Organization of the lateral hypothalamus for control of adrenocorticotropin release in the cat.

Regions in the ventral midbrain that project to the lateral hypothalamus have been implicated in the control of ACTH release. To define further those areas in the lateral hypothalamus through which afferent signals might pass, we electrically stimulated 188 sites in the lateral hypothalamus of 20 cats anesthetized with chloralose-urethane. Stimulations were monophasic pulses of DC (200 microA; 0.2 msec; 100 Hz; 20 sec). Venous samples were drawn over 30 sec 0.5 min before and 1.5 min after stimulation. Equal volumes of warmed isoncotic dextran were infused during sampling to prevent hypovolemia. ACTH was assayed by RIA. Areas were defined in which stimulation led to increased, decreased,, or unchanged ACTH. Mean changes in ACTH were tested by analysis of variance. The present data indicate that the ACTH-active areas defined previously in the midbrain may join the medial forebrain bundle in the subthalamic area and nucleus to traverse the lateral hypothalamus. At the level of the mammillary bodies, a facilitatory area occupied the ventral portion of the medial forebrain bundle. This area extended rostrally and medially to join the medial aspect of the medial forebrain bundle. Continuity with the mediobasal hypothalamus was seen only anteriorly in the area of the supraoptic decussations. An inhibitory area occupied the dorsal extent of the medial forebrain bundle at the level of the mammillary bodies. It extended rostrally and laterally around the caudal pole of the supraoptic nucleus and then medially at the level of the optic chiasm. There appear to be no other medial projections of the lateral lying ACTH-active areas to the mediobasal hypothalamus. The lateral hypothalamus may serve as a site of passage and/or of processing of information that ascends from the midbrain and descends from the limbic system.