Open probability of homomeric murine 5-HT3A serotonin receptors depends onsubunit occupancy

1. The time course of macroscopic current responses of homomeric murine ser otonin 5-HT3A receptors was studied in whole cells and excised membrane pat ches under voltage clamp in response to rapid application of serotonin. 2. Serotonin activated whole cell currents with an EC50 value for the peak response of 2 muM and a Hill slope of 3.0 (n=12), suggesting that the bindi ng of at least three agonist molecules is required to open the channel. 3. Homomeric 5-HT3A receptors in excised membrane patches had a slow activa tion time course (mean +/- S.E.M. 10-90% rise time 12.5 +/-1.6 ms; n=9 patc hes) for 100 muM serotonin. The apparent activation rate was estimated by f itting an exponential function to the rising phase of responses to supramax imal serotonin to be 136 s(-1). 4. The 5-HT3A receptor response to 100 muM serotonin in outside-out patches (n=19) and whole cells (n=41) desensitized with a variable rate that accel erated throughout the experiment. The time course for desensitization was d escribed by two exponential components (for patches tau (slow) 1006 +/- 139 ms, amplitude 31%; tau (fast) 176 +/- 25 ms, amplitude 69%). 5. Deactivation of the response following serotonin removal from excised me mbrane patches (n=8) and whole cells (n=29) was described by a dual exponen tial time course with time constants similar to those for desensitization ( for patches tau (slow) 838 +/- 217 ins, 55% amplitude; tau (fast) 213 +/- 4 4 ms, 45% amplitude). 6. In most patches (6 of 8), the deactivation time course in response to a brief 1-5 ms pulse of serotonin was similar to or slower than desensitizati on. This suggests that the continued presence of agonist can induce desensi tization with a similar or more rapid time course than agonist unbinding. T he difference between the time course for deactivation and desensitization was voltage independent over the range -100 to -40 mV in patches (n=4) and -100 to +50 mV in whole cells (n=4), suggesting desensitization of these re ceptors in the presence of serotonin does not reflect a voltage-dependent b lock of the channel by agonist. 7. Simultaneously fitting the macroscopic 5-HT3A receptor responses in patc hes to submaximal (2 muM) and maximal (100 muM) concentrations of serotonin to a variety of state models suggests that homomeric 5-HT3A receptors requ ire the binding of three agonists to open and possess a peak open probabili ty greater than 0.8. Our modelling also suggests that channel open probabil ity varies with the number of serotonin molecules bound to the receptor, wi th a reduced open probability for fully liganded receptors. Increasing the desensitization rate constants in this model can generate desensitization t hat is more rapid than deactivation, as observed in a subpopulation of our patches.