Effects of prolonged artificial photoperiod on circulating prolactin and melatonin levels in seasonal ewes.

Crossbred ewes exposed to long days for 46 months prior to photoperiod reversal showed no alteration in the duration or amplitude of the circulating melatonin peak between 24 and 46 months of continuous long day exposure. By 3 months after photoreversal to short days, both the amplitude and duration of the peak had adapted to the new scotophase. In short day treated ewes, the melatonin peak was abolished by 46 but not 24 months of short day exposure, and was not fully restored in all ewes 3 months after photoreversal. Mean prolactin levels over 24 h remained high up to 46 months of long day treatment, and declined 3 months after short day exposure. Conversely, mean prolactin levels remained low up to 46 months of short day treatment, increasing 3 months after exposure to long days. Thus: (i) depletion of the melatonin-synthesizing capability of the ovine pineal gland by prolonged exposure to long nights is not completely reversed after 3 months of continuous long day exposure, and (ii) a nocturnal melatonin peak is not essential for maintenance of plasma prolactin levels under these conditions.