Effects of dexamethasone treatment on insulin sensitivity, insulin, thyroxine, and triiodothyronine concentrations, and the MSH and ACTH responses to sulpiride in geldings

Abstract

Two studies were conducted to determine efficacy of cabergoline for suppressing prolactin (PRL) and the possible effects on vernal transition in mares. In experiment 1, six mares each received either vehicle or cabergoline (5 mg, intramuscularly) every 10 days for 12 treatments beginning February 4, 2013. Blood samples were drawn regularly, and mares were challenged with sulpiride periodically to assess PRL suppression. Weekly hair samples were obtained to determine shedding. Prolactin was suppressed (P < .05) by cabergoline, but suppression waned in spring. There was no effect (P > .05) of treatment on day of first ovulation, luteinizing hormone, or follicle stimulating hormone. Hair shedding was generally suppressed (P = .05). In 2014 (experiment 2), eight of the same 12 mares were used in a similar experiment to determine if the rise in PRL observed in experiment 1 was due to refractoriness to cabergoline or perhaps another factor. Treatment began on April 6, 2014, corresponding to the increase in PRL in treated mares in experiment 1. Mares were treated with cabergoline or vehicle until June 5. Prolactin was suppressed (P < .05) by cabergoline, and the pattern of apparent escape from suppression was similar to year 1. We conclude that (1) cabergoline at this dose alters hair shedding but does not alter the time of first ovulation in mares and (2) relative to our previous reports of cabergoline treatment in the fall, there is a seasonal effect on the ability of this dose of cabergoline to suppress unstimulated PRL secretion.