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

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Type 3 inositol 1,4,5-trisphosphate receptor is dispensable for sensory activation of the mammalian vomeronasal organ

Signal transduction in sensory neurons of the mammalian vomeronasal organ (VNO) involves the opening of the canonical transient receptor potential channel Trpc2, a Ca²⁺-permeable cation channel that is activated by diacylglycerol and inhibited by Ca²⁺-calmodulin. There has been a long-standing debate about the extent to which the second messenger inositol 1,4,5-trisphosphate (InsP₃) and type 3 InsP₃ receptor (InsP₃R3) are involved in the opening of Trpc2 channels and in sensory activation of the VNO. To address this question, we investigated VNO function of mice carrying a knockout mutation in the Itpr3 locus causing a loss of InsP₃R3. We established a new method to monitor Ca²⁺ in the endoplasmic reticulum of vomeronasal sensory neurons (VSNs) by employing the GFP-aequorin protein sensor erGAP2. We also performed simultaneous InsP₃ photorelease and Ca²⁺ monitoring experiments, and analysed Ca²⁺ dynamics, sensory currents, and action potential or field potential responses in InsP₃R3-deficient VSNs. Disruption of Itpr3 abolished or minimized the Ca²⁺ transients evoked by photoactivated InsP₃, but there was virtually no effect on sensory activation of VSNs. Therefore, InsP₃R3 is dispensable for primary chemoelectrical transduction in mouse VNO. We conclude that InsP₃R3 is not required for gating of Trpc2 in VSNs.

Aggressive behaviour and physiological responses to pheromones are strongly impaired in mice deficient for the olfactory G-protein -subunit G8

Heterotrimeric G-proteins are critical players in the transduction mechanisms underlying odorant and pheromonal signalling. In the vomeronasal organ (VNO) of the adult mouse, two different G-protein complexes have been identified. Gαoβ2γ8 is preferentially expressed in the basal neurons and coexpresses with type-2 vomeronasal pheromone receptors (V2Rs) whereas Gαi2β2γ2 is found in the apical neurons and coexpresses with type-1 vomeronasal pheromone receptors (V1Rs). V2R-expressing neurons project to the posterior accessory olfactory bulb (AOB) whereas neurons expressing V1Rs send their axon to the anterior AOB. Gγ8 is also expressed in developing olfactory neurons where this protein is probably associated with Go. Here, we generated mice with a targeted deletion of the Gγ8 gene and investigated the behavioural effects and the physiological consequences of this mutation. Gγ8(-/-) mice show a normal development of the main olfactory epithelium; moreover, they do not display major deficits in odour perception. In contrast, the VNO undergoes a slow but remarkable loss of basal neurons starting from the fourth postnatal week, with a 40% reduction of cells at 2 months and 70% at 1 year. This loss is associated with a reduced early-gene expression in the posterior AOB of mice stimulated with pheromones. More interestingly, the Gγ8 deletion specifically leads to a reduced pheromone-mediated aggressiveness in both males and females, all other socio-sexual behaviours remaining unaltered. This study defines a specific role for Gγ8 in maintenance of the neuronal population of the VNO and in the mechanisms of pheromonal signalling that involve the aggressive behaviour towards conspecifics.

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.

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.

From Pheromones to Behavior

In recent years, considerable progress has been achieved in the comprehension of the profound effects of pheromones on reproductive physiology and behavior. Pheromones have been classified as molecules released by individuals and responsible for the elicitation of specific behavioral expressions in members of the same species. These signaling molecules, often chemically unrelated, are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. The standard view of pheromone sensing was based on the assumption that most mammals have two separated olfactory systems with different functional roles: the main olfactory system for recognizing conventional odorant molecules and the vomeronasal system specifically dedicated to the detection of pheromones. However, recent studies have reexamined this traditional interpretation showing that both the main olfactory and the vomeronasal systems are actively involved in pheromonal communication. The current knowledge on the behavioral, physiological, and molecular aspects of pheromone detection in mammals is discussed in this review.

Pregnancy block by MHC class I peptides is mediated via the production of inositol 1,4,5-trisphosphate in the mouse vomeronasal organ

The vomeronasal organ (VNO) has evolved to link an animal's behavior to its environment in a highly species-specific fashion. In mice, it is thought to be the primary sensory system responsible for the detection of pheromones. Pheromones regulate a variety of responses including mate recognition in the context of selective pregnancy failure. MHC (major histocompatibility complex) class I peptides have been identified as compounds that elicit the pregnancy block effect via the VNO. However, the transduction cascade of these molecules is unknown and it is not known if the production of these compounds are androgen dependent. By using male urine and MHC peptides, we show that female mice treated with MHC peptides (in urine or PBS) and urine from castrated males or juvenile mice of different haplotypes respond to the Bruce Effect paradigm in a manner equivalent to female mice exposed to whole urine. In addition to providing new evidence that urine from castrated or juvenile males and MHC peptides can induce pregnancy block, we show correlation of the effect with an increase in inositol 1,4,5-trisphosphate.

Suprasternal gland secretion of male short-tailed opossum induces IP3 generation in the vomeronasal organ

Chemical communication is an important component of mammalian social behaviors. Gray short-tailed opossums (Monodelphis domestica) communicate by scent marking. The male opossum possesses a prominent suprasternal scent gland, extracts of which strongly attract female opossums. This attractivity remains unaltered following repeated lyophilization. The suprasternal gland secretion functions in a sexually dimorphic manner, i.e., it elicits elevated levels of IP(3) in the vomeronasal (VN) sensory epithelium of female opossums, but suppressed the levels of IP(3) in the VN sensory epithelium of male opossums. The elevated levels of IP(3) induced by suprasternal gland secretion in female vomeronasal sensory epithelium is inhibited by the G(i/o) specific inhibitor, NF023, but not its inactive analogue, NF007. It is also suppressed by specific antibodies to the alpha subunits of G(i) and G(o) proteins, by the phospholipase C inhibitor, U73122, as well as by GDPbetaS. Surprisingly, GDPbetaS itself enhances basal levels of IP(3) in female VN sensory epithelium. This GDPbetaS-induced increase in levels of IP(3) is reduced by the PLC inhibitor, U73122, but not by the G(i/o) inhibitor, NF023. In addition, GDP also enhances basal levels of IP(3). GDPbetaS, a known inhibitor of G-protein activation, thus appears to have dual functions: as both stimulator and inhibitor of IP(3) production in the VN sensory epithelium of opossums. In contrast, this nucleotide analogue functions as an inhibitor in the VN sensory epithelium of mice. The mechanism of signal transduction underlying the suprasternal gland secretion-elicited signals in the VN sensory epithelium of opossums appears to involve signals that are generated through activation of G-protein-coupled receptors and transduced via activation of G(i/o)-proteins and the effector, phospholipase C, resulting in an increased production of the second messenger, IP(3). The extracellular signals are thus amplified.

Decreases in urinary pheromonal activities in male mice after exposure to 3-methylchoranthrene

Many classes of environmental pollutants, which are found at significant levels in the environment, affect the reproductive functions. The gonadal functions of various animals are regulated by pheromones excreted from mating partners. Pheromones in male urine play essential roles in the sexual maturation of female mice. Pheromones are received by sensory neurons in the vomeronasal organ, which innervate to the accessory olfactory bulb (AOB). The effects of a typical aromatic environmental pollutant (3-methylchoranthrene) on excretion of pheromones from male mice were explored based on neuronal Fos responses of the AOB of female mice. On days 1 and 3 after intraperitoneal administration of 3-methylchoranthrene (3-MC), the density of Fos-immunoreactive (Fos-ir) cells in the AOB of female mice after exposure to urine excreted from the administered males was lower than that after exposure to urine from non-administered males. These results suggest that 3-MC blocks chemical communication from male to female mice by reducing pheromonal activities.

Pheromone reception in mammals

Pheromonal communication is the most convenient way to transfer information regarding gender and social status in animals of the same species with the holistic goal of sustaining reproduction. This type of information exchange is based on pheromones, molecules often chemically unrelated, that are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. So profound is the relevance of pheromones over the evolutionary process that a specific peripheral organ devoted to their recognition, namely the vomeronasal organ of Jacobson, and a related central pathway arose in most vertebrate species. Although the vomeronasal system is well developed in reptiles and amphibians, most mammals strongly rely on pheromonal communication. Humans use pheromones too; evidence on the existence of a specialized organ for their detection, however, is very elusive indeed. In the present review, we will focus our attention on the behavioral, physiological, and molecular aspects of pheromone detection in mammals. We will discuss the responses to pheromonal stimulation in different animal species, emphasizing the complicacy of this type of communication. In the light of the most recent results, we will also discuss the complex organization of the transduction molecules that underlie pheromone detection and signal transmission from vomeronasal neurons to the higher centers of the brain. Communication is a primary feature of living organisms, allowing the coordination of different behavioral paradigms among individuals. Communication has evolved through a variety of different strategies, and each species refined its own preferred communication medium. From a phylogenetic point of view, the most widespread and ancient way of communication is through chemical signals named pheromones: it occurs in all taxa, from prokaryotes to eukaryotes. The release of specific pheromones into the environment is a sensitive and definite way to send messages to other members of the same species. Therefore, the action of an organism can alter the behavior of another organism, thereby increasing the fitness of either or both. Albeit slow in transmission and not easily modulated, pheromones can travel around objects in the dark and over long distances. In addition, they are emitted when necessary and their biosynthesis is usually economic. In essence, they represent the most efficient tool to refine the pattern of social behaviors and reproductive strategies.

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.

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.

Selective G protein beta gamma-subunit compositions mediate phospholipase C activation in the vomeronasal organ

Chemosensory neurons of the vomeronasal organ (VNO) are supposed to detect pheromones controlling social and reproductive behavior in most terrestrial vertebrates. Recent studies indicate that pheromone signaling in VNO neurons is mediated via phospholipase C (PLC) activation generating the two second messengers inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Since G alpha(i) and G alpha(o) predominantly expressed in VNO neurons are usually not involved in activating PLC, it was explored if PLC activation may be mediated by G beta gamma subunits. It was found that a scavenger for beta gamma dimers reduced the urine-induced IP3 formation in VNO preparations in a dose-dependent manner indicating a role for G beta gamma complexes. Towards an identification of the relevant G beta and G gamma subunit(s), PCR approaches as well as immunohistochemical experiments were performed. It was found that out of the five known G beta subtypes, only G beta2 was expressed in both G alpha(i) as well as G alpha(o) neurons. Experimental approaches focusing on the spatial expression profile of identified G gamma subtypes revealed that G gamma8-positive neurons are preferentially localized to the basal region of the vomeronasal epithelium, whereas G gamma2-reactive cells are restricted to the apical G alpha(i)-positive layer of the sensory epithelium. As IP3 formation induced upon stimulation with volatile urinary compounds was selectively blocked by G gamma2-specific antibodies whereas second messenger formation elicited upon stimulation with alpha2u globulin was inhibited by antibodies recognizing G gamma8, it is conceivable that PLC activation in the two populations of chemosensory VNO neurons is mediated by different G beta gamma complexes.

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.

Ultrastructural localization of G-proteins and the channel protein TRP2 to microvilli of rat vomeronasal receptor cells

Microvilli of vomeronasal organ (VNO) sensory epithelium receptor cells project into the VNO lumen. This lumen is continuous with the outside environment. Therefore, the microvilli are believed to be the subcellular sites of VNO receptor cells that interact with incoming VNO-targeted odors, including pheromones. Candidate molecules, which are implicated in VNO signaling cascades, are shown to be present in VNO receptor cells. However, ultrastructural evidence that such molecules are localized within the microvilli is sparse. The present study provides firm evidence that immunoreactivity for several candidate VNO signaling molecules, notably the G-protein subunits G(ialpha2) and G(oalpha), and the transient receptor potential channel 2 (TRP2), is localized prominently and selectively in VNO receptor cell microvilli. Although G(ialpha2) and G(oalpha) are localized separately in the microvilli of two cell types that are otherwise indistinguishable in their apical and microvillar morphology, the microvilli of both cell types are TRP2(+). VNO topographical distinctions were also apparent. Centrally within the VNO sensory epithelium, the numbers of receptor cells with G(ialpha2)(+) and G(oalpha)(+) microvilli were equal. However, near the sensory/non-sensory border, cells with G(ialpha2)(+) microvilli predominated. Scattered ciliated cells in this transition zone resembled neither VNO nor main olfactory organ (MO) receptor cells and may represent the same ciliated cells as those found in the non-sensory part of the VNO. Thus, this study shows that, analogous to the cilia of MO receptor cells, microvilli of VNO receptor cells are enriched selectively in proteins involved putatively in signal transduction. This provides important support for the role of these molecules in VNO signaling.

Cloning, expression and subcellular localization of two novel splice variants of mouse transient receptor potential channel 2

Transient receptor potential channels (TRPCs) are known as candidate molecular correlates of receptor-activated or store-operated calcium entry. While functional roles for most TRPCs have been suggested, the physiological relevance of TRPC2 remains obscure. Whereas human and bovine TRPC2 are candidate pseudogenes, full-length rodent TRPC2 transcripts have been reported. There is, however, considerable controversy concerning mRNA splicing, tissue distribution and the function of these proteins. We report the molecular cloning of two novel murine TRPC2 splice variants, mTRPC2alpha and mTRPC2beta. mTRPC2alpha RNA is expressed at low levels in many tissues and cell systems, while mTRPC2beta is exclusively and abundantly expressed in the vomeronasal organ (VNO). When expressed in human embryonic kidney (HEK)-293 cells, mTRPC2 did not enhance receptor- or store-activated calcium entry. In order to investigate the basis of such a functional defect, mTRPC2-green fluorescent protein fusion proteins were examined by confocal microscopy. Fusion proteins were retained in endomembranes when expressed in HEK-293 or other cells of epithelial or neuronal origin. Co-expression of TRPC2 with other TRPCs did not restore plasma-membrane trafficking. We conclude that TRPC2 may form functional channels in the cellular context of the VNO, but is unlikely to have a physiological function in other tissues.

Inward current responses to urinary substances in rat vomeronasal sensory neurons

No study has yet demonstrated an inward current in response to pheromonal substances in vomeronasal sensory neurons. Using female rat vomeronasal sensory neurons, we here successfully recorded inward currents in response to urine from various sources. Of the neurons that responded to urine, 77% responded to only one type of urine. Male Wistar urine induced responses preferentially in the apical layer of the sensory epithelium, whilst male Donryu and female Wistar urine induced responses mainly in the basal layer of the epithelium. The amplitude of inward currents induced by application of male Wistar urine was voltage-dependent with average amplitude of -47.1+/-6.2 pA at -74 mV. The average reversal potential for male Wistar urine was -9.3 +/-6.1 mV, which was not apparently different from the reversal potentials for urine from different species. It is likely that the urine-induced inward currents in response to different types of urine are mediated via a similar channel. The simultaneous removal of Na+ and Ca2+ from extracellular solution eliminated the response. The magnitude of the urine-induced inward current in Cl--free external solution was similar to that in normal solution, suggesting that the urine-induced current is cation selective. Removal of external Ca2+ enhanced the amplitude of the urine-induced current and prolonged the response. Application of the constant-field equation indicated a very high permeability coefficient for Ca2+. This study first demonstrated that substances contained in urine elicited inward currents, which induce an excitatory response in vomeronasal sensory neurons, through cation-selective channels.

Cyclic adenosine monophosphate signaling in the rat vomeronasal organ: role of an adenylyl cyclase type VI

The present study indicates that male rat urinary components in female rat vomeronasal organ microvillar preparations not only induce a rapid and transient IP(3) signal, but in addition, the level of cAMP decreases with a delayed and sustained time course. This decrease seems to be a consequence of the preceding activation of the phosphoinositol pathway rather than the result of an enhanced phosphodiesterase activity or an inhibition of adenylyl cyclase (AC) via Galpha(i) or Galpha(o). This notion is supported by the finding that activation of the endogenous protein kinase C suppresses basal as well as forskolin-induced cAMP formation. Furthermore, it was observed that elevated levels of calcium inhibit cAMP formation in rat VNO microvillar preparations. These properties of cAMP signaling in the VNO of rats may be mediated by a calcium- and protein kinase C-inhibited AC VI subtype, which is localized in microvillar preparations of the VNO.

The vomeronasal organ

The vomeronasal organ (VNO) is a chemoreceptor organ enclosed in a cartilaginous capsule and separated from the main olfactory epithelium. The vomeronasal neurons have two distinct types of receptor that differ from each other and from the large family of odorant receptors. The VNO receptors are seven-transmembrane receptors coupled to GTP-binding protein, but appear to activate inositol 1,4,5-trisphosphate signaling as opposed to cyclic adenosine monophosphate. The nature of stimulus access suggests that the VNO responds to nonvolatile cues, leading to activation of the hypothalamus by way of the accessory olfactory bulb and amygdala. The areas of hypothalamus innervated regulate reproductive, defensive, and ingestive behavior as well as neuroendocrine secretion.

Regionalization of Fos immunostaining in rat accessory olfactory bulb when the vomeronasal organ was exposed to urine

The distribution of Fos-immunoreactive (Fos-ir) cells in the accessory olfactory bulb (AOB) of rats following vomeronasal organ exposure to urine was studied. Following exposure to male and female Wistar rat urine, Fos-ir cells were found in the mitral/tufted cell layer, granule cell layer and periglomerular cell layer of the AOB of female Wistar rat, with the highest number in the granule cell layer. Exposure to water or removal of the vomeronasal organ suppressed the expression of Fos-ir cells. These results suggest that female Wistar rats specifically detect urinary substances derived from male or female Wistar rats via the vomeronasal organ. Exposure of the vomeronasal organ of female Wistar rats to male Wistar urine induced the appearance of many more Fos-ir cells in all layers of the AOB than exposure to female Wistar urine. As for the mitral/tufted cell layer, the density of Fos-ir cells in the rostral portion (Gi2alpha-positive) of all regions of the AOB was about twice as high as that in the caudal portion when male urine was given. The distribution pattern of Fos-ir cells in response to female urine was not identical to that in response to male urine. That is, the density of Fos-ir cells in the caudal portion was slightly larger than that in the rostral portion in the lateral region, while in other regions the density in the rostral portion was higher than that in the caudal portion. It is likely that information from different pheromones is transmitted to the higher brain regions through the different regions of the AOB.

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.