HOMOLOGOUS DOWN-REGULATION OF GONADOTROPIN-RELEASING-HORMONE RECEPTOR-SITES AND MESSENGER-RIBONUCLEIC-ACID TRANSCRIPTS IN ALPHA-T3-1 CELLS

GnRH is known to down-regulate its pituitary receptors by mechanisms t hat include endocytosis of the agonist-receptor complex. To evaluate t he extent to which changes in receptor synthesis contribute to this pr ocess, the effects of GnRH and its analogs on GnRH receptor number and messenger RNA (mRNA) levels were analyzed in the alpha T3-1 gonadotro ph cell line. Treatment with GnRH or its potent agonist analog, des-Gl y(10)-[D-Ala(6)]GnRH N-ethylamide, reduced GnRH receptor number in a t ime- and dose-dependent manner, with a half-maximal decrease in respon se to 10(-6) M GnRH or agonist analog by 75 min. The maximum decrease in receptor number (to 31% of the control value) was sustained for up to 72 h. In alpha T3-1 cells incubated with 10(-8) M GnRH or agonist a nalog, the GnRH receptors fell by 28% and 46% after 2 h, respectively; no change in receptors occurred after treatment with 10(-8) M GnRH an tagonist ([D-pGlu(1),D-Phe(2),D-Trp(3,6)]GnRH). Time- and dose-depende nt reductions in the level of receptor mRNA were also observed after t reatment of alpha T3-1 cells with GnRH and the agonist analog. However , the maximal reduction in mRNA levels (to 60-70% of the control value ) was consistently less than the decline in receptor number. These res ults indicate that the mechanism of GnRH receptor down-regulation in a lpha T3-1 gonadotrophs includes reduction of receptor synthesis second ary to decreases in receptor mRNA levels. The finding that reductions in mRNA levels were relatively less than the decreases in receptor num ber is consistent with the involvement of additional mechanisms, inclu ding endocytosis and degradation, in down-regulation of the GnRH recep tor.