Recovery from open channel block by acetylcholine during neuromuscular transmission in zebrafish

At larval zebrafish neuromuscular junctions (NMJs), miniature end plate currents (mEPCs) recorded in vivo have an unusually fast time course. We used fast-flow application of acetylcholine (ACh) onto outside-out patches to mimic the effect of synaptic release onto small numbers of ACh receptor channels (AChRs). Positively charged ACh acted at hyperpolarized potentials and at millimolar concentrations as a fast ("flickering") open channel blocker of AChRs. Because of filtering, the open channel block resulted in reduced amplitude of single channel currents. Immediately after brief (1 msec) application (without significant desensitization) of millimolar ACh at hyperpolarized potentials, a slower, transient current appeared because of delayed reversal of the block. This rebound current depended on the ACh concentration and resembled in time course the mEPC. A simple kinetic model of the AChR that includes an open channel-blocking step accounted for our single channel results, as well as the experimentally observed slowing of the time course of mEPCs recorded at a hyperpolarized compared with a depolarized potential. Recovery from AChR block is a novel mechanism of synaptic transmission that may contribute in part at all NMJs.