Preproenkephalin mRNA in spinal dorsal horn neurons is induced by peripheral inflammation and is co-localized with Fos and Fos-related proteins.

Increased levels of preproenkephalin mRNA in spinal cord neurons induced by peripheral tissue inflammation were examined using in situ hybridization histochemistry. In addition, in situ hybridization histochemistry was combined with immunocytochemistry to determine whether increases in preproenkephalin mRNA were co-localized in spinal cord neurons with increases in immunoreactivity for Fos and Fos-related proteins coded by the immediate-early proto-oncogene, c-fos, and related genes. Dorsal horn laminae I-II, V-VI and VII showed a greater than 200% increase in preproenkephalin mRNA-labeled neurons on the inflamed side as compared to the contralateral control. Inflammation also induced Fos-like immunoreactivity in cell nuclei, mainly in the superficial laminae I-II and the neck of the dorsal horn (laminae V-VI). Few labeled nuclei were detected on the contralateral side. Inflammation resulted in double-labeling of neurons ipsilateral to the inflamed limb whereas they were almost completely absent on the contralateral side. Double-labeled neurons were most frequently found in laminae V-VI. Double-labeled laminae I-II neurons were concentrated in the medial two-thirds of the dorsal horn, the site that receives innervation from the inflamed limb. There were also many double-labeled neurons in laminae VII. Over 90%, 82% and 69% of all neurons expressing preproenkephalin mRNA co-localized Fos immunoreactivity in laminae V-VI, I-II, and VII, respectively. However, the number of neurons expressing increased Fos immunoreactivity was substantially greater than the subpopulation of double-labeled neurons. Our findings indicated that peripheral inflammation induces an increase in preproenkephalin mRNA levels in spinal cord neurons and that most neurons exhibiting preproenkephalin mRNA labeling also co-localized Fos and Fos-related immunoreactivity. These data are consistent with evidence supporting the role of Fos and Fos-related proteins in the regulation of transcription of the preproenkephalin gene in spinal neurons.