Comparative site of action of various anaestetic agents at the mammalian myoneural junction.

Pre- and postjunctional effects of three ethers (enflurane, diethyle ether, and methoxyflurane) and three non-ethers (chloroform, halothane, and trichloroethylene) were studied in the rat phrenic nerve diaphragm preparation using standard microelectrode recording techniques. Depression of postjunctional function included depression of the amplitude of miniature endplate potentials, inhibition of suxamethonium induced depolarization of the muscle endplate, prolongation of duration of the endplate potential, and increase in threshold for generation of the muscle action potential. The last two effects were more marked for the ethers than for the non-ethers. Effects on pre-junctional function included a slight increase in fluctuation of the endplate potential (EPP) amplitude associated with the thers and chloroform, a faster rate of decline of EPP amplitude during a tetanus in presence of the ethers, and prolongation of the normal facilitatory period during paired stimulation in the presence of chloroform. Chloroform had no effect on the rate of decline of EPP amplitude during tetanic stimulation and the ethers had no effect on the facilitatory period during paired stimulation. These results indicate that volatile anaesthetic agents depress synaptic transmission by acting on multiple sites, and that the pattern of this depression is different for each drug or group of similar drugs.