Electrophysiological properties of cat reticular thalamic neurones in vivo.

Abstract

1. The electrophysiological properties of neurones of the reticular thalamic (RE) nucleus were studied in acutely prepared cats under urethane anaesthesia. 2. Two main types of neuronal firing were recorded. At the resting membrane potential (-60 to -65 mV) tonic repetitive firing was elicited when the cell was activated synaptically or by current injection. From membrane potentials more negative than -75 mV, synaptic or direct stimulation generated a burst of action potentials. 3. The burst of RE cells consisted of a discharge of four to eight spikes riding on a slowly growing and decaying depolarization. The discharge rate during the burst showed a characteristic increase, followed by a decrease in frequency. 4. The burst response behaved as a graded phenomenon, as its magnitude was modulated by changing the intensity of the synaptic volley or the intensity of the injected current. 5. Spike-like small potentials presumably of dendritic origin occurred spontaneously and were triggered by synaptic or direct stimulation. They were all-or-none, voltage-dependent events. We postulate that these spikes originate in several hot spots in the dendritic arbor, with no reciprocal refractoriness and may generate multi-component depolarizations at the somatic level. 6. Excitatory postsynaptic potentials (EPSPs) evoked by internal capsule stimulation consisted of two components, the late one being blocked by hyperpolarization. Such compound EPSPs were followed by a period of decreased excitability during which a second response was diminished in amplitude. 7. A series of depolarizing waves at the frequency range of spindle oscillations was triggered by internal capsule stimulation. The individual depolarizing waves constituting the spindle oscillation gradually decreased in amplitude when decreasing the intensity of the stimulation. 8. These results, showing that RE cells are endowed with an excitable dendritic tree and a graded bursting behaviour, support the proposed role of RE nucleus as the generator and synchronizer of spindle rhythmicity.

Spike-and-Wave Oscillations Based on the Properties of GABABReceptors

“…If cortical EPSPs and I PSPs from RE cells were of comparable conductance, cortical feedback could not evoke oscillations in the thalamic circuit because of shunting effects between EPSPs and I PSPs (Destexhe et al, 1998a). The most likely reason for these experimental and modeling evidences for "IPSP dominance" in TC cells is that RE cells are extremely sensitive to cortical EPSPs (Contreras et al, 1993), probably because of a powerf ul T-current in dendrites (Destexhe et al, 1996b). In addition, cortical synapses contact only the distal dendrites of TC cells (Liu et al, 1995) and probably are attenuated for this reason.…”

“…The model also emphasizes a major role for corticothalamic feedback in triggering powerf ul bursts in RE cells, by which the cortex can force the thalamus to generate oscillations at ϳ3 Hz by activating intrathalamic GABA B -mediated inhibition. Because thalamic RE cells may generate bursts through dendritic T-currents (Destexhe et al, 1996b), their sensitivity to corticothalamic feedback EPSPs therefore may be maximal (Contreras et al, 1993), leading to the prediction that this structure should be a major target for a possible suppression of seizures.…”

Inhibitory Interactions between Perigeniculate GABAergic Neurons

“…Perigeniculate and thalamic reticular cells generate action potentials in two distinct firing modes: single spike activity and low threshold C a 2ϩ spike-mediated high-frequency bursts (Mulle et al, 1986;Avanzini et al, 1989;Bal and McCormick, 1993;Contreras et al, 1993). The activation of PGN-mediated IPSPs in PGN cells was capable of inhibiting not only single-spike activity but also determining whether or not the postsynaptic PGN cell generated a low threshold C a 2ϩ spike and a burst of action potentials.…”

Rebound burst firing in the reticular thalamus is not essential for pharmacological absence seizures in mice

“…The TRN is a shell-like structure that covers most of the rostral, lateral, and ventral parts of the thalamus (5) and is composed exclusively of GABAergic interneurons that provide massive inhibitory input to TC neurons (9). The most distinctive feature of thalamocortical circuitry is its intrinsic ability to generate oscillations via the reciprocal circuits between TC and TRN neurons (10-12).Both TC and TRN neurons are able to generate two distinctive patterns of action potential firing: tonic and burst (13,14). Burst firing is mediated by low-voltage-activated (LVA) T-type Ca 2+ channels (15).…”

“…Both TC and TRN neurons are able to generate two distinctive patterns of action potential firing: tonic and burst (13,14). Burst firing is mediated by low-voltage-activated (LVA) T-type Ca 2+ channels (15).…”