Prolactin involvement with the increase in seminal volume after sexual stimulation in stallions

To test the hypothesis that prolactin mediates the increase in seminal volumes induced by sexual stimulation in stallions, semen was collected from six stallions every other day for 26 d. The last eight collection days were treatment days. For each stallion, four treatments were randomly assigned to the first four of the eight treatment collection days, and then repeated in reverse order on the last four collection days; 1) CONTROL: semen collected per normal procedures; 2) Sexually stimulated: stallions were presented to mares in a chute for 10 min before collection; 3) Bromocriptine (dopamine agonist) plus sexual stimulation: stallions were administered bromocriptine 10 min before 10 min of sexual stimulation prior to collection; and 4) Sulpiride: stallions were administered sulpiride (a dopamine antagonist) 25 min before collection. Prolactin concentrations in plasma were increased by sexual stimulation (P < .01) and sulpiride (P < .001) administration and were decreased (P < .01) when bromocriptine was administered before sexual stimulation. Sexual stimulation alone increased (P < .01) volume of gel-free semen relative to control values (102 vs 81 mL), and bromocriptine prevented this response (89 mL; P < .075 relative to sexual stimulation). Sulpiride had no effect (P > .1) on gel-free volume. Volume of gelatinous material, number of mounts, sperm concentration, motility, pH of gel-free semen, number of spermatozoa per ejaculate, and prolactin concentration in gel-free semen were not affected (P > .1) by treatment. Although the effect of bromocriptine indicated that prolactin may mediate the rise in seminal volume due to sexual stimulation, if it does, it must be in conjunction with other hormone(s) or factor(s), because increasing prolactin concentrations alone did not increase seminal volume in these stallions.