Pituitary gonadotropin-releasing hormone receptor regulation in mice. I: Males

The regulation of pituitary GnRH receptors (GnRH-R) has been examined in male mice (C3H/HeH/101H F1 hybrid) after castration and testosterone replacement. GnRH-R were quantified in individual mouse pituitaries by equilibration with 125I(D-Ser(tBut)6) des Gly10 GnRH N ethylamide and compared with serum and pituitary LH and FSH concentrations. The equilibrium association constant was 2.7 X 10(9) M-1 for both intact and castrated male mouse pituitary GnRH-R. Six hours after orchidectomy there was a transient 50% reduction in GnRH-R; 13.6 +/- 3.8 fmol/pituitary (castrate) vs. 25.4 +/- 2.5 (intact). A subsequent partial return of binding sites began at 12 h, reaching a peak value of 18.2 +/- 1.5 fmol/pituitary (33% increase vs. 6 h) at 24-h post orchidectomy. This was followed by a gradual decrease in GnRH-R, reaching a plateau by 72 h. The decrease in GnRH-R was associated with a rapid (6-12 h) increase in serum LH and serum FSH. The pituitary GnRH-R concentration remained 45% below intact control values for up to 3 months and was accompanied by a persistent 5-fold rise in serum LH values. Treatment of male mice with testosterone propionate (TP), 25 micrograms/day, completely prevented the GnRH-R fall and the serum and pituitary LH responses to castration, whereas 12.5 micrograms/day TP produced variable results and 5 micrograms/day TP were ineffective. In another strain of mouse (BALB/c white). GnRH-R values also fell by 66% at 7 days post orchidectomy, with no change in the receptor affinity. In mice with androgen resistance from birth due to absence of androgen receptors (Tfm mice), GnRH-R were 14.45 +/- 0.49 vs. 19.8 +/- 1.67 fmol/pituitary in normal male littermates, and serum LH was 472 +/- 78 ng/ml compared with 52.5 +/- 11.7 ng/ml in normals. These findings are qualitatively similar to those in orchidectomized normal adult mice. Thus, in contrast to reports in rats, pituitary GnRH-R content falls after orchidectomy in mice. Possible explanations for this consistent finding include: persistent receptor occupancy by increased endogenous GnRH secretion, endogenous GnRH-induced receptor loss (down-regulation), or a species difference in the pituitary GnRH-R response to removal of negative steroid feedback, unrelated to changes in endogenous GnRH secretion.