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Vomeronasal Sensory Neuron is a sensory neuron found in many mammals. Its main function is to detect pheromones by encoding behavioral information into action potentials directly to the accessory olfactory bulb [1]. Such sensory neurons allow species to gather information about their environment through pheromones.
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NS neuron
The vomeronasal system (Vomeronasal Sensory Neuron) initiates responses to pheromones, molecules involved in communication and controlling specific behaviors, such as aggression, mating, and the recognition of gender and social status [2]. Pheromones are also considered as semiochemicals that can cause changes in the neuroendocrine system [1].
Anatomy
Mammals have two independent chemosensory systems that are capable of transfering pheromonal information: the main olfactory system and the vomeronasal system [1]. The vomeronasal organ (VNO) of Jacobson is covered in a protective cartilaginous/bony capsule located above the palate in most mammals. Pheromones travel to the VNO, through a duct connecting it to the nasal or oral cavity. There they bind to receptors located on the microvilli on the dendritic knob of vomeronasal sensory neurons, bipolar cells which project its axons to the accessory olfactory bulb in the brain [3]. Then from the accessory olfactory bulb, it travels to the medial amygdala (MeA).The medial amygdala is connected to the nucleus of stria terminalis. Both of these areas projects their axons to the hypothalamus for regulation of neuroendocrine behaviors.[4]. Moreover, the vomeronasal organ can also detect other chemical compounds that are not pheromones with the help of main olfactory system [8].
Physiology
The Vomeronasal Organ sends neuronal signals to the accessory olfactory bulb and then to the amygdala and hypothalamus, which may explain how odors influence aggressive and mating behavior. The main functions of the hypothalamus functions are to monitor the body’s homeostasis and its neuroendocrine system to control the reproductive and behavior physiology [10].Scientists have shown that the characteristics of synapses between accessory olfactory bulb neurons can be influenced through synaptic contacts with vomeronasal neurons [11]. Vomeronasal sensory neurons have been divided into two classes based on their location in the sensory epithelium and type of receptors. Apical neurons V1R and the G protein Gi2, while basal neurons at the sensory epithelium and express V2R and G alpha o. The receptor cells are G-protein-coupled receptors have receptors that can detect pheromones. The receptor neurons have apical microvilli whose axons merge together to form VN nerves that move from the paired olfactory bulbs to the main olfactory bulb, entering the posterior dorsal aspect through the AOB. Researchers have shown two G-protein-coupled receptors found in different areas of VNO: V1 and V2. V1 and V2 are seven transmembrane receptors that are different from the main olfactory receptors [9]. Apical neurons can be stimulated by small urine-derived volatile molecules while basal neurons can be stimulated by nonvolatile peptide ligands of the MHC class I molecules [3]. Other odorants that are not pheromones can also activate vomeronasal sensory neurons; however, their physiological relevance is unclear. There are some evidences of a transient receptor potential channel called TRPC2, expressed in the microvilli. It is important for generating electrical responses to the pheromone stimulation [3,12,13].
Synaptic Connections
The signal transduction cascade, initiated by the binding of pheromones to vomeronasal receptors, produces an excitatory response. Indeed, it has been shown that natural stimuli cause membrane depolarization and increase the action potential firing rate in vomeronasal sensory neurons. Recent studies suggest that vomeronasal neurons respond highly selectively to purified pheromonal compounds. Using techniques such as Ca2+ imaging and cell-attached patch, researchers found that each of six putative mouse pheromones excited very small and non-overlapping vomeronasal neurons [5]. The threshold for these chemicals is very low, approximately 10−11 M. Surprisingly, there is no evidence suggesting that increasing its chemicals concentration help recruit more neurons [5]. Also, additional studies have suggested that there are high levels of selectivity [6]. Such evidences indicate that pheromones are encoded by labeled lines at the level of the vomeronasal organ: each pheromonal compound is represented by the activation of subset receptor neurons.From there, it is transferred to the accessory olfactory bulb through convergent connections [7].
References
1. Halpern M (1987).The organization and function of the vomeronasal system, Annu Rev Neurosci 10 (1987), pp. 325–362.
2. Del Punta K, Leinders-Zufall T, Rodriguez I, Jukam D, Wysocki CJ, Ogawa S, Zufall F, Mombaerts P. (2002) Deficient pheromone responses in mice lacking a cluster of vomeronasal receptor genes. Nature 419:70–74.
3. Shimazaki R, Boccaccio A, Mazzatenta A, Pinato G, Migliore M, Menini A (2006) Electrophysiological Properties and Modeling of Murine Vomeronasal Sensory Neurons in Acute Slice Preparations.PMID16547196
4. Coolen L. and Wood R (1998). Bidirectional connections of the medial amygdaloid nucleus in the Syrian hamster brain: simultaneous anterograde and retrograde tract tracing, J Comp Neurol 399 (1998), pp. 189–209.
5. Leinders-Zufall T, A.P. Lane, A.C. Puche, W. Ma, M.V. Novotny, M.T. Shipley and F. Zufall, Ultrasensitive pheromone detection by mammalian vomeronasal neurons, Nature 405 (2000), pp. 792–796.
6. C. Boschat, C. Pélofi, O. Randin, D. Roppolo, C. Lüscher, M. Broillet and I. Rodriguez, Pheromone detection mediated by a V1r vomeronasal receptor, Nat Neurosci 5 (2002), pp. 1261–1262.
7. Luo M & Katz R (2004). Encoding pheromonal signals in the mammalian vomeronasal system.Current Opinion in Neurobiology [0959-4388] Luo yr:2004 vol:14 iss:4 pg:428 -434.
8. Keller M, Baum MJ, Brock O, Brennan PA, Bakker J. The main and the accessory olfactory systems interact in the control of mate recognition and sexual behavior. Behavioural Brain Research. 2009 Jun 25;200(2):268-76. PMID 19374011
9. Tirindelli R, Dibattista M, Pifferi S, Menini A. From pheromones to behavior. Physiology Reviews. 2009 Jul;89(3):921-56. PMID 19584317
10. Vomeronasal organ. (2009, August 22). In Wikipedia, The Free Encyclopedia. Retrieved August 22, 2009, from http://en.wikipedia.org/w/index.php?title=Vomeronasal_organ&oldid=309371011
11. Moriya-Ito K, Endoh K, Ichikawa M (2008). Vomeronasal neurons promote synaptic formation on dendritic spines but not dendritic shafts in primary culture of accessory olfactory bulb neurons. Neuroscience Letters. Volume 451, Issue 1, 13 February 2009, Pages 20-24. 11
12. Liman ER, Corey DP, Dulac C. (1999) TRP2: a candidate transduction channel for mammalian pheromone sensory signaling. Proc. Natl Acad. Sci. USA 96:5791–5796.TRP2
13. Menco BP, Carr VM, Ezeh PI, Liman ER, Yankova MP. (2001) Ultrastructural localization of G-proteins and the channel protein TRP2 to microvilli of rat vomeronasal receptor cells. J. Comp. Neurol. 438:468–489.Ultrastructural Localization TRP2
Additional information
Structure and function of the vomeronasal system: an update.
Pheromone detection mediated by a V1r vomeronasal receptor
Abstract:Sensory transduction in vomeronasal neurons: evidence for G alpha o, G alpha i2, and adenylyl cyclase II as major components of a pheromone signaling cascade in the air? New insights into mammalian pheromones
