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Salt taste is one of the 5 basic taste qualities. Depending on the concentration, table salt is perceived either as appetitive or aversive, suggesting the contribution of several mechanisms to salt taste, distinguishable by their sensitivity to the epithelial sodium channel (ENaC) blocker amiloride. A taste-specific knockout of the α-subunit of the ENaC revealed the relevance of this polypeptide for low-salt transduction, whereas the response to other taste qualities remained normal. The fully functional ENaC is composed of α-, β-, and γ-subunits. In taste tissue, however, the precise constitution of the channel and the cell population responsible for detecting table salt remain uncertain. In order to examine the cells and subunits building the ENaC, we generated mice carrying modified alleles allowing the synthesis of green and red fluorescent proteins in cells expressing the α- and β-subunit, respectively. Fluorescence signals were detected in all types of taste papillae and in taste buds of the soft palate and naso-incisor duct. However, the lingual expression patterns of the reporters differed depending on tongue topography. Additionally, immunohistochemistry for the γ-subunit of the ENaC revealed a lack of overlap between all potential subunits. The data suggest that amiloride-sensitive recognition of table salt is unlikely to depend on the classical ENaCs formed by α-, β-, and γ-subunits and ask for a careful investigation of the channel composition.

Like other membrane receptor-mediated responses, execution of phagocytosis requires the transduction of signals to cytoplasmic effectors. Signaling in this case is particularly complex as the process involves not only the formation of phagosomes but also their subsequent maturation and resolution. Transient increases in cytosolic calcium, which mediate a variety of other transduction pathways, also feature prominently in phagocytosis. However, despite intensive study over the course of nearly 30 years, the occurrence, source, and functional relevance of such calcium bursts remain the subject of debate. Here, we have attempted to consolidate the information that was reviewed in the past with more recent studies in an effort to shed some light on the existing controversies.

The lingual surface potential (LSP), which hyperpolarizes in response to salt and bitter stimuli, is thought to be a bioelectrical signal associated with taste transduction in humans. In contrast, a recent study reported sweet and sour stimuli to evoke a depolarization of the LSP. We questioned the origin of such a depolarization because liquid junction potentials (JPs), which arise at the interfaces of recording electrode and taste solutions, are neglected in the report. We recorded the LSPs to sucrose and NaCl solutions on the human tongue using an Ag/AgCl electrode. To estimate JPs generated by each taste solution, we made an agar model to simulate the human tongue. The lingual surface was rinsed with a 10 mM NaCl solution that mimics the sodium content of the lingual fluid. In the human tongue, sucrose dissolved in distilled water evoked a depolarizing LSP that could be attributed to JPs, resulting from the change in electrolyte concentration of the taste solution. Sucrose dissolved in 10 mM NaCl solution evoked a hyperpolarizing LSP which became more negative in a concentration-dependent manner (300–1500 mM). Lactisole (3.75 mM), an inhibitor of sweet taste, significantly reduced the LSPs and decreased perceived intensity of sweetness by human subjects. The negative JPs generated by 100 mM NaCl in the agar model were not different from the LSPs to 100 mM NaCl. When the electrolyte environment on the lingual surface is controlled for JPs, the bioelectrical signal associated with sweet taste transduction is a hyperpolarizing potential.

In taste buds, Type I cells represent the majority of cells (50-60%) and primarily have a glial-like function in taste buds. However, recent studies suggest that they have additional sensory and signaling functions including amiloride-sensitive salt transduction, oxytocin modulation of taste, and substance P mediated GABA release. Nonetheless, the overall function of Type I cells in transduction and signaling remains unclear, primarily because of the lack of a reliable reporter for this cell type. GAD65 expression is specific to Type I taste cells and GAD65 has been used as a Cre driver to study Type I cells in salt taste transduction. To test the specificity of transgene-driven expression, we crossed GAD65Cre mice with floxed tdTomato and Channelrhodopsin (ChR2) lines and examined the progeny with immunochemistry, chorda tympani recording, and calcium imaging. We report that while many tdTomato+ taste cells express NTPDase2, a specific marker of Type I cells, we see expression of tdTomato in both Gustducin and SNAP25 positive taste cells. We also see ChR2 in cells just outside the fungiform taste buds. Chorda tympani recordings in the GAD65Cre/ChR2 mice show large responses to blue light, larger than any response to standard taste stimuli. Further, several isolated tdTomato positive taste cells responded to KCl depolarization with increases in intracellular calcium, indicating the presence of voltage-gated calcium channels. Taken together, these data suggest that GAD65Cre mice drive expression in multiple taste cell types and thus cannot be considered a reliable reporter of Type I cell function.

Activation of Type III cells in mammalian taste buds is implicated in the transduction of acids (sour) and salty stimuli. Several lines of evidence suggest that function of Type III cells in the anterior taste fields may differ from that of Type III cells in posterior taste fields. Underlying anatomy to support this observation is, however, scant. Most existing immunohistochemical data characterizing this cell type focus on circumvallate taste buds in the posterior tongue. Equivalent data from anterior taste fields—fungiform papillae and soft palate—are lacking. Here, we compare Type III cells in four taste fields: fungiform, soft palate, circumvallate, and foliate in terms of reactivity to four canonical markers of Type III cells: polycystic kidney disease 2-like 1 (PKD2L1), synaptosomal associated protein 25 (SNAP25), serotonin (5-HT), and glutamate decarboxylase 67 (GAD67). Our findings indicate that while PKD2L1, 5-HT, and SNAP25 are highly coincident in posterior taste fields, they diverge in anterior taste fields. In particular, a subset of taste cells expresses PKD2L1 without the synaptic markers, and a subset of SNAP25 cells lacks expression of PKD2L1. In posterior taste fields, GAD67-positive cells are a subset of PKD2L1 expressing taste cells, but anterior taste fields also contain a significant population of GAD67-only expressing cells. These differences in expression patterns may underlie the observed functional differences between anterior and posterior taste fields.

In retinal photoreceptors, vectorial transport of cargo is critical for transduction of visual signals, and defects in intracellular trafficking can lead to photoreceptor degeneration and vision impairment. Molecular signatures associated with routing of transport vesicles in photoreceptors are poorly understood. We previously reported the identification of a novel rod photoreceptor specific isoform of Receptor Expression Enhancing Protein (REEP) 6, which belongs to a family of proteins involved in intracellular transport of receptors to the plasma membrane. Here we show that loss of REEP6 in mice (Reep6−/−) results in progressive retinal degeneration. Rod photoreceptor dysfunction is observed in Reep6−/− mice as early as one month of age and associated with aberrant accumulation of vacuole-like structures at the apical inner segment and reduction in selected rod phototransduction proteins. We demonstrate that REEP6 is detected in a subset of Clathrin-coated vesicles and interacts with the t-SNARE, Syntaxin3. In concordance with the rod degeneration phenotype in Reep6−/− mice, whole exome sequencing identified homozygous REEP6-E75K mutation in two retinitis pigmentosa families of different ethnicities. Our studies suggest a critical function of REEP6 in trafficking of cargo via a subset of Clathrin-coated vesicles to selected membrane sites in retinal rod photoreceptors.

Among the 5 taste qualities, salt is the least understood. The receptors, their expression pattern in taste cells, and the transduction mechanisms for salt taste are still unclear. Previous studies have suggested that low concentrations of NaCl are detected by the amiloride-sensitive epithelial Na+ channel (ENaC), which in other systems requires assembly of 3 homologous subunits (α, β, and γ) to form a functional channel. However, a new study from Lossow and colleagues, published in this issue of Chemical Senses, challenges that hypothesis by examining expression levels of the 3 ENaC subunits in individual taste cells using gene-targeted mice in combination with immunohistochemistry and in situ hybridization. Results show a lack of colocalization of ENaC subunits in taste cells as well as expression of subunits in taste cells that show no amiloride sensitivity. These new results question the molecular identity of the amiloride-sensitive Na+ conductance in taste cells.

The Pacinian corpuscle (PC) is a dermal mechanoreceptor that responds to high-frequency (20-1000 Hz) vibrations. The PC's structure allows transmission of vibrations through its layers (lamellae) to the centrally-located nerve fiber (neurite). This work combines mechanical models of the PC with an electrochemical model of peripheral nerves to simulate the tactile response of the entire system. A three-stage model of response to a vibratory input was developed, consisting of (1) outer core mechanics, (2) inner core mechanics, and (3) neurite electrochemistry. The model correctly predicts the band-pass nature of the PC's frequency response, showing that the PC structure can amplify oscillatory strains within its target frequency band. Specifically, strain induced by a vibratory stimulus is amplified by a factor of 8-12 from the PC surface to the neurite. Our results also support the hypothesis that PC rapid adaptation is affected by the lamellar structures without requiring neuronal adaptivity. Simulated different-sized PCs showed a shift in frequency response, suggesting that clusters of different-sized PCs could enable more nuanced tactile encoding than uniform clusters. By modeling the PC's mechano-to-neural transduction, we can begin to characterize the mechanosensation of other receptors to understand how multiple receptors interact to create our sensation of touch.

Different parts of the mouth vary in their taste responsiveness and gustatory transduction components. However, there have been few attempts to consider regional variation among areas innervated by a single nerve branch or containing only one type of gustatory papilla. Here, we examined whether taste-elicited responses of a single nerve, the chorda tympani (CT), depend on where taste solutions are delivered on the tongue in mice. In experiment 1, multiunit CT responses to NaCl and sucrose were larger if sapid taste solutions were applied to the tongue tip, which contains the anterior-most fungiform papillae, than if they were flowed over fungiform and foliate papillae on the posterior tongue. Further, the epithelial sodium channel (ENaC) blocker amiloride suppressed NaCl responses to a greater degree for the tongue tip. In experiment 2, CT nerve responses were compared between the tongue tip and a region further back that contained only fungiform papillae. NaCl and sucrose solutions applied to posterior fungiform papillae produced smaller responses than did those elicited by the same taste stimuli applied to anterior fungiform papillae on the tongue tip. Amiloride suppressed the response to NaCl delivered to the anterior fungiform but not posterior fungiform papillae. These results indicate that the CT response is tongue-region dependent in the mouse. Furthermore, the spatial location of a fungiform papilla provides important information about its properties, such as whether sodium taste transduction is mediated by amiloride-sensitive ENaCs.

The main olfactory system of mice contains a small subset of olfactory sensory neurons (OSNs) that are stimulated by CO2. The objective of this study was to record olfactory receptor responses to a range of CO2 concentrations to further elucidate steps in the proposed CO2 transduction pathway in mice. Electro-olfactograms (EOGs) were recorded before and after inhibiting specific steps in the CO 2 transduction pathway with topically applied inhibitors. Inhibition of extracellular carbonic anhydrase (CA) did not significantly affect EOG responses to CO 2 but did decrease EOG responses to several control odorants. Inhibition of intracellular CA or cyclic nucleotide-gated channels attenuated EOG responses to CO2, confirming the role of these components in CO2 sensing in mice. We also show that, like canonical OSNs, CO2-sensitive OSNs depend on Ca 2+ -activated Cl − channels for depolarization of receptor neurons. Lastly, we found that guanylyl cyclase-D knockout mice were still able to respond to CO2, indicating that other pathways may exist for the detection of low concentrations of nasal CO2. We discuss these findings as they relate to previous studies on CO 2-sensitive OSNs in mice and other animals.

Stomatopod crustaceans have complex visual systems containing up to 16 different spectral classes of photoreceptors, more than described for any other animal. A previous molecular study of this visual system focusing on the expression of opsin genes found many more transcripts than predicted on the basis of physiology, but was unable to fully document the expressed opsin genes responsible for this diversity. Furthermore, questions remain about how other components of phototransduction cascades are involved. This study continues prior investigations by examining the molecular function of stomatopods' visual systems using new whole eye 454 transcriptome datasets from two species, Hemisquilla californiensis and Pseudosquilla ciliata. These two species represent taxonomic diversity within the order Stomatopoda, as well as variations in the anatomy and physiology of the visual system. Using an evolutionary placement algorithm to annotate the transcriptome, we identified the presence of nine components of the stomatopods' G-protein-coupled receptor (GPCR) phototransduction cascade, including two visual arrestins, subunits of the heterotrimeric G-protein, phospholipase C, transient receptor potential channels, and opsin transcripts. The set of expressed transduction genes suggests that stomatopods utilize a Gq-mediated GPCR-signaling cascade. The most notable difference in expression between the phototransduction cascades of the two species was the number of opsin contigs recovered, with 18 contigs found in retinas of H. californiensis, and 49 contigs in those of P. ciliata. Based on phylogenetic placement and fragment overlap, these contigs were estimated to represent 14 and 33 expressed transcripts, respectively. These data expand the known opsin diversity in stomatopods to clades of arthropod opsins that are sensitive to short wavelengths and ultraviolet wavelengths and confirm the results of previous studies recovering more opsin transcripts than spectrally distinct types of photoreceptors. Many of the recovered transcripts were phylogenetically placed in an evolutionary clade of crustacean opsin sequences that is rapidly expanding as the visual systems from more species are investigated. We discuss these results in relation to the emerging pattern, particularly in crustacean visual systems, of the expression of multiple opsin transcripts in photoreceptors of the same spectral class, and even in single photoreceptor cells.

Primary cilia have essential roles in transducing signals in eukaryotes. At their core is the ciliary axoneme, a microtubule-based structure that defines cilium morphology and provides a substrate for intraflagellar transport. However, the extent to which axonemal microtubules are specialized for sensory cilium function is unknown. In the nematode Caenorhabditis elegans, primary cilia are present at the dendritic ends of most sensory neurons, where they provide a specialized environment for the transduction of particular stimuli. Here, we find that three tubulin isotypes—the α-tubulins TBA-6 and TBA-9 and the β-tubulin TBB-4—are specifically expressed in overlapping sets of C. elegans sensory neurons and localize to the sensory cilia of these cells. Although cilia still form in mutants lacking tba-6, tba-9, and tbb-4, ciliary function is often compromised: these mutants exhibit a variety of sensory deficits as well as the mislocalization of signaling components. In at least one case, that of the CEM cephalic sensory neurons, cilium architecture is disrupted in mutants lacking specific ciliary tubulins. While there is likely to be some functional redundancy among C. elegans tubulin genes, our results indicate that specific tubulins optimize the functional properties of C. elegans sensory cilia.

In many species, olfactory transduction is triggered by odorant molecules that interact with olfactory receptors coupled to heterotrimeric G-proteins. The role of G-protein-linked transduction in the olfaction of Drosophila is currently under study. Here, we supply a thorough description of the expression in the olfactory receptor organs (antennae and maxillary palps) of all known Drosophila melanogaster genes that encode for G-proteins. Using RT-polymerase chain reaction, we analyzed 6 Galpha (G(s), G(i), G(q), G(o), G(f), and concertina), 3 Gbeta (G(beta5), G(beta13F), and G(beta76C)), and 2 Ggamma genes (G(gamma1) and G(gamma30A)). We found that all Galpha protein-encoding genes showed expression in both olfactory organs, but G(f) mRNA was not detected in palps. Moreover, all the Gbeta and Ggamma genes are expressed in antennae and palps, except for G(beta76C). To gain insight into the hypothesis of different G-protein subunits mediating differential signaling in olfactory receptor neurons (ORNs), we performed immunohistochemical studies to observe the expression of several Galpha and Gbeta proteins. We found that Gs, Gi, Gq, and G(beta13F) subunits displayed generalized expression in the antennal tissue, including ORNs support cells and glial cells. Finally, complete coexpression was found between Gi and Gq, which are mediators of the cyclic adenosine monophosphate and IP3 transduction cascades, respectively.

In mammals, detection of osmolarity by the gustatory system was overlooked until recently. In insects, specific taste receptor neurons detect hypoosmotic stimuli and are commonly called "W" (water) cells. W cells are easy to access in vivo and represent a good model to study the transduction of hypoosmotic stimuli. Using pharmacological and genetic approaches in Drosophila, we show that tarsal W cell firing activity depends on the concentration of external calcium bathing the dendrite. This dependence was confirmed by the strong inhibition of W cell responses to hypoosmotic stimuli by lanthanum (IC(50) = 8 nM), an ion known to inhibit calcium-permeable channels. Downstream, the transduction pathway likely involves calmodulin because calmodulin antagonists such as W-7 (IC(50) = 2 microM) and fluphenazine (IC(50) = 30 microM) prevented the activation of the W cell by hypoosmotic stimuli. A protein kinase C (PKC) may also be involved as W cell responses were blocked by PKC inhibitors, chelerythrine (IC(50) = 20 microM) and staurosporine (IC(50) = 30 microM). It was also reduced when expressing an inhibitory pseudosubstrate of PKC in gustatory receptor neurons. In the rat, the transduction pathway underlying low osmolarity detection involves aquaporin and swelling-activated ion channels. Our study suggests that the transduction pathway of hypoosmotic stimuli in insects differs from mammals.

Odorants cause Ca(2+) to rise in olfactory sensory neurons (OSNs) first within the ciliary compartment, then in the dendritic knob, and finally in the cell body. Ca(2+) not only excites but also produces negative feedback on the transduction pathway. To relieve this Ca(2+)-dependent adaptation, Ca(2+) must be cleared from the cilia and dendritic knob by mechanisms that are not well understood. This work focuses on the roles of plasma membrane calcium pumps (PMCAs) through the use of inhibitors and mice missing 1 of the 4 PMCA isoforms (PMCA2). We demonstrate a significant contribution of PMCAs in addition to contributions of the Na(+)/Ca(2+) exchanger and endoplasmic reticulum (ER) calcium pump to the rate of calcium clearance after OSN stimulation. PMCAs in neurons can shape the Ca(2+) signal. We discuss the contributions of the specific PMCA isoforms to the shape of the Ca(2+) transient that controls signaling and adaptation in OSNs.

Amiloride and its derivatives inhibit a number of sensory transduction processes, including some types of chemosensory transduction. Here, we report that pyrazine derivatives of amiloride reversibly inhibit odorant-evoked activity in lobster olfactory receptor neurons. The potency sequence is as follows-(IC50, mM): 5-(N,N-hexamethylene)amiloride (0.015) approximately 5-(N-methyl-N-isobutyl)amiloride (0.02) approximately 5-(N-ethyl-N-isopropyl)amiloride (0.03)5-(N,N-dimethyl)amiloride (0.48); 3',4'-dichlorobenzamil (0.4), phenamil (0.5), and amiloride itself (2) are ineffective. The same derivatives with the similar potency sequence also block a presumptive transient receptor potential (TRP) channel that is the likely downstream target of phosphoinositide signaling in these cells. Our results suggest that pyrazine derivatives of amiloride are useful probes to study more detailed mechanisms of chemosensory transduction in this system and possibly in other chemosensory systems in which TRP channels are the known or suspected downstream effector.

The gustatory system is essential for almost all animals. However, the signal transduction mechanisms have not yet been fully elucidated. We isolated labellar chemosensilla from blowfly, Phormia regina, and purified calcium binding proteins from the water soluble fraction. The most abundant calcium-binding protein was calmodulin. To investigate the role of calmodulin in taste transduction, electrophysiological responses were recorded with the calmodulin inhibitor, W-7. When we stimulated the labellar chemosensillum with sucrose plus W-7, a dose-dependent decrease of impulse frequency was observed when the concentration was50 microM. In addition, when W-7 at 50 microM or higher concentration was added, an initial short-term impulse generation from the sugar receptor cell was observed, but this was followed by a silent period. When the sensillum was stimulated with W-7 plus a membrane-permeable cGMP analog, dibtyryl-cGMP or 8-bromo-cGMP, impulses of the sugar receptor cell were induced but the frequency was decreased. By the sidewall-recording method, we observed that the receptor potential induced by sucrose stimulation was decreased by W-7 in the sugar receptor cell, and corresponded with a disappearance of impulses. These data strongly suggest that the cGMP-gated channel generating receptor potential in the sugar receptor cell requires calmodulin for its gating.

Olfactory receptor neurons employ a diversity of signaling mechanisms for transducing and encoding odorant information. The simultaneous activation of subsets of receptor neurons provides a complex pattern of activation in the olfactory bulb that allows for the rapid discrimination of odorant mixtures. While some transduction elements are conserved among many species, some species-specificity occurs in certain features that may relate to their particular physiology and ecological niche. However, studies of olfactory transduction have been limited to a relatively small number of vertebrate and invertebrate species. To better understand the diversity and evolution of olfactory transduction mechanisms, we studied stimulus-elicited calcium fluxes in olfactory neurons from a previously unstudied mammalian species, the domestic cat. Isolated cells from cat olfactory epithelium were stimulated with odorant mixtures and biochemical agents, and cell responses were measured with calcium imaging techniques. Odorants elicited either increases or decreases in intracellular calcium; odorant-induced calcium increases were mediated either by calcium fluxes through the cell membrane or by mobilization of intracellular stores. Individual cells could employ multiple signaling mechanisms to mediate responses to different odorants. The physiological features of these olfactory neurons suggest greater complexity than previously recognized in the role of peripheral neurons in encoding complex odor stimuli. The investigation of novel and unstudied species is important for understanding the mechanisms of odorant signaling that apply to the olfactory system in general and suggests both broadly conserved and species-specific evolutionary adaptations.

To clarify the involvement of G protein in denatonium signal transduction, we carried out a whole-cell patch-clamp analysis with isolated taste cells in mice. Two different responses were observed by applying GDP-beta-S, a G-protein inhibitor. One response to denatonium was reduced by GDP-beta-S (G-protein-dependent), whereas the other was not affected (G-protein-independent). These different patterns were also observed by concurrently inhibiting the phospholipase C beta2 and phosphodiesterase pathways via G protein. These data suggest dual, G-protein-dependent and -independent mechanisms for denatonium. Moreover, the denatonium responses were not attenuated by singly inhibiting the phospholipase C beta2 or phosphodiesterase pathway, implying that both pathways were involved in G-protein-dependent transduction. In the G-protein-independent cells, the response was abolished by the depletion of calcium ions within the intracellular store. These results suggest that Ca2+ release from the intracellular store is an important factor. Our data demonstrate multiple transduction pathways for denatonium in mammalian taste cells.

Reactive oxygen species (ROS) are highly reactive reduced oxygen molecules. Recent studies have shown that production of ROS occurs in response to many physiological stimuli in plant cells, including pathogen attack, hormone signaling, polar growth, and gravitropism. Evidence is emerging that ROS

Neural oscillatory activities triggered by odorant stimulation have been often reported at various levels of olfactory nervous systems in vertebrates. To elucidate the origin of neural oscillations, we studied first the oscillatory properties of current responses of isolated olfactory receptor neurons (ORNs) of the rainbow trout to amino acid odorants, using a whole-cell voltage-clamp technique and found that the damped current oscillations were intrinsic in both ciliated and microvillous ORNs and occurred when ORNs were stimulated by odorants at high intensities. Continuous wavelet analysis using the Gabor function revealed that the dominant frequency of oscillations was 1.89 +/- 0.50 Hz (mean +/- SD, n = 92). There was no significant difference in oscillation frequency between the two types of ORNs and between different perfusion conditions with standard and Na(+)-free (choline) Ringer's solutions, but there was a slight difference in oscillation frequency between different holding potential conditions of negative and positive potentials. We then performed a computer simulation of the current responses with a cAMP olfactory transduction model. The model was based on the assumption that the current responses of ORNs were linearly related to the sum of concentrations of active cyclic-nucleotide-gated channels and Ca(2+)-activated Cl(-) channels, and was expressed by 12 differential equations with 44 different parameters. The simulation revealed that the oscillations of current responses of ORNs were mainly due to the oscillatory properties of intracellular cAMP and Ca(2+) concentrations. The necessary reaction component for the oscillations in the transduction model was direct inhibition of adenylate cyclase activity by Ca(2+). High Ca(2+) efflux by the Na(+)-Ca(2+) exchanger and cAMP-phosphodiesterase activity were most influential on the oscillations. The simulation completely represented the characteristics of current responses of ORNs: odorant-intensity-dependent response, intensity-dependent latency and adaptation. Thus, the simulation is generally applicable to current and voltage responses of ORNs equipped with cAMP olfactory transduction pathway in other vertebrate species. The simulation programs for Macintosh (cAMP 9.2.7 and 9.2.8 for MacOS 8.1 or later) and cAMP JAVA applet versions based on cAMP 9.2.8 have been published on the world wide web (http://bio2.sci.hokudai.ac.jp/bio/chinou1/noriyo_home.html).

The transduction of responses to bitter and sweet compounds utilizes guanine nucleotide binding proteins (G proteins) and their coupled receptors. Alpha-gustducin, a transducin-like G protein alpha-subunit, and rod alpha-transducin are expressed in taste receptor cells. Alpha-gustducin knockout mice have profoundly diminished behavioral and electrophysiological responses to many bitter and sweet compounds, although these mice retain residual responses to these compounds. Alpha-gustducin and rod alpha-transducin are biochemically indistinguishable in their in vitro interactions with retinal phosphodiesterase, rhodopsin and G protein betagamma-subunits. To determine if alpha-transducin can function in taste receptor cells and to compare the function of alpha-gustducin versus alpha-transducin in taste transduction in vivo, we generated transgenic mice that express alpha-transducin under the control of the alpha-gustducin promoter in the alpha-gustducin null background. Immunohistochemistry showed that the alpha-transducin transgene was expressed in about two-thirds of the alpha-gustducin lineage of taste receptor cells. Two-bottle preference tests showed that transgenic expression of rod alpha-transducin partly rescued responses to denatonium benzoate, sucrose and the artificial sweetener SC45647, but not to quinine sulfate. Gustatory nerve recordings showed a partial rescue by the transgene of the response to sucrose, SC45647 and quinine, but not to denatonium. These results demonstrate that alpha-transducin can function in taste receptor cells and transduce some taste cell responses. Our results also suggest that alpha-transducin and alpha-gustducin may differ, at least in part, in their function in these cells, although this conclusion must be qualified because of the limited fidelity of the transgene expression.

Understanding how the cochlea works as a system has become increasingly important. We need to know this before integrating new information from genetic, physiological and clinical sources. This chapter will show how the cochlea should be seen as a device for carrying out a frequency analysis built from cells that have been adapted for specialist purposes. Sensory hair cells convert mechanical displacements into the neural code. The transducer channel remains to be identified. The biomechanics of the cochlear duct depends on an energy-dependent feedback from the sensory outer hair cells. The molecular basis for outer hair cell feedback depends on a protein that has recently been identified. The auditory signal encoded by the cochlea is further modified by membrane properties of the hair cells and cochlear supporting cells. The interplay between techniques of genetics, molecular biology and cell physiology has started to reveal which ion channels and transporters in the cochlea are mutated in certain forms of deafness. The interpretation of these mutations requires the cell physiology of the cochlear partition to be better characterised in the future.

Odor transduction mediated by the adenylyl cyclase/cAMP pathway has been well studied, but it is still uncertain whether this pathway mediates the transduction of all odors in vertebrates. We isolated olfactory sensory neurons from the salamander Necturus maculosus and used calcium imaging with the indicator dye fura-2 to examine olfactory responses elicited by amino acids. The properties of approximately two-thirds of the odor responses suggested they were mediated by the adenylyl cyclase/ cAMP pathway, but one-third of the responses were not mimicked by cAMP analogs nor blocked by inhibition of adenylyl cyclase, suggesting that these odor responses were mediated differently. Responses that were unaffected by inhibition of adenylyl cyclase were blocked by neomycin, an inhibitor of phospholipase C, implying that they were transduced by activation of phospholipase C. Some cells which responded to more than one amino acid appeared to employ both pathways, but each was used to transduce different odors. In addition, many responses that were mediated by the adenylyl cyclase/cAMP pathway were enhanced following inhibition of phospholipase C, suggesting that the phospholipase C pathway has a role not only in odor transduction, but also in the modulation of olfactory responses.

Mechanosensation requires the transduction of mechanical stimuli into neuronal impulses. It encompasses not only the sense of touch but also proprioception and hearing. In contrast to sight, smell and taste, relatively little is known about the molecular machinery of mechanosensation. It is already clear, however, that important aspects are conserved across phyla, from Caenorhabditis elegans to humans. Drosophila melanogaster is well placed to make a significant contribution to this field. Its advantages include a sequenced genome allied with powerful genetic techniques, and the ability to conduct electrophysiological recording from mechanoreceptor neurons. For human geneticists, it is expected that Drosophila studies will provide a source of candidate genes whose human homologues can be examined for roles in mechanosensory development, function and disease.

Objectives: To characterize the role that cardiac sensory P1 purinergic (adenosine A1 or A2) receptors play in transducing myocardial ischemia. Methods : Porcine nodose ganglion cardiac sensory neuron adenosine A1 or A2 receptor function was studied in situ during control states as well as in the presence of the peptides bradykinin and substance P or focal ventricular ischemia. The responses of porcine nodose ganglion cardiac and non-cardiac afferent neuronal somata to adenosine were also studied in vitro. Results : Local application of A1 or A2 adenosine receptor agonists modified the activity generated by ventricular sensory neurites associated with 70 and 74% of identified nodose ganglion cardiac afferent somata in situ, respectively, exciting most neurons. In contrast, adenosine reduced the excitability of nodose ganglion cardiac afferent neuronal somata in vitro. Bradykinin and substance P affected 56 and 63%, respectively, of tested afferent neurons. The capacity of ventricular sensory neurites to transduce signals relating to these peptides was virtually eliminated by the presence of P1 purinergic receptor antagonists. So was their capacity to transduce focal ventricular ischemia. Since most cardiac sensory neurites responded differently to adenosine in vivo than did cardiac afferent neuronal somata in vitro, it appears that the transduction properties of cardiac afferent neurons need to be characterized in situ. Conclusions : Most ventricular sensory neurites associated with nodose ganglion afferent neurons possess adenosine A1 and/or A2 receptors that play a primary role in transducing myocardial ischemic events to central neurons. These data support clinical observations implicating cardiac sensory purinoceptors in transducing myocardial ischemic events.

Studies in the last two decades have shown that amiloride-sensitive Na(+) channels play a role in NaCl transduction in rat taste receptors. However, this role is not readily generalized for salt taste transduction in vertebrates, because functional expression of these channels varies across species and also in development in a species. Glossopharyngeal nerve responses to sodium and potassium salts were recorded in larval and metamorphosed salamanders and compared before and after the oral floor was exposed to amiloride, a blocker of Na(+) channels known to be responsible for epithelial ion transport. Pre-exposure to amiloride (100 microM) did not affect salt taste responses in both axolotls (Ambystoma mexicanum) and larval Ezo salamanders (Hynobius retardatus). In contrast, in metamorphosed Ezo salamanders the nerve responses to NaCl were significantly reduced by amiloride. In amphibians amiloride-sensitive components in salt taste transduction seem to develop during metamorphosis.

It has been postulated for decades that ion channels serve as receptors for most sour tasting stimuli. Though many candidates exist, definitive evidence linking any particular channel to sour taste perception has been elusive. Several studies have suggested that two members of the polycystic kidney disease-like family may function as components of an ionotropic taste receptor mediating the transduction of acids. However, the precise role of these proteins in sour taste is controversial. In this issue of Chemical Senses, Nelson et al. use behavioral and electrophysiological approaches in gene-targeted mice to show that one of these putative sour taste receptor subunits, Pkd1l3, is unnecessary for normal taste responses to acids. Their results suggest that other mechanisms and/or other candidate receptors must be contributing to the transduction of acids and the subsequent perception of sour taste.

G protein betagamma (Gbetagamma) complexes are considered to play an important role in second messenger signaling of phospholipase C (PLC). Monitoring the inositol 1,4,5-trisphosphate (IP(3)) response in circumvallate tissue homogenates upon stimulation with denatonium benzoate, it was demonstrated that a glutathione S-transferase-GRK3ct fusion protein-a Gbetagamma scavenger-attenuates the bitter tastant-induced second messenger reaction. Towards an identification of the Gbetagamma complex involved in rat bitter taste transduction, it was found that the G protein beta(3) subtype is specifically expressed in taste receptor cells of circumvallate papillae. Gbeta(3)-specific antibodies blocked the denatonium benzoate-induced IP(3) formation in a dose-dependent manner; the inhibitory effect was reversed by preincubation with the antigenic peptide. A less pronounced inhibition was observed using Gbeta(1)-specific antibodies. Analyzing individual taste cells by single cell reverse transcriptase-polymerase chain reaction approaches, overlapping expression patterns for PLCbeta(2), Galpha(gust), Gbeta(3) and Ggamma(3) could be demonstrated. Furthermore, the co-expression of all profiled signal transduction components in individual taste receptor cells could be detected. These data support the concept that the denatonium benzoate-induced IP(3) response is mediated by an activation of PLCbeta(2) via a Gbetagamma complex, possibly composed of Gbeta(3) as the predominant beta subunit and Ggamma(3), and imply that multiple second messenger pathways may exist in individual taste receptor cells.

through largely unknown pathways into molecular and physiological changes that modulate growth and developPhytochrome is a family of photoreceptors whose ment. Phytochromes posses two photo-interconvertible apoproteins are encoded by divergent genes. Three forms, the R-absorbing form (Pr) and the FR-absorbing modes of action of phytochromes, very-low-fluence form (Pfr) and are synthesised in the Pr form. In recent responses (VLFR), low-fluence responses (LFR) and years it has been established that phytochromes are a high-irradiance responses (HIR), have been considered small family of photoreceptors whose apoproteins are in the literature to define the quantitative relationship encoded by diVerent genes (Clack et al., 1994; Hauser between response and predicted levels of the far-red et al., 1995). Traditionally, three ‘modes of action’ of light absorbing form of phytochrome. The results of phytochromes have been described: very-low-fluence recent experiments with phytochrome mutant and responses ( VLFR); low-fluence responses (LFR) and transgenic seedlings are consistent with the notion high-irradiance responses (HIR). Assigning particular that phytochrome B, and in some cases other stable responses to particular molecular species of phytochromes phytochromes, mediate LFR whereas phytochrome A is one of the important areas of research in the field mediates both VLFR and HIR. The mechanisms of photomorphogenesis. This review seeks to bring involved in phytochrome A inductive responses (VLFR) together data on the modes of action of phytochrome versus HIR appear to be different in some ways. with information on diVerent phytochrome molecular However, VLFR, LFR and HIR can converge to control species. the transcriptional activity of a single gene promoter. The analysis of phytochrome transduction chains is The implications of these findings for the under- currently concentrating the eVorts of many laboratories. standing of phytochrome phototransduction are Experimental approaches include the characterization of discussed. the photoreceptor molecule, in particular the biological significance of diVerent domains (Cherry et al., 1993

SYNOPSIS. Research on the sensory inputs regulating ventilation and perfusion in vertebrates has slowly moved from studies of receptor cells to studies of the receptor proteins on receptor cells involved in signal transduction. With this new emphasis and the insights gained from this work, questions arise about the significance of specific transduction mechanisms and their link to whole animal responses to various stimuli. This is illustrated in the present chapter using two examples. The first describes pulmonary mechanoreceptors in identical locations but with different orientations. As a result of the differences in their orientation, although the receptors have identical signal transduction mechanisms and identical roles in terms of the reflex effects they elicit, they respond to different stimuli at the whole animal level. The second example suggests that different populations of O2 sensitive chemoreceptors may exist that have identical signal transduction mechanisms but, because of differences in their balance of oxygen supply to demand, may respond to different stimuli at the whole animal level. The lesson to be learned from these examples is that accompanying the growing knowledge of receptor cell function at the molecular level is a growing need to integrate this knowledge with empirical observations of whole animal responses.

The olfactory area of the nasal cavity is lined with olfactory receptor cell cilia that come in contact with incoming odor molecules. Ultrastructural immunocytochemical studies in rodents have shown that these cilia contain all the proteins necessary to transduce the odorous message into an electrical signal that can be transmitted to the brain. These signaling proteins include putative odor receptors, GTP binding proteins, type III adenylyl cyclase and cyclic nucleotide-gated channels. The rest of the cells, including dendrites and dendritic knobs, showed no discernible labeling with antibodies to these signaling proteins. Furthermore, freeze-fracture and freeze-etch studies have shown that the membrane morphology of olfactory cilia differs substantially from that of non-sensory cilia. Olfactory cilia have many more membrane particles. Transmembrane signaling proteins, such as odor receptors, adenylyl cyclase and cyclic nucleotide-gated channels, conceivably appear as membrane particles. Thus, the long-standing supposition that olfactory cilia are peculiarly adapted to deal with the reception and initial transduction of odorous messages has now been verified in terms of both ultrastructural morphology and cytochemistry. Emerging studies on vomeronasal receptor cell microvilli indicate that the same is true for this organ, even though the actual signaling components differ from those of the main olfactory system.

We have explored freeze-substitution combined with low-temperature embedding in rat taste buds for postembedding immunocytochemistry. A major difference in taste bud cells that were rapidly frozen without prior chemical fixation and those that were fixed and cryoprotected before freezing was that electron-dense core granules were virtually absent. The antibodies used in these initial studies were directed against calcitonin gene-related peptide (CGRP), a peptide commonly found in nociceptive neurons; the α-subunits of two G-proteins involved in bitter and sweet taste transduction; and choline acetyl transferase (ChAT), an enzyme involved in the synthesis of acetylcholine. Anti-CGRP immunolabeled a subpopulation of unmyelinated perigemmal neurons; anti-Gqα labeled a larger subpopulation of these neurons and the microvilli of cells that were most likely from Type II vallate taste buds. α-Gustducin was found in cytoplasm of Type II and/or III cells and probably in microvilli of Type I cells of vallate taste buds. The best labeling results were obtained with anti-ChAT, which stained microvilli and lateral membranes of some Type II vallate taste bud cells, and the cytoplasm of some other Type II and/or III vallate cells. In addition, anti-ChAT labeled electron-opaque materials inside taste bud pores of vallate papillae, but, under the same conditions, not granules of Type I cells or most of the vesicles in von Ebner's glands. These data suggest that we can not assume a priori that the contents of the electron-dense core granules of Type I cells, or even of those of von Ebner's glands, contain the precursors of the taste bud pore–dense substances.

Single-fiber preparations of the rat chorda tympani (CT) nerve were used to study the mechanism of action of capsaicin on salt-taste transduction. Capsaicin selectively suppressed the responses to NaCl of the CT nerve fibers (N-fibers) that are sodium-specific (insensitive or poorly sensitive to potassium). Among the more broadly responsive, cation-sensitive fibers (E-fibers) there are two subtypes, both of which responded to capsaicin but in different ways ('enhanced' type and 'suppressed' type). In both N- and E-fibers, 5% ethanol (the vehicle for capsaicin) slightly reduced the response to 100 mM NaCl. The suppressive effect of capsaicin on the response of the N-type fibers to 100 mM NaCl was significantly stronger than the effect of 5% ethanol. The suppression lasted for at least 20 s after the simultaneous application of 100 p.p.m. capsaicin-100 nM NaCl. These results indicate that 100 p.p.m. capsaicin can modify the response of CT fibers to NaCl. The observed effect of capsaicin on gustatory fibers could be the net result of opposite suppressive and enhancing processes in the taste buds cells and excited intra- or extragemmal trigeminal nerve endings.

Using the whole cell patch clamp technique, we measured changes in outward K + currents of gerbil taste cells in response to different kinds of sweeteners. Outward K + currents of the taste cell induced by depolarizing pulses were suppressed by sweet stimuli such as 10 mM Na-saccharin. The membrane-permeable analog of cAMP, cpt-cAMP, also decreased outward K + currents. On the other hand, the K + currents were enhanced by amino acid sweeteners such as 10 mM D-tryptophan. The outward K + current was enhanced by external application of Ca 2+ -transporting ionophore, 5 μM ionomycin, and intracellular application of 5 μM inositol-1,4,5-trisphosphate (IP 3 ). The outward K + currents were no longer suppressed by 10 mM Na-saccharin containing 20 μM gurmarin, but were still enhanced by 10 mM D-tryptophan containing 20 μM gurmarin. These results suggest that sweet taste transduction for one group of sweeteners such as Na-saccharin in gerbils is concerned with an increase of the intracellular cAMP level, and that the transduction for the other group of sweeteners such as D-tryptophan is concerned with an increase of the intracellular IP 3 level which releases Ca 2+ from the internal stores.

In this study, ageotropum pea mutant was used to determine the threshold time for perception of an osmotic stimulation in the root cap and the time requirement for transduction and transmission of the hydrotropic signal from the root cap to the elongation region. The threshold time for the perception of an osmotic stimulation was compared to current estimates of threshold times for graviperception in roots. The time required for transduction and transmission in the hydrotropic response of ageotropum was compared to the time requirement in the gravity response of Alaska pea roots. We determined that threshold time for perception of an osmotic stimulation in the root cap is very rapid, occurring in less than 2 min following the application of sorbitol to the root cap. Furthermore, a single 5 min exposure of sorbitol to the root cap fully induced a hydrotropic response. We also found that transduction and transmission of an osmotic stimulus requires 90-120 min for movement from the root cap to more basal tissues involved in differential growth leading to root curvature. The very rapid threshold time for perception of root hydrotropism is similar to those times reported for root gravitropism. However, the time required for the transduction and transmission of an osmotic stimulation from the root cap is significantly longer than the time required in gravitropism. These results suggest that there must exist some differences between root hydrotropism and gravitropism in either the rate or mechanisms of transduction and transmission of the tropistic signal from the root cap.

Sodium-salt transduction in many species may be mediated by both apical and submucosal ion channels on the taste receptor cell membrane. The apical ion channel is blockable by the diuretic amiloride, whereas the submucosal pathway is not. Sodium salts with small anions, such as NaCl, can stimulate submucosal as well as apical ion channels; sodium salts with large anions, such as Na-gluconate, activate primarily the apical channels. In humans, reports on the effects of amiloride on the taste of NaCl are conflicting and no data exist on the effects of amiloride on organic sodium salts. In the present experiment, subjects gave magnitude estimates of the total intensity and of each of the basic taste qualities for NaCl, Na-gluconate and KCl. Five concentrations of each of these stimuli were presented to the anterior tongue following distilled water adaptation and after amiloride treatment. There was a significant decrease in the total taste intensity of NaCl and Na-gluconate after amiloride, but no effect on KCl. The saltiness of all three salts was unaffected, but amiloride decreased the perceived sourness of the sodium salts. KCl sourness was unaffected by amiloride. There was a proportionately larger effect of amiloride on Na-gluconate than on NaCl, which is consistent with a larger role for the apical ion channel in Na-gluconate transduction. However, an appreciable amiloride-insensitive component is present for both NaCl and Na-gluconate, suggesting that an amiloride-insensitive pathway also plays a role in the transduction of both sodium salts. These data support the hypothesis that an amiloride-sensitive transduction component exists in humans, but suggest that it is considerably smaller than in many other species.

Non-hydrolysable analogs of GTP and GDP alter odor-evoked inward and outward currents in voltage-clamped cultured lobster olfactory receptor neurons. Currents of both polarities are pertussis and cholera toxin-insensitive. Antibodies directed against the alpha subunits of G(olf), G(o), G11, an internal Gq sequence, the common carboxyl terminal sequence of Gq and G11 (anti-Gq/11), and the transducin beta subunit, fail to perturb the outward current, but anti-G(o) and anti-Gq/11 selectively block the inward current. Anti-Gq/11 immunolabels a band of approximately 45 kDa by Western blot analysis, but the anti-G(o) immunolabeling is non-specific. These results suggest that the excitatory olfactory signalling pathway that leads to an odor-evoked inward current may be coupled via a member of the Gq family, while the odor-evoked outward current is transduced by a different G protein.

Alcohol consumption is one of the biggest social and medical problems in the world. Alcohol alters function of almost all systems of the organism, first of all – central nervous system. It interferes with the transduction processes, brain plasticity, sensory information processing. Visual …

Recent molecular biological studies have revealed many molecular aspects of taste transduction in receptor cells for each taste categories which are referred to as salty, sweet, sour, bitter and umami in human respectively. (Lindemann, 2001; Gilbertson and Boughter, 2003). For example, ion channels such as ENaC, ASIC and HCN mediate salty and sour taste and G-protein-coupled receptors such as T1Rs and T2Rs play a key role in sweet, bitter and umami taste. However, there is little evidence for involvements of these molecules in taste reception and transduction at the cellular level. It is necessary to examine both molecular and physiological properties in single taste receptor cell. In this study, by use of loose patch recording technique combined with single cell multiplex RT–PCR, we characterized single taste cells by their responses to four basic taste stimuli and expression of taste-related molecules.

Bitter taste perception is innate and induces aversive reactions. Since numerous harmful compounds, including secondary plant metabolites, synthetic chemicals, inorganic ions and rancid fats, do taste bitter, this basic taste modality may be considered as a defence mechanism against the ingestion of potential poisons. For a complete understanding of this defence mechanism it is obligatory to identify and characterize the chemical detectors of the bitter compounds, which display the remarkable ability to recognize thousands of different chemicals. Screening of the genome data bases ultimately led to the discovery of a novel gene family of ∼40 members in mice and ∼30 in humans. Some of the genes identified by this approach are located within chromosomal loci associated with tasting various distinct bitter compounds (Adler et al., 2000; Matsunami et al., 2000; Bufe et al., 2002). Therefore, these genes encoding G-proteincoupled receptors, TAS2Rs (previously referred to as T2Rs or TRBs), have been suggested to represent bitter taste receptors. Several lines of independent evidence further support this assumption (Adler et al., 2000; Chandrashekar et al., 2000; Matsunami et al., 2000). First, the expression pattern of these genes on the rodent tongue was consistent with that of bitter taste receptors. Secondly, functional expression studies identified the bitter compound cycloheximide as an agonist for mTAS2R105. Thirdly, mice strains with impaired cycloheximide tasting have variant mTAS2R105 genes encoding receptors that are less responsive to cycloheximide. Fourthly, mTAS2R105 couples in vitro to α-gustducin (Chandrashekar et al., 2000), a G-protein α-subunit expressed in taste tissue that has amply been shown before to play a role in bitter taste transduction (Margolskee, 2002). Although this question has not been directly addressed, these investigations led to the impression of a narrow tuning of bitter taste receptors and raised the intriguing problem of how organisms that are equipped with a limited number of TAS2R genes are able to perceive numerous chemicals bitter. In the present report we address two questions that are important for the understanding of bitter taste in general and human bitter taste in particular. First, given the largely uncharacterized receptor repertoire, are all TAS2Rs true bitter taste receptors and, secondly, can their broad tuning explain how humans equipped with a fairly small number of TAS2R genes are able to perceive thousands of bitter compounds.

Since their first description in plant cells four decades ago(Ledbetter and Porter 1963), cortical microtubules have beenthe subject of research that has primarily focussed on the rela-tionship between cortical microtubule orientation and cellulosemicrofibril alignment during morphogenesis. This research hasemphasized microtubules behaving as static elements, withtheir structural properties put to useful work as scaffolds andbarriers. Just how microtubules do this, however, is still a mat-ter of debate, and recent conflicting interpretations (compare,for example, Burk and Ye 2002 and Sugimoto et al. 2003) sug-gest that we are far away from understanding this phenomenon.What other functions do plant cortical microtubules have?Their intimate association with the plasma membrane, themajor platform for signal perception and transduction (Gilroyand Trewavas 2001, Wasteneys and Galway 2003), suggeststhat microtubules are targets of various signals and switches.But could the microtubules themselves play an integral role insignalling events that enable plants to adapt to environmentalchanges?Microtubules are far more complex than the hollowcylinders that can assemble in vitro from tubulin dimers.Indeed, the microtubule surface is a busy and congested place,covered with protein complexes, motor proteins and structuralmicrotubule-associated proteins, GDP/GTP, regulatory kinases,phosphatases, and ions. But in addition, we now know thatmicrotubules in plant cells are the landing platforms for somethings less predicable, like elongation factor 1 (Durso and Cyr1994) and the signal transduction enzyme phospholipase D(Gardiner et al. 2001). These discoveries suggest that corticalmicrotubules do much more than just regulate the direction ofcell expansion.Could microtubules act as repositories for signallingenzymes, with their polymer status helping to modulate plantresponses to various stresses and signals? In addition to theirrole in controlling the mechanical properties of cell walls,might cortical microtubules also act as gauges of ambient con-ditions, such as temperature, or the level of nutrients and toxicsubstances in the soil? Three articles published in this issue ofPCP from the laboratories of Tobias Baskin (University ofMissouri), Peter Nick (University of Freiburg) and Jan Marc(University of Sydney) provide glimpses into the behaviourand function of cortical microtubules. These articles suggestthat microtubules are not only downstream targets of signal-ling pathways but also potential agents of signal transduction.

The antimicrotubular drug ethyl-TV-phenylcarbamate (EPC), at 1 to 10 mM, strongly inhibits gravitropism in coleoptiles of Zea mays L. Under the same conditions, phototropism remains essentially unimpaired. Propyzamide, a antimicrotubular drug specific for plant microtubules, at 0.02 to 0.2 mM causes qualitatively identical results, but to a weaker extent. Immunofluorescence data show that even only partial elimination of cortical microtubules in the outer epidermis is correlated with complete inhibition of gravitropism. Phototropism can proceed to some extent even after complete removal of cortical microtubules. Together with previous work the data indicate a complex role of the microtubular cytoskeleton in gravitropism: (1) late transduction of both, photo- and gravitropism and (2) early transduction of gravity, possibly by perceiving and transducing pressure or displacement of statoliths.

Hair cells, the mechanotransducers of the inner ears and lateral lines of vertebrates offer an excellent preparation for the study of presynaptic mechanisms at fast chemical synapses. They offer the compact, almost cylindrical shape of an epithelial cell without entangling dendrites and axons, and make afferent synapses directly from the cell body. The biology of hair cells demands an unusually high performance of the output synapse, requiring continuous, and very quickly modulated transmitter exocytosis. Release follows the small receptor potential generated by the transduction apparatus; more transmitter is released upon depolarization, less after hyperpolarization. In some hair cells, transmission can phase-lock to kilohertz stimulus frequencies, preserving temporal information across the synapse.We have exploited the synaptic terminal-like features of these cells to explore both the physiology and ultrastructure of the machinery underlying neurotransmitter release.

A complex chain of neural events is initiated when a chemical is placed on the tongue that eventually leads to a taste sensation. Beginning with transduction and a receptor potential, the sensory information has to be transformed into action potentials that ascend from the periphery through the central nervous system. This process involves many synaptic junctions where the message is transformed and distributed to a number of different brain areas. While transduction has been a major focus of recent investigations in taste, synaptic processing, and the neurotransmitters and neuromodulators used by the taste system have received less attention. Obviously, an understanding of the role of neurotransmitters and neuromodulators in the gustatory system is very important if progress is to be made in comprehending how taste information is processed by the central nervous system. Thus, it is timely to examine the current knowledge of the role of neurotransmitters and neuromodulators in gustation. A symposium at the Association for Chemoreception Sciences Annual Meeting (AChemS XVII) recently focused on this topic to address the role of neurotransmitters and neuromodulators at the level of the taste bud and at the first central relay in the taste pathway—the nucleus of the solitary tract (NST). The first report of this symposium is a comprehensive review by Steve Roper of the neurotransmitters and neuromodulators at the level of the taste receptors. Once transduction has taken place at the taste cell receptor membrane the information has to pass through a synapse between the taste cells and the primary afferent taste fibers. Suprisingly little is known about this essential event or about the identity of the neurotransmirter/s involved. As detailed in this report several investigators have attempted to study synaptic transmission at the taste bud. Histochemical, immunocytochemical and physiological techniques have been used in a number of different species with many conflicting results. Often an investigator produces a flurry of papers and then apparently abandons the effort. A few years later another investigator tries again, probably because new methods become available, to re-investigate the question of synaptic transmission at the level of the taste bud. Roper describes recent work from his laboratory indicating the significant role played by serotonin as a neuromodulator in the taste bud, but also reports that the identity of the neurotransmitter released at the taste cellprimary afferent fiber synapse is still unknown. In the last three reports information is presented on neurotransmiters and neuromodulators at the level of the NST. Synaptic events at the first relay in the central taste pathway are also little understood, as are the neurotransmiters

A review of body odor-mediated behaviors of hermit crabs, predatory gastropods, barnacle larvae and brachyuran crabs includes a model based on the hypothesis that all the behaviors are mediated by a similar chemical detection mechanism. A key assumption is behaviors evolved due to a selective advantage conferred by detection of information in body odors. In the model, exogenous trypsin-like proteases digest structural proteins to generate neutral-basic peptides. The receptor is postulated to have resulted from a point mutation in a trypsin-like membrane protein. Transduction of the binding event is via second messenger mediated pathways. Examples of body odors functioning as cues, allomones and pheromones are presented. The model is supported by a review of structure-function studies employing synthetic neutral basic peptides and studies with other macromolecules.

Almost all major phyla of invertebrates and lower vertebrates display a direct sensitivity of their chromatophores to light by either dispersing or—in rare cases—aggregating the pigment granules within the cell. This “primary response” of color change is an accessory component of the “dermal light sense” and characterizes the chromatophores as an independent receptor for light and effector of the chromomotor response. Photosensitivity does not seem to be restricted to certain parts of the pigment cell but is rather supposed to be a ubiquitous property of the chromatophore. Experiments on partially illuminated chromatophores show that the photopigment, whose chemical composition is still unknown, is localised within the plasma membrane or the cytoplasmic ground substance. Threshold responses for a just visible reaction are much higher than for background responses and have for some pigment cells been determined to be in a range between 0.2 and 0.5 erg/sec/cm2. The physiological significance of the photosensory reaction is closely related with thermoregulation and the protection of underlying tissue against harmful radiation. The chain of events involved in photosensory transduction remains to be further studied and can at present be interpreted only on the basis of related phenomena like retinal pigment migration and the light-sensitivity of simple non-pigmented contractile systems.

Paramecium tetraurelia can detect and respond to folic acid in the environment. In order tounderstand the initial step in the sensory transduction process, we used covalent crosslinldng of folate todetermine if there were specific binding sites on the cell membrane. Crosslinked cells did not respond to folatebut did respond to other chemoattractants. Membrane preparations were specifically covalently labeled witha chemically active form of folate. This labeling was detected with iodinated folate and a polyclonal antibodyto folate. These results, as well as previous work from this laboratory, support the hypothesis that the folatechemical cue is transduced into a membrane potential change via a receptor. IntroductionA central question of chemosensory transduction concerns the role of receptors, whichare proteins that specifically bind stimuli. The receptor hypothesis is still being examined(Kashiwayanagi and Kurihara, 1985; Dionne, 1988; Lemer etal., 1988). Price (1984)and Kinnamon (1988) have emphasized that olfaction and taste involve a diversity ofmechanisms, not all of which necessarily use receptors. Other chemosensory systemswill presumably show a similar diversity. Thus, any attempt to understand the mechanismof transduction in a chemosensory system must experimentally test the hypothesis thatreceptors are involved in the process.Paramecium tetraurelia is attracted by a chemokinetic mechanism to folic acid (VanHouten, 1978), which is a necesary vitamin (van Wagtendonk, 1974) and probable foodcue (Pan et al., 1972). Folic acid causes membrane hyperpolarization by an as yetunknown mechanism; this in turn leads to the change in ciliary beat that causes apopulation of cells to accumulate (Preston and Van Houten, 1987).Here we present evidence that the transduction process in the chemokinetic responseof Paramecium to folate is a receptor-mediated event and propose a method to identifythe receptor protein. This method includes covalent crosslinking of the ligand to thecell membrane, as performed by Henderson and Zevely (1984), followed by detectionof crosslinked ligand using a radioactive label or an anti-ligand antibody. Folatecan be converted to a chemically active form by incubating with carbodiimide and/V-hydroxysuccinimide as shown in step 1 of Figure 1 (Bauminger and Wilchek, 1980).Cells or cell membrane preparations form a covalent link between the folate moleculeand the adjacent protein when exposed to activated folate (Figure 1, step 2). Our aimis to identify membrane-associated folate binding proteins using the covalently attachedfolate as a marker and thereby to identify a subset of proteins that include the folatechemoreceptor.MethodsCells and culture conditionsParamecium tetraurelia stock 51S was grown in wheat grass medium at 28°C. Mediumwas prepared by boiling 5 g wheat grass (Pines, Lawrence, KS) and 0.3 g proteose© IRL Press 587