Cerebral hypnogenic centers.

Active cerebral processes can initiate and maintain sleep in mammals. Two distinct hypnogenic structures have been documented in the cat: the nucleus of the solitary tract in the medulla and a region in the basal preoptic area. Their electrical or pharmacological activation results in behavioral and EEG sleep. Both can be activated an inhibited in relation to circulatory or thermic homeostasis and to endocrine regulations, or in conditioned paradigms. Bilateral destruction or functional elimination of either hypnogenic region is followed by increased vigilance and insomnia. The tonic influence so revealed is increased by the serotonergic raphe nuclei of the pontine region, which is considered to be hypnotonic rather than hypnogenic. Postsynaptic inhibition of the ascending activating reticular system by the hypnogenic centers is involved in sleep onset and maintenance. Electrophysiological evidence is presented that indicates a mutual tonic inhibitory interaction between the brainstem arousal system and the preoptic hypnogenic center. Such reciprocal inhibitory interaction can facilitate the onset of sleep by a positive-feedback mechanism. In spite of the fact that this retroaction results in break of physiological equilibrium, it should be considered a homeostatic process aimed at maintaining the functional fitness of the forebrain neuronal circuits.