Synaptic late associativity impaired by neuropsin deficiency. A, Two independent inputs (S0 and S1) into the same population of postsynaptic neurons were alternatively stimulated. R, Recording; DG, dentate gyrus. B, A single train of 100 Hz tetanus to S1 after four trains of 100 Hz tetanus in S0 (filled circles) expressed stable L-LTP in S1 (open circles). C, In contrast, in KO, fEPSP values were impaired in S1 (red open circles) with 180 min posttetanus after four trains of 100 Hz tetanus in S0 (red filled circles). D, rmNP administration in KO reversed the induction of L-LTP in weak stimulated S1. The red bar indicates rmNP administration. E, F, The application of NP STOP was ineffective during strong stimuli (four trains of 100 Hz tetanus) at S0 (E) but effective during a single 100 Hz tetanus at S1 (F) in WT mice. The black bar indicates NP STOP administration. Calibration: 1 mV, 10 ms. Inset traces show sample fEPSPs recorded during the baseline (a) period and 180 min (b) after stimulation at S1. Black short vertical bars indicate tetanic trains.