Studies dealing with the effects
Studies dealing with the effects of ischemia on EAATs have been previously reported, but some discrepancies in literature appear between them probably as a consequence of the different models, species, time of reperfusion after ischemia and age of the animals. Thus, in human, Gastrodin stroke-dependent increases in the GLAST (EAAT1) expression, but not in GLT-1 (EAAT2), using immunohistochemical studies have been reported (Beschorner et al., 2007). Also, Western blot and immunohistochemical assays in rat hippocampus and cerebral cortex show modifications in EAAC1, but not in GLAST and GLT-1, at different reperfusion times after transient ischemia (Gottlieb et al., 2000). Using a four vessel occlusion model, modifications in the expression of GLT-1 have been described in rat hippocampal CA1 and striatum but no changes were observed for GLAST (Chen et al., 2005). Decreases in GLT-1 and EAAC1 mRNAs, but not in GLAST ones, at 24 and 72h of reperfusion have been reported in the rat cortex (Rao et al., 2001), as well as decreases in GLT-1 mRNA levels in the hippocampal CA1 region 24h following the ischemic challenge with a return to normal levels 72h after the insult (Yeh et al., 2005). Post-ischemic modifications in EAAC1, GLAST and GLT-1 mRNAs have also been described in a gerbil ischemic model (Fujita et al., 1999).
Regarding the possible role of the glutamate transporters in ischemia–reperfusion (I/R) it should be noted that astroglial uptake of glutamate by GLT-1 and GLAST is a major mechanism in the excitotoxicity and clearance of this neurotransmitter (Rothstein et al., 1996), although other mechanisms such as the “rescue” function of EAAC1 (Kiyama and Kiryu-Seo, 2007) could be involved. The time of glutamate clearance has been reported to increase with age (Nickell et al., 2007), which could be related to changes in the regulation of the EAATs. These age-related alterations in extracellular glutamate regulation may contribute to the increased susceptibility of the aged brain to excitotoxic insults such as stroke. We have previously reported delayed neuronal death following ischemia as well as an outstanding decrease in the mRNA levels of some glutamatergic genes both in 3- and 18-month-old animals undergoing an ischemic insult followed by 48h reperfusion (Dos-Anjos et al., 2009a, Dos-Anjos et al., 2009b, Montori et al., 2010a, Montori et al., 2010b). This time-point seems to be crucial in the time-course of ischemia. Thus, in an attempt to better understand the role of the glutamatergic system in stroke, this study aims at finding out how the glial and neuronal glutamatergic transporter transcription is modified by I/R in brain areas with different vulnerability to ischemia and how age and inflammation alter this response 48h after challenge.
Acknowledgments We thank Alberto Fuertes Puerta for his help in edition and we also thank Marta Fernández and María Rehberger for their technical assistance. This work was supported by Fondo de Investigaciones Sanitarias (Ref. PS0900852) and by Junta de Castilla y León ( Ref. LE009A09). Sheyla Montori was granted in the FPU program (Ref. 2006-01974) by the Ministry of Education and Science.
Introduction Excitatory neurotransmission in the vertebrate central nervous system (CNS) is mediated largely by glutamate (Glu). Two main subtypes of Glu receptors have been defined: ionotropic (iGluRs) and metabotropic receptors (mGluRs). Three iGluRs exist: N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoaxazolepropionate (AMPA) and kainate (KA) receptors (Hollmann and Heinemann, 1994). Metabotropic receptors are divided in terms of sequence similarity, signal transduction mechanisms and pharmacology in three groups. Group I receptors are coupled to the stimulation of phospholipase C with the consequent release of intracellular Ca2+, while Groups II and III are coupled to the inhibition of adenylate cyclase. These three groups are activated preferentially by (RS)-3,5-dihydroxyphenylglycine (DHPG) for Group I, (S)-4-carboxy-3-hydroxyphenylglycine (S)-4C3HPG activates Group II while l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) acts upon Group III (Coutinho and Knopfel, 2002).