A single-unit analysis of reciprocal relationships between the lateral and medial hypothalamus in rats

The spontaneous activity of hypothalamic units was studied in chronic awake and ketamine-anesthetized rats. Ketamine appeared to decrease significantly the spontaneous firing of neurons in the ventromedial hypothalamic nucleus (VMH) but did not influence the activity in either the dorsomedial hypothalamic nucleus (DMH) or the lateral hypothalamic area (LHA). The responses obtained in the medial hypothalamus and the LHA after stimulation of the LHA and VMH, respectively, were not modified in either of the recording conditions. These responses corresponded to a pause in spontaneous activity, sometimes associated with a short-latency excitatory response, orthodromic or antidromic. The possibility of direct reciprocal connections between the LHA and the VMH has been discussed on the basis of electrophysiological and anatomical studies.