Paradoxically sparse chemosensory tuning in broadly-integrating external granule cells in the mouse accessory olfactory bulb

Abstract

The accessory olfactory bulb (AOB) is a critical circuit in the mouse accessory olfactory system (AOS), but AOB processing is poorly understood compared to the main olfactory bulb (MOB). We used 2-photon GCaMP6f Ca 2+ imaging in an ex vivo preparation to study the chemosensory tuning of AOB external granule cells (EGCs), an interneuron population hypothesized to broadly integrate from mitral cells (MCs). We measured MC and EGC tuning to natural chemosignal blends and monomolecular ligands, finding that EGC tuning was far sparser than MC tuning. Simultaneous patch-clamp electrophysiology and Ca 2+ imaging indicated that this was only partially explained by lower GCaMP6f-to-spiking ratios in EGCs compared to MCs. Ex vivo patch-clamp recordings revealed that EGC subthreshold responsivity was broad, but monomolecular ligand responses were insufficient to elicit spiking. These results indicate that EGC spiking is selectively engaged by chemosensory blends, suggesting different roles for EGCs than analogous interneurons in the MOB. 2005; Kaba and Huang, 2005; Araneda and Firestein, 2006; Hendrickson et al., 2008; Cansler et al., 2017; Gao et al., 2017). Cumulatively, these observations suggest that MC-interneuron communication in the AOB is critical for rodent physiology and behavior. Despite recent progress, many gaps in our understanding of AOB MC-interneuron interactions remain. A major limitation for our understanding of AOB function is a lack of information about how MCs interact with several inhibitory neural types. The AOB has a variety of interneurons that have been largely classified based on their location and morphology (Larriva-Sahd, 2008; Moriya-Ito et al., 2013). Three major AOB interneuron types have been identified, along with several minor types. Juxtaglomerular cells (JGCs) are found within the superficial glomerular layer, where excitatory sensory inputs from the vomeronasal organ (VNO) enter the AOB. External granule cells (EGC) are found in the external cell layer alongside the somas of MCs. Internal granule cells (IGCs), the most abundant and well-studied AOB interneuron type, are located in the internal granule layer. JGCs, similar to counterparts in the main olfactory bulb (MOB), are thought to modulate the input to the MCs via their synaptic connection with VNO axon terminals and MC apical dendrites within glomeruli (Geramita and Urban, 2017). EGCs and IGCs, in contrast, are thought to modulate MC activity via reciprocal dendrodendritic connections (Taniguchi and Kaba, 2001; Castro et al., 2007). Relative to JGCs and IGCs, there is very little information about the physiology or function of AOB EGCs; just one targeted study of their intrinsic features has been reported (Maksimova et al., 2019).Cells that appear analogous to AOB EGCs have been studied in the MOB. Specifically, MOB parvalbumin-expressing interneurons in the external plexiform layer (PV-EPL interneurons) resemble EGCs in their morphologies and apparent broad connectivity with MCs. These MOB PV-EPL interneurons were shown to integr...