Fig. 3.

Activation of AMPA receptors by the nondesensitizing agonist domoate indicate that CTZ does not distort the estimated number of agonist binding steps. A, Domoate, (5, 10, and 20 μm) produced a nondesensitizing response in outside-out patches. Ensemble averages from three different patches are shown with their peak amplitudes normalized to facilitate comparison. As for quisqualate, the rise times of the responses were concentration-dependent, with sigmoidal activation at low agonist concentrations. B, The response to a pulse (150 msec) of domoate (10 μm) (dots) was fitted with three different kinetic models incorporating one, two, or three binding sites (lines). All three models gave a good fit late in the response when domoate binding was complete but diverged in the critical rising phase (box). C, An enlargement of the boxed area in B, highlighting the early part of the rising phase. The optimally fitted transients are shown as dashed lines for the one- and three-site models and a solid line for the two-site model. D, Domoate responses in the absence or presence of 100 μm CTZ were compared using the three kinetic models. The mean SSE in the absence of CTZ (−CTZ;n = 6) and presence of CTZ (+CTZ;n = 13) are shown in the histograms. As for quisqualate, the two-site model gave the best fit to the activation phase, and CTZ did not alter the stoichiometry of agonist binding.