Effect of asymmetrical reductions of photoperiod on pineal melatonin, locomotor activity and gonadal condition of male Syrian hamsters.

This study investigated the relationship of two overt circadian rhythms, locomotor activity and melatonin synthesis in the pineal gland, by comparing their responses to asymmetrical reductions in photoperiod. Transfer of male Syrian hamsters from long to short daylengths led to an increase in the duration of both locomotor activity and the period of melatonin synthesis. Over the course of re-entrainment, the two rhythms were held in a stable phase relationship, and the direction of the switch did not influence the rate of decompression or the final phase relationships established after 8 weeks in short daylengths. Decompression of the activity rhythm was not influenced by pinealectomy. Exposure to short photoperiods caused gonadal regression and a consequent decline in serum testosterone levels from 10 to less than 1 nmol/l. The direction of the photoperiodic switch did not affect the time-course of gonadal regression. These data demonstrate the important influence of photoperiod upon the duration of the nocturnal peak of melatonin production by the pineal and also demonstrate that this effect is one example of a more widespread response of the circadian system. A qualitatively similar signal controls both locomotor activity and melatonin synthesis, although the neural basis of this common mechanism is unclear.