The individual functional properties and spatial arrangement of afferent synaptic inputs on dendrites have a critical role in the processing of information by neurons in the mammalian brain. Although recent work has identified visually-evoked local dendritic calcium signals in the rodent visual cortex, sensory-evoked signalling on the level of dendritic spines, corresponding to individual afferent excitatory synapses, remains unexplored. Here we used a new variant of high-resolution two-photon imaging to detect sensory-evoked calcium transients in single dendritic spines of mouse cortical neurons in vivo. Calcium signals evoked by sound stimulation required the activation of NMDA (N-methyl-D-aspartate) receptors. Active spines are widely distributed on basal and apical dendrites and pure-tone stimulation at different frequencies revealed both narrowly and widely tuned spines. Notably, spines tuned for different frequencies were highly interspersed on the same dendrites: even neighbouring spines were mostly tuned to different frequencies. Thus, our results demonstrate that NMDA-receptor-dependent single-spine synaptic inputs to the same dendrite are highly heterogeneous. Furthermore, our study opens the way for in vivo mapping of functionally defined afferent sensory inputs with single-synapse resolution.
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Acknowledgements We thank J. Lou for technical assistance, D. Bayer, F. Bayer and W. Zeitz for building the scanning device, A. Fohr for software support and Y. Kovalchuk and H. Adelsberger for help during the initial experiments. This work was supported by the Schiedel Foundation, the German-Israeli Foundation (GIF grant 1002/2008 to I.N. and A.K.), the Deutsche Forschungsgemeinschaft (IRTG 1373) and the Bundesministerium für Bildung und Forschung (BMBF) in the frame of ERA-NET NEURON.A.K.isa Carl-von-LindeseniorfellowoftheInstitutefor AdvancedStudy ofthe Technische Universität München.