Field potential recordings confirm differential regulation of synaptic plasticity after recovery from MK-801 and AP-5 block. A, Consecutive LTP induced by two tetanic stimuli (100 Hz, twice, 120 s interval). LTP was produced by first tetanic stimulus and the second LTP was further induced on the base of the first one by the second tetanic stimulus. The interval of the two titanic stimuli was 40 min. B, Recovery of LTP induction after removal of AP-5. LTP induction was blocked by AP-5 perfusion, but, after 30 min AP-5 washout, LTP induction could be completely recovered with tetanic stimulation. The extent of field EPSP potentiation was similar to that produced by the first tetanic stimulus in control (A). C, Conversion of synaptic plasticity direction from LTP to LTD after partial recovery from MK-801 block. After 30 min MK-801 washout, the same LTP-producing stimulus protocol induced LTD instead of LTP, indicating conversion of synaptic plasticity direction. D, Superimposed data show differential regulation of synaptic plasticity after recovery from MK-801 and AP-5 block.