Excitotoxin induction of ornithine decarboxylase in cerebral cortex is reduced by phospholipase A2 inhibition.

The enzyme ornithine decarboxylase (ODC) has been shown to be induced by a number of conditions such as cold-injury, kindling, ischaemia and excitotoxin injection. In previous studies we have characterised the cortical response to kainate injection into the nucleus basalis and shown a substantial increase in both ODC mRNA and enzyme activity which reaches a maximum at 8h. This response is completely prevented by treatment with MK-801, indicating the involvement of NMDA receptors in mediating this response. Whilst NMDA receptors are known to gate a cation channel leading to increased calcium entry, an additional effect on the release of arachidonic acid has been reported. The possibility that NMDA receptor mediated activation of phospholipase A2 and release of arachidonic acid might mediate this ODC response was investigated in this study by treatment with the phospholipase inhibitors quinacrine and dexamethasone. Treatment of animals with quinacrine (100 mg/kg) at the time of injection of kainate into the nucleus basalis caused a significant attenuation of the induction of ODC in cerebral cortex of 43%. No further attenuation was seen at higher doses. A similar reduction in ODC induction was seen after treatment with dexamethasone (1 mg/kg) but a greater effect could be obtained (65% attenuation) at higher doses. The possible involvement of arachidonic acid derivatives in mediating ODC induction was further investigated by treatment with the cyclo-oxygenase inhibitor indomethacin and the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA). Indomethacin was able to significantly attenuate the induction of ODC (greater than 60%) whilst NDGA (30 mg/kg) was ineffective. These results indicate the possible role of arachidonic acid derivatives in the regulation of the expression of ODC in cerebral cortex after excitotoxin injection.