Non-quantal acetylcholine release at mouse neuromuscular junction: effects of elevated quantal release and aconitine

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Regulation of acetylcholine release by muscarinic receptors at the mouse neuromuscular junction depends on the activity of acetylcholinesterase

Muscarinic acetylcholine receptors (mAChRs) play an important role in regulating the release of acetylcholine (ACh) in various tissues. We used subtype-specific antibodies and a fluorescent-labelled muscarinic toxin to demonstrate that mammalian neuromuscular junction expresses mAChR subtypes M1 to M4, and that localization of all subtypes is highly restricted to the innervated part of the muscle. To elucidate the roles of the mAChR subtypes regulating ACh release, we measured the mean quantal content of endplate potentials in isolated mouse phrenic--hemidiaphragm preparations in which release was reduced by a low Ca2+/high Mg2+ medium. Muscarine decreased evoked ACh release in normal junctions but, depending on the concentration, reduced or increased transmitter release in collagen Q-deficient junctions completely lacking acetylcholinesterase (AChE). Both effects were also seen in normal junctions when AChE was inhibited by various doses of fasciculin-2. Block of mAChRs by atropine had no effect on evoked release at normal junctions, but decreased release at junctions lacking AChE. The muscarine-elicited depression of ACh release in normal junctions was completely abolished by pertussis toxin or methoctramine pretreatment, but was not affected by muscarinic toxin MT-3, thus indicating the involvement of the M2 mAChR. The muscarine-induced increase of ACh release in AChE-deficient junctions was not affected by pertussis toxin, but was completely blocked by MT-7, a specific M1 mAChR antagonist. Our results show that the M1 and M2 mAChRs have opposite presynaptic functions in modulating quantal ACh release, and that regulation of release by the two receptor subtypes depends on the functional state of AChE at the neuromuscular junction.

Effects of aconitine on neurotransmitter release in the rat neuromuscular junction

Aconitine (ACO), A Na+ channel activator, induces depolarization in skeletal muscle and blocks neuromuscular transmission. We investigated the effects of ACO on neurotransmitter release in the rat isolated phrenic nerve-diaphragm preparation at 24 +/- 1 degrees C. ACO inhibited the twitch responses to nerve stimulation but did not affect direct muscle contractions. ACO, without causing excessive membrane depolarization, increased the frequency of miniature end-plate potential (MEPP)s, but did not alter their amplitude or time course. The increase in MEPP frequency started about 60, 30 and 15 min after the application of 6, 20 and 60 microM ACO, respectively. MEPP frequency reached its maximum (250-400 sec-1), within 10-15 min after it began to increase. ACO, without altering direct muscle action potentials decreased the amplitude and blocked end-plate potential (EPP)s and nerve action potential (NAP)s simultaneously, before the increase in MEPP frequency became evident. ACO did not increase MEPP frequency in Ca(2+)-free media. Prior application of tetrodotoxin (1 microM) inhibited the ACO-induced MEPP frequency increase. Carbamazepine (120 microM) and amiloride (100 microM) did not completely inhibit the MEPP frequency increase but prolonged the latency. ACO-induced alterations in the neuromuscular transmission exhibited minimal recovery upon washing for 2-3 hr. These results indicate that ACO-induced neuromuscular blockade is mainly due to presynaptic mechanisms and can be explained by excessive presynaptic depolarization which leads to the blockade of NAPs and EPPs. Depolarization in turn increases intraterminal Ca2+ concentration and results in an excessive increase in MEPP frequency.

Aconitine-induced increase and decrease of acetylcholine release in the mouse phrenic nerve-hemidiaphragm muscle preparation

The effect of aconitine on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve-diaphragm muscle preparation was studied by a radioisotope method. Both electrical stimulation-evoked release and spontaneous release of 3H-ACh from the preparation preloaded with 3H-choline were measured. The change in the muscle tension was simultaneously recorded in the same preparation. Aconitine (0.1 microM) increased electrically evoked 3H-ACh release, while at higher concentrations (0.3-3 microM) it decreased the evoked release and muscle tension. High concentrations of aconitine (3-30 microM) caused a concentration-dependent increase in spontaneous 3H-ACh release. All these effects were suppressed by tetrodotoxin. The aconitine-induced spontaneous release consisted of two different components: a Ca(2+)-dependent phasic release that was inactivated within a few minutes and a Ca(2+)-independent, long lasting release at a low level. The depression of the Ca(2+)-dependent quantal release seems attributable to the decline of Ca2+ influx into the nerve rather than inactivation of sodium channels. We conclude that aconitine increases and then decreases electrical stimulation-evoked ACh release from the motor nerve through prolonged activation of sodium channels. Further activation of the channels enhances spontaneous release and the subsequent complete inactivation of the quantal release may be due to block of Ca2+ influx.

Quantal and non-quantal ACh release at developing Xenopus neuromuscular junctions in culture

1. Single acetylcholine receptor (AChR) channel openings, detected by the whole-cell patch clamp technique, were used to monitor quantal and non-quantal ACh release at synapses in 1- and 2-day-old co-cultures of Xenopus embryonic motoneurons and muscle cells. motoneuron growth cones in ways that presumably reflect muscle-nerve inductive influences and the development of neurotransmitter release mechanisms. 2. Miniature endplate currents (MEPCs) occurred at a mean frequency of approximately 0.6 s-1 with a skewed distribution and mean amplitude of about twenty channel openings. In addition, occasional brief episodes of rapid deviations in the baseline were observed in some cells, with mean amplitudes of 4-8 pA and durations of a few hundred milliseconds. However, these episodes did not closely resemble summated openings of AChR channels. Moreover, where tested, these episodes were not blocked by curare; and comparable episodes were seen in an uninnervated myocyte. Thus they appear not to reflect ACh release from the nerve terminal. 3. Single-channel openings that might have been responses to non-quantal release of ACh were observed at rates of 0.9-12.3 min-1 (mean 3.0 min-1), only 1-5 times the rate of spontaneous AChR channel openings in uninnervated myocytes (mean 1.4 min-1). 4. We conclude that there is no significant non-quantal ACh leak from the presynaptic contacts in these immature synapses under these culture conditions. This is in disagreement with other, less direct, experimental reports, but consistent with findings in mature frog motor nerve terminals.

Nicotinic agonists antagonize quantal size increases and evoked release at frog neuromuscular junction

1. Previous studies at the frog neuromuscular junction showed that quantal size can be increased two- to fourfold by a variety of treatments, including prior exposure to hypertonic solution (which activates protein kinase A) and insulin (which acts via an unknown pathway). Size increases largely because quanta contain more acetylcholine (ACh). 2. Now the effects of cholinergic agonists on the increases in quantal size have been studied. One muscle from a frog was kept for 2 h in hypertonic sodium gluconate solution. The miniature endplate potential (MEPP) sizes were measured in saline: they had increased about fourfold. The paired muscle went through the same experimental sequence, except that an agonist was added to the hypertonic gluconate solution. Again MEPP sizes were measured in saline. The increase in quantal size was significantly depressed by 0.2 microM 1,1-dimethyl-4-phenyl- piperazinium (DMPP). The sequence of effectiveness of agonists was: DMPP > carbachol > or = ACh = cytisine > oxytremorine. This sequence suggests that the receptor belongs in the nicotinic class. 3. Quantal size is doubled after 1 h in insulin. One micromolar carbachol largely blocked the size increase. 4. The effects of cholinergic antagonists were tested by keeping the experimental muscle in hypertonic gluconate solution containing 1 microM carbachol plus an antagonist. The controls were paired muscles kept in hypertonic gluconate solution (without carbachol or antagonist). MEPP sizes were measured in saline. The depressing action of carbachol on the increase in MEPP size was blocked by 0.2 microM neuronal-bungarotoxin (nBTX). The sequence of effectiveness of antagonists was: nBTX > trimethaphan > d-tubocurarine (dTC). Ten micromolar atropine (without carbachol) depressed the increase in quantal size. Therefore, the antagonist potency of atropine could not be adequately tested. Carbachol action was not blocked by 10 microM hexamethonium or 10 microM mecamylamine. 5. Once quanta are made large they can be converted back to normal size by cholinergic agonists. Muscles in which quantal size had been enlarged were exposed to hypertonic solutions containing the agonist. Quantal size was reduced to a fraction of its former value when the hypertonic solution contained 1 microM carbachol- or 1 microM DMPP. One micromolar oxytremorine had no effect. Carbachol still reduced quantal size when applied in low-Ca2+ solutions, so it does not appear to act by elevating intracellular [Ca2+]. 6. Previous work suggested that the treatments produce a subpopulation of large quanta that are positioned for release.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)