Inhibition by suramin and reactive blue 2 of GABA and glutamate receptor channels in rat hippocampal neurons

100 Years of Suramin

Suramin is 100 years old and is still being used to treat the first stage of acute human sleeping sickness, caused by Trypanosoma brucei rhodesiense Suramin is a multifunctional molecule with a wide array of potential applications, from parasitic and viral diseases to cancer, snakebite, and autism. Suramin is also an enigmatic molecule: What are its targets? How does it get into cells in the first place? Here, we provide an overview of the many different candidate targets of suramin and discuss its modes of action and routes of cellular uptake. We reason that, once the polypharmacology of suramin is understood at the molecular level, new, more specific, and less toxic molecules can be identified for the numerous potential applications of suramin.

Suramin is a novel competitive antagonist selective to α1β2γ2 GABAA over ρ1 GABAC receptors

GABA_A and GABA_C receptors are both GABA-gated chloride channels with distinct pharmacological properties, mainly in their sensitivity to bicuculline and gabazine. In this study, we found that suramin, a purinergic receptor antagonist, is a novel competitive antagonist selective to GABA_A over GABA_C receptors. Specifically, suramin antagonized the GABA-induced current and the spontaneous opening current of the wild type α1β2γ2 GABA_A receptor with high-level expression in Xenopus oocytes. The antagonism was concentration dependent with an IC₅₀ that varied depending on the concentration of GABA, and with the lowest IC₅₀ of 0.43 μM when antagonizing the spontaneous current. Thus, its potency is slightly higher than bicuculline on the same GABA_A receptor. Suramin also antagonized the mouse native brain GABA receptors micro-transplanted into the Xenopus oocytes with its potency depending on the GABA concentration. In addition, in the presence of two fixed concentrations of suramin, the GABA concentration response of the receptor was shifted to the right without reduction of the maximum current. Thus, our results are consistent with that suramin is a competitive antagonist for the α1β2γ2 GABA_A receptor. Interestingly, the rank order of maximum allosteric inhibition (efficacy) of spontaneous current of the GABA_A receptor by three competitive antagonists was suramin > bicuculline > gabazine, similar to the rank order of their molecular weight. In contrast, similar to bicuculline, suramin has much lower potency in antagonizing the GABA-induced current of the ρ1 GABA_C receptor. In conclusion, we have identified a novel GABA_A receptor competitive antagonist, which is selective to the α1β2γ2 over ρ1 GABA receptors.

Cooperation between NMDA-Type Glutamate and P2 Receptors for Neuroprotection during Stroke: Combining Astrocyte and Neuronal Protection

Excitotoxicity is the principle mechanism of acute injury during stroke. It is defined as the unregulated accumulation of excitatory neurotransmitters such as glutamate within the extracellular space, leading to over-activation of receptors, ionic disruption, cell swelling, cytotoxic Ca2+ elevation and a feed-forward loop where membrane depolarisation evokes further neurotransmitter release. Glutamate-mediated excitotoxicity is well documented in neurons and oligodendrocytes but drugs targeting glutamate excitotoxicity have failed clinically which may be due to their inability to protect astrocytes. Astrocytes make up ~50% of the brain volume and express high levels of P2 adenosine triphosphate (ATP)-receptors which have excitotoxic potential, suggesting that glutamate and ATP may mediate parallel excitotoxic cascades in neurons and astrocytes, respectively. Mono-cultures of astrocytes expressed an array of P2X and P2Y receptors can produce large rises in [Ca2+]i; mono-cultured neurons showed lower levels of functional P2 receptors. Using high-density 1:1 neuron:astrocyte co-cultures, ischemia (modelled as oxygen-glucose deprivation: OGD) evoked a rise in extracellular ATP, while P2 blockers were highly protective of both cell types. GluR blockers were only protective of neurons. Neither astrocyte nor neuronal mono-cultures showed significant ATP release during OGD, showing that cell type interactions are required for ischemic release. P2 blockers were also protective in normal-density co-cultures, while low doses of combined P2/GluR blockers where highly protective. These results highlight the potential of combined P2/GluR block for protection of neurons and glia.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

Neuromodulation: purinergic signaling in respiratory control

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.

Potentiation of inhibitory synaptic transmission by extracellular ATP in rat suprachiasmatic nuclei

The hypothalamic suprachiasmatic nuclei (SCN), the circadian master clock in mammals, releases ATP in a rhythm, but the role of extracellular ATP in the SCN is still unknown. In this study, we examined the expression and function of ATP-gated P2X receptors (P2XRs) in the SCN neurons of slices isolated from the brain of 16- to 20-day-old rats. Quantitative RT-PCR showed that the SCN contains mRNA for P2X 1-7 receptors and several G-protein-coupled P2Y receptors. Among the P2XR subunits, the P2X2 > P2X7 > P2X4 mRNAs were the most abundant. Whole-cell patch-clamp recordings from SCN neurons revealed that extracellular ATP application increased the frequency of spontaneous GABAergic IPSCs without changes in their amplitudes. The effect of ATP appears to be mediated by presynaptic P2X2Rs because ATPγS and 2MeS-ATP mimics, while the P2XR antagonist PPADS blocks, the observed enhancement of the frequency of GABA currents. There were significant differences between two SCN regions in that the effect of ATP was higher in the ventrolateral subdivision, which is densely innervated from outside the SCN. Little evidence was found for the presence of P2XR channels in somata of SCN neurons as P2X2R immunoreactivity colocalized with synapsin and ATP-induced current was observed in only 7% of cells. In fura-2 AM-loaded slices, BzATP as well as ADP stimulated intracellular Ca(2+) increase, indicating that the SCN cells express functional P2X7 and P2Y receptors. Our data suggest that ATP activates presynaptic P2X2Rs to regulate inhibitory synaptic transmission within the SCN and that this effect varies between regions.

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.

Purinergic systems, neuropathic pain and the role of microglia

We have learned various data on the role of purinoceptors (P2X4, P2X7, P2Y6 and P2Y12) expressed in spinal microglia and several factors that presumably activate microglia in neuropathic pain after peripheral nerve injury. Purinergic receptor-mediated spinal microglial functions make a critical contribution to pathologically enhanced pain processing in the dorsal horn. Microglial purinoceptors might be promising targets for treating neuropathic pain. A predicted therapeutic benefit of interfering with microglial purinergic receptors may be that normal pain sensitivity would be unaffected since expression or activity of most of these receptors are upregulated or enhanced predominantly in activated microglia in the spinal cord where damaged sensory fibers project.

P2 receptors are involved in the mediation of motivation-related behavior

The importance of purinergic signaling in the intact mesolimbic–mesocortical circuit of the brain of freely moving rats is reviewed. In the rat, an endogenous ADP/ATPergic tone reinforces the release of dopamine from the axon terminals in the nucleus accumbens as well as from the somatodendritic region of these neurons in the ventral tegmental area, as well as the release of glutamate, probably via P2Y₁ receptor stimulation. Similar mechanisms may regulate the release of glutamate in both areas of the brain. Dopamine and glutamate determine in concert the activity of the accumbal GABAergic, medium-size spiny neurons thought to act as an interface between the limbic cortex and the extrapyramidal motor system. These neurons project to the pallidal and mesencephalic areas, thereby mediating the behavioral reaction of the animal in response to a motivation-related stimulus. There is evidence that extracellular ADP/ATP promotes goal-directed behavior, e.g., intention and feeding, via dopamine, probably via P2Y₁ receptor stimulation. Accumbal P2 receptor-mediated glutamatergic mechanisms seem to counteract the dopaminergic effects on behavior. Furthermore, adaptive changes of motivation-related behavior, e.g., by chronic succession of starvation and feeding or by repeated amphetamine administration, are accompanied by changes in the expression of the P2Y₁ receptor, thought to modulate the sensitivity of the animal to respond to certain stimuli.

Pain and purinergic signaling

A growing body of evidence indicates that extracellular nucleotides play important roles in the regulation of neuronal and glial functions in the nervous system through P2 purinoceptors. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (seven types; P2X1-P2X7) contain intrinsic pores that open by binding with ATP, and P2Y receptors (eight types; P2Y1, 2, 4, 6, 11, 12, 13 and 14) are activated by nucleotides and couple to intracellular second-messenger systems through heterotrimeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. Studies have shown that microglia, a type of glial cells known as resident macrophages in the CNS, express several subtypes of P2X and P2Y receptors, and these receptors play a key role in pain signaling in the spinal cord under pathological conditions such as by peripheral nerve injury (called neuropathic pain). Within the spinal dorsal horn, peripheral nerve injury leads to a progressive series of changes in microglia including morphological hypertrophy of the cell body and proliferation, which are considered indicative of activation. These activated microglia upregulate expression of P2X/Y receptors (e.g., P2X4 and P2Y12). Importantly, pharmacological, molecular and genetic manipulations of the function or expression of these microglial molecules strongly suppress neuropathic pain. We expect that further investigation to determine how ATP signaling via P2X receptors participates in the pathogenesis of chronic pain will lead to a better understanding of the molecular mechanisms of pathological pain and provide clues for the development of new therapeutic drugs.

P2X purinoceptors mediate an endothelium-dependent hyperpolarization in longitudinal smooth muscle of anterior mesenteric artery in young chickens

BACKGROUND AND PURPOSE

The chicken anterior mesenteric artery contains an outer longitudinal smooth muscle layer, whose neural regulation remains to be elucidated. ATP evokes a depolarization in the smooth muscle through P2Y purinoceptors. However, there may be an additional inhibitory regulation because blockade of P2Y purinoceptors converts the depolarization to hyperpolarization. The objective of the present study was to examine the mechanism underlying this hyperpolarization.

EXPERIMENTAL APPROACH

Membrane potentials of longitudinal smooth muscle of the chicken mesenteric artery were recorded with a microelectrode technique. Perivascular nerves were stimulated by applying electrical field stimulation (EFS).

KEY RESULTS

EFS induced a hyperpolarization in preparations obtained from 5-week-old chickens, whereas it evoked a depolarization in those from 12-week-old chickens. The EFS-evoked hyperpolarization in 5-week-old chickens was blocked by a non-specific purinoceptor antagonist, suramin, and by a specific P2X purinoceptor antagonist, pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid. Desensitization of the P2X purinoceptor with its agonist alpha,beta-MeATP significantly suppressed EFS-evoked hyperpolarization. Blockade of the P2Y purinoceptor did not affect EFS-evoked hyperpolarization. The application of the NOS inhibitor Nomega-nitro-L-arginine methyl ester or the removal of the endothelium inhibited the hyperpolarization. The application of the nitric oxide (NO) donor sodium nitroprusside mimicked the hyperpolarization. Reverse transcriptase-PCR showed that P2X purinoceptors are expressed in the endothelium of the anterior mesenteric artery.

CONCLUSIONS AND IMPLICATIONS

Hyperpolarization in the longitudinal smooth muscle of the chicken anterior mesenteric artery was induced by ATP. ATP released from perivascular nerves may act on P2X purinoceptors in the endothelium and thereby stimulate NO production.

Endogenous signals initiating inflammation in the injured nervous system

Glial cells are known to respond to a variety of neural injuries and play an important role in tissue damage and repair in the injured nervous system. This glial response, which is initially characterized by the expression of proinflammatory cytokines and chemokines and the attraction of microglial cells toward sites of injury, literally occurs within seconds to minutes of the injury. This suggests that signals that are endogenous to the nervous system are responsible for initiating neuroinflammation. In this review, we summarize the most recent advances made in the identification of these endogenous signals and describe the receptors and signaling pathways by which these ligands stimulate the production of cytokines and chemokines. Among these endogenous damage signals are ligands for toll-like receptors, including several heat shock proteins and extracellular matrix components, as well as self-derived RNA and DNA and associated proteins. Growing evidence also suggests that nucleotides released upon injury and acting through P2 receptors, such as ATP and UTP or their analogues, could serve as endogenous signals for the rapid response of glial cells.

Purinergic type 2 receptors at GABAergic synapses on ventral tegmental area dopamine neurons are targets for ethanol action

The current study investigated whether ethanol alters ATP activation of purinergic type 2 receptors (P2Rs) in the ventral tegmental area (VTA). The VTA is a key region of the brain that has been implicated in the development of alcohol addiction. We investigated the effects of ATP and ethanol on spontaneous inhibitory postsynaptic currents (sIPSCs) and the spontaneous firings in the VTA dopaminergic neurons, obtained using an enzyme-free procedure. These neurons preserved some functional GABA-releasing terminals after isolation. We found that ATP (1-200 microM) either increased or decreased the frequency of sIPSCs and the activity of VTA dopaminergic neurons. The effects of ATP on sIPSC frequency inversely correlated with its effects on dopaminergic neuron activity. The ATP-induced changes in sIPSC frequency were blocked by tetrodotoxin (a sodium channel blocker) and by suramin (a nonselective P2R antagonist). Furthermore, alpha,beta-methylene ATP, a selective P2X(1) and P2X(3) receptor agonist, increased sIPSC frequency, whereas adenosine 5'-[beta-thio]diphosphate, a preferential agonist of P2Y receptors, decreased sIPSC frequency. In experiments testing the effects of ethanol (10 and 40 mM) on sIPSCs, we found that ethanol significantly attenuated ATP-induced increase and enhanced ATP-induced decrease in sIPSC frequency. Taken together, the results demonstrate that multiple subtypes of P2Rs exist on GABA-releasing terminals that make synapses on VTA dopaminergic neurons. It seems that ATP increases sIPSC frequency involving P2X(1) and/or P2X(3) receptors, and ATP decreases sIPSC frequency involving P2YRs. These findings are also consistent with the notion that P2Rs at GABA-releasing terminals on VTA dopaminergic neurons are important targets for ethanol action.

Role of P2 purinergic receptors in synaptic transmission under normoxic and ischaemic conditions in the CA1 region of rat hippocampal slices

The role of ATP and its stable analogue ATPγS [adenosine-5′-o-(3-thio)triphosphate] was studied in rat hippocampal neurotransmission under normoxic conditions and during oxygen and glucose deprivation (OGD). Field excitatory postsynaptic potentials (fEPSPs) from the dendritic layer or population spikes (PSs) from the soma were extracellularly recorded in the CA1 area of the rat hippocampus. Exogenous application of ATP or ATPγS reduced fEPSP and PS amplitudes. In both cases the inhibitory effect was blocked by the selective A₁ adenosine receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) and was potentiated by different ecto-ATPase inhibitors: ARL 67156 (6-N,N-diethyl-D-β,γ-dibromomethylene), BGO 136 (1-hydroxynaphthalene-3,6-disulfonate) and PV4 [hexapotassium dihydrogen monotitanoundecatungstocobaltate(II) tridecahydrate, K₆H₂[TiW₁₁CoO₄₀]·13H₂O]. ATPγS-mediated inhibition was reduced by the P2 antagonist suramin [8-(3-benzamido-4-methylbenzamido)naphthalene-1,3,5-trisulfonate] at the somatic level and by other P2 blockers, PPADS (pyridoxalphosphate-6-azophenyl-2′,4′-disulfonate) and MRS 2179 (2′-deoxy-N⁶-methyladenosine 3′,5′-bisphosphate), at the dendritic level. After removal of both P2 agonists, a persistent increase in evoked synaptic responses was recorded both at the dendritic and somatic levels. This effect was prevented in the presence of different P2 antagonists. A 7-min OGD induced tissue anoxic depolarization and was invariably followed by irreversible loss of fEPSP. PPADS, suramin, MRS2179 or BBG (brilliant blue G) significantly prevented the irreversible failure of neurotransmission induced by 7-min OGD. Furthermore, in the presence of these P2 antagonists, the development of anoxic depolarization was blocked or significantly delayed. Our results indicate that P2 receptors modulate CA1 synaptic transmission under normoxic conditions by eliciting both inhibitory and excitatory effects. In the same brain region, P2 receptor stimulation plays a deleterious role during a severe OGD insult.

ATP induces sustained facilitation of craniofacial nociception through P2X receptors on neck muscle nociceptors in mice

Noxious input from neck muscles probably plays a key role in tension-type headache pathophysiology. ATP selectively excites group III and IV muscle afferents in vitro. Accordingly, ATP infusion into trapezius muscle induces strong pain and local tenderness in healthy man. The present study addresses the impact of ATP on neck muscle nociception in anaesthetized mice. Craniofacial nociceptive processing was tested by the jaw-opening reflex via noxious electrical tongue stimulation. Within 2 h after injection of 100 nmol/l or 1 micromol/l ATP into semispinal neck muscles, reflex integrals significantly increased by 114% or 328%, respectively. Preceding intramuscular administration of the P2X receptor antagonist PPADS (3-100 nmol/l) suppressed the ATP effect. Subsequent application of PPADS (100 nmol/l) caused a total recovery of facilitated reflex to baseline values. ATP induces sustained facilitation of craniofacial nociception by prolonged excitation of P2X receptors in neck muscles.

Long-term depression is not modulated by ATP receptors in the rat CA1 hippocampal region

ATP is an important extracellular messenger in the CNS. In the hippocampus, a brain structure relevant for learning and memory processes, it acts both as a modulator and as a mediator of synaptic transmission, with implications for synaptic plasticity phenomena. Recent evidence suggests that ATP modulates activity-dependent long-term potentiation (LTP) of Schaffer collateral-CA1 synapses. However, it remains unclear if ATP also modulates LTP counterpart's phenomenon, long-term depression (LTD), in the rat hippocampus. This study investigated the effect of ATP analogues on homosynaptic LTD, induced by low-frequency stimulation of the Schaffer collaterals (1 Hz; 900 pulses) in the CA1 region of young rat hippocampal slices. The metabolically stable ATP analogues beta,gamma-ImATP (20 microM), a P2 receptor agonist, and alpha,beta-MeATP (20 microM), a preferential P2X(1,3) receptor agonist, did not modify LTD (LTD values of 14.7+/-0.5% and 14.1+/-3% for aCSF controls and of 15.1+/-4% and 19.0+/-5.2% for beta,gamma-ImATP and alpha,beta-MeATP, respectively). The ATP analogue beta,gamma-ImATP (20 microM) did not modify LTD also in the presence of the adenosine A1 receptor antagonist DPCPX (50 nM) (21.5+/-4.2% for DPCPX only and of 23.8+/-8.9% for DPCPX plus beta,gamma-ImATP). Finally, the preferential P2X(1,3) receptor antagonist NF023 (10 microM) had also no effect on LTD (18.6+/-5.2% for aCSF and of 18.7+/-5.2% for NF023). The present results suggest that ATP does not modulate activity-dependent homosynaptic LTD in the rat CA1 hippocampal region by activating P2 receptors.

Reactive blue prevented caffeine-induced neurotoxicity by an independent mechanism from intracellular calcium currents in cell culture from auditory cortex of rats

Neurotoxicity induced by caffeine in auditory-neuron cultures was studied in rat pups. For possible protective effect, reactive blue (RB) alone and in combination with dantrolene were tested in subsequent doses. RB was found to have a U-shape neuroprotective effect in caffeine neurotoxicity. Dantrolene was also tested in combined application in caffeine neurotoxicity. Despite the existing neuroprotection, no additional protection was obtained with various doses of dantrolene. In conclusion, RB may exert neuroprotective effect by increasing intracellular ATP levels in caffeine toxicity. High ATP levels may postpone the toxic cascade. Dantrolene as an endoplasmic reticulum calcium release blocker had no additional protective effect, suggesting that the increased intracellular calcium levels may be involved in later states of the toxic cascade, occurring after the compensatory phase of the cell death.

P2X7-like receptor subunits enhance excitatory synaptic transmission at central synapses by presynaptic mechanisms

Recent studies demonstrate that P2X7 receptor subunits (P2X7RS) are present at central and peripheral synapses and suggest that P2X7RS can regulate transmitter release. In brainstem slices from 15 to 26 day old pentobarbitone-anesthetized mice, we examined the effect of P2X7RS activation on excitatory postsynaptic currents (EPSCs) recorded from hypoglossal motoneurons using whole-cell patch clamp techniques. After blockade of most P2X receptors with suramin (which is inactive at P2X7RS) and of adenosine receptors with 8-phenyltheophylline (8PT), bath application of the P2X receptor agonist 3'-0-(4-benzoyl)ATP (BzATP) elicited a 40.5+/-16.0% (mean+/-S.E.M., n = 8, P = 0.039) increase in evoked EPSC amplitude and significantly reduced paired pulse facilitation of evoked EPSCs. This response to BzATP (with suramin and 8PT present) was completely blocked by prior application of Brilliant Blue G (200 nM or 2 microM), a P2X7RS antagonist. In contrast, BzATP application with suramin and 8PT present did not alter miniature EPSC frequency or amplitude when action potentials were blocked with tetrodotoxin. These electrophysiological results suggest that P2X7RS activation increases central excitatory transmitter release via presynaptic mechanisms, confirming previous indirect measures of enhanced transmitter release. We suggest that possible presynaptic mechanisms underlying enhancement of evoked transmitter release by P2X7RS activation are modulation of action potential width or an increase in presynaptic terminal excitability, due to subthreshold membrane depolarization which increases the number of terminals releasing transmitter in response to stimulation.

P2 receptors modulate respiratory rhythm but do not contribute to central CO2 sensitivity in vitro

Multiple brainstem sites are proposed to contribute to central respiratory chemosensitivity, however, the underlying molecular mechanisms remain unknown. P2X2 subunit-containing ATP receptors, which mediate pH-sensitive currents, appear to contribute to central chemosensitivity in vivo [J. Physiol. 523 (2000) 441]. However, recent data from P2X2 knockout mice [J. Neurosci. 23 (2003) 11315] indicate that they are not essential. To further explore the role of P2 receptors in central chemosensitivity, we examined the effects of P2 receptor agonists/antagonists on respiratory-related activity and CO2-sensitivity of rhythmically-active in vitro preparations from neonatal rat. Our main findings: (i) that putative chemosensitive regions of the ventrolateral medulla are immunoreactive for the P2X2 subunit; (ii) that ATP potentiates respiratory frequency in a dose-dependent, and PPADS-sensitive (P2 receptor antagonist), manner; and (iii) that the increase in burst frequency produced by increasing CO2 is unaffected by PPADS, indicate that ATP is a potent modulator of respiratory activity, but that P2 receptors do not contribute to central chemosensitivity in vitro.

Purinergic P2 receptors trigger adenosine release leading to adenosine A2A receptor activation and facilitation of long-term potentiation in rat hippocampal slices

Electrophysiological recordings were used to investigate the effects of ATP analogues on theta-burst-induced long-term potentiation (LTP) in rat hippocampal slices. alpha,beta-Methylene ATP (alpha,beta-MeATP; 20 microM) decreased LTP from 36+/-9% to 17+/-5%, an effect prevented by adenosine A(1) receptor blockade in accordance with the localised catabolism of ATP analogues into adenosine, leading to adenosine A(1) receptor activation. Thus, to probe the role of extracellular ATP, all experiments were performed with the A(1) receptor selective antagonist, 1,3-dipropyl-8-cyclopentylxanthine (50 nM). In these conditions, alpha,beta-MeATP or 5'-adenylylimido-diphosphate (beta,gamma-ImATP; 20 microM) facilitated LTP by 120%, an effect prevented by the P2 receptor antagonists, pyridoxalphosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS; 20 microM) or suramin (75 microM), as well as by the P2X(1/3)-selective antagonist 8-(benzamido)naphthalene-1,3,5-trisulfonate (10 microM). The facilitations of LTP by either alpha,beta-MeATP or beta,gamma-ImATP (20 microM) were also prevented by both 4-(2-[7-amino-2-(2-furyl(1,2,4)-triazolo(2,3a)-(1,3,5)triazin-5-yl-amino]ethyl)phenol (50 nM) or 7-2(-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine (50 nM), antagonists of facilitatory adenosine A(2A) receptors, were occluded by the A(2A) receptor agonist, CGS 21680 (10 nM) and were prevented by the protein kinase C inhibitor, chelerythrine (6 microM) and unaffected by the protein kinase A inhibitor, H89 (1 microM). Furthermore, beta,gamma-ImATP (20 microM) enhanced [(3)H]adenosine outflow from rat hippocampal slices by nearly 150%, an effect prevented by PPADS (20 microM) or suramin (75 microM). The adenosine transport inhibitors, nitrobenzylthioinosine (5 microM) and dipyridamole (10 microM) also prevented beta,gamma-ImATP (20 microM)-induced [(3)H]adenosine outflow and facilitation of LTP. These results suggest that ATP analogues facilitate LTP through P2 receptor activation that mainly triggers adenosine release leading to the activation of adenosine A(2A) receptors.

ATP released by astrocytes mediates glutamatergic activity-dependent heterosynaptic suppression

Extracellular ATP released from axons is known to assist activity-dependent signaling between neurons and Schwann cells in the peripheral nervous system. Here we report that ATP released from astrocytes as a result of neuronal activity can also modulate central synaptic transmission. In cultures of hippocampal neurons, endogenously released ATP tonically suppresses glutamatergic synapses via presynaptic P2Y receptors, an effect that depends on the presence of cocultured astrocytes. Glutamate release accompanying neuronal activity also activates non-NMDA receptors of nearby astrocytes and triggers ATP release from these cells, which in turn causes homo- and heterosynaptic suppression. In CA1 pyramidal neurons of hippocampal slices, a similar synaptic suppression was also produced by adenosine, an immediate degradation product of ATP released by glial cells. Thus, neuron-glia crosstalk may participate in activity-dependent synaptic modulation.

Purinergic signalling in the medullary mechanisms of respiratory control in the rat: respiratory neurones express the P2X2 receptor subunit

ATP is involved in central respiratory control and may mediate changes in the activity of medullary respiratory neurones during hypercapnia, thus playing an important role in central chemoreception. The main objective of this study was to explore further the role of ATP-mediated signalling in respiratory control and central chemoreception by characterising the profile of the P2X receptors expressed by physiologically identified respiratory neurones. In particular we determined whether respiratory neurones in the rostral ventrolateral medulla (VLM) express P2X2 receptor subunits of the ATP-gated ion channel, since ATP currents evoked at recombinant P2X2 receptors are potentiated by lowering extracellular pH. Experiments were performed on anaesthetised (pentobarbitone sodium 60 mg kg-1 I.P., then 10 mg kg-1 I.V. as required), gallamine-triethiodide-treated (10 mg kg-1 I.V., then 2-4 mg kg-1 h-1 I.V.) and artificially ventilated rats. The VLM respiratory neurones were classified according to the timing of their discharge pattern in relation to that of the phrenic nerve and by the exclusion of pump cells from the study population; these were labelled with Neurobiotin using the juxtacellular method, and visualised with fluorescence microscopy. It was found that a substantial proportion of the VLM respiratory neurones express the P2X2 receptor subunit. P2X2 receptor subunit immunoreactivity was detected in approximately 50 % (six out of 12) of expiratory neurones and in approximately 20 % (two out of 11) of neurones with inspiratory-related discharge (pre-inspiratory and inspiratory). In contrast, no Neurobiotin-labelled VLM respiratory neurones (n = 19) were detectably immunoreactive for the P2X1 receptor subunit. Microionophoretic application of ATP (0.2 M, 20-80 nA for 40 s) increased the activity of approximately 80 % (13 out of 16) of expiratory neurones and of approximately 30 % (five out of 18) of VLM neurones with inspiratory-related discharge. These effects were abolished by the P2 receptor blocker suramin (0.02 M, 80 nA), which also reduced the baseline firing in some expiratory neurones. These data indicate that modulation of P2X2 receptor function, such as that evoked by acidification of the extracellular environment during hypercapnia, may contribute to the changes in activity of the VLM respiratory neurones that express these receptors.

Suramin inhibits beta-bungarotoxin-induced activation of N-methyl-D-aspartate receptors and cytotoxicity in primary neurons

We demonstrated that beta-bungarotoxin (beta-BuTX), a snake presynaptic neurotoxin, exhibited a potent cytotoxic effect on cultured cerebellar granule neurons. The mechanism of action of beta-BuTX and the cytoprotective agents against beta-BuTX were studied. The neuronal death of cerebellar granule neurons induced by beta-BuTX was manifested with apoptosis and necrosis processes as revealed by neurite fragmentation, morphological alterations, and staining apoptotic bodies with the fluorescent dye Hoechst 33258. By means of microspectrofluorimetry and fura-2, we measured intracellular Ca2+ concentration, [Ca2+]i and found that [Ca2+]i was increased markedly prior to the morphological changes and cytotoxicity. The downstream pathway of the increased [Ca2+]i was investigated: there was increased production of free radicals, decreased mitochondrial membrane potential, and depleted cellular ATP content. MK801 and suramin effectively suppressed these detrimental effects of beta-BuTX. Furthermore, the [3H]MK801 binding was reduced by unlabeled MK801, beta-BuTX, and suramin. Thus, activation of N-methyl-D-aspartate (NMDA) receptors appeared to play a crucial role in the cytotoxic effects following betaBuTX exposure. In conclusion, the novel finding of this study was that a polypeptide beta-BuTX exerted a potent cytotoxic effect through sequential events, including activating NMDA receptors followed by increasing [Ca2+]i, ROS production, and impaired mitochondrial energy metabolism. Suramin, clinically used as a trypanocidal agent, was an effective antagonist against beta-BuTX. Data suggest that suramin might have value to detect the possible pathway of certain neuropathological disorders.

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.

Involvement of P2X7 receptors in the regulation of neurotransmitter release in the rat hippocampus

Although originally cloned from rat brain, the P2X7 receptor has only recently been localized in neurones, and functional responses mediated by these neuronal P2X7 receptors (P2X7 R) are largely unknown. Here we studied the effect of P2X7 R activation on the release of neurotransmitters from superfused rat hippocampal slices. ATP (1-30 mm) and other ATP analogues elicited concentration-dependent [3 H]GABA outflow, with the following rank order of potency: benzoylbenzoylATP (BzATP) > ATP > ADP. PPADS, the non-selective P2-receptor antagonist (3-30 microm), Brilliant blue G (1-100 nm) the P2X7 -selective antagonist and Zn2+ (0.1-30 microm) inhibited, whereas lack of Mg2+ potentiated the response by ATP. In situ hybridization revealed that P2X7 R mRNA is expressed in the neurones of the cell body layers in the hippocampus. P2X7 R immunoreactivity was found in excitatory synaptic terminals in CA1 and CA3 region targeting the dendrites of pyramidal cells and parvalbumin labelled structures. ATP (3-30 microm) and BzATP (0.6-6 microm) elicited concentration-dependent [14 C]glutamate efflux, and blockade of the kainate receptor-mediated transmission by CNQX (10-100 microm) and gadolinium (100 microm), decreased ATP evoked [3 H]GABA efflux. The Na+ channel blocker TTX (1 microm), low temperature (12 degrees C), and the GABA uptake blocker nipecotic acid (1 mm) prevented ATP-induced [3 H]GABA efflux. Brilliant blue G and PPADS also reduced electrical field stimulation-induced [3 H]GABA efflux. In conclusion, P2X7 Rs are localized to the excitatory terminals in the hippocampus, and their activation regulates the release of glutamate and GABA from themselves and from their target cells.

Modulation of phrenic motoneuron excitability by ATP: consequences for respiratory-related output in vitro

On the basis of the high level of P2X receptor expression found in phrenic motoneurons (MN) in rats (Kanjhan et al., J Comp Neurol 407: 11-32, 1999) and potentiation of hypoglossal MN inspiratory activity by ATP (Funk et al., J Neurosci 17: 6325-6337, 1997), we tested the hypothesis that ATP receptor activation also modulates phrenic MN activity. This question was examined in rhythmically active brain stem-spinal cord preparations from neonatal rats by monitoring effects of ATP on the activity of spinal C4 nerve roots and phrenic MNs. ATP produced a rapid-onset, dose-dependent, suramin- and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium-sensitive increase in C4 root tonic discharge and a 22 +/- 7% potentiation of inspiratory burst amplitude. This was followed by a slower, 10 +/- 5% reduction in burst amplitude. ATPgammaS, the hydrolysis-resistant analog, evoked only the excitatory response. ATP induced inward currents (57 +/- 39 pA) and increased repetitive firing of phrenic MNs. These data, combined with persistence of ATP currents in TTX and immunolabeling for P2X2 receptors in Fluoro-Gold-labeled C4 MNs, implicate postsynaptic P2 receptors in the excitation. Inspiratory synaptic currents, however, were inhibited by ATP. This inhibition differed from that seen in root recordings; it did not follow an excitation, had a faster onset, and was induced by ATPgammaS. Thus ATP inhibited activity through at least two mechanisms: 1) a rapid P2 receptor-mediated inhibition and 2) a delayed P1 receptor-mediated inhibition associated with hydrolysis of ATP to adenosine. The complex effects of ATP on phrenic MNs highlight the importance of ATP as a modulator of central motor outflows.

Evidence for the involvement of purinergic signalling in the control of respiration

The ventrolateral medulla has a critical role in the generation and patterning of respiration via an extensive network of respiratory neurones. We have investigated the effects of activating purinergic P2 receptors within the ventrolateral medulla of the anaesthetised rat on the overall pattern of respiratory activity. In addition, using immunohistochemical techniques, we have identified the subtypes of P2X receptors in the ventrolateral medulla. Unilateral microinjection of ATP into the ventrolateral medulla reduced in a dose-dependent manner, or abolished, resting phrenic nerve discharge recorded as an indication of central inspiratory drive. ATP also elicited increases in blood pressure and variable changes in heart rate. These effects were mimicked by microinjection of the P2X receptor agonist alpha,beta-methylene ATP into the ventrolateral medulla. Whilst microinjection of suramin, a P2 receptor antagonist, had no effect on resting cardiorespiratory variables it blocked the respiratory and cardiovascular effects of ATP microinjected into the ventrolateral medulla. Immunohistochemical staining using IgG antibodies showed that P2X1, P2X2, P2X5 and P2X6, but not P2X3, P2X4 or receptor subunits were localised in the rostral ventrolateral medulla.Our results indicate that several P2X receptor subtypes are localised within areas of the ventrolateral medulla that are important for cardiorespiratory control (including the pre-Bötzinger and Bötzinger complexes), and that activation of these receptors can have profound effects on both the cardiovascular and the respiratory networks. Our pharmacological data suggest that different P2X subunits in this region may co-assemble to form hetero-oligomeric assemblies as well as homomultimers within this region.

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.

Fast synaptic transmission mediated by P2X receptors in CA3 pyramidal cells of rat hippocampal slice cultures

1. A fast ATP-mediated synaptic current was identified in CA3 pyramidal cells in organotypic hippocampal slice cultures. In the presence of inhibitors for ionotropic glutamate and GABA receptors, extracellular stimulation in the pyramidal cell layer evoked fast synaptic currents that reversed near 0 mV, reflecting an increase in a non-selective cationic conductance. This response was mimicked by focal application of ATP. Antagonists of ionotropic P2X receptors reduced both synaptic and ATP-induced currents. 2. Using a pharmacological approach, the source of synaptically released ATP was determined. Synaptic ATP responses were insensitive to presynaptic blockade of GABAergic transmission between interneurons and CA3 pyramidal cells with the mu-opioid receptor agonist D-Ala(2),MePhe(4),Met(O)(5)-ol-enkephalin (FK33-824), but were blocked by adenosine, which inhibits glutamate release from synaptic terminals in the hippocampus. However, selective inhibition of mossy fibre glutamatergic transmission with the metabotropic glutamate receptor group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) did not affect the response. This result points to the associational fibres as the source of the ATP-mediated synaptic response. 3. These results suggest that ATP, coreleased with glutamate, induces a synaptic response in CA3 pyramidal cells that is observed mainly under conditions of synchronous discharge from multiple presynaptic inputs.

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.

Protective effect of suramin against cell death in rat cerebellar granular neurons and mouse neuroblastoma cells

In this report, we examined the effect of suramin on the cell death induced by glutamate in cerebellar granule neurons or by staurosporine in NB-2a neuroblastoma cells. Excitotoxicity induced by glutamate was associated with an extensive chromatin condensation, nuclear fragmentation and disintegration of nuclear DNA into the high molecular weight (HMW)-DNA fragments of about 50-100 kb without formation of an oligonucleosomal DNA ladder or caspase activation. Staurosporine-induced cytotoxicity was accompanied by the activation of caspase 3-like protease(s) and disintegration of nuclear DNA into the HMW- and oligonucleosomal DNA fragments. Suramin (100 microM) effectively protected both cultured cerebellar neurons and NB-2a cells against cell death, which appeared as the inhibition of caspase 3-like activity in NB-2a cells, abrogation of both HMW- and internucleosomal DNA fragmentation and maintaining the nuclear morphology indistinguishable of the control cells. Eventually, suramin lead to the marked increase in the cell viability of both cerebellar neurons and NB-2a neuroblastoma cells challenged with glutamate and staurosporine, respectively. We suggest that the novel, neuroprotective activity of suramin may have a therapeutic value in several neuropathological paradigms.

Regulation of the development of allodynia by intrathecally administered P2 purinoceptor agonists and antagonists in mice

Effects of agonists and antagonists of P2X-purinoceptors on the regulation of the development of allodynia were examined in mice; the drugs were administered intrathecally to the spinal cord. Suramin (5, 10 microg) and pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS), antagonists of P2X receptors, inhibited prostaglandin (PG) E(2)-induced allodynia. PPADS did not block glutamate-induced allodynia. alpha,beta-Methylene ATP (alpha, beta-meATP), an agonist of P2X receptor, elicited allodynia. alpha, beta-me ATP-induced allodynia was blocked by co-administration of alpha,beta-meATP with PPADS, MK 801 or N(omega)-nitro-L-arginine methyl ester (L-NAME). Suramin at higher doses (20, 40 microg) induced allodynia, which was inhibited by MK 801 or L-NAME. These results suggest that ATP P2X receptors in the spinal cord are involved in the regulation of tactile allodynia. Glutamate receptor and nitric oxide systems play an important role in the development of allodynia produced by alpha,beta-meATP and suramin.

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.

Novel mechanism of inhibition by the P2 receptor antagonist PPADS of ATP-activated current in dorsal root ganglion neurons

The antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) has been proposed to selectively antagonize the actions of ATP at P2X receptors. Whole cell patch-clamp recording techniques therefore were used to characterize PPADS inhibition of ATP-activated current in bullfrog dorsal root ganglion (DRG) neurons. PPADS, 0.5-10 microM, inhibited ATP-activated current in a concentration-dependent manner with an IC(50) of 2.5 +/- 0.03 microM. PPADS produced a gradual decline of ATP-activated current to a steady state, but this was not an indication of use dependence as the gradual declining component could be eliminated by exposure to PPADS before ATP application. In addition, ATP-activated current recovered completely from inhibition by PPADS in the absence of agonist. The slow onset of inhibition by PPADS was not apparently due to an action at an intracellular site as inclusion of 10 microM PPADS in the recording pipette neither affected the ATP response nor did it alter inhibition of the ATP response when 2.5 microM PPADS was applied externally. PPADS, 2.5 microM, decreased the maximal response to ATP by 51% without changing its EC(50). PPADS inhibition of ATP-activated current was independent of membrane potential between -80 and +40 mV and did not involve a shift in the reversal potential of the current. The magnitude of PPADS inhibition of ATP-activated current was dependent on the duration of the prior exposure to PPADS. The time constants of both onset and offset of PPADS inhibition of ATP-activated current did not differ significantly with changes in ATP concentration from 1 to 5 microM. Recovery of ATP-activated current from PPADS inhibition also exhibited a slow phase that was not accelerated by the presence of agonist and was dependent on the concentration of PPADS. The apparent dissociation rate of PPADS from unliganded ATP-gated ion channels was much greater than the rate of the slow phase of recovery of ATP-activated current from PPADS inhibition. The results suggest that PPADS can inhibit P2X receptor function in a complex noncompetitive manner. PPADS produces a long-lasting inhibition that does not appear to result from open channel block but rather from an action at an allosteric site apparently accessible from the extracellular environment that involves a greatly reduced rate of dissociation from liganded versus unliganded ATP-gated ion channels.

ATP facilitates spontaneous glycinergic IPSC frequency at dissociated rat dorsal horn interneuron synapses

1. The ATP action on spontaneous miniature glycinergic inhibitory postsynaptic currents (mIPSCs) was investigated in rat substantia gelatinosa (SG) neurons mechanically dissociated from the 2nd layer of the dorsal horn in which their presynaptic glycinergic nerve terminals remained intact. 2. ATP reversibly facilitated the frequency of the mIPSCs in a concentration-dependent manner without affecting their amplitude distribution. The ATP agonist, 2-methylthioATP (2MeSATP), mimicked the ATP action, while another ATP receptor agonist, alphabeta-methylene-ATP (alpha,beta-meATP), had no effect on mIPSCs. 3. The ATP receptor antagonists, suramin (1 x 10-6 M) and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (1 x 10-5 M), completely blocked the facilitatory effect of ATP on glycine release (102.0 +/- 11.2 % and 99.3 +/- 16.2 %, n = 6, respectively) without altering the current amplitude distributions. 4. N-Ethylmaleimide (NEM), a sulphydryl alkylating agent, suppressed the inhibitory effect of adenosine on mIPSC frequency (111.2 +/- 13. 3 %, n = 4) without altering the current amplitude distribution. However, ATP still facilitated the mIPSC frequency (693.3 +/- 245.2 %, n = 4) even in the presence of NEM. 5. The facilitatory effect of ATP (1 x 10-5 M) on mIPSC frequency was not affected by adding 1 x 10-4 M Cd2+ to normal external solution but was eliminated in a Ca2+-free external solution. 6. These results suggest that ATP enhances glycine release from nerve terminals, presumably resulting in the inhibition of SG neurons which conduct nociceptive signals to the CNS. This presynaptic P2X-type ATP receptor may function to prevent excess excitability in SG neurons, thus preventing an excessive pain signal and/or SG cell death.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Characterization of P2 receptors modulating neural activity in rat rostral ventrolateral medulla

This study investigated the effects of ATP, and related compounds, on the activity of neurons within the rostral ventrolateral medulla, an area of fundamental importance in reflex control of the cardiovascular system. Extracellular recordings were made from single neurons in anaesthetized, paralysed and artificially ventilated rats. Ionophoretic application of alpha,beta-methylene-ATP, adenosine 5'-O-(2-thiodiphosphate), UTP, 2-methylthio-ATP and ATP altered the ongoing activity in the majority of neurons (>74% of neurons), generally causing increases in the firing rate. Nine of 11 cells with presumed spinal projection were excited by ATP and/or the P2X-selective agonist alpha,beta-methylene-ATP. Desensitization of the excitatory responses to alpha,beta-methylene-ATP was observed in four of 20 rostral ventrolateral medulla neurons. For the remainder of the rostral ventrolateral medulla neurons, the increase in firing rate evoked by alpha,beta-methylene-ATP, and by the other purine compounds tested, did not undergo desensitization. Suramin, a P2 receptor antagonist, blocked excitatory responses to adenosine 5'-O-(2-thiodiphosphate) or alpha,beta-methylene-ATP in five of 16 neurons. These results indicate that ATP can modulate the activity of neurons in the rostral ventrolateral medulla via actions at P2 purine receptors. The data suggest that both P2X and P2Y receptors are involved, and that the functional expression of these receptors within the rostral ventrolateral medulla is not uniform.

Evidence for the involvement of spinal endogenous ATP and P2X receptors in nociceptive responses caused by formalin and capsaicin in mice

1. The aim of the present study is to characterize the role of spinal endogenous ATP and P2X receptors in the generation of neurogenic and inflammatory pain. We examined the effects of intrathecal treatment with P2X receptor antagonists on the formalin- and capsaicin-induced nociceptive behaviours in mice. 2. Intrathecal pretreatment with the general P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS), significantly suppressed both the first and second phases of the formalin-induced nociceptive behaviour. The second phase of the nociceptive response was also suppressed by intrathecal treatment with PPADS after the first phase. Furthermore, pretreatment with the selective antagonist for the P2X1, P2X3 and P2X2+3 receptors, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), significantly reduced the first phase, but not the second phase. The second phase was also not suppressed by intrathecal TNP-ATP after the first phase. 3. Capsaicin-induced nociceptive behaviour that has been shown to be a model for neurogenic pain, was also significantly suppressed by intrathecal pretreatment with PPADS or TNP-ATP. 4. Nociceptive behaviour in the first phase of the formalin test and in the capsaicin test were significantly inhibited by intrathecal pretreatment with alpha, beta-methylene ATP (alpha,betameATP: 5 microg mouse-1) 15 min prior to injection of formalin or capsaicin. This treatment has been previously shown to desensitize spinal P2X3 receptor subtypes in vivo. 5. These findings suggest that spinal endogenous ATP may play a role in (1) the formalin- and capsaicin-induced neurogenic pain via the PPADS- and TNP-ATP-sensitive P2X receptors which are also desensitized by alpha,betameATP (perhaps the P2X3 receptor subtype) and (2) formalin-induced inflammatory pain via PPADS-sensitive, TNP-ATP- and alpha,betameATP-insensitive P2X (and/or P2Y) receptors.

Localisation of P2X receptors in human salivary gland epithelial cells and human embryonic kidney cells by sodium dodecyl sulfate-polyacrylamide gel electrophoresis/Western blotting and immunofluorescence

Human salivary gland epithelial cells, a continuous cell line derived from an irradiated human salivary gland and human embryonic kidney cell line human embryonic kidney (HEK)293 were examined for the purpose of establishing whether they expressed endogenous P2X ionotropic receptors at any stage in their cycles. HSG cells were found to express P2X1-6 subtypes using both Western blotting and immunofluorescence labeling. HEK293 cells had no detectable levels of P2X1-3 and P2X6 under normal circumstances along with very low levels of P2X4 and P2X5 but when the cells were grown past confluence then all subtypes were expressed on the surface membrane with the exception of P2X2. The results are discussed in terms of the likely influence of ATP acting as an intercellular signaling molecule.

Central CO2 chemoreception: a mechanism involving P2 purinoceptors localized in the ventrolateral medulla of the anaesthetized rat

1. The involvement of P2 purinoceptors in chemosensory function in the ventrolateral regions of the medulla oblongata was investigated in the anaesthetized rat. We have investigated the effect of antagonizing, or desensitizing, P2 receptors in the retrofacial area of the ventrolateral medulla on factors modifying respiratory activity. 2. Bilateral microinjection of suramin (50 nl, 0.02 M), a P2 purinoceptor antagonist, into the retrofacial area in the artificially ventilated rat reduced resting phrenic nerve discharge. It also markedly affected the response of the phrenic nerve to increases in arterial CO2. Under conditions of hyperoxic, hypocapnic apnoea, the mean threshold for inducing phrenic nerve activity was raised significantly (from an end-tidal CO2 of 2.5 % to 4.5 %, n = 9). 3. In addition, the slope of the respiratory response curve to increases in CO2 was reduced after suramin. A similar effect was observed after desensitization of certain P2X receptors with alphabeta-methyleneATP. As arterial levels of O2 were greater than 100 mmHg, and an equivalent pattern of response was observed in sino-aortically denervated and vagotomized animals, we believe any contribution of the peripheral chemoreceptors to be minimal. 4. Our data suggest that respiratory neurones within the retrofacial area (Botzinger complex) represent part of the central site of action of CO2 on respiration. Moreover, our observations lead us to suggest that CO2-evoked changes in respiration are mediated at least in part by P2X purinoceptors.

Adenosine 5'-triphosphate-induced dopamine release in the rat nucleus accumbens in vivo

Microdialysis experiments were used to investigate the influence of locally applied 2-methylthioadenosine 5'-triphosphate (2-MeSATP) on extracellular dopamine concentrations in the rat nucleus accumbens (NAc). 2-MeSATP (0.1, 1, 10 mM) infused via the microdialysis probe caused a concentration-dependent stimulation of dopamine release. The P2 receptor antagonists reactive blue 2 and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (30 microM each) depressed the basal release of dopamine when given alone and in addition counteracted the stimulatory effect of 2-MeSATP (1 mM). In contrast, a combination of the excitatory amino acid receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 300 microM) and 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 100 microM) increased the basal release of dopamine by themselves and facilitated the effect of 2-MeSATP (1 mM). The results suggest a physiologically relevant regulation of tonic dopamine release in the NAc by endogenous ATP via P2 receptors. This is due to the combination of a direct and an indirect (via glutamate release) effect of ATP on mesolimbic dopaminergic neurons.

Involvement of P2 purinoceptors and the nitric oxide pathway in [3H]purine outflow evoked by short-term hypoxia and hypoglycemia in rat hippocampal slices

The objective of this study was to study how the outflow of [3H]purines is altered during a brief period of ischemic-like conditions in superfused hippocampal slices and to show whether it is regulated by P2 purinoceptors and the nitric oxide (NO) pathway. The outflow of [3H]purines increased in response to 5 min of combined hypoxia/hypoglycemia. High performance liquid chromatography analysis verified the efflux of [3H]adenosine-triphosphate, [3H]adenosine-diphosphate, [3H]adenosine-monophosphate, [3H]adenosine, [3H]inosine, and [3H]hypoxanthine in response to ischemic-like conditions. The P2 receptor antagonists suramin and pyridoxal-phosphate-6-azophenyl-2'-4'-disulphonic-acid-tetrasodium (PPADS) reduced significantly the [3H]purine efflux evoked by ischemic-like conditions, showing that P2 purinoceptors are involved in the initiation of purine outflow. The NO synthase inhibitor N-nitro-l-arginine-methyl-ester (l-NAME) attenuated significantly the [3H]purine outflow, evoked by ischemic-like conditions, while 7-nitroindazole (7-NI) caused only a mild decrease in the outflow. The NO donor sodium nitroprusside increased significantly the basal efflux of [3H]purines. In summary, a brief period of combined hypoxia/hypoglycemia induced the efflux of ATP in addition to the outflow of other purines. Since P2 receptor antagonists decreased the [3H]purine outflow evoked by ischemic-like conditions we propose that ATP, acting on P2 purinoceptors, is responsible for further efflux of purines after ischemic-like period. It seems likely that NO is also involved in the regulation of purine outflow, since inhibition of NO production attenuated the [3H]purine outflow, evoked by ischemic-like conditions, while exogenous NO facilitated the basal outflow.

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

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

Impaired arterial baroreflex regulation of heart rate after blockade of P2-purinoceptors in the nucleus tractus solitarius

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of ATP mimics baroreflex responses (bradycardia, hypotension); however, the physiological role of these receptors in cardiovascular control remains unclear. We tested whether blockade of these receptors attenuates arterial baroreflex control of heart rate (HR). Baroreflex-induced changes in HR (via graded i.v. infusion of phenylephrine and nitroprusside) were observed in seven alpha-chloralose/urethane anesthetized male Sprague-Dawley rats before and after microinjection of the purinergic P2 receptor antagonist suramin (0.5 nmol in 50 nL) into the subpostremal NTS. Before suramin, typical baroreflex changes in HR were observed (maximum gain, Gmax = 2.94 +/- 0.54 bpm/mmHg). Suramin markedly impaired baroreflex-induced changes in HR (gain = 0.02 +/- 0.08 and 0.18 +/- 0.09 bpm/mmHg for increases and decreases in mean arterial blood pressure, respectively); however, after 90-130 min, HR and baroreflex reactivity returned to control levels. Microinjections of vehicle into the same area did not alter baroreflex function. In addition, suramin did not alter the depressor responses to microinjections of glutamate into the same site of the NTS. We conclude that normal P2x-purinoceptor function in subpostremal NTS may be necessary for baroreflex regulation of HR.

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

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

Inhibition of NMDA-gated ion channels by the P2 purinoceptor antagonists suramin and reactive blue 2 in mouse hippocampal neurones

1. The action of suramin and reactive blue 2 on N-methyl-D-aspartate (NMDA)-activated ion current was studied in mouse hippocampal neurones in culture by use of whole-cell patch-clamp recording. 2. Suramin and reactive blue 2 inhibited steady-state current activated by 25 microM NMDA with IC50 values of 68 and 11 microM, respectively. 3. Reactive blue 2 produced a gradual decline of NMDA-activated current to a steady-state, but this slow onset was not an indication of use-dependence, as it could be eliminated by exposure to reactive blue 2 before NMDA application. In addition, NMDA-activated current recovered completely from inhibition by reactive blue 2 in the absence of agonist. 4. The slow onset of inhibition by reactive blue 2 was not apparently due to an action at an intracellular site, as inclusion of 250 microM reactive blue 2 in the recording pipette did not alter inhibition by 25 microM reactive blue 2 applied externally. 5. Reactive blue 2 and suramin inhibited NMDA-gated channels in a voltage-independent manner. 6. Reactive blue 2, 25 microM, decreased the maximal response to NMDA from 1441 to 598 pA without changing its EC50. In contrast, 75 microM suramin increased the EC50 for NMDA from 13 to 35 microM, and decreased the maximal response to NMDA from 1822 to 1498 pA. Schild analysis of suramin inhibition of NMDA-activated current yielded a nonlinear plot. 7. Both agents decreased the maximal response to glycine without altering its EC50. 8. Suramin and reactive blue 2 appear to inhibit NMDA receptor-channels in a manner that is noncompetitive with respect to both NMDA and glycine. However, inhibition by suramin differed from that by reactive blue 2, in that suramin significantly increased the EC50 of NMDA.