PROLACTIN MAY PLAY A ROLE IN STIMULATING THE EQUINE OVARY DURING THE SPRING REPRODUCTIVE TRANSITION

Plasma prolactin and follicular development increased significantly be tween the 2nd and 5th weeks of March. During this time, follicular dia meters were highly correlated with plasma prolactin (r=.323, df=64, p= .008). There was no significant rise in LH during the experimental per iod (1st week in Jan to the 5th week of March). Plasma prolactin was i ncreased by blocking dopamine receptors (fluphenazine decanoate, 178.6 mug/kg, im, once every 21 days) or by a single injection of ovine pro lactin (200 mg/mare, iv) given during the 1st week in January. Fluphen azine accelerated follicular growth when compared to control mares but did not result in early ovulation. Exogenous ovine prolactin caused g rowth of ovulatory size follicles within 3 days in 3 out of 5 treated mares. One of these mares went on to ovulate 9 weeks following injecti on of prolactin. The remaining 2 mares continued to produce waves of f ollicular growth and regression until they ovulated, coincident with t he control group. The present studies suggest that when endogenous pro lactin secretion is increased during anestrus either by dopamine recep tor blockade or by administering exogenous prolactin, typical transiti onal follicular growth commences.