Mild
NMDA receptor activation is correlated with neuroprotection in models of
cerebral ischemia. Neuroprotection with
NMDA manifests as a form of ischemic tolerance and involves the induction of cellular stress systems sensitive to disturbances in cellular
calcium homeostasis. Unilateral micro-injection of 10, 160 and 320 microM
NMDA into the prefrontal cortex of a rat 30 min prior to permanent occlusion of the middle cerebral artery (MCAO) significantly reduced the area of
infarct observed after 4 h of
ischemia. The highest dose of
NMDA (320 microM) prevented the propagation of ischemic damage through a direct toxicity on neuronal tissue adjacent to the injection site as demonstrated in
thionin-stained sections. As a result, the degree of
ischemia-induced damage was similar to that measured in rats pretreated with the low dose of
NMDA (10 microM). Expression of
heat shock protein (HSP) 70 and
glucose-regulated
protein (GRP) 94 in cortical samples taken from the region of
infarct following MCAO was significantly reduced in rats pretreated with 10 microM
NMDA compared to saline-injected control rats and rats pretreated with higher doses of
NMDA. Furthermore, 10 microM
NMDA did not appear to influence expression of
m-calpain or
GRP78, however, higher doses of
NMDA did significantly induce expression of both
proteins as assessed by Western blotting. In summary, our data demonstrate an in vivo rodent model of ischemic tolerance in which 30 min of neuronal preconditioning with 10 microM
NMDA confers protection against a 4 h period of MCAO-induced
ischemia. This effect may involve modulation of cellular stress signals, in particular HSP70 and
GRP94.