Chemosignal transduction in the vomeronasal organ of garter snakes: Ca(2+)-dependent regulation of adenylate cyclase

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.

Odorant receptors directly activate phospholipase C/inositol-1,4,5-trisphosphate coupled to calcium influx in Odora cells

Mechanisms by which odorants activate signaling pathways in addition to cAMP are hard to evaluate in heterogeneous mixtures of primary olfactory neurons. We used single cell calcium imaging to analyze the response to odorant through odorant receptor (OR) U131 in the olfactory epithelial cell line Odora (Murrell and Hunter 1999), a model system with endogenous olfactory signaling pathways. Because adenylyl cyclase levels are low, agents activating cAMP formation do not elevate calcium, thus unmasking independent signaling mediated by OR via phospholipase C (PLC), inositol-1,4,5-trisphosphate (IP(3)), and its receptor. Unexpectedly, we found that extracellular calcium is required for odor-induced calcium elevation without the release of intracellular calcium, even though the latter pathway is intact and can be stimulated by ATP. Relevant signaling components of the PLC pathway and G protein isoforms are identified by western blot in Odora cells as well as in olfactory sensory neurons (OSNs), where they are localized to the ciliary zone or cell bodies and axons of OSNs by immunohistochemistry. Biotinylation studies establish that IP(3) receptors type 2 and 3 are at the cell surface in Odora cells. Thus, individual ORs are capable of elevating calcium through pathways not directly mediated by cAMP and this may provide another avenue for odorant signaling in the olfactory system.

Male pheromone protein components activate female vomeronasal neurons in the salamander Plethodon shermani

BACKGROUND

The mental gland pheromone of male Plethodon salamanders contains two main protein components: a 22 kDa protein named Plethodon Receptivity Factor (PRF) and a 7 kDa protein named Plethodon Modulating Factor (PMF), respectively. Each protein component individually has opposing effects on female courtship behavior, with PRF shortening and PMF lengthening courtship. In this study, we test the hypothesis that PRF or PMF individually activate vomeronasal neurons. The agmatine-uptake technique was used to visualize chemosensory neurons that were activated by each protein component individually.

RESULTS

Vomeronasal neurons exposed to agmatine in saline did not demonstrate significant labeling. However, a population of vomeronasal neurons was labeled following exposure to either PRF or PMF. When expressed as a percent of control level labeled cells, PRF labeled more neurons than did PMF. These percentages for PRF and PMF, added together, parallel the percentage of labeled vomeronasal neurons when females are exposed to the whole pheromone.

CONCLUSION

This study suggests that two specific populations of female vomeronasal neurons are responsible for responding to each of the two components of the male pheromone mixture. These two neural populations, therefore, could express different receptors which, in turn, transmit different information to the brain, thus accounting for the different female behavior elicited by each pheromone component.

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.

Arachidonic acid plays a role in rat vomeronasal signal transduction

Sensory neurons of the vomeronasal organ (VNO) detect volatile chemicals that are released by conspecific animals and convey information about social and reproductive behavior. The signal transduction pathway in vomeronasal receptor neurons (VRNs) is not known in detail, but is believed to be distinct from that of the sensory neurons of the main olfactory system. Many of the identified olfactory transduction components are not expressed by VRNs. Using Ca2+ imaging and electrophysiological recordings, we investigated the signal transduction pathway of urine perception and the possible role of polyunsaturated fatty acids (PUFAs) as intracellular messengers in freshly dissociated rat VNO neurons. We found that application of urine induced a transient increase in intracellular Ca2+ that was dependent on the activity of phospholipase C and diacylglycerol (DAG) lipase. The Ca2+ transient was not dependent on depletion of intracellular Ca2+ stores but was dependent on the presence of extracellular Ca2+. Furthermore, the urine response was not sensitive to modulators of adenylate cyclase and inhibitors of inositol 1,4,5-trisphosphate receptors. Application of PUFAs (linolenic acid and arachidonic acid, synthesized in living cells from DAG) also elicited Ca2+ transients in fura 2 measurements and inward currents in whole-cell voltage-clamp recordings. Pharmacological inhibition of lipoxygenase and cyclooxygenase induced a transient increase in intracellular Ca2+, possibly by increasing the endogenous level of PUFAs, leading to activation of transduction channels. These data provide evidence for a role of PUFAs in rat vomeronasal signal transduction.

Immunohistochemical identification of components of the chemoattractant signal transduction pathway in vomeronasal bipolar neurons of garter snakes

The chemosignal transduction pathway in the vomeronasal sensory epithelium of garter snakes involves activation of G-protein-coupled receptors and subsequent generation of second messengers leading to production of an electrical signal. Calcium imaging experiments demonstrate that ligand binding to the receptor leads to an increase in intracellular calcium and that the phosphatidylinositol-turnover pathway plays a major role in this Ca(2+) increase. Here, we demonstrate, using immunohistochemistry, that IP(3) receptors are largely distributed in dendritic regions of the epithelium, ryanodine receptors are confined to the somata region, and Na(+)/Ca(2+) exchanger protein is expressed throughout the vomeronasal (VN) sensory epithelium.

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.

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.

Signal transduction in the vomeronasal organ of garter snakes: ligand-receptor binding-mediated protein phosphorylation

The vomeronasal (VN) system of garter snakes plays an important role in several species-typical behaviors, such as prey recognition and responding to courtship pheromones. We (X.C. Jiang et al., J. Biol. Chem. 265 (1990) 8736-8744 and Y. Luo et al., J. Biol. Chem. 269 (1994) 16867-16877) have demonstrated previously that in the snake VN sensory epithelium, the chemoattractant ES20, a 20-kDa glycoprotein derived from electric shock-induced earthworm secretion, binds to its receptor which is coupled to PTX-sensitive G-proteins. Such binding results in elevated levels of IP3. We now report that ES20-receptor binding regulates the phosphorylation of two membrane-bound proteins with molecular masses of 42- and 44-kDa (p42/44) in both intact and cell-free preparations of the VN sensory epithelium. ES20 and DAG regulate the phosphorylation of p42/44 in a similar manner. ES20-receptor binding-mediated phosphorylation of p42/44 is rapid and transient, reaching a peak value within 40 seconds and decaying thereafter. Phosphorylation of p42/44 appears to be regulated by the countervailing actions of a specific membrane-bound protein kinase and a protein phosphatase. The phosphorylation of these membrane-bound proteins significantly reduces the activity of G-proteins as evidenced by a decrease in GTPase activity, but has little effect on ligand-receptor binding. These findings suggest that p42/44 play a role in modulating the signal transduction induced by ES20 in the vomeronasal system.

Cell dynamics in the embryonic and postnatal vomeronasal epithelium of snakes

This review will discuss changes observed in the cell dynamics of the vomeronasal epithelium (VNE) of snakes during embryonic and postnatal growth. Recent work suggests that neuronal differentiation occurs early in VNE development. We have used an antibody to an evolutionarily conserved peptide sequence (the PSTAIRE region) in a family of cell cycle regulatory proteins, the cyclin-dependent kinases, to identify neuronal precursors in the embryonic and postnatal VNE. During prenatal development, the location of neuronal precursors changes in the VNE. Significant postnatal changes occur in cell proliferation in the VNE (as determined by 3H-thymidine autoradiography) and possibly in the larger complement of VNE receptor cell precursors (as determined by anti-PSTAIRE staining). A model is proposed for changes in cell proliferation and death during embryonic development and postnatal maintenance and senescence in VNE of snakes, which may be applicable to the VNE and olfactory epithelium of other vertebrates.