Substance P immunoreactivity in the rat interpeduncular nucleus: synaptic interactions between substance P-positive profiles and choline acetyltransferase- or glutamate decarboxylase-immunoreactive structures.

The subnuclear and synaptic distribution of substance P immunoreactivity was examined in the rat interpeduncular nucleus at the light and electron microscope level. The nucleus possessed a prominent substance P-immunoreactive axonal plexus in the lateral and dorsomedial subnuclei, and in the dorsal cap of the rostral subnucleus. The density of substance P-immunoreactive axons in the remaining subnuclear divisions was sparse to moderate. Terminals of immunoreactive axons contained spherical vesicles and formed asymmetric contacts on dendritic processes exclusively. Immunoreactive neurons, restricted to the rostral subnucleus, possessed long, sparsely branched dendrites. Unlabelled terminals containing either spherical or pleomorphic vesicles contacted substance P-immunoreactive dendritic profiles. Axodendritic and axosomatic synapses containing substance P immunoreactivity pre- and postsynaptically were not observed. Ultrastructural evidence for synaptic relationships between substance P-containing profiles and those containing either choline acetyltransferase or glutamate decarboxylase was obtained by means of double antigen immunohistochemistry. Terminals of fasciculus retroflexus axons stained for choline acetyltransferase immunoreactivity formed asymmetric synaptic contacts with substance P-immunoreactive dendritic profiles. Few substance P-positive dendrites in the rostral subnucleus received terminals possessing glutamate decarboxylase activity. Unlabelled terminals containing either spherical or pleomorphic vesicles contacted substance P- and glutamate decarboxylase-immunoreactive dendritic profiles simultaneously. Terminals possessing either substance P or glutamate decarboxylase immunoreactivity formed synaptic contacts with dendritic processes of neurons in the lateral subnucleus. Many of the neurons within this subnuclear division contained glutamate decarboxylase. This study provides direct evidence of synaptic relationships between choline acetyltransferase-immunoreactive axons and substance P-immunoreactive dendritic profiles, and between substance P-positive axons and glutamate decarboxylase-immunoreactive dendrites. These findings reveal that two types of transmitter-specific axons of the fasciculus retroflexus innervate neuronal populations of the interpeduncular nucleus stained immunohistochemically for either substance P or glutamate decarboxylase.