We wish to thank Mark Evans and Rosie Sammons for help with 3D tracing in Fiji, Annisa Chand for instructions on nose plug manufacture, and Maxim Volgushev for Matlab code. Venki Murthy and all members of the Grubb, Murthy and Galliano laboratories provided helpful discussions, while Juan Burrone and Sue Jones made invaluable comments on the manuscript.

Can alterations in experience trigger different plastic modifications in neuronal structure and function, and if so, how do they integrate at the cellular level? To address this question, we interrogated circuitry in the mouse olfactory bulb responsible for the earliest steps in odour processing. We induced experience-dependent plasticity in mice by blocking one nostril for a day, a minimally-invasive manipulation which leaves the sensory organ undamaged and is akin to the natural transient blockage suffered during common mild rhinal infections. We found that such brief sensory deprivation produced structural and functional plasticity in one highly specialised bulbar cell type: axon-bearing dopaminergic neurons in the glomerular layer. After 24h naris occlusion, the axon initial segment (AIS) in bulbar dopaminergic neurons became significantly shorter, a structural modification that was also associated with a decrease in intrinsic excitability. These effects were specific to the AIS-positive dopaminergic subpopulation, because no experience-dependent alterations in intrinsic excitability were observed in AIS-negative dopaminergic cells. Moreover, 24h naris occlusion produced no structural changes at the AIS of bulbar excitatory neuronsmitral/tufted and external tufted cells -nor did it alter their intrinsic excitability. By targeting excitability in one specialised dopaminergic subpopulation, experience-dependent plasticity in early olfactory networks might act to fine-tune sensory processing in the face of continually fluctuating inputs.