Fig. 3. Dose–response relations for fentanyl-induced inhibition of Ca2+current (ICa) in the three groups of sensory neurons. CAP (+) = capsaicin responsive; CAP (−) = capsaicin unresponsive; Fent (+) = fentanyl sensitive; Fent (−) = fentanyl insensitive. The number of cells studied at each concentration is shown next to each data point. The smooth line through the filled circles represents the best fit of the data from the CAP (+), Fent (+) cells to the Hill equation.