Vomeronasal Receptors and Signal Transduction in the Vomeronasal Organ of Mammals

Excerpt

In most species, there are two chemosensory systems, both located in the nasal cavity but physiologically and anatomically distinct (Figure 10.1). The main olfactory epithelium (MOE) is principally involved in the airborne odor perception, whereas the vomeronasal organ (VNO) of Jacobson in the detection of pheromones that are chemical compounds secreted or excreted by individuals of the same species (conspecifics) (Dulac & Torello 2003; Tirindelli et al. 2009). Deficits in social and reproductive behavior are marked in animals underlying the surgical removal of the VNO but also the chemical inactivation of the MOE (Kiyokawa et al. 2007; Kolunie & Stern 1995; Leypold et al. 2002; Wysocki & Lepri 1991). Furthermore, there is evidence that two olfactory structures, namely the septal organ of Masera and the Grueneberg ganglion also contribute to pheromone detection (Ma 2007; Roppolo et al. 2006; Tirindelli et al. 2009) (Figure 10.1).

Primary sensory VNO neurons (VSNs) send axons to mitral cells in the glomerular region of the accessory olfactory bulb (AOB), whereas olfactory neurons (OSNs) to the mitral cells of the main olfactory bulb (MOB) (Baum & Kelliher 2009; Clapham & Neer 1997) (Figure 10.1). The chemosensory neurons of the VNO and MOE are equipped with specific receptors that activate distinct molecular cascades and mediate sociosexual behaviors (Del Punta et al. 2002; Stowers et al. 2002).

The anatomical components of the VNO firstly developed in a tetrapod ancestor and led to the appearance of a rudimentary structure in amphibians that became highly organized in Squamata and in many mammalian orders as Didelphimorphia, Rodentia, and in primates (prosimians and New World monkeys). In contrast, VNO is virtually absent in birds, bats, Old World monkeys, apes, and humans (Dennis et al. 2004; Grus et al. 2005; Shi & Zhang 2007; Smith et al. 2005; Zhao et al. 2011) (Figure 10.2a).

In Didelphimorphia, Lagomorpha, and Rodentia, the VNO chemosensory epithelium presents two separate neuronal layers (apical and basal) characterized by the expression of different G protein subunits and receptors (Young & Trask 2007). Apical and basal neurons project to two distinct regions (anterior and posterior) of the AOB. From the AOB, the majority of the centripetal projections of the apical and basal neurons remain segregated in the amygdala and hypothalamus (Martinez-Marcos 2009; Yoon et al. 2005).

In other mammalian orders (Carnivora) the dichotomic organization of the vomeronasal projections is not well defined or even absent (Artiodactyla) (Dennis et al. 2003).