Enkephalins may act as sensory transmitters in earthworms

Summary Part of the sensory cells of the earthworm (Lumbricus terrestris) epidermis stain immunocytochemically with enkephalin antisera of different region specificities. The immunocytochemical results suggest the existence of peptides identical with or closely resembling met- and leu-enkephalin in these cells. The processes of enkephalin-immunoreactive cells become collected to form sensory nerves before entering the ventral ganglionic chain where they project as enkephalin-immunoreactive sensory bundles. Injections of the opiate receptor antagonist naloxone in earthworms inhibit their touch-induced withdrawal reflex. Recovery occurs within 2 hours. Moreover, anaesthesia of earthworms in dilute ethanol brings about abolishment of the withdrawal reflex as well as disappearance of enkephalin immunoreactivity from the cell bodies, but not from the sensory hairs. Together, these data suggest that opioid peptides, possibly enkephalins, act as sensory transmitters or modulators in earthworms.