GONADOTROPIN-RELEASING-HORMONE (GNRH) PULSE PATTERN REGULATES GNRH RECEPTOR GENE-EXPRESSION - AUGMENTATION BY ESTRADIOL

GnRH acts via a single cell surface receptor (GnRH-R), and the number of pituitary GnRH-R increases on proestrus, after gonadectomy, or in response to pulsatile GnRH in the rat. Estradiol (E(2)) is known to exert a transient positive action to increase GnRH-R number, and the rise in plasma E(2) contributes to initiation of the midcycle LH surge. The present study was designed to determine the effect of GnRH pulse amplitude and frequency on GnRH-R messenger RNA (mRNA) levels and to assess the relative contributions of GnRH and gonadal steroids to increasing GnRH-R gene expression. These studies were conducted in vivo using previously characterized GnRH-deficient male (castrate testosterone-replaced) and ovariectomized phenoxybenzamine-treated female models. To investigate the effect of GnRH pulse amplitude, adult male and female rats received GnRH iv (5-250 ng/pulse at 30-min intervals; saline pulses to controls) for 12 or 24 h. In males, GnRH-R mRNA was increased by all pulse doses, with maximal effects (3-fold) at 5-25 ng/pulse. In contrast, only lower doses (5-10 ng/pulse) were effective in females (2-fold increase). In a subsequent study, GnRH pulses (25 ng for males; 10 ng for females) were given at 8-, 30-, or 240-min intervals for 12 or 24 h. Some animals received a continuous GnRH infusion (200 ng/h). In males, GnRH-R mRNA levels were stimulated by all GnRH pulse intervals (maximal after 30-min pulses), whereas continuous GnRH was ineffective. In females, only 30- and 240-min pulse intervals increased GnRH-R mRNA levels, with faster (8-min) pulses or continuous GnRH being ineffective. To determine the relative roles of ovarian steroids and GnRH, ovariectomized phenoxybenzamine-treated female animals received GnRH (10 ng/pulse, 30-min interval), E(2) (via sc implants; plasma E(2) levels, similar to 50 pg/ml), or their combination for 12-24 h (saline pulses to controls). In the absence of E(2), GnRH-R concentrations fell by 70% between 12-24 h. E(2) alone tended to increase GnRH-R mRNA at 12 h, with a 2-fold rise observed after 24 h. Pulsatile GnRH alone increased GnRH-R mRNA by 50% at 12 h (compared to saline-pulsed controls; P < 0.05) and by 6-fold after 24 h. When GnRH and E(2) were combined, the magnitude of the increase (vs. saline controls) was greater than that seen for either GnRH or E(2) alone. In E(2)-treated animals, the addition of progesterone (in the absence or presence of GnRH) had no effect. These data reveal that pulsatile GnRH increases GnRH-R mRNA expression in both male and female rats. Female rats appear to display greater sensitivity to alterations in GnRH pulse pattern, particularly pulse amplitude, than males. Further, the stimulatory effect of GnRH pulses on GnRH-R mRNA in the female is markedly enhanced by E(2). These data suggest that the increased pituitary sensitivity to GnRH present before and during the midcycle LH surge, may result from the increased GnRH-R consequent upon enhanced GnRH-R mRNA expression.