TEMPORAL PATTERNS OF GONADOTROPIN-RELEASING-HORMONE (GNRH), C-FOS, AND GALANIN GENE-EXPRESSION IN GNRH NEURONS RELATIVE TO THE LUTEINIZING-HORMONE SURGE IN THE RAT

Gonadotropin-releasing hormone (GnRH) neurons increase their expressio n of Fos and galanin coincident with the luteinizing hormone (LH) surg e in the female rat. To define the temporal relationships between the expression of these genes and the GnRH gene itself and to gain insight about the possible functional interactions of these processes, we com pared levels of c-fos, galanin, and GnRH mRNA in GnRH neurons and plas ma levels of LH in the rat, beginning 6 hr before and continuing for 2 4 hr after a sex steroid-induced LH surge. LH levels were increased si gnificantly by 1600 hr. They increased twofold further by 1800 hr and then returned to baseline by 2400 hr. Using in situ hybridization, we determined that levels of c-fos mRNA in GnRH neurons were elevated sig nificantly at 1600 hr only, whereas levels of galanin mRNA in GnRH neu rons first increased twofold by 1800 hr, increased an additional twofo ld by 2400 hr, and remained elevated al all time points sampled therea fter. There were no significant changes in cellular levels of GnRH mRN A over the time points sampled. These results are consistent with the hypothesis that the induction of c-fos gene expression in GnRH neurons leads to an increase in galanin gene expression, and that the sustain ed increase in galanin mRNA levels in GnRH neurons reflects either the need to replenish galanin stores that are depleted at the time of the LH surge or the involvement of galanin with physiological events that occur on the day of estrus.