Postsynaptic levels of [Ca2+](i) needed to trigger LTD and LTP

Long-term potentiation (LTP) and long-term depression (LTD) in CA1 pyramidal neurons are both triggered by a postsynaptic rise in intracellular Ca2+ concentration ([Ca2+](i)). We used photolysis of postsynaptic caged Ca2+ compounds to search for differential thresholds for activation of these processes. Longlasting potentiation (LLP) resembling LTP, and longlasting depression (LLD) resembling LTD, were evoked by [Ca2+](i) elevations of comparable magnitude and duration in different cells. No distinctions in threshold for these processes were detectable. LLP was occluded by tetanically induced LTP and blocked by calmodulin inhibition, and LLD was occluded by electrically induced LTD and blocked by phosphatase inhibition.