Master of Science (MS)


Animal Science (Animal, Dairy, and Poultry Sciences)

Document Type



Four experiments were conducted in an effort to develop a method, based on prolactin secretion, for assessing the efficacy and duration of activity of dopaminergic agonists for the treatment of pituitary pars intermedia dysfunction (PPID) in horses. In the first experiment, prolactin response to a low dose of the dopamine antagonist, sulpiride, was generally repeatable in estrogen-primed geldings in winter over 8 every-other-day challenges. It was concluded that estrogen-primed, sulpiride-challenged geldings in winter could serve as a model for the study of potential dopaminergic drugs for the treatment of PPID in horses. The second experiment was performed in the summer with mares, and again tested the repeatability of the prolactin responses over a 30-day period. The responses in mares were generally repeatable, and there was no effect due to stage of the estrous cycle. It was concluded that mares could serve as a model for the study of potential dopaminergic drugs as well as geldings, and stage of the estrous cycle did not have to be taken into account. In the third experiment, two formulations of the dopamine agonist, pergolide, were tested (oral administration versus injection) against a single formulation of cabergoline (injected) and control injections (vehicle) for their efficacy to reduce unstimulated plasma prolactin concentrations in geldings. Oral pergolide reduced prolactin concentrations for a few hours, whereas injected pergolide suppressed prolactin concentrations for 24 hours. Cabergoline suppressed prolactin concentrations for up to 5.5 days. It was concluded that the injectable formulations had potential for further study as possible treatments for PPID in horses. The last experiment tested the efficacy of daily pergolide injection versus a single injection of cabergoline, for suppressing the prolactin secretion induced by low dose sulpiride injections in mares. Daily injection of pergolide suppressed prolactin responses as long as the injections were given, plus another 2 days. The single cabergoline injection suppressed prolactin responses for a minimum of 10 days. Based on these results, cabergoline in slow-release vehicle seems to provide an excellent possibility for administering dopaminergic activity to horses with PPID. Whether these results are directly applicable to PPID horses needs to be determined.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Thompson, Donald L., Jr.