Thermoregulatory effects of intrahypothalamic injections of adrenergic and cholinergic substances at different environmental temperatures.

1. Rats were unilaterally implanted with chronic indwelling cannulae directed towards the medial preoptic area of the hypothalamus. Carbamylcholine chloride (carbachol), noradrenaline, and saline were microinjected in volumes of 0.5 mul. at each of three environmental temperatures, 5, 24, and 35 degrees C.2. At an ambient temperature of 24 degrees C injections of carbachol significantly raised core temperature, whereas injections of noradrenaline to the same sites significantly lowered temperature.3. Injections of carbachol did not affect core temperature at 35 degrees C, but injections of noradrenaline defended against the normal hyperthermia observed in this environment.4. Hyperthermia resulted from hypothalamic injections of both substances at an ambient temperature of 5 degrees C.5. The data generally support a neurochemical model for hypothalamic thermoregulatory mechanisms proposed by Myers & Yaksh (1969). According to their theory a cholinergically coded heat production pathway courses from the anterior to the posterior hypothalamus. This pathway is activated by 5-HT and inhibited by noradrenaline in the anterior hypothalamus. The observation in the present experiments of hyperthermia following adrenergic stimulation at 5 degrees C cannot be accounted for by this model and therefore an addition to the model is proposed. It is suggested that a heat loss mechanism is also located, at least partially, in the anterior hypothalamus and that this system is inhibited by noradrenaline. Thus a thermal balance is established via an interaction of the two systems.