g-Aminobutyric acid type B receptor-dependent burst-firing in thalamic neurons: A dynamic clamp study (rebound burstsyoscillationsysynchronization)

Synchronized network responses in thalamus depend on phasic inhibition originating in the thalamic reticular nucleus (nRt) and are mediated by the neurotransmitter g-aminobutyric acid (GABA). A suggested role for intra-nRt connectivity in inhibitory phasing remains controversial. Recently, functional GABA type B (GABAB) receptors were demonstrated on nRt cells, and the slow time course of the GABAB synaptic response seems ideally suited to deinactivate low-threshold calcium channels. This promotes burst firing, a characteristic feature of synchronized responses. Here we investigate GABAB-mediated rebound burst firing in thalamic cells. Whole-cell current-clamp recordings were obtained from nRt cells and somatosensory thalamocortical relay cells in rat brain slices. Synthetic GABAB inhibitory postsynaptic potentials, generated by a hybrid computer– neuron synapse (dynamic clamp), triggered rebound lowthreshold calcium spikes in both cell types when peak inhibitory postsynaptic potential hyperpolarization was greater than292 mV. The threshold inhibitory postsynaptic potential conductance for rebound burst generation was comparable in nRt (7 nS) and thalamocortical (5 nS) cells. However, burst onset in nRt (1 s) was considerably delayed compared with thalamocortical (0.6 s) cells. Thus, GABAB inhibitory postsynaptic potentials can elicit low-threshold calcium spikes in both relay and nRt neurons, but the resultant oscillation frequency would be faster for thalamocortical–nRt networks (3 Hz) than for nRt–nRt networks (1–2 Hz). We conclude, therefore, that fast (>2 Hz) GABAB-dependent thalamic oscillations are maintained primarily by reciprocal connections between excitatory and inhibitory cells. These findings further indicate that when oscillatory neural networks contain both recurrent and reciprocal inhibition, then distinct population frequencies may result when one or the other type of inhibition is favored. The thalamus is capable of generating multiple types of oscillations that act as pacemakers of thalamocortical (TC) rhythms. Among those are spindle waves and delta oscillations (reviewed in refs. 1 and 2), which regulate the global attentive states of the forebrain and are responsible for some pathological forms of spike and wave discharges in epilepsy. The oscillations primarily depend on mutual connectivity of TC relay cells and inhibitory neurons of the adjacent nucleus reticularis thalami (nRt) and the propensity of both cell types to fire bursts of action potentials from a relatively hyperpolarized resting state. Burst firing in thalamic cells is mediated by a low threshold spike (LTS) that is generated by lowvoltage-activated (T-type) calcium channels (1, 2). T channels in TC and nRt cells can be distinguished by their biophysical properties as well as by their somatodendritic distribution. In nRt cells, (i) T channels are thought to be located mainly in the dendrites (3, 4); and (ii) they have a slower time course of inactivation and deinactivation and a higher threshold of activation compared with TC cells (5). nRt cells receive excitatory synaptic input from relay cells and, in turn, generate inhibitory postsynaptic potentials (IPSPs) in TC cells. These IPSPs have both g-aminobutyric acid type A (GABAA) and type B (GABAB) receptor-mediated components and are capable of inducing rebound burst firing in relay cells (6, 7). The GABAA component is critical for spindle generation, whereas the GABAB component mediates slower, more synchronous epileptiform oscillations (6). The slow time course of the GABAB-mediated IPSP is particularly suited to deinactivate T channels in relay cells (reviewed in ref. 8), but a similar action has not yet been shown in nRt cells. The role of nRt proper in synchronizing and maintaining thalamic oscillations is controversial. The surgically isolated nRt sustains spindle activity, suggesting a pacemaker role of this nucleus (9). This view has recently been supported by computer models of nRt showing that assemblies of interconnected nRt cells segregate into different cell clusters, which maintain oscillations by burst firing out of phase (10). In contrast, disconnection of nRt from the relay nuclei abolished spindle waves in slices of the ferret lateral geniculate nucleus, indicating that the spindles result from network interactions between relay and nRt cells (7). In rodents, nRt cells are interconnected by axon collaterals (e.g., ref. 11). These recurrent connections appear to be mediated mainly by GABAA receptors (12–14). As yet, the net effect of this lateral inhibition remains uncertain, although transient hyperpolarizations in nRt cells can trigger rebound bursts of action potentials (15, 16). Direct application of GABA to nRt cells leads to depolarization or weak hyperpolarization (17, 18), leaving some uncertainty about the net effect of a GABAA receptor-mediated increase in chloride conductance in these cells. However, focal application of the GABAA receptor antagonist bicuculline within nRt results in an increased output from this nucleus, suggesting a disinhibitory effect of GABAA receptor blockade (7, 13). In addition, intra-nRt GABAB IPSPs have been investigated by computer simulations, demonstrating their potential role in synchronizing burst firing in this nucleus (19). In a recent in vitro study, we found a weakGABAB-mediated component in a subpopulation of intra-nRt synapses (20). However, the maximally evokable GABAB synaptic responses in our preparation were quite small (,0.2 nS), which prevented us from systematically examining their role on thalamic cell firing. Therefore, in the present study, we investigate GABAB receptor-mediated burst firing in nRt and relay cells by using a hybrid neuron–computer synapse generated by a dynamic clamp (21). This method allowed us to examine in an in vitro The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. Abbreviations: GABA, g-aminobutyric acid; GABAA and GABAB, GABA types A and B, respectively; IPSP, inhibitory postsynaptic potential; LTS, low threshold spike; nRt, nucleus reticularis thalami; TC, thalamocortical. *To whom reprint requests should be addressed at: Department of Neurology andNeurological Sciences, RoomM016, StanfordUniversity School of Medicine, Stanford, CA 94305-5300. e-mail: huguenar@ leland.stanford.edu.