Kainate Receptor Subunit-Positive Gonadotropin-Releasing Hormone Neurons Express c-Fos during the Steroid-Induced Luteinizing Hormone Surge in the Female Rat1

During the preovulatory and estradiol-progesterone-induced GnRH-LH surge, a subpopulation of GnRH neurons transiently expresses the transcription factor c-fos, which is a useful marker of cell activation. To further characterize this subpopulation of GnRH neurons, multiple immunohistochemical procedures were applied to visualize GnRH, c-Fos, KA2, GluR5, GluR6, and GluR7 receptor subunits during different phases of the estrogen-progesterone-induced LH surge. The results show that the LH surge begins at 1400 h and peaks at 1600 h before returning to baseline late in the evening. At 1400 h, about 50% of the GnRH neurons contained c-Fos, and this percentage remained high at 65% at 1600 and 2000 h. During the surge, 50% of the c-Fos-positive GnRH neurons contained KA2 receptor subunit protein at 1400 h, 65% of the c-Fos-positive GnRH neurons expressed the KA2 subunit at 1600 h, and 50% of the c-Fos-positive GnRH neurons expressed the KA2 subunit at 2000 h. As KA2 subunits require other kainate-preferring subunits to form functional receptor channels, we examined GnRH neurons for the presence of GluR5, GluR6, and GluR7 messenger RNA (mRNA) and protein. The results show that the KA2-containing GnRH neurons also contain GluR5 receptor subunit mRNA and protein, and that these GnRH neurons are c-Fos positive during the steroid-induced LH surge. To determine whether administration of kainate is sufficient to induce c-Fos in GnRH neurons, steroid-primed animals received iv injections of subseizure-inducing amounts of kainic acid and were processed for immunohistochemistry and in situ hybridization. The results show that kainic acid causes a significant increase in circulating LH; however, it does not induce c-Fos in GnRH neurons, nor does it cause an increase in GnRH mRNA. Together, the results suggest that a large subset of GnRH neurons expresses KA2 as well as GluR5 receptor subunits, which would allow the formation of functional glutamate receptor channels, and that this subset of GnRH neurons is activated during the steroid-induced LH surge.