Neuroscience Letters, 113 (1990) 281 285 281 Elsevier Scientific Publishers Ireland Ltd. NSL 06904 Changes in the time course of miniature endplate currents induced by bath-applied acetylcholine L.G. Magazanik l, V.A. Snetkov 1, R.A. Giniatullin 2 and R.N. Khazipov 2 I Secheno~ Institute of Evolutionary Physiology and Biochemistry, Leningrad (U.S.S.R.) and 2Ka Medical Institute, Kazan (U.S.S.R.) (Received 22 December 1989; Revised version received 12 February 1990; Accepted 12 Februar Key words." Frog neuromuscular junction; Acetylcholinesterase; Acetylcholine; Endplate currents; Desensitization; Postsynaptic potentiation; e-Bungarotoxin Bath application of 0.5 and 2/LM acetylcholine (ACh) slowed the decay phase of miniature end currents (MEPC) recorded in isolated, voltage-clamped and prostigmine-treated frog sartorius Washout of ACh led to a decrease of the decay time constant of the MEPC to 72+5% (n=5) and 51 (n=6) of initial values, respectively, followed by very slow and incomplete recovery. MEPC ampl changed slightly and recovered relatively fast. This discrepancy in the recovery rates is suggeste due to a 'trapping' ability of desensitized receptors which can compete with the free receptors for molecules and prevent repetitive binding. Thus the high affinity of desensitized receptors to ACh m tially compensate the absence of acetylcholiriesterase activity. Application of acetylcholine (ACh) to muscle treated with anticholinesterase drug slows down the decay phase of miniature endplate currents (MEPCs). This effect is transient and followed by progressive shortening of MEPCs. Two simultaneously oc- curring processes which govern the M E P C time course were suggested: postsynaptic potentiation (PSP) and desensitization (DS) [7, 8, 13, 15, 17]. The changes of the M E P C decay time constant (r) in the presence of agonist and after its withdrawal may indicate that PSP develops and vanishes much faster than DS [7, 8, 13, 15]. This allows the use of the recovery of M E P C parameters after withdrawal of agonist for the elucidation of DS contribution in M E P C generation. Experiments were performed on the frog (R. temporaria) sartorius muscle using the two-microelectrode voltage-clamp technique. The bathing solution contained (in mM): NaCI 115.0, KC1 2.5, CaC12 1.8, NaHCO3 3.0, pH 7.2-7.4. A C h E was inhibited by the permanent presence of prostigmine (2/lg/ml). The perfusion rate was 3 ml/min Correspondence." L.G. Magazanik, Sechenov Institute of Evolutionary Physiology and Biochemistry Academy of Sciences of the U.S.S.R., Thorez pr. 44, Leningrad, 194223, U.S.S.R.