The role of GnRH pulse patterns and gonadal steroids on divergent gonadotropin release throughout the estrous cycle of the rat

Abstract

The object of these studies was to determine the capacity of pulsatile GnRH, estradiol and progesterone to modulate gonadotropin secretion from superfused anterior pituitary cell cultures. Five GnRH pulse patterns were employed. LH responses to R1 ranked as follows: proestrous 1900 > proestrous 1500 > proestrous 0800 = estrous 0800 > diestrous 1 0800 = diestrous 2 0800. No cycl> related differences were observed for FSH secretion with R1. To further pursue FSH responsiveness, GnRH pulses of increased amplitude and duration were employed (R2-5) at proestrous 1900. Only the first pulse of R4 resulted in significantly greater LH release as compared to R1 at P1900. In contrast, FSH release was significantly elevated by all regimens at P1900; however, R3 and R5 elicited greater FSH responses than R2 and R4. At estrous 0800 FSH and LH release were significantly elevated by R3 as compared to R1. When GnRH pulse frequency was reduced (R5) at estrous 0800 FSH release was further elevated while LH release was unaffected as compared to R3. At proestrous 1500 both LH and FSH release were significantly elevated by R3 and when GnRH pulse frequency was reduced (R5), FSH release was further elevated while LH release was suppressed. To study the capacity of estradiol (E) and progesterone (P) to modulate LH and FSH secretion, anterior pituitary cultures derived from two week castrate rats were incubated with or without either 1 or 10 nM E for 48 hours; during the last 12 hours of incubation 100 nM P was added for 3, 6 or 12 hours. Cultures were then stimulated with either R1, R3 or R5. 1 nM E for 48 hours followed by R3 caused FSH release to be significantly elevated while LH was unaffected. Increasing E to 10 nM significantly inhibited LH release in response to R1 and R3. With 1 nM E priming LH release was inhibited with the 3 and 12 hour P incubations and either unaffected or stimulated with 6 hours P regardless of the GnRH regimen employed. When pulsed with R3 and R5 the 6 hour P incubation resulted in a stimulation of FSH release at the first GnRH pulse. With 10 nM E priming LH release was stimulated by the 3 and 6 hour P incubations when pulsed with R3 and R5. When pulsed with R3 FSH release was stimulated by 3 and 6 hours of P while only 3 hours P resulted in elevated FSH when pulsed with R5.