Release and effects of ATP and its derivatives at cholinergic synapses

LY 294002 inhibits adenosine receptor activation by a mechanism independent of effects on PI-3 kinase or casein kinase II

Adenosine reduces both evoked and spontaneous calcium-dependent acetylcholine (ACh) release through a mechanism downstream of calcium entry at amphibian motor nerve endings (Silinsky EM. J Physiol 1984; 346: 243-6). LY 294002 (2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one), an inhibitor of both phosphoinositide-3 kinase (PI-3 kinase) and casein kinase II, has been reported to increase spontaneous ACh release reflected in miniature endplate potential (MEPP) frequencies independently of intraterminal calcium at the frog neuromuscular junction (Rizzoli SO, Betz WJ. J Neurosci 2002; 22: 10680-). It has been suggested that the increase in MEPP frequency caused by LY 294002, is mediated through an action on synaptotagmins, vesicle associated calcium sensors believed to trigger synaptic vesicle exocytosis. We thus examined the effects of adenosine on MEPP frequencies and evoked ACh release reflected as endplate potentials (EPPs) in order to determine if the presumed calcium-independent ACh release is affected by adenosine. We also wanted to determine if PI-3 kinase or casein kinase II is involved in mediating or modulating the inhibitory effects of adenosine. To these ends, we examined the effects of adenosine in the presence of LY 294002, wortmannin (a highly selective the PI-3 kinase inhibitor), or DRB (5,6-dichlorobenzimidazole riboside, an inhibitor of casein kinase II). LY 294002 reduced the sensitivity of both MEPP frequencies and the nerve-evoked calcium dependent EPPs to adenosine. The occlusive effects of LY 294002 on the actions of adenosine on MEPPs and EPPs were overcome by increasing adenosine concentration. Neither wortmannin nor DRB had any effect on the sensitivity of the EPPs to adenosine indicating that neither PI-3 kinase nor casein kinase II inhibition mediates the reduction in motor-nerve terminal sensitivity to adenosine produced by LY 294002. The results indicate a competitive relationship between LY 294002 and adenosine at A₁ receptors at the frog neuromuscular junction. This effect is independent of the previously described effects of LY 294002 on the exocytotic process, and is also independent of PI-3 kinase or casein kinase II.

Modulation of calcium currents is eliminated after cleavage of a strategic component of the mammalian secretory apparatus

Adenosine inhibits neurotransmitter secretion from motor nerves by an effect on the secretory apparatus in amphibia. In contrast, the inhibitory effect of adenosine is associated with decreases in calcium currents at mouse motor nerve endings. To determine if the action of adenosine in the mouse is mediated thorough a direct effect on calcium channels or through the secretory machinery, the effects of cleavage of the SNARE proteins on the action of adenosine were examined. Cleavage of the SNARE syntaxin with botulinum toxin type C (Botx/C) prevented the inhibitory effect of adenosine on nerve terminal calcium currents. Cleavage of the other SNAREs (synaptobrevin with Botx/D or SNAP-25 with Botx/A) failed to affect the inhibitory action of adenosine. The results provide evidence for an intimate coupling of nerve terminal calcium channels with a plasma membrane component of the SNARE complex, such that modulation of calcium currents by a G-protein coupled receptor cannot occur when syntaxin is cleaved.

Adenosine decreases both presynaptic calcium currents and neurotransmitter release at the mouse neuromuscular junction

A controversy currently exists as to the mechanism of action by which adenosine, an endogenous mediator of neurotransmitter depression, reduces the evoked release of the neurotransmitter acetylcholine (ACh) at the skeletal neuromuscular junction. Specifically, it is uncertain whether adenosine inhibits ACh release from mammalian motor nerve endings by reducing Ca(2+) calcium entry through voltage-gated calcium channels or, as is the case at amphibian motor nerve endings, by an effect downstream of Ca(2+) entry. In an attempt to address this controversy, the effects of adenosine on membrane ionic currents and neurotransmitter release were studied at neuromuscular junctions in adult mouse phrenic nerve hemidiaphragm preparations. In wild-type mice, adenosine (500 microm-1 mm) reduced prejunctional Ca(2+) currents simultaneously with a reduction in evoked ACh release. In Rab3A knockout mice, which have been shown to have an increased sensitivity to adenosine, the simultaneous reduction in Ca(2+) currents and ACh secretion occurred at significantly lower adenosine concentrations (< or = 50 microM). Measurements of nerve terminal Na(+) and K(+) currents made simultaneously with evoked ACh release demonstrated that the decreases in Ca(2+) currents were not attributable to changes in cation entry through voltage-gated Na(+) or K(+) channels. Furthermore, no effects of adenosine on residual ionic currents were observed when P/Q-type calcium channels were blocked by Cd(2+) or omega-agatoxin-IVA. The results demonstrate that inhibition of evoked neurotransmitter secretion by adenosine is associated with a reduction in Ca(2+) calcium entry through voltage-gated P/Q Ca(2+) channels at the mouse neuromuscular junction. Whilst it may be that adenosine inhibits ACh release by different mechanisms at amphibia and mammalian neuromuscular junctions, it is also possible that the secretory apparatus is more intimately coupled to the Ca(2+) channels in the mouse such that an effect on the secretory machinery is reflected as changes in Ca(2+) currents.

Phorbol esters and adenosine affect the readily releasable neurotransmitter pool by different mechanisms at amphibian motor nerve endings

Phorbol esters and adenosine have been proposed to interact at common sites downstream of calcium entry at amphibian motor nerve endings. We thus studied the actions and interactions of phorbol esters and adenosine using electrophysiological recording techniques in conjunction with both binomial statistical analysis and high-frequency stimulation at the amphibian neuromuscular junction. To begin this study, we confirmed previous observations that synchronous evoked acetylcholine (ACh) release (reflected as endplate potentials, EPPs) is well described by a simple binomial distribution. We then used binomial analysis to study the effects of the phorbol ester phorbol dibutyrate (PDBu, 100 nM) and adenosine (50 microM) on the binomial parameters n (the number of calcium charged ACh quanta available for release) and p (the average probability of release), where the mean level of evoked ACh release (m) = np. We found that PDBu increased m by increasing the parameter n whilst adenosine reduced m by reducing n; neither agent affected the parameter p. PDBu had no effect on either the potency or efficacy of the inhibition produced by adenosine. Subtle differences between these two agents were revealed by the patterns of EPPs evoked by high-frequency trains of stimuli. Phorbol esters increased ACh release during the early phase of stimulation but not during the subsequent plateau phase. The inhibitory effect of adenosine was maximal at the beginning of the train and was still present with reduced efficacy during the plateau phase. When taken together with previous findings, these present results suggest that phorbol esters increase the immediately available store of synaptic vesicles by increasing the number of primed vesicles whilst adenosine acts at a later stage of the secretory process to decrease the number of calcium-charged primed vesicles.

Regulation by Rab3A of an endogenous modulator of neurotransmitter release at mouse motor nerve endings

Rab3A, a small GTP-binding protein attached to synaptic vesicles, has been implicated in several stages in the process of neurosecretion, including a late stage occurring just prior to the actual release of neurotransmitter. The inhibitory neuromodulator adenosine also targets a late step in the neurosecretory pathway. We thus compared neuromuscular junctions from adult Rab3A(-/-) mutant mice with those from wild-type mice with respect to: (a) the basic electrophysiological correlates of neurotransmitter release at different stimulation frequencies, and (b) the actions of exogenous and endogenous adenosine on neurotransmitter release in normal calcium solutions. Neither the spontaneous quantal release of acetylcholine (ACh) nor basal evoked ACh release (0.05 Hz) differed between the mutant and wild-type mice. At 50-100 Hz stimulation (10-19 stimuli), facilitation of release was observed in the mutant mice but not in wild-type, followed by a depression of ACh release in both strains. ACh release at the end of the stimulus train in the mutant mouse was approximately double that of the wild-type mouse. The threshold concentration for inhibition of ACh release by exogenous adenosine was over 20-fold lower in the mutant mouse than in the wild-type mouse. The adenosine A(1) receptor antagonist 8-cyclopentyltheophylline (CPT) increased ACh release (0.05-1 Hz stimulation) in the mutant mouse under conditions in which it had no effect in the wild-type mouse. CPT had no effect on the pattern of responses recorded during repetitive stimulation in either strain. The results suggest that Rab3A reduces the potency of adenosine as an endogenous mediator of neuromuscular depression.