Topographic representation of the sensory and motor roots of the vagus nerve in the medulla of goldfish, Carassius auratus

Innervation and osteoclast distribution in the inferior pharyngeal jaw of the cichlid Nile tilapia (Oreochromis niloticus)

In addition to an oral jaw, cichlids have a pharyngeal jaw, which is used for crushing and processing captured prey. The teeth and morphology of the pharyngeal jaw bones adapt to changes in prey in response to changes in the growing environment. This study aimed to explore the possible involvement of the peripheral nervous system in remodeling the cichlid pharyngeal jaw by examining the innervation of the inferior pharyngeal jaw in the Nile tilapia, Oreochromis niloticus. Vagal innervation was identified in the Nile tilapia inferior pharyngeal jaw. Double staining with tartrate-resistant acid phosphatase and immunostaining with the neuronal markers, protein gene product 9.5, and acetylated tubulin, revealed that osteoclasts, which play an important role in remodeling, were distributed in the vicinity of the nerves and were in apposition with the nerve terminals. This contact between peripheral nerves and osteoclasts suggests that the peripheral nervous system may play a role in remodeling the inferior pharyngeal jaw in cichlids.

Afferent and efferent connections of the nucleus posterior tuberis in the firemouth cichlid, Thorichthys meeki

The nucleus posterior tuberis (NPT) in teleost fishes, also called posterior tuberal nucleus, is situated in the posterior tuberculum of the diencephalon. It is fused across the midline and densely packed with small cells, but little is known about its connections. In this study, the afferent and efferent connections of the NPT were examined by means of tracer applications of the carbocyanine dye DiI in the firemouth cichlid, Thorichthys meeki. Retrogradely labeled cell bodies were found in the corpus mamillare and nucleus periventricularis of the inferior lobe; and anterogradely labeled terminal fibers were detected in the medial zone of the dorsal telencephalon, medial part of the nucleus lateralis tuberis, dorsal posterior thalamic nucleus, torus lateralis, medial part of the nucleus diffusus of the inferior lobe, and tectum opticum. All these connections show an ipsilateral tendency. The NPT is apparently a significant relay nucleus in the diencephalon of T. meeki, and possibly involved in a variety of feedback circuits. It seems also to be part of a tecto-hypothalamo-telencephalic pathway in cichlids.

Gustatory and general visceral centers and their connections in the brain of adult zebrafish: a carbocyanine dye tract-tracing study

The central connections of the gustatory/general visceral system of the adult zebrafish (Danio rerio) were examined by means of carbocyanine dye tracing. Main primary gustatory centers (facial and vagal lobes) received sensory projections from the facial and vagal nerves, respectively. The vagal nerve also projects to the commissural nucleus of Cajal, a general visceral sensory center. These primary centers mainly project on a prominent secondary gustatory and general visceral nucleus (SGN/V) located in the isthmic region. Secondary projections on the SGN/V were topographically organized, those of the facial lobe mainly ending medially to those of the vagal lobe, and those from the commissural nucleus ventrolaterally. Descending facial lobe projections to the medial funicular nucleus were also noted. Ascending fibers originating from the SGN/V mainly projected to the posterior thalamic nucleus and the lateral hypothalamus (lateral torus, lateral recess nucleus, hypothalamic inferior lobe diffuse nucleus) and an intermediate cell- and fiber-rich region termed here the tertiary gustatory nucleus proper, but not to a nucleus formerly considered as the zebrafish tertiary gustatory nucleus. The posterior thalamic nucleus, tertiary gustatory nucleus proper, and nucleus of the lateral recess gave rise to descending projections to the SGN/V and the vagal lobe. The connectivity between diencephalic gustatory centers and the telencephalon was also investigated. The present results showed that the gustatory connections of the adult zebrafish are rather similar to those reported in other cyprinids, excepting the tertiary gustatory nucleus. Similarities between the gustatory systems of zebrafish and other fishes are also discussed. J. Comp. Neurol. 525:333-362, 2017. © 2016 Wiley Periodicals, Inc.

Cryptic laminar and columnar organization in the dorsolateral pallium of a weakly electric fish

In the weakly electric gymnotiform fish, Apteronotus leptorhynchus, the dorsolateral pallium (DL) receives diencephalic inputs representing electrosensory input utilized for communication and navigation. Cell counts reveal that, similar to thalamocortical projections, many more cells are present in DL than in the diencephalic nucleus that provides it with sensory input. DL is implicated in learning and memory and considered homologous to medial and/or dorsal pallium. The gymnotiform DL has an apparently simple architecture with a random distribution of simple multipolar neurons. We used multiple neurotracer injections in order to study the microcircuitry of DL. Surprisingly, we demonstrated that the intrinsic connectivity of DL is highly organized. It consists of orthogonal laminar and vertical excitatory synaptic connections. The laminar synaptic connections are symmetric sparse, random, and drop off exponentially with distance; they parcellate DL into narrow (60 μm) overlapping cryptic layers. At distances greater than 100 μm, the laminar connections generate a strongly connected directed graph architecture within DL. The vertical connectivity suggests that DL is also organized into cryptic columns; these connections are highly asymmetric, with superficial DL cells preferentially projecting towards deeper cells. Our experimental analyses suggest that the overlapping cryptic columns have a width of 100 μm, in agreement with the minimal distance for strong connectivity. The architecture of DL and the expansive representation of its input, taken together with the strong expression of N-methyl-D-aspartate (NMDA) receptors by its cells, are consistent with theoretical ideas concerning the cortical computations of pattern separation and memory storage via bump attractors.

A hindbrain segmental scaffold specifying neuronal location in the adult goldfish, Carassius auratus

The vertebrate hindbrain develops as a series of well-defined neuroepithelial segments or rhombomeres. While rhombomeres are visible in all vertebrate embryos, generally there is not any visible segmental anatomy in the brains of adults. Teleost fish are exceptional in retaining a rhombomeric pattern of reticulospinal neurons through embryonic, larval, and adult periods. We use this feature to map more precisely the segmental imprint in the reticular and motor basal hindbrain of adult goldfish. Analysis of serial sections cut in three planes and computer reconstructions of retrogradely labeled reticulospinal neurons yielded a segmental framework compatible with previous reports and more amenable to correlation with surrounding neuronal features. Cranial nerve motoneurons and octavolateral efferent neurons were aligned to the reticulospinal scaffold by mapping neurons immunopositive for choline acetyltransferase or retrogradely labeled from cranial nerve roots. The mapping corresponded well with the known ontogeny of these neurons and helps confirm the segmental territories defined by reticulospinal anatomy. Because both the reticulospinal and the motoneuronal segmental patterns persist in the hindbrain of adult goldfish, we hypothesize that a permanent "hindbrain framework" may be a general property that is retained in adult vertebrates. The establishment of a relationship between individual segments and neuronal phenotypes provides a convenient method for future studies that combine form, physiology, and function in adult vertebrates.

Connections of the commissural nucleus of Cajal in the goldfish, with special reference to the topographic organization of ascending visceral sensory pathways

The primary general visceral nucleus of teleosts is called the commissural nucleus of Cajal (NCC). The NCC of goldfish has been divided into the medial (NCCm) and lateral (NCCl) subnuclei that receive inputs from subdiaphragmatic gastrointestinal tract and the posterior pharynx, respectively. Fiber connections of the NCC were examined by tract-tracing methods in the goldfish Carassius auratus. Tracer injections into the NCC suggested that the NCC projects directly not only to the secondary visceral sensory region in the rhombencephalic isthmus and other brain stem centers, but also to the forebrain, similar to the situations in mammals, birds, and the Nile tilapia. Although fiber connections of the NCCm and NCCl were basically similar, the NCCm was the more important source of ascending general visceral fibers to the forebrain. Topographic organization was recognized regarding projections to the isthmic secondary visceral sensory zone; input from the NCCm is represented in the secondary general visceral sensory nucleus, while input from the NCCl in the lateral edge of the secondary gustatory nucleus. Moreover, specific injections into different regions of the vagal lobe revealed that the dorsomedio-ventrolateral axis of the lobe is represented in the lateromedial axis of the secondary gustatory nucleus. These observations suggest fine topographic organization of ascending visceral sensory pathways to the isthmic secondary centers. It should also be noted that the reception of primary afferents from the posterior pharynx and projections to the secondary gustatory nucleus suggest that the NCCl may be regarded as a gustatory rather than a general visceral sensory structure.

Phox2b expression in the taste centers of fish

The homeodomain transcription factor Phox2b controls the formation of the sensory-motor reflex circuits of the viscera in vertebrates. Among Phox2b-dependent structures characterized in rodents is the nucleus of the solitary tract, the first relay for visceral sensory input, including taste. Here we show that Phox2b is expressed throughout the primary taste centers of two cyprinid fish, Danio rerio and Carassius auratus, i.e., in their vagal, glossopharyngeal, and facial lobes, providing the first molecular evidence for their homology with the nucleus of the solitary tract of mammals and suggesting that a single ancestral Phox2b-positive neuronal type evolved to give rise to both fish and mammalian structures. In zebrafish larvae, the distribution of Phox2b²⁺ neurons, combined with the expression pattern of Olig4 (a homologue of Olig3, determinant of the nucleus of the solitary tract in mice), reveals that the superficial position and sheet-like architecture of the viscerosensory column in cyprinid fish, ideally suited for the somatotopic representation of oropharyngeal and bodily surfaces, arise by radial migration from a dorsal progenitor domain, in contrast to the tangential migration observed in amniotes.

Cloning and expression of tachykinins and their association with kisspeptins in the brains of zebrafish

The tachykinins are a family of neuropeptides, including substance P (SP), neurokinin A (NKA), and neurokinin B (NKB), that are encoded by the tac1 (SP and NKA) or tac2/3 (NKB) genes. Tachykinins are widely distributed in the central nervous system and have roles as neurotransmitters and/or neuromodulators. Recent studies in mammals have demonstrated the coexpression of NKB and kisspeptin and their comodulatory roles over the control of reproduction. We have recently identified two kisspeptin-encoding genes, kiss1 and kiss2, in teleosts. However, such relationship between tachykinins and kisspeptins has not been demonstrated in non-mammalian species. To determine the involvement of tachykinins in the reproduction in teleosts, we identified tac1 and two tac2 (tac2a and tac2b) sequences in the zebrafish genome using in silico data mining. Zebrafish tac1 encodes SP and NKA, whereas the tac2 sequences encode NKB and an additional peptide homologous to NKB (NKB-related peptide). Digoxigenin in situ hybridization in the brain of zebrafish showed tac1 mRNA-containing cells in the olfactory bulb, telencephalon, preoptic region, hypothalamus, mesencephalon, and rhombencephalon. The zebrafish tac2a mRNA-containing cells were observed in the preoptic region, habenula, and hypothalamus, whereas the tac2b mRNA-containing cells were predominantly observed in the dorsal telencephalic area. Furthermore, we examined the coexpression of tachykinins and two kisspeptin genes in the brain of zebrafish. Dual fluorescent in situ hybridization showed no coexpression of tachykinins mRNA with kisspeptins mRNA in hypothalamic nuclei or the habenula. These results suggest the presence of independent pathways for kisspeptins and NKB neurons in the brain of zebrafish.

Evolution of gustatory reflex systems in the brainstems of fishes

The great number of species of teleosts permits highly specialized forms to evolve to occupy particular niches. This diversity allows for extreme variations in brain structure according to particular sensory or motor adaptations. In the case of the taste system, goldfish (Carassius auratus L., 1758) and some carps have evolved a specialized intraoral food-sorting apparatus along with corresponding specializations of gustatory centers in the brainstem. A comparison of circuitry within the complex vagal lobe of goldfish, and of the simpler gustatory lobes in catfish (Ictalurus punctatus Rafinesque, 1818) shows numerous similarities in organization and neurotransmitters. Double labeling studies using horseradish peroxidase and biotinylated dextran amine in catfish shows a direct projection from the vagal lobe to the motoneurons of nucleus ambiguous which innervate oropharyngeal musculature. Therefore, a three neuron reflex arc connects gustatory input to motor output. In the vagal lobe of goldfish, a similar three neuron arc can be identified: from primary gustatory afferent, to vagal lobe interneuron, thence to dendrites of the vagal motoneurons that innervate the pharyngeal muscles. Therefore, despite large differences in the gross appearance of the vagal gustatory systems in the brains of catfish and goldfish, the essential connectivity and circuitry is similar. This suggests that evolutionary change in the central nervous system largely proceeds by rearrangement and elaboration of existing systems, rather than by addition of new structures or circuits.

Central regulation of the pharyngeal and upper esophageal reflexes during swallowing in the Japanese eel

We investigated the regulation of the pharyngeal and upper esophageal reflexes during swallowing in eel. By retrograde tracing from the muscles, the motoneurons of the upper esophageal sphincter (UES) were located caudally within the mid-region of the glossopharyngeal-vagal motor complex (mGVC). In contrast, the motoneurons innervating the pharyngeal wall were localized medially within mGVC. Sensory pharyngeal fibers in the vagal nerve terminated in the caudal region of the viscerosensory column (cVSC). Using the isolated brain, we recorded 51 spontaneously active neurons within mGVC. These neurons could be divided into rhythmically (n = 8) and continuously (n = 43) firing units. The rhythmically firing neurons seemed to be restricted medially, whereas the continuously firing neurons were found caudally within mGVC. The rhythmically firing neurons were activated by the stimulation of the cVSC. In contrast, the stimulation of the cVSC inhibited firing of most, but not all the continuously firing neurons. The inhibitory effect was blocked by prazosin in 17 out of 38 neurons. Yohimbine also blocked the cVSC-induced inhibition in five of prazosin-sensitive neurons. We suggest that the neurons in cVSC inhibit the continuously firing motoneurons to relax the UES and stimulate the rhythmically firing neurons to constrict the pharynx simultaneously.

Group III metabotropic glutamate receptors (mGluRs) modulate transmission of gustatory inputs in the brain stem

Glutamate is the principal neurotransmitter at the primary sensory afferent synapse in the medulla for the taste system. At this synapse, glutamate activates N-methyl-D-aspartate (NMDA) and non-NMDA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and kainate) ionotropic receptors to effect a response in the second-order neurons. The current experiment is the first to examine the role of metabotropic glutamate receptors (mGluRs) in the transmission of taste information. In an in vitro slice preparation of the primary vagal gustatory nucleus in goldfish, primary gustatory afferent fibers were stimulated electrically, whereas evoked dendritic field potentials were recorded in the sensory layers. Recordings were made before, during, and after bath application of mGluR agonists for various mGluR groups and subtypes. Whereas L-AP4, a group III agonist, reduced the field potential, group I and group II agonists had no effect. Furthermore, the selective mGluR4 agonist ACPT-III and mGluR8 agonist PPG were effective at reducing the field potential, whereas agonists selective for mGluR6 and 7 were not. MAP4, a group III mGluR antagonist, attenuated frequency-dependent depression, indicating that endogenous glutamate binds to presynaptic mGluRs under normal conditions. Furthermore, polymerase chain reaction showed that mRNA for mGluR4 and 8 is expressed in the vagal ganglia, a prerequisite if those receptors are expressed presynaptically in the vagal lobe. Collectively, these experiments indicate that mGluR4 and 8 are presynaptic at the primary gustatory afferent synapse and that their activation inhibits glutamatergic release.

Evolutionary origins for social vocalization in a vertebrate hindbrain-spinal compartment

The macroevolutionary events leading to neural innovations for social communication, such as vocalization, are essentially unexplored. Many fish vocalize during female courtship and territorial defense, as do amphibians, birds, and mammals. Here, we map the neural circuitry for vocalization in larval fish and show that the vocal network develops in a segment-like region across the most caudal hindbrain and rostral spinal cord. Taxonomic analysis demonstrates a highly conserved pattern between fish and all major lineages of vocal tetrapods. We propose that the vocal basis for acoustic communication among vertebrates evolved from an ancestrally shared developmental compartment already present in the early fishes.

A primitive social circuit: vasotocin-substance P interactions modulate social behavior through a peripheral feedback mechanism in goldfish

At its core, the polyvagal theory proposes that peptides affect simple social behaviors through influences on hindbrain autonomic processes. To test this mechanism, we compared the effects of fore- and hindbrain infusions of vasotocin (VT) on social approach behavior in goldfish. VT infusions into the 4th ventricle, which ink infusions verified did not move rostrally to the forebrain, inhibited social approach at a lower dose than did infusions into the 3rd ventricle, which did diffuse to the hindbrain. Thus, VT actions in the hindbrain appear to modulate this simple social behavior. We then identified a population of substance P (SP)-immunoreactive cells in the hindbrain that are encapsulated by putative VT terminals, and determined that those cells project to the periphery. Injecting SP peripherally, as with infusing VT centrally, inhibited social approach, and peripheral injections of an SP antagonist, but not central infusions, abolished the behavioral effects of central VT infusions. We therefore propose that VT inhibits social approach by activating SP cells in the hindbrain, which then induce changes in body state that feed back to the brain. Central VT infusions did not inhibit feeding, suggesting that this VT mechanism selectively affects appetitive social responses. Because VT projections to the hindbrain are highly conserved in vertebrates, influences on peripheral feedback processes like the one we have described in goldfish may reflect how VT affected simple social behaviors in ancestral vertebrates and thus preadapted members of this peptide family to play increasingly complex roles in social and emotional regulation in modern animals.

Calcium-fluxing glutamate receptors associated with primary gustatory afferent terminals in goldfish (Carassius auratus)

Presynaptic ionotropic glutamate receptors modulate transmission at primary afferent synapses in several glutamatergic systems. To test whether primary gustatory afferent fibers express Ca(2+)-permeable AMPA/kainate receptors, we utilized kainate-stimulated uptake of Co(2+) along with immunocytochemistry for the Ca(2+)-binding proteins (CaBPs) calbindin and calretinin to investigate the primary gustatory afferents in goldfish (Carassius auratus). In goldfish, the primary gustatory nucleus (equivalent to the gustatory portion of the nucleus of the solitary tract) includes the vagal lobe, which is a large, laminated structure protruding dorsally from the medulla. Kainate-stimulated uptake of Co(2+) (a measure of Ca(2+)-fluxing glutamate receptors) shows punctate staining distributed in the distinct laminar pattern matching the layers of termination of the primary gustatory afferent fibers. In addition, CaBP immunocytochemistry, which correlates highly with expression of Ca(2+)-permeable AMPA/kainate receptors, shows a laminar pattern of distribution similar to that found with kainate-stimulated cobalt uptake. Nearly all neurons of the vagal gustatory ganglion show Co(2+) uptake and are immunopositive for CaBPs. Transection of the vagus nerve proximal to the ganglion results in loss of such punctate Co(2+) uptake and of punctate CaBP staining as soon as 4 days postlesion. These results are consonant with the presence of Ca(2+)-fluxing glutamate receptors on the presynaptic terminals of primary gustatory terminals, providing an avenue for modulation of primary gustatory input.

Fiber connections of the corpus glomerulosum pars rotunda, with special reference to efferent projection pattern to the inferior lobe in a percomorph teleost, tilapia (Oreochromis niloticus)

Fiber connections of the corpus glomerulosum pars rotunda (GR) in a teleost, tilapia Oreochromis niloticus, were studied by biotinylated dextran amine injections into the GR and inferior lobe. After tracer injections into the GR, major groups of labeled somata were found bilaterally in the cortical nucleus and ipsilaterally in the nucleus intermedius. Numerous labeled terminals were found ipsilaterally in the central nucleus, nucleus of lateral recess, and diffuse nucleus (NDLI) of the inferior lobe. Some other connections were also elucidated in the present study, although these were less abundant. Notably, efferent projections to the inferior lobe were not evenly distributed within each lobar nucleus. Labeled terminals were confined to the cell body zone of central nucleus and the outer cell-sparse layer of the nucleus of lateral recess. The rostrolateral portion of NDLI and ventrolateral portion of middle to caudal NDLI received few GR fibers, the rostromedial portion of NDLI a moderate density of fibers, and the rest of the nucleus numerous fibers. These different portions of the NDLI, to some extent, also differed in other afferent and efferent connections, suggesting regional specialization of the nucleus. Furthermore, restricted injections to the lobar nuclei suggest different efferent projections of the component cells of the GR: large and small cells. The large cells project only to the central nucleus, whereas the small cells project to the NDLI and nucleus of lateral recess. Therefore, the two types of GR cells appear to constitute parallel pathways from the pretectum to the inferior lobe.

Catecholamines inhibit neuronal activity in the glossopharyngeal-vagal motor complex of the Japanese eel: significance for controlling swallowing water

To clarify neuronal networks controlling swallowing water, inhibitory neurotransmitters were searched on the glossopharyngeal-vagal motor complex (GVC) of the medulla oblongata (MO), which is proposed as a motor nucleus controlling swallowing. Spontaneous firing (20-30 Hz) in the GVC was inhibited by adrenaline (AD), noradrenaline (NA) and dopamine (DA). The inhibitory effects of these catecholamines (CAs) were dose-dependent, and the effects of AD and NA were completely blocked by phenoxybenzamine or yohimbine, indicating that at least these two CAs act on the same receptor, presumably on alpha(2)-adrenoceptor. Even after blocking the alpha(2)-adrenoceptor with yohimbine, the inhibitory effect of DA still remained, indicating separate action of DA from AD or NA. Although DA receptor type was not determined in the present study, these results suggest existence of CA receptors in the GVC neurons. Almost 70% GVC neurons were inhibited by CAs. The CA-sensitive neurons were specifically restricted in the middle part of the GVC area. There were many tyrosine hydroxylase (TH)-immunoreactive somata and fibers in the eel MO. Among these TH-immunoreactive nuclei, the area postrema (AP) and the commissural nucleus of Cajal (NCC) appeared to project to the GVC morphologically. Significance of the catecholaminergic inhibition in the GVC activity is discussed in relation to controlling swallowing water.

Autoradiographic localization and binding study of benzodiazepines receptor sites in carp brain (Cyprinus carpio L.)

This study demonstrates, for the first time, by both autoradiography and binding assay that [3H]Ro 15-1788 binds to carp brain with a high degree of anatomical selectivity. Saturation binding of the radioligand was determined in seven anatomically defined regions and suggested the presence of one class of binding sites (Type I-lke). In general, there was a good correlation between the autoradiographic and the binding data. By far, the optic tectum and the vagal, facial, and glossopharyngeal lobes showed the majority of [3H]Ro 15-1788 binding sites. Low to negative concentration of binding sites was detected in the cerebellum. The location of [3H]Ro 15-1788 binding sites in particular brain regions, indicates that benzodiazepine receptors could be associated with pathways involved in the control of basic central functions as spatial learning acquisition and retention, and feeding behaviour.

Organization of the Sensory and Motor Nuclei of the Glossopharyngeal and Vagal Nerves in Lampreys

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the glossopharyngeal and vagal nerves in the lamprey, Lampetra japonica. Except for the ganglion cells and motoneurons, the distribution patterns of HRP-positive elements differed little between the two nerves. Afferent fibers mainly terminated in the ipsilateral cerebellar area, medial octavolateralis nucleus, and between the ventral octavolateralis nucleus and descending tract and nucleus of the trigeminal nerve (dV). In the cerebellar area, most of the labeled fibers were located in the molecular zone, but some penetrated into the granular zone. In the rostral part of the medial octavolateralis nucleus, labeled fibers coursed from the middle to the lateral area, and in the caudal part, they were localized in the dorsal area of the nucleus. In the area between the dV and ventral octavolateralis nucleus, labeled fibers coursed near the dorsal margin of the rostral part of the dV, and in the caudal part, they shifted dorsally. Ganglion cells and motoneurons of each nerve were also labeled.

Immunohistochemical distribution of neuropeptide Y in the mesencephalon and rhombencephalon of carp, Cyprinus carpio L. (Cyprinidae: Teleostei)

The localization of neuropeptide Y (NPY)-immunoreactive elements was investigated in the mesencephalon and rhombencephalon of carp, Cyprinus carpio, by using antisera raised against porcine NPY and the immunoperoxidase technique. Concurrently, to identify the distribution of NPY-immunoreactivity, we developed an atlas of the studied areas based on Nissl-stained sections. The NPY-immunoreactive (NPY-ir) elements were located in many zones of the mesencephalon and rhombencephalon. In the mesencephalon, positive fibers were the most abundant elements while neurons were scarce. The rhombencephalon rostral part was characterized by a low to moderate fiber density, distributed in the ventro-medial and ventro-lateral region. Differently the caudal part of the rhombencephalon exhibited several NPY-ir elements. In particular, a high density of immunoreactivity was located in the gustatory area at the level of the nucleus (n.) originis nervi glossopharyngei, in the n. nervi vagi, and in the vagal lobe. The latter can be considered a valid neuroanatomical model for the study of gustatory signal processing in vertebrates. Our results regarding the primary gustatory centers give neuroanatomical support to the view that NPY may act as a neurotransmitter and/or a neuromodulator in a wide neural network for feeding behavior control.

Experimental study of the connections of the gustatory system in the rainbow trout, Oncorhynchus mykiss

Salmonids are a group of teleosts with a nonspecialized gustatory system. With the aim of describing the gustatory connections in a member of this group, we carried out tract-tracing experiments using the lipophilic carbocyanine dye 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in fixed brains of the rainbow trout (Oncorhynchus mykiss). The neural tracer was applied to the primary viscerosensory column, secondary gustatory visceral nucleus (SGN), torus lateralis (TL), and tertiary gustatory nucleus (TGN), the dorsal part of the ventral area of the telencephalon (dorsal-Vv), and the medial area of the dorsal telencephalon (Dm). The primary viscerosensory column projects mainly to the SGN. DiI application to the SGN showed a bilateral and reciprocal connection with the TGN and a rostral portion of the nucleus of the lateral hypothalamic recess. The application of DiI in the dorsal-Vv and Dm at levels rostral to the anterior commissure led to labeling of a restricted group of diencephalic neurons in the TGN and sending dendrites to the TL. DiI application to the TL/TGN anterogradely labeled fibers that coursed in the medial forebrain bundle innervating the precommissural portion of the dorsal-Vv and Dm. Caudally, this type of application led to labeling of fibers in the viscerosensory column and perikarya in the SGN. Tract-tracing results showed direct projections from the diencephalic and rhombencephalic gustatory nuclei to the telencephalon. There was a direct and reciprocal connection between the SGN and the ventral telencephalon. The results showed that the gustatory connections of the trout are similar to those of teleosts, with highly specialized gustatory centers as in cyprinids and ictalurids, and to that observed in the percomorph tilapia, thus demonstrating a basic organization that is shared by most teleosts.

Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

Glutamate is a major neurotransmitter of chemoreceptor and baroreceptor afferent pathways in mammals and therefore plays a central role in the development of cardiorespiratory reflexes. In fish, the gills are the major sites of these receptors, and, consequently, the terminal field (sensory area) of their afferents (glossopharyngus and vagus) in the medulla must be an important site for the integration of chemoreceptor and baroreceptor signals. This investigation explored whether fish have glutamatergic mechanisms in the vagal sensory area (Xs) that could be involved in the generation of cardiorespiratory reflexes. The locations of the vagal sensory and motor (Xm) areas in the medulla were established by the orthograde and retrograde axonal transport of the neural tract tracer Fast Blue following its injection into the ganglion nodosum. Glutamate was then microinjected into identified sites within the Xs in an attempt to mimic chemoreceptor- and baroreceptor-induced reflexes commonly observed in fish. By necessity, the brain injections were performed on anaesthetised animals that were fixed by 'eye bars' in a recirculating water system. Blood pressure and heart rate were measured using an arterial cannula positioned in the afferent branchial artery of the 3rd gill arch, and ventilation was measured by impedance probes sutured onto the operculum. Unilateral injection of glutamate (40-100 nl, 10 mmol l(-1)) into the Xs caused marked cardiorespiratory changes. Injection (0.1-0.3 mm deep) in different rostrocaudal, medial-lateral positions induced a bradycardia, either increased or decreased blood pressure, ventilation frequency and amplitude and, sometimes, an initial apnea. Often these responses occurred simultaneously in various different combinations but, occasionally, they appeared singly, suggesting specific projections into the Xs for each cardiorespiratory variable and local determination of the modality of the response. Response patterns related to chemoreceptor reflex activation were predominantly located rostral of obex, whereas patterns related to baroreceptor reflex activation were more caudal, around obex. The glutamate-induced bradycardia was N-methyl-D-aspartate (NMDA) receptor dependent and atropine sensitive. Taken together, our data provide evidence that glutamate is a putative player in the central integration of chemoreceptor and baroreceptor information in fish.

Distribution and quantification of corticotropin-releasing hormone (CRH) in the brain of the teleost fish Oreochromis mossambicus (tilapia)

The recent characterization of the corticotropin-releasing hormone (CRH) prehormone of the fish tilapia (Oreochromis mossambicus) showed that more variation exists between vertebrate CRH amino acid sequences than recognized before. The present study investigates whether the deviating composition of tilapia CRH coincides with an atypical distribution of CRH in the brain. For this purpose we applied immunohistochemistry, as well as radioimmunoassay (RIA) quantification in brain slices. The results are plotted in a new atlas and reconstruction of the tilapia brain. The largest population of CRH-immunoreactive (ir) neurons is present in the lateral part of the ventral telencephalon (Vl). Approximately tenfold less CRH-ir neurons are observed in the preoptic and tuberal region. The CRH-ir neurons observed in the preoptic region are parvocellular and do not, or hardly, display arginine-vasotocin (AVT) immunoreactivity. CRH-ir neurons are also present in the glomerular layer of the olfactory bulb, in the periventricular layer of the optic tectum, and caudal to the glomerular nucleus. A very dense plexus of CRH-ir terminals is located in the most rostral part of the dorsal telencephalon. This region has not been described in other teleosts and is in the present study subdivided into the anterior part of the dorsal telencephalon (Da) and the anterior part of the laterodorsal telencephalon (Dla). High densities of CRH-ir terminals were observed in and around Vl, in the tuberal region, around the rostral part of the lateral recess, and in the caudal part of the vagal lobe. In the pituitary, CRH-ir terminals are concentrated in the neuro-intermediate lobe. Overall, the immunohistochemical and quantitative data correlated well, as the RIA CRH profile in serial 160-microm slices revealed four peaks, which corresponded with major ir-cell groups and terminal fields. Our results strongly suggest that the CRH-ir cells of Vl project to the rostro-dorsal telencephalon. Consequently, they may not be primarily involved in regulation of pituitary cell types but may subserve other functions. The presence of a CRH-containing Vl-Da/Dla projection seems to be restricted to the most modern group of teleosts, i.e., the Acanthopterygians. Further anatomic indications for non-pituitary-related functions of CRH are found in the vagal lobe and the optic tectum of tilapia. Although the low CRH content of the preoptic region reported here for tilapia may be typical for unstressed fish, the fact remains that remarkably few CRH-ir neurons are involved in regulating the pituitary. Overall, the CRH distribution in the brain of tilapia is more widespread than previously reported for other teleosts.

Distribution of cholecystokinin, calcitonin gene-related peptide, neuropeptide Y, and galanin in the primary gustatory nuclei of the goldfish

Cholecystokinin (CCK), neuropeptide Y (NPY), calcitonin gene-related peptide (CGRP), and galanin all are known to have central effects on food intake. Immunocytochemistry was used to examine the presence of these substances within the primary gustatory nuclei of the goldfish, including the vagal lobe, which is a large, laminated structure composed of discrete sensory, fiber, and motor layers. The vagal lobes receive primary afferent input from the gustatory portion of the vagus nerve and contain reflex circuitry involved in the ingestion or rejection of potential food items. Immunohistochemistry indicates a heavy concentration of CCK-, CGRP-, NPY-, and galanin-immunoreactive fibers in the capsular fiber layer as well as in deeper sensory layers of the vagal lobe. CGRP immunoreactivity throughout the sensory layers and capsular immunoreactivity for CCK are greatly reduced 1-2 weeks following vagus nerve transection, indicating that the majority of these fibers are primary sensory afferents. In contrast, NPY and galanin immunoreactivity in the capsular fiber layer and reactivity for CCK, NPY, and galanin in the deeper sensory and fiber layers are relatively unaffected by vagus transection. CCK-, NPY-, and galanin-immunoreactive fibers and puncta also were present in the motor layers, as were CGRP-immunoreactive motor somata. CCK-immunoreactive cell bodies are present in layer III and layer VII/VIII of the vagal lobe and in the superficial granular layer of the lateral subnucleus of the commissural nucleus of Cajal, which is caudally contiguous with the vagal lobe.

Distribution of thyrotropin-releasing hormone (TRH) immunoreactivity in the brain of the zebrafish (Danio rerio)

The distribution of thyrotropin-releasing hormone (TRH) in the brain of the adult zebrafish was studied with immunohistochemical techniques. In the telencephalon, abundant TRH-immunoreactive (TRHir) neurons were observed in the central, ventral, and supra- and postcommissural regions of the ventral telencephalic area. In the diencephalon, TRHir neurons were observed in the anterior parvocellular preoptic nucleus, the suprachiasmatic nucleus, the lateral hypothalamic nucleus, the rostral parts of the anterior tuberal nucleus and torus lateralis, and the posterior tuberal nucleus. Some TRHir neurons were also observed in the central posterior thalamic nucleus and in the habenula. The mesencephalon contained TRHir cells in the rostrodorsal tegmentum, the Edinger-Westphal nucleus, the torus semicircularis, and the nucleus of the lateral lemniscus. Further TRHir neurons were observed in the interpeduncular nucleus. In the rhombencephalon, TRHir cells were observed in the nucleus isthmi and the locus coeruleus, rostrally, and in the vagal lobe and vagal motor nucleus, caudally. In the forebrain, TRHir fibers were abundant in several regions, including the medial and caudodorsal parts of the dorsal telencephalic area, the ventral and commissural parts of the ventral telencephalic area, the preoptic area, the posterior tubercle, the anterior tuberal nucleus, and the posterior hypothalamic lobe. The dorsal thalamus exhibited moderate TRHir innervation. In the mesencephalon, the optic tectum received a rich TRHir innervation between the periventricular gray zone and the stratum griseum centrale. A conspicuous TRHir longitudinal tract traversed the tegmentum and extended to the rhombencephalon. The medial and lateral mesencephalic reticular areas and the interpeduncular nucleus were richly innervated by TRHir fibers. In the rhombencephalon, the secondary gustatory nucleus received abundant TRHir fibers. TRHir fibers moderately innervated the ventrolateral and ventromedial reticular area and richly innervated the vagal lobe and Cajal's commissural nucleus. Some TRHir fibers coursed in the lateral funiculus of the spinal cord. Some TRHir amacrine cells were observed in the retina. The wide distribution of TRHir neurons and fibers observed in the zebrafish brain suggests that TRH plays different roles. These results in the adult zebrafish reveal a number of differences with respect to the TRHir systems reported in other adult teleosts but were similar to those found during late developmental stages of trout (Díaz et al., 2001).

Monoaminergic and peptidergic axonal projections to the vagal motor cell column of a teleost, the filefish Stephanolepis cirrhifer

In an immunohistochemical study, the vagal motor nucleus of a teleost, the filefish Stephanolepis cirrhifer, could be divided into a rostral part and a caudal part, and the former into a dorsolateral group and a ventromedial group. The dorsolateral group consisted of neurons immunoreactive for calcitonin gene-related peptide, whereas the ventrolateral-caudal group was negative for calcitonin gene-related peptide. The latter group was retrogradely labeled after dextran amine injection to the visceral ramus of the vagus nerve, suggesting that it is a general visceral efferent column, made up of parasympathetic preganglionic neurons, whereas the dorsolateral rostral group is a special visceral efferent column. In the general visceral efferent column, a dense concentration of nerve fibers immunoreactive for serotonin, tyrosine hydroxylase, cholecystokinin-8, and substance P, and a small number of fibers immunoreactive for neuropeptide Y was observed. Perikarya in contact with varicose terminals immunoreactive for these substances were frequently seen. In contrast, in the special visceral efferent column, only a moderate concentration of neuropeptide Y-immunoreactive nerve fibers and a sparse distribution of fibers immunoreactive for tyrosine hydroxylase were observed. Perikarya in contact with varicose terminals immunoreactive for these substances were rare. These results suggest that the vagal parasympathetic preganglionic neurons might receive multiple inputs of monoaminergic and peptidergic fibers involved in the regulation of the visceral organs. On the other hand, monoaminergic and peptidergic afferent fibers might be of much less significance in the activity of the special visceral efferent component of the vagus nerve.

Comparative anatomy of the histaminergic and other aminergic systems in zebrafish (Danio rerio)

The histaminergic system and its relationships to the other aminergic transmitter systems in the brain of the zebrafish were studied by using confocal microscopy and immunohistochemistry on brain whole-mounts and sections. All monoaminergic systems displayed extensive, widespread fiber systems that innervated all major brain areas, often in a complementary manner. The ventrocaudal hypothalamus contained all monoamine neurons except noradrenaline cells. Histamine (HA), tyrosine hydroxylase (TH), and serotonin (5-HT) -containing neurons were all found around the posterior recess (PR) of the caudal hypothalamus. TH- and 5-HT-containing neurons were found in the periventricular cell layer of PR, whereas the HA-containing neurons were in the surrounding cell layer as a distinct boundary. Histaminergic neurons, which send widespread ascending and descending fibers, were all confined to the ventrocaudal hypothalamus. Histaminergic neurons were medium in size (approximately 12 microm) with varicose ascending and descending ipsilateral and contralateral fiber projections. Histamine was stored in vesicles in two types of neurons and fibers. A close relationship between HA fibers and serotonergic raphe neurons and noradrenergic locus coeruleus neurons was evident. Putative synaptic contacts were occasionally detected between HA and TH or 5-HT neurons. These results indicate that reciprocal contacts between monoaminergic systems are abundant and complex. The results also provide evidence of homologies to mammalian systems and allow identification of several previously uncharacterized systems in zebrafish mutants.

Kainate-activated cobalt uptake in the primary gustatory nucleus in goldfish: visualization of the morphology and distribution of cells expressing AMPA/kainate receptors in the vagal lobe

Gustatory afferent fibers of the vagus nerve that innervate taste buds of the oropharynx of the goldfish, Carassius auratus, project to the vagal lobe, which is a laminated gustatory nucleus in the dorsal medulla. As in the mammalian gustatory system, responses by second-order cells in the goldfish medulla are mediated by N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic glutamate receptors. We utilized a cobalt uptake technique to label vagal lobe neurons that possess cobalt-permeable ionotropic glutamate receptors. Vagal lobe slices were bathed in kainate (40 microM) or glutamate (0.5 or 1 mM) in the presence of CoCl(2), which can pass into cells through the ligand-gated cation channels of non-NMDA receptors made up of certain subunit combinations. Cobalt-filled cells and dendrites were observed in slices that were activated by kainate or glutamate, but not in control slices that were bathed in CoCl(2) alone, nor in slices that were bathed with the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (10 microM) in addition to an agonist. Likewise, simple depolarization of the cells with KCl failed to induce cobalt loading. Cobalt-filled round unipolar cells, elongate or globular bipolar cells, and multipolar cells with elongate or polygonal perikarya were distributed throughout the cell layers in the sensory zone of the vagal lobe. Numerous labeled neurons had dendrites spanning layers IV and VI, the two principal layers of primary afferent input. Apical and basal dendrites often extended radially through neighboring laminae, but many cells also extended dendrites tangential to the lamination of the sensory zone. In the motor layer, cell bodies and proximal dendrites of small, multipolar neurons, and large motoneurons were regularly loaded with cobalt.

Characterization and distribution of somatostatin binding sites in goldfish brain

Somatostatin (SRIF) binding sites were characterized in goldfish brain. Binding of (125)I-[Tyr(11)]-SRIF-14 to a brain membrane preparation was found to be saturable, reversible, and time-, temperature-, and pH-dependent. Binding was also displaceable by different forms of SRIF. Under optimal conditions (22 degrees C, pH 7.2), the equilibrium binding of (125)I-[Tyr(11)]-SRIF-14 to goldfish brain membranes was achieved after 60 min incubation. Analysis of saturable equilibrium binding revealed a one-site model fit with K(a) of 1.3 nM. SRIF-14, mammalian SRIF-28, and salmon SRIF-25 displaced (125)I-[Tyr(11)]-SRIF-14 binding with similar affinity, whereas other neuropeptides, e.g., substance P, were unable to displace (125)I-[Tyr(11)]-SRIF-14. Autoradiography studies demonstrated that (125)I-[Tyr(11)]-SRIF-14 binding sites are found throughout the goldfish brain. A high density of (125)I-[Tyr(11)]-SRIF-14 binding sites was found in the forebrain, including the nucleus preopticus, nucleus preopticus periventricularis, nucleus anterioris periventricularis, nucleus lateralis tuberis, nucleus dorsomedialis thalami, nucleus dorsolateralis thalami, nucleus ventromedialis thalami, and nucleus diffusus lobi inferioris. In midbrain, (125)I-[Tyr(11)]-SRIF-14 binding sites were found in the optic tectum. The facial and vagal lobes and the mesencephalic-cerebellar tract were found to have a high density of binding sites. This study provides the first characterization and distribution of specific binding sites for SRIF in a fish brain.

Development of immunoreactivity to neuropeptide Y in the brain of brown trout (Salmo trutta fario)

The development of neuropeptide Y-immunoreactive (NPY-ir) neurons in the brain of the brown trout, Salmo trutta fario, was studied by using the streptavidin-biotin immunohistochemical method. Almost all NPY-ir neurons found in the brain of adults already appeared in embryonic stages. The earliest NPY-ir neurons were observed in the laminar nucleus, the locus coeruleus, and the vagal region of 9-mm-long embryos. In the lateral area of the ventral telencephalon, habenula, hypothalamus, optic tectum, and saccus vasculosus, NPY-ir cells appeared shortly after (embryos 12-14 mm in length). The finding of NPY-ir cells in the saccus vasculosus and the vagal region expand the NPY-ir structures known in teleosts. Among the regions of the trout brain most richly innervated by NPY-ir fibers are the hypothalamus, the isthmus, and the complex of the nucleus of the solitary tract/area postrema, suggesting a correlation of NPY with visceral functions. Two patterns of development of NPY-ir populations were observed: Some populations showed a lifetime increase in cell number, whereas, in other populations, cell number was established early in development or even diminished in adulthood. These developmental patterns were compared with those found in other studies of teleosts and with those found in other vertebrates. J. Comp. Neurol. 414:13-32, 1999.

Some forebrain connections of the gustatory system in the goldfish Carassius auratus visualized by separate DiI application to the hypothalamic inferior lobe and the torus lateralis

The neuroanatomical connections of the diencephalic torus lateralis and inferior lobe of the goldfish (Carassius auratus) were studied by retrograde and anterograde labeling with the carbocyanine dye DiI. Both structures have afferents originating in the central zone of the dorsal telencephalic area as well as in the supracommissural nucleus of the ventral telencephalic area, and in the secondary gustatory, tertiary gustatory, and posterior thalamic nuclei. Both structures investigated have efferents to the tertiary gustatory and posterior thalamic nuclei, as well as to the dorsal hypothalamus (dorsal hypothalamic neuropil) and superior reticular formation. The torus lateralis receives additional afferents from the secondary general visceral nucleus and, sparsely, from the dorsal tegmental nucleus. The inferior lobe receives additional afferents from the medial zone of the dorsal telencephalic area, as well as from the suprachiasmatic, posterior pretectal, central posterior thalamic, caudal preglomerular, two tegmental nuclei (T1 and T2), corpus mamillare, and, sparsely, from the cerebellar valvula. The inferior lobe has additional efferents to the dorsal and ventral thalamus and subglomerular nucleus. The lateral torus and inferior lobe are also mutually interconnected. The lateral torus and inferior lobe map topographically onto the vagal-related (intraoral) or onto the facial-related (extraoral) portions, respectively, of both the secondary and tertiary gustatory nuclei. Because the posterior thalamic nucleus is reciprocally connected with the lateral torus and inferior lobe and is further known to project in turn to the area doralis telencephali, it likely represents a quaternary gustatory projection nucleus to the telencephalon in cyprinids. Whereas the lateral torus seems to be exclusively involved with gustatory and general visceral systems, the inferior lobe has inputs from additional sensory (e.g., octavolateralis, visual) systems, and, thus, likely represents a multisensory integration center.

Histochemical and immunocytochemical localization of nitric oxide synthase in the central nervous system of the goldfish, carassius auratus

The distribution of the neuronal type of nitric oxide synthase in the goldfish brain and spinal cord was investigated via NADPH-diaphorase histochemistry and immunocytochemistry using an antiserum raised against the purified mammalian enzyme. Many structures, including magnocellular neurosecretory cells, motoneurons, mesencephalic trigeminal neurons, and radial glial fibers, were stained by the NADPH-diaphorase reaction but were not immunoreactive. This nonspecific NADPH-diaphorase activity was strongly reduced after preincubation of the sections. Therefore, when sections were first reacted for immunofluorescence and, thereafter, stained for NADPH-diaphorase, a corresponding staining pattern was obtained that allowed the reliable localization of neuronal nitric oxide synthase based on both complementary staining methods. In the telencephalon, positive neurons were concentrated in the ventral and posterior parts of the area ventralis. Many intensely stained neurons were present in various diencephalic nuclei, including the nucleus centralis posterior and the ventromedial nucleus of the thalamus, the nucleus tori lateralis, the nucleus recessus lateralis, the nucleus tuberis posterior, and the central nucleus of the inferior lobe. In the midbrain, neurons containing nitric oxide synthase were located in the periventricular zone of the optic tectum, the nucleus vermiformis, and the nucleus reticularis mesencephali. Specific staining in the cerebellum was concentrated in Golgi cells. In the hindbrain, nitroxergic neurons were numerous in all four sensory nuclei of the trigeminus, in the facial lobe, the superior olive, the inferior reticular formation, and the medial general visceral nucleus of the vagus. The dorsal horn of the spinal cord was enriched with positive neurons. A few strongly stained cells were also present in the ventral horn. In conclusion, neurons capable of synthesizing nitric oxide occur throughout the teleost central nervous system. The presence of nitric oxide synthase in projection areas of most afferent nerves suggests a widespread involvement of nitric oxide in sensory information processing. The distribution of nitric oxide synthase-containing neurons in certain areas, e.g., the tectum opticum and the spinal cord, indicates an evolutionarily conserved pattern. Similar to the case in other vertebrates, there appears to be no comprehensive overlap between the distribution of nitric oxide synthase and that of any other chemically characterized neuronal population described thus far. However, strongly positive cell groups in the mesencephalic reticular formation suggest the idea of an evolutionarily conserved mesopontine cholinergic system coexpressing nitric oxide synthase.

The central connections of the vagus nerve in the ferret

The vagus nerve mediates emesis due to gastric irritation. The central representation of the vagus in the ferret was studied to establish how the nerve is connected to areas important in the regulation of emesis. In a series of 10 ferrets, WGA-HRP injections (10 microliters) were made into the nodose ganglion. After 24-48 h, animals were reanesthetized and perfused transcardially. A block extending from the pons to upper cervical spinal cord was cut at 50 microns and sections reacted. Nodose ganglion injections of WGA-HRP produced labeling of vagal preterminal segments in the ipsilateral dorsal vagal complex including all subnuclei of the solitary complex where the medial and subgelatinous subnuclei received the densest input, the area postrema (AP), which contained a modest amount of terminal label, and the dorsal motor nucleus of the vagus (DMX). Contralateral terminal label, quantitatively much less, was similarly distributed except that within the solitary complex it was limited to the medial and subgelatinous subnuclei. Retrogradely labeled cells formed ipsilateral dorsomedial and ventrolateral columns, corresponding, respectively, to the DMX and the nucleus ambiguus (including retrofacial and retroambiguus).

Central projections and motor nuclei of the facial, glossopharyngeal, and vagus nerves in the mormyrid fish Gnathonemus petersii

Most of the information about the anatomy of the fish's cranial nerves was collected in the first two decades of this century. Experimental analysis of the VIIth, IXth, and Xth cranial nerves by modern tract tracing techniques started about 20 years ago. Several species have been investigated to date, including one species of Agnatha (Myxinoidea), two species of elasmobranchs, and species of some orders of Teleostei like Cyprinidae, Siluriformes, Perciformes, and Gadidae. The sensory and motor nuclei of the VIIth, IXth, and Xth cranial nerves of Gnathonemus petersii were studied by anterograde and retrograde axoplasmatic transport of horseradish peroxidase and cobaltous lysine complex. The sensory nuclei form a continuous column of cells in the brain stem extending caudal to the obex. The rostral one-fourth of this column is occupied by the overlapping terminals of the VIIth and IXth nerves. The vagus nerve has 5 roots. The first 4 of these innervate the gills and the fifth supplies viscera. Afferents from the gills terminate ipsilaterally rostral to the obex in topographic order and their terminal fields overlap. Viscerosensory fibers terminate ipsilaterally in the obex region and bilaterally in the commissural nucleus of Cajal. The facial motor nucleus is located rostral to the sensory nucleus. Facial motoneurons have pear-shaped and multipolar perikarya. Their axons form a rostrally directed knee before leaving the brain. The motoneurons of the IXth and Xth nerves have a common cell column. The vagal motoneurons form a periventricular, a medial, and an intermediate cell group rostral to the obex. In the obex region and also caudal to it, a lateral and a caudal group can be distinguished. Vagal motoneurons show a topographic arrangement that is similar to that of the sensory vagal projections. The majority of motoneurons have pear-shaped perikary and ventrolaterally oriented dendrites. In the caudal nucleus the dendrites extend dorsally and overlap the terminals of sensory fibers. The axons form a dorsolaterally directed arch before joining the sensory roots. Since G. petersii uses its electrosensory system primarily for detection of food, its gustatory system is less developed than in other fishes, which possess a large number of taste buds.

Ascending general visceral pathways within the brainstems of two teleost fishes: Ictalurus punctatus and Carassius auratus

The primary general visceral nucleus in goldfish (Carassius auratus) and catfish (Ictalurus punctatus) is located at the ventroposterior boundary of the vagal gustatory lobe and receives coelomic visceral, but not gustatory inputs. The neuronal tracer horseradish peroxidase (HRP) was employed to visualize sources of input to and ascending projections from the primary general visceral nucleus in these species. In addition, immunocytochemical techniques were utilized to define the cytological divisions within the pontine gustatory-visceral complex. The pontine secondary visceral nuclei in both catfish and goldfish contains numerous somata and fibers immunoreactive for calcitonin gene-related peptide (CGRP). In contrast, the secondary gustatory nuclei are devoid of fibers and cells immunoreactive for CGRP. In both the goldfish and the channel catfish, the primary general visceral nucleus receives input from the vagal gustatory lobe, as well as the medullary reticular formation. In the channel catfish, the primary general visceral nucleus projects bilaterally to the secondary visceral nucleus, which lies rostrolateral to the secondary gustatory nucleus in the dorsal pons. Fibers cross the midline via the rostral part of the isthmic commissure. Injection of HRP into the primary general visceral nucleus of a goldfish labels ascending fibers that project to a secondary visceral nucleus situated ventral, lateral, and rostral to the secondary gustatory complex. In general, the results indicate that general visceral systems ascend in parallel to gustatory systems within the brainstem, and that general visceral but not gustatory nuclei are immunoreactive for the peptide CGRP.

Central location of the motoneurons that supply the cucullaris (trapezius) of the clearnose skate, Raja eglanteria

A complex of three muscles (one lateral, one intermediate and one medial in position) in the clearnose skate, Raja eglanteria, is believed to be wholly, or in part, homologous to the cucullaris (trapezius). The retrograde transport of horseradish peroxidase was used to discover the central location of the motoneurons that supply each of these muscles. Motoneurons that project to the lateral muscle occupy the caudal part of the ventral nucleus of X. This nucleus is situated ventrolateral to the dorsal vagal motor column at caudal medullary levels, and lateral to the main ventral motor column of the rostral spinal cord. The axons of these motoneurons exit the medulla within the caudal vagal rootlets and course peripherally within the intestinal (visceral) ramus of the vagus nerve. Motoneurons that innervate the intermediate and medial muscles are located along the ventral border of the ventral column of gray at spinal cord segments 10-15. Their axons course peripherally within the ventral roots of spinal nerves. The caudal ventral nucleus of X, the nerve that supplies the lateral muscle, and the lateral muscle are likely homologues of the accessory nucleus, accessory nerve, and cucullaris (trapezius), respectively, among other fishes and tetrapods. Intermediate and medial muscles, based on the central location of motoneurons that supply them, are part of the longitudinal epaxial musculature and are not part of a trapezius complex.

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.

Functional organization of vagal reflex systems in the brain stem of the goldfish, Carassius auratus

The coordination of secretory and motor responses to food within the alimentary canal requires well organized brain stem reflex systems. In the goldfish, Carassius auratus, three vagal reflex systems control three phases of ingestion and digestion. The orobranchial system sorts food from substrate, the pharyngeal chewing organ prepares items deemed to be food for digestion and absorption, and the abdominal system regulates the digestion of food. Each system is represented in the central nervous system by separate sensory and motor nuclei. The aim of the present study was to determine whether the interrelationships among the vagal sensory and motor nuclei reflect the peripheral organization. The sensory nucleus of each vagal system was injected with the neuronal tracer horseradish peroxidase (HRP), in separate cases. HRP injections into the vagal lobe sensory layers (orobranchial system) labeled fibers projecting topographically to the vagal lobe motor layer, but not at all to the pharyngeal or abdominal motor nuclei. Similarly, injections of HRP into the pharyngeal and abdominal sensory nuclei selectively labeled nerve fibers projecting to the pharyngeal and abdominal motor nuclei, respectively. All injections resulted in labeled fibers and/or cells in the lateral reticular formation, and in fibers ascending in the secondary gustatory-visceral tract. Gustatory information from the pharynx is apparently processed in the same brain stem system as pharyngeal general visceral information, suggesting that functional or regional characteristics of visceral sensory information may be more important for brain stem processing than the traditional "special" (gustatory) versus "general" visceral dichotomy. These results indicate that anatomically and functionally separate reflex systems exist within the goldfish vagal visceral nuclei.

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.

Distributions and colocalization of neuropeptide Y and somatostatin in the goldfish brain

The distributions of single- and double-labelled neuropeptide Y- (NPY-) and somatostatin-immunoreactive (SOM-IR) perikarya and processes were determined in the goldfish brain using immunoperoxidase and immunofluorescence techniques, respectively. In double-labelled material, it was evident that although these two peptides showed markedly similar distributions, they were colocalized in very few instances. A high degree of colocalization of NPY and SOM was noted in the neurons of the ventrolateral telencephalon (VI), the entopenduncular nucleus (NE) and, to a lesser extent, in the dorsocentral nucleus of the telencephalon (Dc). In Vl and NE, neurons showing NPY-IR displayed SOM-IR and vice versa. The only other instance of colocalization was that noted in the brainstem, where SOM and NPY were colocalized in the large cell bodies of the medial column of the vagal motor complex. Single-labelled SOM- and NPY-IR neurons shared a very similar distribution in various nuclei in the diencephalon and in the optic tectum. Colocalization was also noted within fibers throughout many nuclei of the telencephalon and within fibers innervating the swim bladder, one of the peripheral organs to which neurons of the medial column of the vagal motor complex project. Processes in the torus semicircularis and vagal lobe showed single-labelled immunoreactivity for both SOM and NPY in distinct laminar patterns. Large single-labelled SOM-IR terminals appeared to form pericellular baskets in the eminentia granularis of the cerebellum. Single-labelled NPY- or SOM-IR fibers were also found in the secondary gustatory nucleus and tract, the facial lobe, descending trigeminal tract, reticular formation and spinal cord. As in mammalian species, select groups of neurons in teleosts colocalize the neuropeptides SOM and NPY.

Substance P-like immunoreactivity in the brain of the gymnotiform fish Apteronotus leptorhynchus: presence of sex differences

The distribution of substance P-like immunoreactivity (SPli) was charted in the brain of the gymnotiform fish Apteronotus leptorhynchus, and correlated with the circuitry underlying intraspecific electrocommunication. Cell bodies were found predominantly in the lateral hypothalamus and in certain paraventricular organs: nucleus preopticus periventricularis, anterior subdivision; anterior hypothalamus; nucleus posterioris periventricularis; nucleus recessus lateralis, medial subdivision 2; nucleus recessus posterioris and nucleus recessus lateralis, lateral subdivision. Cell bodies were also found in the rostral olfactory nucleus, ventral telencephalon (ventral and central subdivisions), the habenula, the vagal sensory and motor nuclei and in the subtrigeminal nucleus. The distribution of SPli fibers was similar in some respects to that reported for other vertebrates. SPli was found in the rhombencephalon associated with vagal afferent fibers and in the funicular nucleus (possibly related to nociception). In the diencephalon and midbrain SPli fibers were found in the habenular-interpeduncular tract, in the hypothalamus and pituitary. SPli fibers were also found in preoptic and forebrain areas. The most striking result was the sexually dimorphic SPli innervation of certain hypothalamic and septal nuclei, and of the prepacemaker nucleus (PPn), a diencephalic cell group which controls communication ('chirping') in gymnotiforms. The PPn and septal/hypothalamic nuclei were densely innervated by SPli in males but devoid of SPli in females.

Comparative distribution of neuropeptide-immunoreactive systems in the brain of the green molly, Poecilia latipinna

The comparative distribution of peptidergic neural systems in the brain of the euryhaline, viviparous teleost Poecilia latipinna (green molly) was examined by immunohistochemistry. Topographically distinct, but often overlapping, systems of neurons and fibres displaying immunoreactivity (ir) related to a range of neuropeptides were found in most brain areas. Neurosecretory and hypophysiotrophic hormones were localized to specific groups of neurons mostly within the preoptic and tuberal hypothalamus, giving fibre projections to the neurohypophysis, ventral telencephalon, thalamus, and brain stem. Separate vasotocin (AVT)-ir and isotocin (IST)-ir cells were located in the nucleus preopticus (nPO), but many AVT-ir nPO neurons also displayed growth hormone-releasing factor (GRF)-like-ir, and in some animals corticotrophin-releasing factor (CRF)-like-ir. The main group of CRF-ir neurons was located in the nucleus recessus anterioris, where coexistence with galanin (GAL) was observed in some cells. Enkephalin (ENK)-like-ir was occasionally present in a few IST-ir cells of the nPO and was also found in small neurons in the posterior tuberal hypothalamus and in a cluster of large cells in the dorsal midbrain tegmentum. Thyrotrophin-releasing hormone (TRH)-ir cells were found near the rostromedial tip of the nucleus recessus lateralis. Gonadotrophin-releasing hormone (GnRH)-ir cells were present in the nucleus olfactoretinalis, ventral telencephalon, preoptic area, and dorsal midbrain tegmentum. Molluscan cardioexcitatory peptide (FMRF-amide)-ir was colocalized with GnRH-ir in the ganglion cells and central projections of the nervus terminalis. Melanin-concentrating hormone (MCH)-ir neurons were restricted to the tuberal hypothalamus, mostly within the nucleus lateralis tuberis pars lateralis, and somatostatin (SRIF)-ir neurons were numerous throughout the periventricular areas of the diencephalon. A further group of SRIF-ir neurons extending from the ventral telencephalon into the dorsal telencephalon pars centralis also contained neuropeptide Y (NPY)-, peptide YY (PYY)-, and NPY flanking peptide (PSW)-like-ir. These immunoreactivities were, however, also observed in non-SRIF-ir cells and fibres, particularly in the mesencephalon. Calcitonin gene-related peptide (CGRP)-like-ir had a characteristic distribution in cells grouped in the isthmal region and fibre tracts running forward into the hypothalamus, most strikingly into the inferior lobes. Antisera to cholecystokinin (CCK) and neurokinin A (NK) or substance P (SP) stained very extensive, separate systems throughout the brain, with cells most consistently seen in the ventral telencephalon and periventricular hypothalamus. Broadly similar, but much more restricted, distributions of cells and fibres were seen with antisera to neurotensin (NT) and vasoactive intestinal peptide (VIP).(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of dopamine immunoreactivity in the brain of the mormyrid teleost Gnathonemus petersii

The distribution of dopamine-containing cell bodies and fibers was studied with aid of specific antibodies against dopamine in the highly developed brain of the weakly electric fish Gnathonemus petersii. In the telencephalon, dopamine-containing cell bodies were observed in a small area, i.e., area ventralis pars dorsalis and supracommissuralis. In the diencephalon, moderate numbers of dispersed dopamine-immunoreactive cells were present in the preoptic region, while large numbers of dopamine-containing neurons occurred in the hypothalamic paraventricular organ and neighbouring regions. The paraventricular organ, located around small (anterior, intermediate, and posterior) recesses contained many dopamine-immunoreactive cerebrospinal fluid-(CSF)-contacting neurons. Dopamine-containing cells were also observed in a magnocellular hypothalamic cell group, in the nucleus of the lateral recess, and in the nucleus posterior tuberis. In the mesencephalon only a few dopamine-containing cells were observed in a dorsal tegmental (possibly pretectal) area, whereas in ventral mesencephalic regions dopamine-containing cells were lacking. More caudally, dopamine-containing cells were observed in the presumed locus coeruleus, in the caudal region of the reticular formation, and in the presumed area postrema. Dopamine-immunoreactive fiber density was very high in the medioventral hypothalamus and in the preoptic region, where a dense subependymal plexus was observed along the preoptic recess. Such a plexus was also present in the caudal rhombencephalon, where it probably arises from the area postrema. Moderate numbers of dopamine-immunoreactive fibers were present in medioventral parts of the brain along its total rostrocaudal extent as well as in several subnuclei of the torus semicircularis, in the tectum mesencephali, and in the medial part of the dorsal telencephalic area. Other parts of the dorsal telencephalic area, as well as the large cerebellum and the electrosensory lateral line lobe of Gnathonemus, did not contain detectable amounts of dopamine. In spite of the high differentiation of the brain of Gnathonemus, the distribution of catecholamines as visualized with dopamine immunohistochemistry appears to be basically similar to that described in other teleostean and actinopterygian fishes on the basis of formaldehyde-induced fluorescence or tyrosine hydroxylase immunohistochemistry.(ABSTRACT TRUNCATED AT 400 WORDS)