COOPERATIVE INTERACTIONS AMONG AFFERENTS GOVERN THE INDUCTION OF HOMOSYNAPTIC LONG-TERM DEPRESSION IN THE HIPPOCAMPUS

Prolonged periods of low-frequency stimulation have been shown to prod uce a robust, long-term synaptic depression (LTD) in both hippocampus and visual cortex. In the present study we have examined the extent to which interactions among afferents govern the induction of homosynapt ic LTD in young-adult rats in hippocampal region CA1 in vitro. Field e xcitatory postsynaptic potentials were assessed before and after condi tioning stimulation consisting of two 10-min trains of low-frequency s timulation (LFS; 1 Hz) of the Schaffer collateral/commissural pathway. LFS at an intensity producing a 0.5-mV response did not produce signi ficant synaptic depression. However, LFS administered at a higher inte nsity resulted in significant input-specific LTD of a 0.5-mV test resp onse. Picrotoxin, which also facilitates depolarization of CA1 neurons , significantly enhanced the magnitude of LTD after LFS at 0.5 mV. In addition, LFS at 0.5 mV in normal perfusion medium (no picrotoxin) pro duced only small changes in synaptic efficacy when either of two conve rging pathways was conditioned separately but produced a robust LTD wh en both pathways were conditioned simultaneously. This cooperative LTD was reversibly blocked by prior administration of 100 mu M DL-aminoph osphonovaleric acid but not by 20 mu M nimodipine. Taken together, the se results suggest that cooperative interactions among afferents contr ibute to voltage-dependent processes underlying the induction of homos ynaptic LTD.