The effect of twice daily gonadotropin-releasing hormone (GnRH) administration and/or renal pituitary homografts on melatonin-induced gonadal atrophy in male Syrian hamsters.

Adult hamsters were maintained in a 14:10 light:dark cycle and divided into the following experimental groups: 1) controls, 2) daily afternoon (1600h) injections of 25 micrograms of melatonin (MEL), 3) daily MEL (1600) injections and twice daily (0900 and 1600h) injections of 1 microgram GnRH, 4) daily MEL (1600h) injections and two anterior pituitary glands implanted under the kidney capsule, and 5) MEL injections, GnRH injections and pituitary homografts. After 8 and 12 weeks of treatment, MEL injections resulted in testicular and accessory sex organ regression and depressed plasma PRL and testosterone. Twice daily injections of GnRH did not prevent atrophy of the testes and accessory sex organs in MEL-treated animals whereas animals with either homografts alone or the combination of GnRH injections and pituitaries implanted under the kidney capsule had normal or supranormal testicular and accessory sex organ sizes and plasma PRL and testosterone after 8 and 12 weeks of MEL treatment. The results suggest that prolactin alone can act to maintain normal testicular function in hamsters in which MEL-induced gonadal atrophy has taken place.