Functional mechanisms underlying the intrabulbar associational circuit in the olfactory system

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 DESCRIPTION (provided by applicant): Perception relies on the acquisition and processing of sensory stimuli. In olfaction, sniffing presents repetitive odor samples to the olfactory sensory neurons (OSNs). Axons of OSNs expressing the same odorant receptor converge upon a pair of glomeruli located on opposite sides of each olfactory bulb (OB). These two "mirror glomeruli" are interconnected by the intra-bulbar association system (IAS) composed of superficial tufted (ST) cells in the external plexiform layer beneath each corresponding glomerulus. However, the physiological significance of this unique arrangement of sensory neuron projection and the IAS remains unknown. Electron microscopy studies showing synaptic contacts with granule cells lead to the prevailing idea that the IAS is primarily an inhibitory circuit. By contrast, our pilotdata show that IAS functions as a potent excitatory circuit acting at dual levels: (1) providing excitatory input to the OB output neurons mitral/tufted cells (MTCs) via dendrodendritic synapses between their apical dendrites in one mirror glomerulus; (2) providing direct excitatory input via their axons to MTCs in the IPL arising from the second mirror glomerulus on the opposite side of the OB. This suggests that MTCs receive dual feedforward excitation from IAS-STCs affiliating with the two separate mirror glomeruli in response to sensory input. We hypothesize that these two sets of feedforward excitation summate and produce an amplified response in MTCs since mirror glomeruli receive the same odorant-evoked sensory input in a relatively short time window. These novel findings lead to our central hypothesis that the IAS functions as an amplifier of sensory input to MTCs and modulates animal sensitivity to odors. IAS-STCs express glutamate and cholecystokinin (CCK), which provides a specific marker for IAS-STCs and enables us to exploit the powerful opto- and pharmacogenetic approaches to bidirectionally control the IAS and test our central hypothesis at cellular, circuit and behavioral levels. Our pilot data also show that while IAS-STC activation leads to glutamate action at the glomerular level, CCK is released only as input frequency increases. This finding combined with our unpublished observation of CCK action on only GABAergic glomerular interneurons leads to our secondary hypothesis is that the IAS differentially engages glomerular inhibition as input frequency increases. This frequency dependent enhancement of inhibition may counteract the dual excitatory role as input frequency (i.e. sniffing rate) increases. Three specific aims are for testing our novel hypotheses: Aim 1: Determine if the IAS regulates behavioral sensitivity to odors. Aim 2: Test the hypothesis that the IAS functions as a dual feedforward excitatory circuit to amplify sensory input to MTCs. Aim 3: Investigate whether the IAS generates frequency- dependent recruitment of glomerular inhibition. Achievement of these aims will shed crucial light on the physiological significance of the characteristic mirror glomeruli arrangement in the olfactory system and advance our understanding how the OB encodes sensory information conveyed to higher olfactory networks.