Changes in plasma gonadotrophin and prolactin concentrations following castration of the pony stallion

Concentrations of gonadotrophins and prolactin were recorded in pony stalli ons castrated during the early breeding season, to examine the regulatory r ole of the gonad at a time when testosterone has been postulated to exert p ositive feedback on LH secretion. Further, gonadotrophin concentrations in geldings are reported to return to values within the normal range of the en tire stallion. In an attempt to characterize this species-specific reversal , the gonadotrophin concentrations of 6 male ponies castrated on 25 March w ere monitored for 4 months, and 4 stallions were used to generate control d ata. Blood samples were collected daily, from 3 d before to 10 d after cast ration (Day 0), and weekly thereafter until Day 122. The pituitary response to castration was immediate. Castration resulted in a previously unreporte d, dramatic (13-fold) but transient (3 d) surge in circulating concentratio ns of LH. Concentrations of LH and FSH increased in a logarithmically scale d (LH, R-2 = 0.77; FSH, R-2 = 0.93) manner over the subsequent 5 wk, during which temporal changes in concentrations of both hormones were strongly co rrelated (R-2 = 0.97). The ratio of plasma gonadotrophin concentrations was consistent throughout (LH:FSH, 1.43 +/- 0.04). Maximal concentrations of L H (20.58 +/- 1.97 ng/mL, Day 34.8 +/- 3.2) were attained approximately 2 wk before the peak in FSH (16.99 +/- 1.97 ngimL, Day 49.7 +/- 3.0). Plasma go nadotrophin concentrations exceeded those of entire stallions throughout th e study. The equine testes inhibited LH secretion during the early breeding season, and no chronic decrease in plasma gonadotrophin concentrations was recorded. However, the LH surge evident for 3 d immediately afer castratio n, may be related to the dynamic seasonal interaction between gonadal stero ids and the regulation of pituitary gonadotrophin release. (C) 2001 by Else vier Science Inc.