Depression of miniature endplate potential frequency by acetylcholine and its analogues in frog

Abstract

of Tatarstan, U.S.S.R.1 Acetylcholine (ACh), 7.5 x 10-5M, and carbachol, 5 x 10-6M (CCh) depressed the frequency of miniature endplate potentials (m.e.p.ps) in the frog (Rana temporaria) sartorious neuromuscular junction with active acetylcholinesterase to about 50-55% of the controls. 2 A similar depression was produced by the nicotinic agonists, nicotine, suberyldicholine and tetramethylammonium. 3 The muscarinic agonists, oxotremorine, methylfurmethide and methacholine were without effect on m.e.p.p. frequency. The muscarinic antagonist, atropine and the nicotinic antagonist, (+)-tubocurarine, had no effect on the depression of m.e.p.p. frequency evoked by CCh. 4 The ganglionic blockers, benzhexonium and IEM-1119, were also without effect on the CCh-evoked depression of m.e.p.p. frequency. 5 Pretreatment of muscles with anticholinesterases did not prevent the CCh-induced drop in m.e.p.p. frequency. 6 The effect of CCh was proportionally the same as in the controls in preparations where the m.e.p.p. frequency was changed by elevation of K+ and in the presence of theophylline, noradrenaline, dibutyryl adenosine 3': 5'-cyclic monophosphate (db cyclic AMP) and db cyclic GMP.7 An inhibitor of Na+,K+-ATPase, ouabain, 5 x 10-5mollP', prevented or reversed the depression of m.e.p.p. frequency by CCh. However, the depression was present in a nominally K+-free medium. Insulin and adrenaline, which are considered to be Na+,K+-ATPase activators, were without effect on depression of m.e.p.p. frequency. 8 The depression of m.e.p.p. frequency by 5 x 10-6 M CCh was the same at temperatures between 5 and 30°C with a Q10 near to 1.0. When threshold amounts of CCh were used (6 x 10-7 and 3 x 10-7 M), the depression was less at higher temperatures. 9 The receptive structures responsible for the CCh (or ACh)-evoked depression of m.e.p.p. frequency differ pharmacologically from muscarinic, nicotinic ganglionic and neuromuscular junction ACh-receptors as well as from the synaptic cholinesterase, in contrast to previous reports (Duncan & Publicover, 1979). The low temperature-dependence points to the possibility that physical rather than biochemical processes are limiting in this presynaptic effect of cholinomimetics.

The Effect of Carbachol and α‐Bungarotoxin on the Frequency of Miniature Endplate Potentials at the Frog Neuromuscular Junction

“…Subsequent application of CCh led also to a decrease in frequency with good post-washout recovery, in accord with our previous data (Bukharaeva et al 1986;Nikolsky et al 1991a An example of miniature endplate potentials recorded from a single fibre. A: left traces, control; middle traces, 30 min after 1 ² 10¦Ê m á_bungarotoxin (BTX); right traces, 30 min after BTX was washed out and 5 ² 10¦É m carbachol had been added to the Ringer solution (CCh after BTX).…”

“…We demonstrated that CCh decreases the mEPP frequency by 30%, even when Ca¥ was removed from Ringer solution and when Ca¥ was increased to 7 mÒ. (Nikolsky et al 1991a). To compare the CCh-and BTX-induced decrease in spontaneous quantal release, the action of BTX in Ca¥-free solution was tested.…”

“…The decrease in release frequency, which might be as great as 50% of the initial value, is not prevented by the nicotinic acetylcholine receptor antagonist (+)tubocurarine (TC) or the ganglion-affecting drug benzhexonium. This indicates that neither the motor nerve terminal TC-sensitive nicotinic autoreceptor subtype nor the benzhexonium-sensitive autonomic ganglion subtype is involved in the CCh-induced decrease of mEPP frequency (Nikolsky et al 1991a). To examine the properties of the putative receptors at the frog motor nerve terminal, we investigated the possibility that CCh binds to some of the other many subtypes of nicotinic ACh receptors (nAChRs) that are distributed widely throughout the central and peripheral nervous system (Clarke, 1992;Sargent, 1993), namely the group which can bind á_bungarotoxin (BTX).…”

“…The nicotinic (but not muscarinic) agonists acetylcholine (ACh) and its non-hydrolysable analogue carbachol (CCh) and nicotine decrease the frequency of miniature endplate potentials (mEPPs) at the neuromuscular junction of the frog (Duncan & Publicover, 1979;Bukharaeva et al 1986;Nikolsky et al 1991a). The decrease in release frequency, which might be as great as 50% of the initial value, is not prevented by the nicotinic acetylcholine receptor antagonist (+)tubocurarine (TC) or the ganglion-affecting drug benzhexonium.…”

Effects of Carbachol and Adenosine on Neurotransmitter Secretion Induced by Potassium Chloride, Ionomycin,and Sucrose

“…They are released from the motor nerve terminal and perform autoregulation of the neurosecretory apparatus [2,5,6]. However, molecular mechanisms of the inhibitory effect of these neurotransmitters remain unclear.…”