Effects of adrenoceptor blocking agents on body temperature

Abstract

1 The effect on rectal temperature of adrenoceptor blocking agents, injected through a cannula chronically implanted into a lateral cerebral ventricle, was examined in unanaesthetized rabbits, cats and rats, kept at room temperature (19–22° C). 2 In rabbits, the α-adrenoceptor blocking agent phenoxybenzamine (50 or 100 μg) produced marked hypothermia when injected intraventricularly but not when injected intravenously. In some rabbits as little as 1 μg was effective on intraventricular injection. Phentolamine and ergotamine, the other α-adrenoceptor blocking agents examined, had a much weaker hypothermic action when injected intraventricularly, whereas the β-adrenoceptor blocking agents propranolol, pronethalol and Trasicor had no effect. 3 In rabbits in which the noradrenaline stores of the hypothalamus were depleted by intraventricular injections of reserpine, the hypothermic effect of phenoxybenzamine was abolished and remained abolished for a few days. 4 In cats, an intraventricular injection of phenoxybenzamine (200 μg) produced long-lasting hyperthermia, but not in all cats, and only with the first, or the first two or three injections. Injected intraperitoneally, this dose had no effect on temperature. Phentolamine (100 or 200 μg) had a very weak hyperthermic effect and phentolamine (500 μg), a hypothermic effect, but only on intraventricular injection, whereas ergotamine (100 and 200 μg) had a weak hyperthermic effect both on intraventricular and intraperitoneal injection. Propranolol and Trasicor had no effect on temperature when injected intraventricularly. 5 In rats, phenoxybenzamine (5 or 20 μg) produced long-lasting hypothermia on intraventricular injection. 6 Some of the temperature effects produced by intraventricular injections of the α-adrenoceptor blocking agents are explained on the assumption that they prevent the effect on temperature produced by a continuous release of noradrenaline from adrenergic neurones innervating the anterior hypothalamus.