QUANTAL ANALYSIS SUGGESTS PRESYNAPTIC INVOLVEMENT IN EXPRESSION OF NEOCORTICAL SHORT-TERM AND LONG-TERM DEPRESSION

Long-term depression(1,18,26) together with long-term potentiation(3,2 6,30) represent popular experimental models to study synaptic plastici ty. However, analyses of the mechanisms underlying the expression of c ortical long-term depression are in their infancy and have been confin ed to the hippocampus.(4,5,7,8,14,19,22,35) Short-(25,28) and long-ter m(1,26,27,34) depression in neocortex is not well understood. Here we recorded small excitatory postsynaptic potentials intracellularly from rat visual cortex slices. The responses fluctuated between several am plitude levels suggesting a quantal nature of the synaptic transmissio n. Consistent changes in the quantal steps accompanied neither paired- pulse depression (50ms interval within the pair) nor long-term depress ion (induced by 1 Hz, 5 min stimulation). The amplitude distributions shifted to smaller values suggesting decreases in the number of quanta released without essential changes in the postsynaptic quantal effici ency. Both the coefficient of variation of response amplitudes and the number of response failures increased; cases were encountered suggest ing a very low release probability after depression. Changes in quanta l content estimated from the deconvolution analysis were correlated wi th the magnitude of depression. The findings suggest predominantly pre synaptic loci for expression of short- and long-term neocortical depre ssions. The likely underlying mechanism is a decrease in transmitter r elease probability. Long-term depression decreased the probability so strongly that some inputs became virtually silent. (C) 1997 Published by Elsevier Science Ltd.