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

Abstract

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 β and G αi1-3 at the microvillar layer, the presumed site of signal tranduction in these neurons, as evidenced by immunocytochemical techniques. G αo 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 αi1-3 compared with males. Sexually immature (juvenile) turtles showed intense immunolabeling for all three subunits (G β , G αi1-3 , and G αo ) 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. Indexing termsG proteins; GTP-binding protein; vomeronasal organ; turtle; transient receptor potential; transient receptor potential channel Sensory systems capture information from the environment and convey these signals to higher cortical centers in the brain, where the signals are processed to render an internal depiction of the external world (Dulac and Axel, 1995). In several classes of vertebrates, chemical sensory perception is mediated by sensory neurons at two anatomically distinct locations: the main olfactory epithelium (MOE) and the vomeronasal organ (VNO; Winan and Scalia, 1970;Broadwell, 1975;Scalia and Winan, 1975;Kevetter and Winan, 1981;Shepherd, 1988). Although they are of similar embryonic and neuronal origin (Cuschieri and Bannister, 1975), the two olfactory systems appear to diverge in function. (Halpern, 1987): The sensory neurons have unique biophysical properties (Liman and Corey, 1996); the molecular identities of putative transduction proteins that involve G-protein-coupled signaling cascades are notably different (Dulac and Axel, 1995;Ryba and Tirindelli, 1997); and, finally, the sensory information transmitted from each olfactory system is directed to different brain regions (for review, see Keverne, 1999). Hence, the VNO emerges as ...

Vomeronasal sensory neurons fromSternotherus odoratus(stinkpot/musk turtle) respond to chemosignalsviathe phospholipase C system

“…These data imply that the connection between the internal Ca 2+ stores is important to the maintenance of the response in the VNO, and may have a role in the ability of the VNO to respond to chemosignals repetitively without a decrease in action potential firing frequency, a property that is not found in the main olfactory system (Liman and Corey, 1996). Although second messenger dialysis failed to elicit a chemosignal-like current, reversal potential analysis supports the existence of a conductance consistent with a nonspecific cation channel such as TRPC2, the expression of which was found in S. odoratus previously (Murphy et al, 2001). One very interesting result of these studies was that outward currents (as defined at -60·mV) may be due to the closure of an inwardly conducting ion channel.…”

“…In reptiles, it is larger than the main olfactory epithelium (MOE), the sensory organ for the MOS (Ernst et al, 1994;Murphy et al, 2001;Labra et al, 2005). In most animal classes, the VNO is thought to mediate primarily pheromonal signals, but not exclusively (Baxi et al, 2006).…”

“…Vogt et al, 2002), it is generally believed that the initial VNO transduction event is governed by the binding of chemosignals to G-protein coupled receptors (GPCRs), as well described for rodents (Dulac and Axel, 1995;Herrada and Dulac, 1997;Matsunami and Buck, 1997;Ryba and Tirindelli, 1997;Pantages and Dulac, 2000). Although the nucleotide sequence and expression patterns of these receptors have not yet been published for the reptilian VNO, we do know that in the common musk/stinkpot turtle Sternotherus odoratus they must be coupled to the downstream G-protein G ␣i , which has restricted expression in the microvilli of the turtle VSNs, is expressed at greater levels in male versus female turtle VNO, and is developmentally increased in the VNO coincident with reproductive maturity of the turtles (Murphy et al, 2001). More is known for the downstream signaling cascade found in this species, in which there appears to be continuity with that known for rodents.…”

“…This channel is a six-transmembrane domain nonselective cation channel and is a member of the TRP ion channel superfamily (Minke and Cook, 2002). TRPC2 is expressed in the microvilli of rats, mice and turtles (Liman et al, 1999;Murphy et al, 2001;Leypold et al, 2002;Stowers et al, 2002). In both rodents and turtles, TRPC2 shows a developmental pattern of expression, with an increased expression correlated to sexual maturity (Murphy et al, 2001).…”

“…TRPC2 is expressed in the microvilli of rats, mice and turtles (Liman et al, 1999;Murphy et al, 2001;Leypold et al, 2002;Stowers et al, 2002). In both rodents and turtles, TRPC2 shows a developmental pattern of expression, with an increased expression correlated to sexual maturity (Murphy et al, 2001). TRPC2 and the type III inositol 1,4,5-trisphospate receptor (IP 3 R3) demonstrate colocalization in VSN microvilli and can be coimmunoprecipitated from rodent VNO (Brann et al, 2002), but the functional relevance of this protein-protein interaction is yet unknown.…”