Odors activate dual pathways, a TRPC2 and a AA-dependent pathway, in mouse vomeronasal neurons

Noradrenaline modulates sensory information in mouse vomeronasal sensory neurons

During the induction of the body's alert state, the sympathetic system modulates sensory modalities and fine-tunes peripheral organs for improved stimulus detection. We explored noradrenaline (NA)'s role in modulating signaling in vomeronasal sensory neurons (VSNs), responsible for detecting pheromones and other semiochemicals. In current-clamp recordings, NA increased the firing frequency in response to natural stimuli of responsive VSNs and induced spiking activity in previously unresponsive neurons. Current injections into VSNs showed an increase in firing frequency during NA application. Combining transcriptomic analysis, electrophysiology, Ca2+ imaging, and a pharmacological approach, we identified alpha 1 adrenergic receptors as crucial for NA-induced firing frequency increases in VSNs. Immunohistochemistry revealed catecholaminergic fibers in the vomeronasal sensory epithelium, suggesting localized NA release near VSNs. This study unveils NA as a key regulator of VSN signaling, shedding light on the intricate interplay between the sympathetic nervous system and chemosensory processing, advancing our understanding of sensory modulation.

Molecular, Cellular, and Developmental Organization of the Mouse Vomeronasal organ at Single Cell Resolution

We have generated single cell transcriptomic atlases of vomeronasal organs (VNO) from juvenile and adult mice. Combined with spatial molecular imaging, we uncover a distinct, previously unidentified class of cells that express the vomeronasal receptors and a population of canonical olfactory sensory neurons in the VNO. High resolution trajectory and cluster analyses reveal the lineage relationship, spatial distribution of cell types, and a putative cascade of molecular events that specify the V1r, V2r, and OR lineages from a common stem cell population. The expression of vomeronasal and olfactory receptors follow power law distributions, but there is high variability in average expression levels between individual receptor and cell types. Substantial co-expression is found between receptors across clades, from different classes, and between olfactory and vomeronasal receptors, with nearly half from pairs located on the same chromosome. Interestingly, the expression of V2r, but not V1r, genes is associated with various transcription factors, suggesting distinct mechanisms of receptor choice associated with the two cell types. We identify association between transcription factors, surface axon guidance molecules, and individual VRs, thereby uncovering a molecular code that guides the specification of the vomeronasal circuitry. Our study provides a wealth of data on the development and organization of the accessory olfactory system at both cellular and molecular levels to enable a deeper understanding of vomeronasal system function.

Inhaled Volatile Molecules-Responsive TRP Channels as Non-Olfactory Receptors

Generally, odorant molecules are detected by olfactory receptors, which are specialized chemoreceptors expressed in olfactory neurons. Besides odorant molecules, certain volatile molecules can be inhaled through the respiratory tract, often leading to pathophysiological changes in the body. These inhaled molecules mediate cellular signaling through the activation of the Ca2+-permeable transient receptor potential (TRP) channels in peripheral tissues. This review provides a comprehensive overview of TRP channels that are involved in the detection and response to volatile molecules, including hazardous substances, anesthetics, plant-derived compounds, and pheromones. The review aims to shed light on the biological mechanisms underlying the sensing of inhaled volatile molecules. Therefore, this review will contribute to a better understanding of the roles of TRP channels in the response to inhaled molecules, providing insights into their implications for human health and disease.

Sensitivity of olfactory sensory neurons to food cues is tuned to nutritional states by Neuropeptide Y signaling

Modulation of sensory perception by homeostatic feedback from physiological states is central to innate purposive behaviors. Olfaction is an important predictive modality for feeding-related behaviors and its modulation has been associated with hunger-satiety states. However, the mechanisms mapping internal states to chemosensory processing in order to modify behavior are poorly understood. In the zebrafish olfactory epithelium, a subset of olfactory sensory neurons (OSNs) and the terminal nerve projections express neuropeptide Y (NPY). Using a combination of neuronal activity and behavioral evaluation, we find that NPY signaling in the peripheral olfactory system of zebrafish is correlated with its nutritional state and is both necessary and sufficient for the olfactory perception of food-related odorants. NPY activity dynamically modulates the microvillar OSN activation thresholds and acts cooperatively with amino acid signaling resulting in a switch-like increase in OSN sensitivity in starved animals. We suggest that cooperative activation of phospholipase C by convergent signaling from NPY and amino acid receptors is central to this heightened sensitivity. This study provides ethologically relevant, physiological evidence for NPY signaling in the modulation of OSN sensitivity to food-associated amino acid cues. We demonstrate sensory gating directly at the level of OSNs and identify a novel mechanistic framework for tuning olfactory sensitivity to prevailing energy states.

Cyclophosphamide has Long-Term Effects on Proliferation in Olfactory Epithelia

Chemotherapy patients often experience chemosensory changes during and after drug therapy. The chemotherapy drug, cyclophosphamide (CYP), has known cytotoxic effects on sensory and proliferating cells of the taste system. Like the taste system, cells in the olfactory epithelia undergo continuous renewal. Therefore, we asked if a single injection of 75 mg/kg CYP would affect cell proliferation in the anterior dorsomedial region of the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) from 0 to 125 days after injection. Both epithelia showed a decrease in Ki67-labeled cells compared to controls at day 1 and no Ki67+ cells at day 2 postinjection. In the sensory layer of the MOE, cell proliferation began to recover 4 days after CYP injection and by 6 days, the rate of proliferation was significantly greater than controls. Ki67+ cells peaked 30 days postinjection, then declined to control levels at day 45. Similar temporal sequences of initial CYP-induced suppression of cell proliferation followed by elevated rates peaking 30-45 days postinjection were seen in the sustentacular layer of the MOE and all 3 areas (sensory, sustentacular, marginal) of the VNO. CYP affected proliferation in the sensory layer of the MOE more than the sustentacular layer and all 3 areas of the VNO. These findings suggest that chemotherapy involving CYP is capable of affecting cell renewal of the olfactory system and likely contributes to clinical loss of function during and after chemotherapy.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Conditioned odor aversion induces social anxiety towards females in wild-type and TrpC2 knockout male mice

Female-emitted pheromonal inputs possess an intrinsic rewarding value for conspecific males, promoting approach and investigation of the potential mating partner. In mice these inputs are detected mainly by the vomeronasal organ (VNO) and the main olfactory epithelium (MOE). We investigated the role of VNO-mediated inputs in experience-dependent plasticity of reproductive responses. We applied a sex-specific conditioned odor aversion (COA) paradigm on adult, wild-type (WT) male mice and on male mice impaired in VNO-mediated signal transduction (TrpC2^-/- ). We found that WT males, which underwent COA to female-soiled bedding, lost their innate preference to female odors and presented lower motivation to approach a sexually receptive female. COA also abolished the testosterone surge normally seen following exposure to female odors. Moreover, the conditioned males displayed impairments in copulatory behaviors, which lasted for several weeks. Surprisingly, these males also exhibited phobic behaviors towards receptive females, including freezing and fleeing responses. In contrast, WT males which underwent COA specifically to male pheromones showed no change in olfactory preference and only a marginally significant elevation in intermale aggression. Finally, we show that TrpC2^-/- males were able to acquire aversion to female-soiled bedding and presented similar behavioral alterations following COA in their responses to female cues. Our results demonstrate that the intrinsic rewarding value of female pheromones can be overridden through associative olfactory learning, which occurs independently of VNO inputs, probably through MOE signaling.

NHERF1 in Microvilli of Vomeronasal Sensory Neurons

In most mammals, the vomeronasal system detects a variety of (semio)chemicals that mediate olfactory-driven social and sexual behaviors. Vomeronasal chemosensation depends on G protein-coupled receptors (V1R, V2R, and FPR-rs) that operate at remarkably low stimulus concentrations, thus, indicating a highly sensitive and efficient signaling pathway. We identified the PDZ domain-containing protein, Na⁺/H⁺ exchanger regulatory factor-1 (NHERF1), as putative molecular organizer of signal transduction in vomeronasal neurons. NHERF1 is a protein that contains 2 PDZ domains and a carboxy-terminal ezrin-binding domain. It localizes to microvilli of vomeronasal sensory neurons and interacts with V1Rs. Furthermore, NHERF1 and Gα_i2 are closely colocalized. These findings open up new aspects of the functional organization and regulation of vomeronasal signal transduction by PDZ scaffolding proteins.

TRICK or TRP? What Trpc2(-/-) mice tell us about vomeronasal organ mediated innate behaviors

The vomeronasal organ (VNO) plays an important role in mediating semiochemical communications and social behaviors in terrestrial species. Genetic knockout of individual components in the signaling pathways has been used to probe vomeronasal functions, and has provided much insights into how the VNO orchestrates innate behaviors. However, all data do not agree. In particular, knocking out Trpc2, a member of the TRP family of non-selective cationic channel thought to be the main transduction channel in the VNO, results in a number of fascinating behavioral phenotypes that have not been observed in other animals whose vomeronasal function is disrupted. Recent studies have identified signaling pathways that operate in parallel of Trpc2, raising the possibility that Trpc2 mutant animals may display neomorphic behaviors. In this article, I provide a critical analysis of emerging evidence to reconcile the discrepancies and discuss their implications.

Increases in intracellular calcium via activation of potentially multiple phospholipase C isozymes in mouse olfactory neurons

Phospholipase C (PLC) and internal Ca(2+) stores are involved in a variety of cellular functions. However, our understanding of PLC in mammalian olfactory sensory neurons (OSNs) is generally limited to its controversial role in odor transduction. Here we employed single-cell Ca(2+) imaging and molecular approaches to investigate PLC-mediated Ca(2+) responses and its isozyme gene transcript expression. We found that the pan-PLC activator m-3M3FBS (25 μM) induces intracellular Ca(2+) increases in vast majority of isolated mouse OSNs tested. Both the response amplitude and percent responding cells depend on m-3M3FBS concentrations. In contrast, the inactive analog o-3M3FBS fails to induce Ca(2+) responses. The m-3M3FBS-induced Ca(2+) increase is blocked by the PLC inhibitor U73122, while its inactive analog U73433 has no effect. Removal of extracellular Ca(2+) does not change significantly the m-3M3FBS-induced Ca(2+) response amplitude. Additionally, in the absence of external Ca(2+), we found that a subset of OSNs respond to an odorant mixture with small Ca(2+) increases, which are significantly suppressed by U73122. Furthermore, using reverse transcription polymerase chain reaction and real-time quantitative polymerase chain reaction, we found that multiple PLC isozyme gene transcripts are expressed in olfactory turbinate tissue in various levels. Using RNA in situ hybridization analysis, we further show expression of β4, γ1, γ2 gene transcripts in OSNs. Taken together, our results establish that PLC isozymes are potent enzymes for mobilizing intracellular Ca(2+) in mouse OSNs and provide molecular insight for PLC isozymes-mediated complex cell signaling and regulation in the peripheral olfactory epithelium.

Estradiol rapidly modulates odor responses in mouse vomeronasal sensory neurons

In rodents, many social behaviors are driven by the sense of smell. The vomeronasal organ (VNO), part of the accessory olfactory system mediates many of these chemically driven behaviors. The VNO is heavily vascularized, and is readily accessible to circulating peptide or steroid hormones. Potentially, this allows circulating hormones to alter behavior through modulating the output of the primary sensory neurons in the VNO, the vomeronasal sensory neurons (VSNs). Based on this, we hypothesized that steroid hormones, in particular 17β-estradiol, would modulate activity of VSNs. In this paper, we show that the estrogen receptors, GPR30 and ERα, were present in VSNs and that estradiol may be synthesized locally in the VNO. Our results also showed that 17β-estradiol decreased responses of isolated VSNs to dilute urine, a potent natural stimulus, with respect to current amplitudes and depolarization. Further, 17β-estradiol increased the latency of the first action potential (AP) and the AP amplitude. Additionally, calcium responses to sulfated steroids (present in the low molecular weight fraction of urine) that act as ligands for apical vomeronasal receptors were decreased by 17β-estradiol. In conclusion, we show that estradiol modulates odorant responses mediated by VSNs and hence paves the way for future studies to better understand the mechanisms by which odorant mediated behavior is altered by endocrine status of the animal.

Phospholipase C and diacylglycerol mediate olfactory responses to amino acids in the main olfactory epithelium of an amphibian

The semi-aquatic lifestyle of amphibians represents a unique opportunity to study the molecular driving forces involved in the transition of aquatic to terrestrial olfaction in vertebrates. Most amphibians have anatomically segregated main and vomeronasal olfactory systems, but at the cellular and molecular level the segregation differs from that found in mammals. We have recently shown that amino acid responses in the main olfactory epithelium (MOE) of larval Xenopus laevis segregate into a lateral and a medial processing stream, and that the former is part of a vomeronasal type 2 receptor expression zone in the MOE. We hypothesized that the lateral amino acid responses might be mediated via a vomeronasal-like transduction machinery. Here we report that amino acid-responsive receptor neurons in the lateral MOE employ a phospholipase C (PLC) and diacylglycerol-mediated transduction cascade that is independent of Ca²⁺ store depletion. Furthermore, we found that putative transient receptor potential (TRP) channel blockers inhibit most amino acid-evoked responses in the lateral MOE, suggesting that ion channels belonging to the TRP family may be involved in the signaling pathway. Our data show, for the first time, a widespread PLC- and diacylglycerol-dependent transduction cascade in the MOE of a vertebrate already possessing a vomeronasal organ.

TRPC2

Trp2 was the second ortholog of the Drosophila trp gene to be identified. Whereas full-length TRPC2 transcripts have been cloned in a number of species including mice, rats, and New World monkeys, TRPC2 is a pseudogene in humans, apes, Old World monkeys, and in a number of other vertebrates. TRPC2 is highly expressed in the rodent VNO. It is also detectable at the protein level in murine erythroblasts, sperm, and brain and has been detected in other tissues by RT-PCR. Its activation by DAG and by erythropoietin has been described in greatest detail, and inhibition by Ca(2+)-calmodulin has been reported. The major demonstrated functions of TRPC2 are regulation of pheromone-evoked signaling in the rodent VNO, regulation of erythropoietin-stimulated calcium influx in murine erythroid cells, and ZP3-evoked calcium influx into sperm. Depletion of TRPC2 in knockout mice resulted in changes in behavior including altered sex discrimination and lack of male-male aggression. The red cells of TRPC2 knockout mice showed increased mean corpuscular volume, mean corpuscular hemoglobin, and hematocrit and reduced mean corpuscular hemoglobin concentration. TRPC2-depleted red cells were resistant to oxidative stress-induced hemolysis.

TRPs in olfaction

The mammalian olfactory system has become an excellent model system to understand the function of transient receptor potential (TRP) channels within their native cellular and circuit environment. The discovery that the canonical TRP channel TRPC2 is highly expressed in sensory neurons of the vomeronasal organ (VNO) has led to major advances in our understanding of the cellular and molecular processes underlying signal transduction of pheromones and other molecular cues that play an essential role in the control of instinctive decisions and innate social behaviors. TRPC2 knockout mice provide a striking example that the loss of function of a single gene can cause severe alterations in a variety of social interactions including the display of aggression, social dominance, and sexual behaviors. There is mounting evidence that TRPC2 is not the only TRP channel expressed in cells of the olfactory system but that other TRP channel subtypes such as TRPC1, TRPC4, TRPC6, TRPM4, and TRPM5 could also play important functional roles in mammalian olfaction. Here, I review such findings and discuss future areas for investigation.

The genomic basis of vomeronasal-mediated behaviour

The vomeronasal organ (VNO) is a chemosensory subsystem found in the nose of most mammals. It is principally tasked with detecting pheromones and other chemical signals that initiate innate behavioural responses. The VNO expresses subfamilies of vomeronasal receptors (VRs) in a cell-specific manner: each sensory neuron expresses just one or two receptors and silences all the other receptor genes. VR genes vary greatly in number within mammalian genomes, from no functional genes in some primates to many hundreds in rodents. They bind semiochemicals, some of which are also encoded in gene families that are coexpanded in species with correspondingly large VR repertoires. Protein and peptide cues that activate the VNO tend to be expressed in exocrine tissues in sexually dimorphic, and sometimes individually variable, patterns. Few chemical ligand–VR–behaviour relationships have been fully elucidated to date, largely due to technical difficulties in working with large, homologous gene families with high sequence identity. However, analysis of mouse lines with mutations in genes involved in ligand–VR signal transduction has revealed that the VNO mediates a range of social behaviours, including male–male and maternal aggression, sexual attraction, lordosis, and selective pregnancy termination, as well as interspecific responses such as avoidance and defensive behaviours. The unusual logic of VR expression now offers an opportunity to map the specific neural circuits that drive these behaviours.

Responsiveness of vomeronasal cells to a newt peptide pheromone, sodefrin as monitored by changes of intracellular calcium concentrations

A peptide pheromone of the red-bellied male newt, sodefrin was tested for its ability to increase intracellular concentrations of Ca(2+) ([Ca(2+)]i) in the dissociated vomeronasal (VN) cells of females by means of calcium imaging system. The pheromone elicited a marked elevation of [Ca(2+)]i in a small population of VN cells from sexually developed females. The population of cells exhibiting sodefrin-induced elevation of [Ca(2+)]i increased concentration-dependently. A pheromone of a different species was ineffective in this respect. The VN cells from non-reproductive females or from reproductive males scarcely responded to sodefrin in terms of elevating [Ca(2+)]i. In the cells from hypophysectomized and ovariectomized females, the sodefrin-inducible increase of [Ca(2+)]i never occurred. The cells from the operated newts supplemented with prolactin and estradiol exhibited [Ca(2+)]i responses to sodefrin with a high incidence. Thus, sex- and hormone-dependency as well as species-specificity of the responsiveness of the VN cells to sodefrin was evidenced at the cellular level. Subsequently, possibility of involvement of phospholipase C (PLC)-inositol 1,4,5-trisphosphate (IP3) and/or PLC-diacylglycerol (DAG)-protein kinase C (PKC) pathways in increasing [Ca(2+)]i in VN cells in response to sodefrin was explored using pharmacological approaches. The results indicated that PLC is involved in generating the Ca(2+) signal in all sodefrin-responsive VN cells, whereas IP3 in approximately 50% of the cells and DAG-PKC in the remaining cells. In the latter case, the increase of [Ca(2+)]i was postulated to be induced by the influx of Ca(2+) through the L-type channel. The significance of the finding is discussed.

β3GnT2 null mice exhibit defective accessory olfactory bulb innervation

Vomeronasal sensory neurons (VSNs) extend axons to the accessory olfactory bulb (AOB) where they form synaptic connections that relay pheromone signals to the brain. The projections of apical and basal VSNs segregate in the AOB into anterior (aAOB) and posterior (pAOB) compartments. Although some aspects of this organization exhibit fundamental similarities with the main olfactory system, the mechanisms that regulate mammalian vomeronasal targeting are not as well understood. In the olfactory epithelium (OE), the glycosyltransferase β3GnT2 maintains expression of axon guidance cues required for proper glomerular positioning and neuronal survival. We show here that β3GnT2 also regulates guidance and adhesion molecule expression in the vomeronasal system in ways that are partially distinct from the OE. In wildtype mice, ephrinA5(+) axons project to stereotypic subdomains in both the aAOB and pAOB compartments. This pattern is dramatically altered in β3GnT2(-/-) mice, where ephrinA5 is upregulated exclusively on aAOB axons. Despite this, apical and basal VSN projections remain strictly segregated in the null AOB, although some V2r1b axons that normally project to the pAOB inappropriately innervate the anterior compartment. These fibers appear to arise from ectopic expression of V2r1b receptors in a subset of apical VSNs. The homotypic adhesion molecules Kirrel2 and OCAM that facilitate axon segregation and glomerular compartmentalization in the main olfactory bulb are ablated in the β3GnT2(-/-) aAOB. This loss is accompanied by a two-fold increase in the total number of V2r1b glomeruli and a failure to form morphologically distinct glomeruli in the anterior compartment. These results identify a novel function for β3GnT2 glycosylation in maintaining expression of layer-specific vomeronasal receptors, as well as adhesion molecules required for proper AOB glomerular formation.

Brain phospholipase C, diacylglycerol lipase and monoacylglycerol lipase are involved in (±)-epibatidine-induced activation of central adrenomedullary outflow in rats

We previously reported that intracerebroventricularly (i.c.v.) administered (±)-epibatidine (a potent agonist of nicotinic acetylcholine receptors) (1, 5 and 10 nmol/animal) dose-dependently elevated plasma levels of noradrenaline and adrenaline and that this response was reduced by i.c.v. administered indomethacin (cyclooxygenase inhibitor) and abolished by bilateral adrenalectomy, indicating the involvement of brain arachidonic acid, as a substrate of cyclooxygenase, in this alkaloid-induced secretion of both catecholamines from the adrenal medulla in rats. Arachidonic acid is mainly released by the action of phospholipase A(2), but is also released by a phospholipase C-, diacylglycerol lipase- and monoacylglycerol lipase-mediated pathway. In the present study, (±)-epibatidine (5 nmol/animal, i.c.v.)-induced elevation of plasma catecholamines was not influenced by pretreatment with mepacrine (phospholipase A(2) inhibitor) (1.1 and 2.2 μmol/animal, i.c.v.), but was effectively reduced by pretreatment with U-73122 (1-[6-[[(17 β)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione) (phospholipase C inhibitor) (10 and 30 nmol/animal, i.c.v.), RHC-80267 [1,6-bis(cyclohexyloximinocarbonylamino)hexane] (diacylglycerol lipase inhibitor) (1.3 and 2.6 μmol/animal, i.c.v.), MAFP (methyl arachidonoyl fluorophosphonate) (monoacylglycerol lipase inhibitor) (0.7 and 1.4 μmol/animal, i.c.v.) or JZL184 [4-nitrophenyl 4-(dibenzo[d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxylate] (selective monoacylglycerol lipase inhibitor) (0.7 and 1.4 μmol/animal, i.c.v.). Immunohistochemical studies demonstrated that (±)-epibatidine (10 nmol/animal, i.c.v.) activates spinally projecting neurons expressing monoacylglycerol lipase in the rat hypothalamic paraventricular nucleus, a control center of central sympatho-adrenomedullary outflow. Taken together, the brain phospholipase C-, diacylglycerol lipase- and monoacylglycerol lipase-mediated pathway seems to be involved in the centrally administered (±)-epibatidine-induced activation of central adrenomedullary outflow in rats.

Paradoxical contribution of SK3 and GIRK channels to the activation of mouse vomeronasal organ

The vomeronasal organ (VNO) is essential for intraspecies communication in many terrestrial vertebrates. The ionic mechanisms of VNO activation remain unclear. We found that the calcium-activated potassium channel SK3 and the G protein-activated potassium channel GIRK are part of an independent pathway for VNO activation. In slice preparations, the potassium channels attenuated inward currents carried by TRPC2 and calcium-activated chloride channels (CACCs). In intact tissue preparations, paradoxically, the potassium channels enhanced urine-evoked inward currents. This discrepancy resulted from the loss of a high concentration of lumenal potassium, which enabled the influx of potassium ions to depolarize the VNO neurons in vivo. Both Sk3 (also known as Kcnn3) and Girk1 (also known as Kcnj3) homozygous null mice showed deficits in mating and aggressive behaviors, and the deficiencies in Sk3(-/-) mice were exacerbated by Trpc2 knockout. Our results suggest that VNO activation is mediated by TRPC2, CACCs and two potassium channels, all of which contributed to the in vivo depolarization of VNO neurons.

Selective Gαi subunits as novel direct activators of transient receptor potential canonical (TRPC)4 and TRPC5 channels

The ubiquitous transient receptor potential canonical (TRPC) channels function as non-selective, Ca(2+)-permeable channels and mediate numerous cellular functions. It is commonly assumed that TRPC channels are activated by stimulation of Gα(q)-PLC-coupled receptors. However, whether the Gα(q)-PLC pathway is the main regulator of TRPC4/5 channels and how other Gα proteins may regulate these channels are poorly understood. We previously reported that TRPC4/TRPC5 can be activated by Gα(i). In the current work, we found that Gα(i) subunits, rather than Gα(q), are the primary and direct activators of TRPC4 and TRPC5. We report a novel molecular mechanism in which TRPC4 is activated by several Gα(i) subunits, most prominently by Gα(i2), and TRPC5 is activated primarily by Gα(i3). Activation of Gα(i) by the muscarinic M2 receptors or expression of the constitutively active Gα(i) mutants equally and fully activates the channels. Moreover, both TRPC4 and TRPC5 are activated by direct interaction of their conserved C-terminal SESTD (SEC14-like and spectrin-type domains) with the Gα(i) subunits. Two amino acids (lysine 715 and arginine 716) of the TRPC4 C terminus were identified by structural modeling as mediating the interaction with Gα(i2). These findings indicate an essential role of Gα(i) proteins as novel activators for TRPC4/5 and reveal the molecular mechanism by which G-proteins activate the channels.

Emerging roles of canonical TRP channels in neuronal function

Ca(2+) signaling in neurons is intimately associated with the regulation of vital physiological processes including growth, survival and differentiation. In neurons, Ca(2+) elicits two major functions. First as a charge carrier, Ca(2+) reveals an indispensable role in information relay via membrane depolarization, exocytosis, and the release of neurotransmitters. Second on a global basis, Ca(2+) acts as a ubiquitous intracellular messenger to modulate neuronal function. Thus, to mediate Ca(2+)-dependent physiological events, neurons engage multiple mode of Ca(2+) entry through a variety of Ca(2+) permeable plasma membrane channels. Here we discuss a subset of specialized Ca(2+)-permeable non-selective TRPC channels and summarize their physiological and pathological role in the context of excitable cells. TRPC channels are predominately expressed in neuronal cells and are activated through complex mechanisms, including second messengers and store depletion. A growing body of evidence suggests a prime contribution of TRPC channels in regulating fundamental neuronal functions. TRPC channels have been shown to be associated with neuronal development, proliferation and differentiation. In addition, TRPC channels have also been suggested to have a potential role in regulating neurosecretion, long term potentiation, and synaptic plasticity. During the past years, numerous seminal discoveries relating TRPC channels to neurons have constantly emphasized on the significant contribution of this group of ion channels in regulating neuronal function. Here we review the major groundbreaking work that has uniquely placed TRPC channels in a pivotal position for governing neuronal Ca(2+) signaling and associated physiological responses.

Requirement of calcium-activated chloride channels in the activation of mouse vomeronasal neurons

In terrestrial vertebrates, the vomeronasal organ (VNO) detects and transduces pheromone signals. VNO activation is thought to be mediated by the transient receptor potential C2 (TRPC2) channel. The aberrant behavioural phenotypes observed in TRPC2−/− mice are generally attributed to the lost VNO function. Recently, calcium-activated chloride channels have been shown to contribute to VNO activation. Here we show that CACCs can be activated in VNO slice preparations from the TRPC2−/− mice and this activation is blocked by pharmacological agents that inhibit intracellular Ca²⁺ release. Urine-evoked Cl⁻ current is sufficient to drive spiking changes in VNO neurons from both wild-type (WT) and TRPC2−/− mice. Moreover, blocking Cl⁻ conductance essentially abolishes VNO activation in WT neurons. These results suggest a TRPC2-independent signalling pathway in the VNO and the requirement of calcium-activated chloride channels currents to mediate pheromone activation. Our data further suggest that TRPC2−/− mice retain partial VNO function.

The vomeronasal organ detects pheromones, which are thought to activate TRPC2 channels on the surface of vomeronasal neurons. Using TRPC2 knockout mice, the authors show that urinary pheromones can also activate these neurons via calcium-activated chloride channels, suggesting a TRPC2-independent pathway for sensing pheromones.