Prolactin may play a role in stimulating the equine ovry during the spring reproductive transition

Plasma prolactin and follicular development increased significantly between the 2nd and 5th weeks of March. During this time, follicular diameters were highly correlated with plasma prolactin (r=.323, dr=64, p=.008). There was no significant rise in LH during the experimental period (lst week in Jan to the 5th week of March). Plasma prolactin was increased by blocking dopamine receptors (fluphenazine decanoate, 178.6 µg/kg, im, once every 21 days) or by a single injection of ovine prolactin (200 mg/mare, iv) given during the 1st week in January. Fluphenazine accelerated follicular growth when compared to control mares but did not result in early ovulation. Exogenous ovine prolactin caused growth 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 injection of prolactin. The remaining 2 mares continued to produce waves of follicular growth and rcgression until they ovulated, coincident with the control group. The present studies suggest that when endogenous prolactin secretion is increased during anestrus either by dopamine receptor blockade or by administering exogenous prolactin, typical transitional follicular growth commences.