A precise temporal dissection of monosodium glutamate-induced apoptotic events in newborn rat retina in vivo.

Although L-glutamate is the main excitatory neurotransmitter in the retina, excess glutamate level triggers severe neuronal damages. Therefore, monosodium glutamate has been used to probe neurodegenerative mechanisms but precise toxicity schedule is not available in vivo. We report, for the first time, a temporal analysis of apoptotic processes induced by subcutaneously applied monosodium glutamate. We investigated the glutamate triggered subcellular processes over a time scale of 48 h in neonatal retina. We employed immunoblots to measure the level of activated apoptotic factors and immunocytochemistry to reveal the dying cells. Upregulation of active caspase-9 started at 3 h and peaked at 6 h post-injection. Activations of caspase-3, caspase-6 and caspase-7 consistent with their late-phase roles increased at 6 h post-injection. The apoptotic processes were terminated by 24 h post-injection. Caspase 12 and calpain-2 seemed unaffected by subcutaneous monosodium glutamate administration. Uniquely, we found that the ubiquitous calpain-1 is not expressed in newborn rat retina.