Ultrastructural evidence for beta-endorphin-like immunoreactivity in the nervous system of the rat duodenum

Using a modified immunoelectron microscopic staining technique the opioid peptide beta-endorphin could be demonstrated within large dense-core vesicles (LDVs) which are localized in presumptive axons and dendrites of the duodenal myenteric plexus of the rat duodenum. Pre-absorption tests with synthetic beta-endorphin (10(-6)M, final concentration) abolished the beta-endorphin immunoreactivity, resulting in only beta-endorphin unstained vesicles. To demonstrate more clearly electron-dense beta-endorphin material within vesicles the ultrathin sections were not counterstained with heavy metal salts. These sections revealed that the beta-endorphin material is localized as 'cores' with a mean diameter of 35-50 nm within LDVs corresponding to cores of LDVs, which are 70-90 nm in mean diameter.