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Adult Soay rams were housed under long days of 16L:8D for 16 weeks, and then exposed to a lighting regimen of either 8L:40D, 8L:28D, 8L:16D or maintained under 16L:8D. The diameter of the testes and the intensity of the sexual skin flush was recorded every 1-2 weeks, while on four occasions during the 8L:40D and 8L:28D regimens the concentrations of melatonin, prolactin and electrolytes (sodium and potassium) were measured in blood samples collected hourly for 2-4 days. During the initial period of 16L:8D, the testes of all rams regressed in size and the sexual skin flush disappeared. In the animals maintained under 16L:8D for more than 16 weeks, the testes began to redevelop spontaneously and there was a reappearance of the skin flush after 23-29 weeks. Compared to this group, the rams switched to 8L:40D and 8L:16D  after 16 weeks showed a significant acceleration in testicular development and an earlier skin flush. The rams exposed to 8L:28D failed to show this stimulated response and reactivation of the testes was similar to that under prolonged 16L:8D. During the 16 weeks under 8L:40D there was a 3-fold increase in the 24-h mean plasma concentration of melatonin. A clearly defined 24-h rhythm in the levels of melatonin was evident during the treatment. The rhythm was synchronized in its timing for the different individuals in the group, while the peak levels of melatonin occurred earlier relative to the onset of the light period under 8L:40D compared to 16L:8D. Under the 8L:28D regimen there was an increase in the mean levels of melatonin as for 8L:40D; however, there was no consistent daily rhythm. Exposure to the two photoperiodic regimens also resulted in differences in the plasma profiles of prolactin, while no differences were evident for the levels of electrolytes. The combined results are used to develop a model for the photoperiodic control of the timing of the rhythm in secretion of melatonin is considered of central importance.

Photoperiodic Regulation of Reproductive Activity in the Ram: Evidence for the Involvement of Circadian Rhythms in Melatonin and Prolactin Secretion