Effect of maternal treatment with altrenogest on pituitary response to exogenous GnRH in pubertal stallions

Abstract

The pituitary response to exogenous GnRH was studied in 8 colts of Quarter Horse phenotype from 32 to 96 weeks of age. Colts were from dams treatd daily from Day 20 to 325 of gestation with (1) 2 ml neobee oil per 50 kg body weight (controls); or (2) 2 ml altrenogest per 50 kg body weight. GnRH challenges (5 μg/kg body weight) were administered every 8 weeks from 32 to 96 weeks of age to estimate pituitary content of LH. Blood samples were collected every 20 min for 4 h before GnRH and 15, 30, 45, 60, 90, 120, 180, 240 and 360 min after GnRH. Serum concentrations of LH and FSH were determined for the 2 pre-GnRH and all post-GnRH samples. Baseline concentrations (mean of 2 pre-GnRH samples) of LH and FSH were not affected by treatment (P > 0.05). Serum concentrations of LH declined from 40 to 56 weeks and rose again between 72 and 80 weeks. Basal concentrations of FSH declined from 32 to 56 weeks, and varied widely after 56 weeks. The maximum LH response to GnRH (highest concentration after GnRH minus baseline) declined steadily in both groups for 48 to 64 weeks but remained relatively constant in both groups after 64 weeks. The maximum FSH response to GnRH declined from 32 to 64 weeks then remained relatively constant in both groups. The GnRH-induced gonadotrophin release remained low with a transient increase at 72 weeks for both hormones. There was no treatment effect (P > 0.05) on time to maximum gonadotrophin release after GnRH, and the time to maximum release varied widely throughout the study. Changes in basal concentrations of LH were associated with changes in LH release after GnRH; whereas basal concentrations of FSH were associated with changes in FSH release only from 32 to 56 weeks of age. The prepubertal changes in secretion of LH in stallions may therefore be associated with a reduction and restoration of the ability of the pituitary to secrete LH as assessed by discharge following exogenous GnRH. However, this relationship does not hold true for FSH after 56 weeks of age.