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 r esponse to pulsatile GnRH in the rat. Estradiol (E(2)) is known to exe rt a transient positive action to increase GnRH-R number, and the rise in plasma E(2) contributes to initiation of the midcycle LH surge. Th e present study was designed to determine the effect of GnRH pulse amp litude and frequency on GnRH-R messenger RNA (mRNA) levels and to asse ss the relative contributions of GnRH and gonadal steroids to increasi ng GnRH-R gene expression. These studies were conducted in vivo using previously characterized GnRH-deficient male (castrate testosterone-re placed) 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 puls es 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 c ontrast, 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 interva ls (maximal after 30-min pulses), whereas continuous GnRH was ineffect ive. In females, only 30- and 240-min pulse intervals increased GnRH-R mRNA levels, with faster (8-min) pulses or continuous GnRH being inef fective. 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) lev els, similar to 50 pg/ml), or their combination for 12-24 h (saline pu lses 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 1 2 h, with a 2-fold rise observed after 24 h. Pulsatile GnRH alone incr eased 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 t hat seen for either GnRH or E(2) alone. In E(2)-treated animals, the a ddition of progesterone (in the absence or presence of GnRH) had no ef fect. These data reveal that pulsatile GnRH increases GnRH-R mRNA expr ession in both male and female rats. Female rats appear to display gre ater sensitivity to alterations in GnRH pulse pattern, particularly pu lse amplitude, than males. Further, the stimulatory effect of GnRH pul ses on GnRH-R mRNA in the female is markedly enhanced by E(2). These d ata suggest that the increased pituitary sensitivity to GnRH present b efore and during the midcycle LH surge, may result from the increased GnRH-R consequent upon enhanced GnRH-R mRNA expression.