Blockade of the preovulatory LH surge by methadone in rats

The effect of an ovulation-blocking methadone treatment (5mg/kg s.c. at 13:00, 14:00 and 15:00) on preovulatory LH release was studied in Wistar and Sprague-Dawley rats. Estrous cycles were followed by vaginal smears, and on the morning of proestrus a polyvinyl cannula was implanted in the right atrium via the external jugular vein so that blood samples could be collected without the use of anesthesia. Whole blood samples (50 microliter in duplicate) collected every 30 minutes from 13:00 to 18:00 that afternoon were assayed for LH by double antibody radioimmunoassay. Surges of LH release were seen significantly less often in the methadone-treated rats than in untreated or saline-injected controls, supporting the conclusion that methadone exerts its ovulation-blocking effect by preventing the CNS-controlled stimulation of pituitary LH release.