Thermoregulatory effects of methoxamine and isoprenaline following injections into the cerebral ventricles of conscious rabbits

Abstract

Within the hypothalamus noradrenaline [norepinephrine] (NA) appears to be an inhibitory neurotransmitter acting on the heat production and heat loss pathways in the rabbit. The inhibition of the heat loss pathway which results in hyperthermia may be mediated primarily through .alpha.-adrenergic receptors within the anterior hypothalamic-preoptic area. The effects of the .alpha.-receptor agonist methoxamine, administered directly into the cerebral ventricles, on body temperature at various ambient temperatures in both the shorn and unshorn rabbit was examined. At all ambient temperatures tested, administration of methoxamine into a lateral cerebral ventricle produced a gradual dose-dependent hyperthermia. The magnitude of the hyperthermic response diminished with decreasing ambient temperatures. The .beta.-adrenergic agonist isoprenaline produces little or no effect on body temperature following intracranial application at ambient temperatures above 18.degree. C. In the experiments conducted at the lower ambient temperature it produced a pronounced dose-dependent fall in body temperature in the shorn rabbit. The suggestion that NA can act as an inhibitory substance on the heat production or heat loss pathway in the rabbit. Which pathway is inhibited at any one time is dependent on the ambient temperature. Inhibition of the heat loss pathway is probably largely mediated through .alpha.-adrenergic receptors; the inhibition of the heat production pathway is mediated to a large extent through .beta.-adrenergic receptors.