Physiology and pathophysiology of purinergic neurotransmission

Inhibition of Ca2+ signaling and glucagon secretion in mouse pancreatic alpha-cells by extracellular ATP and purinergic receptors

Glucagon secreted from pancreatic alpha-cells plays a critical role in glycemia, mainly by hepatic glucose mobilization. In diabetic patients, an impaired control of glucagon release can worsen glucose homeostasis. Despite its importance, the mechanisms that regulate its secretion are still poorly understood. Since alpha-cells are particularly sensitive to neural and paracrine factors, in this report we studied the role of purinergic receptors and extracellular ATP, which can be released from nerve terminals and beta-cell secretory granules. Using immunocytochemistry, we identified in alpha-cells the P2 receptor subtype P2Y1, as well as the P1 receptors A1 and A2A. In contrast, only P2Y1 and A1 receptors were localized in beta-cells. To analyze the role of purinergic receptors in alpha-cell function, we studied their participation in Ca2+ signaling. At low glucose concentrations, mouse alpha-cells exhibited the characteristic oscillatory Ca2+ signals that lead to secretion. Application of ATP (1-10 microM) abolished these oscillations or reduced their frequency in alpha-cells within intact islets and isolated in culture. ATPgammaS, a nonhydrolyzable ATP derivative, indicated that the ATP effect was mainly direct rather than through ATP-hydrolytic products. Additionally, adenosine (1-10 microM) was also found to reduce Ca2+ signals. ATP-mediated inhibition of Ca2+ signaling was accompanied by a decrease in glucagon release from intact islets in contrast to the adenosine effect. Using pharmacological agonists, we found that only P2Y1 and A2A were likely involved in the inhibitory effect on Ca2+ signaling. All these findings indicate that extracellular ATP and purinergic stimulation are effective regulators of the alpha-cell function.

P2X receptors and synaptic plasticity

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

Receptor webs: can the chunking theory tell us more about it?

Fundamental concepts shared by several classes of ionotropic and metabotropic cell surface receptors, such as receptor mosaic, cooperation, clustering, propensity to oligomerize, all finding expression in the dynamically structured mosaic membrane, will be revisited here in the light of the "combinatorial receptor web model" and the unifying information-processing mechanism defined as "chunking theory". Particularly the ubiquitous and phylogenetically most ancient P2 receptors for extracellular nucleotides will be regarded here as a prototype of receptor family. Whereas up to now we have mainly studied single receptors with the aim to make intelligible their participation to putative functions into wider biological contexts, from now on we should revise our perspective and look more thoroughly at the entire repertoire of expressed cellular receptors, in order to explain complex receptor-function relationships. A way of doing this, is to group the overall receptor web carried by a cell into patterned combinatorial clusters, the "chunks". We deem that the chunk, originally considered an information measure for cognitive systems, from computer science to linguistics, with applications into broad cognitive skills from pianists' finger tapping to chess players' memory retrieval, will rightly become an information measure for receptor webs, thus explaining the numerous receptor subtypes within the same receptor family that are simultaneously expressed on a single cell, as well as the plethora of different, even opposite, biological outputs often triggered by a single ligand. We are confident that the chunking theory will prove to be useful with receptor systems, and it will not be simply a mere speculative exercise.

P2Y12 receptor upregulation in activated microglia is a gateway of p38 signaling and neuropathic pain

Microglia in the spinal cord may play an important role in the development and maintenance of neuropathic pain. A metabotropic ATP receptor, P2Y(12), has been shown to be expressed in spinal microglia constitutively and be involved in chemotaxis. Activation of p38 mitogen-activated protein kinase (MAPK) occurs in spinal microglia after nerve injury and may be related to the production of cytokines and other mediators, resulting in neuropathic pain. However, it remains unknown whether any type of P2Y receptor in microglia is involved in the activation of p38 MAPK and the pain behaviors after nerve injury. Using the partial sciatic nerve ligation (PSNL) model in the rat, we found that P2Y(12) mRNA and protein increased in the spinal cord and peaked at 3 d after PSNL. Double labeling studies revealed that cells expressing increased P2Y(12) mRNA and protein after nerve injury were exclusively microglia. Both pharmacological blockades by intrathecal administration of P2Y(12) antagonist and antisense knockdown of P2Y(12) expression suppressed the development of pain behaviors and the phosphorylation of p38 MAPK in spinal microglia after PSNL. The intrathecal infusion of the P2Y(12) agonist 2-(methythio) adenosine 5'-diphosphate trisodium salt into naive rats mimicked the nerve injury-induced activation of p38 in microglia and elevated pain behaviors. These data suggest a new mechanism of neuropathic pain, in which the increased P2Y(12) works as a gateway of the following events in microglia after nerve injury. Activation of this receptor by released ATP or the hydrolyzed products activate p38 MAPK pathway and may play a crucial role in the generation of neuropathic pain.

Functional evidence for purinergic inhibitory neuromuscular transmission in the mouse internal anal sphincter

The neurotransmitter(s) underlying nitric oxide synthase (NOS)-independent neural inhibition in the internal anal sphincter (IAS) is still uncertain. The present study investigated the role of purinergic transmission. Contractile and electrical responses to electrical field stimulation of nerves (0.1-5 Hz for 10-60 s) were recorded in strips of mouse IAS. A single stimulus generated a 28-mV fast inhibitory junction potential (IJP) and relaxation. The NOS inhibitor N(omega)-nitro-l-arginine (l-NNA) reduced the fast IJP duration by 20%. Repetitive stimulation at 2.5-5 Hz caused a more sustained IJP and sustained relaxation. l-NNA reduced relaxation at 1 Hz and the sustained IJP at 2.5-5 Hz. All other experiments were carried out in the presence of NOS blockade. IJPs and relaxation were significantly reduced by the P2 receptor antagonists 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS) (100 microM), by desensitization of P2Y receptors with adenosine 5'-[beta-thio]diphosphate (ADP-betaS) (10 microM), and by the selective P2Y1 receptor blocker 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS2179) (10 microM). Relaxation and IJPs were also significantly reduced by the K(+) channel blocker apamin (1 microM). Removal of extracellular potassium (K(o)) increased IJP amplitude to 205% of control, whereas return of K(o) 30 min later hyperpolarized cells by 19 mV and reduced IJP amplitude to 50% of control. Exogenous ATP (3 mM) relaxed muscles in the presence of TTX (1 microM) and hyperpolarized cells by 15 mV. In conclusion, these data suggest that purinergic transmission significantly contributes to NOS-independent neural inhibition in the mouse IAS. P2Y1 receptors, as well as at least one other P2 receptor subtype, contribute to this pathway. Purinergic receptors activate apamin-sensitive K(+) channels as well as other apamin-insensitive conductances leading to hyperpolarization and relaxation.

Identification of a vesicular nucleotide transporter

ATP is a major chemical transmitter in purinergic signal transmission. Before secretion, ATP is stored in secretory vesicles found in purinergic cells. Although the presence of active transport mechanisms for ATP has been postulated for a long time, the proteins responsible for its vesicular accumulation remains unknown. The transporter encoded by the human and mouse SLC17A9 gene, a novel member of an anion transporter family, was predominantly expressed in the brain and adrenal gland. The mouse and bovine counterparts were associated with adrenal chromaffin granules. Proteoliposomes containing purified transporter actively took up ATP, ADP, and GTP by using membrane potential as the driving force. The uptake properties of the reconstituted transporter were similar to that of the ATP uptake by synaptic vesicles and chromaffin granules. Suppression of endogenous SLC17A9 expression in PC12 cells decreased exocytosis of ATP. These findings strongly suggest that SLC17A9 protein is a vesicular nucleotide transporter and should lead to the elucidation of the molecular mechanism of ATP secretion in purinergic signal transmission.

New evidence for purinergic signaling in the olfactory bulb: A2A and P2Y1 receptors mediate intracellular calcium release in astrocytes

Purinergic receptors play a key role in neuron-glia and glia-neuron interactions. In the present study, we have recorded cytosolic Ca(2+) responses using confocal imaging in astrocytes of acute olfactory bulb slices from mice (postnatal days 3-8). By application of agonists and antagonists, we identified two types of receptors, P2Y(1) and A(2A), that mediated Ca(2+) responses attributable to Ca(2+) release from intracellular stores in the astrocytes. Both receptor types were activated by application of ATP and ADP; however, when enzymatic ATP degradation was suppressed by the alkaline phosphatase inhibitor levamisole, ATP only activated MRS2179-sensitive P2Y(1) but not ZM241385-sensitive A(2A) receptors. The dose-response curve for A(2A) receptors activated by adenosine revealed an EC(50) of 0.3 microM, one order of magnitude smaller than the EC(50) of 5 microM determined for P2Y(1) receptors activated by ADP. Electrical stimulation of the olfactory nerve in the presence of glutamate receptor blockers to suppress excitation of postsynaptic neurons evoked Ca(2+) responses in most of the astrocytes, which were inhibited by blocking both P2Y(1) and A(2A) receptors. Our results indicate that olfactory nerve terminals release not only glutamate, but also ATP, which activates P2Y(1) receptors and, after degradation of ATP to adenosine, A(2A) receptors in astrocytes.

Drugs and their molecular targets: an updated overview

About 330 targets bind approved drugs, 270 encoded by the human genome and 60 belonging to pathogenic organisms. A large number of druggable targets have been recently proposed from preclinical and first clinical data, but a huge reservoir of putative drug targets, possibly several thousands, remains to be explored. This overview considers the different types of ligands and their selectivity in the main superfamilies of drug targets, enzymes, membrane transporters and ion channels, and the various classes of membrane and nuclear receptors with their signalling pathway. Recently approved drugs such as monoclonal antibodies, tyrosine kinase and proteasome inhibitors, and major drugs under clinical studies are reviewed with their molecular target and therapeutic interest. The druggability of emerging targets is discussed, such as multidrug resistance transporters and cystic fibrosis transmembrane conductance regulator (CFTR), hyperpolarization-activated cyclic nucleotides-gated (HCN), cyclic nucleotide-gated (CNG) and transient receptor potential (TRP) ion channels, tumour necrosis factor (TNF) and receptor activator of NFkappaB (RANK) receptors, integrins, and orphan or recently deorphanized G-protein-coupled and nuclear receptors. Large advances have been made in the therapeutical use of recombinant cytokines and growth factors (i.e. tasonermin, TNFalpha-1a; becaplermin, platelet-derived growth factor (PDGF); dibotermin-alpha, bone morphogenetic proteins (BMP)2; anakinra, interleukin-1 receptor antagonist protein (IRAP), and in enzyme replacement therapy, i.e. algasidase (alpha-galactosidase) and laronidase (alpha-l-iduronidase). New receptor classes are emerging, e.g. membrane aminopeptidases, and novel concepts are stimulating drug research, e.g. epigenetic therapy, but the molecular target of some approved drugs, such as paracetamol and imidazolines, still need to be identified.

Nucleotide- and nucleoside-converting ectoenzymes: Important modulators of purinergic signalling cascade

The involvement of extracellular nucleotides and adenosine in an array of cell-specific responses has long been known and appreciated, but the integrative view of purinergic signalling as a multistep coordinated cascade has emerged recently. Current models of nucleotide turnover include: (i) transient release of nanomolar concentrations of ATP and ADP; (ii) triggering of signalling events via a series of ligand-gated (P2X) and metabotropic (P2Y) receptors; (iii) nucleotide breakdown by membrane-bound and soluble nucleotidases, including the enzymes of ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family, ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family, ecto-5'-nucleotidase/CD73, and alkaline phosphatases; (iv) interaction of the resulting adenosine with own nucleoside-selective receptors; and finally, (v) extracellular adenosine inactivation via adenosine deaminase and purine nucleoside phosphorylase reactions and/or nucleoside uptake by the cells. In contrast to traditional paradigms that focus on purine-inactivating mechanisms, it has now become clear that "classical" intracellular ATP-regenerating enzymes, adenylate kinase, nucleoside diphosphate (NDP) kinase and ATP synthase can also be co-expressed on the cell surface. Furthermore, data on the ability of various cells to retain micromolar ATP levels in their pericellular space, as well as to release other related compounds (adenosine, UTP, dinucleotide polyphosphates and nucleotide sugars) gain another important insight into our understanding of mechanisms regulating a signalling cascade. This review summarizes recent advances in this rapidly evolving field, with particular emphasis on the nucleotide-releasing and purine-converting pathways in the vasculature.

Physiological and pathophysiological roles of extracellular ATP in chemosensory control of breathing

The purine nucleotide ATP mediates several distinct forms of sensory transduction in both the peripheral and central nervous systems. These processes share common mechanisms that involve the release of ATP to activate ionotropic P2X and/or metabotropic P2Y receptors. Extracellular ATP signalling plays an important role in ventilatory control, mediating both peripheral and central chemosensory transduction to changes in arterial levels of oxygen and carbon dioxide. New data also suggest that extracellular ATP may play an important role in mediating certain neurophysiological responses to systemic inflammation. Here, we propose the novel concept that both peripheral and central neurophysiological effects of ATP may contribute to alterations in ventilatory control during inflammatory pathophysiological states.

The neuropathic pain triad: neurons, immune cells and glia

Nociceptive pain results from the detection of intense or noxious stimuli by specialized high-threshold sensory neurons (nociceptors), a transfer of action potentials to the spinal cord, and onward transmission of the warning signal to the brain. In contrast, clinical pain such as pain after nerve injury (neuropathic pain) is characterized by pain in the absence of a stimulus and reduced nociceptive thresholds so that normally innocuous stimuli produce pain. The development of neuropathic pain involves not only neuronal pathways, but also Schwann cells, satellite cells in the dorsal root ganglia, components of the peripheral immune system, spinal microglia and astrocytes. As we increasingly appreciate that neuropathic pain has many features of a neuroimmune disorder, immunosuppression and blockade of the reciprocal signaling pathways between neuronal and non-neuronal cells offer new opportunities for disease modification and more successful management of pain.

Painful purinergic receptors

Multiple P2 receptor-mediated mechanisms exist by which ATP can alter nociceptive sensitivity following tissue injury. Evidence from a variety of experimental strategies, including genetic disruption studies and the development of selective antagonists, has indicated that the activation of P2X receptor subtypes, including P2X(3), P2X(2/3), P2X(4) and P2X(7), and P2Y (e.g., P2Y(2)) receptors, can modulate pain. For example, administration of a selective P2X(3) antagonist, A-317491, has been shown to effectively block both hyperalgesia and allodynia in different animal models of pathological pain. Intrathecally delivered antisense oligonucleotides targeting P2X(4) receptors decrease tactile allodynia following nerve injury. Selective antagonists for the P2X(7) receptor also reduce sensitization in animal models of inflammatory and neuropathic pain, providing evidence that purinergic glial-neural interactions are important modulators of noxious sensory neurotransmission. Furthermore, activation of P2Y(2) receptors leads to sensitization of polymodal transient receptor potential-1 receptors. Thus, ATP acting at multiple purinergic receptors, either directly on neurons (e.g., P2X(3), P2X(2/3), and P2Y receptors) or indirectly through neural-glial cell interactions (P2X(4) and P2X(7) receptors), alters nociceptive sensitivity. The development of selective antagonists for some of these P2 receptors has greatly aided investigations into the nociceptive role of ATP. This perspective highlights some of the recent advances to identify selective P2 receptor ligands, which has enhanced the investigation of ATP-related modulation of pain sensitivity.

Ecto-nucleotidase pathway is altered by different treatments with fluoxetine and nortriptyline

Depression is one of the most disabling diseases and causes a significant burden to both individual and society. Selective serotonin reuptake inhibitors and tricyclic antidepressants, such as fluoxetine and nortriptyline, respectively, are commonly used in treatment for depression. These antidepressants were tested on cerebral cortex and hippocampal synaptosomes after acute and chronic in vivo and in vitro treatments. In chronic treatment, fluoxetine and nortriptyline decreased ATP hydrolysis (17.8% and 16.3%, respectively) in hippocampus. In cerebral cortex, nortriptyline increased ATP (32.3%), ADP (51.8%), and AMP (59.5%) hydrolysis. However, fluoxetine decreased ATP (25.5%) hydrolysis and increased ADP (80.1%) and AMP (33.3%) hydrolysis. Significant activation of ADP hydrolysis was also observed in acute treatment with nortriptyline (49.8%) in cerebral cortex. However, in hippocampus, ATP (24.7%) and ADP (46.1%) hydrolysis were inhibited. Fluoxetine did not alter enzyme activities in acute treatment for both structures. In addition, there were significant changes in NTPDase activities when fluoxetine and nortriptyline (100, 250, and 500 microM) were tested in vitro. There was no inhibitory effect of fluoxetine and nortriptyline on AMP hydrolysis in cerebral cortex and hippocampus. The findings showed that these antidepressant drugs can affect the ecto-nucleotidase pathway, suggesting that the extracellular adenosine levels could be modulated by these drugs.

Purinergic receptor signaling regulates N-cadherin expression in primary astrocyte cultures

Extracellular ATP exerts both short-term and long-term effects in the CNS by stimulating cell-surface purinergic receptors. Here we have examined the effect of purinergic receptor activation on N-cadherin expression, a calcium-dependent cell adhesion molecule involved in many processes, including glia-glia and axon-glia interactions. When primary cultures of rat cortical astrocytes were treated with ATP, N-cadherin protein expression increased in a time- and concentration-dependent manner. In addition, ATP treatment caused an increase in N-cadherin immunoreactivity in both the cytoplasm and on the cell surface membrane. Interestingly, experiments with cycloheximide revealed that relocalization of N-cadherin to the cell surface membrane were independent of protein synthesis. The ATP-induced increase in N-cadherin protein expression was blocked by reactive blue 2 and 8-(p-sulfophenyl)-theophylline, suggesting involvement of both P2 and P1 purinergic receptors, respectively. In addition, N-cadherin expression was partially blocked when signaling from purinergic receptors to extracellular signal regulated protein kinase or Akt was inhibited by 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene or wortmannin, respectively. By using an in vitro model of traumatic CNS injury, we found that N-cadherin expression was increased when astrocytes were subjected to rapid and reversible mechanical strain. The findings presented here demonstrate a role for extracellular ATP, purinergic receptors and protein kinase signaling in regulating N-cadherin expression and suggest a role for this mechanism in cell-cell interactions.

ATP sensitivity of preBötzinger complex neurones in neonatal rat in vitro: mechanism underlying a P2 receptor-mediated increase in inspiratory frequency

P2 receptor (R) signalling plays an important role in the central ventilatory response to hypoxia. The frequency increase that results from activation of P2Y(1)Rs in the preBötzinger complex (preBötC; putative site of inspiratory rhythm generation) may contribute, but neither the cellular nor ionic mechanism(s) underlying these effects are known. We applied whole-cell recording to rhythmically-active medullary slices from neonatal rat to define, in preBötC neurones, the candidate cellular and ionic mechanisms through which ATP influences rhythm, and tested the hypothesis that putative rhythmogenic preBötC neurones are uniquely sensitive to ATP. ATP (1 mm) evoked inward currents in all non-respiratory neurones and the majority of respiratory neurons, which included inspiratory, expiratory and putative rhythmogenic inspiratory neurones identified by sensitivity to substance P (1 microM) and DAMGO (50 microM) or by voltage-dependent pacemaker-like activity. ATP current densities were similar in all classes of preBötC respiratory neurone. Reversal potentials and input resistance changes for ATP currents in respiratory neurones suggested they resulted from either inhibition of a K(+) channel or activation of a mixed cationic conductance. The P2YR agonist 2MeSADP (1 mm) evoked only the latter type of current in inspiratory and pacemaker-like neurones. In summary, putative rhythmogenic preBötC neurones were sensitive to ATP. However, this sensitivity was not unique; ATP evoked similar currents in all types of preBötC respiratory neurone. The P2Y(1)R-mediated frequency increase is therefore more likely to reflect activation of a mixed cationic conductance in multiple types of preBötC neurone than excitation of one, highly sensitive group.

Adenosine is crucial for deep brain stimulation-mediated attenuation of tremor

Deep brain stimulation (DBS) is a widely used neurosurgical approach to treating tremor and other movement disorders. In addition, the use of DBS in a number of psychiatric diseases, including obsessive-compulsive disorders and depression, is currently being tested. Despite the rapid increase in the number of individuals with surgically implanted stimulation electrodes, the cellular pathways involved in mediating the effects of DBS remain unknown. Here we show that DBS is associated with a marked increase in the release of ATP, resulting in accumulation of its catabolic product, adenosine. Adenosine A1 receptor activation depresses excitatory transmission in the thalamus and reduces both tremor- and DBS-induced side effects. Intrathalamic infusion of A1 receptor agonists directly reduces tremor, whereas adenosine A1 receptor-null mice show involuntary movements and seizure at stimulation intensities below the therapeutic level. Furthermore, our data indicate that endogenous adenosine mechanisms are active in tremor, thus supporting the clinical notion that caffeine, a nonselective adenosine receptor antagonist, can trigger or exacerbate essential tremor. Our findings suggest that nonsynaptic mechanisms involving the activation of A1 receptors suppress tremor activity and limit stimulation-induced side effects, thereby providing a new pharmacological target to replace or improve the efficacy of DBS.

Tracking transmitter-gated P2X cation channel activation in vitro and in vivo

We present a noninvasive approach to track activation of ATP-gated P2X receptors and potentially other transmitter-gated cation channels that show calcium fluxes. We genetically engineered rat P2X receptors to carry calcium sensors near the channel pore and tested this as a reporter for P2X(2) receptor opening. The method has several advantages over previous attempts to image P2X channel activation by fluorescence resonance energy transfer (FRET): notably, it reports channel opening rather than a conformation change in the receptor protein. Our FRET-based imaging approach can be used as a general method to track, in real time, the location, regional expression variation, mobility and activation of transmitter-gated P2X channels in living neurons in vitro and in vivo. This approach should help to determine when, where and how different receptors are activated during physiological processes.

Human epididymal and prostatic tracts of vas deferens: Different contraction response to noradrenaline stimulation in isolated organ bath assay

In the present study epididymal and prostatic portions of human vas deferens were separately isolated and stimulated with exogenous noradrenaline to study their contractile properties. The results displayed that the epididymal tract produced a phasic-tonic response, while the prostatic strip produced only a phasic response suggesting a different functional role of each vas deferens segment. Moreover, it has been verified if alpha(1)-adrenoceptor antagonists doxazosin, alphuzosin and terazosin could differently block the noradrenaline response in each segment. Doxazosin, the most potent antagonist, displayed similar potency in epididymal and prostatic tract (pA(2)=8.51 and 8.42, respectively). Analogously, alphuzosin, although less potent than doxazosin, displayed in the same tracts a superimposed potency (pA(2)=7.25 and 7.30, respectively). In contrast with doxazosin and alphuzosin, terazosin displayed higher potency in blocking the contractile response in prostatic tract (pA(2)=7.67) than in epididymal segment (pA(2)=6.43). These results showed that alpha(1)-adrenoceptor antagonists doxazosin and alphuzosin, although with a different potency, did not discriminate between epididymal and prostatic segment while terazosin showed high potency in prostatic tract and only a moderate activity in epididymal section. Moreover, the biological model employed in our experiments could be a valid screening method to test the potential interferences of drugs indicated for bladder outlet obstruction with the peristaltic activity or the global tone of the human vas deferens.

The origin of spontaneous activity in the developing auditory system

Spontaneous activity in the developing auditory system is required for neuronal survival as well as the refinement and maintenance of tonotopic maps in the brain. However, the mechanisms responsible for initiating auditory nerve firing in the absence of sound have not been determined. Here we show that supporting cells in the developing rat cochlea spontaneously release ATP, which causes nearby inner hair cells to depolarize and release glutamate, triggering discrete bursts of action potentials in primary auditory neurons. This endogenous, ATP-mediated signalling synchronizes the output of neighbouring inner hair cells, which may help refine tonotopic maps in the brain. Spontaneous ATP-dependent signalling rapidly subsides after the onset of hearing, thereby preventing this experience-independent activity from interfering with accurate encoding of sound. These data indicate that supporting cells in the organ of Corti initiate electrical activity in auditory nerves before hearing, pointing to an essential role for peripheral, non-sensory cells in the development of central auditory pathways.

Contrasting effects of P2 receptor agonists on spontaneous contractility of human fallopian tubes with and without acute inflammation

Two groups of women were used to compare effects of P2 receptor agonists on the contractile activity of isolated human fallopian tubes with and without acute purulent inflammation. A control group included women with uterine tubes without inflammation (n=15). A study group included women, operated for unsuccessful conservative anti-inflammatory treatment of acute purulent tuboovarial formations (group with acute purulent salpingitis, n=16). Division into two groups was done according to pathohistological diagnosis. Spontaneous contractions of the isolated tubes were registered isometrically with electromechanical transducer before and after incubation of the tissues with agonists of P2 receptors-ATP, UTP, 2-methylthio-ATP and alpha,beta-methylene-ATP. In the control group neither of the agonists produced any significant effect on fallopian tube contractility. In the study group, ATP, 2-methylthio-ATP and alpha,beta-methylene-ATP significantly increased the spontaneous contractility of isolated tubes. It is suggested that higher activity of P2 receptor agonists in the uterine tubes with acute purulent inflammation is due to expression of several subtypes of P2 receptors during inflammation.

Early temporal changes in ecto-nucleotidase activity after cortical stab injury in rat

During a variety of insults to the brain adenine nucleotides are released in large quantities from damaged cells, triggering multiple cellular responses to injury. Here, we evaluated changes in extracellular ATP, ADP and AMP hydrolysis at different times (0-24 hours) after unilateral cortical stab injury (CSI) in adult rats. Results demonstrated that 24 hours following CSI, ATP and ADP hydrolyzing activities were not significantly altered in injured cortex. Based on calculated V (ATP)/V (ADP) ratio it was concluded that ATP/ADP hydrolysis was primarily catalyzed by NTPDase1 enzyme form. In contrast, AMP hydrolysis, catalyzed by 5'-nucleotidase, was significantly reduced at least 4 hours following CSI. Kinetic analysis and Lineweaver-Burk transformation of the enzyme velocities obtained over the range of AMP concentrations (0.05-1.50 mM) revealed that inhibition of 5'-nucleotidase activity after CSI was of the uncompetitive type. Taken together our data suggest that injured tissue has reduced potential for extracellular metabolism of adenine nucleotides in early stages after CSI.

Binding thermodynamic characterization of human P2X1 and P2X3 purinergic receptors

The present study was designed to perform binding and thermodynamic characterization of human P2X1 and P2X3 purinergic receptors expressed in HEK 293 cells. The thermodynamic parameters DeltaG degrees , DeltaH degrees and DeltaS degrees (standard free energy, enthalpy and entropy) of the binding equilibrium of well-known purinergic agonists and antagonists at P2X1 and P2X3 receptors were determined. Saturation binding experiments, performed in the temperature range 4-30 degrees C by using the high affinity purinergic agonist [3H]alphabetameATP, revealed a single class of binding sites with an affinity value in the nanomolar range in both cell lines examined. The affinity changed with the temperature whereas receptor density was essentially independent of it. van't Hoff plots of the purinergic receptors were linear in the range 4-30 degrees C for agonists and antagonists. The thermodynamic parameters of the P2X1 or P2X3 purinergic receptors were in the ranges -31 kJ mol(-1) < or =DeltaH degrees < or =-19 kJ mol(-1) and 17 J K(-1) mol(-1)< or =DeltaS degrees < or =51 J K(-1)mol(-1) or -26 kJ mol(-1)< or =DeltaH degrees < or =36 kJ mol(-1) and 59< or =DeltaS degrees < or =249 JK(-1) mol(-1), respectively. The results of these parameters showed that P2X1 receptors are not thermodynamically discriminated and that the binding of agonists and antagonists was both enthalpy and entropy-driven. P2X3 receptors were thermodynamically discriminated and purinergic agonist binding was enthalpy and entropy-driven while antagonist binding was totally entropy-driven. The analysis of such thermodynamic data makes it possible to obtain additional information on the nature of the forces driving the purinergic binding interaction. These data could be interesting in drug discovery programs aimed at development of novel and potent P2X1 and P2X3 purinergic ligands.

P2 receptors in the regulation of renal transport mechanisms

Extracellular nucleotides (e.g., ATP) regulate physiological and pathophysiological processes through activation of nucleotide P2 receptors in the plasma membrane. Examples include such diverse processes as communication from taste buds to gustatory nerves, platelet aggregation, nociception, or neutrophil chemotaxis. Over approximately the last 15 years, evidence has also accumulated that cells in renal epithelia release nucleotides in response to physiological stimuli and that these nucleotides act in a paracrine and autocrine way to activate P2 receptors and play a significant role in the regulation of transport mechanisms and cell volume regulation. This review discusses potential stimuli and mechanisms involved in nucleotide release in renal epithelia and summarizes the available data on the expression and function of nucleotide P2 receptors along the native mammalian tubular and collecting duct system. Using established agonist profiles for P2 receptor subtypes, significant insights have been gained particularly into a potential role for P2Y(2)-like receptors in the regulation of transport mechanisms in the collecting duct. Due to the lack of receptor subtype-specific antagonists, however, the in vivo relevance of P2 receptor subtypes is unclear. Studies in gene knockout mice provided first insights including an antihypertensive activity of P2Y(2) receptors that is linked to an inhibitory influence on renal Na(+) and water reabsorption. We are only beginning to unravel the important roles of extracellular nucleotides and P2 receptors in the regulation of the diverse transport mechanisms of the kidney.

Combinatorial synthesis of anilinoanthraquinone derivatives and evaluation as non-nucleotide-derived P2Y2 receptor antagonists

A library of anilinoanthraquinone derivatives was synthesized by parallel Ullmann coupling reaction of bromaminic acid with aniline derivatives in solution using a compact parallel synthesizer. The products were purified by HPLC and evaluated as antagonists at mouse and human P2Y2 receptors. 4-Phenylamino-substituted 1-amino-2-sulfoanthraquinones, for example, 1-amino-4-(2-methoxyphenyl)-2-sulfoanthraquinone (PSB-716), were potent P2Y2 antagonists with IC50 values in the low micromolar range.

Involvement of P2X and P2Y receptors in microglial activation in vivo

Microglial cells are the primary immune effector cells in the brain. Extracellular ATP, e.g., released after brain injury, may initiate microglial activation via stimulation of purinergic receptors. In the rat nucleus accumbens (NAc), the involvement of P2X and P2Y receptors in the generation of microglial reaction in vivo was investigated. A stab wound in the NAc increased immunoreactivity (IR) for P2X_1,2,4,7 and P2Y_1,2,4,6,12 receptors on microglial cells when visualized with confocal laser scanning microscopy. A prominent immunolabeling of P2X₇ receptors with antibodies directed against the ecto- or endodomain was found on Griffonia simplicifolia isolectin-B4-positive cells. Additionally, the P2X₇ receptor was colocalized with active caspase 3 but not with the anti-apoptotic marker pAkt. Four days after local application of the agonists α,βmeATP, ADPβS, 2MeSATP, and BzATP, an increase in OX 42- and G. simplicifolia isolectin-IR was observed around the stab wound, quantified both densitometrically and by counting the number of ramified and activated microglial cells, whereas UTPγS appeared to be ineffective. The P2 receptor antagonists PPADS and BBG decreased the injury-induced increase of these IRs when given alone and in addition inhibited the agonist effects. Further, the intra-accumbally applied P2X₇ receptor agonist BzATP induced an increase in the number of caspase-3-positive cells. These results indicate that ATP, acting via different P2X and P2Y receptors, is a signaling molecule in microglial cell activation after injury in vivo. The up-regulation of P2X₇-IR after injury suggests that this receptor is involved in apoptotic rather than proliferative effects.

Responses of rat P2X2 receptors to ultrashort pulses of ATP provide insights into ATP binding and channel gating

To gain insight into the way that P2X(2) receptors localized at synapses might function, we explored the properties of outside-out patches containing many of these channels as ATP was very rapidly applied and removed. Using a new method to calibrate the speed of exchange of solution over intact patches, we were able to reliably produce applications of ATP lasting <200 micros. For all concentrations of ATP, there was a delay of at least 80 micros between the time when ATP arrived at the receptor and the first detectable flow of inward current. In response to 200-micros pulses of ATP, the time constant of the rising phase of the current was approximately 600 micros. Thus, most channel openings occurred when no free ATP was present. The current deactivated with a time constant of approximately 60 ms. The amplitude of the peak response to a brief pulse of a saturating concentration of ATP was approximately 70% of that obtained during a long application of the same concentration of ATP. Thus, ATP leaves fully liganded channels without producing an opening at least 30% of the time. Extensive kinetic modeling revealed three different schemes that fit the data well, a sequential model and two allosteric models. To account for the delay in opening at saturating ATP, it was necessary to incorporate an intermediate closed state into all three schemes. These kinetic properties indicate that responses to ATP at synapses that use homomeric P2X(2) receptors would be expected to greatly outlast the duration of the synaptic ATP transient produced by a single presynaptic spike. Like NMDA receptors, P2X(2) receptors provide the potential for complex patterns of synaptic integration over a time scale of hundreds of milliseconds.

Activation of urothelial transient receptor potential vanilloid 4 by 4alpha-phorbol 12,13-didecanoate contributes to altered bladder reflexes in the rat

The ion channel transient receptor potential vanilloid (TRPV) 4 can be activated by hypo-osmolarity, heat, or certain lipid compounds. Here, we demonstrate expression of functional TRPV4 protein in the urothelium lining the renal pelvis, ureters, urinary bladder, and urethra. Exposure of cultured rat urothelial cells from the urinary bladder to the TRPV4-selective agonist 4alpha-phorbol 12,13-didecanoate (4alpha-PDD) promoted Ca2+ influx, evoked ATP release, and augmented the ATP release evoked by hypo-osmolarity. In awake rats during continuous infusion cystometrograms, intravesical administration of 4alpha-PDD (10-100 microM) increased maximal micturition pressure by 51%, specifically by augmenting the portion of each intravesical pressure wave that follows high-frequency urethral oscillations and voiding. This unusual pharmacological effect was prevented by intravesical pretreatment with the nonselective ATP receptor antagonist, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (100 microM), systemic treatment with the selective P2X3 purinergic antagonist 5-([(3-phenoxybenzyl)[1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino]carbonyl)-1,2,4-benzenetricarboxylic acid (A317491) (250 micromol/kg), or urethane anesthesia, but was unaffected by capsaicin pretreatment (100 mg/kg s.c.) or denervation of the urethral sphincter. 4Alpha-PDD (1-100 microM) did not alter the contractility to electrical stimulation of excised bladder strips. We conclude that activation of urothelial TRPV4 by 4alpha-PDD and release of mediators such as ATP trigger a novel neural mechanism that regulates the late phase of detrusor muscle contraction after micturition. These data raise the possibility that TRPV4 channels in the urothelium could contribute to abnormal bladder activity.

Release of ATP in the central nervous system during systemic inflammation: real-time measurement in the hypothalamus of conscious rabbits

Receptors for extracellular ATP (both ionotropic and metabotropic) are widely expressed in the CNS both in neurones and glia. ATP can modulate neuronal activity in many parts of the brain and contributes to the central nervous control of several physiological functions. Here we show that during the systemic inflammatory response the extracellular concentrations of ATP increase in the anterior hypothalamus and this has a profound effect on the development of the thermoregulatory febrile response. In conscious rabbits we measured ATP release in real time with novel amperometric biosensors and monitored a marked increase in the concentration of ATP (4.0 ± 0.7 μm) in the anterior hypothalamus in response to intravenous injection of bacterial endotoxin – lipopolysaccharide (LPS). No ATP release was observed in the posterior hypothalamus. The release of ATP coincided with the development of the initial phase of the febrile response, starting 18 ± 2 min and reaching its peak 45 ± 2 min after LPS injection. Application of the ATP receptor antagonists pyridoxal-5′-phosphate-6-azophenyl-2′,4′-disulphonic acid, Brilliant Blue G or periodate oxidized ATP dialdehyde to the site of ATP release in the anterior hypothalamus markedly augmented and prolonged the febrile response. These data indicate that during the development of the systemic inflammation, ATP is released in the anterior hypothalamus to limit the magnitude and duration of fever. This release may also have a profound effect on the hypothalamic control of other physiological functions in which ATP and related purines have been implicated to play modulatory roles, such as food intake, hormone secretion, cardiovascular activity and sleep.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

ATP and acetylcholine, equal brethren

Acetylcholine was the first neurotransmitter identified and ATP is the hitherto final compound added to the list of small molecule neurotransmitters. Despite the wealth of evidence assigning a signaling role to extracellular ATP and other nucleotides in neural and non-neural tissues, the significance of this signaling pathway was accepted very reluctantly. In view of this, this short commentary contrasts the principal molecular and functional components of the cholinergic signaling pathway with those of ATP and other nucleotides. It highlights pathways of their discovery and analyses tissue distribution, synthesis, uptake, vesicular storage, receptors, release, extracellular hydrolysis as well as pathophysiological significance. There are differences but also striking similarities. Comparable to ACh, ATP is taken up and stored in synaptic vesicles, released in a Ca(2+)-dependent manner, acts on nearby ligand-gated or metabotropic receptors and is hydrolyzed extracellularly. ATP and acetylcholine are also costored and coreleased. In addition, ATP is coreleased from biogenic amine storing nerve terminals as well as from at least subpopulations of glutamatergic and GABAergic terminals. Both ACh and ATP fulfill the criteria postulated for neurotransmitters. More recent evidence reveals that the two messengers are not confined to neural functions, exerting a considerable variety of non-neural functions in non-innervated tissues. While it has long been known that a substantial number of pathologies originate from malfunctions of the cholinergic system there is now ample evidence that numerous pathological conditions have a purinergic component.

Adenosine A(1) receptor: Functional receptor-receptor interactions in the brain

Over the past decade, many lines of investigation have shown that receptor-mediated signaling exhibits greater diversity than previously appreciated. Signal diversity arises from numerous factors, which include the formation of receptor dimers and interplay between different receptors. Using adenosine A₁ receptors as a paradigm of G protein-coupled receptors, this review focuses on how receptor-receptor interactions may contribute to regulation of the synaptic transmission within the central nervous system. The interactions with metabotropic dopamine, adenosine A_2A, A₃, neuropeptide Y, and purinergic P2Y₁ receptors will be described in the first part. The second part deals with interactions between A₁Rs and ionotropic receptors, especially GABA_A, NMDA, and P2X receptors as well as ATP-sensitive K⁺ channels. Finally, the review will discuss new approaches towards treating neurological disorders.

Mapping P2X and P2Y receptor proteins in striatum and substantia nigra: An immunohistological study

Our work aimed to provide a topographical analysis of all known ionotropic P2X_1–7 and metabotropic P2Y_{1,2,4,6,11–14} receptors that are present in vivo at the protein level in the basal ganglia nuclei and particularly in rat brain slices from striatum and substantia nigra. By immunohistochemistry-confocal and Western blotting techniques, we show that, with the exception of P2Y_11,13 receptors, all other subtypes are specifically expressed in these areas in different amounts, with ratings of low (P2X_5,6 and P2Y_1,6,14 in striatum), medium (P2X₃ in striatum and substantia nigra, P2X_6,7 and P2Y₁ in substantia nigra) and high. Moreover, we describe that P2 receptors are localized on neurons (colocalizing with neurofilament light, medium and heavy chains) with features that are either dopaminergic (colocalizing with tyrosine hydroxylase) or GABAergic (colocalizing with parvalbumin and calbindin), and they are also present on astrocytes (P2Y_2,4, colocalizing with glial fibrillary acidic protein). In addition, we aimed to investigate the expression of P2 receptors after dopamine denervation, obtained by using unilateral injection of 6-hydroxydopamine as an animal model of Parkinson’s disease. This generates a rearrangement of P2 proteins: most P2X and P2Y receptors are decreased on GABAergic and dopaminergic neurons, in the lesioned striatum and substantia nigra, respectively, as a consequence of dopaminergic denervation and/or neuronal degeneration. Conversely, P2X_1,3,4,6 on GABAergic neurons and P2Y₄ on astrocytes augment their expression exclusively in the lesioned substantia nigra reticulata, probably as a compensatory reaction to dopamine shortage. These results disclose the presence of P2 receptors in the normal and lesioned nigro-striatal circuit, and suggest their potential participation in the mechanisms of Parkinson’s disease.

Extracellular nucleotides block bone mineralization in vitro: evidence for dual inhibitory mechanisms involving both P2Y2 receptors and pyrophosphate

Extracellular nucleotides, signaling through P2 receptors, may act as local regulators of bone cell function. We investigated the effects of nucleotide agonists [ATP, ADP, uridine triphosphate (UTP), and uridine diphosphate] and pyrophosphate (PPi, a key physiological inhibitor of mineralization) on the deposition and mineralization of collagenous matrix by primary osteoblasts derived from rat calvariae. Our results show that extracellular ATP, UTP, and PPi strongly and selectively blocked the mineralization of matrix nodules; ADP and uridine diphosphate were without effect. Significant inhibition of mineralization occurred in the presence of relatively low concentrations of ATP, UTP, or PPi (1-10 microm), without affecting production of fibrillar or soluble collagen. In cultures treated with 10 microm ATP or UTP, the expression and activity of alkaline phosphatase, which promotes mineralization by hydrolyzing PPi, was inhibited. The potent inhibitory actions of ATP and UTP on bone mineralization are consistent pharmacologically with mediation by the P2Y(2) receptor, which is strongly expressed by mature osteoblasts. In support of this notion, we found 9-17% increases in bone mineral content of hindlimbs of P2Y(2)-deficient mice. We also found that osteoblasts express ectonucleotide phosphodiesterase/pyrophosphatase-1, an ectonucleotidase that hydrolyzes nucleotide triphosphates to yield PPi, and that addition of 10 microm ATP or UTP to osteoblast cultures generated 2 microm PPi within 10 min. Thus, a component of the profound inhibitory action of ATP and UTP on bone mineralization could be mediated directly by PPi, independently of P2 receptors.

Targeted deletion of ectonucleoside triphosphate diphosphohydrolase 1/CD39 leads to desensitization of pre- and postsynaptic purinergic P2 receptors

We previously reported that ATP coreleased with norepinephrine from cardiac sympathetic nerves activates presynaptic P2X purinoceptors (P2XR), thereby enhancing norepinephrine exocytosis. Blockade of ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1/CD39) potentiates norepinephrine exocytosis, whereas recombinant soluble CD39 (solCD39) in-hibits it. This suggested that CD39 gene (Entpd1) deletion would enhance purinergic and adrenergic signaling by preserving ATP and its norepinephrine-releasing activity. However, we found that the neurogenic contractile response of vasa deferentia from Entpd1-null (CD39(-/-)) mice was attenuated and accompanied by reduced activity of pre- and postsynaptic P2XR, whereas contractile responses to K(+) or norepinephrine remained intact. In addition, the magnitude of ATP and norepinephrine exocytosis from cardiac synaptosomes was decreased in CD39(-/-) mice. Inhibition of E-NTPDase1/CD39, or solCD39 administration, did not affect the attenuated contractile response of vasa deferentia from CD39(-/-) mice. Notably, Entpd1 deletion and pharmacological P2XR desensitization in control mice similarly attenuated vasa deferentia responses. Thus, excessive and prolonged ATP exposure resulting from CD39 deletion desensitizes pre- and postjunctional P2XR at the sympathetic neuromuscular junction. This diminishes purinergic activity directly and adrenergic activity indirectly. It remains to be determined whether this desensitization results from receptor internalization, changes in receptor conformation or phosphorylation. Shutdown of ATP signaling in CD39(-/-) mice may represent a defense mechanism for the prevention of purinergic overstimulation. Our findings emphasize the cardioprotective role of neuronal CD39: by reducing presynaptic facilitatory effects of neurotransmitter ATP, CD39 attenuates norepinephrine release and its dysfunctional consequences. Moreover, by virtue of its antithrombotic action CD39 can potentially prevent the transition from myocardial ischemia to infarction.

A novel method using fluorescence microscopy for real-time assessment of ATP release from individual cells

Many cell types release ATP in response to mechanical or biochemical stimulation. The mechanisms responsible for this release, however, are not well understood and may differ among different cell types. In addition, there are numerous difficulties associated with studying the dynamics of ATP release immediately outside the cell membrane. Here, we report a new method that allows the visualization and quantification of ATP release by fluorescence microscopy. Our method utilizes a two-enzyme system that generates NADPH when ATP is present. NADPH is a fluorescent molecule that can be visualized by fluorescence microscopy using an excitation wavelength of 340 nm and an emission wavelength of 450 nm. The method is capable of detecting ATP concentrations <1 microM and has a dynamic range of up to 100 microM. Using this method, we visualized and quantified ATP release from human polymorphonuclear leukocytes and Jurkat T cells. We show that upon cell stimulation, the concentrations of ATP can reach levels of up to 80 microM immediately outside of the cell membrane. This new method should prove useful for the study of the mechanisms of release and functional role of ATP in various cell systems, including individual cells.

[(3)H]Adenine is a suitable radioligand for the labeling of G protein-coupled adenine receptors but shows high affinity to bacterial contaminations in buffer solutions

[³H]Adenine has previously been used to label the newly discovered G protein-coupled murine adenine receptors. Recent reports have questioned the suitability of [³H]adenine for adenine receptor binding studies because of curious results, e.g. high specific binding even in the absence of mammalian protein. In this study, we showed that specific [³H]adenine binding to various mammalian membrane preparations increased linearly with protein concentration. Furthermore, we found that Tris-buffer solutions typically used for radioligand binding studies (50 mM, pH 7.4) that have not been freshly prepared but stored at 4°C for some time may contain bacterial contaminations that exhibit high affinity binding for [³H]adenine. Specific binding is abolished by heating the contaminated buffer or filtering it through 0.2-μm filters. Three different, aerobic, gram-negative bacteria were isolated from a contaminated buffer solution and identified as Achromobacter xylosoxidans, A. denitrificans, and Acinetobacter lwoffii. A. xylosoxidans, a common bacterium that can cause nosocomial infections, showed a particularly high affinity for [³H]adenine in the low nanomolar range. Structure–activity relationships revealed that hypoxanthine also bound with high affinity to A. xylosoxidans, whereas other nucleobases (uracil, xanthine) and nucleosides (adenosine, uridine) did not. The nature of the labeled site in bacteria is not known, but preliminary results indicate that it may be a high-affinity purine transporter. We conclude that [³H]adenine is a well-suitable radioligand for adenine receptor binding studies but that bacterial contamination of the employed buffer solutions must be avoided.

Abnormalities in neuromuscular junction structure and skeletal muscle function in mice lacking the P2X2 nucleotide receptor

ATP is co-released in significant quantities with acetylcholine from motor neurons at skeletal neuromuscular junctions (NMJ). However, the role of this neurotransmitter in muscle function remains unclear. The P2X2 ion channel receptor subunit is expressed during development of the skeletal NMJ, but not in adult muscle fibers, although it is re-expressed during muscle fiber regeneration. Using mice deficient for the P2X2 receptor subunit for ATP (P2X2(-/-)), we demonstrate a role for purinergic signaling in NMJ development. Whereas control NMJs were characterized by precise apposition of pre-synaptic motor nerve terminals and post-synaptic junctional folds rich in acetylcholine receptors (AChRs), NMJs in P2X2(-/-) mice were disorganized: misapposition of nerve terminals and post-synaptic AChR expression localization was common; the density of post-synaptic junctional folds was reduced; and there was increased end-plate fragmentation. These changes in NMJ structure were associated with muscle fiber atrophy. In addition there was an increase in the proportion of fast type muscle fibers. These findings demonstrate a role for P2X2 receptor-mediated signaling in NMJ formation and suggest that purinergic signaling may play an as yet largely unrecognized part in synapse formation.

Sympathetic vasoconstrictor regulation of mouse colonic submucosal arterioles is altered in experimental colitis

Recent studies suggest that altered neural regulation of the gastrointestinal microvasculature contributes to the pathogenesis of inflammatory bowel disease. Therefore, we employed video microscopy techniques to monitor nerve-evoked vasoconstrictor responses in mouse colonic submucosal arterioles in vitro and examined the effect of 2,4,6-trinitrobenzene sulphonic acid (TNBS) colitis. Nerve stimulation (2-20 Hz) caused frequency-dependent vasoconstrictor responses that were abolished by tetrodotoxin (300 nm) and guanethidine (10 microm). The P2 receptor antagonist suramin (100 microm) or the alpha(1)-adrenoceptor antagonist prazosin (100 nm) reduced the vasoconstriction and the combination of suramin and prazosin completely abolished responses. Nerve-evoked constrictions of submucosal arterioles from mice with TNBS colitis were inhibited by prazosin but not suramin. Superfusion of ATP (10 microm) resulted in large vasoconstrictions in control mice but had no effect in mice with colitis whereas constrictions to phenylephrine (3 microm) were unaffected. P2X(1) receptor immunohistochemistry did not suggest any alteration in receptor expression following colitis. However, Western blotting revealed that submucosal P2X(1) receptor expression was increased during colitis. In contrast to ATP, alphabeta-methylene-ATP (1 microm), which is resistant to catabolism by nucleotidases, constricted control and TNBS arterioles. This indicates that reduced purinergic transmission to submucosal arterioles may be due to increased degradation of ATP during colitis. These data comprise the first description of the neural regulation of mouse submucosal arterioles and identify a defect in sympathetic regulation of the GI vasculature during colitis due to reduced purinergic neurotransmission.

Mice lacking P2Y2 receptors have salt-resistant hypertension and facilitated renal Na+ and water reabsorption

Extracellular nucleotides (e.g., ATP) regulate many physiological and pathophysiological processes through activation of nucleotide (P2) receptors in the plasma membrane. Here we report that gene-targeted (knockout) mice that lack P2Y2 receptors have salt-resistant arterial hypertension in association with an inverse relationship between salt intake and heart rate, indicating intact baroreceptor function. Knockout mice have multiple alterations in their handling of salt and water: these include suppressed plasma renin and aldosterone concentrations, lower renal expression of the aldosterone-induced epithelial sodium channel alpha-ENaC, greater medullary expression of the Na-K-2Cl-cotransporter NKCC2, and greater furosemide-sensitive Na+ reabsorption in association with greater renal medullary expression of aquaporin-2 and vasopressin-dependent renal cAMP formation and water reabsorption despite similar vasopressin levels compared with wild type. Of note, smaller increases in plasma aldosterone were required to adapt renal Na+ excretion to restricted intake in knockout mice, suggesting a facilitation in renal Na+ retention. The results thus identify a previously unrecognized role for P2Y2 receptors in blood pressure regulation that is linked to an inhibitory influence on renal Na+ and water reabsorption. Based on these findings in knockout mice, we propose that a blunting in P2Y2 receptor expression or activity is a new mechanism for salt-resistant arterial hypertension.

ATP release and hydrolysis contribute to rat pial arteriolar dilatation elicited by neuronal activation

Owing to their intimate anatomical relationship with cerebral arterioles, astrocytes have been postulated as signal transducers, transferring information from activated neurones to the cerebral microcirculation. These forwarded signals may involve the release of vasoactive factors from the end-feet of astrocytes. This mechanism is termed 'neurovascular coupling' and its anatomical components (i.e. neurone, astrocyte and vascular cells) are termed the 'neurovascular unit'. The process of neurovascular coupling often involves upstream dilatation. This is necessary during periods of increased metabolic demand, in order to permit more blood to reach dilated downstream vessels, thereby improving nutrient supply to the activated neurones. Without it, that downstream dilatation might be ineffective, placing neurones at risk, especially during episodes of intense neuronal activity, such as seizure. In the brain, pial arterioles represent important 'upstream' vascular segments. The pial arterioles overlie a thick layer of astrocytic processes, termed the glia limitans. This essentially isolates pial arterioles, anatomically, from the neurones below. Vasodilating signals that originate in the neurones therefore reach the pial arterioles via indirect pathways, primarily involving astrocytes and the glia limitans. Here we discuss a process whereby purinergic mechanisms play a key and neuronal activity-dependent role in astrocyte to astrocyte communication, as well as in glia limitans to pial arteriolar signals leading to vasodilatation.