Vomeronasal sensory neurons

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FIGURE 50-6 A model for chemosensory transduction in vomeronasal sensory neurons. The individual steps are detailed in the text. In contrast to the transduction cascade in OSNs, the mechanism of vomeronasal transduction is less well characterized. Vomeronasal neurons express either V1R or V2R receptors and either Got. or Ga0, respectively. The TRPC2 channel subunit is expressed in all vomeronasal neurons, and may be part of a multimeric channel complex. Ca2+ions are represented as purple balls, Na+ ions as blue balls. VR, vomeronasal receptor (VlRorV2R) PIP2, phospha-tidylinositol 4,5-bishphosphate IP, inositol 1,4,5-trisphosphate DAG, diacylglycerol. 

Labra, A., Brann, J. H. and Fadool, D. A. (2005) Heterogeneity of voltage- and chemosignal-activated response profiles in vomeronasal sensory neurons. J. Neurophysiol. 94, 2535-2548. 

Peptides that serve as ligands for major histocompatibility complex (MHC) class I molecules can activate vomeronasal sensory neurons. These peptides contain nine amino acid residues and activate sensory neurons from the V2 receptor... 

Kimoto H, Haga S, Sato K, Touhara K (2005) Sex-specific peptides from exocrine glands stimulate mouse vomeronasal sensory neurons. Nature 437 898-901 Kinnamon SC, Margolskee RF (1996) Mechanisms of taste transduction. Curr Opin Neurobiol 6 506-513... 

Ishii T, Hirota J, Mombaerts P (2003) Combinatorial coexpression of neural and immune multigene families in mouse vomeronasal sensory neurons. Curr Biol 13 394 100 Jia C, Halpern M (1996) Subclasses of vomeronasal receptor neurons differential expression of G proteins (G.a2 and Goa) and segregated projections to the accessory olfactory bulb. Brain Res 719 117-128... 

Kimoto H, Haga S, Sato K, Touhara K (2005) Sex-specific peptides from exocrine glands stimulate mouse vomeronasal sensory neurons. Nature 437 898-901 Kimoto H, Sato K, Nodari F, Haga S, Holy TE, Touhara K (2007) Sex- and strain-specific expression and vomeronasal activity of mouse ESP family peptides. Curr Biol 17 1879-1884 Kobayakawa K, Kobayakawa R, Matsumoto H, Oka Y, Imai T, Ikawa M, Okabe M, Ikeda T, Itohara S, Kikusui T, Mori K, Sakano H (2007) Innate versus learned odour processing in the mouse olfactory bulb. Nature 450 503-508... 

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Martini S., Silvotti L., Shirazi A., Ryba N.J. and Tirindelli R. (2001). Co-expression of putative pheromone receptors in the sensory neurons of the vomeronasal organ. J Neurosci 21, 843-848. 

Rodriguez I., Feinstein P. and Mombaerts P. (1999). Variable patterns of axonal projections of sensory neurons in the mouse vomeronasal system. Cell 97, 199-208. 

Trotier D. and D0ving K. (1996). Inward rectifiying current is activated by hyperpolarisation in Frog vomeronasal receptor cells. Prim Sensory Neuron 1, 245-261. 

By contrast, the accessory olfactory system is thought to be involved in the detection of odors that influence a variety of reproductive and aggressive behaviors (Keverne 1999). Sensory neurons are located in the vomeronasal organ (VNO) and detect pheromones which gain access to the VNO by a pumping mechanism (Meredith and O Connell, 1979). VNO neurons send projections to the accessory olfactory bulb (AOB). Mitral cells of the AOB project in turn to the medial nucleus of the amygdala olfactory information is then dispatched to several hypothalamic regions such as the bed nucleus of the stria terminalis, the medial preoptic area and the ventromedial hypothalamus (Scalia and Winans 1975). 

As with many macrosmatic mammals, rodents have two separate chemosensory systems, the main olfactory system (MOS) and accessory olfactory system (AOS), which respond to social odors. Importantly, these sensory systems differ not only in their peripheral morphology and central projections, but also in the types of chemosignals that they process (Meredith 1991). Sensory neurons of the MOS, which are located in the main olfactory epithelium and project to the main olfactory bulbs, process volatile chemicals and can detect odors at a distance. In contrast, sensory neurons of the AOS, which are located in the vomeronasal organs (VNO) and project to the accessory olfactory bulbs, primarily process large, non-volatile chemicals and require contact for stimulation (Meredith 1991). 

In vertebrates, most of the olfactory neurons of the nasal epithelium protrude from the apical dendritic knob a variety of cilia into the protective mucus layer, thus enlarging the sensory surface area of the cells. A subpopulation of sensory neurons in the main olfactory epithelium and all of the neurons in the vomeronasal organ (VNO) are structurally different their apical region of the dendrite extends in an array of microvilli. 

Second messenger cascades in vomeronasal sensory cells Although the functional implications of the PLC/IP3 cascade in olfactory neurons of nasal epithelium remains mysterious, there is mounting evidence that phospholipase C is a key enzyme for signal transduction in chemosensory neurons... 

Trinh K, Storm DR (2003) Vomeronasal organ detects odorants in absence of signaling through main olfactory epithelium. Nat Neurosci 6(5) 519—525 Tsuboi A, Miyazaki T, Imai T, Sakano H (2006) Olfactory sensory neurons expressing class I odorant receptors converge their axons on an antero-dorsal domain of the olfactory bulb in the mouse. Eur J Neurosci 23(6) 1436-1444... 

Fig. 3 Vomeronasal system. Schematic representation of a rodent nasal cavity and brain (lateral view). Accessory olfactory bulb (AOB) mitral cells project to vomeronasal and extended amygdala. Inset The VNO is a bilateral tubular structure located at the base of the nasal septum. VSNs that express the same V1R or V2R converge on a small number of glomeruli in the AOB. Sensory neurons located in the apical layer of the epithelium project to the anterior part of the AOB, whereas those present in the basal layer project to the posterior part. MOE main olfactory epithelium, MOB main olfactory bulb, BSTMPM posteromedial bed nucleus of the stria terminalis, MEA medial amygdaloid nucleus, BACfF bed nucleus of the accessory olfactory tract, PMCO posteromedial cortical amygdaloid area...

Rodriguez I, Feinstein P, Mombaerts P (1999) Variable patterns of axonal projections of sensory neurons in the mouse vomeronasal system. Cell 97 199-208 Rodriguez I, Greer CA, Mok MY, Mombaerts P (2000) A putative pheromone receptor gene expressed in human olfactory mucosa. Nat Genet 26 18-19 Rodriguez I, Del Punta K, Rothman A, Ishii T, Mombaerts P (2002) Multiple new and isolated families within the mouse superfamily of Vlr vomeronasal receptors. Nat Neurosci 5 134-140... 

In most mammalian species, two functionally distinct classes of chemicals (odorants and pheromones) are detected and processed through anatomically segregated neural pathways the main olfactory system and the vomeronasal (accessory olfactory) system (Buck 2000 Mombaerts 2004). Volatile odorants are received by a large repertoire of ORs expressed on ciliated OSNs in the OE and the information is transmitted to the main OB. By contrast, pheromones are mostly received by two families of vomeronasal receptors (VIRs and V2Rs) expressed on microvillous sensory neurons in the vomeronasal organ that project their axons to the accessory OB. 

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