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

Abstract

There is evidence to suggest that within the hypothalamus noradrenaline (NA) is an inhibitory neurotransmitter acting on both the heat production and heat loss pathways in the rabbit. Further, it has been proposed that the inhibition of the heat loss pathway which results in hyperthermia is mediated primarily through α-adrenergic receptors within the anterior hypothalamic–preoptic area. We have investigated the effects of the α-receptor agonist methoxamine, administered directly into the cerebral ventricles, on body temperature at various ambient temperatures in both the shorn and unshorn rabbit. 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. It is already known that the β-adrenergic agonist isoprenaline produces little or no effect on body temperature following intracranial application at ambient temperatures above 18 °C. In our experiments conducted at the lower ambient temperature, it produced a pronounced dose-dependent fall in body temperature in the shorn rabbit. The results of this work support 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. Further, it would appear that inhibition of the heat loss pathway is largely mediated through α-adrenergic receptors, whilst the inhibition of the heat production pathway is mediated to a large extent through β-adrenergic receptors.