Effects of daily melatonin injections on female reproduction in the white-footed mouse, Peromyscus leucopus

Abstract

Seventy laboratory-reared (16L:8D at 21°) Peromyscus leucopus were injected daily for 7 weeks with either saline, 0.5, 5.0, or 50 μg of melatonin at either 2 or 12 hr into the photophase. Mice injected with 50 μg at 12 hr exhibited a substantial decrease ( P < 0.01) in relative reproductive tract weight (2.3 ± 0.4 mg/g body wt) when compared to saline injected animals (4.6 ± 0.9). Seven out of the nine melatonin-treated mice also exhibited an imperforate vagina. A lesser effect was noted at lower melatonin concentrations. No regression occurred in mice injected 2 hr into the photophase. Thermoregulatory characters (nesting, brown fat, winter molt) were not affected by melatonin injections. In a second experiment 49 mice were injected with 50 μg of melatonin at either 7.5, 10, 14, 15, 18, or 23 hr after lights on. A gradual antigonadal effect was observed with a peak during late photophase. Mice injected during early scotophase (18 hr) exhibited little antigonadal effects of melatonin, but animals injected at 23 hr had reproductive tract weights which were considerably less ( P < 0.05).