Colocalization of TRH mRNA and Fos-like Immunoreactivity in Limbic Structures Following Amygdala Kindling

Previous studies have demonstrated that the immediate-early gene, Fos, and Fos-related antigens (Fras) are increased with discrete anatomical localization following kindled seizures. Other studies have shown that transcription of several neuropeptide genes, which may be regulated by Fos and Fras, is also altered in circumscribed anatomical structures with kindling. The patterns of Fos and thyrotropin-releasing hormone (TRH) mRNA induction following kindling were found to be remarkably similar. The present study examined whether TRH mRNA and Fos-like immunoreactivity (Fos-LI), with an antibody that recognizes both Fos and Fras, are colocalized following amygdala kindling. Rats were sacrificed 5 h after a generalized kindled seizure and the brains processed for in situ hybridization of TRH mRNA and immunohistochemistry of Fos-LI. The percentage of Fos-LI cells with TRH mRNA was 70% in the entorhinal cortex, 62% in the pyriform cortex, and 79% in the perirhinal cortex. Very dense expression of Fos-LI and TRH mRNA was also found in the granule cell layer of the dentate gyrus, but the colocalization was too numerous to count. Sham-kindled control rats had fewer Fos-LI cells and almost no TRH mRNA in any of these regions. In contrast to the limbic cortices and the dentate gyrus, Fos-LI and TRH mRNA were expressed in the hypothalamus of both sham and kindled rats, but were not colocalized. These results demonstrate that the induction of Fos, Fras, and TRH mRNA following kindled seizures are colocalized in limbic structures known to be important for kindling.