Rat brain guanosine binding site. Biological studies and pseudo-receptor construction

Lessons from the physiological role of guanosine in neurodegeneration and cancer: Toward a multimodal mechanism of action?

Neurodegenerative diseases and brain tumours represent important health challenges due to their severe nature and debilitating consequences that require substantial medical care. Interestingly, these conditions share common physiological characteristics, namely increased glutamate, and adenosine transmission, which are often associated with cellular dysregulation and damage. Guanosine, an endogenous nucleoside, is safe and exerts neuroprotective effects in preclinical models of excitotoxicity, along with cytotoxic effects on tumour cells. However, the lack of well-defined mechanisms of action for guanosine hinders a comprehensive understanding of its physiological effects. In fact, the absence of specific receptors for guanosine impedes the development of structure-activity research programs to develop guanosine derivatives for therapeutic purposes. Alternatively, given its apparent interaction with the adenosinergic system, it is plausible that guanosine exerts its neuroprotective and anti-tumorigenic effects by modulating adenosine transmission through undisclosed mechanisms involving adenosine receptors, transporters, and purinergic metabolism. Here, several potential molecular mechanisms behind the protective actions of guanosine will be discussed. First, we explore its potential interaction with adenosine receptors (A1R and A2AR), including the A1R-A2AR heteromer. In addition, we consider the impact of guanosine on extracellular adenosine levels and the role of guanine-based purine-converting enzymes. Collectively, the diverse cellular functions of guanosine as neuroprotective and antiproliferative agent suggest a multimodal and complementary mechanism of action.

Inhibition of uterine contractility by guanine-based purines in non-pregnant rats

Growing evidence pointed out that guanine-based purines are able to modulate smooth muscle contractile activity of blood vessels and gastrointestinal tract. Since, so far, possible guanine-based purine modulation of uterine musculature is unknown, the aim of the present study was to investigate in vitro, using organ bath technique, guanosine and guanine effects on spontaneous uterine contraction, and uterine contraction induced by K⁺-depolarization and oxytocin in a non-pregnant rat. Guanosine, but not guanine, reduced the amplitude of spontaneous contraction of the uterine muscle in a dose-dependent manner. The inhibitory response was antagonized by S-(4-nitrobenzyl)-6-thioinosine (NBTI), a membrane nucleoside transporter inhibitor, but persisted in the presence of theophylline, a nonselective adenosine receptor antagonist, or propanolol, β₁/β₂ adrenoreceptor antagonist or blockers of a nitrergic pathway. In addition, potassium channel blockers did not influence guanosine-induced effects. Guanosine was able to inhibit the external calcium (Ca²⁺) influx-induced contraction, but it did not affect the contraction induced by high-KCl solution, indicating that guanosine does not interact with L-type voltage-gated calcium channel. Guanosine prevented/reduced uterine contractions induced by oxytocin, even in the absence of external calcium. In conclusion, guanosine is able to reduce both spontaneous and oxytocin-induced contractions of rat myometrium, likely subsequently to its intracellular intake. The blockade of extracellular Ca²⁺ influx and reduction of Ca²⁺ release from the intracellular store are the mechanisms involved in the guanosine-induced tocolytic effects.

The antidepressant-like effect of guanosine involves the modulation of adenosine A1 and A2A receptors

Guanosine has been considered a promising candidate for antidepressant responses, but if this nucleoside could modulate adenosine A₁ (A₁R) and A_2A (A_2AR) receptors to exert antidepressant-like actions remains to be elucidated. This study investigated the role of A₁R and A_2AR in the antidepressant-like response of guanosine in the mouse tail suspension test and molecular interactions between guanosine and A₁R and A₂AR by docking analysis. The acute (60 min) administration of guanosine (0.05 mg/kg, p.o.) significantly decreased the immobility time in the tail suspension test, without affecting the locomotor performance in the open-field test, suggesting an antidepressant-like effect. This behavioral response was paralleled with increased A₁R and reduced A_2AR immunocontent in the hippocampus, but not in the prefrontal cortex, of mice. Guanosine-mediated antidepressant-like effect was not altered by the pretreatment with caffeine (3 mg/kg, i.p., a non-selective adenosine A₁R/A_2AR antagonist), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX - 2 mg/kg, i.p., a selective adenosine A₁R antagonist), or 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)-phenol (ZM241385 - 1 mg/kg, i.p., a selective adenosine A_2AR antagonist). However, the antidepressant-like response of guanosine was completely abolished by adenosine (0.5 mg/kg, i.p., a non-selective adenosine A₁R/A_2AR agonist), N-6-cyclohexyladenosine (CHA - 0.05 mg/kg, i.p., a selective adenosine A₁ receptor agonist), and N-6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)ethyl]adenosine (DPMA - 0.1 mg/kg, i.p., a selective adenosine A_2A receptor agonist). Finally, docking analysis also indicated that guanosine might interact with A₁R and A_2AR at the adenosine binding site. Overall, this study reinforces the antidepressant-like of guanosine and unveils a previously unexplored modulation of the modulation of A₁R and A_2AR in its antidepressant-like effect.

Analysis of the interaction between A1 R and A2A R proteins in living cells based on FRET imaging and batch processing method

The quantitative FRET analysis of living cells is a tedious and time-consuming task for freshman lacks technical training. In this study, FRET imaging and batch processing method were combined to analyze reagents-induced interactions of A₁ R and A_2A R on cell membranes. Results showed that the method had taken less time than if cell-by-cell was analyzed. The accuracy and repeatability of FRET efficiency values were likewise improved by removing the interference from anthropogenic factors. Then this method was applied to rapidly analyze acetaldehyde-induced interactions, which analyzed hundreds of single-cell trends by one operation, and the results revealed that interactions were consistently attenuated in LX-2 cells, and statistical differences appeared after 30 min. Combined with batch processing method, procedures of cells FRET analysis have been greatly simplified without additional technical work, which has broad prospects in large-scale analysis of cellar protein interaction.

Guanosine as a promising target for fast-acting antidepressant responses

Although the rapid-onset and sustained antidepressant responses elicited by ketamine have gained considerable attention in recent years, it has some knock-on effects that limit its widespread clinical use. Therefore, ketamine is considered the prototype for the new generation of glutamate-based rapid-acting antidepressants. Within this context, it has been demonstrated that guanosine, an endogenous guanine-based purine, has overlapping mechanisms of action with ketamine and is effective in eliciting fast antidepressant-like responses and even potentiating ketamine's actions in preclinical studies. Here, we review the recent findings regarding the ability of guanosine to produce rapid-acting antidepressant-like effects and we provide an overview of the molecular mechanisms underlying its antidepressant-like actions. Moreover, the neurobiological mechanisms underpinning the ability of guanosine in boosting the antidepressant-like and pro-synaptogenic effects elicited by ketamine are also reported. Taken together, this review opens perspectives for the use of guanosine alone or in combination with ketamine for the management of treatment-resistant depression.

High spatio-temporal resolution measurement of A1 R and A2A R interactions combined with Iem-spFRET and E-FRET methods

A₁ R-A_2A R heterodimers regulate striatal glutamatergic neurotransmission. However, few researches about kinetics have been reported. Here, we combined Iem-spFRET and E-FRET to investigate the kinetics of A₁ R and A_2A R interaction. Iem-spFRET obtains the energy transfer efficiency of the whole cell. E-FRET gets energy transfer efficiency with high spatial resolution, whereas, it was prone to biases because background was easily selected due to manual operation. To study the interaction with high spatio-temporal resolution, Iem-spFRET was used to correct the deviation of E-FRET. In this paper, A₁ R and A_2A R interaction was monitored, and the changes of FRET efficiency of the whole or/and partial cell membrane were described. The results showed that activation of A₁ R or A_2A R leads to rapid aggregation, inhibition of A₁ R or A_2A R leads to slow segregation, and the interaction is reversible. These results demonstrated that combination of Iem-spFRET and E-FRET could measure A₁ R and A_2A R interaction with high spatio-temporal resolution.

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.

Investigating the Role of Guanosine on Human Neuroblastoma Cell Differentiation and the Underlying Molecular Mechanisms

Neuroblastoma arises from neural crest cell precursors failing to complete the process of differentiation. Thus, agents helping tumor cells to differentiate into normal cells can represent a valid therapeutic strategy. Here, we evaluated whether guanosine (GUO), a natural purine nucleoside, which is able to induce differentiation of many cell types, may cause the differentiation of human neuroblastoma SH-SY5Y cells and the molecular mechanisms involved. We found that GUO, added to the cell culture medium, promoted neuron-like cell differentiation in a time- and concentration-dependent manner. This effect was mainly due to an extracellular GUO action since nucleoside transporter inhibitors reduced but not abolished it. Importantly, GUO-mediated neuron-like cell differentiation was independent of adenosine receptor activation as it was not altered by the blockade of these receptors. Noteworthy, the neuritogenic activity of GUO was not affected by blocking the phosphoinositide 3-kinase pathway, while it was reduced by inhibitors of protein kinase C or soluble guanylate cyclase. Furthermore, the inhibitor of the enzyme heme oxygenase-1 but not that of nitric oxide synthase reduced GUO-induced neurite outgrowth. Interestingly, we found that GUO was largely metabolized into guanine by the purine nucleoside phosphorylase (PNP) enzyme released from cells. Taken together, our results suggest that GUO, promoting neuroblastoma cell differentiation, may represent a potential therapeutic agent; however, due to its spontaneous extracellular metabolism, the role played by the GUO-PNP-guanine system needs to be further investigated.

Guanosine-Mediated Anxiolytic-Like Effect: Interplay with Adenosine A1 and A2A Receptors

Acute or chronic administration of guanosine (GUO) induces anxiolytic-like effects, for which the adenosine (ADO) system involvement has been postulated yet without a direct experimental evidence. Thus, we aimed to investigate whether adenosine receptors (ARs) are involved in the GUO-mediated anxiolytic-like effect, evaluated by three anxiety-related paradigms in rats. First, we confirmed that acute treatment with GUO exerts an anxiolytic-like effect. Subsequently, we investigated the effects of pretreatment with ADO or A₁R (CPA, CCPA) or A_2AR (CGS21680) agonists 10 min prior to GUO on a GUO-induced anxiolytic-like effect. All the combined treatments blocked the GUO anxiolytic-like effect, whereas when administered alone, each compound was ineffective as compared to the control group. Interestingly, the pretreatment with nonselective antagonist caffeine or selective A₁R (DPCPX) or A_2AR (ZM241385) antagonists did not modify the GUO-induced anxiolytic-like effect. Finally, binding assay performed in hippocampal membranes showed that [³H]GUO binding became saturable at 100–300 nM, suggesting the existence of a putative GUO binding site. In competition experiments, ADO showed a potency order similar to GUO in displacing [³H]GUO binding, whereas AR selective agonists, CPA and CGS21680, partially displaced [³H]GUO binding, but the sum of the two effects was able to displace [³H]GUO binding to the same extent of ADO alone. Overall, our results strengthen previous data supporting GUO-mediated anxiolytic-like effects, add new evidence that these effects are blocked by A₁R and A_2AR agonists and pave, although they do not elucidate the mechanism of GUO and ADO receptor interaction, for a better characterization of GUO binding sites in ARs.

Apoptotic and anti-proliferative effect of guanosine and guanosine derivatives in HuT-78 T lymphoma cells

The effects of 100 μM of 3',5'-cGMP, cAMP, cCMP, and cUMP as well as of the corresponding membrane-permeant acetoxymethyl esters on anti-CD3-antibody (OKT3)-induced IL-2 production of HuT-78 cutaneous T cell lymphoma (Sézary lymphoma) cells were analyzed. Only 3',5'-cGMP significantly reduced IL-2 production. Flow cytometric analysis of apoptotic (propidium iodide/annexin V staining) and anti-proliferative (CFSE staining) effects revealed that 3',5'-cGMP concentrations > 50 μM strongly inhibited proliferation and promoted apoptosis of HuT-78 cells (cultured in the presence of αCD3 antibody). Similar effects were observed for the positional isomer 2',3'-cGMP and for 2',-GMP, 3'-GMP, 5'-GMP, and guanosine. By contrast, guanosine and guanosine-derived nucleotides had no cytotoxic effect on peripheral blood mononuclear cells (PBMCs) or acute lymphocytic leukemia (ALL) xenograft cells. The anti-proliferative and apoptotic effects of guanosine and guanosine-derived compounds on HuT-78 cells were completely eliminated by the nucleoside transport inhibitor NBMPR (S-(4-Nitrobenzyl)-6-thioinosine). By contrast, the ecto-phosphodiesterase inhibitor DPSPX (1,3-dipropyl-8-sulfophenylxanthine) and the CD73 ecto-5'-nucleotidase inhibitor AMP-CP (adenosine 5'-(α,β-methylene)diphosphate) were not protective. We hypothesize that HuT-78 cells metabolize guanosine-derived nucleotides to guanosine by yet unknown mechanisms. Guanosine then enters the cells by an NBMPR-sensitive nucleoside transporter and exerts cytotoxic effects. This transporter may be ENT1 because NBMPR counteracted guanosine cytotoxicity in HuT-78 cells with nanomolar efficacy (IC50 of 25-30 nM). Future studies should further clarify the mechanism of the observed effects and address the question, whether guanosine or guanosine-derived nucleotides may serve as adjuvants in the therapy of cancers that express appropriate nucleoside transporters and are sensitive to established nucleoside-derived cytostatic drugs.

Adenosine A1-A2A Receptor-Receptor Interaction: Contribution to Guanosine-Mediated Effects

Guanosine, a guanine-based purine nucleoside, has been described as a neuromodulator that exerts neuroprotective effects in animal and cellular ischemia models. However, guanosine's exact mechanism of action and molecular targets have not yet been identified. Here, we aimed to elucidate a role of adenosine receptors (ARs) in mediating guanosine effects. We investigated the neuroprotective effects of guanosine in hippocampal slices from A2AR-deficient mice (A2AR-/-) subjected to oxygen/glucose deprivation (OGD). Next, we assessed guanosine binding at ARs taking advantage of a fluorescent-selective A2AR antagonist (MRS7396) which could engage in a bioluminescence resonance energy transfer (BRET) process with NanoLuc-tagged A2AR. Next, we evaluated functional AR activation by determining cAMP and calcium accumulation. Finally, we assessed the impact of A1R and A2AR co-expression in guanosine-mediated impedance responses in living cells. Guanosine prevented the reduction of cellular viability and increased reactive oxygen species generation induced by OGD in hippocampal slices from wild-type, but not from A2AR-/- mice. Notably, while guanosine was not able to modify MRS7396 binding to A2AR-expressing cells, a partial blockade was observed in cells co-expressing A1R and A2AR. The relevance of the A1R and A2AR interaction in guanosine effects was further substantiated by means of functional assays (i.e., cAMP and calcium determinations), since guanosine only blocked A2AR agonist-mediated effects in doubly expressing A1R and A2AR cells. Interestingly, while guanosine did not affect A1R/A2AR heteromer formation, it reduced A2AR agonist-mediated cell impedance responses. Our results indicate that guanosine-induced effects may require both A1R and A2AR co-expression, thus identifying a molecular substrate that may allow fine tuning of guanosine-mediated responses.

Guanosine and GMP increase the number of granular cerebellar neurons in culture: dependence on adenosine A2A and ionotropic glutamate receptors

The guanine-based purines (GBPs) have essential extracellular functions such as modulation of glutamatergic transmission and trophic effects on neurons and astrocytes. We previously showed that GBPs, such as guanosine-5'-monophosphate (GMP) or guanosine (GUO), promote the reorganization of extracellular matrix proteins in astrocytes, and increase the number of neurons in a neuron-astrocyte co-culture protocol. To delineate the molecular basis underlying these effects, we isolated cerebellar neurons in culture and treated them with a conditioned medium derived from astrocytes previously exposed to GUO or GMP (GBPs-ACM) or, directly, with GUO or GMP. Agreeing with the previous studies, there was an increase in the number of β-tubulin III-positive neurons in both conditions, compared with controls. Interestingly, the increase in the number of neurons in the neuronal cultures treated directly with GUO or GMP was more prominent, suggesting a direct interaction of GBPs on cerebellar neurons. To investigate this issue, we assessed the role of adenosine and glutamate receptors and related intracellular signaling pathways after GUO or GMP treatment. We found an involvement of A_2A adenosine receptors, ionotropic glutamate N-methyl-D-aspartate (NMDA), and non-NMDA receptors in the increased number of cerebellar neurons. The signaling pathways extracellular-regulated kinase (ERK), calcium-calmodulin-dependent kinase-II (CaMKII), protein kinase C (PKC), phosphatidilinositol-3'-kinase (PI3-K), and protein kinase A (PKA) are also potentially involved with GMP and GUO effect. Such results suggest that GMP and GUO, and molecules released in GBPs-ACM promote the survival or maturation of primary cerebellar neurons or both via interaction with adenosine and glutamate receptors.

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.

Neuromodulatory Effects of Guanine-Based Purines in Health and Disease

The function of guanine-based purines (GBPs) is mostly attributed to the intracellular modulation of heteromeric and monomeric G proteins. However, extracellular effects of guanine derivatives have also been recognized. Thus, in the central nervous system (CNS), a guanine-based purinergic system that exerts neuromodulator effects, has been postulated. The thesis that GBPs are neuromodulators emerged from in vivo and in vitro studies, in which neurotrophic and neuroprotective effects of these kinds of molecules (i.e., guanosine) were demonstrated. GBPs induce several important biological effects in rodent models and have been shown to reduce seizures and pain, stabilize mood disorder behavior and protect against gliomas and diseases related with aging, such as ischemia or Parkinson and Alzheimer diseases. In vitro studies to evaluate the protective and trophic effects of guanosine, and of the nitrogenous base guanine, have been fundamental for understanding the mechanisms of action of GBPs, as well as the signaling pathways involved in their biological roles. Conversely, although selective binding sites for guanosine have been identified in the rat brain, GBP receptors have not been still described. In addition, GBP neuromodulation may depend on the capacity of GBPs to interact with well-known membrane proteins in glutamatergic and adenosinergic systems. Overall, in this review article, we present up-to-date GBP biology, focusing mainly on the mechanisms of action that may lead to the neuromodulator role of GBPs observed in neurological disorders.

Guanosine Attenuates Behavioral Deficits After Traumatic Brain Injury by Modulation of Adenosinergic Receptors

Traumatic brain injury (TBI) is a leading cause of disability worldwide, triggering chronic neurodegeneration underlying cognitive and mood disorder still without therapeutic prospects. Based on our previous observations that guanosine (GUO) attenuates short-term neurochemical alterations caused by TBI, this study investigated the effects of chronical GUO treatment in behavioral, molecular, and morphological disturbances 21 days after trauma. Rats subject to TBI displayed mood (anxiety-like) and memory dysfunction. This was accompanied by a decreased expression of both synaptic (synaptophysin) and plasticity proteins (BDNF and CREB), a loss of cresyl violet-stained neurons, and increased astrogliosis and microgliosis in the hippocampus. Notably, chronic GUO treatment (7.5 mg/kg i.p. daily starting 1 h after TBI) prevented all these TBI-induced long-term behavioral, neurochemical, and morphological modifications. This neuroprotective effect of GUO was abrogated in the presence of the adenosine A₁ receptor antagonist DPCPX (1 mg/kg) but unaltered by the adenosine A_2A receptor antagonist SCH58261 (0.05 mg/kg). These findings show that a chronic GUO treatment prevents the long-term mood and memory dysfunction triggered by TBI, which involves adenosinergic receptors.

Uncovering the Signaling Pathway behind Extracellular Guanine-Induced Activation of NO System: New Perspectives in Memory-Related Disorders

Mounting evidence suggests that the guanine-based purines stand out as key player in cell metabolism and in several models of neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases. Guanosine (GUO) and guanine (GUA) are extracellular signaling molecules derived from the breakdown of the correspondent nucleotide, GTP, and their intracellular and extracellular levels are regulated by the fine-tuned activity of two major enzymes, purine nucleoside phosphorylase (PNP) and guanine deaminase (GDA). Noteworthy, GUO and GUA, seem to play opposite roles in the modulation of cognitive functions, such as learning and memory. Indeed GUO, despite exerting neuroprotective, anti-apoptotic and neurotrophic effects, causes a decay of cognitive activities, whereas GUA administration in rats results in working memory improvement (prevented by L-NAME pre-treatment). This study was designed to investigate, in a model of SH-SY5Y neuroblastoma cell line, the signal transduction pathway activated by extracellular GUA. Altogether, our results showed that: (i) in addition to an enhanced phosphorylation of ASK1, p38 and JNK, likely linked to a non-massive and transient ROS production, the PKB/NO/sGC/cGMP/PKG/ERK cascade seems to be the main signaling pathway elicited by extracellular GUA; (ii) the activation of this pathway occurs in a pertussis-toxin sensitive manner, thus suggesting the involvement of a putative G protein coupled receptor; (iii) the GUA-induced NO production, strongly reduced by cell pre-treatment with L-NAME, is negatively modulated by the EPAC-cAMP-CaMKII pathway, which causes the over-expression of GDA that, in turn, reduces the levels of GUA. These molecular mechanisms activated by GUA may be useful to support our previous observation showing that GUA improves learning and memory functions through the stimulation of NO signaling pathway, and underscore the therapeutic potential of oral administration of guanine for treating memory-related disorders.

Guanosine: a Neuromodulator with Therapeutic Potential in Brain Disorders

Guanosine is a purine nucleoside with important functions in cell metabolism and a protective role in response to degenerative diseases or injury. The past decade has seen major advances in identifying the modulatory role of extracellular action of guanosine in the central nervous system (CNS). Evidence from rodent and cell models show a number of neurotrophic and neuroprotective effects of guanosine preventing deleterious consequences of seizures, spinal cord injury, pain, mood disorders and aging-related diseases, such as ischemia, Parkinson’s and Alzheimer’s diseases. The present review describes the findings of in vivo and in vitro studies and offers an update of guanosine effects in the CNS. We address the protein targets for guanosine action and its interaction with glutamatergic and adenosinergic systems and with calcium-activated potassium channels. We also discuss the intracellular mechanisms modulated by guanosine preventing oxidative damage, mitochondrial dysfunction, inflammatory burden and modulation of glutamate transport. New and exciting avenues for future investigation into the protective effects of guanosine include characterization of a selective guanosine receptor. A better understanding of the neuromodulatory action of guanosine will allow the development of therapeutic approach to brain diseases.

Guanosine and its role in neuropathologies

Guanosine is a purine nucleoside thought to have neuroprotective properties. It is released in the brain under physiological conditions and even more during pathological events, reducing neuroinflammation, oxidative stress, and excitotoxicity, as well as exerting trophic effects in neuronal and glial cells. In agreement, guanosine was shown to be protective in several in vitro and/or in vivo experimental models of central nervous system (CNS) diseases including ischemic stroke, Alzheimer's disease, Parkinson's disease, spinal cord injury, nociception, and depression. The mechanisms underlying the neurobiological properties of guanosine seem to involve the activation of several intracellular signaling pathways and a close interaction with the adenosinergic system, with a consequent stimulation of neuroprotective and regenerative processes in the CNS. Within this context, the present review will provide an overview of the current literature on the effects of guanosine in the CNS. The elucidation of the complex signaling events underlying the biochemical and cellular effects of this nucleoside may further establish guanosine as a potential therapeutic target for the treatment of several neuropathologies.

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.

Guanosine Anxiolytic-Like Effect Involves Adenosinergic and Glutamatergic Neurotransmitter Systems

Accumulating evidences indicate that endogenous modulators of excitatory synapses in the mammalian brain are potential targets for treating neuropsychiatric disorders. Indeed, glutamatergic and adenosinergic neurotransmissions were recently highlighted as potential targets for developing innovative anxiolytic drugs. Accordingly, it has been shown that guanine-based purines are able to modulate both adenosinergic and glutamatergic systems in mammalian central nervous system. Here, we aimed to investigate the potential anxiolytic-like effects of guanosine and its effects on the adenosinergic and glutamatergic systems. Acute/systemic guanosine administration (7.5 mg/kg) induced robust anxiolytic-like effects in three classical anxiety-related paradigms (elevated plus maze, light/dark box, and round open field tasks). These guanosine effects were correlated with an enhancement of adenosine and a decrement of glutamate levels in the cerebrospinal fluid. Additionally, pre-administration of caffeine (10 mg/kg), an unspecific adenosine receptors' antagonist, completely abolished the behavioral and partially prevented the neuromodulatory effects exerted by guanosine. Although the hippocampal glutamate uptake was not modulated by guanosine (both ex vivo and in vitro protocols), the synaptosomal K⁺-stimulated glutamate release in vitro was decreased by guanosine (100 μM) and by the specific adenosine A₁ receptor agonist, 2-chloro-N ⁶-cyclopentyladenosine (CCPA, 100 nM). Moreover, the specific adenosine A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM) fully reversed the inhibitory guanosine effect in the glutamate release. The pharmacological modulation of A_2a receptors has shown no effect in any of the evaluated parameters. In summary, the guanosine anxiolytic-like effects seem closely related to the modulation of adenosinergic (A₁ receptors) and glutamatergic systems.

Absence epileptic activity changing effects of non-adenosine nucleoside inosine, guanosine and uridine in Wistar Albino Glaxo Rijswijk rats

Adenosine (Ado) and non-adenosine (non-Ado) nucleosides such as inosine (Ino), guanosine (Guo) and uridine (Urd) may have regionally different roles in the regulation of physiological and pathophysiological processes in the central nervous system (CNS) such as epilepsy. It was demonstrated previously that Ino and Guo decreased quinolinic acid (QA)-induced seizures and Urd reduced penicillin-, bicuculline- and pentylenetetrazole (PTZ)-induced seizures. It has also been demonstrated that Ino and Urd may exert their effects through GABAergic system by altering the function of GABA(A) type of gamma-aminobutyric acid receptors (GABAA receptors) whereas Guo decreases glutamate-induced excitability through glutamatergic system, which systems (GABAergic and glutamatergic) are involved in pathomechanisms of absence epilepsy. Thus, we hypothesized that Ino and Guo, similarly to the previously described effect of Urd, might also decrease absence epileptic activity. We investigated in the present study whether intraperitoneal (i.p.) application of Ino (500 and 1000mg/kg), Guo (20 and 50mg/kg), Urd (500 and 1000mg/kg), GABA(A) receptor agonist muscimol (1 and 3mg/kg), GABA(A) receptor antagonist bicuculline (2 and 4mg/kg), non-selective Ado receptor antagonist theophylline (5 and 10mg/kg) and non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5,10-imine maleate (MK-801, 0.0625 and 0.1250mg/kg) alone and in combination have modulatory effects on absence epileptic activity in Wistar Albino Glaxo Rijswijk (WAG/Rij) rats. We found that Guo decreased the number of spike-wave discharges (SWDs) whereas Ino increased it dose-dependently. We strengthened that Urd can decrease absence epileptic activity. Our results suggest that Guo, Urd and their analogs could be potentially effective drugs for treatment of human absence epilepsy.

The guanosine-adenosine interaction exists in vivo

In cultured renal cells and isolated perfused kidneys, extracellular guanosine augments extracellular adenosine and inosine (the major renal metabolite of adenosine) levels by altering the extracellular disposition of these purines. The present study addressed whether this "guanosine-adenosine mechanism" exists in vivo. In rats (n = 15), intravenous infusions of adenosine (1 µmol/kg per minute) decreased mean arterial blood pressure (MABP) from 114 ± 4 to 83 ± 5 mm Hg, heart rate (HR) from 368 ± 11 to 323 ± 9 beats/min), and renal blood flow (RBF) from 6.2 ± 0.5 to 5.3 ± 0.6 ml/min). In rats (n = 15) pretreated with intravenous guanosine (10 µmol/kg per minute), intravenous adenosine (1 µmol/kg per minute) decreased MABP (from 109 ± 4 to 58 ± 5 mm Hg), HR (from 401 ± 10 to 264 ± 20 beats/min), and RBF (from 6.2 ± 0.7 to 1.7 ± 0.3). Two-factor analysis of variance (2F-ANOVA) revealed a significant interaction (P < 0.0001) between guanosine and adenosine for MABP, HR, and RBF. In control rats, the urinary excretion rate of endogenous inosine was 211 ± 103 ng/30 minutes (n = 9); however, in rats treated with intravenous guanosine (10 µmol/kg per minute), the excretion rate of inosine was 1995 ± 300 ng/30 minutes (n = 12; P < 0.0001 versus controls). Because adenosine inhibits inflammatory cytokine production, we also examined the effects of intravenous guanosine on endotoxemia-induced increases in tumor necrosis factor-α (TNF-α). In control rats (n = 7), lipopolysaccharide (LPS; Escherichia coli 026:B6 endotoxin; 30 mg/kg) increased plasma TNF-α from 164 ± 56 to 4082 ± 730 pg/ml, whereas in rats pretreated with intravenous guanosine (10 µmol/kg per minute; n = 6), LPS increased plasma TNF-α from 121 ± 45 to 1821 ± 413 pg/ml (2F-ANOVA interaction effect, P = 0.0022). We conclude that the guanosine-adenosine mechanism exists in vivo and that guanosine may be a useful therapeutic for reducing inflammation.

Guanosine regulates adenosine levels in the kidney

In cell culture, extracellular guanosine increases extracellular adenosine by attenuating the disposition of extracellular adenosine (American Journal of Physiology – Cell Physiology 304: C406–C421, 2013). The goal of this investigation was to determine whether this “guanosine–adenosine mechanism” is operative in an intact organ. Twenty‐seven isolated, perfused mouse kidneys were subjected to metabolic poisons (iodoacetate plus 2,4‐dinitrophenol) to cause energy depletion and thereby stimulate renal adenosine production. Adenosine levels in the renal venous perfusate increased from a baseline of 36 ± 8 to 499 ± 96, 258 ± 50, and 71 ± 13 nmol/L at 15, 30, and 60 min, respectively, after administering metabolic poisons (% of basal; 1366 ± 229, 715 ± 128, and 206 ± 33, respectively). Changes in renal venous levels of guanosine closely mirrored the time course of changes in adenosine: baseline of 15 ± 2 to 157 ± 13, 121 ± 8, and 50 ± 5 nmol/L at 15, 30, and 60 min, respectively (% of basal; 1132 ± 104, 871 ± 59, and 400 ± 51, respectively). Freeze‐clamp experiments in 12 kidneys confirmed that metabolic poisons increased kidney tissue levels of adenosine and guanosine. In eight additional kidneys, we examined the ability of guanosine to reduce the renal clearance of exogenous adenosine; and these experiments revealed that guanosine significantly decreased the renal extraction of adenosine. Because guanosine is metabolized by purine nucleoside phosphorylase (PNPase), in another set of 16 kidneys we examined the effects of 8‐aminoguanine (PNPase inhibitor) on renal venous levels of adenosine and inosine (adenosine metabolite). Kidneys treated with 8‐aminoguanine showed a more robust increase in both adenosine and inosine in response to metabolic poisons. We conclude that in the intact kidney, guanosine regulates adenosine levels.

In cell culture, extracellular guanosine increases extracellular adenosine by attenuating the disposition of extracellular adenosine (American Journal of Physiology – Cell Physiology 304: C406–C421, 2013). The goal of this study was to determine whether the “guanosine–adenosine mechanism” is operative in an intact organ. In isolated, perfused mouse kidneys, inhibition of energy production induced changes in renal venous levels of guanosine that closely mirrored the time course of changes in adenosine, and freeze‐clamp experiments confirmed that metabolic poisons similarly increased kidney tissue levels of adenosine and guanosine. Moreover, exogenous guanosine significantly decreased the renal extraction of exogenous adenosine, and inhibition of purine nucleoside phosphorylase (metabolizes guanosine) augmented the effects of energy depletion on renal levels of both guanosine and adenosine. We conclude that in the intact kidney, guanosine regulates adenosine levels.

Guanosine negatively modulates the gastric motor function in mouse

The aim of the present study was to evaluate if guanine-based purines may affect the gastric motor function in mouse. Thus, the influence of guanosine on the gastric emptying rate in vivo was determined and its effects on spontaneous gastric mechanical activity, detected as changes of the intraluminal pressure, were analyzed in vitro before and after different treatments. Gastric gavage of guanosine (1.75-10 mg/kg) delayed the gastric emptying. Guanosine (30 μM-1 mM) induced a concentration-dependent relaxation of isolated stomach, which was not affected by the inhibition of the purine nucleoside phosphorylase enzyme by 4'-deaza-1'-aza-2'-deoxy-1'-(9-methylene)-immucillin-H. The inhibitory response was antagonized by S-(4-nitrobenzyl)-6-thioinosine, a membrane nucleoside transporter inhibitor, but not affected by 9-chloro-2-(2-furanyl)-[1,2,4]triazolo[1,5-c]quinazolin-5-amine, a nonselective adenosine receptor antagonist, or by tetrodotoxin, a blocker of neuronal voltage-dependent Na(+) channels. Moreover, guanosine-induced effects persisted in the presence of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase or tetraethylammonium, a nonselective potassium channel blocker, but they were progressively reduced by increasing concentrations of 2'5'dideoxyadenosine, an adenylyl cyclase inhibitor. Lastly, the levels of cyclic adenosine monophosphate (cAMP), measured by ELISA, in gastric full thickness preparations were increased by guanosine. In conclusion, our data indicate that, in mouse, guanosine is able to modulate negatively the gastric motor function, reducing gastric emptying and inducing muscular relaxation. The latter is dependent by its cellular uptake and involves adenylyl cyclase activation and increase in cAMP intracellular levels, while it is independent on neural action potentials, adenosine receptors, and K(+) channel activation.

Regulation of Cell Proliferation by the Guanosine-Adenosine Mechanism: Role of Adenosine Receptors

A recent study (American Journal of Physiology – Cell Physiology 304:C406–C421, 2013) suggests that extracellular guanosine increases extracellular adenosine by modifying the disposition of extracellular adenosine (“guanosine–adenosine mechanism”) and that the guanosine–adenosine mechanism is not mediated by classical adenosine transport systems (SLC28 and SLC29 families) nor by classical adenosine-metabolizing enzymes. The present investigation had two aims (1) to test the hypothesis that the “guanosine–adenosine mechanism” affects cell proliferation; and (2) to determine whether the transporters SLC19A1, SLC19A2, SLC19A3, or SLC22A2 (known to carrier guanosine analogs) might be responsible for the guanosine–adenosine mechanism. In the absence of added adenosine, guanosine had little effect on the proliferation of coronary artery vascular smooth muscle cells (vascular conduit cells) or preglomerular vascular smooth muscle cells (vascular resistance cells). However, in the presence of added adenosine (3 or 10 μmol/L), guanosine (10–100 μmol/L) decreased proliferation of both cell types, thus resulting in a highly significant (P < 0.000001) interaction between guanosine and adenosine on cell proliferation. The guanosine–adenosine interaction on cell proliferation was abolished by 1,3-dipropyl-8-(p-sulfophenyl)xanthine (adenosine receptor antagonist). Guanosine (30 μmol/L) increased extracellular levels of adenosine when adenosine (3 μmol/L) was added to the medium. This effect was not reproduced by high concentrations of methotrexate (100 μmol/L), thiamine (1000 μmol/L), chloroquine (1000 μmol/L), or acyclovir (10,000 μmol/L), archetypal substrates for SLC19A1, SLC19A2, SLC19A3, and SLC22A2, respectively; and guanosine still increased adenosine levels in the presence of these compounds. In conclusion, the guanosine–adenosine mechanism affects cell proliferation and is not mediated by SLC19A1, SLC19A2, SLC19A3, or SLC22A2.

Extracellular guanosine regulates extracellular adenosine levels

The aim of this investigation was to test the hypothesis that extracellular guanosine regulates extracellular adenosine levels. Rat preglomerular vascular smooth muscle cells were incubated with adenosine, guanosine, or both. Guanosine (30 μmol/l) per se had little effect on extracellular adenosine levels. Extracellular adenosine levels 1 h after addition of adenosine (3 μmol/l) were 0.125 ± 0.020 μmol/l, indicating rapid disposition of extracellular adenosine. Extracellular adenosine levels 1 h after addition of adenosine (3 μmol/l) plus guanosine (30 μmol/l) were 1.173 ± 0.061 μmol/l, indicating slow disposition of extracellular adenosine. Cell injury increased extracellular levels of endogenous adenosine and guanosine, and the effects of cell injury on endogenous extracellular adenosine were modulated by altering the levels of endogenous extracellular guanosine with exogenous purine nucleoside phosphorylase (converts guanosine to guanine) or 8-aminoguanosine (inhibits purine nucleoside phosphorylase). Extracellular guanosine also slowed the disposition of extracellular adenosine in rat preglomerular vascular endothelial cells, mesangial cells, cardiac fibroblasts, and kidney epithelial cells and in human aortic and coronary artery vascular smooth muscle cells and coronary artery endothelial cells. The effects of guanosine on adenosine levels were not mimicked or attenuated by 5-iodotubericidin (adenosine kinase inhibitor), erythro-9-(2-hydroxy-3-nonyl)-adenine (adenosine deaminase inhibitor), 5-aminoimidazole-4-carboxamide (guanine deaminase inhibitor), aristeromycin (S-adenosylhomocysteine hydrolase inhibitor), low sodium (inhibits concentrative nucleoside transporters), S-(4-nitrobenzyl)-6-thioinosine [inhibits equilibrative nucleoside transporter (ENT) type 1], zidovudine (inhibits ENT type 2), or acadesine (known modulator of adenosine levels). Guanosine also increases extracellular inosine, uridine, thymidine, and cytidine, yet decreases extracellular uric acid. In conclusion, extracellular guanosine regulates extracellular adenosine levels.

Purine nucleosides: endogenous neuroprotectants in hypoxic brain

Even a short blockade of oxygen flow in brain may lead to the inhibition of oxidative phosphorylation and depletion of cellular ATP, which results in profound deficiencies in cellular function. Following ischemia, dying, injured, and hypoxic cells release soluble purine-nucleotide and -nucleoside pools. Growing evidence suggests that purine nucleosides might act as trophic factors in the CNS and PNS. In addition to equilibrative nucleoside transporters (ENTs) regulating purine nucleoside concentrations intra- and extracellularly, specific extracellular receptor subtypes for these compounds are expressed on neurons, glia, and endothelial cells, mediating stunningly diverse effects. Such effects range from induction of cell differentiation, apoptosis, mitogenesis, and morphogenetic changes, to stimulation of synthesis and/or release of cytokines and neurotrophic factors under both physiological and pathological conditions. Multiple signaling pathways regulate the critical balance between cell death and survival in hypoxia-ischemia. A convergent pathway for the regulation of multiple modalities involved in O₂ sensing is the mitogen activated protein kinase (p42/44 MAPK) or (ERK1/2 extracellular signal-regulated kinases) pathway terminating in a variety of transcription factors, for example, hypoxia-inducible factor 1α. In this review, the coherence of purine nucleoside-related pathways and MAPK activation in the endogenous neuroprotective regulation of the nervous system's development and neuroplasticity under hypoxic stress will be discussed.

Systemic administration of guanosine promotes functional and histological improvement following an ischemic stroke in rats

Previously we have found that extracellular guanosine (Guo) has neuroprotective properties in in vitro and in vivo. Moreover, extracellular Guo significantly increased in the ipsilateral hemisphere within 2h following focal stroke in rats, and remained elevated for one week. Therefore, we hypothesized that Guo could be a potential candidate for a non-toxic neuroprotective agent. In the present study, we examined the effects of Guo on rats following permanent middle cerebral artery occlusion (MCAO). We also determined whether Guo can precondition neurons by modulating endoplasmic reticulum (ER) stress proteins. As most therapies employ a combination treatment regimen, we optimized the neuroprotection by combining pre- and post-MCAO treatments with Guo, attempting to reduce both ischemic cell death and improve functional recovery. A combination of 4mg/kg Guo given 30min pre-stroke and 8mg/kg Guo given 3, 24 and 48h post-stroke exerted the most significant decrease in infarct volume and sustainable improvement in neurological function. Moreover, these effects are not attributable to Guo metabolites. Measurements taken 6h post-MCAO from animals pre-treated with Guo did not reveal any significant changes in ER stress proteins (GRP 78 and 94) or HSP 70, but did reveal significantly increased levels of m-calpain. Thus, our data indicate that there is a treatment regimen for Guo as a neuroprotectant following ischemic stroke. The mechanism by which Guo confers neuroprotection may involve an increase in m-calpain, possibly resulting from a mild increase in intracellular calcium. M-calpain may be involved in the preconditioning response to ischemia by upregulating endogenous pro-survival mechanisms in neurons.

Evidence for the existence of a specific g protein-coupled receptor activated by guanosine

Guanosine, released extracellularly from neurons and glial cells, plays important roles in the central nervous system, including neuroprotection. The innovative DELFIA Eu-GTP binding assay was optimized for characterization of the putative guanosine receptor binding site at rat brain membranes by using a series of novel and known guanosine derivatives. These nucleosides were prepared by modifying the purine and sugar moieties of guanosine at the 6- and 5'-positions, respectively. Results of these experiments prove that guanosine, 6-thioguanosine, and their derivatives activate a G protein-coupled receptor that is different from the well-characterized adenosine receptors.

Can guanine-based purines be considered modulators of intestinal motility in rodents?

Adenine-based purines play a pivotal role in the control of gastrointestinal motility in rodents. Recently, guanine-based purines have been also shown to exert extracellular effects in the central nervous system raising the possibility of the existence of distinct receptors for guanine-based purines. Thus, it seems likely to speculate that also guanine-based purines may play a role in the modulation of the intestinal contractility. Spontaneous and neurally-evoked mechanical activity was recorded in vitro as changes in isometric tension in circular muscle strips from mouse distal colon. Guanosine up to 3mM or guanine up to 1mM failed to affect the spontaneous mechanical activity, but reduced the amplitude of the electrical field stimulation (EFS)-induced cholinergic contractions, without affecting the early nitrergic relaxation. Both compounds failed to affect the direct contractile responses evoked by carbachol. No desensitization of the response was observed. Guanine-based purine effects were not altered by theophylline, P1 purinoceptor antagonist, by PPADS or suramin, P2 purinoceptor antagonists, by ODQ, guanilyl cyclase inhibitor, or by DDA, adenylyl cyclase inhibitor. Nucleoside uptake inhibitors, dipyridamole or 6-[(4-Nitrobenzyl)thio]-9-β-D-ribofuranosylpurine (NBTI), antagonized the inhibitory effects induced by guanosine without interfering with guanine. On the contrary, adenine, a competitive inhibitor of nucleobase uptake, antagonized guanine-induced effects. In conclusion, our data indicate that guanosine and guanine are able to modulate negatively the excitatory cholinergic neurotransmission in the circular muscle layer of mouse colon. Guanine-based purines appear to interfere with prejunctional acethylcoline release. Their effects are dependent by their cellular uptake, and independent by adenine-based purine receptors.

Expression and contribution of satellite glial cells purinoceptors to pain transmission in sensory ganglia: an update

The role of adenosine-5'-triphosphate (ATP) and of the ligand-gated P2X3 receptor in neuronal dorsal root ganglia (DRG) pain transmission is relatively well established. Much less is known about the purinergic system in trigeminal ganglia (TG), which are involved in certain types of untreatable neuropathic and inflammatory pain, as well as in migraine. Emerging data suggest that purinergic metabotropic P2Y receptors on both neurons and satellite glial cells (SGCs) may also participate in both physiological and pathological pain development. Here, we provide an updated literature review on the role of purinergic signaling in sensory ganglia, with special emphasis on P2Y receptors on SGCs. We also provide new original data showing a time-dependent downregulation of P2Y2 and P2Y4 receptor expression and function in purified SGCs cultures from TG, in comparison with primary mixed neuron-SGCs cultures. These data highlight the importance of the neuron-glia cross-talk in determining the SGCs phenotype. Finally, we show that, in mixed TG cultures, both adenine and guanosine induce intracellular calcium transients in neurons but not in SGCs, suggesting that also these purinergic-related molecules can participate in pain signaling. These findings may have relevant implications for the development of new therapeutic strategies for chronic pain treatment.

Mechanisms involved in the antinociception induced by systemic administration of guanosine in mice

BACKGROUND AND PURPOSE

It is well known that adenine-based purines exert multiple effects on pain transmission. However, less attention has been given to the potential effects of guanine-based purines on pain transmission. The aim of this study was to investigate the effects of intraperitoneal (i.p.) and oral (p.o.) administration of guanosine on mice pain models. Additionally, investigation into the mechanisms of action of guanosine, its potential toxicity and cerebrospinal fluid (CSF) purine levels were also assessed.

EXPERIMENTAL APPROACH

Mice received an i.p. or p.o. administration of vehicle (0.1 mM NaOH) or guanosine (up to 240 mg x kg(-1)) and were evaluated in several pain models.

KEY RESULTS

Guanosine produced dose-dependent antinociceptive effects in the hot-plate, glutamate, capsaicin, formalin and acetic acid models, but it was ineffective in the tail-flick test. Additionally, guanosine produced a significant inhibition of biting behaviour induced by i.t. injection of glutamate, AMPA, kainate and trans-ACPD, but not against NMDA, substance P or capsaicin. The antinociceptive effects of guanosine were prevented by selective and non-selective adenosine receptor antagonists. Systemic administration of guanosine (120 mg x kg(-1)) induced an approximately sevenfold increase on CSF guanosine levels. Guanosine prevented the increase on spinal cord glutamate uptake induced by intraplantar capsaicin.

CONCLUSIONS AND IMPLICATIONS

This study provides new evidence on the mechanism of action of the antinociceptive effects after systemic administration of guanosine. These effects seem to be related to the modulation of adenosine A(1) and A(2A) receptors and non-NMDA glutamate receptors.

Vital role of protein kinase C-related kinase in the formation and stability of neurites during hypoxia

Exposure of pheochromocytoma cells to hypoxia (1% O(2)) favors differentiation at the expense of cell viability. Additional incubation with nerve growth factor (NGF) and guanosine, a purine nucleoside with neurotrophin characteristics, rescued cell viability and further enhanced the extension of neurites. In parallel, an increase in the activity of protein kinase C-related kinase (PRK1), which is known to be involved in regulation of the actin cytoskeleton, was observed in hypoxic cells. NGF and guanosine further enhanced PRK1 in normoxic and hypoxic cells. To study the role of PRK1 during cellular stress response and neurotrophin-mediated signaling, pheochromocytoma cells were transfected with small interfering RNA directed against PRK1. Loss of functional PRK1 initiated a significant loss of viability and inhibited neurite formation. SiRNA-mediated knockdown of PRK1 also completely stalled guanosine-mediated neuroprotective effects. Additionally, the F-actin-associated cytoskeleton and the expression of the plasticity protein growth associated protein-43 were disturbed upon PRK1 knockdown. A comparable dependency of neurite formation and growth associated protein-43 immunoreactivity on functional PRK1 expression was observed in cerebellar granule neurons. Based on these data, a putative role of PRK1 as a key-signaling element for the successive NGF- and purine nucleoside-mediated protection of hypoxic neuronal cells is hypothesized.

Guanosine prevents thermal hyperalgesia in a rat model of peripheral mononeuropathy

It is well known that adenine-based purines exert multiple effects on pain transmission. Less attention has been given, however, to the antinociceptive effects of guanine-based purines. The aim of this study was to investigate the effects of intraperitoneal administration of guanosine on a rat model of peripheral mononeuropathy. Additionally, investigation of the mechanism of action of guanosine, its general toxicity and measurements of central nervous system purine levels were performed. Rats received an intraperitoneal administration of vehicle (0.1 mM NaOH) or guanosine (up to 120 mg.kg(-1)) in an acute or chronic regimen. Guanosine significantly reduced thermal hyperalgesia on the ipsilateral side of the sciatic nerve ligation. Additionally, guanosine prevented locomotor deficits and body weight loss induced by the mononeuropathy. Acute systemic administration of guanosine caused an approximately 11-fold increase on central nervous system guanosine levels, but this effect was not observed after chronic treatment. Chronic guanosine administration prevented the increase on cortical glutamate uptake but not the decrease in spinal cord glutamate uptake induced by the mononeuropathy. No significant general toxicity was observed after chronic exposure to guanosine. This study provides new evidence on the mechanism of action of guanine-based purines, with guanosine presenting antinociceptive effects against a chronic pain model.

PERSPECTIVE

This study provides a new role for guanosine: chronic pain modulation. Guanosine presents as a new target for future drug development and might be useful for treatment of neuropathic pain.

Antinociceptive effects of intracerebroventricular administration of guanine-based purines in mice: evidences for the mechanism of action

It is well known that adenine-based purines exert multiple effects on pain transmission. However, less attention has been given to the potential effects of guanine-based purines (GBPs) on pain transmission. The aim of this study was to investigate the effects of intracerebroventricular (i.c.v.) guanosine and GMP on mice pain models. Mice received an i.c.v. injection of vehicle (saline or 10 muM NaOH), guanosine (5 to 400 nmol), or GMP (240 to 960 nmol). Additional groups were also pre-treated with i.c.v. injection of the A(1)/A(2A) antagonist caffeine (15 nmol), the non-selective opioid antagonist naloxone (12.5 nmol), or the 5'-nucleotidase inhibitor AOPCP (1 nmol). Measurements of CSF purine levels and cortical glutamate uptake were performed after treatments. Guanosine and GMP produced dose-dependent antinociceptive effects. Neither caffeine nor naloxone affected guanosine antinociception. Pre-treatment with AOPCP completely prevented GMP antinociception, indicating that conversion of GMP to guanosine is required for its antinociceptive effects. Intracerebroventricular administration of guanosine and GMP induced, respectively, a 180- and 1800-fold increase on CSF guanosine levels. Guanosine was able to prevent the decrease on cortical glutamate uptake induced by intraplantar capsaicin. This study provides new evidence on the mechanism of action of GBPs, with guanosine and GMP presenting antinociceptive effects in mice. This effect seems to be independent of adenosine and opioid receptors; it is, however, at least partially associated with modulation of the glutamatergic system by guanosine.

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.

MPP(+)-induced cytotoxicity in neuroblastoma cells: Antagonism and reversal by guanosine

Guanosine exerts neuroprotective effects in the central nervous system. Apoptosis, a morphological form of programmed cell death, is implicated in the pathophysiology of Parkinson's disease (PD). MPP(+), a dopaminergic neurotoxin, produces in vivo and in vitro cellular changes characteristic of PD, such as cytotoxicity, resulting in apoptosis. Undifferentiated human SH-SY5Y neuroblastoma cells had been used as an in vitro model of Parkinson's disease. We investigated if extracellular guanosine affected MPP(+)-induced cytotoxicity and examined the molecular mechanisms mediating its effects. Exposure of neuroblastoma cells to MPP(+) (10 muM-5 mM for 24-72 h) induced DNA fragmentation in a time-dependent manner (p < 0.05). Administration of guanosine (100 muM) before, concomitantly with or, importantly, after the addition of MPP(+) abolished MPP(+)-induced DNA fragmentation. Addition of MPP(+) (500 muM) to cells increased caspase-3 activity over 72 h (p < 0.05), and this was abolished by pre- or co-treatment with guanosine. Exposure of cells to pertussis toxin prior to MPP(+) eliminated the anti-apoptotic effect of guanosine, indicating that this effect is dependent on a Gi protein-coupled receptor, most likely the putative guanosine receptor. The protection by guanosine was also abolished by the selective inhibitor of the enzyme PI-3-K/Akt/PKB (LY294002), confirming that this pathway plays a decisive role in this effect of guanosine. Neither MPP(+) nor guanosine had any significant effect on alpha-synuclein expression. Thus, guanosine antagonizes and reverses MPP(+)-induced cytotoxicity of neuroblastoma cells via activation of the cell survival pathway, PI-3-K/Akt/PKB. Our results suggest that guanosine may be an effective pharmacological intervention in PD.

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.

Multifocal atrophy of cerebellar internal granular neurons in lesch-nyhan disease: case reports and review

The neuropathologic findings in 31 cases (aged 6 months to 33 years) of Lesch-Nyhan disease (hypoxanthine-guanine phosphoribosyltransferase deficiency) have been previously reported. Herein 2 additional cases, a 10-year-old boy and a 21-year-old man, are described. Both cases had unusual cerebellar abnormalities comprising multifocal internal granular layer atrophy with sparing of the Purkinje layer, one had a slightly small brain, and neither had striatal abnormalities. Careful review of the literature indicates that the most prevalent neuropathologic abnormalities are small cerebrum (13 of 33 cases) and multifocal cerebellar lesions (9 of 33 cases), although these could be underreported. Other authors have disregarded these abnormalities, focusing on the apparently normal basal nuclei, and they have suggested that the clinical neurologic abnormalities are based solely on changes in neurotransmitters. We discuss potential mechanisms of cerebellar damage, suggest that the cerebellar abnormality could in part explain the clinical syndrome, and recommend that cerebellar structure and function should be more carefully studied in Lesch-Nyhan disease.

P2Y2 receptor up-regulation induced by guanosine or UTP in rat brain cultured astrocytes

Among P2 metabotropic ATP receptors, P2Y2 subtype seems to be peculiar as its upregulation triggers important biological events in different cells types. In non-stimulated cells including astrocytes, P2Y2 receptors are usually expressed at levels lower than P2Y1 sites, however the promoter region of the P2Y2 receptors has not yet been studied and little is known about the mechanisms underlying the regulation of the expression of this ATP receptor. We showed that not only UTP and ATP are the most potent and naturally occurring agonist for P2Y2 sites, but also guanosine induced an up-regulation of astrocyte P2Y2 receptor mRNA evaluated by Northern blot analysis. We also focused our attention on this nucleoside since in our previous studies it was reported to be released by cultured astrocytes and to exert different neuroprotective effects. UTP and guanosine-evoked P2Y2 receptor up-regulation in rat brain cultured astrocytes was linked to an increased P2Y2-mediated intracellular calcium response, thus suggesting an increased P2Y2 activity. Actinomycin D, a RNA polymerase inhibitor, abrogated both UTP and guanosine-mediated P2Y2 up-regulation, thus indicating that de novo transcription was required. The effect of UTP and guanosine was also evaluated in astrocytes pretreated with different inhibitors of signal transduction pathways including ERK, PKC and PKA reported to be involved in the regulation of other cell surface receptor mRNAs. The results show that ERK1-2/MAPK pathway play a key role in the P2Y2 receptor up-regulation mediated by either UTP or guanosine. Moreover, our data suggest that PKA is also involved in guanosine-induced transcriptional activation of P2Y2 mRNA and that increased intracellular calcium levels and PKC activation may also mediate P2Y2 receptor up-regulation triggered by UTP. The extracellular release of ATP under physiological and pathological conditions has been widely studied. On the contrary, little is known about the release of pyrimidines and in particular of UTP. Here we show that astrocytes are able to release UTP, either at rest or during and following hypoxia/hypoglycemia obtained by submitting the cells to glucose-oxygen deprivation (OGD). Interestingly, also P2Y2 receptor mRNA increased by about two-fold the control values when the cultures were submitted to OGD. It has been recently reported that P2Y2 receptors can play a protective role in astrocytes, thus either guanosine administration or increased extracellular concentrations of guanosine and UTP reached locally following CNS injury may increase P2Y2-mediated biological events aimed at promoting a protective astrocyte response.

Guanosine promotes the up-regulation of inward rectifier potassium current mediated by Kir4.1 in cultured rat cortical astrocytes

Guanosine (Guo) is an endogenous neuroprotective molecule of the CNS, which has various acute and long-term effects on both neurones and astroglial cells. Whether Guo also modulates the activity/expression of ion channels involved in homeostatic control of extracellular potassium by the astrocytic syncytium is still unknown. Here we provide electrophysiological evidence that chronic exposure (48 h) to Guo (500 microm) promotes the functional expression of an inward rectifier K+ (Kir) conductance in primary cultured rat cortical astrocytes. Molecular screening indicated that Guo promotes the up-regulation of the Kir4.1 channel, the major component of the Kir current in astroglia in vivo. Furthermore, the properties of astrocytic Kir current overlapped those of the recombinant Kir4.1 channel expressed in a heterologous system, strongly suggesting that the Guo-induced Kir conductance is mainly gated by Kir4.1. In contrast, the expression levels of two other Kir channel proteins were either unchanged (Kir2.1) or decreased (Kir5.1). Finally, we showed that inhibition of translational process, but not depression of transcription, prevents the Guo-induced up-regulation of Kir4.1, indicating that this nucleoside acts through de novo protein synthesis. Because accumulating data indicate that down-regulation of astroglial Kir current contributes to the pathogenesis of neurodegenerative diseases associated with dysregulation of extracellular K+ homeostasis, these results support the notion that Guo might be a molecule of therapeutic interest for counteracting the detrimental effect of K+-buffering impairment of the astroglial syncytium that occurs in pathological conditions.

Guanosine effect on cholesterol efflux and apolipoprotein E expression in astrocytes

The main source of cholesterol in the central nervous system (CNS) is represented by glial cells, mainly astrocytes, which also synthesise and secrete apolipoproteins, in particular apolipoprotein E (ApoE), the major apolipoprotein in the brain, thus generating cholesterol-rich high density lipoproteins (HDLs). This cholesterol trafficking, even though still poorly known, is considered to play a key role in different aspects of neuronal plasticity and in the stabilisation of synaptic transmission. Moreover, cell cholesterol depletion has recently been linked to a reduction in amyloid beta formation. Here we demonstrate that guanosine, which we previously reported to exert several neuroprotective effects, was able to increase cholesterol efflux from astrocytes and C6 rat glioma cells in the absence of exogenously added acceptors. In this effect the phosphoinositide 3 kinase/extracellular signal-regulated kinase 1/2 (PI3K/ERK1/2) pathway seems to play a pivotal role. Guanosine was also able to increase the expression of ApoE in astrocytes, whereas it did not modify the levels of ATP-binding cassette protein A1 (ABCA1), considered the main cholesterol transporter in the CNS. Given the emerging role of cholesterol balance in neuronal repair, these effects provide evidence for a role of guanosine as a potential pharmacological tool in the modulation of cholesterol homeostasis in the brain.

Application of solid phase extraction and high-performance liquid chromatography to qualitative and quantitative analysis of nucleotides and nucleosides in human cerebrospinal fluid

New method of qualitative and quantitative analysis of nucleotides in human cerebrospinal fluid (CSF), based on the combination of extraction of purines and pyrimidines to the solid phase (SPE) and high-performance liquid chromatography (HPLC), was proposed. Use of SPE and lyophilization of samples allowed for the first time to detect the presence of di- and triphosphonucleotides in human CSF. Concentration of those compounds varied from 0.003 to 5.0 microM. Differences in the nucleotide mixture composition in human CSF detected with the new method are coupled with the neurological disorders and might be a basis for an efficient diagnostic tool.