Guanosine Enhances Glutamate Transport Capacity in Brain Cortical Slices

Neuroprotective Effects of Guanosine in Ischemic Stroke-Small Steps towards Effective Therapy

Guanosine (Guo) is a nucleotide metabolite that acts as a potent neuromodulator with neurotrophic and regenerative properties in neurological disorders. Under brain ischemia or trauma, Guo is released to the extracellular milieu and its concentration substantially raises. In vitro studies on brain tissue slices or cell lines subjected to ischemic conditions demonstrated that Guo counteracts destructive events that occur during ischemic conditions, e.g., glutaminergic excitotoxicity, reactive oxygen and nitrogen species production. Moreover, Guo mitigates neuroinflammation and regulates post-translational processing. Guo asserts its neuroprotective effects via interplay with adenosine receptors, potassium channels, and excitatory amino acid transporters. Subsequently, guanosine activates several prosurvival molecular pathways including PI3K/Akt (PI3K) and MEK/ERK. Due to systemic degradation, the half-life of exogenous Guo is relatively low, thus creating difficulty regarding adequate exogenous Guo distribution. Nevertheless, in vivo studies performed on ischemic stroke rodent models provide promising results presenting a sustained decrease in infarct volume, improved neurological outcome, decrease in proinflammatory events, and stimulation of neuroregeneration through the release of neurotrophic factors. In this comprehensive review, we discuss molecular signaling related to Guo protection against brain ischemia. We present recent advances, limitations, and prospects in exogenous guanosine therapy in the context of ischemic stroke.

Guanosine enhances glutamate uptake and oxidation, preventing oxidative stress in mouse hippocampal slices submitted to high glutamate levels

Glutamate (Glu) is the main mammalian brain neurotransmitter. Concerning the glutamatergic neurotransmission, excessive levels of glutamate in the synaptic cleft are extremally harmful. This phenomenon, named as excitotoxicity is involved in various acute and chronic brain diseases. Guanosine (GUO), an endogenous guanine nucleoside, possesses neuroprotective effects in several experimental models of glutamatergic excitotoxicity, an effect accompanied by an increase in astrocytic glutamate uptake. Therefore, the objective of this study was to investigate the involvement of an additional putative parameter, glutamate oxidation to CO₂, involved in ex-vivo GUO neuroprotective effects in mouse hippocampal slices submitted to glutamatergic excitotoxicity. Mice were sacrificed by decapitation, the hippocampi were removed and sliced. The slices were incubated for various times and concentrations of Glu and GUO. First, the concentration of Glu that produced an increase in L-[¹⁴C(U)]-Glu oxidation to CO₂ without cell injury was determined at different time points (between 0 and 90 min); 1000 μM Glu increased Glu oxidation between 30 and 60 min of incubation without cell injury. Under these conditions (Glu concentration and incubation time), 100 μM GUO increased Glu oxidation (35%). Additionally, 100 μM GUO increased L-[3,4-3H]-glutamate uptake (45%) in slices incubated with 1000 μM Glu (0-30 min). Furthermore, 1000  μM Glu increased reactive species levels, SOD activity, and decreased GPx activity, and GSH content after 30 and 60 min; 100 μM GUO prevented these effects. This is the first study demonstrating that GUO simultaneously promoted an increase in the uptake and utilization of Glu in excitotoxicity-like conditions preventing redox imbalance.

Novel Positive Allosteric Modulators of Glutamate Transport Have Neuroprotective Properties in an in Vitro Excitotoxic Model

Dysfunction of excitatory amino acid transporters (EAATs) has been implicated in the pathogenesis of various neurological disorders, such as stroke, brain trauma, epilepsy, and several neurodegenerative disorders. EAAT2 is the main transporter subtype responsible for glutamate clearance in the brain, and plays a key role in regulating neurotransmission and preventing excitotoxicity. Therefore, compounds that increase the activity of EAAT2 have therapeutic potential for neuroprotection. In previous studies, we used virtual screening approaches to identify novel positive allosteric modulators (PAMs) of EAAT2. These compounds were shown to selectively increase the activity of EAAT2 and increase V_max of transport, without changing substrate affinity. In this work, our major effort was to investigate whether increasing the activity of EAAT2 by allosteric modulation would translate to neuroprotection in in vitro primary culture models of excitotoxicity. To investigate potential neuroprotective effects of one EAAT2 PAM, GT949, we subjected cultures to acute and prolonged excitotoxic insults by exogenous application of glutamate, or oxidative stress by application of hydrogen peroxide. GT949 administration did not result in neuroprotection in the oxidative stress model, likely due to damage of the glutamate transporters. However, GT949 displayed neuroprotective properties after acute and prolonged glutamate-mediated excitotoxicity. We propose that this compound prevents excess glutamate signaling by increasing the rate of glutamate clearance by EAAT2, thereby preventing excitotoxic damage and cell death. This novel class of compounds is therefore an innovative approach for neuroprotection with potential for translation in in vivo animal models of excitotoxicity.

Guanosine Prevents against Glutamatergic Excitotoxicity in C. elegans

Glutamatergic neurotransmission is present in most mammalian excitatory synapses and plays a key role in central nervous system homeostasis. When over-activated, it can induce excitotoxicity, which is present in several neuropathologies. The nucleoside guanosine (GUO) is a guanine-based purine known to have neuroprotective effects by modulating glutamatergic system during glutamate excitotoxicity in mammals. However, GUO action in Caenorhabditis elegans, as well as on C. elegans glutamatergic excitotoxicity model, is not known. The GUO effects on behavioral parameters in Wild Type (WT) and knockouts worms for glutamate transporters (GLT-3, GLT-1), glutamate vesicular transporter (EAT-4), and NMDA and non-NMDA receptors were used to evaluate the GUO modulatory effects. The GUO tested concentrations did not alter the animals' development, but GUO reduced pharyngeal pumps in WT animals in a dose-dependent manner. The same effect was observed in pharyngeal pumps, when the animals were treated with 4 mM of GUO in glr-1, nmr-1 and eat-4, but not in glt-3 and glt-3;glt-1 knockouts. The double mutant glt-3; glt-1 for GluTs had decreased body bends and an increased number of reversions. This effect was reverted after treatment with GUO. Furthermore, GUO did not alter the sensory response in worms with altered glutamatergic signaling. Thus, GUO seems to modulate the worm's glutamatergic system in situations of exacerbated glutamatergic signaling, which are represented by knockout strains to glutamate transporters. However, in WT animals, GUO appears to reinforce glutamatergic signaling in specific neurons. Our findings indicate that C. elegans strains are useful models to study new compounds that could be used in glutamate-associated neurodegenerative diseases.

Novel Targets for Fast Antidepressant Responses: Possible Role of Endogenous Neuromodulators

The available medications for the treatment of major depressive disorder have limitations, particularly their limited efficacy, delayed therapeutic effects, and the side effects associated with treatment. These issues highlight the need for better therapeutic agents that provide more efficacious and faster effects for the management of this disorder. Ketamine, an N-methyl-D-aspartate receptor antagonist, is the prototype for novel glutamate-based antidepressants that has been shown to cause a rapid and sustained antidepressant effect even in severe refractory depressive patients. Considering the importance of these findings, several studies have been conducted to elucidate the molecular targets for ketamine's effect. In addition, efforts are under way to characterize ketamine-like drugs. This review focuses particularly on evidence that endogenous glutamatergic neuromodulators may be able to modulate mood and to elicit fast antidepressant responses. Among these molecules, agmatine and creatine stand out as those with more published evidence of similarities with ketamine, but guanosine and ascorbic acid have also provided promising results. The possibility that these neuromodulators and ketamine have common neurobiological mechanisms, mainly the ability to activate mechanistic target of rapamycin and brain-derived neurotrophic factor signaling, and synthesis of synaptic proteins in the prefrontal cortex and/or hippocampus is presented and discussed.

The Guanine-Based Purinergic System: The Tale of An Orphan Neuromodulation

Guanine-based purines (GBPs) have been recently proposed to be not only metabolic agents but also extracellular signaling molecules that regulate important functions in the central nervous system. In such way, GBPs-mediated neuroprotection, behavioral responses and neuronal plasticity have been broadly described in the literature. However, while a number of these functions (i.e., GBPs neurothophic effects) have been well-established, the molecular mechanisms behind these GBPs-dependent effects are still unknown. Furthermore, no plasma membrane receptors for GBPs have been described so far, thus GBPs are still considered orphan neuromodulators. Interestingly, an intricate and controversial functional interplay between GBPs effects and adenosine receptors activity has been recently described, thus triggering the hypothesis that GBPs mechanism of action might somehow involve adenosine receptors. Here, we review recent data describing the GBPs role in the brain. We focus on the involvement of GBPs regulating neuronal plasticity, and on the new hypothesis based on putative GBPs receptors. Overall, we expect to shed some light on the GBPs world since although these molecules might represent excellent candidates for certain neurological diseases management, the lack of putative GBPs receptors precludes any high throughput screening intent for the search of effective GBPs-based drugs.

Neuroprotective Effects of the Glutamate Transporter Activator (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153) following Traumatic Brain Injury in the Adult Rat

Traumatic brain injury (TBI) in humans and in animals leads to an acute and sustained increase in tissue glutamate concentrations within the brain, triggering glutamate-mediated excitotoxicity. Excitatory amino acid transporters (EAATs) are responsible for maintaining extracellular central nervous system glutamate concentrations below neurotoxic levels. Our results demonstrate that as early as 5 min and up to 2 h following brain trauma in brain-injured rats, the activity (Vmax) of EAAT2 in the cortex and the hippocampus was significantly decreased, compared with sham-injured animals. The affinity for glutamate (KM) and the expression of glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST) were not altered by the injury. Administration of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a GLT-1 activator, beginning immediately after injury and continuing for 24 h, significantly decreased neurodegeneration, loss of microtubule-associated protein 2 and NeuN (+) immunoreactivities, and attenuated calpain activation in both the cortex and the hippocampus at 24 h after the injury; the reduction in neurodegeneration remained evident up to 14 days post-injury. In synaptosomal uptake assays, MS-153 up-regulated GLT-1 activity in the naïve rat brain but did not reverse the reduced activity of GLT-1 in traumatically-injured brains. This study demonstrates that administration of MS-153 in the acute post-traumatic period provides acute and long-term neuroprotection for TBI and suggests that the neuroprotective effects of MS-153 are related to mechanisms other than GLT-1 activation, such as the inhibition of voltage-gated calcium channels.

The antidepressant-like effect of chronic guanosine treatment is associated with increased hippocampal neuronal differentiation

Guanosine is a purine nucleoside that occurs naturally in the central nervous system, exerting trophic effects. Given its neuroprotective properties, the potential of guanosine as an antidepressant has been recently examined. Within this context, the present study sought to investigate the effects of chronic treatment with guanosine on the tail suspension test (TST), open field test and adult hippocampal neurogenesis. Swiss mice were administered guanosine for 21 days (5 mg/kg/day, p.o.) and subsequently submitted to the TST and open-field test. Following behavioural testing, animals were killed and the brains were processed for immunohistochemical analyses of hippocampal cell proliferation and neuronal differentiation. Animals treated with guanosine showed a reduction in immobility time in the TST without alterations in locomotor activity, confirming the antidepressant-like effect of this compound. Quantitative microscopic analysis did not reveal significant alterations in the numbers of Ki-67- and proliferating cell nuclear antigen (PCNA)-positive cells in the hippocampal dentate gyrus (DG) of guanosine-treated mice. However, guanosine treatment resulted in a significant increase in the number of immature neurons, as assessed by immunohistochemistry for the neurogenic differentiation protein. Interestingly, this effect was localized to the ventral hippocampal DG, a functionally distinct region of this structure known to regulate emotional and motivational behaviours. Taken together, our results suggest that the antidepressant-like effect of chronic guanosine treatment is associated with an increase in neuronal differentiation, reinforcing the notion that this nucleoside may be an endogenous mood modulator.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Guanosine produces an antidepressant-like effect through the modulation of NMDA receptors, nitric oxide-cGMP and PI3K/mTOR pathways

Guanosine is an extracellular signaling molecule implicated in the modulation of glutamatergic transmission and neuroprotection. The present study evaluated the antidepressant-like effect of guanosine in the forced swimming test (FST) and in the tail suspension test (TST) in mice. The contribution of NMDA receptors as well as l-arginine-NO-cGMP and PI3K-mTOR pathways to this effect was also investigated. Guanosine administered orally produced an antidepressant-like effect in the FST (0.5-5 mg/kg) and TST (0.05-0.5 mg/kg). The anti-immobility effect of guanosine in the TST was prevented by the treatment of mice with NMDA (0.1 pmol/site, i.c.v.), d-serine (30 μg/site, i.c.v., a co-agonist of NMDA receptors), l-arginine (750 mg/kg, i.p., a substrate for nitric oxide synthase), sildenafil (5 mg/kg, i.p., a phosphodiesterase 5 inhibitor), LY294002 (10 μg/site, i.c.v., a reversible PI3K inhibitor), wortmannin (0.1 μg/site, i.c.v., an irreversible PI3K inhibitor) or rapamycin (0.2 nmol/site, i.c.v., a selective mTOR inhibitor). In addition, the administration of ketamine (0.1 mg/kg, i.p., a NMDA receptor antagonist), MK-801 (0.001 mg/kg, i.p., another NMDA receptor antagonist), 7-nitroindazole (50 mg/kg, i.p., a neuronal nitric oxide synthase inhibitor) or ODQ (30 pmol/site i.c.v., a soluble guanylate cyclase inhibitor) in combination with a sub-effective dose of guanosine (0.01 mg/kg, p.o.) reduced the immobility time in the TST when compared with either drug alone. None of the treatments affected locomotor activity. Altogether, results firstly indicate that guanosine exerts an antidepressant-like effect that seems to be mediated through an interaction with NMDA receptors, l-arginine-NO-cGMP and PI3K-mTOR pathways.

Effects of chronic guanosine treatment on hippocampal damage and cognitive impairment of rats submitted to chronic cerebral hypoperfusion

Chronic cerebral hypoperfusion contributes to a cognitive decline related to brain disorders. Its experimental model in rats is a permanent bilateral common carotid artery occlusion (2VO). Overstimulation of the glutamatergic system excitotoxicity due to brain energetic disturbance in 2VO animals seems to play a pivotal role as a mechanism of cerebral damage. The nucleoside guanosine (GUO) exerts extracellular effects including antagonism of glutamatergic activity. Accordingly, our group demonstrated several neuroprotective effects of GUO against glutamatergic excitotoxicity. Therefore, in this study, we evaluated a chronic GUO treatment effects in rats submitted to 2VO. We evaluated the animals performance in the Morris water maze and hippocampal damage by neurons and astrocytes immunohistochemistry. In addition, we investigated the cerebrospinal fluid (CSF) brain derived neurotrophic factor (BDNF) and serum S100B levels. Additionally, the purine CSF and plasma levels were determined. GUO treatment did not prevent the cognitive impairment promoted by 2VO. However, none of the 2VO animals treated with GUO showed differences in the hippocampal regions compared to control, while 20% of 2VO rats not treated with GUO presented loss of pyramidal neurons and increased glial labeling cells in CA1 hippocampal region. In addition, we did not observe differences in CSF BDNF nor serum S100B levels among the groups. Of note, both the 2VO surgery and GUO treatment changed the purine CSF and plasma profile. In conclusion, GUO treatment did not prevent the cognitive impairment observed in 2VO animals, but our data suggest that GUO could be neuroprotective against hippocampal damage induced by 2VO.

Effects of 3 weeks GMP oral administration on glutamatergic parameters in mice neocortex

Overstimulation of the glutamatergic system (excitotoxicity) is involved in various acute and chronic brain diseases. Several studies support the hypothesis that guanosine-5'-monophosphate (GMP) can modulate glutamatergic neurotransmission. The aim of this study was to evaluate the effects of chronically administered GMP on brain cortical glutamatergic parameters in mice. Additionally, we investigated the neuroprotective potential of the GMP treatment submitting cortical brain slices to oxygen and glucose deprivation (OGD). Moreover, measurements of the cerebrospinal fluid (CSF) purine levels were performed after the treatment. Mice received an oral administration of saline or GMP during 3 weeks. GMP significantly decreases the cortical brain glutamate binding and uptake. Accordingly, GMP reduced the immunocontent of the glutamate receptors subunits, NR2A/B and GluR1 (NMDA and AMPA receptors, respectively) and glutamate transporters EAAC1 and GLT1. GMP treatment significantly reduced the immunocontent of PSD-95 while did not affect the content of Snap 25, GLAST and GFAP. Moreover, GMP treatment increased the resistance of neocortex to OGD insult. The chronic GMP administration increased the CSF levels of GMP and its metabolites. Altogether, these findings suggest a potential modulatory role of GMP on neocortex glutamatergic system by promoting functional and plastic changes associated to more resistance of mice neocortex against an in vitro excitotoxicity event.

Enhanced subcortical spreading depression in familial hemiplegic migraine type 1 mutant mice

Familial hemiplegic migraine type 1, a monogenic migraine variant with aura, is linked to gain-of-function mutations in the CACNA1A gene encoding Ca(V)2.1 channels. The S218L mutation causes severe channel dysfunction, and paroxysmal migraine attacks can be accompanied by seizures, coma, and hemiplegia; patients expressing the R192Q mutation exhibit hemiplegia only. Familial hemiplegic migraine knock-in mice expressing the S218L or R192Q mutation are highly susceptible to cortical spreading depression, the electrophysiological surrogate for migraine aura, and develop severe and prolonged motor deficits after spreading depression. The S218L mutants also develop coma and seizures and sometimes die. To investigate underlying mechanisms for these symptoms, we used multielectrode electrophysiological recordings, diffusion-weighted magnetic resonance imaging, and c-fos immunohistochemistry to trace spreading depression propagation into subcortical structures. We showed that unlike the wild type, cortical spreading depression readily propagated into subcortical structures in both familial hemiplegic migraine type 1 mutants. Whereas the facilitated subcortical spread appeared limited to the striatum in R192Q, hippocampal and thalamic spread was detected in the S218L mutants with an allele-dosage effect. Both strains exhibited increased susceptibility to subcortical spreading depression and reverberating spreading depression waves. Altogether, these data show that spreading depression propagates between cortex, basal ganglia, diencephalon, and hippocampus in genetically susceptible brains, which could explain the prolonged hemiplegia, coma, and seizure phenotype in this variant of migraine with aura.

Neuroprotective effect of guanosine against glutamate-induced cell death in rat hippocampal slices is mediated by the phosphatidylinositol-3 kinase/Akt/ glycogen synthase kinase 3β pathway activation and inducible nitric oxide synthase inhibition

Excitotoxicity and cell death induced by glutamate are involved in many neurodegenerative disorders. We have previously demonstrated that excitotoxicity induced by millimolar concentrations of glutamate in hippocampal slices involves apoptotic features and glutamate-induced glutamate release. Guanosine, an endogenous guanine nucleoside, prevents excitotoxicity by its ability to modulate glutamate transport. In this study, we have evaluated the neuroprotective effect of guanosine against glutamate-induced toxicity in hippocampal slices and the mechanism involved in such an effect. We have found that guanosine (100 μM) was neuroprotective against 1 mM glutamate-induced cell death through the inhibition of glutamate release induced by glutamate. Guanosine also induced the phosphorylation and, thus, activation of protein kinase B (PKB/Akt), a downstream target of phosphatidylinositol-3 kinase (PI3K), as well as phosphorylation of glycogen synthase kinase 3β, which has been reported to be inactivated by Akt after phosphorylation at Ser9. Glutamate treated hippocampal slices showed increased inducible nitric oxide synthase (iNOS) expression that was prevented by guanosine. Slices preincubated with SNAP (an NO donor), inhibited the protective effect of guanosine. LY294002 (30 μM), a PI3K inhibitor, attenuated guanosine-induced neuroprotection, guanosine prevention of glutamate release, and guanosine-induced GSK3β(Ser9) phosphorylation but not guanosine reduction of glutamate-induced iNOS expression. Taken together, the results of this study show that guanosine protects hippocampal slices by a mechanism that involves the PI3K/Akt/GSK3β(Ser9) pathway and prevention of glutamate-induced glutamate release. Furthermore, guanosine also reduces glutamate-induced iNOS by a PI3K/Akt-independent mechanism.

A purine-selective nucleobase/nucleoside transporter in PK15NTD cells

Nucleoside and nucleobase transporters are important for salvage of purines and pyrimidines and for transport of their analog drugs into cells. However, the pathways for nucleobase translocation in mammalian cells are not well characterized. We identified an Na-independent purine-selective nucleobase/nucleoside transport system in the nucleoside transporter-deficient PK15NTD cells. This transport system has 1,000-fold higher affinity for nucleobases than nucleosides with K(m) values of 2.5 +/- 0.7 microM for [(3)H]adenine, 6.4 +/- 0.5 microM for [(3)H]guanine, 1.1 +/- 0.1 mM for [(3)H]guanosine, and 4.2 +/- 0.5 mM [(3)H]adenosine. The uptake of [(3)H]guanine (0.05 microM) was inhibited by other nucleobases and nucleobase analog drugs (at 0.5-1 mM in the order of potency): 6-mercaptopurine = thioguanine = guanine > adenine >>> thymine = fluorouracil = uracil. Cytosine and methylcytosine had no effect. Nucleoside analog drugs with modification at 2' and/or 5 positions (all at 1 mM) were more potent than adenosine in competing the uptake of [(3)H]guanine: 2-chloro-2'-deoxyadenosine > 2-chloroadenosine > 2'3'-dideoxyadenosine = 2'-deoxyadenosine > 5-deoxyadenosine > adenosine. 2-Chloro-2'-deoxyadenosine and 2-chloroadenosine inhibited [(3)H]guanine uptake with IC(50) values of 68 +/- 5 and 99 +/- 10 microM, respectively. The nucleobase/nucleoside transporter was resistant to nitrobenzylthioinosine {6-[(4-nitrobenzyl) thiol]-9-beta-D-ribofuranosylpurine}, dipyridamole, and dilazep, but was inhibited by papaverine, the organic cation transporter inhibitor decynium-22 (IC(50) of approximately 1 microM), and by acidic pH (pH = 5.5). In conclusion, we have identified a mammalian purine-selective nucleobase/nucleoside transporter with high affinity for purine nucleobases. This transporter is potentially important for transporting naturally occurring purines and purine analog drugs into cells.

Guanosine promotes B16F10 melanoma cell differentiation through PKC-ERK 1/2 pathway

Malignant melanoma is one of the most lethal cancers. Nowadays, several anti-melanoma therapies have been employed. However, the poor prognosis and/or the increased toxicity of those treatments clearly demonstrate the requirement of searching for new drugs or novel combined chemotherapeutic protocols, contemplating both effectiveness and low toxicity. Guanosine (Guo) has been used in combination with acriflavina to potentiate the latter's antitumor activity, through still unknown mechanisms. Here, we show that Guo induces B16F10 melanoma cell differentiation, attested by growth arrest, dendrite-like outgrowth and increased melanogenesis, and also reduced motility. A sustained ERK 1/2 phosphorylation was observed after Guo treatment and ERK inhibition led to blockage of dendritogenesis. Intracellular cyclic AMP was not involved in ERK activation, since its levels remained unchanged. Protein kinase C (PKC), in contrast to phospholipase C (PLC), inhibition completely prevented ERK activation. While the classical melanoma differentiation agent forskolin activates cAMP-PKA-Raf-MEK-ERK pathway in B16F10 cells, here we suggest that a cAMP-independent, PKC-ERK axis is involved in Guo-induced B16F10 differentiation. Altogether, our results show that Guo acts as a differentiating agent, with cytostatic rather than cytotoxic properties, leading to a decreased melanoma malignancy. Thus, we propose that Guo may be envisaged in combination with lower doses of conventional anti-melanoma drugs, in an attempt to prevent or diminish their adverse effects.

Proposal of a guanine-based purinergic system in the mammalian central nervous system

Guanine-based purines have been traditionally studied as modulators of intracellular processes, mainly G-protein activity. However, they also exert several extracellular effects not related to G proteins, including modulation of glutamatergic activity, trophic effects on neural cells, and behavioral effects. In this article, the putative roles of guanine-based purines on the nervous system are reviewed, and we propose a specific guanine-based purinergic system in addition to the well-characterized adenine-based purinergic system. Current evidence suggest that guanine-based purines modulate glutamatergic parameters, such as glutamate uptake by astrocytes and synaptic vesicles, seizures induced by glutamatergic agents, response to ischemia and excitotoxicity, and are able to affect learning, memory and anxiety. Additionally, guanine-based purines have important trophic functions affecting the development, structure, or maintenance of neural cells. Although studies addressing the mechanism of action (receptors and second messenger systems) of guanine-based purines are still insufficient, these findings point to the guanine-based purines (nucleotides and guanosine) as potential new targets for neuroprotection and neuromodulation.

Enhancing glutamate transport: mechanism of action of Parawixin1, a neuroprotective compound from Parawixia bistriata spider venom

Previous studies have shown that a compound purified from the spider Parawixia bistriata venom stimulates the activity of glial glutamate transporters and can protect retinal tissue from ischemic damage. To understand the mechanism by which this compound enhances transport, we examined its effects on the functional properties of glutamate transporters after solubilization and reconstitution in liposomes and in transfected COS-7 cells. Here, we demonstrate in both systems that Parawixin1 promotes a direct and selective enhancement of glutamate influx by the EAAT2 transporter subtype through a mechanism that does not alter the apparent affinities for the cosubstrates glutamate or sodium. In liposomes, we observed maximal enhancement by Parawixin1 when extracellular sodium and intracellular potassium concentrations are within physiological ranges. Moreover, the compound does not enhance the reverse transport of glutamate under ionic conditions that favor efflux, when extracellular potassium is elevated and the sodium gradient is reduced, nor does it alter the exchange of glutamate in the absence of internal potassium. These observations suggest that Parawixin1 facilitates the reorientation of the potassium-bound transporter, the rate-limiting step in the transport cycle, a conclusion further supported by experiments showing that Parawixin1 does not stimulate uptake by an EAAT2 transport mutant (E405D) defective in the potassium-dependent reorientation step. Thus, Parawixin1 enhances transport through a novel mechanism targeting a step in the transport cycle distinct from substrate influx or efflux and provides a basis for the design of new drugs that act allosterically on transporters to increase glutamate clearance.

Neuroprotective agents: is effective on toxicity in glial cells?

1. Glial cells are the most abundant cell population in the central nervous system. The aim of this study was to examine the effects of melatonin, 7-nitroindazole, and riluzole on glutamate toxicity in primary glial cell culture. 2. Glutamate toxicity was induced by addition of 100 microM glutamate to the cultures, and then 100 microM melatonin, 500 microM 7-nitroindazole, and 10 (M riluzole were administered in different groups. Lactate Dehydrogenase activity and nitrite levels were determined at the 1st, 6th, and 24th h. 3. Melatonin, 7-nitroindazole, and riluzole decrease Lactate Dehydrogenase activity at the 1st, 6th, and 24th h (at all hours, p<0.05). Nitrite levels were decreased by melatonin and riluzole at the 1st, 6th, and 24th h. 4. In this study, we observed that melatonin, 7-nitroindazole, and riluzole are effective as protective agents on glutamate toxicity in mixed glial cells.