INDUCTION OF GALANIN MESSENGER-RNA IN GNRH NEURONS BY ESTRADIOL AND ITS FACILITATION BY PROGESTERONE

On the day of proestrus in the rat, rising plasma levels of estradiol (E) act in concert with progesterone (P) to trigger a preovulatory rel ease of gonadotropins. Cellular levels of galanin mRNA in GnRH neurons are increased in association with the proestrous surge of gonadotropi n secretion; however, the relative contribution made by E and P to the induction of galanin mRNA expression in GnRH neurons is unknown. We i nvestigated the role of E and P in the induction of galanin gene expre ssion in GnRH neurons by examining the effects of different combinatio ns of E (estradiol benzoate; 50 mu g and P (5 mg)) on the LH surge and the concomitant induction of galanin mRNA in GnRH neurons. We sacrifi ced ovariectomized adult rats after 1 of 4 treatments: Group 1: vehicl e control (n=6); Group 2: P alone (n=7) Group 3: E alone (n=7); Group 4: combined E/P (n=6); the animals were killed at 18.00 h at the time of the LH surge. The brains from these animals were processed by doubl e-label in situ hybridization to allow measurement of galanin mRNA lev els in GnRH neurons. GnRH neurons were identified with a digoxigenin-l abeled cRNA probe for GnRH mRNA, and galanin mRNA was detected and mea sured simultaneously with an S-35-labeled cRNA probe coupled with comp uterized grain counting. Estimation of cellular levels of GnRH mRNA wa s accomplished with single-label in situ hybridization, an S-35-labele d GnRH cRNA probe and computerized grain counting. We observed a 3-fol d induction of galanin mRNA in the GnRH neurons of animals treated wit h E alone compared with those treated with the vehicle alone (vehicle: 13+/-2 vs E: 42+/-4 grains/cell (g/c); P<0.01); LH levels in the E-tr eated animals were elevated, albeit moderately, with respect to the ve hicle controls. Compared with vehicle-treated animals, those treated w ith the combination of E and P showed a 5-fold induction of galanin mR NA in GnRH neurons (68+/-9 g/c), which was significantly (P<0.01) grea ter than that observed in the animals treated with E alone; in additio n, the magnitude of the LH surge was much greater (P<0.05) in the E/P- treated group compared with the E alone group. In contrast, compared t o the vehicle controls, animals treated with P alone (15+/-2 g/c) show ed no discernable effect on galanin mRNA levels; moreover, no LH surge occurred in the P alone group. Neither the number of identified GnRH cells nor their content of GnRH mRNA differed significantly among the experimental groups (GnRH mRNA signal: vehicle controls: 153+/-6 vs E: 159+/-6 vs E/P: 153+/-3 vs P: 148+/-8 g/c). We conclude that while E is the primary ovarian signal inducing galanin mRNA expression in GnRH neurons and the LH surge itself, P plays a facilitatory role in both of these processes.