same as that of an IPSC evoked by 1.5 times threshold stimuli (e.g., see Fig. 4, A and C). Although the GABA responses varied in amplitude from cell to cell, the decay rate of the response was relatively constant among neurons within a given nucleus. These GABA-evoked currents were GABAA receptor-mediated, because they reversed at the same potential as the electrically evoked IPSCs (Fig. 4, A, C, and D) and were also blocked by 20 µM bicuculline (Fig. 4D). The reversal potentials were 0.9 ± 2.2 mV (n = 4) in nRt and 1.0 ± 0.8 mV (n = 4) in VB neurons. The decay phases of GABA responses were best fitted with single exponential functions (Fig. 6, A1 and A2) and, as was the case for sIPSCs and eIPSCs, the decay time constants were significantly longer in nRt (224.8 ± 22.2 ms, n = 7) versus VB neurons (115.0 ± 15.4 ms, n = 8, P = 0.001; Fig. 6B).
The principal finding in these experiments is a heterogeneity of functional inhibition in rat somatosensory thalamus. Most inhibition within this structure originates from neurons of the nucleus reticularis thalami (Yen et al. 1985 Suggested basis of IPSC difference: heterogeneity of GABAA receptors
The suggestion that there are fundamental differences in GABAA receptors (channels) in nRt and VB neurons gains support from anatomic studies of GABAA receptors in the thalamus and also from pharmacological and single channel studies. For example, immunocytochemical experiments have shown that Functional implications
Recurrent inhibitory circuitry within nRt is a powerful regulator of synchronized network activity. Focal application of BMI within nRt increases the inhibitory output from nRt onto VB (Huguenard and Prince 1994a
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
).
-Aminobutyric acid-A (GABAA) and/or type B (GABAB) receptor-mediated inhibitory postsynaptic currents (IPSCs) in thalamic relay cells, which result from activation of nucleus reticularis (nRt) neurons, evoke rebound bursts that can reactivate nRt (Huguenard and Prince 1994a
; von Krosigk et al. 1993
). This recurrent circuitry is active during sleep spindles and perhaps in absence epilepsy. We demonstrated that a benzodiazepine (clonazepam) paradoxically decreases theGABAergic output of nRt (Huguenard and Prince 1994b
), probably by enhancing the recurrent inhibitory circuitry within this nucleus. Clonazepam appeared to exert more powerful actions within nRt than in ventrobasal (VB) neurons, suggesting a heterogeneity of GABAA receptors, with those within nRt being more susceptible to benzodiazepine modulation than those on thalamic relay neurons (Zhang et al. 1994
).
; Gutiérrez et al. 1994
; Wisden et al. 1992
), implying that GABAA receptors with different functional properties might be present in these structures (Levitan et al. 1988
; Verdoorn 1994
; Verdoorn et al. 1990
). Heterogeneity of GABA responses within a specific brain region was reported in cerebellar cortex where spontaneous inhibitory synaptic currents and GABA-evoked responses are longer lasting in granule cells than in Purkinje cells (Puia et al. 1994
).
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
and Zhang and Jackson (1995)
. Briefly, 8- to 13-day old Sprague-Dawley rats were anesthetized with 50 mg/kg ip pentobarbital sodium, decapitated, and the brains rapidly removed and placed in chilled (4°C) artificial cerebral spinal fluid (aCSF) solution. Horizontal slices (300 µm) of thalamus were prepared with a vibratome (TPI, St. Louis, MO) and incubated for
1 h in gassed aCSF before being placed in a recording chamber. Thalamic neurons were viewed with an upright microscope (Zeiss, Thornwood, NY) equipped with differential interference contrast Nomarski optics and an electrically insulated ×40 water immersion objective with a working distance of 1.75 mm.
-aminoethyl ether)-N,N,N
,N
-tetraacetic acid (EGTA), and 10 N-2-hydroxyethylpiperazine-N
-2-ethanesulfonic acid (HEPES). pH was adjusted to 7.2 with CsOH. GABA, BMI, and QX-314 were obtained from Sigma, CNQX and APV from RBI.
). Neurons were held at a holding potential of
60 mV. Electrodes were pulled from KG-33 borosilicate glass capillaries (1.5 mm OD, 1.0 mm ID; Garner Glass, Claremont, CA) that had resistances of 2-4 M
when filled with the internal solution. A bipolar tungsten electrode (impedance 0.1-1 M
; FHC, Brunswick, ME) placed within nRt was used to deliver electrical stimuli. GABA pulses were applied at 0.066 Hz via pressure ejection from a micropipette with a tip size of ~1.5 µm OD. Pressure and duration of GABA applications were adjusted so that the peak of the GABA-evoked current was comparable in amplitude to that of the standard electrically evoked IPSC.
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FIG. 1.
Spontaneous inhibitory postsynaptic currents (sIPSCs) decay more slowly in nucleus reticularis thalami (nRt) than in ventrobasal (VB) neurons. A and B: single sIPSCs were recorded in nRt (A1-3) and VB neurons (B1-3) in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 µM) and amino-5-phosphonobutyric acid (AP5) (200 µM). Patch pipette contained the 135 mM CsCl internal solution. Note difference in time calibration in A and B. C: averages of 30 consecutive single isolated sIPSCs from nRt (C1) and VB neurons (C2). Superimposed on the averaged IPSCs are best-fitted double exponential curves. Note different time base in C1 and C2. Equation describing inhibitory postsynaptic potential (IPSP) decay was -34.7*e t/21.4
35.0*e
t/107 for nRt and
26.6*e
t/7.4 - 87.3*e
t/25.8 for VB. D: averaged IPSCs in C1 and C2 are scaled to same amplitude and superimposed on same time base. E: average peak amplitudes of sIPSCs from 9 nRt and 15 VB neurons. Error bars represent mean ± SE. F-H: average rise time (F), half-width (G), and charge (H) from same neurons as in E. Charge transferred during sIPSCs was obtained by integrating IPSC trace over time (see text). ***P
0.001.
3 dB, 8 pole, low-pass Bessel filter) and stored in digitized form (Neurocorder, Neurodata, NY) on VHS tape. CDR and SCAN software were used to collect continuous records and detect spontaneous events. Evoked IPSCs (eIPSCs) and GABA responses were recorded with pClamp 5.6 and analyzed with pClamp 6.1. Curve fittings were performed with Origin (MicroCal Software, Northampton, MA) and Clampfit 6.1.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
100 µm medial to nRt. Much of the large dendritic tree of individual relay neurons could usually be observed in various focal planes above and below the soma. In a few cases, neurons of each type were filled with biocytin and subsequently processed immunohistochemically to confirm cell identity (Tseng et al. 1991
). Care was taken to study cells 50-100 µm below the slice surface so that stable recordings could be obtained from apparently healthy neurons. The access resistance ranged from 5 to 17 M
(typically ~10 M
) and input resistance was ~400 M
in these cesium-loaded cells. Only those recordings with stable and low access resistance were included in the final analysis. Our results are derived from a total population of 45 nRt and 61 VB neurons.
60 mV. Typical sIPSCs, inward currents under these recording conditions (see METHODS), are shown for nRt and VB neurons in the top panels of Fig. 1, A1-3 and B1-3, respectively). Averages of 30 consecutive sIPSCs recorded from single nRt and VB neurons are shown in Fig. 1, C1 and C2, respectively. Responses scaled to the same amplitude are shown in Fig. 1D to illustrate differences in time course.
; Puia et al. 1994
), spontaneous GABAA receptor-mediated IPSCs were highly variable in amplitude, ranging from 20 to 200 pA. The peak amplitudes of the averaged sIPSCs were slightly lower in nRt (51.2 ± 7.7 pA, mean ± SE, n = 9) than in VB neurons (61.4 ± 7.6 pA, n = 15; Fig. 1E), but this difference was not significant (P = 0.4). Bath perfusion with GABAA receptor antagonist BMI (20 µM) completely abolished sIPSCs in both nRt and VB neurons (Fig. 2). With an expected reversal potential of 0 mV, the peak IPSC current amplitudes in symmetric Cl
corresponded to conductances of ~0.85 nS in nRt and 1.0 nS in VB neurons. These values are similar to those in other types of neurons such as dentate gyrus (Edwards et al. 1990
; Otis and Mody 1992
) and cerebellar granule cells (Puia et al. 1994
).
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FIG. 2.
sIPSCs in VB and nRt are -aminobutyric acid-A (GABAA) receptor-mediated. sIPSCs in nRt (A) and VB neuron (B) were blocked completely by bicuculline methiodide (BMI; 20 µM). Access resistance before and after addition of BMI was 14 M
in A and 15 M
in B.
0.1) in the two groups of neurons (Fig. 3, B and D). The lack of such correlations has been used to argue against a strong influence of electrotonic filtering on the shape of recorded sIPSCs (Silver et al. 1992
). However, it was also suggested that a linear relationship between rise time and amplitude would not necessarily be expected in a complex dendritic arbor (Major et al. 1994
; Ulrich and Luscher 1993
). Furthermore, recent evidence from dentate granule cells suggests that only proximal (i.e., perisomatic) IPSCs are obtained with whole cell somatic recordings (Soltesz et al. 1995
). Assuming similar IPSC activation kinetics, the equivalent rise times in VB and nRt neurons suggest that events at similar electrotonic distances were recorded in each cell type.
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FIG. 3.
Plots of rise time (A, C) and half-width (B, D) vs. amplitude of sIPSCs in nRt (A-B) and VB neuron (C-D). Linear regressions fittings generated the correlation coefficients (R) indicated. Each point was derived from single sIPSC. Correlations were low in each case suggesting that dendritic filtering played little role in shaping IPSCs. There are no differences in these parameters between nRt and VB neurons, which suggests that the differences in decay time constants are not due to the geometric differences between these 2 types of neurons.
7; Fig. 1G). Although sIPSC decay kinetics could not be well fitted with a single exponential in either VB or nRt neurons, double exponential curves closely approximated the data (Fig. 1, C1 and C2). The two time constants of decay (
1, fast;
2, slow) were both larger in nRt (
1, 37.5 ± 3.8 ms;
2, 186.1 ± 18.3 ms) than VB neurons (
1, 16.7 ± 1.8 ms;
2, 39.0 ± 2.9 ms; Table 1).
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TABLE 1.
Exponential decay properties of sIPSCs
charge movement. This measure takes into account both amplitude and time course of the inhibitory events and was obtained by integrating the averaged sIPSC from each cell over time. As shown in Fig. 1H, the efficacy of sIPSCs was much greater in nRt neurons (5.0 ± 0.7 pC, n = 9) than in VB cells (1.8 ± 0.3 pC, n = 15, P = 4 × 10
5). Furthermore, analysis of the slowly and rapidly decaying sIPSC components demonstrated that most of the charge (~4 pC) arose from the slow component in nRt cells, compared with about one-half (~1 pC) in VB cells (see values for C1 and C2 in nRt and VB given in Table 1).
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FIG. 5.
Electrically evoked IPSCs recorded from thalamic neurons. A and B: averages of 20 consecutive single isolated IPSCs evoked in nRt (A) and VB neuron (B) by 1.5 times threshold stimuli ( ). Double exponential fits are superimposed on actual averaged traces. Time calibration bar: 200 (A) and 100 ms (B). Fitted curves are described by
45.3*e
t/15.5
131*e
t/158 for the nRt and
222*e
t/22.7
136*e
t/61.1 for the VB neuron. C: averaged traces from nRt (from A, dark line) and VB (from B, gray line) neurons are scaled to the same amplitude and superimposed on same time base for direct comparison. D-F: graphs of average peak eIPSC amplitude (D), eIPSC half-width (E), and eIPSC total charge (F) for nRt vs. VB neurons. **P < 0.01; ***P
0.001.
ions (0.5 mV; [Cl]i/[Cl]o = 139/136.5). The conductance of the IPSCs evoked by 1.5 times threshold stimuli, obtained from linear regression of the I-V curves, was 2.4 ± 0.6 nS in nRt compared with 5.7 ± 0.9 nS in VB (Fig. 4B). Evoked IPSCs were completely abolished by the specific GABAA antagonist BMI (20 µM; Fig. 4B,
).
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FIG. 4.
Evoked IPSCs (eIPSCs) and GABA responses are GABAA receptor-mediated. A and C: electrically evoked IPSCs ( ) and responses to pressure pulses of GABA (100 µM;
) at a range of membrane potentials between
60 and +40 mV (20 mV steps) in nRt (A) and VB neuron (C). Responses of both types reversed near 0 mV in both neurons. Note different time calibrations in A and C. B and D: I-V curves for IPSCs (B) and GABA-evoked responses (D) from nRt (
) and VB neurons (
) reversed at ~0 mV. Addition of 20 µM BMI in the perfusion solution completely blocked IPSCs in VB (B,
) and nRt (B,
) as well as GABA responses (D,
and
). Recordings obtained with symmetrical [Cl
]; calculated ECl = 0 mV. PSC, postsynaptic current.
6; Fig. 5, C and E). In each case the decay phase of eIPSCs was best described by the sum of two exponentials (Fig. 5, A and B), with
1 = 42.5 ± 6.1 ms and
2 = 177.6 ± 16.4 ms in nRt neurons (n = 10) and
1 = 23.0 ± 3.4 ms and
2 = 68.3 ± 6.7 ms in VB neurons (n = 11; Table 2). Both fast and slow components were significantly different between the two cell groups, with P-values of 0.01 and 4.0 × 10
6, respectively. The slow component was always present in both cell types, but more prominent in nRt neurons. The average contributions of both fast and slow components to the eIPSC peak current are shown in Table 2 by the values of A1 and A2, respectively.
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TABLE 2.
Decay properties of evoked IPSCs
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FIG. 6.
GABA-evoked GABAA receptor-mediated responses in nRt and VB neurons. A: averaged traces of 20 responses evoked by 100 µM GABA pressure pulses in nRt (A1) and VB (A2) neuron. Single exponential function was fitted to decay phase of averaged traces and is superimposed. Fitted curves were described by 433*e
t/268 for nRt and
917*e
t/141 for VB. B: decay time constants for GABA responses from nRt and VB neurons were significantly different (see text). **P < 0.01.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
), although an extrinsic inhibitory pathway has been described (Keller et al. 1992
). Thus in these experiments spontaneous and evoked synaptic currents primarily reflect GABAA receptor-mediated postsynaptic events triggered by activity in nRt axons and terminals. It is interesting to note that similar presynaptic nRt activity results in very different responses that depend on the postsynaptic target neuron. IPSCs in relay neurons that arise from afferent nRt fibers are relatively short-lived (half-width ~20 ms), whereas IPSCs in nRt neurons that result from activity in recurrent local collaterals (Cox et al. 1996
; Spreafico et al. 1988
; Yen et al. 1985
) are prolonged by comparison (~60 ms). In addition, the responses elicited by pressure-ejection of GABA are almost two times longer lasting in nRt (decay time constant ~225 ms) than in VB (~115 ms) neurons. The decay of IPSCs and GABA-elicited responses in nRt cells are much slower than those reported for most other types of neurons (Edwards et al. 1990
; Puia et al. 1994
; Zhang and Jackson 1993
), with the exception of hippocampal inhibitory cells in culture (Jones and Westbrook 1995
). The decay phase of IPSCs in both nuclei is composed of two exponentials, which likely reflect the intrinsic properties of the GABAA receptor channels, as was suggested in cerebellar granule neurons (Puia et al. 1994
).
), the single reversal potential of the postsynaptic currents at ~ECl (Fig. 4, A-C), and the complete blockade in BMI.
, 1996) and zinc (Kang et al. 1996
).
). Inhibition of GABA uptake does not alter the time course of inhibitory synaptic responses, at least in hippocampal slices (Isaacson et al. 1993
), making it unlikely that slowed GABA uptake prolongs IPSPs at intra-nRt synapses. Also, it has been proposed that diffusion is a very rapid process in the synaptic cleft (Busch and Sakmann 1990
; Clements 1996
; Korn and Faber 1987
; Wathey et al. 1979
), suggesting that the differences in IPSC time course at nRt versus VB synapses are not due to diffusional differences. The finding that a benzodiazepine compound alters the decay time constant of the GABA responses (Zhang et al. 1994
) is consistent with the idea that the kinetics of the response are largely determined by the properties of the GABA receptors themselves and not to other factors such as reuptake, diffusion, etc. Thus the most likely explanation for these findings is a heterogeneity in the properties of GABA receptor Cl
channels. Channel reopenings during bursts (Bormann and Clapham 1985
; De Koninck and Mody 1994
; Weiss and Magleby 1989
; Zhang and Jackson 1995
) can lead to an extended total response (Zhang and Jackson 1994
), so one possibility is that burst duration is longer for GABAA channels in nRt neurons than for those in VB cells. Single channel data showing longer open times in nRt than VB suggest that this is the case (Kang et al. 1996
). It is also possible that GABA receptor desensitization (Puia et al. 1994
) may be less prominent in nRt neurons and/or that reopenings from the desensitized state (Jones and Westbrook 1995
) are more likely in these cells.
1,
2, and
3 subunit proteins are abundant in VB but absent in nRt, whereas the
3 subunit is prominent in nRt and absent in VB (Fritschy and Mohler 1995
). In situ hybridization experiments also indicate a heterogeneity of subunits in different nuclei so that
1,
4, and
mRNAs are abundant in VB but only weakly present in nRt (Wisden et al. 1992
). The precise composition of native GABAA receptors is not known in thalamus or other CNS structures, nor has a detailed correlation between subunit composition and the kinetic properties of GABAA receptor channels been possible (Macdonald and Olsen 1994). However, data from studies of recombinant receptors make it likely that the structural differences detected to date will be reflected in different receptor properties (Dominguez-Perrot et al. 1995; Gingrich et al. 1995
; Verdoorn 1994
; Verdoorn et al. 1990
). For example,
3 subunits when coexpressed with
2 and
2 in HEK293 cells give rise to slower activation, deactivation, and desensitization than comparable receptors containing the
1 subunit (Gingrich et al. 1995
; Verdoorn 1994
). Differences in sensitivity of IPSCs in VB versus nRt to benzodiazepines (Zhang et al. 1996) and zinc (Kang et al. 1996
), as well as different kinetics of single channels in the two structures (Kang et al. 1996
) support this assumption. Single channel studies of native receptors whose subunit composition is known will be required to determine the bases for the physiological differences in IPSCs in neurons of VB and nRt described here.
) and bath BMI application enhances the extent and duration of 3-Hz network oscillations in thalamic slices (Huguenard and Prince 1994b
; von Krosigk et al. 1993
). In addition, BMI induces a very large increase in the duration of calcium-dependent bursts during synchronized network activity in perigeniculate (PGN) neurons (Bal et al. 1995
), which in ferret dorsal lateral geniculate are analogous to nRt cells. This latter finding suggests that recurrent inhibition in nRt/PGN can hyperpolarize neurons in these structures and/or shunt the calcium-dependent burst-firing response that is induced byexcitatory synaptic currents during network activity. The long-lasting nature of the intra-nRt IPSP described here may then enhance shunting and lead to early termination of the bursts. On the other hand, if ECl is sufficiently negative, IPSPs in nRt cells may lead to calcium channel deinactivation and generation of rebound burst discharge.
) IPSCs have comparable sizes in the two nuclei. Because miniature events presumably reflect quantal release, this difference in evoked response amplitude suggests that the total number of functional synapses or release sites is larger in VB cells than in nRt neurons. Support for this conclusion is also provided by the finding that frequency of miniature IPSCs was approximately three times higher in VB than in nRt neurons (Ulrich and Huguenard 1996
). This inference from electrophysiological data is consistent with anatomic observations. The axonal projection patterns of nRt neurons (Cox et al. 1996
; Scheibel and Scheibel 1966
; Yen et al. 1985
) suggest widespread connections with large numbers of putative release sites in relay nuclei but few axon collaterals within nRt. In spite of this, the net effective inhibition, as measured by total Cl
flux during the evoked IPSC, was approximately equal in nRt and VB cells (Fig. 5F), indicating that the long duration of the intra-nRt IPSC compensates for the lower level of connectivity to produce equivalent inhibitory output.
), at 37°C slow decay would be 67% complete in ~70 ms, thus restricting network repetitive activity to frequencies less than ~14 Hz. The contribution of intra-nRt connectivity to rhythmic activity during sleep spindles remains controversial (McCormick and Bal 1994
; Steriade 1995
), but the prolonged IPSC decay demonstrated here is consistent with theoretical studies of spindlelike synchrony in an interconnected network of inhibitory cells (Destexhe et al. 1993
; Golomb et al. 1994
).
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ACKNOWLEDGEMENTS |
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CDR and SCAN software was graciously provided by Dr. J. Dempster.
This work was supported by National Institutes of Health Grant NS-06477 and training grant HL-07740 and Pimley Research and Training funds. S. J. Zhang was supported by Dr. Phyllis Gardner during completion of data analysis and manuscript preparation.
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FOOTNOTES |
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Present address of S. J. Zhang: Dept. of Molecular Pharmacology, Stanford University School of Medicine, Stanford, CA, 94305.
Address for reprint requests: J. R. Huguenard, Dept. of Neurology and Neurological Sciences, Room M016, Stanford University Medical Center, Stanford, CA 94305-5300.
Received 28 January 1997; accepted in final form 8 July 1997.
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REFERENCES |
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