Inhibitory effects of melatonin on testosterone but not on androstenedione production during winter in the vespertilionid bat, Scotophilus heathi

Abstract: The possible effects of melatonin on testosterone and androstenedione production in vitro by testes of wild-caught bats, Scotophilus heathi , during different reproductive phases were investigated. Bats were captured during reproductive quiescent (April-August), recrudescent (September-October), breeding (November-February), and winter dormancy (late December-January) phases. Testes were incubated in media-199 for 2 h at 37°C with luteinizing hormone (LH) and with or without melatonin. Melatonin had no effect on LH-induced testosterone (T) or androstenedione (A) production during the quiescent, recrudescent, and breeding phases. However, it significantly (P<0.05) suppressed LH-induced T production but had no effect on A during winter dormancy. Testicular 17β-hydroxy steroid dehydrogenase (17β-HSD) activity was then measured in the testes from bats trapped during quiescence, breeding, and winter dormancy. Interestingly, melatonin along with LH caused suppression of 17β-HSD activities (3.56 ± 0.03 unit/min/mg protein) when compared with levels of LH treated testes (7.10 ± 1.15 unit/min/mg protein) during winter dormancy, while it had no significant effect on 17β-HSD activity during quiescence and breeding phases. These results suggest that in S. heathi , melatonin during winter dormancy suppresses LH-induced T production by the testes via the suppression of 17β-HSD activity. This may be the reason for the decline in testicular activity during winter dormancy.