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

Abstract

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_Na+= 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.Significance statement 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.