Spines and dendrites of central neurons represent an important site of syna
ptic signaling and integration. Here we identify a new, synaptically mediat
ed spine signal with unique properties. Using two-photon Na+ imaging, we sh
ow that suprathreshold synaptic stimulation leads to transient increases in
Na+ concentration in postsynaptic spines and their adjacent dendrites. Thi
s local signal is restricted to a dendritic domain near the site of synapti
c input. In presumed active spines within this domain, the Na+ level increa
ses by 30-40 mM even 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 similar to 100 mM. We find th
at the Na+ transients are mainly mediated by Na+ entry through NMDA recepto
r channels and are detected during the coincident occurrence of synaptic po
tentials and backpropagating action potentials. The large amplitudes of the
Na+ transients and their location on dendritic spines suggest that this si
gnal is an important determinant of electrical and biochemical spine charac
teristics.