Endomorphin-2 is an endogenous opioid in primary sensory afferent fibers

Evidence is presented that the recently discovered endogenous mu-selective agonist, endomorphin-2, is localized in primary sensory afferents. Endomorp hin-2-like immunoreactivity was found to be colocalized in a subset of subs tance P- and mu opiate receptor-containing fibers in the superficial lamina e of the spinal cord and spinal trigeminal nucleus. Disruption of primary s ensory afferents by mechanical (deafferentation by dorsal rhizotomy) or che mical (exposure to the primary afferent neurotoxin, capsaicin) methods virt ually abolished endomorphin-2-like immunoreactivity in the dorsal horn. The se results indicate that endomorphin-2 is present in primary afferent fiber s where it can serve as the endogenous ligand for pre- and postsynaptic mu receptors and as a major modulator of pain perception. (C) 1998 Elsevier Sc ience Inc.