Dynamics of dendritic calcium transients evoked by quantal release at excitatory hippocampal synapses

Synaptic N-methyl-D-aspartate (NMDA) receptors detect coincident pre- and p ostsynaptic activity and play a critical role in triggering changes in syna ptic strength at central synapses, Despite intensive study of synaptic plas ticity, relatively little is known about the magnitude and duration of calc ium accumulation caused by unitary events at individual synapses. We used f luorescence imaging to detect NMDA receptor-mediated miniature synaptic cal cium transients (MSCTs) caused by spontaneous release of synaptic vesicles in dendrites of cultured hippocampal neurons. MSCTs originated focally in d endritic regions <1 Ccm in length and decayed with a time constant of 0.35 +/- 0.03 s, Multiple occurrences of MSCTs recorded at single sites had fluc tuating amplitudes, with a coefficient of variation of 0.34, From the reduc tion in the spatial spread of MSCTs with decreasing concentration of indica tor dye, we estimated that the dominant endogenous calcium buffer in dendri tes is relatively immobile (diffusion coefficient between 10 and 50 mu m(2) /s). We conclude that calcium rise caused by spontaneous quantal synaptic N MDA receptor activation (i) is variable, (ii) lasts for a time period brief er than previous measurements indicate, and (iii) is confined by endogenous calcium buffers to local dendritic regions even when synapses are not on s pines.