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

At larval zebrafish neuromuscular junctions (NMJs), miniature end plate cur rents (mEPCs) recorded in vivo have an unusually fast time course. We used fast-flow application of acetylcholine (ACh) onto outside-out patches to mi mic the effect of synaptic release onto small numbers of ACh receptor chann els (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) appl ication (without significant desensitization) of millimolar ACh at hyperpol arized potentials, a slower, transient current appeared because of delayed reversal of the block. This rebound current depended on the ACh concentrati on and resembled in time course the mEPC. A simple kinetic model of the ACh R that includes an open channel-blocking step accounted for our single chan nel results, as well as the experimentally observed slowing of the time cou rse of mEPCs recorded at a hyperpolarized compared with a depolarized poten tial. Recovery from AChR block is a novel mechanism of synaptic transmissio n that may contribute in part at all NMJs.