Gustatory Nucleus Research Articles

A single-cell resolution gene expression atlas of the larval zebrafish brain.

The advent of multimodal brain atlases promises to accelerate progress in neuroscience by allowing in silico queries of neuron morphology, connectivity, and gene expression. We used multiplexed fluorescent in situ RNA hybridization chain reaction (HCR) technology to generate expression maps across the larval zebrafish brain for a growing set of marker genes. The data were registered to the Max Planck Zebrafish Brain (mapzebrain) atlas, thus allowing covisualization of gene expression, single-neuron tracings, and expertly curated anatomical segmentations. Using post hoc HCR labeling of the immediate early gene cfos, we mapped responses to prey stimuli and food ingestion across the brain of freely swimming larvae. This unbiased approach revealed, in addition to previously described visual and motor areas, a cluster of neurons in the secondary gustatory nucleus, which express the marker calb2a, as well as a specific neuropeptide Y receptor, and project to the hypothalamus. This discovery exemplifies the power of this new atlas resource for zebrafish neurobiology.

Spexin in the European sea bass, Dicentrarchus labrax: Characterization, brain distribution, and interaction with Gnrh and Gnih neurons.

Spexin (Spx) is a recently characterized neuropeptide implicated in multiple physiological processes in vertebrates, including reproduction, food intake, and regulation of anxiety and stress. Two orthologs (Spx1 and Spx2) are present in some nonmammalian vertebrates, including teleosts. However, information on the distribution of Spx in the brain and its interactions with other neuroendocrine systems in fish is still scarce. In this work, we cloned and sequenced the sea bass (Dicentrarchus labrax) Spx1, which included a 27 aa signal peptide and a mature peptide of 14 aa that is C-terminal amidated. spx1 transcripts were higher in the diencephalon/caudal preoptic area/hypothalamus and medulla but were also detected in the olfactory bulbs, telencephalon/rostral preoptic area, optic tectum/tegmentum, cerebellum/pons, and pituitary. The immunohistochemical study revealed Spx1-immunoreactive (ir) cells in different nuclei of the preoptic area, habenula, prethalamus, mesencephalic tegmentum and in the proximal pars distalis (PPD) and pars intermedia of the pituitary. Spx1-ir fibers were widely distributed throughout the brain being particularly abundant in the midbrain and hindbrain, in close contact with tegmental gonadotropin-releasing hormone 2 (Gnrh2) cells and isthmic gonadotropin-inhibitory hormone (Gnih) cells of the secondary gustatory nucleus. Moreover, Gnih fibers were observed innervating Spx1-ir cells lying in several subdivisions of the magnocellular preoptic nucleus and in the lateral nucleus of the valvula, whereas ventrolateral prethalamic Spx1-ir cells received immunopositive Gnrh2 fibers. In the pituitary, Gnrh1-ir fibers were observed closely associated with Spx1-ir cells of the PPD. These results suggest that Spx1 could be involved in both reproductive and nonreproductive (i.e., food intake, behavior) functions in sea bass.

Organization of the Melanin concentrating hormone secreting neuronal system in the brain of the cichlid fish Oreochromis mossambicus

Melanin concentrating hormone (MCH) is a highly conserved cyclic peptide present in vertebrates. In this study, we describe the organization of MCH-immunoreactive (MCH-ir) cells and fibres in different regions of the brain in the cichlid fish Oreochromis mossambicus. Only MCH-ir fibres were observed in dorsal and ventral subdivisions of the telencephalon, the preoptic area and magnocellular and parvocellular divisions of the nucleus preopticus, and in hypothalamic areas such as the suprachiasmatic nucleus and tuberal area. Distinctly labelled MCH-ir perikarya were observed in the paraventricular organ, lateral and medial subdivisions of the nucleus lateralis tuberis, nucleus recessus lateralis and in the nucleus posterior tuberis. The pituitary gland showed MCH-ir fibres in the proximal pars distalis, neurohypophyseal ramifications and in pars intermedia where the dark accumulations of MCH-ir content corresponded to enlarged axon terminals. In the diencephalon, MCH-ir fibres were also labelled in the pretectal area, thalamic nuclei and preglomerular complex. In the midbrain tegmentum, a cluster of MCH-ir neurons was detected in the dorsal tegmental nucleus, whereas MCH-ir fibres were distributed in the torus semicircularis and optic tectum. In the rhombencephalon, MCH-ir fibres were located in the nucleus lateralis valvulae, cerebellum and secondary gustatory nucleus. Overall, the widespread distribution of MCH-ir cells and fibres in the brain suggests diverse roles for MCH such as regulation of sensorimotor and neuroendocrine functions in the tilapia.

Influences of ethanol and temperature on sucrose-evoked response of gustatory neurons in the hamster solitary nucleus.

Taste-responsive neurons in the nucleus of the solitary tract (NST), the first gustatory nucleus, often respond to thermal or mechanical stimulation. Alcohol, not a typical taste modality, is a rewarding stimulus. In this study, we aimed to investigate the effects of ethanol (EtOH) and/or temperature as stimuli to the tongue on the activity of taste-responsive neurons in hamster NST. In the first set of experiments, we recorded the activity of 113 gustatory NST neurons in urethane-anesthetized hamsters and evaluated responses to four basic taste stimuli, 25% EtOH, and 40°C and 4°C distilled water (dH2O). Sixty cells responded to 25% EtOH, with most of them also being sucrose sensitive. The response to 25% EtOH was significantly correlated with the sucrose-evoked response. A significant correlation was also observed between sucrose- and 40°C dH2O- and between 25% EtOH- and 40°C dH2O-evoked firings. In a subset of the cells, we evaluated neuronal activities in response to a series of EtOH concentrations, alone and in combination with 32 mM sucrose (EtOH/Suc) at room temperature (RT, 22°C–23°C), 40°C, and 4°C. Neuronal responses to EtOH at RT and 40°C increased as the concentrations increased. The firing rates to EtOH/Suc were greater than those to EtOH or sucrose alone. The responses were enhanced when solutions were applied at 40°C but diminished at 4°C. In summary, EtOH activates most sucrose-responsive NST gustatory cells, and the concomitant presence of sucrose or warm temperatures enhance this response. Our findings may contribute to elucidate the neural mechanisms underlying appetitive alcohol consumption.

Neuroanatomical organization of methionine-enkephalinergic system in the brain of the Mozambique tilapia Oreochromis mossambicus

Enkephalins are a class of opioid peptides implicated in several physiological and neuroendocrine responses in vertebrates. In this study, using immunocytochemical or immunofluorescence technique, we examined the neuroanatomical distribution of methionine enkephalin (M-ENK) immunoreactivity in the central nervous system (CNS) of the cichlid fish Oreochromis mossambicus. In the telencephalon, no M-ENK-like-immunoreactive (M-ENK-L-ir) perikarya, but sparsely distributed fibres were detected in the glomerular layer and the granule cell layer of the olfactory bulb. Although intensely labeled M-ENK-L-ir cells and fibres were found in the pallium, no M-ENK immunoreactivity was observed in the subpallium. The preoptic area showed a few M-ENK-L-ir somata and dense innervations of fibres. In the hypothalamic area, M-ENK-L-ir cells and fibres were located in magnocellular and parvocellular subdivisions of the nucleus preopticus, and medial and lateral subdivisions of the nucleus lateralis tuberis. Surrounding the recessus lateralis of the third ventricle, several intensely stained and packed M-ENK-L-ir cells and fibres were seen in dorsal, lateral and ventral subdivisions of the nucleus recessus lateralis. In the diencephalon, M-ENK immunoreactivity was restricted to the habenula, the thalamus, the pretectal area and the nucleus posterior tuberis. Dense aggregations of M-ENK-L-ir fibres were seen in the mesencephalic subdivisions, the optic tectum and the torus semicircularis, whereas a few fusiform M-ENK-L-ir cells and fibres were scattered in the midbrain tegmentum. In the rhombencephalon, different populations of ovoid or spindle shaped M-ENK-L-ir cells were observed in the secondary gustatory nucleus, the sensory trigeminal nerve nucleus, the nucleus reticularis medialis and the vagal motor nucleus, whereas bands of fibres were seen in the rostral spinal cord. Collectively, the widespread distribution of M-ENK immunoreactivity in the CNS suggests a role for this opioid peptide in regulation of neuroendocrine control of reproduction and modulation of sensorimotor functions in fish.

Regional chemoarchitecture of the brain of lungfishes based on calbindin D-28K and calretinin immunohistochemistry.

Lungfishes are the closest living relatives of land vertebrates, and their neuroanatomical organization is particularly relevant for deducing the neural traits that have been conserved, modified, or lost with the transition from fishes to land vertebrates. The immunohistochemical localization of calbindin (CB) and calretinin (CR) provides a powerful method for discerning segregated neuronal populations, fiber tracts, and neuropils and is here applied to the brains of Neoceratodus and Protopterus, representing the two extant orders of lungfishes. The results showed abundant cells containing these proteins in pallial and subpallial telencephalic regions, with particular distinct distribution in the basal ganglia, amygdaloid complex, and septum. Similarly, the distribution of CB and CR containing cells supports the division of the hypothalamus of lungfishes into neuromeric regions, as in tetrapods. The dense concentrations of CB and CR positive cells and fibers highlight the extent of the thalamus. As in other vertebrates, the optic tectum is characterized by numerous CB positive cells and fibers and smaller numbers of CR cells. The so-called cerebellar nucleus contains abundant CB and CR cells with long ascending axons, which raises the possibility that it could be homologized to the secondary gustatory nucleus of other vertebrates. The corpus of the cerebellum is devoid of CB and CR and cells positive for both proteins are found in the cerebellar auricles and the octavolateralis nuclei. Comparison with other vertebrates reveals that lungfishes share most of their features of calcium binding protein distribution with amphibians, particularly with salamanders.

5-HT3A -driven green fluorescent protein delineates gustatory fibers innervating sour-responsive taste cells: A labeled line for sour taste?

Taste buds contain multiple cell types with each type expressing receptors and transduction components for a subset of taste qualities. The sour sensing cells, Type III cells, release serotonin (5-HT) in response to the presence of sour (acidic) tastants and this released 5-HT activates 5-HT3 receptors on the gustatory nerves. We show here, using 5-HT3A GFP mice, that 5-HT3 -expressing nerve fibers preferentially contact and receive synaptic contact from Type III taste cells. Further, these 5-HT3 -expressing nerve fibers terminate in a restricted central-lateral portion of the nucleus of the solitary tract (nTS)-the same area that shows increased c-Fos expression upon presentation of a sour tastant (30 mM citric acid). This acid stimulation also evokes c-Fos in the laterally adjacent mediodorsal spinal trigeminal nucleus (DMSp5), but this trigeminal activation is not associated with the presence of 5-HT3 -expressing nerve fibers as it is in the nTS. Rather, the neuronal activation in the trigeminal complex likely is attributable to direct depolarization of acid-sensitive trigeminal nerve fibers, for example, polymodal nociceptors, rather than through taste buds. Taken together, these findings suggest that transmission of sour taste information involves communication between Type III taste cells and 5-HT3 -expressing afferent nerve fibers that project to a restricted portion of the nTS consistent with a crude mapping of taste quality information in the primary gustatory nucleus.

Distribution of endomorphin-like-immunoreactive neurones in the brain of the cichlid fish Oreochromis mossambicus.

Endomorphins (EMs) are tetrapeptides involved in pain and neuroendocrine responses with a high affinity for μ-opioid receptors in mammals. In the present study, we investigated the distribution of EM-like-immunoreactive (EM-L-IR) neurones in the brain of the cichlid fish Oreochromis mossambicus. Application of antisera against EM-1 and 2 (EM-1-2) revealed the presence of EM-L-IR somata and fibres throughout the different subdivisions of the olfactory bulb, such as the olfactory nerve layer and the granule cell layer. Although the extensions of EM-L-IR fibres were seen along the medial olfactory tract, intensely labelled EM-L-IR somata were found in different subdivisions of the telencephalon. In the diencephalon, intensely stained EM-L-IR neurones were noted in the preoptic area, the nucleus preopticus pars magnocellularis, the suprachiasmatic nucleus, the nucleus lateralis tuberis pars lateralis and the nucleus lateralis tuberis pars medialis regions, whereas projections of EM-L-IR fibres were also seen along the hypothalamic-hypophyseal tract, suggesting a possible hypophysiotrophic role for these neurones. Intense to moderately stained EM-L-IR neurones were noted in different subdivisions of thalamic nucleus, such as the dorsal posterior thalamic nucleus, commissura posterior, ventromedial thalamic nucleus, nucleus posterior tuberis, ventrolateral thalamic nucleus and medial preglomerular nucleus. Numerous intensely stained perikarya and axonal fibres were also noted throughout the inferior lobe, along the periventricular margin of the reccessus lateralis and in the nucleus recesus lateralis regions. In addition, numerous moderately labelled EM-like neuronal populations were found in the secondary gustatory nucleus and rostral spinal cord. The widespread distribution of EM-L-IR neurones throughout the brain and spinal cord indicates the diverse roles for these cells in neuroendocrine and neuromodulatory responses for the first time in fish. The present study provides further insights into the possible existence of EM-like peptides in early vertebrate lines and suggests that these peptides might have been well-conserved during the course of evolution.

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.

Descending projections from the nucleus accumbens shell excite activity of taste-responsive neurons in the nucleus of the solitary tract in the hamster.

The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second relays in the rodent central taste pathway. A series of electrophysiological experiments revealed that spontaneous and taste-evoked activities of brain stem gustatory neurons are altered by descending input from multiple forebrain nuclei in the central taste pathway. The nucleus accumbens shell (NAcSh) is a key neural substrate of reward circuitry, but it has not been verified as a classical gustatory nucleus. A recent in vivo electrophysiological study demonstrated that the NAcSh modulates the spontaneous and gustatory activities of hamster pontine taste neurons. In the present study, we investigated whether activation of the NAcSh modulates gustatory responses of the NST neurons. Extracellular single-unit activity was recorded from medullary neurons in urethane-anesthetized hamsters. After taste response was confirmed by delivery of sucrose, NaCl, citric acid, and quinine hydrochloride to the anterior tongue, the NAcSh was stimulated bilaterally with concentric bipolar stimulating electrodes. Stimulation of the ipsilateral and contralateral NAcSh induced firings from 54 and 37 of 90 medullary taste neurons, respectively. Thirty cells were affected bilaterally. No inhibitory responses or antidromic invasion was observed after NAcSh activation. In the subset of taste cells tested, high-frequency electrical stimulation of the NAcSh during taste delivery enhanced taste-evoked neuronal firing. These results demonstrate that two-thirds of the medullary gustatory neurons are under excitatory descending influence from the NAcSh, which is a strong indication of communication between the gustatory pathway and the mesolimbic reward pathway.

Distribution of choline acetyltransferase (ChAT) immunoreactivity in the CNS of the common carp Cyprinus carpio

Cholinergic systems play a role in basic cerebral functions and a number of human neurodegenerative disorders. Mechanisms involved in human brain diseases, including Parkinson’s disease (1), are often approached by using fish models, especially cyprinids, given basic similarities of the fish brain to that of mammals. In the present paper, the organization of central cholinergic systems have been described in the cyprinid Cyprinus carpio , the common carp, by using specific polyclonal antibodies against ChAT, the synthetic enzyme of acetylcholine, that is currently used as a specific marker for cholinergic neurons in all vertebrates. In this work, serial transverse and sagittal sections of the brain and the spinal cord were immunostained for ChAT. Results showed that positive neurons are present in several nuclei. In particular, ChAT-immunoreactive (ir) neurons were found in the forebrain (preoptic region, habenula), the midbrain (optic tectum, oculomotor nucleus, rostral tegmental nucleus), the hindbrain and the spinal cord (reticular formation, nucleus isthmi, secundary gustatory nucleus, cranial nerve motor nuclei from IV to X, spinal cord motoneurons). Moreover, ChAT-ir neurons were detected in the synencephalon (nucleus of the medial longitudinal fascicle) and in the cerebellum. In addition to neuronal bodies, afferent varicose fibers were stained for ChAT in the ventral telencephalon, the preoptic area, the hypothalamus and the posterior tuberculum. No neuronal cell bodies were present in the telencephalon. The comparison of ChAT-ir distribution observed in the present study with that reported in other CNS of cyprinids (2,3) has revealed a number of similarities and also some interesting differences. Our results provide additional information on the cholinergic system from a phylogenetic point of view, suggesting that cholinergic systems of the common carp show many primitive features that have been conserved during evolution, together with characteristics that are exclusive. In addition, the present study may add new perspectives to physiological roles of cholinergic system during evolution and the neuroanatomical basis of neurological diseases.

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.

Restoration of quinine-stimulated Fos-immunoreactive neurons in the central nucleus of the amygdala and gustatory cortex following reinnervation or cross-reinnervation of the lingual taste nerves in rats.

Remarkably, when lingual gustatory nerves are surgically rerouted to inappropriate taste fields in the tongue, some taste functions recover. We previously demonstrated that quinine-stimulated oromotor rejection reflexes and neural activity (assessed by Fos immunoreactivity) in subregions of hindbrain gustatory nuclei were restored if the posterior tongue, which contains receptor cells that respond strongly to bitter compounds, was cross-reinnervated by the chorda tympani nerve. Such functional recovery was not seen if instead, the anterior tongue, where receptor cells are less responsive to bitter compounds, was cross-reinnervated by the glossopharyngeal nerve, even though this nerve typically responds robustly to bitter substances. Thus, recovery depended more on the taste field being reinnervated than on the nerve itself. Here, the distribution of quinine-stimulated Fos-immunoreactive neurons in two taste-associated forebrain areas was examined in these same rats. In the central nucleus of the amygdala (CeA), a rostrocaudal gradient characterized the normal quinine-stimulated Fos response, with the greatest number of labeled cells situated rostrally. Quinine-stimulated neurons were found throughout the gustatory cortex, but a "hot spot" was observed in its anterior-posterior center in subregions approximating the dysgranular/agranular layers. Fos neurons here and in the rostral CeA were highly correlated with quinine-elicited gapes. Denervation of the posterior tongue eliminated, and its reinnervation by either nerve restored, numbers of quinine-stimulated labeled cells in the rostralmost CeA and in the subregion approximating the dysgranular gustatory cortex. These results underscore the remarkable plasticity of the gustatory system and also help clarify the functional anatomy of neural circuits activated by bitter taste stimulation.

Origin of Palatability Coding in Medial Prefrontal Cortex

Gustatory processing starts with activation of taste receptors located in the oral cavity. Axons of these receptor cells project to the primary gustatory nucleus in the brainstem, and from there to the pons. In the following stage, taste information flows in two independent channels: through the

Transgenic labeling of higher order neuronal circuits linked to phospholipase C‐β2–expressing taste bud cells in medaka fish

The sense of taste plays a pivotal role in the food-selecting behaviors of vertebrates. We have shown that the fish ortholog of the phospholipase C gene (plc-β2) is expressed in a subpopulation of taste bud cells that transmit taste stimuli to the central nervous system to evoke favorable and aversive behaviors. We generated transgenic medaka expressing wheat germ agglutinin (WGA) under the control of a regulatory region of the medaka plc-β2 gene to analyze the neuronal circuit connected to these sensory cells. Immunohistochemical analysis of the transgenic fish 12 days post fertilization revealed that the WGA protein was transferred to cranial sensory ganglia and several nuclei in the hindbrain. WGA signals were also detected in the secondary gustatory nucleus in the hindbrain of 3-month-old transgenic fish. WGA signals were observed in several diencephalic and telencephalic regions in 9-month-old transgenic fish. The age-dependent increase in the labeled brain regions strongly suggests that labeling occurred at taste bud cells and progressively extended to cranial nerves and neurons in the central nervous system. These data are the first to demonstrate the tracing of higher order gustatory neuronal circuitry that is associated with a specific subpopulation of taste bud cells. These results provide insight into the basic neuronal architecture of gustatory information processing that is common among vertebrates.

Descending projections from the nucleus accumbens shell suppress activity of taste-responsive neurons in the hamster parabrachial nuclei

The parabrachial nuclei (PbN), the second central relay for the gustatory pathway, transfers taste information to various forebrain gustatory nuclei and to the gustatory cortex. The nucleus accumbens is one of the critical neural substrates of the reward system, and the nucleus accumbens shell region (NAcSh) is associated with feeding behavior. Taste-evoked neuronal responses of PbN neurons are modulated by descending projections from the gustatory nuclei in the forebrain. In the present study, we investigated whether taste-responsive neurons in the PbN project to the NAcSh and whether pontine gustatory neurons are subject to modulatory influence from the NAcSh in urethane-anesthetized hamsters. Extracellular single-unit activity was recorded in the PbN, and taste responses were confirmed by the delivery of 32 mM sucrose, NaCl, quinine hydrochloride, and 3.2 mM citric acid to the anterior tongue. The NAcSh was then stimulated (0.5 ms, ≤100 μA) bilaterally using concentric bipolar stimulating electrodes. A total of 98 taste neurons were recorded from the PbN. Eighteen neurons were antidromically invaded from the NAcSh, mostly the ipsilateral NAcSh (n = 16). Stimulation of the ipsilateral and contralateral NAcSh suppressed the neuronal activity of 88 and 55 neurons, respectively; 52 cells were affected bilaterally. In a subset of pontine neurons tested, electrical stimulation of the NAcSh during taste stimulation also suppressed taste-evoked neuronal firing. These results demonstrated that taste-responsive neurons in the PbN not only project to the NAcSh but also are under substantial descending inhibitory influence from the bilateral NAcSh.

Ascending gustatory pathways to the telencephalon in goldfish

Ascending pathways to the telencephalon from the secondary gustatory nucleus (SGN), preglomerular tertiary gustatory nucleus (pTGN), and medial preglomerular nucleus (PGm) were examined by tract-tracing experiments in goldfish Carassius auratus. Tracer injections to the SGN suggest the presence of direct ascending pathways to the supracommissural and the dorsal parts of the ventral telencephalic area, and the medial part of the dorsal telencephalic area (Dm), restricted to its ventral region. The SGN experiments also suggest projections to the pTGN and PGm, and several neuronal types in the primary gustatory centers were newly found to give rise to ascending fibers to the SGN. Injections to the pTGN suggest reciprocal connections of the nucleus with the dorsal region of the Dm (dDm). Injections to the PGm resulted in labeled cells in the dorsal part of the SGN, the secondary general visceral nucleus, and the posterior part of the dorsal telencephalic area, suggesting that this preglomerular nucleus receives gustatory, general visceral, and olfactory inputs. Fibers labeled from the PGm terminated in the central part of the dorsal telencephalic area and the dDm; the latter region contained many labeled somata. The terminal zone of PGm fibers in the dDm is located laterally adjacent to that from the pTGN. Injection experiments to the pTGN and PGm also suggest connections of these nuclei with the inferior lobar nuclei and torus lateralis. Based on the results of the present as well as recent studies, an updated map is provided that shows by and large distinct sensory representation within the goldfish dorsal telencephalic area.

Vagal gustatory reflex circuits for intraoral food sorting behavior in the goldfish: Cellular organization and neurotransmitters

The sense of taste is crucial in an animal's determination as to what is edible and what is not. This gustatory function is especially important in goldfish, who utilize a sophisticated oropharyngeal sorting mechanism to separate food from substrate material. The computational aspects of this detection are carried out by the medullary vagal lobe, which is a large, laminated structure combining elements of both the gustatory nucleus of the solitary tract and the nucleus ambiguus. The sensory layers of the vagal lobe are coupled to the motor layers via a simple reflex arc. Details of this reflex circuit were investigated with histology and calcium imaging. Biocytin injections into the motor layer labeled vagal reflex interneurons that have radially directed dendrites ramifying within the layers of primary afferent terminals. Axons of reflex interneurons extend radially inward to terminate onto both vagal motoneurons and small, GABAergic interneurons in the motor layer. Functional imaging shows increases in intracellular Ca++ of vagal motoneurons following electrical stimulation in the sensory layer. These responses were suppressed under Ca(++)-free conditions and by interruption of the axons bridging between the sensory and motor layers. Pharmacological experiments showed that glutamate acting via (+/-)-alpha-amino-3-hydroxy- 5-ethylisoxazole-4-propioinc acid (AMPA)/kainate and N-methyl-D-aspartic acid (NMDA) receptors mediate neurotransmission between reflex interneurons and vagal motoneurons. Thus, the vagal gustatory portion of the viscerosensory complex is linked to branchiomotor neurons of the pharynx via a glutamatergic interneuronal system.

Lesions of the gustatory parabrachial nucleus but not the thalamic orosensory area eliminate anticipatory contrast for sucrose and corn oil in sham feeding rats

Lesions of the gustatory parabrachial nucleus but not the thalamic orosensory area eliminate anticipatory contrast for sucrose and corn oil in sham feeding rats

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.