Cultures of glial cells release purines under field electrical stimulation: the possible ionic mechanisms

Selective Antagonism of A1 Adenosinergic Receptors Strengthens the Neuromodulation of the Sensorimotor Network During Epidural Spinal Stimulation

Although epidural spinal stimulation (ESS) results in promising therapeutic effects in individuals with spinal cord injury (SCI), its potential to generate functional motor recovery varies between individuals and remains largely unclear. However, both preclinical and clinical studies indicate the capacity of electrical and pharmacological interventions to synergistically increase the engagement of spinal sensorimotor networks and regain motor function after SCI. This study explored whether selective pharmacological antagonism of the adenosine A1 receptor subtype synergizes with ESS, thereby increasing motor response. We hypothesized that selective pharmacological antagonism of A1 receptors during ESS would produce facilitatory effects in spinal sensorimotor networks detected as an increased amplitude of spinally-evoked motor potentials and sustained duration of ESS induced activity. Terminal experiments were performed in adult rats using trains of stereotyped pulses at 40 Hz delivered at L5 with the local administration to the cord of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). We demonstrated that ESS combined with the blockage of A1 receptors increased the magnitude of the endogenous modulation and postponed the decay of responses that occur during ESS alone. Although DPCPX significantly increased the yield of repetitive stimulation in intact spinal cords, the effects of A1 antagonism on motor evoked responses after an acute spinal transection was not detected. These studies support the future investigation of the optimal dosage, methods of delivery, and systemic effects of the synergistic application of A1 antagonists and spinal stimulation in the intact and injured spinal cord.

And yet it moves: Recovery of volitional control after spinal cord injury

Preclinical and clinical neurophysiological and neurorehabilitation research has generated rather surprising levels of recovery of volitional sensory-motor function in persons with chronic motor paralysis following a spinal cord injury. The key factor in this recovery is largely activity-dependent plasticity of spinal and supraspinal networks. This key factor can be triggered by neuromodulation of these networks with electrical and pharmacological interventions. This review addresses some of the systems-level physiological mechanisms that might explain the effects of electrical modulation and how repetitive training facilitates the recovery of volitional motor control. In particular, we substantiate the hypotheses that: (1) in the majority of spinal lesions, a critical number and type of neurons in the region of the injury survive, but cannot conduct action potentials, and thus are electrically non-responsive; (2) these neuronal networks within the lesioned area can be neuromodulated to a transformed state of electrical competency; (3) these two factors enable the potential for extensive activity-dependent reorganization of neuronal networks in the spinal cord and brain, and (4) propriospinal networks play a critical role in driving this activity-dependent reorganization after injury. Real-time proprioceptive input to spinal networks provides the template for reorganization of spinal networks that play a leading role in the level of coordination of motor pools required to perform a given functional task. Repetitive exposure of multi-segmental sensory-motor networks to the dynamics of task-specific sensory input as occurs with repetitive training can functionally reshape spinal and supraspinal connectivity thus re-enabling one to perform complex motor tasks, even years post injury.

P2Y1 and Cysteinyl Leukotriene Receptors Mediate Purine and Cysteinyl Leukotriene Co-Release in Primary Cultures of Rat Microglia

Inflammation is widely recognized as contributing to the pathology of acute and chronic neurodegenerative conditions. Microglial cells are pathologic sensors in the brain and activated microglia have been viewed as detrimental. Leukotriene, including cysteinyl leukotrienes (CysLTs) are suggested to be involved in brain inflammation and neurological diseases and ATP, by its receptors is a candidate for microglia activation. A23187 (10μM) stimulated microglia to co-release CysLTs and [3H]adenine based purines ([3H]ABPs), mainly ATP. The biosynthetic production of CysLTs was abolished by 10μM MK-886, an inhibitor of 5-lipoxygenase-activating protein activity. RT-PCR analysis showed that microglia expressed both CysLT1 / CysLT2 receptors, P2Y1 ATP-receptors and several members of the ATP binding cassette (ABC) transporters including MRP1, MRP4 and Pgp. The increase in [Ca2+]i elicited by LTD4 (0.1 μM) and 2MeSATP (100μM), agonists for CysLT- and P2Y1-receptors, was abolished by the respective antagonists, BAYu9773 (0.5 μM) and suramin (50 μM). The stimulation of both receptor subtypes, induced a concomitant increase in the release of both [3H]ABPs and CysLTs that was blocked by the antagonists and significantly reduced by a cocktail of ABC transporter inhibitors, BAPTA/AM (intracellular Ca2+ chelator) and staurosporine (0.1 μM, PKC blocker). P2Y antagonist was unable to antagonise the effects of LTD4 and BAYu9773 did not reduce the effects of 2MeSATP. These data suggest that: i) the efflux of purines and cysteinyl-leukotrienes is specifically and independently controlled by the two receptor types, ii) calcium, PKC and the ABC transporter system can reasonably be considered common mechanisms underlying the release of ABPs and CysLTs from microglia. The blockade of P2Y1 or CysLT1/CysLT2 receptors by specific antagonists that abolished the raise in [Ca2+]i and drastically reduced the concomitant efflux of both compounds, as well as the effects of BAPTA and staurosporine support this hypothesis. In conclusion, the data of the present study suggest a cross talk between the purine and leukotriene systems in a possible autocrine/paracrine control of the microglia-mediated initiation and progression of an inflammatory response.

Contribution of extrasynaptic N-methyl-D-aspartate and adenosine A1 receptors in the generation of dendritic glutamate-mediated plateau potentials

Thin basal dendrites can strongly influence neuronal output via generation of dendritic spikes. It was recently postulated that glial processes actively support dendritic spikes by either ceasing glutamate uptake or by actively releasing glutamate and adenosine triphosphate (ATP). We used calcium imaging to study the role of NR2C/D-containing N-methyl-d-aspartate (NMDA) receptors and adenosine A1 receptors in the generation of dendritic NMDA spikes and plateau potentials in basal dendrites of layer 5 pyramidal neurons in the mouse prefrontal cortex. We found that NR2C/D glutamate receptor subunits contribute to the amplitude of synaptically evoked NMDA spikes. Dendritic calcium signals associated with glutamate-evoked dendritic plateau potentials were significantly shortened upon application of the NR2C/D receptor antagonist PPDA, suggesting that NR2C/D receptors prolong the duration of calcium influx during dendritic spiking. In contrast to NR2C/D receptors, adenosine A1 receptors act to abbreviate dendritic and somatic signals via the activation of dendritic K(+) current. This current is characterized as a slow-activating outward-rectifying voltage- and adenosine-gated current, insensitive to 4-aminopyridine but sensitive to TEA. Our data support the hypothesis that the release of glutamate and ATP from neurons or glia contribute to initiation, maintenance and termination of local dendritic glutamate-mediated regenerative potentials.

Role of glycogenolysis in stimulation of ATP release from cultured mouse astrocytes by transmitters and high K+ concentrations

This study investigates the role of glycogenolysis in stimulated release of ATP as a transmitter from astrocytes. Within the last 20 years our understanding of brain glycogenolysis has changed from it being a relatively uninteresting process to being a driving force for essential brain functions like production of transmitter glutamate and homoeostasis of potassium ions (K⁺) after their release from excited neurons. Simultaneously, the importance of astrocytic handling of adenosine, its phosphorylation to ATP and release of some astrocytic ATP, located in vesicles, as an important transmitter has also become to be realized. Among the procedures stimulating Ca²⁺-dependent release of vesicular ATP are exposure to such transmitters as glutamate and adenosine, which raise intra-astrocytic Ca²⁺ concentration, or increase of extracellular K⁺ to a depolarizing level that opens astrocytic L-channels for Ca²⁺ and thereby also increase intra-astrocytic Ca²⁺ concentration, a prerequisite for glycogenolysis. The present study has confirmed and quantitated stimulated ATP release from well differentiated astrocyte cultures by glutamate, adenosine or elevated extracellular K⁺ concentrations, measured by a luciferin/luciferase reaction. It has also shown that this release is virtually abolished by an inhibitor of glycogenolysis as well as by inhibitors of transmitter-mediated signaling or of L-channel opening by elevated K⁺ concentrations.

Stimulation of Ca²⁺-dependent astrocytic ATP release by glutamate, adenosine or high K⁺ concentration is abolished by inhibition of glycogenolysis, probably due to signaling inhibition. This is consistent with the K⁺-mediated pathway, but glycogenolysis-sensitive steps in the transmitter pathways are unknown.

Glial cells modulate the synaptic transmission of NTS neurons sending projections to ventral medulla of Wistar rats

There is evidence that sympathoexcitatory and respiratory responses to chemoreflex activation involve ventrolateral medulla-projecting nucleus tractus solitarius (NTS) neurons (NTS-VLM neurons) and also that ATP modulates this neurotransmission. Here, we evaluated whether or not astrocytes is the source of endogenous ATP modulating the synaptic transmission in NTS-VLM neurons. Synaptic activities of putative astrocytes or NTS-VLM neurons were recorded using whole cell patch clamp. Tractus solitarius (TS) stimulation induced TS-evoked excitatory postsynaptic currents (TS-eEPSCs) in NTS-VLM neurons as well in NTS putative astrocytes, which were also identified by previous labeling. Fluoracetate (FAC), an inhibitor of glial metabolism, reduced TS-eEPSCs amplitude (-85.6 ± 16 vs. -39 ± 7.1 pA, n = 12) and sEPSCs frequency (2.8 ± 0.5 vs. 1.8 ± 0.46 Hz, n = 10) in recorded NTS-VLM neurons, indicating a gliomodulation of glutamatergic currents. To verify the involvement of endogenous ATP a purinergic antagonist was used, which reduced the TS-eEPSCs amplitude (-207 ± 50 vs. -149 ± 50 pA, n = 6), the sEPSCs frequency (1.19 ± 0.2 vs. 0.62 ± 0.11 Hz, n = 6), and increased the paired-pulse ratio (PPR) values (∼20%) in NTS-VLM neurons. Simultaneous perfusion of Pyridoxalphosphate-6-azophenyl-2',5'-disulfonic acid (iso-PPADS) and FAC produced reduction in TS-eEPSCs similar to that observed with iso-PPADS or FAC alone, indicating that glial cells are the source of ATP released after TS stimulation. Extracellular ATP measurement showed that FAC reduced evoked and spontaneous ATP release. All together these data show that putative astrocytes are the source of endogenous ATP, which via activation of presynaptic P2X receptors, facilitates the evoked glutamate release and increases the synaptic transmission efficacy in the NTS-VLM neurons probably involved with the peripheral chemoreflex pathways.

FACS array profiling identifies Ecto-5' nucleotidase as a striatopallidal neuron-specific gene involved in striatal-dependent learning

The striatopallidal (STP) and striatonigral (STN) neurons constitute the main neuronal populations of the striatum. Despite the increasing knowledge concerning their involvement in multiple tasks associated with the striatum, it is still challenging to understand the precise differential functions of these two neuronal populations and to identify and study new genes involved in these functions. Here, we describe a reliable approach, applied on adult mouse brain, to generate specific STP and STN neuron gene profiles. STP and STN neurons were identified in the same animal using the transgenic Adora2A-Cre × Z/EG mouse model combined with retrograde labeling, respectively. Gene profiling was generated from FACS-purified neurons leading to the identification of new STP and STN neuron-specific genes. Knock-down models based on Cre-dependent lentiviral vector were developed to investigate their function either in striatal or in STP neurons. Thereby, we demonstrate that ecto-5'-nucleotidase (NT5e) is specifically expressed in STP neurons and is at the origin of most of the extracellular adenosine produced in the striatum. Behavioral analysis of striatal and STP neuron knock-down mouse models as well as NT5e knock-out mice demonstrates the implication of this STP neuron enzyme in motor learning.

Activation of P2X(7) receptors stimulates the expression of P2Y(2) receptor mRNA in astrocytes cultured from rat brain

Under pathological conditions brain cells release ATP at concentrations reported to activate P2X(7) ionotropic receptor subtypes expressed in both neuronal and glial cells. In the present study we report that the most potent P2X(7) receptor agonist BzATP stimulates the expression of the metabotropic ATP receptor P2Y(2) in cultured rat brain astrocytes. In other cell types several kinds of stimulation, including stress or injury, induce P2Y(2) expression that, in turn, is involved in different cell reactions. Similarly, it has recently been found that in astrocytes and astrocytoma cells P2Y(2) sites can trigger neuroprotective pathways through the activation of several mechanisms, including the induction of genes for antiapoptotic factors, neurotrophins, growth factors and neuropeptides. Here we present evidence that P2Y(2) mRNA expression in cultured astrocytes peaks 6 h after BzATP exposure and returns to basal levels after 24 h. This effect was mimicked by high ATP concentrations (1 mM) and was abolished by P2X(7)-antagonists oATP and BBG. The BzATP-evoked P2Y(2) receptor up-regulation in cultured astrocytes was coupled to an increased UTP-mediated intracellular calcium response. This effect was inhibited by oATP and BBG and by P2Y(2)siRNA, thus supporting evidence of increased P2Y(2) activity. To further investigate the mechanisms by which P2X(7) receptors mediated the P2Y(2) mRNA up-regulation, the cells were pre-treated with the chelating agent EGTA, or with inhibitors of mitogen-activated kinase (MAPK) (PD98059) or protein kinase C, (GF109203X). Each inhibitor significantly reduced the extent to which BzATP induced P2Y(2) mRNA. Both BzATP and ATP (1 mM) increased ERK1/2 activation. P2X(7)-induced ERK1/2 phosphorylation was unaffected by pre-treatment of astrocytes with EGTA whereas it was inhibited by GF109203X. Phorbol-12-myristate-13-acetate (PMA), an activator of PKCs, rapidly increased ERK1/2 activation. We conclude that activation of P2X(7) receptors in astrocytes enhances P2Y(2) mRNA expression by a mechanism involving both calcium influx and PKC/MAPK signalling pathways.

Adenosine released by astrocytes contributes to hypoxia-induced modulation of synaptic transmission

Astrocytes play a critical role in brain homeostasis controlling the local environment in normal as well as in pathological conditions, such as during hypoxic/ischemic insult. Since astrocytes have recently been identified as a source for a wide variety of gliotransmitters that modulate synaptic activity, we investigated whether the hypoxia-induced excitatory synaptic depression might be mediated by adenosine release from astrocytes. We used electrophysiological and Ca2+ imaging techniques in hippocampal slices and transgenic mice, in which ATP released from astrocytes is specifically impaired, as well as chemiluminescent and fluorescence photometric Ca2+ techniques in purified cultured astrocytes. In hippocampal slices, hypoxia induced a transient depression of excitatory synaptic transmission mediated by activation of presynaptic A1 adenosine receptors. The glia-specific metabolic inhibitor fluorocitrate (FC) was as effective as the A1 adenosine receptor antagonist CPT in preventing the hypoxia-induced excitatory synaptic transmission reduction. Furthermore, FC abolished the extracellular adenosine concentration increase during hypoxia in astrocyte cultures. Several lines of evidence suggest that the increase of extracellular adenosine levels during hypoxia does not result from extracellular ATP or cAMP catabolism, and that astrocytes directly release adenosine in response to hypoxia. Adenosine release is negatively modulated by external or internal Ca2+ concentrations. Moreover, adenosine transport inhibitors did not modify the hypoxia-induced effects, suggesting that adenosine was not released by facilitated transport. We conclude that during hypoxia, astrocytes contribute to regulate the excitatory synaptic transmission through the release of adenosine, which acting on A1 adenosine receptors reduces presynaptic transmitter release. Therefore, adenosine release from astrocytes serves as a protective mechanism by down regulating the synaptic activity level during demanding conditions such as transient hypoxia.

Cysteinyl-leukotrienes are released from astrocytes and increase astrocyte proliferation and glial fibrillary acidic protein via cys-LT1 receptors and mitogen-activated protein kinase pathway

Cysteinyl-leukotrienes (cys-LTs), potent mediators in inflammatory diseases, are produced by nervous tissue, but their cellular source and role in the brain are not very well known. In this report we have demonstrated that rat cultured astrocytes express the enzymes (5'-lipoxygenase and LTC(4) synthase) required for cys-LT production, and release cys-LTs in resting condition and, to a greater extent, in response to calcium ionophore A23187, 1 h combined oxygen-glucose deprivation or 2-methyl-thioATP, a selective P2Y(1)/ATP receptor agonist. MK-886, a LT synthesis inhibitor, prevented basal and evoked cys-LT release. In addition, 2-methyl-thioATP-induced cys-LT release was abolished by suramin, a P2 receptor antagonist, or by inhibitors of ATP binding cassette proteins involved in cys-LT release. We also showed that astrocytes express cys-LT(1) and not cys-LT(2) receptors. The stimulation of these receptors by LTD(4) activated the mitogen-activated protein kinase (MAPK) pathway. This effect was: (i) insensitive to inhibitors of receptor-coupled Gi protein (pertussis toxin) or tyrosine kinase receptors (genistein); (ii) abolished by MK-571, a cys-LT(1) selective receptor antagonist, or PD98059, a MAPK inhibitor; (iii) reduced by inhibitors of calcium/calmodulin-dependent kinase II (KN-93), Ca(2+)-dependent and -independent (GF102903X) or Ca(2+)-dependent (Gö6976) protein kinase C isoforms. LTD(4) also increased astrocyte proliferation and glial fibrillary acidic protein content, which are considered hallmarks of reactive astrogliosis. Both effects were counteracted by cell pretreatment with MK-571 or PD98059. Thus, cys-LTs released from astrocytes might play an autocrine role in the induction of reactive astrogliosis that, in brain injuries, contributes to the formation of a reparative glial scar.

Adenosine acting via A1 receptors, controls the transition to status epilepticus-like behaviour in an in vitro model of epilepsy

Adenosine has powerful inhibitory effects in the central nervous system. In this study, we aim to understand how adenosine controls the progression of seizure-like events (SLEs) in a seizure-prone region of the brain, the entorhinal cortex. We chose to use a low Mg(2+) model of epilepsy in an in vitro slice preparation where, in the entorhinal cortex, SLEs progress into a type of epileptiform activity called late recurrent discharges (LRDs) that bear resemblance to status epilepticus. Adenosine, acting via its A1 receptor, exerted powerful inhibitory effects to prevent the spontaneous progression to LRDs while the potent A1 receptor antagonist, DPCPX, accelerated the progression in a concentration dependent manner. The spontaneous progression from SLEs to LRDs was associated with a decline in total cellular ATP levels and studies with metabolic inhibitors indicated a key role for the production of endogenous adenosine from ATP. We therefore hypothesise that when ATP becomes rate limiting, extracellular adenosine levels fall, the normal inhibitory brake is removed and the progression from SLEs to LRDs or status epilepticus-like activity can ensue. Moreover, under these conditions, inhibition of the adenine nucleotide salvage pathways reversed the status epilepticus-like activity. Our findings suggest a powerful role for adenosine for the control of the progression to status epilepticus-like activity in an epilepsy model that is refractory to most anti-epileptic drugs. On this basis, manipulation of adenine nucleotide metabolism may represent a potential therapeutic approach for the treatment of status epilepticus.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

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

Activity-dependent release of adenosine contributes to short-term depression at CA3-CA1 synapses in rat hippocampus

High-frequency stimulation results in a transient, presynaptically mediated decrease in synaptic efficacy called short-term depression (STD). Stimulation of Schaffer-collateral axons at 10 Hz for 5 s resulted in approximately 75% depression of excitatory postsynaptic current (EPSC) slope recorded from CA1 cells in rat organotypic slice cultures. An adenosine A(1) receptor antagonist decreased the magnitude of STD elicited with 10-Hz stimulation by approximately 30%. The A(1) receptor antagonist had no effect on STD elicited with 3-Hz stimulation. The activation of A(1) receptors during 10-Hz stimulation was not due to the extracellular conversion of released ATP to adenosine, because block of 5'-ectonucleotidases did not significantly affect STD. The adenosine transport inhibitor dipyridamole did not reduce STD, indicating that adenosine was not released by facilitated transport. We conclude that 10-Hz, but not 3-Hz, stimulation causes the vesicular release of adenosine and the rapid (<3 s) activation of presynaptic inhibitory A(1) receptors, which account for approximately 40% of homosynaptic EPSC depression.

Effect of Climbing Fibre Deprivation on the K+-evoked Release of Endogenous Adenosine from Rat Cerebellar Slices

We report the identification of a compound whose K+-induced Ca2+-dependent release in rat cerebellar slices was reduced following climbing fibre deprivation by 3-acetylpyridine (3-AP) treatment. Based on HPLC retention time, UV absorption spectrum, and mass spectrometry, this compound was identified as adenosine. The K+-induced, Ca2+-dependent release of adenosine was subsequently quantified in control and 3-AP-treated rats. It decreased by 60 - 70% in both the cerebellar vermis and hemispheres following climbing fibre deprivation, while 3-AP treatment had no effect on adenosine release in the cerebral cortex. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal and stimulated efflux of adenosine in the cerebellum by 50 - 60%, indicating that a significant proportion of adenosine was derived from the extracellular metabolism of released nucleotides. Taken with the reports of other groups on adenosine in cerebellum, these results suggest that climbing fibre activity increases the extracellular level of adenosine, probably through the metabolism of released nucleotides. This adenosine could then cause presynaptic inhibition of the release of the parallel fibre transmitter, which is presumably glutamate. This may account for the climbing fibre-evoked depression of Purkinje cell sensitivity to parallel fibre input.

Involvement of astrocytes in purine‐mediated reparative processes in the brain

Astrocytes are involved in multiple brain functions in physiological conditions, participating in neuronal development, synaptic activity and homeostatic control of the extracellular environment. They also actively participate in the processes triggered by brain injuries, aimed at limiting and repairing brain damages. Purines may play a significant role in the pathophysiology of numerous acute and chronic disorders of the central nervous system (CNS). Astrocytes are the main source of cerebral purines. They release either adenine-based purines, e.g. adenosine and adenosine triphosphate, or guanine-based purines, e.g. guanosine and guanosine triphosphate, in physiological conditions and release even more of these purines in pathological conditions. Astrocytes express several receptor subtypes of P1 and P2 types for adenine-based purines. Receptors for guanine-based purines are being characterised. Specific ecto-enzymes such as nucleotidases, adenosine deaminase and, likely, purine nucleoside phosphorylase, metabolise both adenine- and guanine-based purines after release from astrocytes. This regulates the effects of nucleotides and nucleosides by reducing their interaction with specific membrane binding sites. Adenine-based nucleotides stimulate astrocyte proliferation by a P2-mediated increase in intracellular [Ca2+] and isoprenylated proteins. Adenosine also, via A2 receptors, may stimulate astrocyte proliferation, but mostly, via A1 and/or A3 receptors, inhibits astrocyte proliferation, thus controlling the excessive reactive astrogliosis triggered by P2 receptors. The activation of A1 receptors also stimulates astrocytes to produce trophic factors, such as nerve growth factor, S100beta protein and transforming growth factor beta, which contribute to protect neurons against injuries. Guanosine stimulates the output of adenine-based purines from astrocytes and in addition it directly triggers these cells to proliferate and to produce large amount of neuroprotective factors. These data indicate that adenine- and guanine-based purines released in large amounts from injured or dying cells of CNS may act as signals to initiate brain repair mechanisms widely involving astrocytes.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

Adenosine and the nervous system: pharmacological data and therapeutic perspectives

1. Adenosine acts on a family of G-protein-coupled receptors called purinoreceptors. 2. Four subtypes have been cloned and pharmacologically characterized. 3. The principal pharmacological data and structure-function relations for agonist interactions with P1 receptors are presented. 4. We conclude that the potent role of adenosine in the nervous system may be interesting for the development of drugs targeted at purines and their receptors.

Release of ATP from cultured rat astrocytes elicited by glutamate receptor activation

The release of ATP was studied in cultures of astrocytes derived from the brain hemispheres of newborn rats. There was a basal efflux of ATP, which was increased up to 19-fold by glutamate (300-1000 microM). N-methyl-D-aspartate (20-500 microM), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 30-100 microM) and kainate (20 microM). The N-methyl-D-aspartate receptor-selective antagonist 2-amino-5-phosphonopentanoate (100 microM) blocked the effect of N-methyl-D-aspartate but not the effects of AMPA, kainate and glutamate. The AMPA receptor-selective antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (30 microM) blocked the effect of AMPA and also of glutamate and N-methyl-D-aspartate, but not the effect of kainate. The kainate receptor-selective antagonist D-glutamyl-amino-methanesulfonate (30 microM) blocked the effect of kainate but not of glutamate. Glutamate (1000 microM) did not increase the release of lactate dehydrogenase from astrocytes. Excitatory amino acids are known to release adenyl compounds in the brain. The present results identify one adenyl compound thus released, namely ATP, and identify astrocytes as one source. The release is brought about by activation of any of the three ionotropic glutamate receptor types-N-methyl-D-aspartate, AMPA and kainate receptors. AMPA receptors seem to mediate at least a part of the effect of glutamate itself, but the involvement of other receptors cannot be ruled out. ATP and its degradation products, such as adenosine, once released, may exert acute as well as trophic effects on neurons and glial cells.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

Influence of Ca2+ channel modulators on [3H]purine release from rat cultured glial cells

[3H]Purine release from rat striatum astrocyte cultures was studied at 14 days in vitro (DIV). Superfusion of cultures with a Ca(2+)-free medium + 0.5 mM ethylene glycol-bis(beta-aminoethylether)N,N,N',N'-tetracetic acid (EGTA) reduced the electrically evoked [3H]purine release. Nimodipine only at the concentration of 10 microM modified [3H]purine outflow whereas 0.1 microM omega-conotoxin and 0.03-0.1 microM nitrendipine reduced the evoked one. Superfusion of cultures with 0.1 microM omega-conotoxin + 0.1 microM nitrendipine antagonized the evoked [3H]purine release similarly to each drug given alone. Neither nitrendipine nor omega-conotoxin influenced the uptake of 45Ca2+ by the cultures. The treatment of cells with the Ca2+ agonist Bay K 8644 did not affect [3H]purine release or the 45Ca2+ uptake. The drug did not either alter [Ca2+]i, evaluated by loading the cells with 3 microM Fura-2/AM. 10-30 microM 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), a blocker of intracellular Ca2+ discharge, significantly reduced the evoked [3H]purine release. On the other hand, 2 microM thapsigargin, an inhibitor of the ion store Ca2+ ATPase, was able to increase either the culture [3H]purine release or the [Ca2+]i. Together, the findings indicate that voltage-sensitive calcium channels (VSCCs) of the neuronal N and L-types are not involved in the modulation of [3H]purine release from rat cultured astrocytes whereas Ca2+ coming from intracytoplasmic stores seems to play a prevailing role. Moreover, agents which block VSCC, seem to be able to affect [3H]purine outflow with mechanisms other than VSCC gating.

Effects of exogenous ATP and related analogues on the proliferation rate of dissociated primary cultures of rat astrocytes

The effects of ATP (5-500 microM) were evaluated on the proliferation rate of cultured astrocytes by measuring 3H-thymidine incorporation and by flow cytometric analysis of the cell cycle. Determinations after 16 hours showed that ATP present in the culture medium for the whole period caused a dose-dependent reduction of cell proliferation, while if the exposure to ATP was limited to the first 8 hours, the proliferation was increased (always in a dose-dependent manner). A time course study of 3H-thymidine incorporation showed that, in the presence of ATP, 3H-thymidine was incorporated at a slower rate than in controls; the replacement of the culture medium with an ATP-free fresh medium, at the 8th hour, was followed by a 3H-thymidine incorporation occurring at such a fast rate to overshoot the control values. High performance liquid chromatography (HPLC) analysis, carried out to identify purine compounds present in the culture medium during cell exposure to ATP, indicated that more than 95% of the added ATP was metabolized within 1 hr. Conversely, an increase of purine metabolites was measured, this accumulation being greater at the highest concentrations of added ATP. The presence of high levels of extracellular ATP catabolites suggested that these compounds may act on the regulation of cell replication via the different purine receptors. This hypothesis was tested and confirmed by using agonists and antagonists selective for the P1 and the P2 sites. One hundred microM 2methylthio-ATP (2MeSATP), a P2Y agonist metabolized as fast as ATP, reproduced effects very similar to the ATP-induced ones. On the other hand, the nonhydrolisable ATP analogue, adenosine 5'-(beta, gamma-imido)-triphosphate (AMP-PNP) at 100 microM, induced a mitogenic effect as well as the A2 site stimulation. On the contrary, the activation of A1 receptors by 5 microM R-phenyl-isopropyladenosine (R-PIA) inhibited astrocyte proliferation; moreover, 100 nM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an A1 site antagonist, reversed the ATP-induced inhibition of cell proliferation. These results indicate that exogenous ATP, as a consequence of its rapid extracellular breakdown, exerts a dual influence on astrocyte proliferation by the involvement of both P1 and P2Y receptors. These findings might be relevant to such pathological conditions of the central nervous system (CNS), as seizures, hypoxia or ischemia, in which great amounts of purines released in the brain can influence a reactive astrocyte proliferative response to injury.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

N-methyl-D-aspartate- and non-N-methyl-D-aspartate-evoked adenosine release from rat cortical slices: distinct purinergic sources and mechanisms of release

Excitatory amino acids, acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors, release the inhibitory neuromodulator adenosine from superfused rat cortical slices. This study was initiated to investigate the possible purinergic sources and mechanisms of release for the adenosine release evoked by NMDA and non-NMDA receptor activation. Inhibition of the bidirectional nucleoside transporter with dipyridamole greatly enhanced adenosine release evoked by glutamate. NMDA, kainate, and (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Inhibition of ecto-5'-nucleotidase with alpha, beta-methylene ADP and GMP had no effect on either kainate- or AMPA-evoked adenosine release, but it decreased glutamate- and NMDA-evoked adenosine release by 23 and 68%, respectively. A similar inhibition of NMDA-evoked adenosine release was observed with alpha, beta-methylene ADP alone, indicating that the inhibitory effect was not due to the reported competitive inhibition of NMDA receptors by GMP. Finally, NMDA-evoked adenosine release, but not kainate- or AMPA-evoked release, was Ca2+ dependent. These results indicate that activation of non-NMDA receptors releases adenosine per se in a Ca(2+)-independent manner. In contrast, NMDA receptor activation releases primarily a nucleotide that is subsequently converted extracellularly to adenosine; in this case, release is Ca2+ dependent. Although neither NMDA- nor non-NMDA-evoked adenosine release occurs via the nucleoside transporter, this transporter does appear to be a major route for removal of adenosine from the extracellular space.

Role of excitatory amino acid receptors in K+- and glutamate-evoked release of endogenous adenosine from rat cortical slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.

N-methyl-D-aspartate, kainate and quisqualate release endogenous adenosine from rat cortical slices

N-Methyl-D-aspartate, kainate, and quisqualate released endogenous adenosine from superfused slices of rat parietal cortex. N-Methyl-D-aspartate-evoked adenosine release was blocked by D,L-2-amino-5-phosphono-valeric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), indicating that it was receptor-mediated, although it did not show the expected potentiation in the absence of Mg2+. In contrast, N-methyl-D-aspartate-evoked release of [3H]noradrenaline from the same slices was markedly potentiated in Mg2(+)-free medium. Therefore, the lack of Mg2+ modulation of N-methyl-D-aspartate-evoked adenosine release was not due to depolarization-induced alleviation of the Mg2+ block in the slices. Kainate-evoked adenosine release was diminished by the non-specific excitatory amino acid antagonist, gamma-D-glutamyl-glycine, and kainate- and quisqualate-evoked adenosine release was diminished by 6,7-dinitroquinoxaline-2,3-dione, indicating that these agonists release adenosine by acting at non-N-methyl-D-aspartate receptors. Tetrodotoxin decreased N-methyl-D-aspartate- and kainate-evoked adenosine release by 40% and 19% respectively, indicating that release was mediated in part by propagated action potentials in the slices. Total release of adenosine by N-methyl-D-aspartate, kainate or quisqualate was not diminished in the absence of Ca2+. A second exposure to kainate following restoration of Ca2+ to slices previously depolarized in the absence of Ca2+ resulted in an amount of adenosine release equal to an initial release by slices in the presence of Ca2+, a result suggesting the presence of separate Ca2(+)-dependent and Ca2(+)-independent pools of adenosine. The present experiments demonstrate that activation of all three major subtypes of excitatory amino acid receptors in the cortex releases adenosine, possibly from separate Ca2(+)-dependent and -independent pools. Adenosine released from the cortex following excitatory amino acid stimulation may, by acting at inhibitory P1 purinoceptors, diminish excitatory neurotransmission and protect against excitotoxicity.

Influence of PLA2-PG system on purine release and cAMP content in dissociated primary glial cultures from rat striatum

Purine release and prostaglandin (PG) outflow were simultaneously evaluated from untreated glial primary cultures of rat striatum, at rest and under field electrical stimulation. Purine release was also assayed from sister cultured cells in which a suitable pharmacological treatment with 1 x 10(-6) M dexamethasone or 1 x 10(-4) M indomethacin had produced a complete inhibition of the phospholipase A2-prostaglandin (PLA2-PG) system. Purine release from untreated cells seems to be regulated by specific receptor sites for released adenosine (Ado); A1 receptors exert an inhibitory control on purine release while A2 receptors facilitate it. PG release appears to be related to A1-mediated Ado activity, since culture treatment with 1 x 10(-10) M 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or 1 x 10(-4) M N-ethylmaleimide (NEM), A1 receptor inhibitory agents able to increase purine release, induced a significant reduction of the evoked PG outflow. Purine amount, released from glial cells with inhibited PLA2-PG system, was remarkably greater than that one assayed from control cultured cells. In so treated cultures, no additive effect, NEM-induced, was detected, while the addition of a mixture of PGs partially reduced the increased purine outflow. An electrically evoked cAMP accumulation, significantly greater than that found in controls, was even detected in cultured cells with inhibited PLA2-PG system. Since 10 micrograms/ml adenosine deaminase (ADA) reduced while DPCPX enhanced the evoked cAMP accumulation, it seems partially due to released Ado and accounts for a prevalent A2-stimulating rather than an A1-inhibitory control on adenylate cyclase activity. Thus, in cultured glial cells, the PLA2-PG system, likely linked to A1 receptor sites, concurs to control purine release and seems to affect less directly cAMP accumulation.