Fig. 2. Responses to capsaicin (CAP) and fentanyl in three different sensory neurons. The cell in A  was sensitive to both CAP and fentanyl; the cell in B  was sensitive to CAP but not to fentanyl; the cell in C  was sensitive to neither CAP nor fentanyl. The top panels show inward currents induced by CAP (10 μm; time of CAP application denoted by bar above each trace). The middle panels show representative high-voltage–activated Ca²⁺current (I^Ca) traces, and the bottom panels show the Ca²⁺current–voltage (I-V) relations, before (open squares), during (filled circles), and after (open triangles) washout of fentanyl. Fentanyl concentrations were 100 nm in A  and B  and 1 μm in C . For the CAP-induced current traces, the horizontal scale bars denote 1 s, and the vertical scale bars denote 1 nA. For I^Catraces, the horizontal scale bars denote 10 ms, and the vertical scale bars denote 500 pA.