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Notes: Abstract: The major cholinergic innervation of the rat cerebral cortex arises from the nucleus basalis in the basal fore-brain. Introduction of the excitotoxins kainate or ibotenate into the nucleus basalis by stereotaxic injecton results in degeneration of the cholinergic cells. We have investigated the effect of this excitotoxic action on ornithine decarboxylase (ODC) activity and cholinergic responsiveness in the cerebral cortex. A massive and rapid induction of ODC activity was seen in ipsilateral cortex after injection of excitotoxin. A maximal increase in ODC activity of 268 times the control value was seen in ipsilateral cerebral cortex 8 h after lesioning. Thereafter, ODC activity declined but remained significantly greater than control levels for 32 h. Pretreatment of animals with the irreversible ODC inhibitor difluoromethylornithine prevented the induction of ODC by kainate. Tissue content of the ODC product putrescine showed a marked increase in cerebral cortex ipsilateral to the lesion, increasing sevenfold at 24 h, the maximal concentration reached. After 24 h, the level of putrescine decreased but remained significantly elevated above control values for 5 days. Levels of the poly-amines spermidine and spermine were unaffected by lesioning. Increases in ODC activity of much smaller magnitude were also seen in brain regions not directly innervated from the ipsilateral nucleus basalis. However, the response in ipsilateral cortex was found to be dependent on an intact projection from nucleus basalis to cortex. The induction of ODC was shown to be prevented by treatment of rats with MK-801, a result indicating the involvement of N-Methyl-d-Aspartate (NMDA) receptors. The effect of MK-801 was dose dependent, with the maximal effect being obtained at a dose of 1 mg/kg, and also critically dependent on the time of administration, with its effectiveness being significantly reduced if given as late as 4 h after lesioning. The anticonvulsant barbiturate pentobarbital produced an effect similar to that of MK-801. The potentiation of cortical cholinergic responsiveness (assessed by measuring carbachol-stimulated phosphoinositide turnover in cortical slices) was also prevented by MK-801. Thus, both the induction of ODC and the enhanced postsynaptic responsiveness appear to be mediated by the involvement of NMDA receptors, in view of the efficacy of MK-801.