Effects of mesencephalic central gray and raphe nuclei lesions on hypothalamically induced escape.

Central gray (CG) lesions, in particular those located in its anterior part, provoked in about half of the lesioned rats a decrease in switch-off responses (SOR) induced by medial hypothalamic stimulation. Such a decrease in performance, followed by a partial or even total recovery, occurred only when the stimulation site was located in the postero-medial part of the medial hypothalamus. Occurrence and duration of the decrease in SOR did not depend on a possible severance of noradrenergic fibers that ascend from the locus coeruleus and course along the CG, since bilateral destruction of the locus coeruleus did not alter SOR. On the other hand, a lasting facilitation of SOR was observed in those rats in which the CG lesion extended more caudalle fully explained by an interruption of serotonergic fibers ascending from the raphé nuclei since lesions limited to the dorsal nucleus as well as combined lesions of the dorsal and medial raphé nuclei induced only a transient facilitation, whereas a lasting one was provoked both by raphé lesions that encroached upon the ventro-medial tegmentum and by CG lesions that only partially destroyed the dorsal raphé nucleus.