Recommendations for clinical GnRH challenge testing of stallions

Eight mature Light-breed stallions with normal testes size, sperm output an d semen quality were used to evaluate response to 3 GnRH challenge regimens in the summer in southeast Texas. Gonadotropin releasing hormone (50 mug) was administered intravenously once to each of eight stallions after three days of sexual rest (50 mug GnRH-1X). The same stallions were administered either 5 mug GnRH intravenously once hourly for three injections (5 mug GnR H-3X) and 15 mug GnRH intravenously once (15 mug GnRH-1X) one and two weeks later. Blood samples were collected prior to and at intervals after GnRH a dministration. Plasma was immediately separated from blood samples and was frozen until assayed for LH, FSH, estradiol and testosterone concentrations . Percentage changes in hormone concentrations from pre-treatment values (b aseline) were analyzed by paired student's t-test to detect significant ris es in hormone concentrations. Group mean percentage changes in hormone conc entrations were analyzed by analysis of variance to compare responses among treatments. A computerized peak-detection algorithm (PC Pulsar) was used t o detect peaks in LH and testosterone concentrations following 5 mug GnRH-3 X and 15 mug GnRH-1X treatment. No differences (P > 0.10) were detected in percentage change from baseline concentration for LH, FSH, or testosterone at one or two hours after admini stration of any of the three regimens of GnRH. When more frequent sampling intervals were analyzed for 5 mug GnRH-3X or 15 mug GnRH-1X treatments, no differences were detected in percentage change from baseline concentration for any hormone at 15, 30 or 60 minutes. Thereafter, percentage changes in concentrations of LH and FSH remained increased for 5 mug GnRH-3X compared to 15 mug GnRH-1X treated stallions (P < 0.05). Percentage changes in conce ntrations of testosterone were increased for 5 <mu>g GnRH-3X compared to 15 mug GnRH-1X treated stallions from 180 - 300 min (P < 0.05), while no diff erences (P > 0.10) were detected between 5 mug GnRH-3X and 15 mug GnRH-1X t reated stallions for changes in concentrations of estradiol throughout the experiment. For 15 mug GnRH-1X treated stallions, maximum concentrations of LH in PC Pu lsar-detected peaks occurred most commonly at 15 to 30 minutes (7/8 treatme nt periods) after GnRH injection. Maximum concentrations of testosterone in PC Pulsar-detected peaks occurred most commonly at 60 -120 min (7/8 treatm ent periods) after GnRH injection. A protocol of blood sampling prior to, and 15, 30, 60 and 120 minutes after , intravenous administration of small doses of GnRH would be practical for challenge testing of stallions during the breeding season. In order to redu ce cost of hormone assays, we suggest assay of the pre-challenge blood samp le (baseline) could include LH, FSH, testosterone and estradiol concentrati ons (to assess overall hypothalamic-pituitary-testicular function), while o nly LH and testosterone concentrations need be determined after GnRH admini stration (to assess pituitary and testicular responsiveness). Assay for LH could be done on only the 15 and 30 minute post-GnRH samples, and assay for testosterone could be done on only the 60 and 120 minute post-GnRH samples . Failure to achieve approximately a 50%; increase in LH concentration by 3 0 minutes after GnRH administration, and/or failure to achieve approximatel y a 100% increase in testosterone concentration by two hours after GnRH adm inistration, could be further pursued either by treatment with increasing d osages of GnRH, or repeated administration of GnRH at hourly intervals, as has been suggested by other workers.