P2 receptors and neuronal injury

Differential effects of undernourishment on the differentiation and maturation of rat enteric neurons

The colocalization, number, and size of various classes of enteric neurons immunoreactive (IR) for the purinergic P2X2 and P2X7 receptors (P2X2R, P2X7R) were analyzed in the myenteric and submucosal plexuses of control, undernourished, and re-fed rats. Pregnant rats were exposed to undernourishment (protein-deprivation) or fed a control diet, and their offspring comprised the following experimental groups: rats exposed to a normal diet throughout gestation until postnatal day (P)42, rats protein-deprived throughout gestation and until P42, and rats protein-deprived throughout gestation until P21 and then given a normal diet until P42. Immunohistochemistry was performed on the myenteric and submucosal plexuses to evaluate immunoreactivity for P2X2R, P2X7R, nitric oxide synthase (NOS), choline acetyltransferase (ChAT), calbindin, and calretinin. Double-immunohistochemistry of the myenteric and submucosal plexuses demonstrated that 100% of NOS-IR, calbindin-IR, calretinin-IR, and ChAT-IR neurons in all groups also expressed P2X2R and P2X7R. Neuronal density increased in the myenteric and submucosal plexuses of undernourished rats compared with controls. The average size (profile area) of some types of neurons in the myenteric and submucosal plexuses was smaller in the undernourished than in the control animals. These changes appeared to be reversible, as animals initially undernourished but then fed a normal diet at P21 (re-feeding) were similar to controls. Thus, P2X2R and P2X7R are present in NOS-positive inhibitory neurons, calbindin- and calretinin-positive intrinsic primary afferent neurons, cholinergic secretomotor neurons, and vasomotor neurons in rats. Alterations in these neurons during undernourishment are reversible following re-feeding.

Microglial calcium signaling in the adult, aged and diseased brain

Microglial cells are the resident immune cells of the CNS. They mediate innate immune response of the brain to injury, inflammation and neurodegenerative diseases. Apart from their role in disease they are critically involved in the development and plasticity-driven reorganization of neuronal networks and the homeostatic maintenance of brain tissue. Accumulating in vitro evidence suggests that executive functions of microglia are coupled to the intracellular Ca(2+) signaling of these cells. So far, however, very little is known about microglial Ca(2+) signaling in situ or in vivo, both in the healthy and in the diseased brain. Here, we summarize the recent in vivo/in situ findings and compare the properties of surveillant microglia in these preparations with those of microglia in vitro. The data suggest that surveillant microglia rarely show spontaneous Ca(2+) transients, express fewer functional receptors directly coupled to changes in the intracellular free Ca(2+) concentration on their surface, but vividly respond with Ca(2+) transients to cell or tissue damage in their microenvironment. Interestingly, some of these properties microglia share with monocytes engrafting in the brain under pathological conditions.

The role of non-synaptic extracellular glutamate

Although there are some mechanisms which allow the direct crossing of substances between the cytoplasm of adjacent cells (gap junctions), most substances use the extracellular space to diffuse between brain cells. The present work reviews the behavior and functions of extracellular glutamate (GLU). There are two extracellular pools of glutamate (GLU) in the brain, a synaptic pool whose functions in the excitatory neurotransmission has been widely studied and an extrasynaptic GLU pool although less known nonetheless is gaining attention among a growing number of researchers. Evidence accumulated over the last years shows a number of mechanisms capable of releasing glial GLU to the extracellular medium, thus modulating neurons, microglia and oligodendrocytes, and regulating the immune response, cerebral blood flow, neuronal synchronization and other brain functions. This new scenario is expanding present knowledge regarding the role of GLU in the brain under different physiological and pathological conditions. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Spatial and developmental heterogeneity of calcium signaling in olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are specialized glial cells in the mammalian olfactory system supporting growth of axons from the olfactory epithelium into the olfactory bulb. OECs in the olfactory bulb can be subdivided into OECs of the outer nerve layer and the inner nerve layer according to the expression of marker proteins and their location in the nerve layer. In the present study, we have used confocal calcium imaging of OECs in acute mouse brain slices and olfactory bulbs in toto to investigate physiological differences between OEC subpopulations. OECs in the outer nerve layer, but not the inner nerve layer, responded to glutamate, ATP, serotonin, dopamine, carbachol, and phenylephrine with increases in the cytosolic calcium concentration. The calcium responses consisted of a transient and a tonic component, the latter being mediated by store-operated calcium entry. Calcium measurements in OECs during the first three postnatal weeks revealed a downregulation of mGluR(1) and P2Y(1) receptor-mediated calcium signaling within the first 2 weeks, suggesting that the expression of these receptors is developmentally controlled. In addition, electrical stimulation of sensory axons evoked calcium signaling via mGluR(1) and P2Y(1) only in outer nerve layer OECs. Downregulation of the receptor-mediated calcium responses in postnatal animals is reflected by a decrease in amplitude of stimulation-evoked calcium transients in OECs from postnatal days 3 to 21. In summary, the results presented reveal striking differences in receptor responses during development and in axon-OEC communication between the two subpopulations of OECs in the olfactory bulb.

G protein-coupled adenosine (P1) and P2Y receptors: ligand design and receptor interactions

The medicinal chemistry and pharmacology of the four subtypes of adenosine receptors (ARs) and the eight subtypes of P2Y receptors (P2YRs, activated by a range of purine and pyrimidine mono- and dinucleotides) has recently advanced significantly leading to selective ligands. X-ray crystallographic structures of both agonist- and antagonist-bound forms of the A(2A)AR have provided unprecedented three-dimensional detail concerning molecular recognition in the binding site and the conformational changes in receptor activation. It is apparent that this ubiquitous cell signaling system has implications for understanding and treating many diseases. ATP and other nucleotides are readily released from intracellular sources under conditions of injury and organ stress, such as hypoxia, ischemia, or mechanical stress, and through channels and vesicular release. Adenosine may be generated extracellularly or by cellular release. Therefore, depending on pathophysiological factors, in a given tissue, there is often a tonic activation of one or more of the ARs or P2YRs that can be modulated by exogenous agents for a beneficial effect. Thus, this field has provided fertile ground for pharmaceutical development, leading to clinical trials of selective receptor ligands as imaging agents or for conditions including cardiac arrhythmias, ischemia/reperfusion injury, diabetes, pain, thrombosis, Parkinson's disease, rheumatoid arthritis, psoriasis, dry eye disease, pulmonary diseases such as cystic fibrosis, glaucoma, cancer, chronic hepatitis C, and other diseases.

Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Glaucoma is a common optic neuropathy with a complex etiology often linked to sensitivity to intraocular pressure. Though the precise mechanisms that mediate or transduce this sensitivity are not clear, the axon of the retinal ganglion cell appears to be vulnerable to disease-relevant stressors early in progression. One reason may be because the axon is generally thin for both its unmyelinated and myelinated segment and much longer than the thicker unmyelinated axons of other excitatory retinal neurons. This difference may predispose the axon to metabolic and oxidative injury, especially at distal sites where pre-synaptic terminals form connections in the brain. This idea is consistent with observations of early loss of anterograde transport at central targets and other signs of distal axonopathy that accompany physiological indicators of progression. Outright degeneration of the optic projection ensues after a critical period and, at least in animal models, is highly sensitive to cumulative exposure to elevated pressure in the eye. Stress emanating from the optic nerve head can induce not only distal axonopathy with aspects of dying back neuropathy, but also Wallerian degeneration of the optic nerve and tract and a proximal program involving synaptic and dendritic pruning in the retina. Balance between progressive and acute mechanisms likely varies with the level of stress placed on the unmyelinated axon as it traverses the nerve head, with more acute insult pushing the system toward quicker disassembly. A constellation of signaling factors likely contribute to the transduction of stress to the axon, so that degenerative events along the length of the optic projection progress in retinotopic fashion. This pattern leads to well-defined sectors of functional depletion, even at distal-most sites in the pathway. While ganglion cell somatic drop-out is later in progression, some evidence suggests that synaptic and dendritic pruning in the retina may be a more dynamic process. Structural persistence both in the retina and in central projection sites offers the possibility that intrinsic self-repair pathways counter pathogenic mechanisms to delay as long as possible outright loss of tissue.

Purinergic signaling promotes proliferation of adult mouse subventricular zone cells

In adult mammalian brains, neural stem cells (NSCs) exist in the subventricular zone (SVZ), where persistent neurogenesis continues throughout life. Those NSCs produce neuroblasts that migrate into the olfactory bulb via formation of transit-amplifying cells, which are committed precursor cells of the neuronal lineage. In this SVZ niche, cell-cell communications conducted by diffusible factors as well as physical cell-cell contacts are important for the regulation of the proliferation and fate determination of NSCs. Previous studies have suggested that extracellular purinergic signaling, which is mediated by purine compounds such as ATP, plays important roles in cell-cell communication in the CNS. Purinergic signaling also promotes the proliferation of adult NSCs in vitro. However, the in vivo roles of purinergic signaling in the neurogenic niche still remain unknown. In this study, ATP infusion into the lateral ventricle of the mouse brain resulted in an increase in the numbers of rapidly dividing cells and Mash1-positive transit-amplifying cells (Type C cells) in the SVZ. Mash1-positive cells express the P2Y1 purinergic signaling receptor and infusion of the P2Y1 receptor-specific antagonist MRS2179 decreased the number of rapidly dividing bromodeoxyuridine (BrdU)-positive cells and Type C cells. Moreover, a 17% reduction of rapidly dividing BrdU-positive cells and a 19% reduction of Mash1-positive cells were observed in P2Y1 knock-out mice. Together, these results suggest that purinergic signaling promotes the proliferation of rapidly dividing cells and transit-amplifying cells, in the SVZ niche through the P2Y1 receptor.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Chronic renal denervation increases renal tubular response to P2X receptor agonists in rats: implication for renal sympathetic nerve ablation

BACKGROUND

Kidney noradrenergic innervation regulates tubular function. Adenosine triphosphate (ATP)-a co-transmitter of norepinephrine-acts on purinoceptors, including ion channel receptor, P2X. P2X receptor agonists, α,β-methylene ATP (α,β-meATP) and β,γ-methylene ATP (β,γ-meATP), induce natriuresis. Regarding the functional co-localization of adrenoceptors and P2X receptors, we evaluated rat renal tubular system sensitivity to natriuretic action of P2X receptor agonists in chronically denervated kidney.

METHODS

Clearance studies with α,β-meATP and β,γ-meATP (intravenous infusion rate, 2 µmol/kg + 20 nmol/kg/min) were performed after bilateral surgical kidney denervation (DNx) and sham-operation (Sham). Na/K-ATPase activity was measured in isolated rat renal proximal tubules.

RESULTS

In DNx compared with Sham, saline infusion significantly increased renal sodium and urine excretion and P2X receptor agonist infusion was significantly more natriuretic and diuretic. In DNx and Sham, respectively, α,β-meATP increased fractional excretion of sodium (FE(Na)) by 2 ± 0.3 and 0.6 ± 0.1% and urine (FE(V)) by 1.6 ± 0.3 and 0.9 ± 0.2%; β,γ-meATP had similar effects. In both groups of rats, natriuretic and diuretic actions were abolished by P2 receptor blocker (pyridoxal-phosphate-6-azophenyl-2',4'-disulphonate, PPADS), mean arterial blood pressure and glomerular filtration rate remained unchanged during infusion of P2X receptor agonists and antagonist and basal Na/K-ATPase activities in isolated proximal tubules were similar. Both α,β-meATP and β,γ-me-ATP decreased the Na/K-ATPase activity, with 20% inhibition (P < 0.05) in denervated and innervated rats; these inhibitory effects were abolished in the presence of PPADS.

CONCLUSIONS

Decreased renal sympathetic activity enhances the natriuretic effect of P2X receptor stimulation. This effect is probably not related to altered Na/K-ATPase activity in renal proximal tubules.

Psychophysical and cerebral responses to heat stimulation in patients with central pain, painless central sensory loss, and in healthy persons

Patients with central pain (CP) typically have chronic pain within an area of reduced pain and temperature sensation, suggesting an impairment of endogenous pain modulation mechanisms. We tested the hypothesis that some brain structures normally activated by cutaneous heat stimulation would be hyperresponsive among patients with CP but not among patients with a central nervous system lesion causing a loss of heat or nociceptive sensation with no pain (NP). We used H(2)(15)O positron emission tomography to measure, in 15 healthy control participants, 10 NP patients, and 10 CP patients, increases in regional cerebral blood flow among volumes of interest (VOI) from the resting (no stimulus) condition during bilateral contact heat stimulation at heat detection, heat pain threshold, and heat pain tolerance levels. Both patient groups had a reduced perception of heat intensity and unpleasantness on the clinically affected side and a bilateral impairment of heat detection. Compared with the HC group, both NP and CP patients had more hyperactive and hypoactive VOI in the resting state and more hyperresponsive and hyporesponsive VOI during heat stimulation. Compared with NP patients, CP patients had more hyperresponsive VOI in the intralaminar thalamus and sensory-motor cortex during heat stimulation. Our results show that focal CNS lesions produce bilateral sensory deficits and widespread changes in the nociceptive excitability of the brain. The increased nociceptive excitability within the intralaminar thalamus and sensory-motor cortex of our sample of CP patients suggests an underlying pathophysiology for the pain in some central pain syndromes.

The role of purinergic receptors in retinal function and disease

Extracellular ATP acts as a neurotransmitter in the central and peripheral nervous systems. In this review, the role of purinergic receptors in neuronal signaling and bi-directional glial-neuronal communication in the retina will be considered. There is growing evidence that a range of P2X and P2Y receptors are expressed on most classes of retinal neurons and that activation of P2 receptors modulates retinal function. Furthermore, neuronal control of glial function is achieved through neuronal release of ATP and activation of P2Y receptors expressed by Müller cells. Altered purinergic signaling in Müller cells has been implicated in gliotic changes in the diseased retina and furthermore, elevations in extracellular ATP may lead to apoptosis of retinal neurons.

Impact of ectoenzymes on p2 and p1 receptor signaling

P2 receptors that are activated by extracellular nucleotides (e.g., ATP, ADP, UTP, UDP, Ap(n)A) and P1 receptors activated by adenosine control a diversity of biological processes. The activation of these receptors is tightly regulated by ectoenzymes that metabolize their ligands. This review presents these enzymes as well as their roles in the regulation of P2 and P1 receptor activation. We focus specifically on the role of ectoenzymes in processes of our interest, that is, inflammation, vascular tone, and neurotransmission. An update on the development of ectonucleotidase inhibitors is also presented.

Purinergic signaling in the cerebellum: Bergmann glial cells express functional ionotropic P2X7 receptors

Astrocytes constitute active networks of intercommunicating cells that support the metabolism and the development of neurons and affect synaptic functions via multiple pathways. ATP is one of the major neurotransmitters mediating signaling between neurons and astrocytes. Potentially acting through both purinergic metabotropic P2Y receptors (P2YRs) and ionotropic P2X receptors (P2XRs), up until now ATP has only been shown to activate P2YRs in Bergmann cells, the radial glia of the cerebellar cortex that envelopes Purkinje cell afferent synapses. In this study, using multiple experimental approaches in acute cerebellar slices we demonstrate the existence of functional P2XRs on Bergmann cells. In particular, we show here that Bergmann cells express uniquely P2X7R subtypes: (i) immunohistochemical analysis revealed the presence of P2X7Rs on Bergmann cell processes, (ii) in whole cell recordings P2XR pharmacological agonists induced depolarizing currents that were blocked by specific antagonists of P2X7Rs, and could not be elicited in slices from P2X₇R-deficient mice and finally, (iii) calcium imaging experiments revealed two distinct calcium signals triggered by application of exogenous ATP: a transient signal deriving from release of calcium from intracellular stores, and a persistent one following activation of P2X7Rs. Our data thus reveal a new pathway by which extracellular ATP may affect glial cell function, thus broadening our knowledge on purinergic signaling in the cerebellum.

Effects of ischemia and reperfusion on P2X2 receptor expressing neurons of the rat ileum enteric nervous system

PURPOSE

We investigated the effects of ischemia/reperfusion in the intestine (I/R-i) on purine receptor P2X2-immunoreactive (IR) neurons of the rat ileum.

METHODS

The superior mesenteric artery was occluded for 45 min with an atraumatic vascular clamp and animals were sacrificed 4 h later. Neurons of the myenteric and submucosal plexuses were evaluated for immunoreactivity against the P2X2 receptor, nitric oxide synthase (NOS), choline acetyl transferase (ChAT), calbindin, and calretinin.

RESULTS

Following I/R-i, we observed a decrease in P2X2 receptor immunoreactivity in the cytoplasm and surface membranes of neurons of the myenteric and submucosal plexuses. These studies also revealed an absence of calbindin-positive neurons in the I/R-i group. In addition, the colocalization of the P2X2 receptor with NOS, ChAT, and calretinin immunoreactivity in the myenteric plexus was decreased following I/R-i. Likewise, the colocalization between P2X2 and calretinin in neurons of the submucosal plexus was also reduced. In the I/R-i group, there was a 55.8% decrease in the density of neurons immunoreactive (IR) for the P2X2 receptor, a 26.4% reduction in NOS-IR neuron, a 25% reduction in ChAT-IR neuron, and a 47% reduction in calretinin-IR neuron. The density of P2X2 receptor and calretinin-IR neurons also decreased in the submucosal plexus of the I/R-i group. In the myenteric plexus, P2X2-IR, NOS-IR, ChAT-IR and calretinin-IR neurons were reduced in size by 50%, 49.7%, 42%, and 33%, respectively, in the I/R-i group; in the submucosal plexus, P2X2-IR and calretinin-IR neurons were reduced in size by 56% and 72.6%, respectively.

CONCLUSIONS

These data demonstrate that ischemia/reperfusion of the intestine affects the expression of the P2X2 receptor in neurons of the myenteric and submucosal plexus, as well as density and size of neurons in this population. Our findings indicate that I/R-i induces changes in P2X2-IR enteric neurons that could result in alterations in intestinal motility.

Neurotransmitters couple brain activity to subventricular zone neurogenesis

Adult neurogenesis occurs in two privileged microenvironments, the hippocampal subgranular zone of the dentate gyrus and the subventricular zone (SVZ) along the lateral ventricle. This review focuses on accumulating evidence suggesting that the activity of specific brain regions or bodily states influences SVZ cell proliferation and neurogenesis. Neuromodulators such as dopamine and serotonin have been shown to have long-range effects through neuronal projections into the SVZ. Local γ-aminobutyric acid and glutamate signaling have demonstrated effects on SVZ proliferation and neurogenesis, but an extra-niche source of these neurotransmitters remains to be explored and options will be discussed. There is also accumulating evidence that diseases and bodily states such as Alzheimer's disease, seizures, sleep and pregnancy influence SVZ cell proliferation. With such complex behavior and environmentally-driven factors that control subregion-specific activity, it will become necessary to account for overlapping roles of multiple neurotransmitter systems on neurogenesis when developing cell therapies or drug treatments.

The relationship between the neuromodulator adenosine and behavioral symptoms of autism

The neuromodulator adenosine is an endogenous sleep promoter, neuroprotector and anticonvulsant, and people with autism often suffer from sleep disruption and/or seizures. We hypothesized that increasing adenosine can decrease behavioral symptoms of autism spectrum disorders, and, based on published research, specific physiological stimuli are expected to increase brain adenosine. To test the relationship between adenosine and autism, we developed a customized parent-based questionnaire to assess child participation in activities expected to influence adenosine and quantify behavioral changes following these experiences. Parents were naive to study hypotheses and all conditions were pre-assigned. Results demonstrate significantly better behavior associated with events pre-established as predicted to increase rather than decrease or have no influence on adenosine. Understanding the physiological relationship between adenosine and autism could open new therapeutic strategies--potentially preventing seizures, improving sleep, and reducing social and behavioral dysfunction.

Blockade of P2 nucleotide receptors after spinal cord injury reduced the gliotic response and spared tissue

Spinal cord injury (SCI) triggers a sequel of events commonly associated with cell death and dysfunction of glias and neurons surrounding the lesion. Although astrogliosis and glial scar formation have been involved in both damage and repair processes after SCI, their role remains controversial. Our goal was to investigate the effects of the P2 receptors antagonists, PPADS and suramin, in the establishment of the reactive gliosis and the formation of the glial scar. Molecular biology, immunohistochemistry, spared tissue, and locomotor behavioral studies were used to evaluate astrogliosis, in adult female Sprague-Dawley rats treated with P2 antagonists after moderate injury with the NYU impactor device. Semi-quantitative RT-PCR confirmed the presence of P2Y(1,) P2Y(2,) P2Y(4,) P2Y(6,) P2Y(12), and P2X(2) receptors in the adult spinal cord. Immunohistochemistry studies confirmed a significant decrease in GFAP-labeled cells at the injury epicenter as well as a decrease in spared tissue after treatment with the antagonists. Functional open field testing revealed no significant locomotor score differences between treated and control animals. Our work is consistent with studies suggesting that astrogliosis is an important event after SCI that limits tissue damage and lesion spreading.

Dynamic changes in the expression pattern of ecto-5'-nucleotidase in the rat model of cortical stab injury

Traumatic injury induces massive release of ATP in the extracellular space, where it influences numerous aspects of neuronal, astrocytic, and microglial responses to injury by activating P2X and P2Y receptors. The extracellular ATP actions are controlled by the ectonucleotidase enzyme pathway, which hydrolyses ATP to adenosine at all neuronal and nonneuronal cell types. Adenosine activates its P1 receptors, which have important neuroprotective roles. The rate-limiting enzyme in the ectonucleotidase pathway is ecto-5'-nucleotidase (e-5NT), which catalyzes the final step of dephosphorylation of AMP to adenosine. The aim of the present study was to characterize the expression pattern and cellular distribution of e-5NT in the perilesioned cortex at 4 hr and 1, 2, 7, and 15 days after unilateral cortical stab injury (CSI). Immunoblot and immunohistochemical studies showed that overall e-5NT expression was lower 4 hr and 1 day postinjury and then gradually increased above the control levels. Double-immunofluorescence studies further showed in control tissue the presence of the enzyme in the membranes surrounding neuronal somata and apical dendrites and less frequently in astrocytes. CSI caused a rapid (after 4 hr) and irreversible loss of the enzyme from neurons, accounting for a decrease in the overall enzyme expression. This was accompanied with a gradual increase in e-5NT-positive astrocytes, accounting for up-regulation of the enzyme levels in the injured area. Thus, CSI induced dynamic changes in the expression pattern of e-5NT that modify the ATP/adenosine ratio and the extent of P1 and P2 receptors activation and, therefore, outcome of the pathological processes after CSI.

P2X receptors in health and disease

Seven P2X receptor subunits have been cloned which form functional homo- and heterotrimers. These are cation-selective channels, equally permeable to Na(+) and K(+) and with significant Ca(2+) permeability. The three-dimensional structure of the P2X receptor is described. The channel pore is formed by the α-helical transmembrane spanning region 2 of each subunit. When ATP binds to a P2X receptor, the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed on both central and peripheral neurons, where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion, and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

Regional changes in purines and selected purinergic receptors in immature rat brain exposed to lead

Lead (Pb) toxicity still remains a significant health problem, since it was recognized as a potent neurodevelopmental toxin. Regarding the fact that in the nervous system ATP is not only the energy source but also acts as a signaling molecule outside the cell, it was of interest to investigate both the level of purines and expression of purinergic receptors in different regions of immature rat brain under Pb toxicity conditions. We examined the expression of A₁ receptor which is involved in neuroprotective mechanisms, and P2X₇R receptor related to the inflammatory and neurodegenerative processes. Expression of receptors' protein was analysed using immunoblotting method whereas HPLC method was used to measure the levels of purines. We observed the features of energetic stress in all examined brain structures expressed by decrease in ATP and ADP levels and AEC ratio. However, in forebrain cortex, the observed changes were milder than in cerebellum and hippocampus. Enhanced expression of A₁R and high increase of adenosine (Ado) level, suggest the proper function of protective mechanisms mediated by Ado. We have found that hippocampus is most vulnerable to Pb toxicity, both due to the high energy depletion and the pattern of expression of investigated receptors. Enhanced expression of P2X₇R and connexin 43 (Cx43) in glial fraction (GPV), suggests the involvement of astrocytic pool of cells into the pathological changes observed in this structure of Pb-exposed immature rat brains.

Extracellular high dosages of adenosine triphosphate induce inflammatory response and insulin resistance in rat adipocytes

Adenosine triphosphate (ATP), an important signaling molecule, participates in various pathophysiological processes via the activation of purinergic-receptors. Recent studies have shown that the expression and function of purinergic-receptors (P2-receptors) could be altered in diabetic or hyperinsulinemia conditions. To characterize the effect of ATP on insulin signaling, we treated primary rat adipocytes with varied concentrations of ATP. The pre-treatment led to impaired insulin signaling, i.e., blunted phosphorylation in Insulin Receptor Substrate-1 (IRS-1) tyrosine and Protein Kinase B (PKB) Ser473 in response to insulin treatment, when ATP concentration reached 1mM. We then observed that ATP dose-dependently reduced the level of IκB, a negative regulator of inflammatory response. Consistently, IRS-1 Ser307 phosphorylation in response to insulin treatment, a site for inflammatory pathway to interfere insulin signaling, was enhanced by ATP. Furthermore, effects of ATP on insulin signaling and IκB content were blocked by P2-receptor inhibition. Finally, insulin-stimulated glucose uptake was impaired by ATP in adipocytes but not in the L6 muscle cells. This study therefore shows for the first time the involvement of ATP-evoked P2-receptor activation in mediating the inflammatory response and the generation of insulin resistance in adipocytes.

Neuronal soma-satellite glial cell interactions in sensory ganglia and the participation of purinergic receptors

It has been known for some time that the somata of neurons in sensory ganglia respond to electrical or chemical stimulation and release transmitters in a Ca2+-dependent manner. The function of the somatic release has not been well delineated. A unique characteristic of the ganglia is that each neuronal soma is tightly enwrapped by satellite glial cells (SGCs). The somatic membrane of a sensory neuron rarely makes synaptic contact with another neuron. As a result, the influence of somatic release on the activity of adjacent neurons is likely to be indirect and/or slow. Recent studies of neuron-SGC interactions have demonstrated that ATP released from the somata of dorsal root ganglion neurons activates SGCs. They in turn exert complex excitatory and inhibitory modulation of neuronal activity. Thus, SGCs are actively involved in the processing of afferent information. In this review, we summarize our understanding of bidirectional communication between neuronal somata and SGCs in sensory ganglia and its possible role in afferent signaling under normal and injurious conditions. The participation of purinergic receptors is emphasized because of their dominant roles in the communication.

Bradykinin increases resensitization of purinergic receptor signaling in glioma cells

BACKGROUND

Purinergic receptor-mediated signaling plays an important role in the function of glial cells, including glial tumor cells. Bradykinin is also an important paracrine mediator which is highly expressed in brain tumors and may correlate with their pathological grade. Interaction between bradykinin and purinergic signaling may therefore be involved in the regulation of glial tumor cells.

RESULTS

We examined the effect of bradykinin on glial purinergic signaling in an immortalized glioma cell line. Confocal calcium imaging revealed that ATP evokes an increase in [Ca2+]i in the U87 human astrocytoma cell line. This response was reduced with repetitive application of ATP, likely due to receptor desensitization. However exposure to bradykinin increased the Ca2+ response to a second application of ATP, consistent with increased resensitization. The bradykinin effect on resensitization was similar in the absence of extracellular Ca2+ or in the presence of the PKC activator PMA, but was inhibited by the protein phosphatase inhibitor okadaic acid and the PI3K inhibitor LY294002.

CONCLUSIONS

Modulation of protein phosphatases and the PI3K pathway may represent a mechanism by which bradykinin potentiates purinergic signaling in glial cells.

Temporal expression of P2X7 purinergic receptor during the course of experimental autoimmune encephalomyelitis

Purinergic P2X(7) receptors are nucleotide-gated ion channels widely distributed in brain. Strong evidence suggests that they are involved in cross-talk between glial and neuronal cells. These receptors activated under pathological conditions may participate in regulation of inflammatory response and cell death. In this study we show the expression of P2X(7) protein and mRNA during the course of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), in different stages of the disease (4, 6, 8, 10 post-immunization). The enhanced expression of the receptor at the level of both mRNA and protein was observed in the peak of neurological symptoms and was connected mostly with neurons. However, early overexpression of receptor protein was observed also in an asymptomatic phase of EAE and was tightly related to astrocytic pool of cells. This suggests the early involvement of this kind of receptor into pathological mechanisms leading for symptoms characteristic for EAE.

Caffeine and the control of cerebral hemodynamics

While the influence of caffeine on the regulation of brain perfusion has been the subject of multiple publications, the mechanisms involved in that regulation remain unclear. To some extent, that uncertainty is a function of a complex interplay of processes arising from multiple targets of caffeine located on a variety of different cells, many of which have influence, either directly or indirectly, on cerebral vascular smooth muscle tone. Adding to that complexity are the target-specific functional changes that may occur when comparing acute and chronic caffeine exposure. In the present review, we discuss some of the mechanisms behind caffeine influences on cerebrovascular function. The major effects of caffeine on the cerebral circulation can largely be ascribed to its inhibitory effects on adenosine receptors. Herein, we focus mostly on the A1, A2A, and A2B subtypes located in cells comprising the neurovascular unit (neurons, astrocytes, vascular smooth muscle); their roles in the coupling of increased neuronal (synaptic) activity to vasodilation; how caffeine, through blockade of these receptors, may interfere with the "neurovascular coupling" process; and receptor-linked changes that may occur in cerebrovascular regulation when comparing acute to chronic caffeine intake.

Effects of protein deprivation and re-feeding on P2X2 receptors in enteric neurons

AIM: To investigate the effects of malnutrition and re-feeding on the P2X₂ receptor, nitric oxide synthase (NOS), calretinin, calbindin and choline acetyltransferase (ChAT) in neurons of the rat ileum.

METHODS: We analyzed the co-localization, numbers and sizes of P2X₂-expressing neurons in relation to NOS-immunoreactive (IR), calbindin-IR, ChAT-IR, and calretinin-IR neurons of the myenteric and submucosal plexus. The experimental groups consisted of: (1) rats maintained on normal feed throughout pregnancy until 42 d post-parturition (N); (2) rats deprived of protein throughout pregnancy and 42 d post-parturition (D); and (3) rats undernourished for 21 d post-parturition and then given a protein diet from days 22 to 42 (DR). The myenteric and submucosal plexuses were evaluated by double labeling by immunohistochemical methods for P2X₂ receptor, NOS, ChAT, calbindin and calretinin.

RESULTS: We found similar P2X₂ receptor immunoreactivity in the cytoplasm and surface membranes of myenteric and submucosal neurons from the N, D and DR groups. Double labeling of the myenteric plexus demonstrated that approximately 100% of NOS-IR, calbindin-IR, calretinin-IR and ChAT-IR neurons in all groups also expressed the P2X₂ receptor. In the submucosal plexus, the calretinin-IR, ChAT-IR and calbindin-IR neurons were nearly all immunoreactive for the P2X₂ receptor. In the myenteric plexus, there was a 19% increase in numbers per cm² for P2X₂ receptor-IR neurons, 64% for NOS-IR, 84% for calretinin-IR and 26% for ChAT-IR neurons in the D group. The spatial density of calbindin-IR neurons, however, did not differ among the three groups. The submucosal neuronal density increased for calbindin-IR, calretinin-IR and ChAT-IR neurons. The average size of neurons in the myenteric plexus neurons in the D group was less than that in the controls and, in the re-fed rats; there was a 34% reduction in size only for the calretinin-IR neurons.

CONCLUSION: This work demonstrates that expression of the P2X₂ receptor is present in inhibitory, intrinsic primary afferent, cholinergic secretomotor and vasomotor neurons. Undernutrition affected P2X₂ receptor expression in the submucosal plexus, and neuronal and size. These changes were rescued in the re-fed rats.

P2Y2 receptor expression is altered in rats after spinal cord injury

Spinal cord injury increases inhibitory factors that may restrict neurite outgrowth after trauma. The expression of repulsive molecules in reactive astrocytes and the formation of the glial scar at the injury site produce the non-permissive environment for axonal regeneration. However, the mechanism that triggers this astrogliotic response is unknown. The release of nucleotides has been linked to this hypertrophic state. Our goal is to investigate the temporal profile of P2Y(2) nucleotide receptor after spinal cord injury in adult female Sprague-Dawley rats. Molecular biology, immunofluorescence studies, and Western Blots were used to evaluate the temporal profile (2, 4, 7, 14, and 28 days post-injury) of this receptor in rats injured at the T-10 level using the NYU impactor device. Real time RT-PCR showed a significant increase of P2Y(2) mRNA after 2 days post-injury that continues throughout 28 days post-injury. Double labeling studies localized P2Y(2) immunoreactivity in neuronal cell bodies, axons, macrophages, oligodendrocytes and reactive astrocytes. Immunofluorescence studies also demonstrated a low level of P2Y(2) receptor in sham samples, which increased after injury in glial fibrillary acidic protein positive cells. Western Blot performed with contused spinal cord protein samples revealed an upregulation in the P2Y(2) 42 kDa protein band expression after 4 days post-injury that continues until 28 days post-injury. However, a downregulation of the 62 kDa receptor protein band after 2 days post-injury that continues up to 28 days post-injury was observed. Therefore, the spatio-temporal pattern of P2Y(2) gene expression after spinal cord injury suggests a role in the pathophysiology response generated after trauma.

P2Y12 receptor-mediated integrin-beta1 activation regulates microglial process extension induced by ATP

Microglia are the primary immune surveillance cells in the brain, and when activated they play critical roles in inflammatory reactions and tissue repair in the damaged brain. Microglia rapidly extend their processes toward the damaged areas in response to stimulation of the metabotropic ATP receptor P2Y(12) by ATP released from damaged tissue. This chemotactic response is a highly important step that enables microglia to function properly at normal and pathological sites in the brain. To investigate the molecular pathways that underlie microglial process extension, we developed a novel method of modeling microglial process extension that uses transwell chambers in which the insert membrane is coated with collagen gel. In this study, we showed that ATP increased microglial adhesion to collagen gel, and that the ATP-induced process extension and increase in microglial adhesion were inhibited by integrin blocking peptides, RGD, and a functional blocking antibody against integrin-beta1. An immunoprecipitation analysis with an antibody against the active form of integrin-beta1 showed that P2Y(12) mediated the integrin-beta1 activation by ATP. In addition, time-lapse imaging of EGFP-labeled microglia in mice hippocampal slices showed that RGD inhibited the directional process extension toward the nucleotide source, and immunohistochemical staining showed that integrin-beta1 accumulated in the tips of the microglial processes in rat hippocampal slices stimulated with ADP. These findings indicate that ATP induces the integrin-beta1 activation in microglia through P2Y(12) and suggest that the integrin-beta1 activation is involved in the directional process extension by microglia in brain tissue.

Release of adenosine and ATP during ischemia and epilepsy

Eighty years ago Drury & Szent-Györgyi described the actions of adenosine, AMP (adenylic acid) and ATP (pyrophosphoric or diphosphoric ester of adenylic acid) on the mammalian cardiovascular system, skeletal muscle, intestinal and urinary systems. Since then considerable insight has been gleaned on the means by which these compounds act, not least of which in the distinction between the two broad classes of their respective receptors, with their many subtypes, and the ensuing diversity in cellular consequences their activation invokes. These myriad actions are of course predicated on the release of the purines into the extracellular milieu, but, surprisingly, there is still considerable ambiguity as to how this occurs in various physiological and pathophysiological conditions. In this review we summarise the release of ATP and adenosine during seizures and cerebral ischemia and discuss mechanisms by which the purines adenosine and ATP may be released from cells in the CNS under these conditions.

P2Y1 receptors inhibit long-term depression in the prefrontal cortex

Long-term depression (LTD) is a form of synaptic plasticity that may contribute to information storage in the central nervous system. Here we report that LTD can be elicited in layer 5 pyramidal neurons of the rat prefrontal cortex by pairing low frequency stimulation with a modest postsynaptic depolarization. The induction of LTD required the activation of both metabotropic glutamate receptors of the mGlu1 subtype and voltage-sensitive Ca(2+) channels (VSCCs) of the T/R, P/Q and N types, leading to the stimulation of intracellular inositol trisphosphate (IP3) receptors by IP3 and Ca(2+). The subsequent release of Ca(2+) from intracellular stores activated the protein phosphatase cascade involving calcineurin and protein phosphatase 1. The activation of purinergic P2Y(1) receptors blocked LTD. This effect was prevented by P2Y(1) receptor antagonists and was absent in mice lacking P2Y(1) but not P2Y(2) receptors. We also found that activation of P2Y(1) receptors inhibits Ca(2+) transients via VSCCs in the apical dendrites and spines of pyramidal neurons. In addition, we show that the release of ATP under hypoxia is able to inhibit LTD by acting on postsynaptic P2Y(1) receptors. In conclusion, these data suggest that the reduction of Ca(2+) influx via VSCCs caused by the activation of P2Y(1) receptors by ATP is the possible mechanism for the inhibition of LTD in prefrontal cortex.

Functional ligand-gated purinergic receptors (P2X) in rat vestibular ganglion neurons

The expression of purinergic receptors (P2X) on rat vestibular ganglion neurons (VGNs) was examined using whole-cell patch-clamp recordings. An application of adenosine 5'-triphosphate (ATP; 100microM) evoked inward currents in VGNs at a holding potential of -60mV. The decay time constant of the ATP-evoked currents was 2-4s, which is in between the values for rapidly desensitizing subgroups (P2X1 and P2X3) and slowly desensitizing subgroups (P2X2, P2X4, etc.), suggesting the heterogeneous expression of P2X receptors. A dose-response experiment showed an EC(50) of 11.0microM and a Hill's coefficient of 0.82. Suramin (100microM) reversibly inhibited the ATP-evoked inward currents. Alpha, beta-methylene ATP (100microM), a P2X-specific agonist, also evoked inward currents but less extensively than ATP. An application of adenosine 5'-dihosphate (ADP; 100microM) evoked similar, but much smaller, currents. The current-voltage relationship of the ATP-evoked conductance showed pronounced inward rectification with a reversal potential more positive than 0mV, suggesting non-selective cation conductance. However, the channel was not permeable to a large cation (N-methyl-d-glucamine) and acidification (pH 6.3) had little effect on the ATP-evoked conductance. RT-PCR confirmed the expression of five subtypes (P2X2-P2X6) in VGNs. The physiological role of P2X receptors includes the modulation of excitability at the synapses between hair cells and dendrites and/or trophic support (or also neuromodulation) from supporting cells surrounding the VGNs.

Trimethyltin intoxication up-regulates nitric oxide synthase in neurons and purinergic ionotropic receptor 2 in astrocytes in the hippocampus

Nitric oxide (NO) and purinergic ionotropic receptors (P2X) mediate cellular events in the central nervous system (CNS) under physiological conditions as well as during pathological events, and they have been recently proposed to interact in mediating CNS response to injury (Viscomi et al. [2004] Neuroscience 123:393-404; Florenzano et al. [2008] Pflugers Arch. 452:622-644). Trimethyltin (TMT) is an organotin compound that generates neurotoxic effects, and it has been used in a model of neurodegenerative disease and memory dysfunction. TMT causes neuronal death and reactive gliosis primarily in the hippocampus and other limbic regions. In the present study, we examined the degenerative events and the expression of nitric oxide synthase (NOS) and P2X receptor subtypes (P2X(1,2,4,7)Rs) that were induced by TMT administration at different time points (3, 7, 14, and 21 days) by conventional and confocal microscopy and Western blotting. Massive glial activation and neuronal death in the CA1 and CA3 regions were observed after TMT treatment. In these areas, astrocytic P2X(2)R and neuronal NOS were temporarily enhanced in association with the progression of neuronal death. In the hippocampus, the physiological expression of P2X(1)R, P2X(4)R, and P2X(7)R was not modified by TMT. The present data demonstrate that, as in other neurodegenerative models, TMT-induced hippocampal degeneration is associated with nitrergic and purinergic activations. Nevertheless, at odds with previous data, in this model the two systems are active in segregated cell populations, namely, P2XR in astrocytes and NOS in neurons. Finally, the temporal relations between P2XR and NOS expression and neuronal degeneration suggest interactions between P2XR/NO signaling and cell survival.

Ca2+-dependent modulation of GABAA and NMDA receptors by extracellular ATP: implication for function of tripartite synapse

The importance of communication between neuronal and glial cells for brain function is recognized by a modern concept of 'tripartite synapse'. Astrocytes enwrap synapses and can modulate their activity by releasing gliotransmitters such as ATP, glutamate and D-serine. One of the regulatory pathways in the tripartite synapse is mediated by P2X purinoreceptors. Release of ATP from synaptic terminals and astrocytes activates Ca(2+) influx via P2X purinoreceptors which co-localize with NMDA (N-methyl-D-aspartate) and GABA (gamma-aminobutyric acid) receptors and can modulate their activity via intracellular cascades which involve phosphatase II and PKA (protein kinase A).

NF546 [4,4'-(carbonylbis(imino-3,1-phenylene-carbonylimino-3,1-(4-methyl-phenylene)-carbonylimino))-bis(1,3-xylene-alpha,alpha'-diphosphonic acid) tetrasodium salt] is a non-nucleotide P2Y11 agonist and stimulates release of interleukin-8 from human monocyte-derived dendritic cells

The G protein-coupled P2Y(11) receptor is involved in immune system modulation. In-depth physiological evaluation is hampered, however, by a lack of selective and potent ligands. By screening a library of sulfonic and phosphonic acid derivatives at P2Y(11) receptors recombinantly expressed in human 1321N1 astrocytoma cells (calcium and cAMP assays), the selective non-nucleotide P2Y(11) agonist NF546 [4,4'-(carbonylbis(imino-3,1-phenylene-carbonylimino-3,1-(4-methyl-phenylene)carbonylimino))-bis(1,3-xylene-alpha,alpha'-diphosphonic acid) tetrasodium salt] was identified. NF546 had a pEC(50) of 6.27 and is relatively selective for P2Y(11) over P2Y(1), P2Y(2), P2Y(4), P2Y(6), P2Y(12), P2X(1), P2X(2), and P2X(2)-X(3). Adenosine-5'-O-(3-thio)triphosphate (ATPgammaS), a nonhydrolyzable analog of the physiological P2Y(11) agonist ATP, and NF546 use a common binding site as suggested by molecular modeling studies and their competitive behavior toward the nanomolar potency antagonist NF340 [4,4'-(carbonylbis(imino-3,1-(4-methyl-phenylene)carbonylimino))bis(naphthalene-2,6-disulfonic acid) tetrasodium salt] in Schild analysis. The pA(2) of NF340 was 8.02 against ATPgammaS and 8.04 against NF546 (calcium assays). NF546 was further tested for P2Y(11)-mediated effects in monocyte-derived dendritic cells. Similarly to ATPgammaS, NF546 led to thrombospondin-1 secretion and inhibition of lipopolysaccharide-stimulated interleukin-12 release, whereas NF340 inhibited these effects. Further, for the first time, it was shown that ATPgammaS or NF546 stimulation promotes interleukin 8 (IL-8) release from dendritic cells, which could be inhibited by NF340. In conclusion, we have described the first selective, non-nucleotide agonist NF546 for P2Y(11) receptors in both recombinant and physiological expression systems and could show a P2Y(11)-stimulated IL-8 release, further supporting the immunomodulatory role of P2Y(11) receptors.

Osmosensitive response of glutamate in the substantia nigra

Previous studies have suggested the increase of extracellular glutamate (GLU) in the substantia nigra (SN) as a cause of dopamine-cell degeneration (excitotoxicity) in Parkinson's disease (PD). However, the mechanisms involved in this increase remain unknown. The present work studied osmoregulation as a cause of GLU release in the SN. Microdialysis was used to change extracellular osmolarity, to administer drugs and to quantify the extracellular non-synaptic GLU (EnS-GLU). Two osmolarity modifications were performed, a moderate decrease (5%) resembling physiological modifications and a substantial decrease (>or=20% decrease) similar to that observed under pathological conditions. Hypo-osmolarity induced a dose-response (285-80 mOsm) increase of EnS-GLU which was detected after small osmolarity modifications (15 mOsm) and which was very marked (>1000%) after more intense osmolarity changes. This response disappeared after pre-treating rats with a P2 purinergic-receptor antagonist (pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid; 1 mM) suggesting ATP involvement in the osmosensitive EnS-GLU response. The EnS-GLU increase observed after administration of ATP (0.1-100 microM) and 2-methylthioadenosine triphosphate tetrasodium (P2-receptor agonist; 100 microM) and the lack of effects of adenosine administration (1 mM) suggest that the ATP action on P2 receptors is an amplificatory mechanism in the osmosensitive EnS-GLU response. The marked action of osmolarity on extracellular Glu suggests osmolarity regulation as a possible source for excitotoxicity in the SN.

Metabolic inhibition increases activity of connexin-32 hemichannels permeable to Ca2+ in transfected HeLa cells

Numerous cell types express functional connexin (Cx) hemichannels (HCs), and membrane depolarization and/or exposure to a divalent cation-free bathing solution (DCFS) have been shown to promote HC opening. However, little is known about conditions that can promote HC opening in the absence of strong depolarization and when extracellular divalent cation concentrations remain at physiological levels. Here the effects of metabolic inhibition (MI), an in vitro model of ischemia, on the activity of mouse Cx32 HCs were examined. In HeLa cells stably transfected with mouse Cx32 (HeLa-Cx32), MI induced an increase in cellular permeability to ethidium (Etd). The increase in Etd uptake was directly related to an increase in levels of Cx32 HCs present at the cell surface. Moreover, MI increased membrane currents in HeLa-Cx32 cells. Underlying these currents were channels exhibiting a unitary conductance of approximately 90 pS, consistent with Cx32 HCs. These currents and Etd uptake were blocked by HC inhibitors. The increase in Cx32 HC activity was preceded by a rapid reduction in mitochondrial membrane potential and a rise in free intracellular Ca(2+) concentration ([Ca(2+)](i)). The increase in free [Ca(2+)](i) was prevented by HC blockade or exposure to extracellular DCFS and was virtually absent in parental HeLa cells. Moreover, inhibition of Cx32 HCs expressed by HeLa cells in low-confluence cultures drastically reduced cell death induced by oxygen-glucose deprivation, which is a more physiological model of ischemia-reperfusion. Thus HC blockade could reduce the increase in free [Ca(2+)](i) and cell death induced by ischemia-like conditions in cells expressing Cx32 HCs.

Roles of purines in synaptic modulation evoked by hypercapnia in isolated spinal cord of neonatal rat in vitro

BACKGROUND AND PURPOSE

The purine compounds, adenosine 5'-triphosphate (ATP) and adenosine, are known to accumulate in the extracellular space and to elicit various cellular responses during hypoxia/ischemia, whereas the roles of purines during hypercapnia are poorly understood. In this study, we examined the effects of various drugs affecting purine turnover on the responses to hypercapnia in the spinal cord.

EXPERIMENTAL APPROACH

Electrically evoked reflex potentials were measured in an in vitro preparation of the isolated spinal cord of the neonatal rat by extracellular recording. Extracellular adenosine concentrations were assayed by high performance liquid chromatography (HPLC) methods.

KEY RESULTS

Hypercapnia (20% CO2) depressed the reflex potentials, which were partially reversed by an adenosine A1 receptor antagonist, 8-cyclopentyl theophylline, but not by a P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid. Exogenous adenosine and ATP also depressed the reflex potentials via adenosine A1 receptors. The hypercapnia-evoked depression was not reversed by inhibitors of gap junction hemichannels, anion channels, P2X7 receptors or equilibrative nucleoside transporters, all of which might be involved in purine efflux pathways. The adenosine accumulation evoked by hypercapnia was not inhibited by tetrodotoxin, ethylene glycol-bis(beta-amino ethyl ether) tetraacetic acid (EGTA) or an ecto-ATPase inhibitor, ARL 67156. Homocysteine thiolactone, used to trap intracellular adenosine, significantly reduced extracellular adenosine accumulation during hypercapnia.

CONCLUSIONS AND IMPLICATIONS

These results suggest that hypercapnia released adenosine itself from intracellular sources, using pathways different from the conventional exocytotic mechanism, and that this adenosine depressed spinal synaptic transmission via adenosine A1 receptors.

Endogenous signals initiating inflammation in the injured nervous system

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

Therapeutic potential of extracellular ATP and P2 receptors in nervous system diseases

Extracellular adenosine 5 inch-triphosphate (ATP) is a key signaling molecule present in the central nervous system (CNS), and now is receiving greater attention due to its role as a messenger in the CNS during different physiological and pathological events. ATP is released into the extracellular space through vesicular exocytosis or from damaged and dying cells. Once in the extracellular environment, ATP binds to the specific receptors termed P2, which mediate ATP effects and are present broadly in both neurons and glial cells. There are P2X, the ligand-gated ionotropic receptors, possessing low affinity for ATP and responsible for fast excitatory neurotransmission, and P2Y, the metabotropic G-protein-coupled receptors, possessing high affinity for ATP. Since massive extracellular release of ATP often occurs after stress, brain ischemia and trauma, the extracellular ATP is considered relating to or involving in the pathological processes of many nervous system diseases. Conversely, the trophic functions have also been extensively described for the extracellular ATP. Therefore, extracellular ATP plays a very complex role in the CNS and its binding to P2 receptors can be related to toxic and/or beneficial effects. In this review, we described the extracellular ATP acting via P2 receptors as a potent therapeutic target for treatment of nervous system diseases.

The P2Y-like receptor GPR17 as a sensor of damage and a new potential target in spinal cord injury

Upon central nervous system injury, the extracellular concentrations of nucleotides and cysteinyl-leukotrienes, two unrelated families of endogenous signalling molecules, are markedly increased at the site of damage, suggesting that they may act as 'danger signals' to alert responses to tissue damage and start repair. Here we show that, in non-injured spinal cord parenchyma, GPR17, a P2Y-like receptor responding to both uracil nucleotides (e.g. UDP-glucose) and cysteinyl-leukotrienes (e.g. LTD4 and LTC4), is present on a subset of neurons and of oligodendrocytes at different stages of maturation, whereas it is not expressed by astrocytes. GPR17 immunoreactivity was also found on ependymal cells lining the central canal that still retain some of the characteristics of stem/progenitor cells during adulthood. Induction of spinal cord injury (SCI) by acute compression resulted in marked cell death of GPR17+ neurons and oligodendrocytes inside the lesion followed by the appearance of proliferating GPR17+ microglia/macrophages migrating to and infiltrating into the lesioned area. Moreover, 72 h after SCI, GPR17+ ependymal cells started to proliferate and to express GFAP, suggesting their activation and 'de-differentiation' to pluripotent progenitor cells. The in vivo knock down of GPR17 by an antisense oligonucleotide strategy during SCI induction markedly reduced tissue damage and related histological and motor deficits, thus confirming the crucial role played by this receptor in the early phases of tissue damage development. Taken together, our findings suggest a dual and spatiotemporal-dependent role for GPR17 in SCI. At very early times after injury, GPR17 mediates neuronal and oligodendrocyte death inside the lesioned area. At later times, GPR17+ microglia/macrophages are recruited from distal parenchymal areas and move toward the lesioned zone, to suggest a role in orchestrating local remodelling responses. At the same time, the induction of the stem cell marker GFAP in GPR17+ ependymal cells suggests initiation of repair mechanisms. Thus, GPR17 may act as a 'sensor' of damage that is activated by nucleotides and cysteinyl-leukotrienes released in the lesioned area, and could also participate in post-injury responses. Moreover, its presence on spinal cord pre-oligodendrocytes and precursor-like cells suggests GPR17 as a novel target for therapeutic manipulation to foster remyelination and functional repair in SCI.

Key determinants of nucleotide-activated G protein-coupled P2Y(2) receptor function revealed by chemical and pharmacological experiments, mutagenesis and homology modeling

The P2Y(2) receptor, which is activated by UTP, ATP, and dinucleotides, was studied as a prototypical nucleotide-activated GPCR. A combination of receptor mutagenesis, determination of its effects on potency and efficacy of agonists and antagonists, homology modeling, and chemical experiments was applied. R272 (extracellular loop EL3) was found to play a gatekeeper role, presumably responsible for recognition and orientation of the nucleotides. R272 is also directly involved in binding of dinucleotides, which behaved as partial agonists. Y118A (3.37) mutation led to dramatically reduced efficacy of agonists; it is part of the entry channel as well as the triphosphate binding site. While the Y114A (3.33) mutation did not have any effect on agonist activities, the antagonist Reactive Blue 2 (6) was completely inactive at that mutant. The disulfide bridge Cys25-Cys278 was found to be important for agonist potency but neither for agonist efficacy nor for antagonist potency.

Extracellular ATP induces retinal photoreceptor apoptosis through activation of purinoceptors in rodents

We have previously demonstrated that photoreceptors express P2X(7) purinoceptors. These excitatory receptors are activated by extracellular adenosine 5'-triphosphate (ATP) and have been implicated in neurodegeneration in other parts of the central nervous system (CNS). In this study we examined whether extracellular ATP could contribute to photoreceptor degeneration in rodents through excessive activation of P2 purinoceptors. Intravitreal injection of high concentrations of extracellular ATP into normal rat eyes induced extensive and selective apoptosis of photoreceptors within 18 hours of injection. Five days after injection the outer nuclear layer was severely degenerated and electroretinographic responses were impaired. Preinjection of the purinergic antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) protected against ATP-mediated apoptosis. The initial phase of ATP-induced photoreceptor death did not temporally coincide with retinal pigment epithelium degeneration or microglial activation, suggesting that cell death was due to direct activation of purinergic receptors on photoreceptors. Finally, we demonstrate that intravitreal injection of PPADS results in a 30% increase in photoreceptor survival in the rd1 mouse, a model of human recessive retinitis pigmentosa (RP). These findings highlight the importance of extracellular ATP in retinal neurodegeneration and provide a potential new avenue for therapeutic intervention in RP.

Trophic functions of nucleotides in the central nervous system

In addition to short-term effects, one of the fundamental roles of extracellular nucleotides in the central nervous system involves long-term trophic effects. Physiological outcomes include neurogenesis, neuronal differentiation, glial proliferation, migration, growth arrest and apoptosis. Nucleotides exert these functions via P2-receptor-mediated mechanisms that can also interact with polypeptide-growth-factor-mediated or integrin-mediated signaling pathways. In addition, pathogenic roles for extracellular nucleotides in response to central nervous system injury including trauma and ischemia have been observed after the release of nucleotides by damaged and dying cells and in the development of neuropathic and inflammatory pain. Here, we illuminate the contribution of extracellular nucleotides to the development, growth, cellular plasticity and death of neural cells and the mechanisms regulating these trophic effects.

Ependymal cells along the lateral ventricle express functional P2X(7) receptors

Ependymal cells line the cerebral ventricles and are located in an ideal position to detect central nervous system injury and inflammation. The signaling mechanisms of ependymal cells, however, are poorly understood. As extracellular adenosine 5'-triphosphate is elevated in the context of cellular damage, experiments were conducted to determine whether ependymal cells along the mouse subventricular zone (SVZ) express functional purinergic receptors. Using whole-cell patch clamp recording, widespread expression of P2X(7) receptors was detected on ependymal cells based on their antagonist sensitivity profile and absence of response in P2X(7) (-/-) mice. Immunocytochemistry confirmed the expression of P2X(7) receptors, and electron microscopy demonstrated that P2X(7) receptors are expressed on both cilia and microvilli. Ca(2+) imaging showed that P2X(7) receptors expressed on cilia are indeed functional. As ependymal cells are believed to function as partner cells in the SVZ neurogenic niche, P2X(7) receptors may play a role in neural progenitor response to injury and inflammation.

Subsets of retinal neurons and glia express P2Y1 receptors

Recent evidence suggests that extracellular ATP modulates retinal processing and could play a role in modulating glial cells during retinal diseases. Here, we evaluated the localization of P2Y(1) receptors in the rat retina using indirect immunofluorescence immunocytochemistry. We observed labeling within defined populations of inner retinal neurons and Müller cell processes and end feet. Double labeling of P2Y(1) receptor with choline acetyltransferase revealed extensive colocalization indicating the expression of this receptor by cholinergic amacrine cells. Ganglion cell labeling for P2Y(1) receptors was also observed. Having established the normal pattern of immunolabeling within the retina, we next examined whether immunolabeling was altered by retinal disease. P2Y(1) receptor immunolabeling of Müller cells was of greater intensity following light-induced retinal degeneration, suggesting that Müller cell gliosis is accompanied by changes in P2Y(1) receptor expression. Overall, these data provide further evidence for a role of extracellular ATP in retinal signaling within subsets of retinal neurons as well as glia.