How Does Melatonin Code for Day Length in the Ewe: Duration of Nocturnal Melatonin Release or Coincidence of Melatonin with a Light-Entrained Sensitive Period?' NANCY L. WAYNE: BENOIT MALPAUX: and FRED

The main objective of the study was to test the hypothesis that the phase of melatonin release with respect to the light-dark cycle mediates the effects of photoperiod on the reproductive response of the ewe. To test the phase hypothesis, we eliminated endogenous melatonin secretion by pinealectomy and then restored physiological levels of serum melatonin with rises of the same duration but a t different phases of the light-dark cycle (either a t night or in the middle of the day). Serum melatonin patterns were determined by radioirnmunoassay in samples taken hourly for 24 h. The reproductive state was monitored by measuring serum luteinizing hormone ( L H ) in ovariectomized ewes treated with constant-release estradiol implants. Infusion of a long-day pattern of melatonin was equally effective in maintaining reproductive suppression when given during the night or the middle of the day. LH remained low for approximately 150 days and then rose as ewes became refractory to the inhibitory melatonin signal. These results d o not support the phase hypothesis. Rather, they are consistent with the hypothesis that the duration of the nocturnal secretion of melatonin codes for day length.