How does plasticity in target interneurons influence functional recovery during naturally occurring neuronal regeneration?

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In the mammalian olfactory system, olfactory sensory neurons (OSNs) in the nose send axons to their central target, the olfactory bulb (OB). There, initial stages of sensory processing in modular units called glomeruli control the gain of information transmission to the OB's output neurons, mitral/tufted cells (M/TCs). After the wholescale death of OSNs due to injury, infection, or toxic damage, the entire OSN population can be restored, and their axons can grow into the brain in order to re-establish olfactory function. Using a reliable, reproducible, and selective means of inducing such de- and regeneration with a single dose of the olfactotoxin methimazole (MMZ), we have identified key milestones in this anatomical and functional recoveryHowever, in this well-described, entirely naturally occurring mammalian model of successful neuronal regeneration, we currently know almost nothing about the role played by plasticity in target circuits. The OB is an extremely plastic part of the brain throughout life, and our recent work has identified OB cell-type-specific plasticity occurring after even brief periods of sensory deprivation. This involves a specific subset of inhibitory interneurons in the OB's glomerular layer, which release both dopamine and GABA to influence the gain of information transmission between OSN inputs and M/TC outputs. These dopaminergic (DA) cells can receive direct input from OSNs, and undertake two main inhibitory functions in glomerular networks: feedback inhibition of OSN release, and feedforward inhibition of glomerular neurons. They are also heterogeneous, with our previous work identifying two major subtypes: 1) a predominant (>98%) small-soma type, which entirely lacks an axon, ramifies locally, and can be generated via both embryonic and adult neurogenesis; and 2) a rarer, large-soma type, which is axon-bearing and broadly ramifying, and is exclusively generated in early embryonic developmentBoth types undergo experience-dependent plastic changes: we and others have described experience-dependent alterations in their structure, function, and gene expression. This includes our preliminary findings that levels of tyrosine hydroxylase (TH) - the rate-limiting enzyme for dopamine synthesis - are decreased in OB DA neurons during MMZ-induced OSN regeneration, potentially reducing these cells' inhibitory influence on the nose-to-brain flow of olfactory information. We therefore postulate that plasticity in these neurons, especially within the predominant anaxonic subtype whose small arbours limit their sphere of influence very locally, influences intra-glomerular circuit function during OSN re-connection. Specifically, we hypothesise that plastic mechanisms that reduce the overall inhibitory action of OB DA neurons on local glomerular networks produce an increase in those networks' input output gain, in order to facilitate the downstream use of initially weak regenerating input.Does plasticity in target circuits help or hinder functional recovery? Do re-grown axons encounter a network which is adapted to and can appropriately process the new information they provide? Or do plastic central mechanisms produce aberrant circuit activity which turns newly re-connected inputs into dysfunctional output? We will ask precisely these questions here, taking advantage of the unique natural regenerative capacity of the olfactory system, as well as the renowned plastic capabilities of OB DA neurons. We will study and manipulate these cells both ex vivo and in vivo, to test our primary hypothesis: 'That plasticity in target interneurons contributes to functional recovery during naturally occurring neuronal regeneration'.