INHIBITION OF STEROID-INDUCED GALANIN MESSENGER-RNA EXPRESSION IN GNRH NEURONS BY SPECIFIC NMDA-RECEPTOR BLOCKADE

Galanin mRNA levels in GnRH neurons increase in association with a ste roid-induced LH surge in female rats. Both the steroid-induced LH surg e and the concomitant increase of galanin mRNA in GnRH neurons are blo cked by non-specific inhibition of central nervous system activity imp osed by pentobarbital and specific central alpha-adrenergic receptor b lockade. Based on these observations, we hypothesized that galanin gen e expression in GnRH neurons is induced whenever GnRH neurons become a ctivated to generate an LH surge. If this were the case, then any neur otransmitter receptor blocking agent that inhibits the LH surge by cen tral mechanisms would likewise block the associated increase in galani n mRNA in GnRH neurons. We tested this hypothesis by examining the eff ects of an N-methyl-D-aspartate (NMDA) receptor antagonist on the ster oid-induced LH surge and on levels of galanin mRNA in GnRH neurons. Th ree groups of ovariectomized rats were used: Group 1-treated with estr adiol and progesterone (EP) and sacrificed at the peak of the LH surge ; Group 2-treated the same as Group 1 except that dizocilpine (MK801, an NMDA receptor antagonist) was used to block the LH surge; and Group 3-treated the same as Group 1 except they received vehicle instead of E/P. Double-and single-label in situ hybridization followed by comput erized image analysis were used to measure levels of galanin mRNA and GnRH mRNA in GnRH neurons [as grains/cell (g/c)]. E/P treatment induce d a 3-fold increase in LH levels and a 5-fold increase in the galanin mRNA signal content of GnRH neurons. Treatment with MK801 completely p revented the LH surge in all animals and also blocked the steroid-indu ced increase in galanin mRNA in GnRH neurons. As assessed by 2 indepen dent GnRH single-labeled assays, neither GnRH message content nor the number of identifiable GnRH neurons differed among the experimental gr oups. We conclude that the increase in galanin mRNA levels in GnRH neu rons is tightly coupled to the occurrence of a steroid-evoked LH surge , and we infer that induction of galanin gene expression in GnRH neuro ns is induced as a consequence of synaptic activation of GnRH neurons.