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closed conformation via a mechanism independent from free radical scavenging (Papucci et al., 2003).

Retinal ischemia and glutamate

Glutamate is the major excitatory neurotransmitter in the retina and is removed from the extracellular space by an energy-dependent process involving neuronal and glial cell transporters (Barnett et al., 2001). Control of extracellular glutamate levels under physiological and pathophysiological conditions is a prerequisite for the prevention of neurodegeneration, particularly in the retina where elevated glutamate levels are associated with experimental glaucoma (Dreyer and Grosskreutz, 1997; Osborne et al., 1999, 2004; Nucci et al., 2005). Clinical studies also support a role for excitotoxicity in the development of the RGC damage in subjects suffering from glaucoma (Manabe et al., 2005). Excessive activation of glutamate receptors, mainly of the NMDA subtype, results in calcium overload, free radical formation, and activation of proteases, including caspases, and transcription factors, leading to excitotoxic neuronal death (Joo et al., 1999; Osborne et al., 2004; Lipton, 2006). Several studies suggest that excitotoxicity occurs during retinal ischemia with subsequent loss of RGCs and that this process plays a role in the pathogenesis of ischemic retinopathy (Sucher et al., 1997). In vitro exposure of the retina to the ionotropic glutamate receptor agonist NMDA produces histological changes similar to those caused by experimental ischemia (see Sucher et al., 1997). Subcutaneous injections of glutamate into young mice cause severe damage of the inner layers of the retina, most notably in the ganglion cell layer (Lucas and Newhouse, 1957). A similar pattern of damage was observed when an acute bolus of glutamate was injected intravitreally to adult rats (Sisk and Kuwabara, 1985) or following chronic administration (Samy et al., 1994). The pivotal role of excessive glutamate in the mechanisms of retinal damage is also documented by the evidence that NMDA and non-NMDA receptor antagonists afford protection in experimental models of RGC death, both in vitro (see Joo et al., 1999; Osborne

et al., 1999) and in vivo (Adachi et al., 1998; Nucci et al., 2005). Under normal conditions, in the retina glutamate is removed from the extracellular space by an energy-dependent process involving neuronal and glial cell transporters (excitatory amino acid transporters, EAATs), thus terminating the excitatory signal and preventing excitotoxic neuronal damage. During ischemia, glutamate levels are thought to rise by a combination of vesicular release and reversal of the EAATs (Billups and Attwell, 1996; Maguire et al., 1998; Jabaudon et al., 2000; Rossi et al., 2000). It has been reported that glutamate levels are elevated in the vitreous humor of glaucoma patients (Dreyer et al., 1996; but see also Honkanen et al., 2003) and the progressive deterioration of visual field in glaucomatous patients is a consequence of progressive RGC death (Osborne et al., 1999).

Coenzyme Q10 minimizes glutamate increase induced by ischemia/reperfusion

However, direct evidence of accumulation of toxic levels of glutamate in experimental glaucoma has only recently been gathered by using a microdialysis technique in high IOP-induced retinal ischemia in living rat (Nucci et al., 2005). These data demonstrate that elevation of glutamate occurs during the first 10 min after induction of the ischemic insult, while a significant increase is seen during reperfusion (Fig. 2). A similar observation has been reported in the rabbit and cat retina where a much larger increase in extracellular glutamate was also seen during the reperfusion as compared to the ischemic phase (Louzada-Junior et al., 1992; Adachi et al., 1998). The lower levels of glutamate observed during the period of ischemia might reflect the tolerant nature of the retina to the ischemic insult as compared to the brain (Neal et al., 1994). In addition, the prolonged accumulation of glutamate that occurs after the ischemic insult may stem from a defective glutamate uptake system in the retinal neurons known to be devoid of high affinity for glutamate (White and Neal, 1976). Previous microdialysis experiments carried out in anesthetized rats undergone high IOPinduced ischemia have documented that glutamate levels increase in the retina and these are reversed