Immunohistochemical demonstration of the serotonin-containing subependymal cells in the frog hypothalamus

Organization of serotonergic system in Sphaerotheca breviceps (Dicroglossidae) tadpole brain

The monoaminergic neurotransmitter 5-hydroxytryptamine (5-HT) is known to be involved in several physiological, behavioural and neuroendocrine functions in vertebrates. In this study, we investigated the distribution of 5-HT neuronal system in the central nervous system (CNS) of Sphaerotheca breviceps tadpoles at metamorphic climax stage. In the telencephalon, there was no 5-HT-immunoreactive (5-HT-ir) perikarya, but conspicuous fibres were observed in the olfactory bulb, pallium, subpallium and amygdala complexes. The preoptic area showed dense 5-HT-ir somata and cerebrospinal fluid contacting fibres, whereas a few varicose 5-HT-ir fibres were noticed in the suprachiasmatic nucleus. 5-HT-ir cells and fibres were found in the ventral, lateral dorsal subdivisions of the hypothalamus and in the nucleus tuberculi posterioris, but only 5-HT-ir fibres were localised in the periventricular area and pituitary gland. Numerous 5-HT-ir cells and/or fibres were detected in the thalamus, entopeduncular area and mesencephalic subdivisions. In the rhombencephalon, although 5-HT-ir cells and fibres were noticed in the subdivisions of the raphe nucleus and reticular formation, a moderate plexus of fibres was observed in the cerebellum, parabrachial nucleus and solitary tract. Distinct 5-HT-ir fibres, but no perikarya, were observed in the rostral spinal cord. Overall, extensively labelled 5-HT-ir cells and fibres in the CNS of the metamorphic tadpole suggest possible roles for the involvement of 5-HT in various somatosensory, behavioural and neuroendocrine functions during final stages of development.

Actual problems of the cerebrospinal fluid-contacting neurons

Cerebrospinal fluid (CSF)-contacting neurons form a part of the circumventricular organs of the central nervous system. Represented by different cytologic types and located in different regions, they constitute a CSF-contacting neuronal system, the most central periventricular ring of neurons in the brain organized concentrically according to our concept. Because the central nervous system of deuterostomian echinoderm starfishes and the prochordate lancelet is composed mainly of CSF-contacting-like neurons, we hypothesize that this cell type represents ancient cells, or protoneurons, in the vertebrate brain. Neurons may contact the ventricular CSF via their dendrites, axons, or perikarya. Most of the CSF-contacting nerve cells send their dendritic processes into the ventricular cavity, where they form ciliated terminals. These ciliated endings resemble those of known sensory cells. By means of axons, the CSF-contacting neurons also may contact the external CSF space, where the axons form terminals of neurohormonal type similar to those known in the neurohemal areas. The most simple CSF-contacting neurons of vertebrates are present in the terminal filum, spinal cord, and oblongate medulla. The dendritic pole of these medullospinal CSF-contacting neurons terminates with an enlargement bearing many stereocilia in the central canal. These cells are also supplied with a 9 x 2 + 2 kinocilium that may contact Reissner's fiber, the condensed secretory material of the subcommissural organ. The Reissner's fiber floating freely in the CSF leaves the central canal at the caudal open end of the terminal filum in lower vertebrates, and open communication is thus established between internal CSF and the surrounding tissue spaces. Resembling mechanoreceptors cytologically, the spinal CSF-contacting neurons send their axons to the outer surface of the spinal cord to form neurosecretory-type terminals. They also send collaterals to local neurons and to higher spinal segments. In the hypothalamic part of the diencephalon, neurons of two circumventricular organs, the paraventricular organ and the vascular sac, of the magnocellular neurosecretory nuclei and several parvocellular nuclei, form CSF-contacting dendritic terminals. A CSF-contacting neuronal area also was found in the telencephalon. The CSF-contacting dendrites of all these areas bear solitary 9 x 2 + 0 cilia and resemble chemoreceptors and developing photoreceptors cytologically. In electrophysiological experiments, the neurons of the paraventricular organ are highly sensitive to the composition of the ventricular CSF. The axons of the CSF-contacting neurons of the paraventricular organ and hypothalamic nuclei terminate in hypothalamic synaptic zones, and those of magno- and parvocellular neurosecretory nuclei also form neurohormonal terminals in the median eminence and neurohypophysis. The axons of the CSF-contacting neurons of the vascular sac run in the nervus and tractus sacci vasculosi to the nucleus (ganglion) sacci vasculosi. Some hypothalamic CSF-contacting neurons contain immunoreactive opsin and are candidates to represent the "deep encephalic photoreceptors." In the newt, cells derived from the subependymal layer develop photoreceptor outer segments protruding to the lumen of the infundibular lobe under experimental conditions. Retinal and pineal photoreceptors and some of their secondary neurons possess common cytologic features with CSF-contacting neurons. They contact the retinal photoreceptor space and pineal recess, respectively, both cavities being derived from the third ventricle. In addition to ciliated dendritic terminals, there are intraventricular axons and neuronal perikarya contacting the CSF. Part of the CSF-contacting axons are serotoninergic; their perikarya are situated in the raphe nuclei. Intraventricular axons innervate the CSF-contacting dendrites, intraventricular nerve cells, and/or the ventricular surface of the ependyma. (ABSTRACT TRUNCATED)

Immunocytochemical identification of serotonergic supraependymal nerve fibers in the third ventricle of the house musk shrew, Suncus murinus

Supraependymal fibers of the house musk shrew (Suncus murinus) were examined by conventional scanning electron microscopy (SEM), backscattered electron (BSE) imaging of enzyme immunohistochemistry and by immunotransmission electron microscopy in the dorsal part of the third ventricular wall. In this region, ependymal cells were not so heavily ciliated and conventional SEM studies showed two main categories of supraependymal fibers. The first type consisted of long fibers fasciculated which were distributed over the ventricular surface between the anterior commissure and the subfornical organ. The second category was a thin fiber which was observed on the ependymal luminal surface. Some of these fibers had varicoses or terminal-like swellings. This type of supraependymal fiber seemed to originate in the first type of fiber bundles. To confirm the nature and the distribution of serotonin-immunoreactive supraependymal fibers, BSE imaging using immunohistochemical reactions was used. Serotonin-immunoreactive structures were shown as highlighted structures by means of a backscattered electron mode. These investigations revealed that the majority of both types of supraependymal fibers observed by conventional SEM contained serotonin. A moderate number of serotonergic supraependymal fibers was observed on the ventricular surface of the subfornical organ. Immunohistochemical studies using Vibratome sections of identical ventricular regions revealed the presence of serotonin-immunoreactive processes, with the use of light- and electron-microscopy. They were distributed in in the third ventricle just adjacent to the ependymal luminal surfaces. These fibers contained immunoreactive large cored vesicles and immunonegative small clear vesicles.

Distribution and synaptic relations of NOS neurons in the dorsal raphe nucleus: a comparison to 5-HT neurons

Anti-nitric oxide synthase antibody was used to study the distribution, cytoarchitecture, and synaptic relations of nitric oxide synthase-like immunoreactive neurons in the whole rostral-caudal length of the dorsal raphe nucleus of the rat and compared them with serotonergic neurons. Results showed that the distribution of the nitric oxide synthase in the dorsal raphe nucleus was similar to that of the serotonergic neurons at the rostral part of the dorsal raphe nucleus, including the mediodorsal and the medioventral cell groups, and changed at the middle and caudal parts of the dorsal raphe nucleus. The cytoarchitecture of the nitric oxide synthase-like immunoreactive neurons in the medioventral cell group of the dorsal raphe nucleus was similar to that of the serotonergic neurons. Similar to the serotonergic neurons there, nitric oxide synthase-like immunoreactive neurons also received synapses from axon terminals that contained round, or flattened vesicles, or both kinds. Different to the serotonergic neurons, the few nitric oxide synthase-like immunoreactive axon terminals that were in this area formed synapses.

Organization of the serotoninergic system in the brain of two amphibian species, Ambystoma mexicanum (Urodela) and Typhlonectes compressicauda (Gymnophiona)

An immunocytochemical investigation was made of the distribution of serotonin (5-HT) in the brain of larval and adult Ambystoma mexicanum and adult Typhlonectes compressicauda. Immunoreactive perikarya can be identified in the caudal diencephalon (paraventricular organ and infundibular nucleus), in the ventral mesencephalon (interpeduncular nucleus) and in the raphe of the rhombencephalon. Immunopositive fibers and terminal arborizations are widely distributed, extending from the whole telencephalon to the spinal lemniscus area. However, the retinorecipient structures of the thalamus and mesencephalon are either very weakly innervated (Ambystoma) or completely immunonegative (Typhlonectes). The habenular system also exhibits very few 5-HT-positive structures. The major serotoninergic neuron clusters, in both Urodela and Gymnophiona, tend to gather, from the paraventricular organ to the raphe, on both sides of the sagittal plane, showing no tendency to "lateralization". A new interpretation of the limited development of the serotoninergic system in amphibians is given.

GABAergic innervation of serotonergic neurons in the dorsal raphe nucleus of the rat studied by electron microscopy double immunostaining

A double immunocytochemical method combining the preembedding PAP technique and the postembedding immunogold technique was used to examine interactions between GABAergic and serotonergic neurons in the same tissue sections of the dorsal raphe nucleus of the rat. A large number of immunogold stained GABAergic axon terminals were found to be presynaptic to strongly PAP immunostained serotonergic perikarya and dendrites. The types of synapses were mostly symmetrical although a few asymmetrical ones were also found. No axo-axonic synapse between the GABAergic axon terminals and the serotonergic neuronal profiles was found. These results suggest that GABAergic neurons could modulate serotonergic neurons in the dorsal raphe nucleus through synaptic relations.

Immunocytochemical demonstration of serotonin-immunoreactive cerebrospinal fluid-contacting neurons in the paraventricular organ of pigeons and domestic chickens

The paraventricular organs (PVO) of the pigeon and domestic chicken contain at least three types of serotonin-immunoreactive (serotonin-ir) CSF-contacting neurons. Type 1 neurons were predominant. They had two bipolar extending processes. The somata were mostly found in the pars hypendymalis. Type 2 neurons were characterized by thin and long apical processes. Their perikarya were found in the pars distalis of the PVO or the more lateral area of this organ. Type 3 neurons were considerably smaller and had round somata. They were mostly bipolar with thin and short dendritic processes and thin basal processes. A small number of this type was conspicuous along the cranial peripheral region of the PVO. In addition to the PVO area, aggregations of small, bipolar serotonin-ir CSF-contacting neurons were shown in the most caudal wall of the third ventricle of both species, distributed medially or paramedially. Immunoelectron microscopy revealed many dense granules in apical ventricular processes and perikarya. Synaptic connections were frequently observed on basal processes.

Enkephalinergic synaptic axon terminals on serotoninergic neurons in the dorsal raphe nucleus of the rat: an electron microscopic study by the double immunostaining method

Met-enkephalinergic synaptic axon terminals on serotoninergic neurons in the dorsal raphe nucleus of the rat were observed with double immunostaining at the electron microscopic level. Met-enkephalin-like immunoreactive axon terminals were found to make asymmetrical or symmetrical synaptic contacts with serotonin-like immunoreactive nerve cell bodies and dendrites. The findings suggest that opioid-containing neurons modulate serotoninergic neurons through synapses in the dorsal raphe nucleus.

Paraventricular organ of the lungfish Protopterus dolloi: morphology and projections of CSF-contacting neurons

The morphology and projections of neurons in the paraventricular organ (PVO) were studied by means of silver impregnation after intraocular application of cobaltous lysine in the lungfish Protopterus dolloi. Cobalt-labeled neurons were found exclusively in the PVO in the dorsal and infundibular hypothalamus. These bipolar neurons possess one CSF-contacting process that protrudes into the ventricular lumen with a club-shape ending and a thick, ramifying process directed into the hypothalamic neuropil; the ependymofugal processes form intra- and extrahypothalamic projections. Impregnated fibers from paraventricular neurons cross in infundibular and hypothalamic commissures, the commissure of the posterior tuberculum, the postoptic, the habenular, and the anterior commissures. Projections to the infundibulum and the median eminence are relatively sparse; no fibers are labeled in the pituitary gland. Ascending projections to the forebrain are extensive. Major targets include the dorsal hypothalamus, the periventricular preoptic nuclei, the habenula, the subhabenular region, the anterodorsal thalamus, and the medial telencephalic hemisphere (septum). Most ascending fibers follow the medial forebrain bundle; others course in the fasciculus retroflexus and terminate in rostral parts of the ipsilateral habenula. Descending fibers run caudally along the ventral floor of the brainstem. They terminate in the neuropil of the mesencephalic tegmentum, ventral tectum, isthmic region, ventral portions of the reticular formation throughout the rhombencephalon, and extend into the spinal cord. Intraocular application of cobaltous lysine results in selective impregnation of neurons in the PVO and their ascending and descending projections, presumably via uptake of tracer from vascular circulation. These projections do not represent retinofugal or retinopetal projections. We provide conclusive evidence for the existence of a PVO in Protopterus. On the basis of PVO location and acetylcholinesterase histochemistry, we propose subdivisions of the infundibular hypothalamus corresponding to those in amphibians. Ascending PVO projections appear to be particularly well developed in lungfish compared with other species and may be related to specialized endocrine mechanisms in this group of vertebrates.

Comparative neuroanatomy of the histaminergic system in the brain of the frog Xenopus laevis

The distribution of the histaminergic neuronal system in the brain of the clawed frog Xenopus laevis was mapped with an antiserum against carbodiimide-fixed histamine and compared to that in mammals. The histamine-immunoreactive cell bodies were located in a small area of the posterolateral hypothalamus, close to the dorsal infundibular nucleus, which contains catecholaminergic and serotonergic neurons. This area may be homologous to the tuberomammillary nucleus in mammals. A thick process extended from each cell between the ependymal cell layer and terminated in the ventricle lumen. The number of histaminergic cell bodies in adult Xenopus brain was relatively low, as compared with the mammalian brain. Preliminary analysis of adjacent sections stained with antisera against GABA or serotonin indicated that the histamine cells were not immunoreactive for these. The pathways and distribution of histaminergic fibers in Xenopus brain showed many similarities to mammals. The densest fiber networks were present in the medial basal forebrain, particularly in the medial amygdala and septum. A distinct cluster of fibers was concentrated around the cell bodies of nucleus accumbens. In most pallial areas, the density was moderate to low. In the primordial piriform cortex and the striatum, very few fibers were seen. In diencephalon, highest fiber densities were found in the anterior and ventral thalamus and posterior and lateral hypothalamus. In hindbrain, the density was highest in the medullary central gray, as in some mammals. The results suggest that the general pattern of the histaminergic system in vertebrate brain is conserved from amphibians to mammals.

Bulbospinal serotoninergic pathways in the frog Rana pipiens

We combined retrograde fluorescent tracing with rhodamine immunofluorescence to identify the origin of serotoninergic neurons with descending projections to the spinal cord of frogs. After injections of Fluoro-gold into the spinal cord, retrogradely labeled immunoreactive serotoninergic neurons were detected in the caudal part of the brainstem from the level of the obex through the level of the VIII nerve. These doubly labeled cells were distributed along the midline throughout the rostrocaudal extent of the dorsal portion of the raphe nuclear region. Doubly labeled neurons were more numerous in the rostral than in the caudal part of the raphe area. The fluorescent tracer 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) was then placed in and around the middle and rostral raphe nuclear region. Anterogradely labeled fibers could be traced bilaterally in the lateral portion of the dorsal funiculus and the lateral and ventral funiculi. These fibers were seen terminating in the dorsal and ventral horns, as well as in the intermediate grey matter. After placement of DiI in the caudal raphe area, labeled fibers were found only in the intermediate grey and ventral horn. These findings suggest that the organization of bulbospinal serotoninergic pathways in the frog is similar to that of mammals, and that an isolated amphibian spinal cord preparation could be a useful model for pharmacological and physiological studies of the action of serotonin (5HT) in the spinal cord.

Distribution of serotonin immunoreactivity in the forebrain and midbrain of the lizard Gekko gecko

The distribution of serotonin (5-hydroxytryptamine, 5-HT) in the forebrain and midbrain of the lizard Gekko gecko was studied by means of antibodies against serotonin. In the diencephalon, serotonin-immunoreactive (5-HTi) cell bodies were found in the hypothalamic periventricular organ and the ependymal wall of the infundibular recess. In the midbrain, 5-HTi cells were observed in the nucleus raphes superior and the lateral portion of the nucleus reticularis superior. In addition, 5-HTi cell bodies were found lateral to the ventral interpeduncular nucleus and around the ventral aspect of the medial longitudinal fasciculus. Serotonin-immunoreactive fibers and varicosities are present throughout the forebrain and the midbrain, but particularly in the nucleus accumbens, the septal area, the dorsal cortex, the dorsal thalamus, the lateral geniculate body, the ventromedial hypothalamic nucleus, the pretectal nucleus, and the basal optic nucleus. The medial habenular nucleus contains a dense 5-HTi plexus that shows a patchlike pattern. A laminar organization of 5-HTi fibers and varicosities is present in the midbrain tectum. When compared with data obtained in other vertebrates, the present study has confirmed that in the phylogenetic series fishes-amphibians-reptiles-birds-mammals there appears to be (1) a gradual decrease in the number of cerebrospinal-fluid-contacting serotoninergic cells in the hypothalamic periventricular layer and (2) a remarkable increase in number of serotoninergic cells in the midbrain tegmentum. As in mammals, a strong serotoninergic innervation of structures related to sensory, in particular visual, pathways could be recognized.

Evidence for presence of kynurenine in lung and brain of neonate hamsters

Using immunocytochemical techniques, we report here direct evidence of kynurenine (Kyn) presence and localization in the lung and brain. Kyn is a metabolite of tryptophan and 5-hydroxytryptophan, produced by indoleamine 2,3-dioxygenase (IDO). Whereas IDO has been quantitated in tissues from lung, brain, and other organs, Kyn has only been identified in brain (by HPLC), and its specific localization has not been determined. We reacted alternate serial paraffin sections with antisera raised in rabbits against a L-Kyn-albumin conjugate, and with anti-5HT (serotonin, 5-hydroxytryptamine), using the PAP method. Kyn-like immunoreactivity in the lung was specifically localized to cells of the bronchiolar epithelium resembling basal cells. Taller epithelial cells in the bronchi and dorsal trachea were likewise positive whereas neuroepithelial bodies were negative. Immunoreactivity in the brain was typically localized to cells localized in the ependyma of the walls of all ventricles, and to nerve fibers. The cellular Kyn-like reactivity was totally separate from that of anti-5HT, the latter uniquely staining argyrophil lung neuroendocrine cells and raphae neurons of the brain. Our findings suggest a route of tryptophan metabolism in the lung and brain alternate to the common pathway leading to 5-hydroxyindoleacetic acid via 5-HT. This route is of physiologic and pathologic significance as many metabolites are pharmacologically active.

Immunohistochemical demonstration of the serotonin neuron system in the central nervous system of the bullfrog, Rana catesbeiana

The distribution of serotonin immunoreactivity in the brain of the bullfrog (Rana catesbeiana) was studied, using the peroxidase-antiperoxidase (PAP) immunohistochemical method with serotonin antiserum. The somata of the serotonin neurons were mainly located in the raphe regions of the brain stem from the level of the caudal mesencephalon to that of the spinomedullary junction. A small number of serotonin neurons were also distributed as cerebrospinal-fluid contacting neurons in the preoptic recess organ (PRO), the paraventricular organ (PVO), and the nucleus infundibularis dorsalis (Nid). In the raphe region, these serotonin neurons formed nearly-continuous bilaterally-symmetrical cell columns along the midline of the brain stem, divided into lateral and medial groups. The medial group was further subdivided into rostral and caudal parts. Processes of the serotonin neurons were widely distributed in the central nervous system, forming dense networks in various regions. The greatest concentrations of these fibers were in the nucleus medialis septi, lateral portion of striatum, nucleus corporis geniculi, nucleus entopeduncularis, periventricular gray of ventral hypothalamus, optic tectum, nucleus isthmi, nucleus interpeduncularis, dorsal edge of medulla oblongata, and fasciculus solitarius.