Fig. 1.

Two-component dose–response curves of human α4β2 nAChRs. A, Increasing ACh concentrations (200 msec pulses) were delivered every 15 sec. Typical traces of ACh-evoked currents are presented at the top. Horizontal bars indicate the ACh pulses with the concentration values (in micromolar). The fast desensitization and the current rebound observed at the end of the application pulse (1000 μm ACh) might indicate an open-channel block by the high ACh concentration. Mean current amplitudes were plotted as a function of the ACh concentration on a semilogarithmic scale (n = 12). Thedashed line corresponds to the best fit that can be drawn using a single Hill equation (EC₅₀ = 30 μm and nH = 0.8). A better fit is obtained with the sum of two Hill equations (continuous line) yielding high-affinity coefficients of EC₅₀H = 1.60 μm and nH1 = 0.92, whereas the low-affinity values are EC₅₀L = 68 μm and nH2 = 1.60. The fraction of high- and low-affinity states are 25 and 75%, respectively. B, The same protocol as inA was repeated for the determination of the α4β2 nAChR sensitivity toward nicotine. Currents were then elicited with increasing concentrations of nicotine (top).Horizontal bars indicate the nicotine pulses with the concentration values (in micromolar). Mean current amplitudes were plotted as a function of the nicotine concentration on a semilogarithmic scale (n = 11). The dashed line corresponds to the best fit that can be drawn using a single Hill equation (EC₅₀ = 10 μm and nH = 1.30). A better fit is obtained with the sum of two Hill equations (thick line) yielding high-affinity coefficients of EC₅₀H = 2.4 μm and nH1 = 0.91, whereas the low-affinity values are EC₅₀L = 14.5 μm and nH2 = 1.53. The fractions of high- and low-affinity states are 25 and 75%, respectively. For comparison, the ACh dose–response profile is scaled up to the maximal nicotine-evoked current (gray line). Note that the low-affinity component is much more sensitive to nicotine than to ACh.