Fig. 6. Action of DAMGO on AMPA receptor–mediated (A ) and NMDA receptor–mediated EPSCs (B ) and effects of fentanyl on NMDA receptor–mediated EPSCs. (A ) Voltage-clamp recordings of AMPA–EPSCs in the absence (control) and presence of DAMGO and DAMGO plus naltrexone. DAMGO reduced the amplitude of the EPSC by 33%. This inhibition was reversed by naltrexone (2 μm). (B ) Voltage-clamp recordings of NMDA–EPSCs in the absence (control) and presence of DAMGO and DAMGO plus naltrexone. DAMGO reduced the amplitude of the EPSC by 21%. This inhibition was also naltrexone reversible. (C ) Voltage-clamp recordings of NMDA–EPSCs in the absence (control) and presence of fentanyl and fentanyl plus naloxone. Fentanyl reduced the amplitude of the EPSC by 13%. This inhibition was also naloxone reversible. Each trace represents the average of five consecutive single recordings.