Fig. 4.

Activation of κ-opioid receptors attenuates immediate and slow C_m responses evoked by steps inducing smaller Ca²⁺ influx.C_m increases (A₁,A₂, B₁,B₂) and corresponding Ca²⁺currents (A₃, B₃) recorded in response to 200 msec depolarizations to +10 mV (A) and +30 mV (B) from a holding potential of −90 mV. Administration of U69,593 appreciably reduced Ca²⁺ influx (32.1%, from 18.53 to 12.58 pC) evoked by depolarization to +10 mV but had little effect on the corresponding immediate C_m response (9% reduction, from 49 to 44 fF, indicated by dotted lines). Depolarization to +30 mV induced much smaller Ca²⁺influx (9.1 pC) that was attenuated further by activation of κ-opioid receptors (to 6.23 pC, representing 31.5% blockade). Under this condition of reduced driving force for Ca²⁺ influx, inhibition of the immediate C_m response by the κ-opioid agonist was markedly increased (47%, from 36 to 19 fF). Opioid administration also reduced the slowly increasingC_m component recorded in this nerve ending, and this effect of U69,593 was greater on the response measured at +30 mV (33.9%, from 118 to 78 fF) compared with that recorded at +10 mV (17.7%, from 192 to 158 fF). C, Bar graph summarizes the effects of U69,593 on the immediate step and slowly increasing components of C_m responses. Bars represent the mean ± SEM of the normalized C_mresponses pooled from κ-opioid-responsive terminals (number shown in numerator, out of the total number of terminals examined, shown in denominator) for immediate and slowC_m responses evoked at command potentials of +10 or +30 mV, expressed as a percent of the corresponding predrug control values. Reduction of depolarization-evoked Ca²⁺ influx to submaximal levels resulted in an increased probability for modulation of the immediateC_m response by κ-opioid receptor activation from 32% (13 of 41) at +10 mV to 100% (6 of 6) at +30 mV, without significantly (p = 0.19,NS indicates not significant) altering the magnitude of the inhibitory effect of U69,593 on these initial exocytotic events. In contrast, the inhibitory effect of U69,593 on the slowC_m responses was significantly increased under conditions in which the driving force for Ca²⁺influx and the net amount of depolarization-evoked Ca²⁺ influx was reduced (asteriskindicates p < 0.001).