FIGURE 3.

Determination of the IC₅₀ for picrotoxin. (A) GABA-activated currents (using an EC₅₀ concentration of GABA) in the presence of increasing concentrations of PTX are shown. Note that the mutations imparted varying amounts of picrotoxin resistance. (B) The PTX inhibition relationship for wild-type receptors (open circles), as well as the receptors where all five subunits had been mutated to serine (solid circles), valine (squares), or tyrosine (diamonds) are shown. The continuous lines are fits of Eq. 2 to the dose-response relationships. This yielded IC₅₀ values of 0.8 ± 0.1, 38.3 ± 2.5, 361 ± 26, and 1614 ± 48 for wild-type, T6′S, T6′V, and T6′Y, respectively. For these experiments, current measurements were made at the end of agonist-antagonist application.