Endogenous adenosine modulates stimulation-induced depression at the frog neuromuscular junction

Endogenous purines modulate K+ -evoked ACh secretion at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP) is co-released with the neurotransmitter acetylcholine (ACh), and once in the synaptic cleft, it is hydrolyzed to adenosine. Both ATP/adenosine diphosphate (ADP) and adenosine modulate ACh secretion by activating presynaptic P2Y₁₃ and A₁ , A_2A , and A₃ receptors, respectively. To elucidate the action of endogenous purines on K⁺ -dependent ACh release, we studied the effect of purinergic receptor antagonists on miniature end-plate potential (MEPP) frequency in phrenic diaphragm preparations. At 10 mM K⁺ , the P2Y₁₃ antagonist N-[2-(methylthio)ethyl]-2-[3,3,3-trifluoropropyl]thio-5'-adenylic acid, monoanhydride with (dichloromethylene)bis[phosphonic acid], tetrasodium salt (AR-C69931MX) increased asynchronous ACh secretion while the A₁ , A₃ , and A_2A antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), (3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(±)-dihydropyridine-3,5-, dicarboxylate (MRS-1191), and 2-(2-Furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine (SCH-58261) did not modify neurosecretion. The inhibition of equilibrative adenosine transporters by S-(p-nitrobenzyl)-6-thioinosine provoked a reduction of 10 mM K⁺ -evoked ACh release, suggesting that the adenosine generated from ATP is being removed from the synaptic space by the transporters. At 15 and 20 mM K⁺ , endogenous ATP/ADP and adenosine bind to inhibitory P2Y₁₃ and A₁ and A₃ receptors since AR-C69931MX, DPCPX, and MRS-1191 increased MEPP frequency. Similar results were obtained when the generation of adenosine was prevented by using the ecto-5'-nucleotidase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt. SCH-58261 only reduced neurosecretion at 20 mM K⁺ , suggesting that more adenosine is needed to activate excitatory A_2A receptors. At high K⁺ concentration, the equilibrative transporters appear to be saturated allowing the accumulation of adenosine in the synaptic cleft. In conclusion, when motor nerve terminals are depolarized by increasing K⁺ concentrations, the ATP/ADP and adenosine endogenously generated are able to modulate ACh secretion by sequential activation of different purinergic receptors.

Transmitter release site organization can predict synaptic function at the neuromuscular junction

We have investigated the impact of transmitter release site (active zone; AZ) structure on synaptic function by physically rearranging the individual AZ elements in a previously published frog neuromuscular junction (NMJ) AZ model into the organization observed in a mouse NMJ AZ. We have used this strategy, purposefully without changing the properties of AZ elements between frog and mouse models (even though there are undoubtedly differences between frog and mouse AZ elements in vivo), to directly test how structure influences function at the level of an AZ. Despite a similarly ordered ion channel array substructure within both frog and mouse AZs, frog AZs are much longer and position docked vesicles in a different location relative to AZ ion channels. Physiologically, frog AZs have a lower probability of transmitter release compared with mouse AZs, and frog NMJs facilitate strongly during short stimulus trains in contrast with mouse NMJs that depress slightly. Using our computer modeling approach, we found that a simple rearrangement of the AZ building blocks of the frog model into a mouse AZ organization could recapitulate the physiological differences between these two synapses. These results highlight the importance of simple AZ protein organization to synaptic function. NEW & NOTEWORTHY A simple rearrangement of the basic building blocks in the frog neuromuscular junction model into a mouse transmitter release site configuration predicted the major physiological differences between these two synapses, suggesting that transmitter release site structure and organization is a strong predictor of function.

P2Y13 receptors mediate presynaptic inhibition of acetylcholine release induced by adenine nucleotides at the mouse neuromuscular junction

It is known that adenosine 5'-triphosphate (ATP) is released along with the neurotransmitter acetylcholine (ACh) from motor nerve terminals. At mammalian neuromuscular junctions (NMJs), we have previously demonstrated that ATP is able to decrease ACh secretion by activation of P2Y receptors coupled to pertussis toxin-sensitive Gi/o protein. In this group, the receptor subtypes activated by adenine nucleotides are P2Y12 and P2Y13. Here, we investigated, by means of pharmacological and immunohistochemical assays, the P2Y receptor subtype that mediates the modulation of spontaneous and evoked ACh release in mouse phrenic nerve-diaphragm preparations. First, we confirmed that the preferential agonist for P2Y12-13 receptors, 2-methylthioadenosine 5'-diphosphate trisodium salt hydrate (2-MeSADP), reduced MEPP frequency without affecting MEPP amplitude as well as the amplitude and quantal content of end-plate potentials (EPPs). The effect on spontaneous secretion disappeared after the application of the selective P2Y12-13 antagonists AR-C69931MX or 2-methylthioadenosine 5'-monophosphate triethylammonium salt hydrate (2-MeSAMP). 2-MeSADP was more potent than ADP and ATP in reducing MEPP frequency. Then we demonstrated that the selective P2Y13 antagonist MRS-2211 completely prevented the inhibitory effect of 2-MeSADP on MEPP frequency and EPP amplitude, whereas the P2Y12 antagonist MRS-2395 failed to do this. The preferential agonist for P2Y13 receptors inosine 5'-diphosphate sodium salt (IDP) reduced spontaneous and evoked ACh secretion and MRS-2211 abolished IDP-mediated modulation. Immunohistochemical studies confirmed the presence of P2Y13 but not P2Y12 receptors at the end-plate region. Disappearance of P2Y13 receptors after denervation suggests the presynaptic localization of the receptors. We conclude that, at motor nerve terminals, the Gi/o protein-coupled P2Y receptors implicated in presynaptic inhibition of spontaneous and evoked ACh release are of the subtype P2Y13. This study provides new insights into the types of purinergic receptors that contribute to the fine-tuning of cholinergic transmission at mammalian neuromuscular junction.

Transmitter release is evoked with low probability predominately by calcium flux through single channel openings at the frog neuromuscular junction

The quantitative relationship between presynaptic calcium influx and transmitter release critically depends on the spatial coupling of presynaptic calcium channels to synaptic vesicles. When there is a close association between calcium channels and synaptic vesicles, the flux through a single open calcium channel may be sufficient to trigger transmitter release. With increasing spatial distance, however, a larger number of open calcium channels might be required to contribute sufficient calcium ions to trigger vesicle fusion. Here we used a combination of pharmacological calcium channel block, high-resolution calcium imaging, postsynaptic recording, and 3D Monte Carlo reaction-diffusion simulations in the adult frog neuromuscular junction, to show that release of individual synaptic vesicles is predominately triggered by calcium ions entering the nerve terminal through the nearest open calcium channel. Furthermore, calcium ion flux through this channel has a low probability of triggering synaptic vesicle fusion (∼6%), even when multiple channels open in a single active zone. These mechanisms work to control the rare triggering of vesicle fusion in the frog neuromuscular junction from each of the tens of thousands of individual release sites at this large model synapse.

Redox-sensitive synchronizing action of adenosine on transmitter release at the neuromuscular junction

The kinetics of neurotransmitter release was recognized recently as an important contributor to synaptic efficiency. Since adenosine is the ubiquitous modulator of presynaptic release in peripheral and central synapses, in the current project we studied the action of this purine on the timing of acetylcholine quantal release from motor nerve terminals in the skeletal muscle. Using extracellular recording from frog neuromuscular junction we tested the action of adenosine on the latencies of single quantal events in the pro-oxidant and antioxidant conditions. We found that adenosine, in addition to previously known inhibitory action on release probability, also synchronized release by removing quantal events with long latencies. This action of adenosine on release timing was abolished by oxidants whereas in the presence of the antioxidant the synchronizing action of adenosine was further enhanced. Interestingly, unlike the timing of release, the inhibitory action of adenosine on release probability was redox-independent. Modulation of release timing by adenosine was mediated by purinergic A1 receptors as it was eliminated by the specific A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and mimicked by the specific A1 agonist N(6)-cyclopentyl-adenosine. Consistent with data obtained from dispersion of single quantal events, adenosine also reduced the rise-time of multiquantal synaptic currents. The latter effect was reproduced in the model based on synchronizing effect of adenosine on release timing. Thus, adenosine which is generated at the neuromuscular junction from the breakdown of the co-transmitter ATP induces the synchronization of quantal events. The effect of adenosine on release timing should preserve the fidelity of synaptic transmission via "cost-effective" use of less transmitter quanta. Our findings also revealed important crosstalk between purinergic and redox modulation of synaptic processes which could take place in the elderly or in neuromuscular diseases associated with oxidative stress like lateral amyotrophic sclerosis.

Effect of purines on calcium-independent acetylcholine release at the mouse neuromuscular junction

At the mouse neuromuscular junction, activation of adenosine A(1) and P2Y receptors inhibits acetylcholine release by an effect on voltage dependent calcium channels related to spontaneous and evoked secretion. However, an effect of purines upon the neurotransmitter-releasing machinery downstream of Ca(2+) influx cannot be ruled out. An excellent tool to study neurotransmitter exocytosis in a Ca(2+)-independent step is the hypertonic response. Intracellular recordings were performed on diaphragm fibers of CF1 mice to determine the action of the specific adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentyl-adenosine (CCPA) and the P2Y(12-13) agonist 2-methylthio-adenosine 5'-diphosphate (2-MeSADP) on the hypertonic response. Both purines significantly decreased such response (peak and area under the curve), and their effect was prevented by specific antagonists of A(1) and P2Y(12-13) receptors, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and N-[2-(methylthioethyl)]-2-[3,3,3-trifluoropropyl]thio-5'-adenylic acid, monoanhydride with dichloromethylenebiphosphonic acid, tetrasodium salt (AR-C69931MX), respectively. Moreover, incubation of preparations only with the antagonists induced a higher response compared with controls, suggesting that endogenous ATP/ADP and adenosine are able to modulate the hypertonic response by activating their specific receptors. To search for the intracellular pathways involved in this effect, we studied the action of CCPA and 2-MeSADP in hypertonicity in the presence of inhibitors of several pathways. We found that the effect of CPPA was prevented by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) while that of 2-MeSADP was occluded by the protein kinase C antagonist chelerythrine and W-7. On the other hand, the inhibitors of protein kinase A (N-(2[pbromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide, H-89) and phosphoinositide-3 kinase (PI3K) (2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one hydrochloride, LY-294002) did not modify the modulatory action in hypertonicity of both purines. Our results provide evidence that activation of A(1) and P2Y(12-13) receptors by CCPA and 2-MeSADP inhibits ACh release from mammalian motor nerve terminals through an effect on a Ca(2+)-independent step in the cascade of the exocytotic process. Since presynaptic calcium channels are intimately associated with components of the synaptic vesicle docking and fusion processes, further experiments could clarify if the actions of purines on calcium channels and on secretory machinery are related.

Selective disruption of the mammalian secretory apparatus enhances or eliminates calcium current modulation in nerve endings

Modulation of secretion via G protein-coupled receptors (GPCRs) serves an important regulatory function in neuronal and nonneuronal secretory cells. Most secretory cells possess voltage-gated calcium channels, share homologues of the core complex of three proteins (the SNAREs) that constitute the secretory apparatus, and are modulated by GPCR activation. Activators of GPCRs generally inhibit the release of neurotransmitter substances to a maximum of only 50-60% of the control level, suggesting that complex protein-protein interactions may govern the efficacy of this form of modulation. In this article, molecular genetic approaches are used in combination with botulinum toxins (selective molecular scalpels that cleave the SNAREs at highly restricted loci) to address this issue. The results suggest that the cleavage of either of the plasma membrane SNAREs (syntaxin or SNAP-25) prevents modulation of calcium currents by A(1) adenosine receptors at mammalian motor nerve endings. In contrast, cleavage of the synaptic vesicle SNARE (synaptobrevin) in conjunction with deletion of the vesicle-docking protein Rab3A greatly enhances the efficacy of calcium current modulation.

Presynaptic inhibition of spontaneous acetylcholine release mediated by P2Y receptors at the mouse neuromuscular junction

At the neuromuscular junction, ATP is co-released with the neurotransmitter acetylcholine (ACh) and once in the synaptic space, it is degraded to the presynaptically active metabolite adenosine. Intracellular recordings were performed on diaphragm fibers of CF1 mice to determine the action of extracellular ATP (100 muM) and the slowly hydrolysable ATP analog 5'-adenylylimidodiphosphate lithium (betagamma-imido ATP) (30 muM) on miniature end-plate potential (MEPP) frequency. We found that application of ATP and betagamma-imido ATP decreased spontaneous secretion by 45.3% and 55.9% respectively. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) adenosine receptor antagonist and alpha,beta-methylene ADP sodium salt (alphabeta-MeADP), which is an inhibitor of ecto-5'-nucleotidase, did not prevent the inhibitory effect of ATP, demonstrating that the nucleotide is able to modulate spontaneous ACh release through a mechanism independent of the action of adenosine. Blockade of Ca(2+) channels by both, Cd(2+) or the combined application of nitrendipine and omega-conotoxin GVIA (omega-CgTx) (L-type and N-type Ca(2+) channel antagonists, respectively) prevented the effect of betagamma-imido ATP, indicating that the nucleotide modulates Ca(2+) influx through the voltage-dependent Ca(2+) channels related to spontaneous secretion. betagamma-Imido ATP-induced modulation was antagonized by the non-specific P2 receptor antagonist suramin and the P2Y receptor antagonist 1-amino-4-[[4-[[4-chloro-6-[[3(or4)-sulfophenyl] amino]-1,3,5-triazin-2-yl]amino]-3-sulfophenyl] amino]-9,10-dihydro-9,10-dioxo-2-anthracenesulfonic acid (reactive blue-2), but not by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt (PPADS), which has a preferential antagonist effect on P2X receptors. Pertussis toxin and N-ethylmaleimide (NEM), which are blockers of G(i/o) proteins, prevented the action of the nucleotide, suggesting that the effect is mediated by P2Y receptors coupled to G(i/o) proteins. The protein kinase C (PKC) antagonist chelerythrine and the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) occluded the effect of betagamma-imido ATP, while the protein kinase A (PKA) antagonist KT-5720 and the inhibitor of the calcium/calmodulin-dependent protein kinase II (CAMKII) KN-62 failed to do so. betagamma-Imido ATP did not affect 10, 15 and 20 mM K(+)-evoked release and application of reactive blue-2 before incubation in high K(+) induced a higher asynchronous secretion. Thus, our results show that at mammalian neuromuscular junctions, ATP induces presynaptic inhibition of spontaneous ACh release due to the modulation of Ca(2+) channels related to tonic secretion through the activation of P2Y receptors coupled to G(i/o) proteins. We also demonstrated that at increasing degrees of membrane depolarization evoked by K(+), endogenously released ATP induces presynaptic inhibition as a means of preventing excessive neurotransmitter secretion.

The effects of presynaptic calcium channel modulation by roscovitine on transmitter release at the adult frog neuromuscular junction

Calcium (Ca2+) influx through presynaptic calcium channels triggers transmitter release, and any alterations in the gating of these calcium channels results in changes in the magnitude of transmitter released. We used (R)-roscovitine, a cyclin-dependent kinase inhibitor that also appears to act directly on calcium channels, as a tool to modulate presynaptic calcium influx and study effects on transmitter release. We show that this compound increased the quantal content of acetylcholine released from the Rana frog motor nerve terminal (by 149%) without changing paired-pulse facilitation (under low calcium conditions). In contrast, exposure to 3,4-diaminopyridine (DAP; which similarly affects transmitter release by partially blocking potassium channels, altering the shape of the presynaptic action potential, and indirectly increasing calcium entry) increased paired-pulse facilitation (by 23%). In addition, we show that (R)-roscovitine predominately slowed deactivation kinetics of calcium current (by 427%) recorded from Xenopus frog motoneurons, and as a result, increased the integral of calcium channel current evoked by a physiological action potential waveform (by 44%). Because we did not observe any significant effects of structurally related cyclin-dependent kinase inhibitors [(S)-roscovitine or olomoucine] on evoked transmitter release or calcium current kinetics, it appears that these effects of (R)-roscovitine are independent of cyclin-dependent kinases (cdks). In summary, we hypothesize that (R)-roscovitine effects on transmitter release at the adult frog neuromuscular junction (NMJ) are mediated by its effects on calcium channel gating, and these effects increase our understanding of calcium triggered secretion at this synapse.

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.

Mechanisms of ATP action on motor nerve terminals at the frog neuromuscular junction

We have shown previously that ATP inhibits transmitter release at the neuromuscular junction through the action on metabotropic P2Y receptors coupled to specific second messenger cascades. In the present study we recorded K(+) or Ca(2+) currents in motor nerve endings or blocked K(+) or Ca(2+) channels in order to explore the nature of downstream presynaptic effectors. Endplate currents were presynaptically depressed by ATP. Blockers of Ca(2+)-activated K(+)-channels, such as iberiotoxin, apamin or tetraethylammonium, did not change the depressant action of ATP. By contrast, K(+) channel blocker 4-aminopyridine (4-AP) and raised extracellular Ca(2+) attenuated the effect of ATP. However, these effects of 4-AP and high Ca(2+) were reversed by Mg(2+), suggesting Ca(2+)-dependence of the ATP action. Ba(2+) promoted the depressant action of ATP as did glibenclamide, a blocker of ATP-sensitive K(+) channels, or mild depolarization produced by 7.5 mm K(+). None of the K(+) channel blockers affected the depressant action of adenosine. Focal recording revealed that neither ATP nor adenosine affected the fast K(+) currents of the motor nerve endings. However, unlike adenosine, ATP or UTP, an agonist of P2Y receptors, reversibly reduced the presynaptic Ca(2+)-current. This effect was abolished by suramin, an antagonist of P2 receptors. Depressant effect of ATP on the endplate and Ca(2+)-currents was mimicked by arachidonate, which precluded the action of ATP. ATP reduced acetylcholine release triggered by ionomycin or sucrose, suggesting inhibition of release machinery. Thus, the presynaptic depressant action of ATP is mediated by inhibition of Ca(2+) channels and by mechanism acting downstream of Ca(2+) entry.

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.

Presynaptic inhibition of spontaneous acetylcholine release induced by adenosine at the mouse neuromuscular junction

1. At the mouse neuromuscular junction, adenosine (AD) and the A(1) agonist 2-chloro-N(6)-cyclopentyl-adenosine (CCPA) induce presynaptic inhibition of spontaneous acetylcholine (ACh) release by activation of A(1) AD receptors through a mechanism that is still unknown. To evaluate whether the inhibition is mediated by modulation of the voltage-dependent calcium channels (VDCCs) associated with tonic secretion (L- and N-type VDCCs), we measured the miniature end-plate potential (mepp) frequency in mouse diaphragm muscles. 2. Blockade of VDCCs by Cd(2+) prevented the effect of the CCPA. Nitrendipine (an L-type VDCC antagonist) but not omega-conotoxin GVIA (an N-type VDCC antagonist) blocked the action of CCPA, suggesting that the decrease in spontaneous mepp frequency by CCPA is associated with an action on L-type VDCCs only. 3. As A(1) receptors are coupled to a G(i/o) protein, we investigated whether the inhibition of PKA or the activation of PKC is involved in the presynaptic inhibition mechanism. Neither N-(2[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide (H-89, a PKA inhibitor), nor 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine (H-7, a PKC antagonist), nor phorbol 12-myristate 13-acetate (PHA, a PKC activator) modified CCPA-induced presynaptic inhibition, suggesting that these second messenger pathways are not involved. 4. The effect of CCPA was eliminated by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-acetoxymethyl ester epsilon6TDelta-BM, which suggests that the action of CCPA to modulate L-type VDCCs may involve Ca(2+)-calmodulin. 5. To investigate the action of CCPA on diverse degrees of nerve terminal depolarization, we studied its effect at different external K(+) concentrations. The effect of CCPA on ACh secretion evoked by 10 mm K(+) was prevented by the P/Q-type VDCC antagonist omega-agatoxin IVA. 6. CCPA failed to inhibit the increases in mepp frequency evoked by 15 and 20 mm K(+). We demonstrated that, at high K(+) concentrations, endogenous AD occupies A1 receptors, impairing the action of CCPA, since incubation with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A(1) receptor antagonist) and adenosine deaminase (ADA), which degrades AD into the inactive metabolite inosine, increased mepp frequency compared with that obtained in 15 and 20 mm K(+) in the absence of the drugs. Moreover, CCPA was able to induce presynaptic inhibition in the presence of ADA. It is concluded that, at high K(+) concentrations, the activation of A(1) receptors by endogenous AD prevents excessive neurotransmitter release.

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.

Distinct receptors and different transduction mechanisms for ATP and adenosine at the frog motor nerve endings

Corelease of ATP with ACh from motor endings suggests a physiological role for ATP in synaptic transmission. We previously showed that, on skeletal muscle, ATP directly inhibited ACh release via presynaptic P2 receptors. The receptor identification (P2X or P2Y) and its transduction mechanism remained, however, unknown. In the present study using the voltage-clamp technique we analyzed the properties of presynaptic ATP receptors and subsequent effector mechanisms. ATP or adenosine presynaptically depressed multiquantal end-plate currents, with longer latency for ATP action. ATPgammaS, agonist at P2X receptors, or Bz-ATP, agonist at P2X7 receptors, were ineffective. The action of ATP was prevented by suramin and unchanged by PPADS or TNP-ATP, antagonists of P2X receptors, or RB-2, a blocker of certain P2Y receptors. The depressant action of ATP was reproduced by UTP, metabotropic P2Y receptor agonist. Pertussis toxin (PTX), antagonist of Gi/o-proteins, and inhibitors of phosphatidylcholine specific PLC (D609) and PKC (staurosporine or chelerythrine) prevented the effect of ATP while blockers of PLA2 (OBAA) and COX (aspirin or indomethacin) attenuated it. Inhibitors of phosphatidylinositide-specific PLC (U73122), guanylylcyclase (ODQ), PKA (Rp-cAMPS) or PLD (1-butanol) did not affect the action of ATP. No inhibitor of second messengers (except PTX) changed the action of adenosine. Our data indicate, for motor nerve endings, the existence of inhibitory P2Y receptors coupled to multiple intracellular cascades including phosphatidylinositide-specific PLC/PKC/PLA2/COX. This divergent presynaptic P2 signalling (unlike the single effector mechanism for P1 receptors) could provide feedback inhibition of transmitter release and perhaps be involved in presynaptic plasticity.

Adenosine depresses a Ca(2+)-independent step in transmitter exocytosis at frog motor nerve terminals

The depressant action of adenosine on acetylcholine release at frog motor nerve terminals was studied by intracellular recording and Ca(2+)-imaging techniques. Adenosine (200 microm) quickly and reversibly decreased the amplitude and quantal content of end-plate potentials (EPPs) with no change in quantal size in a low-Ca(2+), high-Mg(2+) solution, and EPP amplitude in normal Ringer containing d-tubocurarine. Likewise, adenosine (200 microm) reduced miniature EPP (MEPP) frequency, but not amplitude, in a high-K(+) (6 mm) solution. Adenosine (40-200 microm), however, did not affect single or repetitive impulse(s)-induced rises in Ca(2+) in the nerve terminals or its basal level. Adenosine (100-200 microm) reduced the Ca(2+)-independent enhancement of MEPP frequency caused by hypertonicity. EPPs induced by tetanic stimulation (33 Hz) in Ringer with d-tubocurarine initially increased in amplitude within 10 stimuli and then declined to the minimum. Adenosine (200 microm) decreased EPP amplitude in the initial phase of the tetanus, but enhanced it in the middle phase, thus prolonging the decay of EPP amplitude. The total sum of these EPPs, reflecting the readily releasable pool of vesicles and its refilling, however, was not changed. The results suggest that adenosine inhibits a Ca(2+)-independent step of transmitter exocytosis at frog motor nerve terminals.

Differential frequency-dependent regulation of transmitter release by endogenous nitric oxide at the amphibian neuromuscular synapse

Nitric oxide (NO) is a potent neuromodulator in the CNS and PNS. At the frog neuromuscular junction (nmj), exogenous application of NO reduces neurotransmitter release, and NO synthases (NOSs), the enzymes producing NO, are present at this synapse. This work aimed at studying the molecular mechanisms by which NO modulates synaptic efficacy at the nmj using electrophysiological recordings and Ca(2+)-imaging techniques. Bath application of the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside decreased end plate potential (EPP) amplitude as well as the frequency of miniature EPPs but not their amplitude. Ca(2+) responses elicited in presynaptic terminals by single action potentials were unaffected by NO, but responses evoked by a short train of stimuli were increased. Tonic endogenous production of NO was observed as suggested by the increase in EPP amplitude by bath application of the NO scavenger hemoglobin and the neuronal NOS inhibitor 3-bromo-7-nitroindazole sodium salt. A soluble guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY-83583), increased EPP amplitude and occluded the effects of the NO donor, suggesting that NO acts via a cGMP-dependent mechanism. High-frequency-induced depression was reduced in the presence of the NO scavenger but not by LY-83583. However, adenosine-induced depression was significantly reduced after bath perfusion of SNAP and in the presence of LY-83583. Our results indicate that NO regulates transmitter release and adenosine-induced depression via a cGMP-dependent mechanism that occurs after Ca(2+) entry and that high-frequency-induced synaptic depression is regulated by NO in a cGMP-independent manner.

ATP but not adenosine inhibits nonquantal acetylcholine release at the mouse neuromuscular junction

The postsynaptic membrane of the neuromuscular synapse treated with antiacetylcholinesterase is depolarized due to nonquantal release of acetylcholine (ACh) from the motor nerve ending. This can be demonstrated by the hyperpolarization produced by the application of curare (H-effect). ATP (1 x 10-5 M) decreased the magnitude of the H-effect from 5 to 1.5 mV. The membrane input resistance and the ACh sensitivity were unchanged, and so changes in these cannot explain the ATP effect. Adenosine alone was without effect on the nonquantal release. On the other hand, both ATP and adenosine depressed the frequency of spontaneous miniature endplate potentials, to 56% and 43% respectively. The protein kinase A inhibitor Rp-cAMP or the guanylyl cyclase inhibitor 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one did not affect the inhibitory influence of ATP on the H-effect, whereas staurosporine, an inhibitor of protein kinase C, completely abolished the action of ATP. Suramin, an ATP antagonist, enhanced the H-effect to 8.6 mV and, like staurosporine, prevented the inhibitory effect of ATP. ATP thus suppresses the nonquantal release via a direct action on presynaptic metabotropic P2 receptors coupled to protein kinase C, whilst adenosine exerts its action mainly by affecting the mechanisms underlying quantal release. These data, together with earlier evidence, show that nonquantal release of ACh can be modulated by several distinct regulatory pathways, in particular by endogenous substances which may or may not be present in the synaptic cleft at rest or during activity.

Effect of Temperature on Endplate Potential Rundown and Recovery in Rat Diaphragm

The amplitude of neuromuscular junction end-plate potentials (EPPs) decreases quickly within a train but recovers nearly completely from train to train during intermittent stimulation. Rundown has been shown to be dependent not only on the rate of transmitter release but also on the rate of replenishment of the depleted neurotransmitter at the site of release. Two groups of processes have been proposed for synaptic vesicle recycling, both of which involve multiple energy-requiring steps and enzymatic reactions and which therefore would be expected to be very temperature-sensitive. The present study tested the hypothesis that low temperature therefore increases the rate of EPP amplitude rundown. Studies were performed in vitro on rat diaphragm and used μ-conotoxin to allow normal-sized EPPs to be recorded from intact fibers. EPP amplitude rundown during intermittent stimulation at 20 and 50 Hz (duty cycle 333 ms) was greater at 20°C than it was at 37°C. Initially, temperature affected only intra-train rundown but, over longer periods of stimulation, both intra- and inter-train rundown were significantly accelerated by cold temperature. Cumulative EPP amplitudes were calculated by successively adding the amplitudes of each EPP during the stimulation period to provide an estimate of total neurotransmitter release in the neuromuscular junction. The cumulative EPP amplitude was significantly lower at 20°C than it was at 37°C during both 20 and 50 Hz stimulation. These data indicate that the mechanism involved in EPP amplitude rundown and recovery is temperature-sensitive, with a greater decrement in EPP amplitude at cold than at warm temperatures.

Depression of transmitter release at synapses in the rat superior cervical ganglion: the role of transmitter depletion

The characteristics of depression of the excitatory postsynaptic potential (EPSP) during a short train of impulses to the rat superior cervical ganglion (SCG) have been ascertained with the object of determining the relative contributions of transmitter depletion and autoreceptors to depression. Successive EPSPs in a short train were depressed after the first (Vo) up to about the fourth impulse when a steady-state depressed EPSP level (Vss) was reached. Vss increased with the stimulation frequency between 1 and 30 Hz. Vo recovered after a short train with a time constant of about 2.8 s in the frequency range from 5 to 30 Hz. In order to determine if depression was related to changes in calcium influx with successive impulses in the train. preganglionic boutons were loaded with the calcium indicator Oregon Green 488 BAPTA-1 and line scans taken through individual boutons with a confocal laser microscope. Successive calcium transients were of about the same amplitude in boutons during short trains of impulses at 5 Hz. The contribution of autoreceptors activated by the action of endogenously derived adenosine on the extent of depression of the EPSP during short trains was ascertained by blocking these receptors with 8-phenyltheophylline (10 microM). There was no change in the extent or time course of development of depression. Similar results were obtained with the opioid receptor antagonist naloxone (10 microM) and the adrenergic receptor antagonist yohimbine (10 microM). Factors, which increased the extent of transmitter release during a train, such as increasing the external calcium concentration from 0.8 to 2.5 mM, increased depression. Factors. which decreased the extent of transmitter release such as increasing the exogenous adenosine concentration between 1 and 200 microM decreased depression. These results are interpreted in terms of a model in which vesicles are mobilised by a calcium-dependent process from a store into an available pool of docked vesicles. Depletion of the docked vesicles during exocytosis then leads to depression of transmitter release during a train of impulses.

Differential frequency-dependent regulation of transmitter release by endogenous nitric oxide at the amphibian neuromuscular synapse

Nitric oxide (NO) is a potent neuromodulator in the CNS and PNS. At the frog neuromuscular junction (nmj), exogenous application of NO reduces neurotransmitter release, and NO synthases (NOSs), the enzymes producing NO, are present at this synapse. This work aimed at studying the molecular mechanisms by which NO modulates synaptic efficacy at the nmj using electrophysiological recordings and Ca(2+)-imaging techniques. Bath application of the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside decreased end plate potential (EPP) amplitude as well as the frequency of miniature EPPs but not their amplitude. Ca(2+) responses elicited in presynaptic terminals by single action potentials were unaffected by NO, but responses evoked by a short train of stimuli were increased. Tonic endogenous production of NO was observed as suggested by the increase in EPP amplitude by bath application of the NO scavenger hemoglobin and the neuronal NOS inhibitor 3-bromo-7-nitroindazole sodium salt. A soluble guanylate cyclase inhibitor, 6-anilino-5,8-quinolinedione (LY-83583), increased EPP amplitude and occluded the effects of the NO donor, suggesting that NO acts via a cGMP-dependent mechanism. High-frequency-induced depression was reduced in the presence of the NO scavenger but not by LY-83583. However, adenosine-induced depression was significantly reduced after bath perfusion of SNAP and in the presence of LY-83583. Our results indicate that NO regulates transmitter release and adenosine-induced depression via a cGMP-dependent mechanism that occurs after Ca(2+) entry and that high-frequency-induced synaptic depression is regulated by NO in a cGMP-independent manner.

Stimulation-dependent release, breakdown, and action of endogenous ATP in mouse hemidiaphragm preparation: the possible role of ATP in neuromuscular transmission

In this study the in vitro mouse phrenic nerve- hemidiaphragm preparation was utilized to study the release and extracellular catabolism of endogenous ATP and its action on the postsynaptic site, i.e. on the contraction force evoked by nerve stimulation. ATP, measured by the luciferin-luciferase assay, was released stimulation-dependently from the mouse hemidiaphragm in response to electrical field stimulation at 10 Hz. Blockade of the Na(+) channel activity by tetrodotoxin inhibited the majority of the release of ATP in response to stimulation, showing that it is related to neuronal activity. The nicotinic receptor antagonists d-tubocurarine, and alpha-bungarotoxin and cooling the bath temperature to 7 degrees C also reduced stimulation-induced ATP outflow, suggesting that nicotinic receptors are responsible for the part of the release of ATP that is released from postsynaptic sites in a carrier-mediated manner. Exogenous ATP (20-500 microM) added to the bath was degraded to ADP and AMP by the action of ectoATPase and ectoATPdiphosphohydrolase; the K(m) and v(max) values of these enzymes were 185.8 microM and 55.16 nmol/min.g respectively. However, the total amount of nucleotides ([ATP+ADP+AMP]) was increased after the addition of ATP, indicating that ATP itself promoted further adenine nucleotide release. Twitch contractions of the rat hemidiaphragm preparation evoked by low frequency electrical stimulation was blocked concentration-dependently by the non-depolarizing muscle relaxants d-tubocurarine and pancuronium. Suramin (100 microM-1 mM) reversed neuromuscular blockade by d-tubocurarine and pancuronium; i.e., it shifted their concentration-response curves to the right Taken together our data, that endogenous ATP is released by stimulation and subsequently catabolized in the hemidiaphragm preparation and that suramin inhibits ecto-ATPase activity could be interpreted as meaning that suramin prolongs the action of endogenous ATP to elicit twitch contraction, which points to a new, undefined role of ATP in neuromuscular transmission. The source of ATP is partly postsynaptic, released from the muscle in response to activation of nicotinic ACh receptors expressed on the muscle.

Optimization of rhythmic behaviors by modulation of the neuromuscular transform

We conclude our study of the properties and the functional role of the neuromuscular transform (NMT). The NMT is an input-output relation that formalizes the processes by which patterns of motor neuron firing are transformed to muscle contractions. Because the NMT acts as a dynamic, nonlinear, and modifiable filter, the transformation is complex. In the two preceding papers we developed a framework for analysis of the NMT and identified with it principles by which the NMT transforms different firing patterns to contractions. We then saw that, with fixed properties, the NMT significantly constrains the production of functional behavior. Many desirable behaviors are not possible with any firing pattern. Here we examine, theoretically as well as experimentally in the accessory radula closer (ARC) neuromuscular system of Aplysia, how this constraint is alleviated by making the properties of the NMT variable by neuromuscular plasticity and modulation. These processes dynamically tune the properties of the NMT to match the desired behavior, expanding the range of behaviors that can be produced. For specific illustration, we continue to focus on the relation between the speed of the NMT and the speed of cyclical, rhythmic behavior. Our analytic framework emphasizes the functional distinction between intrinsic plasticity or modulation of the NMT, dependent, like the contraction itself, on the motor neuron firing pattern, and extrinsic modulation, independent of it. The former is well suited to automatically optimizing the performance of a single behavior; the latter, to multiplying contraction shapes for multiple behaviors. In any case, to alleviate the constraint of the NMT, the plasticity and modulation must be peripheral. Such processes are likely to play a critical role wherever the nervous system must command, through the constraint of the NMT, a broad range of functional behaviors.

Effects of adenosine on Ca2+ entry in the nerve terminal of the frog neuromuscular junction

This study aimed to test whether nerve-evoked and adenosine-induced synaptic depression are due to reduction in Ca2+ entry in nerve terminals of the frog neuromuscular junction. Nerve terminals were loaded with the fluorescent Ca2+ indicator fluo 3 (fluo 3-AM) or loaded with dextran-coupled Ca2+ green-1 transported from the cut end of the nerve. Adenosine (10-50 µM) did not change the resting level of Ca2+ in the presynaptic terminal, whereas it induced large Ca2+ responses in perisynaptic Schwann cells, indicating that adenosine was active and might have induced changes in the level of Ca2+ in the nerve terminal. Ca2+ responses in nerve terminals could be induced by nerve stimulation (0.5 or 100 Hz for 100 ms) over several hours. In the presence of adenosine (10 µM), the size and duration of the nerve-evoked Ca2+ responses were unchanged. When extracellular Ca2+ concentration was lowered to produce the same reduction in transmitter release as the application of adenosine, Ca2+ responses induced by nerve stimulations were reduced by 40%. This indicates that changes in Ca2+ responsible for the decrease in release should have been detected if the mechanism of adenosine depression involved partial block of Ca2+ influx. Ca2+ responses evoked by prolonged high frequency trains of stimuli (50 Hz for 10 or 30 s), which caused profound depression of transmitter release, were sustained during the whole duration of the stimulation, and adenosine had no effect on these responses. These data indicate that neither adenosine induced synaptic depression nor stimulation-induced synaptic depression are caused by reductions in Ca2+ entry into the presynaptic terminal in the frog neuromuscular junction.Key words: adenosine, Ca2+, nerve terminal, transmitter release, synaptic depression.

Kinetics of synaptic depression and vesicle recycling after tetanic stimulation of frog motor nerve terminals

We measured the time courses of two key components of the synaptic vesicle cycle during recovery from synaptic depression under different conditions, and used this and other information to create a kinetic model of the vesicle cycle. End plate potential (EPP) amplitudes were used to follow recovery from synaptic depression after different amounts of tetanic stimulation. This provided an estimate of the time course of vesicle mobilization from the reserve pool to the docked (readily releasable) pool. In addition, FM1-43 was used to measure the rate of membrane retrieval after tetanic stimulation, and the amount of membrane transferred to the surface membrane. This provided a measure of the rate of refilling of the reserve pool with recycled vesicles. The time courses of both synaptic depression and endocytosis were slowed by prolonged tetanic stimulation. This behavior could be fitted by a simple model, assuming a first-order kinetics for both vesicle endocytosis and mobilization. The results show that a nearly 20-fold decrease in the rate constant of endocytosis greatly delays refilling of the depleted reserve pool. However, to fully account for the slower recovery of depression, a decrease in the rate constant of vesicle mobilization from the reserve pool of about sixfold is also required.

ATP and adenosine inhibit transmitter release at the frog neuromuscular junction through distinct presynaptic receptors

1. The effects of exogenous ATP or adenosine on end-plate currents (e.p.cs; evoked by simultaneous action of a few hundred quanta of ACh) or on miniature e.p.cs (m.e.p.cs) were studied under voltage clamp conditions on frog sartorius muscle fibres. 2. ATP or adenosine (100 microM(-1) mM) reduced the e.p.c. amplitude but did not affect m.e.p.c. amplitude, decay time constant and voltage-dependence of m.e.p.c., suggesting that e.p.c. depression induced by these purines had presynaptic origin only. 3. The action of ATP, unlike that of adenosine, was prevented by the P2-purinoceptor antagonist suramin (100 microM). The stable ATP analogue alpha,beta-methylene ATP (100 microM), known to be desensitizing agent on P2X receptors, also abolished the depressant effect of ATP while sparing the action of adenosine. Concanavalin A, an inhibitor of ecto-5'-nucleotidase, did not affect the presynaptic action of exogenously applied ATP. 4. The presynaptic action of adenosine was prevented by theophylline (1 mM), a blocker of adenosine receptors, while the effect of ATP was not changed under these conditions. The selective blocker of A1 adenosine receptors, 8-cyclopentyl-1,3,dipropylxanthine (DPCPX; 0.1 microM), abolished the presynaptic action of adenosine but did not prevent the depressant effect of ATP. 5. The effects of ATP and adenosine (at nearly saturating concentration) were additive suggesting that these purines activated not only distinct receptors but also different intracellular signalling mechanisms. 6. In contrast to the hypothesis that at the neuromuscular junction ATP reduces transmitter release via enzymatic degradation to presynaptically active adenosine, our data suggest that ATP (through its own presynaptic receptors) directly inhibits ACh release. Thus, ATP and adenosine might be almost equipotent as endogenous prejunctional neuromodulators at the neuromuscular junction.

P2 receptor excitation of rodent hypoglossal motoneuron activity in vitro and in vivo: a molecular physiological analysis

The role of P2 receptors in controlling hypoglossal motoneuron (XII MN) output was examined (1) electrophysiologically, via application of ATP to the hypoglossal nucleus of rhythmically active mouse medullary slices and anesthetized adult rats; (2) immunohistochemically, using an antiserum against the P2X2 receptor subunit; and (3) using PCR to identify expression of P2X2 receptor subunits in micropunches of tissue taken from the XII motor nucleus. Application of ATP to the hypoglossal nucleus of mouse medullary slices and anesthetized rats produced a suramin-sensitive excitation of hypoglossal nerve activity. Additional in vitro effects included potentiation of inspiratory hypoglossal nerve output via a suramin- and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive mechanism, XII MN depolarization via activation of a suramin-sensitive inward current, decreased neuronal input resistance, and a slow-onset theophylline-sensitive reduction of inspiratory output likely resulting from hydrolysis of extracellular ATP to adenosine and activation of P1 receptors. Immunohistochemically, P2X2 receptors were detected in inspiratory XII MNs that were labeled with Lucifer yellow. These data, combined with identification of mRNA for three P2X2 receptor subunit isoforms within the hypoglossal nucleus (two of which have not been localized previously in brain) and the previous demonstration that P2X receptors are ubiquitously expressed in cranial and spinal motoneuron pools, support not only a role of P2 receptors in modulating inspiratory hypoglossal activity but a general role of P2 receptors in modulating motor outflow from the CNS.

P2 receptor excitation of rodent hypoglossal motoneuron activity in vitro and in vivo: a molecular physiological analysis

The role of P2 receptors in controlling hypoglossal motoneuron (XII MN) output was examined (1) electrophysiologically, via application of ATP to the hypoglossal nucleus of rhythmically active mouse medullary slices and anesthetized adult rats; (2) immunohistochemically, using an antiserum against the P2X2 receptor subunit; and (3) using PCR to identify expression of P2X2 receptor subunits in micropunches of tissue taken from the XII motor nucleus. Application of ATP to the hypoglossal nucleus of mouse medullary slices and anesthetized rats produced a suramin-sensitive excitation of hypoglossal nerve activity. Additional in vitro effects included potentiation of inspiratory hypoglossal nerve output via a suramin- and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive mechanism, XII MN depolarization via activation of a suramin-sensitive inward current, decreased neuronal input resistance, and a slow-onset theophylline-sensitive reduction of inspiratory output likely resulting from hydrolysis of extracellular ATP to adenosine and activation of P1 receptors. Immunohistochemically, P2X2 receptors were detected in inspiratory XII MNs that were labeled with Lucifer yellow. These data, combined with identification of mRNA for three P2X2 receptor subunit isoforms within the hypoglossal nucleus (two of which have not been localized previously in brain) and the previous demonstration that P2X receptors are ubiquitously expressed in cranial and spinal motoneuron pools, support not only a role of P2 receptors in modulating inspiratory hypoglossal activity but a general role of P2 receptors in modulating motor outflow from the CNS.

Presynaptic excitability

Based on functional characterizations with electrophysiological techniques, the channels in nerve terminals appear to be as diverse as channels in nerve cell bodies (Table I). While most presynaptic Ca2+ channels superficially resemble either N-type or L-type channels, variations in detail have necessitated the use of subscripts and other notations to indicate a nerve terminal-specific subtype (e.g., Wang et al., 1993). Variations such as these pose a serious obstacle to the identification of presynaptic channels based solely on the effects of channel blockers on synaptic transmission. Pharmacological sensitivity alone is not likely to help in determining functional properties. Crucial details, such as voltage sensitivity and inactivation, require direct examination. It goes without saying that every nerve terminal membrane contains Ca2+ channels as an entry pathway so that Ca2+ can trigger secretion. However, there appears to be no general specification of channel type, other than the exclusion of T-type Ca2+ channels. T-type Ca2+ channels are defined functionally by strong inactivation and low threshold. Some presynaptic Ca2+ channels inactivate (posterior pituitary and Xenopus nerve terminals), and others have a somewhat reduced voltage threshold (retinal bipolar neurons and squid giant synapse). Perhaps it is just a matter of time before a nerve terminal Ca2+ channel is found with both of these properties. The high threshold and strong inactivation of T-type Ca2+ channels are thought to be adaptations for oscillations and the regulation of bursting activity in nerve cell bodies. The nerve terminals thus far examined have no endogenous electrical activity, but rather are driven by the cell body. On functional grounds, it is then reasonable to anticipate finding T-type Ca2+ channels in nerve terminals that can generate electrical activity on their own. The rarity of such behavior in nerve terminals may be associated with the rarity of presynaptic T-type Ca2+ channels. In four of the five preparations reviewed in this chapter--motor nerve, squid giant synapse, ciliary ganglion, and retina bipolar neurons--evidence was presented that supports a location for Ca2+ channels that is very close to active zones of secretion. All of these synapses secrete from clear vesicles, and the speed and specificity of transduction provided by proximity may be a common feature of these rapid synapses. In contrast, the posterior pituitary secretion apparatus may be triggered by higher-affinity Ca2+ receptors and lower concentrations of Ca2+ (Lindau et al., 1992). This would correspond with the slower performance of peptidergic secretion, but because of the large stimuli needed to evoke release from neurosecretosomes, the possibility remains that the threshold for secretion is higher than that reported. While the role of Ca2+ as a trigger of secretion dictates a requirement for voltage-activated Ca2+ channels as universal components of the presynaptic membrane, the presence of other channels is more difficult to predict. Depolarizations caused by voltage-activated Na+ channels activate the presynaptic Ca2+ channels, but whether this depolarization requires Na+ channels in the presynaptic membrane itself may depend on the electrotonic length of the nerve terminal. Variations in density between motor nerve terminals may reflect species differences in geometry. The high Na+ channel density in the posterior pituitary reflects the great electrotonic length of this terminal arbor. Whether Na+ channels are abundant or not in a presynaptic membrane, K+ channels provide the most robust mechanism for limiting depolarization-induced Ca2+ entry. K+ channel blockers enhance transmission at most synapses. In general, K+ channels are abundant in nerve terminals, although their apparent lower priority compared to Ca2+ channels in the eyes of many investigators leaves us with fewer detailed investigations in some preparations. Most nerve terminals have more than

Signalling via ATP in the nervous system

Strong evidence has been provided that ATP can act as a transmitter not only in smooth muscle but also in peripheral ganglia and in brain. The cloning and molecular identification of two putative ATP receptors supports the previously established pharmacological receptor classifications. This review places into perspective the evidence for ATP as a neural signalling substance by examining sites of storage, release and hydrolysis, as well as potential actions and targets. The action of ATP is related to that of the nucleoside adenosine, and the potential of additional nucleotides to function as neural messenger is examined briefly.

Changes in MEPP frequency during depression of evoked release at the frog neuromuscular junction

1. Endplate potentials (EPPs) and miniature endplate potentials (MEPPs) were recorded from frog neuromuscular junctions bathed in Ringer solutions containing normal (1.8 mM) or high (3.6 mM) Ca2+. The peptide toxin mu-conotoxin GIIIA was added to the Ringer solution to prevent muscle action potentials and contraction. 2. The nerve was stimulated with conditioning trains of 200-4800 impulses applied at 20 impulses s-1 to characterize the effects of repetitive stimulation on changes in EPP amplitude and MEPP frequency under high quantal conditions. 3. MEPP frequency was dramatically increased during and immediately following repetitive stimulation under high quantal conditions, whereas EPP amplitude was greatly depressed. There was no effect of repetitive stimulation on MEPP amplitude. 4. Following the conditioning stimulation the increase in MEPP frequency decayed back to the control level with a time course that could be described by four exponentials. The time constants of these exponentials were very similar to those that describe the components of stimulation-induced increases in EPP amplitude and MEPP frequency observed under low quantal conditions when depression is absent. 5. The results of this study indicate that depression and the components of stimulation-induced increases in release (facilitation, augmentation and potentiation) can be present at the same time, suggesting that the mechanism of depression involves different underlying factors from the mechanism(s) responsible for increases in release. They also indicate either that depression selectively affects only those quanta destined to be released in direct response to the nerve action potential, which would suggest that EPPs and MEPPs arise from different pools of transmitter, or that depression in some way affects a step in the release process involved only in evoked release, and not asynchronous (spontaneous) release.

ATP released together with acetylcholine as the mediator of neuromuscular depression at frog motor nerve endings

1. The hypothesis that ATP released by presynaptic stimulation is hydrolysed to adenosine and mediates prejunctional neuromuscular depression was tested at vertebrate neuromuscular junctions. Electrophysiological recordings of evoked acetylcholine (ACh) release and perineural ionic currents at motor nerve endings were made using the frog cutaneous pectoris nerve-muscle preparation. Either tubocurarine or alpha-bungarotoxin was used to block muscle contractions. 2. Either alpha,beta-methylene ADP (which inhibits ecto-5'nucleotidases and thus prevents the degradation of ATP to adenosine) or selective adenosine receptor antagonists (8-cyclo-pentyl alkyl xanthines) prevented the inhibitory effects of exogenous ATP on ACh release in response to low-frequency nerve stimulation. These results confirm earlier findings that ATP must be hydrolysed to adenosine to inhibit ACh release. 3. The presence of alpha,beta-methylene ADP completely prevented neuromuscular depression in response to repetitive high-frequency nerve stimulation (0.5-1 Hz). alpha,beta-Methylene ADP had no effect on ACh secretion under conditions where ACh release is well maintained (low-frequency stimulation, 0.05 Hz). 4. Selective adenosine receptor antagonists completely eliminated neuromuscular depression produced by repetitive high-frequency nerve stimulation (1.0 Hz) but had no effect on ACh release at low frequencies of stimulation (0.05 Hz). 5. Exogenous adenosine deaminase (5 i.u. ml-1), which degrades adenosine to its inactive nucleoside inosine, also eliminated neuromuscular depression but had no significant effect on ACh release at frequencies of nerve stimulation too low to produce prejunctional depression. 6. During maximal neuromuscular depression, the effects of exogenous adenosine or 2-chloroadenosine, an adenosine agonist, were occluded. 7. The calcium-sensitive component of perineurial recordings of motor nerve terminal currents did not change during depression or during application of adenosine receptor antagonists and adenosine deaminase, suggesting that neuromuscular depression in this species was not associated with changes in presynaptic Ca2+ currents. 8. These results suggest that, under the conditions of these experiments, endogenous ATP, after hydrolysis to adenosine, causes prejunctional neuromuscular depression. This inhibitory effect of endogenous adenosine occurs at a site distal to the locus of Ca2+ entry in the frog.

Calcium currents at motor nerve endings: absence of effects of adenosine receptor agonists in the frog

1. The effects of adenosine (50 microM) and 2-chloroadenosine (1-25 microM) were studied on Ca2+ currents in frog motor nerve endings. 2. Ca2+ currents associated with the synchronous, neurally evoked release of acetylcholine (ACh) were measured using either perineural or patch recording methods. Tetraethylammonium and/or 3,4-diaminopyridine were employed to block K+ currents. 3. Ca2+ currents were depressed by omega-conotoxin (1.5-2.5 microM), Cd2+ (100 microM-2 mM), Co2+ (500 microM-5 mM) or by a reduction of the extracellular calcium concentration. Such currents were also observed when Sr2+ was substituted for Ca2+. Both ACh release and Ca2+ currents at motor nerve endings have been reported to be insensitive to 1,4-dihydropyridine antagonists in this species. 4. Adenosine receptor agonists did not affect Ca2+ currents at concentrations that produced maximal inhibition of ACh release. 5. The effects of adenosine receptor agonists were examined on asynchronous K(+)-dependent ACh release under conditions in which the Ca2+ concentration gradient is likely to be reversed (Ca(2+)-free Ringer solution containing 1 mM EGTA). ACh release was measured by monitoring the frequency of occurrence of miniature endplate potentials (MEPPs). In Ca(2+)-free solutions containing 1 mM EGTA, high K+ depolarization caused a decrease in MEPP frequency, presumably because it elicits the efflux of Ca2+ from the nerve ending via membrane Ca2+ channels in a reverse Ca2+ gradient. 6. The Ca2+ channel blocker Co2+, which blocks the exit of Ca2+ from the nerve ending, increased the frequency of MEPPs in a concentration-dependent manner in a reverse Ca2+ gradient. 7. Adenosine or 2-chloroadenosine inhibited ACh release in a reverse Ca2+ gradient. 8. The results suggest that blockade of Ca2+ entry is not responsible for the inhibitory effects of adenosine at frog motor nerve endings.