THE ROSTRAL HYPOTHALAMUS - AN AREA FOR THE INTEGRATION OF AUTONOMIC AND SENSORY RESPONSIVENESS

1. An investigation has been made of the effects of chemical stimulati on of neurons in the rostral hypothalamus on cardiovascular and respir atory parameters and on a nociceptive viscerosomatic reflex (reflex ac tivity recorded from a lumbar spinal nerve in response to electrical s timulation of visceral afferent fibers in the splanchnic nerve) in alp haxalone/alphadolone-anesthetised rats. 2. Neurons were stimulated by microinjection of DL-homocysteic acid (DLH) at ventromedial forebrain sites between 2 mm caudal and 3 mm rostral to bregma. Any changes were monitored in viscerosomatic reflex activity and in the rate and depth of respiration, arterial blood pressure, heart rate, and hindlimb mus cle blood flow. 3. Viscero-somatic reflex activity was depressed to va rying degrees after injection of DLH at 20/96 sites and this was alway s accompanied by some form of cardiovascular change. At 8 sites inhibi tion of the reflex was accompanied by a cardiovascular response that i ncluded an initial pressor response (type I response) and at the other 12 sites there was an initial depressor response (type II response). Injection of DLH at a further 30 sites evoked a depressor response and no change in viscerosomatic reflex activity and at the remaining 45 s ites there was no detectable change in any of the variables measured. 4. The degree of inhibition evoked from type I sites (mean 90%) was si gnificantly greater (P < 0.001) than that evoked from type II sites (m ean 58%). Type I responses were only evoked from an extremely localize d region in the rostral anterior hypothalamus that corresponds to the lateral area of the anterior hypothalamus. 5. The results of this stud y suggest that neurons within a circumscribed region of the anterior h ypothalamus may integrate descending control of nociceptive processing with sympathoexcitation. The functional significance of these finding s is discussed.