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

Effect of endogenous purines on electrically evoked ACh release at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP), which is co-released with the neurotransmitter acetylcholine (ACh), and its metabolite adenosine, modulate neurotransmitter release by activating presynaptic inhibitory P2Y₁₃ receptors (a subtype of ATP/adenosine diphosphate [ADP] receptor), inhibitory A₁ and A₃ adenosine receptors, and excitatory A_2A adenosine receptors. To study the effect of endogenous purines, when phrenic-diaphragm preparations are depolarized by different nerve stimulation patterns, we analyzed the effect of the antagonists for P2Y₁₃ , A₁ , A₃ , and A_2A receptors (AR-C69931MX, 8-cyclopentyl-1,3-dipropylxanthine, MRS-1191, and SCH-58261, respectively) on the amplitude of the end-plate potentials of the trains, and contrasted these results with those obtained with the selective agonists of these receptors (2-methylthioadenosine 5'-diphosphate trisodium salt hydrate, 2-chloro-N⁶ -cyclopentyl-adenosine, inosine, and PSB-0777, respectively). During continuous 0.5-Hz stimulation, the amount of endogenous purines was not enough to activate purinergic receptors, while at continuous 5-Hz stimulation, an incipient action of endogenous purines on P2Y₁₃ , A₁ and A3 receptors might be evident just at the end of the trains. During continuous 50-Hz stimulation, the concentration of endogenous ATP/ADP and adenosine exerted an inhibitory action on ACh release after of the initial phase of the train, but when the nerve was stimulated at intermittent 50 Hz (5 bursts), this behavior was not observed. Excitatory A_2A receptors were only activated when continuous 100-Hz stimulation was applied. In conclusion, when motor nerve terminals are depolarized by repetitive stimulation of the phrenic nerve, endogenous ATP/ADP and adenosine are able to fine-tune neurosecretion depending on the frequency and pattern of stimulation.

Physiological involvement of presynaptic L-type voltage-dependent calcium channels in GABA release of cerebellar molecular layer interneurons

While high threshold voltage-dependent Ca²⁺ channels (VDCCs) of the N and P/Q families are crucial for evoked neurotransmitter release in the mammalian CNS, it remains unclear to what extent L-type Ca²⁺ channels (LTCCs), which have been mainly considered as acting at postsynaptic sites, participate in the control of transmitter release. Here, we investigate the possible role of LTCCs in regulating GABA release by cerebellar molecular layer interneurons (MLIs) from rats. We found that BayK8644 (BayK) markedly increases mIPSC frequency in MLIs and Purkinje cells (PCs), suggesting that LTCCs are expressed presynaptically. Furthermore, we observed (1) a potentiation of evoked IPSCs in the presence of BayK, (2) an inhibition of evoked IPSCs in the presence of the LTCC-specific inhibitor Compound 8 (Cp8), and (3) a strong reduction of mIPSC frequency by Cp8. BayK effects are reduced by dantrolene, suggesting that ryanodine receptors act in synergy with LTCCs. Finally, BayK enhances presynaptic AP-evoked Ca²⁺ transients and increases the frequency of spontaneous axonal Ca²⁺ transients observed in TTX. Taken together, our data demonstrate that LTCCs are of primary importance in regulating GABA release by MLIs.

β3 Adrenoceptor-induced cholinergic inhibition in human and rat urinary bladders involves the exchange protein directly activated by cyclic AMP 1 favoring adenosine release

BACKGROUND AND PURPOSE

The mechanism by which β₃ receptor agonists (e.g. mirabegron) control bladder overactivity may involve adenosine release from human and rat detrusor smooth muscle. Retrograde activation of adenosine A₁ receptors reduces ACh release from cholinergic bladder nerves. β₃ -Adrenoceptors usually couple to adenylyl cyclase. Here we investigated, which of the cAMP targets, protein kinase A or the exchange protein directly activated by cAMP (EPAC) could be involved in this cholinergic inhibition of the bladder.

EXPERIMENTAL APPROACH

[³ H]ACh and adenosine release from urothelium-denuded detrusor strips of cadaveric human organ donors and rats were measured by liquid scintillation spectrometry and HPLC, respectively. In vivo cystometry was also performed in urethane-anaesthetized rats.

KEY RESULTS

The exchange protein directly activated by cAMP (EPAC) inhibitor, ESI-09, prevented mirabegron- and isoprenaline-induced adenosine release from human and rat detrusor strips respectively. ESI-09, but not the PKA inhibitor, H-89, attenuated inhibition of [³ H]ACh release from stimulated (10 Hz) detrusor strips caused by activating β₃ -adrenoceptors, AC (forskolin) and EPAC1 (8-CTP-2Me-cAMP). Isoprenaline-induced inhibition of [³ H]ACh release was also prevented by inhibitors of PKC (chelerythrine and Go6976) and of the equilibrative nucleoside transporter 1 (ENT1; dipyridamole and NBTI), but not by PLC inhibition with U73122. Pretreatment with ESI-09, but not with H-89, prevented the reduction of the voiding frequency caused by isoprenaline and forskolin in vivo.

CONCLUSION AND IMPLICATIONS

Data suggest that β₃ -adrenoceptor-induced inhibition of cholinergic neurotransmission in human and rat urinary bladders involves activation of an EPAC1/PKC pathway downstream cAMP production resulting in adenosine outflow via ENT1.

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.

Bothropstoxin-I reduces evoked acetylcholine release from rat motor nerve terminals: radiochemical and real-time video-microscopy studies

Understanding the biological activity profile of the snake venom components is fundamental for improving the treatment of snakebite envenomings and may also contribute for the development of new potential therapeutic agents. In this work, we tested the effects of BthTX-I, a Lys49 PLA(2) homologue from the Bothrops jararacussu snake venom. While this toxin induces conspicuous myonecrosis by a catalytically independent mechanism, a series of in vitro studies support the hypothesis that BthTX-I might also exert a neuromuscular blocking activity due to its ability to alter the integrity of muscle cell membranes. To gain insight into the mechanisms of this inhibitory neuromuscular effect, for the first time, the influence of BthTX-I on nerve-evoked ACh release was directly quantified by radiochemical and real-time video-microscopy methods. Our results show that the neuromuscular blockade produced by in vitro exposure to BthTX-I (1 μM) results from the summation of both pre- and postsynaptic effects. Modifications affecting the presynaptic apparatus were revealed by the significant reduction of nerve-evoked [(3)H]-ACh release; real-time measurements of transmitter exocytosis using the FM4-64 fluorescent dye fully supported radiochemical data. The postsynaptic effect of BthTX-I was characterized by typical histological alterations in the architecture of skeletal muscle fibers, increase in the outflow of the intracellular lactate dehydrogenase enzyme and progressive depolarization of the muscle resting membrane potential. In conclusion, these findings suggest that the neuromuscular blockade produced by BthTX-I results from transient depolarization of skeletal muscle fibers, consequent to its general membrane-destabilizing effect, and subsequent decrease of evoked ACh release from motor nerve terminals.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

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.

Tetanic depression is overcome by tonic adenosine A(2A) receptor facilitation of L-type Ca(2+) influx into rat motor nerve terminals

Motor nerve terminals possess multiple voltage-sensitive calcium channels operating acetylcholine (ACh) release. In this study, we investigated whether facilitation of neuromuscular transmission by adenosine generated during neuronal firing was operated by Ca(2+) influx via 'prevalent' P-type or via the recruitment of 'silent' L-type channels. The release of [(3)H]ACh from rat phrenic nerve endings decreased upon increasing the stimulation frequency of the trains (750 pulses) from 5 Hz (83 +/- 4 x 10(3) disintegrations per minute per gram (d.p.m. g(-1)); n = 11) to 50 Hz (30 +/- 3 x 10(3) d.p.m. g(-1); n = 5). The P-type Ca(2+) channel blocker, omega-agatoxin IVA (100 nm) reduced (by 40 +/- 10%; n = 6) the release of [(3)H]ACh evoked by 50-Hz trains, while nifedipine (1 microM, an L-type blocker) was inactive. Tetanic depression was overcome (88 +/- 6 x 10(3) d.p.m. g(-1); n = 12) by stimulating the phrenic nerve with 50-Hz bursts (five bursts of 150 pulses, 20 s interburst interval). In these conditions, omega-agatoxin IVA (100 nM) failed to affect transmitter release, but nifedipine (1 microM) decreased [(3)H]ACh release by 21 +/- 7% (n = 4). Inactivation of endogenous adenosine with adenosine deaminase (ADA, 0.5 U ml(-1)) reduced (by 54 +/- 8%, n = 5) the release of [(3)H]ACh evoked with 50-Hz bursts. This effect was opposite to the excitatory actions of adenosine (0.5 mm), S-(p-nitrobenzyl)-6-thioinosine (5 microM, an adenosine uptake blocker) and CGS 21680C (3 nM, a selective A(2A) receptor agonist); as the A(1) receptor agonist R-N(6)-phenylisopropyl adenosine (R-PIA, 300 nM) failed to affect the release of [(3)H]ACh, the results indicate that adenosine generated during 50-Hz bursts exerts an A(2A)-receptor-mediated tonus. The effects of ADA (0.5 U ml(-1)) and CGS 21680C (3 nm) were prevented by nifedipine (1 microM). Blocking tonic A(2A) receptor activation, with ADA (0.5 U ml(-1)) or 3,7-dimethyl-1-propargyl xanthine (10 microM, an A(2A) antagonist), recovered omega-agatoxin IVA (100 nM) inhibition and caused the loss of function of nifedipine (1 microM). Data indicate that, in addition to the predominant P-type Ca(2+) current triggering ACh release during brief tetanic trains, motoneurones possess L-type channels that may be recruited to facilitate transmitter release during high-frequency bursts. The fine-tuning control of Ca(2+) influx through P- or L-type channels is likely to be mediated by endogenous adenosine. Therefore, tonic activation of presynaptic A(2A) receptors operating Ca(2+) influx via L-type channels may contribute to overcome tetanic depression during neuronal firing.

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

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

Iberiotoxin-induced block of Ca2+-activated K+ channels induces dihydropyridine sensitivity of ACh release from mammalian motor nerve terminals

The role which Ca(2+)-activated K(+) (K(Ca)) channels play in regulating acetylcholine (ACh) release was examined at mouse motor nerve terminals. In particular, the ability of the antagonist iberiotoxin to recruit normally silent L-type Ca(2+) channels to participate in nerve-evoked release was examined using conventional intracellular electrophysiological techniques. Incubation of cut hemidiaphragm preparations with 10 microM nimodipine, a dihydropyridine L-type Ca(2+) channel antagonist, had no significant effect on quantal content of end-plate potentials. Nevertheless, 1 microM S-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester (Bay K 8644) enhanced quantal content to 134.7 +/- 3.5% of control. Iberiotoxin (150 nM) increased quantal content to 177.5 +/- 9.9% of control, whereas iberiotoxin plus nimodipine increased quantal content to only 145.7 +/- 10.4% of control. Coapplication of 1 microM Bay K 8644 with iberiotoxin did not significantly increase quantal content further than did treatment with iberiotoxin alone. The effects of iberiotoxin and nimodipine alone or in combination on the miniature end-plate potential (MEPP) frequency following KCl-induced depolarization were examined using uncut hemidiaphragm preparations. Nimodipine alone had no effect on MEPP frequency from preparations incubated in physiological saline containing 5 to 20 mM KCl. Moreover, iberiotoxin alone or combined with nimodipine also had no effect on MEPP frequency in physiological salines containing 5 to 15 mM KCl. At 20 mM KCl, however, iberiotoxin significantly increased MEPP frequency to 125.6% of iberiotoxin-free values; combined treatment with nimodipine and iberiotoxin prevented this increase in MEPP frequency. Thus, loss of functional K(Ca) channels unmasks normally silent L-type Ca(2+) channels to participate in ACh release from motor nerve terminals, particularly under conditions of intense nerve terminal depolarization.

The surgical management of facial nerve injury

Treatment of facial nerve injuries depends upon a detailed understanding of its anatomic course, accurate clinical examination, and timely and appropriate diagnostic studies. Reconstruction depends upon the extent of injury, the availability of the proximal stump. and the time since injury and duration of muscle denervation. Although no alternative is perfect, these techniques, in combination with static and ancillary procedures. can protect the eye, prevent drooling, restore the smile, and improve facial symmetry. New techniques (including single-stage free tissue transfers and bioengineered nerve grafts), further research on the characteristics of the facial musculature, and methods of preserving the neuromuscular junction will undoubtedly manifest themselves as further refinements of established surgical techniques.

Adenosine inhibits N-type calcium channels at the rat neuromuscular junction

1. In earlier studies, it has been reported that under in vitro conditions transmitter release at the rat neuromuscular junction is normally suppressed due to the effect of adenosine release from the isolated tissue. In the present study we wanted to determine whether this action may involve the inhibition of calcium influx through adenosine-sensitive calcium channels. 2. In order to test this hypothesis, we examined the role of N-type calcium channels in regulating nerve-evoked transmitter release by using the N-type calcium channel-specific blocker omega-conotoxin GVIA (CTX). In order to control the inhibitory action of adenosine, we also used the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). We tested the effect of blocking N-type calcium channels with CTX in the presence and absence of DPCPX. We examined the effects of these drugs on quantal transmitter release in the transected preparation of the phrenic nerve-hemidiaphragm of the rat using intracellular recording techniques. 3. At 10 nmol/L, CTX alone had no effect on nerve-evoked transmitter release; however, in the presence of 0.1 micro mol/L DPCPX, CTX significantly depressed nerve-evoked transmitter release. 4. These data support the view that adenosine inhibits nerve-evoked transmitter release by inhibiting N-type calcium channels on nerve terminals.

Calcium channel subtypes contributing to acetylcholine release from normal, 4-aminopyridine-treated and myasthenic syndrome auto-antibodies-affected neuromuscular junctions

1 Acetylcholine release at the neuromuscular junction relies on rapid, local and transient calcium increase at presynaptic active zones, triggered by the ion influx through voltage-dependent calcium channels (VDCCs) clustered on the presynaptic membrane. Pharmacological investigation of the role of different VDCC subtypes (L-, N-, P/Q- and R-type) in spontaneous and evoked acetylcholine (ACh) release was carried out in adult mouse neuromuscular junctions (NMJs) under normal and pathological conditions. 2 omega-Agatoxin IVA (500 nM), a specific P/Q-type VDCC blocker, abolished end plate potentials (EPPs) in normal NMJs. However, when neurotransmitter release was potentiated by the presence of the K(+) channel blocker 4-aminopyridine (4-AP), an omega-agatoxin IVA- and omega-conotoxin MVIIC-resistant component was detected. This resistant component was only partially sensitive to 1 micro M omega-conotoxin GVIA (N-type VDCC blocker), but insensitive to any other known VDCC blockers. Spontaneous release was dependent only on P/Q-type VDCC in normal NMJs. However, in the presence of 4-AP, it relied on L-type VDCCs too. 3 ACh release from normal NMJs was compared with that of NMJs of mice passively injected with IgGs obtained from patients with Lambert-Eaton myasthenic syndrome (LEMS), a disorder characterized by a compromised neurotransmitter release. Differently from normal NMJs, in LEMS IgGs-treated NMJs an omega-agatoxin IVA-resistant EPP component was detected, which was only partially blocked by calciseptine (1 micro M), a specific L-type VDCC blocker. 4 Altogether, these data demonstrate that multiple VDCC subtypes are present at the mouse NMJ and that a resistant component can be identified under 'pharmacological' and/or 'pathological' conditions.

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.

Synergism between A(2A)-adenosine receptor activation and vasoactive intestinal peptide to facilitate [3H]-acetylcholine release from the rat motor nerve terminals

The effect of vasoactive intestinal peptide (VIP) on evoked [(3)H]-acetylcholine ([(3)H]-ACh) release from motor nerve terminals, and its interaction with presynaptic facilitatory A(2A)-adenosine receptors was investigated in the rat phrenic nerve-hemidiaphragms. Facilitation of [(3)H]-ACh release by VIP (100 nM) only becomes apparent when high frequency (50 Hz) or long lasting pulses (1 ms) were delivered to the phrenic nerve; VIP excitation was prevented by removal of endogenous adenosine tonus, with adenosine deaminase (2.5 units/ml) or with the A(2A)-receptor antagonist, 3,7-dimethyl-1-propargyl xanthine, (10 microM). Pretreatment with the selective A(2A)-receptor agonist, CGS 21680C (2 nM), potentiated the neurofacilitatory action of VIP (100 nM). The results suggest that tonic A(2A)-receptors activation by endogenous adenosine is required to trigger the facilitatory action of VIP on evoked [(3)H]-ACh release from motor nerve endings.