Immunoreactivity for endomorphin-2 occurs in primary afferents in rats and monkey.

Antisera were raised against endomorphin-2, a recently isolated endogenous opioid peptide that binds potently and selectively to the mu-opioid receptor. When sections of spinal cord were stained immunocytochemically, a dense plexus of fibres and varicosities was visualized in the superficial dorsal horn of rats and one monkey. Following unilateral multiple dorsal rhizotomy, labeling for endomorphin-2 was markedly reduced ipsilateral to the lesion. In sections stained for both endomorphin-2 and CGRP, double-labeling was observed. Taken together, these data suggest that endomorphin-2 occurs in small diameter primary afferent fibres in rodents and primates. It appears possible that the release of neurotransmitters from nociceptive primary afferents might be regulated by release of endomorphin-2 from primary afferent terminals.