Immunohistochemical localization of enkephalin in peripheral sensory axons in the rat

Dorsal root ganglia (DRG) contain measurable amounts of met-enkephalin (ENK), and a significant number of DRG cells contain mRNA for the manufacture of ENK. Yet almost no DRG cells are immunostained for ENK and dorsal rhizotomy does not diminish ENK staining in the dorsal horn. A hypothesis which would explain these seemingly discrepant results is the phenomenon of differential transport, where DRG cells making ENK rapidly transport the peptide only to their peripheral sensory axons. Evidence consistent with this hypothesis would be the demonstration of ENK-containing peripheral sensory axons. The present study demonstrates that approximately 17% of peripheral cutaneous axons label for ENK. The presence of a significant number of ENK-containing axons suggests an endogenous neural source of opiate ligand in the periphery and, in addition to ENK-containing inflammatory cells, this neural source may be functionally important in responses to physiologic as well as inflammatory pain.