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Detection of NaCl by the gustatory system is fundamental for salt intake and tissue homeostasis. Yet, signal transduction mechanisms for salty taste have remained obscure. In this issue of Neuron, Nomura et al. (2020) report that the epithelial sodium channel ENaC, which serves as the salty receptor, is co-expressed with the voltage-activated ATP release channel CALHM1/3 in a subset of taste cells and that these cells mediate amiloride-sensitive salty taste.

The sense of touch is fundamental as it provides vital, moment-to-moment information about the nature of our physical environment. Primary sensory neurons provide the basis for this sensation in the periphery; however, recent work demonstrates that touch transduction mechanisms also occur upstream of the sensory neurons via non-neuronal cells such as Merkel cells and keratinocytes. Within the spinal cord, deep dorsal horn circuits transmit innocuous touch centrally and also transform touch into pain in the setting of injury. Here non-neuronal cells play a key role in the induction and maintenance of persistent mechanical pain. This review highlights recent advances in our understanding of mechanosensation, including a growing appreciation for the role of non-neuronal cells in both touch and pain.

Touch sensation or proprioception requires the transduction of mechanical stimuli into electrical signals by mechanoreceptors in the periphery. These mechanoreceptors are equipped with various transducer channels. Although Piezo1 and 2 are mechanically activated (MA) channels with rapid inactivation, MA molecules with other inactivation kinetics have not been identified. Here we report that heterologously expressed Tentonin3 (TTN3)/TMEM150C is activated by mechanical stimuli with distinctly slow inactivation kinetics. Genetic ablation of Ttn3/Tmem150c markedly reduced slowly adapting neurons in dorsal-root ganglion neurons. The MA TTN3 currents were inhibited by known blockers of mechanosensitive ion channels. Moreover, TTN3 was localized in muscle spindle afferents. Ttn3-deficient mice exhibited the loss of coordinated movements and abnormal gait. Thus, TTN3 appears to be a component of a mechanosensitive channel with a slow inactivation rate and contributes to motor coordination. Identification of this gene advances our understanding of the various types of mechanosensations, including proprioception.

Transmembrane channel-like (TMC) 1 and 2 are required for the mechanotransduction of mouse inner ear hair cells and localize to the site of mechanotransduction in mouse hair cell stereocilia. However, it remains unclear whether TMC1 and TMC2 are indeed ion channels and whether they can sense mechanical force directly. Here we express TMC1 from the green sea turtle (CmTMC1) and TMC2 from the budgerigar (MuTMC2) in insect cells, purify and reconstitute the proteins, and show that liposome-reconstituted CmTMC1 and MuTMC2 proteins possess ion channel activity. Furthermore, by applying pressure to proteoliposomes, we demonstrate that both CmTMC1 and MuTMC2 proteins can indeed respond to mechanical stimuli. In addition, CmTMC1 mutants corresponding to human hearing loss mutants exhibit reduced or no ion channel activity. Taken together, our results show that the CmTMC1 and MuTMC2 proteins are pore-forming subunits of mechanosensitive ion channels, supporting TMC1 and TMC2 as hair cell transduction channels.

SummaryThe electrical activity of mammalian osmosensory neurons (ONs) is increased by plasma hypertonicity to command thirst, antidiuretic hormone release, and increased sympathetic tone during dehydration. Osmosensory transduction is a mechanical process whereby decreases in cell volume cause the activation of transient receptor potential vanilloid type-1 (TRPV1) channels to induce depolarization and increase spiking activity in ONs. However, it is not known how cell shrinking is mechanically coupled to channel activation. Using superresolution imaging, we found that ONs are endowed with a uniquely interweaved scaffold of microtubules throughout their somata. Microtubules physically interact with the C terminus of TRPV1 at the cell surface and provide a pushing force that drives channels activation during shrinking. Moreover, we found that changes in the density of these interactions can bidirectionally modulate osmosensory gain. Microtubules are thus an essential component of the vital neuronal mechanotransduction apparatus that allows the brain to monitor and correct body hydration.

The identity of the inner ear's transduction channel has bedeviled auditory neuroscientists for decades. In this issue of Neuron, Pan et al. (2018) report the most convincing evidence to date implicating the transmembrane channel-like (TMC) proteins as forming the pore of the transduction channel.

SummarySensory transduction in auditory and vestibular hair cells requires expression of transmembrane channel-like (Tmc) 1 and 2 genes, but the function of these genes is unknown. To investigate the hypothesis that TMC1 and TMC2 proteins are components of the mechanosensitive ion channels that convert mechanical information into electrical signals, we recorded whole-cell and single-channel currents from mouse hair cells that expressed Tmc1, Tmc2, or mutant Tmc1. Cells that expressed Tmc2 had high calcium permeability and large single-channel currents, while cells with mutant Tmc1 had reduced calcium permeability and reduced single-channel currents. Cells that expressed Tmc1 and Tmc2 had a broad range of single-channel currents, suggesting multiple heteromeric assemblies of TMC subunits. The data demonstrate TMC1 and TMC2 are components of hair cell transduction channels and contribute to permeation properties. Gradients in TMC channel composition may also contribute to variation in sensory transduction along the tonotopic axis of the mammalian cochlea.

SummaryMany animals use their olfactory systems to learn to avoid dangers, but how neural circuits encode naive and learned olfactory preferences, and switch between those preferences, is poorly understood. Here, we map an olfactory network, from sensory input to motor output, which regulates the learned olfactory aversion of Caenorhabditis elegans for the smell of pathogenic bacteria. Naive animals prefer smells of pathogens but animals trained with pathogens lose this attraction. We find that two different neural circuits subserve these preferences, with one required for the naive preference and the other specifically for the learned preference. Calcium imaging and behavioral analysis reveal that the naive preference reflects the direct transduction of the activity of olfactory sensory neurons into motor response, whereas the learned preference involves modulations to signal transduction to downstream neurons to alter motor response. Thus, two different neural circuits regulate a behavioral switch between naive and learned olfactory preferences.

It is a central tenet of cochlear neurobiology that mammalian ears rely on a local, mechanical amplification process for their high sensitivity and sharp frequency selectivity. While there is general agreement that outer hair cells provide the amplification, two mechanisms have been proposed: stereociliary motility and somatic motility. The latter is driven by the motor protein prestin. Electrophysiological phenotyping of a prestin knockout mouse intimated that somatic motility is the amplifier. However, outer hair cells of knockout mice have significantly altered mechanical properties, which makes this mouse model unsatisfactory. Here we study a new mouse model without alteration to outer hair cell and organ of Corti mechanics or to mechano-electric transduction, but with diminished prestin function. These animals have knockout-like behavior, demonstrating that prestin-based electromotility is required for cochlear amplification.

SummaryIncreases in core body temperature promote thermoregulatory cooling by stimulating sweat production and preemptive renal water reabsorption through the release of vasopressin (VP, antidiuretic hormone). The mechanism by which the hypothalamus orchestrates this anticipatory VP release during hyperthermia is unknown but has been linked to a central thermosensory mechanism. Here, we report that thermal stimuli spanning core body temperatures activate a calcium-permeable, ruthenium red- and SB366791-sensitive nonselective cation conductance in hypothalamic VP neurons. This response is associated with a depolarizing receptor potential and an increase in action potential firing rate, indicating that these neurons are intrinsically thermosensitive. The thermosensitivity of VP neurons isolated from trpv1 knockout (Trpv1−/−) mice was significantly lower than that of wild-type counterparts. Moreover, Trpv1−/− mice showed an impaired VP response to hyperthermia in vivo. Channels encoded by the trpv1 gene thus confer thermosensitivity in central VP neurons and contribute to the thermal control of VP release in vivo.

SummaryThe molecular mechanisms regulating the sensitivity of sensory circuits to environmental stimuli are poorly understood. We demonstrate here a central role for stem cell factor (SCF) and its receptor, c-Kit, in tuning the responsiveness of sensory neurons to natural stimuli. Mice lacking SCF/c-Kit signaling displayed profound thermal hypoalgesia, attributable to a marked elevation in the thermal threshold and reduction in spiking rate of heat-sensitive nociceptors. Acute activation of c-Kit by its ligand, SCF, resulted in a reduced thermal threshold and potentiation of heat-activated currents in isolated small-diameter neurons and thermal hyperalgesia in mice. SCF-induced thermal hyperalgesia required the TRP family cation channel TRPV1. Lack of c-Kit signaling during development resulted in hypersensitivity of discrete mechanoreceptive neuronal subtypes. Thus, c-Kit can now be grouped with a small family of receptor tyrosine kinases, including c-Ret and TrkA, that control the transduction properties of sensory neurons.

In this issue of Neuron, Kwan et al. demonstrate that TRPA1 is critical for the transduction of noxious cold and mechanical stimuli, as well as in mediating the activation of nociceptors by endogenous and natural irritants. Differences between the present report and a previous study indicate that further study is needed to reach a consensus on the role of TRPA1 in the transduction of mechanical and noxious cold stimuli.

TRPA1, a member of the transient receptor potential (TRP) family of ion channels, is expressed by dorsal root ganglion neurons and by cells of the inner ear, where it has proposed roles in sensing sound, painful cold, and irritating chemicals. To test the in vivo roles of TRPA1, we generated a mouse in which the essential exons required for proper function of the Trpa1 gene were deleted. Knockout mice display behavioral deficits in response to mustard oil, to cold ( approximately 0 degrees C), and to punctate mechanical stimuli. These mice have a normal startle reflex to loud noise, a normal sense of balance, a normal auditory brainstem response, and normal transduction currents in vestibular hair cells. TRPA1 is apparently not essential for hair-cell transduction but contributes to the transduction of mechanical, cold, and chemical stimuli in nociceptor sensory neurons.

SummaryIn sensory hair cells of the inner ear, mechanical amplification of small stimuli requires fast adaptation, the rapid closing of mechanically activated transduction channels. In frog and mouse vestibular hair cells, we found that the rate of fast adaptation depends on both channel opening and stimulus size and that it is modeled well as a release of a mechanical element in series with the transduction apparatus. To determine whether myosin-1c molecules of the adaptation motor are responsible for the release, we introduced the Y61G mutation into the Myo1c locus and generated mice homozygous for this sensitized allele. Measuring transduction and adaptation in the presence of NMB-ADP, an allele-specific inhibitor, we found that the inhibitor not only blocked slow adaptation, as demonstrated previously in transgenic mice, but also inhibited fast adaptation. These results suggest that mechanical activity of myosin-1c is required for fast adaptation in vestibular hair cells.

A threshold-like nonlinearity in signal transfer from mouse rod photoreceptors to rod bipolar cells dramatically improves the absolute sensitivity of the rod signals. The work described here reaches three conclusions about the mechanisms generating this nonlinearity. (1) The nonlinearity is caused primarily by saturation of the feedforward rod-to-rod bipolar synapse and not by feedback from horizontal or amacrine cells. This saturation renders the rod bipolar current insensitive to small changes in transmitter release from the rod. (2) Saturation occurs within the G protein cascade that couples receptors to channels in the rod bipolar dendrites, with little or no contribution from presynaptic mechanisms or saturation of the postsynaptic receptors. (3) Between 0.5 and 2 bipolar transduction channels are open in darkness at each synapse, compared to the approximately 30 channels open at the peak of the single photon response.

Sensory signals regulate multiple developmental and behavioral circuits in C. elegans, providing a genetically tractable system in which to investigate the mechanisms underlying the acquisition and integration of sensory information. kin-29 mutants are defective in the expression of a set of chemoreceptor genes, and exhibit characteristics associated with altered sensory signaling, including increased lifespan, decreased body size, and deregulated entry into the dauer developmental stage. kin-29 encodes a Ser/Thr kinase with similarity to the MARK and AMPK/SNF1 family of kinases. We show that KIN-29 acts cell-autonomously and non-cell-autonomously in sensory neurons to regulate chemoreceptor expression, body size, and the dauer decision, suggesting that kin-29 function is essential for the correct acquisition and transduction of sensory information.

Summary Hair cells are the mechanosensory cells of the inner ear. Mechanotransduction channels in hair cells are gated by tip links. The molecules that connect tip links to transduction channels are not known. Here we show that the transmembrane protein TMIE forms a ternary complex with the tip-link component PCDH15 and its binding partner TMHS/LHFPL5. Alternative splicing of the PCDH15 cytoplasmic domain regulates formation of this ternary complex. Transducer currents are abolished by a homozygous Tmie -null mutation, and subtle Tmie mutations that disrupt interactions between TMIE and tip links affect transduction, suggesting that TMIE is an essential component of the hair cell's mechanotransduction machinery that functionally couples the tip link to the transduction channel. The multisubunit composition of the transduction complex and the regulation of complex assembly by alternative splicing is likely critical for regulating channel properties in different hair cells and along the cochlea's tonotopic axis.

Mutations in the no-mechanoreceptor-potential A (nompA) gene, which eliminate transduction in Drosophila mechanosensory organs, disrupt contacts between neuronal sensory endings and cuticular structures. nompA encodes a transmembrane protein with a large, modular extracellular segment that includes a zona pellucida (ZP) domain and several plasminogen N-terminal (PAN) modules. It is specifically expressed in type I sense organs of the peripheral nervous system by the support cells that ensheath the neuronal sensory process. A green fluorescent protein (GFP)-NompA fusion protein is localized to the dendritic cap, an extracellular matrix that covers the ciliary outer segment of the sensory process and that shows organizational defects in nompA mutants. The structure and location of NompA suggest that it forms part of a mechanical linkage required to transmit mechanical stimuli to the transduction apparatus.

Spontaneous fluctuations in the electrical signals of the retina's photoreceptors impose a fundamental limit on visual sensitivity. While noise in the rods has been studied extensively, relatively little is known about the noise of cones. We show that the origin of the dark noise in salamander cones varies with cone type. Most of the noise in long wavelength–sensitive (L) cones arose from spontaneous activation of the photopigment, which is a million-fold less stable than the rod photopigment rhodopsin. Most of the noise in short wavelength–sensitive (S) cones arose in a later stage of the transduction cascade, as the photopigment was relatively stable. Spontaneous pigment activation effectively light adapted L cones in darkness, causing them to have a smaller and briefer dim flash response than S cones.

The molecular basis of sensory hair cell mechanotransduction is largely unknown. In order to identify genes that are essential for mechanosensory hair cell function, we characterized a group of recently isolated zebrafish motility mutants. These mutants are defective in balance and swim in circles but have no obvious morphological defects. We examined the mutants using calcium imaging of acoustic-vibrational and tactile escape responses, high resolution microscopy of sensory neuroepithelia in live larvae, and recordings of extracellular hair cell potentials (microphonics). Based on the analyses, we have identified several classes of genes. Mutations in sputnik and mariner affect hair bundle integrity. Mutant astronaut and cosmonaut hair cells have relatively normal microphonics and thus appear to affect events downstream of mechanotransduction. Mutant orbiter, mercury, and gemini larvae have normal hair cell morphology and yet do not respond to acoustic-vibrational stimuli. The microphonics of lateral line hair cells of orbiter, mercury, and gemini larvae are absent or strongly reduced. Therefore, these genes may encode components of the transduction apparatus.

SummaryThe response of cortical neurons to a sensory stimulus is shaped by the network in which they are embedded. Here we establish a role of parvalbumin (PV)-expressing cells, a large class of inhibitory neurons that target the soma and perisomatic compartments of pyramidal cells, in controlling cortical responses. By bidirectionally manipulating PV cell activity in visual cortex we show that these neurons strongly modulate layer 2/3 pyramidal cell spiking responses to visual stimuli while only modestly affecting their tuning properties. PV cells' impact on pyramidal cells is captured by a linear transformation, both additive and multiplicative, with a threshold. These results indicate that PV cells are ideally suited to modulate cortical gain and establish a causal relationship between a select neuron type and specific computations performed by the cortex during sensory processing.

By adapting to sustained stimuli, hair cells of the internal ear maintain their optimal sensitivity to minute displacements. Biophysical experiments have suggested that adaptation is mediated by a molecular motor, most likely a member of the myosin family. To provide direct evidence for the presence of myosin isozymes in hair bundles, we used photoaffinity labeling with vanadate-trapped uridine and adenine nucleotides to identify proteins of 120, 160, and 230 kd in a preparation of hair bundles purified from the bullfrog's sacculus. The photoaffinity labeling properties of these proteins, particularly the 120 kd protein, resembled those of other well-characterized myosins. A 120 kd hair-bundle protein was also recognized by a monoclonal antibody directed against a vertebrate myosin I isozyme. Immunofluorescence microscopy localized this protein near the beveled top edge of the hair bundle, the site of mechano-electrical transduction and adaptation.

An attractive hypothesis for hair-cell transduction is that fine, filamentous "tip links" pull directly on mechanically sensitive ion channels located at the tips of the stereocilia. We tested the involvement of tip links in the transduction process by treating bundles with a BAPTA-buffered, low-Ca2+ saline (10(-9) M). BAPTA abolished the transduction current in a few hundred milliseconds. BAPTA treatment for a few seconds eliminated the tip links observed by either scanning or transmission electron microscopy. BAPTA also eliminated the voltage-dependent movement and caused a positive bundle displacement of 133 nm, in quantitative agreement with a model for regulation of tension. We conclude that tip links convey tension to the transduction channels of hair cells.

In order to understand how the hair cell's mechanoelectrical transduction channels are gated during mechanical stimulation, it is essential to determine their location with respect to the hair bundle's constituent stereocilia. We localized the transduction channels by focally blocking receptor currents with iontophoretically ejected gentamicin, an aminoglycoside antibiotic that acts as a reversible channel blocker. The drug was most effective when directed at the top of a hair bundle, whereas application at the bundle's bottom or at the cuticular plate had little or no effect. Computer simulations of blocking agreed with experimental data only when the transduction channels were hypothesized to occur near the bundle's top. These results confirm that the hair cell's transduction channels are located near the stereociliary tips.

Calcium channels transduce natural voltage transients, like action potentials, into functionally important intracellular calcium transients. We have used digitally constructed waveforms that simulate natural action potentials as voltage-clamp commands to study channel function in transduction. Whole-cell calcium currents elicited by several action potential waveforms (APWs) were studied. The currents were subdivided into T (or low voltage-activated) and high voltage-activated components. Calcium entry through T channels constituted a disproportionately large fraction of the total during normal, brief APWs. Entry through high voltage-activated channels was much more responsive to APW, increasing more significantly as APW duration increased. Thus the results indicate that differences in the gating properties of these two channel classes combine to endow them with strikingly different transducer properties.

Though a fair amount is known about the transduction of individual tastants, there is scant evidence linking these specific transduction mechanisms with stimulus-evoked behavioral responses such as discrimination or rejection. The one exception to this involves the role of ASSCs in the transduction of sodium salts (and acids), which has been investigated in rats and hamsters at all levels along the gustatory pathway from receptor cell to behavior. The results of these various studies confirm that a significant portion of NaCl taste is mediated via an influx of sodium ions through ASSCs (references can be found in Lindemann 1996xLindemann, B. Physiol. Rev. 1996; 76: 719–766See all ReferencesLindemann 1996).Electrophysiological studies of the permeability properties of ASSCs in mammalian taste cells reveal that these channels are significantly permeable to Na+, Li+ and H+, have single channel conductances of ∼5 pS, and are blocked by submicromolar concentrations of amiloride. Based on these properties, it has become increasingly clear that the ASSCs in taste cells are similar molecularly and functionally to the heterooligomeric epithelial sodium channels (ENaCs) found in a variety of sodium-transporting epithelia. Recent RT–PCR and immunocytochemical studies confirm the presence of ENaC subunits in rat TRCs (e.g.Lindemann et al. 1998xLindemann, B, Barbry, P, Kretz, O, and Bock, R. Ann. N. Y. Acad. Sci. 1998; 855: 116–125Crossref | PubMedSee all ReferencesLindemann et al. 1998). Similar to their epithelial counterparts, the activity of ASSCs in TRCs is regulated by a number of natriferic hormones, hormones that control Na+ transport (Gilbertson 1998bxGilbertson, T.A. Ann. N. Y. Acad. Sci. 1998; 855: 860–868Crossref | PubMedSee all ReferencesGilbertson 1998b).Recent studies using amiloride to inhibit physiological and behavioral responses to sodium salts have provided a basis for a model of sodium salt taste linking the ASSC transduction mechanism to taste-guided behavior in mammals. Single gustatory nerve fibers and their primary target neurons in the NST are, as a rule, rather broadly responsive to stimuli of different taste qualities. However, a subset of individual fibers and neurons can be classified as “Na+-best” because they have a strong response to sodium and lithium salts relative to other stimuli. Others respond to sodium salts, but, importantly, they also respond well or better to nonsodium salts and acids. It was appreciated in early taste research that activity in different fiber types may contribute to a neural code for taste discrimination (Pfaffmann 1959xPfaffmann, C. Am. Psychol. 1959; 14: 226–232CrossrefSee all ReferencesPfaffmann 1959).The effects of lingual amiloride on taste responses in different neuron types of the NST, whose second-order gustatory neurons are a potential substrate for taste discrimination and other taste processes, have been studied in rats (Scott and Giza 1990xScott, T.R and Giza, B.K. Science. 1990; 249: 1585–1587Crossref | PubMedSee all ReferencesScott and Giza 1990) and hamsters (Boughter and Smith 1998xBoughter, J.D Jr. and Smith, D.V. J. Neurophysiol. 1998; 80: 1362–1372PubMedSee all ReferencesBoughter and Smith 1998). In both species, responses to NaCl in Na+-best neurons were significantly reduced or eliminated by micromolar concentrations of amiloride. Responses to NaCl in those cells broadly responsive to both salts and acids were completely unaffected. Although there is convergence of peripheral fibers onto gustatory neurons in the NST, input from different receptor mechanisms ultimately activates separate populations of neurons.In order to understand how the activity of afferent fibers and CNS neurons that are sensitive to amiloride may provide a basis for taste discrimination, Spector and colleagues tested the ability of amiloride to disrupt the discrimination between NaCl and KCl (Spector et al. 1996xSpector, A.C, Guagliardo, N.A, and St. John, S.J. J. Neurosci. 1996; 16: 8115–8122PubMedSee all ReferencesSpector et al. 1996). They conducted elegant behavioral experiments in the rat using an operant conditioning paradigm in which water-restricted rats were trained to discriminate between two different taste stimuli, NaCl and a nonsodium salt (KCl). Because there were both positive and negative consequences of each identification (i.e., presence or absence of water reward), the rats were more likely to report subtle differences between taste stimuli than in standard two-bottle preference tests. When the amiloride, which itself is tasteless to rats, was added to the salts, rats trained to distinguish between the two salts could no longer make the discrimination, performing at a level no better than chance. The effect of amiloride was significant at 10 μM, a concentration effective in receptor cell physiology experiments. The pattern of behavioral responses suggested that amiloride predominantly affected the taste quality of NaCl rather than KCl, which is consistent with electrophysiological observations that KCl does not elicit a strong response in the amiloride-sensitive Na+-best cells (Scott and Giza 1990xScott, T.R and Giza, B.K. Science. 1990; 249: 1585–1587Crossref | PubMedSee all ReferencesScott and Giza 1990). After amiloride treatment, the information contained in the neural pattern of activity is insufficient to discriminate sodium from a nonsodium salt. Despite the fact that the distribution of ASSC inputs to NST neurons is specific, the physiological data may, in fact, argue against a particular neuron type functioning as a “labeled line” for sodium salts. Na+-best neurons respond to multiple stimuli, and both NaCl and acid responses were blocked by amiloride in these cells in the hamster NST (Boughter and Smith 1998xBoughter, J.D Jr. and Smith, D.V. J. Neurophysiol. 1998; 80: 1362–1372PubMedSee all ReferencesBoughter and Smith 1998). This organization suggests that any one neuron type alone or any one transduction mechanism alone may be insufficient for the discrimination among different-tasting stimuli.As more taste transduction mechanisms are elucidated, it will be necessary to attempt to link the effects of tastants at the receptor cell level with, ultimately, the behavior of the organism. For example, bitter taste in humans is stimulated by a diverse array of compounds, ranging from simple salts to toxic plant alkaloids. Multiple transduction mechanisms for bitter stimuli have been proposed (e.g.Gilbertson 1998axGilbertson, T.A. Front. Oral Biol. 1998; 9: 1–28CrossrefSee all ReferencesGilbertson 1998a). But behavioral studies with rodents indicate that while many of these compounds provoke an avoidance response, others do not. To understand how the mammalian CNS encodes bitter and other taste qualities will require the integration of research at the receptor cell, afferent nerve fiber, central taste nuclei, and behavioral levels.‡To whom correspondence should be addressed (e-mail: tim.gilbertson@tasteful.com).

SummaryFood palatability and hedonic value play central roles in nutrient intake. However, postingestive effects can influence food preferences independently of palatability, although the neurobiological bases of such mechanisms remain poorly understood. Of central interest is whether the same brain reward circuitry that is responsive to palatable rewards also encodes metabolic value independently of taste signaling. Here we show that trpm5−/− mice, which lack the cellular machinery required for sweet taste transduction, can develop a robust preference for sucrose solutions based solely on caloric content. Sucrose intake induced dopamine release in the ventral striatum of these sweet-blind mice, a pattern usually associated with receipt of palatable rewards. Furthermore, single neurons in this same ventral striatal region showed increased sensitivity to caloric intake even in the absence of gustatory inputs. Our findings suggest that calorie-rich nutrients can directly influence brain reward circuits that control food intake independently of palatability or functional taste transduction.

SummaryIn vertebrate olfactory receptor neurons (ORNs), the odorant-triggered receptor current flows through two distinct ion channels on the sensory cilia: Ca2+ influx through a cyclic nucleotide-gated (CNG) channel followed by Cl− efflux through a Ca2+-activated anion channel. The excitatory Cl− current amplifies the small CNG current and crucially depends on a high intracellular Cl− concentration. We show here that a Na+-K+-2Cl− cotransporter, NKCC1, is required for this Cl− current, in that ORNs deficient in Nkcc1 or incubated with an NKCC blocker (bumetanide) lack the Cl− current. Surprisingly, immunocytochemistry indicates that NKCC1 is located on the somata and dendrites of ORNs rather than the cilia, where transduction occurs. This topography is remarkably similar to the situation in secretory epithelial cells, where basolateral Cl− uptake and apical Cl− efflux facilitate transepithelial fluid movement. Thus, a single functional architecture serves two entirely different purposes, probably underscoring the epithelial origin of the ORNs.

After opening in response to mechanical stimuli, hair cell transduction channels adapt with fast and slow mechanisms that each depend on Ca(2+). We demonstrate here that transduction and adaptation require phosphatidylinositol 4,5-bisphosphate (PIP(2)) for normal kinetics. PIP(2) has a striking distribution in hair cells, being excluded from the basal region of hair bundles and apical surfaces of frog saccular hair cells. Localization of a phosphatidylinositol lipid phosphatase, Ptprq, to these PIP(2)-free domains suggests that Ptprq maintains low PIP(2) levels there. Depletion of PIP(2) by inhibition of phosphatidylinositol 4-kinase or sequestration by aminoglycosides reduces the rates of fast and slow adaptation. PIP(2) and other anionic phospholipids bind directly to the IQ domains of myosin-1c, the motor that mediates slow adaptation, permitting a strong interaction with membranes and likely regulating the motor's activity. PIP(2) depletion also causes a loss in transduction current. PIP(2) therefore plays an essential role in hair cell adaptation and transduction.

Variability in the single photon responses of rod photoreceptors limits the accuracy with which the number and timing of photon absorptions are encoded. We investigated how much single photon responses of mammalian rods fluctuate and what mechanisms control these fluctuations. Mammalian rods, like those of toads, generated responses to single photons with trial-to-trial fluctuations 3–4 times smaller than other familiar signals produced by single molecules. We used the properties of the measured fluctuations to constrain models for how the single photon responses are regulated. Neither feedback control of rhodopsin's activity nor saturation within the transduction cascade were consistent with experiment. The measured responses, however, could be explained by multistep shutoff of rhodopsin or a combination of multistep shutoff and saturation.

Frequency tuning in the vertebrate cochlea requires a continuously varying amplifier in auditory hair cells. In this issue of Neuron, Ricci et al. show that the mechanosensitive transduction channel, a possible amplifier component, has a single-channel conductance that increases systematically along the frequency axis of the turtle cochlea.

Single photon responses were compared in wild-type and transgenic retinal rods with and without guanylate cyclase activating protein (GCAP) to disrupt Ca(2+)-dependent feedback regulation of guanylate cyclase (see Burns et al. in this issue of Neuron). The results provided new insights into the molecular mechanisms underlying phototransduction.

Summary Central serotonin (5-hydroxytryptophan, 5-HT) modulates somatosensory transduction, but how it achieves sensory modality-specific modulation remains unclear. Here we report that enhancing serotonergic tone via administration of 5-HT potentiates itch sensation, whereas mice lacking 5-HT or serotonergic neurons in the brainstem exhibit markedly reduced scratching behavior. Through pharmacological and behavioral screening, we identified 5-HT1A as a key receptor in facilitating gastrin-releasing peptide (GRP)-dependent scratching behavior. Coactivation of 5-HT1A and GRP receptors (GRPR) greatly potentiates subthreshold, GRP-induced Ca 2+ transients, and action potential firing of GRPR + neurons. Immunostaining, biochemical, and biophysical studies suggest that 5-HT1A and GRPR may function as receptor heteromeric complexes. Furthermore, 5-HT1A blockade significantly attenuates, whereas its activation contributes to, long-lasting itch transmission. Thus, our studies demonstrate that the descending 5-HT system facilitates GRP-GRPR signaling via 5-HT1A to augment itch-specific outputs, and a disruption of crosstalk between 5-HT1A and GRPR may be a useful antipruritic strategy. Video Abstract

Dark adaptation requires timely deactivation of phototransduction and efficient regeneration of visual pigment. No previous study has directly compared the kinetics of dark adaptation with rates of the various chemical reactions that influence it. To accomplish this, we developed a novel rapid-quench/mass spectrometry-based method to establish the initial kinetics and site specificity of light-stimulated rhodopsin phosphorylation in mouse retinas. We also measured phosphorylation and dephosphorylation, regeneration of rhodopsin, and reduction of all-trans retinal all under identical in vivo conditions. Dark adaptation was monitored by electroretinography. We found that rhodopsin is multiply phosphorylated and then dephosphorylated in an ordered fashion following exposure to light. Initially during dark adaptation, transduction activity wanes as multiple phosphates accumulate. Thereafter, full recovery of photosensitivity coincides with regeneration and dephosphorylation of rhodopsin.

Light adaptation is thought to be orchestrated by a Ca2+ feedback signal that desensitizes the response by speeding recovery. To evaluate the role of Ca2+ in adaptation, we compared the effect of lowered Ca2+ on response properties in darkness and during adaptation. Internal Ca2+ was reduced from its normal resting dark level (535 nM) by either background illumination or exposure to Ringer's solution containing low Ca2+ and/or cyclic GMP–gated channel blockers in darkness. Ca2+ reductions in light decreased the activation gain of the transduction process and speeded recovery kinetics, while equivalent Ca2+ reductions in darkness caused similar gain reduction without accelerating recovery. This indicates that adaptational changes in the response are not due purely to feedback effects on recovery.

SummaryAuditory receptor cells rely on mechanically gated channels to transform sound stimuli into neural activity. Several TRP channels have been implicated in Drosophila auditory transduction, but mechanistic studies have been hampered by the inability to record subthreshold signals from receptor neurons. Here, we develop a non-invasive method for measuring these signals by recording from a central neuron that is electrically coupled to a genetically defined population of auditory receptor cells. We find that the TRPN family member NompC, which is necessary for the active amplification of sound-evoked motion by the auditory organ, is not required for transduction in auditory receptor cells. Instead, NompC sensitizes the transduction complex to movement and precisely regulates the static forces on the complex. In contrast, the TRPV channels Nanchung and Inactive are required for responses to sound, suggesting they are components of the transduction complex. Thus, transduction and active amplification are genetically separable processes in Drosophila hearing.

Heat and cold transduction by peripheral sensory neurons is a fundamental step in the avoidance of dangerous thermal extremes. In this issue of Neuron, Dhaka et al. and Colburn et al. report that mice lacking the cold- and menthol-gated ion channel TRPM8 exhibit deficient behavioral responses to cold temperatures.

Mechanical stimuli are thought to open the transduction channels of a hair cell by tensing elastic components, the gating springs, that pull directly on the channels. To test this model, we measured the stiffness of hair bundles during mechanical stimulation. A bundle's compliance increased by about 40% at the position where half of the channels opened. This we attribute to conformational changes of transduction channels as they open and close. The magnitude and displacement dependence of the gating compliance provide quantitative information about the molecular basis of mechanoelectrical transduction: the force required to open each channel, the number of transduction channels per hair cell, the stiffness of a gating spring, and the swing of a channel's gate as it opens.