NMDA and glutamate evoke excitotoxicity at distinct cellular locations in rat cortical neurons in vitro

The development of cortical neurons in vivo and in vitro is accompanied by alterations in NMDA receptor subunit expression and concomitant modificatio ns in the pharmacological profile of NMDA-activated ionic currents. For exa mple, we observed that with decreasing NR2B/NR2A subunit expression ratio, the block of NMDA receptor-mediated whole-cell responses by the NR2B-select ive antagonist haloperidol was also decreased. In mature cultures (>22 d in vitro), however, NMDA responses obtained from excised nucleated macropatch es, which comprised a large portion of the soma, remained strongly antagoni zed by haloperidol. These results suggest that in more mature neurons NR1/N R2B receptors appear to be preferentially expressed in the cell body. As pr edicted from the whole-cell recording pharmacological profile, NMDA-induced toxicity was largely unaffected by haloperidol in mature cultures. However , haloperidol effectively blocked glutamate toxicity in the same cultures, suggesting that the neurotoxic actions of this amino acid were mostly due t o the activation of somatic NMDA receptors. In experiments in which the pot ency of glutamate toxicity was increased by the transport inhibitor L-trans -pyrrolidine-2,4-dicarboxylic acid, the neuroprotective effects of haloperi dol were significantly diminished. This was likely because of the fact that glutamate, now toxic at much lower concentrations, was able to reach and a ctivate dendritic receptors under these conditions. These results strongly argue that exogenous glutamate and NMDA normally induce excitotoxicity at d istinct cellular locations in mature mixed neuronal cultures and that NR1/N R2B receptors remain an important component in the expression of glutamate, but not NMDA-induced excitotoxicity.