The ultrastructure and organization of methionine-enkephalin immunoreactive profiles in the interpeduncular nucleus of the cat

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.

Analysis of the rat interpeduncular subnuclei by immunocytochemical double-staining for enkephalin and substance P, with some reference to the coexistence of both peptides

The rat interpeduncular nucleus (IPN) was immunocytochemically double-stained for enkephalin (ENK) and substance P (SP) on the same sections. On the basis of both peptidergic distribution patterns and topographic relationship, the IPN was divided into nine subnuclei and one cap: the rostral subnucleus (IP-R), the central subnucleus (IP-C), the rostral-lateral subnucleus (IP-RL), the main lateral subnucleus (IP-L), the caudal-lateral subnucleus (IP-CL), the dorsal-lateral subnucleus (IP-DL), the dorsal-medial subnucleus (IP-DM), the apical subnucleus (IP-A), the intermediate subnucleus (IP-I), and the dorsal cap (IP-Cap). As the descriptions of the IP-RL, IP-L, and IP-CL were inconsistent with previous reports, they were reevaluated; the IP-RL was proposed as the region situated in the lateral portion at rostral levels and characterized by the lack of ENK and SP immunoreactive structures, the IP-L as the region situated throughout the rostrocaudal extent in the lateral portion of the IPN and containing the highest density of SP immunoreactive fibers but no ENK immunoreactive fibers, and the IP-CL as the region situated just laterocaudal to the IP-L in the caudal pole of the IPN and containing ENK immunoreactive cells and fibers but no SP immunoreactive structures. Our results also showed that some cells in the IP-R have both ENK and SP immunoreactivity. This coexistence was observed in some small spherical cells of the IP-R, but rarely in larger oval-shaped cells, which occasionally showed only ENK immunoreactivity. In addition, paired ENK immunoreactive fiber bundles entering the IP-R were found to run just rostral to the paired SP immunoreactive columns, both of which composed parts of the interpedunculotegmental tract. A three-dimensional model representing the subnuclear organization of the IPN was proposed on the basis of the present results.

Opiate binding sites in the interpeduncular nucleus of the rat: normal distribution and the effects of fasciculus retroflexus lesions

Opiate receptors have been localized autoradiographically to many regions of the rat central nervous system. The interpeduncular nucleus has an especially high concentration of these receptors. We used [3H]naloxone and [125I] [D-Ala2,MePhe4,Glyol5]enkephalin as ligands to map the distribution of opiate receptors among the subnuclei of the interpeduncular nucleus. The rostral subnucleus contains label that is densest dorsally. More caudally, high densities of opiate binding sites are found in the lateral, rostral, and central subnuclei. The dorsal subnucleus contains a moderate density of binding sites and the intermediate subnuclei contain almost none. Opiate receptors have also been localized to the medial habenulae and the fasciculi retroflexi, which provide a major afferent input to the interpeduncular nucleus. Lesions of the fasciculi retroflexi decreased the density of opiate binding sites in the rostral and lateral subnuclei of the interpeduncular nucleus. There were no changes seen in the dorsal, intermediate or central subnuclei. These results suggest that a minority of opiate receptors in the interpeduncular nucleus are located presynaptically on fasciculi retroflexi axons. Immunocytochemical studies have demonstrated that the rat interpeduncular nucleus contains substance P, serotonin and enkephalin, and each has a distinct subnuclear distribution. Although the opiate binding sites have a wider distribution than substance P, serotonin, or enkephalin individually, the pattern of opiate binding most closely parallels substance P distribution. The combined distribution of substance P, serotonin, and enkephalin is equivalent to that of the opiate binding sites.

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

The interpeduncular nucleus of the rat is a complex structure, displaying diverse immunocytochemical and ultrastructural features. This nucleus contains opiate receptors, enkephalin-positive cell bodies and enkephalin-positive fibers. The ultrastructure of rat interpeduncular enkephalinergic neurons has not been described, nor has the role that these neurons play in the internal organization of the interpeduncular nucleus been established. The purpose of the present study was to describe the ultrastructure of enkephalinlike-immunoreactive (ELI) cells with particular emphasis on the subnuclear organization of their dendritic and terminal fields. Enkephalinlike-immunoreactive (ELI) cell bodies are present in the rostral and apical subnuclei of the interpeduncular nucleus (IPN), but are absent from the other subnuclei of the IPN. The rostral subnucleus also contains immunoreactive dendrites that are postsynaptic to nonreactive terminal boutons. Numerous ELI axon terminals were observed in the central and intermediate subnuclei. The results of our study suggest that enkephalinergic interneurons link the rostral IPN with more caudal regions of this nucleus.

Choline acetyltransferase immunoreactivity is localized to four types of synapses in the rat interpeduncular nucleus

The cholinergic synapses of the rat interpeduncular nucleus (IPN) were demonstrated by immunostaining that utilized a monoclonal antibody directed against choline acetyltransferase. The rostral, central, intermediate and lateral subnuclei of the IPN each contained a single type of immunoreactive terminal. Immunoreactivity was localized to synaptic vesicle membranes (especially at the contact zones), and longitudinal microtubules in preterminal portions of axons. Terminals were identified by comparison to previous studies of the synaptic organization of the IPN. In the rostral subnucleus, the immunoreactive terminals were characterized by their content of spherical vesicles, 45 nm in diameter, intermixed with moderate numbers of dense-cored vesicles, 75-100 nm in diameter. These terminals formed asymmetrical contacts. They correspond to the more numerous of the two types of axodendritic terminals described in this subnucleus, i.e. those which degenerate after lesions of the habenula. The moderate number of immunoreactive terminals in the lateral subnucleus contained pleomorphic vesicles, 30-45 nm in diameter. Up to three of these formed symmetrical contacts with individual dendrites, which ranged in diameter from 0.35 to 0.55 micron. The other types of axodendritic terminal in this subnucleus, which often contacted the same dendrites, were unstained. These latter terminals have been interpreted as being those which contain substance P. The immunoreactive terminals in the diameter, and formed markedly asymmetrical en passant contacts with small dendritic processes or spines. The immunoreactive terminals in the intermediate subnucleus had the same presynaptic and contact morphology. Many were clearly crest synapses. The remainder appeared to be such, but seen only partially within the plane of section. In the intermediate subnucleus there were up to several hundred immunostained terminals per grid square in some sections. These findings are consistent with the existence of a dense cholinergic projection to the IPN. The habenular region, are shown to crest and S synapses, both of which generate after lesions of the cholinergic innervation of other subnuclei of the IPN increases understanding of the relation of cholinergic to other transmitters localized to various portions of this nucleus.