Morphine inhibition of the preovulatory surges of plasma luteinizing hormone and follicle stimulating hormone in the rat

A study was made of the effects on the preovulatory surges of LH and FSH exerted by morphine sulfate (M) administered at 1400 h on proestrus, the beginning of the critical period for gonadotropin release in cycling rats in our colony. Various doses of M were given ip and jugular venous blood was collected under ether anesthesia for radioimmunoassay of plasma levels of LH and FSH. To verify ovulation, oviducts were examined the following day for the presence of ova. A high dose of M (60 mg/kg) totally abolished the LH surge and partially prevented the FSH surge while completely blocking ovulation. With intermediate doses of M (20-30 mg/kg) about half of the rats showed undiminished elevations of plasma LH at 1800 h and complete ovulation the next morning, whereas the other half revealed almost total blockade of LH release and no ovulation. Following a low dose of M (10 mg/kg) a small but significant (P < 0.05) augmentation of LH levels was observed in two of three experiments and all animals receiving this dose ovulated. Pretreatment with naloxone (10 mg/kg) did not itself affect LH or FSH release or ovulation but reversed the effect of the high dose of M and permitted complete ovulation. At 1800 h the pituitaries of control and Mblocked rats responded about equally to exogenous LHRH (100 ng iv) in 15 min increments of plasma LH and FSH and all of the rats ovulated. A timecourse study indicates that the onset of the LH surge may be somewhat delayed but the amplitude was undiminished following administration of a low dose of M. These findings indicate that M exerts dose-dependent effects on the preovulatory discharge of pituitary gonadotropins in the cycling rat.