Factors affecting basal and post-exercise prolactin secretion in horses

Abstract

There has been thorough documentation to support the role of dopamine in the control of prolactin production and secretion in various mammalian species, including the horse. However, there is evidence that other factors are involved in prolactin secretion. Seven experiments were conducted to assess factors that potentially might affect prolactin secretion in the horse. The first two experiments were conducted (separately) to test whether arginine vasopressin (AVP) or vasoactive intestinal polypeptide (VIP) affected prolactin secretion. In each experiment, AVP or VIP was administered intravenously and blood samples were collected to determine the effect on prolactin secretion. Neither peptide produced any alteration in plasma prolactin concentrations compared to simultaneous saline-injected controls (P > 0.1). Five subsequent experiments were conducted to assess the effects of various drugs on prolactin secretion in response to acute exercise. Pre-exercise treatments included dexamethasone (a glucocorticoid analog, administered 15 h before exercise), naloxone (an opiod antagonist, administered 2 min before exercise), cabergoline (a dopaminergic agonist, administered 15 h before exercise), flunixin meglumine (a prostaglandin inhibitor, administered 15 min before exercise), and sulpiride (a dopamine antagonist that causes the release of prolactin, administered 1.5 h before exercise). In all experiments, exercise induced an immediate increase (P < 0.05) in plasma prolactin concentrations in control horses. Pretreatment with dexamethasone, naloxone, or flunixin meglumine did not alter (P > 0.1) plasma prolactin concentrations relative to saline-treated controls. Pretreatment with cabergoline completely obliterated (P < 0.01) the exercise induced rise in prolactin concentrations. Pretreatment with sulpiride caused an immediate increase (P < 0.001) in prolactin concentrations relative to controls, but resulted in no change in prolactin response to exercise 90 min later relative to controls. It is concluded that the only drug that had a significant effect on prolactin secretion was the dopaminergic agonist cabergoline. Direct administration of AVP or VIP, or perturbations of the adrenal cortical axis, the opioid system, or the prostaglandin system, had no effect on prolactin secretion as has been reported previously for other species.