DIFFERENTIAL-EFFECTS OF GONADOTROPIN-RELEASING-HORMONE (GNRH) PULSE FREQUENCY ON GONADOTROPIN SUBUNIT AND GNRH RECEPTOR MESSENGER-RIBONUCLEIC-ACID LEVELS IN-VITRO

The hypothalamic hormone, GnRH, is released and transported to the ant erior pituitary in a pulsatile manner, where it binds to specific high -affinity receptors and regulates gonadotropin biosynthesis and secret ion. The frequency of GnRH pulses changes under various physiological conditions, and varying GnRH pulse frequencies have been shown to regu late differentially the secretion of LH and FSH and the expression of the gonadotropin alpha, LH beta, and FSH beta subunit genes in vivo. W e demonstrate differential effects of varying GnRH pulse frequency in vitro in superfused primary monolayer cultures of rat pituitary cells. Cells were treated with 10 nM GnRH pulses for 24 h at a frequency of every 0.5, 1, 2, or 4 h. alpha, LH beta, and FSH beta messenger RNA (m RNA) levels were increased by GnRH at all pulse frequencies. alpha and LH beta mRNA levels and LH secretion were stimulated to the greatest extent at a GnRH pulse frequency of every 30 min, whereas FSH beta mRN A levels and FSH secretion were stimulated maximally at a lower GnRH p ulse frequency, every 2 h. GnRH receptor (GnRHR) mRNA levels also were increased by GnRH at all pulse frequencies and were stimulated maxima lly at a GnRH pulse frequency of every 30 min. Similar results were ob tained when the dose of each pulse of GnRH was adjusted to maintain a constant total cumulative dose of GnRH over 24 h. These data show that gonadotropin subunit gene expression is regulated differentially by v arying GnRH pulse frequencies in vitro, suggesting that the differenti al effects of varying GnRH pulse frequencies on gonadotropin subunit g ene expression occur directly at the level of the pituitary. The patte rn of regulation of GnRHR mRNA levels correlated with that of alpha an d LH beta but was different from that of FSH beta. This suggests that alpha and LH beta mRNA levels are maximally stimulated when GnRHR leve ls are relatively high, whereas FSH beta mRNA levels are maximally sti mulated at lower levels of GnRHR expression, and that the mechanism fo r differential regulation of the gonadotropins by varying pulse freque ncies of GnRH may involve levels of GnRHR. Furthermore, these data sug gest that the mechanisms whereby varying GnRH pulse frequencies stimul ate alpha, LH beta, and GnRHR gene expression are similar, whereas the stimulation of FSH beta mRNA levels may be different.