Title page for ETD etd-11172010-095959


Type of Document Master's Thesis
Author Clavier, Sarah Case
URN etd-11172010-095959
Title Dopaminergic Input to the Equine Pituitary: Seasonal and Estradiol Effects
Degree Master of Science (M.S.)
Department Dairy Science (Animal, Dairy, & Poultry Sciences)
Advisory Committee
Advisor Name Title
Thompson, Jr., Donald L. Committee Chair
Bondioli, Kenneth R. Committee Member
Williams, Cathleen C. Committee Member
Keywords
  • dopamine
  • equine
  • seasonal
  • estradiol
Date of Defense 2010-11-02
Availability unrestricted
Abstract
Two experiments were conducted to study the effects of season and estradiol administration on dopaminergic input to the equine adenohypophysis. Experiment 1 began in the spring with 8 mares and 8 stallions and was repeated again in the summer, fall, and winter. Horses were given sulpiride injections of eight incremental doses (0.25 to 32 g/kg BW). Within each group of 8 horses, half received the sulpiride in an increasing manner, the other half in a decreasing manner. Prolactin concentrations peaked in the first 15 to 30 min in all horses in all seasons. Prolactin areas under the curve increased (P < 0.001) with increasing doses of sulpiride, and were highest (P < 0.05) in March for stallions, but in June for mares. The calculated half-maximum values, which should be proportional to the dopaminergic input to the pituitary, were lowest (P < 0.05) in June and greatest in September. The variation in half-maximum values with season indicated a change in dopaminergic input to the pituitary, with lowest input occurring in June in both sexes. Experiment 2 was designed to determine if the degree of dopaminergic input to the adenohypophysis is altered by estradiol administration. Twelve geldings were used. On day 0, geldings received an i.m. injection of either estradiol cypionate (ECP; n = 6) or vegetable oil (n = 6) at one of two dose volumes (2 vs. 10 mL). On day 6, all geldings received an injection of sulpiride at 0.082 g/kg BW in saline. Sulpiride injections were repeated every other day with the dose increasing each day, from 0.164 to 100 g/kg BW. Estradiol pretreatment increased (P < 0.05) the prolactin response to sulpiride at the 2.56 g/kg dose and higher. However, the half-maximum values for ECP-treated and control geldings did not differ, indicating that the amount of sulpiride needed to counterbalance the amount of dopamine reaching the pituitary was unaltered by estradiol treatment. It was concluded that estradiol likely stimulates prolactin production and secretion after sulpiride directly at the lactotrope level, rather than by decreasing hypothalamic dopamine input to the lactotropes.

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