A current-source density analysis of the long-term potentiation in the hippocampus

Tetanic stimulation of presynaptic fibres induces long-term potentiation (LTP) which means enhancement of synaptic efficacy in the stimulated pathway for hours or days. In addition to that component, a permanent change occurs in the postsynaptic cells promoting their discharges. This latter effect called 'EPSP-to-spike (E-S) potentiation' is thought to be mediated by voltage-sensitive channels in the dendrites. Current-source density (CSD) analysis was made in the CA1 area of hippocampal slice preparations to find if LTP causes changes of the transmembrane currents in the stratum radiatum which can be detected with this technique. Some increase of currents associated with synaptic transmission itself at distant dendritic areas was accompanied by a disproportional enhancement of other currents attributed to activation of dendritic membranes at approximately 150 microns away from the pyramidal layer. When this current grew sufficiently large, it propagated towards the cell body layer. In slices where LTP had less E-S potentiation component, the increase in CSD at distant and more proximal portions of the dendrites remained proportional. Paired pulse facilitation induced in the same slices did not produce disproportional enhancement of proximal dendritic currents either. Our findings support the assumptions that during LTP associated with E-S potentiation the probability of activation of voltage-sensitive channel is enhanced on the dendrites of CA1 pyramidal cells.