Effect of progesterone on galanin mRNA levels in the hypothalamus and the pituitary: Correlation with the gonadotropin surge

Galanin has been shown to be an important neuropeptide associated with the preovulatory surge of gonadotropins. Since the preovulatory surge of gonadotropins is induced by the action of estrogens and progesterone, the present study examined the effect of estrogens and progesterone on hypothalamic and pituitary galanin mRNA levels. Galanin mRNA was present as the 900-base pair message in the hypothalamus and the pituitary. In the 27-day-old ovariec-tomized immature rat, treatment with 2 ug of estradiol for 2 days did not bring about any changes in hypothalamic galanin mRNA levels. In adult rats, ovariectomy also did not bring about any changes in hypothalamic galanin mRNA levels. However, 14 days of estradiol replacement increased hypothalamic galanin mRNA levels. Progesterone treatment in the estrogen-primed ovariecto-mized immature rat resulted in a significant increase in hypothalamic galanin mRNA levels at 12:00 and 14:00 h. This increase occurred at the time of the initiation of the progesterone-induced LH and FSH surges. Estrogen treatment enhanced pituitary galanin mRNA levels which were further increased by progesterone treatment at 12:00 h, the time of the initiation of the gonadotropin surge by progesterone. In the PMSG-primed immature rat, pituitary galanin mRNA levels were increased due to ovarian estrogen and progesterone secretion as a result of PMSG treatment. Pituitary galanin mRNA levels were decreased by the anti-progestin RU486 at 12:00 and 20:00 h, times that coincided with a decrease in serum LH by RU486 as compared to the PMSG-treated controls. Thus, galanin is an important neuropeptide that acts at the level of the hypothalamus and the pituitary in modulating the gonadotropin surge and is regulated by progesterone in both tissues.