NMDA receptors have been widely implicated as mediators of necrosis and apoptosis following excitotoxic insult. Both processes have been described in the retina where they are linked to the clinical disorders of ischemia and glaucoma, respectively. Although both processes are initiated by NMDA receptor activation, recent data now suggest that the two pathways diverge and occur independently of the other. Using a rodent retinal model, we will investigate the mechanism(s) through which NMDA-induced toxicity leads to necrotic and apoptotic cell death. Necrosis will be modeled by blocking apoptosis prior to an acute exposure to high levels of NMDA or GABA, whereas apoptosis will be modeled by 1) blocking necrosis prior to low-level exposure to NMDA. We hypothesize that NMDA induced apoptosis is Ca+2 dependent and requires the NR2B subunit whereas necrosis involves non-NR2B-containing NMDA receptor channels and GABA release by reverse transport with subsequent CI- ion toxicity on GABA receptive neurons.
Three specific aims are proposed to test this hypothesis: 1) Much if not all of NMDA-induced necrosis in the retina requires not only NMDA-receptive neurons but also the participation of GABA receptors; 2) Death to retinal neurons via NMDA induced necrosis involves the influx of Na+ through NMDA receptor channels, causing reversed polarity of the GABA transporter (GAT-1) leading to release of GABA and subsequent CI- toxicity; 3) The NR2B subunit is directly responsible for initiation of apoptosis but is not linked to necrosis. Using biochemical and cytochemical approaches to measure the extent of cell death by apoptosis and necrosis we will elucidate the pathways that lead to NMDA-mediated apoptosis and necrosis with the goal of providing new insight regarding the applicability of specific drugs for treating necrotic and apoptotic visual disorders.
