Doctor of Philosophy (PhD)


Animal Sciences

Document Type



A series of experiments studied the effects and practical applications of dopaminergic and antidopaminergic compounds in equine metabolic physiology. The first experiment was performed to assess the long-term effects of repeated cabergoline injections (every 10 days for a total of seven injections) on prolactin and α-melanocyte stimulating hormone concentrations in insulin insensitive mares. Additionally, the experiment also evaluated the use of cabergoline for improving insulin sensitivity. Plasma prolactin and α-melanocyte stimulating hormone concentrations were suppressed throughout the duration of the experiment even when the mares were challenged with a low-dose dopamine antagonist, sulpiride, the day previous to their subsequent cabergoline injection. Insulin sensitivity was unaffected by cabergoline administration. To further investigate the effect of dopaminergic activity on metabolism, a second set of experiments compared the long-term and short-term effects of cabergoline and sulpiride in both insulin resistant and insulin sensitive horses during the spring and summer months. The results confirmed that neither dopaminergic or antidopaminergic drugs have any effect on insulin sensitivity in horses. The third series of experiments studied the prolactin response to different modes of administration and concentrations of sulpiride. Prolactin response to sulpiride in horses was found to be enhanced and extended by using a higher dosage (1g compared to 3g) and hydrophobic vehicles (vegetable shortening or sucrose acetate isobutyrate. The final series of experiments evaluated the effects of dopaminergic (bromocriptine, cabergoline and pergolide) and antidopaminergic (sulpiride) drugs on heart rate after a brief period of exercise (two minutes at a trot). A noticeable reduction in heart rate was found in horses that received bromocriptine or cabergoline treatment but not when the horses were treated with pergolide or sulpiride. Over all experiments, it was concluded that changes in dopaminergic activity, whether stimulatory or inhibitory, has no effect on insulin sensitivity in horses. In addition, strong dopamine agonists were found to alter the heart rate in lightly exercised healthy horses.



Committee Chair

Thompson, Jr., Donald L.