Intracerebroventricular capsaicin and food intake in the rat

To investigate whether capsaicin affected food intake and body weight and/or interfered with the mechanisms regulating the body temperature, capsaicin was administered intracerebroventricularly to rats at a daily dose of 25 micrograms of 5 microliters of vehicle for 10 consecutive days. The vehicle consisted of 5% ethanol plus 5% Tween 80 in isotonic saline. The ambient temperature (Ta) was 22 degrees C. Two control groups were used: the first group was treated with the vehicle but its concentration was lowered 10 times in order to avoid the wellknown brain toxicity of Tween 80; the second group was injected with NaCl 0.9% alone. The subsequent observation period lasted 38 days and the rats were exposed to a Ta of 22, 32, 35, 10 and 22 degrees C for different times. Food intake and body weight showed a remarkable decrease during the treatment period in both the capsaicin- and vehicle-treated groups. During the observation period, food intake behavior and body weight evolution were different among the three groups of treated rats. The influence of Ta was most apparent in the capsaicin-treated rats that showed a long-lasting deficit in their body weight and a clear inability to regulate their body temperature (Tb) in a warm environment (Ta 35 degrees C). However, the capsaicin-treated rats began to recover from the weight loss when they were finally housed at Ta 22 degrees C. Tween 80-treated rats began to recover from the weight loss earlier, the pretreatment body weight having been reached during Ta 32 degrees C exposure, i.e., 12 days after the end of the treatment. Since capsaicin did not reduce food intake for a long time during the recovery process, the effect of capsaicin on Ta and body weight seems to be related to metabolic processes. Under the conditions of the present study, neither capsaicin nor Tween 80 seem to have permanently impaired the animals' energy balance regulation.