NMDA Receptor-Mediated Na+Signals in Spines and Dendrites

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Abstract
Spines and dendrites of central neurons represent an important site of synaptic signaling and integration. Here we identify a new, synaptically mediated spine signal with unique properties. Using two-photon Na+imaging, we show that suprathreshold synaptic stimulation leads to transient increases in Na+concentration in postsynaptic spines and their adjacent dendrites. This local signal is restricted to a dendritic domain near the site of synaptic input. In presumed active spines within this domain, the Na+level increases by 30–40 mmeven during short bursts of synaptic stimulation. During a long-term potentiation induction protocol (100 Hz, 1 sec), the Na+level in the active spines reaches peak amplitudes of ∼100 mm. We find that the Na+transients are mainly mediated by Na+entry through NMDA receptor channels and are detected during the coincident occurrence of synaptic potentials and backpropagating action potentials. The large amplitudes of the Na+transients and their location on dendritic spines suggest that this signal is an important determinant of electrical and biochemical spine characteristics.