[Circannual rhythms in the secretion of prolactin in the Peulh ewe]

Plasma prolactin was measured by radioimmunoassay in 6 bicoloured Fulani ewes for 2 consecutive years. The highest concentrations (greater than 200 ng/ml) were observed in April/May when the days were the longest and the average temperature the highest. The lowest concentrations of prolactin (less than 100 ng/ml) occurred concomitantly with the shortest days and lowest temperatures in December/January. Regression coefficients between prolactin levels and photoperiod or temperature, respectively indicated that the correlations were stronger with photoperiod than temperature. These results therefore suggest (1) that the Sahelian climate has little influence on the rhythm of prolactin secretion in the ewe, and that even slight variations in day length can stimulate prolactin secretion; and (2) that there are marked individual variations in prolactin secretion levels in response to the various levels in photoperiod.