Antigonadal Effects of Melatonin in White-footed Mice (Peromyscus leucopus)

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Abstract
Melatonin administered via s.c. Silastic capsules caused the testes of white-footed mice to regress; after 9 weeks of treatment with a 25 mm capsule, spermatogenesis was arrested, and testicular and seminal vesicle weights reduced to 32% and 21% of control values, respectively. The testes of mice bearing melatonin capsules recrudesced after 30 weeks and spermatogenesis resumed, indicating that the neuroendocrine axis eventually becomes unresponsive to melatonin’s antigonadal effects. Other mice maintained on a 15L:9D photoperiod (lights on at 0700 h) were injected once daily for 10 weeks with melatonin (5 or 25 µg per injection) or with the sesame oil vehicle. There was a diurnal rhythm of responsiveness to melatonin. Males injected at 1800 h had regressed testes and reduced seminal vesicle weights; injections at 1100 h or 1530 h had no detectable effects on spermatogenesis, testicular size, or seminal vesicle weight. These findings suggest that melatonin exerts antigonadal effects in Peromyscus leucopus.