Signal Detection and Coding in the Accessory Olfactory System

Abstract

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Figure 1

Schematic overview of the mouse AOS. Shown is a sagittal view of a mouse head indicating the locations of the two major olfactory subsystems, including 1) main olfactory epithelium (MOE) and main olfactory bulb (MOB), as well as 2) the vomeronasal organ (VNO) and accessory olfactory bulb (AOB). Not shown are the septal organ and Grueneberg ganglion. The MOE lines the dorsolateral surface of the endoturbinates inside the nasal cavity. The VNO is built of two bilaterally symmetrical blind-ended tubes at the anterior base of the nasal septum, which are connected to the nasal cavity by the vomeronasal duct. Apical (red) and basal (green) VSNs project their axons to glomeruli located in the anterior (red) or posterior (green) aspect of the AOB, respectively. AOB output neurons (mitral cells) project to the vomeronasal amygdala (blue), from which connections exist to hypothalamic neuroendocrine centers (orange). The VNO resides inside a cartilaginous capsule that also encloses a large lateral blood vessel (BV), which acts as a pump to allow stimulus entry into the VNO lumen following vascular contractions (see main text). In the diagram of a coronal VNO section, the organizational dichotomy of the crescent-shaped sensory epithelium into an “apical” layer (AL) and a “basal” layer (BL) becomes apparent.

Figure 2

Diagram illustrating the current model of VSN primary signal transduction. Known vomeronasal chemoreceptors—formyl peptide receptor-like (FPR-rs) proteins, V1R, and V2R receptors—initiate G protein–coupled phospholipase C type β (PLCβ) signaling that results in phosphoinositide turnover and elevations in both inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). Notably, a given VSN only expresses one member of either receptor family and, accordingly, either G_αi2 or G_αo. DAG-mediated Ca²⁺ entry via transient receptor potential canonical type 2 (TRPC2) channels underlies initial depolarization as well as gating of a Ca²⁺-activated Cl⁻ channel (anoctamin1 [ANO1]). Bound to calmodulin (CaM), Ca²⁺ also triggers negative feedback inhibition of TRPC2.

Figure 3

General and VSN-specific (top left) members of the cellular Ca²⁺ signaling “toolkit.” Low cytoplasmic Ca²⁺ levels at rest (~100 nM) are maintained by 1) extrusion via active transport across either the plasma membrane (plasma membrane Ca²⁺ ATPase [PMCA]) or the endoplasmic reticulum (smooth endoplasmic reticular Ca²⁺ ATPase [SERCA]), 2) facilitated transport via the electrogenic Na⁺/Ca²⁺ exchanger (NCX) in the plasma membrane, and 3) mitochondrial uptake by the mitochondrial Ca²⁺ “uniporter” (mCU), a high affinity–low capacity ion channel. Both in the extracellular medium and inside storage organelles (ER and mitochondria), Ca²⁺ concentrations reach millimolar levels. The resulting steep gradient underlies the massive, but transient cytoplasmic Ca²⁺ increase upon opening of voltage- and/or ligand-gated ion channels, including voltage-activated Ca²⁺ (Ca_V) channels, transient receptor potential canonical type 2 (TRPC2) channels as well as endoplasmic reticulum IP₃ receptors (IP3R) and ryanodine receptors (RyR). Note that, in VSNs, TRPC2 and the Ca²⁺-activated Cl⁻ channel (anoctamin1 [ANO1]) are highly enriched in the plasma membrane of the microvillar compartment. By contrast, VSN storage organelles (endoplasmic reticulum and mitochondria) are likely restricted to other subcellular areas, creating functionally distinct Ca²⁺ signaling compartments. The precise location of the many diverse “toolkit” components in VSNs, however, is still missing.

Figure 4

Visualization of the intact mouse AOB. In cleared brains from adult mice (CLARITY technique [Chung and Deisseroth 2013; Chung et al. 2013]), AMCs are specifically labeled with the fluorescent protein tdTomato (offspring from crossing Tbet-Cre [Haddad et al. 2013] and Ai9 reporter mice [Madisen et al. 2010]). (A and B) 3D rendering in which fluorescent cells that reside inside the mitral cell layer (MCL) are shown in green, whereas the lateral olfactory tract (LOT) and putative mitral cells adjacent to the AOB are shown in red. Perspectives implement a sagittal lateral-to-medial view (A) as well as the view from deep in the granule cell layer (B). Scale bars indicate 150 µm. A total of 21 203 nuclei were identified within the MCL. Of these, 6842 nuclei were also tdTomato-positive. (C) Single confocal section through the AOB from six stitched z-stacks. Nuclei are stained using DRAQ5 (blue; C_i); putative AMCs and LOT fibers are shown in red (C_ii). GL = glomerular layer; GCL = granule cell layer.

Figure 5

Simplified circuit diagram of the AOB. VSN axon bundles comprise the vomeronasal nerve layer (VNL) and innervate relatively small, loosely defined glomeruli (dashed circles) in the glomerular layer (GL). AOB periglomerular cells (PGCs) are sparser than in the MOB. The large mitral cell layer (MCL) contains juxtaglomerular neurons (JGNs) in an apical subglomerular zone as well as widely distributed projection neurons (i.e., AOB mitral cells [AMCs]) that each innervate several glomeruli. In the MCL, some external granule cells (eGCs) are also found. The LOT, a complex fiber tract that pierces the AOB between its external and internal cellular layers, receives efferent axons from both main bulb projection neurons and AMCs. The internal cellular layer mainly harbors axonless GABAergic internal granule cells (iGCs) and is thus designated as the granule cell layer (GCL).