Post-synaptic N-methyl-d-aspartate signalling in hippocampal neurons of rat: spillover increases the impact of each spike in a short burst discharge.

High-frequency burst discharges in hippocampus typically consist of less than ten spikes fired at frequencies too high to be followed by a post-synaptic neuron. How significant are these numbers for synaptic signalling? We have measured the N-methyl-d-aspartate (NMDA) component of the excitatory post-synaptic current (EPSC(NMDA)) in hippocampal CA1 neurons of rat after burst discharge of variable duration. The synaptic facilitation is accompanied by a slow-down of the EPSC(NMDA) which develops on a spike-to-spike basis. Consequently the charge transferred by the after-burst EPSC(NMDA) is increased with each spike. The phenomenon is most probably due to the spillover-mediated recruitment of extrasynaptic NMDA receptors. In terms of post-synaptic signalling it dramatically increases the impact of each spike in a short burst discharge.