[Involvement of serotoninergic mechanisms in the circling behaviour induced by apomorphine and LSD 25 in rats (author's transl)].

Studies with selective lesions of cerebral structures have revealed the involvement of serotonin in sleep and wakefulness. However, the mechanisms underlying the role of serotonin remain obscure. An appreciation of the interrelationships between serotonin and other transmitter systems might lead to a better understanding of the functions subserved by serotonin. A serotoninergic projection from the median raphé nucleus to dopamine neurons in the substantia nigra has been reported. We have therefore investigated the possible influence of the raphé system on the nigro striatal dopamine pathway in the rat using the "turning" model described by UNGERSTEDT (1971). An unilateral lesion in the median raphé nucleus induces contralateral turning similar to that observed following apomorphine administration to rats with a unilaterally degenerated nigro striatal pathway. Similarly, unilateral application of 2.5 micrograms of LSD 25 to the median raphé nucleus of normal rats also provokes contralateral turning. On the other hand, unilateral injection of LSD 25 into the zona compacta of the substantia nigra induces turning ipsilateral to the site of injection. The above results, and also those from several experiments in which apomorphine or LSD 25 was applied systematically, suggest that serotonin neurons in the median raphé nucleus exert an inhibitory influence on the dopaminergic nigro striatal system. These studies demonstrate that the serotoninergic median raphé system, in addition to its involvement in sleep and wakefulness, also exerts an important influence on motor functions.