Thermoregulatory deficits in the monkey produced by 5,6-dihydroxytryptamine injected into the hypothalamus

Abstract

In 7 male rhesus monkeys ( Macaco mulatta), an array of 4–8 guide cannulae was implanted stereotaxically so that the tips rested just above the intended sites of microinjection in the rostral and other regions of the hypothalamus. To examine the thermoregulatory capacity of each monkey, the air in their chair chamber was elevated to 40°C or cooled to 4°C by a stream of warm or cold air, respectively, for an interval of 3 hr. After a baseline temperature was recorded colonically, serotonin was microinjected in 1.0–1.5 μl volumes in doses of 3.5 to 7.5 μg at a total of 96 sites in order to determine the sensitive loci at which the indoleamine evoked hyperthermia. 5,6-Dihydroxytryptamine (5,6-DHT), an agent which destroys serotonergic nerve endings, was injected on a different day, in doses of 2–7 to 12. μg in a similar volume at the serotonin-sensitive sites. The initial effect of 5,6-DHT was dose-dependent hyperthermia similar to that evoked earlier by serotonin. Severe thermoregulatory deficits in response to the cold challenge were observed on subsequent days after low doses of 5,6-DHT were injected primarily in the region of the anterior hypothalamus. Impairment in thermoregulation to the warm temperature also occurred after higher doses of the neurotoxin were given. However, serotonin still evoked a temperature response nearly identical to that seen before 5,6-DHT treatment. These results support the theory that serotonergic neurones in the hypothalamus are involved in the central control of body temperature in the primate.