Here we demonstrate that cerebellar stellate cells diffusionally isolate synaptically evoked signals in dendrites and are capable of input-specific synaptic plasticity. Sustained activity of parallel fibers induces a form of long-term depression that requires opening of calcium (Ca2+)-permeable AMPA-type glutamate receptors (CP-AMPARs) and signaling through class 1 metabotropic glutamate receptors (mGluR1) and CB1 receptors. This depression is induced by postsynaptic increases in Ca2+ concentration ([Ca2+]) and is limited to activated synapses. To understand how synapse-specific plasticity is induced by diffusible second messengers in aspiny dendrites, we examined diffusion of Ca2+ and small molecules within stellate cell dendrites. Activation of a single parallel fiber opened CP-AMPARs, generating long-lived Ca2+ transients that were confined to submicron dendritic stretches. The diffusion of Ca2+ was severely retarded due to interactions with parvalbumin and a general restriction of small molecule mobility. Thus stellate cell dendrites spatially restrict signaling cascades that lead from CP-AMPAR activation to endocannabinoid production and trigger the selective regulation of active synapses.
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