Interaction between the baroreflex and anterior hypothalamic stimulation. Demonstration of a noradrenergic involvement.

Cats, anaesthetized with alpha-chloralose, were used to investigate an interaction between the baroreceptor reflex and stimulation of the anterior hypothalamus (AH). In these cats, stimulation of the anterior hypothalamus gave autonomic responses that characterize the defense reaction. Stimulation of the anterior hypothalamus also interacted with the baroreceptor reflex without altering the effectiveness of the vasodepressor component of the reflex (induced by stimulation of the aortic nerves). However, in spinal cats the bradycardia component (induced by angiotensin at 0.097 nmol/kg pe min, i.v.) was inhibited by stimulation of the anterior hypothalamus and the inhibition was augmented by pretreatment with phentolamine (17.7 nmol) in the nucleus tractus solitarius (NTS). Pretreatment with phentolamine (17.7 nmol) in the nucleus tractus solitarius also reduced the sensitivity of the bradycardia component of the reflex and attenuated (3 and 6 nmol) the induced inhibition of the bradycardia induced by stimulation of the anterior hypothalamus. This attenuation induced by NE was abolished by pretreatment with phentolamine (17.7 nmol), but not by pretreatment with sotalol (16 nmol) or haloperidol (12 nmol) in the nucleus tractus solitarius. Additionally, NE was released from the nucleus tractus solitarius during activation of the baroreceptor and during stimulation of the anterior hypothalamus in cats with intact baroreceptor nerves, but not in baroreceptor-denervated cats. Moreover, the baroreceptor reflex attenuated the pressor response evoked by stimulation of the anterior hypothalamus, and also caused the release of NE from the anterior hypothalamus. At this site, injection of NE (3 and 6 nmol) attenuated the pressor response evoked by stimulation of the anterior hypothalamus and this attenuation was abolished by pretreatment with phentolamine (17.7 nmol), but not by pretreatment with sotalol (16 nmol) or haloperidol (12 nmol) in the anterior hypothalamus. It is concluded that NE participates at the nucleus tractus solitarius and the anterior hypothalamus as an attenuator in the interaction between the baroreflex and stimulation of nerve fibers in the anterior hypothalamus that mediate the defense reaction.