Long-Term Treatments with Morphine and Naloxone have Sex-Differentiated Effects on Luteinizing Hormone Secretion in Chronically Catheterized Fetal Pigs

Short‐ (one bolus injection) and long‐term (repeated injections over a period of at least 7 days) effects of drugs on pituitary function in pig fetuses were studied to investigate the influence of morphine and naloxone on luteinizing hormone secretion in the chronically catheterized pig fetus between days 102 and 110 of gestation (term: 113±1 SD day). Both substances were intravenously administered in two doses: 0.1 mg and 1 mg/fetus. Repeated injections at 2‐day intervals enabled us to study short‐ as well as long‐term effects. Morphine acutely inhibited luteinizing hormone secretion both in male and female fetuses. Long‐term treatment with morphine at both doses caused an inhibition of basal luteinizing hormone (levels before treatment on each day) in females (0.1 mg: r=−0.60, P