Anterior hypothalamic lesions impair reflex bradycardia selectively in rats

To determine whether removal of the anterior hypothalamus would alter baroreflex activity, reflex inhibition of heart rate and renal nerve activity during pressor responses to intravenous phenylephrine was examined in rats with bilateral anterior hypothalamic lesions. Although there were no differences in basal blood pressure and heart rate 10 days after surgery, reflex bradycardia was significantly reduced in awake lesioned rats compared with sham-operated controls. The differences in reflex bradycardia were abolished by cholinergic blockade with atropine but unaltered by beta-adrenergic blockade with propranolol. Because attenuation of chronotropic reflexes by anterior hypothalamic lesions resembled that produced by cholinergic blockade, vagal inhibition must be at least partly responsible. Reflex bradycardia was still attenuated even in lesioned rats anesthetized with alpha-chloralose, but renal nerve activity was as inhibited as in sham-operated rats. These results suggest that the anterior hypothalamus participates in baroreflex regulation of heart rate by altering parasympathetic tone but without affecting cardiac or renal sympathetic nerve activity.