Neural architecture of the primary gustatory center of Drosophila melanogaster visualized with GAL4 and LexA enhancer‐trap systems

Gustatory information is essential for animals to select edible foods and avoid poisons. Whereas mammals detect tastants with their taste receptor cells, which convey gustatory signals to the brain indirectly via the taste sensory neurons, insect gustatory receptor neurons (GRNs) send their axons directly to the primary gustatory center in the suboesophageal ganglion (SOG). In spite of this relatively simple architecture, the precise structure of the insect primary gustatory center has not been revealed in enough detail. To obtain comprehensive anatomical knowledge about this brain area, we screened the Drosophila melanogaster GAL4 enhancer‐trap strains that visualize specific subsets of the gustatory neurons as well as putative mechanosensory neurons associated with the taste pegs. Terminals of these neurons form three branches in the SOG. To map the positions of their arborization areas precisely, we screened newly established LexA::VP16 enhancer‐trap strains and obtained a driver line that labels a large subset of peripheral sensory neurons. By double‐labeling specific and landmark neurons with GAL4 and LexA strains, we were able to distinguish 11 zones in the primary gustatory center, among which 5 zones were identified newly in this study. Arborization areas of various known GRNs on the labellum, oesophagus, and legs were also mapped in this framework. The putative mechanosensory neurons terminate exclusively in three zones of these areas, supporting the notion of segregated primary centers that are specialized for chemosensory and mechanosensory signals associated with gustatory sensation. J. Comp. Neurol. 518:4147–4181, 2010. © 2010 Wiley‐Liss, Inc.     

Two closely located areas in the suboesophageal ganglion and the tritocerebrum receive projections of gustatory receptor neurons located on the antennae and the proboscis in the moth Heliothis virescens

Sucrose stimulation of gustatory receptor neurons on the antennae, the tarsi, and the mouthparts elicits the proboscis extension reflex in many insect species, including lepidopterans. The sensory pathways involved in this reflex have only partly been investigated, and in hymenopterans only. The present paper concerns the pathways of the gustatory receptor neurons on the antennae and on the proboscis involved in the proboscis extension reflex in the moth Heliothis virescens (Lepidoptera; Noctuidae). Fluorescent dyes were applied to the contact chemosensilla, sensilla chaetica on the antennae, and sensilla styloconica on the proboscis, permitting tracing of the axons of the gustatory receptor neurons in the central nervous system. The stained axons showed projections from the two appendages in two closely located but distinct areas in the suboesophageal ganglion (SOG)/tritocerebrum. The projections of the antennal gustatory receptor neurons were located posterior-laterally to those from the proboscis. Electrophysiological recordings from the receptor neurons in s. chaetica during mechanical and chemical stimulation were performed, showing responses of one mechanosensory and of several gustatory receptor neurons. Separate neurons showed excitatory responses to sucrose and sinigrin. The effect of these two tastants on the proboscis extension reflex was tested by repeated stimulations with solutions of the two compounds. Whereas sucrose elicited extension in 100% of the individuals in all repetitions, sinigrin elicited extension in fewer individuals, a number that decreased with repeated stimulation.     

Role of brain-derived neurotrophic factor in target invasion in the gustatory system.

Brain-derived neurotrophic factor (BDNF) is a survival factor for different classes of neurons, including gustatory neurons. We have studied innervation and development of the gustatory system in transgenic mice overexpressing BDNF under the control of regulatory sequences from the nestin gene, an intermediate filament gene expressed in precursor cells of the developing nervous system and muscle. In transgenic mice, the number and size of gustatory papillae were decreased, circumvallate papillae had a deranged morphology, and there was also a severe loss of lingual taste buds. Paradoxically, similar deficits have been found in BDNF knock-out mice, which lack gustatory neurons. However, the number of neurons in gustatory ganglia was increased in BDNF-overproducing mice. Although gustatory fibers reached the tongue in normal numbers, the amount and density of nerve fibers in gustatory papillae were reduced in transgenic mice compared with wild-type littermates. Gustatory fibers appeared stalled at the base of the tongue, a site of ectopic BDNF expression, where they formed abnormal branches and sprouts. Interestingly, palatal taste buds, which are innervated by gustatory neurons whose afferents do not traverse sites of ectopic BDNF expression, appeared unaffected. We suggest that lingual gustatory deficits in BDNF overexpressing mice are a consequence of the failure of their BDNF-dependent afferents to reach their targets because of the effects of ectopically expressed BDNF on fiber growth. Our findings suggest that mammalian taste buds and gustatory papillae require proper BDNF-dependent gustatory innervation for development and that the correct spatial expression of BDNF in the tongue epithelium is crucial for appropriate target invasion and innervation.     

A Golgi analysis of the gustatory zone of the nucleus of the solitary tract in the adult hamster

The somal shapes, dendritic features, and orientations of the neurons within the gustatory zone of the nucleus of the solitary tract were studied with the rapid Golgi method in the adult hamster. These Golgi studies complement previous quantitative morphometric analyses of the distributions of large and small neurons within the gustatory zone. Class 1 neurons are usually fusiform and possess long, relatively unbranched dendrites that often extend beyond the cytoarchitectonic boundaries of the gustatory zone. Class II neurons are multipolar and possess more dendrites that are significantly shorter than those of class I neurons. Both classes of neurons are spine poor. Computer-generated three-dimensional rotational analyses demonstrate that the dendritic arborizations of neurons of the gustatory zone are oriented preferentially in the horizontal plane. Dendrites extend in parallel or perpendicular to the solitary tract, the source of peripheral gustatory inputs, and appear to be positioned spatially to maximize synaptic interactions with these peripheral fibers. These Golgi studies also suggest that individual gustatory neurons may be influenced by incoming gustatory fibers that innervate separate populations of taste buds, a finding that is not predictable from the topographical organization of the gustatory zone.     

Adult-like complexity of the larval antennal lobe of D. melanogaster despite markedly low numbers of odorant receptor neurons

We provide a detailed analysis of the larval head chemosensory system of Drosophila melanogaster, based on confocal microscopy of cell-specific reporter gene expression in P[GAL4] enhancer trap lines. In particular, we describe the neuronal composition of three external and three pharyngeal chemosensory organs, the nerve tracts chosen by their afferents, and their central target regions. With a total of 21 olfactory and 80 gustatory neurons, the sensory level is numerically much simpler than that of the adult. Moreover, its design is different than in the adult, showing an association between smell and taste sensilla. In contrast, the first-order relay of the olfactory afferents, the larval antennal lobe (LAL), exhibits adult-like features both in terms of structure and cell number. It shows a division into approximately 30 subunits, reminiscent of glomeruli in the adult antennal lobe. Taken together, the design of the larval chemosensory system is a "hybrid," with larval-specific features in the periphery and central characteristics in common with the adult. The largely reduced numbers of afferents and the similar architecture of the LAL and the adult antennal lobe, render the larval chemosensory system of Drosophila a valuable model system, both for studying smell and taste and for examining the development of its adult organization.     

Glutamate-induced cobalt uptake reveals non-NMDA receptors in rat taste cells

Taste receptor cells are chemical detectors in the oral cavity. Taste cells form synapses with primary afferent neurons that convey the gustatory information to the central nervous system. Taste cells may also synapse with other taste cells within the taste buds. Furthermore, taste cells may receive efferent connections. However, the neurotransmitters at these synapses have not been identified. Glutamate, a major excitatory neurotransmitter in other sensory organs, might act at synapses in taste buds. We used a cobalt staining technique to detect Ca(2+)-permeable glutamate receptors in taste buds and thus establish whether there might be glutamatergic synapses in gustatory end organs. When 500 microm slices of foliate and vallate papillae were briefly exposed to 1 mM glutamate in the presence of CoCl(2), a subset of spindle-shaped taste cells accumulated Co(2+). Cobalt uptake showed concentration-dependency in the range from 10 microm to 1 mM glutamate. Interestingly, higher glutamate concentrations depressed cobalt uptake. This concentration-response relation for cobalt uptake suggests that synaptic glutamate receptors, not receptors for glutamate taste, were activated. Sensory axons and adjacent non-sensory epithelium were not affected by these procedures. Glutamate-stimulated cobalt uptake in taste cells was antagonized by the non-NMDA receptor antagonist CNQX. Depolarization with 50 mM K(+) and application of NMDA (300 microM) did not increase the number of stained taste cells. This pharmacological characterization of the cobalt uptake suggests that non-NMDA receptors are present in taste cells. These receptors might be autoreceptors at afferent synapses, postsynaptic receptors of a putative efferent system, or postsynaptic receptors at synapses with other taste cells.     

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. J. Comp. Neurol. 522:2498–2517, 2014. © 2014 Wiley Periodicals, Inc.     

Variable Branching Characteristics of Peripheral Taste Neurons Indicates Differential Convergence

Taste neurons are functionally and molecularly diverse, but their morphologic diversity remains completely unexplored. Using sparse cell genetic labeling, we provide the first reconstructions of peripheral taste neurons. The branching characteristics across 96 taste neurons show surprising diversity in their complexities. Individual neurons had 1–17 separate arbors entering between one and seven taste buds, 18 of these neurons also innervated non-taste epithelia. Axon branching characteristics are similar in gustatory neurons from male and female mice. Cluster analysis separated the neurons into four groups according to branch complexity. The primary difference between clusters was the amount of the nerve fiber within the taste bud available to contact taste-transducing cells. Consistently, we found that the maximum number of taste-transducing cells capable of providing convergent input onto individual gustatory neurons varied with a range of 1–22 taste-transducing cells. Differences in branching characteristics across neurons indicate that some neurons likely receive input from a larger number of taste-transducing cells than other neurons (differential convergence). By dividing neurons into two groups based on the type of taste-transducing cell most contacted, we found that neurons contacting primarily sour transducing cells were more heavily branched than those contacting primarily sweet/bitter/umami transducing cells. This suggests that neuron morphologies may differ across functional taste quality. However, the considerable remaining variability within each group also suggests differential convergence within each functional taste quality. Each possibility has functional implications for the system.SIGNIFICANCE STATEMENT Taste neurons are considered relay cells, communicating information from taste-transducing cells to the brain, without variation in morphology. By reconstructing peripheral taste neuron morphologies for the first time, we found that some peripheral gustatory neurons are simply branched, and can receive input from only a few taste-transducing cells. Other taste neurons are heavily branched, contacting many more taste-transducing cells than simply branched neurons. Based on the type of taste-transducing cell contacted, branching characteristics are predicted to differ across (and within) quality types (sweet/bitter/umami vs sour). Therefore, functional differences between neurons likely depends on the number of taste-transducing cells providing input and not just the type of cell providing input.     

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. J. Comp. Neurol. 521:1781–1802, 2013. © 2012 Wiley Periodicals, Inc.     

Hunger Modulates the Responses to Gustatory Stimuli of Single Neurons in the Caudolateral Orbitofrontal Cortex of the Macaque Monkey

1. In order to determine whether the responsiveness of neurons in the caudolateral orbitofrontal cortex (a secondary cortical gustatory area) is influenced by hunger, the activity evoked by prototypical taste stimuli (glucose, NaCl, HCl, and quinine hydrochloride) and fruit juice was recorded in single neurons in this cortical area before, while, and after cynomolgous macaque monkeys were fed to satiety with glucose or fruit juice. 2. It was found that the responses of the neurons to the taste of the glucose decreased to zero while the monkey ate it to satiety during the course of which his behaviour turned from avid acceptance to active rejection. 3. This modulation of responsiveness of the gustatory responses of the neurons to satiety was not due to peripheral adaptation in the gustatory system or to altered efficacy of gustatory stimulation after satiety was reached, because modulation of neuronal responsiveness by satiety was not seen at earlier stages of the gustatory system, including the nucleus of the solitary tract, the frontal opercular taste cortex, and the insular taste cortex. 4. The decreases in the responsiveness of the neurons were relatively specific to the food with which the monkey had been fed to satiety. For example, in seven experiments in which the monkey was fed glucose solution, neuronal responsiveness decreased to the taste of the glucose but not to the taste of blackcurrant juice. Conversely, in two experiments in which the monkey was fed to satiety with fruit juice, the responses of the neurons decreased to fruit juice but not to glucose. 5. These and earlier findings lead to a proposed neurophysiological mechanism for sensory-specific satiety in which the information coded by single neurons in the gustatory system becomes more specific through the processing stages consisting of the nucleus of the solitary tract, the taste thalamus, and the frontal opercular and insular taste primary taste cortices, until neuronal responses become relatively specific for the food tasted in the caudolateral orbitofrontal cortex (secondary) taste area. Then sensory-specific satiety occurs because in this caudolateral orbitofrontal cortex taste area (but not earlier in the taste system) it is a property of the synapses that repeated stimulation results in a decreased neuronal response. 6. Evidence was obtained that gustatory processing involved in thirst also becomes interfaced to motivation in the caudolateral orbitofrontal cortex taste projection area, in that neuronal responses here to water were decreased to zero while water was drunk until satiety was produced.     

Origin and distribution of noradrenergic innervation in the habenula: A neurochemical study

Retrograde axonal transport of fluorescent dyes was used to demonstrate collateral projections from neurons of the pontine taste area (PTA) to gustatory-responsive areas of the posterior ventromedial thalamic nucleus (VPM), and to the gustatory neocortex (GN) of the rat. Dual-labeled PTA neurons were reliably observed following application of two different fluorescent dyes to the GN and to VPM thalamus. Dye injections into the GN and into thalamic regions surrounding the VPM nucleus, the bed nucleus of stria terminalis or the infralimbic neocortex, did not result in dual-labeled cells within the PTA. This finding suggests that gustatory information may be relayed simultaneously and specifically to VPM thalamus and to the GN via collateral axons of PTA neurons.     

Axon collaterals of pontine taste area neurons project to the posterior ventromedial thalamic nucleus and to the gustatory neocortex

Retrograde axonal transport of fluorescent dyes was used to demonstrate collateral projections from neurons of the pontine taste area (PTA) to gustatory-responsive areas of the posterior ventromedial thalamic nucleus (VPM), and to the gustatory neocortex (GN) of the rat. Dual-labeled PTA neurons were reliably observed following application of two different fluorescent dyes to the GN and to VPM thalamus. Dye injections into the GN and into thalamic regions surrounding the VPM nucleus, the bed nucleus of stria terminalis or the infralimbic neocortex, did not result in dual-labeled cells within the PTA. This finding suggests that gustatory information may be relayed simultaneously and specifically to VPM thalamus and to the GN via collateral axons of PTA neurons.     

Functional redundancy and gustatory development in bdnf null mutant mice

In the mouse nasopalate papilla and in the trenches of the foliate and vallate papillae, taste buds accumulated primarily during the first 2 weeks after birth. Null mutation for brain-derived neurotrophic factor caused extensive death of embryonic taste neurons, with the secondary outcome that most taste buds failed to form. However not all taste neurons died; functional redundancy rescued a variable number. The primary research objective was to identify the likely site of the taste neuron rescue factor that substituted for BDNF. In this quest taste bud abundance served as a useful gauge of taste neuron abundance. The proportion of taste buds that developed was variable and uncorrelated among the nasopalate, vallate, and foliate gustatory papillae within each bdnf null mutant mouse. Thus, in spite of shared IXth nerve innervation, the vallate and foliate papillae independently varied in residual gustatory innervation. This variation rules against the rescue of gustatory neurons by system-wide factors or by factors acting on the IXth ganglion or nerve trunk. Therefore it is likely that surviving BDNF-deprived taste neurons were stochastically rescued by a redundant neurotrophic factor at the level of the local gustatory epithelium. These findings broaden the classic expectation that target tissue supplies only a single neurotrophic factor that can sustain sensory (taste) neurons.     

Forebrain connections of the gustatory system in ictalurid catfishes

Horseradish peroxidase tracing and extracellular electrophysiological recording techniques were employed to delineate prosencephalic connections of the gustatory system in ictalurid catfishes. The isthmic secondary gustatory nucleus projects rostrally to several areas of the ventral diencephalon including the nucleus lobobulbaris and the nucleus lateralis thalami. Injections of HRP in the vicinity of the nucleus lobobulbaris reveal an ascending projection to the telencephalon terminating in the area dorsalis pars medialis (Dm) and the medial region of area dorsalis pars centralis (Dc). Conversely, injections of HRP into the gustatory region of area dorsalis pars medialis label small neurons in the nucleus lobobulbaris. Gustatory neurons in the telencephalon send descending projections via the medial and lateral forebrain bundles to several nuclei in the anterior and ventroposterior diencephalon. The nucleus lateralis thalami, a diencephalic nucleus, receives ascending gustatory projections from the secondary gustatory nucleus but does not project to the telencephalon. Neurons in both the nucleus lateralis thalami and the telencephalic gustatory target exhibit multiple extraoral and oral receptive fields and complex responses to chemical (taste) and tactile stimulation.     

Age‐related changes of gustatory function depend on alteration of neuronal circuits

Studies on age‐related gustatory function report a reduction of the taste function, but the degeneration of the peripheral papillae alone cannot explain this reduction. In the present study, we apply psychophysics and gustatory event–related potentials (gERPs) to explore age‐related differences in the processing of gustatory information as indicated by the cerebral sources of the gERP. A total of 96 subjects (47 female), subdivided into four groups with increasing age, participated in the study. After olfactory and gustatory screening for normal function, the subjects were invited to two sessions of gERP acquisition. They received a randomized combination of five isointense basic tastants that were presented at a medium level. At the same time, we recorded scalp electroencephalography (EEG) from 128 scalp locations. Psychophysical testing for smell and taste function exhibited a significant decrease with age. Topographical analyses of the EEG delineated four basic topographical maps that explained the processing of taste in the pre‐decline age range, with sources inside the relevant gustatory areas. The age‐related change of gustatory processing was associated with the absence of a specific map with sources inside the cerebellum and posterior insula, and the temporal broadening of a map with sources in the bilateral inferior frontal gyrus. These results confirm the hypothesis that the reduction of taste function with aging is not only due to degradation of gustatory peripheral tissues but is also related to different neural signatures in the central nervous system.     

Taste-evoked Fos expression in nitrergic neurons in the nucleus of the solitary tract and reticular formation of the rat

The current investigation used double labeling for NADPHd and Fos-like immunoreactivity to define the relationship between nitric oxide synthase-containing neural elements and taste-activated neurons in the nucleus of the solitary tract (NST) and subjacent reticular formation (RF). Stimulation of awake rats with citric acid and quinine resulted in significant increases in the numbers of double-labeled neurons in both the NST and RF, suggesting that some medullary gustatory neurons utilize nitric oxide (NO) as a transmitter. Overall, double-labeled neurons were most numerous in the caudal reaches of the gustatory zone of the NST, where taste neurons receive inputs from the IXth nerve, suggesting a preferential role for NO neurons in processing gustatory inputs from the posterior oral cavity. However, double-labeled neurons also exhibited a preferential distribution depending on the gustatory stimulus. In the NST, double-labeled neurons were most numerous in the rostral central subnucleus after either stimulus but had a medial bias after quinine stimulation. In the RF, after citric acid stimulation, there was a cluster of double-labeled neurons with distinctive large soma in the parvicellular division of the lateral RF, subjacent to the rostral tip of NST. In contrast, in response to quinine, there was a cluster of double-labeled neurons with much smaller soma in the intermediate zone of the medial RF, a few hundred micrometers caudal to the citric acid cluster. These differential distributions of double-labeled neurons in the NST and RF suggest a role for NO in stimulus-specific gustatory autonomic and oromotor reflex circuits.     

The morphogenesis of mouse vallate gustatory epithelium and taste buds requires BDNF-dependent taste neurons

The developmental absence of brain-derived neurotrophic factor (BDNF) in null mutant mice caused three interrelated defects in the vallate gustatory papilla: sparse innervation, a reduction in the area of the gustatory epithelium, and fewer taste buds. On postnatal day 7, the stunted vallate papilla of bdnf null mutant mice was 30% narrower, the trench walls 35% reduced in area, and the taste buds 75% less abundant compared with wild-type controls. Quantitative assessment of innervation density was carried out to determine if the small trench walls and shortage of taste buds could be secondary consequences of the depletion of gustatory neurons. The diminished gustatory innervation was linearly associated with a reduced trench wall area (r=+0.94) and fewer taste buds (r=+0.96). Residual taste buds were smaller than normal and were innervated by a few surviving taste neurons. We conclude that BDNF-dependent taste neurons contribute to the morphogenesis of lingual gustatory epithelia and are necessary for both prenatal and postnatal mammalian taste bud formation. The gustatory system provides a conspicuous example of impaired sense organ morphogenesis that is secondary to sensory neuron depletion by neurotrophin gene null mutation.     

Parallel somatotopic maps of gustatory and mechanosensory neurons in the central nervous system of an insect

Relatively little is still known about the sense of taste, or contact chemoreception, compared with other sensory modalities, despite its importance to many aspects of animal behaviour. The central projections of the sensory neurons from bimodal contact chemoreceptors (basiconic sensilla) were compared with those from mechanosensory tactile hairs located on similar regions of the middle leg of the locust. Basiconic sensilla are multiply innervated, containing one mechanosensory and several chemosensory neurons, whereas tactile hairs are innervated by a single mechanosensory neuron. We show that the sensory neurons from tactile hairs form a complete 3-dimensional somatotopic map in the mesothoracic ganglion. Sensory neurons from hairs located on the coxa projected to a region near the midline of the ganglion with neurons from hairs located on progressively more distal parts of the leg arborizing in successively more lateral regions of neuropil. All the neurons from basiconic sensilla, both mechanosensory and chemosensory, also projected in a similar, strictly somatotopic, manner, and the arbors from these neurons overlapped considerably with those from tactile hairs on equivalent parts of the leg to form a continuous region. Thus, the position of a receptor on the leg is preserved in the central nervous system not only for the mechanosensory neurons from both tactile hairs and basiconic sensilla but also for chemosensory neurons. We could observe no anatomical features or small differences in projection region between sensory neurons from individual basiconic sensilla consistent with differences in modality.     

Telencephalic ascending gustatory system in a cichlid fish,Oreochromis (Tilapia) niloticus

Central fiber connections of the gustatory system were examined in a percomorph fish Oreochromis (Tilapia) niloticus by means of the horseradish peroxidase (HRP), biocytin, and carbocyanine dye tracing methods. The primary gustatory areas in tilapia are the facial, glossopharyngeal, and vagal lobes of the medulla. The secondary gustatory nucleus (SGN) is a dumb-bell-shaped structure located in the isthmic region. In the SGN, there are two or three layers of neurons lining the ventromedial periphery of the nucleus and a molecular layer constituting of the major part of the nucleus. The SGN receives bilateral projections from the facial lobes and ipsilateral projections from the glossopharyngeal and vagal lobes. Ascending fibers originating from the SGN form the ipsilateral tertiary gustatory tract. A major part of the tract courses rostrally and terminates ipsilaterally in several diencephalic nuclei: the preglomerular tertiary gustatory nucleus (pTGN), the posterior thalamic nucleus, the nucleus diffusus lobi inferioris, the nucleus centralis of inferior lobe, and the nucleus recessus lateralis. The remaining small fiber bundle enters the medial and lateral forebrain bundles and terminates directly in two telencephalic regions; the area ventralis pars intermedia (Vi) and the area dorsalis pars posterior (Dp). Ascending fibers from the pTGN pass through the lateral forebrain bundle and terminate ipsilaterally in the dorsal region of area dorsalis pars medialis (dDm) of the telencephalon. Following biocytin injections into the dDm, small, round cells were labeled in the pTGN. After biocytin injections into the Vi and Dp of the telencephalon, retrogradely labeled cells were found in the ipsilateral SGN. The results show that the ascending fiber connections of the central gustatory system in the percomorph teleost tilapia are essentially similar to those of mammals. That is, the pathway from the primary gustatory areas (facial, glossopharyngeal, and vagal lobes) through the SGN and pTGN to the dDm in tilapia corresponds with the mammalian gustatory pathway from the solitary nucleus through the pontine taste areas (nucleus parabrachialis) and the thalamic relay nucleus (ventral posteromedial nucleus) to gustatory neocortices. In addition, the pathway from the primary gustatory areas through the SGN to the Vi and Dp in tilapia corresponds with the pathway from the solitary nucleus through the pontine taste areas to the amygdala in mammals. J. Comp. Neurol. 392:209–226, 1998. © 1998 Wiley-Liss, Inc.     

Structure and development of the subesophageal zone of the Drosophila brain. II. Sensory compartments

The subesophageal zone (SEZ) of the Drosophila brain processes mechanosensory and gustatory sensory input from sensilla located on the head, mouth cavity and trunk. Motor output from the SEZ directly controls the movements involved in feeding behavior. In an accompanying paper (Hartenstein et al., 2017 ), we analyzed the systems of fiber tracts and secondary lineages to establish reliable criteria for defining boundaries between the four neuromeres of the SEZ, as well as discrete longitudinal neuropil domains within each SEZ neuromere. Here we use this anatomical framework to systematically map the sensory projections entering the SEZ throughout development. Our findings show continuity between larval and adult sensory neuropils. Gustatory axons from internal and external taste sensilla of the larva and adult form two closely related sensory projections, (a) the anterior central sensory center located deep in the ventromedial neuropil of the tritocerebrum and mandibular neuromere, and (b) the anterior ventral sensory center (AVSC), occupying a superficial layer within the ventromedial tritocerebrum. Additional, presumed mechanosensory terminal axons entering via the labial nerve define the ventromedial sensory center (VMSC) in the maxilla and labium. Mechanosensory afferents of the massive array of chordotonal organs (Johnston's organ) of the adult antenna project into the centrolateral neuropil column of the anterior SEZ, creating the antenno‐mechanosensory and motor center (AMMC). Dendritic projections of dye back‐filled motor neurons extend throughout a ventral layer of the SEZ, overlapping widely with the AVSC and VMSC. Our findings elucidate fundamental structural aspects of the developing sensory systems in Drosophila.