In this set of experiments, the morphine concentration-response relationship was determined. Figure 3 shows the time courses of peak Ca2+ current amplitude obtained before and during application of morphine (10 μM) in 118AA and 118GG sensory neurons, respectively. The Ca2+ currents were evoked every 10 s with a 50 ms test pulse to -15 mV from a holding potential of -80 mV (shown in Figure 3B). The peak current amplitude was measured isochronally 10 ms after the initiation of the test pulse. Figure 3B shows the numbered Ca2+ current traces evoked with this paradigm and correspond to those plotted in Figure 3A. Exposure of the neuron to morphine (trace 2) resulted in approximately 45% inhibition of the Ca2+ current. Removal of morphine resulted in a rapid recovery of the current amplitude. The time course of a 118GG neuron in Figure 3C shows the effect of morphine on the peak Ca2+ current and the corresponding numbered traces are shown in Figure 3D. The application of morphine to the neuron led to a 27% inhibition of the current, and the time of recovery after agonist removal was fast.
