Concentration-response relationships at mutated nAChRs measured by two-electrode voltage clamp on <i>X. laevis</i> oocytes.

<p>(<b>A</b>) α4β2 nAChRs with 3α:2β stoichiometry have three binding sites for ACh (black arrows): two with high sensitivity (HS) and one with low sensitivity (LS). Point-mutation of a central tryptophan residue in the α4 subunit will change the complementary (−) side of the LS site (red circle and arrow). Point-mutation of the corresponding tryptophan in the β2 subunit will change the complementary side of both HS binding sites (blue circles and arrows). (<b>B</b>) Concentration-response relationships (CRRs) of ACh at α4^(W88A)β2 and α4β2^(W82A) receptors. The black curve is drawn from previously published data for the ACh CRR at type α4β2 nAChRs with 3α:2β stoichiometry <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091232#pone.0091232-Harpse1" target="_blank">[22]</a>, with EC₅₀ and fraction values listed below the figure. ‘Fraction’ describes the fraction of the maximum response that is elicited by the high-sensitivity phase. Numbers in parenthesis refer to 95% confidence intervals. ‘n’ is the range of the number of measurements that were made of each point on a curve. An F test was carried out in GraphPad Prism 4 against the null hypothesis of a monophasic fit, which was rejected for α4^(W88A)β2 (F = 120, DFnN = 2, DFnD = 288) and accepted for α4β2^(W82A) (F = 0.73, DFnN = 2, DFnD = 72), where DFnN and DFnD are the degrees of freedom of the numerator and denominator in the F test, respectively.