Prolactin secretion in lactating mares before and after treatment with bromocriptine

The effect of treating lactating mares with the dopamine agonist bromocriptine was investigated. Seven pony and 4 Thoroughbred lactating mares were given a single intramuscular (i.m.) injection of 100 mg bromocriptine between Days 18 and 28 after foaling when the secretion rate of prolactin was elevated. Prolactin and luteinizing hormone (LH) concentrations were measured in serial peripheral plasma samples taken before and after the treatment and, in 5 of the pony mares, levels of these hormones were also measured in pituitary venous effluent obtained by cannulation of the cavernous sinus. In all the mares, prolactin concentrations fell sharply after treatment (Day 0) and they started to increase again on Day 2 in the Thoroughbreds and Day 3 in the ponies. Pituitary LH secretion rate also declined significantly for 2 days after treatment but there were no measureable influences on cyclic ovarian activity.