Non-sensory Epithelium Research Articles

Histological and scanning electron microscopic organization and functional aspects of the surface olfactory epithelium of the freshwater minor carp, Puntius sophore (Hamilton)

The architecture and fuction of different cells lining the olfactory epithelium of Puntius sophore (Hamilton) have been systematically studied by light and scanning electron microscope. The olfactory organ is ovoid in shape and is complicated by the presence of 16-18 lamellae on both side of the median raphe. The large area of the middle part of the olfactory lamella is covered with sensory epithelium, whereas the distal part mainly occupies by non-sensory epithelium. The sensory epithelium is composed two types of receptor cells (flagellated and microvillous) and stratified epithelial cells. The non-sensory epithelium is made up of stratified epithelial cells and mucous cells. Different cells on the olfactory epithelium have been correlated with the functional significance of the fish concerned.

Fine structure of the vomeronasal organ in the grass lizard, Takydromus tachydromoides

The squamates are composed of many taxa, among which there is morphological variation in the vomeronasal organ (VNO). To elucidate the evolution of chemoreception in squamate reptiles, morphological data from the VNO from a variety of squamate species is required. In this study, the morphology of the VNO of the grass lizard Takydromus tachydromoides was examined using light and electron microscopy. The VNO consists of a pair of dome-shaped structures, which communicate with the oral cavity. There are no associated glandular structures. Microvilli are present on the apical surfaces of receptor cells in its sensory epithelium, as well as on supporting cells, and there are centrioles and ciliary precursor bodies on the dendrites. In addition to ciliated cells and basal cells in the non-sensory epithelium, there is a novel type of non-ciliated cell in T. tachydromoides. They have constricted apical cytoplasm and microvilli instead of cilia, and are sparsely distributed in the epithelium. Based on these results, the variation in the morphology of the VNO in scincomorpha, a representative squamate taxon, is discussed.

Histoarchitecture and scanning electron microscopic studies of the olfactory epithelium in the exotic fish Puntius javanicus (Bleeker)

The olfactory epithelium of Puntius javanicus (Bleeker) was examined methodically with a light microscope (LM) and a scanning electron microscope (SEM). The ovoid olfactory apparatus consists of 25 to 26 primary lamellae arranged on both sides of the narrow median raphe. The sensory epithelium occupies the upper middle and apical half of the olfactory lamellae and consists of receptor cells (both ciliated and microvillus) and mucous cells. The region from basal part of the lamellae to the junction of the median raphe is covered with non-sensory epithelium. The non-sensory epithelium is composed of non-sensory supporting cells, stratified epithelial cells with fingerprint like microridges, and mucous cells.

Na+Homeostasis in the Inner Ear

homeostasis in the inner ear is important to maintain normal hearing and balance. transport in the inner ear is reported to be occurred in non-sensory epithelium of inner ear which forms a barrier between endolymphatic space and perilymphatic space. Functionally identified and constitutively active absorption sites in the inner ear are Reissner's membrane, outer sulcus cells, vestibular transitional cells, saccular nonsensory epithelial cells, and endolymphatic sac epithelial cells. transport in these epithelial cells is mediated by apically located epithelial channels (ENaC), nonselective cation channels and basolaterally located , -ATPase. absorption is increased by glucocorticoid through glucocorticoid receptor or ATP through purinergic receptors depending on cell types.

Histological Studies Of The Vomeronasal Organ Of African Giant Rat (Cricetomys gambianus, Waterhouse)

The vomeronasal organ (VNO), a chemoreceptive organ was studied in African giant rat (Cricetomys gambianus, Waterhouse), a macrosmatic animal, by means of gross dissection and light microscopy. The VNO was located in the rostral part of the base of the nasal septum. It was tubular in shape, about 10.6 mm in length and opened in the rostral region of the nasal cavity, with a blind caudal end that terminated in glandular branches. Its lumen in the middle segment was lined by sensory epithelium on the medial wall and non-sensory epithelium on the lateral wall. The sensory epithelium showed sensory, supporting and basal cells, whereas the non-sensory wall contained psendostratified columnar cells with ciliated epithelium. Vomeronasal glands were present in the lateral wall and on the dorso-lateral region between the sensory and non-sensory epithela. Intraepithelial blood vessels were observed on the medial sensory epithelium. Nerve bundles were also apparent in the medial sensory wall. The vomeronasal capsule incompletely housed the organ and showed ossified areas. The histological observations suggest that the VNO isan important organ in sexual behaviours as in other rodents and will throw more light on future studies of the vomeronasal organ. Keywords: African giant rat, Histology, Chemoreception, Vomeronasal organ

Calcium-dependent Binding of HCN1 Channel Protein to Hair Cell Stereociliary Tip Link Protein Protocadherin 15 CD3

The cytoplasmic amino terminus of HCN1, the primary full-length HCN isoform expressed in trout saccular hair cells, was found by yeast two-hybrid protocols to bind the cytoplasmic carboxyl-terminal domain of a protocadherin 15a-like protein. HCN1 was immunolocalized to discrete sites on saccular hair cell stereocilia, consistent with gradated distribution expected for tip link sites of protocadherin 15a. HCN1 message was also detected in cDNA libraries of rat cochlear inner and outer hair cells, and HCN1 protein was immunolocalized to cochlear hair cell stereocilia. As predicted by the trout hair cell model, the amino terminus of rat organ of Corti HCN1 was found by yeast two-hybrid analysis to bind the carboxyl terminus of protocadherin 15 CD3, a tip link protein implicated in mechanosensory transduction. Specific binding between HCN1 and protocadherin 15 CD3 was confirmed with pull-down assays and surface plasmon resonance analysis, both predicting dependence on Ca(2+). In the presence of calcium chelators, binding between HCN1 and protocadherin 15 CD3 was characterized by a K(D) = 2.39 x 10(-7) m. Ca(2+) at 26.5-68.0 microm promoted binding, with K(D) = 5.26 x 10(-8) m (at 61 microm Ca(2+)). Binding by deletion mutants of protocadherin 15 CD3 pointed to amino acids 158-179 (GenBank accession number XP_238200), with homology to the comparable region in trout hair cell protocadherin 15a-like protein, as necessary for binding to HCN1. Amino terminus binding of HCN1 to HCN1, hypothesized to underlie HCN1 channel formation, was also found to be Ca(2+)-dependent, although the binding was skewed toward a lower effective maximum [Ca(2+)] than for the HCN1 interaction with protocadherin 15 CD3. Competition may therefore exist in vivo between the two binding sites for HCN1, with binding of HCN1 to protocadherin 15 CD3 favored between 26.5 and 68 microm Ca(2+). Taken together, the evidence supports a role for HCN1 in mechanosensory transduction of inner ear hair cells.

Lmx1a is required for segregation of sensory epithelia and normal ear histogenesis and morphogenesis

At embryonic day 8.5, the LIM-homeodomain factor Lmx1a is expressed throughout the otic placode but becomes developmentally restricted to non-sensory epithelia of the ear (endolymphatic duct, ductus reuniens, cochlea lateral wall). We confirm here that the ears of newborn dreher (Lmx1a (dr)) mutants are dysmorphic. Hair cell markers such as Atoh1 and Myo7 reveal, for the first time, that newborn Lmx1a mutants have only three sensory epithelia: two enlarged canal cristae and one fused epithelium comprising an amalgamation of the cochlea, saccule, and utricle (a "cochlear-gravistatic" endorgan). The enlarged anterior canal crista develops by fusion of horizontal and anterior crista, whereas the posterior crista fuses with an enlarged papilla neglecta that may extend into the cochlear lateral wall. In the fused endorgan, the cochlear region is distinguished from the vestibular region by markers such as Gata3, the presence of a tectorial membrane, and cochlea-specific innervation. The cochlea-like apex displays minor disorganization of the hair and supporting cells. This contrasts with the basal half of the cochlear region, which shows a vestibular epithelium-like organization of hair cells and supporting cells. The dismorphic features of the cochlea are also reflected in altered gene expression patterns. Fgf8 expression expands from inner hair cells in the apex to most hair cells in the base. Two supporting cell marker proteins, Sox2 and Prox1, also differ in their cellular distribution between the base and the apex. Sox2 expression expands in mutant canal cristae prior to their enlargement and fusion and displays a more diffuse and widespread expression in the base of the cochlear region, whereas Prox1 is not detected in the base. These changes in Sox2 and Prox1 expression suggest that Lmx1a expression restricts and sharpens Sox2 expression, thereby defining non-sensory and sensory epithelium. The adult Lmx1a mutant organ of Corti shows a loss of cochlear hair cells, suggesting that the long-term maintenance of hair cells is also disrupted in these mutants.

Molecular Identity and Functional Properties of a Novel T-Type Ca2+ Channel Cloned From the Sensory Epithelia of the Mouse Inner Ear

The molecular identity of non-Cav1.3 channels in auditory and vestibular hair cells has remained obscure, yet the evidence in support of their roles to promote diverse Ca2+-dependent functions is indisputable. Recently, a transient Cav3.1 current that serves as a functional signature for the development and regeneration of hair cells has been identified in the chicken basilar papilla. The Cav3.1 current promotes spontaneous activity of the developing hair cell, which may be essential for synapse formation. Here, we have isolated and sequenced the full-length complementary DNA of a distinct isoform of Cav3.1 in the mouse inner ear. The channel is derived from alternative splicing of exon14, exon25A, exon34, and exon35. Functional expression of the channel in Xenopus oocytes yielded Ca2+ currents, which have a permeation phenotype consistent with T-type channels. However, unlike most multiion channels, the T-type channel does not exhibit the anomalous mole fraction effect, possibly reflecting comparable permeation properties of divalent cations. The Cav3.1 channel was expressed in sensory and nonsensory epithelia of the inner ear. Moreover, there are profound changes in the expression levels during development. The differential expression of the channel during development and the pharmacology of the inner ear Cav3.1 channel may have contributed to the difficulties associated with identification of the non-Cav1.3 currents.

Analysis of mouse kreisler mutants reveals new roles of hindbrain-derived signals in the establishment of the otic neurogenic domain

The inner ear, the sensory organ responsible for hearing and balance, contains specialized sensory and non-sensory epithelia arranged in a highly complex three-dimensional structure. To achieve this complexity, a tight coordination between morphogenesis and cell fate specification is essential during otic development. Tissues surrounding the otic primordium, and more particularly the adjacent segmented hindbrain, have been implicated in specifying structures along the anteroposterior and dorsoventral axes of the inner ear. In this work we have first characterized the generation and axial specification of the otic neurogenic domain, and second, we have investigated the effects of the mutation of kreisler/MafB – a gene transiently expressed in rhombomeres 5 and 6 of the developing hindbrain – in early otic patterning and cell specification. We show that kr/kr embryos display an expansion of the otic neurogenic domain, due to defects in otic patterning. Although many reports have pointed to the role of FGF3 in otic regionalisation, we provide evidence that FGF3 is not sufficient to govern this process. Neither Krox20 nor Fgf3 mutant embryos, characterized by a downregulation or absence of Fgf3 in r5 and r6, display ectopic neuroblasts in the otic primordium. However, Fgf3−/−Fgf10−/− double mutants show a phenotype very similar to kr/kr embryos: they present ectopic neuroblasts along the AP and DV otic axes. Finally, partial rescue of the kr/kr phenotype is obtained when Fgf3 or Fgf10 are ectopically expressed in the hindbrain of kr/kr embryos. These results highlight the importance of hindbrain-derived signals in the regulation of otic neurogenesis.

Photoperiod and feeding behavior of the Antarctic fish Notothenia rossii (Perciformes: Nototheniidae) and functional morphology of chemical and visual sensory structures used in foraging

The influence of photoperiod on the ability of Notothenia rossii Richardson, 1844 to detect prey was studied experimentally. In 12L/12D there was the highest number of individuals stimulated visually and chemically while in 24D chemical perception was the same as that observed in 12L/12D, the latter being lower than in 24L. The ultrastructure of chemo- and photo-sensory structures, involved in the detection of food, was studied. The photoreceptor cells of the retina are single and double cones, and rods. Taste buds were concentrated in the central part of upper and lower lips. Their shape is even, but the size is variable. The olfactory rosette has a sensory and a non-sensory epithelium, being rich in different mucosubstances. The conclusion was that in different photoperiods there is a difference in the response to visual and chemical stimulation for prey detection and that all sensory systems are more stimulated in periods of more luminosity, mainly when there is an alternation between light and dark periods. Sensory organs are complex when compared to some other Antarctic fish such as Gobionotothen gibberifrons Lönnberg, 1905 and Ophthalmolycus amberensis Tomo, Marshoff & Torno, 1977 but similar to those of N. coriiceps Rchardson, 1844.

Immunolocalization of Water Channel Aquaporins in the Vomeronasal Organ of the Rat: Expression of AQP4 in Neuronal Sensory Cells

The vomeronasal organ comprises a pair of narrow tubes in the mammalian nasal septum, serving as a chemosensory system for pheromones. We examined the expression and localization of water channel aquaporins (AQPs) in the rat vomeronasal organ. AQP1 was localized in blood vessels, being particularly abundant in cavernous tissues of the nonsensory mucosa. AQP5 was found in the apical membrane of the gland acinar cells in the vomeronasal organ. AQP3 was detected in the basal cells of the nonsensory epithelium, whereas it was absent in the sensory epithelium. AQP4 was found in both the sensory and the nonsensory epithelia. Interestingly, AQP4 was highly concentrated in the sensory cells of the sensory epithelium. Immunoelectron microscopic examination clearly showed that AQP4 was localized at the plasma membrane in the cell body and lateral membrane of the dendrite, except for the microvillous apical membrane. Nerve fiber bundles emanating from neuronal sensory cells were positive for AQP4, whereby the plasma membrane of each axon was positive for AQP4. These observations clearly show that neuronal sensory cells in the vomeronasal organ are unique in that they express abundant AQP4 at their plasma membrane. This is in marked contrast to the olfactory and central nervous systems, where AQPs are not detectable in neurons, and instead, AQP4 is abundant in the supporting cells and astrocytes surrounding them. The present findings suggest a unique water-handling feature in neuronal sensory cells in the vomeronasal organ.

GPR expression in the rat taste bud relating to fatty acid sensing

We investigated the expression of G protein-coupled receptor GPR40 and GPR120 in the rat tongue. Using reverse transcription polymerase chain reaction, we detected a significant expression of GPR120 mRNA in the epithelium of the circumvallate papillae but not in the nonsensory epithelium, while the expression of GPR40 mRNA was undetectable in the sensory papillae. Western blotting analysis of colon and circumvallate papillae for GPR120 showed a protein band with a molecular weight that corresponds to that of GPR120, indicating that this antibody could recognize a native form of GPR120. Immunohistochemistry using anti-GPR120 antibody revealed GPR120 immunoreactivity in the enteroendocrine cells of the colon. Furthermore, some cells in each taste bud were stained positively with more intense labeling in the apical part of the cells. These results suggested that GPR120 is expressed in the taste cells of the circumvallate papillae to sense dietary fat, like the receptor expressed in the enteroendocrine cells.

Hindbrain signals in otic regionalization: walk on the wild side

The inner ear, the sensory organ responsible for hearing and balance, contains specialized sensory and non-sensory epithelia arranged in a highly complex three-dimensional structure. To achieve this level of complexity, a tight coordination between morphogenesis and cell fate specification is essential during otic development. Tissues surrounding the otic primordium and more particularly the adjacent segmented hindbrain, have been implicated in conferring signals required for inner ear development. In this review, we present the current view on the role of hindbrain signals in axial specification of the inner ear. The functional analysis of mutants of hindbrain segmentation genes, as well as the investigation of signaling pathways potentially involved, all point to an essential role of FGF, Wnt and Hh signaling in otic regionalization. However, these data provide conflicting evidence regarding the involvement of hindbrain signals in otic regionalization in fish and in amniotes. We discuss the possible origin of these differences.

Distribution of Notch1-expressing cells and proliferating cells in mouse vomeronasal organ

Vomeronasal receptor neurons (VRNs) proliferate and differentiate continuously in the vomeronasal organ (VNO) throughout life. In adult mice, new VRNs are generated mainly in the marginal region, located in the boundary region between sensory and nonsensory epithelia. The Notch signaling pathway is involved in differentiation in the developing nervous system. To understand the Notch signaling pathway involved in generating VRNs, we focused on the relationship between the expression pattern of Notch1 and the localization of proliferating cells in both developing and regenerating mice VNO, and examined the Notch signaling pathway involved in the development of VNO by in situ hybridization of Notch1 and immunocytochemistry of 5-bromo-2′-deoxyuridine. During embryonic and neonatal development, proliferating cells and Notch1-expressing (+) cells were observed evenly throughout VNO. A large number of proliferating cells and Notch1 (+) cells were observed in embryonic VNO, but gradually decreased during development. The localization of proliferating cells was similar to that of Notch1 (+) cells at each developmental stage. In adult VNO, there are a few proliferating cells and Notch1 (+) cells, which were only in the marginal region of VNO. Seven days after removal of the accessory olfactory bulb (AOB), VRNs proliferated throughout VNO. Although the number of Notch1 (+) cells also increased in VNO, the majority of these were concentrated in the dorsal region of VNO, suggesting that it has two types of differentiating cell. These results suggest that Notch1 plays a role in the differentiation of VRNs during development and regeneration of VRNs after removal of AOB.

Comparative analysis of Gata3 and Gata2 expression during chicken inner ear development

The inner ear is a complex sensory organ with hearing and balance functions. Gata3 and Gata2 are expressed in the inner ear, and to gain more insight into their roles in otic development, we made a detailed expression analysis in chicken embryos. At early stages, their expression was highly overlapping. At later stages, Gata2 expression became prominent in vestibular and cochlear nonsensory epithelia. In contrast to Gata2, Gata3 was mainly expressed in the developing sensory epithelia, reflecting the importance of this factor in the sensory-neural development of the inner ear. While the later expression patterns of both Gata3 and Gata2 were highly conserved between chicken and mouse, important differences were observed especially with Gata3 during early otic development, providing indications of divergent molecular control during placode invagination in mice and chickens. We also found indications that the regulatory hierarchy observed in mouse, where Gata3 is upstream of Gata2 and Fgf10, could be conserved in chicken.

Pituitary adenylyl cyclase-activating polypeptide (PACAP) and its receptor (PAC1-R) are positioned to modulate afferent signaling in the cochlea

Pituitary adenylyl cyclase-activating polypeptide (PACAP), via its specific receptor pituitary adenylyl cyclase-activating polypeptide receptor 1 (PAC1-R), is known to have roles in neuromodulation and neuroprotection associated with glutamatergic and cholinergic neurotransmission, which, respectively, are believed to form the primary basis for afferent and efferent signaling in the organ of Corti. Previously, we identified transcripts for PACAP preprotein and multiple splice variants of its receptor, PAC1-R, in microdissected cochlear subfractions. In the present work, neural localizations of PACAP and PAC1-R within the organ of Corti and spiral ganglion were examined, defining sites of PACAP action. Immunolocalization of PACAP and PAC1-R in the organ of Corti and spiral ganglion was compared with immunolocalization of choline acetyltransferase (ChAT) and synaptophysin as efferent neuronal markers, and glutamate receptor 2/3 (GluR2/3) and neurofilament 200 as afferent neuronal markers, for each of the three cochlear turns. Brightfield microscopy giving morphological detail for individual immunolocalizations was followed by immunofluorescence detection of co-localizations. PACAP was found to be co-localized with ChAT in nerve fibers of the intraganglionic spiral bundle and beneath the inner and outer hair cells within the organ of Corti. Further, evidence was obtained that PACAP is expressed in type I afferent axons leaving the spiral ganglion en route to the auditory nerve, potentially serving as a neuromodulator in axonal terminals. In contrast to the efferent localization of PACAP within the organ of Corti, PAC1-R immunoreactivity was co-localized with afferent dendritic neuronal marker GluR2/3 in nerve fibers passing beneath and lateral to the inner hair cell and in fibers at supranuclear and basal sites on outer hair cells. Given the known association of PACAP with catecholaminergic neurotransmission in sympathoadrenal function, we also re-examined the issue of whether the organ of Corti receives adrenergic innervation. We now demonstrate the existence of nerve fibers within the organ of Corti which are immunoreactive for the adrenergic marker dopamine β-hydroxylase (DBH). DBH immunoreactivity was particularly prominent in nerve fibers both at the base and near the cuticular plate of outer hair cells of the apical turn, extending to the non-sensory Hensen’s cell region. Evidence was obtained for limited co-localization of DBH with PAC1-R and PACAP. In the process of this investigation, we obtained evidence that efferent and afferent nerve fibers, in addition to adrenergic nerve fibers, are present at supranuclear sites on outer hair cells and distributed within the non-sensory epithelium of the apical cochlear turn for rat, based upon immunoreactivity for the corresponding neuronal markers. Overall, PACAP is hypothesized to act within the organ of Corti as an efferent neuromodulator of afferent signaling via PAC1-R that is present on type I afferent dendrites, in position to afford protection from excitotoxicity. Additionally, PACAP/PAC1-R may modulate secretion of catecholamines from adrenergic terminals within the organ of Corti.

BMP-signaling regulates the generation of hair-cells

Bone morphogenetic proteins (BMPs) are diffusible molecules involved in a variety of cellular interactions during development. Bmp4 expression accompanies the development of the ear sensory organs during patterning and specification of sensory cell fates, yet there is no understanding of the role of BMP4 in this process. The present work was aimed at exploring the effects of BMP-signaling on the development of hair-cells. For this purpose, we studied gene expression, cell proliferation and cell death in isolated chick otic vesicles that were grown in vitro in the presence of recombinant BMP4 or the BMP-inhibitor Noggin. Cath1 was used as a marker for hair-cell specification. BMP4 reduced the number of Cath1-cells and, conversely, Noggin increased the size of the sensory patches and the number of Cath1-positive cells. The effect of BMP4 was irreversible and occurred before hair-cell specification. Lfng and Fgf10 were expressed in the prosensory domain before Cath1, and their expression was expanded by Noggin. At these stages, modifications of BMP activity did not respecify non-sensory epithelium of the otic vesicle. The expression of Bmp4 at sensory patches was suppressed by BMP4 and induced by Noggin suggesting an autoregulatory loop. Analysis of BrdU incorporation during 6 and 18 h indicated that the effects of BMP4 were due to its ability to reduce the number of actively proliferating progenitors and inhibit cell fate specification. BMP4 induced cell death within the prosensory domain of the otic vesicle, along with the expression of Msx1, but not Msx2. On the contrary, BMP-inhibition with Noggin favored hair-cell specification without changes in the overall cell proliferation. We propose that about the stage of terminal division, the balance between BMP and BMP-inhibitory signals regulates survival and specification of hair-cell precursors, the final number of sensory hair-cells being limited by excess levels of BMPs. The final size of sensory patches would hence depend on the balance between BMP4 and opposing signals.

Structural organization of the olfactory epithelium of a spotted snakehead fish, Channa punctatus

Background. Enormous diversities exist regarding shape, number, and arrangement of the olfactory lamellae, distribution of the sensory and non-sensory epithelium as well as the abundance of various receptor cells among different teleosts. This study was aimed at describing the morphology, histology and SEM ultrastructure of the olfactory epithelium of a spotted snakehead fish, Channa punctatus (Bloch, 1793). Materials and methods. OE of C. punctatus was examined through LM and SEM. Results. Olfactory epithelium of C. punctatus containing 18 to 20 lamellae provides a large surface area. Sensory epithelium possesses both ciliated and microvillous olfactory receptor cells (ORC), which are intermingled and distributed throughout the epithelium. ORCs are arranged in alternate rows with supporting cells (SCs). ORC is columnar cell with a round cell body, an axonal process and a long dendrite. The apical surfaces of the ORCs are provided either with 10–12 cilia or numerous microvilli and the SCs with fingerprint like microridges. ORCs with rod-shaped dendrite ending were found in small numbers. Non-sensory epithelium is composed of ciliated nonsensory cells (cNSC) and covered with a dense carpet of cilia, which help in ventilating the olfactory chambers. Conclusions. A large surface area of OE, the high density of both ciliated and microvillous ORC and adequate arrangement for ventilating the olfactory chambers indicate that C. punctatus is very much dependent on its olfactory sense.

Expression of bHLH Transcription Factors and IGFs in the Non-sensory Patches, Olfactory Epithelium and Vomeronasal Organ

Non-sensory patches in the posterior dorsal fossa of the rodent nasal cavity are formed between postnatal (P) days 10 and 21 due to programmed death of ORNs and GBCs (Suzuki et al., 2000). In a previous study, we found bHLH transcription factors Hes6 and NeuroD to be expressed in the GBC layer in the olfactory epithelium (OE) of mice. During embryonic days, there were no regional differences in the expression of these two genes, whereas at postnatal days 3–7, expression disappeared in the region corresponding to the presumptive non-sensory patches (Suzuki et al., 2003). Hes 6 and NeuroD are known to promote neuronal differentiation, NeuroD acting downstream of Mash1 and Neurogenin 1, which are determination genes for ORNs. Mash 1 null mutant mice failed to produce progenitor cells and Neurogenin 1 null mutant mice, progenitors of ORNs are generated but their differentiation is blocked (Cau et al., 2002). It is not yet known whether Mash1 and Neurogenin1 are related to the formation of non-sensory patches. On the other hand, insulin-like growth factor (IGF) family and related molecules were reported to be expressed in the rat OE where IGFs may have functions of differentiation and survival of ORNs (Suzuki and Takeda, 2002). Six high-affinity IGF binding proteins (IGFBP-1 to -6) modulate IGF effects. In the present study, we examined whether the expression of bHLH transcription factors and IGFBPs changes in the region where the programmed death of ORNs and GBCs occurs.

Acid-Sensitive Two-Pore Domain Potassium (K2P) Channels in Mouse Taste Buds

Sour (acid) taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K(2)P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca(2+)-, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 microM), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (< or = approximately 17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 microM; Gd(3+), 1 mM; arachidonic acid, 100 microM; quinidine, 200 microM; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.