(D-Met2,Pro5)-enkephalinamide-induced blockade of ovulation and its reversal by naloxone in the rat.

The effects of (D-Met2,Pro5)-enkephalinamide--a superactive enkephalin analogue--on the preovulatory discharge of gonadotrophic hormones and on ovulation were investigated. Injection of the opioid (16 nmol) into a lateral cerebral ventricle of regularly cycling female rats immediately before the critical period on the day of proestrus resulted in a blockade of ovulation and in a concomitant depression of the preovulatory plasma luteinizing hormone and follicle-stimulating hormone levels. A smaller dose of the opioid (2 nmol) did not inhibit ovulation. The inhibition of the preovulatory gonadotrophic hormone discharge and the blockade of ovulation by the opioid were reversed by naloxone administration. Our data are consistent with the view that the endogenous opioid peptides may be involved in the physiological regulation of the central neural events which lead to ovulation in the rat.