Global ischemia induces downregulation of GluR2 mRNA and increases AMPA receptor-mediated CA²⁺ influx in hippocampal CA1 neurons of gerbil

Transient, severe forebrain or global ischemia leads to delayed cell death of pyramidal neurons in the hippocampal CA1. The precise molecular mechanisms underlying neuronal cell death after global ischemia are as yet unknown. Glutamate receptor-mediated Ca²⁺ influx is thought to play a critical role in this cell death. In situ hybridization revealed that the expression of mRNA encoding GluR2 (the subunit that limits Ca²⁺ permeability of AMPA-type glutamate receptors) was markedly and specifically reduced in gerbil CA1 pyramidal neurons after global ischemia but before the onset of neurodegeneration. To determine whether the change in GluR2 expression is functionally significant, we examined the AMPA receptor- mediated rise in cytoplasmic free Ca²⁺ level ([Ca²⁺](i)) in individual CA1 pyramidal neurons by optical imaging with the Ca²⁺ indicator dye fura- 2 and by intracellular recording. Seventy-two hours after ischemia, CA1 neurons that retained the ability to fire action potentials exhibited a greatly enhanced AMPA-elicited rise in [Ca²⁺](i). Basal [Ca²⁺](i) in these neurons was unchanged. These findings provide evidence for Ca²⁺ entry directly through AMPA receptors in pyramidal neurons destined to die. Downregulation of GluR2 gene expression and an increase in Ca²⁺ influx through AMPA receptors in response to endogenous glutamate are likely to contribute to the delayed neuronal death after global ischemia.