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

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

Inhibition of G protein-coupled P2Y2 receptor induced analgesia in a rat model of trigeminal neuropathic pain

BACKGROUNDS

ATP and P2X receptors play important roles in the modulation of trigeminal neuropathic pain, while the role of G protein-coupled P2Y₂ receptors and the underlying mechanisms are less clear. The threshold and frequency of action potentials, fast inactivating transient K+ channels (IA) are important regulators of membrane excitability in sensory neurons because of its vital role in the control of the spike onset. In this study, pain behavior tests, QT-RT-PCR, immunohistochemical staining, and patch-clamp recording, were used to investigate the role of P2Y₂ receptors in pain behaviour.

RESULTS

In control rats: 1) UTP, an agonist of P2Y₂/P2Y₄ receptors, caused a significant decrease in the mean threshold intensities for evoking action potentials and a striking increase in the mean number of spikes evoked by TG neurons. 2) UTP significantly inhibited IA and the expression of Kv1.4, Kv3.4 and Kv4.2 subunits in TG neurons, which could be reversed by the P2 receptor antagonist suramin and the ERK antagonist U0126. In ION-CCI (chronic constriction injury of infraorbital nerve) rats: 1) mRNA levels of Kv1.4, Kv3.4 and Kv4.2 subunits were significantly decreased, while the protein level of phosphorylated ERK was significantly increased. 2) When blocking P2Y₂ receptors by suramin or injection of P2Y2R antisense oligodeoxynucleotides both led to a time- and dose-dependent reverse of allodynia in ION-CCI rats. 3) Injection of P2Y₂ receptor antisense oligodeoxynucleotides induced a pronounced decrease in phosphorylated ERK expression and a significant increase in Kv1.4, Kv3.4 and Kv4.2 subunit expression in trigeminal ganglia.

CONCLUSIONS

Our data suggest that inhibition of P2Y₂ receptors leads to down-regulation of ERK-mediated phosphorylation and increase of the expression of I(A)-related Kv channels in trigeminal ganglion neurons, which might contribute to the clinical treatment of trigeminal neuropathic pain.

Exocytosis of ATP from astrocytes modulates phasic and tonic inhibition in the neocortex

Astrocytes secrete ATP by exocytosis from synaptic-like vesicles, activating neuronal P2X receptors, which contribute to postsynaptic GABA receptor down-regulation, ultimately mediating the communication between astrocytes and neurons required for brain function.

Communication between neuronal and glial cells is important for many brain functions. Astrocytes can modulate synaptic strength via Ca²⁺-stimulated release of various gliotransmitters, including glutamate and ATP. A physiological role of ATP release from astrocytes was suggested by its contribution to glial Ca²⁺-waves and purinergic modulation of neuronal activity and sleep homeostasis. The mechanisms underlying release of gliotransmitters remain uncertain, and exocytosis is the most intriguing and debated pathway. We investigated release of ATP from acutely dissociated cortical astrocytes using “sniff-cell” approach and demonstrated that release is vesicular in nature and can be triggered by elevation of intracellular Ca²⁺ via metabotropic and ionotropic receptors or direct UV-uncaging. The exocytosis of ATP from neocortical astrocytes occurred in the millisecond time scale contrasting with much slower nonvesicular release of gliotransmitters via Best1 and TREK-1 channels, reported recently in hippocampus. Furthermore, we discovered that elevation of cytosolic Ca²⁺ in cortical astrocytes triggered the release of ATP that directly activated quantal purinergic currents in the pyramidal neurons. The glia-driven burst of purinergic currents in neurons was followed by significant attenuation of both synaptic and tonic inhibition. The Ca²⁺-entry through the neuronal P2X purinoreceptors led to phosphorylation-dependent down-regulation of GABAA receptors. The negative purinergic modulation of postsynaptic GABA receptors was accompanied by small presynaptic enhancement of GABA release. Glia-driven purinergic modulation of inhibitory transmission was not observed in neurons when astrocytes expressed dn-SNARE to impair exocytosis. The astrocyte-driven purinergic currents and glia-driven modulation of GABA receptors were significantly reduced in the P2X4 KO mice. Our data provide a key evidence to support the physiological importance of exocytosis of ATP from astrocytes in the neocortex.

Brain function depends on the interaction between two major types of cells: neurons transmitting electrical signals and glial cells, which control cerebral circulation and neuronal homeostasis. There is a growing evidence of the participation of astrocytes in regulating neuronal excitability and synaptic plasticity via the release of “gliotransmitters,” which include glutamate and ATP. The importance of ATP release from astrocytes was suggested by studies that demonstrated its contribution to neuronal activity and sleep homeostasis via modulation of known “purinergic” receptors. But the mechanisms underlying gliotransmitter release and the physiological significance of direct glia-to-neuron communication remain unknown and intensively debated. Here, we investigate the release of ATP from astrocytes of brain neocortex and demonstrate that astrocytes can release ATP by Ca²⁺-dependent exocytosis, most likely from synaptic-like microvesicles. We also find that vesicular release of ATP from astrocytes can directly activate excitatory signaling in the neighboring neurons, operating through purinergic P2X receptors. We saw that activation of these P2X receptors by astrocyte-driven ATP down-regulated the inhibitory synaptic signaling in the neocortical neurons. Our results imply that exocytosis of gliotransmitters is important for the communication between astrocytes and neurons in the neocortex.

Interaction of medullary P2 and glutamate receptors mediates the vasodilation in the hindlimb of rat

In the nucleus tractus solitarii (NTS) of rats, blockade of extracellular ATP breakdown to adenosine reduces arterial blood pressure (AP) increases that follow stimulation of the hypothalamic defense area (HDA). The effects of ATP on NTS P2 receptors, during stimulation of the HDA, are still unclear. The aim of this study was to determine whether activation of P2 receptors in the NTS mediates cardiovascular responses to HDA stimulation. Further investigation was taken to establish if changes in hindlimb vascular conductance (HVC) elicited by electrical stimulation of the HDA, or activation of P2 receptors in the NTS, are relayed in the rostral ventrolateral medulla (RVLM); and if those responses depend on glutamate release by ATP acting on presynaptic terminals. In anesthetized and paralyzed rats, electrical stimulation of the HDA increased AP and HVC. Blockade of P2 or glutamate receptors in the NTS, with bilateral microinjections of suramin (10 mM) or kynurenate (50 mM) reduced only the evoked increase in HVC by 75 % or more. Similar results were obtained with the blockade combining both antagonists. Blockade of P2 and glutamate receptors in the RVLM also reduced the increases in HVC to stimulation of the HDA by up to 75 %. Bilateral microinjections of kynurenate in the RVLM abolished changes in AP and HVC to injections of the P2 receptor agonist α,β-methylene ATP (20 mM) into the NTS. The findings suggest that HDA-NTS-RVLM pathways in control of HVC are mediated by activation of P2 and glutamate receptors in the brainstem in alerting-defense reactions.

Spatiotemporal and anatomical analyses of P2X receptor-mediated neuronal and glial processing of sensory signals in the rat dorsal horn

Extracellularly released adenosine triphosphate (ATP) modulates sensory signaling in the spinal cord. We analyzed the spatiotemporal profiles of P2X receptor-mediated neuronal and glial processing of sensory signals and the distribution of P2X receptor subunits in the rat dorsal horn. Voltage imaging of spinal cord slices revealed that extracellularly applied ATP (5-500 μM), which was degraded to adenosine and acting on P1 receptors, inhibited depolarizing signals and that it also enhanced long-lasting slow depolarization, which was potentiated after ATP was washed out. This post-ATP rebound potentiation was mediated by P2X receptors and was more prominent in the deep than in the superficial layer. Patch clamp recording of neurons in the superficial layer revealed long-lasting enhancement of depolarization by ATP through P2X receptors during the slow repolarization phase at a single neuron level. This depolarization pattern was different from that in voltage imaging, which reflects both neuronal and glial activities. By immunohistochemistry, P2X(1) and P2X(3) subunits were detected in neuropils in the superficial layer. The P2X(5) subunit was found in neuronal somata. The P2X(6) subunit was widely expressed in neuropils in the whole gray matter except for the dorsal superficial layer. Astrocytes expressed the P2X(7) subunit. These findings indicate that extracellular ATP is degraded into adenosine and prevents overexcitation of the sensory system, and that ATP acts on pre- and partly on postsynaptic neuronal P2X receptors and enhances synaptic transmission, predominantly in the deep layer. Astrocytes are involved in sensitization of sensory network activity more importantly in the superficial than in the deep layer.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Spinal P2X receptor modulates muscle pressor reflex via glutamate

Static contraction of skeletal muscle evokes reflex increases in blood pressure and heart rate. Previous studies showed that P2X receptors located at the dorsal horn of the spinal cord play a role in modulating the muscle pressor reflex. P2X stimulation can alter release of the excitatory amino acid, glutamate (Glu). In this report, we tested the hypothesis that stimulation of P2X receptors enhances the concentrations of Glu ([Glu]) in the dorsal horn, and that blocking P2X receptors attenuates contraction-induced Glu increases and the resultant reflex pressor response. Contraction was elicited by electrical stimulation of the L(7) and S(1) ventral roots of 14 cats. Glu samples were collected from microdialysis probes inserted in the L(7) level of the dorsal horn of the spinal cord, and dialysate [Glu] was determined using the HPLC method. First, microdialyzing alpha,beta-methylene ATP (0.4 mM) into the dorsal horn significantly increased [Glu]. In addition, contraction elevated [Glu] from baseline of 536 +/- 53 to 1,179 +/- 192 nM (P < 0.05 vs. baseline), and mean arterial pressure by 39 +/- 8 mmHg in the control experiment. Microdialyzing the P2X receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (10 mM) into the dorsal horn attenuated the contraction induced-Glu increase (610 +/- 128 to 759 +/- 147 nM; P > 0.05) and pressor response (16 +/- 3 mmHg, P < 0.05 vs. control). Our findings demonstrate that P2X modulates the cardiovascular responses to static muscle contraction by affecting the release of Glu in the dorsal horn of the spinal cord.

P2X receptors and synaptic plasticity

Adenosine triphosphate (ATP) is released in many synapses in the CNS either together with other neurotransmitters, such as glutamate and GABA, or on its own. Postsynaptic action of ATP is mediated through metabotropic P2Y and ionotropic P2X receptors abundantly expressed in neural cells. Activation of P2X receptors induces fast excitatory postsynaptic currents in synapses located in various brain regions, including medial habenula, hippocampus and cortex. P2X receptors display relatively high Ca2+ permeability and can mediate substantial Ca2+ influx at resting membrane potential. P2X receptors can dynamically interact with other neurotransmitter receptors, including N-methyl-D-aspartate (NMDA) receptors, GABA(A) receptors and nicotinic acetylcholine (ACh) receptors. Activation of P2X receptors has multiple modulatory effects on synaptic plasticity, either inhibiting or facilitating the long-term changes of synaptic strength depending on physiological context. At the same time precise mechanisms of P2X-dependent regulation of synaptic plasticity remain elusive. Further understanding of the role of P2X receptors in regulation of synaptic transmission in the CNS requires dissection of P2X-mediated effects on pre-synaptic terminals, postsynaptic membrane and glial cells.

Direct Excitation of Hypocretin/Orexin Cells by Extracellular ATP at P2X Receptors

Hypocretin/orexin (hcrt) neurons play an important role in hypothalamic arousal and energy homeostasis. ATP may be released by neurons or glia or by pathological conditions. Here we studied the effect of extracellular ATP on hypocretin cells using whole cell patch-clamp recording in hypothalamic slices of transgenic mice expressing green fluorescent protein (GFP) exclusively in hcrt-producing cells. Local application of ATP induced a dose-dependent increase in spike frequency. In the presence of TTX, ATP (100 μM) depolarized the cells by 7.8 ± 1.2 mV. In voltage clamp under blockade of synaptic activity with the GABAA receptor antagonist bicuculline, and ionotropic glutamate receptor antagonists dl-2-amino-5-phosphonopentanoic acid (AP-5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), ATP (100 μM) evoked an 18 pA inward current. The inward current was blocked by extracellular choline substitution for Na+, had a reversal potential of −27 mV, and was not affected by nominally Ca2+-free external buffer, suggesting that ATP activated a nonselective cation current. All excitatory effects of ATP showed rapid attenuation. ATP-induced excitatory actions were mimicked by nonhydrolyzable ATP-γ-S but not by α,β-MeATP and inhibited by the purinoceptor antagonists suramin and pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt (PPADS). The current was potentiated by a decrease in bath pH, suggesting P2X2 subunit involvement. Frequency and amplitude of spontaneous and miniature synaptic events were not altered by ATP. Suramin, but not PPADS, caused a small suppression of evoked excitatory synaptic potentials. Together, these results show a depolarizing response to extracellular ATP that would lead to an increased activity of the hypocretin arousal system.

Divergent effects of the purinoceptor antagonists suramin and pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors

Suramin is a large naphthyl-polysulfonate compound that inhibits an array of receptors and enzymes, and it has also been reported to block currents mediated by glutamate receptors. This study shows that suramin and several structurally related compounds [8,8'-[carbonylbis(imino-3,1-phenylenecarbonylamino)]bis-(1,3,5-naphthalenetrisulfonic acid), 6Na (NF023), 8,8'-(carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino))bis-1,3,5-naphthalenetrisulfonic acid, Na (NF279), and 4,4',4'',4'''-[carbonyl-bis[imino-5,1,3-benzenetriyl-bis-(carbonylimino)]]tetrakis-benzene-1,3-disulfonic acid, 8Na (NF449)] reduce binding of [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and [3H]fluorowillardiine to rat brain membranes and homomeric GluR1-4 receptors, with IC50 values in the range of 5 to 180 microM. Inhibition often was less than complete at saturating drug concentrations and thus seems to be noncompetitive in nature. Pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) is a potent antagonist of purinoceptors that shares some structural elements with suramin yet is smaller than the latter. PPNDS also had potent effects on AMPA receptors (EC50 value of 4 microM) but of a kind not seen with the other compounds in that it increased binding affinity for radioagonists severalfold. In addition, PPNDS slowed association and dissociation rates more than 10 times. In physiological experiments with GluR2 receptors, PPNDS at 50 microM reduced the peak current by 30 to 50% but had only small effects on other waveform aspects such desensitization and steady-state currents. This pattern of effects differentiates PPNDS from other compounds such as thiocyanate and up-modulators, which increase agonist binding by enhancing desensitization or slowing deactivation, respectively. Receptor model simulations indicate that most effects can be accounted for by assuming that PPNDS slows agonist binding/unbinding and stabilizes the bound-closed state of the receptor. By extension, suramin is proposed to stabilize the unbound state and thereby to reduce affinity for agonists. These drugs thus act through a novel type of noncompetitive antagonism.

P2 receptor-mediated effects on the open field behaviour of rats in comparison with behavioural responses induced by the stimulation of dopamine D2-like and by the blockade of ionotrophic glutamate receptors

The effects of the P2 receptor ligands 2-methylthio ATP (2-MeSATP; 10 pmol)--as a non-specific agonist--and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10 pmol)--as a non-selective antagonist--after bilateral intra-accumbens injection on the locomotor response were investigated in an open field situation. The P2 receptor-mediated effects on the pattern of locomotor activity were compared with the effects caused by the dopamine D2-like receptor agonist quinpirole (10 pmol) and by the combination of the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 10 pmol) with the alpha-amino-3-hydro-5-methyl-4-isoxazolpropionic acid (AMPA) and kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 30 pmol). The intra-accumbens injection of all tested compounds elicited an increase in the locomotor activity over a test period of 20 min when compared with the controls. No statistically significant differences could be evaluated between the different drug-treated groups. However, a more detailed analysis--using further behavioural parameters such as the number of movement direction changes, the effective running time and the running speed--revealed two basically different patterns of locomotor activity. The locomotor response induced by the injection of 2-MeSATP or quinpirole was characterised by a continuous and consistent locomotion, whereas the enhanced locomotor activity elicited by PPADS or CPP/CNQX was determined by an increased running speed accompanied by more disruptions and more changes of movement direction. The coadministration of 2-MeSATP and quinpirole led to an enhancement of locomotor activity in a limited post-treatment interval. The effects of both compounds could be abolished by the pre-treatment with the D2/D3 receptor antagonist sulpiride (100 pmol). Coadministration of PPADS and CPP/CNQX caused additive effects suggesting that the pathway mediated by P2 and ionotrophic glutamate receptors is different. The stimulation of P2 receptors in the nucleus accumbens (NAc) modulates the locomotion in the direction to be to be longer lasting, more consistent and more goal directed.

Modulation of feeding behaviour by blocking purinergic receptors in the rat nucleus accumbens: a combined microdialysis, electroencephalographic and behavioural study

The nonspecific P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), the nonspecific P1 receptor antagonist 8-(p-sulphophenyl)-theophylline (8-SPT) and the combination of both were applied by retrograde microdialysis into the nucleus accumbens (NAc) before and during feeding of 18-h food-deprived rats. In addition to the registration of behavioural parameters, such as the amount and duration of food intake, the feeding-induced changes in dopamine (DA) concentration and the concomitant changes of neuronal activity in the NAc and the ventral tegmental area (VTA) were simultaneously determined. The perfusion with PPADS (20 microm) diminished the amount of food intake and the duration of feeding. Furthermore, the P2 receptor antagonist blocked the feeding-induced DA release and prevented the feeding-elicited changes of the electroencephalography (EEG) power distribution which was characterised by an increase in the power of the 8.0-13.0-Hz frequency band in the NAc and the VTA. The effects of PPADS could be completely prevented by the concomitantly perfused adenosine receptor antagonist 8-SPT (100 microm). When given alone, 8-SPT increased the amount of food ingested, the duration of feeding and the EEG power of the higher frequency range, particularly between 19.0 and 30.0 Hz, in both the NAc and the VTA. The feeding-elicited DA release was supplemented to the enhanced DA level caused by the perfusion with 8-SPT in an additive manner. The P2 and P1 receptor antagonists interact antagonistically in the modulation of feeding behaviour and the feeding-induced changes of EEG activity suggesting that both endogenous extracellular ATP and adenosine are involved in the regulation of the feeding-associated mesolimbic neuronal activity in a functionally antagonistic manner.

Stimulation of P2Y1 receptors causes anxiolytic-like effects in the rat elevated plus-maze: implications for the involvement of P2Y1 receptor-mediated nitric oxide production

The widespread and abundant distribution of P2Y receptors in the mammalian brain suggests important functions for these receptors in the CNS. To study a possible involvement of the P2Y receptors in the regulation of fear and anxiety, the influences of the P2Y(1,11,12) receptor-specific agonist adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), the P2X(1,3) receptor agonist alpha,beta-methylene ATP (alpha,betameATP), the unspecific P2 receptor antagonist pyridoxalphosphate-6-azopheny l-2',4'-disulfonic acid (PPADS), and the specific P2Y(1) receptor antagonist N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS 2179) on the elevated plus-maze behavior of the rat were investigated. All tested compounds were given intracerebroventricularly (0.5 microl). ADPbetaS (50 and 500 fmol) produced an anxiolytic-like behavioral profile reflected by an increase of the open arm exploration. The anxiolytic-like effects were antagonized by pretreatment with PPADS (5 pmol) or MRS 2179 (5 pmol). Both compounds caused anxiogenic-like effects when given alone. Furthermore, the anxiolytic-like effects of ADPbetaS could be antagonized by pretreatment with the nitric oxide synthase (NOS) inhibitor N(w)-nitro-L-arginine methyl ester (L-NAME). In addition, the anxiogenic-like effects of PPADS were reversed by the pretreatment with L-arginine (500 pmol), which is the natural substrate for NOS, but not by D-arginine (500 pmol), which is not. Immunofluorescence staining revealed the presence of P2Y(1) receptors on neurons in different brain regions such as hypothalamus, amygdala, hippocampus and the periaqueductal gray. Furthermore, the colocalization of P2Y(1) receptors and neuronal NOS (nNOS) on some neurons in these regions could be demonstrated. The highest density of P2Y(1)- and nNOS-immunoreactivity was detected in the dorsomedial hypothalamic nucleus. Taken together, the present results suggest that P2Y(1) receptors are involved in the modulation of anxiety in the rat. The anxiolytic-like effects after stimulation of P2Y(1) receptors seem to be in close connection with the related nitric oxide production.

Molecular physiology of P2X receptors

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

Complementary role of extracellular ATP and adenosine in ischemic preconditioning in the rat heart

Although adenosine is an important mediator of ischemic preconditioning (IPC), its relative contribution to IPC remains unknown. Because adenosine is formed through the hydrolysis of ATP, the present study investigated the role of ATP and adenosine in IPC. Isolated and buffer-perfused rat hearts underwent IPC by three cycles of 5-min ischemia and 5-min reperfusion before 25 min of global ischemia. The rate-pressure product (RPP) 30 min after reperfusion was taken as an endpoint of functional protection. Interstitial fluid (ISF) adenine nucleotides and adenosine were measured by cardiac microdialysis techniques. Inhibition of IPC-induced recovery of RPP was partial by the adenosine receptor antagonist 8-(p-sulfophenyl)theophylline (SPT; 100 microM) or by the structurally distinct P2Y purinoceptor antagonists suramin (300 microM) or reactive blue (RB; 10 microM) but was additive when SPT was given with suramin or RB. The P2X antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (50 microM) had no effect on functional protection. The improved functional recovery was not significantly affected by an ecto-5'-nucleotidase inhibitor, alpha,beta-methylene adenosine diphosphate (AMP-CP; 100 microM), alone but was inhibited by AMP-CP plus SPT, suramin, or RB. ISF ATP and adenosine increased temporarily by 10-fold during IPC. AMP-CP augmented the increase in ISF ATP associated with the decrease in ISF adenosine. There was a reciprocal correlation between the ISF concentration of ATP and adenosine in preconditioned hearts. In addition, there was a significant correlation between ISF adenosine and ATP and the inhibitory potency of SPT and suramin or RB against functional protection conferred by IPC. These results suggest that extracellular ATP and adenosine play a complementary role in IPC through P2Y purinoceptors and adenosine receptors, respectively.

The anesthetic interaction between adenosine triphosphate and N-methyl-D-aspartate receptor antagonists in the rat

Modulation of synaptic neurotransmission through the ligand-gated ion channel is probably involved in the mechanisms of analgesic and anesthetic actions. In the central nervous system, adenosine triphosphate and glutamate are fast excitatory neurotransmitters through their effects on P2X and N-methyl-D-aspartate (NMDA) receptors respectively. To examine the anesthetic interaction between adenosine triphosphate and NMDA receptor antagonists, we studied the effect of intracerebroventricular administration of P2 and/or NMDA antagonists on the minimum alveolar concentration (MAC) of sevoflurane in rats. Intracerebro- ventricular administration of phosphonopentanoic acid azophenyl-2',4'-disulfonate and D (-)-2-anino-5-phophonopentanoic acid, P2 and NMDA antagonists, significantly reduced the MAC of sevoflurane. The reduction of the MAC by both phosphonopentanoic acid azophenyl-2',4'-disulfonate and D (-)-2-anino-5-phophonopentanoic acid was dose-dependent. The effect of coadministration of both antagonists was additive in the reduction of sevoflurane minimum alveolar concentration. These results suggest that P2 and NMDA receptors mediate nociceptive/anesthetic processing as inhibition of these receptors resulted in analgesic and anesthetic effects. However the pathway mediated through each receptor may be different postsynaptically and/or one of these presynaptic receptors may modulate the neurotransmitter release of the other.

Effect of P2 purinoceptor antagonists on kainate-induced currents in rat cultured neurons

The action of purinergic antagonists on kainate-induced currents was studied in rat cortical neurons in primary culture using the whole-cell configuration of the patch-clamp technique. The amplitude of the currents induced by kainate in cortical neurons was concentration-dependent (EC(50)=106 microM). Pyridoxal-phosphate-6-azophenyll-2',4'-disulphonic acid 4-sodium (PPADS), a P2X antagonist, was ineffective in the reduction of the kainate-induced current in cortical neurons, while 2, 2'-pyridylisatogen (PIT), basilen blue (BB) and suramin, respectively two selective P2Y and a non-selective P2 receptor antagonist, caused a reduction in the amplitude of the current induced by kainate. BB decreased the inward current induced by kainate at all holding potentials and the reduction was dose-dependent (EC(50)=34 microM). The total conductance of the neurons for the kainate-induced current was significantly reduced (P<0.01) and the effect was completely reversible. BB furthermore reduced the kainate-induced current in granule and hippocampal neurons and decreased the amplitude of the alpha-amino-3-hydroxy-5-methyl-4-isoxalepropionic acid (AMPA)-evoked current in cortical neurons. Cholera toxin (ChTx) did not affect the action of BB on the kainate-induced currents in cortical neurons and moreover, when guanosine 5'-o-(3-thiotriphosphate) (GTPgammaS) was added to the electrode solution, the kainate-induced currents were still reduced by 100 microM BB. The maximal response to kainate decreased in the presence of 20 microM BB without changing its EC(50), indicating a non-competitive mechanism of inhibition. These results demonstrate that preferential P2Y receptor antagonists are able to modulate the kainate and AMPA-induced currents in central neurons, suggesting a potential use of these compounds as neuroprotective agents.

Warm-coding deficits and aberrant inflammatory pain in mice lacking P2X3 receptors

ATP activates damage-sensing neurons (nociceptors) and can evoke a sensation of pain. The ATP receptor P2X3 is selectively expressed by nociceptors and is one of seven ATP-gated, cation-selective ion channels. Here we demonstrate that ablation of the P2X3 gene results in the loss of rapidly desensitizing ATP-gated cation currents in dorsal root ganglion neurons, and that the responses of nodose ganglion neurons to ATP show altered kinetics and pharmacology resulting from the loss of expression of P2X(2/3) heteromultimers. Null mutants have normal sensorimotor function. Behavioural responses to noxious mechanical and thermal stimuli are also normal, although formalin-induced pain behaviour is reduced. In contrast, deletion of the P2X3 receptor causes enhanced thermal hyperalgesia in chronic inflammation. Notably, although dorsal-horn neuronal responses to mechanical and noxious heat application are normal, P2X3-null mice are unable to code the intensity of non-noxious 'warming' stimuli.

ATP, P2X receptors and pain pathways

A role for ATP in nociception and pain induction was proposed on the basis of human psychophysical experiments shortly after the formulation of the purinergic hypothesis. Following the pharmacological definition of distinct P2X and P2Y purinergic receptor subtypes by Burnstock and his collaborators, molecular cloning studies have identified the gene products that underlie the effects of ATP on peripheral sensory neurons. One particular receptor, P2X(3), is of particular interest in the context of pain pathways, because it is relatively selectively expressed at high levels by nociceptive sensory neurons. Evidence that this receptor may play a role in the excitation of sensory neurons has recently been complemented by studies that suggest an additional presynaptic role in the regulation of glutamate release from primary afferent neurons in the dorsal horn of the spinal cord. In this brief review, we discuss the present state of knowledge of the role of ATP in pain induction through its action on peripheral P2X receptors.

P2-purinergic receptor antagonists reduce the minimum alveolar concentration of inhaled volatile anesthetics

In the central nervous system (CNS), adenosine triphosphate (ATP) is reported to serve as a fast excitatory neurotransmitter via P2X receptor. To examine possible involvement of inhibition of ATP signal-transmission in anesthetic mechanism, the effect of intracerebroventricular (ICV) administration of P2 receptor antagonists on the minimum alveolar concentration (MAC) of sevoflurane and isoflurane was studied in rat. ICV administration of P2 receptor antagonists, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), significantly reduced MAC of both anesthetics. The reduction of the MAC by both suramin and PPADS was dose-dependent and reached plateau at 150 microgram/rat. These results suggest that the inhibition of ATP-signal transmission may be involved in analgesic or anesthetic effect in brain.

Modulatory roles of the adenosine triphosphate P2x-purinoceptor in generation of the persistent nociception induced by subcutaneous bee venom injection in the conscious rat

To study the role of adenosine triphosphate (ATP) P2x-purinoceptor in the persistent nociceptive response induced by subcutaneous (s.c.) bee venom injection, we used a selective P2x receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), to evaluate whether spinal P2x receptor play a role in development of spontaneous persistent pain. Injection s.c. of bee venom into the plantar surface of one hindpaw in the conscious rat produces a monophasic, prolonged persistent nociception characterized by continuously flinching reflex of the injected paw for 1-2 h. Intrathecal (i.t.) pretreatment with PPADS at two lower doses of 5 and 10 microg resulted in suppression of the flinching reflex in a dose dependent manner with the inhibitory rate 37 and 44%, respectively, when compared with the control group; whereas i.t. PPADS at a higher dose of 30 microg failed to produce any inhibitory effect. This result suggests that activation of P2x-purinoceptor in the spinal cord contributes to the induction of bee venom-induced prolonged persistent pain. However, the antinociceptive effect of ATP P2x-purinoceptor antagonist such as PPADS on clinical pathological pain seems to be limited due to its lack of effectiveness at higher dose.

Effects of spinally administered P2X receptor agonists and antagonists on the responses of dorsal horn neurones recorded in normal, carrageenan-inflamed and neuropathic rats

1. The function and role of P2X receptors in the spinal transmission of nociception was investigated using the selective P2X receptor agonists, alpha,beta-methylene ATP (alpha,beta-me ATP) and beta, gamma-methylene-L-ATP (beta,gamma-me-L-ATP) and the P2X receptor antagonists pyridoxal-phosphate-6-azophenyl-2',4'-disulphonate (PPADS) and suramin. 2. Intrathecal administration of 5 and 50 microg of beta,gamma-me-L-ATP produced a significant facilitation of the C-fibre evoked response and a tendency towards increased excitability of the post-discharge, but not Abeta-fibre evoked response of dorsal horn neurones recorded in normal animals. Administration of similar doses of alpha,beta-me ATP did not produce an overall change in the response of the neuronal population. 3. Peripheral administration of 20 microg of these agonists into the paw of the rat evoked firing in the dorsal horn neurones. 4. Intrathecal administration of the antagonists, suramin (50 and 500 microg) and PPADS (5, 50 and 500 microg), to normal animals and to animals with a model of neuropathy induced by spinal nerve ligation did not alter the evoked neuronal responses. In contrast, intrathecal administration of 500 microg of suramin to animals 3 h after the induction of carrageenan inflammation produced a significant inhibition of the C-fibre evoked response of the neurones. Similar inhibitions were also seen following high doses of intrathecal PPADS, although this did not reach significance. 5. These results suggest that spinal P2X receptors may play a role in the modulation of spinal nociceptive transmission following the development of inflammation, but that these receptors play at most a minor role in spinal nociceptive processing in normal and neuropathic animals.