ENKEPHALIN NEURONS IN THE GUINEA PIG DUODENUM: A LIGHT AND ELECTRON MICROSCOPIC IMMUNOCYTOCHEMICAL STUDY USING AN ANTISERUM TO METHIONINE-ENKEPHALIN-ARG6-GLY7-LEU8

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Abstract
The occurrence, distribution and ultrastructure of nerve elements containing opioid peptide-like immunoreactivity (enkephalin neurons) in the guinea pig duodenum were investigated using an antiserum (R-0171) to methionine-enkephalin-Arg6-Gly7-Leu8 (Met-Enk-Arg-Gly-Leu). Whole mount preparations of duodenal tissues perfusion-fixed Leu-like immunoreactivity. The morphology of these enkephalin neurons was different from cell to cell. Immunopositive Dogiel type 1 (characterized by short, broad buttonlike processes and a single, long axon-like process), Dogiel type 2 (characterized by smooth perikaryon and a few tapering processes) and Dogiel type 3 neurons (characterized by dendrite-like processes branching around other ganglion cells) were demonstrated. The single axon-like process of many Dogiel type 1 neurons was anally directed and ran in the primary fasciculi across the ganglia. Non-varicose processes and beaded terminals of the enkephalin neurons were demonstrated not only in the myenteric plexus but also in the longitudinal muscle layer, in the circular muscle layer, in the deep muscular plexus, in the submucous plexus, in the muscularis mucosae and around the duodenal gland. No immunopositive perikarya were found in the submucous plexus. No nerve elements in the perivascular plexus and in the mucosa showed immunoreactivity. In the electron microscopic immunocytochemistry by the protein A-colloidal gold method, Met-Enk-Arg-Gly-Leu-like immunoreactivity was demonstrated in the largecored vesicles in the nerve fiber varicosities and the perikarya. Synapse formation between enkephalin-immunoreactive nerve terminals and either immunopositive or immunonegative perikarya was found in the myenteric plexus.