The effects of long-term treatment with cabergoline on the vernal transition in mares

Abstract

The purpose of this studywas to determine the effects of prolactin inhibition on the vernal transition in mares. Twelve light horse mares, determined by plasma progesterone and ultrasonography to be in anestrus, were blocked by age (4 to 23 years old), body condition score (4-7) and ovarian activity prior to start of experiment (February 4), then randomly assigned to either treatment (n1⁄46) or control (n1⁄46). Inhibition of prolactin was achieved by administration of the dopamine agonist, cabergoline, which has proven to be very effective at long term suppression of prolactin. On February 4 (day 0) and every 10 days after until May 25 (day 110), treatment mares received cabergoline (5 mg, intramuscular) while control mares received vehicle only (intramuscular). Beginning on February 5 (day 1) and every four days after until May 28 (day 113), blood was drawn via jugular venipuncture and plasma stored to determine circulating prolactin concentrations. On days 29, 59, 79, 89 and 119, mares were challenged with intravenous sulpiride (0.1 mg/kg BW) and blood drawn at 0, 5, 10 and 20 min relative to injection. Ovaries of each mare were scanned weekly until a follicle >25 mm emerged, and then daily thereafter until ovulation. Once ovulation occurred, blood was drawn for 16 successive days to determine circulating concentrations of LH and FSH. Estrous behavior was assessed by teasing with one of two stallions once a follicle reached 25 mm; estrous