This study is aimed at the investigation of the morphological and electrophysiological characteristics of neurons from the nucleus reticularis thalami in rat thalamic slices incubated in vitro. Ten neurons were recorded in the ventrobasal complex, four of which were successfully injected following horseradish peroxidase injection. Two main types of reticular thalamic neurons were morphologically identified: (1) the small fusiform "f" cells characterized by a very elongated perikaryon, dendritic arborization prevalent in the rostrocaudal and dorsoventral planes and an axon without any collaterals branching within the nucleus reticularis thalami; and (2) the large fusiform 'F' neurons with dendrites arborizing mainly in the horizontal plane and with axonal branches within the nucleus reticularis thalami. The electrophysiological properties of the neurons were similar in F and f cells. The reticular neurons showed, in resting conditions, a single spike response followed by a postexcitatory hyperpolarizing potential. The hyperpolarization of these neurons transformed the single spike response into a burst discharge similar to that observed in thalamic relay neurons at resting membrane potential. The same phenomenon was observed when bicuculline was administered by perfusion to the slices and, in this case, a recovery to a single spike response was obtained by a depolarizing d.c. current injection. By contrast, the local administration of GABA induced a depolarization with a pronounced decrease in input resistance. The present data demonstrate the presence of at least two neuronal subtypes within the nucleus reticularis thalami, suggesting that only one is responsible for the phenomenon of auto-inhibition by means of intrinsic axon collaterals. Moreover, it is hypothesized that intranuclear GABAergic collaterals could control neuronal excitability of reticular thalamic cells by both shunting the membrane and shifting the burst firing to a single spike firing mode.
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