Calcium Transients and Transmitter Secretion in Different Parts of Frog Nerve Endings in Different Conditions of Calcium Ion Influx

Neuroscience and Behavioral Physiology - Experiments on frog neuromuscular preparations were performed to study the characteristics of the calcium response and the quantum secretion of...     

Effect of cholinergic compounds on spontaneous quantal mediator release at the neuromuscular synapse of the frog

The action of cholinergic drugs on spontaneous quantal transmitter release has been investigated in the frog sartorius muscle. Acetylcholine and carbacholine decreased the miniature end-plate potential frequency. These presynaptic effects had no dependence on the potassium concentration in the bath solution. Nicotinic agonists--nicotine, tetramethylammonium and suberyldicholine had a similar effect, while muscarinic agents--methylfurmetide, oxotremorine and F-2268 (L- and D-stereoisomers) did not affect the transmitter release. The presynaptic effects of carbacholine and acetylcholine were abolished neither by atropine nor by d-tubocurarine and bensohexonium. It is suggested that there are nicotinic receptors on the frog motor nerve terminals that modify spontaneous quantal transmitter release and differ pharmacologically from nicotinic end-plate, ganglionic and presynaptic receptors of higher vertebrates.     

The effect of carbacholine on spontaneous quantum secretion of a mediator from frog motor nerve endings in the presence of ouabain and in a potassium-free medium

The carbacholine (Cch) effect on the frequency of miniature end-plate potentials (MEPP) has been studied in the frog muscle in the presence of ouabain and in the potassium-free solution. The depressing presynaptic effect of Cch on the MEPP frequency was eliminated by ouabain, whereas the inactivation of ATPase by the potassium-free solution did not affect the degree of the MEPP frequency decrease. The possibility of the spontaneous quantal release regulation in cholinomimetic drugs by means of the ouabain-sensitive mechanism not related to the pumping function of the Na+, K+ and ATPase active transport is discussed.     

The Effects of Carbachol on the Proximal and Distal Parts of Frog Motor Nerve Endings

Cholinomimetics not only activate postsynaptic cholinoreceptors in neuromuscular synapses, but also alter the process of acetylcholine secretion from nerve endings. However, the mechanism of action of cholinomimetics on the secretory process remains unidentified. We approached the question of the mechanism of the presynaptic action of cholinomimetics in the present study by investigating the effects of the n,m-cholinomimetic carbachol on nerve ending currents and postsynaptic membrane currents. Carbachol induced decreases in the postsynaptic response, without affecting the duration and amplitude of the nerve ending current in both the central and distal part of the nerve ending. However, carbachol increased the time between the arrival of the presynaptic action potential and the start of transmitter secretion. This effect on synaptic delay was more marked in the distal parts of the ending. The action of another potential modulator, extracellular potassium, was accompanied by decreases in presynaptic currents and also by increases in synaptic delay. These data provide evidence for the suppressive effect of carbachol on acetylcholine secretion acting via presynaptic metabotropic cholinoreceptors which control the level and time course of secretion of neurotransmitter quanta.     

Reactivating and cholinolytic action of dipyroxime in the myoneural synapse of warm-blooded animals

Reactivating (RA) and cholinolytic (Chl) effects of dipyroxime in soleus and diaphragm muscles of the rat were estimated by amplitudes and durations of miniature end-plate potentials (MEPP) and currents (MEPC). After armin-induced inhibition of acetylcholinesterase (AchE), the action of dipyroxime in concentrations of 5.10(-6)-5.10(-4) mol/l on the amplitude and duration of MEPPs and MEPCs represents the combination of RA and Chl effects. Separate study of the RA effect (after washing out of the reactivator) has shown that this effect increased within the whole range of concentrations used. Complete reactivation of phosphorylated AchE was achieved at dipyroxime concentrations of 2-5.10(-4) mol/l. Separate analysis of Chl effect (in voltage-clamp experiments with intact AchE) has shown that dipyroxime suppressed acetylcholine-induced responses due to a block of cholinoreceptors in the open conformation. This block was characterized by prolongation of MEPCs decay without affecting its exponential nature. It is concluded that dipyroxime is a "very fast blocker".     

Effect of tetraalkylammonium derivative of 6-methyluracil on amplitude and temporal parameters of miniature endplate potentials in frog neuromuscular junction

The effect of C-547, a tetraalkylammonium derivative of 6-methyluracil, a novel highly selective acetylcholinesterase inhibitor, on frog neuromuscular junction was studied. In concentrations 10^-9-10^-7 M the preparation increased the amplitude and temporal parameters of miniature endplate potentials. In contrast to the effect of C-574 on purified acetylcholinesterase from mammals, the effect of this agent on frog neuromuscular junction was reversible. In a concentration of 10^-6 M the preparation apart from anticholinesterase activity produced a parasympatholytic effect manifested in a decrease in amplitude and decay time constant of miniature endplate potentials accompanied by a decrease in spontaneous transmitter secretion. After washout, the parasympatholytic effect recovered more slowly, but disappeared more rapidly compared to anticholinesterase activity. These findings suggest that parasympatholytic effect of C-547 results from direct action on receptor-channel complexes in the endplate membrane.     

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

1. Acetylcholine (ACh), 7.5 x 10(-5) M, and carbachol, 5 x 10(-6) M (CCh) depressed the frequency of miniature endplate potentials (m.e.p.ps) in the frog (Rana temporaria) sartorius 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(-5) mol l-1, 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 degrees C with a Q10 near to 1.0.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synchronization of evoked secretion of quanta of mediator as a mechanism facilitating the action of sympathomimetics

Experiments on frog neuromuscular junction preparations with extracellular recording of nerve terminal action potentials and single-quantum end-plate currents (EPC) were used to assess the time course of evoked quantum secretion of mediator by analyzing histograms of the distribution of true synaptic delays. These studies showed that noradrenaline, isoproterenol, and dobutamine change the kinetics of secretion of quanta, leading to synchronization of the process of mediator release; substances blocking β-adrenoceptors (atenolol, propranolol) blocked this effect. Clonidine and phenylephrine, which activate α-receptors, had no effect on the kinetics of secretion, while the α-blocker phentolamine had no effect on the synchronizing action of noradrenaline. Reconstruction of multiquantum EPC from changes in the level of synchronization in the release of individual quanta, showed that EPC amplitude increased in response to noradrenaline by 17%, and that this was due only to alterations in the time course of secretion. These data led to the conclusion that these is a special presynaptic mechanism which facilitates the action of sympathomimetics, acting via β-adrenoceptors. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 84, No. 10, pp. 1121–1131, October, 1998.     

Changes in the Kinetics of Evoked Secretion of Transmitter Quanta – an Effective Mechanism Modulating the Synaptic Transmission of Excitation

The main presynaptic mechanisms of synaptic plasticity are generally believed to consist of changes in the numbers of neurotransmitter quanta released in response to a nerve spike (the quantum composition of postsynaptic responses) and quantum size. However, studies in recent years have demonstrated the existence of a further, previously unconsidered but effective mechanism modulating the synaptic transmission of excitation, which is associated with changes in the secretion time course (kinetics) of the release of the individual quanta forming the multiquantum postsynaptic response. This review discusses current data (including our own results) on the kinetics of the evoked release of neurotransmitter quanta from motor nerve endings in peripheral synapses, the mechanisms of its modulation, and quantitative methods for its analysis.