Apoptosis and necrosis occurring in excitotoxic cell death in isolated chick embryo retina

Excitotoxic studies using isolated chick embryo retina indicated that such an in vitro model provides a valid tool to characterize the effect of different agonists for subtypes of glutamate ionotropic receptors. In retinas maintained for 24 h in a Krebs medium, after a brief exposure (30 min) to glutamate agonists, we compared the effects produced by NMDA and non-NMDA-agonists, such as kainic acid (KA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Delayed retinal damage was assessed by measuring lactate dehydrogenase (LDH) present in the medium after exposure to the previously named agonists. Although at high concentrations, both KA and AMPA produced more relevant release than NMDA, 7-8% of total retinal LDH was released after exposure to a 50 microM concentration of non-NMDA agonists. These values were similar to those obtained after 100 microM NMDA. In this regard, retinal tissue appeared to be less sensitive to excitotoxicity based on the activation of NMDA receptor subtype. All three agents produced histopathological lesions typical for excitotoxic damage. A delayed form of excitotoxicity observed in retina segments was predominated by necrotic features. However, the activation of apoptotic machinery early during the incubation period subsequent to brief exposure to NMDA (100 microM) was also present. The activation of caspase enzymes was studied by a fluorometric protease activity assay as well as by western blot analysis. Caspase-3-like activity reached the highest value within 3 h of incubation after exposure to excitotoxin, then the level of enzyme activity declined to lower values. As confirmed by a time-related appearance of TUNEL-positive nuclei, apoptotic features appeared to be specific for retina response to NMDA. In contrast, the exposure to a 50 microM concentration of KA or AMPA induced necrotic cell damage which was evident through the incubation, leading to a delayed mechanism of excitotoxicity. These observations provide evidence that in the retinal model, with regard to agonist concentrations and subtype of glutamate receptors, the cascade of events leading to excitotoxicity may result in either apoptotic or necrotic neuronal cell damage.