PATTERN AND INHIBITION-DEPENDENT INVASION OF PYRAMIDAL CELL DENDRITESBY FAST SPIKES IN THE HIPPOCAMPUS IN-VIVO

The invasion of sodium spikes from the soma into dendrites was studied in hippocampal pyramidal cells by simultaneous extracellular and intr acellular recordings in anesthetized rats and by simultaneous extracel lular recordings of the somatic and dendritic potentials in freely beh aving animals. During complex-spike patterns, recorded in the immobile or sleeping animal, dendritic invasion of successive spikes was subst antially attenuated. Complex spike bursts occurred in association with population discharge of CA3-CA1 pyramidal cells (sharp wave field eve nts). Synaptic inhibition reduced the amplitude of sodium spikes in th e dendrites and prevented the occurrence of calcium spikes. These find ings indicate that (i) the voltage-dependent calcium influx into the d endrites is under the control of inhibitory neurons and (ii) the tempo ral coincidence of synaptic depolarization and activation of voltage-d ependent calcium conductances by the backpropagating spikes during sha rp wave bursts may be critical for synaptic plasticity in the intact h ippocampus.