Fig. 3. Opioid peptides regulate LTP induction by a mechanism that depends on GABAergic inhibition. A, HFS was applied in the presence of standard ACSF (control;n = 8) or ACSF containing picrotoxin (PTX, 50 μm; n = 4). Input–output curves were obtained during perfusion with standard or PTX containing ACSF (baseline 1, dotted line), 20 min after further perfusion in this medium (baseline 2, solid line), and 40 min after HFS (post-HFS, dashed line) applied to the LPP. B, HFS was applied in the presence of naloxone (NLX, 5 μm) in slices continuously perfused with standard ACSF (n= 8) or PTX containing ACSF (n = 5). Plots are input–output curves obtained during perfusion with standard or PTX containing ACSF (predrug, dotted line), 20 min after addition of naloxone (drug baseline, solid line), and 40 min after HFS (post-HFS, dashed line). PTX perfusion commenced 80 min before the addition of naloxone.Inset shows representative traces taken before (solid line) and 40 min post-HFS (dashed line). All input–output curves are group mean ± SEM of fEPSP slope values normalized relative to the maximum value of the input–output curve collected immediately before HFS. Horizontal bar, 2 msec; vertical bar, 3 mV. C, Time course of changes in LPP-evoked fEPSPs. Plots are group mean ± SEM changes in fEPSP slope expressed in percentage of baseline. The period of drug perfusion is indicated by the stippled bar, and delivery of HFS to the LPP is indicated by an arrow. Perfusion with PTX abolished the ability of naloxone to block LTP. D, Time course of changes in MPP-evoked fEPSPs after HFS of the LPP in the presence of naloxone in slices perfused in standard ACSF or PTX containing ACSF. Note that LTP was induced selectively in LPP fibers in PTX-containing medium.