Presynaptic nicotine receptors mediating a positive feed-back on transmitter release from the rat phrenic nerve

Homeostatic synaptic plasticity at the neuromuscular junction in myasthenia gravis

A number of studies in the past 20 years have shown that perturbation of activity of the nervous system leads to compensatory changes in synaptic strength that serve to return network activity to its original level. This response has been termed homeostatic synaptic plasticity. Despite the intense interest in homeostatic synaptic plasticity, little attention has been paid to its role in the prototypic synaptic disease, myasthenia gravis. In this review, we discuss mechanisms that have been shown to mediate homeostatic synaptic plasticity at the mammalian neuromuscular junction. A subset of these mechanisms have been shown to occur in myasthenia gravis. The homeostatic changes occurring in myasthenia gravis appear to involve the presynaptic nerve terminal and may even involve changes in the excitability of motor neurons within the spinal cord. The finding of presynaptic homeostatic synaptic plasticity in myasthenia gravis leads us to propose that changes in the motor unit in myasthenia gravis may be more widespread than previously appreciated.

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.

Amplification of neuromuscular transmission by methylprednisolone involves activation of presynaptic facilitatory adenosine A2A receptors and redistribution of synaptic vesicles

The mechanisms underlying improvement of neuromuscular transmission deficits by glucocorticoids are still a matter of debate despite these compounds have been used for decades in the treatment of autoimmune myasthenic syndromes. Besides their immunosuppressive action, corticosteroids may directly facilitate transmitter release during high-frequency motor nerve activity. This effect coincides with the predominant adenosine A2A receptor tonus, which coordinates the interplay with other receptors (e.g. muscarinic) on motor nerve endings to sustain acetylcholine (ACh) release that is required to overcome tetanic neuromuscular depression in myasthenics. Using myographic recordings, measurements of evoked [(3)H]ACh release and real-time video microscopy with the FM4-64 fluorescent dye, results show that tonic activation of facilitatory A2A receptors by endogenous adenosine accumulated during 50 Hz bursts delivered to the rat phrenic nerve is essential for methylprednisolone (0.3 mM)-induced transmitter release facilitation, because its effect was prevented by the A2A receptor antagonist, ZM 241385 (10 nM). Concurrent activation of the positive feedback loop operated by pirenzepine-sensitive muscarinic M1 autoreceptors may also play a role, whereas the corticosteroid action is restrained by the activation of co-expressed inhibitory M2 and A1 receptors blocked by methoctramine (0.1 μM) and DPCPX (2.5 nM), respectively. Inhibition of FM4-64 loading (endocytosis) by methylprednisolone following a brief tetanic stimulus (50 Hz for 5 s) suggests that it may negatively modulate synaptic vesicle turnover, thus increasing the release probability of newly recycled vesicles. Interestingly, bulk endocytosis was rehabilitated when methylprednisolone was co-applied with ZM241385. Data suggest that amplification of neuromuscular transmission by methylprednisolone may involve activation of presynaptic facilitatory adenosine A2A receptors by endogenous adenosine leading to synaptic vesicle redistribution.

Adenosine A(2A) receptor antagonists are broad facilitators of antinicotinic neuromuscular blockade monitored either with 2 Hz train-of-four or 50 Hz tetanic stimuli

1. The 2 Hz train-of-four ratio (TOF(ratio)) is used to monitor the degree of patient curarization. Using a rat phrenic nerve-hemidiaphragm preparation, we showed that antinicotinic agents, such as hexamethonium, d-tubocurarine and pancuronium, but not cisatracurium, decreased contractions produced by physiological nerve activity patterns (50 Hz) more efficiently than those caused by 2 Hz trains. Uncertainty about the usefulness of the TOF(ratio) to control safe recovery from curarization prompted us to investigate the muscarinic and adenosine neuromodulation of tetanic (50 Hz) fade induced by antinicotinic agents at concentrations that cause a 25% reduction in the TOF(ratio) (TOF(fade)). 2. Tetanic fade caused by d-tubocurarine (1.1 μmol/L), pancuronium (3 μmol/L) and hexamethonium (5.47 mmol/L) was attenuated by blocking presynaptic inhibitory muscarinic M(2) and adenosine A(1) receptors with methoctramine (1 μmol/L) and 1,3-dipropyl-8-cyclopentylxanthine (2.5 nmol/L), respectively. These compounds enhanced rather than decreased tetanic fade induced by cisatracurium (2.2 μmol/L), but they consistently attenuated cisatracurium-induced TOF(fade). The effect of the M(1) receptor antagonist pirenzepine (10 nmol/L) on fade produced by antinicotinic agents at 50 Hz was opposite to that observed with TOF stimulation. Blockade of adenosine A(2A) receptors with ZM 241385 (10 nmol/L) attenuated TOF(fade) caused by all antinicotinic drugs tested, with the exception of the 'pure' presynaptic nicotinic antagonist hexamethonium. ZM 241385 was the only compound tested in this series that facilitated recovery from tetanic fade produced by cisatracurium. 3. The data suggest that distinct antinicotinic relaxants interfere with fine-tuning neuromuscular adaptations to motor nerve stimulation patterns via activation of presynaptic muscarinic and adenosine receptors. These results support the use of A(2A) receptor antagonists together with atropine to facilitate recovery from antinicotinic neuromuscular blockade.

Reevaluation of indirect field stimulation technique to demonstrate oxime effectiveness in OP-poisoning in muscles in vitro

Organophosphorus (OP) pesticides or nerve agents cause severe intoxication by inhibition of acetylcholinesterase, finally resulting in death due to respiratory failure. The phrenic nerve diaphragm preparation is considered as the classic model to investigate the effect of OP intoxications and oxime treatment at the neuromuscular junction. However, this preparation is unsuitable for larger species or for muscle strips from biopsies where no nerve is available for stimulation. An alternative technique is the indirect field stimulation of muscles containing intramuscular nerve branches only. The proposed method by Wolthuis et al. [Wolthuis, O.L., Vanwersch, R.A.P., Van Der Wiel, H.J., 1981. The efficacy of some bis-pyridinium oximes as antidotes to soman in isolated muscles of several species including man. Eur. J. Pharmacol. 70, 355-369] was modified and experimentally reevaluated in isolated mouse diaphragms. To confirm that electrical field stimulation technique induced muscle contraction only via the neuromuscular endplate the nicotinic antagonists pancuronium or d-tubocurarine (1microM) were given. In the presence of a nicotinic antagonist hardly any contraction was blocked after indirect field stimulation technique with very short pulses (5micros, <0.6A), in contrast to direct muscle stimulation (broader pulse width, or higher amplitude >0.6A). During paraoxon circumfusion (20min, 1micromol/l) muscle force generation by indirect stimulation was almost completely blocked. Restoration of paralyzed muscle function to 80% of initial values could be achieved after paraoxon wash out (20min) and circumfusion with obidoxime (1micromol/l, 20min). This data correspond quite well to data shown earlier when using conventional nerve stimulation techniques.

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.

Protein kinase A and Ca(v)1 (L-Type) channels are common targets to facilitatory adenosine A2A and muscarinic M1 receptors on rat motoneurons

At the rat motor endplate, pre-synaptic facilitatory adenosine A2A and muscarinic M1 receptors are mutually exclusive. We investigated whether these receptors share a common intracellular signalling pathway. Suppression of McN-A-343-induced M1 facilitation of [3H]ACh release was partially recovered when CGS21680C (an A2A agonist) was combined with the cyclic AMP antagonist Rp-cAMPS. Forskolin, rolipram and 8-bromo-cyclic AMP mimicked CGS21680C blockade of M1 facilitation. Both Rp-cAMPs and nifedipine reduced augmentation of [3H]ACh release by McN-A-343 and CGS21680C. Activation of M1 and A2A receptors enhanced Ca2+ recruitment through nifedipine-sensitive channels. Nifedipine inhibition revealed by McN-A-343 was prevented by chelerythrine (a PKC inhibitor) and Rp-cAMPS, suggesting that Ca(v)1 (L-type) channels phosphorylation by PKA and PKC is required. Rp-cAMPS inhibited [3H]ACh release in the presence of phorbol 12-myristate 13-acetate, but PKC inhibition by chelerythrine had no effect on release in the presence of 8-bromo-cyclic AMP. This suggests that the involvement of PKA may be secondary to M1-induced PKC activation. In conclusion, competition of M1 and A2A receptors to facilitate ACh release from motoneurons may occur by signal convergence to a common pathway involving PKA activation and Ca2+ influx through Ca(v)1 (L-type) channels.

Cholinergic modulation of nociceptive responses in vivo and neuropeptide release in vitro at the level of the primary sensory neuron

Muscarinic acetylcholine receptors (mAChRs) have been widely reported as pharmacological targets for the treatment of pain. However, most of these efforts have focused on CNS mAChRs and their role in modulating nociception at the level of the spinal cord. The present study examines the contribution of peripheral mAChRs in trigeminal nociceptive pathways using a combination of in vivo and in vitro approaches. In the formalin model of orofacial nociception in rats, a peri-oral co-injection of the M2 agonist arecaidine dose-dependently inhibited phase 2 nocifensive behavior up to approximately 50% at 5 nmol. This effect was blocked by co-treatment with the mAChR antagonist atropine and was not seen when arecaidine was administered under the skin of the back, a site distant from that of the formalin injection. In vitro superfusion of isolated rat buccal mucosa with the non-selective mAChR agonist muscarine or arecaidine led to a concentration-dependent inhibition of capsaicin-evoked CGRP release to 39% (EC50=255 nM) and 28% (EC50=847 nM) of control values, respectively. Both responses were blocked by the non-selective mAChR antagonist atropine or the M2 antagonist gallamine. Further, the endogenous ligand ACh produced a bi-phasic response, potentiating evoked CGRP release to 195% of control (EC50= 918nM) and inhibiting evoked CGRP release to 45% of control (EC50=255 microM), effects that were shown to be mediated by nAChRs and mAChRs, respectively. Finally, combined in situ hybridization/immunofluorescence demonstrated that m2 mRNA was present in 20% of trigeminal ganglion neurons between 30 and 60 microm in diameter and that 5-9% of these also expressed CGRP or VR1 immunoreactivity. These results show that activation of peripheral M2 receptors produces antinociception in vivo and the inhibition of nociceptor activity in vitro. While histological analyses at the level of the trigeminal neuronal cell bodies leave open the question of whether the effects of M2 agonists are direct or indirect, these data indicate that primary sensory neuronal M2 receptors may represent a viable peripheral target for the treatment of pain and inflammation.

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.

Pharmacological characterization of mikatoxin, an alpha-neurotoxin isolated from the venom of the New-Guinean small-eyed snake Micropechis ikaheka

Symptoms of envenomation by the New-Guinean small-eyed snake Micropechis ikaheka (Elapidae) include peripheral neurotoxicity and myotoxicity. We have now purified to homogeneity a long-chain neurotoxin, mikatoxin, from M. ikaheka venom by successive gel filtration and reverse-phase chromatography. Electrospray ionization mass spectrometry showed mikatoxin to be a homogenous peptide of MW 7775.6. Mikatoxin was devoid of any phospholipase A(2) activity associated with the crude venom and did not exhibit any intrinsic anticholinesterase activity. In the chick biventer cervicis muscle, it produced an irreversible, concentration-dependent block of responses to exogenously applied acetylcholine and carbachol as well as twitches evoked by nerve, but not by direct muscle stimulation. Moreover, mikatoxin, like alpha-bungarotoxin and erabutoxin-b, did not show significant fade response to train-of-four stimulation of the mouse phrenic nerve-hemi diaphragm muscle. It also failed to block ganglionic transmission in the guinea pig ileum and muscarinic responses in the rat anococcygeus muscle. Our study provides strong evidence for the presence of a neurotoxin (mikatoxin) in M. ikaheka venom that produces neuromuscular blockade in skeletal muscle attributable to selective and irreversible antagonism of postsynaptic nicotinic acetylcholine receptors of the neuromuscular junction and likely contributes to the peripheral neurotoxicity observed in M. ikaheka envenomation.

Modulation by adenosine of both muscarinic M1-facilitation and M2-inhibition of [3H]-acetylcholine release from the rat motor nerve terminals

The crosstalk between adenosine and muscarinic autoreceptors regulating evoked [3H]-acetylcholine ([3H]-ACh) release was investigated on rat phrenic nerve-hemidiaphragm preparations. Motor nerve terminals possess facilitatory M1 and inhibitory M2 autoreceptors that can be activated by McN-A-343 (1-30 microm) and oxotremorine (0.3-100 microm), respectively. The muscarinic receptor antagonist, dicyclomine (3 nm-10 microm), caused a biphasic (inhibitory/facilitatory) effect, indicating that M1-facilitation prevails during 5 Hz stimulation trains. Concomitant activation of AF-DX 116-sensitive M2 receptors was partially attenuated, as pretreatment with M1 antagonists, muscarinic toxin 7 (MT-7, 0.1 nm) and pirenzepine (1 nm), significantly enhanced inhibition by oxotremorine. Activation of A2A-adenosine receptors with CGS 21680C (2 nm) (i) potentiated oxotremorine inhibition, and (ii) shifted McN-A-343-induced facilitation into a small inhibitory effect. Conversely, the A1-receptor agonist, R-N6-phenylisopropyl adenosine (R-PIA, 100 nm), attenuated the inhibitory effect of oxotremorine, without changing facilitation by McN-A-343. Synergism between A2A and M2 receptors is regulated by a reciprocal interaction with facilitatory M1 receptors, which may be prevented by pirenzepine (1 nm). During 50 Hz-bursts, facilitation (M1) of [3H]-ACh release by McN-A-343 disappeared, while the inhibitory (M2) effect of oxotremorine became predominant. This muscarinic shift results from the interplay with A2A receptors, as it was precluded by the selective A2A receptor antagonist, ZM 241385 (10 nm). In conclusion, when the muscarinic M1 positive feedback loop is fully operative, negative regulation of ACh release is mediated by adenosine A1 receptors. During high frequency bursts, tonic activation of A2A receptors promotes M2 autoinhibition by braking the M1 receptor operated counteraction.

Presynaptic inhibitory effects of rocuronium and SZ1677 on [3H]acetylcholine release from the mouse hemidiaphragm preparation

It has been shown that nondepolarizing muscle relaxants may have effects on nicotinic acetylcholine receptors (nAChRs) other than those located on the skeletal muscle: some of them possess inhibitory effects on neuronal nAChRs [Anesth. Analg. 59 (1980) 935; Trends Pharmacol. Sci. 9 (1988) 16; Pharmacol. Ther. 73 (1997) 75]. It was shown that, e.g. (+)-tubocurarine and pancuronium are able to inhibit ACh release from the axon terminals of hemidiaphragm preparations and produce tetanic fade indicating their presynaptic effect. In this study rocuronium, a nondepolarizing steroidal muscle relaxant with shorter onset of action, and SZ1677 [1-(3alpha-hydroxy-17beta-acetyloxy)-2beta-(1.4-dioxa-8-azaspiro-[4,5]-dec-8-yl)-(5alpha-androstane-16beta-yl)-1-(2-propenyl) pyrrolidinium bromide], a short-acting muscle relaxant [Ann. New York Acad. Sci. 757 (1995b) 84] inhibited the release of ACh in response to axonal stimulation, while alpha-bungarotoxin failed to reduce the stimulation evoked release of ACh and did not produce tetanic fade. These results indicate that in addition to their postsynaptic effect, rocuronium and SZ1677 have presynaptic inhibitory effects on neuronal nAChRs at the neuromuscular junction. The finding that alpha-bungarotoxin does not inhibit the release and does not produce tetanic fade indicates that it possesses affinity only for the postsynaptic muscle nAChRs.

The role of nicotinic acetylcholine receptors in the mechanisms of anesthesia

Nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors result from the association of five subunits each contributing to the pore lining. The major neuronal nicotinic acetylcholine receptors are heterologous pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic ganglia). Another class of neuronal receptors that are found both in the central and peripheral nervous system is the homomeric alpha7 receptor. The muscle receptor subtypes comprise of alphabetadeltagamma (embryonal) or alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine receptors are not directly involved in the hypnotic component of anesthesia, it is possible that modulation of central nicotinic transmission by volatile agents contributes to analgesia. The main effect of anesthetic agents on nicotinic acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors are more sensitive to anesthetics than their muscle counterparts, with the exception of the alpha7 receptor. Several intravenous anesthetics such as barbiturates, etomidate, and propofol exert also an inhibitory effect on the nicotinic acetylcholine receptors, but only at concentrations higher than those necessary for anesthesia. Usual clinical concentrations of curare cause competitive inhibition of muscle nicotinic acetylcholine receptors while higher concentrations may induce open channel blockade. Neuronal nAChRs like alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, but at low concentrations the alpha4beta2 receptor is activated. Inhibition of sympathetic transmission by clinically relevant concentrations of some anesthetic agents is probably one of the factors involved in arterial hypotension during anesthesia.

The effects of lignocaine on actions of the venom from the yellow scorpion "Leiurus quinquestriatus" in vivo and in vitro

Many toxins from scorpion venoms activate sodium channels, thereby enhancing neurotransmitter release. The aim of the present work was to determine if the in vivo and in vitro effects of Leiurus quinquestriatus venom (LQQ) could be ameliorated by lignocaine, a sodium channel blocker. In urethane anaesthetised rabbits, LQQ venom (0.5 mg kg(-1), i.v.) caused initial hypotension and bradycardia followed by hypertension, pulmonary oedema, electrocardiographic changes indicating conduction defects, ischaemia, infarction, and then hypotension and death. Lignocaine (1 mg kg(-1) i.v. bolus initially, followed by i.v. infusion of 50 microg kg(-1) min(-1)) significantly attenuated the majority of the venom-evoked effects and reduced mortality. Addition of LQQ venom (1, 3 and 10 microg ml(-1)) to chick biventer cervicis, guinea pig ileum, and rat vas deferens preparations, increased the height of electrically-induced twitches, elevated resting tension, and caused autorhythmic oscillations. Lignocaine (3 x 10(-4)-1.2 x 10(-3) M) greatly attenuated these venom-evoked actions in the three preparations. Antagonists of appropriate neurotransmitters were also tested to determine the contribution of released transmitters to LQQ effects. Atropine significantly decreased the venom-elicited effects on guinea pig ileum preparations, while prazosin and guanethidine significantly reduced the venom's actions on rat vas deferens. In chick biventer cervicis preparations, tubocurarine and hexamethonium significantly attenuated the venom-induced effects. This study supports the hypothesis that many effects of LQQ venom involve the release of neurotransmitters and may be ameliorated by treatment with lignocaine.

No involvement of nicotinic receptors in the facilitation of acetylcholine outflow in mouse cortex in the presence of neostigmine and atropine

The role of nicotinic and muscarinic receptors in the modulation of acetylcholine release was studied using field stimulated mouse cortex slices incubated with [(3)H]-choline. Both acetylcholine (100 microM) and the cholinesterase inhibitor neostigmine (100 microM) inhibited the stimulation-induced (S-I) outflow of radioactivity but in the presence of atropine (0.3 microM) an enhancement was seen, which may be indicative of facilitatory nicotinic receptors. Mecamylamine (100 microM) was unable to antagonize the enhancement seen in the presence of acetylcholine and atropine. The nicotinic agonist dimethylphenylpiperazinium (30 microM) did not facilitate S-I outflow of radioactivity. A range of nicotinic blockers had no effect on the enhancement seen in the presence of neostigmine and atropine, nor did indomethacin, the 5HT(3) antagonist MDL 7222 nor the NMDA antagonist MK-801. The inability to block this effect suggests that nicotinic receptors are not involved. We postulate, at least for neostigmine, that the facilitation is an artefact because of the use of [(3)H]-choline as a radiotracer whereby the efflux of radioactivity is enhanced because the radiolabelled acetylcholine is not metabolized to choline and therefore flows out of the tissue more readily.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Temperature-sensitive neuromuscular transmission in Kv1.1 null mice: role of potassium channels under the myelin sheath in young nerves

In mammalian myelinated nerves, the internodal axon that is normally concealed by the myelin sheath expresses a rich repertoire of K channel subtypes thought to be important in modulating action potential propagation. The function of myelin-covered K channels at transition zones, however, has remained unexplored. Here we show that deleting the voltage-sensitive potassium channel Kv1.1 from mice confers a marked temperature-sensitivity to neuromuscular transmission in postnatal day 14 (P14)-P21 mice. Using immunofluorescence and electrophysiology, we examined contributions of four regions of the peripheral nervous system to the mutant phenotype: the nerve trunk, the myelinated segment preceding the terminal, the presynaptic terminal membrane itself, and the muscle. We conclude that the temperature-sensitive neuromuscular transmission is accounted for solely by a deficiency in Kv1.1 normally concealed in the myelinated segments just preceding the terminal. This paper demonstrates that under certain situations of physiological stress, the functional role of myelin-covered K channels is dramatically enhanced as the transition zone at the neuromuscular junction is approached.

Prejunctional effects of the nicotinic ACh receptor agonist dimethylphenylpiperazinium at the rat neuromuscular junction

1. We have studied the effects of the nicotinic acetylcholine (ACh) receptor agonist dimethylphenylpiperazinium (DMPP) on the evoked release of ACh from motor terminals in the rat isolated hemidiaphragm using an electrophysiological approach. 2. DMPP (1-4 microM) had no effect on the rate of spontaneous quantal ACh release but increased the number of quanta of ACh released per impulse during 50 Hz stimulation. The DMPP-induced increase in evoked ACh release was dependent on the frequency of stimulation, being absent when it was reduced to 0.5 Hz, but was not Ca2+ dependent, being unaffected at 50 Hz by a 4-fold decrease in the extracellular Ca2+ concentration. 3. The facilitation of evoked ACh release at 50 Hz by 2 microM DMPP was abolished by 10 microM of the calmodulin antagonist W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide hydrochloride) and, in the presence of W7, 2 microM DMPP depressed evoked ACh release at 0.5 Hz. The ability of the nicotinic ACh receptor antagonist vecuronium (1 microM) to depress evoked ACh release at 50 Hz was also abolished by 10 microM W7. 4. The present findings demonstrate, using an electrophysiological technique, that DMPP can produce changes in the evoked ACh release from rat motor nerve terminals that are consistent with the existence of facilitatory nicotinic ACh receptors on the motor nerve endings. Further, they indicate a role for calmodulin-dependent systems in this facilitatory effect of the compound.

Hexamethonium- and methyllycaconitine-induced changes in acetylcholine release from rat motor nerve terminals

1. The neuronal nicotinic receptor antagonists hexamethonium and methyllycaconitine (MLA) have been used to study the putative prejunctional nicotinic ACh receptors (AChRs) mediating a negative-feedback control of ACh release from motor nerve terminals in voltage-clamped rat phrenic nerve/ hemidiaphragm preparations. 2. Hexamethonium (200 microM), but not MLA (0.4-2.0 microM), decreased the time constant of decay of both endplate currents (e.p.cs) and miniature endplate currents (m.e.p.cs), indicating endplate ion channel block with hexamethonium. However, driving function analysis and reconvolution of e.p.cs and m.e.p.cs indicated that this ion channel block did not compromise the analysis of e.p.c. quantal content. 3. At low frequencies of stimulation (0.5-2 Hz), hexamethonium (200 microM) and MLA (2.0 microM) increased e.p.c. quantal content by 30-40%. At high frequencies (50-150 Hz) neither compound affected e.p.c. quantal content. All effects on quantal content were paralleled by changes in the size of the pool of quanta available for release. 4. The low frequency augmentation of e.p.c. quantal content by hexamethonium was absent when extracellular [Ca2+] was lowered from 2.0 to 0.5 mM. 5. At the concentrations studied, MLA and hexamethonium produced a small (10-20%) decrease in the peak amplitude of m.e.p.cs. 6. Neither apamin (100 nM) nor charybdotoxin (80 nM) had effects on spontaneous or nerve evoked current amplitudes at any frequency of stimulation. Thus the ability of nicotinic antagonists to augment e.p.c. quantal content is not due to inhibition of Ca(2+)-activated K(+)-channels. 7. We suggest that hexamethonium and MLA increase evoked ACh release by blocking prejunctional nicotinic AChRs. These receptors exert a negative feedback control over evoked ACh release and are probably of the alpha-bungarotoxin-insensitive neuronal type.

Effects of feeding conditions on sensitivity to tubocurarine of nerve-muscle preparations from the mouse

1. The effects of feeding conditions on the sensitivity to the effect of tubocurarine (dTc) in vitro were compared among various nerve-muscle preparations from mice. The mice were fed under conditions that restricted or compelled their movement for 64 days and controls were fed conventionally. 2. The sensitivity to the effect of dTc differed considerably among preparations. It was much higher in the sciatic nerve-extensor digitorum longus muscle (EDL), moderately higher in the sciatic nerve-soleus muscle (SOL) and lower in the phrenic nerve-diaphragm (DPH) in control mice. 3. The order of the sensitivity was not altered by either type of conditioning. Constant restriction of movement or compelled movement did not modify the sensitivity of DPH to the effect of dTc in vitro. 4. Compulsion facilitated the sensitivity in both SOL and EDL. Restriction selectively increased the sensitivity of EDL. Both types of conditioning selectively and significantly reduced twitch development in EDL. 5. These results indicate that the sensitivity to dTc of neuromuscular transmission reflects constant states of motor activity.

Effects of feeding condition after birth on the sensitivity of neuromuscular transmission to d-tubocurarine in vitro in mice

1. Effects of feeding condition from birth were examined on the sensitivity of neuromuscular transmission to d-tubocurarine (dTc) in vitro in male mice of the ddY strain. 2. Mice were trained to climb two separated cylindrical steel-wire tubes for feeding and drinking, respectively, from 16 days of age. Some mice were conventionally fed, from 99 days of age. Nerve-muscle preparations were made from the left phrenic nerve diaphragm muscle (DPH), the sciatic nerve soleus muscle (SOL), and the sciatic nerve extensor digitorum longus muscle (EDL) of 99-day-old and 155-day-old mice. The nerve trunk was electrically activated with trains of four pulses and tetanic pulses. 3. The sensitivity to the effects of dTc decreased in the order EDL, SOL, and DPH. This result held true in all mice tested. 4. This sensitivity was significantly potentiated by the compulsory movement. 5. The supersensitivity remained even when mice were conventionally fed after 99 days of age. 6. The compulsion rendered EDL antifatigable on tetanic stimulation. This property was also retained after a return to conventional feeding. 7. These results suggest that the effects of feeding condition from birth might remain on neuromuscular functions after termination of the conditioning.

The effect of rate of stimulation on force of contraction in a partially paralyzed rat phrenic nerve hemidiaphragm preparation

This study was performed to determine whether presynaptic receptor blockade could be differentiated from postsynaptic blockade by examining the effect of increasing rates of indirect stimulation on twitch height depression (THD) on partially paralyzed in vitro rat diaphragm preparations. We calculated the T200/T1 ratio (force of the 200th stimuli divided by the force of the first stimuli) at rates of 0.2 Hz, 0.5 Hz, 1 Hz, and 2 Hz using a drug concentration which provided approximately 20% THD during stimulation at 0.1 Hz. Markedly different T200/T1 ratios were demonstrated when hexamethonium, a drug with predominantly presynaptic effects, was compared with alpha bungarotoxin, a drug with predominantly postsynaptic effects. These results were then compared with those from vecuronium, rocuronium, mivacurium, and tubocurarine. Both hexamethonium and rocuronium caused a marked decrease in T200/T1 ratio at higher rates of stimulation; alpha bungarotoxin caused a slight increase in T200/T1 ratio at higher rates of stimulation. The T200/T1 ratios produced by vecuronium, mivacurium, and tubocurarine lay intermediate between hexamethonium and alpha bungarotoxin. Significant differences in T200/T1 ratios were found when alpha bungarotoxin was compared with all other drugs at 2 Hz. Hexamethonium and rocuronium produced significant differences in T200/T1 ratio from those of all the other drugs at 1 Hz and 2 Hz. There were significant differences in the T200/T1 ratio found after hexamethonium and rocuronium compared to alpha bungarotoxin at 0.5 Hz. No significant differences at any rate of stimulation were found between hexamethonium and rocuronium. No difference was observed in the effect of vecuronium, mivacurium, and tubocurarine. We conclude that, if the observed effect is the result of hexamethonium acting predominantly at presynaptic sites and alpha bungarotoxin acting predominantly at postsynaptic sites, the relative contribution of small doses of nondepolarizing drugs at each site can be differentiated by determining the T200/T1 ratio at rates of 1 Hz or 2 Hz. Our results are consistent with the suggestion that small doses of rocuronium have marked presynaptic activity, but that vecuronium, mivacurium, and tubocurarine have both pre- and postsynaptic effects.

Neuromuscular transmission and its pharmacological blockade. Part 1: Neuromuscular transmission and general aspects of its blockade

Blockade of neuromuscular transmission is an important feature during anaesthesia and intensive care treatment of patients. The neuromuscular junction exists in a prejunctional part where acetylcholine is synthesized, stored and released in quanta via a complicated vesicular system. In this system a number of proteins is involved. Acetylcholine diffuses across the junctional cleft and binds to acetylcholinereceptors at the postjunctional part, and is thereafter metabolized by acetylcholinesterase in the junctional cleft. Binding of acetylcholine to its postjunctional receptor evokes muscle contraction. Normally a large margin of safety exists in the neuromuscular transmission. In various situations, apart from up-and-down regulation of acetylcholine receptors, adjustment of acetylcholine release can occur. Pharmacological interference can interrupt the neuromuscular transmission and causes muscle relaxation. For this reason both depolarizing and non-depolarizing muscle relaxants are clinically used. The characteristics of an ideal clinical muscle relaxant are defined. In the description of the pharmacology of the relaxants the importance of pharmacodynamic and pharmacokinetic parameters are defined. Stereoisomerism plays a role with the relaxants. Toxins and venoms also interfere with neuromuscular transmission, through both pre- and postjunctional mechanisms.

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.

Simultaneous comparison of nicotinic receptor antagonists on three nicotinic acetylcholine receptors

The relative potencies of several nicotinic cholinoceptor antagonists in producing tetanic fade and reduction of striated muscle contraction were investigated in the isolated guinea-pig oesophagus as well as the guinea-pig and rat phrenic nerve-diaphragm preparations. Contractile smooth muscle responses to vagal stimulation, which involves ganglionic activation, were also measured simultaneously with striated muscle responses in the oesophagus. The relative potency for inhibiting the response of oesophageal smooth muscle to vagal stimulation (20 Hz) was trimetaphan > mecamylamine > hexamethonium > tubocurarine > pancuronium. For oesophageal striated muscle, production of tetanic fade at 100 Hz and reduction in peak tetanic tension at 20 or 100 Hz showed a similar relative potency; pancuronium > tubocurarine > mecamylamine > trimetaphan > hexamethonium and similar results were obtained in the guinea-pig diaphragm for the antagonists investigated (pancuronium, tubocurarine and mecamylamine). In the rat phrenic nerve-diaphragm preparation, production of tetanic fade at 50 Hz and reduction in twitch or tetanic tension all showed the relative potency; tubocurarine > pancuronium > mecamylamine > trimetaphan > hexamethonium. These findings indicate differences in the nicotinic cholinoceptor subtypes involved in vagal ganglionic responses and those in tetanic fade.

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.

Interaction of decamethonium with hexamethonium or vecuronium in the rat: an isobolographic analysis

We used isobolographic analysis to investigate the interaction of decamethonium with either hexamethonium or vecuronium in the rat phrenic nerve hemidiaphragm preparation. EC50 values of decamethonium, hexamethonium, and vecuronium were (mean +/- SEM) 47.36 +/- 9.58 microM, 4.27 +/- 0.53 mM, and 5.19 +/- 1.17 microM, respectively. Combinations of drugs in concentrations corresponding to the 1:2, 1:1, and 2:1 ratios of their EC50 values were used to determine three points of each isobole. Decamethonium and hexamethonium showed antagonism: significant deviations from the line of additivity were found at EC50 ratios of 2:1 and 1:1 (P < 0.01 and P < 0.05, respectively) indicating that hexamethonium is a potent antagonist of decamethonium. For decamethonium and vecuronium none of the three points on the isobole was significantly different from the corresponding point on the line of additivity. Hexamethonium is known to be a weak antagonist at the postsynaptic nicotinic acetylcholine receptor but a potent antagonist at the presynaptic nicotinic receptor. Vecuronium is a more potent antagonist at the postsynaptic nicotinic receptor but a much weaker antagonist at the presynaptic site. It was postulated that in the rat the primary site of action of decamethonium is at the presynaptic nerve terminal. Our findings suggest that presynaptic rather than postsynaptic potency of a nondepolarizing drug determines ability to antagonize the effect of a depolarizing drug in the rat.

A quantitative study of the pancuronium antagonism at the motor endplate in human organophosphorus intoxication

Nine patients with organophosphorus (OP) intoxication developing neuromuscular transmission defects were given pancuronium 1, 2, or 4 mg intravenously (IV). Thirteen patient controls with hypoxic encephalopathy received similar dosages. The responses were monitored electrophysiologically using single and repetitive nerve stimulation (20 and 50 Hz). In OP patients, pancuronium did not alter the amplitude of the single CMAP, whereas its repetitive discharges were reduced. Severe neuromuscular blocks were reversed only partially by pancuronium 4 mg. In less severe blocks, 1 and 2 mg resulted in marked improvement. In the patient controls, pancuronium 4 mg induced a severe neuromuscular block but not with 1 and 2 mg. Pancuronium dosages necessary to reverse severe OP-induced neuromuscular blockade produce a neuromuscular block when AChE activity is normal. Low dosages have little effect on normal neuromuscular transmission, but improve the block to a mild degree and may be useful as part of treatment in OP intoxications.

Immunohistochemical localization of neuronal nicotinic receptor subtypes at the pre- and postjunctional sites in mouse diaphragm muscle

The existence of neuronal nicotinic acetylcholine receptor (nAChR) subunits was investigated in the cryostat sections of mouse diaphragm muscles using the indirect immunofluorescence technique. The specific immunolabelings with monoclonal antibodies (mAbs) to beta 2 and to alpha 8 subunits of neuronal nAChR were observed at the endplate determined by labeling with a fluorescent dye (BODIPY)-conjugated alpha-bungarotoxin. The immunoreactivity of mAb to the alpha 3 subunit of neuronal nAChR was detected on the motor nerve fibers including the nerve terminals. These results provide evidence that the subtypes of postsynaptic nAChR, recognized by the anti-beta 2 and/or anti-alpha 8 mAbs, and the presynaptic nAChR recognized by the anti-alpha 3 mAb, are present at the neuromuscular junction, in addition to the classical muscle nAChR.

Improvement in performance of a delayed matching-to-sample task by monkeys following ABT-418: a novel cholinergic channel activator for memory enhancement

ABT-418, a newly characterized centrally acting cholinergic channel activator (ChCA), was evaluated for its ability to improve performance in a delayed matching-to-sample (DMTS) task by mature macaques well trained in the task. Previous studies in rodents have indicated that ABT-418 shares the memory/cognitive enhancing actions of nicotine, but without many of nicotine's dose-limiting side effects. As DMTS provides a measure both of general cognitive function (the matching concept) and of recent memory, it was hypothesized that some doses of ABT-418 would enhance the monkeys' ability to correctly perform the DMTS task. Intramuscular administration of ABT-418 significantly enhanced DMTS performance at low (2-32.4 nmol/kg) doses. In fact, the drug was slightly more potent that nicotine in this regard, and all eight animals tested in this study exhibited enhanced performance at one or more doses. ABT-418 produced the greatest improvement in DMTS performance at the longest delay interval. In animals repeatedly tested with their individualized "Best Dose", DMTS performance increased on average by 10.1 +/- 3.5 percentage points correct, which was equivalent to an increase of 16.2% over baseline performance. ABT-418 did not significantly affect response times, i.e., latencies to make a choice between stimuli, or latencies to initiate new trials. Whereas nicotine enhanced DMTS performance both on the day of administration and on the following day (in the absence of drug), ABT-418-induced enhanced performance was detected only on the day of administration. Finally, single daily administration of the individualized best dose in three monkeys over a period of 8 days generally maintained enhancement of DMTS performance. Thus, the data were not consistent with the development of significant tolerance to the drug's mnemonic actions. In contrast to nicotine, no overt toxicity or side effects to acute or repeated administration of the drug were noted. Thus, ABT-418 represents a prototype of a new class of nicotinic agonists designed for the potential treatment of human dementias having a low profile of toxicity.

Prejunctional actions of muscle relaxants: synaptic vesicles and transmitter mobilization as sites of action

1. Nicotinic antagonists such as tubocurarine affect acetylcholine release from motor nerve terminals at the neuromuscular junction. 2. Electrophysiological studies comparing the prejunctional actions of tubocurarine to those of vesamicol and vecuronium have been used to provide an insight into the mechanisms involved in the prejunctional effects of tubocurarine-like compounds. 3. The observed prejunctional actions of tubocurarine can be accounted for by a model in which the compound has two separately identifiable effects on the nerve terminal. At low frequencies of nerve stimulation tubocurarine augments acetylcholine release while at high frequencies of nerve stimulation tubocurarine depresses acetylcholine release. 4. Both of the effects of tubocurarine on acetylcholine release are a consequence of a change in the number of quanta within the nerve terminal immediately available for release upon nerve stimulation. 5. On the basis of our experimental observations, we suggest that the two prejunctional effects of tubocurarine are mediated through two pharmacologically distinct prejunctional nAChRs.

P-type Ca2+ channels trigger stimulus-evoked [3H]acetylcholine release from mammalian motor endplates

In the present experiments it was tested whether omega-agatoxin-IVA, a peptide blocking P-type voltage-dependent Ca2+ channels, inhibits the evoked release of newly synthesized [3H]acetylcholine from the rat phrenic nerve. Release of [3H]acetylcholine was evoked by electrical stimulation of the isolated phrenic nerve (100 or 750 pulses at 5 Hz). omega-Agatoxin-IVA inhibited evoked [3H]acetylcholine release in a concentration-related manner; inhibition started at a concentration of 30 nM with complete block occurring at 500 nM. In conclusion, the present experiments demonstrate that omega-agatoxin-IVA-sensitive P-type Ca2+ channels are critically involved in the regulation of stimulus-induced transmitter release at mammalian motor endplates.

Inhibitory action of nicotinic antagonists on transmitter release at the neuromuscular junction of the rat

The effects of two nicotinic antagonists, d-tubocurarine (TC) and hexamethonium (HEX) were tested on the rat diaphragm neuromuscular junction during train-of-six stimuli to determine if a second action of these antagonists on evoked release could be demonstrated, in addition to its known impact of blocking the autoreceptor pathway. To minimize the autoreceptor pathway, the preparations were examined under low transmitter release conditions. It was observed that both compounds significantly depressed the end-plate potential amplitudes more than the miniature end-plate potential amplitudes, while also significantly depressing quantal release output. This inhibitory action is contrary to what is observed when transmitter release is high, where feedback regulation via the autoreceptors serves a prominent role. It is concluded that this depressive action on transmitter output contributes to onset of tetanic fade and that when higher concentrations of these antagonists are used this inhibitory action of TC and HEX may override autoreceptor feedback regulation.

Tonic adenosine A2A receptor activation modulates nicotinic autoreceptor function at the rat neuromuscular junction

The influence of the activation of presynaptic adenosine receptors on nicotinic autofacilitation of electrically evoked [3H]acetylcholine release from rat phrenic motor nerve terminals was investigated. Blocking the adenosine A2A receptor with 3,7-dimethyl-1-propargylxanthine (DMPX, 10 microM) greatly potentiated, whereas the adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 2.5 nM), partially prevented the facilitatory effect of the nicotinic receptor agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 1 microM, 3 min), on evoked [3H]acetylcholine release. The adenosine A2A receptor agonist, 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamideadeno sine (CGS 21680C, 3 nM), but not the adenosine A1 receptor agonist, R-N6-phenylisopropyl adenosine (R-PIA, 300 nM), partially blocked the DMPP (1 microM) facilitation. Forskolin (3 microM) mimicked the attenuation caused by CGS 21680C; inhibition of adenylate cyclase with N-(as-2-phenylcyclopentyl)azacyclo-tridecan-2-imine hydrochloride (MDL 12,330A, 10 microM) markedly enhanced the facilitatory effect of DMPP (1 microM). Prolonged exposure to a high concentration of DMPP (10 microM, 15 min) decreased evoked tritium outflow. The decrease in evoked [3H]acetylcholine release following prolonged exposure to DMPP was augmented by pretreatment with CGS 21680C (3 nM) and forskolin (3 microM), and was abolished by inactivating endogenous adenosine with adenosine deaminase (0.5 U/ml). It is concluded that tonic adenosine A2A receptor activation regulates nicotinic acetylcholine autofacilitation. This action is likely to be mediated through an adenylate cyclase/cyclic AMP-dependent mechanism.

Nicotinic antagonist-produced frequency-dependent changes in acetylcholine release from rat motor nerve terminals

1. The frequency (0.5-150 Hz) and calcium dependence (0.5-2.0 mM) of the effects of the nicotinic antagonist tubocurarine (0.2 microM) on acetylcholine (ACh) liberation from motor nerve terminals has been examined using binomial analysis of quantal transmitter release. 2. At an extracellular calcium ion concentration ([Ca2+]o) of 2.0 mM, tubocurarine produced a decrease in the endplate current (EPC) quantal content of approximately 30% at high frequencies of motor nerve stimulation (50-150 Hz). In contrast, at low frequencies of stimulation (0.5-1.0 Hz), tubocurarine enhanced the EPC quantal content by approximately 20%. 3. The enhancement of EPC quantal content produced by tubocurarine at low frequencies of motor nerve stimulation was [Ca2+]o dependent, being abolished when [Ca2+]o was lowered from 2.0 to 0.5 mM. In contrast, the decrease in quantal content produced by tubocurarine at high frequencies of motor nerve stimulation was independent of [Ca2+]o, being approximately 30% at all calcium ion concentrations studied. 4. In direct contrast to tubocurarine, the nicotinic antagonist vecuronium (1.0 microM) produced no increase in EPC quantal content at low frequencies of nerve stimulation. However, at high frequencies of nerve stimulation it decreased EPC quantal content to a similar extent to 0.2 microM tubocurarine. The frequency-dependent decrease in EPC quantal content produced by 1.0 microM vecuronium in 2.0 mM [Ca2+]o was very similar to that seen with 0.2 microM tubocurarine in 0.5 mM [Ca2+]o. 5. Binomial analysis revealed that all the changes in EPC quantal content associated with both nicotinic antagonists were due to changes in the size of the pool of quanta in the nerve terminal available for immediate release with no effect on the probability of release of an individual quantum. 6. The results are interpreted in terms of two separately identifiable prejunctional actions of the nicotinic antagonists, both involving an action at nicotinic ACh receptors situated on the motor nerve terminal. Thus, at high frequencies of motor nerve stimulation tubocurarine and vecuronium produce a [Ca2+]o-independent decrease in ACh release, probably through an inhibitory action on a positive-feedback prejunctional nicotinic autoreceptor closely related to the muscle-type nicotinic ACh autoreceptor. However, at low frequencies of motor nerve stimulation we suggest that tubocurarine, but not vecuronium, produces a [Ca2+]o-dependent increase in ACh release through an action at a negative-feedback prejunctional neuronal-type nicotinic ACh autoreceptor.

Potentiation by tonic A2a-adenosine receptor activation of CGRP-facilitated [3H]-ACh release from rat motor nerve endings

1. The effect of calcitonin gene-related peptide (CGRP) on [3H]-acetylcholine ([3H]-ACh) release from motor nerve endings and its interaction with presynaptic facilitatory A2a-adenosine and nicotinic acetylcholine receptors was studied on rat phrenic nerve-hemidiaphragm preparations loaded with [3H]-choline. 2. CGRP (100-400 nM) increased electrically evoked [3H]-ACh release from phrenic nerve endings in a concentration-dependent manner. 3. The magnitude of CGRP excitation increased with the increase of the stimulation pulse duration from 40 microseconds to 1 ms, keeping the frequency, the amplitude and the train length constants. With 1 ms pulses, the evoked [3H]-ACh release was more intense than with 40 microseconds pulse duration. 4. Both the nicotinic acetylcholine receptor agonist, 1,1-dimethyl-4-phenylpiperazinium, and the A2a adenosine receptor agonist, CGS 21680C, increased evoked [3H]-ACh release, but only CGS 21680C potentiated the facilitatory effect of CGRP. This potentiation was prevented by the A2a adenosine receptor antagonist, PD 115,199. 5. Adenosine deaminase prevented the excitatory effect of CGRP (400 nM) on [3H]-ACh release. This effect was reversed by the non-hydrolysable A2a-adenosine receptor agonist, CGS 21680C. 6. The nicotinic antagonist, tubocurarine, did not significantly change, whereas the A2-adenosine receptor antagonist, PD 115,199, blocked the CGRP facilitation. The A1-adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine, potentiated the CGRP excitatory effect. 7. The results suggest that the facilitatory effect of CGRP on evoked [3H]-ACh release from rat phrenic motor nerve endings depends on the presence of endogenous adenosine which tonically activates A2a-adenosine receptors. Since both CGRP and A2a-adenosine receptors are positively coupled to the adenylate cyclase/cyclic AMP system, cooperation between these receptors might occur at the second messenger transduction system level.

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.

Enhancement by calcitonin gene-related peptide of nicotinic receptor-operated noncontractile Ca2+ mobilization at the mouse neuromuscular junction

1. The involvement of calcitonin gene-related peptide (CGRP) in the mechanism of nicotinic acetylcholine receptor-operated noncontractile Ca2+ mobilization (not accompanied by twitch tension) was investigated by measuring Ca(2+)-aequorin luminescence at the neuromuscular junction of mouse diaphragm muscle treated with neostigmine. 2. Noncontractile Ca2+ transients were enhanced by 4-aminopyridine (100 microM), a K+ channel blocker, and inhibited by botulinum toxin (1-100 micrograms, i.p.) and hexamethonium (10-100 microM), a neuronal nicotinic receptor antagonist. 3. Noncontractile Ca2+ transients were diminished by CGRP8-37 (10-20 microM), a CGRP antagonist. CGRP (0.3-10 nM) prolonged the duration of noncontractile Ca2+ transients. The effect of CGRP was suppressed by CGRP8-37 (0.1 microM). 4. Noncontractile Ca2+ transients were inhibited by H-89 (0.1-1 microM), a protein kinase-A inhibitor. The catalytic subunit of protein kinase-A and AA373 (300 microM), a protein kinase-A activator, prolonged the duration of noncontractile transients. The prolongations either by CGRP or by AA373 were not observed in the presence of H-89 (0.1 microM). 5. Contractile (accompanied by twitch tension) but not noncontractile Ca2+ transients were decreased by 12-O-tetradecanoyl phorbol 13-acetate (TPA, 0.3-1 microM), a protein kinase-C activator. Phospholipase A2 increased only contractile Ca2+ transients. Calmodulin-related agents affected neither type of Ca2+ transients. 6. These results provide the first evidence that nicotinic acetylcholine receptor-operated noncontractile Ca2+ mobilization is promoted by nerve-released CGRP activating protein kinase-A, and is dependent on the accumulated amounts of acetylcholine at the neuromuscular junction where desensitization might readily develop.

Adenosine and adenine nucleotides are independently released from both the nerve terminals and the muscle fibres upon electrical stimulation of the innervated skeletal muscle of the frog

The independent release of adenosine and adenine nucleotides upon electrical stimulation was studied in the innervated sartorius muscle of the frog after blockade of the extracellular catabolism of adenosine monophosphate (AMP) through exo-AMP deaminase and ecto-5'-nucleotidase. Nerve stimulation (30 min, 0.2Hz) induced the release of both adenosine (19 +/- 3 pmol) and adenine nucleotides (101 +/- 7 pmol). Experiments performed in the presence of tubocurarine (5 microM) to prevent purine release due to nerve-evoked muscle twitching, or under direct stimulation of the muscle in low calcium solutions to prevent pre-synaptic release of purines, showed that there was an evoked release of adenosine and adenine nucleotides both from the nerve endings and from the twitching muscle fibres. Removal of ecto-5'-nucleotidase inhibition shows that the catabolism of adenine nucleotides released during stimulation contributes in about 50% to the amount of endogenous extracellular adenosine. When only one of the enzymes catabolizing AMP (ecto-5'-nucleotidase or exo-AMP deaminase) was inhibited, the evoked release of adenine nucleotides was undetectable, suggesting that each enzyme is able to catabolize all the AMP formed from adenine nucleotides released upon stimulation. It is concluded that the concentration of endogenous extracellular adenosine is under the control of the relative activities of exo-AMP deaminase and ecto-5'-nucleotidase.

Pharmacological studies on cutaneous inflammation induced by ultraviolet irradiation (1): quantification of erythema by reflectance colorimetry and correlation with cutaneous blood flow

This study was conducted to quantify the intensity of ultraviolet (UV) erythema in guinea pigs, a method for evaluating anti-inflammatory drugs, and to clarify any correlation of erythema with cutaneous blood flow. Skin color and cutaneous blood flow in non-administered and indomethacin-administered animals were measured by a colorimeter and a laser Doppler flowmeter over time after UV-irradiation treatment. Skin color was indicated by a XYZ colorimetric system and L*a*b* color space. In either colorimetric system, the values of two indices, x and y or a* and b*, increased along with the intensification of erythema. The increase in the chroma (C*) value calculated from a* and b* was UV-dose-dependent. This value was significantly suppressed by indomethacin 0.5-4 hr after irradiation, and it was found to be a clear and sensitive index for evaluating the suppressive effect of drugs. Cutaneous blood flow also increased with UV irradiation. Indomethacin significantly suppressed this increase 2-3 hr after UV irradiation. The changes of cutaneous blood flow correlated with those of C*. These results suggested C* was a suitable parameter to quantify UV erythema, and the change of skin color in UV erythema reflected the change of cutaneous blood flow.

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)

Effects of hexamethonium on transmitter release from the rat phrenic nerve

Hexamethonium (HEX) was applied to isolated transected diaphragm-phrenic nerve preparations of the rat in order to further elucidate the functional role of the presynaptic nicotinic autoreceptors. End-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) were recorded from the neuromuscular junctions in the presence and absence of HEX to determine the relative effect of this nicotinic antagonist on end-plate sensitivity and evoked release. In this study we show that HEX enhances transmitter release for the first few stimuli, but this action is not maintained during a train-of-six stimulation. While these results support the hypothesis that transmitter released from the nerve terminal normally has a negative feedback effect by depressing transmitter release it is proposed that HEX also has secondary actions on the neuromuscular junction that are unrelated to autoreceptor blockage. The results with HEX suggests that the presynaptic receptors may differ pharmacologically from the postsynaptic receptors.

Effects of tetrodotoxin, Ca2+ absence, d-tubocurarine and vesamicol on spontaneous acetylcholine release from rat muscle

1. Rat hemidiaphragms were incubated in a physiological low-K+ medium without stimulation and the amount of acetylcholine (ACh) released was measured radioenzymatically. Cholinesterases were inhibited by paraoxon. 2. In the presence of 1 microM tetrodotoxin (TTX), the amount of ACh released during a 2 h incubation was lowered by 40%. A similar decrease was observed in the absence of Ca2+ and in the presence of 10 microM-d-tubocurarine (dTC). The effects of TTX combined with Ca2+ removal, and of TTX combined with dTC were no greater than those of TTX, dTC or Ca2+ removal alone. TTX and dTC had no effect on the release of ACh from diaphragms 4 days after denervation. 3. The reduction of spontaneous ACh release observed in the presence of TTX or dTC or in the absence of Ca2+ is best interpreted on the assumption that about 40% of the ACh release was due to the impulse activity known to be generated in intramuscular motor nerve branches by the ACh which accumulates after the inhibition of cholinesterases. 4. In the presence of 1 and 10 microM vesamicol (AH5183, 2-(4-phenylpiperidino)-cyclohexanol), the release of ACh was also diminished by approximately 40%. Vesamicol did not augment the inhibition of release produced by TTX or by the omission of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)