The role of desensitisation in decay time of miniature endplate currents in frogs Rana ridibunda and Rana temporaria

A new comparative characteristic of endplate microphysiology has been intro duced. It is the feasibility of receptors to become desensitised as demonst rated on two frog species, Rana temporaria and Rana ridibunda: the decay ti mes (tau (MEPC)) of single quantum miniature endplate currents (MEPCs) in t he sartorius muscles of both species were about 1 ms and were not affected by the desensitisation-promoting agent proadifen when AChE was active. Howe ver: when the desensitisation was induced by anticholinesterase neostigmine and promoted by proadifen, the prolongation of tau (MEPC) from 1 ms was al most twice as great in Rann tempoiaria (tau (MEPC) = 4.4 ms) than in Rana r idibunda (tau (MEPC) = 3.1). This indicates that desensitisation reduces th e number of available receptors and lowers the number of available ACh mole cules for repetitive binding by trapping them by desensitised, high-affinit y receptors significantly more in Rana ridibunda than in Rana temporaria. T he application of proadifen, a promoter of desensitisation, decreased the p rolongation of MEPCs in both species, but this shortening was more rapid in Rana ridibunda than in Rana temporaria. It is concluded that the desensiti sation-induced reduction in the density, and the number of postsynaptic rec eptors is significantly higher at Rann ridibunda than in Rana tempo,aria en dplates. (C) 2001 Elsevier Science Ireland Ltd and the Japan Neuroscience S ociety. All rights reserved.