Prolactin response to thyrotrophin-releasing hormone stimulation in normal and agalactic mares

Serum prolactin concentration was determined before and after TRH administration to normal mares at 10 months of gestation, 2 and 4 months post partum and during a -7- to +14-day peri-parturient period. The serum prolactin concentration increased significantly (P less than 0.05) at 15, 30 and 60 min after TRH administration in the normal mares regardless of the season of the year, pregnancy or lactation status. However, during the periparturient period, the basal prolactin concentration was increased 4-fold and there was only a marginal increase after TRH administration. Of 9 agalactic mares, 6 had a decreased basal serum prolactin concentration; the other 3 agalactic mares had a normal to increased basal concentration suggestive of a peripheral resistance to prolactin.