Detailed diurnal rhythm of sensitivity to melatonin injections in Turkish hamsters, Mesocricetus brandti.

A diurnal rhythm of sensitivity to exogenous melatonin was defined in adult male Turkish hamsters, Mesocricetus brandti. Melatonin was administered daily by subcutaneous injections (15 micrograms in 0.1 ml 10% ethanolic saline) for 10 weeks in animals exposed to 16 L:8 D. As in golden and Djungarian hamsters, two periods of melatonin sensitivity were identified. The first, in late afternoon, persisted for 6 hr, from 7 hr to 1 hr before lights off. The second period was briefer, of only 2 hr duration in the late night, terminating at the time of lights on. Melatonin injections given during these sensitive periods promoted testicular regression in most animals; melatonin administered at other times of the day was without effect on testicular function in most animals of these groups. Gonadal regression induced by properly timed melatonin injections was rapid, in many groups nearly complete in 6 to 7 weeks. The results are discussed in relation to the function of pineal melatonin in photoperiodic time measurement in hamsters.