Spatio-temporal dynamics of lateral Na diffusion in apical dendrites of mouse CA1 pyramidal neurons.

Sodium ions (Na) are major charge carriers mediating neuronal excitation and play a fundamental role in brain physiology. Glutamatergic synaptic activity is accompanied by large transient Na increases, but the spatio-temporal dynamics of Na signals and properties of Na diffusion within dendrites are largely unknown. To address these questions, we employed multi-photon Na imaging combined with whole-cell patch-clamp in dendrites of CA1 pyramidal neurons in tissue slices from mice of both sexes. Fluorescence lifetime microscopy revealed a dendritic baseline Na concentration of ∼10 mM. Using intensity-based line-scan imaging we found that local, glutamate-evoked Na signals spread rapidly within dendrites, with peak amplitudes decreasing and latencies increasing with increasing distance from the site of stimulation. Spread of Na along dendrites was independent of dendrite diameter, order or overall spine density in the ranges measured. Our experiments also show that dendritic Na readily invades spines and suggest that spine necks may represent a partial diffusion barrier. Experimental data were well reproduced by mathematical simulations assuming normal diffusion with a diffusion coefficient of D+= 600 µm²/s. Modeling moreover revealed that lateral diffusion is key for the clearance of local Na increases at early time points, whereas when diffusional gradients are diminished, Na/K-ATPase becomes more relevant. Taken together, our study thus demonstrates that Na influx causes rapid lateral diffusion of Na within spiny dendrites. This results in an efficient redistribution and fast recovery from local Na transients which is mainly governed by concentration differences. Activity of excitatory glutamatergic synapses generates large Na transients in postsynaptic cells. Na influx is a main driver of energy consumption and modulates cellular properties by modulating Na-dependent transporters. Knowing the spatio-temporal dynamics of dendritic Na signals is thus critical for understanding neuronal function. To study propagation of Na signals within spiny dendrites, we performed fast Na imaging combined with mathematical simulations. Our data shows that normal diffusion, based on a diffusion coefficient of 600 µm²/s, is crucial for fast clearance of local Na transients in dendrites, whereas Na export by the Na/K-ATPase becomes more relevant at later time points. This fast diffusive spread of Na will reduce the local metabolic burden imposed by synaptic Na influx.