Transmission processes in the ventrobasal complex of rat during the sleep-waking cycle

Abstract

The thalamocortical reactivity and the transmission processes in the thalamic ventrobasal complex were studied during the different stages of sleep and waking. The mass responses of somesthetic S1 cortex and diencephalic relay nucleus were elicited by stimulation of the medial lemniscus with two shocks of identical parameters. The thalamic afférents were studied by the amplitude of the lemniscal response induced by antidromic stimulation. The thalamocortical population response (tested by the amplitude of the positive surface 4 component of evoked potential) and the thalamic one (quantified by the amplitude of the postsynaptic r1 wave) varied in a parallel manner. During waking without theta activity, the thalamocortical and thalamic responses were maximal. They significantly decreased in the course of waking with theta (active and/or attentive) for a part, at least, because of a presynaptic inhibition of lemniscal terminals. During the different stages of slow sleep, thalamic transmission diminished to its minimal value during the intermediate stage which preceded and followed rapid sleep. During rapid sleep, thalamic transmission and cortical excitability significantly increased when the eye movements occurred in bursts, and the thalamic and thalamocortical excitability fell in part because of a depolarization of lemniscal afferents. The recovery cycle of the thalamocortical and thalamic responsiveness was long, reaching 500 msec. Its variations in the course of the different stages of sleep and waking give some complementary indications of the functional modulation of diencephalic and telencephalic levels. The results are interpreted with respect to data in the literature pertaining to sleep-waking mechanisms in the rat.