THE TIMING OF CHANNEL OPENING DURING MINIATURE END-PLATE CURRENTS AT THE FROG AND MOUSE NEUROMUSCULAR-JUNCTIONS - EFFECTS OF FASCICULIN-2, OTHER ANTI-CHOLINESTERASES AND VESAMICOL

Fluctuation analysis was used to estimate the mean single-channel cond uctance and the mean channel duration of opening. Miniature endplate c urrents (MEPCs) were measured with the voltage-clamp technique The tim ing of endplate channel opening during the generation of the MEPC was estimated by a deconvolution method. Often all of the channels opened during the rise of the MEPC, but in about half of the examples some 10 % of the channels opened after the peak. We studied the effects of ace tylcholinesterase (AChE) inhibition with neostigmine, diisopropyl fluo rophosphate (DFP) and fasciculin-2. With AChE largely inhibited, the n umber of channels opening increased as much as fourfold, largely by ch annels opening in the ''tail'' that follows the peak of the MEPC. The results were compared to models of MEPC generation. Models did not acc ount well for the pattern of channel opening, particularly after AChE inhibition. In the presence of fasciculin-2, the addition of 2 mu M (- )-vesamicol reduced the number of channels opening and shortened the p eriod over which channels were open. One interpretation is that quanta l ACh release is not almost instantaneous, but that some of the ACh is released over a period of a millisecond or more and that some of the release is blocked by (-)-vesamicol.