TABLE 3

Radioligand binding parameters for the interaction between [3H]QNB, WIN 62,577, and C7/3-phth, alcuronium, or brucine at the M4 mAChR

Parameters derived from nonlinear regression analysis are presented as the mean ± S.E.M. (n = 3-5). LogKB is the logarithm of the modulator equilibrium dissociation constant at the free receptor. Logα is the logarithm of the cooperativity factor for the interaction between WIN 62,577 and [3H]QNB. Antilogarithm (geometric mean) is shown in parentheses. Logβ is the logarithm of the cooperativity factor for the interaction between the indicated prototypical modulator and [3H]QNB. Antilogarithm (geometric mean) is shown in parentheses. Logγ is the logarithm of the cooperativity factor for the interaction between WIN 62,577 and the indicated prototypical modulator on the [3H]QNB-free receptor. Antilogarithm (geometric mean) is shown in parentheses. Logδ is the logarithm of the cooperativity factor for the interaction between WIN 62,577 and the indicated prototypical modulator on the [3H]QNB-occupied receptor. Antilogarithm (geometric mean) is shown in parentheses. In all instances, the value for Logδ was determined by an extra-sum-of squares test (F test) to not be significantly different from 0 (i.e., δ = 1) and was constrained as such in the analyses

Allosteric Modulator LogKB Logα Logβ Logγ Logδ
C7/3-phth −5.81 ± 0.22 −2.69 ± 0.03 (0.002) −1.02 ± 0.04 (0.096) 0 (1)
Alcuronium −4.50 ± 0.06 −1.49 ± 0.04 (0.032) −0.60 ± (0.25) 0 (1)
Brucine −4.64 ± 0.03 −100a (~0) −0.31 ± 0.06 (0.49) 0 (1)
WIN 62,577 −5.32 ± 0.07 −0.79 ± 0.01 (0.16)
  • a Irrespective of initial values, the nonlinear regression algorithm always converged to the arbitrarily assigned boundary value of logβ = −100, indicating that the degree of negative cooperativity between brucine and [3H]QNB was so high that it could not be distinguished from competition (β = 0).