Injection of capsaicin into the nucleus raphe dorsalis elicits heat loss in the rat.

The acute thermoregulatory effect of microinjection of capsaicin into the nucleus raphe dorsalis (NRD) has been studied in anesthetised rats. Application of capsaicin (20 micrograms) into the NRD increased the tail skin temperature and decreased the body temperature. In animals that had previously been treated with capsaicin (50 mg/kg s.c.) the thermolytic effect of capsaicin microinjection was completely abolished. Additionally, the effects of NRD capsaicin injection on the monoamines in the preoptic region + hypothalamus (RPO-H) were studied. Levels of noradrenaline, dopamine, serotonin, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were determined by means of liquid chromatography. Neurochemical analysis of RPO-H showed no changes in the serotonin metabolism, but an elevated DOPAC level 30 min after the injection of capsaicin into the NRD.