Effects of adrenalectomy, propranolol and methylatropine on the increase in heart rate induced by injection of dermorphin in the rat anterior hypothalamic nucleus.

Anterior hypothalamic injections of 40 pmol dermorphin, a potent opiate receptor agonist, increased heart rate 17% and had no effect on blood pressure in halothane-anesthetized rats. Administration of the beta-receptor antagonist, propranolol, during the peak response to dermorphin, reduced the heart rate to levels not different from pretreatment control; pretreatment with propranolol completely blocked the tachycardia produced by a subsequent injection of dermorphin. In contrast, neither adrenalectomy nor pretreatment with methylatropine altered the response to dermorphin. These data suggest that increased activity of the sympathetic nervous system, primarily to the heart, and not increased release of adrenal catecholamines or inhibition of parasympathetic nervous system activity, is responsible for the increase in heart rate resulting from injection of dermorphin into the anterior hypothalamic nucleus.