Cerebral Energy Metabolism and Immediate Early Gene Induction Following Severe Incomplete Ischaemia in Transgenic Mice Overexpressing the Human Ornithine Decarboxylase Gene: Evidence that Putrescine is not Neurotoxic In Vivo

Cerebral ischaemia causes activation of ornithine decarboxylase followed by accumulation of putrescine, and these biochemical phenomena have been thought to contribute to the development of neuronal damage. We have used a transgenic mouse line overexpressing the human ornithine decarboxylase gene in their neurons with constitutively high putrescine to study the possible role of putrescine in development of neuronal damage in forebrain ischaemia. An incomplete forebrain ischaemia model was developed in which common carotid arteries were bilaterally occluded and reduction of blood pressure caused by orthostatic reaction was used as a way of decreasing cerebral circulation. Cerebral high‐energy metabolites, intracellular pH and lactate were monitored by means of 31 P and 1 H nuclear magnetic resonance spectroscopy respectively. Incomplete ischaemia for 15 min resulted in severe energy failure, as indicated by an increase in the inorganic phosphate/phosphocreatine ratio, intracellular acidification from a pH of ∼7.1 to ∼6.5 and an increase in lactate concentration from <1 to ∼10 mmol/kg in both syngenic and transgenic mice. Following deocclusion, recovery of energy metabolites, intracellular pH and lactate were identical in both animal groups. Ornithine decarboxylase activity rose 9‐ and 3‐fold in syngenic and transgenic mice respectively 6 h after ischaemia. Activation of ornithine decarboxylase resulted in extensive accumulation of putrescine in the brains of transgenic animals 12–24 h after ischaemia, which was ∼50‐fold greater than the basal level in syngenic mice. In situ hybridization experiments revealed induction of transcription factors c‐Fos and zif‐268 in the hippocampus, throughout the cerebral cortex and striatum 1–3 h after ischaemia. Messenger RNA of heat shock protein 70 was induced in dentate gyrus and CA3 and CA4 subfields of the hippocampus 1 h after ischaemia. The distribution and extent of induced mRNAs were similar in syngenic and transgenic animals. Histological evaluation did not reveal any difference between the two animal groups despite large variation in their cerebral putrescine content. Neuronal necroses were observed in the CA4 layer of hippocampi in both syngenic and transgenic mice. These data suggest that ornithine decarboxylase activation and accumulation of endogenous putrescine are indicative of an ischaemic insult and that these changes reflect an adaptive response rather than acting as causative factors of neuronal damage.