Identification of purine binding sites on Torpedo acetylcholine receptor

Functional interactions between nicotinic and P2X receptors in celiac ganglia neurons

Here we characterized the cross-inhibitory interactions between nicotinic and P2X receptors of celiac neurons from the guinea pig by recording whole-cell currents induced by 1mM ACh (I(ACh)), 1mM ATP (I(ATP)) and by the simultaneous application of both agonists (I(ACh)(+ATP)). I(ACh) and I(ATP) were inhibited by hexamethonium (nicotinic channel blocker) and PPADS (P2X receptor antagonist), respectively. The amplitude of I(ACh)(+ATP) was equal to the current induced by the most effective agonist, indicating a current occlusion. Various observations indicate that I(ACh)(+ATP) is carried out through both nicotinic (nACh) and P2X channels: i) I(ACh)(+ATP) desensitisation kinetics were in between that of I(ACh) and I(ATP); ii) application of ATP+ACh, decreased I(ACh) and I(ATP), whereas no cross-desensitisation was observed between nACh and P2X receptors; iii) ATP did not affect I(ACh) in the presence of PPADS or after P2X receptor desensitisation; and iv) ACh did not affect I(ATP) when nACh channels were blocked with hexamethonium or after nACh receptor desensitisation. Current occlusion is not mediated by activation of metabotropic receptors as it is: i) voltage dependent (was not observed at + 5 mV); ii) present at low temperature (10 degrees C) and after inhibition of protein kinase activity (with staurosporine); and iii) absent at 30 microM ATP and 30 microM ACh (concentrations that should activate metabotropic receptors). In conclusion, current occlusion described here is similar to the previously reported myenteric neurons. This occlusion is likely the result of allosteric interactions between these receptors.

Acetylcholine nicotinic receptors: finding the putative binding site of allosteric modulators using the "blind docking" approach

Allosteric potentiation of acetylcholine nicotinic receptors is considered to be one of the most promising approaches for the treatment of Alzheimer's disease. However, the exact localization of the allosteric binding site and the potentiation mechanism at the molecular level are presently unknown. We have performed the "blind docking" of three known allosteric modulators (galanthamine, codeine and eserine) with the Acetylcholine Binding Protein and models of human alpha7, alpha3beta4 and alpha4beta2 nicotinic receptors, created by homology modeling. Three putative binding sites were identified in the channel pore, each one showing different affinities for the ligands. One of these sites is localized opposite to the agonist binding site and is probably implicated in the potentiation process. On the basis of these results, a possible mechanism for nicotinic acetylcholine receptor (nAChRs) activation is proposed. The present findings may represent an important advance for understanding the allosteric modulation mechanism of nAChRs. [Figure: see text].

Effects of catecholamines and purines on luminescence in the brittlestar Amphipholis squamata (Echinodermata)

The effects of catecholamines (dopamine, adrenaline, noradrenaline and its derivatives), 5-hydroxytryptamine and purines (adenosine, ATP and their derivatives) on the acetylcholine-induced luminescence of isolated arms and dissociated photocytes of the luminescent ophiuroid Amphipholis squamata were tested. The results showed that catecholamines and 5-hydroxytryptamine (10(−)(5) to 10(−)(3)mol l(−)(1)) had a strong dose-dependent inhibitory effect on acetylcholine-induced luminescence. In contrast, purines (10(−)(4) and 10(−)(3)mol l(−)(1)) triggered luminescence in the absence of acetylcholine and/or potentiated acetylcholine-induced luminescence. The results with specific purinergic agonists and antagonists indicated the involvement of P(1)- and P(2)-like purinoceptors in the control of luminescence. Our study suggests that, in addition to the previously described cholinergic system in Amphipholis squamata, there may be a purinergic system, acting in synergy with acetylcholine, and an inhibitory neuromodulatory catecholaminergic system, all associated with the control of luminescence.

Mutual occlusion of P2X ATP receptors and nicotinic receptors on sympathetic neurons of the guinea-pig

1. The interaction of ion channels activated by nicotinic receptor agonists with ion channels gated by extracellular ATP (i.e. P2X receptors) was studied on sympathetic neurons acutely dissociated from coeliac ganglia of the guinea-pig. Patch clamp methods were used to measure the inward current generated through these non-selective cationic channels under voltage clamp. 2. At the whole cell level, the specific nicotinic receptor agonists nicotine (5-100 microM) or cytisine (50-75 microM) and the P2X receptor agonists ATP (0.1-7 microM) or alpha,beta-methylene ATP (6 microM) were examined separately and in the presence of the other receptor activator. When a nicotinic and P2X receptor agonist were applied together, mutually occlusive effects were generally observed. This occurred even with concentrations of agonists that in themselves generated little to no inward current. 3. The occlusive effects of nicotinic agonists on ATP-gated currents were blocked by the nicotinic receptor/ion channel blocker hexamethonium (150 microM). The occlusive effects of ATP analogues on inward currents generated by nicotinic agonists were blocked by the P2X receptor antagonist suramin (100 microM). 4. Mutual occlusion of the effects of nicotinic agonists and ATP analogues were also observed when currents through single channels were studied in excised (outside-out) patches. 5. The results suggest that nicotinic receptors and P2X ATP receptors do not act independently in these sympathetic neurons.

Mapping of a binding site for ATP within the extracellular region of the Torpedo nicotinic acetylcholine receptor beta-subunit

Using 2,8,5'-[3H]ATP as a direct photoaffinity label for membrane-bound nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata, we have identified a binding site for ATP in the extracellular region of the beta-subunit of the receptor. Photolabeling was completely inhibited in the presence of saturating concentrations of nonradioactive ATP, whereas neither the purinoreceptor antagonists suramin, theophyllin, and caffeine nor the nAChR antagonists alpha-bungarotoxin and d-tubocurarine affected the labeling reaction. Competitive and noncompetitive nicotinic agonists and Ca2+ increased the yield of the photoreaction by up to 50%, suggesting that the respective binding sites are allosterically linked with the ATP site. The dissociation constant KD of binding of ATP to the identified site on the nAChR was of the order of 10(-4) M. Sites of labeling were found in the sequence regions Leu11-Pro17 and Asp152-His163 of the nAChR beta-subunit. These regions may represent parts of a single binding site for ATP, which is discontinuously distributed within the primary structure of the N-terminal extracellular domain. The existence of an extracellular binding site for ATP confirms, on the molecular level, that this nucleotide can directly act on nicotinic receptors, as has been suggested from previous electrophysiological and biochemical studies.

Calcium flux through predominantly independent purinergic ATP and nicotinic acetylcholine receptors

Ligand-gated nicotinic acetylcholine receptors (nAChRs) and purinergic ATP receptors are often expressed in the same peripheral and central neurons, and ATP and acetylcholine (ACh) are stored together in some synaptic vesicles. Evidence has suggested that nAChRs and ATP receptors are not independent and that some agonists strongly cross-activate and desensitize both receptor types. Rat sympathetic neurons and nAChRs expressed in Xenopus oocytes were studied to determine the significance of the interactions caused by the two agonist types. Current amplitudes included with separate or combined applications of ATP and nicotine are > 90% additive and independent. Half of all neurons tested responded to either ATP or nicotine but not to both, indicating differences in the expression of the two receptors. In neurons that expressed both receptors types, the nAChRs were inhibited by the activity-dependent open-channel blocker chlorisondamine. If the purinergic and nicotinic receptors were significantly dependent and coactivated, then blocking the ion channels opened by a nicotinic agonist should diminish the current activated by a purinergic agonist. That result was not seen; rather, complete open-channel block of nAChRS with chlorisondamine did not significantly alter the amplitude or kinetics of ATP-induced currents in the same neurons. Finally, when cloned nAChR subunits were expressed in oocytes, ATP activated only very small currents compared with the current activated by Ach. For the 13 different nAChR subunit combinations that were studied, ATP (50-500 microM) activated a current that ranged from 0 to 4% of the size of the current activated by 100 microM ACh. In summary, we find that there is little cross reactivity, and nAChRs and purinergic ATP receptors are predominantly independent, acting with separable physiological characteristics. Therefore the quantitative Ca2+ flux could be separately determined for nAChRs and ATP receptors. The fraction of total current that is carried by Ca2+ was quantitatively determined by simultaneously measuring the whole cell current and the associated change in intracellular Ca2+ with fura-2. For sympathetic neurons bathed in 2.5 mM Ca2+ at a holding potential of -50 mV, Ca2+ carries 4.8 +/- 0.3% (mean +/- SE) of the inward current through neuronal nAChRs and 6.5 +/- 0.1% of the current through purinergic ATP receptors. In conclusion, activity-dependent Ca2+ influx through predominately independent populations of nAChRS and ATP neurons can produce different intracellular signals at purinergic and cholinergic synapses.

Reduction of acetylcholine-activated current by low concentrations of extracellular adenosine 5'-triphosphate

Effects of adenosine 5'-triphosphate (ATP) on ionic currents activated by acetylcholine (ACh) were investigated using rat pheochromocytoma PC12 cells and Xenopus oocytes expressing nicotinic receptors. In PC12 cells, ATP (10 nM to 1 microM) inhibited an inward current activated by ACh in not all but about 60% of cell batches. The ACh-activated current was also inhibited by ATP in Xenopus oocytes and, with a subunit combination of alpha 3 plus beta 4, the inhibition was observed at concentration as low as 100 fM. Uridine 5'-triphosphate (UTP) induced a similar inhibition of the ACh-activated current both in PC12 cells and Xenopus oocytes. These and other properties suggest that the current reduction by ATP is not mediated through conventional P2-purinoceptors.