Measurement of equine prolactin with an equine-canine radioimmunoassay: seasonal effects on the prolactin response to thyrotropin releasing hormone

A radioimmunoassay (RIA) based on anti-equine prolactin antiserum and radioiodinated canine prolactin was used to assess the dose response of plasma prolactin to thyrotropin releasing hormone (TRH) in mares in the nonbreeding season (winter) and in mares in estrus in the breeding season (summer). Mares were administered TRH intravenously and blood samples were collected via jugular catheters at ?15, 0, 15, 30, 45, 60, 90, 120, 180 and 240 min relative to injection. Doses of TRH were 0, .08, .40, 2.0 and 10.0 mg per mare (n = 3 per dose within each season). The prolactin response was assessed by absolute hormonal concentrations before and after TRH injection and by net area under the curve. Prolactin concentrations in plasma before injection of TRH were higher (P < .01) in estrous mares in summer than in anestrous mares in winter (4.8 vs 1.3 ng/ml). Moreover, there was a greater (P < .01) response to TRH injection in estrous mares than in anestrous mares. Based on areas under the curve, there was an effect of season (P < .01) and of TRH dose (P < .01) as well as a season-dose interaction (P < .01). In general, there was little or no prolactin response to any dose of TRH in anestrous mares in winter when pre-TRH concentrations were low. In contrast, there was an increase in the prolactin response with increasing doses of TRH up to 2.0 mg in estrous mares in summer; 2.0 and 10.0 mg of TRH resulted in similar prolactin secretion. We conclude 1) that prolactin secretion in the horse is stimulated by TRH as has been reported for other species and 2) that prolactin concentrations and the TRH-induced secretion of prolactin are greater in estrous mares in summer than in anestrous mares in winter.