A CURRENT-SOURCE DENSITY ANALYSIS OF THE LONG-TERM POTENTIATION IN THE HIPPOCAMPUS

Tetanic stimulation of presynaptic fibres induces long-term potentiati on (LTP) which means enhancement of synaptic efficacy in the stimulate d pathway for hours or days. In addition to that component, a permanen t change occurs in the postsynaptic cells promoting their discharges. This latter effect called 'EPSP-to-spike (E-S) potentiation' is though t to be mediated by voltage-sensitive channels in the dendrites. Curre nt-source density (CSD) analysis was made in the CA1 area of hippocamp al slice preparations to find if LTP causes changes of the transmembra ne currents in the stratum radiatum which can be detected with this te chnique. Some increase of currents associated with synaptic transmissi on itself at distant dendritic areas was accompanied by a disproportio nal enhancement of other currents attributed to activation of dendriti c membranes at similar to 150 mu m 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, t he increase in CSD at distant and more proximal portions of the dendri tes remained proportional. Paired pulse facilitation induced in the sa me slices did not produce disproportional enhancement of proximal dend ritic currents either. Our findings support the assumptions that durin g LTP associated with E-S potentiation the probability of activation o f voltage-sensitive channel is enhanced on the dendrites of CA1 pyrami dal cells.