Nucleotide receptors

Age-related changes in adenosine 5'-triphosphate-induced constriction of isolated, perfused mesenteric arteries of rats

In isolated mesenteric arteries of rats, dose-dependent increase in perfusion pressure by adenosine 5'-triphosphate (ATP, 0.1 approximately 3000 nmole) diminished with age. ATP responses of both 4- and 32-week-old rats were enhanced by indomethacin (5 microM), and further by the combination of indomethacin and N(G)-nitro-L-arginine methyl ester (L-NAME, 5 microM). The enhancement with each of the treatments was less in 32-week-old rats than that in 4-week-old rats, and there was no enhancement in 75-week-old rats. The ATP response was enhanced by removing the endothelium only in 4-week-old rats. The constrictions in response to ATP (1000 nmole) in both 4- and 32-week-old rats were equally enhanced by reactive blue 2 (30 micromole) and were inhibited by pyridoxal-phosphate-6-azophenyl-2',4-disulphonic acid (PPADS, 30 microM) and alpha, beta-methylene ATP (alpha, beta-mATP, 100 nmole) to a similar extent. The increased tone which was produced by the perfusion with physiological solution containing 100 mM potassium chloride was greater in older animals. This age-related change in the vascular tone disappeared when the responses were potentiated by L-NAME. These results demonstrate that in rat mesenteric arteries, ATP-induced constriction decreases with age. The age-related decline of vasoconstriction is not likely to arise from the changes in the contractility of smooth muscle, from the counterbalancing regulation by the endothelium, or from the cooperation of P2 purinoceptor subtypes. The density of purinoceptors and some post-receptor signal transduction mechanisms in the vascular smooth muscle cells may change with age. The enhanced ATP response might have special physiological significance in rats during development.

Functional and molecular diversity of purinergic ion channel receptors

P2X receptors are membrane ion channels gated by extracellular adenosine 5'-triphosphate (ATP); nucleotides also activate a family of seven transmembrane G protein-coupled receptors (P2Y). P2X receptors are widely expressed on mammalian cells, where they can be broadly differentiated into three groups. The first group is almost equally well activated by ATP and its analog alpha beta methyleneATP (alpha beta meATP), whereas a second group is not activated by alpha beta meATP. A third-group type of receptor (termed P2Z) is distinguished by the fact that the channel opening is followed by cell permeabilization and lysis if the agonist application is continued for more than a few seconds. Seven cDNAs have been cloned that encode P2X receptor subunits. When expressed individually in heterologous systems, P2X1 and P2X3 subunits form channels activated by ATP or alpha beta meATP; whereas P2X2, P2X4, and P2X5 form channels activated by ATP but not alpha beta meATP. P2X6 receptors do not express readily, and P2X7 receptors correspond closely in their properties to P2Z. Further phenotypes can be produced when two subunits are coexpressed, indicating hetero-multimerization. This chapter compares the properties of the native P2X receptors with those of the cloned and expressed subunits.

Sympathetic neuronal oscillators are capable of dynamic synchronization

In this paper we show that the discharges of sympathetic neurons innervating an identified peripheral target are driven by multiple oscillators that undergo dynamic synchronization when an entraining force, central respiratory drive (CRD), is increased. Activity was recorded from postganglionic sympathetic neurons (PGNs) innervating the caudal ventral artery of the rat tail: (1) at the population level from the ventral collector nerve (VCN); and (2) from pairs of single PGNs recorded simultaneously using a focal recording technique. Autospectral analysis of VCN activity revealed a more prominent rhythmical component in the presence of CRD than in its absence, suggesting that (1) multiple oscillators drive the discharges of PGNs and (2) these oscillators can be entrained and therefore synchronized by CRD. This interpretation was supported by analysis of the firing behavior of PGN pairs. Autocorrelation and cross-correlation analysis showed that pairs were not synchronized in the absence of CRD but showed significant synchronization when CRD was enhanced. Time-evolving spectral analysis and raster plots demonstrated that the temporal stability of PGN-to-PGN and CRD-to-PGN interactions at a given level of CRD were also dynamic in nature, with stable constant phase relationships predominating as CRD was increased. This is the first reported example of dynamic synchronization in populations of single postganglionic sympathetic neurons, and we suggest that, as in sensory processing and motor control, temporal pattern coding may also be an important feature of neuronal discharges in sympathetic pathways.

Pore dilation of neuronal P2X receptor channels

P2X receptors are ligand-gated ion channels activated by the binding of extracellular adenosine 5'-triphosphate (ATP). Brief (< 1 s) applications of ATP to nodose ganglion neurons or to cells transfected with P2X2 or P2X4 receptor cDNAs induce the opening of a channel selectively permeable to small cations within milliseconds. We now show that, during longer ATP application (10-60 s), the channel also becomes permeable to much larger cations such as N-methyl-D-glucamine and the propidium analog YO-PRO-1. This effect is enhanced in P2X2 receptors carrying point mutations in the second transmembrane segment. Progressive dilation of the ion-conducting pathway during prolonged activation reveals a mechanism by which ionotropic receptors may alter neuronal function.

Ion channels of nociception

Nociceptors are the first cells in the series of neurons that lead to the sensation of pain. The essential functions of nociceptors--transducing noxious stimuli into depolarizations that trigger action potentials, conducting the action potentials from the peripheral sensory site to the synapse in the central nervous system, and converting the action potentials into neurotransmitter release at the presynaptic terminal--all depend on ion channels. This review discusses recent results in the converging fields of nociception and ion channel biology. It focuses on (a) the capsaicin receptor and its possible role in thermosensation, (b) ATP-gated channels, (c) proton-gated channels, and (d) nociceptor-specific Na+ channels.

IFN-γ Up-Regulates the A2B Adenosine Receptor Expression in Macrophages: A Mechanism of Macrophage Deactivation

Adenosine is a potent endogenous anti-inflammatory agent released by cells in metabolically unfavorable conditions, such as hypoxia or ischemia. Adenosine modulates different functional activities in macrophages. Some of these activities are believed to be induced through the uptake of adenosine into the macrophages, while others are due to the interaction with specific cell surface receptors. In murine bone marrow-derived macrophages, the use of different radioligands for adenosine receptors suggests the presence of A2B and A3 adenosine receptor subtypes. The presence of A2B receptors was confirmed by flow cytometry using specific Abs. The A2B receptor is functional in murine macrophages, as indicated by the fact that agonists of A2B receptors, but not agonists for A1, A2A, or A3, lead to an increase in cAMP levels. IFN-γ up-regulates the surface protein and gene expression of the A2B adenosine receptor by induction of de novo synthesis. The up-regulation of A2B receptors correlates with an increase in cAMP production in macrophages treated with adenosine receptor agonist. The stimulation of A2B receptors by adenosine or its analogues inhibits the IFN-γ-induced expression of MHC class II genes and also the IFN-γ-induced expression of nitric oxide synthase and of proinflammatory cytokines. Therefore, the up-regulation of the A2B adenosine receptor expression induced by IFN-γ could be a feedback mechanism for macrophage deactivation.

Ca2+ response of rat mesangial cells to ATP analogues

The aim of this investigation was to characterise the effects of ATP analogues and UTP on the single cell intracellular Ca2+ concentration ([Ca2+]i) in cultured rat mesangial cells. Typically, there were two phases in the Ca2+ response to the agonists, an initial fast transient peak and a subsequent slower decline, or plateau, phase. For the peak amplitude in [Ca2+]i the agonists had about equal effect. But when taking in consideration the percentage of responding cells and the integrated Ca2+ response over 1 min, the order of efficacy of nucleotide agonists (100 microM) was UTP = ATP > ATPgammaS > ADP = 2MeS-ATP (2-methylthio-ATP). Adenosine, AMP and beta,gamma-Me-ATP (100 microM) had no effect. Suramine (100 microM) and reactive blue (50 microM) decreased the number of responding cells. Removing Ca2+ from the bath diminished neither the peak in [Ca2+]i nor the percentage of responding cells, but the average [Ca2+]i increase in 1 min was significantly reduced. The results indicate that P2Y2 receptors are present in rat mesangial cells but it cannot be excluded that there are receptors distinct from P2Y2 which also mediate a rise in [Ca2+]i.

Benzoyl ATP is an antagonist of rat and human P2Y1 receptors and of platelet aggregation

The effects of 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) on intracellular Ca2+ mobilization and cyclic AMP accumulation were investigated using rat brain capillary endothelial cells which express an endogenous P2Y1 receptor, human platelets which are known to express a P2Y1 receptor, and Jurkat cells stably transfected with the human P2Y1 receptor. In endothelial cells, BzATP was a competitive inhibitor of 2-methylthio ADP (2-MeSADP) and ADP induced [Ca2+]i responses (Ki = 4.7 microM) and reversed the inhibition by ADP of adenylyl cyclase (Ki = 13 microM). In human platelets, BzATP inhibited ADP-induced aggregation (Ki = 5 microM), mobilization of intracellular Ca2+ stores (Ki = 6.3 microM), and inhibition of adenylyl cyclase. In P2Y1-Jurkat cells, BzATP inhibited ADP and 2-MeSADP-induced [Ca2+]i responses (Ki = 2.5 microM). It was concluded that BzATP is an antagonist of rat and human P2Y1 receptors and of platelet aggregation. In contrast to other P2Y1 receptor antagonists (A2P5P and A3P5P) which inhibit only ADP-induced Ca2+ mobilization, BzATP inhibits both the Ca2+- and the cAMP-dependent intracellular signaling pathways of ADP. These results provide further evidence that P2Y1 receptors contribute to platelet ADP responses.

ATP modulates anti-IgE-induced release of histamine from human lung mast cells

Adenosine 5'-triphosphate (ATP) is released from the cytoplasm under physiologic and pathophysiologic conditions and enters the extracellular space, where it acts on a group of recently cloned cell-surface receptors termed P2-purinoceptors (subtypes P2X and P2Y). We examined the effects of extracellular ATP, uridine triphosphate (UTP), the stable ATP analogues alpha,betamethylene-ATP (alpha,betamATP), beta,gammamethylene-ATP (beta,gammamATP), and 2-methylthio-ATP (2mSATP), and adenosine (10(-6)-10(-3) M) on histamine release from human lung mast cells (HLMC) induced by anti-IgE and the calcium ionophore A23187. None of the nucleotides or adenosine directly induced histamine release. Adenosine exhibited a bimodal effect, enhancing histamine release at 10(-6) to 10(-4) M (P > 0.05, NS) and inhibiting it at 10(-3) M (P < 0.05). ATP (10(-4) M) enhanced anti-IgE-induced histamine release (10.9 +/- 2.7% to 19. 2 +/- 2.9%, n = 20, P < 0.01), but not ionophore A23187-induced histamine release (n = 10). The adenine nucleotides consistently enhanced anti-IgE-induced histamine release; the rank order for this action was: ATP > 2mSATP > alpha,betamATP > beta,gammamATP, suggesting mediation by a P2Y-purinoceptor subtype. The selective P2X purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid failed to influence the effect of ATP, further supporting P2Y-purinoceptor mediation of anti-IgE-induced histamine release. UTP, an agonist at P2Y-purinoceptors, also significantly enhanced anti-IgE-induced histamine release. Application of the reverse transcription-polymerase chain reaction indicated that HLMC constitutively express the messenger RNAs encoding the P2Y1- and P2Y2-purinoceptor subtypes, and not that encoding the P2X7-purinoceptor (i.e., P2Z), a subtype implicated in ATP-induced histamine release in rodent peritoneal mast cells. The data produced in the study suggest that ATP plays an important modulatory role in histamine release from HLMC, and that it may therefore be mechanistically involved in human allergic/asthmatic reactions.

Confocal calcium imaging reveals an ionotropic P2 nucleotide receptor in the paranodal membrane of rat Schwann cells

1. The paranodal Schwann cell region is of major importance for the function of a myelinated axon. In the present study we searched for a possible ionotropic effect of extracellular ATP in this Schwann cell compartment. 2. Whole-cell patch-clamp recordings from cultured rat Schwann cells revealed that ATP and 2'-3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP) induced a non-specific cation current. The effect of ATP was much enhanced in a Ca2+- and Mg2+-free solution. ADP, UTP and alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP) had no effect. 3. Confocal Ca2+ imaging of myelinating Schwann cells in isolated rat spinal roots showed a BzATP-induced rise in the free intracellular Ca2+ concentration in the paranodal Schwann cell cytoplasm whereas alpha,beta-meATP and 2-(methylthio)-adenosine 5'-triphosphate were without effect. In contrast to the known metabotropic effect of UTP on these Schwann cell regions, the BzATP-induced Ca2+ signal was not transient, was unaffected by depletion of intracellular Ca2+ stores and dependent on the presence of extracellular Ca2+. 4. These results suggest that an ionotropic ATP receptor with electrophysiological and pharmacological characteristics of the P2X7 subtype of nucleotide receptors is functionally active in myelinating Schwann cells of peripheral nerves. Such a receptor might contribute to Schwann cell reactions in nerve injury or neuropathy.

Synaptic corelease of ATP and GABA in cultured spinal neurons

In the spinal dorsal horn (DH), transmission and modulation of peripheral nociceptive (pain-inducing) messages involve classical neurotransmitters and neuropeptides. We show that approximately half of DH neurons use ATP as a fast excitatory neurotransmitter acting at ionotropic P2X postsynaptic receptors. ATP was not codetected with glutamate but was coreleased with the inhibitory neurotransmitter GABA. Moreover, adenosine, probably generated by extracellular metabolism of ATP, finely tuned GABAergic inhibitory postsynaptic currents. Differential modulation of excitatory versus inhibitory components of this mixed cotransmission may help to explain changes in sensory message processing in the DH during mechanical hyperalgesia and neuropathic pain.

Expression and a role of functionally coupled P2Y receptors in human dendritic cells

We investigated the physiology and function of P2Y receptors expressed in human dendritic cells (DCs) differentiated in vitro from CD14+ cells (DC-14). These were obtained after a 10 day stimulation period in GM-CSF, IL-4 and monocyte conditioned medium. DC-14 were found to express high amounts of MHC class II, B7, CD40 as well as CD83. The functional analysis, using single cell Ca2+ imaging, demonstrated the expression of at least three subtypes of P2Y receptors. We further found using patch-clamp measurements that ATP evoked a pertussis toxin insensitive non-selective cation current with a peak current amplitude of -276+/-43 pA (holding potential -80 mV, n = 23). This current was not Ca(2+)-activated, since it was still observed under conditions of high intracellular Ca2+ buffering and could be blocked by Gd3+ (0.5 mM). In addition, intracellular application of GTP-gamma-S (0.3 mM) also activated the current. Interestingly, DC-14 redirected the orientation of their dendrites as well as cell shape towards a pipette containing ATP as observed with time lapse microscopy. These data suggest that in human DCs, ATP acts via P2Y receptors and induces chemokine effects.

ATP released from astrocytes mediates glial calcium waves

Calcium waves represent a widespread form of intercellular communication. Although they have been thought for a long time to require gap junctions, we recently demonstrated that mouse cortical astrocytes use an extracellular messenger for calcium wave propagation. The present experiments identify ATP as a major extracellular messenger in this system. Medium collected from astrocyte cultures during (but not before) calcium wave stimulation contains ATP. The excitatory effects of medium samples and of ATP are blocked by purinergic receptor antagonists and by pretreatment with apyrase; these same purinergic receptor antagonists block propagation of electrically evoked calcium waves. ATP, applied at the concentration measured in medium samples, evokes responses that are qualitatively and quantitatively similar to those evoked by those medium samples. These data implicate ATP as an important transmitter between CNS astrocytes.

ATP stimulates sympathetic transmitter release via presynaptic P2X purinoceptors

ATP is a fast transmitter in sympathetic ganglia and at the sympathoeffector junction. In primary cultures of dissociated rat superior cervical ganglion neurons, ATP elicits noradrenaline release in an entirely Ca2+-dependent manner. Nevertheless, ATP-evoked noradrenaline release was only partially reduced (by approximately 50%) when either Na+ or Ca2+ channels were blocked, which indicates that ATP receptors themselves mediated transmembrane Ca2+ entry. An "axonal" preparation was obtained by removing ganglia from explant cultures, which left a network of neurites behind; immunostaining for axonal and dendritic markers revealed that all of these neurites were axons. In this preparation, ATP raised intraaxonal Ca2+ and triggered noradrenaline release, and these actions were not altered when Ca2+ channels were blocked by Cd2+. Hence, Ca2+-permeable ATP-gated ion channels, i.e., P2X purinoceptors, are located at presynaptic sites and directly mediate Ca2+-dependent transmitter release. These presynaptic P2X receptors displayed a rank order of agonist potency of ATP >/= 2-methylthio-ATP > ATPgammaS >> alpha,beta-methylene-ATP approximately beta,gamma-methylene-L-ATP and were blocked by suramin or PPADS. ATP, 2-methylthio-ATP, and ATPgammaS also evoked inward currents measured at neuronal somata, but there these agonists were equipotent. Hence, presynaptic P2X receptors resemble the cloned P2X2 subtype, but they appear to differ from somatodendritic P2X receptors in terms of agonist sensitivity. Suramin reduced depolarization-evoked noradrenaline release by up to 20%, when autoinhibitory mechanisms were inactivated by pertussis toxin. These results indicate that presynaptic P2X purinoceptors mediate a positive, whereas G-protein-coupled P2Y purinoceptors mediate a negative, feedback modulation of sympathetic transmitter release.

Selectivity of diadenosine polyphosphates for rat P2X receptor subunits

The pharmacological activity of diadenosine polyphosphates was investigated at three recombinant P2X receptors (rat P2X1, rat P2X3, rat P2X4) expressed in Xenopus oocytes and studied under voltage-clamp conditions. For the rat P2X1 receptor, only P1,P6-diadenosine hexaphosphate (Ap6A) was a full agonist yet 2-3 folds less potent than ATP. At rat P2X3, P1,p4-diadenosine tetraphosphate (Ap4A), P1,P5-diadenosine pentaphosphate (Ap5A) and Ap6A were full agonists and more potent than ATP. Ap4A alone was equipotent with ATP at rat P2X4, but only as a partial agonist. Compared to known data for rat P2X2 and human P2X1 receptors, our findings contrast with rat P2X2 where only Ap4A is a full agonist although four folds less potent than ATP. At rat and human orthologues of P2X1, Ap5A was a partial agonist with similar potency. These data provide a useful basis for selective agonists of P2X receptor subunits.

Molecular cloning, functional characterization and possible cooperativity between the murine P2X4 and P2X4a receptors

We have cloned and functionally characterised the mouse orthologue of the P2X4 receptor, mP2X4, and a splice variant of this receptor, mP2X4a. mP2X4 is 388 amino acids in length and shares 94% and 87% identity with the rat and human P2X4 receptors, respectively, while mP2X4a is 361 amino acids in length and lacks a 27-amino acid region in the extracellular domain corresponding to exon 6 of the known P2X receptor gene structures. When expressed in Xenopus laevis oocytes, mP2X4 produces a rapid inward current in response to ATP with an EC50 of 1.68+/-0.2 microM, consistent with the affinity of the rat and human P2X4 receptors for ATP. This agonist response is potentiated by the P2X receptor antagonists suramin, Reactive blue 2 and, over a limited concentration range, by PPADS. Although mP2X4a forms a poorly functional homomeric receptor, it appears able to interact with the full-length mP2X4 subunit to result in a functional channel with a reduced affinity for ATP. These results suggest a possible role for splice variants of P2X receptors in the formation of functional heteromeric ion channels.

Modulation of ATP-responses at recombinant rP2X4 receptors by extracellular pH and zinc

The modulatory effects of extracellular H+ and Zn2+ were tested against ATP-responses at rat P2X4 (rP2X4) receptors expressed in Xenopus oocytes under voltage-clamp conditions. ATP (0.1-100 microM, at pH 7.5), evoked inward currents via rP2X4 receptors (EC50 value, 4.1+/-0.98 microM; nH, 1.2+/-0.1). ATP potency was reduced 2 fold, at pH 6.5, without altering maximal activity. ATP potency was reduced by a further 4 fold, at pH 5.5, and the maximal activity of ATP was also reduced. Alkaline conditions (pH 8.0) had no effect on ATP-responses. Zn2+ (100 nM - 10 microM) potentiated ATP-responses at the rP2X4 receptor by 2 fold, whereas higher concentrations (30 microM - 1 mM) inhibited ATP-responses. Zn2+ potentiation was due to an increase in ATP potency, whereas its inhibitory action was due to a reduction in ATP efficacy. Zn2+ modulation of ATP-responses was pH-dependent. At pH 6.5, the bell-shaped curve for Zn2+ was shifted to the right by 1 log unit. At pH 5.5, Zn2+ potentiation was abolished and its inhibitory effect reduced considerably. Suramin (50 microM) also potentiated ATP-responses at rP2X4 receptors. Neither H+ (pH 6.5 and 5.5), Zn2+ (10-100 microM) or a combination of both failed to reveal an inhibitory action of suramin at rP2X4 receptors. In conclusion, H+ and Zn2+ exerted opposite effects on the rP2X4 receptor by lowering and raising agonist potency, respectively. H+ (> or = 3 microM) and Zn2+ (> or = 30 microM) also reduces agonist efficacy by lowering the number of rP2X4 receptors available for activation. The striking differences between the modulatory actions of H+ and Zn2+ at rP2X4 and rP2X2 receptors are discussed.

Human primary fibroblasts in vitro express a purinergic P2X7 receptor coupled to ion fluxes, microvesicle formation and IL-6 release

We have investigated reponses to extracellular ATP in human fibroblasts obtained by skin biopsies. Our data show that these cells express a P2X7 purinergic receptor, as judged by (1) RT-PCR with specific primers, (2) reactivity with a specific anti-P2X7 antiserum, (3) activation by the selective P2X agonist benzoylbenzoylATP and (4) stimulation of transmembrane ion fluxes. Stimulation with benzoylbenzoylATP, and to a lesser extent with ATP, also caused striking morphological changes and increased formation of cytoplasmic microvesicles. These changes were fully reversible upon nucleotide removal. Two known blockers of P2X receptors, oxidised ATP and pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid, inhibited the morphological changes fully and the ion fluxes partially. The residual rise in intracellular Ca2+ levels and membrane depolarization observed in the presence of the inhibitors were dependent upon activation of a P2Y-type receptor exhibiting a peculiar pharmacological profile, in that CTP was the preferred agonist. ATP stimulation triggered release of the pro-inflammatory cytokine IL-6 in fibroblasts pre-treated with PMA and bacterial endotoxin. These observations reveal a novel pathway for fibroblast activation and for their recruitment in the inflammatory response.

Dual coupling of heterologously-expressed rat P2Y6 nucleotide receptors to N-type Ca2+ and M-type K+ currents in rat sympathetic neurones

1. The P2Y6 receptor is a uridine nucleotide-specific G protein-linked receptor previously reported to stimulate the phosphoinositide (PI) pathway. We have investigated its effect in neurones, by micro-injecting its cRNA into dissociated rat sympathetic neurones and recording responses of N-type Ca2+ (I(Ca(N))) and M-type K+ (I(K(M))) currents. 2. In P2Y6 cRNA-injected neurones, UDP or UTP produced a voltage-dependent inhibition of I(Ca(N)) by approximately 53% in whole-cell (disrupted-patch) mode and by 73% in perforated-patch mode; no inhibition occurred in control cells. Mean IC50 values (whole-cell) were: UDP, 5.9+/-0.3 nM; UTP, 20+/-1 nM. ATP and ADP (1 microM) had no significant effect. Pertussis toxin (PTX) substantially (approximately 60%) reduced UTP-mediated inhibition in disrupted patch mode but not in perforated-patch mode. 3. Uridine nucleotides also inhibited I(K(M)) in P2Y6 cRNA-injected cells (by up to 71% at 10 microM UTP; perforated-patch). Mean IC50 values were: UDP, 30+/-3 nM; UTP, 115+/-12 nM. ATP (10 microM) again had no effect. No significant inhibition occurred in control cells. Inhibition was PTX-resistant. 4. Thus, the P2Y6 receptor, like the P2Y2 subtype studied in this system, couples to both of these two neuronal ion channels through at least two different G proteins. However, the P2Y6 receptor displays a much higher sensitivity to its agonists than the P2Y2 receptor in this expression system and higher than previously reported using other expression methods. The very high sensitivity to both UDP and UTP suggests that it might be preferentially activated by any locally released uridine nucleotides.

Evidence that multiple P2X purinoceptors are functionally expressed in rat supraoptic neurones

1. The expression, distribution and function of P2X purinoceptors in the supraoptic nucleus (SON) were investigated by reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, and Ca2+-imaging and whole-cell patch-clamp techniques, respectively. 2. RT-PCR analysis of all seven known P2X receptor mRNAs in circular punches of the SON revealed that mRNAs for P2X2, P2X3, P2X4, P2X6 and P2X7 receptors were expressed in the SON, and mRNAs for P2X3, P2X4 and P2X7 were predominant. 3. In situ hybridization histochemistry for P2X3 and P2X4 receptor mRNAs showed that both mRNAs were expressed throughout the SON and in the paraventricular nucleus (PVN). 4. ATP caused an increase in [Ca2+]i in a dose-dependent manner with an ED50 of 1.7 x 10-5 M. The effects of ATP were mimicked by ATPgammaS and 2-methylthio ATP (2MeSATP), but not by AMP, adenosine, UTP or UDP. alphabeta-Methylene ATP (alphabetaMeATP) and ADP caused a small increase in [Ca2+]i in a subset of SON neurones. 5. The P2X7 agonist 2'- & 3'-O-(4-benzoylbenzoyl)-ATP (BzATP) at 10-4 M increased [Ca2+]i, but the potency of BzATP was lower than that of ATP. In contrast, BzATP caused a more prominent [Ca2+]i increase than ATP in non-neuronal cells in the SON. 6. The effects of ATP were abolished by extracellular Ca2+ removal or by the P2 antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), and inhibited by extracellular Na+ replacement or another P2 antagonist, suramin, but were unaffected by the P2X7 antagonist oxidized ATP, and the inhibitor of Ca2+-ATPase in intracellular Ca2+ stores cyclopiazonic acid. 7. Two patterns of desensitization were observed in the [Ca2+]i response to repeated applications of ATP: some neurones showed little or moderate desensitization, while others showed strong desensitization. 8. Whole-cell patch-clamp analysis showed that ATP induced cationic currents with marked inward rectification. The ATP-induced currents exhibited two patterns of desensitization similar to those observed in the [Ca2+]i response. 9. The results suggest that multiple P2X receptors, including P2X3, are functionally expressed in SON neurones, and that activation of these receptors induces cationic currents and Ca2+ entry. Such ionic and Ca2+-signalling mechanisms triggered by ATP may play an important role in the regulation of SON neurosecretory cells.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

ATP receptors for the protection of hippocampal functions

The inhibitory effects of extracellular adenosine 5'-triphosphate (ATP) are reviewed in the present paper. ATP inhibits the release of the excitatory transmitter glutamate and stimulates the release of the inhibitory transmitter GABA from hippocampal neurons. Also, ATP activates potassium conductance directly through G protein, resulting in hyperpolarization of membrane potential. ATP activates microglia to secrete plasminogen that promotes the development of mesencephalic dopaminergic neurons and enhances neurite outgrowth from explants of neocortical tissue. Moreover, ATP may protect hippocampal neurons from excitotoxic cell death by preserving mitochondrial function. Thus, ATP may have a role in the protection of the function of hippocampus from over-stimulation by glutamate.

Relations between intracellular Ca2+ stores and store-operated Ca2+ entry in primary cultured human glioblastoma cells

1. In primary cultured human glioblastoma cells extracellular application of ATP triggered elevation in cytoplasmic calcium concentration ([Ca2+]i) mediated entirely by generation of inositol 1,4,5-trisphosphate (InsP3)-dependent Ca2+ release from endoplasmic reticulum Ca2+ stores followed by the activation of store-operated Ca2+ entry into the cells. 2. Sensitivity of P2Y purinoceptors to extracellular ATP was regulated by extracellular Ca2+: in Ca2+-free extracellular solution the threshold concentration of ATP that induced an increase in [Ca2+]i was reduced by one order of magnitude. 3. Activation of Ca2+ release and store-operated Ca2+ entry was dissociated: low concentrations of ATP induced substantial Ca2+ release without activation of Ca2+ entry; activation of the latter required higher ATP concentrations. 4. Mitochondria participated in buffering Ca2+ loads that resulted from store-operated Ca2+ influx; in contrast Ca2+ released from intracellular stores was not accumulated by the mitochondrial depot. 5. We conclude that ATP-induced Ca2+ responses are governed by several pathways with different sensitivities to the agonist. This enables cells to respond either with pure Ca2+ release from intracellular stores (at low ATP concentrations) or (at high ATP concentrations) the response is amplified by plasmalemmal Ca2+ influx. Store-operated Ca2+ entry increases mitochondrial Ca2+ content providing a link between cellular activation and mitochondrial function.

The P2Y1 receptor, necessary but not sufficient to support full ADP-induced platelet aggregation, is not the target of the drug clopidogrel

Recently we showed that the P2Y1 receptor coupled to calcium mobilization is necessary to initiate ADP-induced human platelet aggregation. Since the thienopyridine compound clopidogrel specifically inhibits ADP-induced platelet aggregation, it was of interest to determine whether the P2Y1 receptor was the target of this drug. Therefore we studied the effects of clopidogrel and of the two specific P2Y1 antagonists A2P5P and A3P5P on ADP-induced platelet events in rats. Although clopidogrel treatment (50 mg/kg) greatly reduced platelet aggregation in response to ADP as compared to untreated platelets, some residual aggregation was still detectable. In contrast, A2P5P and A3P5P totally abolished ADP-induced shape change and aggregation in platelets from both control and clopidogrel-treated rats. A2P5P and A3P5P (100 microM) totally inhibited the [Ca2+]i rise induced by ADP (0.1 microM) in control and clopidogrel-treated platelets, whereas clopidogrel treatment had no effect. Conversely, the inhibition of adenylyl cyclase induced by ADP (5 microM) was completely blocked by clopidogrel but not modified by A2P5P or A3P5P (100 microM). A3P5P (1 mM) reduced the number of [33P]2MeSADP binding sites on control rat platelets from 907 +/- 50 to 611 +/- 25 per platelet. After clopidogrel treatment, binding of [33P]2MeSADP decreased to 505 +/- 68 sites per platelet and further decreased to 55 +/- 12 sites in the presence of A3P5P (1 mM). In summary, these results demonstrate that the platelet P2Y1 receptor responsible for the initiation of aggregation in response to ADP is not the target of clopidogrel. Platelets may express another, as yet unidentified, P2Y receptor, specifically coupled to the inhibition of adenylyl cyclase and necessary to induce full platelet aggregation, which could be the target of this drug.

Expression and trans-synaptic regulation of P2x4 and P2z receptors for extracellular ATP in parotid acinar cells. Effects of parasympathetic denervation

Trans-synaptic regulation of muscarinic, peptidergic, and purinergic responses after denervation has been reported previously in rat parotid acinar cells (McMillian, M. K., Soltoff, S. P., Cantley, L. C., Rudel, R., and Talamo, B. R. (1993) Br. J. Pharmacol. 108, 453-461). Characteristics of the ATP-mediated responses and the effects of parasympathetic denervation were further analyzed through assay of Ca2+ influx, using fluorescence ratio imaging methods, and by analysis of P2x receptor expression. ATP activates both a high affinity and a low affinity response with properties corresponding to the recently described P2x4 and the P2z (P2x7)-type purinoceptors, respectively. Reverse transcription-polymerase chain reaction analysis reveals mRNA for P2x4 as well as P2x7 subtypes but not P2x1, P2x2, P2x3, P2x5, or P2x6. P2x4 protein also is detected by Western blotting. Distribution of the two types of ATP receptor responses on individual cells was stochastic, with both high and low affinity responses on some cells, and only a single type of response on others. Sensitivity to P2x4-type activation also varied even among cells responsive to low concentrations of ATP. Parasympathetic denervation greatly enhanced responses, tripling the proportion of acinar cells with a P2x4-type response and increasing the fraction of highly sensitive cells by 7-fold. Moreover, P2x4 mRNA is significantly increased following parasympathetic denervation. These data indicate that sensitivity to ATP is modulated by neurotransmission at parasympathetic synapses, at least in part through increased expression of P2x4 mRNA, and suggest that similar regulation may occur at other sites in the nervous system where P2x4 receptors are widely expressed.

Central P2X4 and P2X6 channel subunits coassemble into a novel heteromeric ATP receptor

Ionotropic ATP receptors are widely expressed in mammalian CNS. Despite extensive functional characterization of neuronal homomeric P2X receptors in heterologous expression systems, the subunit composition of native central P2X ATP-gated channels remains to be elucidated. P2X4 and P2X6 are major central subunits with highly overlapping mRNA distribution at both regional and cellular levels. When expressed alone in Xenopus oocytes, P2X6 subunits do not assemble into surface receptors responsive to ATP applications. On the other hand, P2X4 subunits assemble into bona fide ATP-gated channels, slowly desensitizing and weakly sensitive to the partial agonist alpha,beta-methylene ATP and to noncompetitive antagonists suramin and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulfonic acid. We demonstrate here that the coexpression of P2X4 and P2X6 subunits in Xenopus oocytes leads to the generation of a novel pharmacological phenotype of ionotropic ATP receptors. Heteromeric P2X4+6 receptors are activated by low-micromolar alpha, beta-methylene ATP (EC50 = 12 microM) and are blocked by suramin and by Reactive Blue 2, which has the property, at low concentrations, to potentiate homomeric P2X4 receptors. The assembly of P2X4 with P2X6 subunits results from subunit-dependent interactions, as shown by their specific copurification from HEK-293 cells transiently transfected with various epitope-tagged P2X channel subunits. Our data strongly suggest that the numerous cases of neuronal colocalizations of P2X4 and P2X6 subunits observed in mammalian CNS reflect the native expression of heteromeric P2X4+6 channels with unique functional properties.

Time course of the initial [Ca2+]i response to extracellular ATP in smooth muscle depends on [Ca2+]e and ATP concentration

In response to extracellular application of 50 microM ATP, all individual porcine aortic smooth muscle cells respond with rapid rises from basal [Ca2+]i to peak [Ca2+]i within 5 s. The time from stimulus to the peak of the [Ca2+]i response increases with decreasing concentration of ATP. At ATP concentrations of 0.5 microM and below, the time to the [Ca2+]i peak varies more significantly from cell to cell than at higher concentrations, and each cell shows complicated initiation and decay kinetics. For any individual cell, the lag phase before a response decreases with increasing concentration of ATP. An increase in lag time with decreasing ATP concentration is also observed in the absence of extracellular Ca2+, but the lag phase is more pronounced, especially at concentrations of ATP below 0.5 microM. Whole-cell patch-clamp electrophysiology shows that in porcine aortic smooth muscle cells, ATP stimulates an inward current carried mainly by Cl- ion efflux with a time course similar to the [Ca2+]i changes and no detectable current from an ATP-gated cation channel. A simple signal cascade initiation kinetics model, starting with nucleotide receptor activation leading to IP3-mediated Ca2+ release from IP3-sensitive internal stores, fits the data and suggests that the kinetics of the Ca2+ response are dominated by upstream signal cascade components.

Synthesis and Structure-Activity Relationships of Pyridoxal-6-arylazo-5'-phosphate and Phosphonate Derivatives as P2 Receptor Antagonists

Novel analogs of the P2 receptor antagonist pyridoxal-5'-phosphate-6-phenylazo-2',4'-disulfonate (PPADS) were synthesized. Modifications were made through functional group substitution on the sulfophenyl ring and at the phosphate moiety through the inclusion of phosphonates, demonstrating that a phosphate linkage is not required for P2 receptor antagonism. Substituted 6-phenylazo and 6-naphthylazo derivatives were also evaluated. Among the 6-phenylazo derivatives, 5'-methyl, ethyl, propyl, vinyl, and allyl phosphonates were included. The compounds were tested as antagonists at turkey erythrocyte and guinea-pig taenia coli P2Y(1) receptors, in guinea-pig vas deferens and bladder P2X(1) receptors, and in ion flux experiments by using recombinant rat P2X(2) receptors expressed in Xenopus oocytes. Competitive binding assay at human P2X(1) receptors in differentiated HL-60 cell membranes was carried out by using [(35)S]ATP-γ-S. A 2'-chloro-5'-sulfo analog of PPADS (C(14)H(12)O(9)N(3)ClPSNa), a vinyl phosphonate derivative (C(15)H(12)O(11)N(3)PS(2)Na(3)), and a naphthylazo derivative (C(18)H(14)O(12)N(3)PS(2)Na(2)), were particularly potent in binding to human P2X(1) receptors. The potencies of phosphate derivatives at P2Y(1) receptors were generally similar to PPADS itself, except for the p-carboxyphenylazo phosphate derivative C(15)H(13)O(8)N(3)PNa and its m-chloro analog C(15)H(12)O(8)N(3)ClPNa, which were selective for P2X vs. P2Y(1) receptors. C(15)H(12)O(8)N(3)ClPNa was very potent at rat P2X(2) receptors with an IC(50) value of 0.82 μM. Among the phosphonate derivatives, [4-formyl-3-hydroxy-2-methyl-6-(2-chloro-5-sulfonylphenylazo)-pyrid-5-yl]methylphosphonic acid (C(14)H(12)-O(8)N(3)ClPSNa) showed high potency at P2Y(1) receptors with an IC(50) of 7.23 μM. The corresponding 2,5-disulfonylphenyl derivative was nearly inactive at turkey erythrocyte P2Y(1) receptors, whereas at recombinant P2X(2) receptors had an IC(50) value of 1.1 μM. An ethyl phosphonate derivative (C(15)H(15)O(11)N(3)PS(2)Na(3)), whereas inactive at turkey erythrocyte P2Y(1) receptors, was particularly potent at recombinant P2X(2) receptors.

Analysis of the influence of nucleotidases on the apparent activity of exogenous ATP and ADP at P2Y1 receptors

1. ADP is a potent agonist of rat and human P2Y1 purinoceptors. ATP is a weak competitive antagonist. This study analyses the situation in which P2Y1 receptors are exposed to ATP in the presence of exogenous ecto-nucleotidases (apyrases) that have high or low ATPase/ADPase activity ratio. 2. Rat brain capillary endothelial cells of the B10 clone express P2Y1 receptors that couple to intracellular Ca2+ mobilization. They have low endogenous ecto-ATPase and ecto-ADPase activities. 3. ATP did not raise intracellular Ca2+ in B10 cells. Addition of apyrases III or VII (1 u ml(-1)) to ATP treated cells induced large intracellular Ca2+ transients. Apyrases had no action in the absence of ATP. 4. A 1 u ml(-1) apyrase III solution generated 20 microM ADP from 0.1 mM ATP within 15 s. This concentration of ADP was sufficient to produce maximal activation of P2Y1 receptors. 5. ATP was a full agonist of P2Y1 receptors in the presence of 1 u ml(-1) apyrase III. Dose response curves for the apparent actions of ATP were bell shaped in the presence of 0.1 u ml(-1) apyrase III. Apyrase III did not alter ADP dose response curves when coincubated with ADP for 15 s. 6. Apyrase VII (1 u ml(-1)) shifted dose response curves for the actions of ADP to larger concentrations. It induced a bell shaped ATP dose response curve. 7. Results suggest that ATPDases prevent P2Y1 receptor activation by degrading ADP but may contribute to P2Y1 receptor activation by generating ADP from ATP.

Effects of the P2-purinoceptor antagonists suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid on glutamatergic synaptic transmission in rat dorsal horn neurons of the spinal cord

The effects of suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) on glutamatergic synaptic transmission were studied on dorsal horn lamina II neurons of rat spinal cord slice preparation and cultured dorsal horn neurons. Suramin at 100 microM significantly suppressed the amplitude of the evoked excitatory postsynaptic currents (EPSCs) by 33%, miniature EPSC (mEPSC) amplitude was decreased by 46% and the mEPSC frequency also decreased by 41%. PPADS at 50 microM had little effect on either the evoked EPSCs or mEPSCs. The lack of effect of PPADS on glutamatergic synaptic transmission suggests that the effect of suramin is less likely to be mediated by P2x receptors. When whole-cell (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) currents evoked by glutamate were examined, both suramin and PPADS showed no inhibition of peak amplitude. However, the onset of glutamate-evoked whole-cell currents became significantly slowed by suramin but not by PPADS. The suppression of synaptic transmission by suramin may be due, in part, to the slowed onset of glutamate-evoked AMPA currents. These results suggest that the analgesic effects of suramin shown in cancer patients and animal pain models may not be solely due to its antagonism to purinoceptors. PPADS is probably a more suitable antagonist for the study of synaptic P2x receptor function at excitatory synapses mediated by AMPA receptors.

Regulation of ocular mucin secretion by P2Y2 nucleotide receptors in rabbit and human conjunctiva

The effects of adenine analogues on secretion of high molecular weight, mucin-like glycoproteins (mucins) by conjunctival goblet cells were investigated using isolated rabbit and human conjunctiva. Mucin secretion was assayed using a quantitative dot-blot assay of Helix pomatia agglutinin-horseradish peroxidase binding to mucins absorbed to nitrocellulose filters. In rabbit conjunctiva, exogenous ATP (10(-7)-10(-3) m) induced a concentration-dependent, four-fold increase in mucin secretion that reached a plateau 15 min after drug addition. The rank order of potency of agonists was UTP>=ATPgammaS>ATP>ITP>ADP>>AMP. Adenosine, alpha,beta-methylene- ATP and beta,gamma-methylene-ATP were ineffective in stimulating mucin release. The response to ATP was unmodified by the putative P2 purinergic antagonists suramin or reactive blue (5x10(-5) m). In human conjunctiva, ATP and UTP were nearly equipotent in stimulating mucin secretion with an EC50 congruent with5x10(-6) m. These findings demonstrate that rabbit and human conjunctival cells contain functional P2Y2 (formerly designated as P2U) nucleotide receptors that govern mucin secretion. These receptors may provide useful pharmacological targets for therapeutic modulation of tear film mucins in dry-eye disorders and/or corneal wound healing.

P2X receptor-mediated excitation of nociceptive afferents in the normal and arthritic rat knee joint

1. We tested the hypothesis that functional P2X receptors are present on peripheral terminals of primary afferent articular nociceptors in the rat knee joint. Neural activity was recorded extracellularly from the medial articular nerve innervating the knee joint in rats anaesthetized with pentobarbitone. 2. The selective P2X receptor agonist, alphabeta methylene ATP (alphabetameATP), and the endogenous ligand, ATP, caused a rapid short-lasting excitation of a sub-population of C and Adelta nociceptive afferent nerves innervating normal knee joints when injected intra-arterially or intra-articularly, and this effect was antagonized by the non-selective P2 receptor antagonist PPADS. 3. Induction of a chronic (14-21 days) unilateral inflammatory arthritis of the knee joint using locally injected Freund's adjuvant neither increased or decreased responsiveness of joint nociceptors to alphabetameATP or ATP. 4. Our results support the hypothesis that alphabetameATP-sensitive P2X receptors are expressed on peripheral nociceptive afferents in the rat knee joint suggesting that they may be involved in the initiation of nociception and pain.

Calcium signalling through nucleotide receptor P2Y2 in cultured human vascular endothelium

Microfluorometric measurements in Fura-2-loaded single cultured human vascular endothelial cells were used to characterize the intracellular calcium [Ca2+]i responses triggered by extracellular application of adenosine 5'-triphosphate (ATP) and other nucleotides. Application of ATP or uridine 5'-triphosphate (UTP) gave rise to dose-dependent elevations of [Ca2+]i in all the cells tested. At saturating concentrations of agonist, the [Ca2+]i response was biphasic, with an early peak and a sustained plateau. Unlike peak responses, the sustained Ca2+ plateau was sensitive to removal of Ca2+ from the external medium. Mn2+ quenching revealed the presence of Ca2+ influx during the agonist-induced calcium plateau. The agonist-evoked calcium plateau was inhibited in a dose-dependent manner by the Cl-channel blocker NPPB, by the divalent cation Ni2+ and by the imidazole antimycotic econazole. Previously, these compounds have been shown to block store-operated Ca2+ entry. The two phases of the agonist-evoked [Ca2+]i response were blocked by the specific phospholipase C inhibitor U-73122 and by intracellular injection of low molecular weight heparin, suggesting the involvement of IP3-sensitive intracellular Ca2+ stores. The pharmacological profile of the response, using different nucleotides and analogues, ATP = UTP > ADP = UDP, and no responses to P2X1 and P2Y1 agonists, suggested the involvement of P2Y2 receptors. The expression of mRNA for the P2Y2 receptor was detected by RT-PCR analysis. These results indicate that P2Y2 receptors linked to intracellular Ca2+ mobilization are present in human vascular endothelial cells. The initial [Ca2+]i mobilization is followed by a phase of elevated [Ca2+]i influx.

ATP receptor-mediated enhancement of fast excitatory neurotransmitter release in the brain

ATP-gated cation channels (P2X receptors) exist on the soma of proprioceptive neurons in the trigeminal mesencephalic nucleus (MNV) in the brain stem. However, these pseudomonopolar neurons seem to receive no synaptic input to their soma; we therefore hypothesized that in MNV neurons, the P2X receptors of importance may be those located on their central terminal projections. Here, we show in trigeminal mesencephalic motor nucleus neurons, which receive their major input from the MNV, that both exogenous ATP (1 mM) and high frequency focal stimulation to evoke endogenous ATP release enhanced the frequency of spontaneous fast excitatory postsynaptic currents (EPSCs) with no change in their amplitude. The enhancement was reduced by the antagonists suramin (300 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (30 microM) and persisted when action potential conduction was blocked with tetrodotoxin (1 microM). Thus, functional P2X receptors are expressed on nerve terminals in the brain stem, where they increase the spontaneous release of glutamate onto trigeminal mesencephalic motor nucleus neurons.

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.

Ion channels in microglia (brain macrophages)

Microglia are immunocompetent cells in the brain that have many similarities with macrophages of peripheral tissues. In normal adult brain, microglial cells are in a resting state, but they become activated during inflammation of the central nervous system, after neuronal injury, and in several neurological diseases. Patch-clamp studies of microglial cells in cell culture and in tissue slices demonstrate that microglia express a wide variety of ion channels. Six different types of K+ channels have been identified in microglia, namely, inward rectifier, delayed rectifier, HERG-like, G protein-activated, as well as voltage-dependent and voltage-independent Ca2+-activated K+ channels. Moreover, microglia express H+ channels, Na+ channels, voltage-gated Ca2+ channels, Ca2+-release activated Ca2+ channels, and voltage-dependent and voltage-independent Cl- channels. With respect to their kinetic and pharmacological properties, most microglial ion channels closely resemble ion channels characterized in other macrophage preparations. Expression patterns of ion channels in microglia depend on the functional state of the cells. Microglial ion channels can be modulated by exposure to lipopolysaccharide or various cytokines, by activation of protein kinase C or G proteins, by factors released from astrocytes, by changes in the concentration of internal free Ca2+, and by variations of the internal or external pH. There is evidence suggesting that ion channels in microglia are involved in maintaining the membrane potential and are also involved in proliferation, ramification, and the respiratory burst. Further possible functional roles of microglial ion channels are discussed.

Cooling sensitive [Ca2+]i response associated with signaling of G protein-coupled receptors

The influence of cooling on the intracellular concentration of Ca2+ ([Ca2+]i) was tested in cell lines expressing chemical receptors. First, when ATP was externally added to rat basophilic leukemia (RBL-2H3) cells, cooling from 37 degrees C to 27 degrees C induced a transient rapid increase in [Ca2+]i. In the absence of extracellular Ca2+, the [Ca2+]i response was induced whereas an inhibitor of microsomal Ca2+ ATPase, thapsigargin, largely abolished the [Ca2+]i response, suggesting that the internal Ca2+ store liberate the Ca2+. A purinergic receptor antagonist, suramin, completely inhibited the [Ca2+]i response to the cooling. Secondly, when serotonin (5-HT) was added to rat glioma C6BU-1 cells, the cooling induced a transient increase in [Ca2+]. This [Ca2+]i response was induced in the absence of external Ca2+, suggesting that the internal Ca2+ stores liberate the Ca2+. These results raise the possibility that some G protein-coupled receptors are sensitive to cooling in the presence of agonist for the receptor.

Phenotype changes of the vascular smooth muscle cell regulate P2 receptor expression as measured by quantitative RT-PCR

Studies using selective agonists have suggested that the contractile effect of extracellular nucleotides, such as ATP and UTP, in blood vessels is mediated mainly by P2X1 receptors with a smaller contribution of P2Y receptors while the mitogenic effect is mediated by P2Y (P2Y1, P2Y2, P2Y4, and P2Y6) receptors with no effect of P2X1 receptors. This indicates a difference in P2 receptor expression between the contractile and the synthetic phenotype of the SMC. To measure the expression of mRNA for these receptors a competitive RT-PCR assay was developed that utilised synthetic RNA-competitors allowing determination of the number of mRNA copies for each receptor in the samples. In the synthetic phenotype the mitogenic P2Y1 and P2Y2 receptor transcripts were upregulated by 342- and 8-fold, respectively, while the contractile P2X1 receptor is totally downregulated and the P2Y4 and P2Y6 receptors were unchanged. This plasticity of the receptor expression may be important in the transition from the contractile to the synthetic SMC phenotype.

P2Y2 nucleotide receptors expressed heterologously in sympathetic neurons inhibit both N-type Ca2+ and M-type K+ currents

The P2Y2 receptor is a uridine/adenosine triphosphate (UTP/ATP)-sensitive G-protein-linked nucleotide receptor that previously has been reported to stimulate the phosphoinositide signaling pathway. Messenger RNA for this receptor has been detected in brain tissue. We have investigated the coupling of the molecularly defined rat P2Y2 receptor to neuronal N-type Ca2+ channels and to M-type K+ channels by heterologous expression in rat superior cervical sympathetic (SCG) neurons. After the injection of P2Y2 cRNA, UTP inhibited the currents carried by both types of ion channel. As previously reported [Filippov AK, Webb TE, Barnard EA, Brown DA (1997) Inhibition by heterologously expressed P2Y2 nuerones. Br J Pharmacol 121:849-851], UTP inhibited the Ca2+ current (ICa(N)) by up to 64%, with an IC50 of approximately 0.5 microM. We now find that UTP also inhibited the K+M current (IK(M)) by up to 61%, with an IC50 of approximately 1.5 microM. UTP had no effect on either current in neurons not injected with P2Y2 cRNA. Structure-activity relations for the inhibition of ICa(N) and IK(M) in P2Y2 cRNA-injected neurons were similar, with UTP >/= ATP > ITP >> GTP,UDP. However, coupling to these two channels involved different G-proteins: pretreatment with Pertussis toxin (PTX) did not affect UTP-induced inhibition of IK(M) but reduced inhibition of ICa(N) by approximately 60% and abolished the voltage-dependent component of this inhibition. In unclamped neurons, UTP greatly facilitated depolarization-induced action potential discharges. Thus, the single P2Y2 receptor can couple to at least two G-proteins to inhibit both Ca2+N and K+M channels with near-equal facility. This implies that the P2Y2 receptor may induce a broad range of effector responses in the nervous system.

P2X7 receptor activation-induced contraction and lysis in human saphenous vein smooth muscle

In cutaneous veins where purinergic neurotransmission is more prominent compared with in deep vessels, physiological and pathological roles of nerve-released ATP have been described. Neuronally released ATP has been reported to act through activation of unidentified ionotropic P2X receptor(s). This study analyzed P2X receptor subtypes expressed in human saphenous vein smooth muscle and their physiological functions. Transcripts for both hP2X1 receptors, already identified in other smooth muscles, and, surprisingly, hP2X7 receptors known to be responsible for the cytotoxic effect of ATP in macrophages were detected by Northern blot analysis in total RNA from saphenous vein smooth muscle. ATP and other P2X receptor agonists [alphabeta-methylene-ATP, 2-methylthio-ATP, and 2',3'-(4-benzoyl)benzoyl-ATP] dose-dependently contracted venous rings, but the contraction induced by 2-methylthio-ATP was more transient than that evoked by the other P2X agonists. The effect of hP2X1 agonists involved the activation of a rapidly desensitizing cation current recorded in freshly isolated myocytes. The action of hP2X7 receptor agonists was related to a maintained nondesensitizing cation current. In addition, hP2X7 receptor activation formed membrane pores that were permeable to large molecules. hP2X1 and hP2X7 receptors coexpressed in COS cells did not associate to form heteromultimers. Our data indicate that both hP2X1 and hP2X7 receptors are expressed as 2 separated populations of channels in human saphenous vein myocytes and are involved in ATP-induced tension. We suggest that cell lysis consequent to hP2X7 receptor-induced pore formation contributes to the disorganization and decrease in the amount of contractile myocytes in the media of varicose veins.

Pharmacological and histochemical evidence for P2X receptors in human umbilical vessels

The presence of P2X purinoceptors in human umbilical vessels were studied with organ bath recording, radioligand binding assays, autoradiography, and immunohistochemistry. In isolated umbilical arteries and veins from normal term pregnancy, both ATP and alpha,beta-methylene ATP caused concentration-dependent contractions. ATP-induced responses were blocked by desensitisation with alpha,betamethylene ATP. However, both the ATP- and alpha,beta-methylene ATP-induced responses were not antagonised by suramin. No significant difference in responses was observed in the vessels with or without endothelial cells. Radioligand binding assays using [3H]alpha,beta-methylene ATP showed the presence of a population of high-affinity binding sites in both the arteries and veins. The Kd values of the binding sites were 2.77 + 1.10 nM for the arteries, and 3.23+/-1.22 nM for the veins. The maximum binding site densities were 634+/-237 and 947+/-308 fmol/mg protein for the arteries and the veins, respectively. Autoradiographic localisation with [3H]alpha,beta-methylene ATP demonstrated that the specific binding sites were only distributed over the smooth muscle cells of the vessels. Immunohistochemical studies with specific polyclonal antibodies against P2X1-6 receptors showed that positive immunostaining was also restricted to smooth muscle cells. Antibodies against P2X1 receptors produced the strongest signals, while antibodies against the other five P2X subtypes produced much weaker signals. The results in the present study indicate the existence of P2X purinoceptors in the smooth muscle of human umbilical vessels. Their physiological functions remain to be studied.

Selective inhibition of M-type potassium channels in rat sympathetic neurons by uridine nucleotide preferring receptors

1. UTP and UDP depolarize rat superior cervical ganglion neurons and trigger noradrenaline release from these cells. The present study investigated the mechanisms underlying this excitatory action of uridine nucleotides by measuring whole-cell voltage-dependent K+ and Ca2+ currents. 2. Steady-state outward (holding) currents measured in the amphotericin B perforated-patch configuration at a potential of -30 mV were reduced by 10 microM UTP in a reversible manner, but steady-state inward (holding) currents at -70 mV were not affected. This action of UTP was shared by the muscarinic agonist oxotremorine-M. In current-voltage curves between -20 and -100 mV, UTP diminished primarily the outwardly rectifying current components arising at potentials positive to -60 mV. 3. Slow relaxations of muscarinic K+ currents (IM) evoked by hyperpolarizations from -30 to -55 mV were also reduced by 10 microM UTP (37% inhibition) and oxotremorine-M (81% inhibition). In contrast, transient K+-currents, delayed rectifier currents, fast and slow Ca2+-dependent K+ currents, as well as voltage-dependent Ca2+ currents were not altered by UTP. 4. In conventional (open-tip) whole-cell recordings, replacement of GTP in the pipette by GDPbetaS abolished the UTP-induced inhibition of IM, whereas replacement by GTPgammaS rendered it irreversible. 5. The UTP-induced reduction of IM was half maximal at 1.5 microM with a maximum of 37% inhibition; UDP was equipotent and equieffective, while ADP was less potent (half maximal inhibition at 29 microM). ATP had no effect at < or = 30 microM. 6. The inhibition of IM induced by 10 microM UTP was antagonized by pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) at > or = 30 microM and by reactive blue 2 at > or = 10 microM, but not by suramin at concentrations up to 30 microM. 7. These results show that rat superior cervical ganglion neurons possess uridine nucleotide preferring P2Y receptors which inhibit KM channels. This effect presumably forms the basis of the excitatory action of uridine nucleotides in rat sympathetic neurons.

Calcium permeability and block at homomeric and heteromeric P2X2 and P2X3 receptors, and P2X receptors in rat nodose neurones

1. Whole-cell recordings were made from HEK 293 (human embryonic kidney) cells stably transfected with cDNAs encoding P2X2, P2X3 or both receptors (P2X2/3) and from cultured rat nodose neurones. Nodose neurones all showed immunoreactivity for both P2X2 and P2X3, but not P2X1, receptors. 2. Reversal potentials were measured in extracellular sodium, N-methyl-D-glucamine (NMDG) and NMDG containing 5 mM Ca2+; the values were used to compute relative permeabilities (PNMDG/PNa and PCa/PNa). PNMDG/PNa was not different for P2X2, P2X2/3 and nodose neurones (0.03) but was significantly higher (0.07) for P2X3 receptors. PCa/PNa was not different among P2X3, P2X2/3 and nodose neurones (1.2-1.5) but was significantly higher (2.5) for P2X2 receptors. 3. External Ca2+ inhibited purinoceptor currents with half-maximal concentrations of 5 mM at the P2X2 receptor, 89 mM at the P2X3 receptor and 15 mM at both the P2X2/3 heteromeric receptor and nodose neurones. In each case, the inhibition was voltage independent and was overcome by increasing concentrations of agonist. 4. These results may indicate that Ca2+ permeability of the heteromeric (P2X2/3) channel is dominated by that of the P2X3 subunit, while Ca2+ block of the receptor involves both P2X2 and P2X3 subunits. The correspondence in properties between P2X2/3 receptors and nodose ganglion neurones further supports the conclusion that the native alpha,beta-methylene ATP-sensitive receptor is a P2X2/3 heteromultimer.

G protein-coupled receptors in gastrointestinal physiology. IV. Neural regulation of gastrointestinal smooth muscle

G protein-coupled receptors receive many of the neural, hormonal, and paracrine inputs to gastrointestinal (GI) smooth muscle cells. This article examines the major G protein-coupled receptors, G proteins, and effectors that mediate responses to enteric neuromuscular transmitters. Excitatory transmitters primarily couple through Gq/11 and Gi/Go proteins and elicit responses via formation of inositol trisphosphate and diacylglycerol and inhibition of adenylyl cyclase. Several inhibitory transmitters couple through Gs and activation of adenylyl cyclase. There are interesting examples, however, of inhibitory transmitters apparently using pathways regulated by Gq/11 to elicit responses by localized Ca2+ release and activation of Ca2+-dependent ion channels. G protein-coupled receptors may also be differentially expressed by smooth muscle cells and interstitial cells of Cajal, which may increase the diversity of responses and allow specialized innervation of GI muscle tissues.

Ligand specificity and ticlopidine effects distinguish three human platelet ADP receptors

Human platelets express adenosine 5'-diphosphate (ADP)-specific purinoceptors of the P2X and P2Y receptor superfamily, but their structure, diversity, and precise pharmacological profile is not well understood. Here, functional assays with intact platelets and well-characterized nucleotide derivatives were performed in order to characterize the ligand specificity of these platelet-specific purinoceptors. For the signalling pathways investigated (aggregation, rapid Ca2+-influx, desensitization of Ca2+-influx, Ca2+-mobilization, inhibition of adenylyl cyclase), significant differences in ligand specificity were demonstrated. ADP activated all purinoceptors of human platelets, while adenosine 5'-triphosphate (ATP) was a weak agonist for the P2X receptor and an antagonist for the P2Y receptors. The ADP-receptor pathway-antagonist ticlopidine inhibited ADP-evoked aggregation and adenylyl cyclase inhibition but did not affect platelet purinoceptors associated with Ca2+-influx and Ca2+-mobilization. These results indicate the presence of three distinct ADP-selective purinoceptors on human platelets.

A pyridoxine cyclic phosphate and its 6-azoaryl derivative selectively potentiate and antagonize activation of P2X1 receptors

Analogues of the P2 receptor antagonists pyridoxal-5'-phosphate and the 6-azophenyl-2',4'-disulfonate derivative (PPADS), in which the phosphate group was cyclized by esterification to a CH2OH group at the 4-position, were synthesized. The cyclic pyridoxine-alpha4, 5-monophosphate, compound 2 (MRS 2219), was found to be a selective potentiator of ATP-evoked responses at rat P2X1 receptors with an EC50 value of 5.9 +/- 1.8 microM, while the corresponding 6-azophenyl-2',5'-disulfonate derivative, compound 3 (MRS 2220), was a selective antagonist. The potency of compound 3 at the recombinant P2X1 receptor (IC50 10.2 +/- 2.6 microM) was lower than PPADS (IC50 98.5 +/- 5.5 nM) or iso-PPADS (IC50 42.5 +/- 17.5 nM), although unlike PPADS its effect was reversible with washout and surmountable. Compound 3 showed weak antagonistic activity at the rat P2X3 receptor (IC50 58.3 +/- 0.1 microM), while at recombinant rat P2X2 and P2X4 receptors no enhancing or antagonistic properties were evident. Compounds 2 and 3 were found to be inactive as either agonists or antagonists at the phospholipase C-coupled P2Y1 receptor of turkey erythrocytes, at recombinant human P2Y2 and P2Y4 receptors, and at recombinant rat P2Y6 receptors. Similarly, compounds 2 and 3 did not have measurable affinity at adenosine A1, A2A, or A3 receptors. The lack of an aldehyde group in these derivatives indicates that Schiff's base formation with the P2X1 receptor is not necessarily required for recognition of pyridoxal phosphate derivatives. Thus, compounds 2 and 3 are relatively selective pharmacological probes of P2X1 receptors, filling a long-standing need in the P2 receptor field, and are also important lead compounds for future studies.

Membrane topology of an ATP-gated ion channel (P2X receptor)

Western blots of Xenopus oocyte membrane preparations showed that the apparent molecular mass of the wild type P2X2 receptor (about 65 kDa) was reduced by pretreatment with endoglycosidase H. Mutagenesis of one or more of three potential asparagines (N182S, N239S, and N298S) followed by Western blots showed that each of the sites was glycosylated in the wild type receptor. Functional channels were formed by receptors lacking any single asparagine, but not by channels mutated in two or three positions. Artificial consensus sequences (N-X-S/T) introduced into the N-terminal region (asparagine at position 9, 16, or 26) were not glycosylated. Asparagines were glycosylated when introduced at the C-terminal end of the first hydrophobic domain (positions 62 and 66) and at the N-terminal end of the second hydrophobic domain (position 324). A protein in which the C terminus of one P2X2 subunit was joined to the N terminus of a second P2X2 subunit (from a concatenated cDNA) had twice the molecular mass of the P2X2 receptor subunit, and formed fully functional channels. The experiments provide direct evidence for the topology originally proposed for the P2X receptor, with intracellular N and C termini, two membrane-spanning domains, and a large extracellular loop.

Regional distribution of [35S]2'-deoxy 5'-O-(1-thio) ATP binding sites and the P2Y1 messenger RNA within the chick brain

The distribution of the P2Y1 receptor protein and transcript in the one-day-old chick brain were determined by quantitative in vitro ligand autoradiography and in situ hybridization histochemistry. We have previously used [35S]2'-deoxy 5'-O-(1-thio) ATP as a radioligand for the recombinant P2Y1 receptor transiently expressed in COS-7 cells and have also shown that such sites are present at high density (Bmax: approximately 37 pmol radioligand bound/mg protein) in chick brain membranes. Here we report the macroscopic localization of these [35S]2'-deoxy 5'-O-(1-thio) ATP binding sites within the chick brain. They were found to be widely distributed there (within the range of 0.047 +/- 0.012 to 0.309 +/- 0.035 pmol bound/mg wet tissue). The affinities of P2 agonists and antagonists at these binding sites was comparable to that found previously for the recombinant P2Y1 receptor. In parallel experiments, the regional and cellular localization of the P2Y1 receptor messenger RNA was examined by in situ hybridization. The transcript was also found to be widely distributed throughout the brain. High levels of hybridization were detected in the cortex piriformis, ectostriatum, hippocampus, cerebellum and in a range of discrete nuclei throughout the brain, including the ovoidalis, isthmo-opticus and spiriformis lateralis nuclei. Localization at cellular level indicates that this receptor transcript is expressed in neurons and also at non-neuronal sites. Furthermore, the distribution of the P2Y1 transcript and the [35S]2'-deoxy 5'-O-(1-thio) ATP binding sites matched in a number of the regions and structures mentioned above. The present study clarifies the anatomical distribution of the P2Y1 receptor within the chick brain. Its broad distribution coupled with its neuronal expression suggest an important role for this type of metabotropic nucleotide receptor within the brain.

The antagonist trinitrophenyl-ATP reveals co-existence of distinct P2X receptor channels in rat nodose neurones

1. Whole-cell recordings were made from rat nodose ganglion neurones in culture and from human embryonic kidney (HEK293) cells stably transfected to express P2X2, P2X3 or both receptor subunits. We examined the blocking actions of 2',3'-O-trinitrophenyl-ATP (TNP-ATP) on currents evoked by the agonists ATP and alpha, beta-methylene ATP. 2. In cells expressing only P2X2 or P2X3 receptor subunits, the inhibition by TNP-ATP was fitted by a single binding site model with half-maximal concentrations of about 3 microM and 3 nM, respectively. In cells expressing both P2X2 and P2X3 receptor subunits, currents showed little or no desensitization, thus excluding contributions from homomeric P2X3 receptors. When alpha,beta-methylene ATP was the agonist (activating heteromeric P2X2/3 receptors), the inhibition by TNP-ATP conformed to a single binding site (half-maximal concentration about 3 nM). When ATP (30 microM) was the agonist, activating both heteromeric P2X2/3 as well as homomeric P2X2 receptors, the inhibition curve was biphasic (half-maximal concentrations about 3 nM and 3 microM); the proportion of high affinity sites in all six cells tested was about 40 %. 3. In nodose ganglion neurones, the inhibition by TNP-ATP of currents evoked by ATP (30 microM) was also clearly biphasic. In this case, individual neurones showed more variability in the proportion of high and low affinity sites for TNP-ATP. 4. We conclude that more than one form of multimeric P2X receptor channels are functionally expressed on the cell bodies of individual nodose ganglion neurones. On the basis of sensitivity to TNP-ATP, and other properties, one of these may correspond to the homomeric P2X2 receptor and the other(s) to heteromeric P2X2/3 receptors.