Immunoreactive pro-enkephalin and prodynorphin products are differentially distributed within the nucleus of the solitary tract of the rat

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

The ins and outs of the caudal nucleus of the solitary tract: An overview of cellular populations and anatomical connections

The body and brain are in constant two-way communication. Driving this communication is a region in the lower brainstem: the dorsal vagal complex. Within the dorsal vagal complex, the caudal nucleus of the solitary tract (cNTS) is a major first stop for incoming information from the body to the brain carried by the vagus nerve. The anatomy of this region makes it ideally positioned to respond to signals of change in both emotional and bodily states. In turn, the cNTS controls the activity of regions throughout the brain that are involved in the control of both behaviour and physiology. This review is intended to help anyone with an interest in the cNTS. First, I provide an overview of the architecture of the cNTS and outline the wide range of neurotransmitters expressed in subsets of neurons in the cNTS. Next, in detail, I discuss the known inputs and outputs of the cNTS and briefly highlight what is known regarding the neurochemical makeup and function of those connections. Then, I discuss one group of cNTS neurons: glucagon-like peptide-1 (GLP-1)-expressing neurons. GLP-1 neurons serve as a good example of a group of cNTS neurons, which receive input from varied sources and have the ability to modulate both behaviour and physiology. Finally, I consider what we might learn about other cNTS neurons from our study of GLP-1 neurons and why it is important to remember that the manipulation of molecularly defined subsets of cNTS neurons is likely to affect physiology and behaviours beyond those monitored in individual experiments.

Kappa-opioid receptors in the caudal nucleus tractus solitarius mediate 100 hz electroacupuncture-induced sleep activities in rats

Previous results demonstrated that 10 Hz electroacupuncture (EA) of Anmian acupoints in rats during the dark period enhances slow wave sleep (SWS), which involves the induction of cholinergic activity in the caudal nucleus tractus solitarius (NTS) and subsequent activation of opioidergic neurons and μ-receptors. Studies have shown that different kinds of endogenous opiate peptides and receptors may mediate the consequences of EA with different frequencies. Herein, we further elucidated that high-frequency (100 Hz)-EA of Anmian enhanced SWS during the dark period but exhibited no direct effect on rapid eye movement (REM) sleep. High-frequency EA-induced SWS enhancement was dose-dependently blocked by microinjection of naloxone or κ-receptor antagonist (nor-binaltorphimine) into the caudal NTS, but was affected neither by μ- (naloxonazine) nor δ-receptor antagonists (natatrindole), suggesting the role of NTS κ-receptors in the high-frequency EA-induced SWS enhancement. Current and previous results depict the opioid mechanisms of EA-induced sleep.

Monosynaptic input from Leu5-enkephalin-immunoreactive terminals to vagal motor neurons in the nucleus ambiguus: comparison with the dorsal motor nucleus of the vagus

Vagal motor neurons in the rat dorsal motor nucleus of the vagus (DMN) are known to receive direct synaptic input from enkephalin-containing terminals. We examined 1) whether the vagal motor neurons within the nucleus ambiguus (NA) also received monosynaptic input from enkephalin-immunoreactive terminals and 2), if so, whether their ultrastructural relations differed from those in the DMN. In both regions, terminals containing Leu5-enkephalin-like immunoreactivity (LE-LI) were examined in relation to motor neurons identified by retrograde transport of wheat germ-agglutinated horseradish peroxidase (WGA-HRP) applied to the cut end of the cervical vagus nerve in single sections of the medulla oblongata of adult rats. By light microscopy, the most significant overlap between varicose processes with LE-LI and WGA-HRP-containing neurons was seen in the rostral compact portion of the NA and the DMN at the level of the obex. Thus, only these regions were examined by electron microscopy. The most distinguishing ultrastructural feature of WGA-HRP-labeled neurons in the NA compared to the DMN was their higher incidence of nonsynaptic appositions with other neurons. In both the NA and the DMN, terminals with LE-LI formed primarily symmetric synapses on smaller (presumably distal) dendrites; many of these dendrites, as well as most target perikarya, contained WGA-HRP. Additionally, in the compact portion of the NA compared to the DMN 1) multiple LE-labeled terminals more frequently contacted single perikarya or dendrites and 2) single terminals with LE-LI more commonly showed two contacts or active zones and contained more abundant LE-immunoreactive large (80-100 nm) dense-core vesicles (dcvs). In contrast to small (40-50 nm), clear vesicles, which were usually aggregated near active zones, the immunoreactive dcvs were usually located near glial processes distal to these zones. These results indicate that enkephalin immunoreactivity is intensely localized to dcvs within terminals that may have direct inhibitory (symmetric synapses) actions on vagal motor neurons in both the compact portion of the NA and the DMN. Moreover, because numbers of dcvs and active zones have been equated with synaptic strength, our findings suggest enhanced potencies of enkephalin-immunoreactive terminals in the compact portion of the NA. Our findings support a prominent role for enkephalin in the coordinated activity of esophageal motor neurons located in the compact portion of the NA.

Neuropeptides in the human dorsal vagal complex: an immunohistochemical study

The distribution of twelve biologically active neuropeptides, i.e., thyrotropin-releasing hormone, corticotropin-releasing factor, pro-opiomelanocortin-derived peptides (adrenocorticotropic hormone, beta-endorphin, alpha-melanocyte-stimulating hormone), leucine-enkephalin, dynorphin A, dynorphin B, cholecystokinin, substance P, galanin and calcitonin gene-related peptide, was examined by immunohistochemistry in the human dorsal vagal complex including the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the area postrema. Immunoreactivity of all the twelve neuropeptides was found widely distributed in the various subdivisions of the nucleus of the solitary tract, showing a unique distribution for every peptide. Neuronal cell bodies immunostained with leucine-enkephalin, galanin and dynorphin B were found in this region. There were no immunopositive perikarya for any of the peptides in the other structures studied. Fibers containing galanin, corticotropin-releasing factor, substance P, dynorphin B, thyrotropin-releasing hormone and calcitonin gene-related peptide were observed at a relatively high density in the nucleus of the solitary tract. In the same structure, a moderately dense network of fibers immunostained with dynorphin A, cholecystokinin and leucine-enkephalin, but only solitary pro-opiomelanocortin-derived peptides-containing fiber fragments were observed. In the dorsal motor nucleus of the vagus the most prominent network of fibers was found to contain thyrotropin-releasing hormone, galanin and substance P. In contrast to these, no beta-endorphin immunoreactivity was detected. The area postrema contained only moderate to low densities of galanin-, substance P-, calcitonin gene-related peptide-, dynorphin B- and cholecystokinin-immunoreactive fibers.

Opioid peptide gene expression in the nucleus tractus solitarius of rat brain and increases induced by unilateral cervical vagotomy: implications for role of opioid neurons in respiratory control mechanisms

Neurons expressing messenger RNA encoding the opioid peptide precursors, preproenkephalin and preprodynorphin were localized in the medulla oblongata of the rat by in situ hybridization of specific DNA oligonucleotide probes. Neurons containing preproenkephalin messenger RNA were found throughout the medullary reticular formation in the gigantocellular and paragigantocellular reticular nuclei, the parvicellular and lateral reticular nuclei; commissural, medial and ventrolateral subnuclei in the nucleus tractus solitarius and the nucleus of the spinal trigeminal tract. Labelled cells were also concentrated in the more medial regions of the area postrema. In contrast, neurons containing preprodynorphin messenger RNA had a more restricted distribution and were detected in the commissural and ventrolateral nucleus tractus solitarius and nucleus of the spinal trigeminal tract, especially in the more dorsal regions. Expression of preproenkephalin and preprodynorphin messenger RNA was also examined in the dorsal vagal complex of rats that had undergone a unilateral nodose ganglionectomy or cervical vagotomy. Twenty-four hours after both cervical vagotomy and nodose ganglionectomy, there was a specific 1.5-2-fold elevation in preproenkephalin and preprodynorphin messenger RNA levels in the ventrolateral subnucleus of the contralateral nucleus tractus solitarius relative to levels in the ipsilateral nucleus tractus solitarius and in the nucleus tractus solitarius of sham-operated animals. Previous immunohistochemical studies demonstrating the co-localization of enkephalin and dynorphin in the ventrolateral nucleus tractus solitarius suggest that these changes occurred in the same population of neurons. In light of the suggested role of the ventrolateral nucleus tractus solitarius as a central respiratory centre and the activation of the intact pulmonary afferents that innervate this area following a unilateral vagotomy (which increases inspiration volume and expiratory time by affecting the Hering-Breuer reflex), our results suggest a specific involvement of enkephalin- and dynorphin-containing neurons in the ventrolateral nucleus tractus solitarius in central respiratory control mechanisms.

Presence of a dynorphin-like peptide in a restricted subpopulation of catecholaminergic neurons in rat nucleus tractus solitarii

Immunofluorescence colocalization techniques were used to examine the extent of coexistence of the endogenous opioid peptide dynorphin with catecholamines and the related opioid peptide enkephalin within neurons of the rat medulla oblongata. Immunoreactivities for dynorphin and the catecholamine-synthesizing enzyme tyrosine hydroxylase were found to coexist within a limited subpopulation of A2 catecholamine cells, localized to the medial nucleus of the nucleus tractus solitarii. Colocalization of the two opioid peptides was found mainly within perikarya situated in the medial and ventrolateral nuclei of the nucleus tractus solitarii. Triple-labeling studies revealed only rare cases of catecholamine/dynorphin/enkephalin coexistence. These data demonstrate that dynorphin is present within a restricted subpopulation of catecholamine neurons in the dorsal medulla oblongata. In addition, the content of either of the opioids enkephalin or dynorphin appears to distinguish subsets of medullary catecholamine neurons.

Immunohistochemical localization of serotonin, leu-enkephalin, tyrosine hydroxylase, and substance P within the visceral sensory area of cartilaginous fish

We examined the distribution of immunoreactivity to serotonin (5-HT), leu-enkephalin (LENK), tyrosine-hydroxylase (TH), and substance P (SP) within the primary visceral sensory region of cartilaginous fish. Two genera of sharks, Squalus and Heterodontus, a skate, Raja, a ray, Myliobatis, and a holocephalian, Hydrolagus, were used. Cranial nerves, VII, IX, and X enter the visceral sensory complex from the lateral aspect and divide it into lobes. Based on sagittally cut sections, there are four lobes in Hydrolagus and five in Squalus, corresponding to the number of gill arches. The neurochemicals are differentially distributed within each lobe. LENK+ and 5-HT+ fibers are located in all regions within the visceral sensory complex. SP+ fibers are extremely dense in a dorsolateral subdivision and do not extend as far ventrally as 5-HT+ and LENK+ fibers. The lobes lack 5-HT+ cells, but contain a few LENK+ and SP+ cells. Many TH+ cells are distributed in dorsomedial portions of the complex, but there are few TH+ fibers. Thus, the visceral sensory area of cartilaginous fish contains several divisions that can be distinguished by their neurochemical content.

Neuropharmacologic correlates of deglutition: lessons from fictive swallowing

Pharmacologic investigations into the transmission processes underlying fictive swallowing in the rat have disclosed the potential diversity of chemical signals used in central deglutitive pathways. Monoaminergic mechanisms appear to serve as links between subcortical structures and the medullary pattern generator of swallowing (PGS), and may play a critical role in maintaining internal facilitatory drive, required by the PGS for optimal responsivity to peripheral sensory input. Cholinergic bulbar interneurons form an integral component of the PGS subnetwork controlling esophageal peristalsis. Local GABA neurons exert a tonic inhibition of the buccopharyngeal stage, may regulate buccopharyngeal-esophageal coupling, and may contribute to peristaltic rhythmic generation at both the premotoneuronal and motoneuronal level. Receptor subtypes for excitatory amino acids (glutamate, aspartate) are differentially associated with deglutitive premotoneurons for both the buccopharyngeal and esophageal stage, as well as with ambiguus motoneurons. Preliminary evidence suggests the existence of excitatory peptidergic mechanisms involving thyrotropin-releasing hormone, vasopressin, oxytocin, and somatostatin, a probable candidate for excitatory transmitter in the solitarioambigual internuncial projection to motoneurons innervating esophageal striated musculature. Further validation of this experimental model may ultimately help to establish a framework for the clinical recognition, management, and exploitation of drug actions on central deglutitive neuroeffectors.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical localization of serotoninergic, enkephalinergic, and catecholaminergic cells in the brainstem and diencephalon of a cartilaginous fish, Hydrolagus colliei

We localized serotonin (5-HT), leu-enkephalin (LENK), and tyrosine hydroxylase (TH) immunoreactive cells in the brain of a holocephalian, Hydrolagus colliei, by use of antibodies made in rabbit and the peroxidase-antiperoxidase technique. Only three locations contained TH+ cells, the caudal myelencephalon, the locus coeruleus, and the diencephalon. Of these locations, the diencephalon contained the most cells and the locus coeruleus the least cells. The caudal TH+ myelencephalic cells formed a single large group that spanned both the dorsal and ventral portions of the brain (A1A2). The diencephalic TH+ cells were located in the posterior tuberculum, in the ventromedial and ventrolateral thalamic nuclei, and in the inferior lobe of the hypothalamus. Hydrolagus differed from mammals and the elasmobranchs, their sister group, in that no substantia nigra (A9), ventral tegmental area (A10), or A5 cell group was found. Distribution of LENK+ and 5-HT+ cells were similar to each other; the raphe nuclei contained most of the 5-HT+ and LENK+ cells. These 5-HT+ and LENK+ cells were found at all rostrocaudal levels of the myelencephalon. The nucleus reticularis magnocellularis, reticularis paragigantocellularis lateralis, the ventral met- and mesencephalon (B7 and B9 cell groups), the hypothalamus, and the pretectal area contained additional 5-HT+ and LENK+ cells. The solitary complex contained LENK+ cells but not but 5-HT+ cells. A dorsal raphe nucleus, which is the largest 5-HT+ cell group in mammals, was absent in Hydrolagus. A dorsal raphe nucleus is present in one galeomorph shark radiation but is absent in three radiations of batoids (rays, skates, and guitarfish). Thus even within cartilaginous fish, there are differences in the distribution of neurochemicals and possibly nuclei within their brains.

Localization of serotonin, tyrosine hydroxylase, and leu-enkephalin immunoreactive cells in the brainstem of the horn shark, Heterodontus francisci

In previous studies on reptiles and elasmobranchs, we determined that some reticular groups are either absent or may be displaced compared to their locations in mammals. For example, nucleus raphe dorsalis, the largest serotoninergic cell group in mammals, is not present in rays, skates, or guitarfish. In the present study, we chose heterodontid sharks, a sister group to these batoids, for an out-group comparison of this and other characters. We identified cells in the brainstem of Heterodontus francisci by use of antibodies against tyrosine hydroxylase, serotonin, or leu-enkephalin and compared the distribution of these nuclei to descriptions in mammals and other elasmobranchs. The majority of tyrosine hydroxylase-positive cells were found in the midbrain tegmentum (A8-A10) and the hypothalamus. In addition, putative A1, A2, A5, A7 (noradrenergic) groups were found in the metencephalon and myelencephalon. Serotonin-positive cells were found in raphe nuclei and scattered lateral to the raphe. We identified probable homologues to raphe pallidus, raphe obscurus, raphe magnus, and raphe centralis superior (B8) cell groups, which have been described in mammals. A cluster of cells dorsomedial to the medial longitudinal fasciculus was identified as raphe dorsalis. The distributions of leu-enkephalin and serotonin immunoreactive cells were similar to each other, but the tyrosine-hydroxylase immunoreactive cells rarely intermingle with the former two immunoreactive cell types. Other reticular groups that contained both serotonin- and leu-enkephalin-positive cells included reticularis (r.) ventralis, r. magnocellularis, r. paragigantocellularis lateralis, r. pontis caudalis, and r. pontis oralis medialis and lateralis. Thus, this shark contains many of the major brainstem raphe and catecholaminergic cell groups described for rats, but the relative distribution of the immunopositive cell groups differs in mammals and cartilaginous fish.

Relationship of Met-enkephalin-like immunoreactivity to vagal afferents and motor dendrites in the nucleus of the solitary tract: a light and electron microscopic dual labeling study

Methionine (Met5)-enkephalin has been implicated in autonomic functions involving vagal reflexes within the nucleus of the solitary tract (NTS). We examined the light and electron microscopic relationships between neurons containing methionine (Met5)-enkephalin-like immunoreactivity (MELI) and vagal afferents and motor dendrites in the rat NTS. A polyclonal antibody raised against Met5-enkephalin and showing maximal cross-reactivity with this peptide was localized by immunoautoradiography. In the same sections, vagal afferents and motor neurons were identified by histochemical detection of anterogradely and retrogradely transported horseradish peroxidase (HRP). By light microscopy, the MELI was detected in perikarya distributed principally in the dorsomedial, intermediate and parasolitary subdivisions of the NTS. These subnuclei as well as medial and commissural divisions of the NTS also showed: (1) aggregates of silver grains thought to overlie terminals containing MELI, and (2) anterogradely transported HRP in varicose processes. Electron microscopic analysis of the dorsomedial NTS at the level of the area postrema established that MELI was detectable in perikarya, dendrites, and axon terminals. Most of the MELI was associated with large dense core vesicles (dcvs). These opioid terminals formed primarily symmetric synapses on proximal and asymmetric synapses on distal dendrites. Analysis of the dendritic targets of terminals containing MELI revealed that 13/222 were in synaptic contact with dendrites also containing MELI. The remainder of the terminals containing MELI either lacked recognized junctions or formed synapses with unlabeled dendrites. In comparison to the terminals containing MELI in the same series of sections, anterogradely labeled vagal terminals extensively formed asymmetric junctions with distal dendrites and spines. Of the observed anterogradely labeled terminals 6/84 formed synapses with dendrites containing MELI and 3/84 with dendrites containing retrogradely transported HRP. The remainder of the junctions were with dendrites lacking detectable immunoautoradiographic or HRP-labeling. The majority of the recognized synapses on labeled dendrites were at more proximal sites possibly reflecting more limited detection of both MELI and retrogradely transported HRP in smaller dendrites. However, the presence of even a few junctions at proximal sites on dendrites where synaptic transmission is known to be more effective suggests a potentially strong modulation of both opioid and vagal motor neurons by visceral afferents in the NTS. In addition to forming synapses on dendrites, both vagal afferents and terminals containing MELI showed frequent synaptic associations with unlabeled terminals, but not with each other. This finding suggests that the previously demonstrated opiate binding sites on vagal afferents is most likely attributed to other endogenous opiates.

Fluorescent histochemical localization of neutral endopeptidase-24.11 (enkephalinase) in the rat brainstem

Characterization of the distribution of the peptide-degrading enzyme neutral endopeptidase-24.11 (E.C. 3.4.24.11; NEP; enkephalinase) in the rat brainstem was examined by means of a unique fluorescent histochemical method. Enzyme staining was completely blocked by three potent NEP inhibitors (thiorphan, phosphoramidon, and JHF-26) at a concentration of 50 nM, supporting the specificity of this method to visualize sites of NEP activity selectively. At all levels of the brainstem, NEP was localized to cell bodies, cell processes or terminal-like fields and was localized to more than 90 distinct nuclei or subnuclei. In the mesencephalon these included the central gray, cuneiform n., dorsal and lateral tegmental n., inferior colliculus, interpeduncular n., lateral and medial geniculate n., central linear raphe n., mesencephalic n. of the trigeminal nerve, mammillary nuclei, occulomotor n., red n., superior colliculus, ventral n. of the lateral lemniscus, substantia nigra-ventral tegmental area, and the zona incerta. In the pons, NEP staining was restricted to fewer regions or nuclei, including the dorsal and ventral cochlear n., facial n., motor trigeminal n., principal sensory trigeminal n., parabrachial nuclei, pontine n., the oral and caudal pontine reticular n., pontine olivary nuclei, several pontine tegmental nuclei, pontine raphe nuclei, and the trapezoid n. In the cerebellum, staining was localized largely to the granule cell layer of the cerebellar cortex. Scattered staining was observed in the molecular cell layer. The medulla contained extensive NEP staining localized to nuclei that included the ambiguous n., dorsal motor n. of the vagus, hypoglossal n., inferior olivary n., prepositus hypoglossus n., solitary tract n., nuclei of the spinal tract of the trigeminal n., and the lateral, medial, and superior vestibular nuclei. Nuclei of the medullary reticular formation that were also richly stained for NEP included the raphe magnus n., raphe obscurus n., raphe pallidus n., dorsal, lateral, and ventral reticular nuclei of the medulla, and the gigantocellular, lateral paragigantocellular, linear, paramedian and parvicellular reticular nuclei. The widespread distribution of NEP in the brainstem suggests the existence of a number of functional systems, including the pathways involved in the mechanisms of pain and analgesia, which are potential targets of NEP inhibitors. In most regions, the distribution of NEP closely overlapped with that reported for the enkephalins, and showed a more restricted overlap with the reported distribution of substance P.

Neuropeptides and catecholamines in efferent projections of the nuclei of the solitary tract in the rat

This study focuses on the involvement of catecholamines and nine different peptides in efferents of the nucleus of the solitary tract to the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and different parabrachial and hypothalamic nuclei in the rat. A double-labeling technique was used that combines a protein-gold complex as the retrograde tracer with immunohistochemistry. Catecholaminergic projection neurons were the most numerous type observed and projected mainly ipsilaterally to all targets studied. Most projections arose from areas overlying the dorsal motor nucleus, mainly the medial nucleus. Neurons synthesizing somatostatin, met-enkephalin-Arg-Gly-Leu, dynorphin B, neuropeptide Y, and neurotensin projected to all structures examined. Somatostatin and enkephalin immunoreactive projection cells were the most numerous. They were located in close proximity to each other, including all subnuclei immediately surrounding the solitary tract, bilaterally. Most dynorphin and neuropeptide Y immunoreactive projection cells were found rostral to that of enkephalinergic and somatostatinergic projections, and mainly in the ipsilateral medial nucleus. Neurotensinergic projections were sparse and from dorsal and dorsolateral nuclei. Substance P and cholecystokinin contribute to parabrachial afferents. The location of substance P immunoreactive projection cells closely resembled that of enkephalinergic and somatostatinergic projections. Projecting cholecystokinin immunoreactive cells were observed in dorsolateral nucleus. Bombesin immunoreactive cells in dorsal nucleus projected to either the parabrachial or hypothalamic nuclei. No vasoactive intestinal polypeptide-containing cells were detected. Thus, most catecholaminergic and neuropeptidergic efferents originated from different populations of cells. It is proposed that catecholaminergic neurons constitute the bulk of solitary efferents and that they may contribute to autonomic neurotransmission. Peptidergic neurons mainly form other subgroups of projections and may play a role in modulating the physiological state of the target nuclei.

Postnatal development of preproenkephalin mRNA containing neurons in the rat lower brainstem

Postnatal developmental changes of preproenkephalin (PPE) gene expression in rat brainstem neurons were studied by in situ hybridization histochemistry. On the basis of PPE mRNA expression, brainstem neurons were categorized into three types: 1) type I neurons were characterized by constant or increasing expression of PPE mRNA during postnatal development; 2) type II neurons started to express PPE mRNA several days after birth and continued to do so thereafter; and 3) type III neurons showed transient expression of PPE mRNA or stopped expressing the mRNA during early postnatal development. Type I PPE neurons were observed in diverse brainstem structures including the mesencephalic and pontine central gray matter, various reticular and raphe nuclei, the ventral tegmental area of Tsai, the interpeduncular nucleus, the nucleus of the brachium of the inferior colliculus, the ventral and dorsal tegmental nuclei of Gudden, the sphenoid nucleus, the laterodorsal tegmental nucleus, Barrington's nucleus, the parabrachial region, the lateral lemniscus and its related nuclei, the trapezoid nucleus, the rostral and ventromedial periolivary nuclei, the mesencephalic trigeminal and principal sensory trigeminal nuclei, the locus coeruleus, the subcoeruleus nucleus, the medial and spinal vestibular nuclei, the dorsal and ventral cochlear nuclei, the medial and lateral cerebellar nuclei, the Roller nucleus, and the intermedius nucleus of the medulla. Type II PPE neurons were found in the superior colliculus, the inferior colliculus, the central part of the dorsal tegmental nucleus, and as Golgi neurons in the granular layer of the cerebellum. Type III PPE neurons were located in the substantia nigra, the red nucleus, the superior olive, the motor trigeminal nucleus, the facial nucleus, the inferior olive, the dorsal motor nucleus of the vagus, and the hypoglossal nucleus. Such region-specific expression of the PPE gene during postnatal ontogeny suggests that rat brainstem PPE neurons may be involved in a variety of developmental events, such as cell proliferation, differentiation, and migration.

Segregation of coarse and fine glossopharyngeal axons in the visceral nucleus of the tractus solitarius of the cat

The projections of coarse and fine axons of the glossopharyngeal (IX) nerve upon the caudal two thirds of the nucleus of the tractus solitarius (NTS) were studied in the cat. These afferents convey the chemo- and baroreceptor activities from the carotid receptors. We applied the Fink-Heimer method on brainstem sections, at different survival times, after a petrosal ganglionectomy. A segregation of fine and coarse fibered components was observed. Degeneration of coarse axons was mostly found in the lateral NTS, while fine fiber degeneration was predominant in regions of the medial and commissural NTS. The injection of WGA-HRP in the different NTS divisions demonstrated that the lateral NTS was mainly innervated by the set of largest neurons of the petrosal ganglion and that the medial and the commissural NTS were innervated by the set of smaller neurons of the ganglia. These results were discussed in relation to cytoarchitecture, myeloarchitecture, distribution of normal axons, and known central connectivity of the different NTS divisions. We concluded that coarse and fine visceral afferents of the IX nerve, which includes the afferents of the carotid body and the carotid sinus, represent different afferent populations that project to particular divisions of the NTS and connect to different central pathways.

Neural mechanisms of swallowing: neurophysiological and neurochemical studies on brain stem neurons in the solitary tract region

Neurophysiological studies of the nuclei of the tractus solitarius (NTS) and adjacent regions have provided a partial understanding of the integrative brainstem network underlying swallowing and related functions such as respiration. The NTS is also richly endowed with an abundance of neuropeptides and other neuroactive substances, but only limited information is available on their influences on neurons involved specifically in swallowing. Since dysfunction of these neurophysiological and neurochemical regulatory mechanisms in the NTS region may be important in pathophysiological conditions such as dysphagia, increased awareness of and focus on these mechanisms are warranted. This paper outlines recent neurophysiological and neurochemical data that provide information on the afferent inputs and neurophysiological properties of neurons in NTS and adjacent caudal brainstem regions implicated in swallowing, respiration, and respiratory-related reflexes.

Distribution of glycine receptor immunoreactivity in the spinal cord of the rat: cytochemical evidence for a differential glycinergic control of lamina I and V nociceptive neurons

In this study we characterized the distribution of glycine receptor immunoreactivity in the spinal cord of the rat by using monoclonal antisera directed against the purified glycine receptor. There was dense, punctate glycine receptor immunoreactive staining in all regions of the gray matter ventral to the substantia gelatinosa. The densest staining was found in laminae III and IV of the dorsal horn. There were also distinct, tributarylike bands of punctate staining that extended well into the white matter of the lateral and ventral funiculi. The only consistent cell body staining was found in small neurons of the ventral horn. The labelled neurons were distributed among larger, unlabelled motoneurons. In general, the pattern of glycine receptor immunoreactivity was similar at all levels of the spinal cord and was comparable to that seen with binding of a tritiated glycine receptor antagonist, strychnine, to sections of rat spinal cord (Zarbin et al.: J. Neurosci. 1:532-547, '81). Two important exceptions, however, were observed. In contrast to the high levels of strychnine binding reported in the substantia gelatinosa, we found almost no glycine receptor immunoreactivity in laminae I and II of the superficial dorsal horn of the spinal cord or of the trigeminal nucleus caudalis. There was also a notable absence of antibody staining in the intermediolateral cell column of the thoracic cord. The presence of dense glycine receptor immunoreactivity in the region of lamina V and its absence in the superficial dorsal horn are discussed in terms of a possible differential glycinergic control of nociceptive neurons of laminae I and V.

Catecholamine-synthesizing neuronal projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to catecholamine-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine if the labelled neurons were immunoreactive for the catecholamine-synthesizing enzymes, dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyl transferase (PNMT). Approximately 60% of the DBH-immunoreactive neurons that projected to the nucleus tractus solitarii belonged to the A1/C1 cell group, while an additional 20% belonged to the A5 cell group. Thus, these two ventrolateral rhombencephalic cell groups accounted for nearly 80% of the total number of rhodamine-bead-labelled DBH-immunoreactive neurons in this series of experiments. Only a small number of DBH-immunoreactive neurons of the A2/C2 cell group contained rhodamine-filled latex microspheres. Rarely, DBH-immunoreactive neurons in the locus coeruleus and the nucleus subcoeruleus were found to project to the nucleus tractus solitarii. The majority of the PNMT-immunoreactive neurons that projected to the nucleus tractus solitarii belonged to the C1 cell group. Only small numbers of PNMT-immunoreactive neurons of the C2 and C3 groups were found to contain rhodamine-filled latex microspheres. It is concluded that neurons in the ventrolateral medulla and pons, some of which presumably utilize norepinephrine and/or epinephrine as a transmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.

Serotonin- and substance P-containing projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to serotonin- and substance P-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine whether the labelled neurons contained serotonin or substance P immunoreactivity. Serotonin-immunoreactive neurons that projected to the nucleus tractus solitarii were found in the nucleus raphe magnus, nucleus raphe obscurus, nucleus raphe pallidus, and in the ventral medulla, lateral to the pyramidal tract. Substance P-immunoreactive neurons that projected to the nucleus tractus solitarii were found in similar areas but were proportionately less numerous in the nucleus raphe magnus and proportionately more numerous in the nucleus raphe pallidus. It is concluded that neurons in the medullary raphe nuclei, some of which presumably utilize serotonin or substance P as a neurotransmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.

Immunocytochemical localization of proenkephalin-derived peptides in the central nervous system of the rat

Most of the early studies on the immunohistochemical distribution of enkephalin pentapeptide-like immunoreactivity used antisera that stained both proenkephalin- and prodynorphin-containing neurons. The present study used the peroxidase-antiperoxidase method, thick Vibratome sections and antisera specific for the carboxyl termini of [Met]enkephalin, [Met]enkephalyl-Arg6-Phe7, [Met]enkephalyl-Arg6-Gly7-Leu8, and metorphamide and for BAM 22P in order to obtain a detailed description of the distribution of authentic proenkephalin-containing perikarya and nerve processes. The peroxidase-antiperoxidase reaction product was intensified by the selective deposition of silver crystals in order to display the morphology of proenkephalin-containing neurons with great fidelity. The results indicate that the magnocellular perikarya in the supraoptic and paraventricular nuclei contain prodynorphin rather than proenkephalin as had been suggested by earlier investigators. The coarse fibers in the internal zone of the median eminence and the granule cell-mossy fiber pathway in the hippocampus also contain prodynorphin rather than proenkephalin. The number of proenkephalin-containing perikarya and/or the density of proenkephalin-containing nerve terminals in several other areas of the brain, e.g. the substantia nigra, the central amygdaloid nucleus, the periaqueductal gray and the parabrachial nuclei, were overestimated by earlier investigators. The distribution of authentic proenkephalin-containing perikarya and nerve processes is, despite these errors, similar to the distribution of enkephalin pentapeptide-like immunoreactivity described by earlier investigators. Proenkephalin-containing perikarya were identified for the first time in the medial and lateral habenular nuclei of the adult rat. Antisera specific for [Met]enkephalin, [Met]enkephalyl-Arg6-Phe7, [Met]enkephalyl-Arg6-Gly7-Leu8 and BAM 22P stain perikarya and nerve terminals with a similar distribution. The metorphamide antiserum also stains the same perikarya and nerve terminals; however, it also stains magnocellular perikarya in the zona incerta and the lateral hypothalamus that are not stained by any of the other proenkephalin-specific antisera.

Colocalization of immunoreactive proenkephalin and prodynorphin products in medullary neurons of the rat

This study addressed the possible coexistence of products of the proenkephalin and prodynorphin opioid peptide precursors in single neurons of the central nervous system of the rat. Antisera directed against met-enkephalin-arg-gly-leu and against Dyn B were used in immunohistochemical preparations of sections through the rat medulla. Examination of serial three micron frozen sections stained alternately with the two different antisera revealed that the majority of labelled neurons stain with only one of the two antisera. In specific area, however, immunoreactive m-enk and Dyn B could be detected in the same neuron. This was particularly true of the caudal ventrolateral nucleus of the solitary tract, where the two peptides were colocalized in most neurons. Other areas where the two peptides coexist include the midline raphe and the nucleus reticularis paragigantocellularis. These data provide the first evidence for colocalization of different opioid peptide families in single CNS neurons.