Intracellular injection of inositol 1,4,5-trisphosphate increases a conductance in membranes of turtle vomeronasal receptor neurons in the slice preparation

Somatic ion channels and action potentials in olfactory receptor cells and vomeronasal receptor cells

Olfactory receptor cells are primary sensory neurons that catch odor molecules in the olfactory system, and vomeronasal receptor cells catch pheromones in the vomeronasal system. When odor or pheromone molecules bind to receptor proteins expressed on the membrane of the olfactory cilia or vomeronasal microvilli, receptor potentials are generated in their receptor cells. This initial excitation is transmitted to the soma via dendrites, and action potentials are generated in the soma and/or axon and transmitted to the central nervous system. Thus, olfactory and vomeronasal receptor cells play an important role in converting chemical signals into electrical signals. In this review, the electrophysiological characteristics of ion channels in the somatic membrane of olfactory receptor cells and vomeronasal receptor cells in various species are described and the differences between the action potential dynamics of olfactory receptor cells and vomeronasal receptor cells are compared.

Ectopic expression of olfactory receptors and associated G-protein subunits in the head integument of the amphihaline migratory fish hilsa Tenualosa ilisha

The chemosensory nature of the tissue from the dorsal surface of the head (also termed sensory pad; SP) of the amphihaline diadromous fish hilsa Tenualosa ilisha was investigated for odorant receptor (OR), olfactory marker protein (OMP) and G-protein subunits (Gαs-olf, Gαq, Gαo, Gαi3) through immunolocalization and immunoblotting techniques. The immunolocalization of OR, OMP and G-protein subunits showed clear expression of these proteins in the tissues of the SP. Robust expressions of these proteins in the SP were detected with immunoblot analysis. The strong expression of these proteins in the SP indicates that the tissues from this area in riverine T. ilisha may play significant role in chemosensing and signalling through ectopic expression of olfactory receptor proteins which are otherwise reported in olfactory organs in vertebrates. Being migratory in nature, ectopic expression of these receptors in T. ilisha probably helps them to prevent damage to epidermal tissues of the SP, or they may also utilize them as a chemo and mechanosensory tool to optimize chemo-communications during migration.

Localization of G protein alpha subunits and morphology of receptor neurons in olfactory and vomeronasal epithelia in Reeve's turtle, Geoclemys reevesii

Most vertebrates have two nasal epithelia: the olfactory epithelium (OE) and the vomeronasal epithelium (VNE). The apical surfaces of OE and VNE are covered with cilia and microvilli, respectively. In rodents, signal transduction pathways involve G alpha olf and G alpha i2/G alpha o in OE and VNE, respectively. Reeve's turtles (Geoclemys reevesii) live in a semiaquatic environment. The aim of this study was to investigate the localization of G proteins and the morphological characteristics of OE and VNE in Reeve's turtle. In-situ hybridization analysis revealed that both G alpha olf and G alpha o are expressed in olfactory receptor neurons (ORNs) and vomeronasal receptor neurons (VRNs). Immunocytochemistry of G alpha olf/s and G alpha o revealed that these two G proteins were located at the apical surface, cell bodies, and axon bundles in ORNs and VRNs. Electron microscopic analysis revealed that ORNs had both cilia and microvilli on the apical surface of the same neuron, whereas VRNs had only microvilli. Moreover G alpha olf/s was located on only the cilia of OE, whereas G alpha o was not located on cilia but on microvilli. Both G alpha olf/s and G alpha o were located on microvilli of VNE. These results imply that, in Reeve's turtle, both G alpha olf/s and G alpha o function as signal transduction molecules for chemoreception in ORNs and VRNs.

Chemosensory cues from the lacrimal and preputial glands stimulate production of IP3 in the vomeronasal organ and aggression in male mice

The social and reproductive behaviors of most mammals are modulated by chemosensory cues. The perception of some of these cues is mediated by the vomeronasal organ, which is a cartilage-encased elongated organ associated with the vomer bone in the rostral nasal cavity. Several studies have shown that chemosensory cues are present in urine, seminal fluid or vaginal secretions but only a few studies have focused on exocrine glands as a source of chemosensory cues. Here we show that chemosensory cues present in two exocrine glands, i.e., the preputial gland located at the caudal region and the lacrimal gland located at the rostral region, are capable of stimulating aggression in male mice. We further show that these extracts can stimulate the production of inositol-(1,4,5)-trisphosphate in the vomeronasal organ.

Vomeronasal sensory neurons from Sternotherus odoratus (stinkpot/musk turtle) respond to chemosignals via the phospholipase C system

The mammalian signal transduction apparatus utilized by vomeronasal sensory neurons (VSNs) in the vomeronasal organ (VNO) has been richly explored, while that of reptiles, and in particular, the stinkpot or musk turtle Sternotherus odoratus, is less understood. Given that the turtle's well-known reproductive and mating behaviors are governed by chemical communication, 247 patch-clamp recordings were made from male and female S. odoratus VSNs to study the chemosignal-activated properties as well as the second-messenger system underlying the receptor potential. Of the total neurons tested, 88 (35%) were responsive to at least one of five complex natural chemicals, some of which demonstrated a degree of sexual dimorphism in response selectivity. Most notably, male VSNs responded to male urine with solely outward currents. Ruthenium Red, an IP3 receptor (IP3R) antagonist, failed to block chemosignal-activated currents, while the phospholipase C (PLC) inhibitor, U73122, abolished the chemosignal-activated current within 2 min, implicating the PLC system in the generation of a receptor potential in the VNO of musk turtles. Dialysis of several second messengers or their analogues failed to elicit currents in the whole-cell patch-clamp configuration, negating a direct gating of the transduction channel by cyclic adenosine monophosphate (cAMP), inositol 1,4,5-trisphosphate (IP3), arachidonic acid (AA), or diacylglycerol (DAG). Reversal potential analysis of chemosignal-evoked currents demonstrated that inward currents reversed at -5.7+/-7.8 mV (mean +/- s.e.m.; N=10), while outward currents reversed at -28.2+/-2.4 mV (N=30). Measurements of conductance changes associated with outward currents indicated that the outward current represents a reduction of a steady state inward current by the closure of an ion channel when the VSN is exposed to a chemical stimulus such as male urine. Chemosignal-activated currents were significantly reduced when a peptide mimicking a domain on canonical transient receptor potential 2 (TRPC2), to which type 3 IP3 receptor (IP3R3) binds, was included in the recording pipette. Collectively these data suggest that there are multiple transduction cascades operational in the VSNs of S. odoratus, one of which may be mediated by a non-selective cation conductance that is not gated by IP3 but may be modulated by the interaction of its receptor with the TRPC2 channel.

Heterogeneity of voltage- and chemosignal-activated response profiles in vomeronasal sensory neurons

Liolaemus lizards were explored to ascertain whether they would make an amenable model to study single-cell electrophysiology of neurons in the vomeronasal organ (VNO). Despite a rich array of chemosensory-related behaviors chronicled for this genus, no anatomical or functional data exist for the VNO, the organ mediating these types of behaviors. Two Liolaemus species (L. bellii and L. nigroviridis) were collected in Central Chile in the Farellones Mountains and transported to the United States. Lizards were subjected to hypothermia and then a lethal injection of sodium pentabarbitol prior to all experiments described in the following text. Retrograde dye perfusion combined with histological techniques demonstrated a compartmentalization of the proportionally large VNO from the main olfactory epithelium (MOE) in cryosections of L. bellii. SDS-PAGE analysis of the VNO of both species demonstrated the expression of three G protein subunits, namely, G(alphao), G(alphai2), and G(beta), and the absence of G(alphaolf), G(alpha11), and G(q), the latter of which are traditionally found in the MOE. Vomeronasal (VN) neurons were enzymatically isolated for whole cell voltage-clamp electrophysiology of single neurons. Both species demonstrated a tetrodotoxin (TTX)-sensitive, rapidly inactivating sodium current and a tetraethylammonium (TEA)-sensitive potassium current that had a transient and sustained component. VN neurons were classified into two types dependent on the ratio of sodium over sustained potassium current. VN neurons exhibited outward and inward chemosignal-evoked currents when stimulated with pheromone-containing secretions taken from the feces, skin, and precloacal pores. Fifty-nine percent of the neurons were responsive to at least one compound when presented with a battery of five different secretions. The breadth of responsiveness (H metric) demonstrated a heterogeneous population of tuning with a mean of 0.29.

A diacylglycerol-gated cation channel in vomeronasal neuron dendrites is impaired in TRPC2 mutant mice: mechanism of pheromone transduction

Vomeronasal sensory neurons play a crucial role in detecting pheromones, but the chemoelectrical transduction mechanism remains unclear and controversial. A major barrier to the resolution of this question has been the lack of an activation mechanism of a key transduction component, the TRPC2 channel. We have identified a Ca(2+)-permeable cation channel in vomeronasal neuron dendrites that is gated by the lipid messenger diacylglycerol (DAG), independently of Ca(2+) or protein kinase C. We demonstrate that ablation of the TRPC2 gene causes a severe deficit in the DAG-gated channel, indicating that TRPC2 encodes a principal subunit of this channel and that the primary electrical response to pheromones depends on DAG but not Ins(1,4,5)P(3), Ca(2+) stores, or arachidonic acid. Thus, a previously unanticipated mechanism involving direct channel opening by DAG underlies the transduction of sensory cues in the accessory olfactory system.

Photolysis of caged inositol 1,4,5-trisphosphate induces action potentials in frog vomeronasal microvillar receptor neurones

To study the effect of inositol 1,4,5-trisphosphate (IP(3)) in isolated frog vomeronasal microvillar receptor neurones, whole-cell recordings were performed with 0.5 microM caged IP(3) dissolved in the pipette solution. IP(3) was released by photolysis of caged IP(3) initiated by a 0.8-ms ultraviolet flash from a xenon flash lamp 70 s after the start of dialysis of caged IP(3) into the cell. Flash illuminating the whole receptor neurone with caged IP(3) triggered action potentials when the current was clamped at zero and a series of transient inward currents of 12-55 pA at a holding potential of -70 mV. The average number of spikes during the first 40 s after release of IP(3) was 7.2+/-2.5 (n=6, mean+/-S.E.M.). The average maximum current and the total inward transport of charge during the first 40 s after photolysis of caged IP(3) were -24+/-8.0 pA and -1.7+/-0.8 pC, respectively (n=5, mean+/-S.E.M.). Inward membrane currents of 12-55 pA after release of IP(3) were not observed with 50 microM La(3+) in the bath. Notably, flash focused on the terminal vesicle also triggered action potentials. No action potentials were observed following flash focused on the soma or outside the dendrite. The average number of spikes during the first 40 s after release of IP(3) initiated by flash spatially restricted to the terminal vesicle was 5.0+/-2.0 (n=4, mean+/-S.E.M.).The present study indicates that local release of IP(3) in the terminal vesicle of the vomeronasal neurones triggers transient depolarizations and induces action potentials. We suggest that IP(3) might be a second messenger in the vomeronasal microvillar receptor neurones.

Involvement of G(q/11) in signal transduction in the mammalian vomeronasal organ

Social behaviors of most mammals are profoundly affected by pheromones. Pheromones are detected by G-protein coupled receptors in the vomeronasal organ (VNO). To investigate the role of G alpha(q/11) in vomeronasal signal transduction pathways, microvillar membranes from murine VNO were prepared. Incubation of such membranes from prepubertal females with adult male urine results in an increase in production of inositol-(1,4,5)-trisphosphate (IP(3)). This stimulation is mimicked by GTP gamma S, blocked by GDP beta S and is tissue specific. Furthermore, use of bacterial toxins such as pertussis that lead to ADP-ribosylation of the G-protein alpha subunits of G(o) and G(i2) do not block the increase in IP(3) levels but U-73122, a PLC inhibitor, blocks the production of IP(3). Studies with monospecific antibodies revealed the presence of three G-proteins, G alpha(o), G alpha(i2) and G alpha(q/11)-related protein, in vomeronasal neurons, concentrated on their microvilli. Our observations indicate that pheromones in male urine act on vomeronasal neurons in the female VNO via a receptor-mediated, G alpha(q/11)-protein-dependent increase in IP(3) levels.

Type-specific inositol 1,4,5-trisphosphate receptor localization in the vomeronasal organ and its interaction with a transient receptor potential channel, TRPC2

The vomeronasal organ (VNO) is the receptor portion of the accessory olfactory system and transduces chemical cues that identify social hierarchy, reproductive status, conspecifics and prey. Signal transduction in VNO neurons is apparently accomplished via an inositol 1,4,5-trisphosphate (IP3)-activated calcium conductance that includes a different set of G proteins than those identified in vertebrate olfactory sensory neurons. We used immunohistochemical (IHC) and SDS-PAGE/western analysis to localize three IP3 receptors (IP3R) in the rat VNO epithelium. Type-I IP3R expression was weak or absent. Antisera for type-II and -III IP3R recognized appropriate molecular weight proteins by SDS-PAGE, and labeled protein could be abolished by pre-adsorption of the respective antibody with antigenic peptide. In tissue sections, type-II IP3R immunoreactivity was present in the supporting cell zone but not in the sensory cell zone. Type-III IP3R immunoreactivity was present throughout the sensory zone and overlapped that of transient receptor potential channel 2 (TRPC2) in the microvillar layer of sensory epithelium. Co-immunoprecipitation of type-III IP3R and TRPC2 from VNO lysates confirmed the overlapping immunoreactivity patterns. The protein-protein interaction complex between type-III IP3R and TRPC2 could initiate calcium signaling leading to electrical signal production in VNO neurons.

Inositol 1,4,5-trisphosphate transduction cascade in taste reception of the fleshfly, Boettcherisca peregrina

The role of an inositol 1,4,5-trisphosphate (IP3)-mediated transduction cascade in the response of taste receptor cells of the fleshfly Boettcherisca peregrina was investigated by using the following reagents: neomycin (an inhibitor of IP3 production), U73122 (an inhibitor of phospholipase C), adenophostin A (an agonist of the IP3-gated channel), IP3, ruthenium red (a blocker of the IP3-gated channel), and 2-aminoethoxydiphenylborate (2-APB; an antagonist of the IP3-gated channel). For introduction into the receptor cell, the reagents were mixed with a detergent, deoxycholate (DOC). After treatment with neomycin + DOC or U73122 + DOC, the response of the sugar receptor cell to sugars was depressed compared with responses after treatment with DOC alone. During the treatment of adenophostin A + DOC, the response of the sugar receptor cell was elicited. After treatment with IP3 + DOC, the response of the sugar receptor cell to sugars and to amino acids was apparently enhanced. When taste stimuli were administered in the presence of ruthenium red or 2-APB, the response of the sugar receptor cell to glucose were inhibited. The expression of genes for substances involved in the IP3 transduction cascade, such as G protein alpha subunit (dGqalpha), phospholipase C (norpA), and IP3 receptor (itpr), were examined in the taste receptor cell of the fruitfly Drosophila melanogaster by using the pox-neuro70 mutant (poxn70), which lacks taste receptor cells. The expressed levels of dGqalpha and itpr in the tarsus of poxn70 mutant flies were reduced compared with those of wild-type flies. These results suggest that the IP3 transduction cascade is involved in the response of the sugar receptor cell of the fly.

Calcium transients in the garter snake vomeronasal organ

The signaling cascade involved in chemosensory transduction in the VN organ is incompletely understood. In snakes, the response to nonvolatile prey chemicals is mediated by the vomeronasal (VN) system. Using optical techniques and fluorescent Ca(2+) indicators, we found that prey-derived chemoattractants produce initially a transient cytosolic accumulation of [Ca(2+)](i) in the dendritic regions of VN neurons via two pathways: Ca(2+) release from IP(3)-sensitive intracellular stores and, to a lesser extent, Ca(2+) influx through the plasma membrane. Both components seem to be dependent on IP(3) production. Chemoattractants evoke a short-latency Ca(2+) elevation even in the absence of extracellular Ca(2+), suggesting that in snake VN neurons, Ca(2+) release from intracellular stores is independent of a preceding Ca(2+) influx, and both components are activated in parallel during early stages of chemosensory transduction. Once the response develops in apical dendritic segments, other mechanisms can also contribute to the amplification and modulation of these chemoattractant-mediated cytosolic Ca(2+) transients. In regions close to the cell bodies of the VN neurons, the activation of voltage-sensitive Ca(2+) channels and a Ca(2+)-induced Ca(2+) release from intracellular ryanodine-sensitive stores secondarily boost initial cytosolic Ca(2+) elevations increasing their magnitude and durations. Return of intracellular Ca(2+) to prestimulation levels appears to involve a Ca(2+) extrusion mediated by a Na(+)/Ca(2+) exchanger mechanism that probably plays an important role in limiting the magnitude and duration of the stimulation-induced Ca(2+) transients.

Patch-clamp analysis of voltage-activated and chemically activated currents in the vomeronasal organ of Sternotherus odoratus (stinkpot/musk turtle)

The electrophysiological basis of chemical communication in the specialized olfactory division of the vomeronasal (VN) organ is poorly understood. In total, 198 patch-clamp recordings were made from 42 animals (Sternotherus odoratus, the stinkpot/musk turtle) to study the electrically and chemically activated properties of VN neurons. The introduction of tetramethylrhodamine-conjugated dextran into the VN orifice permitted good visualization of the vomeronasal neural epithelium prior to dissociating it into single neurons. Basic electrical properties of the neurons were measured (resting potential, -54.5 +/- 2.7 mV, N=11; input resistance, 6.7 +/- 1.4 G Omega, N=25; capacitance, 4.2 +/- 0.3 pF, N=22; means +/- S.E.M.). The voltage-gated K(+) current inactivation rate was significantly slower in VN neurons from males than in those from females, and K(+) currents in males were less sensitive (greater K(i)) to tetraethylammonium. Vomeronasal neurons were held at a holding potential of -60 mV and tested for their response to five natural chemicals, female urine, male urine, female musk, male musk and catfish extract. Of the 90 VN neurons tested, 33 (34 %) responded to at least one of the five compounds. The peak amplitude of chemically evoked currents ranged from 4 to 180 pA, with two-thirds of responses less than 25 pA. Urine-evoked currents were of either polarity, whereas musk and catfish extract always elicited only inward currents. Urine applied to neurons harvested from female animals evoked currents that were 2-3 times larger than those elicited from male neurons. Musk-evoked inward currents were three times the magnitude of urine- or catfish-extract-evoked inward currents. The calculated breadth of responsiveness for neurons presented with this array of five chemicals indicated that the mean response spectrum of the VN neurons is narrow (H metric 0.11). This patch-clamp study indicates that VN neurons exhibit sexual dimorphism in function and specificity in response to complex natural chemicals.iol

Sexual dimorphism and developmental expression of signal-transduction machinery in the vomeronasal organ

We have explored the use of a new model to study the transduction of chemosignals in the vomeronasal organ (VNO), for which the functional pathway for chemical communication is incompletely understood. Because putative vomeronasal receptors in mammalian and other vertebrate models belong to the superfamily of G-protein-coupled receptors, the objective of the present study was to define which G-protein subunits were present in the VNO of Sternotherus odoratus (stinkpot or musk turtle) in order to provide directionality for future functional studies of the downstream signaling cascades. The turtle vomeronasal epithelium (VNE) was found to contain the G-proteins G(beta) and G(alphail-3) at the microvillar layer, the presumed site of signal tranduction in these neurons, as evidenced by immunocytochemical techniques. G(alphao) labeled the axon bundles in the VNE and the somata of the vomeronasal sensory neurons but not the microvillar layer. Densitometric analysis of Western blots indicated that the VNO from females contained greater concentrations of G(alphai1-3) compared with males. Sexually immature (juvenile) turtles showed intense immunolabeling for all three subunits (G(beta), G(alphai1-3), and G(alphao)) in the axon bundles and an absence of labeling in the microvillar layer. Another putative signaling component found in the microvilli of mammalian VNO, transient receptor potential channel, was also immunoreactive in S. odoratus in a gender-specific manner, as quantified by Western blot analysis. These data demonstrate the utility of Sternotherus for discerning the functional signal transduction machinery in the VNO and may suggest that gender and developmental differences in effector proteins or cellular signaling components may be used to activate sex-specific behaviors.

Inositol 1,4,5-trisphosphate and adenophostin analogues induce responses in turtle olfactory sensory neurons

Using the whole-cell mode of the patch-clamp technique, we recorded inward currents in response to inositol-1,4,5-trisphosphate (IP3) and adenophostin analogues in turtle olfactory sensory neurons. Dialysis of IP3 into the neurons induced inward currents with an increase in membrane conductance in a dose-dependent manner under the voltage-clamp conditions (holding potential -70 mV). The application of Ca2+-free Ringer solution to neurons previously dialysed with IP3 induced inward currents that were reversibly inhibited by application of Na+, Ca2+-free Ringer solution, normal Ringer solution or 10 microM ruthenium red. Dialysis of the adenophostin analogues, novel IP3 receptor ligands, also induced inward currents with an increase in membrane conductance. The magnitude of the responses to the adenophostin analogues varied among these derivatives. The application of Ca2+-free Ringer solution to neurons previously dialysed with the adenophostin analogues induced inward currents that were inhibited by the application of normal Ringer solution. The reversal potential of inward currents induced by an adenophostin analogue was similar to that induced by IP3, suggesting that inward currents induced by the adenophostin analogue were generated by a similar ionic mechanism to that induced by IP3. The present study demonstrated that IP3-mediated transduction pathways exist in turtle olfactory receptor neurons and that adenophostin analogues act as agonists of IP3.

Protease-sensitive urinary pheromones induce region-specific Fos-expression in rat accessory olfactory bulb

Vomeronasal organs of female Wistar rats were exposed with sprayed urine preparations of male Wistar rats prior to sacrifice. Exposure to crude urine and ultrafiltrated urine preparation (<5000 Da) induced significant Fos expression, which is correlated with cellular activity, in the mitral/tufted cell layer of the accessory olfactory bulb (AOB), while exposure to the remaining substances after ultrafiltration (>5000 Da) and control salt solution did not. Exposure to urine preparation treated with papain induced expression of Fos-immunoreactive cells in the rostral region of the AOB, but did not induce such expression in the caudal region. Exposure to urine preparation treated with pronase induced urine-specific Fos immunoreactivity neither in the rostral nor in the caudal region. These results suggest that at least two different peptides carrying pheromonal activities are contained in male Wistar rat urine.

Laminar distribution of pheromone-receptive neurons in rat vomeronasal epithelium

1. Responses of vomeronasal sensory neurons to urine excreted from rats, mice and hamsters were studied by the on-cell patch clamp method in slices of sensory epithelium from female Wistar rats. 2. The urine excreted from male and female Wistar rats, male Donryu rats and male C57BL/6 mice induced relatively large responses, while urine from male Sprague-Dawley rats and male Syrian hamsters induced small responses. 3. Of the 62 neurons responding to urine, 57 responded to only one of the urine preparations. 4. The sensory neurons that responded to the male Wistar urine were localized in the apical position of the epithelium where one type of GTP-binding protein, Gi2alpha, is selectively expressed. The neurons in the basal position of the epithelium, which express Goalpha, responded to urine from the other animals. 5. This study demonstrates that sensory neurons responsive to different urinary pheromones are localized in a segregated layer in the rat vomeronasal sensory epithelium.

Inositol-1,4,5-trisphosphate accumulation induced by urinary pheromones in female rat vomeronasal epithelium

The mechanisms involved in pheromone-induced responses in the vomeronasal neurons, especially in mammals, are still unclear. In the present study, we examined the effects of rat urine samples containing various types of pheromones regulating gonadal functions on the accumulation of cAMP and inositol 1,4,5-trisphosphate (IP3) in a vomeronasal membrane preparation from the female Wistar rat. Stimulation of the preparation with forskolin induced cAMP accumulation, but stimulation with urine samples excreted from the male Wistar rat, the female Wistar rat, and the male Donryu rat did not change cAMP levels. These results were consistent with the electrophysiological results showing that dialysis of a high concentration of cAMP into the vomeronasal neuron does not induce currents. Stimulation with the three urine samples induced the accumulation of IP3 in the membrane preparation. These results are consistent with previous electrophysiological results [K. Inamura, M. Kashiwayanagi, K. Kurihara, Inositol-1,4,5-trisphosphate induces responses in receptor neurons in rat vomeronasal sensory slices, Chem. Senses 22 (1997) 93-103; K. Inamura, M. Kashiwayanagi, K. Kurihara, Blockage of urinary responses by inhibitors for IP3-mediated pathway in rat vomeronasal sensory neurons, Neurosci. Lett. 233 (1997) 129-132]. After the treatment with Pertussis toxin (PTX), the male Wistar urine did not induce IP3 accumulation significantly. Application of the male Wistar urine decreased ADP-ribosylation of Gi with PTX, while that of the male Donryu urine decreased ADP-ribosylation of Go. Thus, the present results support a mechanism by which the responses of the rat vomeronasal neurons to urinary pheromones are mediated by IP3, Gi and/or Go.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

Blockage of urinary responses by inhibitors for IP3-mediated pathway in rat vomeronasal sensory neurons

The mammalian vomeronasal system is involved in the effects of urinary chemicals on gonadal functions and sexual behaviors. For example, exposure to urine affects the timing of oestrous cycles in rats. Rat vomeronasal sensory neurons in slice preparation were studied under on-cell patch clamp conditions. We found that urine excreted from male Wistar rats increased impulse frequency in vomeronasal sensory neurons of female Wistar rats. The urinary responses were blocked by an inositol-1,4,5-trisphosphate (IP3)-channel inhibitor (10 microM ruthenium red) or phospholipase C inhibitors (10 microM U-73122 and 1 mM neomycin), suggesting that pheromone-like substances in the urine induce the response in the rat vomeronasal sensory neurons via the IP3-dependent transduction pathway.

Pheromone regulated production of inositol-(1, 4, 5)-trisphosphate in the mammalian vomeronasal organ

Social behaviors of most mammals are profoundly affected by chemical signals, pheromones, exchanged between conspecifics. Pheromones interact with dendritic microvilli of bipolar neurons in the vomeronasal organ (VNO). To investigate vomeronasal signal transduction pathways, microvillar membranes from porcine VNO were prepared. Incubation of such membranes from prepubertal females with boar seminal fluid or urine results in an increase in production of inositol-(1, 4, 5)-trisphosphate (IP3). The dose response for IP3 production is biphasic with a GTP-dependent component at low stimulus concentrations and a nonspecific increase in IP3 at higher stimulus concentrations. The GTP-dependent stimulation is mimicked by GTPgammaS and blocked by GDPbetaS. Furthermore, the GTP-dependent component of the stimulation of IP3 production is sex specific and tissue dependent. Studies with monospecific antibodies reveal a G alpha(q/11)-related protein in vomeronasal neurons, concentrated at their microvilli. Our observations indicate that pheromones in boar secretions act on vomeronasal neurons in the female VNO via a receptor mediated, G protein-dependent increase in IP3. These observations set the stage for further investigations on the regulation of stimulus-excitation coupling in vomeronasal neurons. The pheromone-induced IP3 response also provides an assay for future purification of mammalian reproductive pheromones.

Urine-derived compound evokes membrane responses in mouse vomeronasal receptor neurons

Sensory neurons of the vomeronasal organ (VNO) are thought to detect species-specific chemical signals important for reproductive function. The electrical properties of VNO neurons have begun to be characterized in a variety of species; however, the response of VNO neurons to possible physiological ligands has not yet been reported. One physiological effector, dehydro-exo-brevicomin (DHB), is found in the urine of intact male mice and affects the estrous cycle of female mice. In the present study, dissociated VNO neurons were voltage- or current-clamped and their response to DHB was determined. Approximately 26% of VNO neurons responded to DHB with an outward current at negative holding potentials; the current reversed at approximately +4 mV. Application of DHB in current-clamp mode produced membrane hyperpolarization and/or a reduction in the firing of action potentials. Because membrane conductance was shown to be decreased during application of DHB, the results suggest that the outward current associated with DHB application is a reflection of a reduction in inward current caused by closing an ion channel. This study provides the first evidence that a compound found in male urine directly affects VNO neurons.

Electrophysiological characterization of chemosensory neurons from the mouse vomeronasal organ

The mechanism of sensory transduction in chemosensory neurons of the vomeronasal organ (VNO) is not known. Based on molecular data, it is likely to be different from that mediating sensory transduction in the main olfactory system. To begin to understand this system, we have characterized the electrophysiological properties of dissociated mouse VNO neurons with patch-clamp recording. Sensory neurons were distinguished from nonsensory neurons by the presence of a dendrite, by immunoreactivity for olfactory marker protein, and by the firing of action potentials. The resting potential of VNO neurons was approximately -60 mV, and the average input resistance was 3 Gomega. Current injections as small as 1-2 pA elicited steady trains of action potentials that showed no sign of elicited steady trains of action potentials that showed no sign of adaptation during a 2 sec stimulus duration. The voltage-gated conductances in VNO neurons are distinct from those in olfactory neurons. The Na+ current is composed of two components; the major component was TTX-sensitive (Ki = 3.6 nM). The outward K+ current activates at -30 mV with kinetics 10 times slower than for K+ currents in olfactory neurons. The Ca2+ current is composed of at least two components: an L-type current and a T-type current that activates at -60 mV and is not found in olfactory neurons. We find no evidence for cyclic nucleotide-gated channels in VNO neurons under a variety of experimental conditions, including those that produced large responses in mouse olfactory neurons, which is further evidence for a novel transduction pathway.

Pheromone transduction in the vomeronasal organ

Single-cell physiology and cloning efforts have extended studies of the vomeronasal organ to cellular and molecular levels. Recent work has shown that transduction in the vomeronasal organ is probably mediated by signalling pathways distinct from those that mediate transduction in the main olfactory system. An advance in understanding transduction has come with the cloning from rat vomeronasal organ of a family of putative pheromone receptor genes that bear no sequence similarity to previously cloned receptors. Other work has examined the expression of putative signalling components and found a zonal organization of the epithelium. Patch-clamp studies have described the basic electrical properties of vomeronasal neurons and explored second-messenger pathways.

When is a butterfly like an elephant?

The behaviours of organisms as diverse as elephants and butterflies are affected by pheromones with identical or similar structures. Recent developments in the molecular biology of pheromone detection suggest why.

Subclasses of vomeronasal receptor neurons: differential expression of G proteins (Gi alpha 2 and G(o alpha)) and segregated projections to the accessory olfactory bulb

Differential expression of G proteins (Gi alpha 2 and G(o alpha) and the separate central projections of Gi alpha 2- and G(o alpha)-immunoreactive (ir) vomeronasal receptor neurons were investigated in the mouse and rat using immunocytochemical methods. In the vomeronasal organ (VNO), receptor neurons with their cell bodies located in the middle layer (middle 1/3) of the vomeronasal sensory epithelium express Gi alpha 2. Axons of these Gi alpha 2-ir neurons can be followed from VNO to the anterior part, but not the posterior part, of the nerve-glomerular (N-GL) layer of the accessory olfactory bulb (AOB). Another population of receptor neurons, which are located in the deep layer (basal 1/3) of the vomeronasal sensory epithelium, express G(o alpha), and axons of the G(o alpha)-ir neurons can be traced to the posterior part, but not the anterior part, of the N-GL layers of the AOB. The axons of the two subclasses of receptor neurons are intermingled near the VNO and become segregated as they enter the AOB. Removal of the AOB results in retrograde degeneration of both Gi alpha 2-ir and G(o alpha)-ir receptor neurons in the VNO. These results suggest that at least two subclasses of receptor neurons exist in the VNO: the Gi alpha 2-ir neurons in the middle layer and the G(o alpha)-ir neurons in the deep layer of the VNO. The Gi alpha 2-ir neurons in the middle layer of the VNO project to the anterior part of the AOB, while the G(o alpha)-ir neurons in the deep layer of the VNO project to the posterior half of the AOB. These results are similar to our previous observations in the gray short-tailed opossum, suggesting that the existence of at least two subclasses of receptor neurons in the vomeronasal epithelium with differential projections to the AOB is a conserved feature among mammals.

Evidence for distinct signaling mechanisms in two mammalian olfactory sense organs

In mammals, olfactory stimuli are detected by sensory neurons at two distinct sites: the olfactory epithelium (OE) of the nasal cavity and the neuroepithelium of the vomeronasal organ (VNO). While the OE can detect volatile chemicals released from numerous sources, the VNO appears to be specialized to detect pheromones that are emitted by other animals and that convey information of behavioral or physiological importance. The mechanisms underlying sensory transduction in the OE have been well studied and a number of components of the transduction cascade have been cloned. Here, we investigated sensory transduction in the VNO by asking whether VNO neurons express molecules that have been implicated in sensory transduction in the OE. Using in situ hybridization and Northern blot analyses, we found that most of the olfactory transduction components examined, including the guanine nucleotide binding protein alpha subunit (G-alpha-olf), adenylyl cyclase type III, and an olfactory cyclic nucleotide-gated (CNG) channel subunit (oCNC1), are not expressed by VNO sensory neurons. In contrast, VNO neurons do express a second olfactory CNG channel subunit (oCNC2). These results indicate that VNO sensory transduction is distinct from that in the OE but raise the possibility that, like OE sensory transduction, sensory transduction in the VNO might involve cyclic nucleotide-gated ion channels.