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

Characterisation of P2Y receptor subtypes mediating vasodilation and vasoconstriction of rat pulmonary artery using selective antagonists

Pulmonary vascular tone is modulated by nucleotides, but which P2 receptors mediate these actions is largely unclear. The aim of this study, therefore, was to use subtype-selective antagonists to determine the roles of individual P2Y receptor subtypes in nucleotide-evoked pulmonary vasodilation and vasoconstriction. Isometric tension was recorded from rat intrapulmonary artery rings (i.d. 200-500 µm) mounted on a wire myograph. Nucleotides evoked concentration- and endothelium-dependent vasodilation of precontracted tissues, but the concentration-response curves were shallow and did not reach a plateau. The selective P2Y₂ antagonist, AR-C118925XX, inhibited uridine 5'-triphosphate (UTP)- but not adenosine 5'-triphosphate (ATP)-evoked relaxation, whereas the P2Y₆ receptor antagonist, MRS2578, had no effect on UTP but inhibited relaxation elicited by uridine 5'-diphosphate (UDP). ATP-evoked relaxations were unaffected by the P2Y₁ receptor antagonist, MRS2179, which substantially inhibited responses to adenosine 5'-diphosphate (ADP), and by the P2Y_12/13 receptor antagonist, cangrelor, which potentiated responses to ADP. Both agonists were unaffected by CGS1593, an adenosine receptor antagonist. Finally, AR-C118925XX had no effect on vasoconstriction elicited by UTP or ATP at resting tone, although P2Y₂ receptor mRNA was extracted from endothelium-denuded tissues using reverse transcription polymerase chain reaction with specific oligonucleotide primers. In conclusion, UTP elicits pulmonary vasodilation via P2Y₂ receptors, whereas UDP acts at P2Y₆ and ADP at P2Y₁ receptors, respectively. How ATP induces vasodilation is unclear, but it does not involve P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₃, or adenosine receptors. UTP- and ATP-evoked vasoconstriction was not mediated by P2Y₂ receptors. Thus, this study advances our understanding of how nucleotides modulate pulmonary vascular tone.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Prostaglandin E2 glyceryl ester is an endogenous agonist of the nucleotide receptor P2Y6

Cyclooxygenase-2 catalyses the biosynthesis of prostaglandins from arachidonic acid but also the biosynthesis of prostaglandin glycerol esters (PG-Gs) from 2-arachidonoylglycerol. Previous studies identified PG-Gs as signalling molecules involved in inflammation. Thus, the glyceryl ester of prostaglandin E2, PGE2-G, mobilizes Ca2+ and activates protein kinase C and ERK, suggesting the involvement of a G protein-coupled receptor (GPCR). To identify the endogenous receptor for PGE2-G, we performed a subtractive screening approach where mRNA from PGE2-G response-positive and -negative cell lines was subjected to transcriptome-wide RNA sequencing analysis. We found several GPCRs that are only expressed in the PGE2-G responder cell lines. Using a set of functional readouts in heterologous and endogenous expression systems, we identified the UDP receptor P2Y6 as the specific target of PGE2-G. We show that PGE2-G and UDP are both agonists at P2Y6, but they activate the receptor with extremely different EC50 values of ~1 pM and ~50 nM, respectively. The identification of the PGE2-G/P2Y6 pair uncovers the signalling mode of PG-Gs as previously under-appreciated products of cyclooxygenase-2.

Signaling mechanism for modulation by ATP of glycine receptors on rat retinal ganglion cells

ATP modulates voltage- and ligand-gated channels in the CNS via the activation of ionotropic P2X and metabotropic P2Y receptors. While P2Y receptors are expressed in retinal neurons, the function of these receptors in the retina is largely unknown. Using whole-cell patch-clamp techniques in rat retinal slice preparations, we demonstrated that ATP suppressed glycine receptor-mediated currents of OFF type ganglion cells (OFF-GCs) dose-dependently, and the effect was in part mediated by P2Y1 and P2Y11, but not by P2X. The ATP effect was abolished by intracellular dialysis of a Gq/11 protein inhibitor and phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor, but not phosphatidylcholine (PC)-PLC inhibitor. The ATP effect was accompanied by an increase in [Ca(2+)]i through the IP3-sensitive pathway and was blocked by intracellular Ca(2+)-free solution. Furthermore, the ATP effect was eliminated in the presence of PKC inhibitors. Neither PKA nor PKG system was involved. These results suggest that the ATP-induced suppression may be mediated by a distinct Gq/11/PI-PLC/IP3/Ca(2+)/PKC signaling pathway, following the activation of P2Y1,11 and other P2Y subtypes. Consistently, ATP suppressed glycine receptor-mediated light-evoked inhibitory postsynaptic currents of OFF-GCs. These results suggest that ATP may modify the ON-to-OFF crossover inhibition, thus changing action potential patterns of OFF-GCs.

P2Y Receptors in Synaptic Transmission and Plasticity: Therapeutic Potential in Cognitive Dysfunction

ATP released from neurons and astrocytes during neuronal activity or under pathophysiological circumstances is able to influence information flow in neuronal circuits by activation of ionotropic P2X and metabotropic P2Y receptors and subsequent modulation of cellular excitability, synaptic strength, and plasticity. In the present paper we review cellular and network effects of P2Y receptors in the brain. We show that P2Y receptors inhibit the release of neurotransmitters, modulate voltage- and ligand-gated ion channels, and differentially influence the induction of synaptic plasticity in the prefrontal cortex, hippocampus, and cerebellum. The findings discussed here may explain how P2Y₁ receptor activation during brain injury, hypoxia, inflammation, schizophrenia, or Alzheimer's disease leads to an impairment of cognitive processes. Hence, it is suggested that the blockade of P2Y₁ receptors may have therapeutic potential against cognitive disturbances in these states.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

Coupling of P2Y receptors to G proteins and other signaling pathways

P2Y receptors are G protein-coupled receptors (GPCRs) that are activated by adenine and uridine nucleotides and nucleotide sugars. There are eight subtypes of P2Y receptors (P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄), which activate intracellular signaling cascades to regulate a variety of cellular processes, including proliferation, differentiation, phagocytosis, secretion, nociception, cell adhesion, and cell migration. These signaling cascades operate mainly by the sequential activation or deactivation of heterotrimeric and monomeric G proteins, phospholipases, adenylyl and guanylyl cyclases, protein kinases, and phosphodiesterases. In addition, there are numerous ion channels, cell adhesion molecules, and receptor tyrosine kinases that are modulated by P2Y receptors and operate to transmit an extracellular signal to an intracellular response.

Cellular localization of P2Y₆ receptor in rat retina

Extracellular nucleotides exert their actions via two subfamilies of purinoceptors: P2X and P2Y. Eight mammalian P2Y receptor subtypes (P2Y(1,2,4,6,11,12,13,14)) have been identified. In this work, the localization of P2Y(6) was studied in rat retina using double immunofluorescence labeling and confocal scanning microscopy. Immunostaining for P2Y(6) was strong in the outer plexiform layer and was diffusely distributed throughout the full thickness of the inner plexiform layer. In addition, P2Y(6) immunoreactivity was clearly observed in many cells in the inner nuclear layer and the ganglion cell layer. In the outer retina photoreceptor terminals, labeled by VGluT1, and horizontal cells, labeled by calbindin, were P2Y(6)-positive. However, no P2Y(6) immunostaining was detected in bipolar cells, labeled by homeobox protein Chx10. In the inner retina P2Y(6) was localized to most of GABAergic amacrine cells, including dopaminergic and cholinergic ones, stained by tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) respectively. Some of glycinergic amacrine cells, but not glycinergic AII amacrine cells, were also labeled by P2Y(6). Moreover, P2Y(6) immunoreactivity was seen in almost all ganglion cells, labeled by Brn3a. In Müller glial cells, stained by cellular retinaldehyde binding protein (CRALBP), however, no P2Y(6) expression was found in both somata and processes. We speculate that P2Y(6) may be involved in retinal information processing in different ways, probably by regulating the release of transmitters and/or modulating the radial flow of visual signals and lateral interaction mediated by horizontal and amacrine cells.

The impact of commercially available purinergic ligands on purinergic signalling research

Due to the extremely wide-spread expression of purinergic receptors, purinergic signalling has been implicated in numerous physiological and pathophysiological areas. To better understand the involvement of purinergic receptors in such areas, the researcher's requirement for diverse and varied purinergic receptor ligands has greatly increased. This has generated increased commercial opportunities for life science suppliers, and ultimately, has led to a rapid expansion in the number of commercially available purinergic receptor ligands. The wide-spread availability of ligands to researchers has greatly benefited the scientific community, nurturing the rapid and continued expansion of the purinergic signalling field.

P2Y receptors as regulators of lung endothelial barrier integrity

Endothelial cells (ECs), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, control such diverse processes as vascular tone, homeostasis, adhesion of platelets, and leukocytes to the vascular wall and permeability of vascular wall for cells and fluids. Mechanisms which govern the highly clinically relevant process of increased EC permeability are under intense investigation. It is well known that loss of this barrier (permeability increase) results in tissue inflammation, the hall mark of inflammatory diseases such as acute lung injury and its severe form, acute respiratory distress syndrome. Little is known about processes which determine the endothelial barrier enhancement or protection against permeability increase. It is now well accepted that extracellular purines and pyrimidines are promising and physiologically relevant barrier-protective agents and their effects are mediated by interaction with cell surface P2Y receptors which belong to the superfamily of G-protein-coupled receptors. The therapeutic potential of P2Y receptors is rapidly expanding field in pharmacology and some selective agonists became recently available. Here, we present an overview of recently identified P2Y receptor agonists that enhance the pulmonary endothelial barrier and inhibit and/or reverse endothelial barrier disruption.

Regulation of neuronal ion channels via P2Y receptors

Within the last 15 years, at least 8 different G protein-coupled P2Y receptors have been characterized. These mediate slow metabotropic effects of nucleotides in neurons as well as non-neural cells, as opposed to the fast ionotropic effects which are mediated by P2X receptors. One class of effector systems regulated by various G protein-coupled receptors are voltage-gated and ligand-gated ion channels. This review summarizes the current knowledge about the modulation of such neuronal ion channels via P2Y receptors. The regulated proteins include voltage-gated Ca²⁺ and K⁺ channels, as well as N-methyl-d-aspartate, vanilloid, and P2X receptors, and the regulating entities include most of the known P2Y receptor subtypes. The functional consequences of the modulation of ion channels by nucleotides acting at pre- or postsynaptic P2Y receptors are changes in the strength of synaptic transmission. Accordingly, ATP and related nucleotides may act not only as fast transmitters (via P2X receptors) in the nervous system, but also as neuromodulators (via P2Y receptors). Hence, nucleotides are as universal transmitters as, for instance, acetylcholine, glutamate, or γ-aminobutyric acid.

Inositol triphosphate-mediated Ca2+ signals direct purinergic P2Y receptor regulation of neuronal ion channels

Purinergic P2Y receptors are one of four types of G(q/11)-coupled receptors in rat superior cervical ganglia (SCG) sympathetic neurons. In cultured SCG neurons, purinergic and bradykinin suppression of I(M) were similar in magnitude and somewhat less than that by muscarinic agonists. The effects of the P2Y receptor agonist UTP on neuronal excitability and discharge properties were studied. Under current clamp, UTP increased action potential (AP) firing in response to depolarizing current steps, depolarized the resting potential, decreased the threshold current required to fire an AP, and decreased spike-frequency adaptation. These effects were very similar to those resulting from bradykinin stimulation and not as profound as from muscarinic stimulation or full M-current blockade. We then examined the P2Y mechanism of action. Like bradykinin, but unlike muscarinic, purinergic stimulation induced rises in intracellular [Ca(2+)](i). Tests using expression of IP(3)"sponge" or IP(3) phosphatase constructs implicated IP(3) accumulation as necessary for purinergic suppression of I(M). Overexpression of wild-type or dominant-negative calmodulin (CaM) implicated Ca(2+)/CaM in the purinergic action. Both sets of results were similar to bradykinin, and opposite to muscarinic, suppression. We also examined modulation of Ca(2+) channels. As for bradykinin, purinergic stimulation did not suppress I(Ca), unless neuronal calcium sensor-1 (NCS-1) activity was blocked by a dominant-negative NCS-1 construct. Our results indicate that P2Y receptors modulate M-type channels in SCG cells via IP(3)-mediated [Ca(2+)](i) signals in concert with CaM and not by depletion of phosphatidylinositol-4, 5-biphosphate. We group purinergic P2Y and bradykinin B(2) receptors together as having a common mode of action.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Functions of neuronal P2Y receptors

Within the last 15 years, at least eight different G protein-coupled nucleotide receptors, i.e., P2Y receptors, have been characterized by molecular means. While ionotropic P2X receptors are mainly involved in fast synaptic neurotransmission, P2Y receptors rather mediate slower neuromodulatory effects. This P2Y receptor-dependent neuromodulation relies on changes in synaptic transmission via either pre- or postsynaptic sites of action. At both sites, the regulation of voltage-gated or transmitter-gated ion channels via G protein-linked signaling cascades has been identified as the predominant underlying mechanisms. In addition, neuronal P2Y receptors have been found to be involved in neurotoxic and neurotrophic effects of extracellular adenosine 5-triphosphate. This review provides an overview of the most prominent actions mediated by neuronal P2Y receptors and describes the signaling cascades involved.

Extracellular ATP stimulates NO production in rat thick ascending limb

NO produced by NO synthase (NOS) 3 acts as an autacoid to regulate NaCl absorption in the thick ascending limb. ATP induces NO production by NOS 3 in endothelial cells. We hypothesized that extracellular ATP activates NOS in thick ascending limbs through P2 receptors. To test this, we measured intracellular NO production using the NO-selective fluorescent dye DAF-2 in suspensions of rat medullary thick ascending limbs. We found that ATP increased DAF-2 fluorescence in a concentration-dependent manner, reaching saturation at &200 micromol/L with an EC50 of 37 micromol/L. The increase was blunted by 74% by the nonselective NOS inhibitor L-omega-nitro-arginine-methyl-ester (2 mmol/L; 60+/-7 versus 16+/-6 arbitrary fluorescence units; P<0.02; n=5). In the presence of the P2 receptor antagonist suramin (300 micromol/L), ATP-induced NO production was reduced by 64% (101+/-11 versus 37+/-5 arbitrary fluorescence units; P<0.002; n=5). Blocking ATP hydrolysis with a 5'-ectonucleotidase inhibitor, ARL67156 (30 micromol/L) enhanced the response to ATP and shifted the EC(50) to 0.8 micromol/L. In the presence of ARL67156, the EC50 of the P2X-selective agonist beta,gamma-methylene-adenosine 5'-triphosphate was 4.8 micromol/L and the EC50 for the P2Y-selective agonist UTP was 40.4 micromol/L. The maximal responses for both agonists were similar. Taken together, these data indicate that ATP stimulates NO production in the thick ascending limb primarily through P2X receptor activation and that ATP hydrolysis may regulate NO production.

Multiple P2Y receptors couple to calcium-dependent, chloride channels in smooth muscle cells of the rat pulmonary artery

BACKGROUND

Uridine 5'-triphosphate (UTP) and uridine 5'-diphosphate (UDP) act via P2Y receptors to evoke contraction of rat pulmonary arteries, whilst adenosine 5'-triphosphate (ATP) acts via P2X and P2Y receptors. Pharmacological characterisation of these receptors in intact arteries is complicated by release and extracellular metabolism of nucleotides, so the aim of this study was to characterise the P2Y receptors under conditions that minimise these problems.

METHODS

The perforated-patch clamp technique was used to record the Ca2+-dependent, Cl- current (I(Cl,Ca)) activated by P2Y receptor agonists in acutely dissociated smooth muscle cells of rat small (SPA) and large (LPA) intrapulmonary arteries, held at -50 mV. Contractions to ATP were measured in isolated muscle rings. Data were compared by Student's t test or one way ANOVA.

RESULTS

ATP, UTP and UDP (10(-4) M) evoked oscillating, inward currents (peak = 13-727 pA) in 71-93% of cells. The first current was usually the largest and in the SPA the response to ATP was significantly greater than those to UTP or UDP (P < 0.05). Subsequent currents tended to decrease in amplitude, with a variable time-course, to a level that was significantly smaller for ATP (P < 0.05), UTP (P < 0.001) and UDP (P < 0.05) in the SPA. The frequency of oscillations was similar for each agonist (mean approximately to 6-11.min(-1)) and changed little during agonist application. The non-selective P2 receptor antagonist suramin (10(-4) M) abolished currents evoked by ATP in SPA (n = 4) and LPA (n = 4), but pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (10(-4) M), also a non-selective P2 antagonist, had no effect (n = 4, 5 respectively). Currents elicited by UTP (n = 37) or UDP (n = 14) were unaffected by either antagonist. Contractions of SPA evoked by ATP were partially inhibited by PPADS (n = 4) and abolished by suramin (n = 5). Both antagonists abolished the contractions in LPA.

CONCLUSION

At least two P2Y subtypes couple to ICl,Ca in smooth muscle cells of rat SPA and LPA, with no apparent regional variation in their distribution. The suramin-sensitive, PPADS-resistant site activated by ATP most resembles the P2Y11 receptor. However, the suramin- and PPADS-insensitive receptor activated by UTP and UDP does not correspond to any of the known P2Y subtypes. These receptors likely play a significant role in nucleotide-induced vasoconstriction.

Identification of the P2Y(12) receptor in nucleotide inhibition of exocytosis from bovine chromaffin cells

Nucleotides are released from bovine chromaffin cells and take part in a feedback loop to inhibit further exocytosis. To identify the nucleotide receptors involved, we measured the effects of a range of exogenous nucleotides and related antagonists on voltage-operated calcium currents (I(Ca)), intracellular calcium concentration ([Ca(2+)](i)), and membrane capacitance changes. In comparative parallel studies, we also cloned the bovine P2Y(12) receptor from chromaffin cells and determined its properties by coexpression in Xenopus laevis oocytes with inward-rectifier potassium channels made up of Kir3.1 and Kir3.4. In both systems, the agonist order of potency was essentially identical (2-methylthio-ATP approximately 2-methylthio-ADP >> ATP approximately ADP > UDP). alphabeta-Methylene-ATP and adenosine were inactive. UTP inhibited I(Ca) in chromaffin cells (pEC(50) = 4.89 +/- 0.11) but was essentially inactive at the cloned P2Y(12) receptor. The relatively nonselective P2 antagonist pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid blocked nucleotide responses in both chromaffin cells and X. laevis oocytes, whereas the P2Y(12)- and P2Y(13)-selective antagonist N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene ATP (ARC69931MX) blocked responses to ATP in both chromaffin cells and X. laevis oocytes but not to UTP in chromaffin cells. These results identify the P2Y(12) purine receptor as a key component of the nucleotide inhibitory pathway and also demonstrate the involvement of a UTP-sensitive G(i/o) -coupled pyrimidine receptor.

Inhibition of N-type voltage-activated calcium channels in rat dorsal root ganglion neurons by P2Y receptors is a possible mechanism of ADP-induced analgesia

Patch-clamp recordings from small-diameter rat dorsal root ganglion (DRG) neurons maintained in culture demonstrated preferential inhibition by ATP of high-voltage-activated, but not low-voltage-activated, Ca2+ currents (I(Ca)). The rank order of agonist potency was UTP > ADP > ATP. ATP depressed the omega-conotoxin GVIA-sensitive N-type current only. Pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate tetraammonium, two P2Y1 receptor antagonists, almost abolished the ATP-induced inhibition. Both patch-clamp recordings and immunocytochemistry coupled with confocal laser microscopy indicated a colocalization of functional P2X3 and P2Y1 receptors on the same DRG neurons. Because the effect of ATP was inhibited by intracellular guanosine 5'-O-(2-thiodiphosphate) or by applying a strongly depolarizing prepulse, P2Y1 receptors appear to block I(Ca) by a pathway involving the betagamma subunit of a G(q/11) protein. Less efficient buffering of the intracellular Ca2+ concentration ([Ca2+]i) by reducing the intrapipette EGTA failed to interfere with the ATP effect. Fura-2 microfluorimetry suggested that ATP raised [Ca2+]i by a Galpha-mediated release from intracellular pools and simultaneously depressed the high external potassium concentration-induced increase of [Ca2+]i by inhibiting I(Ca) via Gbetagamma. Adenosine 5'-O-(2-thiodiphosphate) inhibited dorsal root-evoked polysynaptic population EPSPs in the hemisected rat spinal cord and prolonged the nociceptive threshold on intrathecal application in the tail-flick assay. These effects were not antagonized by PPADS. Hence, P2Y receptor activation by ADP, which is generated by enzymatic degradation of ATP, may decrease the release of glutamate from DRG terminals in the spinal cord and thereby partly counterbalance the algogenic effect of ATP.

Adenine nucleotides inhibit recombinant N-type calcium channels via G protein-coupled mechanisms in HEK 293 cells; involvement of the P2Y13 receptor-type

1. N-type Ca(2+) channel modulation by an endogenous P2Y receptor was investigated by the whole-cell patch-clamp method in HEK 293 cells transfected with the functional rabbit N-type calcium channel. 2. The current responses (I(Ca(N))) to depolarizing voltage steps were depressed by ATP in a concentration-dependent manner. Inclusion of either guanosine 5'-O-(3-thiodiphosphate) or pertussis toxin into the pipette solution as well as a strongly depolarizing prepulse abolished the inhibitory action of ATP. 3. In order to identify the P2Y receptor subtype responsible for this effect, several preferential agonists and antagonists were studied. Whereas the concentration-response curves of ADP and adenosine 5'-O-(2-thiodiphosphate) indicated a higher potency of these agonists than that of ATP, alpha,beta-methylene ATP, UTP and UDP were considerably less active. The effect of ATP was abolished by the P2Y receptor antagonists suramin and N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, 2'deoxy-N(6)-methyladenosine-3',5'-diphosphate or 2-methylthio AMP. 4. Using reverse transcription and polymerase chain reaction, mRNA for the P2Y(1), P2Y(4), P2Y(6), P2Y(11) and P2Y(13) receptor subtypes, but not the P2Y(2), and P2Y(12) subtypes, was detected in HEK 293 cells. 5. Immunocytochemistry confirmed the presence of P2Y(1), and to a minor extent that of P2Y(4), but not of P2Y(2) receptors. 6. Hence, it is tempting to speculate that P2Y(13) receptors may inhibit N-type Ca(2+) channels via the betagamma subunits of the activated G(i) protein.

Extracellular ATP-induced calcium channel inhibition mediated by P1/P2Y purinoceptors in hamster submandibular ganglion neurons

1. The presence and profile of purinoceptors in neurons of the hamster submandibular ganglion (SMG) have been studied using the whole-cell configuration of the patch-clamp technique. 2. Extracellular application of adenosine 5'-triphosphate (ATP) reversibly inhibited voltage-dependent Ca(2+) channel (VDCC) currents (I(Ca)) via G(i/o)-protein in a voltage-dependent manner. 3. Extracellular application of uridine 5'-triphosphate (UTP), 2-methylthioATP (2-MeSATP), alpha,beta-methylene ATP (alpha,beta-MeATP) and adenosine 5'-diphosphate (ADP) also inhibited I(Ca). The rank order of potency was ATP=UTP>ADP>2-MeSATP=alpha,beta-MeATP. 4. The P2 purinoceptor antagonists, suramin and pyridoxal-5-phosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS), partially antagonized the ATP-induced inhibition of I(Ca), while coapplication of suramin and the P1 purinoceptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), virtually abolished I(Ca) inhibition. DPCPX alone partially antagonized I(Ca) inhibition. 5. Suramin antagonized the UTP-induced inhibition of I(Ca), while DPCPX had no effect. 6. Extracellular application of adenosine (ADO) also inhibited I(Ca) in a voltage-dependent manner via G(i/o)-protein activation. 7. Mainly N- and P/Q-type VDCCs were inhibited by both ATP and ADO via G(i/o)-protein betagamma subunits in seemingly convergence pathways.

Long-term potentiation of glutamatergic synaptic transmission induced by activation of presynaptic P2Y receptors in the rat medial habenula nucleus

A novel form of long-term potentiation of glutamatergic synaptic transmission is described in the rat medial habenula nucleus. It occurs when uridine 5'-triphosphate is bath applied at low micromolar concentrations and is prevented by Reactive Blue 2, suggesting that it is mediated by P2Y4 receptors. Uridine 5'-diphosphate can also cause such a Reactive Blue 2-sensitive potentiation, but at higher concentrations (200 microm), suggesting that this might also be an effect on the relatively uridine 5'-diphosphate-insensitive P2Y4 receptor. The potentiation is due to an increase in presynaptic release probability. It requires neither depolarization nor calcium influx postsynaptically and is thus probably non-Hebbian. When potentiation due to low concentrations of uridine 5'-triphosphate is inhibited in the presence of Reactive Blue 2, uridine 5'-triphosphate causes instead a significant inhibition of glutamate release. We suggest that this inhibition may be mediated by a Reactive Blue 2-insensitive P2Y2-like receptor. At higher concentrations of uridine 5'-triphosphate (200 micro m), the inhibitory effect dominates such that even in the absence of Reactive Blue 2 no potentiation is seen.

Coupling of the nucleotide P2Y4 receptor to neuronal ion channels

1. G protein-linked P2Y nucleotide receptors are known commonly to stimulate the phosphoinositide signalling pathway. However, we have previously demonstrated that the cloned P2Y(2), P2Y(6) and P2Y(1) receptors couple to neuronal N-type Ca(2+) channels and to M-type K(+) channels. Here we investigate the coupling of recombinant, neuronally expressed rat- and human P2Y(4) receptors (rP2Y(4), hP2Y(4)) to those channels. 2. Rat sympathetic neurones were nuclear-injected with a P2Y(4) cDNA plasmid. A subsequent activation of rP2Y(4) or hP2Y(4) by UTP (100 micro M) in whole-cell (ruptured-patch) mode produced only about 12% inhibition of the N-type Ca(2+) current (I(Ca(N))). Surprisingly, in perforated patch mode, UTP produced much more inhibition of I(Ca(N)) (maximally 51%), with an IC(50) value of 273 nM. This inhibition was voltage-dependent and was blocked by co-expression of the betagamma-binding transducin Galpha-subunit. Pertussis toxin (PTX) pretreatment also suppressed I(Ca(N)) inhibition. 3. UTP inhibited the M-current, recorded in perforated patch mode, by (maximally) 52%, with IC(50) values of 21 nM for rP2Y(4) and 28 nM for hP2Y(4). This inhibition was not affected by PTX pretreatment. 4. With rP2Y(4), ATP inhibited the M-current (IC(50) 524 nM, 26 times weaker than UTP), whereas ATP had no agonist activity at hP2Y(4). This suggests a difference in agonist binding site between rP2Y(4) and hP2Y(4). 5. We conclude that, in contrast to other nucleotide receptors studied, the P2Y(4) receptor couples much more effectively to M-type K(+) channels than to Ca(2+) channels. Coupling to the Ca(2+) channels involves the betagamma-subunits of G(i/o)-proteins and requires a diffusible intracellular component that is lost in ruptured-patch recording.

Regional variation in P2 receptor expression in the rat pulmonary arterial circulation

The P2 receptors that mediate contraction of the rat isolated small (SPA, 200-500 micro m i.d.) and large (LPA, 1-1.5 mM i.d.) intrapulmonary arteries were characterized. 2 In endothelium-denuded vessels the contractile order of potency was alpha,beta-methyleneATP (alpha,beta-meATP)>>UDP=UTP=ATP=2-methylthioATP>ADP in the SPA and alpha,beta-meATP=UTP>or=UDP>2-methylthioATP, ATP>>ADP in the LPA. alpha,beta-meATP, 2-methylthioATP and ATP had significantly greater effects in the SPA than the LPA (P<0.001), but there was no difference in the potency of UTP or UDP between the vessels. 3 In the SPA, P2X1 receptor desensitisation by alpha,beta-meATP (100 microM) inhibited contractions to alpha,beta-meATP (10 nM-300 microM), but not those to UTP or UDP (100 nM-300 microM). In the LPA, prolonged exposure to alpha,beta-meATP (100 microM) did not desensitize P2X receptors. 4 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), suramin and reactive blue 2 (RB2) (30-300 microM) inhibited contractions evoked by alpha,beta-meATP. UTP and UDP were potentiated by PPADS, unaffected by RB2 and inhibited, but not abolished by suramin. 1 and 3 mM suramin produced no further inhibition, indicating suramin-resistant components in the responses to UTP and UDP. 5 Thus, both P2X and P2Y receptors mediate contraction of rat large and small intrapulmonary arteries. P2Y agonist potency and sensitivity to antagonists were similar in small and large vessels, but P2X agonists were more potent in small arteries. This indicates differential expression of P2X, but not P2Y receptors along the pulmonary arterial tree.

Characterization and channel coupling of the P2Y(12) nucleotide receptor of brain capillary endothelial cells

Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.

Increase in intracellular Ca(2+) and calcitonin gene-related peptide release through metabotropic P2Y receptors in rat dorsal root ganglion neurons

We examined the effects of the activation of metabotropic P2Y receptors on the intracellular Ca(2+) concentration and the release of neuropeptide calcitonin gene-related peptide (CGRP) in isolated adult rat dorsal root ganglion neurons. In small-sized dorsal root ganglion neurons (soma diameter<30 microm) loaded with fura-2, a bath application of ATP (100 microM) evoked an increase in intracellular Ca(2+) concentration, while the removal of extracellular Ca(2+) partly depressed the response to ATP, thus suggesting that the ATP-induced increase in intracellular Ca(2+) concentration is due to both the release of Ca(2+) from intracellular stores and the influx of extracellular Ca(2+). Bath application of uridine 5'-triphosphate (UTP; 100 microM) also caused an increase in intracellular Ca(2+) concentration in small-sized dorsal root ganglion neurons and the P2 receptor antagonists suramin (100 microM) and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 10 microM) virtually abolished the response, indicating that the intracellular Ca(2+) elevation in response to UTP is mediated through metabotropic P2Y receptors. This intracellular Ca(2+) increase was abolished by pretreating the neurons with thapsigargin (100 nM), suggesting that the UTP-induced increase in intracellular Ca(2+) is primarily due to the release of Ca(2+) from endoplasmic reticulum Ca(2+) stores. An enzyme-linked immunosorbent assay showed that an application of UTP (100 microM) significantly stimulated the release of CGRP and that suramin (100 microM) totally abolished the response, suggesting that P2Y receptor-mediated increase in intracellular Ca(2+) is accompanied by CGRP release from dorsal root ganglion neurons. These results suggest that metabotropic P2Y receptors contribute to extracellular ATP-induced increase in intracellular Ca(2+) concentration and subsequent release of neuropeptide CGRP in rat dorsal root ganglion neurons.

An interaction between ATP and high K+: mutual impairment of ATP- and high K(+)-evoked [Ca2+]i increase in NG 108-15 cells

The interaction between ATP- and high K(+)-evoked increase in intracellular free calcium concentration ([Ca2+]i) was investigated to gain an insight into the mechanism of interaction of ATP with voltage-sensitive calcium channels. [Ca2+]i was measured in the neuronal model, neuroblastoma x glioma hybrid cells (NG 108-15), using the fluorescence indicator fura-2. In the presence of 1.8 mM extracellular Ca2+, ATP induced a rapid, concentration-dependent increase in [Ca2+]i. High K+ (50 mM) evoked a [Ca2+]i rise from 109 +/- 11 nM to 387 +/- 81 nM (n = 16). The application of either of these two [Ca2+]i-increase provoking agents in sequence with the other caused impairment of the latter effect. The mutual desensitization of the responses to ATP and high K+ strongly suggests that both agents rely at least in part on the same source of Ca2+ for elevation of [Ca2+]i in NG 108-15 cells.

A retrospective of recombinant P2Y receptor subtypes and their pharmacology

Since the first cloning of P2Y receptor sequences in 1993 it has become apparent that this family of G-protein-coupled receptors is omnipresent. At least 25 individual sequences entered in the GenBank sequence database encode P2Y receptors from a variety of species ranging from the little skate Raja erinacea to man. In man, six receptor subtypes have been cloned and found to be functionally active (P2Y(1), P2Y(2), P2Y(4), P2Y(6), P2Y(11), and P2Y(12)). In this article a review of the P2Y receptor subtypes is presented considering both their sequences and the pharmacological profiles of the encoded receptors expressed in heterologous expression systems.

Evidence for basolateral P2Y(6) receptors along the rat proximal tubule: functional and molecular characterization

In this study, the distribution of P2Y(6) receptor mRNA in rat nephron segments was investigated and a functional approach was used to analyze basolateral protein expression. Reverse transcription-PCR studies revealed more intense expression of P2Y(6) receptor mRNA in the proximal tubule and the thick ascending limb of Henle's loop, less intense expression in the thin descending limb and the cortical and outer medullary collecting ducts, and no detectable expression in either the thin ascending limb or the inner medullary collecting duct. Dose-dependent calcium responses to basolateral administration of UDP (a selective agonist for the P2Y(6) receptor) were observed in the proximal tubule but not in any of the other segments studied. In the proximal tubule, intracellular calcium concentration changes induced by UDP were associated with increased production of inositol phosphates, as were those induced by ATP and norepinephrine. However, UDP-induced intracellular calcium concentration changes were different, exhibiting no plateau after the initial peak; moreover, a single stimulation with a high concentration of UDP induced full desensitization of the UDP-sensitive calcium pathway but did not alter the responsiveness of the proximal tubule to ADP (a specific P2Y(1) receptor agonist), ATP or norepinephrine. In summary, this report demonstrates that P2Y(6) receptor mRNA is expressed in most segments of the rat nephron but that basolateral expression of the protein is restricted to the proximal tubule, where the receptor is coexpressed with the P2Y(1) receptor. The differences in the distributions of P2Y(6) receptor mRNA and UDP responses may indicate the presence of luminal receptors in other nephron segments.

Mechanisms of adenosine 5'-triphosphate-induced dopamine release in the rat nucleus accumbens in vivo

The endogenous mechanisms modulating ATP-induced dopamine release in the nucleus accumbens (NAc) were studied by microdialysis in freely moving rats. The ATP analog 2-Methylthio ATP (2-MeSATP) facilitated the release of dopamine in a manner sensitive to pertussis toxin and tetrodotoxin. It is suggested that G-protein-coupled P2Y receptors and voltage-gated sodium channels are involved in this process. N-methyl-D-aspartate (NMDA) applied in a concentration of 100 microM decreased the extracellular dopamine level, whereas 1 and 10 mM NMDA enhanced it. The endogenous agonist glutamate (10 microM) inhibited the basal and facilitated release of dopamine. Infusion with a combination of the ionotropic glutamate receptor antagonists (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), as well as with the metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) increased the basal level of dopamine and potentiated the 2-MeSATP-facilitated dopamine release, suggesting an ATP-mediated glutamate release. The GABA(A) receptor antagonist bicuculline infused into the NAc also enhanced the basal level of dopamine; however, the application of 2-MeSATP in the presence of bicuculline caused an early decrease and a subsequent increase of dopamine release. The facilitatory phase of the 2-MeSATP effect was comparable with that measured in the absence of bicuculline. By contrast, when bicuculline was infused into the ventral tegmental area (VTA) it elevated the accumbal basal dopamine level and in addition facilitated the 2-MeSATP- and the glutamate-induced dopamine release above that measured in the absence of bicuculline. These results suggest that ATP in the NAc has a physiologically relevant function in modulating dopaminergic transmission depending on the mesolimbic neuronal activity. The first component of the ATP effect involves a direct stimulation of the terminals of VTA neurons, while the second inhibitory component involves a sequential activation of glutamate and, finally, via ionotropic and metabotropic glutamate receptors, of GABA neurons projecting to the VTA.

Pharmacological characterisation of pyrimidinoceptor responses in NG108-15 cells

In the present study, the P2Y receptor(s) mediating the effects of the pyrimidines UTP and UDP on phospholipase C activation in the mouse neuroblastoma x rat glioma hybrid cell line NG108-15 was investigated. Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) analysis detected transcripts for the P2Y(6) and P2Y(2) receptors, but not for P2Y(1) and P2Y(4.) UTP and UDP were equipotent agonists and their effects were partially additive. Suramin, reactive blue 2 and pyridoxal phosphate-6-azophenyl-2',4'disulfonic acid (PPADS) antagonised the phospholipase C response to both UTP and UDP. High micromolar concentrations of adenosine, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680), 2',3'-O-isopropylideneadenosine (iPAdo) and adenosine 3':5'-cyclic monophosphate (3',5'-cAMP) were able to antagonise the effect of UTP on phospholipase C but not that of UDP. The additivity of the UTP and UDP responses, novel P2 receptor antagonist profile and the distinguishing action of adenosine may indicate the expression of a pyrimidine selective P2Y receptor in addition to the P2Y(6) type in these cells.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

P2 purinoceptor-mediated control of rat cerebral (pial) microvasculature; contribution of P2X and P2Y receptors

Purine and pyrimidine nucleotides evoke changes in the vascular tone of medium to large cerebral vessels through the activation of P2 purinoceptors. We have applied P2 receptor drugs to rat pial arterioles and measured changes in arteriole diameter (o.d. 40-84 micrometer at rest), and recorded currents from arteriolar smooth muscle cells using patch-clamp techniques. Transient vasoconstrictions and rapidly inactivating currents were evoked by alpha,beta-methylene ATP (0.1-30 micrometer) and were sensitive to the P2 receptor antagonists suramin and iso-PPADS. UTP and UDP (0.1-1000 micrometer) evoked sustained suramin-sensitive vasoconstrictions. ATP (0.1-1000 micrometer) and 2-methylthioATP (2MeSATP, 300 micrometer) evoked transient vasoconstrictions followed by sustained vasodilatations. ADP application resulted in only vasodilatation (EC50 approximately 4 micrometer). Vasodilator responses to ATP, 2MeSATP or ADP were unaffected by suramin (100 micrometer). RT-PCR analysis indicated that P2X1-7 and P2Y1,2,6 RNA can be amplified from the pial sheet. Our results provide direct evidence for the presence of functional P2X receptors with a phenotype resembling the P2X1 receptor subtype on cerebral resistance arterioles. The pharmacological properties of the pyrimidine-evoked responses suggest that a combination of P2Y2- and P2Y6-like receptors are responsible for the sustained vasoconstrictions. It is therefore likely that the nucleotides and their associated receptors are involved in a complicated regulatory system to control cerebral blood pressure.

Imagination and reality in the search for the P2Y receptors

A great body of evidence based on tissue and organ physiology and pharmacology led to the recognition, widespread by about 1990, that there must be cell membrane receptors for extracellular nucleotides to transduce their effects. This evidence was provided by the pioneering work of Geoffrey Burnstock and those who worked with him, or was developed by others starting from that information. This article will review how we could start from that foundation to clone the first known gene for such a receptor, P2Y(1). Some unusual properties of that receptor were revealed. I will consider further the P2Y receptors as a class - its definition, now that many such genes have become known. Imagination and reality have been intertwined in this saga.

Inhibition of potassium and calcium currents in neurones by molecularly-defined P2Y receptors

Messenger RNAs and cDNAs for individual cloned P2Y(1), P2Y2 and P2Y(6) nucleotide receptors have been expressed by micro-injection into dissociated rat superior cervical sympathetic neurones and the effects of stimulating the expressed receptors on voltage-activated N-type Ca(2+) currents and M-type K(+) currents recorded. Both currents were reduced by stimulating all three receptors, with the following mean IC(50) values: P2Y(1) (agonist: ADP) - I(K(M)) 6.9 nM, I(Ca) 8.2 nM; P2Y(2) (agonist: UTP) - I(K(M)) 1.5 microM, I(Ca) 0.5 microM; P2Y(6) (agonist: UDP) - I(K(M)) 30 nM, I(Ca) 5.9 nM. Inhibition of I(K(M)) was voltage-independent and insensitive to Pertussis toxin; inhibition of I(Ca) showed both voltage-sensitive and insensitive, and Pertussis toxin-sensitive and insensitive components. It is concluded that these P2Y receptors can couple to more than one G protein and thereby modulate more than one ion channel. It is suggested that these effects on K(M) and Ca(N) channels may induce both postsynaptic excitory and presynaptic inhibitory responses.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Heteromeric assembly of GABA(B)R1 and GABA(B)R2 receptor subunits inhibits Ca(2+) current in sympathetic neurons

Neuronal GABA(B) receptors regulate calcium and potassium currents via G-protein-coupled mechanisms and play a critical role in long-term inhibition of synaptic transmission in the CNS. Recent studies have demonstrated that assembly of GABA(B) receptor GABA(B)R1 and GABA(B)R2 subunits into functional heterodimers is required for coupling to potassium channels in heterologous systems. However whether heterodimerization is required for the coupling of GABA(B) receptors to effector systems in neurons remains to be established. To address this issue, we have studied the coupling of recombinant GABA(B) receptors to endogenous Ca(2+) channels in superior cervical ganglion (SCG) neurons using nuclear microinjection to introduce both sense and antisense expression constructs. Patch-clamp recording from neurons injected with both GABA(B)R1a/1b and GABA(B)R2 cDNAs or with GABA(B)R2 alone produced marked baclofen-mediated inhibition of Ca(2+) channel currents via a pertussis toxin-sensitive mechanism. The actions of baclofen were blocked by CGP62349, a specific GABA(B) antagonist, and were voltage dependent. Interestingly, SCGs were found to express abundantly GABA(B)R1 but not GABA(B)R2 at the protein level. To determine whether heterodimerization of GABA(B)R1 and GABA(B)R2 subunits was required for Ca(2+) inhibition, the GABA(B)R2 expression construct was microinjected with a GABA(B)R1 antisense construct. This resulted in a dramatic decrease in the levels of the endogenous GABA(B)R1 protein and a marked reduction in the inhibitory effects of baclofen on Ca(2+) currents. Therefore our results suggest that in neurons heteromeric assemblies of GABA(B)R1 and GABA(B)R2 are essential to mediate GABAergic inhibition of Ca(2+) channel currents.

Heteromeric assembly of GABA(B)R1 and GABA(B)R2 receptor subunits inhibits Ca(2+) current in sympathetic neurons

Neuronal GABA(B) receptors regulate calcium and potassium currents via G-protein-coupled mechanisms and play a critical role in long-term inhibition of synaptic transmission in the CNS. Recent studies have demonstrated that assembly of GABA(B) receptor GABA(B)R1 and GABA(B)R2 subunits into functional heterodimers is required for coupling to potassium channels in heterologous systems. However whether heterodimerization is required for the coupling of GABA(B) receptors to effector systems in neurons remains to be established. To address this issue, we have studied the coupling of recombinant GABA(B) receptors to endogenous Ca(2+) channels in superior cervical ganglion (SCG) neurons using nuclear microinjection to introduce both sense and antisense expression constructs. Patch-clamp recording from neurons injected with both GABA(B)R1a/1b and GABA(B)R2 cDNAs or with GABA(B)R2 alone produced marked baclofen-mediated inhibition of Ca(2+) channel currents via a pertussis toxin-sensitive mechanism. The actions of baclofen were blocked by CGP62349, a specific GABA(B) antagonist, and were voltage dependent. Interestingly, SCGs were found to express abundantly GABA(B)R1 but not GABA(B)R2 at the protein level. To determine whether heterodimerization of GABA(B)R1 and GABA(B)R2 subunits was required for Ca(2+) inhibition, the GABA(B)R2 expression construct was microinjected with a GABA(B)R1 antisense construct. This resulted in a dramatic decrease in the levels of the endogenous GABA(B)R1 protein and a marked reduction in the inhibitory effects of baclofen on Ca(2+) currents. Therefore our results suggest that in neurons heteromeric assemblies of GABA(B)R1 and GABA(B)R2 are essential to mediate GABAergic inhibition of Ca(2+) channel currents.

The P2Y(1) receptor closes the N-type Ca(2+) channel in neurones, with both adenosine triphosphates and diphosphates as potent agonists

The rat P2Y(1) nucleotide receptor, the P2Y subtype abundant in the brain, was heterologously expressed in rat superior cervical ganglion neurones by micro-injection of the receptor cRNA or cDNA. ADP inhibited the N-type Ca(2+) current by 64%, with EC(50) 8.2 nM, an action blocked competitively by the P2Y(1) receptor antagonist adenosine 3', 5'-bis-phosphate (K(i) 0.7 microM). 2-Methylthio-ADP inhibited the Ca(2+) current likewise, but with EC(50) 0.57 nM, giving the highest potency reported therewith for P2Y(1). Significantly, ATP and 2-methylthio-ATP were also agonists, the latter again at a very high potency (EC(50) 2.5 nM). We propose that this neuronal receptor, when present in brain at a high density as at synapses, can respond to very low concentrations of ATP and ADP as agonists, and that this would result in inhibition of N-type Ca(2+) currents and hence can reduce transmitter release or increase neuronal excitability.

M-type K+ currents in rat cultured thoracolumbar sympathetic neurones and their role in uracil nucleotide-evoked noradrenaline release

Cultured sympathetic neurones are depolarized and release noradrenaline in response to extracellular ATP, UDP and UTP. We examined the possibility that, in neurones cultured from rat thoracolumbar sympathetic ganglia, inhibition of the M-type potassium current might underlie the effects of UDP and UTP. Reverse transcriptase-polymerase chain reaction indicated that the cultured cells contained mRNA for P2Y(2)-, P2Y(4)- and P2Y(6)-receptors as well as for the KCNQ2- and KCNQ3-subunits which have been suggested to assemble into M-channels. In cultures of neurones taken from newborn as well as from 10 day-old rats, oxotremorine, the M-channel blocker Ba(2+) and UDP all released previously stored [(3)H]-noradrenaline. The neurones possessed M-currents, the kinetic properties of which were similar in neurones from newborn and 9 - 12 day-old rats. UDP, UTP and ATP had no effect on M-currents in neurones prepared from newborn rats. Oxotremorine and Ba(2+) substantially inhibited the current. ATP also had no effect on the M-current in neurones prepared from 9 - 12 day-old rats. Oxotremorine and Ba(2+) again caused marked inhibition. In contrast to cultures from newborn animals, UDP and UTP attenuated the M-current in neurones from 9 - 12 day-old rats; however, the maximal inhibition was less than 30%. The results indicate that inhibition of the M-current is not involved in uracil nucleotide-induced transmitter release from rat cultured sympathetic neurones during early development. M-current inhibition may contribute to release at later stages, but only to a minor extent. The mechanism leading to noradrenaline release by UDP and UTP remains unknown.

Only pyrimidinoceptors are functionally expressed in mouse neuroblastoma cell lines

The ability of UTP, UDP, ATP, and ADP to influence inositol phospholipid hydrolysis in neuroblastoma origin cell lines was assessed. The mouse neuroblastoma lines N1E 115, Neuro 2a, and NB4 1A3 and the rat glioma/mouse neuroblastoma hybrid line NG108-15 gave robust responses to both UTP and UDP, which were essentially equipotent. Thus a range of cell lines of mouse neuroblastoma origin express a pyrimidine-selective P2Y receptor. The NG108-15 cells were the only cell type tested at which ATP and ADP displayed activity with EC50 values of greater than 100 microM, compared with values of 0.58 and 1.25 microM for UTP and UDP, respectively. In contrast to the cell lines derived from mouse neuroblastoma, the human neuroblastoma lines SH-SY5Y and SK-N-SH did not respond to any nucleotides, although both responded well to carbachol.