DECREASED FREQUENCY BUT NOT AMPLITUDE OF QUANTAL SYNAPTIC RESPONSES ASSOCIATED WITH EXPRESSION OF CORTICOSTRIATAL LONG-TERM DEPRESSION

We have investigated the site of expression of striatal long-term syna ptic depression (LTD) using analysis of Sr2+-induced asynchronous rele ase of quanta from stimulated synapses. The cumulative amplitude distr ibution of Sr2+-induced asynchronous synaptic responses overlaps with that of miniature EPSCs (mEPSCs), suggesting that Sr2+-induced asynchr onous responses are quantal. Quantal amplitude at stimulated synapses is not significantly altered after LTD induction, whereas quantal freq uency decreases after LTD induction. The decrease in quantal frequency is prevented when LTD expression is blocked by dialyzing 10 mM EGTA i nto the postsynaptic neuron. Our findings are most consistent with the idea that expression of striatal LTD involves decreased neurotransmit ter release with no change in quantal amplitude, despite the fact that induction of striatal LTD involves postsynaptic mechanisms.