INVOLVEMENT OF SILENT SYNAPSES IN THE INDUCTION OF LONG-TERM POTENTIATION AND LONG-TERM DEPRESSION IN NEOCORTICAL AND HIPPOCAMPAL-NEURONS

Changes in the latency of small excitatory postsynaptic potentials wer e observed in association with induction of long-term modifications of synaptic transmission in slices of rat neocortex and guinea-pig hippo campus. After potentiation response latency decreased in 3/10 cases in the neocortex and in 6/24 cases in the hippocampus, and increased aft er depression in 4/8 cases in the neocortex. These latency changes cou ld not be attributed to changes in presynaptic fibre excitability, mon osynaptic inhibition, release kinetics or activation kinetics of posts ynaptic ion channels. We conclude therefore that potentiation led to t he activation of previously silent synapses of fast-conducting afferen ts and depression to the inactivation of previously functional synapse s. Thus, neocortical and hippocampal synapses can be in a non-function al state, and regimes that induce long-term potentiation and depressio n not only change the efficacy of synapses but also alter their functi onal state. Copyright (C) 1996 IBRO. Published by Elsevier Science Ltd .