Cardiovascular responses elicited by hypothalamic stimulation in rabbits reveal a mediolateral organization

Heart rate and blood pressure response patterns were elicited by relatively low intensity (approximately 0.2 mA) stimulation (10 s trains; 100 pulses/s; 0.25 ms pulse duration) in different regions of the hypothalamus in anesthetized rabbits. A bradycardia/depressor pattern was elicited from both anterior and posterior stimulating electrode sites in the far lateral hypothalamus. Medial sites including the ventromedial hypothalamus elicited a tachycardia/pressor pattern. A bradycardia/pressor pattern was elicited from both the anterior and posterior hypothalamus at sites between the lateral hypothalamic (bradycardia/depressor) and the medial hypothalamic (tachycardia/pressor) zones. Paralysis by injection of decamethonium in conjunction with artificial ventilation did not reveal systematic differences in cardiovascular response threshold, form, or magnitude between the paralyzed and non-paralyzed state, indicating that the cardiovascular response patterns were not secondary to changes in respiration. Bilateral vagotomy abolished the bradycardia responses evoked by hypothalamic stimulation indicating that the cardiodeceleration was mediated by the vagus nerves; vagotomy attenuated tachycardia responses indicating that the responses in part reflected a decrease in vagal restraint. Bradycardia responses to ipsilateral but not contralateral hypothalamic stimulation were attenuated or abolished when examined 10-14 days after unilateral destruction of the central nucleus of the amygdala. This indicates that the bradycardia responses elicited by hypothalamic stimulation are in part mediated by fibers of passage. Stimulation of lateral hypothalamic sites eliciting a bradycardia/depressor pattern in anesthetized rabbits, evoked mild orienting in the same animals when conscious, whereas medial hypothalamic sites producing the tachycardia/pressor response pattern in anesthetized animals elicited intense excitement with autonomic concomitants in the nonanesthetized state. The results indicate that the rabbit hypothalamus has a mediolateral organization in terms of at least some cardiovascular and behavioral responses to intracranial stimulation.