NMDA spikes in basal dendrites of cortical pyramidal neurons

Basal dendrites are a major target for synaptic inputs innervating cortical pyramidal neurons(1). At present little is known about signal processing i n these fine dendrites. Here we show that coactivation of clustered neighbo uring basal inputs initiated local dendritic spikes, which resulted in a 5. 9 +/- 1.5 mV (peak) and 64.4 +/- 19.8 ms (half-width) cable-filtered voltag e change at the soma that amplified the somatic voltage response by 226 +/- 46%. These spikes were accompanied by large calcium transients restricted to the activated dendritic segment. In contrast to conventional sodium or c alcium spikes, these spikes were mediated mostly by NMDA (N-methyl-D-aspart ate) receptor channels, which contributed at least 80% of the total charge. The ionic mechanism of these NMDA spikes may allow 'dynamic spike-initiati on zones', set by the spatial distribution of glutamate pre-bound to NMDA r eceptors, which in turn would depend on recent and ongoing activity in the cortical network. In addition, NMDA spikes may serve as a powerful mechanis m for modification of the cortical network by inducing long-term strengthen ing of co-activated neighbouring inputs.