Expression of SP-mediated potentiation requires test stimulation. (A) The potentiation by SP requires test stimulations and is input-specific. Only SC-CA2 synapses, but not the EC-CA2 synapses, showed potentiation when the EC-CA2 test stimulation was suspended at the time of SP application and for a subsequent 1 h (n = 8). (B) In a reverse scenario, i.e., suspension of SC-CA2 test stimulation during and 45 min after drug application, no potentiation was observed in SC-CA2, unlike the case for EC-CA2 synapses, in which potentiation was expressed (n = 7). (C) Suspended baseline stimulation of SC-CA2 and EC-CA2 synaptic inputs at the time of SP application and for a subsequent 1 h prevented potentiation in either synaptic input (n = 8). (D) A control experiment showing no potentiation in SC-CA2 and EC-CA2 synaptic inputs in response to the suspension of baseline stimulation and in the absence of drug application (n = 6). Representative fEPSP traces 15 min before (closed line), 95 min after (dotted line), and 180 min after (hatched line) SP application are depicted. Calibration bars for fEPSP traces in all panels are 2 mV/3 ms.