NMDA spikes in basal dendrites of cortical pyramidal neurons

Basal dendrites are a major target for synaptic inputs innervating cortical pyramidal neurons1. At present little is known about signal processing in these fine dendrites. Here we show that co−activation of clustered neighbouring basal inputs initiated local dendritic spikes, which resulted in a 5.9 plusminus 1.5 mV (peak) and 64.4 plusminus 19.8 ms (half−width) cable−filtered voltage change at the soma that amplified the somatic voltage response by 226 plusminus 46%. These spikes were accompanied by large calcium transients restricted to the activated dendritic segment. In contrast to conventional sodium or calcium spikes, these spikes were mediated mostly by NMDA (N −methyl−D−aspartate) receptor channels, which contributed at least 80% of the total charge. The ionic mechanism of these NMDA spikes may allow 'dynamic spike−initiation zones', set by the spatial distribution of glutamate pre−bound to NMDA receptors, which in turn would depend on recent and ongoing activity in the cortical network. In addition, NMDA spikes may serve as a powerful mechanism for modification of the cortical network by inducing long−term strengthening of co−activated neighbouring input