Surplus acetylcholine and acetylcholine release in the rat diaphragm

L-citrulline inhibits [3H]acetylcholine release from rat motor nerve terminals by increasing adenosine outflow and activation of A1 receptors

BACKGROUND AND PURPOSE

Nitric oxide (NO) production and depression of neuromuscular transmission are closely related, but little is known about the role of L-citrulline, a co-product of NO biosynthesis, on neurotransmitter release.

EXPERIMENTAL APPROACH

Muscle tension recordings and outflow experiments were performed on rat phrenic nerve-hemidiaphragm preparations stimulated electrically.

KEY RESULTS

L-citrulline concentration-dependently inhibited evoked [(3)H]ACh release from motor nerve terminals and depressed nerve-evoked muscle contractions. The NO synthase (NOS) substrate, L-arginine, and the NO donor, 3-morpholinosydnonimine chloride (SIN-1), also inhibited [(3)H]ACh release with a potency order of SIN-1>L-arginine>L-citrulline. Co-application of L-citrulline and SIN-1 caused additive effects. NOS inactivation with N(omega)-nitro-L-arginine prevented L-arginine inhibition, but not that of L-citrulline. The NO scavenger, haemoglobin, abolished inhibition of [(3)H]ACh release caused by SIN-1, but not that caused by L-arginine. Inactivation of guanylyl cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) fully blocked SIN-1 inhibition, but only partially attenuated the effects of L-arginine. Reduction of extracellular adenosine accumulation with adenosine deaminase or with the nucleoside transport inhibitor, S-(p-nitrobenzyl)-6-thioinosine, attenuated the effects of L-arginine and L-citrulline, while not affecting inhibition by SIN-1. Similar results were obtained with the selective adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. L-citrulline increased the resting extracellular concentration of adenosine, without changing that of the adenine nucleotides.

CONCLUSIONS AND IMPLICATIONS

NOS catalyses the formation of two neuronally active products, NO and L-citrulline. While, NO may directly reduce transmitter release through stimulation of soluble guanylyl cyclase, the inhibitory action of L-citrulline may be indirect through increasing adenosine outflow and subsequently activating inhibitory A(1) receptors.

Blockade of neuronal facilitatory nicotinic receptors containing alpha 3 beta 2 subunits contribute to tetanic fade in the rat isolated diaphragm

Nicotinic receptor (nAChR) subtypes involved in pre- and postjunctional actions underlying tetanic fade were studied in rat phrenic-nerve hemidiaphragms. We investigated the ability of subtype-specific nAChR antagonists to depress nerve-evoked contractions and [(3)H]-acetylcholine ([(3)H]-ACh) release. Muscle tension was transiently increased during brief high frequency trains (50 Hz for 5 sec). The rank potency order of nAChR antagonists to reduce tetanic peak tension was alpha-bungarotoxin > d-tubocurarine >> mecamylamine > hexamethonium. Reduction of maximal tetanic tension produced by dihydro-beta-erythroidine (0.03-10 microM), methyllycaconitine (0.003-3 microM), and alpha-conotoxin MII (0.001-0.3 microM) did not exceed 30%. Besides reduction of peak tension d-tubocurarine (0.1-0.7 microM), mecamylamine (0.1-300 microM), and hexamethonium (30-3,000 microM) also caused tetanic fading. With alpha-conotoxin MII (0.001-0.3 microM) and dihydro-beta-erythroidine (0.03-10 microM), tetanic fade was evident only after decreasing the safety factor of neuromuscular transmission (with high magnesium ions, 6-7 mM). The antagonist rank potency order to reduce evoked (50 Hz for 5 sec) [(3)H]-ACh release from motor nerve terminals was alpha-conotoxin MII (0.1 microM) > dihydro-beta-erythroidine (1 microM) approximately d-tubocurarine (1 microM) > mecamylamine (100 microM) > hexamethonium (1,000 microM). When applied in a concentration (0.3 microM) above that producing tetanic paralysis, alpha-bungarotoxin failed to affect [(3)H]-ACh release. Data obtained suggest that postjunctional neuromuscular relaxants interact with alpha-bungarotoxin-sensitive nicotinic receptors containing alpha1-subunits, whereas blockade of neuronal alpha3beta2-containing receptors produce tetanic fade by breaking nicotinic autofacilitation of acetylcholine release.

Stimuli that induce a cholinergic neuronal phenotype of NG108-15 cells upregulate ChAT and VAChT mRNAs but fail to increase VAChT protein

The vesicular acetylcholine transporter (VAChT) and choline acetyltransferase (ChAT) are encoded by genes organized in a single gene locus, and coregulation of the transcription of the two genes has been repeatedly reported in cholinergic tissues. In the present study, different stimuli were used to induce the differentiation of the hybridoma cells NG108-15 and we examined their effects on the modulation of VAChT and ChAT expression at the mRNA and protein levels. All agents upregulated the VAChT and ChAT mRNA levels, but to a different extent. ChAT activity was increased by retinoic acid, dexamethasone, and dibutyrylcyclic AMP (dbcAMP), and a synergistic effect was observed with a combined dexamethasone and dbcAMP treatment. Nonetheless, no changes in the VAChT protein level could be observed, as judged from ligand binding studies as well as from immunochemical detection. Hemicholinium-3-sensitive choline uptake, hemicholinium-3 binding, and acetylcholine content were increased by differentiating agents, with a rank order of potency comparable to their effects on ChAT activity. Prominent changes were observed in the expression of vesicular protein markers, particularly with the associated treatment dexamethasone and dbcAMP. Thus, it appears that although the different stimuli we have been using are able to stimulate neuronal features and activate the transcription of cholinergic genes, they did not contrive to increase the level of VAChT protein in these cells.

Influence of stimulation on Ca(2+) recruitment triggering [3H]acetylcholine release from the rat motor-nerve endings

The influence of rat phrenic nerve stimulation frequency (5-50 Hz) and of pulse duration (0.04-1 ms) on Ca(2+) mobilization triggering [3H]acetylcholine release was investigated. The P-type voltage-dependent Ca(2+) channel (VDCC) blocker, omega-agatoxin IVA (100 nM), decreased [3H]acetylcholine release evoked by pulses of 0. 04-ms duration delivered at 5 Hz frequency. When the stimulus pulse duration was increased to 1 ms (5 Hz frequency) or the stimulation frequency to 50 Hz (0.04-ms duration), inhibition of [3H]acetylcholine release became evident after blockade of L-type VDCC, with nifedipine (1 microM), and/or depletion of thapsigargin-sensitive internal stores. The inhibitory effect of thapsigargin (2 microM) was still observed in Ca(2+)-free medium. Neither omega-conotoxin GVIA (1 microM) nor omega-conotoxin MVIIC (150 nM) modified neurotransmitter release. The results suggest that, depending on the stimulus paradigm, both internal (thapsigargin-sensitive) and external (either P- or L-type channels) Ca(2+) pools can be mobilized to promote acetylcholine release from motor nerve terminals.

A(2A) adenosine receptor facilitation of neuromuscular transmission: influence of stimulus paradigm on calcium mobilization

The influence of stimulus pulse duration on calcium mobilization triggering facilitation of evoked [(3)H]acetylcholine ([(3)H]ACh) release by the A(2A) adenosine receptor agonist CGS 21680C was studied in the rat phrenic nerve-hemidiaphragm. The P-type calcium channel blocker omega-agatoxin IVA (100 nM) decreased [(3)H]ACh release evoked with pulses of 0.04-ms duration, whereas nifedipine (1 microM) inhibited transmitter release with pulses of 1-ms duration. Depletion of intracellular calcium stores by thapsigargin (2 microM) decreased [(3)H]ACh release evoked by pulses of 1 ms, an effect observed even in the absence of extracellular calcium. With short (0.04-ms) stimulation pulses, when P-type calcium influx triggered transmitter release, facilitation of [(3)H]ACh release by CGS 21680C (3 nM) was attenuated by both thapsigargin (2 microM) and nifedipine (1 microM). With longer stimuli (1 ms), a situation in which both thapsigargin-sensitive internal stores and L-type channels are involved in ACh release, pretreatment with either omega-agatoxin IVA (100 nM) or nifedipine (1 microM) reduced the facilitatory effect of CGS 21680C (3 nM). The results suggest that A(2A) receptor activation facilitates ACh release from motor nerve endings through alternatively mobilizing the available calcium pools (thapsigargin-sensitive internal stores and/or P- or L-type channels) that are not committed to the release process in each stimulation condition.

Synthesis and release of an acetylcholine-like compound by human myoblasts and myotubes

1. Exogenously applied acetylcholine (ACh) is a modulator of human myoblast fusion. Using a chemiluminescent method, we examined whether an endogenous ACh-like compound (ACh-lc) was present in, and released by, pure human myogenic cells. 2. Single, freshly isolated satellite cells and proliferating myoblasts contained 15 and 0.5 fmol ACh-lc, respectively. Cultured myotubes contained ACh-lc as well. Also, ACh-like immunoreactivity was detected in all myogenic cells. 3. Part of the ACh-lc was synthesized by choline acetyltransferase (ChAT), as indicated by the reduction of ACh-lc content when bromoACh was present in the culture medium, and by direct measurements of ChAT activity. Also, ChAT-like immunoreactivity was observed in all myogenic cells. 4. Myoblasts and myotubes released ACh-lc spontaneously by a partially Ca(2+)-dependent mechanism. 5. The application by microperfusion of medium conditioned beforehand by myoblasts (thus presumably containing ACh-lc) onto a voltage-clamped myotube induced inward currents resembling ACh-induced currents in their kinetics, reversal potential, and sensitivity to nicotinic antagonists. 6. In vitro, the spontaneously released ACh-lc promoted myoblast fusion but only in the presence of an anticholinesterase. 7. Our observations indicate that human myogenic cells synthesize and release an ACh-lc and thereby promote the fusion process that occurs in muscle during growth or regeneration.

Suppression by cholinesterase inhibition of a Ca(2+)-independent efflux of [3H]acetylcholine from the neuromuscular junction of the isolated rat diaphragm

Endplate preparations of the left rat hemidiaphragm were incubated with [3H]choline to label neuronal acetylcholine stores. Elevation of the concentration (13.5-135 mmol/l) of extracellular potassium chloride (KCl) stimulated the release of [3H]acetylcholine in a concentration-dependent manner. KCl (27 mmol/l) still caused a significant efflux of [3H]acetylcholine in a Ca(2+)-free medium. Inhibitors of cholinesterase (physostigmine, diisopropylfluorophosphate) suppressed by 80% this Ca(2+)-independent efflux of [3H]acetylcholine. Vesamicol (10 mumol/l), the blocker of the vesicular acetylcholine carrier, also suppressed the stimulated, Ca(2+)-independent efflux of [3H]acetylcholine. The inhibitory effect of physostigmine was not prevented by muscarine or nicotine receptor antagonists, but the inhibitory effect was lost when the stimulus strength was increased (81 mmol/l KCl). The present experiments showed cholinesterase inhibition to suppress a Ca(2+)-independent efflux of [3H]acetylcholine, probably by interference with a membrane-bound acetylcholine carrier.

The synthesis and release of acetylcholine by depolarized hippocampal slices is increased by increased choline available in vitro prior to stimulation

The objective of these experiments was to determine whether preincubating hippocampal slices with choline provides precursor that can be used during a subsequent incubation to support or enhance the synthesis of acetylcholine (ACh). Slices were preincubated for 60 min with 0, 10, 25, or 50 microM choline, washed, resuspended, and then incubated for 10 min in choline-free buffer containing 4.74 (Krebs-Ringer bicarbonate, KRB) or 25 mM KCl. The tissue contents of ACh and choline were determined prior to and after the preincubation, as well as after the incubation; the amounts of ACh and choline released were measured, and ACh synthesis was calculated. Preincubation in the absence of choline increased the tissue content of ACh to 242% of original levels; preincubation with 10 microM choline did not lead to a further increase, but preincubation with 25 or 50 microM choline increased the ACh content to 272% of original levels, significantly greater than that of slices preincubated with either 0 or 10 microM choline. When tissues were subsequently incubated for 10 min with either KRB or 25 mM KCl, ACh release from slices preincubated with 50 microM choline was greater than from slices preincubated with 0, 10, or 25 microM choline. Incubation of slices with KRB did not alter the tissue content of ACh, but when tissues were incubated with 25 mM KCl, the ACh content of slices preincubated with 0 or 10 microM choline decreased significantly, whereas that of slices preincubated with 25 or 50 microM choline did not.(ABSTRACT TRUNCATED AT 250 WORDS)

A non-immunogenic myasthenia gravis model and its application in a study of transsynaptic regulation at the neuromuscular junction

A non-immunological model for myasthenia gravis was developed in rats: 'toxin-induced myasthenia gravis'. Rats were injected once every 48 h with 3-5 micrograms alpha-bungarotoxin for periods of up to 5 weeks. This treatment caused weakness, especially of facial muscles. Respiration, however, was unaffected. Miniature endplate potentials and 125I-alpha-bungarotoxin binding in the extensor digitorum longus muscles were severely reduced. Acetylcholine release evoked by electrical and chemical (50 mM KCl) stimulation was higher in diaphragms from alpha-bungarotoxin-treated rats than in those from control animals. Histological investigation of the tibialis anterior muscle provided no evidence that the endplates were enlarged. It is concluded that the activity of acetylcholine receptors influences the rate of transmitter release in the neuromuscular junction and it is suggested that a transsynaptic regulation process may be active in myasthenia gravis. The present animal model for myasthenia gravis seems very suitable for studying such a regulation of transmitter release.

The dependence of non-quantal acetylcholine release on the choline-uptake system in the mouse diaphragm

The time course of local end-plate hyperpolarization after d-tubocurarine application measured by an intracellular microelectrode was followed in vitro in anticholinesterase-treated mouse diaphragm pinned to the bottom of the perfusion chamber. The d-tubocurarine-induced hyperpolarization, which served as an indicator of non-quantal acetylcholine release, started to decline from 6 mV after 1 h and was negligible after 3 h in continuously perfused preparations. This decline was slowed down by 10 mumol l-1 choline and almost completely prevented by long-term nerve stimulation with a frequency of 3 Hz. The rapid decrease of the d-tubocurarine-induced hyperpolarization was observed within 10-15 min after the application of 1 mumol l-1 hemicholinium-3 and substitution of lithium for sodium. Both these procedures inhibit the fast choline uptake into nerve terminals. Our data suggest that the amount of available acetylcholine for non-quantal release is proportional to the rate of its synthesis and to the number of available carriers in the nerve terminals. Some of our observations might also be explained by postulating that the choline-uptake system as such is responsible for the non-quantal release.

The effects of nerve terminal activity on non-quantal release of acetylcholine at the mouse neuromuscular junction

1. Local endplate depolarization induced by anticholinesterase application to mouse nerve-diaphragm preparations was taken as a measure of non-quantal release of acetylcholine. 2. Non-quantal acetylcholine release occurred within 20-60 s after anticholinesterase application, either spontaneously or evoked by nerve stimulation. Non-quantal release declined with time and disappeared after 3-5 min. 3. The amplitude of stimulation-evoked non-quantal release increased with the frequency of stimulation and was maximal at frequencies above 50 Hz. Two stimuli were sufficient to evoke the maximal effect. 4. Micromolar concentrations of atropine, pirenzepine and vesamicol reduced the amplitude and shortened the duration of non-quantal release. Oxotremorine (10(-8) M) enhanced the amplitude and ouabain (10(-4) M) prolonged the duration of non-quantal release. 5. Our results support the idea that the non-quantal release is due to the vesicular acetylcholine transport system which becomes transiently a part of the nerve terminal during exocytotic release of quantal acetylcholine.

Direct measurement of ACh release from exposed frog nerve terminals: constraints on interpretation of non-quantal release

1. Acetylcholine (ACh) release from enzymatically exposed frog motor nerve terminals has been measured directly with closely apposed outside-out clamped patches of Xenopus myocyte membrane, rich in ACh receptor channels. When placed close to the synaptic surface of the terminal, such a membrane patch detects both nerve-evoked patch currents (EPCs) and spontaneous quantal 'miniature' patch currents (MPCs), from a few micrometres length of the terminal, in response to ACh release from the nearest three to five active zones. 2. Chemical measurements of ACh efflux from whole preparations revealed a spontaneous release rate of 4.1 pmol (2 h)-1, and no significant difference in resting efflux between enzyme-treated and control preparations. The ratio of enzyme-treated to contralateral control muscle efflux averaged 1.17, indicating that enzyme treatment did not affect spontaneous ACh release. Vesamicol (1.7 microM), which blocks the ACh transporter in synaptic vesicles, decreased the spontaneous release of ACh to 67% of control. 3. In the absence of nerve stimulation, the frequency of single-channel openings recorded by outside-out patch probes adjacent to nerve terminals was very low (1-2 min-1), and little different at a distance of hundreds of micrometres, suggesting that if ACh was continually leaking from the terminal in a non-quantal fashion, the amount being released near active zone regions on the terminal was below the limit of detection with the patches. 4. Direct measurements of the sensitivity of the patches, coupled with calculated ACh flux rates, lead to the conclusion that the amount of ACh released non-quantally from the synaptic surface of the frog nerve terminal is less than one-tenth the amount expected if all non-quantal release is from this region of the terminal membrane. 5. Following a series of single nerve shocks or a 50 Hz train of nerve stimuli, the frequency of asynchronous single-channel openings increased for several seconds. This transient increase in channel openings was not sensitive to movement of the patch electrode a significant distance (4 microns) away from the active sites, or to manipulations previously reported to block non-quantal transmitter leakage, including addition of 10 mM-Ca2+ or 1.7 microM-vesamicol to the bath. These channel openings appear to be due to an accumulation of ACh which originated from many evoked quanta, and not the effect of locally increased non-quantal ACh release due to nerve stimulation. 6. We conclude that transmitter leakage at adult frog terminals is either localized to a source other than the synaptic surface of the nerve terminal, or released in a widespread and diffuse fashion from many sources, which may include the nerve terminal.

Accumulation of extracellular calcium at the endplate of mouse diaphragm after ecothiopate in vitro

1. Experiments were carried out to investigate the accumulation from the extracellular medium of 45Ca2+ by the endplate region of skeletal muscle. 2. Mouse diaphragm muscle was incubated in physiological saline labelled with 45Ca at 37 degrees C for periods of up to 1.5 h. 3. The muscle was divided into junctional and non-junctional portions and the Ca from the extracellular fluid accumulated at the endplate determined from the 45Ca content of the portions. 4. The accumulation of extracellular Ca at the endplate region of muscles incubated in pysiological saline alone was nil, but there was accumulation in the presence of the anticholinesterase ecothiopate iodide 0.5 x 10(-6) M (ECO). Stimulation of the phrenic nerve at 0.02 Hz caused no further increase in accumulation but reduced the amount of spontaneous fasciculation. In tetrodotoxin (TTX) 10(-6) M, the accumulation was halved, and in 3.5 mM Mg2+ the accumulation was nil. Carbachol 10(-4) M resulted in an accumulation of Ca similar to that in ECO. 5. It is concluded that there was an accumulation of extracellular Ca following excitation of the nerve by stimulation at a low frequency and during the spontaneous fasciculations, and about half of the accumulation of extracellular Ca after ECO in the experiments was due to the postsynaptic action of ACh released non-quantally from the nerve terminals.

Control of transmitter release from the motor nerve by presynaptic nicotinic and muscarinic autoreceptors

Until recently, release studies have failed to indicate the existence of autoreceptors on motor nerves. Ignaz Wessler now reports on a refinement of the technique - the measurement of newly synthesized [3H]acetylcholine released from the phrenic nerve - which provides clear evidence in support of release-modulating autoreceptors. Presynaptic nicotinic receptors mediate a positive feedback mechanism, can rapidly be desensitized and appear to differ in their pharmacological profile from the postsynaptic receptors. In addition, inhibitory and facilitatory muscarinic receptors appear to be involved in the presynaptic control of transmitter release from the phrenic nerve.

Constraints on the interpretation of nonquantal acetylcholine release from frog neuromuscular junctions

In the frog nerve-muscle preparation, there is evidence for nonquantal release of acetylcholine (ACh) at a level 100 times that attributable to spontaneous quantal events (miniature endplate potentials). It is widely assumed that nonquantal release occurs near the sites of quantal release (active zones) in the nerve terminal, close to the postsynaptic muscle membrane. This high level of nonquantal ACh release has led to the suggestion that it may serve a trophic function at the nerve-muscle junction. However, the precise origin and mechanism of nonquantal release have not been determined. We have used outside-out patches of ACh receptor-rich membrane as a sensitive technique for the direct measurement of ACh release from highly localized regions of enzymatically treated nerve terminals and have found little detectable ACh "leakage" from active-zone regions. If all of the nonquantal ACh release were localized to the under surface of the nerve terminal, we estimate that we would have detected more than 10 times the low level detected with our patch probe. Furthermore, although quantal release was easily measurable, vesicular exocytosis (hypothesized to insert ACh transport proteins into the plasma membrane, thereby producing the leak) did not increase single-channel activity in the patch probe above that attributable to quantal release. We conclude that, at rest, the active-zone region of nerve terminals is not a major source of nonquantal ACh release and that vesicular exocytosis does not noticeably increase the level of nonquantal release from the nerve terminal. Thus, with biochemical measurements that indicate that spontaneous ACh release is relatively unchanged by prior denervation, these results question the assumed source and mechanism of nonquantal release and the suggestion that leakage of ACh from active-zone regions plays a trophic role in nerve-muscle interaction.

Acetylcholine and choline in rat adrenals and brain cortex prisms incubated at elevated concentrations of choline in the medium

Experiments were performed with rat adrenals and brain cortex prisms incubated in vitro in order to clarify whether it is possible to increase their acetylcholine (ACh) content by adding a high concentration of choline to the medium and whether the additional ACh formed can be released by subsequent depolarization. After 60 min incubation with 0.5 mmol/l choline, the concentration of ACh in the adrenals was increased by 116% (compared to the incubation without added choline), while in cortical prisms the observed increase (by 37%) was statistically non-significant. The content of ACh in both tissues was raised by paraoxon during incubations without added choline, but paraoxon did not augment the increased concentration of ACh in tissues incubated with added 0.5 mmol/l choline. The ACh that accumulated in the adrenals during 60 min preincubations with added choline could be released during subsequent depolarizing incubations; the release was Ca2+ independent. In contrast to brain cortex prisms and to the adrenals preincubated without choline, no resynthesis of ACh occurred during the period of depolarization in the adrenals preincubated with 0.5 mmol/l choline. Large amounts of choline accumulated in both tissues during incubations with 0.5 mmol/l choline and the accumulated choline could be released by depolarization; the release of choline from the adrenals was Ca2+ independent. Free choline was produced in the adrenals (presumably from choline esters) during the periods of depolarization. The reason for differences between the effects of increased concentrations of choline on ACh in the adrenals and in brain cortex is not known.(ABSTRACT TRUNCATED AT 250 WORDS)