ATP-induced in vivo neurotoxicity in the rat striatum via P2 receptors

Glia from the central and peripheral nervous system are differentially affected by paclitaxel chemotherapy via modulating their neuroinflammatory and neuroregenerative properties

Glia are critical players in defining synaptic contacts and maintaining neuronal homeostasis. Both astrocytes as glia of the central nervous system (CNS), as well as satellite glial cells (SGC) as glia of the peripheral nervous system (PNS), intimately interact with microglia, especially under pathological conditions when glia regulate degenerative as well as regenerative processes. The chemotherapeutic agent paclitaxel evokes peripheral neuropathy and cognitive deficits; however, the mechanisms underlying these diverse clinical side effects are unclear. We aimed to elucidate the direct effects of paclitaxel on the function of astrocytes, microglia, and SGCs, and their glia-glia and neuronal-glia interactions. After intravenous application, paclitaxel was present in the dorsal root ganglia of the PNS and the CNS of rodents. In vitro, SGC enhanced the expression of pro-inflammatory factors and reduced the expression of neurotrophic factor NT-3 upon exposure to paclitaxel, resulting in predominantly neurotoxic effects. Likewise, paclitaxel induced a switch towards a pro-inflammatory phenotype in microglia, exerting neurotoxicity. In contrast, astrocytes expressed neuroprotective markers and increasingly expressed S100A10 after paclitaxel exposure. Astrocytes, and to a lesser extent SGCs, had regulatory effects on microglia independent of paclitaxel exposure. Data suggest that paclitaxel differentially modulates glia cells regarding their (neuro-) inflammatory and (neuro-) regenerative properties and also affects their interaction. By elucidating those processes, our data contribute to the understanding of the mechanistic pathways of paclitaxel-induced side effects in CNS and PNS.

Purinergic signaling as a new mechanism underlying physical exercise benefits: a narrative review

In the last years, it has become evident that both acute and chronic physical exercise trigger responses/adaptations in the purinergic signaling and these adaptations can be considered one important mechanism related to the exercise benefits for health improvement. Purinergic system is composed of enzymes (ectonucleotidases), receptors (P1 and P2 families), and molecules (ATP, ADP, adenosine) that are able to activate these receptors. These components are widely distributed in almost all cell types, and they respond/act in a specific manner depending on the exercise types and/or intensities as well as the cell type (organ/tissue analyzed). For example, while acute intense exercise can be associated with tissue damage, inflammation, and platelet aggregation, chronic exercise exerts anti-inflammatory and anti-aggregant effects, promoting health and/or treating diseases. All of these effects are dependent on the purinergic signaling. Thus, this review was designed to cover the aspects related to the relationship between physical exercise and purinergic signaling, with emphasis on the modulation of ectonucleotidases and receptors. Here, we discuss the impact of different exercise protocols as well as the differences between acute and chronic effects of exercise on the extracellular signaling exerted by purinergic system components. We also reinforce the concept that purinergic signaling must be understood/considered as a mechanism by which exercise exerts its effects.

Roles of Cytokines in the Temporal Changes of Microglial Membrane Currents and Neuronal Excitability and Synaptic Efficacy in ATP-Induced Cortical Injury Model

Cytokines are important neuroinflammatory modulators in neurodegenerative brain disorders including traumatic brain injury (TBI) and stroke. However, their temporal effects on the physiological properties of microglia and neurons during the recovery period have been unclear. Here, using an ATP-induced cortical injury model, we characterized selective effects of ATP injection compared to needle-control. In the damaged region, the fluorescent intensity of CX₃CR1-GFP (+) cells, as well as the cell density, was increased and the maturation of newborn BrdU (+) cells continued until 28 day-post-injection (dpi) of ATP. The excitability and synaptic E/I balance of neurons and the inward and outward membrane currents of microglia were increased at 3 dpi, when expressions of tumor necrosis factor (TNF)-α/interleukin (IL)-1β and IL-10/IL-4 were also enhanced. These changes of both cells at 3 dpi were mostly decayed at 7 dpi and were suppressed by any of IL-10, IL-4, suramin (P2 receptor inhibitor) and 4-AP (K⁺ channel blocker). Acute ATP application alone induced only small effects from both naïve neurons and microglial cells in brain slice. However, TNF-α alone effectively increased the excitability of naïve neurons, which was blocked by suramin or 4-AP. TNF-α and IL-1β increased and decreased membrane currents of naïve microglia, respectively. Our results suggest that ATP and TNF-α dominantly induce the physiological activities of 3 dpi neurons and microglia, and IL-10 effectively suppresses such changes of both activated cells in K⁺ channel- and P2 receptor-dependent manner, while IL-4 suppresses neurons preferentially.

Edaravone attenuates H2O2 or glutamate-induced toxicity in hippocampal neurons and improves AlCl3/D-galactose induced cognitive impairment in mice

Edaravone (Eda) is a free radical scavenger used in clinical trials for the treatment of ischemic stroke and amyotrophic lateral sclerosis. However, how Eda exerts its neuroprotective effects remains to be elucidated. We investigated the neuroprotective effects of Eda in cultured hippocampal neurons and in a mouse model of AlCl₃/D-galactose-induced cognitive impairment. Eda protected hippocampal neurons by eliminating H₂O₂ or glutamate-induced toxicity, leading to decreased cell viability and neurite shortening. Consistently, Eda restored impaired levels of BDNF, FGF2 and their associated signaling axes (including TrkB, p-Akt and Bcl-2) to attenuate neuronal death. In a mouse model of chemically-induced cognitive impairment, Eda restored the levels of BDNF, FGF2 and TrkB/Akt signaling axis to attenuate neuronal apoptosis, thereby ameliorating cognitive impairment. Meanwhile, the pro-inflammation was eliminated due to the restoration of pro-inflammatory factors such as TNF-α, IL-6, IL-1β, and NOS2. In summary, Eda is an effective drug for protecting neurons from neurotoxic injury. BDNF, FGF2, and their regulated pathways may be potential therapeutic targets for neuroprotection.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Enhanced ATP release and CD73-mediated adenosine formation sustain adenosine A2A receptor over-activation in a rat model of Parkinson's disease

BACKGROUND AND PURPOSE

Parkinson's disease (PD) involves an initial loss of striatal dopamine terminals evolving into degeneration of dopamine neurons in substantia nigra (SN), which can be modelled by 6-hydroxydopamine (6-OHDA) administration. Adenosine A_2A receptor blockade attenuates PD features in animal models, but the source of the adenosine causing A_2A receptor over-activation is unknown. As ATP is a stress signal, we have tested if extracellular catabolism of adenine nucleotides into adenosine (through ecto-5'-nucleotidase or CD73) leads to A_2A receptor over-activation in PD.

EXPERIMENTAL APPROACH

Effects of blocking CD73 with α,β-methylene ADP (AOPCP) were assayed in 6-OHDA-treated rats and dopamine-differentiated neuroblastoma SH-SY5Y cells.

KEY RESULTS

6-OHDA increased ATP release and extracellular conversion into adenosine through CD73 up-regulation in SH-SY5Y cells. Removing extracellular adenosine with adenosine deaminase, blocking CD73 with AOPCP, or blocking A_2A receptors with SCH58261 were equi-effective in preventing 6-OHDA-induced damage in SH-SY5Y cells. In vivo striatal exposure to 6-OHDA increased ATP release and extracellular formation of adenosine from adenosine nucleotides and up-regulated CD73 and A_2A receptors in striatal synaptosomes. Intracerebroventricular administration of AOPCP phenocopied effects of SCH58261, attenuating 6-OHDA-induced (a) increase of contralateral rotations after apomorphine, (b) reduction of dopamine content in striatum and SN, (c) loss of TH staining in striatum and SN, (d) motor dysfunction in the cylinder test, and (e) short-term memory impairment in the object recognition test.

CONCLUSION AND IMPLICATIONS

Our data indicate that increased ATP-derived adenosine formation is responsible for A_2A receptor over-activation in PD, suggesting CD73 as a new target to manage PD.

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

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

Purinergic Antagonism Prevents Mitochondrial Dysfunction and Behavioral Deficits Associated with Dopaminergic Toxicity Induced by 6-OHDA in Rats

Purinoceptors are present in neurons, microglia and oligodendrocytes and regulate dopamine (DA) release, striatal-related function and striatal neuronal and DA cells damage. Therefore, purinoceptors may be involved in the pathology of Parkinson's disease (PD) and purinergic antagonism may show neuroprotective effect. The study investigated the role of the non-selective purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2^', 4^'-disulfonic acid (PPADS) and a selective purinergic receptor P2X7 receptor antagonist Brilliant Blue G (BBG) against 6-OHDA induced dopaminergic neurotoxicity in rats; while adenosine triphosphate (ATP) was used as a P2X receptor agonist. Behavioral parameters like spontaneous motor activity, narrow beam walk, footprint, bar catalepsy, grip strength and rotarod tests were performed to evaluate motor deficits in PD. Striatal DA contents were estimated as neurochemical measures of PD. Mitochondrial studies and oxidative status were assessed to investigate the mechanism of purinergic system antagonists. Involvement of purinergic receptors in apoptosis was assessed by expressing cytochrome-C, caspase-9 and caspase-3. Both the antagonists not only attenuated 6-OHDA induced motor deficits but also protected against 6-OHDA induced DA depletion in the striatum. Oxidative stress, mitochondrial integrity and dysfunction were attenuated by purinergic antagonists. Further, they attenuated mitochondrial-linked apoptosis as observed by a decrease in expression of cytochrome-C, caspase-9 and caspase-3. Therefore, purinoceptor antagonism shows neuroprotective effect in 6-OHDA induced dopamine toxicity through preservation of mitochondrial bioenergetics and anti-apoptotic activities.

P2X7 receptor-pannexin 1 interaction mediates extracellular alpha-synuclein-induced ATP release in neuroblastoma SH-SY5Y cells

Abnormalities of alpha-synuclein (ASN), the main component of protein deposits (Lewy bodies), were observed in Parkinson’s disease (PD), dementia with Lewy bodies, Alzheimer’s disease, and other neurodegenerative disorders. These alterations include increase in the levels of soluble ASN oligomers in the extracellular space. Numerous works have identified several mechanisms of their toxicity, including stimulation of the microglial P2X7 receptor leading to oxidative stress. While the significant role of purinergic signaling—particularly, P2 family receptors—in neurodegenerative disorders is well known, the interaction of extracellular soluble ASN with neuronal purinergic receptors is yet to be studied. Therefore, in this study, we have investigated the effect of ASN on P2 purinergic receptors and ATP-dependent signaling. We used neuroblastoma SH-SY5Y cell line and rat synaptoneurosomes treated with exogenous soluble ASN. The experiments were performed using spectrofluorometric, radiochemical, and immunochemical methods. We found the following: (i) ASN-induced intracellular free calcium mobilization in neuronal cells and nerve endings depends on the activation of purinergic P2X7 receptors; (ii) activation of P2X7 receptors leads to pannexin 1 recruitment to form an active complex responsible for ATP release; and (iii) ASN greatly decreases the activity of extracellular ecto-ATPase responsible for ATP degradation. Thus, it is concluded that purinergic receptors might be putative pharmacological targets in the molecular mechanism of extracellular ASN toxicity. Interference with P2X7 signaling seems to be a promising strategy for the prevention or therapy of PD and other neurodegenerative disorders.

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

ATP as a multi-target danger signal in the brain

ATP is released in an activity-dependent manner from different cell types in the brain, fulfilling different roles as a neurotransmitter, neuromodulator, in astrocyte-to-neuron communication, propagating astrocytic responses and formatting microglia responses. This involves the activation of different ATP P2 receptors (P2R) as well as adenosine receptors upon extracellular ATP catabolism by ecto-nucleotidases. Notably, brain noxious stimuli trigger a sustained increase of extracellular ATP, which plays a key role as danger signal in the brain. This involves a combined action of extracellular ATP in different cell types, namely increasing the susceptibility of neurons to damage, promoting astrogliosis and recruiting and formatting microglia to mount neuroinflammatory responses. Such actions involve the activation of different receptors, as heralded by neuroprotective effects resulting from blockade mainly of P2X7R, P2Y1R and adenosine A_2A receptors (A_2AR), which hierarchy, cooperation and/or redundancy is still not resolved. These pleiotropic functions of ATP as a danger signal in brain damage prompt a therapeutic interest to multi-target different purinergic receptors to provide maximal opportunities for neuroprotection.

Different mechanisms of hydroxyl radical production susceptible to purine P2 receptor antagonists between carbon monoxide poisoning and exogenous ATP in rat striatum

Previous studies have suggested that carbon monoxide (CO) poisoning stimulates cAMP production via purine P2Y11-like receptors in the rat striatum, activating cAMP signaling pathways, resulting in hydroxyl radical ((•)OH) production. Extracellular ATP was thought likely to trigger the cascade, but the present study has failed to demonstrate a clear increase in the extracellular ATP due to CO poisoning. The CO-induced (•)OH production was attenuated by the P2Y11 receptor antagonist NF157, in parallel with its abilities to suppress the CO-induced cAMP production. The (•)OH production was more strongly suppressed by a non-selective P2 receptor antagonist, PPADS, which had no effect on cAMP production. More selective antagonists toward the respective P2 receptors susceptible to PPADS, including NF279, had little or no effect on the CO-induced (•)OH production. The intrastriatal administration of exogenous ATP dose-dependently stimulated (•)OH production, which was dose-dependently antagonized by PPADS and NF279 but not by NF157. Exogenous GTP and CTP dose-dependently stimulated (•)OH production, though less potently. The GTP-induced (•)OH production was susceptible to both of NF279 and PPADS, but the CTP-induced (•)OH production was resistant to PPADS. The mechanism of (•)OH production may differ between CO poisoning and exogenous ATP, while multiple P2 receptors could participate in (•)OH production. The CO-induced (•)OH production was susceptible to the inhibition of NADPH oxidase, but not xanthine oxidase. Also, the NADPH oxidase inhibition suppressed (•)OH production induced by forskolin, a stimulator of intracellular cAMP formation. It is likely that (•)OH is produced by NADPH oxidase activation via cAMP signaling pathways during CO poisoning.

Pathological potential of astroglial purinergic receptors

Acute brain injury and neurodegenerative disorders may result in astroglial activation. Astrocytes are able to determine the progression and outcome of these neuropathologies in a beneficial or detrimental way. Nucleotides, e.g. adenosine 5'-triphosphate (ATP), released after acute or chronic neuronal injury, are important mediators of glial activation and astrogliosis.Acute injury may cause significant changes in ATP balance, resulting in (1) a decline of intracellular ATP levels and (2) an increase in extracellular ATP concentrations via efflux from the intracellular space. The released ATP may have trophic effects, but can also act as a proinflammatory mediator or cytotoxic factor, inducing necrosis/apoptosis as a universal "danger" signal. Furthermore, ATP, primarily released from astrocytes, is a means of communication between neurons, glial cells, and intracerebral blood vessels.Astrocytes express a heterogeneous battery of purinergic ionotropic and metabotropic receptors (P2XRs and P2YRs, respectively) to respond to extracellular nucleotides.In this chapter, we summarize the contemporary knowledge on the pathological potential of P2Rs in relation to changes of astrocytic functions, determined by distinct molecular signaling cascades, in a variety of diseases. We discuss specific aspects of reactive astrogliosis, with respect to the involvement of prominent receptor subtypes, such as the P2X7 and P2Y1/2Rs. Examples of purinergic signaling of microglia, oligodendrocytes, and blood vessels under pathophysiological conditions will also be presented.The understanding of the pathological potential of purinergic signaling in "controlling and fine-tuning" of astrocytic responses is important for identifying possible therapeutic principles to treat acute and chronic central nervous system diseases.

P2X7 receptor activation regulates microglial cell death during oxygen-glucose deprivation

Brain-resident microglia may promote tissue repair following stroke but, like other cells, they are vulnerable to ischemia. Here we identify mechanisms involved in microglial ischemic vulnerability. Using time-lapse imaging of cultured BV2 microglia, we show that simulated ischemia (oxygen-glucose deprivation; OGD) induces BV2 microglial cell death. Removal of extracellular Ca(2+) or application of Brilliant Blue G (BBG), a potent P2X7 receptor (P2X7R) antagonist, protected BV2 microglia from death. To validate and extend these in vitro findings, we assessed parenchymal microglia in freshly isolated hippocampal tissue slices from GFP-reporter mice (CX3CR1(GFP/+)). We confirmed that calcium removal or application of apyrase, an ATP-degrading enzyme, abolished OGD-induced microglial cell death in situ, consistent with involvement of ionotropic purinergic receptors. Indeed, whole cell recordings identified P2X7R-like currents in tissue microglia, and OGD-induced microglial cell death was inhibited by BBG. These pharmacological results were complemented by studies in tissue slices from P2X7R null mice, in which OGD-induced microglia cell death was reduced by nearly half. Together, these results indicate that stroke-like conditions induce calcium-dependent microglial cell death that is mediated in part by P2X7R. This is the first identification of a purinergic receptor regulating microglial survival in living brain tissues. From a therapeutic standpoint, these findings could help direct novel approaches to enhance microglial survival and function following stroke and other neuropathological conditions.

Loss of calcium/calmodulin-dependent protein kinase II activity in cortical astrocytes decreases glutamate uptake and induces neurotoxic release of ATP

The extent of calcium/calmodulin-dependent protein kinase II (CaMKII) inactivation in the brain after ischemia correlates with the extent of damage. We have previously shown that a loss of CaMKII activity in neurons is detrimental to neuronal viability by inducing excitotoxic glutamate release. In the current study we extend these findings to show that the ability of astrocytes to buffer extracellular glutamate is reduced when CaMKII is inhibited. Furthermore, CaMKII inhibition in astrocytes is associated with the rapid onset of intracellular calcium oscillations. Surprisingly, this rapid calcium influx is blocked by the N-type calcium channel antagonist, ω-conotoxin. Although the function of N-type calcium channels within astrocytes is controversial, these voltage-gated calcium channels have been linked to calcium-dependent vesicular gliotransmitter release. When extracellular glutamate and ATP levels are measured after CaMKII inhibition within our enriched astrocyte cultures, no alterations in glutamate levels are observed, whereas ATP levels in the extracellular environment significantly increase. Extracellular ATP accumulation associated with CaMKII inhibition contributes both to calcium oscillations within astrocytes and ultimately cortical neuron toxicity. Thus, a loss of CaMKII signaling within astrocytes dysregulates glutamate uptake and supports ATP release, two processes that would compromise neuronal survival after ischemic/excitotoxic insults.

Mitochondrial dysfunction is involved in P2X7 receptor-mediated neuronal cell death

P2X7 receptor (P2X7R) is known to be a 'death receptor' in immune cells, but its functional expression in non-immune cells such as neurons is controversial. Here, we examined the involvement of P2X7R activation and mitochondrial dysfunction in ATP-induced neuronal death in cultured cortical neurons. In P2X7R- and pannexin-1-expressing neuron cultures, 5 or more mM ATP or 0.1 or more mM BzATP induced neuronal death including apoptosis, and cell death was prevented by oxATP, P2X7R-selective antagonists. ATP-treated neurons exhibited Ca(2+) entry and YO-PRO-1 uptake, the former being inhibited by oxATP and A438079, and the latter by oxATP and carbenoxolone, while P2X7R antagonism with oxATP, but not pannexin-1 blocking with carbenoxolone, prevented the ATP-induced neuronal death. The ATP treatment induced reactive oxygen species generation through activation of NADPH oxidase and activated poly(ADP-ribose) polymerase, but both of them made no or negligible contribution to the neuronal death. Rhodamine123 efflux from neuronal mitochondria was increased by the ATP-treatment and was inhibited by oxATP, and a mitochondrial permeability transition pore inhibitor, cyclosporine A, significantly decreased the ATP-induced neuronal death. In ATP-treated neurons, the cleavage of pro-caspase-3 was increased, and caspase inhibitors, Q-VD-OPh and Z-DEVD-FMK, inhibited the neuronal death. The cleavage of apoptosis-inducing factor was increased, and calpain inhibitors, MDL28170 and PD151746, inhibited the neuronal death. These findings suggested that P2X7R was functionally expressed by cortical neuron cultures, and its activation-triggered Ca(2+) entry and mitochondrial dysfunction played important roles in the ATP-induced neuronal death.

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.

Microglia and neuronal cell death

Microglia, the brain's innate immune cell type, are cells of mesodermal origin that populate the central nervous system (CNS) during development. Undifferentiated microglia, also called ameboid microglia, have the ability to proliferate, phagocytose apoptotic cells and migrate long distances toward their final destinations throughout all CNS regions, where they acquire a mature ramified morphological phenotype. Recent studies indicate that ameboid microglial cells not only have a scavenger role during development but can also promote the death of some neuronal populations. In the mature CNS, adult microglia have highly motile processes to scan their territorial domains, and they display a panoply of effects on neurons that range from sustaining their survival and differentiation contributing to their elimination. Hence, the fine tuning of these effects results in protection of the nervous tissue, whereas perturbations in the microglial response, such as the exacerbation of microglial activation or lack of microglial response, generate adverse situations for the organization and function of the CNS. This review discusses some aspects of the relationship between microglial cells and neuronal death/survival both during normal development and during the response to injury in adulthood.

cAMP production mediated through P2Y(11)-like receptors in rat striatum due to severe, but not moderate, carbon monoxide poisoning

We examined the effect of carbon monoxide (CO) poisoning on the production of cAMP, an intracellular second messenger, in rat striatum in terms of extracellular cAMP, which is highly correlated with intracellular cAMP, by using microdialysis. Severe poisoning due to 3000ppm CO, but not moderate poisoning due to 1000ppm CO, caused an increase in cAMP, which was susceptible to a voltage-dependent Na(+) channel blocker, tetrodotoxin, and more profound than that under comparable hypoxia caused by 5% O(2). These results were similar to our previous findings on the production of hydroxyl radical ((•)OH), suggesting a close relationship between cAMP and (•)OH production. The increase in cAMP was suppressed by a non-selective purine P2 receptor antagonist, suramin. However, other non-selective P2 receptor antagonists, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid and reactive blue 2, exhibited no effect and weak non-significant suppression, respectively. A P2Y(11) receptor antagonist, NF157, dose-dependently suppressed the increase in cAMP, although rats lack the P2Y(11) receptor. These results suggest that a threshold for cAMP production mediated through P2Y(11)-like receptors following depolarization triggered by Na(+) influx exists in rat striatum during CO poisoning, and that the threshold is reached only in cases of severe CO poisoning. It is also likely that the threshold is related to the generation of (•)OH, contributing to the toxicity of CO in the brain.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

The P2 receptor antagonist PPADS supports recovery from experimental stroke in vivo

BACKGROUND

After ischemia of the CNS, extracellular adenosine 5'-triphosphate (ATP) can reach high concentrations due to cell damage and subsequent increase of membrane permeability. ATP may cause cellular degeneration and death, mediated by P2X and P2Y receptors.

METHODOLOGY/PRINCIPAL FINDINGS

The effects of inhibition of P2 receptors by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on electrophysiological, functional and morphological alterations in an ischemia model with permanent middle cerebral artery occlusion (MCAO) were investigated up to day 28. Spontaneously hypertensive rats received PPADS or vehicle intracerebroventricularly 15 minutes prior MCAO for up to 7 days. The functional recovery monitored by qEEG was improved by PPADS indicated by an accelerated recovery of ischemia-induced qEEG changes in the delta and alpha frequency bands along with a faster and sustained recovery of motor impairments. Whereas the functional improvements by PPADS were persistent at day 28, the infarct volume measured by magnetic resonance imaging and the amount of TUNEL-positive cells were significantly reduced by PPADS only until day 7. Further, by immunohistochemistry and confocal laser scanning microscopy, we identified both neurons and astrocytes as TUNEL-positive after MCAO.

CONCLUSION

The persistent beneficial effect of PPADS on the functional parameters without differences in the late (day 28) infarct size and apoptosis suggests that the early inhibition of P2 receptors might be favourable for the maintenance or early reconstruction of neuronal connectivity in the periinfarct area after ischemic incidents.

Tissue-nonspecific alkaline phosphatase promotes axonal growth of hippocampal neurons

Axonal growth is essential for establishing neuronal circuits during brain development and for regenerative processes in the adult brain. Unfortunately, the extracellular signals controlling axonal growth are poorly understood. Here we report that a reduction in extracellular ATP levels by tissue-nonspecific alkaline phosphatase (TNAP) is essential for the development of neuritic processes by cultured hippocampal neurons. Selective blockade of TNAP activity with levamisole or specific TNAP knockdown with short hairpin RNA interference inhibited the growth and branching of principal axons, whereas addition of alkaline phosphatase (ALP) promoted axonal growth. Neither activation nor inhibition of adenosine receptors affected the axonal growth, excluding the contribution of extracellular adenosine as a potential hydrolysis product of extracellular ATP to the TNAP-mediated effects. TNAP was colocalized at axonal growth cones with ionotropic ATP receptors (P2X₇ receptor), whose activation inhibited axonal growth. Additional analyses suggested a close functional interrelation of TNAP and P2X₇ receptors whereby TNAP prevents P2X₇ receptor activation by hydrolyzing ATP in the immediate environment of the receptor. Furthermore inhibition of P2X₇ receptor reduced TNAP expression, whereas addition of ALP enhanced P2X₇ receptor expression. Our results demonstrate that TNAP, regulating both ligand availability and protein expression of P2X₇ receptor, is essential for axonal development.

Role of purinergic receptors in CNS function and neuroprotection

The purinergic receptor family contains some of the most abundant receptors in living organisms. A growing body of evidence indicates that extracellular nucleotides play important roles in the regulation of neuronal and glial functions in the nervous system through purinergic receptors. Nucleotides are released from or leaked through nonexcitable cells and neurons during normal physiological and pathophysiological conditions. Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system (CNS), participate in the synaptic processes, and mediate intercellular communications between neuron and gila and between glia and other glia. Glial cells in the CNS are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of purinergic receptors, which are integral to their activation. Astrocytes release adenosine triphosphate (ATP) as a "gliotransmitter" that allows communication with neurons, the vascular walls of capillaries, oligodendrocytes, and microglia. Oligodendrocytes are myelin-forming cells that construct insulating layers of myelin sheets around axons, and using purinergic receptor signaling for their development and for myelination. Microglia also express many types of purinergic receptors and are known to function as immunocompetent cells in the CNS. ATP and other nucleotides work as "warning molecules" especially by activating microglia in pathophysiological conditions. Studies on purinergic signaling could facilitate the development of novel therapeutic strategies for disorder of the CNS.

The hydrolysis of striatal adenine- and guanine-based purines in a 6-hydroxydopamine rat model of Parkinson's disease

Parkinson's disease (PD) is characterized by a progressive neurodegeneration in the substantia nigra and a striatal dopamine decrease. Striatal extracellular adenosine and ATP modulate the dopaminergic neurotransmission whereas guanosine has a protective role in the brain. Therefore, the regulation of their levels by enzymatic activity may be relevant to the clinical feature of PD. Here it was evaluated the extracellular nucleotide hydrolysis from striatal slices 4 weeks after a unilateral infusion with 6-OHDA into the medial forebrain bundle. This infusion increased ADP, AMP, and GTP hydrolysis by 15, 25, and 41%, respectively, and decreased GDP hydrolysis by 60%. There was no change in NTPDases1, 2, 3, 5, 6, and 5'-nucleotidase transcription. Dopamine depletion changes nucleotide hydrolysis and, therefore, alters the regulation of striatal nucleotide levels. These changes observed in 6-OHDA-lesioned animals may contribute to the symptoms observed in the model and provide evidence to indicate that extracellular purine hydrolysis is a key factor in understanding PD, giving hints for new therapies.

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.

Functional aspects of redox control during neuroinflammation

Neuroinflammation is a CNS reaction to injury in which some severe pathologies, regardless of their origin, converge. The phenomenon emphasizes crosstalk between neurons and glia and reveals a complex interaction with oxidizing agents through redox sensors localized in enzymes, receptors, and transcription factors. When oxidizing pressures cause reversible molecular changes, such as minimal or transitory proinflammatory cytokine overproduction, redox couples provide a means of translating the presence of reactive oxygen or nitrogen species into useful signals in the cell. Additionally, thiol-based redox sensors convey information about localized changes in redox potential induced by physiologic or pathologic situations. They are susceptible to oxidative changes and become key events during neuroinflammation, altering the course of a signaling response or the behavior of specific transcription factors. When oxidative stress augments the pressure on the intracellular environment, the effective reduction potential of redox pairs diminishes, and cell signaling shifts toward proinflammatory and proapoptotic signals, creating a vicious cycle between oxidative stress and neuroinflammation. In addition, electrophilic compounds derived from the oxidative cascade react with key protein thiols and interfere with redox signaling. This article reviews the relevant functional aspects of redox control during the neuroinflammatory process.

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.

Inflammation and brain injury: acute cerebral ischaemia, peripheral and central inflammation

Inflammation is a classical host defence response to infection and injury that has many beneficial effects. However, inappropriate (in time, place and magnitude) inflammation is increasingly implicated in diverse disease states, now including cancer, diabetes, obesity, atherosclerosis, heart disease and, most relevant here, CNS disease. A growing literature shows strong correlations between inflammatory status and the risk of cerebral ischaemia (CI, most commonly stroke), as well as with outcome from an ischaemic event. Intervention studies to demonstrate a causal link between inflammation and CI (or its consequences) are limited but are beginning to emerge, while experimental studies of CI have provided direct evidence that key inflammatory mediators (cytokines, chemokines and inflammatory cells) contribute directly to ischaemic brain injury. However, it remains to be determined what the relative importance of systemic (largely peripheral) versus CNS inflammation is in CI. Animal models in which CI is driven by a CNS intervention may not accurately reflect the clinical condition; stroke being typically induced by atherosclerosis or cardiac dysfunction, and hence current experimental paradigms may underestimate the contribution of peripheral inflammation. Experimental studies have already identified a number of potential anti-inflammatory therapeutic interventions that may limit ischaemic brain damage, some of which have been tested in early clinical trials with potentially promising results. However, a greater understanding of the contribution of inflammation to CI is still required, and this review highlights some of the key mechanism that may offer future therapeutic targets.

Diadenosine tetraphosphate protects sympathetic terminals from 6-hydroxydopamine-induced degeneration in the eye

AIMS

To examine diadenosine tetraphosphate (Ap(4)A) for its ability to protect the eye from neurodegeneration induced by subconjunctival application of 6-hydroxydopamine (6-OHDA).

METHODS

Intraocular neurodegeneration of anterior structures was induced by subconjunctival injections of 6-OHDA. Animals were pre-treated with topical corneal applications of Ap(4)A or saline.

RESULTS

6-OHDA caused miosis, abnormal pupillary light reflexes, a precipitous drop in intraocular pressure and loss of VMAT2-labelled (vesicle monoamine transporter-2, a marker for sympathetic neurones) intraocular neurones. Pre-treatment with Ap(4)A prevented all of these changes from being induced by 6-OHDA, demonstrably preserving the sympathetic innervation of the ciliary processes. This neuroprotective action of Ap(4)A was not shared with the related compounds adenosine, ATP or diadenosine pentaphosphate. P2-receptor antagonists showed that the effects of Ap(4)A were mediated via a P2-receptor.

CONCLUSION

Ap4A is a natural component of tears and aqueous humour, and its neuroprotective effect indicates that one of its physiological roles is to maintain neurones within the eye. Ap(4)A can prevent the degeneration of intraocular nerves, and it is suggested that this compound may provide the basis for a therapeutic intervention aimed at preventing or ameliorating the development of glaucoma associated with neurodegenerative diseases. Furthermore, subconjunctival application of 6-OHDA provides a useful model for studying diseases that cause ocular sympathetic dysautonomia.

A neuroprotective exercise protocol reduces the adenine nucleotide hydrolysis in hippocampal synaptosomes and serum of rats

Regular and moderate exercise has been considered as an interesting neuroprotective strategy. However, the molecular mechanisms by which physical exercise alters brain function are unclear. Purinergic signaling seems to modulate the pathophysiology of ischemic neuronal damage, since it has been described a neuroprotective activity of adenosine and a dual role of ATP. In the present study, we investigated the effect of daily moderate intensity exercise on ectonucleotidase activities in synaptosomes from hippocampus and the soluble nucleotidases from blood serum of rats. Adult male Wistar rats were assigned to non-exercised (sedentary) group and exercised during 20-min sessions on different programs. The effects of physical activity on hydrolysis of ATP, ADP and AMP were assayed in the synaptosomal fraction obtained from the hippocampus and serum approximately 16 h after the last training session. Our data demonstrated that a neuroprotective exercise protocol, daily 20 min of training in treadmill during 2 weeks, diminished significantly the ADP hydrolysis and there is a trend to reduce the ATP hydrolysis in both hippocampal synaptosomes and blood serum of rats. We suggest that the neuroprotective exercise protocol may modulate nucleotidase activities.

Expression and functional analysis of Na(+)-dependent glutamate transporters from zebrafish brain

High-affinity excitatory amino acid transporters (EAATs) regulate extracellular glutamate levels. Zebrafish (Danio rerio) provides an excellent model to study the function of different neurotransmitter systems. Although the identification of the EAAT family is well established in the mammalian central nervous system (CNS), EAAT-related genes and their expression profile in zebrafish have not yet been reported. Here we identify and describe the expression profile of EAATs-related genes and functional properties of glutamate uptake in three major brain structures from zebrafish (telencephalon, optic tectum and cerebellum). Searches on zebrafish genome databases and a phylogenetic analysis confirmed the presence of several EAAT-related genes (EAAT2, EAAT3, three EAAT1 paralogs and two EAAT5 sequences). All sequences identified were expressed in the structures analyzed. EAAT2 and EAAT3 were the most prominent glutamate transporters expressed in all brain areas. A uniform expression was observed for EAAT1A, whereas higher EAAT1B transcript levels were detected in telencephalon. Lower amounts of EAAT1C transcripts were observed in cerebellum when compared to other structures. No EAAT4-related sequence was found in the zebrafish genome. The EAAT5A expression was similar to EAAT5B in the telencephalon, while EAAT5B was less expressed than EAAT5A in optic tectum and cerebellum. Moreover, the glutamate uptake was significantly higher in optic tectum, which indicates functional differences within zebrafish brain structures. Altogether, the study of glutamate uptake in zebrafish could be important to evaluate the modulation of glutamatergic signaling through pharmacological and toxicological studies.

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.

P2Y1 receptor signaling enhances neuroprotection by astrocytes against oxidative stress via IL-6 release in hippocampal cultures

Cell survival is a critical issue in the onset and progression of neurodegenerative diseases and following pathological events including ischemia and traumatic brain injury. Oxidative stress is the main cause of cell damage in such pathological conditions. Here, we report that adenosine 5'-triphosphate (ATP) protects hippocampal astrocytes from hydrogen peroxide (H(2)O(2))-evoked oxidative injury in astrocyte monocultures. The effect of ATP was prevented by a selective antagonist of or siRNAs against P2Y(1)R. Interestingly, in astrocyte-neuron cocultures, ATP also produced neuroprotective effects against H(2)O(2)-evoked neuronal cell death, whereas ATP did not produce any neuroprotective effects in monocultures. The ATP-induced neuroprotection in cocultures was completely inhibited by silencing of astrocytic P2Y(1)R expression, indicating that ATP acts on astrocytes and enhances their neuroprotective functions by activating P2Y(1)R. Furthermore, this neuroprotective effect was mimicked by applying conditioned medium from astrocytes that had been stimulated by ATP, implying an involvement of diffusible factors from astrocytes. We found that, in both purified astrocyte cultures and astrocyte-neuronal cocultures, ATP and the P2Y(1)R agonist 2-methylthioadenosine 5' diphosphate (2MeSADP) induced the release of interleukin-6 (IL-6), but this did not occur in neuron monocultures. Moreover, exogenous IL-6 produced a neuroprotective effect, and the neuroprotection induced by P2Y(1)R-stimulated astrocytes was prevented in the presence of an anti-IL-6 antibody. Taken together, these results suggest that P2Y(1)R-stimulated astrocytes protect against neuronal damage induced by oxidative stress, and that IL-6 is a crucial signaling molecule released from astrocytes. Thus, activation of P2Y(1)R in astrocytes may rescue neurons from secondary cell death under pathological conditions.

Do ATP and NO interact in the CNS?

Enzymatically derived NO and extracellular ATP are receiving greater attention due to their role as messengers in the CNS during different physiological and pathological processes. Ionotropic (P2XR) and metabotropic (P2YR) purinergic receptors mediate ATP effects and are present throughout the body. Particularly P2XR are crucial for brain plasticity mechanisms, and are involved in the pathogenesis of different CNS illnesses. NO does not have a specific receptor and its actions are directly dependent on the production on demand by different nitric oxide synthase isoforms. NO synthesizing enzymes are present virtually in all tissues, and NO influences multifarious physiological and pathological functions. Interestingly, various are the tissue and organs modulated by both ATP and NO, such as the immune, brain and vascular systems. Moreover, direct interactions between purinergic and nitrergic mechanisms outside the CNS are well documented, with several studies also indicating that ATP and NO do participate to the same CNS functions. In the past few years, further experimental evidence supported the physiological and pathological relevance of ATP and NO direct interactions in the CNS. The aim of the present review is to provide an account of the available information on the interplay between purinergic and nitrergic systems, focussing on the CNS. The already established relevance of ATP and NO in different pathological processes would predict that the knowledge of ATP/NO cross-talk mechanisms would support pharmacological approaches toward the development of novel ATP/NO combined pharmacological agents.

The role of ATP and adenosine in the brain under normoxic and ischemic conditions

By taking advantage of some recently synthesized compounds that are able to block ecto-ATPase activity, we demonstrated that adenosine triphosphate (ATP) in the hippocampus exerts an inhibitory action independent of its degradation to adenosine. In addition, tonic activation of P2 receptors contributes to the normally recorded excitatory neurotransmission. The role of P2 receptors becomes critical during ischemia when extracellular ATP concentrations increase. Under such conditions, P2 antagonism is protective. Although ATP exerts a detrimental role under ischemia, it also exerts a trophic role in terms of cell division and differentiation. We recently reported that ATP is spontaneously released from human mesenchymal stem cells (hMSCs) in culture. Moreover, it decreases hMSC proliferation rate at early stages of culture. Increased hMSC differentiation could account for an ATP-induced decrease in cell proliferation. ATP as a homeostatic regulator might exert a different effect on cell trophism according to the rate of its efflux and receptor expression during the cell life cycle. During ischemia, adenosine formed by intracellular ATP escapes from cells through the equilibrative transporter. The protective role of adenosine A(1) receptors during ischemia is well accepted. However, the use of selective A(1) agonists is hampered by unwanted peripheral effects, thus attention has been focused on A(2A) and A(3) receptors. The protective effects of A(2A) antagonists in brain ischemia may be largely due to reduced glutamate outflow from neurones and glial cells. Reduced activation of p38 mitogen-activated protein kinases that are involved in neuronal death through transcriptional mechanisms may also contribute to protection by A(2A) antagonism. Evidence that A(3) receptor antagonism may be protective after ischemia is also reported.

Adenosine deaminase-related genes: molecular identification, tissue expression pattern and truncated alternative splice isoform in adult zebrafish (Danio rerio)

Adenosine deaminase (ADA) is responsible for cleaving the neuromodulator adenosine to inosine. Two members of ADA subfamilies, known as ADA1 and ADA2, were described and evidence demonstrated another similar protein group named ADAL (adenosine deaminase "like"). Although the identification of ADA members seems to be consistent, the expression profile of ADA1, ADA2 and ADAL genes in zebrafish has not yet been reported. The aim of the present study was to map the expression pattern of ADA-related genes in various tissues of adult zebrafish (Danio rerio). An extensive search on zebrafish genome followed by a phylogenetic analysis confirmed the presence of distinct ADA-related genes (ADA1, ADAL and two orthologous genes of ADA2). Specific primers for each ADA member were designed, optimized semi-quantitative RT-PCR experiments were conducted and the relative amount of transcripts was determined. The tissue samples (brain, gills, heart, liver, skeletal muscle and kidney) were collected and the expression of ADA1, ADAL and ADA2 genes was characterized. ADA1 had a similar expression pattern, whereas ADAL was less expressed in the heart. The highest relative amount of ADA2-1 transcripts was observed in the brain, liver and gills and it was less expressed in the heart. RT-PCR assays revealed that the other ADA2 form (ADA2-2) was expressed ubiquitously and at comparable levels in zebrafish tissues. The strategy adopted also allowed the identification of an ADA2-1 truncated alternative splice isoform (ADA2-1/T), which was expressed at different intensities. These findings demonstrated the existence of different ADA-related genes, their distinct expression pattern and a truncated ADA2-1 isoform, which suggest a high degree of complexity in zebrafish adenosinergic system.

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

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

Modulation by D1 and D2 dopamine receptors of ATP-induced release of intracellular Ca2+ in cultured rat striatal neurons

The aim of the present study was to investigate, whether dopamine D1 and/or D2 receptors are able to interfere with the ATP-induced increase of the intracellular Ca2+ concentration ([Ca2+]i) in cultured striatal neurons identified by their morphological characteristics and their [Ca2+]i transients in response to a high-K+ superfusion medium. ATP appeared to release Ca2+ mostly from an intracellular pool, since its effect was markedly depressed in the presence of cyclopiazonic acid, which is known to deplete such storage sites [Rubini, P., Pinkwart, C., Franke, H., Gerevich, Z., Nörenberg, W., Illes, P., 2006. Regulation of intracellular Ca2+ by P2Y1 receptors may depend on the developmental stage of cultured rat striatal neurons. J. Cell. Physiol. 209, 81-93]. The mixed D1/D2 receptor agonist dopamine increased the ATP-induced [Ca2+]i transients in a subpopulation of neurons. At the same time, dopamine did not alter the responses to K+ in these cells. The selective D1 (SKF 83566) and D2 (sulpiride) receptor antagonists failed to modify the effect of ATP, but unmasked in the previously unresponsive neurons an inhibitory and facilitatory effect of dopamine, respectively. A combination of the two antagonists resulted in a failure of dopamine to modulate the [Ca2+]i responses in any cell investigated. In conclusion, D1 and D2 receptors may modulate in an opposite manner the signalling pathways of P2Y1 receptors in striatal neurons and thereby alter their development/growth or their cellular excitability and/or the release of GABA from their terminals.

Purinergic receptor antagonism prevents cold preservation-induced cell death independent of cellular ATP levels

BACKGROUND

Purinergic (P2Y) receptors play an important role in intracellular Ca(2+) regulation in hepatocytes. Prevention of mitochondrial Ca(2+) (mCa(2+)) overload during ischemic conditions prevents cellular cell death during the early reperfusion period. P2Y antagonists are cytoprotective in other settings. We studied the effect of P2Y receptor antagonism on mitochondrial associated cell death during the period of cold storage.

METHODS

HepG2 cells were stored in UW with or without 300 muM reactive blue 2 (RB2) or 10 muM ruthenium red (RR) under either normoxic-hypothermic or hypoxic-hypothermic conditions. Cytoplasmic cytochrome c levels were studied by transfection of cytochrome c-GFP. Immunofluorescence determined the intracellular, spatio-temporal distribution of Bax, and terminal deoxynucleotidyl transferase mediated dUTP nick end labeling staining was used to evaluate cell death. Intracellular compartmental ATP levels were assayed by transfecting with luciferase vectors specific for cytoplasm (PcDNA3-luciferase-LL/V) and mitochondria (PcDNA3-COX8-luciferase).

RESULTS

Bax translocation to the mitochondria occurred immediately following cold storage and was followed by cytochrome c-GFP redistribution to the cytosol during rewarming. RB2 treatment significantly attenuated Bax translocation, cytochrome c-GFP redistribution, and cell death following both storage conditions. Both RR and RB2 provided cytoprotection despite ongoing cytoplasmic ATP consumption during cold ischemia.

CONCLUSION

These data indicate that the cytoprotective effects of mCa(2+) uptake inhibition and P2Y receptor antagonism are independent of cytoplasmic ATP levels during cold ischemia.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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

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

Purinergic modulation of microglial cell activation

Microglial cells are resident macrophages in the brain and their activation is an important part of the brain immune response and the pathology of the major CNS diseases. Microglial activation is triggered by pathological signals and is characterized by morphological changes, proliferation, phagocytosis and the secretion of various cytokines and inflammatory mediators, which could be both destructive and protective for the nervous tissue. Purines are one of the most important mediators which regulate different aspects of microglial function. They could be released to the extracellular space from neurons, astrocytes and from the microglia itself, upon physiological neuronal activity and in response to pathological stimuli and cellular damage. Microglial activation is regulated by various subtypes of nucleotide (P2X, P2Y) and adenosine (A₁, A(₂A) and A₃) receptors, which control ionic conductances, membrane potential, gene transcription, the production of inflammatory mediators and cell survival. Among them, the role of P2X₇ receptors is especially well delineated, but P2X₄, various P2Y, A₁, A(₂A) and A₃ receptors also powerfully participate in the microglial response. The pathological role of microglial purine receptors has also been demonstrated in disease models; e.g., in ischemia, sclerosis multiplex and neuropathic pain. Due to their upregulation and selective activation under pathological conditions, they provide new avenues in the treatment of neurodegenerative and neuroinflammatory illnesses.

The co-treatments of vigabatrin and P2X receptor antagonists protect ischemic neuronal cell death in the gerbil hippocampus

During transient global ischemia, the excessive accumulation of intracellular Ca2+ induced by several episodes triggers delayed neuronal death within the vulnerable CA1 region of the hippocampus after ischemia-reperfusion insults. Although P2X receptors provide an additional source of Ca2+ entry, little data are available that these receptors could modulate the performance of the ischemic neuronal death. Therefore, we investigated the roles of the P2X receptor in the ischemic neuronal damage associated with various sequelae of transient ischemia, and the effects of their antagonist on the ischemic insults. As the results, ischemic insults increased P2X receptor expression in the gerbil hippocampus. Neither vigabatrin (VGB) nor P2X receptor antagonists (suramin, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid) protected against the delayed neuronal death in the CA1 region of the hippocampus after ischemia. However, the co-treatments of VGB and P2X receptor antagonists effectively prevent ischemia-induced neurodegeneration. Therefore, these findings suggest that blockade of the P2X receptor accompanied by activation of GABAergic inhibition may play an important role in the neuroprotection against ischemic insults.

Neuroprotective effects of the P2 receptor antagonist PPADS on focal cerebral ischaemia-induced injury in rats

After acute injury of the central nervous system extracellular adenosine 5'-triphosphate (ATP) can reach high concentrations as a result of cell damage and subsequent increase in membrane permeability. Released ATP may act as a toxic agent, which causes cellular degeneration and death, mediated through P2X and P2Y receptors. Mechanisms underlying the various effects of purinoceptor modulators in models of cerebral damage are still uncertain. In the present study the effect of P2 receptor inhibition after permanent middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats was investigated. Rats received either the non-selective P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) or artificial cerebrospinal fluid (ACSF) as control by the intracerebroventricular route. First, these treatments were administered 10 min before MCAO and subsequently twice daily for 1 or 7 days after MCAO. The functional recovery of motor and cognitive deficits was tested at an elevated T-labyrinth. The PPADS-treated group showed a significant reduction of paresis-induced sideslips compared with ACSF-treated animals. Infarct volume was reduced in the PPADS group in comparison with the ACSF group. A significant decrease in intermediately and profoundly injured cells in favour of intact cells in the PPADS group was revealed by quantification of celestine blue/acid fuchsin-stained cells in the peri-infarct area. The data provide further evidence for the involvement of P2 receptors in the pathophysiology of cerebral ischaemia in vivo. The inhibition of P2 receptors at least partially reduces functional and morphological deficits after an acute cerebral ischaemic event.

P2X7 receptor modulation on microglial cells and reduction of brain infarct caused by middle cerebral artery occlusion in rat

Adenosine 5'-triphosphate outflow increases after an ischemic insult in the brain and may induce the expression of P2X7 receptors in resting microglia, determining its modification into an activated state. To assess the effects of P2X7 receptor blockade in preventing microglia activation and ameliorating brain damage and neurological impairment, we delivered the P2 unselective antagonist Reactive Blue 2 to rats after middle cerebral artery occlusion. In sham-operated animals, devoid of brain damage, double immunofluorescence verified the absence of P2X7 immunoreactivity on resting microglia, astrocytes, and neurons, identified, respectively, by OX-42, glial fibrillary acid protein, and neuronal nuclei (NeuN) immunoreactivity. After ischemia, vehicle-treated rats showed monolateral sensorimotor deficit and tissue damage in striatum and frontoparietal cortex. Moreover, P2X7 immunoreactivity was de novo expressed on activated microglia in infarcted and surrounding areas, as well as on a reactive form of microglia, resting in shape but P2X7 immunoreactive, present in ipsi- and contralateral cingulate and medial frontal cortex. Reactive Blue 2 improved sensorimotor deficit and restricted the volume of infarction, without preventing the expression of P2X7, but inducing it in the microglia of contralateral frontal and parietal cortex and striatum, which had lost reciprocal connections with the remote infarct area. De novo expression of P2X7 occurred in both activated and reactive microglia, suggesting their differentiated roles in the area of infarct and in remote regions. Reactive Blue 2 reduced ischemic brain damage, likely blocking the function of activated microglia in the infarct area, but in the remote brain regions promoted the expression of P2X7 on reactive microglia, developing defense and reparative processes.

Methanol alters ecto-nucleotidases and acetylcholinesterase in zebrafish brain

Methanol is a neurotoxic compound that is responsible for serious damage on CNS. Besides being found as an environmental contaminant, this alcohol is also employed as a component of cryoprotector solutions for zebrafish embryos. Here we tested the acute effect of methanol on ecto-nucleotidase (NTPDase, ecto-5'-nucleotidase) and acetylcholinesterase (AChE) activities in zebrafish brain. After acute treatment, there were significant decreases on ATP (26% and 45%) and ADP hydrolysis (26% and 30%) at 0.5% and 1.0%, respectively. However, no significant alteration on ecto-5'-nucleotidase activity was verified in zebrafish brain. A significant inhibition on AChE activity (39%, 33% and 30%) was observed at the range of 0.25% to 1.0% methanol exposure. Four NTPDase sequences were identified from phylogenetic analyses, which one is similar to NTPDase1 and the others to NTPDase2. Methanol was able to inhibit NTPDase1, two isoforms of NTPDase2 and AChE transcripts. To evaluate if methanol affects directly these enzymes activities, we have performed in vitro assays. ATP hydrolysis presented a significant inhibition (19% and 34%) at 1.5% and 3.0%, respectively, and ADP hydrolysis decreased only at 3.0% (29.2%). Nevertheless, AMP hydrolysis and AChE were not altered after in vitro exposure. The inhibitory effect observed on these enzymes could contribute to the neurodegenerative events promoted by methanol in zebrafish brain.

P2 receptors and neuronal injury

Extracellular adenosine 5'-triphosphate (ATP) was proposed to be an activity-dependent signaling molecule that regulates glia-glia and glia-neuron communications. ATP is a neurotransmitter of its own right and, in addition, a cotransmitter of other classical transmitters such as glutamate or GABA. The effects of ATP are mediated by two receptor families belonging either to the P2X (ligand-gated cationic channels) or P2Y (G protein-coupled receptors) types. P2X receptors are responsible for rapid synaptic responses, whereas P2Y receptors mediate slow synaptic responses and other types of purinergic signaling involved in neuronal damage/regeneration. ATP may act at pre- and postsynaptic sites and therefore, it may participate in the phenomena of long-term potentiation and long-term depression of excitatory synaptic transmission. The release of ATP into the extracellular space, e.g., by exocytosis, membrane transporters, and connexin hemichannels, is a widespread physiological process. However, ATP may also leave cells through their plasma membrane damaged by inflammation, ischemia, and mechanical injury. Functional responses to the activation of multiple P2 receptors were found in neurons and glial cells under normal and pathophysiological conditions. P2 receptor-activation could either be a cause or a consequence of neuronal cell death/glial activation and may be related to detrimental and/or beneficial effects. The present review aims at demonstrating that purinergic mechanisms correlate with the etiopathology of brain insults, especially because of the massive extracellular release of ATP, adenosine, and other neurotransmitters after brain injury. We will focus in this review on the most important P2 receptor-mediated neurodegenerative and neuroprotective processes and their beneficial modulation by possible therapeutic manipulations.

Enhancement of Excitatory Synaptic Transmission in Spiny Neurons After Transient Forebrain Ischemia

Spiny neurons in the neostriatum are highly vulnerable to ischemia. Enhancement of excitatory synaptic transmissions has been implicated in ischemia-induced excitotoxic neuronal death. Here we report that evoked excitatory postsynaptic currents in spiny neurons were potentiated after transient forebrain ischemia. The ischemia-induced potentiation in synaptic efficacy was associated with an enhancement of presynaptic release as demonstrated by an increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs) and a decrease in the paired-pulse ratio. The amplitude of inward currents evoked by exogenous application of glutamate did not show significant changes after ischemia, suggesting that postsynaptic mechanism is not involved. The ischemia-induced increase in mEPSCs frequency was not affected by blockade of voltage-gated calcium channels, but it was eliminated in the absence of extracellular calcium. Bath application of ATP P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS) significantly reduced mEPSC frequency in ischemic neurons but had no effects on the control ones. Furthermore, the inhibitory effect of PPADS on ischemic neurons was abolished in Ca2+-free external solution. These results indicate that excitatory synaptic transmissions in spiny neurons are potentiated after ischemia via presynaptic mechanisms. Activation of P2X receptors and the consequent Ca2+ influx might contribute to the ischemia-induced facilitation of glutamate release.