No subject

 We have examined the effects of high concentrations of cholinergic
agonists on currents through single acetylcholine receptor (AChR)
channels  on clonal BC3H1 cells. We find that raised concentrations of
acetylcholine   (ACh; above 300 microM) or carbamylcholine (Carb; above
1,000 microM)   produce a voltage- and concentration-dependent reduction
in the mean       single-channel current. Raised concentrations of
suberyldicholine (Sub;    above 3 microM) produce a voltage- and
concentration-dependent increase in  the number of brief duration
low-conductance interruptions of open-channel currents. These
observations can be quantitatively described by a model in which agonist
molecules enter and transiently occlude the ion-channel of  the AChR.