N. Torii et al., QUANTAL ANALYSIS SUGGESTS PRESYNAPTIC INVOLVEMENT IN EXPRESSION OF NEOCORTICAL SHORT-TERM AND LONG-TERM DEPRESSION, Neuroscience, 79(2), 1997, pp. 317-321
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.