RESUMEN
AIM To study the analgesic effects of melatonin (MT) and related mechanisms. METHODS The threshold was measured by hot plate test.? endorphine (? EP) was determined by radioimmunoassay (RIA). RESULTS MT (40～160 mg?kg -1 , ip) had significant dose and time dependent analgesic effects in hot plate test. The analgesia was also induced by injection (icv) of MT (0 25 mg?kg -1 ). The day night rhythm of pain threshold was not observed in 7 d after pinealectomy, but it appeared in sham operated rats.? EP content of rat brain measured by RIA indicated that the level of ? EP in hypothalamus and pituitary decreased significantly 1 h after ip MT(100 mg?kg -1 ). The analgesic effect of MT was attenuated by pretreatment of reserpine (3 mg?kg -1 ip) or ? receptor blocker phentolamine (100 mg?kg -1 , sc). CaCl 2 (230 mg?kg -1 ) antagonized the analgesic effect of MT, and EGTA and verapamil significantly enhanced the analgesia of MT in the same situation. CONCLUSION MT has significant analgesic action which may be related to opiate system, adrenergic system, and Ca 2+ content.