Effects of Melatonin and 6‐Methoxybenzoxazolinone on Photoperiodic Control of Testis Size in Adult Male Golden Hamsters

Consumption of young plants containing 6‐methoxybenzoxazolinone (6‐MBOA) appears to play an important role in the initiation of reproduction each spring in wild populations of the montane vole. Following its identification, 6‐MBOA has been found to stimulate the reproductive system in a number of rodent species, but the mechanism of action remains unknown. The chemical structure of 6‐MBOA is similar to melatonin, which, in addition to its well‐known antigonadal effects, can exert a progonadal influence under certain experimental conditions. To determine if 6‐MBOA might act as a melatonin agonist, four experiments were conducted to compare the effect of these two compounds on testis size in the golden hamster, a rodent whose responses to melatonin are well characterized. 1) Hamsters exposed to 14 h light per day (14L: 10D) received a daily injection of melatonin (25.0 μg) or 6‐MBOA (17.8 μg). 2) Hamsters exposed to 6L: 18D received Silastic capsules (50 or 200 mm) containing melatonin or 6‐MBOA. 3) Hamsters exposed to 6L‐.18D received chow containing melatonin (21.1 or 42.2 μg/gm chow) or 6‐MBOA (15.0 or 30.0 μg/gm). 4) Hamsters exposed to 6L: 18D received drinking water containing melatonin (15.5 μg/ml) or 6‐MBOA (11.0 μg/ml). Testis widths were determined at 2–3 week intervals, and after 66–73 days testes were removed and weighed. Melatonin significantly influenced testis size in each experiment, but treatment with 6‐MBOA had no effect in any of these experimental paradigms, indicating that 6‐MBOA does not act as a melatonin agonist in the hamster. However, these results indicate that the consumption of melatonin (and presumably melatonin agonists) could serve as an environmental stimulus for reproductive activity.