Effects of bromocriptine administration during the follicular phase of theoestrous cycle on prolactin and gonadotrophin secretion and follicular dynamics in Merino monovular ewes

Two experiments using Spanish Merino ewes were conducted to investigate whe ther the secretion of prolactin during the follicular phase of the sheep oe strous cycle was involved in the patterns of growth and regression of folli cle populations. In both experiments, oestrus was synchronized with two clo prostenol injections which were administered 10 days apart. Concurrent with the second injection (time 0), ewes (n = 6 per group) received one of the following treatments every 12 h from time 0 to 72 h: group 1: vehicle injec tion (control); group 2: 0.6 mg bromocriptine (0.03 mg per kg per day); and group 3: 1.2 mg bromocriptine (0.06 mg per kg per day). In Expt 1, blood s amples were collected every 3 h from 0 to 72 h, and also every 20 min from 38 to 54 h to measure prolactin, LH and FSH concentrations. In Expt 2, tran srectal ultrasonography was carried out every 12 h from time 0 until oestru s, and blood samples were collected every 4 h to measure prolactin, LH and FSH concentrations. Ovulation rates were determined by laparoscopy on day 4 after oestrus. Bromocriptine markedly decreased prolactin secretion, but d id not affect FSH concentrations, the mean time of the LH preovulatory surg e or LH concentrations in the preovulatory surge. Both doses of bromocripti ne caused a similar decrease in LH pulse frequency before the preovulatory surge. The highest bromocriptine dose led to a reduction (P < 0.01) in the number of 2-3 mm follicles detected in the ovaries at each time point. Howe ver, bromocriptine did not modify the total number or the number of newly d etected 4-5 mm follicles at each time point, the number of follicles >5 mm or the ovulation rate. In conclusion, the effects of bromocriptine on gonad otrophin and prolactin secretion and on the follicular dynamics during the follicular phase of the sheep oestrous cycle indicate that prolactin may in fluence the viability of gonadotrophin-responsive follicles shortly after l uteolysis.