Effects of intracerebroventricular capsaicin on thermoregulatory behavior in the rat

Abstract

To clarify the action of capsaicin on the thermoregulatory system of rat, behavioral and autonomic responses were studied following intracerebroventricular (ICV) injection. Rats were chronically implanted with a lateral cerebral ventricular guide cannula. After the recovery period they were placed in a climatic chamber at ambient temperature (T a) of 20, 30 or 35°C. In the first series of experiments, they had access to a lever which activated a fan that drew cool outside air into the chamber. After ICV capsaicin (23 μg), the rats increased bar-pressing behavior for fresh air at T a ranging from 20°C to 35°C. In the second series of experiment, the rats had no access to fanning. ICV capsaicin produced a fall in rectal and hypothalamic temperature (T hy) and increased n cutaneous temperature. These changes depended on T a. At a T a of 30°C T hy fell slightly (mean of 0.2±0.16°C. At a T a of 20°C T hy fell to a mean of 1°C±0. 17°C. The conclusion drawn is ICV capsaicin activated behavioral as well as autonomic thermoregulatory heat-loss responses. The effect of capsaicin resembles the effect of local heating of the hypothalamus. However, since hypothalamic temperature decreased the drug may have lowered the thermal set point, or excited directly hypothalamic warm-sensitive neurons.