Substance P in the interpeduncular nucleus of the rat: normal distribution and the effects of deafferentation

Atypical Course of the Habenulo-Interpeduncular Tract in Chick Embryos

Classical studies of the avian diencephalon hardly mention the habenulo-interpeduncular tract (a.k.a. retroflex tract), although both the habenula (HB) (its origin) and the interpeduncular nuclear complex (its target) are present. Retroflex tract fibers were described at early embryonic stages but seem absent in the adult in routine stains. However, this tract is a salient diencephalic landmark in all other vertebrate lineages. It typically emerges out of the caudal HB, courses dorsoventrally across thalamic alar and basal plates just in front of the thalamo-pretectal boundary, and then sharply bends 90° caudalwards at paramedian basal plate levels (this is the "retroflexion"), to approach longitudinally via paramedian pretectum and midbrain the rostralmost hindbrain, specifically the prepontine median interpeduncular complex across isthmus and rhombomere 1. We systematize this habenulo-interpeduncular course into four parts named subhabenular, retrothalamic, tegmental, and interpeduncular. We reexamined the chicken habenulo-interpeduncular fibers at stages HH30 and HH35 (6.5- and 9-day incubation) by mapping them specifically with immunoreaction for BEN protein, a well-known marker. We found that only a small fraction of the stained retroflex tract fibers approaches the basal plate by coursing along the standard dorsoventral pathway in front of the thalamo-pretectal boundary. Many other habenular fibers instead diverge into atypical dispersed courses across the thalamic cell mass (implying alteration of the first subhabenular part of the standard course) before reaching the basal plate; this dispersion explains their invisibility. A significant number of such transthalamic habenular fibers cross orthogonally the zona limitans (ZLI) (the rostral thalamic boundary) and invade the caudal alar prethalamus. Here, they immediately descend dorsoventrally, just rostrally to the ZLI, until reaching the prethalamic basal plate, where they bend (retroflex) caudalwards, entering the thalamic basal paramedian area. These atypical fibers gradually fasciculate with the other groups of habenular efferent fibers in their final longitudinal approach to the hindbrain interpeduncular complex. We conclude that the poor visibility of this tract in birds is due to its dispersion into a diversity of atypical alternative routes, though all components eventually reach the interpeduncular complex. This case merits further analysis of the diverse permissive versus nonpermissive guidance mechanisms called into action, which partially correlate distinctly with successive diencephalic, mesencephalic, and hindbrain neuromeric fields and their boundaries.

The role of the habenula in the transition from reward to misery in substance use and mood disorders

The habenula (Hb) is an evolutionary well-conserved structure located in the epithalamus. The Hb receives inputs from the septum, basal ganglia, hypothalamus, anterior cingulate and medial prefrontal cortex, and projects to several midbrain centers, most importantly the inhibitory rostromedial tegmental nucleus (RMTg) and the excitatory interpeduncular nucleus (IPN), which regulate the activity of midbrain monoaminergic nuclei. The Hb is postulated to play a key role in reward and aversion processing across species, including humans, and to be implicated in the different stages of transition from recreational drug intake to addiction and co-morbid mood disorders. The Hb is divided into two anatomically and functionally distinct nuclei, the lateral (LHb) and the medial (MHb), which are primarily involved in reward-seeking (LHb) and misery-fleeing (MHb) behavior by controlling the RMTg and IPN, respectively. This review provides a neuroanatomical description of the Hb, discusses preclinical and human findings regarding its role in the development of addiction and co-morbid mood disorders, and addresses future directions in this area.

Circuits Regulating Pleasure and Happiness: The Evolution of the Amygdalar-Hippocampal-Habenular Connectivity in Vertebrates

Appetitive-searching (reward-seeking) and distress-avoiding (misery-fleeing) behavior are essential for all free moving animals to stay alive and to have offspring. Therefore, even the oldest ocean-dwelling animal creatures, living about 560 million years ago and human ancestors, must have been capable of generating these behaviors. The current article describes the evolution of the forebrain with special reference to the development of the misery-fleeing system. Although, the earliest vertebrate ancestor already possessed a dorsal pallium, which corresponds to the human neocortex, the structure and function of the neocortex was acquired quite recently within the mammalian evolutionary line. Up to, and including, amphibians, the dorsal pallium can be considered to be an extension of the medial pallium, which later develops into the hippocampus. The ventral and lateral pallium largely go up into the corticoid part of the amygdala. The striatopallidum of these early vertebrates becomes extended amygdala, consisting of centromedial amygdala (striatum) connected with the bed nucleus of the stria terminalis (pallidum). This amygdaloid system gives output to hypothalamus and brainstem, but also a connection with the cerebral cortex exists, which in part was created after the development of the more recent cerebral neocortex. Apart from bidirectional connectivity with the hippocampal complex, this route can also be considered to be an output channel as the fornix connects the hippocampus with the medial septum, which is the most important input structure of the medial habenula. The medial habenula regulates the activity of midbrain structures adjusting the intensity of the misery-fleeing response. Within the bed nucleus of the stria terminalis the human homolog of the ancient lateral habenula-projecting globus pallidus may exist; this structure is important for the evaluation of efficacy of the reward-seeking response. The described organization offers a framework for the regulation of the stress response, including the medial habenula and the subgenual cingulate cortex, in which dysfunction may explain the major symptoms of mood and anxiety disorders.

Cocaine reduces cytochrome oxidase activity in the prefrontal cortex and modifies its functional connectivity with brainstem nuclei

Cocaine-induced psychomotor stimulation may be mediated by metabolic hypofrontality and modification of brain functional connectivity. Functional connectivity refers to the pattern of relationships among brain regions, and one way to evaluate this pattern is using interactivity correlations of the metabolic marker cytochrome oxidase among different regions. This is the first study of how repeated cocaine modifies: (1) mean cytochrome oxidase activity in neural areas using quantitative enzyme histochemistry, and (2) functional connectivity among brain regions using inter-correlations of cytochrome oxidase activity. Rats were injected with 15 mg/kg i.p. cocaine or saline for 5 days, which lead to cocaine-enhanced total locomotion. Mean cytochrome oxidase activity was significantly decreased in cocaine-treated animals in the superficial dorsal and lateral frontal cortical association areas Fr2 and Fr3 when compared to saline-treated animals. Functional connectivity showed that the cytochrome oxidase activity of the noradrenergic locus coeruleus and the infralimbic cortex were positively inter-correlated in cocaine but not in control rats. Positive cytochrome oxidase activity inter-correlations were also observed between the dopaminergic substantia nigra compacta and Fr2 and Fr3 areas and the lateral orbital cortex in cocaine-treated animals. In contrast, cytochrome oxidase activity in the interpeduncular nucleus was negatively correlated with that of Fr2, anterior insular cortex, and lateral orbital cortex in saline but not in cocaine groups. After repeated cocaine specific prefrontal areas became hypometabolic and their functional connectivity changed in networks involving noradrenergic and dopaminergic brainstem nuclei. We suggest that this pattern of hypofrontality and altered functional connectivity may contribute to cocaine-induced psychomotor stimulation.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

A novel monoclonal antibody recognizes a previously unknown subdivision of the habenulo-interpeduncular system in zebrafish

The habenulo-interpeduncular system is an evolutionarily conserved structure found in the brain of almost all vertebrates. We prepared a monoclonal antibody (6G11) which very specifically recognizes only a part of this system. 6G11 is a monoclonal antibody prepared from a neuronal membrane protein in adult zebrafish brain. In western blot analysis of the adult zebrafish brain, the antibody recognized a 95 kDa protein, and the class of the antibody was determined to be IgM. The 6G11 antigen was not detected in zebrafish muscle, intestine, testis or ovary. A group of neurons stained by the 6G11 antibody was located in the caudomedial part of the zebrafish habenula. The 6G11-immunopositive neurons extended their axons into the fasciculus retroflexus (FR). One group of immunopositive neurons projected toward the interpeduncular nucleus (IPN), especially to the intermediate and the central subnucleus (type 1 neuron). The other group projected to the ventral midline at the level of the raphe nucleus; these axons passed ipsilaterally beside the IPN and converged in the ventral midline under the raphe nucleus (type 2 neuron). Both type 1 and type 2 fibers are relatively minor components of the FR. Little has previously been known about this topological pattern in any species. The 6G11 monoclonal antibody could be a useful tool for expanding knowledge of the habenulo-interpeduncular system.

Neuroanatomical localisation of Substance P in the CNS and sensory neurons

The anatomical distribution of Substance P (SP) has been investigated since the development of antibodies against it in the 1970s. Although initial studies were performed with antibodies that also recognised the other endogenous neurokinins, most of the initial descriptions are surprisingly still valid today. In this review, we provide an integrated overview of the pathways containing SP in the central and peripheral nervous systems. The highest densities of SP immunoreactivity occur in the superficial dorsal horn of the spinal cord, in the substantia nigra and in the medial amygdaloid nucleus. In the peripheral nervous system, SP occurs in high concentrations in small diameter primary sensory fibres and in the enteric nervous system. SP is extensively co-localised with classical transmitters and other neuropeptides. In the spinal cord, SP immunoreactive axonal boutons are preferentially presynaptic to neurons expressing the SP receptor, suggesting that the neurokinin acts at a short distance from the release site. In contrast, in the periphery, the situation probably differs in the autonomic ganglia, where the targets are directly innervated by SP, and in other peripheral territories, where SP has to diffuse through the connective tissue to reach the structures expressing the receptor.

Widespread expression of rat collapsin response-mediated protein 4 in the telencephalon and other areas of the adult rat central nervous system

The rat collapsin response-mediated protein 4 (rCRMP-4) is a member of a family of proteins that are involved in axonal growth. It is found transiently in postmitotic neurons, such as those that are generated in the adult hippocampus. The authors used immunocytochemistry to investigate whether areas of the rat central nervous system (CNS) that retain postnatal neurogenesis express this protein. They found pronounced rCRMP-4 immunoreactivity in recently generated cells in the dentate granular layer, the subventricular zone, the olfactory bulbs, and the rostral migratory stream, four areas in which the production or migration of neurons occurs in adulthood. However, rCRMP-4 immunoreactivity also is expressed in many other regions of the rat brain in which there is no record of adult neurogenesis or neuronal migration, e.g., in the olfactory glomeruli and in neurons of the cerebral cortex. In the hypothalamus, intensely rCRMP-4-labeled neurons populated the supraoptic, paraventricular, and periventricular nuclei as well as the median eminence and the arcuate nucleus. Immunoreactivity for rCRMP-4 also was present in certain neurons of the interpeduncular nucleus, median raphe, superior colliculus, and scattered granule cerebellar neurons. Many of these regions are known to display axonal outgrowth and/or synaptic rearrangement in adulthood and to coexpress the polysialylated form of the neural cell adhesion molecule. Thus, the results of this study suggest that rCRMP-4 expression in the CNS is associated with cells that are migrating or are undergoing axonal growth. Nevertheless, small, rCRMP-4-immunoreactive cells were seen throughout the brain. These cells did not express neuronal, astroglial, or microglial markers, although some of them also were immunoreactive for rip antibody, suggesting an oligodendroglial lineage.

Ovarian cycle-related changes of glial fibrillary acidic protein (GFAP) immunoreactivity in the rat interpeduncular nucleus

The interpeduncular nucleus (IPN) of female rats was studied across the estrous cycle to observe whether the expression of the astroglial marker, glial fibrillary acidic protein (GFAP) reacts to hormonal changes in an area not belonging to the 'endocrine brain'. A marked reduction of immunoreactive GFAP was observed in estrus as compared to the immunoreactivities in met- and proestrus. This finding is consistent with earlier observations in the endocrine hypothalamus, but also proves that gonadal steroids influence astroglia in brain regions not involved in neuroendocrine regulation. Since cyclic fluctuations of synaptic numbers in the female have been described only for the endocrine hypothalamus, decrease of immunoreactive GFAP in the IPN during estrus may reflect a down-regulation of GFAP synthesis.

Lesion of the habenular efferent pathway produces anxiety and locomotor hyperactivity in rats: a comparison of the effects of neonatal and adult lesions

Recent studies have implicated the habenula in modulating states of arousal and chronic responses to stress. We examined whether lesion of the habenula efferent pathway, the fasciculus retroflexus (FR), at either 3 (P3) or 70 (P70) days of age affects stress-related anxiety (elevated plus-maze test) and activity levels (open-field test) in rats tested as adults. Both P3- and P70-lesioned rats showed chronically elevated plasma levels of corticosterone. Rats receiving FR lesions as neonates (P3) exhibited greater open arm avoidance on the elevated plus-maze than controls 2 months postoperatively, suggesting a heightened state of anxiety. In contrast, P70-lesioned rats behaved similarly to controls on the plus-maze, but showed increased locomotion and increased grooming in the open field, effects not observed in P3-lesioned rats. When an additional stressful condition was imposed (5 days of social isolation plus 24 h food deprivation) before testing, both FR-lesion groups showed an attenuation of the normal behavioral responses (decreased open-arm entries/time in open arms, increased freezing). The effects of FR lesions on activity and behavioral indices of anxiety may be due to disruption of lateral habenular projections to dopaminergic neurons in the ventral tegmentum and/or projections to regions containing high concentrations of benzodiazepine receptors, the median and dorsal raphe and dorsal periaqueductal gray. Behavioral differences observed as a function of lesion age suggest differential capabilities of P3- and P70-lesioned rats to utilize compensatory mechanisms to correct FR lesion-induced deficits.

Distribution of calretinin-immunoreactive septal axons in the normal and deafferented medial habenula of adult rats

To characterize the neural circuitry and plasticity of the septohabenular pathway, the present study analyzes the distribution of calretinin-immunoreactive fibers within the normal and deafferented medial habenula (MHb) at the light and ultrastructural levels. In the adult rat, a dense plexus of calretinin-positive fibers was found throughout the entire MHb neuropil; these immunoreactive terminals formed asymmetric synaptic contacts with unstained dendritic profiles. Calretinin-positive axons that innervate the MHb originated from neurons of the ipsilateral posterior septum, specifically those of the nucleus septofimbrialis and the nucleus triangularis. Unilateral deafferentation of the MHb resulted in the complete loss of calretinin-immunostained fibers within the ipsilateral MHb after 7 days; no reduction was apparent on the contralateral side. Four weeks after unilateral MHb deafferentation, new calretinin-immunoreactive fibers were found confined to the caudal regions of the MHb, these axons again formed asymmetrical contacts with unstained dendritic profiles. No calretinin-positive axons, however, were found within the MHb at 4 weeks following bilateral deafferentation, thus suggesting that the source of these new fibers within the long-term deafferented MHb arises from the contralateral septal neurons. Supporting this idea, injections of biotinylated dextran amine into the 4-week deafferented MHb resulted in retrogradely labeled somata observed in the contralateral posterior septum. These data reveal that septal projections to the MHb, which are normally ipsilateral, respond to a unilateral deafferentation by extending contralateral fibers that cross the midline at the habenular commissure and reinnervate the caudal regions of the nucleus.

Expression of a phosphorylated isoform of MAP1B is maintained in adult central nervous system areas that retain capacity for structural plasticity

Microtubule-associated protein IB (MAP1B) is the first MAP to be detected in the developing nervous system, and it becomes markedly down-regulated postnatally. Its expression, particularly that of its phosphorylated isoform, is associated with axonal growth. To determine whether adult central nervous system (CNS) areas that retain immunoreactivity for MAP1B are associated with morphological plasticity, we compared the distribution of a phosphorylated MAP1B isoform (MAP1B-P) to the distribution of total MAP1B protein and MAP1B-mRNA. Although they were present only at very low levels, both protein and message were found ubiquitously in almost all adult CNS neurons. The intensity of staining, however, varied markedly among different regions, with only a few nuclei retaining relatively high levels. MAP1B-P was restricted to axons, whereas total MAP1B was present in cell bodies and processes. Relatively to total MAP1B protein and its mRNA, MAP1B-P levels decreased more dramatically with maturation, and they were detectable in only a few specific areas that underwent structural modifications. These included primary afferents and motor neurons, olfactory tubercles, habenular and raphe projections to interpeduncular nuclei, septum, and the hypothalamus. The distribution pattern of MAP1B-P was compared to that of the embryonic N-CAM rich in polysialic acid (PSA-NCAM). We found that the PSA-NCAM immunostaining was largely overlapped with that of MAP1B-P in the adult CNS. These results suggest that, like PSA-NCAM, MAP1B may be one of the molecules expressed during brain development that also plays a role in structural remodeling in the adult.

Terminal patterns of the fasciculus retroflexus in the interpeduncular nucleus of the mouse: a Golgi study

The courses and terminal patterns of the fasciculus retroflexus (FR) in the interpeduncular nucleus (IP) were studied in the mouse, using the rapid Golgi method. Mainly on the basis of the distribution areas and terminal patterns, the FR fibers are divided into two types. The type 1 FR fibers are coarse in contour and take zigzag courses to distribute throughout the entire rostral half and core region of the caudal IP. In contrast, the type 2 fibers are fine, travel caudally along the lateral boundary of the IP and terminate in the lateral regions of the caudal half, forming a dense fiber plexus. The distribution areas of the type 1 and type 2 fibers are clearly differentiated from each other, from the cytoarchitectural as well as the fibroarchitectural viewpoint. Thus, the type 1 and type 2 FR fibers form different fiber systems in the IP. These results are discussed in the light of the known hodological, histochemical and ultrastructural studies.

Distribution of the substance P receptor (NK-1 receptor) in the central nervous system

The overall distribution of mRNA for the substance P receptor (NK-1) was investigated in the rat brain by in situ hybridization histochemistry using synthetic oligonucleotide probes. The NK-1 positive cells were distributed throughout the brain. Among the positive cells an intense hybridization signal was observed in the basal ganglia and the dorsal tegmental areas. In the hippocampus, hypothalamus, midbrain, and medulla oblongata, weak to moderate positive signal were recorded in various areas. Comparing our results with those of previous workers on the localization of substance P using both immunohistochemistry and in situ hybridization various relationships between the ligand and its receptor localization were characterized. Generally the areas containing the abundant substance P immunoreactive (SP-IR) terminals were associated with NK-1 mRNA positive cells, some examples of this can be seen in the medial amygdaloid nucleus, the locus coeruleus, and the dorsal raphe nucleus. Contrary to these matched areas, we could not detect NK-1 mRNA positive cells in the substantia nigra, where there is a strong projection from the caudate putamen containing the substance P. Similarly the substantia gelatinosa in the trigeminal tract nucleus and the dorsal horn of the spinal cord where numerous SP-IR fibers are located, did not show abundant NK-1 mRNA positive cells. Only a few weakly labeled positive cells could be found.

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.

The development of substance P-like immunoreactivity in the goldfish brain

The development of substance P-like immunoreactivity (SPLI) in the goldfish brain was studied by means of the indirect peroxidase-antiperoxidase technique and an antibody to substance P. By 80 h after fertilization, the first SPLI-cell bodies appear in the ventricular zone of the future diencephalon and the first SPLI-fibers appear in the olfactory bulbs. Two days after hatching (which occurs at 100 h after fertilization), SPLI fibers connecting the olfactory bulbs and hypothalamus are seen. In the optic tectum SPLI-fibers appear for the first time 5 days after hatching. In the brain stem, SPLI-cell bodies appear in juvenile animals 40 days after hatching. The highest number and intensity of SPLI-cell bodies and fibers are found in the area postrema. SPLI-cell bodies are also seen in the gustatory nucleus, nucleus ambiguous, reticular formation of the medulla, dorsal motor nucleus of the vagus and commissural nucleus of Cajal. The significant information gained from the present study is: 1. The rostro-caudal sequence in which the SPLI appears in the developing nuclei of the goldfish brain 2. The reduction of SPLI-cell bodies in some nuclei with age Thus, in the brain stem, SPLI-cell bodies that were labeled in juvenile goldfish were not seen in adults. This might be due to changes in the rate of axonal transport, changes of the SP phenotype during development or cell death. The developmental sequence and relative timing in which SPLI-cell bodies appear in the goldfish, rat and mice are similar.

Plasticity of spinal systems after unilateral lumbosacral dorsal rhizotomy in the adult rat

Plasticity of spinal systems in response to lumbosacral deafferentation has previously been described for the cat, by using immunocytochemistry to demonstrate plasticity of tachykinin systems and degeneration methods to demonstrate plasticity of descending systems. In this study, we describe the response to lumbosacral deafferentation in the adult rat. Application of immunocytochemical methods to visualize tachykinins (predominantly substance P magnitude of SP), serotonin (5-HT), and dopamine B-hydroxylase (DBH), the synthesizing enzyme for norepinephrine, permits us to compare the response of SP systems in rat and cat spinal cord and to examine the response of two descending systems, serotoninergic and noradrenergic, to deafferentation. We used image analysis of light microscopic preparations to quantify the immunoreaction product in the spinal cord in order to estimate the magnitude, time course and localization of changes induced by the lesion. The distribution of SP, serotoninergic (5-HT), and noradrenergic staining in the spinal cord of rat is very similar to that of the cat. Unilateral lumbosacral rhizotomy elicits a partial depletion, followed by a partial replacement of tachykinin immunoreactivity in laminae I and II. This response was similar to that described for the cat, although characterized by a longer time course, and, as in the cat, is likely due to plasticity of tachykinin containing interneurons. The same lesion elicits no depletion but a marked and permanent increase in 5-HT immunoreactivity in laminae I and II, which develops more rapidly than the response by the SP system. These results indicate sprouting or increased production of SP and 5-HT in response to deafferentation. No change was seen in DBH immunoreactivity, indicating that the noradrenergic system does not show plasticity in response to deafferentation. Our results demonstrate that dorsal rhizotomy evokes different effects in different systems in the adult spinal cord of the rat and thus suggests that the response of undamaged pathways to partial denervation of their target is regulated rather than random.

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.

Differential effects of fasciculus retroflexus lesions on serotonin, glutamate and gamma-aminobutyrate content and choline acetyltransferase activity in the interpeduncular nucleus

After placing bilateral electrolytic lesions in the fasciculus retroflexus (FR) of the rat, the endogenous content of serotonin, glutamate and gamma-aminobutyrate (GABA) as well as choline acetyltransferase activity (ChAT) were measured in the interpeduncular nucleus (IPN) at the 7th, 28th and 120th survival days. Confirming earlier results, an almost total depletion of ChAT was obtained in the IPN following complete FR lesions at any survival day studied. In such cases, the following changes were observed; 1) the serotonin level increased consistently and roughly doubled at the 120th survival day, suggesting heterotypic sprouting of serotonergic fibers and/or enhanced serotonin synthesis in the serotonergic neurons in the IPN, 2) the glutamate level decreased by approximately one-half, while the activity of high affinity uptake of glutamate remained unaltered, at the 7th survival day, suggesting a lowered glutamate formation coupled with lowered glucose utilization in the IPN, and 3) the GABA level decreased at a slower rate and reached one-third of the control at the 120th survival day, for which either transsynaptic degeneration of GABA neurons in the IPN or a suppressed metabolic rate in the GABA shunt following the lowered glutamate formation is a possible explanation.

A comparison of the subnuclear and ultrastructural distribution of acetylcholinesterase and choline acetyltransferase in the rat interpeduncular nucleus

The subnuclear and synaptic staining patterns for acetylcholinesterase (AChE) activity and choline acetyltransferase (ChAT) activity were studied in the rat interpeduncular nucleus (IPN) using histochemical and immunohistochemical methods. AChE reactivity was prominent in the neuropil of the rostral, lateral and dorsomedial subnuclei, whereas ChAT immunoreactivity was confined to axons and terminals in the rostral, intermediate and central subnuclei. AChE-positive somata were evident in all the subnuclear divisions of the IPN, and possessed reaction product in the rough endoplasmic reticulum and nuclear envelope. ChAT-positive somata were not present in the IPN. Characteristic axodendritic synapses in the rostral, intermediate and central subnuclei possessed ChAT immunoreactivity presynaptically, and AChE reactivity both pre- and postsynaptically. Other synaptic arrangements in the lateral subnucleus lacked ChAT-immunoreactive terminals, yet possessed prominent AChE reactivity. The results of the present study reveal that AChE reactivity and ChAT immunoreactivity are heterogeneously distributed among the subnuclear divisions of the rat IPN, and that AChE reactivity is present in both the cholinoceptive and noncholinoceptive subnuclei. Although neuronal colocalization of ChAT and AChE activity is not evident in the IPN, AChE-positive neurons are in receipt of putative cholinergic, as well as peptidergic, afferent inputs.

Substance P-containing neurons in the pontomesencephalic tegmentum of the human brain

We have employed immunohistochemical and computerized morphometric procedures to study substance P-containing neurons in the tegmentum of adult humans. An estimated 192,500 +/- 40,500 substance P-containing neurons were found in three main cytoarchitectural regions: the mesencephalic reticular formation, the central gray, and the pontine reticular formation. The morphology of the immunoreactive neurons varied according to the region in which they were found. On the basis of size alone two types of substance P-containing neurons, large and small, were readily distinguishable by eye and measurement. Within each of the three main regions it was possible to distinguish distinct subgroups using cell size, morphology and position. Large neurons were concentrated in the caudal midbrain (pedunculopontine tegmental nuclei), in the oral pontine reticular nucleus and in the lateral dorsal tegmental nucleus. In contrast, small neurons were concentrated in the rostral mesencephalic reticular formation (cuniform nuclei). Both small and large neurons were found in the midbrain and pontine raphe nuclei. In addition, small neurons were concentrated in discrete midline regions (the periaqueductal gray, the tegmental nuclei of the pontine central gray, and the interpeduncular nucleus). The findings suggest that the majority of neurons in the brainstem tegmental nuclei previously identified as cholinergic also contain substance P in humans.

Cellular localization of substance P- and neurokinin A-encoding preprotachykinin mRNA in the female rat brain

To determine the locations of neurons in the rat brain expressing substance P and neurokinin A mRNA, we performed in situ hybridization with a radiolabeled cRNA probe that was complementary to alpha-, beta-, and gamma-preprotachykinin mRNA. Several types of controls indicated specificity of the labeling. Brain regions containing many labeled neurons include the anterior olfactory nucleus, layer II of the olfactory tubercle, the islands of Calleja, the nucleus accumbens, the caudate-putamen, portions of the amygdala and hypothalamus, the medial habenular nucleus, nuclei of the pontine tegmentum, several raphe nuclei, several portions of the reticular formation, and the nucleus of the solitary tract. Less frequent labeled neurons were also found in many other regions of the brain. These results extend many previous immunocytochemical studies of the locations of neurons containing immunoreactive substance P, neurokinin A, and neuropeptide K.

Neurokinin B and substance P genes are co-expressed in a subset of neurons in the rat habenula

Colocalization of neurokinin B (NKB) and substance P (SP) mRNAs in neurons of the habenula was examined on thin, adjacent sections in the rat. Extensive colocalization was found in the medial habenula. In its dorsolateral part, most of the neurons contained both transcripts, with high levels of SP, while in the dorsomedial part fewer instances of colocalization were found. The ventral half could be divided into two parts, a dorsal part with most of the neurons containing both messages, having low levels of SP mRNA, and a ventral part with most of the cells containing only NKB mRNA. Cells in the medial habenula had low levels of NKB mRNA. These results suggest a structural and functional heterogeneity of the medial habenula.

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.

Heterogeneous distribution of polysialylated neuronal-cell adhesion molecule during post-natal development and in the adult: an immunohistochemical study in the rat brain

A monoclonal antibody raised against the capsular polysaccharides of meningococcus B was used for immunohistochemical studies in the rat brain, with particular focus on the substantia nigra. This antibody recognizes polysialic acid residues specifically associated with the neuronal-cell adhesion molecule, and reacts with the highly sialylated embryonic neuronal-cell adhesion molecule, but not with the weakly sialylated adult form of the molecule. Immunoreactivity to this monoclonal antibody was intense and widespread in the brain of 1-10-day-old hooded rats. Immunolabeling was associated with cell membranes and present in the intersomata space. In sections from 16- and 25-day-old rats, marked heterogeneity in the level of immunostaining appeared among individual brain nuclei. Areas devoid of labeling with the anti-meningococcus antibody still expressed immunoreactivity to a polyclonal anti-neuronal-cell adhesion molecule antibody. This suggests that the loss of immunostaining with the monoclonal antibody did not correspond to a loss of expression of neuronal-cell adhesion molecule, but to a maturation from the embryonic to the adult form of the molecule, occurring at different rates in various brain regions. In 2-month-old rats, immunolabeling with the monoclonal antibody was still present in discrete brain areas, including the substantia nigra, suggesting that the presence of highly sialylated neuronal-cell adhesion molecule outlasts post-natal development in those brain regions. It is proposed that neuronal-cell adhesion molecule associated polysialic residues may play a role in neuronal plasticity in restricted areas of the adult brain.

Topographical and ultrastructural investigation of the habenulo-interpeduncular pathway in the rat: a wheat germ agglutinin-horseradish peroxidase anterograde study

The topographical and ultrastructural organization of the habenular projection to the interpeduncular nucleus (IPN) of the rat was examined employing the anterogradely transported tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and the chromogen tetramethylbenzidine (TMB). Unilateral placements of WGA-HRP in the habenular complex resulted in heavy terminal labelling in the rostral, central, and intermediate subnuclei bilaterally, and in the lateral subnuclei ipsilaterally. The apical subnucleus possessed only a sparse amount of label. Placements confined to the medial habenula (mH) produced similar results to those observed when the entire habenula was filled, suggesting that the afferent contribution made by the lateral habenula (lH) to the IPN is small. Unilateral placements of WGA-HRP in the dorsal portion of the mH resulted in heavy, predominantly ipsilateral labelling in the lateral subnucleus and the dorsal cap of the rostral subnucleus. In the lateral subnucleus labelled habenular terminals consistently contacted single dendritic processes shared by one or more other boutons, possibly of nonhabenular origin. Labelled habenular terminals in the rostral subnucleus normally contacted one or two dendrites. Labelled terminals in both subnuclei possessed clear, spherical vesicles and a variable number of dense-core vesicles. Unilateral placements of WGA-HRP in the ventral portion of the mH resulted in heavy labelling in the rostral half of the rostral subnucleus with a slight ipsilateral predominance, and in the central and intermediate subnuclei bilaterally. Terminal labelling was observed in crest and S synapses in the intermediate and central subnuclei respectively. Crest synapses, which consist of two parallel habenular terminals contacting an attenuated dendritic process, normally possessed label in only one of the two boutons. In the central subnucleus labelled horizontal axons formed several en passant S synapses with dendritic processes of small and medium diameter. These synaptic specializations of habenular axons contained numerous clear, spherical vesicles. This study demonstrates that a major topographically organized projection to the IPN originates from two distinct subpopulations of habenular neurons which comprise a dorsal sector and a ventral sector of the mH. Ultrastructural examination demonstrated that axons originating from neurons in the ventral and dorsal mH form characteristic contacts in the various IPN subnuclei.

Tachykinin binding sites in the interpeduncular nucleus of the rat: normal distribution, postnatal development and the effects of lesions

Tachykinin binding sites in the basal midbrain were labeled in adult and neonatal rats using 125I-Bolton Hunter (BH) substance P (SP) and 125I-BH eledoisin as ligands. In the adult, binding was very low in the tegmentum and raphe adjacent to the interpeduncular nucleus (IPN). Within the IPN, no binding with either ligand was seen in the target subnuclei of the habenular SP and substance K projections, the lateral subnuclei and the cap of the rostral subnucleus. Labeling with 125I-BH-SP was very light and was restricted primarily to the central subnucleus of the IPN while 125I-BH-eledoisin labeling was very dense over the dorsal, the ventral sector of the rostral, the intermediate and the central subnuclei. Lesions of major afferents to the IPN, the fasciculus retroflexus or the locus coeruleus, had no effect on the distribution or density of the binding of either ligand. In rats 0, 4 or 7 days or age, 125I-BH-SP binding was very dense in the ventral tegmental region, the raphe and in the dorsal, rostral and central subnuclei. 125I-BH-eledoisin binding was extremely dense in the raphe and in the dorsal, rostral, intermediate and central subnuclei but was less dense in the ventral tegmentum. Adult levels of binding in the midbrain were established by 11 days of age. Neonatal lesions restricted to the fasciculus retroflexus had no effect on the density of labeling with either ligand in animals allowed to reach adulthood.

Quantitative radioimmunocytochemical evidence that haloperidol and SCH 23390 induce opposite changes in substance P levels of rat substantia nigra

Chronic blockade of the dopamine (DA) D2 receptor by repeated systemic administration of the butyrophenone neuroleptic, haloperidol (HAL), is known to lead to a decrease in levels of the neuroactive peptide, substance P (SP), in the rat striatum and substantia nigra (SN). Using a high-resolution, quantitative radioimmunocytochemistry (RIC) technique, we have shown the HAL-induced decrease in rat nigral SP to be both dose- and time-dependent. In addition, chronic administration of the highly selective D2 antagonist, S(-)-sulpiride, also decreased nigral SP. Following blockade of the dopamine D1 receptor by chronic administration of the selective D1 antagonist, SCH 23390, we found, in contrast, that levels of SP in SN were increased in a dose- and time-dependent fashion. The magnitude of the maximum SCH 23390-induced elevation (20-30%) of nigral SP was approximately equal to that of the maximum HAL-induced decrease. The opposite response of nigral SP levels to repeated injections of a D1 or D2 antagonist suggests that the two DA receptor subtypes exert tonic, opposing, modulatory influences on the SP content of the striatonigral pathway.

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.

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.

Prenatal and postnatal development of substance P immunocytochemistry within subnuclei of the rat interpeduncular nucleus

In the present study, the temporal appearance and distribution of substance P within individual subnuclei has been examined during the development of the rat interpeduncular nucleus (IPN). The prenatal organization as well as migration pattern of individual IPN subnuclei are also described. The IPN was distinguishable on embryonic day (E) 19, near the ventral mesencephalon. At this age, the IPN was organized into individual subnuclei like the adult, except for a bilateral distribution of presumptive rostral neurons. Rostral neurons were merged into a single, midline subnucleus by the day of birth, thereby completing an adult pattern of subnuclear organization. SP immunoreactivity, restricted to the lateral subnuclei, was first detected at E20. The intensity of SP-positive fibers in the lateral subnucleus increased with age, and appeared to become selectively distributed along both the medial and lateral borders of this subnucleus. Additional SP-positive fibers became evident postnatally in a thin band overlying both central and intermediate subnuclei, and within the dorsal medial, central and apical subnuclei. SP-positive cell bodies were present in the rostral subnucleus on postnatal day 28, thereby completing the development of an adult pattern of SP immunoreactivity within the IPN.

Distribution of somatostatinlike immunoreactivity in the brain of the little brown bat (Myotis lucifugus)

The distribution of somatostatinlike immunoreactive (SLI) perikarya, axons, and terminals was mapped in subcortical areas of the brain of the little brown bat, Myotis lucifugus, using light microscopic immunocytochemistry. A preponderance of immunoreactivity was localized in reticular, limbic, and hypothalamic areas including: 1) in the forebrain: the bed nucleus of the stria terminalis; lateral preoptic, dorsal, anterior, lateral and posterior hypothalamic areas; amygdaloid, periventricular, arcuate, supraoptic, suprachiasmatic, ventromedial, dorsomedial, paraventricular, lateral and medial mammillary, and lateral septal nuclei; the nucleus of the diagonal band of Broca and nucleus accumbens septi; 2) in the midbrain: the periaqueductal gray, interpeduncular, dorsal and ventral tegmental, pretectal, and Edinger-Westphal nuclei; and 3) in the hindbrain: the superior central and parabrachial nuclei, nucleus incertus, locus coeruleus, and nucleus reticularis gigantocellularis. Other areas containing SLI included the striatum (caudate nucleus and putamen), zona incerta, infundibulum, supramammillary and premammillary nuclei, medial and dorsal lateral geniculate nuclei, entopeduncular nucleus, lateral habenular nucleus, central medial thalamic nucleus, central tegmental field, linear and dorsal raphe nuclei, nucleus of Darkschewitsch, superior and inferior colliculi, nucleus ruber, substantia nigra, mesencephalic nucleus of V, inferior olivary nucleus, inferior central nucleus, nucleus prepositus, and deep cerebellar nuclei. While these results were similar in some respects to those previously reported in rodents, they also provided interesting contrasts.

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.

Norepinephrine in the interpeduncular nucleus of the rat: normal distribution and the effects of deafferentation

We used correlative biochemical and histochemical methods to examine (1) the norepinephrine (NE) projection from the paired locus coeruleus (LC) to the midline interpeduncular nucleus (IPN) of the adult rat and (2) the ability of the LC to respond to denervation of their target following removal of noradrenergic afferents (6-hydroxydopamine lesions of the LC) or non-noradrenergic afferents (lesion of the paired fasciculi retroflexi(FR]. Histofluorescence revealed that the NE innervation from the two LC to the IPN is symmetric and overlapping. This projection is confined to rostral, central, and intermediate subnuclei and is absent from lateral and dorsal subnuclei. We found no evidence for homotypic collateral sprouting of undamaged LC neurons into the IPN following unilateral LC lesion. Bilateral LC lesions also did not induce sprouting by NE-containing neurons from other systems (e.g. the superior cervical ganglion or the lateral tegmental group) or from those LC neurons that survived the 6-hydroxydopamine lesion. Histofluorescence following bilateral FR lesions confirmed an earlier observation that apparent hyperinnervation of the IPN by LC afferents is elicited following removal of non-noradrenergic afferents. Measurements of the turnover rate of NE in the IPN of control animals and those that received bilateral FR lesions indicate an increased NE content and increased turnover rate of NE in the IPN of lesioned animals. Taken together these results suggest an increased number of NE terminals and an increase in the activity of tyrosine hydroxylase. No change in NE content or turnover rate was seen in the frontal cortex from these same animals. This is consistent with a target-dependent regulation of heterotypic collateral sprouting.

Normal development and effects of early deafferentation on choline acetyltransferase, substance P and serotonin-like immunoreactivity in the interpeduncular nucleus

The normal postnatal development and response to neonatal fasciculus retroflexus (FR) lesions of serotonin, substance P (SP), and choline acetyltransferase (ChAT) distribution are described for the rat interpeduncular nucleus (IPN). Serotonin-, SP- and ChAT-containing axons differed in development, distribution, and response to deafferentation. Serotonergic axons and cell bodies were present at birth. SP was present in the FR and in the lateral subnuclei by 3 days of age but did not appear in the rostral or dorsal subnuclei until 7-14 days. Intrinsic SP perikarya were not seen until 17 days of age. The development of ChAT was late, appearing only during the second week of life and not reaching adult patterns and density until after 21 days of age. The pattern of development of cytochrome oxidase and Bodian silver staining are also described. Both cytochrome oxidase and Bodian staining paralleled the patterns of localization and development of ChAT staining. Bilateral neonatal FR lesions resulted in a permanent loss of ChAT and cytochrome oxidase staining throughout the IPN and of SP in the lateral and rostral subnuclei. No changes were seen in the serotonergic system. Following unilateral lesions, the pattern of SP loss and replacement paralleled that seen after adult lesions. The pattern of replacement of ChAT differed from that after adult lesions in that there was partial replacement in the ipsilateral intermediate subnucleus following neonatal lesions. This result suggests that late developing cholinergic axons can innervate the contralateral intermediate nucleus to a much greater extent following infant lesions than following adult lesions.

Acetylcholine in the interpeduncular nucleus of the rat: normal distribution and effects of deafferentation

We studied the cholinergic projection to the interpeduncular nucleus (IPN) by examining localization of choline acetyltransferase (ChAT) in the habenula, fasciculus retroflexus (FR) and among the subnuclei of the IPN of the rat, using and antibody raised against ChAT. ChAT-containing neurons were present in the ventral portion of the medial habenula, ChAT-stained axons were present in the FR and ChAT-stained axons and terminals were present in the rostral, central and intermediate subnuclei of the IPN. No ChAT staining was seen in the lateral or dorsal subnuclei. The pattern of ChAT localization was thus complementary to the pattern of the habenular substance P projection to the IPN. Lesions of the FR eliminated all ChAT from the IPN while lesions of the stria medullaris produced a modest decrease. Unilateral FR lesions indicated that the FR projection to the central and rostral subnuclei is largely bilateral and symmetrical and that to the intermediate subnuclei is largely ipsilateral. We found no evidence of lesion-induced plasticity, i.e. replacement of ChAT immunoreactivity, by surviving FR axons in these adult brains.

Topography of cholinergic and substance P pathways in the habenulo-interpeduncular system of the rat. An immunocytochemical and microchemical approach

The topography of cholinergic and substance P containing habenulo-interpeduncular projections has been studied in the rat. The research has been carried out by combining choline acetyltransferase and substance P immunohistochemistry to experimental lesions and biochemical assays in microdissected brain areas. In addition, computer-assisted image analysis has been performed in order to obtain quantification of immunohistochemical data. The results show that cholinergic and substance P containing neurons have a different localization in the medial habenula and project to essentially different areas of the interpeduncular nucleus. Cholinergic neurons are crowded in the ventral two-thirds of the medial habenula while substance P containing cells are exclusively localized in the dorsal part of the nucleus. In most parts of the interpeduncular nucleus, choline acetyltransferase and substance P containing fibres and terminals are similarly segregated and no overlapping is apparent except for the rostralmost and the caudalmost ends of the nucleus. Cholinergic activity is largely concentrated in the central core of the nucleus, while substance P is preferentially localized in the peripheral subnuclei of the interpeduncular nucleus. In addition, both substance P and choline acetyltransferase levels show peculiar regional variations along the rostrocaudal axis of the interpeduncular nucleus. The results of experimental lesions demonstrate that the substance P projection carried by each fasciculus retroflexus is prevailingly ipsilateral in the rostral part of the interpeduncular nucleus and becomes progressively bilateral as far as more caudal regions of the nucleus are reached. By contrast, the cholinergic projections carried by each fasciculus retroflexus intermingle more rapidly and only show a slight ipsilateral dominance in the interpeduncular nucleus. The results of the study are discussed with reference to previous anatomical and neurochemical data which, in several instances, had given rise to discrepant interpretations.

Analysis of the habenulopetal enkephalinergic system in the rat brain: an immunohistochemical study

The enkephalinergic afferent system in the rat habenula was examined by the indirect immunofluorescence method with antibodies against leucine-enkephalin. Leucine-enkephalin-like immunoreactive (L-ENKI) fibers were observed in the dorsal portion of the medial habenular nucleus (MHb), the intermediate portion of the lateral habenular nucleus (LHb), and the border zone between the MHb and the LHb (BZHb). Knife-cut studies demonstrated that almost all the fibers were supplied via the stria medullaris. Also two discrete ENKergic afferents to the MHb and the LHb were found by several kinds of lesion studies: the MHb was ipsilaterally innervated from L-ENKI neurons in "septoperiventricular area," which is the junctional area between the ventral supracommissural septum and the rostral thalamic periventricular region. The LHb was ipsilaterally innervated from L-ENKI neurons in the rostral portion of the lateral hypothalamus. Our findings also suggested that the L-ENKI fibers in the BZHb are supplied via the stria medullaris with an ipsilateral predominance and that, at the most caudal level, they arise not only from the ipsilateral stria medullaris but also from the contralateral stria medullaris via the habenular commissure.

Cytoarchitecture, fiber connections, and some histochemical aspects of the interpeduncular nucleus in the rat

The organization of afferent and efferent connections of the interpeduncular nucleus (IP) has been examined in correlation with its subnuclear parcellation by using anterograde and retrograde tracing techniques. Based on Nissl, myelin, and acetylcholinesterase staining five paired and three unpaired IP subnuclei are distinguished. The unpaired division includes the rostral subnucleus (IP-R), the apical subnucleus (IP-A), and the central subnucleus (IP-C). The subnuclei represented bilaterally are the paramedian dorsal medial (IP-DM) and intermediate subnuclei (IP-I) and the laterally placed rostral lateral (IP-RL), dorsal lateral (IP-DL), and lateral subnuclei (IP-L). Immunohistochemical techniques showed cell bodies and fibers and terminals immunoreactive for substance P, leu-enkephalin, met-enkephalin, or serotonin to be differentially distributed over the different IP subnuclei. Substance P-positive perikarya were found in IP-R, enkephalin neurons in IP-R, IP-A, and the caudodorsal part of IP-C, and serotonin-containing cell bodies in IP-A and the caudal part of IP-L. Efferent IP projections were studied both by injecting tritiated leucine in IP and by injecting HRP or WGA-HRP in the presumed termination areas. The results indicate that the major outflow of IP is directed caudal-ward to the median and dorsal raphe nuclei and the caudal part of the central gray substance, i.e., the dorsal tegmental region. The projection appears to terminate mainly in the raphe nuclei, around the ventral and dorsal tegmental nuclei of Gudden, and in the dorsolateral tegmental nucleus. The descending projection to the dorsal tegmental region originates in virtually all IP subnuclei, but the main contribution comes from IP-R and the lateral subnuclei IP-RL, IP-DL, and IP-L. Sparser projections to the dorsal tegmental region originate in IP-C and IP-I, whereas the contribution of IP-A is only minimal. The projections from IP-R are mainly ipsilateral and those from IP-DM are mainly contralateral. IP fibers to the median and dorsal raphe nuclei originate predominantly in IP-R and IP-DM, and to a lesser extent in IP-C, IP-I, IP-RL, and IP-DL. A much smaller contingent of IP fibers ascends to diencephalic and telencephalic regions. A relatively minor projection, stemming from IP-RL and IP-DL, reaches the lateral part of the mediodorsal nucleus, the nucleus gelatinosus, and some midline thalamic nuclei. These IP fibers follow either the habenulo-interpeduncular pathway or the mammillothalamic tract.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent projections to the interpeduncular nucleus in the rat, as studied by retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase

We examined the afferent projections to the subnuclei of the interpeduncular nucleus (IPN) in the rat by means of retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). We observed locations of retrogradely labeled cells following injections of WGA-HRP into the IPN, and distributions of anterogradely labeled fibers and terminals within the IPN following injections into the areas that contain cells of origin of afferents. Results of the retrograde and anterograde experiments have clarified the detailed organization of the IPN afferents. A part of the nucleus incertus, located dorsomedial to the dorsal tegmental nucleus, projects to the contralateral half of the rostral subnucleus of the IPN; the pars caudalis of the dorsal tegmental nucleus projects sparsely to the rostral lateral, dorsal lateral, lateral, caudal, and apical subnuclei predominantly contralaterally; the laterodorsal tegmental nucleus, to most of the subnuclei predominantly contralaterally; the ventromedial central gray rostral to the dorsal tegmental nucleus and lateral to the dorsal raphe nucleus projects to the rostral lateral and dorsal lateral subnuclei predominantly contralaterally; the median raphe nucleus, substantially to all subnuclei; the medial habenular nucleus, in a topographic manner, to the rostral, central, and intermediate subnuclei, to the rostral lateral and lateral subnuclei predominantly ipsilaterally, and to the dorsal lateral subnucleus predominantly contralaterally; the supramammillary nucleus and areas around the origin of the mammillothalamic tract and near the third ventricle project sparsely to the ventral part of the rostral subnucleus and to the central, lateral, caudal and apical subnuclei; the nucleus of the diagonal band, sparsely to the rostral, central, dorsal lateral, caudal, and apical subnuclei. These differential projections of the afferents to the subnuclei of the IPN may reflect its complex functions within the limbic midbrain circuit.

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.

Neurogenesis in subnuclei of the rat interpeduncular nucleus and medial habenula

Development of the rat habenulo-interpeduncular system is of interest because of its highly ordered adult structure, including seven subnuclei within the interpeduncular nucleus (IPN), localization of synapses of different types and medial habenular (MH) sites of origin within the subnuclei, and differential localization of neurochemicals amongst the subnuclei. In order to further investigate the mechanisms by which these features are produced, the birthdays of neurons within IPN and MH were investigated. All IPN neurons were formed on embryonic days (E) 12 to 16. Birthdays varied for the subnuclei of IPN with the earliest being the serotonin containing apical subnucleus (p less than 0.0001) and the latest being the rostral subnucleus (p less than 0.0001). Rostral lateral and lateral subnuclei were approximately simultaneous with apical, and the intermediate and central subnuclei, related to each other in a number of other ways, had simultaneous birthdays earlier than the rostral subnucleus (p less than 0.0001) and later than the other subnuclei (p less than 0.0001). The medial habenula was found to consist of three regions, termed medial, lateral and dorsal, whose neurons undergo their final mitoses on E15-16, 16-17 and 17-18, respectively. These findings provide additional support for the validity of IPN subnuclei as currently delineated. They suggest mechanisms of development which involve early interactions between specific groups of MH neurons and specific groups of IPN neurons. It is proposed that a sequence of several control mechanisms operates during development to produce this complex system.

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.

Antidromic discharge property of meso-accumbens dopaminergic VTA neurons in rats

Three groups of meso-accumbens (Acc) neurons in the ventral tegmental area were differentiated by their antidromic discharge property; dopaminergic type 1 (n = 10), non-dopaminergic type 2 (n = 2) and unclassified (n = 2) neurons. During repetitive activation at 10 Hz, the latency of the initial segment (IS) spike, which was often not followed by the somadendritic (SD) spike, was gradually prolonged in type 1, but not in type 2 and unclassified neurons. The latency prolongation of type 1 neurons was reduced to about a half of the normal in rats treated with kainic acid plus haloperidol, but only slightly when treated with kainic acid or picrotoxin. The rate of SD invasion tended to increase after all kinds of chemical treatment. Stimulation of the medial forebrain bundle in type 1 neurons gave responses comparable to Acc stimulation. It is suggested that the latency prolongation of IS spike is produced mainly by axonal mechanism. But additional somatic mechanisms such as dopaminergic self-inhibition and GABAergic and non-GABAergic inputs from the Acc would make some contribution, and at the same time produce frequent suppression of the antidromic SD spike.