Effect of capsaicin on morphine analgesia--possible involvement of hypothalamic structures.

1. Rats were treated with 50 mg/kg capsaicin on the second day of life or at the age of 2--3 months. The effect of morphine on the nociceptive threshold, as determined by the reaction time in tail withdrawal test, was measured 3--4 and 1--2 months after capsaicin pretreatment, respectively. 2. The analgesic effect of morphine was markedly attenuated in rats treated with capsaicin in the adult age, while neonatal capsaicin treatment did not affect morphine analgesia. 3. Pretreatment of adult rats with capsaicin results in the impairment of certain hypothalamic preoptic neurones, while neonatal capsaicin treatment induces selective degeneration of chemosensitive primary sensory neurones without affecting hypothalamic neurones. Therefore, it is suggested that in the analgesic effect of morphine the capsaicin-sensitive neurones of the preoptic area are involved, and the contribution of spinal mechanisms might be of minor importance. Thus, the preoptic region may be an important link in endogenous pain controlling systems.