Olfaction and Pheromones: Uncanonical Sensory Influences and Bulbar Interactions

Abstract

The rodent main and accessory olfactory systems (AOS) are considered functionally and anatomically segregated information-processing pathways. Each system is devoted to the detection of volatile odorants and pheromones, respectively. However, a growing number of evidences supports a cooperative interaction between them. For instance, at least four non-canonical receptor families (i.e., different from olfactory and vomeronasal receptor families) have been recently discovered. These atypical receptor families are expressed in the sensory organs of the nasal cavity and furnish parallel processing-pathways that detect specific stimuli and mediate specific behaviors as well. Aside from the receptor and functional diversity of these sensory modalities, they converge into a poorly understood bulbar area at the intersection of the main- main olfactory bulb (MOB) and accessory olfactory bulb (AOB) that has been termed olfactory limbus (OL). Given the intimate association the OL with specialized glomeruli (i.e., necklace and modified glomeruli) receiving uncanonical sensory afferences and its interactions with the MOB and AOB, the possibility that OL is a site of non-olfactory and atypical vomeronasal sensory decoding is discussed.

Keywords: olfaction; olfactory limbus; pheromone; sensory convergence.

Figure 1

(A) Diagramatic representation of putative bulbar and sensory inputs to the olfactory limbus (OL) and medial nucleus of the amygdala (MeA). AON, anterior olfactory nuclei; AOB, accessory olfactory bulb; dMOB, dorsal part of the main olfactory bulb; MOB, main olfactory bulb. Arrows designate putative afferences to the OL. (B) Diagram of the OL (pink-colored) with the main (blue) and accessory (orange) olfactory bulbs. Insert at the bottom left. Light micrograph of a biocytin-injected large principal cell whose apical dendrites diverge to resolve, in the anterior accessory olfactory bulb (aAOB; orange) and necklace (deep purple) glomeruli. (C) Slice recordings of the large principal cell seen in “B”, insert. To note is the numerous spikes grouped into episodic bursts in a similar fashion to that observed by pacemaker neurons. Adult rat olfactory bulb.

Figure 2

Position, interactions and putative connectivity of the olfactory limbus (OL, pale green). The OL lies between the caudal region of the main olfactory bulb (dMOB; pale blue) and the anterior part of the accessory olfactory bulb (aAOB; pink-colored). To note is that the necklace glomerulus (NG, turquois) receives dendrites from both a modified mitral cell in the dMOB and a tufted cell in the OL. A modified glomerulus (MG, dark green) receives dendrites from large principal cells (red) in the aAOB. Forked arrows designate axon collaterals of a large principal- (deep red) and a mitral-cell (black) that possibly (?) synapse with mitral- and granule-cells in the dMOB and with granule cells in the aAOB, respectively.