Single channel properties of P2X ATP receptors in outside-out patches from rat hippocampal granule cells

Enlightening activation gating in P2X receptors

P2X receptors are trimeric nonselective cation channels gated by ATP. They assemble from seven distinct subunit isoforms as either homo- or heteromeric complexes and contain three extracellularly located binding sites for ATP. P2X receptors are expressed in nearly all tissues and are there involved in physiological processes like synaptic transmission, pain, and inflammation. Thus, they are a challenging pharmacological target. The determination of crystal and cryo-EM structures of several isoforms in the last decade in closed, open, and desensitized states has provided a firm basis for interpreting the huge amount of functional and biochemical data. Electrophysiological characterization in conjugation with optical approaches has generated significant insights into structure–function relationships of P2X receptors. This review focuses on novel optical and related approaches to better understand the conformational changes underlying the activation of these receptors.

A gold nano-electrode for single ion channel recordings

The artificial bilayer single channel recording technique is commonly used to observe the detailed physiological properties of various ion channel proteins. It permits easy control of the solution and membrane lipid composition, and is also compatible with pharmacological screening devices. However, its use is limited due to low measurement efficiency. Here, we developed a novel artificial bilayer single channel recording technique in which solubilized ion channel proteins immobilized on a gold nano-electrode are directly incorporated into a lipid bilayer at the same time as the bilayer is formed at the tip of it on coming in contact with an aqueous-oil interface. Using this technique, we measured the single channel currents of several types of channels including KcsA, MthK, hBK and P2X4. This technique requires only one action to simultaneously form the bilayers and reconstitute the channels into the membranes. This simplicity greatly increases the measurement efficiency and allows the technique to potentially be combined with high-throughput screening devices.

A fluorescent approach for identifying P2X1 ligands

There are no commercially available, small, receptor-specific P2X1 ligands. There are several synthetic derivatives of the natural agonist ATP and some structurally-complex antagonists including compounds such as PPADS, NTP-ATP, suramin and its derivatives (e.g. NF279, NF449). NF449 is the most potent and selective ligand, but potencies of many others are not particularly high and they can also act at other P2X, P2Y and non-purinergic receptors. While there is clearly scope for further work on P2X1 receptor pharmacology, screening can be difficult owing to rapid receptor desensitisation. To reduce desensitisation substitutions can be made within the N-terminus of the P2X1 receptor, but these could also affect ligand properties. An alternative is the use of fluorescent voltage-sensitive dyes that respond to membrane potential changes resulting from channel opening. Here we utilised this approach in conjunction with fragment-based drug-discovery. Using a single concentration (300 μM) we identified 46 novel leads from a library of 1443 fragments (hit rate = 3.2%). These hits were independently validated by measuring concentration-dependence with the same voltage-sensitive dye, and by visualising the competition of hits with an Alexa-647-ATP fluorophore using confocal microscopy; confocal yielded k_on (1.142 × 10⁶ M⁻¹ s⁻¹) and k_off (0.136 s⁻¹) for Alexa-647-ATP (K_d = 119 nM). The identified hit fragments had promising structural diversity. In summary, the measurement of functional responses using voltage-sensitive dyes was flexible and cost-effective because labelled competitors were not needed, effects were independent of a specific binding site, and both agonist and antagonist actions were probed in a single assay. The method is widely applicable and could be applied to all P2X family members, as well as other voltage-gated and ligand-gated ion channels.

This article is part of the Special Issue entitled ‘Fluorescent Tools in Neuropharmacology’.

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A novel fluorescence-based screening approach for identifying P2X1 receptor ligand candidates.

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Fragment-based drug discovery applied to ligand-gated ion channels.

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The use of confocal microscopy to determine the kinetics and affinity of Alexa-647-ATP binding to P2X1 receptors.

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Alexa-647-ATP for imaging P2X1 receptors on live cells.

Conductance simulation of the purinergic P2X2, P2X4, and P2X7 ionic channels using a combined Brownian dynamics and molecular dynamics approach

This paper investigates the application of an original combined approach of molecular and Brownian dynamic methods with quantum chemistry calculations for modeling the process of conductance of ion channels using purinergic P2X family receptors P2X2, P2X4, and P2X7 as a case study. A simplified model of the ionic channel in the lipid bilayer has been developed. A high level of conductance (30 pS) of P2X2 ionic channel together with the key role of Asp349 in forming the selectivity filter of P2X2 has been shown by using this approach. Calculated P2X2 permeability to monovalent cations Li(+), Na(+), and K(+) conforms to the free diffusion coefficient of these ions, which shows the low selectivity of P2X2 ionic channel.

Pharmacological properties of a pore induced by raising intracellular Ca2+

Recent studies on the P2X(7) receptor in 2BH4 cells and peritoneal macrophages have demonstrated that the raise in intracellular Ca(2+) concentration induces a pore opening similar to P2X(7) receptor pore. Herein, we have investigated whether the pore activated by the elevation of intracellular Ca(2+) concentration is associated to P2X(7) receptor. Using patch clamp in cell attached, whole cell configuration, and dye uptake, we measured the pore opening in cell types that express the P2X(7) receptor (2BH4 cells and peritoneal macrophages) and in cells that do not express this receptor (HEK-293 and IT45-RI cells). In 2BH4 cells, the stimulation with ionomycin (5-10 microM) increased intracellular free Ca(2+) concentration and induced pore formation with conductance of 421 +/- 14 pS, half-time (t(1/2)) for ethidium bromide uptake of 118 +/- 17 s, and t(1/2) for Lucifer yellow of 122 +/- 11 s. P2X(7) receptor antagonists did not block these effects. Stimulation of HEK-293 and IT45-RI cells resulted in pore formation with properties similar to those found for 2BH4 cells. Connexin hemichannel inhibitors (carbenoxolone and heptanol) also did not inhibit the pore-induced effect following the increase in intracellular Ca(2+) concentration. However, 5-(N,N-hexamethylene)-amiloride, a P2X(7) receptor pore blocker, inhibited the induced pore. Moreover, intracellular signaling modulators, such as calmodulin, phospholipase C, mitogen-activated protein kinase, and cytoskeleton components were important for the pore formation. Additionally, we confirmed the results obtained for electrophysiology by using the flow cytometry, and we discarded the possibility of cellular death induced by raising intracellular Ca(2+) at the doses used by using lactate dehydrogenase release assay. In conclusion, increased concentration in intracellular Ca(+2) induces a novel membrane pore pharmacologically different from the P2X(7) associated pore and hemigap-junction pore.

Activation kinetics of single P2X receptors

After the primary structure of P2X receptors had been identified, their function had to be characterized on the molecular level. Since these ligand-gated ion channels become activated very quickly after binding of ATP, methods with adequate time resolution have to be applied to investigate the early events induced by the agonist. Single-channel recordings were performed to describe conformational changes on P2X(2), P2X(4), and P2X(7) receptors induced by ATP and also by allosteric receptor modifiers. The main results of these studies and the models of P2X receptor kinetics derived from these observations are reviewed here. The investigation of purinoceptors by means of the patch clamp technique following site-directed mutagenesis will probably reveal more details of P2X receptor function at the molecular level.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Biophysics of P2X receptors

The P2X receptor is the baby brother of the ligand-gated ion channel super-family. An understanding of its role in human physiology is still developing, and no one truly knows how it works to transport ions across the membrane. In this study, we review some aspects of P2X channel biophysics, concentrating on ion permeation and gating. P2X channels transport both small and large cations and anions across cell membranes in a manner that depends on both the subunit composition of the receptor and the experimental conditions. We describe the pore properties of wild-type receptors and use the altered phenotypes of mutant receptors to point the way towards a structural model of the pore.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Memory in astrocytes: a hypothesis

Background

Recent work has indicated an increasingly complex role for astrocytes in the central nervous system. Astrocytes are now known to exchange information with neurons at synaptic junctions and to alter the information processing capabilities of the neurons. As an extension of this trend a hypothesis was proposed that astrocytes function to store information. To explore this idea the ion channels in biological membranes were compared to models known as cellular automata. These comparisons were made to test the hypothesis that ion channels in the membranes of astrocytes form a dynamic information storage device.

Results

Two dimensional cellular automata were found to behave similarly to ion channels in a membrane when they function at the boundary between order and chaos. The length of time information is stored in this class of cellular automata is exponentially related to the number of units. Therefore the length of time biological ion channels store information was plotted versus the estimated number of ion channels in the tissue. This analysis indicates that there is an exponential relationship between memory and the number of ion channels. Extrapolation of this relationship to the estimated number of ion channels in the astrocytes of a human brain indicates that memory can be stored in this system for an entire life span. Interestingly, this information is not affixed to any physical structure, but is stored as an organization of the activity of the ion channels. Further analysis of two dimensional cellular automata also demonstrates that these systems have both associative and temporal memory capabilities.

Conclusion

It is concluded that astrocytes may serve as a dynamic information sink for neurons. The memory in the astrocytes is stored by organizing the activity of ion channels and is not associated with a physical location such as a synapse. In order for this form of memory to be of significant duration it is necessary that the ion channels in the astrocyte syncytium be electrically in contact with each other. This function may be served by astrocyte gap junctions and suggests that agents that selectively block these gap junctions should disrupt memory.

Functional regulation of P2X6 receptors by N-linked glycosylation: identification of a novel alpha beta-methylene ATP-sensitive phenotype

Investigation of rat recombinant P2X(6) receptors has been limited because of the difficulty in obtaining functional expression in heterologous systems. In this study, we demonstrate glycosylation-dependent regulation of recombinant P2X(6) receptor function and associated conferral of a novel phenotype that is sensitive to the P2X(1) and P2X(3) receptor agonist, alphabeta-methylene ATP. In cells functionally expressing P2X(6) receptors, ATP and alphabeta-methylene ATP evoked slowly desensitizing inward currents (EC(50) values, 0.5 and 0.6 microM, respectively) with slow kinetics of current decay on agonist washout. 2',3'-O-(2,4,6-trinitrophenyl ATP) and iso-pyridoxalphosphate-6-azophenyl-2'-5'-disulfonate were effective antagonists (IC(50) values, 0.8 and 22 microM, respectively); however, suramin was relatively ineffective. Reverse transcription-polymerase chain reaction analysis confirmed the absence of other P2X receptor subunits. Western analysis of membrane fractions from functional and nonfunctional clones confirmed the presence of P2X(6) at the cell membrane but revealed a difference in apparent molecular mass of immunoreactive products ( approximately 70 and approximately 60 kDa, respectively). N-glycosidase F treatment of both functional and nonfunctional receptor cell membranes increased the electrophoretic mobilities of immunoreactive products, with both proteins migrating at approximately 55 kDa, demonstrating an increased level of glycosylation of the P2X(6) receptor in functional compared with nonfunctional cells. This study demonstrates that nonfunctional rat recombinant P2X(6) receptors 1) are expressed on the membrane surface of human embryonic kidney cells and 2) are glycosylated. Expression of the novel functional receptor phenotype is associated with further glycosylation, resulting in an apparently larger molecular mass. These results suggest that P2X(6) receptor subunits contribute to alphabeta-methylene ATP sensitivity.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

ATP inhibits NMDA receptors after heterologous expression and in cultured hippocampal neurons and attenuates NMDA-mediated neurotoxicity

We investigated the potential of ATP to inhibit heterologously expressed NMDA receptor subunit combinations, NMDA-induced currents in cultured hippocampal cells, and NMDA-induced neurotoxicity. The effect of ATP on diheteromeric NR1a/NR2A-D NMDA receptor (NR) combinations expressed in Xenopus laevis oocytes was studied by voltage-clamp recording. ATP strongly inhibited NMDA-induced inward currents only at the NR1a/NR2B receptor combination. At NMDA concentrations corresponding to the EC50 value (20 microm), ATP revealed an IC50 value of 135 microm. Mutation studies suggest that ATP exerts its inhibition via the glutamate-binding pocket of the NR2B subunit. Inosine 5'-triphosphate (ITP), GTP, and AMP also inhibited the recombinant NR1a/NR2B receptor, whereas UTP and CTP, ADP, or adenosine had no or only a small effect. Correspondingly, ATP inhibited NMDA-induced but not kainate-induced currents at cultured hippocampal neurons. An abundant expression of the NR2B subunit in the cultured neurons was verified by immunocytochemistry and blockade of NMDA-induced currents by the NR2B-selective antagonist ifenprodil. In addition we studied the role of ATP in NMDA-mediated neurotoxicity using cultured rat hippocampal cells. ATP exhibited a dose-dependent rescue effect when coapplied with the excitotoxicant NMDA, in contrast to ADP, AMP, and adenosine. The effect of ATP was mimicked by GTP and ITP but not by UTP and CTP. ATP had no effect on kainate-elicited neurotoxicity. Our results suggest that ATP can act as an inhibitor of NMDA receptors depending on receptor subunit composition and that it can attenuate NMDA-mediated neurotoxicity that is mediated neither by ATP nor by adenosine receptors.

ATP inhibits NMDA receptors after heterologous expression and in cultured hippocampal neurons and attenuates NMDA-mediated neurotoxicity

We investigated the potential of ATP to inhibit heterologously expressed NMDA receptor subunit combinations, NMDA-induced currents in cultured hippocampal cells, and NMDA-induced neurotoxicity. The effect of ATP on diheteromeric NR1a/NR2A-D NMDA receptor (NR) combinations expressed in Xenopus laevis oocytes was studied by voltage-clamp recording. ATP strongly inhibited NMDA-induced inward currents only at the NR1a/NR2B receptor combination. At NMDA concentrations corresponding to the EC50 value (20 microm), ATP revealed an IC50 value of 135 microm. Mutation studies suggest that ATP exerts its inhibition via the glutamate-binding pocket of the NR2B subunit. Inosine 5'-triphosphate (ITP), GTP, and AMP also inhibited the recombinant NR1a/NR2B receptor, whereas UTP and CTP, ADP, or adenosine had no or only a small effect. Correspondingly, ATP inhibited NMDA-induced but not kainate-induced currents at cultured hippocampal neurons. An abundant expression of the NR2B subunit in the cultured neurons was verified by immunocytochemistry and blockade of NMDA-induced currents by the NR2B-selective antagonist ifenprodil. In addition we studied the role of ATP in NMDA-mediated neurotoxicity using cultured rat hippocampal cells. ATP exhibited a dose-dependent rescue effect when coapplied with the excitotoxicant NMDA, in contrast to ADP, AMP, and adenosine. The effect of ATP was mimicked by GTP and ITP but not by UTP and CTP. ATP had no effect on kainate-elicited neurotoxicity. Our results suggest that ATP can act as an inhibitor of NMDA receptors depending on receptor subunit composition and that it can attenuate NMDA-mediated neurotoxicity that is mediated neither by ATP nor by adenosine receptors.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Adenosine triphosphate accelerates recovery from hypoxic/hypoglycemic perturbation of guinea pig hippocampal neurotransmission via a P(2) receptor

The present study was designed to assess the effects of adenosine triphosphate (ATP) on hippocampal neurotransmissions under the normal and hypoxic/hypoglycemic conditions. ATP reversely depressed population spikes (PSs), which were monitored in the dentate gyrus of guinea pig hippocampal slices, in a dose-dependent manner at concentrations ranged from 0.1 micro M to 1 mM. A similar depression was obtained with the P(2) receptor agonist, alpha,beta-methylene ATP (alpha,beta-MeATP), and the effect was inhibited by the P(2) receptor antagonists, suramin and PPADS. The inhibitory action of ATP or alpha,beta-MeATP was inhibited by the gamma-aminobutyric acid(A) (GABA(A)) receptor antagonist, bicuculline, but it was not affected by theophylline, a broad inhibitor of adenosine (P(1)) receptors, tetraethylammonium, a broad inhibitor of K(+) channels, or ecto-protein kinase inhibitors. ATP or alpha,beta-MeATP enhanced GABA release from guinea pig hippocampal slices, that was inhibited by deleting extracellular Ca(2+) or in the presence of tetrodotoxin, while ATP had no effect on GABA release from cultured rat hippocampal astrocytes or postsynaptic GABA-gated channel currents in cultured rat hippocampal neurons. Twenty-minutes deprivation of glucose and oxygen from extracellular solution abolished PSs, the amplitude recovering to about 30% of basal levels 50 min after returning to normal conditions. ATP or alpha,beta-MeATP accelerated the recovery after hypoxic/hypoglycemic insult (approximately 80% of basal levels). Adenosine diphosphate and adenosine monophosphate accelerated the recovery, but to a much lesser extent, and adenosine had no effect. The results of the present study thus suggest that ATP inhibits neuronal activity by enhancing neuronal GABA release via a P(2) receptor, perhaps a P2X receptor, thereby protecting against hypoxic/hypoglycemic perturbation of hippocampal neurotransmission.

P2X purinergic receptor channel expression and function in bovine aortic endothelium

We examined bovine aortic endothelial cells (BAECs) for the functional expression of P2X receptors, the ATP-gated cation channels. We identified the P2X subtypes present in BAECs using RT-PCR. mRNA was present for only three of seven family members: P2X4, P2X5, and P2X7. We then characterized agonist-activated currents in whole cell and outside-out patch recordings using 2-methyl-thio-ATP (MeSATP) as a P2X4 and P2X5 receptor agonist and 2',3'-O-(4-benzoylbenzoyl)ATP (BzATP) as a P2X7 receptor agonist. MeSATP (10-20 microM) produced current with characteristics of P2X4 receptors. The current was an inwardly rectifying current, reversed near 0 mV, slowly desensitized, was not blocked by suramin (300 microM) or reactive blue (60 microM), and had a single channel conductance of 36 pS. BzATP (10-100 microM), on the other hand, activated a 9-pS channel with sustained activity in the continued presence of the agonist. BzATP-activated current was blocked by reactive blue (60 microM) and by suramin (approximately 50% block at 300 microM). We confirmed, by immunocytochemistry, the presence of P2X4 and P2X7 protein. The agonists failed, however, to induce significant uptake of the large molecule YO-PRO, indicating the lack of pore development that has been demonstrated for P2X7 and P2X4 in response to agonist in some cell types.

Distribution of P2X receptors on astrocytes in juvenile rat hippocampus

Recent evidence suggested that ATP acting via ionotropic (P2X) and metabotropic (P2Y) purinergic receptors might be involved in signaling between glial cells and within glial-neuronal networks. In contrast to their neuronal counterpart, the identity of P2X receptors in CNS glial cells is largely unknown. In the present study, antibodies recognizing the subunits P2X1-P2X7 were applied together with the astroglial marker S100beta and nuclear labeling with Hoechst 33342 to investigate semiquantitatively the distribution of the whole set of P2X receptors in astrocytes of the juvenile rat hippocampus. Expression of P2X1-P2X4, P2X6, and P2X7 subunits was observed in astrocytes of various hippocampal subregions, but the cells were completely devoid of P2X5 protein. S100beta-positive cells expressing subunits P2X3-P2X7 occurred evenly in the different subfields, while P2X1- and P2X2-positive astrocytes were distributed more heterogeneously. The staining pattern of P2X subunits also differed at the subcellular level. Antibodies against P2X2 and P2X4 labeled both astroglial cell bodies and processes. Immunoreactivity for P2X1 and P2X6 was mainly confined to somatic areas of S100beta-positive cells, whereas the subunit P2X3 was primarily localized along astroglial processes. Knowledge of the distribution of P2X receptors might provide a basis for a better understanding of their specific role in cell-cell signaling.

ATP modulates Na+ channel gating and induces a non-selective cation current in a neuronal hippocampal cell line

Extracellular ATP evoked two excitatory responses in hippocampal neuroblastoma cells (HN2). The first, an opening of a receptor-operated non-selective cation channel and the second was a leftward shift in Na+ channel activation. Both ATP (5-1000 microM) and 2',3'-(4-benzoyl)-benzoyl-ATP (Bb-ATP, 50 microM) activated a non-selective cation current reversing near 0 mV and shifted the Na+ activation and inactivation curves to the left. Based on a comparison of a series of agonists and antagonists, the inward current appeared to be partially mediated by activation of a P2X7 receptor, although hybrid channels cannot be ruled out. The shift in Na+ channel gating could be separated from the opening of the cation channel, as application of the P2Y antagonist Reactive Blue-2 and GTP shifted the Na+ current activation to the left but failed to elicit the inward cation current. Both responses to ATP and Bb-ATP were insensitive to block by the P2X antagonist suramin (300 microM) but were prevented by incubation in oxidized ATP (200 microM); a putative P2X7 receptor antagonist. Prior screening of the surface negative charge of the membrane with a high concentration of divalent cations prevented both responses. We suggest that ATP4- activates a P2X receptor and becomes trapped on a site, on or near the Na+ channel. Activation of the P2X receptor leads to the opening of a non-specific cation channel, while the binding of ATP4- leads to a modified charge sensed by the Na+ channel, similar to what occurs in the presence of charged amphiphiles as well as a number of beta-scorpion toxins.

Synaptic P2X receptors

Over the past two years, ATP has clearly been shown to act as a co-transmitter with GABA, glycine and probably glutamate in the central nervous system. Our understanding of the ATP-gated P2X receptors is progressing rapidly, and the pharmacology, stoichiometry and subunit combinations of heteropolymeric P2X channels has been substantially elucidated.

Activation-dependent changes in receptor distribution and dendritic morphology in hippocampal neurons expressing P2X2-green fluorescent protein receptors

ATP-gated P2X(2) receptors are widely expressed in neurons, but the cellular effects of receptor activation are unclear. We engineered functional green fluorescent protein (GFP)-tagged P2X(2) receptors and expressed them in embryonic hippocampal neurons, and report an approach to determining functional and total receptor pool sizes in living cells. ATP application to dendrites caused receptor redistribution and the formation of varicose hot spots of higher P2X(2)-GFP receptor density. Redistribution in dendrites was accompanied by an activation-dependent enhancement of the ATP-evoked current. Substate-specific mutant T18A P2X(2)-GFP receptors showed no redistribution or activation-dependent enhancement of the ATP-evoked current. Thus fluorescent P2X(2)-GFP receptors function normally, can be quantified, and reveal the dynamics of P2X(2) receptor distribution on the seconds time scale.