Capsaicin pretreatment interferes with the thermoregulatory effect of morphine.

Body temperature changes after the administration of 8 mg/kg morphine sulphate were studied in freely-moving and restrained rats, which were pretreated with capsaicin (300 mg/kg). Morphine caused a hyperthermic response irrespective of the previous capsaicin treatment in freely-moving rats; the lag time from the application of morphine to the hyperthermic maximum was, however, significantly delayed in capsaicin pretreated rats. A careful habituation to the experimental procedure (including injections, taking temperature, etc.) facilitated the hyperthermic response both in the pretreated and the control groups. Restrained rats typically showed a hypothermic response to the same dose of morphine. The drop of body temperature in habituated animals was significantly larger in the case of capsaicin pretreated rats. This potentiation of the hypothermic effect of morphine can be regarded as the primary site of interaction between capsaicin and morphine. Since capsaicin pretreated rats also showed an exaggerated thermoregulatory response to experimental stress, we conclude that the thermoregulatory effects of morphine and endogenous opiates are facilitated after capsaicin pretreatment.