The ultrastructure of enkephalin-immunoreactive neurons in the interpeduncular nucleus of the rat

Habenulo-interpeduncular descending pathways and their relationship to enkephalin- and somatostatin-immunoreactive neurons in the interpeduncular nucleus of human fetuses

The interpeduncular nucleus of six human fetuses aged 15 (one specimen), 26 (one specimen), 38 (one specimen) and 40 (three specimens) gestation weeks was studied by immunohistochemistry for enkephalin and somatostatin localization and immunohistochemistry coupled with silver staining. Enkephalin-positive and somatostatin-positive cells were detected, the former initially at 15 weeks gestation and the latter at 26 weeks gestation. They appeared to receive long afferents from the habenular region and projected short efferents to adjacent cells devoid of enkephalin and somatostatin positivity. We postulate that these enkephalin- and somatostatin-positive neurons function as modulatory interneurons in the habenulo-interpeduncular and related pathways.

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.

Ultrastructural identification of neuropeptides in the central nervous system

A number of different neuropeptides have been described within presynaptic terminals at the ultrastructural level in the central nervous system. The majority of these neuropeptides share a common morphology with one another. Terminals containing neuropeptides have a population of small, clear vesicles associated with the active zone of the synapse and a lesser number of large, granular vesicles that are located at a distance from the active site of the synapse. It is believed that the large, granular vesicles act as a mechanism for the transport/storage of the neuropeptides, while the small, clear vesicles are thought to be acting as structures responsible for the release of the neurotransmitter/neuropeptide into the synaptic cleft. The neuropeptide containing terminals most often have asymmetrical junctions associated with their presynaptic membranes, although symmetrical junctions have been described with peptide containing terminals in a number of areas in the central nervous system. Neuropeptide containing terminals contact every part of the neuronal membrane; however, the majority of synaptic contacts involve portions of the dendritic shafts. Evidence is beginning to accumulate to indicate that for certain neuropeptides there is a specific spatial arrangement to their termination along the neuronal membrane.

Glutamate decarboxylase immunoreactivity in the rat interpeduncular nucleus: a light and electron microscope investigation

The distribution of immunohistochemically demonstrable glutamate decarboxylase, the synthetic enzyme for GABA, was examined in the rat interpeduncular nucleus at the light and electron microscope levels. Immunoreactive perikarya were distributed in a characteristic pattern among the subnuclear divisions. The rostral, ventral and caudal portions of the nucleus possessed numerous immunoreactive perikarya, while few immunoreactive somata were observed in the subnuclei of the dorsal aspect. A dense field of immunostained axons and terminals was also present throughout. Ultrastructural examination of glutamate decarboxylase immunoreactivity revealed numerous labelled somata, dendritic processes, axons and boutons. Axodendritic and axosomatic synapses with immunoreactive postsynaptic profiles were numerous throughout those subnuclei with large numbers of immunoreactive somata. Immunostained terminals in contact with both immunoreactive and non-immunoreactive somatic and dendritic profiles were also present. An abundance of immunostained terminals was observed in the subnuclei that possessed a sparse population of immunoreactive somata. Immunoreactive myelinated axons of unknown origin were also present. This investigation demonstrates that the rat interpeduncular nucleus possesses a large population of glutamate decarboxylase-immunoreactive neurons coextensive with a plexus of immunostained axons and terminals. The results suggest that the immunoreactive neurons give rise to axons which contribute to an intrinsic circuit interconnecting the different subnuclear divisions. These immunoreactive neurons are in receipt of non-immunoreactive afferent inputs of variable morphology, as well as projections from intrinsic immunoreactive neurons.

Behavioral and metabolic alterations in the opiate withdrawal syndrome induced by lesions of fasciculus retroflexus

The interpeduncular nucleus (IPN) appears to be an important integrative center within the limbic system based on its extensive afferent and efferent connections and the presence of numerous neurotransmitters and peptides. Opiate receptors are present within particular subregions of IPN, which is one of the limbic structures showing an increase in regional glucose utilization (RGU) during withdrawal of morphine-addicted rats. The possible role of neural connections in withdrawal was studied by lesioning the main afferent pathway to IPN, the fasciculus retroflexus (FR) bilaterally. Four subnuclei of IPN, lateral, central, rostral and intermediate and FR showed significantly smaller increases in RGU during naloxone-induced withdrawal when compared to sham-operated controls. No difference was found in the apical, dorsal medial or dorsal lateral subnuclei. This metabolic effect of the lesions is not related in any simple way to the localization of opiate receptors or other neurochemical features of IPN. The lesioned animals also had greater weight loss due to diarrhea during withdrawal, consistent with IPN's presumed connection to the vagal nuclei. IPN appears to exhibit local and independent effects of FR lesions during opiate withdrawal.