Pituitary Gonadotropin-Releasing Hormone Receptor Regulation in the Hypogonadotrophic Hypogonadal (hpg) Mouse*

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Abstract
Recent evidence indicates that endogenous GnRH is required for maintenance of its own pituitary receptors (GnRH-R). We have measured GnRH-R in pituitaries of hypogonadotrophic hypogonadal (hpg) mice, in whom hypothalamic GnRH is deficient or absent. The GnRH-R concentration in hpg male mouse pituitaries was 10.6 ± 1 fmol/pituitary vs. 30.9 ± 1 fmol/pituitary in normal male littermate pituitaries. Similarly, GnRH-R in female hpg mice (15.2 ± 1.7 fmol/pituitary) were 30% those of normal random cycling females (51.4 ± 3.5 fmol/pituitary). There was no difference in receptor affinity (Ka = 1.5−3 × 109 M−1) of hpg mouse pituitaries. The pituitary LH content in hpg male and female mice was very similar (range 3.4−4.8 μg/pituitary) representing 5% and 19% of normal male (95 ± 7.2 μg/pituitary) and female (18.1 ± 1.5 μg/pituitary) values, respectively. The administration of 50 ng GnRH sc 10 times daily to male hpg mice, increased GnRH-R to 80% of normal values within 3 days. Serum FSH and pituitary FSH content rose to normal male values after 7 days of GnRH injections. However, serum LH remained undetectable and pituitary LH reached only 20% of normal male levels, even after 15 days of GnRH administration. Treatment of hpg male mice with 60 ng GnRH either once daily for 6 days, or 12 times daily for 5 days, increased GnRH-R to 50% of normal male values. Twelve daily injections of GnRH elevated serum FSH to above the normal male range, whereas daily GnRH only doubled untreated hpg levels. Pituitary FSH was stimulated to 50% of normal with 12 daily injections, whereas once daily administration elevated pituitary FSH to 30% of normal values. Both pulsatile regimes depleted pituitary LH. These data demonstrate that: 1) despite absence of bioactive GnRH, GnRH-R values are only reduced to 30% of normal in hpg mouse pituitaries, suggesting that little, if any, endogenous GnRH is required for expression of GnRH receptors. 2) Pituitary GnRH-R number rapidly increase when GnRH is administered to hpg male mice indicating that, as in the rat, GnRH positively regulates its own receptor concentration. 3) The pituitary FSH and LH responses to GnRH treatment in hpg mice depends to a different extent on the frequency and duration of GnRH administration. 4) The hpg mouse provides an ideal animal model for investigating the interaction of defined regimens of exogenous GnRH and gonadal steroids on pituitary GnRH receptor and gonadotroph function. (Endocrinology113: 55, 1983)

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