The effect of estrogen on luteinizing hormone-releasing hormone binding sites in hypothalamic membranes

The binding sites for [¹²⁵I]LHRH were characterized in membranes from the hypthalamus and the effect of estrogen on the binding characteristics was studied in ovariectomized female rats. The radioligand, [¹²⁵I]LHRH, was found to bind specifically to membranes from the hypothalamus at a maximal level, with an optimal temperature of 0°C and a pH between 7 and 8. The binding was enhanced by NaCl at a concentration of 0.1-0.2 M. The specifically bound [[¹²⁵I]LHRH was only displaced by LHRH, but not by sodium iodide (NaI), bovine serum albumin and other hormones, such as thyrotropin-releasing hormone, bradykinin, oxytocin, prolactin, luteinizing hormone and growth hormone. The divalent metal ions, copper (Cu²⁺) and mercury (Hg²⁺), inhibited the specific binding of [¹²⁵I]LHRH completely, whereas magnesium (Mg²⁺) and calcium (Ca²⁺) caused a decrease in binding. As revealed from Scatchard plot analysis, the binding sites for [¹²⁵I]LHRH in the hypothalamus had a dissociation constant of 0.40 ± 0.03 μM and the maximum number of binding sites was 98.55 ± 4.34 pmol/mg protein. Treatment of female rates (ovariectomized for 3 weeks) with 4 μg of estradiol benzoate caused a statistically significant decrease in the maximal number of binding sites without any significant effect on the dissociation constant. However, the direct addition of estradiol hemisuccinate to the membrane preparations had no statistically significant effect on the specific binding off [¹²⁵I]LHRH. The present study provides the evidence that estrogen decreases the density of binding sites for [¹²⁵I]LHRH in the hypothalamus in vivo.