Adult, male hamsters exposed to a photoperiod of 1 h light: 23 h darkness for either 2, 3, 4, 6, 8, or 10 weeks exhibited significantly lower testicular weights than did controls after 3 and 4 weeks of exposure to 14 h light: 10 h darkness. Liver Δ4-reductase activity was significantly decreased by 4 weeks of exposure to the short photoperiod. Exogenous melatonin decreased Δ4-reductase activity when incubated with either hamster liver (at 10−7 and 10−5 mol melatonin/l) or hamster hypothalamus (at 10−6 mol melatonin/l). In contrast, exogenous melatonin stimulated Δ4-reductase activity in preparations of rat liver (at 10−7 and 10−6 mol/l) and rat hypothalamus (at 10−9, 10−8, 10−7, and 10−6 mol/l). The possible role of melatonin in influencing the testosterone: dihydrotestosterone ratio is discussed in relation to possible regulation of gonadotrophin release.
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