Fig. 4. The effects of the membrane potential on the extent of opioid inhibition. ( A ) The current–voltage relationships of the ε2/ζ1 channel before and during perfusion of 300 μM meperidine. The measured current responses were normalized to the control current responses at −70 mV before meperidine perfusion (n = 7 or 8). ( B ) The current–voltage relationships of the ε2/ζ1 channel before and during perfusion of 300 μM morphine (n = 7 or 8). ( C ) The effects of the membrane potential on the extent of inhibition of the ε2/ζ1 channel by 300 μM meperidine, 300 μM morphine, and 1 mM naloxone. The Kd(0)values (mM) for meperidine, morphine, and naloxone were 2.6 ± 0.5, 3.8 ± 0.5, and 10.7 ± 1.6, respectively, and the zδ values for those were 0.9 ± 0.05, 1.0 ± 0.04, and 1.0 ± 0.06, respectively (n = 7 or 8).