CA2-TERM DEPRESSION IN THE HIPPOCAMPUS( SIGNALING REQUIREMENTS FOR LONG)

It has been hypothesized that the direction of synaptic weight change elicited by synaptic activity depends on the magnitude of the activity -dependent rise in intracellular Ca2+ concentration. Several aspects o f this hypothesis were examined at the Schaffer collateral CA1 synapse , where both long-term depression (LTD) and long-term potentiation (LT P) can be elicited and are Ca2+ dependent. Brief tetanic stimulation, which normally generated LTP, could induce LTD when Ca2+ entry via NMD A receptors was limited either by moderate concentrations of D-APV or by voltage clamping cells at negative membrane potentials. Repetitive activation of voltage-dependent Ca2+ channels in the absence of affere nt stimulation could also elicit an LTD that was Ca2+ dependent and wa s occluded by prior generation of homosynaptic LTD using prolonged low frequency stimulation. These results provide strong evidence that the minimal requirements for inducing LTD involve simply a transient infl ux of Ca2+ into the postsynaptic cell, via either NMDA receptors or vo ltage-dependent Ca2+ channels.