CCKergic Tufted Cells Differentially Drive Two Anatomically Segregated Inhibitory Circuits in the Mouse Olfactory Bulb

Abstract

Delineation of functional synaptic connections is fundamental to understanding sensory processing. Olfactory signals are synaptically processed initially in the olfactory bulb (OB) where neural circuits are formed among inhibitory interneurons and the output neurons mitral cells (MCs) and tufted cells (TCs). TCs function in parallel with but differently from MCs and are further classified into multiple subpopulations based on their anatomic and functional heterogeneities. Here, we combined optogenetics with electrophysiology to characterize the synaptic transmission from a subpopulation of TCs, which exclusively express the neuropeptide cholecystokinin (CCK), to two groups of spatially segregated GABAergic interneurons, granule cells (GCs) and glomerular interneurons in mice of both sexes with four major findings. First, CCKergic TCs receive direct input from the olfactory sensory neurons (OSNs). This monosynaptic transmission exhibits high fidelity in response to repetitive OSN input. Second, CCKergic TCs drive GCs through two functionally distinct types of monosynaptic connections: (1) dendrodendritic synapses onto GC distal dendrites via their lateral dendrites in the superficial external plexiform layer (EPL); (2) axodendritic synapses onto GC proximal dendrites via their axon collaterals or terminals in the internal plexiform layer (IPL) on both sides of each bulb. Third, CCKergic TCs monosynaptically excite two subpopulations of inhibitory glomerular interneurons via dendrodendritic synapses. Finally, sniff-like patterned activation of CCKergic TCs induces robust frequency-dependent depression of the dendrodendritic synapses but facilitation of the axodendritic synapses. These results demonstrated important roles of the CCKergic TCs in olfactory processing by orchestrating OB inhibitory activities.

Dynamics of Glutamatergic Drive Underlie Diverse Responses of Olfactory Bulb OutputsIn Vivo

“…Mitral cells and sTCs have distinct odorant response properties ( Fukunaga et al, 2012 ; Igarashi et al, 2012 ). Many sTCs express the peptide neurotransmitter cholecystokinin (CCK; Seroogy et al, 1985 ), these neurons are strongly driven by monosynaptic input from OSNs ( Sun et al, 2020 ), and we have previously shown that they have faster-onset and less diverse odorant-evoked responses than the general MT cell population ( Economo et al, 2016 ; Short and Wachowiak, 2019 ; Eiting and Wachowiak, 2020 ). We compared glutamate dynamics onto CCK + sTCs and piriform cortex-projecting MT cells (pcMTs), using Flex.AAV9.iGluSnFR.A184S virus injection into the superficial EPL of CCK-IRES-Cre mice or retrograde viral expression via injection into anterior piriform cortex of Tbet-Cre mice ( Fig.…”

Diverse dynamics of glutamatergic input from sensory neurons underlie heterogeneous responses of olfactory bulb outputs in vivo

“…Mitral cells and superficial tufted cells (sTCs) have distinct odorant response properties 26,27 . Many sTCs express the peptide neurotransmitter cholecystokinin (CCK) 28 ; these neurons are strongly driven by monosynaptic input from OSNs 29 , and we have previously shown that they have faster-onset and less diverse odorant-evoked responses than the general MT cell population 6,24,30 . We compared glutamate dynamics onto CCK+ sTCs and piriform cortex-projecting MT cells (pcMTs), using Flex.AAV9.iGluSnFR.A184S virus injection into the superficial EPL of CCK-IRES-Cre mice or retrograde viral expression via injection into anterior piriform cortex of Tbet-Cre mice (Fig.…”

“…Sensory-evoked glutamatergic input onto MT cells can arise from OSNs or from multisynaptic excitation involving dendritic glutamate release from ET cells and, possibly, sTCs 16,29,41-43 . In addition, inhibitory circuits can modulate multisynaptic glutamate signaling 11,12,17 .…”

“…To confirm postsynaptic blockade, we tested the ability of APV+NBQX to block activation of CCK-expressing sTCs, as these, like ET cells, are strongly driven by monosynaptic OSN input 29 . We imaged activation of CCK+ OB neurons in CCK-IRES-Cre mice crossed to a Cre-dependent GCaMP6f reporter line.…”

Molecular characterization of projection neuron subtypes in the mouse olfactory bulb

“…For OB projection neurons, however, only little information about chemotypes is available at this point: Pcdh1 and Tbx21 (Haddad et al, 2013; Nagai et al, 2005) have been shown to be selectively expressed in a subset of OB projection neurons. CCK distinguishes a subset of TCs (superficial TCs, (Liu and Shipley, 1994; Seroogy et al, 1985; Short and Wachowiak, 2019; Sun et al, 2020). Vasopressin expressing cells might constitute a further subset of superficial TCs (Lukas et al, 2019), and recently the Lbhd2 gene has been used to obtain more specific genetic access to MCs (Koldaeva et al, 2020).…”