Regeneration of taste buds in tongue grafts after reinnervation by neurons in transplanted lumbar sensory ganglia

Taste buds disappear after denervation and reappear after nerve regeneration. Sensory neurons are responsible since reinnervation by motor or autonomic fibers of peripheral nerve fail to induce bud regeneration. However, we do not know whether some neurons in all sensory ganglia can support buds or whether gustatory (i.e., taste bud inducing) neurons are localized to specific cranial ganglia. The present study was therefore pefrormed to determine whether neurons in transplanted spinal ganglia could support taste buds similarly to those in transplanted cranial ganglia. Grafts of lumbar or vagal nodose ganglia were combined with grafts of tongue's vallate papillae in the anterior chamber of rats' eyes and the papillae examined for taste buds 35 days later. Neurons were present in all transplanted ganglia, and all papillae reinnervated by them contained regenerated taste buds. Nerve fibers could be traced from the transplanted ganglia to the epithelium of the tongue grafts which bore the regenerated taste buds. Papillae transplanted without ganglia lacked buds. These findings indicate that some neurons in all sensory ganglia can induce taste bud formation. The present results could occur if gustatory neurons are intrinsically present in all sensory ganglia, but an alternative interpretation is that the tongue grafts transformed some neurons into gustatory neurons and, hence, that neuronal plasticity is involved.     

The zebrafish cerebellar upper rhombic lip generates tegmental hindbrain nuclei by long‐distance migration in an evolutionary conserved manner

The upper rhombic lip (URL) of the developing mammalian cerebellum produces different neuronal cell types in a temporal sequence. The first neuronal populations arising from this proliferation zone include the progenitors of the parabrachial, parabigeminal, and laterodorsal‐pedunculopontine tegmental hindbrain nuclei. By means of expression analysis, histology, and retrograde neuronal tracing, we have identified the zebrafish homologues of these nuclei, namely, the secondary gustatory/viscerosensory nucleus, the nucleus isthmi, and the superior reticular nucleus, respectively, in the embryonic and larval brain of a stable transgenic wnt1:Gal4‐VP16‐14 × UAS:GFP zebrafish strain. Combining time‐lapse confocal imaging with individual cell tracing, we characterize the migratory behavior of these neuronal precursor populations in detail by revealing their migration path, velocity, and directionality. In addition, we identify neuronal progenitors of the secondary gustatory/viscerosensory nucleus and nucleus isthmi/superior reticular nucleus as belonging to the polysialic acid (PSA)‐expressing cell population in the cerebellar plate that migrates in a PSA‐dependent manner. Finally, we reveal that circuitries involved in the processing of sensory information (visual, gustatory, general viscerosensory) are already established in the zebrafish larva at day 4 of development. Also the wnt1‐expressing pretectal neuronal precursors (not originating from the URL) sending mossy fiber‐like projections into the cerebellar corpus are established at that time. In sum, our results show that the origin of neurons of some tegmental hindbrain nuclei, namely, nucleus isthmi/superior reticular nucleus and secondary gustatory/viscerosensory nucleus is in the URL, and that the temporal order of cell types produced by the URL and their developmental program are conserved among vertebrate species. J. Comp. Neurol. 518:2794–2817, 2010. © 2010 Wiley‐Liss, Inc.     

Taste buds: development and evolution

The gustatory system in vertebrates comprises peripheral receptors (taste buds), innervated by three cranial nerves (VII, IX, and X), and a series of central neural centers and pathways. All vertebrates, with the exception of hagfishes, have taste buds. These receptors vary morphologically in different vertebrates but usually consist of at least four types of cells (dark, light, basal, and stem cells). An out-group analysis indicates that taste buds were restricted to the oropharynx, primitively, and that external taste buds, distributed over the head and, in some cases, even the trunk, evolved a number of times independently. The sensory neurons of the cranial nerves that innervate taste buds are believed to arise from epibranchial placodes, which are induced by pharyngeal endoderm, but it has never been demonstrated experimentally that these sensory neurons do, in fact, arise from these placodes. Although many details of the development of the innervation of taste buds are still unknown, it is now clear that taste buds are induced from either ecto- or endodermal epithelia, rather than arising from either placodes or neural crest. At present, there are two developmental models of taste bud induction: The neural induction model claims that peripheral nerve fibers induce taste buds, whereas the early specification model claims that oropharyngeal epithelium is specified by or during gastrulation and that taste buds arise from cell-cell interactions within the specified epithelium. There is now substantial evidence that the early specification model best describes the induction of taste buds.     

Genetic tracing of the gustatory and trigeminal neural pathways originating from T1R3-expressing taste receptor cells and solitary chemoreceptor cells

We established transgenic mouse lines expressing a transneuronal tracer, wheat germ agglutinin (WGA), under the control of mouse T1R3 gene promoter/enhancer. In the taste buds, WGA transgene was faithfully expressed in T1R3-positive sweet/umami taste receptor cells. WGA protein was transferred not laterally to the synapse-bearing, sour-responsive type III cells in the taste buds but directly to a subset of neurons in the geniculate and nodose/petrosal ganglia, and further conveyed to a rostro-central region of the nucleus of solitary tract. In addition, WGA was expressed in solitary chemoreceptor cells in the nasal epithelium and transferred along the trigeminal sensory pathway to the brainstem neurons. The solitary chemoreceptor cells endogenously expressed T1R3 together with bitter taste receptors T2Rs. This result shows an exceptional signature of receptor expression. Thus, the t1r3-WGA transgenic mice revealed the sweet/umami gustatory pathways from taste receptor cells and the trigeminal neural pathway from solitary chemoreceptor cells.     

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.     

drd2‐cre:ribotag mouse line unravels the possible diversity of dopamine d2 receptor‐expressing cells of the dorsal mouse hippocampus

Increasing evidences suggest that dopamine facilitates the encoding of novel memories by the hippocampus. However, the role of dopamine D2 receptors (D2R) in such regulations remains elusive due to the lack of the precise identification of hippocampal D2R‐expressing cells. To address this issue, mice expressing the ribosomal protein Rpl22 tagged with the hemagglutinin (HA) epitope were crossed with Drd2‐Cre mice allowing the selective expression of HA in D2R‐containing cells (Drd2‐Cre:RiboTag mice). This new transgenic model revealed a more widespread pattern of D2R‐expressing cells identified by HA immunoreactivity than the one initially reported in Drd2‐EGFP mice, in which the hilar mossy cells were the main neuronal population detectable. In Drd2‐Cre:RiboTag mice, scattered HA/GAD67‐positive neurons were detected throughout the CA1/CA3 subfields, being preferentially localized in stratum oriens and stratum lacunosum‐moleculare. At the cellular level, HA‐labeled cells located in CA1/CA3 subfields co‐localized with calcium‐binding proteins (parvalbumin, calbindin, and calretinin), neuropeptides (neuropeptide Y, somatostatin), and other markers (neuronal nitric oxide synthase, mGluR1α, reelin, coupTFII, and potassium channel‐interacting protein 1). These results suggest that in addition to the glutamatergic hilar mossy cells, D2R‐expressing cells constitute a subpopulation of GABAergic hippocampal interneurons. © 2014 Wiley Periodicals, Inc.     

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.     

Expression of the voltage‐gated potassium channel KCNQ1 in mammalian taste bud cells and the effect of its null‐mutation on taste preferences

Vertebrate taste buds undergo continual cell turnover. To understand how the gustatory progenitor cells in the stratified lingual epithelium migrate and differentiate into different types of mature taste cells, we sought to identify genes that were selectively expressed in taste cells at different maturation stages. Here we report the expression of the voltage‐gated potassium channel KCNQ1 in mammalian taste buds of mouse, rat, and human. Immunohistochemistry and nuclear staining showed that nearly all rodent and human taste cells express this channel. Double immunostaining with antibodies against type II and III taste cell markers validated the presence of KCNQ1 in these two types of cells. Co‐localization studies with cytokeratin 14 indicated that KCNQ1 is also expressed in type IV basal precursor cells. Null mutation of the kcnq1 gene in mouse, however, did not alter the gross structure of taste buds or the expression of taste signaling molecules. Behavioral assays showed that the mutant mice display reduced preference to some umami substances, but not to any other taste compounds tested. Gustatory nerve recordings, however, were unable to detect any significant change in the integrated nerve responses of the mutant mice to umami stimuli. These results suggest that although it is expressed in nearly all taste bud cells, the function of KCNQ1 is not required for gross taste bud development or peripheral taste transduction pathways, and the reduced preference of kcnq1‐null mice in the behavioral assays may be attributable to the deficiency in the central nervous system or other organs. J. Comp. Neurol. 512:384–398, 2009. © 2008 Wiley‐Liss, Inc.     

Impaired glutamate recycling and GluN2B‐mediated neuronal calcium overload in mice lacking TGF‐β1 in the CNS

Transforming growth factor β1 (TGF‐β1) is a pleiotropic cytokine expressed throughout the CNS. Previous studies demonstrated that TGF‐β1 contributes to maintain neuronal survival, but mechanistically this effect is not well understood. We generated a CNS‐specific TGF‐β1‐deficient mouse model to investigate the functional consequences of TGF‐β1‐deficiency in the adult mouse brain. We found that depletion of TGF‐β1 in the CNS resulted in a loss of the astrocyte glutamate transporter (GluT) proteins GLT‐1 (EAAT2) and GLAST (EAAT1) and decreased glutamate uptake in the mouse hippocampus. Treatment with TGF‐β1 induced the expression of GLAST and GLT‐1 in cultured astrocytes and enhanced astroglial glutamate uptake. Similar to GLT‐1‐deficient mice, CNS‐TGF‐β1‐deficient mice had reduced brain weight and neuronal loss in the CA1 hippocampal region. CNS‐TGF‐β1‐deficient mice showed GluN2B‐dependent aberrant synaptic plasticity in the CA1 area of the hippocampus similar to the glutamate transport inhibitor DL‐TBOA and these mice were highly sensitive to excitotoxic injury. In addition, hippocampal neurons from TGF‐β1‐deficient mice had elevated GluN2B‐mediated calcium signals in response to extrasynaptic glutamate receptor stimulation, whereas cells treated with TGF‐β1 exhibited reduced GluN2B‐mediated calcium signals. In summary, our study demonstrates a previously unrecognized function of TGF‐β1 in the CNS to control extracellular glutamate homeostasis and GluN2B‐mediated calcium responses in the mouse hippocampus.     

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.     

TGF‐β1 Pathway as a New Target for Neuroprotection in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder that affects more than 37 million people worldwide. Current drugs for AD are only symptomatic, but do not interfere with the underlying pathogenic mechanisms of the disease. AD is characterized by the presence of ß‐amyloid (Aβ) plaques, neurofibrillary tangles, and neuronal loss. The identification of the molecular determinants underlying AD pathogenesis is a fundamental step to design new disease‐modifying drugs. Recently, a specific impairment of transforming‐growth‐factor‐β1 (TGF‐β1) signaling pathway has been demonstrated in AD brain. The deficiency of TGF‐β1 signaling has been shown to increase both Aβ accumulation and Aβ‐induced neurodegeneration in AD models. The loss of function of TGF‐ß1 pathway seems also to contribute to tau pathology and neurofibrillary tangle formation. Growing evidence suggests a neuroprotective role for TGF‐β1 against Aβ toxicity both in vitro and in vivo models of AD. Different drugs, such as lithium or group II mGlu receptor agonists are able to increase TGF‐β1 levels in the central nervous system (CNS), and might be considered as new neuroprotective tools against Aβ‐induced neurodegeneration. In the present review, we examine the evidence for a neuroprotective role of TGF‐β1 in AD, and discuss the TGF‐β1 signaling pathway as a new pharmacological target for the treatment of AD.     

Transforming growth factor‐β1 primes proliferating adult neural progenitor cells to electrophysiological functionality

The differentiation of adult neural progenitors (NPCs) into functional neurons is still a limiting factor in the neural stem cell field but mandatory for the potential use of NPCs in therapeutic approaches. Neuronal function requires the appropriate electrophysiological properties. Here, we demonstrate that priming of NPCs using transforming growth factor (TGF)‐β1 under conditions that usually favor NPCs' proliferation induces electrophysiological neuronal properties in adult NPCs. Gene chip array analyses revealed upregulation of voltage‐dependent ion channel subunits (Kcnd3, Scn1b, Cacng4, and Accn1), neurotransmitters, and synaptic proteins (Cadps, Snap25, Grik4, Gria3, Syngr3, and Gria4) as well as other neuronal proteins (doublecortin [DCX], Nrxn1, Sept8, and Als2cr3). Patch‐clamp analysis demonstrated that control‐treated cells expressed only voltage‐dependent K⁺‐channels of the delayed‐rectifier type and the A‐type channels. TGF‐β1‐treated cells possessed more negative resting potentials than nontreated cells owing to the presence of delayed‐rectifier and inward‐rectifier channels. Furthermore, TGF‐β1‐treated cells expressed voltage‐dependent, TTX‐sensitive Na⁺ channels, which showed increasing current density with TGF‐β1 treatment duration and voltage‐dependent (+)BayK8644‐sensitive L‐Type Ca²⁺ channels. In contrast to nontreated cells, TGF‐β1‐treated cells responded to current injections with action‐potentials in the current‐clamp mode. Furthermore, TGF‐β1‐treated cells responded to application of GABA with an increase in membrane conductance and showed spontaneous synaptic currents that were blocked by the GABA‐receptor antagonist picrotoxine. Only NPCs, which were treated with TGF‐β1, showed Na⁺ channel currents, action potentials, and GABAergic currents. In summary, stimulation of NPCs by TGF‐β1 fosters a functional neuronal phenotype, which will be of relevance for future cell replacement strategies in neurodegenerative diseases or acute CNS lesions. GLIA 2013;61:1767–1783     

Smad3‐dependent signaling underlies the TGF‐β1‐mediated enhancement in astrocytic iNOS expression

We previously demonstrated that transforming growth factor‐β1 (TGF‐β1), while having no effect alone, enhances nitric oxide (NO) production in primary, purified mouse astrocytes induced by lipopolysaccharide (LPS) plus interferon‐γ (IFN‐γ), by recruiting a latent population of astrocytes to respond, thereby enhancing the total number of cells that express Nos2. In this investigation, we evaluated the molecular signaling pathway by which this occurs. We found that purified murine primary astrocytes express mRNA for TGFβRII as well as the TGFβRI subunit activin‐like kinase 5 (ALK5), but not ALK1. Immunofluorescence microscopy confirmed the expression of TGFβRII and ALK5 protein in astrocytes. Consistent with ALK5 signaling, Smad3 accumulated in the nucleus of astrocytes as early as 30 min after TGF‐β1 (3 ng/mL) treatment and persisted upto 32 hr after TGF‐β1 administration. Addition of ALK5 inhibitors prevented TGF‐β1‐mediated Smad3 nuclear accumulation and NO production when given prior to the Nos2 induction stimuli, but not after. Finally, astrocyte cultures derived from Smad3 null mutant mice did not exhibit a TGF‐β1‐mediated increase in iNOS expression. Overall, this data suggests that ALK5 signaling and Smad3 nuclear accumulation is required for optimal enhancement of LPS plus IFNγ‐induced NO production in astrocytes by TGF‐β1. © 2010 Wiley‐Liss, Inc.     

Neuron/target plasticity in the peripheral gustatory system

Taste bud volume on the anterior tongue in adult rats is matched by an appropriate number of innervating geniculate ganglion cells. The larger the taste bud, the more geniculate ganglion cells that innervate it. To determine if such a match is perturbed in the regenerated gustatory system under different dietary conditions, taste bud volumes and numbers of innervating neurons were quantified in adult rats after unilateral axotomy of the chorda tympani nerve and/or maintenance on a sodium-restricted diet. The relationship between taste bud size and innervation was eliminated in rats merely fed a sodium-restricted diet; individual taste bud volumes were smaller than predicted by the corresponding number of innervating neurons. Surprisingly, the relationship was disrupted in a similar way on the intact side of the tongue in unilaterally sectioned rats, with no diet-related differences. The mismatch in these groups was due to a decrease in average taste bud volumes and not to a change in numbers of innervating ganglion cells. In contrast, individual taste bud volumes were larger than predicted by the corresponding number of innervating neurons on the regenerated side of the tongue; again, with no diet-related differences. However, the primary variable responsible for disrupting the function on the regenerated side was an approximate 20% decrease in geniculate ganglion cells available to innervate taste buds. Therefore, the neuron/target match in the peripheral gustatory system is susceptible to surgical and/or dietary manipulations that act through multiple mechanisms. This system is ideally suited to model sensory plasticity in adults.     

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. J. Comp. Neurol. 522:2446–2464, 2014. © 2014 Wiley Periodicals, Inc.     

Postlarval growth of the peripheral gustatory system in the channel catfish, Ictalurus punctatus.

The phenomenon of postlarval cell addition to the peripheral nervous system of fish has been reported for some sensory systems, but has yet to be characterized for the gustatory system. Many fishes, such as catfish, possess taste buds scattered across their body surface, and presumably, the number of taste buds increases during growth of the animal. The present study was undertaken in order to examine the process of growth in the peripheral gustatory system and to determine whether the degree of convergence of receptors onto primary sensory afferents changes during growth. The recurrent facial nerve of channel catfish was used for these studies since this nerve contains no general cutaneous components and innervates taste buds along the fish's body surface. Electron micrographs were made of cross sections of this nerve taken from individuals ranging in size from 5.1 to 39.5 cm standard length. In addition, estimates were made of the number of taste buds innervated by this nerve by determining taste bud density along selected regions of the flank and fins in large and small fish. As catfish get larger, the number of both myelinated and unmyelinated axon profiles in the recurrent facial nerve increases, but at a slower rate than the number of taste buds innervated by this nerve. Thus, on average, the number of taste buds innervated by each fiber increases as the fish enlarges; on average there are 2 taste buds per axons profile in small fish and nearly 14 taste buds per axon profile in large fish. The rate of addition of new axon profiles to the nerve is estimated at roughly 70 per day over the range of sizes studied. Although generation of new ganglion cells and axons may contribute to this increase, several lines of evidence indicate that axonal branching occurs. In addition, the mean axon diameter for both myelinated and unmyelinated axons increases during postlarval growth. The finest myelinated fibers (0.2 micron) in small animals were significantly smaller than the finest myelinated fibers (0.7 micron) in larger animals.     

Three urocortins in medaka: identification and spatial expression in the central nervous system

The urocortin (UCN) group of neuropeptides includes urocortin 1/sauvagine/urotensin 1 (UTS1), urocortin 2 (UCN2) and urocortin 3 (UCN3). In recent years, evidence has accumulated showing that UCNs play pivotal roles in mediating stress response and anxiety in mammals. Evidence has also emerged regarding the evolutionary conservation of UCNs in vertebrates, but very little information is available about UCNs in non‐mammalian vertebrates. Indeed, at present, there are no reports of the empirical identification of ucn2 in non‐mammalian vertebrates or of the distribution of ucn2 and ucn3 expression in the adult central nervous system (CNS) of these animals. To gain insight into the evolutionary nature of UCNs in vertebrates, we cloned uts1, ucn2 and ucn3 in a teleost fish, medaka and examined the spatial expression of these genes in the adult brain and spinal cord. Although all known UCN2 genes except those in rodents have been reported to likely lack the necessary structural features to produce a functional pre‐pro‐protein, all three UCN genes in medaka, including ucn2, displayed all of these features, suggesting their functionality. The three UCN genes exhibited distinct spatial expression patterns in the medaka brain: uts1 was primarily expressed in broad regions of the dorsal telencephalon, ucn2 was expressed in restricted regions of the thalamus and brainstem and ucn3 was expressed in discrete nuclei throughout many regions of the brain. We also found that these genes were all expressed throughout the medaka spinal cord, each with a distinct spatial pattern. Given that many of these regions have been implicated in stress responses and anxiety, the three UCNs may serve distinct physiological roles in the medaka CNS, including those involved in stress and anxiety, as shown in the mammalian CNS.     

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.