ACTIVATION-DEPENDENT REGULATION OF GALANIN GENE-EXPRESSION IN GONADOTROPIN-RELEASING-HORMONE NEURONS IN THE FEMALE RAT

In rats, galanin is colocalized in GnRH neurons, and galanin mRNA in G nRH neurons is increased coincidentally with the preovulatory gonadotr opin surge. Whether the induction of galanin mRNA in GnRH neurons at p roestrus reflects the action of sex steroids is unknown. We tested thi s hypothesis by challenging ovariectomized rats (n = 7) with estrogen and progesterone (E/P) to induce a LH surge and measuring galanin mRNA in GnRH neurons to determine whether there was an associated inductio n of galanin message in these cells. We used single and double label i n situ hybridization and image analysis to compare among groups the le vels of both galanin mRNA and GnRH mRNA in GnRH neurons. We found that steroid-primed animals showed an approximately 400% induction of gala nin mRNA signal in GnRH neurons over that in vehicle-treated animals. Second, we hypothesized that steroid-dependent events which induce the expression of galanin mRNA in GnRH neurons depend on transsynaptic in put to GnRH neurons. We tested this hypothesis by examining the effect of a pharmacological blockade of the steroid-induced activation of Gn RH neurons on levels of galanin mRNA in these cells. We killed groups of ovariectomized adult female rats at the peak of a E/P-primed LH sur ge (n = 7) and after steroid priming followed by blockade of the LH su rge with either the general anesthetic pentobarbital (n = 7) or the sp ecific ar-adrenergic receptor blocker phenoxybenzamine (n = 7). When w e examined signal levels representing galanin mRNA content in GnRH neu rons, we observed a 4-fold increase in signal for galanin mRNA in; the GnRH neurons of steroid-primed (E/P surge) animals compared with that in oil-treated controls (P < 0.0004). This increase in galanin mRNA w as prevented when the LH surge was blocked by treatment with either pe ntobarbital or phenoxybenzamine (P < 0.03 and P < 0.0001 vs. E/P surge controls, respectively). Cellular levels of GnRH mRNA were not differ ent among control, E/P, and E/P plus pentobarbital groups (P > 0.2). T hese observations suggest that an increase in galanin mRNA levels in G nRH neurons is tightly coupled to the occurrence of a LH surge. By inf erence, induction of galanin mRNA in GnRH neurons reflects their activ ation, possibly via afferent neurons that transduce the steroid signal to GnRH neurons.