Fig. 2.
The time course of desensitization varies with nAChR subtype. A, Responses to brief (∼3 min) applications of nicotine at near EC50 concentrations in calcium-containing media for oocytes injected with different subunit combinations. For comparison, the responses are normalized to their peaks. The time course of desensitization for responses obtained from oocytes expressing β4-containing receptors (left two traces) were well described by a single exponential component. For β2-containing receptors (right two traces), the decay phase was well described by the sum of two exponentials.B, Responses to 3 min applications of nicotine in calcium-free (left trace) and calcium-containing (right trace) media for an oocyte expressing α4β4 receptors. The traces are normalized to the peak currents. Thesolid lines are single exponential functions fit to the desensitizing phase of the responses, and their associated time constants (τs) are shown. C,Responses to 3 min applications of nicotine in calcium-free (left trace) and calcium-containing (right trace) media for an oocyte expressing α3β4 receptors. Thesolid lines are single exponential functions fit to the desensitizing phase of the responses. In calcium-containing media the data were fit with two separate single exponentials, designated τf and τs. The exponential functions contained a steady-state component, which was constrained not to fall below baseline current. The final steady-state values were generally in the range of 0–20% peak current.