Metabotropic glutamate receptor subtypes are differentially expressed after transient cerebral ischemia without, during and after tolerance induction in the gerbil hippocampus

Postischemic delayed neuronal death (DND) of hippocampal CA1 neurons can be prevented by a preconditioning sublethal ischemic stimulus. To check for possible participation of metabotropic glutamate receptors (mGluRs) in neuronal death or survival, we analyzed postischemic protein expression of subtypes 1b and 5 of group I mGluRs, which are thought to exert neurotoxic effects after pathological activation due to ischemia, and subtypes 2 and 3 of group II mGluRs, which in contrast are thought to be neuroprotective in this state, respectively. Therefore, three groups of gerbils with reperfusion intervals between 8 h and 4 days (n=5 each) were investigated: one group was subjected to 5 min ischemia, resulting in DND of CA1 neurons, a second group to a tolerance inducing 2.5 min period of ischemia and a third group to 5 min ischemia after prior tolerance induction. The major finding was a transient postischemic reduction of mGluR1b and 5 expression in the ischemic tolerant CA1 subfield at 8 h. This downregulation of neurotoxic mGluRs may indicate a contribution to the survival of highly vulnerable CA1 neurons in the ischemic tolerant state.