Bath-applied AAIs neither shift I-O curves nor alter PPF. (A) I-O curves were generated at stimulation intensities that evoked threshold, 30–50%, 50%, and maximal pEPSP amplitudes. PPF was tested at 20, 50, 100, and 200 ms intervals with intensities evoking 30–50% of maximal pEPSP amplitude. I-O curves and PPF protocols were performed during control recordings and compared with protocols taken 40 and 80 min after the start of AAI superfusion. No significant changes in I-Os or PPFs were detectable. (B Left) Traces of a typical I-O curve. In the enlarged section, note the increasing amplitude of the presynaptic volley after the stimulation artifact (arrowhead). (Right) Single traces from a PPF protocol. Although the presynaptic volley did not change in size, the amplitude of the second pEPSP was greatly potentiated at all intervals tested.