Ultrastructural identification of substance P cells and processes in the rat myenteric plexus.

Substance P containing perikarya and processes were identified by electron microscopic immunocytochemistry in the rat small intestine. The labelled perikarya possessed a cytoplasm of high electron density rich in organelles most of which contained numerous 80 to 130 nm large granulated vesicles. Labelled processes were found in pre- and postsynaptic positions. The presynaptic terminals contained 80 to 130 nm granular vesicles and 30 to 40 nm clear vesicles. These findings provide a morphological basis for the assumption that the peptide substance P may act as a transmitter and exerts postsynaptic effect on other intrinsic neurons and that the labelled cell bodies are links to a local neuronal circuit in the small intestine.