Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


School of Animal Science

First Advisor

Donald L. Thompson, Jr


In Experiment I, stallion exposure to an estrous mare on d 2 caused cortisol and PRL concentrations to increase (P $$.05) on d 1, 3, 4, 5 or 6 when no exposure occurred. In Experiment II, cortisol concentrations increased (P $<$.05) within 10 min after sexual stimulation (SS), physical exercise (PE), restraint via a twitch (RT) and epinephrine administration (EA) but not during control bleedings. PRL concentrations increased (P $<$.05) following SS, PE and RT but not after EA or during control bleedings. In Experiment III, cortisol concentrations in geldings increased (P $<$.05) following EA, PE and RT but not following SS or control bleedings. PRL concentrations increased (P $<$.05) following PE and SS. In Experiment IV, sulpiride injection was used in stallions in an attempt to increase PRL concentrations. Prior to sulpiride treatment, concentrations of both cortisol and PRL increased (P $<$.05) by 40 to 80% in response to exercise; concentrations of LH and FSH also increased (P $<$.05) by approximately 10%. Sulpiride treatment stimulated (P $<$.05) daily PRL secretion by approximately 5-fold, and caused a decrease (P $<$.05) in daily FSH secretion whereas only isolated differences (P $<$.05) occurred between treatment groups for daily LH and cortisol secretion. At the end of treatment, there was no PRL response to exercise for stallions treated with sulpiride, but there was (P $<$.05) for control stallions; only control stallions exhibited a marginal cortisol response (P $<$.05) to exercise. The PRL response to TRH was increased (P $<$.05) 4-fold in stallions treated with sulpiride but was unchanged in control stallions. Sulpiride treatment did not affect (P $>$.05) the LH or FSH response to exogenous GnRH. We conclude that: (1) returning stallions to the site of a previous period of sexual excitement does not cause a memory-mediated rise in cortisol or prolactin; (2) cortisol and PRL are secreted rapidly in response to some but not all forms of physical and chemical stress in stallions and geldings; (3) treatment with sulpiride during winter increases daily secretion and perhaps production of PRL in stallions; and (4) the increase in PRL characteristics due to sulpiride does not stimulate the PRL response to exercise, but does increase the PRL response to TRH.