Interaction of daily injections and subcutaneous reservoirs of melatonin on the reproductive physiology of female Syrian hamsters.

This study investigated the ability of continuously available melatonin (from sc melatonin-beeswax pellets that were implanted bi-weekly) in overcoming the antigonadotrophic action of single daily melatonin (25 microgram) injections given late in the light period to female Syrian hamsters kept in light:dark cycles of 14:10. The melatonin-beeswax pellets contained either 1, 50, 500 microgram, 1 or 10 mg melatonin and 25 mg beeswax. Daily melatonin injections into hamsters implanted with beeswax only caused vaginal acyclicity in 60% of the animals within 6 weeks.

Interaction of luteinizing hormone-releasing hormone, cyproterone acetate and arginine vasotocin on plasma levels of luteinizing hormone in intact and castrated adult male rats.

Treatment of unanesthetized castrated adult male rats every 3 h for 48 h with either 5 microgram of arginine vasotocin (AVT) and/or 1 microgram luteinizing hormone-releasing hormone (LRH) caused a significant inhibition of plasma levels of luteinizing hormone (LH) and compared to castrated control rats receiving diluent only. However, the intravenous (iv) injection of 1 microgram of AVT into urethane-anesthetized male rats which had been castrated for 0, 24 or 48 h did not affect plasma levels of LH at 10, 20 or 60 min following injection compared to their respective diluent-treated castrated control rats. Similarly, the iv injection of either 100 ng, 1 microgram or 10 microgram AVT was unable to acutely affect plasma levels of LH in intact male rats.

Effect of several androgens, cyproterone acetate or estrogen-progesterone on the prolactin-releasing activity of arginine vasotocin in castrated male rats.

Intravenous (iv) administration of 5 microgram of arginine vasotocin (AVT) into urethane-anesthetized, castrated male rats had no effect on plasma prolactin titers as compared to the rise in prolactin levels observed in intact AVT-treated rats. However, when castrated rats were first treated for two days with 2.5 mg/day of testosterone propionate and then challenged with a 5-microgram dose of AVT, the prolactin surge values obtained were comparable to those seen in intact AVT-treated rats.