RESUMO
Methylmercury (MeHg) is a highly neurotoxic compound and several studies have reported intoxication signs in children whose mothers were exposed to this environmental toxicant. Although it is well established that the in utero exposure to MeHg causes neurological deficits in animals and humans, there is no evidence of the exclusive contribution of lactational exposure to MeHg as a possible cause of neurotoxicity in the offspring. In this study, we investigated the exclusive contribution of MeHg exposure through maternal milk on biochemical parameters related to the glutamatergic homeostasis (glutamate uptake by slices) and to the oxidative stress (total and nonprotein sulfhydryl groups, nonprotein hydroperoxides, glutathione peroxidase and catalase activities) in the cerebellum of suckling mice (Swiss albino). The same parameters were also evaluated in the cerebellum of mothers. Our results showed, for the first time, that lactational exposure to MeHg caused a high percent of inhibition (50%) on glutamate uptake by cerebellar slices in pups. Contrarily, this effect was not observed in mothers, which were submitted to a direct oral exposure to MeHg (15 mg/l in drinking water). In addition, behavioral/functional changes were observed in the weaning mice exposed to MeHg. It was observed an increase in the levels of nonprotein hydroperoxides in cerebellum, and this increase was negatively correlated to the glutamate uptake by cerebellar slices. This study indicates that (1) the exposure of lactating mice to MeHg causes inhibition of the glutamate uptake by cerebellar slices in the offspring; (2) this inhibitory effect seems to be related to increased levels of hydroperoxide.
Assuntos
Cerebelo/fisiopatologia , Ácido Glutâmico/fisiologia , Compostos de Metilmercúrio/intoxicação , Leite/química , Síndromes Neurotóxicas/fisiopatologia , Animais , Animais Lactentes , Antioxidantes/metabolismo , Comportamento Animal/efeitos dos fármacos , Peso Corporal/efeitos dos fármacos , Catalase/metabolismo , Cerebelo/enzimologia , Cerebelo/metabolismo , Feminino , Ácido Glutâmico/metabolismo , Glutationa Peroxidase/metabolismo , Peróxido de Hidrogênio/metabolismo , Técnicas In Vitro , Masculino , Camundongos , Proteínas do Tecido Nervoso/metabolismo , Síndromes Neurotóxicas/enzimologia , Síndromes Neurotóxicas/psicologia , Compostos de Sulfidrila/metabolismoRESUMO
A predominantly neurological presentation is common in patients with glutaric acidemia type I (GA-I). 3-hydroxyglutaric acid (3-OHGA), which accumulates in affected patients, has recently been demonstrated to play a central role in the neuropathogenesis of this disease. In the present study, we investigated the in vitro effects of 3-OHGA at concentrations ranging from 10 to 1000 microM on various parameters of the glutamatergic system, such as the basal and potassium-induced release of [3H]glutamate by synaptosomes, as well as on Na+-dependent [3H]glutamate uptake by synaptosomes and astrocytes and Na+-independent [3H]glutamate uptake by synaptic vesicles from cerebral cortex of 30-day-old Wistar rats. First, we observed that exposure of cultured astrocytes to 3-OHGA for 20 h did not reduce their viability. Furthermore, 3-OHGA significantly increased Na+-dependent [3H]glutamate uptake by astrocytes by up to 80% in a dose-dependent manner at doses as low as 30 microM. This effect was not dependent on the presence of the metabolite during the uptake assay, since it occurred even when 3-OHGA was withdrawn from the medium after cultured cells had been exposed to the acid for approximately 1 h. All other parameters investigated were not influenced by this organic acid, indicating a selective action of 3-OHGA on astrocyte transporters. Although the exact mechanisms involved in 3-OHGA-stimulatory effect on astrocyte glutamate uptake are unknown, the present findings contribute to the understanding of the pathophysiology of GA-I, suggesting that astrocytes may protect neurons against excitotoxic damage caused by 3-OHGA by increasing glutamate uptake and therefore reducing the concentration of this excitatory neurotransmitter in the synaptic cleft.
Assuntos
Astrócitos/metabolismo , Córtex Cerebral/metabolismo , Ácido Glutâmico/metabolismo , Glutaratos/farmacologia , Animais , Sobrevivência Celular/efeitos dos fármacos , Células Cultivadas , Córtex Cerebral/citologia , Córtex Cerebral/efeitos dos fármacos , Masculino , Proteínas do Tecido Nervoso/biossíntese , Ratos , Ratos Wistar , Estimulação Química , Vesículas Sinápticas/efeitos dos fármacos , Vesículas Sinápticas/metabolismo , Sinaptossomos/efeitos dos fármacos , Sinaptossomos/metabolismoRESUMO
During the early postnatal period the brain is extremely sensitive to external agents. Here, we examined the effect of subcutaneous injections of methylmercury (MeHg; 2 mg/kg) during the suckling period (postnatal days [PND] 3-10, 3-17, or 3-24) on glutamate release from brain synaptosomal preparations and on glutamate uptake by brain cortical slices of rat pups. The possible antagonist effect of ebselen against MeHg effect was also examined at PND 24. MeHg increased the basal (but not K+-stimulated) glutamate release and glutamate uptake at PND 24. A strong tendency of increase in the basal glutamate release from synaptosomes (p= 0.088) was observed at PND 17. Ebselen, which did not affect glutamate release and uptake per se, prevented both effects of MeHg. This study indicates that (1) the effect of MeHg on glutamate release could be involved in its toxicity; (2) the increase in the glutamate uptake could represent a pathophysiological response to MeHg-induced glutamate release; (3) the inhibitory effect of ebselen on MeHg-induced glutamate release could be related to its reported neuroprotective effects.
Assuntos
Animais Lactentes/metabolismo , Azóis/farmacologia , Química Encefálica/efeitos dos fármacos , Córtex Cerebral/metabolismo , Ácido Glutâmico/metabolismo , Compostos de Metilmercúrio/antagonistas & inibidores , Compostos de Metilmercúrio/toxicidade , Fármacos Neuroprotetores/farmacologia , Compostos Organosselênicos/farmacologia , Sinaptossomos/metabolismo , Envelhecimento/fisiologia , Animais , Córtex Cerebral/efeitos dos fármacos , Técnicas In Vitro , Isoindóis , L-Lactato Desidrogenase/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Terminações Pré-Sinápticas/efeitos dos fármacos , Terminações Pré-Sinápticas/metabolismo , Ratos , Ratos Wistar , Sinaptossomos/efeitos dos fármacosRESUMO
Intraperitoneal guanosine has been shown to prevent quinolinic acid-induced seizures in mice. In this study, we investigated the effect of orally administered guanosine on seizures induced by the glutamate agonists quinolinic acid and kainate, and the endogenous glutamate releaser alpha-dendrotoxin. Guanosine (7.5 mg/kg, per os), administered 75 min in advance, prevented 70% of seizures induced by i.c.v. quinolinic acid, being as efficient as the NMDA channel blocker MK-801 administered intraperitoneally. Guanosine was ineffective against kainate-induced seizures, but significantly reversed the potentiation of seizures and death caused by the concomitant injection of MK-801. Guanosine also significantly prevented seizures and death induced by i.c.v. alpha-dendrotoxin, whereas MK-801 and phenobarbital only prevented death. Altogether, our findings underscore the therapeutic potential of oral administration of guanosine for treating diseases involving glutamatergic excitotoxicity, including epilepsy.
Assuntos
Encéfalo/efeitos dos fármacos , Morte , Epilepsia/tratamento farmacológico , Agonistas de Aminoácidos Excitatórios/farmacologia , Ácido Glutâmico/metabolismo , Guanosina/farmacologia , Fármacos Neuroprotetores/farmacologia , Animais , Encéfalo/metabolismo , Encéfalo/fisiopatologia , Cafeína/farmacologia , Maleato de Dizocilpina/farmacologia , Relação Dose-Resposta a Droga , Venenos Elapídicos/farmacologia , Epilepsia/induzido quimicamente , Epilepsia/fisiopatologia , Antagonistas de Aminoácidos Excitatórios/farmacologia , Ácido Caínico/farmacologia , Masculino , Camundongos , Fenobarbital/farmacologia , Inibidores de Fosfodiesterase/farmacologia , Ácido Quinolínico/farmacologia , Receptores Purinérgicos P1/efeitos dos fármacos , Receptores Purinérgicos P1/metabolismoRESUMO
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.
Assuntos
Astrócitos/metabolismo , Ácido Glutâmico/farmacocinética , Guanosina/farmacologia , Animais , Astrócitos/citologia , Astrócitos/efeitos dos fármacos , Transporte Biológico/efeitos dos fármacos , Células Cultivadas , Córtex Cerebral/citologia , Relação Dose-Resposta a Droga , Ratos , Ratos WistarRESUMO
Dendrotoxin (DTX), a well characterized IA-type potassium channel blocker, directly added to the culture medium had no effect on survival of cultured cortical neurons at 6 or 14 days in vitro. On the contrary, neurons exposed to DTX remained in better condition than untreated ones. In an attempt to demonstrate the mechanisms by which DTX may affect neuronal survival we studied its effect in co-cultures of cortical neurons and astrocytes submitted to successive medium changes. After the second change of medium, at 9 days in vitro, the neuronal number in controls decreased by 43%, while in cultures receiving astrocyte-conditioned medium the cell loss was significantly reduced (15%, p < 0.01) with respect to control conditions. When DTX was added to the culture medium neuronal loss was also significantly prevented (25% for 1 microM DTX, p < 0.01) with respect to control conditions. 4-Aminopyridine (4-AP) and 21 mM K+ also preserved neurons. The L-type calcium channel antagonist nifedipine (5 microM) abolished the protective effect of DTX and 4-AP. These results show that K+ channel blockade induces protection against damage produced by repetitive medium change and that this effect is mediated by L-type Ca2+ channels.