Fig. 6.

X/XO-induced potentiation and LTP occlude each other. A, X/XO-induced potentiation occludes LTP. Stable baseline responses of the fEPSP slope were recorded for 20 min before the slices were incubated with X/XO (20 and 2 μg/ml) as indicated by the bar. At 60 min after the washout of X/XO the stimulus intensity was reduced (indicated by thedown arrow) to match the fEPSP slope before exposure to X/XO. LTP-inducing HFS (indicated by the up arrow) then was delivered to the slices as described in Materials and Methods. Additional incubations of the slices with X/XO before HFS did not produce additional potentiation, indicating that the X/XO-induced potentiation was saturated (data not shown). Error bars are SEM for six determinations. B, LTP can be induced after a blockade of X/XO-induced potentiation by SOD. Slices were incubated with X/XO and SOD (25 μg/ml) as indicated by the bar. At 60 min after the washout of X/XO and SOD, LTP-inducing HFS (indicated by thearrow) was delivered to the slices. Error bars are SEM for six determinations. C, LTP occludes X/XO-induced potentiation. Stable baseline responses in hippocampal slices were recorded for 20 min before the delivery of LTP-inducing HFS as indicated by the up arrow. At 30 min after the last train of HFS the stimulus intensity was reduced (indicated by thedown arrow) to match the fEPSP slope before the HFS. Slices then were incubated with X/XO (20 and 2 μg/ml) as indicated by the bar. Additional delivery of HFS before incubation of the slices with X/XO did not produce additional potentiation, indicating that LTP was saturated (data not shown). Error bars are SEM for six determinations. D, X/XO can induce potentiation after a blockade of LTP by APV. HFS (indicated by thearrow) was delivered to the slices in the presence of APV (indicated by the bar). At 30 min after HFS the slices were incubated with X/XO as indicated by the bar. Error bars are SEM for six determinations.