Although NMDA and non-NMDA receptor antagonists blocked glutamate-induced increase in extracellular ATP, only kainate was capable of inducing nucleotide accumulation in medium. No increase was observed by incubating buy BLU9931 cells with NMDA. Both antagonists also blocked the increase in extracellular ATP levels induced by kainate. At least two possibilities could account for this observation. The first would be that NMDA receptor antagonist MK-801 blocked non-NMDA receptor stimulation. This possibility however, does not seem plausible since no evidences for such non-specific effect of MK-801 were found so far. Another possibility would be that

kainate induced the release of endogenous glutamate as already suggested by Uckermann et al. (2006) in the rat retina. In this scenario, released glutamate would stimulate NMDA receptors that together with the activation of non-NMDA receptors by glutamate or VX 770 kainate would

induce the release of ATP from cultured Müller cells. This possibility is particularly interesting since a kainate-induced, calcium-dependent release of [3H]-d-aspartate was previously demonstrated in mixed chick retinal cultures (Duarte et al., 1996) as well as in the retina of other species (Ohia et al., 2000). Since Müller cells seems to take up and release glutamate (Gadea et al., 2004, Newman and Zahs, 1998 and Reis et al., 2008), one interesting point that deserves further investigation is whether glutamate itself can induce the release of d-aspartate or glutamate from cultured chick Müller

cells. Previous evidences have shown that glutamate does not induce calcium mobilization in Müller cells from adult rodent retinas (Newman, 2005, Newman and Zahs, 1997, Rillich et al., 2009 and Uckermann et al., 2004). Moreover, in same preparations, the release of ATP from Müller cells was shown to be a calcium-independent, non-exocytotic process (Uckermann et al., 2006 and Wurm et al., 2008). In the present study, glutamate-induced accumulation of extracellular ATP was blocked by BAPTA-AM, a chelator of intracellular calcium and by bafilomycin A1, a v-ATPase many inhibitor. The discrepancies between our findings and those mentioned above may have several explanations, including species or age differences. An interesting hypothesis is that the glutamate-induced calcium-dependent exocytotic release of ATP observed in the present study occurred only in cultured Müller cells, but not in freshly dissociated or non-dissociated Müller cells as those used in the mentioned studies. It is known that Müller cells in purified cultures can dedifferentiate to progenitors and express different sets of signaling components (Reis et al., 2008 and Bringmann et al., 2009).