Effects of "non-quantal" acetylcholine on postsynaptic membrane senstitivity: Effects of ouabain,and mimicry by exogenous acetylcholine

The development of postsynaptic potentiation (PSP) and desensitization due to "non-quantal" acetylcholine that occurs when acetylcholinesterase (AChE) is inhibited was studied using the Na,K-ATPase inhibitor, ouabain, to alter (initially increasing, then decreasing) the level of non-quantal acetylcholine secretion, and exogenous acetylcholine. When ouabain increased non-quantal secretion the time constant () of the miniature end-plate current (MEPC) decay increased, i.e., PSP developed. The later the application of ouabain relative to inhibition of AChE, the greater the degree of PSP. During the next phase when non-quantal secretion was inhibited the MEPC time course shortened more rapidly than in the controls, i.e., desensitization occurred. If ouabain abolished non-quantal secretion before AChE had been inhibited did not change, and neither PSP nor desensitization developed. When AChE was not inhibited ouabain had no effect on . When ACh was continuously applied at 20 nmol·liter^–1, similar to the nonquantal concentration, the shortening of slowed down, and the signal amplitude declined more rapidly than in controls. Addition of exogenous ACh (50 nmol·liter^–1) after acceleration of MEPC decay had developed caused to increase to its initial value. The combined appearance of PSP and desensitization during the action of non-quantal ACh, and the sustained desensitization after removal of released ACh from the synaptic cleft are discussed.S. V. Kurashova Institute of Medicine, Russian Federation Ministry of Public Health, Kzan. Translated from Neirofiziologiya, Vol. 24, No. 4, pp. 396–404, July–August, 1992.     

Effects of "non-quantal" acetylcholine on miniature endplate currents in mice

The possibility of postsynaptic potentiation (PSP) and desensitization developing due to nonquantal acetylcholine (ACh) secretion was investigated in mouse diaphragm with reference to time-amplitude relationships of miniature endplate currents (MEPC). The H effect (which characterizes nonquantal secretion (NS) of ACh) fell to zero over 3 h under the action of armine-induced inhibition of acetylcholinesterase (AChE) at a temperature of 20°C. A decline in the decay time constant () of MEPC unaccompanied by observable alteration in MEPC amplitude occurred at the same time. This accelerated decay of MEPC was not observed in the absence of NS (the early stages of denervation). Start of NS did not show any effect on maximum retardation of MEPC decay due to AChE inhibition, indicating that no PSP sets in under the effects of non-quantal secretion. The effect of decline in accelerated with a rise in temperature; it could be reproduced with neostigmine replacing armine, while remained unchanged in the time spells investigated with AChE in its active state. Non-quantal ACh is not thought to produce substantial retardation of MEPC decay, although it does bring about desensitization, signs of which may be partially masked owing to concurrent onset of PSP.S. V. Kurashov Medical Institute, Kazan'. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 507–513, July–August, 1990.     

Electrotropic acetylcholine action on the mammalian auricle by means of the "phase plane" trajectories of the action potential

A single sucrose gap techniques has been used to study action potentials and phase plane trajectories of them in atrial trabeculae of the rabbit. Using polynomial representations of current-voltage relationships a model of membrane action potential of atrial myocardial fibres is described and allows an interpretation of recording data from the phase plane trajectories. Our findings show: 1. Increasing extracellular calcium concentration increases a potassium conductivity of the atrial membrane. 2. An anomalous rectification concerning repolarizing currents in atrial fibres decreases with increasing extracellular calcium. 3. Acetylcholine (3.10(-4) g.cm-3) abolishes the anomalous rectification. These results are discussed in relation to previous electrophysiological studies of negative electrotropic effects of acetylcholine in cardiac muscle.     

Molybdic and tungstic heteropolyanions for "ionic fixation" of acetylcholine in cholinergic motor nerve terminals

The treatment of neuromuscular junctions with phosphomolybdic acid (PMA) and silicotungstic acid (STA) heteropolyanions permits the visualization of electron dense precipitates in the synaptic vesicles of the cholinergic motor nerve terminals. At the light microscopic level, the uncolored molybdenum salt is visualized after reduction to molybdenum blue. The blue coloration is confined to the nerve terminals. Since PMA and STA are known as strong precipitating agents of quaternary ammonium compounds (cations) it is supposed that they have insolubilized in situ the acetylcholine (Ach) of the synaptic vesicles by means of a rapid ionic interaction. Furthermore, in spite of the strong acidity of PMA and STA solutions, the ultrastructure of the treated tissue is not significantly altered but on the contrary seems to be well preserved. The ionic insolubilization of Ach, added to the good preservation of the ultrastructure prompted us to use the term "ionic fixation".     

Habenula "cholinergic" neurons co-release glutamate and acetylcholine and activate postsynaptic neurons via distinct transmission modes

Acetylcholine is an important neurotransmitter, and the habenulo-interpeduncular projection is a major cholinergic pathway in the brain. To study the physiological properties of cholinergic transmission in the interpeduncular nucleus (IPN), we used a transgenic mouse line in which the light-gated cation channel ChannelRhodopsin-2 is selectively expressed in cholinergic neurons. Cholinergic axonal terminals were activated by light pulses, and postsynaptic responses were recorded from IPN neurons. Surprisingly, brief photostimulation produces fast excitatory postsynaptic currents that are mediated by ionotropic glutamate receptors, suggesting wired transmission of glutamate. By contrast, tetanic photostimulation generates slow inward currents that are largely mediated by nicotinic acetylcholine receptors, suggesting volume transmission of acetylcholine. Finally, vesicular transporters for glutamate and acetylcholine are coexpressed on the same axonal terminals in the IPN. These results strongly suggest that adult brain "cholinergic" neurons can corelease glutamate and acetylcholine, but these two neurotransmitters activate postsynaptic neurons via different transmission modes.     

Effects of acetylcholine on the coronary artery

Acetylcholine acts on the different components of the coronary arterial wall by 1) initiating endothelium-dependent relaxation of the smooth muscle cells; 2) inhibiting the exocytotic release of norepinephrine (NE), which could result in either vasodilator or vasoconstrictor effects depending on whether the main action of NE is alpha- or beta-adrenergic, respectively; and 3) activating the contractile process of the smooth muscle cells. These different effects of the cholinergic transmitter are muscarinic in nature. Their relative importance varies among species, or when acetylcholine is given exogenously rather than released from cholinergic nerves.     

Exchangeability of radioactive acetylcholine with the bound acetylcholine of synaptosomes and synaptic vesicles

1. The exchangeability with added radioactive acetylcholine of the acetylcholine in isolated presynaptic nerve terminals (synaptosomes) and isolated synaptic vesicles was studied by a Sephadex-column method. 2. A substantial proportion of the synaptosomal acetylcholine is exchangeable with added radioactive acetylcholine. It is liberated by hypo-osmotic shock and ultrasonic treatment, and behaves as though it occupies the cytoplasmic compartment of synaptosomes. 3. Methods of isolating vesicles from hypo-osmotically ruptured synaptosomes in optimum yield are discussed. 4. The acetylcholine of synaptic vesicles isolated on a sucrose density gradient is released by hypo-osmotic conditions, suggesting that it is enclosed by a semi-permeable membrane; however, it is not easily released by ultrasonic treatment. 5. Added radioactive acetylcholine does not exchange with vesicular acetylcholine under a variety of different conditions. These include addition of ATP and Mg(2+), and pre-loading of the synaptosome with radioactive acetylcholine before hypo-osmotic rupture. This failure to exchange is discussed in terms of the possible storage mechanism of vesicular acetylcholine.     

Effects of inhibitors of acetylcholine synthesis on brain acetylcholine and survival in soman-intoxicated animals

The effects of hemicholinium-3 (HC-3) or 4-(1-naphthylvinyl)pyridine (4-NVP) alone and together with cholinolytics and/or cholinesterase inhibitors on brain acetylcholine (ACh) levels and survival were studied. Intracerebroventricular (ICVT) injection of 10 μg HC-3 280 min before euthanasia by microwave irradiation reduced rat cerebral ACh levels from 28.4 to 5.4 nmoles ACh/g wet tissue. In rats pretreated with HC-3 alone or with other pretreatment drugs prior to giving up to 2.7 LD50 of soman, iv, cerebral ACh levels increased very little, but in animals not receiving HC-3, brain ACh levels increased to 67.1 nmoles. Treatment of unpoisoned rats with 4-NVP resulted in a significant (26%) reduction in ACh. The inclusion of atropine with 4-NVP resulted in a further reduction in ACh. Pretreatment with 4-NVP caused sign-free doses of physostigmine to produce toxic signs in rabbits and did not enhance the efficacy of carbamate pretreatment against soman. Pretreatment of rabbits with pyridostigmine and atropine methyl nitrate (AMN) failed to provide any protection against soman, but when HC-3, ICVT, was included with those drugs, the protective ratio (PR) against soman was increased from 0.8 to 7.3. These data are consistent with the hypothesis that excess ACh is a primary lesion in organophosphorus anticholinesterase intoxication and that the central nervous system is quite sensitive to excesses of ACh.     

CNDO calculations on the conformational structure of acetylcholine

Molecular orbital theory in the CNDO framework has been used to calculate the torsional angles which lead to minimum energy conformations in acetylcholine. The calculated angles agree well with the experimental observations on acetylcholine and its derivatives. The results have been compared with the earlier predictions based on extended Hückel theory and van der Waal pairwise interactions.