Acetylcholine at motor nerves: storage, release, and presynaptic modulation by autoreceptors and adrenoceptors

Adrenoceptors Modulate Cholinergic Synaptic Transmission at the Neuromuscular Junction

Adrenoceptor activators and blockers are widely used clinically for the treatment of cardiovascular and pulmonary disorders. More recently, adrenergic agents have also been used to treat neurodegenerative diseases. Recent studies indicate a location of sympathetic varicosities in close proximity to neuromuscular junctions. The pressing question is whether there could be any effects of endo- or exogenous catecholamines on cholinergic neuromuscular transmission. It was shown that the pharmacological stimulation of adrenoceptors, as well as sympathectomy, can affect both acetylcholine release from motor nerve terminals and the functioning of postsynaptic acetylcholine receptors. In this review, we discuss the recent data regarding the effects of adrenergic drugs on neurotransmission at the neuromuscular junction. The elucidation of the molecular mechanisms by which the clinically relevant adrenomimetics and adrenoblockers regulate quantal acetylcholine release from the presynaptic nerve terminals and postsynaptic sensitivity may help in the design of highly effective and well-tolerated sympathomimetics for treating a number of neurodegenerative diseases accompanied by synaptic defects.

Sympathomimetics regulate quantal acetylcholine release at neuromuscular junctions through various types of adrenoreceptors

The studies of the interaction between the sympathetic and motor nervous systems are extremely relevant due to therapy for many neurodegenerative and cardiovascular disorders involving adrenergic compounds. Evidences indicate close contact between sympathetic varicosities and neuromuscular synapses. This raises questions about the effects of catecholamines on synaptic transmission. The currently available information is contradictory, and the types of adrenoreceptors responsible for modulation of neurotransmitter release have not been identified in mammalian neuromuscular synapses. Our results have shown that the α1A, α1B, α2A, α2B, α2C, and β1 adrenoreceptor subtypes are expressed in mouse diaphragm muscle containing neuromuscular synapses and sympathetic varicosities. Pharmacological stimulation of adrenoreceptors affects both spontaneous and evoked acetylcholine quantal secretion. Agonists of the α1, α2 and β1 adrenoreceptors decrease spontaneous release. Activation of the α2 and β1 adrenoreceptors reduces the number of acetylcholine quanta released in response to a nerve stimulus (quantal content), but an agonist of the β2 receptors increases quantal content. Activation of α2 and β2 adrenoreceptors alters the kinetics of acetylcholine quantal release by desynchronizing the neurosecretory process. Specific blockers of these receptors eliminate the effects of the specific agonists. The action of blockers on quantal acetylcholine secretion indicates possible action of endogenous catecholamines on neuromuscular transmission. Elucidating the molecular mechanisms by which clinically utilized adrenomimetics and adrenoblockers regulate synaptic vesicle release at the motor axon terminal will lead to the creation of improved and safer sympathomimetics for the treatment of various neurodegenerative diseases with synaptic defects.

Nicotinic α7 receptor-induced adenosine release from perisynaptic Schwann cells controls acetylcholine spillover from motor endplates

Acetylcholine (ACh) spillover from motor endplates occurs after neuronal firing bursts being potentiated by cholinesterase inhibitors (e.g., neostigmine). Nicotinic α7 receptors (α7nAChR) on perisynaptic Schwann cells (PSCs) can control ACh spillover by unknown mechanisms. We hypothesized that adenosine might be the gliotransmitter underlying PSCs-nerve terminal communication. Rat isolated hemidiaphragm preparations were used to measure (1) the outflow of [³ H]ACh, (2) real-time transmitter exocytosis by video-microscopy with the FM4-64 fluorescent dye, and (3) skeletal muscle contractions during high-frequency (50 Hz) nerve stimulation bursts in the presence of a selective α7nAChR agonist, PNU 282987, or upon inhibition of cholinesterase activity with neostigmine. To confirm our prediction that α7nAChR-mediated effects require direct activation of PSCs, we used fluorescence video-microscopy in the real-time mode to measure PNU 282987-induced [Ca²⁺ ]_i transients from Fluo-4 NW loaded PSCs in non-stimulated preparations. The α7nAChR agonist, PNU 282987, decreased nerve-evoked diaphragm tetanic contractions. PNU 282987-induced inhibition was mimicked by neostigmine and results from the reduction of ACh exocytosis measured as decreases in [³ H]ACh release and FM4-64 fluorescent dye unloading. Methyllycaconitine blockage of α7nAChR and the fluoroacetate gliotoxin both prevented inhibition of nerve-evoked ACh release and PSCs [Ca²⁺ ]_i transients triggered by PNU 282987 and neostigmine. Adenosine deamination, inhibition of the ENT1 nucleoside outflow, and blockage of A₁ receptors prevented PNU 282987-induced inhibition of transmitter release. Data suggest that α7nAChR controls tetanic-induced ACh spillover from the neuromuscular synapse by promoting adenosine outflow from PSCs via ENT1 transporters and retrograde activation of presynaptic A₁ inhibitory receptors.

Interactions between glycopyrronium and indacaterol on cholinergic neurotransmission and contractile response in bovine trachealis

Background

Muscarinic-receptor antagonists and β-adrenoceptor agonists are used, alone or in combination, as first-line treatment for chronic obstructive pulmonary disease. Both drugs decrease airway smooth muscle tone by post-junctional mechanisms but they may have opposing effects on pre-junctional acetylcholine (ACh)-release.

Methods

We studied the effects of the muscarinic-receptor antagonist glycopyrronium (GLY), the β-adrenoceptor agonist indacaterol (IND) and their combination on electrically-induced ACh-release and contractile response in isolated bovine trachealis. Data were analyzed by paired t-test and analysis of variance for repeated or independent measures with Newmann-Keuls post-hoc test when appropriate.

Results

GLY 10⁻⁸ M decreased contractile response by 19 ± 6% (p = 0.010) without altering ACh-release. GLY 10⁻⁷ M and 10⁻⁶ M almost abolished contractile responses even if the ACh-release was increased by 27 ± 19% (p < 0.001) and 20 ± 8% (p = 0.004), respectively. IND 10⁻⁷ M had no significant effects on contractile response and ACh-release, whereas IND 10⁻⁶ M reduced contractile response by 24 ± 12% (p = 0.002) without altering ACh-release. IND 10⁻⁵ M decreased contractile response by 51 ± 17% (p < 0.001) and ACh-release by 22 ± 11% (p = 0.004). Co-incubation with GLY 10⁻⁸ M and IND 10⁻⁷ M did not alter ACh-release but inhibited contractile response by 41 ± 8% (p < 0.001). The latter effect was greater than with GLY 10⁻⁸ M, or IND 10⁻⁷ M, or IND 10⁻⁶ M given separately (p < 0.001 for all). The increment of ACh-release caused by GLY was attenuated by IND 10⁻⁵ M, though this did not affect contractile response.

Conclusions

At equimolar concentration, GLY alone attenuates airway smooth muscle contraction more than IND, despite an increased ACh-release. Combination of GLY with IND at submaximal concentrations has more than additive effect suggesting a synergistic post-junctional effect. Adding GLY to IND provides a greater inhibitory effect on airway smooth muscle contraction than increasing IND concentration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12931-017-0627-5) contains supplementary material, which is available to authorized users.

Alzheimer's disease: Targeting the Cholinergic System

Acetylcholine (ACh) has a crucial role in the peripheral and central nervous systems. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing ACh from acetyl-CoA and choline in the cytoplasm and the vesicular acetylcholine transporter (VAChT) uptakes the neurotransmitter into synaptic vesicles. Following depolarization, ACh undergoes exocytosis reaching the synaptic cleft, where it can bind its receptors, including muscarinic and nicotinic receptors. ACh present at the synaptic cleft is promptly hydrolyzed by the enzyme acetylcholinesterase (AChE), forming acetate and choline, which is recycled into the presynaptic nerve terminal by the high-affinity choline transporter (CHT1). Cholinergic neurons located in the basal forebrain, including the neurons that form the nucleus basalis of Meynert, are severely lost in Alzheimer’s disease (AD). AD is the most ordinary cause of dementia affecting 25 million people worldwide. The hallmarks of the disease are the accumulation of neurofibrillary tangles and amyloid plaques. However, there is no real correlation between levels of cortical plaques and AD-related cognitive impairment. Nevertheless, synaptic loss is the principal correlate of disease progression and loss of cholinergic neurons contributes to memory and attention deficits. Thus, drugs that act on the cholinergic system represent a promising option to treat AD patients.

Block of postjunctional muscle-type acetylcholine receptors in vivo causes train-of-four fade in mice

BACKGROUND

Train-of-four (TOF) fade during nerve-mediated muscle contraction is postulated to be attributable to inhibition of prejunctional nicotinic α3β2 acetylcholine receptors (nAChRs), while decrease of twitch tension is attributable to block of postjunctional muscle nAChRs. The validity of these presumptions was tested using specific prejunctional and postjunctional nAChR antagonists, testing the hypothesis that fade is not always a prejunctional phenomenon.

METHODS

Pentobarbital anaesthetized mice had TOF fade measured after administration of: either 0.9% saline; the prejunctional α3β2 nAChR antagonist, dihydro-β-erythroidine (DHβE); the postjunctional nAChR antagonists, α-bungarotoxin (α-BTX) or α-conotoxin GI; and a combination of α-BTX and DHβE; or a combination of α-conotoxin GI and DHβE.

RESULTS

Saline caused no neuromuscular changes. Administration of muscle nAChR antagonists, α-BTX or α-conotoxin GI caused significant decrease of twitch tension and TOF fade compared with baseline (P<0.01). DHβE alone caused no change of twitch tension or fade even after 90 min, but its coadministration with α-BTX or α-conotoxin GI significantly accelerated the onset of paralysis and degree of fade compared with α-BTX or α-conotoxin GI alone (P<0.01).

CONCLUSIONS

Occupation of postjunctional nAChRs alone by α-BTX or α-conotoxin GI causes fade. As the prejunctional effects of DHβE on fade became manifest only when co-administered with α-BTX or α-conotoxin GI, specific inhibition of prejunctional nAChR alone is not necessary and sufficient to cause fade. Fade observed during repetitive nerve stimulation can be because of block of either postjunctional nAChRs alone, or block of prejunctional and postjunctional nAChRs together.

Beneficial effects of albuterol in congenital endplate acetylcholinesterase deficiency and Dok-7 myasthenia

INTRODUCTION

Congenital myasthenic syndromes (CMS) are disabling but treatable disorders. Anticholinesterase therapy is effective in most of them, but is contraindicated in endplate (EP) acetylcholinesterase (AChE) deficiency, the slow-channel syndrome, Dok-7 myasthenia, and β(2) -laminin deficiency, and is not useful in CMS due to defects in muscle-specific kinase (MuSK), agrin, and plectin. EP AChE, Dok-7, and β(2)-laminin deficiencies respond favorably to ephedrine, but ephedrine can no longer be prescribed in the USA.

METHODS

We used albuterol, another sympathomimetic agent, to treat 3 patients with EP AChE deficiency and 15 with Dok-7 myasthenia. Response to therapy was evaluated by a 9-point questionnaire pertaining to activities of daily life.

RESULTS

Comparison of the pre- and posttreatment responses indicated a beneficial response to albuterol (P < 0.001) in both patient groups. The adverse effects of therapy were like those of ephedrine.

CONCLUSION

Our observations should spur controlled, prospective clinical trials of albuterol in these as well as other CMS.

Acetylcholine negatively regulates development of the neuromuscular junction through distinct cellular mechanisms

Emerging evidence suggests that the neurotransmitter acetylcholine (ACh) negatively regulates the development of the neuromuscular junction, but it is not clear if ACh exerts its effects exclusively through muscle ACh receptors (AChRs). Here, we used genetic methods to remove AChRs selectively from muscle. Similar to the effects of blocking ACh biosynthesis, eliminating postsynaptic AChRs increased motor axon branching and expanded innervation territory, suggesting that ACh negatively regulates synaptic growth through postsynaptic AChRs. However, in contrast to the effects of blocking ACh biosynthesis, eliminating postsynaptic AChRs in agrin-deficient mice failed to restore deficits in pre- and postsynaptic differentiation, suggesting that ACh negatively regulates synaptic differentiation through nonpostsynaptic receptors. Consistent with this idea, the ACh agonist carbachol inhibited presynaptic specialization of motorneurons in vitro. Together, these data suggest that ACh negatively regulates axon growth and presynaptic specialization at the neuromuscular junction through distinct cellular mechanisms.

Role of presynaptic nicotinic acetylcholine receptors in the regulation of gastrointestinal motility

Presynaptic nicotinic acetylcholine receptors (nAChRs) located on cholinergic terminals facilitate the release of acetylcholine (ACh), thereby constituting a fail-safe mechanism at strategic locations, such as the neuromuscular junction, where reliable transmission is vital. Accumulating data indicate that myenteric neurons in the enteric nervous system possess not only somatodendritic nAChRs, which mediate cholinergic transmission between neurons, but also presynaptic nAChRs. Functional evidence shows that these receptors mediate a positive feedback with respect to ACh release from myenteric motoneurons, and might therefore play an important role in the regulation of gastrointestinal motility. These presynaptic nAChRs were found to be more sensitive to nicotinic ligands than somatodendritic nAChRs and could therefore be primary targets of exogenous compounds, such as nicotine. This interaction might provide a neurochemical basis for the effect of smoking on gastrointestinal motility. Another important human pharmacological implication is based on our recent observation that monoamine uptake inhibitor-type antidepressant drugs are able to inhibit presynaptic nAChRs in the enteric nervous system. The disruption of the nAChR-mediated positive feedback modulation by antidepressants might explain the frequent occurrence of constipation, a common side effect, attributed to these drugs. Clarification of the role of presynaptic nAChRs in feedback mechanisms in the enteric nervous system might be instrumental in the development of new drugs affecting gastrointestinal motility.

Influence of nicotine on brain reward systems: study of intracranial self-stimulation

We used the rate-frequency curve-shift procedure to evaluate the effects of nicotinic blockers, locally infused into the mesopontine tegmentum or ventral tegmentum, on the threshold of brain stimulation reward. Mecamylamine, the nicotinic acetylcholine receptor blocker, was infused one hour before the self-stimulation of the medial forebrain bundle. When injected into the mesopontine tegmentum, mecamylamine shifted rate-frequency curves to the right. Similar effects were also observed when the drug was injected into the ventral tegmentum. Thus, in both the mesopontine tegmentum and the ventral tegmentum, nicotinic receptors appear to facilitate the rewarding effect of the self-stimulation.

Development of ultra short-acting muscle relaxant agents: History, research strategies, and challenges

Author has reviewed the literature and his own work related to the chemistry, pharmacology, and clinical aspects of new muscle relaxants. Emphasis has been placed on the basic science concepts and technologies (e.g. structure-activity relationships, nicotinic receptor pharmacology, and investigation of side effects) behind the development of rapidly and short acting nondepolarizing muscle relaxants.

Cellular mechanisms of atracurium-induced tetanic fade in the isolated rat muscle

Although atracurium is a widely used neuromuscular blocker, we still lack knowledge regarding some of its cellular mechanisms of action. Thus, similar to other clinically used blockers atracurium induces, both in vivo and in vitro, fade of the tetanic contraction. However, the cellular mechanisms underlying this tetanic fade have never been systematically studied. In the present work these mechanisms were investigated in vitro. A sciatic nerve extensor digitorum longus muscle preparation of the rat was used. A combination of myographical and electrophysiological techniques was employed. Indirect twitches were evoked at 0.1 Hz and tetanic contractions at 50 Hz. Trains of end-plate potentials were evoked at a frequency of 50 Hz. The electrophysiological variables used in the analysis of the trains of end-plate potentials were: peak amplitude of the first end-plate potential in the train, peak amplitude of plateau end-plate potentials in the train, tetanic run-down of the end-plate potentials' train, quantal content of first and plateau end-plate potentials in the train, quantal size. In the myographical study atracurium, at a concentration of 2.4 microM, induced a complete fade of the tetanic contraction while only slightly affected the twitch. In the electrophysiological study atracurium, at the same 2.4 microM concentration, significantly decreased the amplitude of both first end-plate potentials in the train (control: 14.4 mV; atracurium: 3.2 mV) and plateau end-plate potentials (control: 10.8 mV; atracurium: 2.4 mV) and reinforced the tetanic run-down of the train of end-plate potentials, evaluated as the percent loss in amplitude of plateau end-plate potentials compared to first end-plate potentials in the trains (control: 25.2%; atracurium: 33.2%). Atracurium also significantly decreased the quantal content of first end-plate potentials in the train (control: 231; atracurium: 68), the quantal content of plateau end-plate potentials (control: 159; atracurium: 42) and the quantal size (control: 0.119 mV; atracurium: 0.075 mV). In relative terms the decrease in quantal content was about twice as large as the decrease in quantal size. This indicates that the fade of the tetanic contraction induced by atracurium (2.4 microM) is due to both pre- and postsynaptic blocking effects, the presynaptic one being stronger.

L- and DL-carnitine induce tetanic fade in rat neuromuscular preparation

Carnitine, a structurally choline-like metabolite, has been used to increase athletic performance, although its effects on neuromuscular transmission have not been investigated. It is present in skeletal muscle and its plasma levels are about 30 to 90 micro M. Using rat phrenic nerve diaphragm preparations indirectly and directly stimulated with high rate pulses, D-carnitine (30 and 60 micro M), L-carnitine (60 micro M) and DL-carnitine (60 micro M) were shown to induce tetanic fade (D-carnitine = 19.7 +/- 3.1%, N = 6; L-carnitine = 16.6 +/- 2.4%, N = 6; DL-carnitine = 14.9 +/- 2.1%, N = 6) without any reduction of maximal tetanic tension. D-carnitine induced tetanic fade in neuromuscular preparations previously paralyzed with d-tubocurarine and directly stimulated. The effect was greater than that obtained by indirect muscle stimulation. Furthermore, previous addition of atropine (20 to 80 micro M) to the bath did not reduce carnitine isomer-induced tetanic fade. In contrast to D-carnitine, the tetanic fade induced by L- and DL-carnitine was antagonized by choline (60 micro M). The combined effect of carnitine isomers and hemicholinium-3 (0.01 nM) was similar to the effect of hemicholinium-3 alone. The data suggest that L- and DL-carnitine-induced tetanic fade seems to depend on their transport into the motor nerve terminal.

Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus)

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Early postdenervation depolarization is controlled by acetylcholine and glutamate via nitric oxide regulation of the chloride transporter

Resting non-quantal acetylcholine (ACh) and probably glutamate (Glu) release from nerve endings activates M1- and NMDA receptor-mediated Ca2+ entry into the sarcoplasm with following activation of NOS and production of NO. This is a trophic message from motoneurons, which keeps the Cl- transport inactive in the innervated sarcolemma. After denervation, the secretion of ACh and Glu at the neuromuscular junction is eliminated within 3-4 h and the production of NO in the sarcoplasm is lowered. As a result, the Cl- influx is probably activated by dephosphorylation of the Cl- transporter with subsequent elevation of intracellular Cl- concentration. The equilibrium Cl- potential becomes more positive and the muscle membrane becomes depolarized.

Aberrant patterning of neuromuscular synapses in choline acetyltransferase-deficient mice

In this study we examined the developmental roles of acetylcholine (ACh) by establishing and analyzing mice lacking choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh. As predicted, ChAT-deficient embryos lack both spontaneous and nerve-evoked postsynaptic potentials in muscle and die at birth. In mutant embryos, abnormally increased nerve branching occurs on contact with muscle, and hyperinnervation continues throughout subsequent prenatal development. Postsynaptically, ACh receptor clusters are markedly increased in number and occupy a broader muscle territory in the mutants. Concomitantly, the mutants have significantly more motor neurons than normal. At an ultrastructural level, nerve terminals are smaller in mutant neuromuscular junctions, and they make fewer synaptic contacts to the postsynaptic muscle membrane, although all of the typical synaptic components are present in the mutant. These results indicate that ChAT is uniquely essential for the patterning and formation of mammalian neuromuscular synapses.

Mechanism of nicotine-evoked release of 3H-noradrenaline in human cerebral cortex slices

1. The mechanism of stimulation of noradrenaline (NA) release by nicotine (NIC) was investigated in human cerebral cortex slices preloaded with 3H-noradrenaline. 2 NIC (10-1000 micro M) increased 3H-NA release in a concentration-dependent manner. 3. NIC (100 micro M)-evoked 3H-NA release was largely dependent on external Ca2+, and was attenuated by omega-conotoxin GVIA (0.1 micro M) but not by nitrendipine (1 micro M). 4. Tetrodotoxin (1 micro M) and nisoxetine (0.1 micro M) attenuated the NIC (100 micro M)-evoked release of 3H-NA. 5. Mecamylamine (10 micro M), dihydro-beta-erythroidine (10 micro M) and d-tubocurarine (30 micro M), but not alpha-bungarotoxin (alpha-BTX, 0.1 micro M), attenuated the NIC (100 micro M)-evoked release of 3H-NA. 6. NIC (100 micro M)-evoked release of 3H-NA was not affected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 30 micro M) and D(-)-2-amino-5-phosphonopentanoic acid (D-AP5, 100 micro M), but attenuated by MK-801 (10 micro M). MK-801 (0.1-1000 micro M) displaced the specific binding of 3H-nisoxetine with K(i) values of 91.2 micro M. NIC (100, 300 and 1000 micro M) did not induce 3H-D-aspartate release in human cerebral cortex slices. 7. NIC (100 micro M)-evoked release of 3H-NA was attenuated by 7-nitroindazole (10 micro M), N(G)-nitro-L-arginine methyl ester HCl (L-NAME, 30 micro M), N(G)-monomethyl-L-arginine acetate (L-NMMA, 300 micro M). [(3)H]-NA release induced by NIC (100 micro M) was attenuated by methylene blue (3 micro M) and 1H-[1,2,4]oxadiazole[4,3-alpha]quinoxalin-1-one (ODQ, 10 micro M), and enhanced by zaprinast (30 micro M). 8. In conclusion, NIC stimulates the release of 3H-NA through activation of alpha-BTX-insensitive nicotinic acetylcholine receptors in the human cerebral cortex slices and this action of NIC is associated with modulation of the NO/cGMP pathway.

Channelopathies can cause epilepsy in man

Idiopathic epilepsies, which account for up to 40% of all epilepsies, are mainly caused by genetic factors. Most idiopathic epilepsies are due to oligogenic or multifactorial rather than monogenetic inheritance. Nevertheless, most of what is known today about the molecular genetics of idiopathic epilepsies has been found by analysing large families with rare monogenetic forms of the disease. For the first time, gene defects can be linked to certain epilepsies. Mutations in the CHRNA4 or CHRNB subunits of the neuronal nicotinic acetylcholine receptor lead to familial nocturnal frontal lobe epilepsy, while defects in the voltage-gated potassium channels KCNQ2 and KCNQ3 have recently been found to cause benign familial neonatal convulsions. The voltage-gated sodium channel subunits SCN1B, SCN1A and SCN2A as well as the GABRG2 subunit of the GABA(A) receptor are involved in the pathology of the newly described syndrome generalized epilepsy with febrile seizures plus. These rare monogenetic epilepsies can serve as models for further genetic analysis of the common forms of idiopathic epilepsies.

Neurotransmitter release mechanisms in sympathetic neurons: past, present, and future perspectives

In 1969, Paton and Vizi described the inhibitory actions of noradrenaline on acetylcholine release from the innervation of the guinea-pig ileum longitudinal muscle. They concluded "that acetylcholine output by the nervous networks of the longitudinal strip is under the normal control of the sympathetic by a species of presynaptic inhibition mediated by <==> receptors". This work was carried out in the Pharmacology Department at Oxford University. Clearly, a period in the 'Dreaming Spires' of Oxford sufficiently inspired Sylvester to take up a life long career in scientific research. He has published more than 300 papers on a wide range of topics but clearly has a strong interest in neurotransmitter release mechanisms and recently, non-synaptic interactions between neurons. It seems fitting therefore to write a brief review on the continuing studies on neurotransmitter release mechanisms in sympathetic neurons in a volume honoring the now distinguished Professor Vizi.

Carbachol and acetylcholine delay the early postdenervation depolarization of muscle fibres through M1-cholinergic receptors

The resting membrane potential (RMP) of denervated muscle fibres of rat diaphragm muscle is depolarized by approximately 8-10 mV during the first 3 h after nerve section and this early postdenervation depolarization is reduced substantially by the presence of 5x10(-8) M acetylcholine (ACh) or carbachol (CB). The muscarinic antagonist atropine (Atr; 5x10(-9) to 5x10(-6) M) reduced the effect of CB in a dose-dependent manner (K(i)=7x10(-8) M) and increased the rate of the early postdenervation depolarization. In lower doses (5x10(-7) M), Atr acted only in the presence of an allosteric stabilizator hexamethylene-bis-[dimethyl-(3-phtalimidopropyl)ammonium] (W-84). Also pirenzepine, a specific inhibitor of the M1 subtype of muscarinic receptor, blocked the action of CB in a dose-dependent manner with an apparent inhibition constant K(i)=1x10(-7) microM. DAMP, a specific M3 antagonist, was without effect on the muscle hyperpolarization induced by CB. CB also hyperpolarized the membrane potentials of muscles which were denervated for 1-3 days. It is concluded that ACh and CB protect the muscle fibres from early depolarization through M1-cholinergic receptors on the muscle membrane. These particular receptors can apparently mediate the 'trophic', non-impulse regulation of RMP in skeletal muscles when they are activated by acetylcholine released non-quantally.

Mechanisms underlying the vecuronium-induced tetanic fade in the isolated rat muscle

The cellular mechanisms underlying the effects of vecuronium on the tetanic contraction were studied in vitro with a combination of myographic and electrophysiologic techniques. We used the isolated sciatic nerve extensor digitorum longus muscle preparation of the rat. Indirect twitches were evoked at 0.1 Hz pulses and tetani at 50 Hz pulses. Trains of end-plate potentials were generated at 50 Hz. The electrophysiological variables used in the analysis of the end-plate potentials were: amplitude, tetanic run-down, quantal size and quantal content. The myographic study demonstrated that vecuronium at 0.4 microM caused tetanic fade, but left the twitch unaffected. Regarding electrophysiology, vecuronium (0.4 microM) decreased the amplitude of end-plate potentials and increased their tetanic run-down. These changes were due to significant reductions in both the quantal content of the end-plate potentials and the quantal size. It is concluded that vecuronium has both pre- and postsynaptic effects at the neuromuscular junction, and that it induces fade of the tetanic contraction via a summation of these effects.

One hundred years of adrenaline: the discovery of autoreceptors

The active principle of suprenal extract that produces its pressor effects was isolated by the joint research of John Abel in 1899 and Jokichi Takamine in 1901. Within three years Elliott, working in Langley's laboratory, suggested that this active principle, referred to by British physiologists as "adrenaline" and named "Adrenalin" by Takamine, was released from sympathetic nerve terminals to act on smooth muscle cells. However, it was not until 1946 that von Euler showed that demythelated adrenaline (noradrenaline) rather than adrenaline is a sympathetic transmitter. The possibility that this sympathetic transmitter could also act on nerve terminals was not developed until 1971. Research on autoreceptors culminated in the identification of adrenergic receptors on nerve terminals different to those on muscle cells. This paper assesses the contributions that established the idea of the adrenergic autoreceptor, 100 years after the discovery of adrenaline.

Presynaptic ionotropic receptors and the control of transmitter release

The quantity of neurotransmitter released into the synaptic cleft, the reliability with which it is released, and the response of the postsynaptic cell to that transmitter all contribute to the strength of a synaptic connection. The presynaptic nerve terminal is a major regulatory site for activity-dependent changes in synaptic function. Ionotropic receptors for the inhibitory amino acid GABA, expressed on the presynaptic terminals of crustacean motor axons and vertebrate sensory neurons, were the first well-defined mechanism for the heterosynaptic transmitter-mediated regulation of transmitter release. Recently, presynaptic ionotropic receptors for a large range of transmitters have been found to be widespread throughout the central and peripheral nervous systems. In this review, we first consider some general theoretical issues regarding whether and how presynaptic ionotropic receptors are important regulators of presynaptic function. We consider the criteria that should be met to identify a presynaptic ionotropic receptor and its regulatory function and review several examples of presynaptic receptors that meet at least some of those criteria. We summarize the classic studies of presynaptic inhibition mediated by GABA-gated Cl channels and then focus on presynaptic nicotinic ACh receptors and presynaptic glutamate receptors. Finally, we briefly discuss evidence for other types of presynaptic ionotropic receptors.

ATP stimulates sympathetic transmitter release via presynaptic P2X purinoceptors

ATP is a fast transmitter in sympathetic ganglia and at the sympathoeffector junction. In primary cultures of dissociated rat superior cervical ganglion neurons, ATP elicits noradrenaline release in an entirely Ca2+-dependent manner. Nevertheless, ATP-evoked noradrenaline release was only partially reduced (by approximately 50%) when either Na+ or Ca2+ channels were blocked, which indicates that ATP receptors themselves mediated transmembrane Ca2+ entry. An "axonal" preparation was obtained by removing ganglia from explant cultures, which left a network of neurites behind; immunostaining for axonal and dendritic markers revealed that all of these neurites were axons. In this preparation, ATP raised intraaxonal Ca2+ and triggered noradrenaline release, and these actions were not altered when Ca2+ channels were blocked by Cd2+. Hence, Ca2+-permeable ATP-gated ion channels, i.e., P2X purinoceptors, are located at presynaptic sites and directly mediate Ca2+-dependent transmitter release. These presynaptic P2X receptors displayed a rank order of agonist potency of ATP >/= 2-methylthio-ATP > ATPgammaS >> alpha,beta-methylene-ATP approximately beta,gamma-methylene-L-ATP and were blocked by suramin or PPADS. ATP, 2-methylthio-ATP, and ATPgammaS also evoked inward currents measured at neuronal somata, but there these agonists were equipotent. Hence, presynaptic P2X receptors resemble the cloned P2X2 subtype, but they appear to differ from somatodendritic P2X receptors in terms of agonist sensitivity. Suramin reduced depolarization-evoked noradrenaline release by up to 20%, when autoinhibitory mechanisms were inactivated by pertussis toxin. These results indicate that presynaptic P2X purinoceptors mediate a positive, whereas G-protein-coupled P2Y purinoceptors mediate a negative, feedback modulation of sympathetic transmitter release.

Nerve terminal currents induced by autoreception of acetylcholine release

The activation of autoreceptors is known to be important in the modulation of presynaptic transmitter secretion in peripheral and central neurons. Using whole-cell recordings made from the free growth cone of myocyte-contact motoneurons of Xenopus cell cultures, we have observed spontaneous nerve terminal currents (NTCs). These spontaneous NTCs are blocked by d-tubocurarine (d-TC) and alpha-bungarotoxin (alpha-BuTx), indicating that endogenously released acetylcholine (ACh) can produce substantial membrane depolarization in the nerve terminals. Local application of NMDA to the growth cone increased the frequency of spontaneous NTCs. When the electrical stimulations were applied at the soma to initiate evoked-release of ACh, evoked ACh-induced potentials were recorded in the nerve terminals, which were inhibited by d-TC and hexamethonium but not by atropine. Replacement of normal Ringer's solution with high-Mg2+, low-Ca2+ solution also reversibly inhibited evoked ACh-induced potentials. The possible regulatory role of presynaptic nicotinic autoreceptors on the synaptic transmission was also examined. When the innervated myocyte was whole-cell voltage-clamped to record synaptic currents, application of hexamethonium inhibited the amplitude of evoked synaptic currents at a higher degree than that of iontophoretic ACh-induced currents. Furthermore, hexamethonium markedly reduced the frequency of spontaneous synaptic currents at high-activity synapses. Pretreatment of neurons with alpha-BuTx also inhibited the evoked synaptic currents in manipulated synapses. These results suggest that ACh released spontaneously or by electrical stimulation may act on the presynaptic nicotinic autoreceptors of the same nerve terminals to produce membrane potential change and to regulate synaptic transmission.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Non-neuronal acetylcholine, a locally acting molecule, widely distributed in biological systems: expression and function in humans

Acetylcholine acts as a neurotransmitter in the central and peripheral nervous systems in humans. However, recent experiments demonstrate a widespread expression of the cholinergic system in non-neuronal cells in humans. The synthesizing enzyme choline acetyltransferase, the signalling molecule acetylcholine, and the respective receptors (nicotinic or muscarinic) are expressed in epithelial cells (human airways, alimentary tract, epidermis). Acetylcholine is also found in mesothelial, endothelial, glial, and circulating blood cells (platelets, mononuclear cells), as well as in alveolar macrophages. The existence of non-neuronal acetylcholine explains the widespread expression of muscarinic and nicotinic receptors in cells not innervated by cholinergic neurons. Non-neuronal acetylcholine appears to be involved in the regulation of important cell functions, such as mitosis, trophic functions, automaticity, locomotion, ciliary activity, cell-cell contact, cytoskeleton, as well as barrier and immune functions. The most important tasks for the future will be to clarify the multiple biological roles of non-neuronal acetylcholine in detail and to identify pathological conditions in which this system is up- or down-regulated. This could provide the basis for the development of new therapeutic strategies to target the non-neuronal cholinergic system.

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.

Presynaptic nicotinic ACh receptors

Nicotinic ACh (nACh) receptors in the CNS are composed of a diverse array of subunits and have a range of pharmacological properties. However, despite the fact that they are ligand-gated cation channels, their physiological functions have not been determined. This has led to increased interest in presynaptic nACh receptors that act to modulate the release of transmitter from presynaptic terminals.

Side effects of nondepolarizing muscle relaxants: relationship to their antinicotinic and antimuscarinic actions

Since acetylcholine (ACh) is the 'master key' to different subtypes of nicotinic and muscarinic receptors, and muscle relaxants (MRs) available in clinical practice are structurally related to it, MRs may exert their unwanted effects through inhibition of these receptors. Since the subunit composition of nicotinic ACh receptors (nAChRs) of pre- and/or postsynaptic location and the binding potency of MRs to these and muscarinic receptors are different, a search for selective muscle nAChR antagonists without or with less side effects seems to be promising and important for clinical practice.

alpha-Bungarotoxin, kappa-bungarotoxin, alpha-cobratoxin and erabutoxin-b do not affect [3H]acetylcholine release from the rat isolated left hemidiaphragm

Endplate preparations of the rat left hemidiaphragm were incubated with [3H]choline to label neuronal transmitter stores. Nerve evoked release of newly-synthesized [3H]acetylcholine was measured in the absence of cholinesterase inhibitors to investigate whether snake venom neurotoxins by blocking presynaptic nicotinic autoreceptors affect evoked transmitter release. Contractions of the indirectly stimulated hemidiaphragm were recorded to characterize the blocking effect of alpha-neurotoxins at the post-synaptic nicotinic receptors. Neither the long chain neurotoxins alpha-cobratoxin (1 microgram ml-1) and alpha-bungarotoxin (5 microgram ml-1) nor the short chain neurotoxin erabutoxin-b (0.1, 1 and 10 micrograms ml-1) affected the nerve-evoked release of [3H]acetylcholine. kappa-Bungarotoxin (1 and 5 micrograms ml-1), a toxin preferentially blocking neuronal nicotinic receptors, did also not affect evoked [3H]acetylcholine release, whereas (+)-tubocurarine (1 microM) under identical conditions reduced the release by about 50%. alpha-Bungarotoxin, alpha-cobratoxin and erabutoxin-b concentration-dependently (0.01-0.6 micrograms ml-1) inhibited nerve-evoked contractions of the hemidiaphragm. All neurotoxins except erabutoxin-b enhanced the basal tritium efflux immediately when applied to the endplate preparation or to a non-innervated muscle strip labelled with [3H]choline. This effect was attributed to an enhanced efflux of [3H]phosphorylcholine, whereas the efflux of [3H]choline and [3H]acetylcholine was not affected. It is concluded that the alpha-neurotoxins and kappa-bungarotoxin do not block presynaptic nicotinic receptors of motor nerves. These nicotinic autoreceptors differ from nicotinic receptors localized at the muscle membrane and at autonomic ganglia.

Beta-adrenoceptors mediate inhibition of [3H]-acetylcholine release from the isolated rat and guinea-pig trachea: role of the airway mucosa and prostaglandins

1. Rat or guinea pig isolated tracheae were labelled with [3H]-choline to measure evoked tritium outflow, which reflects neuronal release of [3H]-acetylcholine. Tritium outflow was evoked either by electrical stimulation of the extrinsic vagal nerve (rat tracheae) or by 27 mM potassium (guinea pig tracheae). 2. In rat tracheae isoprenaline (0.01, 0.1 microM) inhibited evoked [3H]-acetylcholine release, whereas beta 2-adrenoceptor-selective agonists (fenoterol, formoterol, salbutamol) were ineffective. 3. The inhibitory effect of isoprenaline was abolished under the following conditions: (i) presence of propranolol (1 microM) or of the beta 1-selective antagonist CGP 20712 A (0.1 microM); (ii) removal of the mucosa at the start of the experiments; (iii) blockade of cyclooxygenase activity by 3 microM indomethacin. 4. In rat isolated tracheae prelabelled with [3H]-arachidonic acid, isoprenaline (0.1 microM) but not formoterol (0.01 microM) enhanced the outflow of [3H]-prostaglandins (PGD2, PGE2). This effect was blocked by 0.1 microM CGP 20712 A. 5. In guinea pig tracheae electrical stimulation of the extrinsic vagal nerve did not cause a constant release of [3H]-acetylcholine, but 27 mM potassium elicited a reproducible release of [3H]-acetylcholine. In this species both isoprenaline (0.1 microM) and formoterol (0.01 microM) inhibited evoked [3H]-acetylcholine release. Inhibition was abolished under the following conditions: (i) presence of propranolol (1 microM) or of the beta 2-selective antagonist ICI 118551 (0.3 microM); (ii) removal of the mucosa at the start of the experiments; (iii) blockade of cyclooxygenase activity by 3 microM indomethacin. 6. In conclusion, the present experiments have demonstrated that activation of beta-adrenoceptors localized in the mucosa mediates inhibition of [3H]-acetylcholine release from the neuroeffector junctions of the pulmonary, parasympathetic nerves most probably by the liberation of inhibitory prostaglandins from the airway mucosa. The adrenoceptor subtype involved differs in rat (beta 1 subtype) and guinea pig (beta 2 subtype) airways.

The effect of acetylcholine and related drugs on currents at the frog motor nerve terminal

Acetylcholine, acetylthiocholine, carbachol, suberyldicholine, propionylcholine, succinylcholine, methylfurmethide and F 2268 were tested on motor nerve ending currents recorded with an extracellular microelectrode. The isolated and transversally cut cutaneous pectoris muscle of frog Rana ridibunda was used. Only acetylcholine and acetylthiocholine affected the spike waveforms in a concentration-dependent manner. Lower concentrations (1-6 x 10(-4) M) prolonged the inward Na+ current and increased the outward K+ current at the proximal and central parts of the nerve terminal. Most remote parts of the terminal were not affected. At 7 x 10(-4) M and higher, both drugs further prolonged the Na+ current and eliminated the K+ component of the spike. The potentiating effect of acetylcholine and acetylthiocholine on the K+ phase of nerve terminal current disappeared after treatment with tetraethylammonium and 4-aminopyridine. The effect also disappeared when synaptic cholinesterase was inhibited by the anticholinesterases or by treatment with collagenase. Reactivation of cholinesterase by dipyroxime restored the facilitating effect of acetylcholine. Choline and slight acidification to pH 6.8 did not mimic the acetylcholine action on the terminal currents. Facilitation of the K+ current by acetylcholine was not calcium-dependent. The results indicate that lower acetylcholine concentrations inhibit the delayed rectifier only, whereas 7 x 10(-4) M and higher concentrations of acetylcholine depress all outward currents of the terminal.

Postsynaptic effects of methoctramine at the mouse neuromuscular junction

Functional studies were performed to evaluate the effects of methoctramine at the neuromuscular junction of the mouse. The presynaptic control of acetylcholine release and the postsynaptic activation of the nicotinic receptor have been analysed by means of the extracellular recording with an EPC7 Patch Clamp amplifier. This electrophysiological method revealed a dose-related inhibitory effect of methoctramine on the studied parameters. The dramatic reduction of the kinetics of the quantal conductance change indicates an action at the postsynaptic level. The effects of methoctramine have been compared with those of the muscarinic agonist oxotremorine. Concentration/response curves for the two drugs were obtained and the apparent EC50 values calculated. The effects of oxotremorine were not antagonized by 1 microM methoctramine. These findings suggest an interaction of some muscarinic agents on the postsynaptic receptor-ion-channel complex at the mouse neuromuscular junction.

Muscarinic modulation of neurotransmission: the effects of some agonists and antagonists

1. Functional studies were performed to evaluate the effects of some muscarinic agents at the neuromuscular junction of the mouse. 2. The presynaptic control of acetylcholine release and the postsynaptic activation of the nicotinic receptor have been analyzed by means of extracellular recording. The amplitude of spontaneous and of evoked acetylcholine release, the frequency of spontaneous acetylcholine release and the time course of the quantal release have been measured by means of an EPC7 patch clamp amplifier. 3. This electrophysiological method revealed multiple dose-related effects of some agonists and antagonists on the above parameters. Concentration-response curves related to the parameters underlying the function of this cholinergic synapse were obtained and the apparent EC50 values calculated. 4. Many of the interactions of the agonists and antagonists could inhibit neuromuscular transmission. The rank order potencies related to the inhibition of the evoked signals were carbachol > oxotremorine > d,l-muscarine for the agonists and methoctramine > 4-DAMP > l-hyoscyamine > AFDX-116 > ipratropium > pirenzepine for the antagonists. 5. These findings suggest a more complicated pattern related to the muscarinic action at the mouse neuromuscular junction with the involvement of some post-synaptic located sites.

Effect of N,N'-dicyclohexylcarbodiimide on compartmentation and release of newly synthesized and preformed acetylcholine in Torpedo synaptosomes

Using isolated cholinergic synaptosomes prepared from Torpedo electric organ, we studied the effects of N,N'-dicyclohexylcarbodiimide (DCCD) on acetylcholine (ACh) synthesis, compartmentation, and release after stimulation. Whereas ACh synthesis was unchanged, ACh compartmentation inside synaptosomes was affected by the presence of DCCD. In resting conditions, the uptake into the synaptic vesicle pool of newly synthesized ACh (i.e., [14C]ACh synthesized in the presence of the drug) was progressively and markedly inhibited as the duration of DCCD preincubation was increased, whereas compartmentation of endogenous ACh was unchanged in the presence of DCCD. After stimulation, the release of endogenous ACh from DCCD-treated synaptosomes was similar to that of control, in contrast to the release of [14C]ACh, which was markedly inhibited. This inhibition was observed whatever the conditions of stimulation used (gramicidin D, calcium ionophore A23187, or KCl depolarization). The study of the compartmentation of [14C]ACh during stimulation revealed a transfer of highly labeled ACh from the free to the bound ACh compartment in the presence of DCCD, suggesting the existence of several ACh subcompartments within the free and bound ACh pools. The present results are discussed in comparison with the previously reported effects of vesamicol (AH5183) on ACh compartmentation and release.

Cromakalim inhibits electrically-evoked [3H]acetylcholine release from a tube-preparation of the rat isolated trachea by an epithelium-dependent mechanism

Rat isolated tracheae were labelled by incubation with [3H]choline to measure the tritium efflux elicited by electrical stimulation of the extrinsic parasympathetic nerves in vitro. Stimulated tritium efflux reflects the neuronal release of newly synthesized acetylcholine; the effects of potassium channel openers on the stimulated tritium efflux were investigated. In tracheae opened longitudinally neither cromakalim nor its 3S,4R-enantiomer, BRL 38227, reduced the stimulated tritium efflux, whereas in intact tube-preparations cromakalim (0.01-1 mumol/l) mediated a concentration-dependent inhibition. The inhibitory effect of 1 mumol/l cromakalim was prevented by 0.1 mumol/l glibenclamide. Likewise, BRL 38227 (0.01 and 0.1 mumol/l) inhibited the stimulated tritium efflux, but the inhibitory effect vanished at high concentrations (1 and 10 mumol/l). The 3R,4S-enantiomer of cromakalim, BRL 38226 (0.1, 1 and 10 mumol/l), on its own did not significantly inhibit the stimulated tritium efflux, but a combination of both enantiomers (0.5 or 1 mumol/l of each) produced an inhibition similar to that caused by 1 mumol/l cromakalim. In epithelium-denuded tube-preparations neither cromakalim nor BRL 38227 reduced the stimulated tritium efflux. The mucosal/submucosal microenvironment is better preserved in intact tube-preparations than in longitudinally-opened tracheae which are cut along their whole length so that the luminal surface is exposed directly to the surrounding medium. The present experiments show an neuronal inhibitory effect of cromakalim which is mediated by an epithelium-dependent mechanism.

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.