The opioid octapeptide Met5-enkephalin-Arg6-Gly7-Leu8: characterization and distribution in rat spinal cord

The regional quantitation, immunohistochemical localization and molecular heterogeneity of Met5-enkephalin-Arg6-Gly7-Leu8 were examined in rat spinal cord with a specific radioimmunoassay. A rostrocaudal gradient in Met5-enkephalin-Arg6-Gly7-Leu8 content was observed; the highest levels occurred in sacral cord. Dorsal cord content was higher than that of ventral cord at all spinal segments. Immunohistochemical staining supported and refined the latter observation: a dense network of perikarya and fibers was found in Laminae I and II of the dorsal horn. Cell bodies were frequently observed in lamina IV. Additional terminals were seen around the central canal and in the ventral gray matter, often outlining perikarya of motor neurons. Total Met5-enkephalin-Arg6-Gly7-Leu8 immunoreactivity could be fractionated into two main components using gel filtration chromatography. Nearly half of the total immuno-reactivity eluted as a high molecular weight peptide; the other half which co-eluted with Met5-enkephalin-Arg6-Gly7-Leu8 was further identified to be authentic Met5-enkephalin-Arg6-Gly7-Leu8 on reverse phase high pressure liquid chromatography. The present data, in conjunction with our previous study of Met5-enkephalin and Met5-enkephalin-Arg6-Phe7 indicates that all opioid peptides derived from preproenkephalin A are present in spinal cord and most likely are stored in the same neurons. Immunohistochemical localization of Met5-enkephalin-Arg6-Gly7-Leu8 in dorsal and ventral cord suggest a role for this peptide in both sensory and motor integration.