Changes in the pattern of prolactin secretion in the postpartum ewe and daily rhythms of prolactinemia during different reproductive states

In order to study the relationship between plasma prolactin (PRL) levels and the secretion pattern of this hormone, blood samples were performed every 10 minutes for six hours on dry, restricted and unrestricted suckling Texel ewes on Days 7 and 30 after October or November lambings. On Day 7 post partum, mean plasma PRL concentration, frequency and amplitude of PRL pulses were higher in unrestricted suckling ewes than those in other groups (P < 0.01). Moreover, mean plasma PRL concentrations irrespective of the managing mode were positively related to PRL pulse frequency on Day 7 (R = 0.74, P = 0.0001) and Day 30 (R = 0.43, P = 0.087) post partum. In a second experiment, hourly blood sampling was performed for a period of 24 consecutive hours on cyclic ewes at the luteal or estrual phase (in December), and on seasonal anestrius (in March) or pregnant (in May) ewes in order to study daily variations in PRL secretion. Day/night variations in prolactinemia were only detected in cyclic ewes in December (P < 0.05) not in seasonal anestrous or pregnant ewes in March or May. During the estrual phase of the cycle, the results clearly showed a circahemidian rhythm in PRL secretion with a significant increase at the beginning of daylight or darkness (P < 0.01).