Reproductive Responses of Male Microtus montanus to Photoperiod, Melatonin, and 6‐MBOA

Juvenile male Microtus montanus were examined for the effects of photoperiod, melatonin, and the naturally occurring reproductive stimulant 6‐MBOA on growth and sexual maturation. 6‐MBOA, present in sprouting grass, is an important environmental cue used for the initiation of reproduction in natural populations of this species. Long photoperiod (16:8) was stimulatory to body, testes, and seminal vesicle growth, while short photoperiod (8:16) inhibited these parameters. The pineal hormone melatonin, administered via daily afternoon injections (5 μg), was also inhibitory to all of the above parameters as well as to serum testosterone. 6‐MBOA, administered via injection (0.0001–100 μg) or dietary means (0.1 or 1.0 μg/gm unsifted chow), appeared unable to augment the rate of maturation in long‐photoperiod‐stimulated animals. When short‐photoperiod animals were treated with high doses of the compound (100 μg injected or 1.0 μg/gm sifted chow), body and gonadal growth was inhibited to a greater extent than when animals were exposed to short photoperiod alone, and serum LH was reduced. Lower doses of the compound had no effect. Melatonin‐treated animals experienced less maturational inhibition when simultaneously given a low dose of 6‐MBOA‐coated chow (1.0 μg/gm unsifted chow). A higher dose of 6‐MBOA (1.0 μg/gm sifted chow) was ineffective in preventing the response to melatonin. These results indicate that 1) male M. montanus utilize photoperiod, rather than 6‐MBOA, as a primary environmental cue, 2) high doses of 6‐MBOA can be inhibitory under short photoperiod, 3) juvenile male voles are highly sensitive to the inhibitory effects of exogenously administered melatonin, and 4) 6‐MBOA can partially prevent the inhibitory effects of melatonin on growth and sexual maturation.