Pliable synaptic fidelity and excitatory inter-synaptic crosstalk in the intact brain

Memory formation in neural circuits may involve changes in synaptic efficacy and in cell intrinsic excitability, yet how this process unfolds in the living brain has remained elusive. Here, we employed multiplexed imaging of genetically encoded indicators of glutamate and Ca2+ in mouse barrel cortex to detect increased fidelity coupled with reduced excitation of thalamocortical connections that undergo whisker-stimulation induced LTP. High-resolution imaging revealed that whisker stimuli trigger excitatory synaptic activity that generates extrasynaptic glutamate transients reaching the bulk of neighbouring synapses in the target cortical area. Our findings pave the way to understanding basic plasticity features of the synaptic connectome while revealing a significant component of volume-transmitted glutamatergic signalling among cells in the intact brain. One Sentence SummarySensory-stimulation LTP increases fidelity while reducing excitation at individual thalamocortical connections which generate spatially intersecting glutamate discharges