ATP P2X receptor-mediated enhancement of glutamate release and evoked EPSCs in dorsal horn neurons of the rat spinal cord

Expression of P2X7 receptor immunoreactivity in distinct subsets of synaptic terminals in the ventral horn of rat lumbar spinal cord

Adenosine 5'-triphosphate (ATP) may regulate neurotransmission in the CNS by activating presynaptic and/or postsynaptic P2X (P2X1-P2X7) ionotropic receptors. P2X7 purinergic receptors have been shown to modulate transmitter release at excitatory synapses in the hippocampus and have been localized in glutamatergic terminals in several CNS regions. Here, we analyze P2X7-immunoreactivity (IR) in a variety of immunohistochemically identified excitatory and inhibitory presynaptic terminals in the spinal cord ventral horn, including cholinergic C-terminals and motor axon collaterals and glutamatergic terminals that express VGLUT1- or VGLUT2-IR. Whereas there is widespread colocalization of P2X7-IR and VGLUT2-IR ( approximately 94%), there is little colocalization (< or =15%) with VGLUT1, monoaminergic or inhibitory terminals. Furthermore, although P2X7-IR is present in motor axon terminals at the neuromuscular junction (NMJ), only about 32% of the presumed motor axon terminals in the ventral horn exhibit P2X7-IR; in contrast, almost all large cholinergic C-terminals contacting motoneurons (91%) express P2X7-IR. The results suggest that distinct populations of synapses involved in spinal cord motor control circuits may be differentially regulated by the activation of P2X7 receptors.

Purinergic modulation of extracellular glutamate levels in the nucleus accumbens in vivo

In the present study, the P2 receptor-mediated modulation of the extracellular glutamate concentration was investigated by microdialysis in the nucleus accumbens (NAc) of freely moving rats. Because of the known interference of dopaminergic and glutamatergic mechanisms in this area the experiments were performed with animals intra-accumbally treated with 6-hydroxydopamine (6-OHDA) to deplete dopamine pools. Perfusion of the NAc with the prototypic P2 receptor agonist 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 0.1, 1 and 10mM) concentration-dependently increased the extracellular level of glutamate in this area. Pretreatment with the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 0.1mM) decreased the basal extracellular glutamate concentration and inhibited the 2-MeSATP-induced outflow of glutamate. In rats treated with 6-OHDA, 2-MeSATP increased the total extracellular glutamate to an extent about fivefold larger than in sham-lesioned rats. The perfusion of the dopamine-depleted NAc with the D(2)/D(3) dopamine receptor agonist quinpirole (0.1mM) diminished the basal concentration of glutamate and reduced the effect of 2-MeSATP on the extracellular glutamate. These results provide evidence that the stimulation of P2 receptors is involved in the increase of accumbal extracellular glutamate in vivo. This behaviourally relevant mechanism depends on a dopamine D(2) receptor-mediated tone in the nucleus accumbens. Furthermore, the inhibition of P2 receptors may prevent, at least partly, glutamate-mediated neurodegeneration.

ATP- and adenosine-mediated signaling in the central nervous system: synaptic purinoceptors: the stage for ATP to play its "dual-role"

The studies aiming to understand the function of purinoceptors in the central nervous system (CNS), which has been explored mostly in isolated and cultured cell systems, are now at the stage of identifying their physiological and pathophysiological significance in the native organs, tissues, and whole animals. The results of our recent studies made in brain slice preparations are not in full accordance with what have been demonstrated in isolated cells, mostly due to strong interplay between ATP receptors, adenosine receptors, and ecto-nucleotidases. This suggests that these proteins form coordinated regulation systems in the native tissue, controlling the local network behaviors through regulating the balance between the effects of ATP and adenosine on synaptic transmissions. We propose that this tripartite regulation system by extracellular purines may be an important target of CNS drugs.

ATP- and adenosine-mediated signaling in the central nervous system: chronic pain and microglia: involvement of the ATP receptor P2X4

We have been studying the role of ATP receptors in pain and already reported that activation of P2X(2/3) heteromeric channel/receptor in primary sensory neurons causes acutely tactile allodynia, one hallmark of neuropathic pain. We report here that tactile allodynia under the chronic pain state requires an activation of the P2X(4) ionotropic ATP receptor and p38 mitogen-activated protein kinase (MAPK) in spinal cord microglia. Two weeks after L5 spinal nerve injury, rats displayed a marked mechanical allodynia. In the rats, activated microglia were detected in the injured side of the dorsal horn and the level of the dually-phosphorylated active form of p38MAPK (phospho-p38MAPK) in these microglia was increased. Moreover, intraspinal administration of a p38MAPK inhibitor, SB203580, suppressed the allodynia. We also found that the expression level of P2X(4) was increased strikingly in spinal cord microgila after nerve injury and that pharmacological blockade or inhibition of the expression of P2X(4) reversed the allodynia. Taken together, our results demonstrate that activation of P2X(4) or p38MAPK in spinal cord microglia is necessary for tactile allodynia after nerve injury.

[ATP receptors in pain sensation]

We reported that activation of P2X2/3 heteromeric channels in A delta-DRG neurons causes tactile allodynia and activation of P2X3 in C-fiber causes nocifensive behavior. We also found that tactile allodynia under the chronic pain state requires an activation of P2X4 ionotropic ATP receptor and p38 mitogen-activated protein kinase in spinal microglia.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

Purinergic modulation of synaptic input to Purkinje neurons in rat cerebellar brain slices

Adenosine triphosphate (ATP) is a cotransmitter and an extracellular neuromodulator in nervous systems, and it influences neural circuits and synaptic strength. We have studied a stimulating effect of ATP (100 micro m) on the synaptic input of Purkinje neurons in acute cerebellar brain slices of juvenile rats (p14-19). Bath application of ATP increased the frequency of spontaneous postsynaptic currents (sPSCs) almost twofold, and increased their amplitude. These effects were fully suppressed by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2'4'-disulphonic acid (PPADS; 10 microm), or after blocking action potentials with tetrodotoxin (TTX; 0.5 microm), but were not impaired by inhibiting ionotropic, non-NMDA glutamate receptors with 2,3-dioxo-6-nitro-1,2,3,4,-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX; 5 microm). The frequency of sPSCs was reduced by 35% by PPADS and increased by 50% after inhibiting ectonucleotidase with ARL67156 (50 microm), suggesting intrinsic, 'tonic', stimulation of synaptic activity via P2 receptors. The pharmacological profile indicated that the ATP effect was mediated by both P2X and P2Y receptors, most probably of the P2X5- and P2Y(2,4)-like subtypes. The action potential frequency in the inhibitory basket cells was increased by 65%, and decreased in Purkinje neurons by 25%, in the presence of ATP. Our results suggest that ATP continuously modulates the cerebellar circuit by increasing the activity of inhibitory input to Purkinje neurons, and thus decreasing the main cerebellar output activity, which contributes to locomotor coordination.

P2X receptor subtype-specific modulation of excitatory and inhibitory synaptic inputs in the rat brainstem

The role of P2 receptors in synaptic transmission to the rat medial nucleus of the trapezoid body (MNTB) was studied in an in vitro brain slice preparation. Whole-cell patch recordings were made and spontaneous synaptic responses studied under voltage clamp during application of P2X receptor agonists. ATPgammaS (100 microm) had no effect on holding current, but facilitated spontaneous excitatory postsynaptic current (sEPSC) frequency in 41% of recordings and facilitated spontaneous inhibitory postsynaptic currents (sIPSCs) in 20% of recordings. These were blocked by the P2 receptor antagonist suramin (100 microm). alpha,beta-meATP also facilitated sEPSC and sIPSC frequency, while l-beta,gamma-meATP facilitated only sIPSCs. The sEPSC facilitation by ATPgammaS was blocked by TTX (but did not block facilitation of sIPSCs). sEPSC facilitation was blocked by PPADS (30 microm) and the selective P2X(3) receptor antagonist A-317491 (3 microm), suggesting that modulation of sEPSCs involves P2X(3) receptor subunits. alpha,beta-meATP-facilitated sIPSCs were also recorded in wild-type mouse MNTB neurones, but were absent in the MNTB from P2X(1) receptor-deficient mice demonstrating a functional role for P2X(1) receptors in the CNS.

P2X3 receptors and peripheral pain mechanisms

ATP released from damaged or inflamed tissues can act at P2X receptors expressed on primary afferent neurones. The resulting depolarization can initiate action potentials that are interpreted centrally as pain. P2X(3) subunits are found in a subset of small-diameter, primary afferent neurones, some of which are also sensitive to capsaicin. They can form homo-oligomeric channels, or they can assemble with P2X(2) subunits into hetero-oligomers. Studies with antagonists selective for P2X(3)-containing receptors, experiments with antisense oligonucleotides to reduce P2X(3) subunit levels, and behavioural testing of P2X(3) knock-out mice, all suggest a role for the P2X(2/3) receptor in the signalling of chronic inflammatory pain and some features of neuropathic pain. The availability of such tools and experimental approaches promises to accelerate our understanding of the other physiological roles for P2X receptors on primary afferent neurones.

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.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

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

Synaptic transmission of chaotic spike trains between primary afferent fiber and spinal dorsal horn neuron in the rat

Primary sensory neurons can generate irregular burst firings in which the existence of significant deterministic behaviors of chaotic dynamics has been proved with nonlinear time series analysis. But how well the deterministic characteristics and neural information of presynaptic chaotic spike trains were transmitted into postsynaptic spike trains is still an open question. Here we investigated the synaptic transmission of chaotic spike trains between primary Adelta afferent fiber and spinal dorsal horn neuron. Two kinds of basic stimulus unit, brief burst and single pulse, were employed by us to comprise chaotic stimulus trains. For time series analysis, we defined "events" as the longest sequences of spikes with all interspike intervals less than or equal to a certain threshold and extracted the interevent intervals (IEIs) from spike trains. Return map analysis of the IEI series showed that the main temporal structure of chaotic input trains could be detected in postsynaptic output trains, especially under brief-burst stimulation. Using correlation dimension and nonlinear prediction methods, we found that synaptic transmission could influence the nonlinear characteristics of chaotic trains, such as fractal dimension and short-term predictability, with greater influence made under single-pulse stimulation. By calculating the mutual information between input and output trains, we found the information carried by presynaptic spike trains could not be completely transmitted at primary afferent synapses, and that brief bursts could more reliably transmit the information carried by chaotic input trains across synapses. These results indicate that although unreliability exists during synaptic transmission, the main deterministic characteristics of chaotic burst trains can be transmitted across primary afferent synapses. Moreover, brief bursts that come from the periphery can more reliably transmit neural information between primary afferent fibers and spinal dorsal horn neurons.

Presence of functional ATP and dinucleotide receptors in glutamatergic synaptic terminals from rat midbrain

Glutamatergic terminals from rat midbrain were characterized by immunolocalization of synaptophysin and the vesicular glutamate transporters, either VGLUT1 or VGLUT2. Terminals containing these markers represent about 31% (VGLUT1) and 16% (VGLUT2) of the total synaptosomal population. VGLUT1-positive glutamatergic terminals responded to ATP or P1,P 5-di(adenosine-5') pentaphosphate (Ap5A) with an increase in the intrasynaptosomal calcium concentration as measured by a microfluorimetric technique in single synaptosomes. Roughly 20% of the VGLUT1-positive terminals responded to ATP, 13% to Ap5A and 11% to both agonists. Finally 56% of the terminals labeled with the anti-VGLUT1 antibody did not show any calcium increase in response to ATP or Ap5A. A similar response distribution was also observed in the VGLUT2-positive terminals. The Ca2+ responses induced by ATP and Ap5A in the glutamatergic terminals could be selectively inhibited by pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 80 micro m) and P1,P 5-di(inosine-5') pentaphosphate (Ip5I, 100 nm), respectively. Both ATP and Ap5A, once assayed in the presence of extrasynaptosomal calcium, were able to induce a concentration-dependent glutamate release from synaptosomal populations, EC50 values being 21 micro m and 38 micro m for ATP and Ap5A, respectively. Specific inhibition of glutamate release was obtained with PPADS on the ATP effect and with Ip5I on the dinucleotide response, indicating that separate receptors mediate the secretory effects of both compounds.

Crossing the pain barrier: P2 receptors as targets for novel analgesics

In 1995 the P2X3 receptor was found to be expressed at high levels in nociceptive sensory neurones, consistent with earlier reports that ATP induced pain in humans and animals. At first it was thought that ATP was most likely to play a role in acute pain, following its release from damaged or stressed cells and since then a wide variety of experimental techniques and approaches have been used to study this possibility. Whilst it is clear that exogenous and endogenous ATP can indeed acutely stimulate sensory neurones, more recent reports using gene knockout and antisense oligonucleotide technologies, and a novel, selective P2X3 antagonist, A-317491, all indicate that ATP and P2X3 receptors are more likely to be involved in chronic pain conditions, particularly chronic inflammatory and neuropathic pain. These reports indicate that P2X3 receptors on sensory nerves may be tonically activated by ATP released from nearby damaged or stressed cells, or perhaps from the sensory nerves themselves. This signal, when transmitted to the CNS, will be perceived consciously as chronic pain. In addition, it is now clear that several subtypes of P2Y receptor are also expressed in sensory neurones. Although their distribution and functions have not been as widely studied as P2X receptors, the effects that they mediate indicate that they might also be considered as therapeutic targets in the treatment of pain. Although our ability to treat persistent painful conditions, such as chronic inflammatory and neuropathic pain, has improved in recent years, these conditions are often resistant to currently available therapies, such as opioids or non-steroidal anti-inflammatory drugs. This reflects a limited understanding of the underlying pathophysiology. It is now clear that the development and maintenance of chronic pain are mediated by multiple factors, but many of these factors, and the receptors and mechanisms through which they act, remain to be identified. Chronic pain is debilitating and can greatly decrease quality of life, not just due to the pain per se, but also because of the depression that can often ensue. Thus a greater understanding of the mechanisms that underlie chronic pain will help identify new targets for novel analgesics, which will be of great therapeutic benefit to many people.

P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury

Pain after nerve damage is an expression of pathological operation of the nervous system, one hallmark of which is tactile allodynia-pain hypersensitivity evoked by innocuous stimuli. Effective therapy for this pain is lacking, and the underlying mechanisms are poorly understood. Here we report that pharmacological blockade of spinal P2X4 receptors (P2X4Rs), a subtype of ionotropic ATP receptor, reversed tactile allodynia caused by peripheral nerve injury without affecting acute pain behaviours in naive animals. After nerve injury, P2X4R expression increased strikingly in the ipsilateral spinal cord, and P2X4Rs were induced in hyperactive microglia but not in neurons or astrocytes. Intraspinal administration of P2X4R antisense oligodeoxynucleotide decreased the induction of P2X4Rs and suppressed tactile allodynia after nerve injury. Conversely, intraspinal administration of microglia in which P2X4Rs had been induced and stimulated, produced tactile allodynia in naive rats. Taken together, our results demonstrate that activation of P2X4Rs in hyperactive microglia is necessary for tactile allodynia after nerve injury and is sufficient to produce tactile allodynia in normal animals. Thus, blocking P2X4Rs in microglia might be a new therapeutic strategy for pain induced by nerve injury.

ATP modulation of excitatory synapses onto interneurons

Inhibitory interneurons play important roles in neuronal circuits, but the synaptic mechanisms that regulate excitatory input onto interneurons remain to be fully understood. We show that ATP-gated presynaptic P2X2 channels facilitate excitatory transmission onto stratum radiatum interneurons but not onto CA1 pyramidal neurons. ATP released endogenously during carbachol-induced oscillations facilitates excitatory synapses onto interneurons. Overall, these data provide evidence for the molecular identity, synaptic function, and interneuron synapse specificity of a presynaptic neurotransmitter-gated cation channel. The findings highlight a novel form of presynaptic facilitation for hippocampal interneurons and suggest a role for extracellular ATP in neuronal networks.

Distinct roles of P2X receptors in modulating glutamate release at different primary sensory synapses in rat spinal cord

Using spinal cord slice preparations and patch-clamp recordings in lamina II and lamina V regions, we tested a hypothesis that P2X receptor subtypes differentially modulate glutamate release from primary afferent terminals innervating different sensory regions. We found that activation of P2X receptors by alpha,beta-methylene-ATP increased glutamate release onto >80% of DH neurons in both lamina regions. However, two distinct types of modulation, a transient and a long-lasting enhancement of glutamate release were observed. In lamina II recordings, >70% of the modulation was transient. In contrast, P2X receptor-mediated modulation was always long-lasting in lamina V. Pharmacologically, both transient and long-lasting types of modulation were blocked by 10 microM pyridxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium, a broad-spectrum P2X receptor antagonist. Transient modulation was not observed in the presence of 1 microM trinitrophenyl-ATP (TNP-ATP), a subtype-selective P2X receptor antagonist, suggesting that homomeric P2X3 receptors may be involved in the transient modulation in lamina II. The long-lasting modulation remained in the presence of 1 microM TNP-ATP. Selective removal of P2X3-expressing afferent terminals by the targeting toxin saporin-conjugated isolectin B4 or surgical removal of superficial DH did not affect P2X receptor-mediated long-lasting modulation in lamina V. Taken together, these results suggest that P2X receptor subtypes play distinct roles in sensory processing in functionally different sensory regions.

Caffeine-mediated presynaptic long-term potentiation in hippocampal CA1 pyramidal neurons

We report a new form of long-term potentiation (LTP) in Schaffer collateral (SC)-CA1 pyramidal neuron synapses that originates presynaptically and does not require N-methyl-d-aspartate (NMDA) receptor activation nor increases in postsynaptic-free Ca2+. Using rat hippocampal slices, application of a brief "pulse" of caffeine in the bath evoked a nondecremental LTP (CAFLTP) of SC excitatory postsynaptic currents. An increased probability of transmitter release paralleled the CAFLTP, suggesting that it originated presynaptically. The P1 adenosine receptor antagonist 8-cyclopentyltheophylline and the P2 purinoreceptor antagonists suramin and piridoxal-5'-phosphate-azophenyl 2',4'-disulphonate blocked the CAFLTP. Inhibition of Ca2+ release from caffeine/ryanodine stores by bath-applied ryanodine inhibited the CAFLTP, but ryanodine in the pipette solution was ineffective, suggesting a presynaptic effect of ryanodine. Previous induction of the "classical" LTP did not prevent the CAFLTP, suggesting that the LTP and the CAFLTP have different underlying cellular mechanisms. The CAFLTP is insensitive to the block of NMDA receptors by 2-amino-5-phosphonopentanoic acid and to Ca2+ chelation with intracellular 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, indicating that neither postsynaptic NMDA receptors nor increases in cytosolic-free Ca2+ participate in the CAFLTP. We conclude that the CAFLTP requires the interaction of caffeine with presynaptic P1, P2 purinoreceptors, and ryanodine receptors and is caused by an increased probability of glutamate release at SC terminals.

TNP-ATP-resistant P2X ionic current on the central terminals and somata of rat primary sensory neurons

P2X receptors have been suggested to be expressed on the central terminals of A delta-afferent fibers innervating dorsal horn lamina V and play a role in modulating sensory synaptic transmission. These P2X receptors have been widely thought to be P2X2+3 receptors. However, we have recently found that P2X receptor-mediated modulation of sensory transmission in lamina V is not inhibited by trinitrophenyl-adenosine triphosphate (TNP-ATP), a potent antagonist of P2X1, P2X3 homomers, and P2X2+3 heteromers. To provide direct evidence for the presence of TNP-ATP-resistant P2X receptors on primary afferent fibers, we examined alpha,beta-methylene-ATP (alpha beta meATP)-evoked currents and their sensitivity to TNP-ATP in rat dorsal root ganglion (DRG) neurons. alpha beta meATP evoked fast currents, slow currents, and mixed currents that contained both fast and slow current-components. Fast currents and fast current components in the mixed currents were both completely inhibited by 0.1 microM TNP-ATP (n = 14). Both slow currents and slow-current components in the mixed currents showed broad spectrum of sensitivity to 1 microM TNP-ATP, ranging from complete block (TNP-ATP-sensitive) to little block (TNP-ATP-resistant). TNP-ATP-resistant currents evoked by 10 microM alpha beta meATP could be largely inhibited by 10 microM iso-pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid. Cells with P2X currents that were highly resistant to TNP-ATP were found to be insensitive to capsaicin. These results suggest that TNP-ATP-resistant P2X receptor subtypes are expressed on capsaicin-insensitive A delta-afferent fibers and play a role in modulating sensory transmission to lamina V neurons.

Localization and function of ATP and GABAA receptors expressed by nociceptors and other postnatal sensory neurons in rat

The role of endogenous GABA and ATP in regulating transmitter release from primary afferent terminals in the superficial dorsal horn of the spinal cord is still controversial. ATP is co-released with GABA from some inhibitory dorsal horn neurons raising the possibility that ATP could act in concert with GABA to regulate transmitter release from primary afferent terminals if receptors to both transmitters are expressed there. Using electrophysiology together with immunocytochemistry, we have investigated the expression of ATP-gated P2X and GABAA receptors by identified subpopulations of dorsal root ganglion (DRG) neurons known to project primarily to the superficial dorsal horn. Expression of the heat-sensitive vanilloid receptor 1 (VR1) and sensitivity to capsaicin were used to characterize DRG neurons sensitive to noxious heat. Both P2X and GABAA receptors were expressed on the majority of DRG neurons examined. Recording compound action potentials (CAPs) from dorsal roots in the presence of muscimol, alpha,beta-methylene-ATP (alpha,beta-meATP) or capsaicin resulted in depression of CAP in the slow and medium conducting fibres, indicating cognate receptor expression on the small diameter axons. Dorsal root-evoked dorsal root potentials (DR-DRPs), reflecting depolarization of primary afferent terminals by endogenously released substances, were depressed by the GABAA receptor antagonist SR95531 and alpha,beta-meATP. These results suggest that GABAA and P2X receptors are expressed on DRG cell bodies and slow fibre axons, many of which are heat-nociceptive. These fibres project to the superficial lamina of the dorsal horn where the receptors may function to modulate transmitter release near their central terminals.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Expression and function of P2X purinoceptors in rat histaminergic neurons

(1) The pharmacology of ATP responses and the expression pattern of seven known subunits of the P2X receptor were investigated in individual histaminergic neurons of the tuberomamillary nucleus (TM). (2) ATP (3-1000 micro M) evoked fast non-desensitizing inward currents in TM neurons. 2-methylthioATP (2MeSATP) displayed the same efficacy but a lower potency, EC(50)s 84 micro M versus 48 micro M, when compared with ATP. Adenosine-diphosphate (ADP), uridine-triphosphate (UTP) and alpha beta methylene-ATP (alphabeta-meATP) were inactive. (3) ATP-mediated whole cell currents were potentiated by acidification of the recording solution (pH 7.5 and 6.6 were compared). (4) Single-cell RT-PCR (scRT-PCR) analysis revealed that the P2X(2) receptor is expressed in all PCR-positive neurons. Each of the P2X(1), P2X(3), P2X(4), P2X(5) and P2X(6) mRNAs were detected in less than 35% of the cells. (5) Suramin antagonized ATP responses with an IC(50) of 4.2 micro M and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 micro M) reduced ATP responses to 43% of control, when antagonists were pre-applied 90s before the agonist. Cibacron blue (3 micro M) given together with ATP potentiated control responses by 67%, but inhibited it to 10% after pre-application. (6) 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) antagonized ATP responses with an IC(50) of 7 micro M. (7) Pharmacological properties of ATP responses together with scRT-PCR data suggest that P2X(2) is the major purinoceptor on the soma of TM neurons, however the presence of heteromeric P2X(2/5) receptors in some neurons cannot be excluded.

Modulation of inhibitory synaptic activity by a non-alpha4beta2, non-alpha7 subtype of nicotinic receptors in the substantia gelatinosa of adult rat spinal cord

The GABA/glycine-mediated inhibitory activity in the substantia gelatinosa (SG) of the spinal cord is critical in the control of nociceptive transmission. We examined whether and how SG inhibitory activity might be regulated by neuronal nicotinic receptors (nAChRs). Patch-clamp recordings were performed in SG neurons of spinal slice preparations from adult rats. We provided electrophysiological evidence that inhibitory presynaptic terminals in the SG expressed nAChRs and their activation resulted in large increases in the frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in over 90% SG neurons tested. The enhancement of inhibitory activity was mediated by increases in the release of GABA/glycine, and direct Ca(2+) entry through SG presynaptic nAChRs appeared to be involved. Miniature IPSC frequency could be enhanced by the nAChR agonists nicotine or cytisine. Nicotine could still elicit large increases in mIPSC frequency in the presence of the alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (5 microM) and the alpha7 nAChR-selective antagonist methyllycaconitine (40 nM). However, nicotine did not produce a significant enhancement of mIPSC frequency in the presence of the broad spectrum nAChR antagonist mecamylamine (5 microM). Nicotinic agonist-evoked whole-cell currents from SG neurons and the antagonist profiles also indicated the presence of a subtype of nAChRs, which were different from the major central nervous system nAChR subtypes, i.e. alpha4beta2* or alpha7 nAChRs. Together, our results suggest that a subtype of nAChR, possibly alpha3beta4* nAChR or a new nAChR type, is highly expressed at the inhibitory presynaptic terminals in SG of adult rats and play a role in the control of inhibitory activity in SG.

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.

P2X(1)-mediated activation of extracellular signal-regulated kinase 2 contributes to platelet secretion and aggregation induced by collagen

Adenosine triphosphate (ATP) and its stable analog, alpha,beta-methylene ATP, activate the platelet P2X(1) ion channel, causing a rapid Ca(++) influx. Here, we show that, in washed apyrase-treated platelets, alpha,beta-methylene ATP elicits reversible extracellular signal-regulated kinase 2 (ERK2) phosphorylation through a Ca(++)- and protein kinase C-dependent pathway. In contrast, high-performance liquid chromatography-purified adenosine diphosphate (ADP) did not trigger ERK2 phosphorylation. alpha,beta-Methylene ATP also activated the ERK2 pathway in P2X(1)-transfected HEK293 cells but not in cells expressing mutated P2X(1)delL nonfunctional channels. Because ATP released from the dense granules during platelet activation contributes to platelet aggregation elicited by low doses of collagen, and because collagen causes ERK2 phosphorylation, we have investigated the role of P2X(1)-mediated ERK2 activation in these platelet responses. We found that the antagonism of P2X(1) with ADP or desensitization of this ion channel with alpha,beta-methylene ATP both resulted in impaired ERK2 phosphorylation, ATP secretion, and platelet aggregation induced by low concentrations of collagen (< or = 1 microg/mL) without affecting the minor early dense granule release. Selective MEK1/2 inhibition by U-0126 and Ca(++) chelation with EGTA (ethyleneglycoltetraacetic acid) behaved similarly, whereas the PKC inhibitor GF109203-X totally prevented collagen-induced secretion and ERK2 activation. In contrast, when elicited by high collagen concentrations (2 microg/mL), platelet aggregation and secretion no longer depended on P2X(1) or ERK2 activation, as shown by the lack of their inhibition by alpha,beta-methylene ATP or U-0126. We thus conclude that mild platelet stimulation with collagen rapidly releases ATP, which activates the P2X(1)-PKC-ERK2 pathway. This process enhances further degranulation of the collagen-primed granules allowing platelet aggregation to be completed.

P2X receptors in trigeminal subnucleus caudalis modulate central sensitization in trigeminal subnucleus oralis

This study investigated the role of trigeminal subnucleus caudalis (Vc) P2X receptors in the mediation of central sensitization induced in nociceptive neurons in subnucleus oralis (Vo) by mustard oil (MO) application to the tooth pulp in anesthetized rats. MO application produced a long-lasting central sensitization reflected in neuroplastic changes (i.e., increases in neuronal mechanoreceptive field size and responses to innocuous and noxious mechanical stimuli) in Vo nociceptive neurons. Twenty minutes after MO application, the intrathecal (i.t.) administration to the rostral Vc of the selective P2X(1), P2X(3), and P2X(2/3) receptor antagonist, 2'-(or 3'-)O-trinitrophenyl-ATP (TNP-ATP), significantly and reversibly attenuated the MO-induced central sensitization for more than 15 min; saline administration had no effect. Administration to the rostral Vc of the selective P2X(1), P2X(3), and P2X(2/3) receptor agonist, alpha,beta-methylene ATP (alpha,beta-meATP, i.t.) produced abrupt and significant neuroplastic changes in Vo nociceptive neurons, followed by neuronal desensitization as evidenced by the ineffectiveness of a second i.t. application of alpha,beta-meATP and subsequent MO application to the pulp. Administration to the rostral Vc of the selective P2X(1) receptor agonist beta,gamma-methylene ATP (beta,gamma-meATP, i.t.) produced no significant neuroplastic changes per se and did not affect the subsequent MO-induced neuroplastic changes in Vo nociceptive neurons. These results suggest that P2X(3) and possibly also the P2X(2/3) receptor subtypes in Vc may play a role in the initiation and maintenance of central sensitization in Vo nociceptive neurons induced by MO application to the pulp.

Extracellular ATP and P2X7 receptors in neurodegeneration

Neuronal injury and cell death in the central nervous system (CNS) are underlying features of neurodegenerative disorders. However, our understanding of the fundamental mechanisms involved is still limited. Inflammatory processes mediated by cytokines, and interleukin-1 (IL-1) in particular, play a significant role in neuronal death following pathological insults. Despite this growing area of research, very little is known about the factors regulating the expression, cleavage and release of interleukin-1 in the brain. Recent studies on immune cells demonstrate that extracellular ATP can act as a potent stimulus for the maturation and release of interleukin-1beta, via activation of P2X7 receptors. Stimulation of P2X7 receptors with ATP has dramatic cytotoxic properties and a wider role in neurodegenerative processes is possible. This review discusses the potential involvement of extracellular ATP and P2X7 receptors as regulators of interleukin-1-mediated neuropathologies and thus as a mediator of cell death following pathological insults.

P2X receptor trafficking in neurons is subunit specific

P2X receptors within the CNS mediate excitatory synaptic transmission and also act presynaptically to modulate neurotransmitter release. We have studied the targeting and trafficking of P2X4 and P2X2 receptors heterologously expressed in cultured olfactory bulb neurons. Homomeric P2X4 receptors had a punctate distribution, and many of the puncta colocalized with early endosomes. In contrast, P2X2 receptors were primarily localized at the plasma membrane. By antibody-labeling of surface receptors in living neurons, we showed that P2X4 receptors undergo rapid constitutive internalization and subsequent reinsertion into the plasma membrane, whereas P2X2 receptors were not regulated in such a way. The internalization of P2X4 receptors was dynamin-dependent, and the binding of ATP enhanced the basal rate of retrieval in a Ca2+-independent manner. The presence of the P2X4 subunit in a P2X4/6 heteromer governed the trafficking properties of the receptor. P2X receptors acted presynaptically to enhance the release of glutamate, suggesting that the regulated cycling of P2X4-containing receptors might provide a mechanism for modulation of synaptic transmission.

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.

Activation of central terminal vanilloid receptor-1 receptors and alpha beta-methylene-ATP-sensitive P2X receptors reveals a converged synaptic activity onto the deep dorsal horn neurons of the spinal cord

Using a spinal cord slice preparation and patch-clamp recordings from spinal cord dorsal horn neurons, we examined excitatory and inhibitory circuits connecting to lamina V neurons after the activation of afferent central terminal vanilloid receptor-1 (VR1) receptors and P2X receptors. We found that single neurons in lamina V often received excitatory inputs from two chemically defined afferent pathways. One of these pathways was polysynaptic from capsaicin-sensitive afferent terminals. In this pathway the capsaicin-sensitive afferent input first activated interneurons in superficial laminas, and then the excitatory activity was transmitted onto lamina V neurons. The second excitatory input was monosynaptic from alpha(beta)m-ATP-sensitive/capsaicin-insensitive afferent terminals. Both capsaicin-sensitive and alpha(beta)m-ATP-sensitive/capsaicin-insensitive pathways also recruited polysynaptic inhibitory inputs to lamina V neurons. Furthermore, we demonstrated that simultaneous activation of both capsaicin-sensitive afferent pathways and alpha(beta)m-ATP-sensitive/capsaicin-insensitive pathways could generate a temporal summation of excitatory inputs onto single lamina V neurons. These convergent pathways may provide a mechanism of sensory integration for two chemically defined sensory inputs and may have implications in different sensory states.

Peripheral inflammation sensitizes P2X receptor-mediated responses in rat dorsal root ganglion neurons

ATP-gated P2X receptors in nociceptive sensory neurons participate in transmission of pain signals from the periphery to the spinal cord. To determine the role of P2X receptors under injurious conditions, we examined ATP-evoked responses in dorsal root ganglion (DRG) neurons isolated from rats with peripheral inflammation, induced by injections of complete Freund's adjuvant (CFA) into the hindpaw. Application of ATP induced both fast- and slow-inactivating currents in control and inflamed neurons. CFA treatment had no effect on the affinity of ATP for its receptors or receptor phenotypes. On the other hand, inflammation caused a twofold to threefold increase in both ATP-activated currents, altered the voltage dependence of P2X receptors, and enhanced the expression of P2X2 and P2X3 receptors. The increase in ATP responses gave rise to large depolarizations that exceeded the threshold of action potentials in inflamed DRG neurons. Thus, P2X receptor upregulation could account for neuronal hypersensitivity and contribute to abnormal pain responses associated with inflammatory injuries. These results suggest that P2X receptors are useful targets for inflammatory pain therapy.