Differential effects of guanine nucleotides on kainic acid binding and on adenylate cyclase activity in chick optic tectum

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.

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: 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.

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.

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.

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.

Systemic administration of GMP induces anxiolytic-like behavior in rats

The glutamatergic system has received considerable attention over the last few years as potential target to develop anxiolytic drugs. Guanine based purines (GBPs) play an important neurmodulatory effect in the glutamatergic system. Several studies have shown the ability of the GBPs to reduce glutamatergic activity. In the present study, we investigated the anxiolytic effect of GBPs - by Guanosina Monophosphate (GMP) administration - in rodents. Adult male Wistar rats were pretreated with GMP (10, 25, 50, 100 and 150mg/kg: i.p.); or saline (NaCl 0.9%; i.p.) (control); or, diazepam (2mg/kg: i.p.) (positive control). One hour after the injection, the anxiety-related behaviors for each animal was evaluated in the light/dark, elevated plus-maze, and open field tasks. Additionally, purines concentration in the cerebrospinal fluid (CSF) was verified. The administration of 25 and 50mg/kg GMP was able to promote anxiolytic-like behavior, in the light/dark and elevated plus-maze task, similar to diazepam effect. However, no changes in the open field task, or CSF purines concentration were found for either GMP or diazepam treated animals, when compared with saline group. Thus, this study suggests that acute administration of GMP was able to decrease the levels of anxiety in classical behavioral tasks.

Guanosine-5'-monophosphate induces cell death in rat hippocampal slices via ionotropic glutamate receptors activation and glutamate uptake inhibition

Guanine derivatives modulate the glutamatergic system through displacement of binding of glutamate to its receptors acting as antagonist of glutamate receptors in moderate to high micromolar concentrations. Guanosine-5'-monophosphate (GMP) is shown to be neuroprotective against glutamate- or oxygen/glucose deprivation-induced neurotoxicity and also against NMDA-induced apoptosis in hippocampal slices. However, in this study we are showing that high extracellular GMP concentrations (5mM) reduced cell viability in hippocampal brain slices. The toxic effect of GMP was not blocked by dipyridamole, a nucleoside transport inhibitor, nor mimicked by guanosine, suggesting an extracellular mode of action to GMP which does not involve its hydrolysis to guanosine. GMP-dependent cell damage was not blocked by P1 purinergic receptor antagonists, neither altered by adenosine A(1) or A(2A) receptor agonists. The blockage of the ionotropic glutamate receptors AMPA or NMDA, but not KA or metabotropic glutamate receptors, reversed the toxicity induced by GMP. GMP (5mM) induced a decrease in glutamate uptake into hippocampal slices, which was reversed by dl-TBOA. Therefore, GMP-induced hippocampal cell damage involves activation of ionotropic glutamate receptors and inhibition of glutamate transporters activity.

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.

Quinolinic Acid-induced Seizures Stimulate Glutamate Uptake into Synaptic Vesicles from Rat Brain: Effects Prevented by Guanine-based Purines

Glutamate uptake into synaptic vesicles is a vital step for glutamatergic neurotransmission. Quinolinic acid (QA) is an endogenous glutamate analog that may be involved in the etiology of epilepsy and is related to disturbances on glutamate release and uptake. Guanine-based purines (GBPs) guanosine 5'-monophosphate (GMP and guanosine) have been shown to exert anticonvulsant effects against QA-induced seizures. The aims of this study were to investigate the effects of in vivo administration of several convulsant agents on glutamate uptake into synaptic vesicles and investigate the role of MK-801, guanosine or GMP (anticonvulsants) on glutamate uptake into synaptic vesicles from rats presenting QA-induced seizures. Animals were treated with vehicle (saline 0.9%), QA 239.2 nmoles, kainate 30 mg/kg, picrotoxin 6 mg/kg, PTZ (pentylenetetrazole) 60 mg/kg, caffeine 150 mg/kg or MES (maximal transcorneal electroshock) 80 mA. All convulsant agents induced seizures in 80-100% of animals, but only QA stimulated glutamate uptake into synaptic vesicle. Guanosine or GMP prevented seizures induced by QA (up to 52% of protection), an effect similar to the NMDA antagonist MK-801 (60% of protection). Both GBPs and MK-801 prevented QA-induced glutamate uptake stimulation. This study provided additional evidence on the role of QA and GBPs on glutamatergic system in rat brain, and point to new perspectives on seizures treatment.

Mechanism of guanosine-induced neuroprotection in rat hippocampal slices submitted to oxygen-glucose deprivation

Guanine derivates have been implicated in many relevant extracellular roles, such as modulation of glutamate transmission, protecting neurons against excitotoxic damage. Guanine derivatives are spontaneously released to the extracellular space from cultured astrocytes during oxygen-glucose deprivation (OGD) and may act as trophic factors, glutamate receptors blockers or glutamate transport modulators, thus promoting neuroprotection. The aim of this study was to evaluate the mechanisms involved in the neuroprotective role of the nucleoside guanosine in rat hippocampal slices submitted to OGD, identifying a putative extracellular binding site and the intracellular signaling pathways related to guanosine-induced neuroprotection. Cell damage to hippocampal slices submitted to 15 min of OGD followed by 2 h of reperfusion was decreased by the addition of guanosine (100 microM) or guanosine-5'-monophosphate (GMP, 100 microM). The neuroprotective effect of guanosine was not altered by the addition of adenosine receptor antagonists, nucleosides transport inhibitor, glutamate receptor antagonists, glutamate transport inhibitors, and a non-selective Na(+) and Ca(2+) channel blocker. However, in a Ca(2+)-free medium (by adding EGTA), guanosine was ineffective. Nifedipine (a Ca(2+) channel blocker) increased the neuroprotective effect of guanosine and 4-aminopyridine, a K(+) channel blocker, reversed the neuroprotective effect of guanosine. Evaluation of the intracellular signaling pathways associated with guanosine-induced neuroprotection showed the involvement of PKA, PKC, MEK and PI-3 K pathways, but not CaMKII. Therefore, this study shows guanosine is acting via K(+) channels activation, depending on extracellular Ca(2+) levels and via modulation of the PKA, PKC, MEK and/or PI-3 K pathways.

In vivo quinolinic acid increases synaptosomal glutamate release in rats: reversal by guanosine

Glutamate, the main excitatory neurotransmitter in the mammalian central nervous system (CNS), plays important role in brain physiological and pathological events. Quinolinic acid (QA) is a glutamatergic agent that induces seizures and is involved in the etiology of epilepsy. Guanine-based purines (GBPs) (guanosine and GMP) have been shown to exert neuroprotective effects against glutamatergic excitotoxic events. In this study, the influence of QA and GBPs on synaptosomal glutamate release and uptake in rats was investigated. We had previously demonstrated that QA "in vitro" stimulates synaptosomal L-[3H]glutamate release. In this work, we show that i.c.v. QA administration induced seizures in rats and was able to stimulate synaptosomal L-[3H]glutamate release. This in vivo neurochemical effect was prevented by i.p. guanosine only when this nucleoside prevented QA-induced seizures. I.c.v. QA did not affect synaptosomal L-[3H]glutamate uptake. These data provided new evidence on the role of QA and GBPs on glutamatergic system in rat brain.

Intracerebroventricular guanine-based purines protect against seizures induced by quinolinic acid in mice

Acute and chronic administration of the nucleoside guanosine have been shown to prevent quinolinic acid (QA) and alpha-dendrotoxin-induced seizures, as well as to impair memory and anxiety in rats and mice. In this study, we investigated the effect of i.c.v. administration of guanine-based purines (GTP, GDP, GMP, and guanosine) against seizures induced by the NMDA agonist and glutamate releaser quinolinic acid in mice. We also aimed to study the effects of the poorly hydrolysable analogs of GTP (GppNHp and GTPgammaS) and GDP (GDPbetaS) in this seizure model. QA produced seizures in 100% of mice, an effect partially prevented by guanine-based purines. In contrast to GTP (480 nmol), GDP (320-640 nmol), GMP (320-480 nmol) and guanosine (300-400 nmol), the poorly hydrolysable analogs of GTP and GDP did not affect QA-induced seizures. Thus, the protective effects of guanine nucleotides seem to be due to their conversion to guanosine. Altogether, these findings suggest a potential role of guanine-based purines for treating diseases involving glutamatergic excitotoxicity.

Guanosine selectively inhibits locomotor stimulation induced by the NMDA antagonist dizocilpine

Guanosine has been shown to modulate glutamate system by stimulating astrocytic glutamate uptake. Recent evidence suggest that the locomotor effects of NMDA receptor antagonists, an animal model of schizophrenia, is associated with activation of non-NMDA glutamatergic receptors caused by increased glutamate release. The present work was undertaken to evaluate whether guanosine could have influence on the hyperlocomotion induced in mice by dizocilpine (MK-801), a NMDA antagonist. We also evaluated the effect of guanosine on the hyperlocomotion induced by the indirect dopamine agonist amphetamine, and by the non-selective adenosine receptor antagonist caffeine. Guanosine (7.5 mg/kg) produced an attenuation of about 60% on the hyperlocomotion induced by dizocilpine (0.25 mg/kg), whereas it did not affect the hyperlocomotion induced by amphetamine (5 mg/kg) or caffeine (30 mg/kg). Guanosine pre-treatment did not affect total spontaneous locomotion in all experiments. To test neuronal pathway selectivity, we evaluated MK-801 against guanosine in a working memory paradigm (spontaneous alternation task). Guanosine did not reverted the impairment caused by MK-801 in the spontaneous alternation test, and when administered alone also presented an amnesic effect. The results are discussed based on the current hypothesis of locomotor activation induced by the psychoactive drugs studied. Further studies are necessary to evaluate if guanosine could have clinical utility for the treatment of schizophrenia.

Mechanisms intrinsic to 5-HT2B receptor-induced potentiation of NMDA receptor responses in frog motoneurones

In the presence of NMDA receptor open-channel blockers [Mg(2+); (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801); 1-amino-3,5-dimethyladamantane (memantine)] and TTX, high concentrations (30-100 microm) of either 5-hydroxytryptamine (5-HT) or alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT) significantly potentiated NMDA-induced depolarizations of frog spinal cord motoneurones. Potentiation was blocked by LY-53,857 (10-30 microm), SB 206553 (10 microm), and SB 204741 (30 microm), but not by spiroxatrine (10 microm), WAY 100,635 (1-30 microm), ketanserin (10 microm), RS 102221 (10 microm), or RS 39604 (10-20 microm). Therefore, alpha-Me-5-HT's facilitatory effects appear to involve 5-HT(2B) receptors. These effects were G-protein dependent as they were prevented by prior treatment with guanylyl-5'-imidodiphosphate (GMP-PNP, 100 microm) and H-Arg-Pro-Lys-Pro-Gln-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH(2) (GP antagonist 2A, 3-6 microm), but not by pertussis toxin (PTX, 3-6 ng ml(-1), 48 h preincubation). This potentiation was not reduced by protein kinase C inhibition with staurosporine (2.0 microm), U73122 (10 microm) or N-(2-aminoethyl)-5-isoquinolinesulfonamide HCl (H9) (77 microm) or by intracellular Ca(2+) depletion with thapsigargin (0.1 microm) (which inhibits Ca(2+)/ATPase). Exposure of the spinal cord to the L-type Ca(2+) channel blockers nifedipine (10 microm), KN-62 (5 microm) or gallopamil (100 microm) eliminated alpha-Me-5-HT's effects. The calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide (W7) (100 microm) diminished the potentiation. However, the calcium/calmodulin-dependent protein kinase II (CaM Kinase II) blocker KN-93 (10 microm) did not block the 5-HT enhancement of the NMDA responses. In summary, activation of 5-HT(2B) receptors by alpha-Me-5-HT facilitates NMDA-depolarizations of frog motoneurones via a G-protein, a rise in [Ca(2+)](i) from the entry of extracellular Ca(2+) through L-type Ca(2+) channels, the binding of Ca(2+) to calmodulin and a lessening of the Mg(2+) -produced open-channel block of the NMDA receptor.

Characterization of imido [8-(3)H] guanosine 5'-triphosphate binding sites to rat brain membranes

Besides their well-defined intracellular roles in transmembrane signals transduction, guanine derivatives play important roles by acting from the outside of neural cell membranes. These roles are mediated by two different pool sites in cell membranes: G proteins, which bind to specific (GDP and GTP) intracellular guanine derivatives, and sites that bind to extracellular guanine derivatives. In this study we investigated some methodological characteristics of both guanine derivatives binding sites (intracellular and extracellular) in rat brain neural membranes. By investigating the binding of a poorly hydrolyzed GTP analogue and the adenylate cyclase activity in neural membranes, we observed some distinctiveness of guanine derivatives binding sites: stability to washing procedures (extracellular) and modulation of adenylate cyclase activity (intracellular). These results allow dealing with each site separately, which could be useful for discriminating the roles of extracellular and intracellular guanine derivatives in the central nervous system.

Guanine derivatives modulate L-glutamate uptake into rat brain synaptic vesicles

Glutamate uptake into synaptic vesicles is driven by a proton electrochemical gradient generated by a vacuolar H(+)-ATPase and stimulated by physiological concentrations of chloride. This uptake plays an important role in glutamatergic transmission. We show here that vesicular glutamate uptake is selectively inhibited by guanine derivatives, in a time- and concentration-dependent manner. Guanosine, GMP, GDP, guanosine-5'-O-2-thiodiphosphate, GTP, or 5'-guanylylimidodiphosphate (GppNHp) inhibited glutamate uptake in 1.5 and 3 min incubations, however, when incubating for 10 min, only GTP or GppNHp displayed such inhibition. By increasing ATP concentrations, the inhibitory effect of GTP was no longer observed, but GppNHp still inhibited glutamate uptake. In the absence of ATP, vesicular ATPase can hydrolyze GTP in order to drive glutamate uptake. However, 5mM GppNHp inhibited ATP hydrolysis by synaptic vesicle preparations. GTP or GppNHp decreased the proton electrochemical gradient, whereas the other guanine derivatives did not. Glutamate saturation curves were assayed in order to evaluate the specificity of inhibition of the vesicular glutamate carrier by the guanine derivatives. The maximum velocity of the initial rate of glutamate uptake was decreased by all guanine derivatives. These results indicate that, although GppNHp can inhibit ATPase activity, guanine derivatives are more likely to be acting through interaction with vesicular glutamate carrier.

Oral administration of guanosine impairs inhibitory avoidance performance in rats and mice

Extracellular guanine-based purines, mainly the nucleoside guanosine, have recently been shown to exert neuroprotective effects, which seem to be related to antagonism of the glutamatergic system. In this study, we investigated the effects of acute oral administration of guanosine on inhibitory avoidance task in rats and mice. We also studied its effects on locomotor activity, anxiety-related behaviors and mechanisms of action involving the purinergic system. Guanosine (2.0 and 7.5mg/kg, per os), administered 75min pretraining, dose-dependently impaired retention of the inhibitory avoidance task in rats and mice, an effect not prevented by the adenosine receptor antagonist caffeine. Guanosine presented no effects on locomotor activity and anxiety-related behaviors. This amnesic effect of guanosine may be compatible with inhibition of glutamatergic system and seems to be not mediated by adenosine.

Effect of undernutrition on GMP-PNP binding and adenylate cyclase activity from rat brain

1. Undernutrition is an insult that affects brain development and functioning. Considering that signaling through metabotropic receptors/G proteins is critical for normal synaptic transmission and contributes to CNS development and synaptic plasticity, the present study investigated the effects of pre- and postnatal protein deprivation (diet: 8% protein; normonourished group: 25% protein) on brain signal transduction by G proteins. 2. Undernutrition decreased the [3H] GMP-PNP binding to G proteins and AC activity, in neural plasma synaptic membranes of 21- and 75-day-old rats. This effect was less pronounced or even absent in old rats. 3. Ontogenetically, the dietary treatment effect might be interpreted as a retarded development associated with protein malnutrition.

Neuroprotective effect of GMP in hippocampal slices submitted to an in vitro model of ischemia

1. Guanosine-5'-monophosphate (GMP) was evaluated as a neuroprotective agent against the damage observed in rat hippocampal slices submitted to an in vitro model of ischemia with or without the presence of the ionotropic glutamate receptor agonist, Kainic acid (KA). 2. Cellular injury was evaluated by MTT reduction, lactate dehydrogenase(LDH) release assay, and measurement of intracellular ATP levels. 3. In slices submitted to ischemic conditions, 1 mM GMP partially prevented the decrease in cell viability induced by glucose and oxygen deprivation and the addition of KA. 4. KA or N-methyl-D-aspartate (NMDA) receptor antagonists, gamma-D-glutamylamino-methylsulfonate (GAMS) or (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801, 20 microM) also prevented toxicity in hippocampal slices under ischemic conditions, respectively. 5. The association of GMP with GAMS or MK-801 did not induce additional protection than that observed with GMP or that classical glutamate receptor antagonists alone. 6. GMP, probably by interacting with ionotropic glutamate receptors, attenuated the damage caused by glucose and oxygen deprivation in hippocampal slices. This neuroprotective action of GMP in this model of excitotoxicity is of outstanding interest in the search for effective therapies against ischemic injury.

Guanine based purines inhibit [(3)H]glutamate and [(3)H]AMPA binding at postsynaptic densities from cerebral cortex of rats

Extracellular guanine-based purines (GBPs) have been implicated in neuroprotective effects against glutamate toxicity by modulating the glutamatergic system through mechanisms without the involvement of G proteins. Accordingly, GBPs have been shown to inhibit the binding of glutamate and its analogs in different brain membrane preparations. However, brain membrane preparations used for these studies are comprised of both post- and pre-neuronal and glial synaptic components. In this study we investigated the ability of GBPs to displaced glutamate and AMPA binding at postsynaptic densities (PSDs). PSDs are markedly prominent in glutamatergic synapses and retains the native apposition of membrane components and post synaptic receptors. The PSD fraction was prepared from cerebral cortex of Wistar rats and it was characterized as PSDs by electron microscopy and by an enrichment of PSD-95, a protein marker of PSDs (90% of immunodetection). Moreover, we detected an enrichment of glutamate receptors subunits that including NR1 subunit of NMDA receptors and GluR1 subunit of AMPA receptors. GppNp (poor hydrolyzable GTP analog) and GMP displaced 40 and 36% of glutamate binding, respectively, and guanosine only 23%. AMPA binding was not affected by guanosine and was inhibited 21 and 25% by GppNp and GMP, respectively. Hence, this study demonstrates that guanine based purines inhibited glutamate and AMPA binding at postsynaptic membrane preparations, contributing for a better understanding of the mechanisms by which GBPs antagonize glutamatergic neurotoxicicity, e.g. the possible involvement of glutamatergic postsynaptic receptors in their neuroprotective roles.

Diphenyl diselenide and diphenyl ditelluride affect the rat glutamatergic system in vitro and in vivo

The aim of this study was to investigate the possible involvement of the glutamatergic system in the toxicity of organochalcogens, since this is an important neurotransmitter system for signal transduction and neural function. The results indicated that 100 microM diphenyl diselenide (PhSe)(2) and diphenyl ditelluride (PhTe)(2) inhibit by 50 and 70% (P<0.05), respectively, [(3)H]glutamate binding in vitro. Acute administration of 25 micromol/kg (PhSe)(2) or 3 micromol/kg (PhTe)(2) caused a significant reduction in [(3)H]glutamate (30%, P<0.05) or [(3)H]MK-801 binding (30%, P<0.05) to rat synaptic membranes. These results suggest that (PhSe)(2) and (PhTe)(2) affect, in a rather complex way, the glutamatergic system after acute in vivo exposure in rats. In vitro, total [(3)H]GMP-PNP binding was inhibited about 40% at 100 microM (PhSe)(2) and (PhTe)(2). Acute exposure in vivo to (PhSe)(2) decreased the stable [(3)H]GMP-PNP binding to 25% and (PhTe)(2) to 68% of the control value (P<0.05, for both compounds). Simultaneously, the unstable binding of [(3)H]GMP-PNP was decreased about 30 and 50% (P<0.05, for both compounds) after exposure to (PhSe)(2) and (PhTe)(2), respectively. GMP-PNP stimulated adenylate cyclase (AC) activity significantly in control animals. (PhSe)(2)- and (PhTe)(2)-treated animals increased the basal activity of this enzyme, but GMP-PNP stimulation was totally abolished. These results suggest that the toxic effects of organochalcogens could result from action at different levels of neural signal transduction pathways, possibly involving other neurotransmitters besides the glutamatergic system.

Activation of glutamate uptake by guanosine in primary astrocyte cultures

Guanine-based purines have been shown to modulate the effects of glutamate, which is essential for brain function and mediates excitotoxicity. In the search for a mechanism involving the interaction between purine nucleoside guanosine and glutamate, we found that guanosine dose-dependently, significantly (63%) and potently (EC50 =2.47 microM) enhanced glutamate uptake in cultured astrocytes. This effect was not inhibited by the blocker of nucleoside transporter dipyridamole nor by the adenosine antagonist theophylline, suggesting an extracellular site of action without the involvement of adenosine receptors. These results indicate a regulatory role of guanosine on extracellular levels of glutamate, possibly contributing for protecting neural cells against glutamate-induced excitotoxicity.

Investigations into the mechanism of 2,3-dimercaptopropanol neurotoxicity

2,3 dimercaptopropanol (BAL), is a dithiol chelating agent, used for the treatment of heavy metal intoxication; however, this compound has low therapeutic efficacy and in some situations may cause neurotoxic effects. In experimental models, administration of high doses of BAL produces seizures that culminate in animal death. However, investigations on the modulation of neurotransmitter system(s) involved in BAL-induced seizures are still lacking in the literature. In the present study, the neurotoxicity of BAL, as measured by the manifestation of seizures was examined and the modulation of glutamatergic and GABAergic receptors and ion channels potentially involved in BAL-induced seizures was investigated. The results demonstrated that BAL (18.6 mg/kg) induced seizures and all mice died within one day. GABAergic allosteric modulators (3 or 12 mg/kg diazepam and 50 mg/kg phenobarbital) blocked the appearance of seizure and reduced almost completely the death caused by BAL. Carbamazepine (5 mg/kg) significantly reduced the incidence of BAL-induced seizures, while sodium valproate and MK-801 were not effective in reducing the incidence of seizures. Valproate (300 mg/kg) and MK-801(0.5 mg/kg) prolonged the latencies for onset of seizures; however, all animals died within one day after BAL administration. High doses of ZnCl2 (135 mg/kg) blocked the appearance of seizures episodes, but no animal survived more than one day. The content of total non-protein -SH in brain of mice treated with 18.6 and 124 mg/kg BAL increased from 0.9+/-0.3 nmol/g (control animals) to 1.7+/-0.3 and 3.5+/-0.8 nmol/g, respectively. In vitro, 0.1-1 mM concentrations of BAL inhibited [3H]glutamate and [3H]MK-801 binding, but increased the binding of [3H]muscimol to brain synaptic plasma membrane. The results reported here demonstrate that GABAergic allosteric modulators (diazepam and phenobarbital) and carbamazepine, a compound that acts by prolonging the recovery of voltage-activated ion channels from inactivation, are able to abolish BAL-induced seizures, while the NMDA antagonist (MK-801) prolonged the latencies for onset of seizures suggesting that modulators of this subtype of glutamate receptor have a modest role on BAL-induced seizures. The results of the present study suggest that allosteric modulators of GABAergic system and carbamazepine, a voltage-gated Na+-channel antagonist, should be considered for the treatment of animals or patients intoxicated with BAL.

Extracellular guanosine 5' triphosphate enhances nerve growth factor-induced neurite outgrowth via increases in intracellular calcium

Extracellular guanosine 5' triphosphate (GTP) enhances nerve growth factor-dependent neurite outgrowth from rat pheochromocytoma (PC12) cells; cultures of PC12 cells exposed to GTP and nerve growth factor together contain significantly more neurite-bearing cells than do those exposed to either nerve growth factor or GTP alone [Gysbers J. W. and Rathbone M. P. (1996) Int. J. devl Neurosci. 14, 19-34]. PC12 cells contain specific cell surface binding sites for extracellular GTP, which do not bind ATP or uridine 5' triphosphate. Exposure of PC12 cells to extracellular GTP (300microM) produced a robust and sustained increase in intracellular Ca(2+) ([Ca(2+)](i)), different from the transient response to the addition of ATP. The GTP-induced [Ca(2+)](i) increase was blocked by the L-type calcium channel inhibitor, nifedipine. The L-type Ca(2+) channel inhibitors, nifedipine or verapamil, also inhibited the enhancement of neurite outgrowth by GTP, but did not affect neurite outgrowth stimulated by nerve growth factor alone. Pre-treatment of PC12 cells with ryanodine (0.5-50microM) depleted calcium from internal stores and prevented the further release of calcium by GTP. Similarly, pre-treatment of PC12 cells with thapsigargin (an inhibitor of internal store Ca(2+)/ATPase) or dantrolene (which blocks Ca(2+) release from some of these stores) also reduced the enhancement of neurite outgrowth by GTP. Therefore, Ca(2+)-induced Ca(2+) release from specific stores, present in PC12 cells, is involved in the enhancement of nerve growth factor-induced neurite outgrowth by GTP, possibly acting at specific binding sites on the cell surface. GTP is proving to be an important extracellular trophic modulator in the central nervous system. These studies show that the neuritogenic actions of GTP involve moderate but sustained increases in intracellular Ca(2+) which are likely due to activation of L-type Ca(2+) channels and Ca(2+)-induced Ca(2+) release from intracellular stores. These effects of extracellular GTP are likely mediated at the cell surface and may be related to specific GTP binding sites which are distinct from G-proteins and from hitherto described purine nucleotide (P2) receptors. These data indicate a mechanism whereby the neuritogenic effects of GTP are mediated and emphasize the importance of considering GTP as a neurotrophic mediator.

Guanosine and GMP prevent seizures induced by quinolinic acid in mice

In the mammalian CNS, glutamate and GABA are the principal neurotransmitters mediating excitatory and inhibitory synaptic events, respectively, and have been implicated in the neurobiology of seizures. Guanine-based purines, including the nucleoside guanosine and the nucleotide GMP, have been shown to antagonize glutamatergic activity at the receptor level and the other purine nucleoside adenosine is a well-known modulator of seizure threshold. In the present study we investigated the anticonvulsant effect of i. p. guanosine and GMP against seizures induced by the glutamate agonist quinolinic acid (QA) or the GABA(A) antagonist picrotoxin in mice. Animals were pretreated with an i.p. injection of saline, guanosine or GMP 30 min before either an i.c.v. injection of 4 microliter QA (36.8 nmol) or a subcutaneous injection of picrotoxin (3.2 mg/kg). All animals pretreated with vehicle followed by QA or picrotoxin presented seizures, which were completely prevented by the NMDA antagonist MK-801 and the GABA agonist phenobarbital, respectively. Guanosine and GMP dose-dependently protected against QA-induced seizures, up to 70 and 80% at 7.5 mg/kg, with ED(50)=2. 6+/-0.4 and 1.7+/-0.6 mg/kg, respectively. Conversely, neither guanosine, GMP nor MK-801 affected picrotoxin-induced seizures, indicating some degree of specificity towards the glutamatergic system. This study suggests anticonvulsant properties of i.p. guanosine and GMP, which may be related with antagonism of glutamate receptors.

Compounds extracted from Phyllantus and Jatropha elliptica inhibit the binding of [3H]glutamate and [3H]GMP-PNP in rat cerebral cortex membrane

Glutamate is to be considered a nociceptive neurotransmitter and glutamatergic antagonists present antinoceptive activity. In this study we investigated the effects of the naturally occurring antinociceptive compounds rutin, geraniin and quercetine extracted from Phyllanthus, as well as the diterpene jatrophone, extracted from Jatropha elliptica on the binding of [3H]glutamate and [3H]GMP-PNP [a GTP analogue which binds to extracellular site(s), modulating the glutamatergic transmission] in rat brain membrane. Jatrophone inhibited [3H]glutamate binding and geraniin inhibited [3H]GMP-PNP binding. Quercetine inhibited the binding of both ligands. These results may indicate a neurochemical parameter possibly related to the antinoceptive activity of these natural compounds.

Interaction of adenosine and guanine derivatives in the rat hippocampus: effects on cyclic AMP levels and on the binding of adenosine analogues and GMP

Guanine nucleotides (GN) have been implicated in many intracellular mechanisms. Extracellular actions, probably as glutamate receptor antagonists, have also been recently attributed to these compounds. GN may have a neuroprotective role by inhibiting excitotoxic events evoked by glutamate. Effects of extracellular GN on adenosine-evoked cellular responses have also been reported. However, the exact mechanism of such interaction is not known. In the present study, we showed that GN potentiated adenosine-induced cAMP accumulation in slices of hippocampus from young rats. However, neither GMP nor the metabotropic glutamate receptor agonist, 1S,3R-ACPD, inhibited the binding of the adenosine receptor agonist [3H]NECA (when binding to adenosine A2 receptors), or the binding of the adenosine A2a receptor agonist [3H]CGS 21680 in hippocampal membrane preparations. GppNHp, probably by interacting with G-proteins, decreased [3H]CGS 21680 binding. [3H]GMP binding was assayed in order to evaluate the GN sites which are not G-proteins. [3H]GMP binding was inhibited by GMP and GppNHp, but not by IS,3R-ACPD. The interaction of endogenous adenosine with the GMP-binding sites was determined by incubating membranes in the presence or absence of adenosine deaminase (ADA). NECA, CADO, CGS 21680 and CPA (only at the highest concentration used) increased GMP binding in the presence of ADA. However, in the absence of ADA, the control levels of GMP binding were as high as in the presence of added ADA plus adenosine agonists, indicating that endogenous adenosine modulates the binding of GMP. If this site has a neuroprotective role, adenosine may be increasing its neuromodulator and proposed protective action.

Study of adenosine A2 receptors in membrane preparations from optic tectum of chicks

Binding properties of the subtypes of adenosine A2 receptors in membrane preparations and the effects of adenosine receptor ligands on cAMP accumulation in slices from the optic tectum of neonatal chicks have been investigated. [3H]2-[4-(2-p-carboxyethyl)phenylamino]-5'-N-ethylcarboxaminoadenosin e (CGS 21680), a selective ligand for adenosine A2a receptors, did not bind to optic tectal membranes, as observed with rat striatal membranes. CGS 21680 also did not induce cyclic AMP accumulation in optic tectum slices. However, 5'-N-ethylcarboxamidoadenosine (NECA), 2-chloro-adenosine or adenosine induced a 2.5- to 3-fold increase on cyclic AMP accumulation in this preparation. [3H]NECA binds to fresh non-washed-membranes obtained from optic tectum of chicks, displaying one population of binding sites, which can be displaced by NECA, 8-phenyltheophylline, 2-chloro-adenosine, but is not affected by CGS 21680. The estimated K(D) value was 400.90 +/- 80.50 nM and the Bmax was estimated to be 2.51 +/- 0.54 pmol/mg protein. Guanine nucleotides, which modulate G-proteins activity intracellularly, are also involved in the inhibition of glutamate responses by acting extracellularly. Moreover, we have previously reported that guanine nucleotides potentiate, while glutamate inhibits, adenosine-induced cyclic AMP accumulation in slices from optic tectum of chicks. However, the guanine nucleotides, GMP or GppNHp and the metabotropic glutamate receptors agonist, 1S,3R-ACPD did not alter the [3H]NECA binding observed in fresh non-washed-membranes. Therefore, the adenosine A2 receptor found in the optic tectum must be the adenosine A2b receptor which is available only in fresh membrane preparations, and its not modulated by guanine nucleotides or glutamate analogs.

Effects of lead on adenylate cyclase activity in rat cerebral cortex

Lead decreased in a dose dependent manner the basal AC activity in membranes of rat cerebral cortex (IC50 = 2.5 +/- 0.1 microM). In membranes preincubated under basal conditions, AC activity was stimulated by approximately two and fourfold by 10 microM Gpp(NH)p or forskolin, respectively. Under basal conditions, lead (3 microM) inhibited enzyme activity up to 50%, but was not able to inhibit the Gpp(NH)p- or the forskolin-stimulated AC activity. However, in membranes preincubated with Gpp(NH)p (10 microM), lead (3 microM) had no significant effect on enzyme activity, but it partly blocked the stimulation of AC activity elicited by forskolin (10 microM). In membranes preincubated with 10 microM lead, the addition of 10 microM Gpp(NH)p or forskolin in the incubation medium did not stimulate AC activity. However, when added together in the incubation medium Gpp(NH)p + forskolin produced an increase in enzyme activity. In membranes preincubated with 10 microM lead + 10 microM Gpp(NH)p, Gpp(NH)p (10 microM) or forskolin (10 microM) added alone or in combination to the incubation medium did not stimulate AC activity. Moreover, under these latter conditions lead had no further effect on enzyme activity. These results indicate that lead may interact with G-proteins and with the catalytic subunit of cerebral cortical AC to produce inhibition of the enzyme activity.

Effects of guanine nucleotides on adenosine and glutamate modulation of cAMP levels in optic tectum slices from chicks

Glutamate and adenosine both modulate adenylyl cyclase activity through interaction of their specific receptors with stimulatory or inhibitory G-proteins. Guanine nucleotides (GN), which modulate G-protein activity intracellularly, are also involved in the inhibition of glutamate responses, acting from the outside of the cells. We had previously reported that glutamate inhibits adenosine-induced cyclic AMP (cAMP) accumulation in slices obtained from the optic tectum of chicks. In the present study we investigated the interaction of GN with these two neurotransmitters and found that GN inhibit the inhibitory effect of glutamate on adenosine-induced cAMP accumulation and potentiate adenosine-induced cAMP accumulation. These effects were observed with 5'-guanylylimidodiphosphate (GppNHp) or GMP, but not with guanosine (the nucleoside). Besides, these interactions of GN occur via a metabotropic glutamate receptor (mGluR) sensitive to (1 S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 S,3R-ACPD) but not to L-2-amino-4-phosphonobutyrate (L-AP4). These effects were partially modulated by a mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine ((RS)M-CPG), and by an adenosine receptor antagonist, 8-phenyltheophylline. GN only potentiated the adenosine response when adenosine was acting through its receptor positively linked to adenylyl cyclase. Therefore, the data show that guanine nucleotides not only inhibit glutamate-induced responses, but also stimulate adenosine-induced responses, a fact that may contribute to the understanding of the physiological functions of guanine nucleotides.

Effects of guanine nucleotides on glutamate-induced chemiluminescence in rat hippocampal slices submitted to hypoxia

Glutamate significantly increased levels of spontaneous chemiluminescence (CL) in rat hippocampal slices incubated under hypoxic conditions. Although it has been previously shown that guanine nucleotides (GN) displace glutamate from several of its receptors, in our study only GMP, as well as the glutamate antagonist MK-801, was able to reverse the increase in CL provoked by glutamate. On the other hand, not only GTP or Gpp(NH)p failed to reverse the action of glutamate, but they increased CL production like glutamate. This effect of GTP/Gpp(NH)p was also reversed by GMP. We concluded that, under neurotoxic conditions, GMP acted as an antagonist and GTP or Gpp(NH)p acted as agonists of glutamate. These results reinforced the evidence of the existence of extracellular site(s) for GN and indicated a possible role for GN in excitotoxicity.

Guanine nucleotides inhibit cAMP accumulation induced by metabotropic glutamate receptor activation

Metabotropic glutamate receptors (mGluRs) have been shown to modulate adenylate cyclase activity via G-proteins. In the present study we report similar results to the previously observed in the literature, showing that glutamate and the metabotropic agonists, 1S,3R-ACPD or quisqualate induced cAMP accumulation in hippocampal slices of young rats. Moreover, guanine nucleotides GTP, GDP or GMP, inhibited the glutamate-induced cAMP accumulation. By measuring LDH activity in the buffer surrounding the slices, we showed that the integrity of the slices was maintained, indicating that the effect of guanine nucleotides was extracellular. GMP, GDPbeta-S or Gpp(NH)p abolished quisqualate-induced cAMP accumulation. GDPbeta-S or Gpp(NH)p but not GMP inhibited 1S,3R-ACPD-induced cAMP accumulation. The response evoked by glutamate was also abolished by the mGluR antagonists: L-AP3 abolished glutamate-induced cAMP accumulation in a dose-dependent manner and MCPG was effective only at the 2 mM dose. DNQX was ineffective. We are reporting here, an inhibition induced by guanine nucleotides, via an extracellular site (s), similar to the observed with classical glutamate antagonists on a cellular response evoked by mGluR agonists.

GMP protects against quinolinic acid-induced loss of NADPH-diaphorase-positive cells in the rat striatum

When injected into the rat striatum, quinolinic acid causes dose-dependent widespread cell death. All cell types, including the NADPH-diaphorase-positive neurons appear to be sensitive to the toxin. The latter cells are destroyed by quinolinic acid injections of 180 nmol per striatum, this effect being blocked by the concomitant administration of 5 mg/kg of the non-competitive N-methyl-D-aspartate antagonist MK-801. We report that guanosine-5'-monophosphate (GMP), at a dose of 360 nmol, is equally effective in protecting the diaphorase-positive cells against quinolinate toxicity.

Specific binding of [3H]GppNHp to extracellular membrane receptors in chick cerebellum: possible involvement of kainic acid receptors

Guanine nucleotides (GNs), including GMP, displace [3H]kainic acid binding to chick cerebellar lysed and vesiculated membranes. Saturation studies of [3H]GppNHp binding, under conditions that prevent the occupation of the nucleotide binding sites in G-proteins, demonstrate the existence of extracellular membrane receptors specific for guanine nucleotides. Affinity-labeling of a vesicle preparation with [alpha-32P]GTP gives one single labeled band, upon electrophoresis, with an apparent molecular mass of 50 kDa. Additional experiments with partially purified kainate receptors suggest that the GN extracellular sites may overlap, at least partially, the kainic acid binding sites, being then responsible for the displacement of [3H]kainic acid by GNs. The physiological significance of these findings remains unclear.

Effect of guanine nucleotides on [3H]glutamate binding and on adenylate cyclase activity in rat brain membranes

GMP-PNP, a non-hydrolyzable analog of GTP binds tightly to G-protein in the presence of Mg2+, so that the binding is stable even after exhaustive washings. This property was exploited to prepare membrane samples of rat brain where G-protein GTP-binding sites were saturated with GMP-PNP. Experiments carried out with these membranes showed that GTP, GMP-PNP, GDP-S and GMP (1 mM) inhibit the sodium-independent [3H]glutamate binding by 30-40% [F(4,40) = 5.9; p < .001], whereas only GMP-PNP activates adenylate cyclase activity [F(6,42) = 3.56; p < .01]. The inhibition of sodium-independent [3H]glutamate binding occurred in the absence of Mg2+. These findings suggest that guanine nucleotides may inhibit glutamate binding and activate adenylate cyclase through distinct mechanisms by acting on different sites.

Effects of Psychotria colorata alkaloids in brain opioid system

An ethnopharmacological survey showed that home remedies prepared with flowers and fruits of Psychotria colorata are used by Amazonian peasants as pain killers. Psychopharmacological in vivo evaluation of alkaloids obtained from leaves and flowers of this species showed a marked dose-dependent naloxone-reversible analgesic activity, therefore suggesting an opioid-like pharmacological profile. This paper reports an inhibitory effect of P. colorata flower alkaloids on [3H]naloxone binding in rat striata as well as a decrease in adenylate cyclase basal activity. The alkaloids did not affect [3H] GMP-PNP binding. These findings provide a neurochemical basis for the opioid-like activity previously detected in vivo and point to Psychotria alkaloids as a potential source of new bioactive opiate derivatives.

Neurochemical effects of L-pyroglutamic acid

The effect of L-pyroglutamic acid, a metabolite that accumulates in pyroglutamic aciduria, on different neurochemical parameters was investigated in adult male Wistar rats. Glutamate binding, adenylate cyclase activity and G protein coupling to adenylate cyclase were assayed in the presence of the acid. L-pyroglutamic acid decreased Na(+)-dependent and Na(+)-independent glutamate binding. Basal and GMP-PNP stimulated adenylate cyclase activity were not affected by the acid. Furthermore, rats received unilateral intrastriatal injections of 10-300 nmol of buffered L-pyroglutamic acid. Vehicle (0.25 M Tris-Cl, pH 7.35-7.4) was injected into the contralateral striatum. Neurotoxic damage was assessed seven days after the injection by histological examination and by weighing both cerebral hemispheres. No difference in histology or weight could be identified between hemispheres. These results suggest that, although capable of interfering with glutamate binding, pyroglutamate did not cause a major lesion in the present model of neurotoxicity.