In rats, galanin is colocalized in GnRH neurons, and galanin mRNA in GnRH neurons is increased coincidentally with the preovulatory gonadotropin surge. Whether the induction of galanin mRNA in GnRH neurons at proestrus reflects the action of sex steroids is unknown. We tested this 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 induction of galanin message in these cells. We used single and double label in situ hybridization and image analysis to compare among groups the levels of both galanin mRNA and GnRH mRNA in GnRH neurons. We found that steroid-primed animals showed an approximately 400% induction of galanin 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 input to GnRH neurons. We tested this hypothesis by examining the effect of a pharmacological blockade of the steroid-induced activation of GnRH 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 surge (n = 7) and after steroid priming followed by blockade of the LH surge with either the general anesthetic pentobarbital (n = 7) or the specific ±-adrenergic receptor blocker phenoxybenzamine (n = 7). When we examined signal levels representing galanin mRNA content in GnRH neurons, 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 was prevented when the LH surge was blocked by treatment with either pentobarbital or phenoxybenzamine (P < 0.03 and P < 0.0001 vs. E/P surge controls, respectively). Cellular levels of GnRH mRNA were not different among control, E/P, and E/P plus pentobarbital groups (P > 0.2). These observations suggest that an increase in galanin mRNA levels in GnRH neurons is tightly coupled to the occurrence of a LH surge. By inference, induction of galanin mRNA in GnRH neurons reflects their activation, possibly via afferent neurons that transduce the steroid signal to GnRH neurons.
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