Death of septal cholinergic neurons produced by chronic exposure to glutamate is prevented by the noncompetitive NMDA receptor/channel antagonist, MK‐801: Role of nerve growth factor and nitric oxide

Abstract

To study the sequence of degenerative events possibly associated with cholinergic cell death in Alzheimer's disease, septal cholinergic neurons derived from rat embryonic brains were exposed to chronic excitotoxic stress by glutamate. Counts of choline acetyltransferase (ChAT)-immunopositive neurons and measurement of ChAT activity revealed that concentrations of glutamate on the order of 70 microM killed 50% of cholinergic neurons after 24 hr of treatment. Neurotoxic effects were not aimed at cholinergic neurons specifically, since other populations of cells present in these cultures were also affected at similar concentrations. The noncompetitive N-methyl-D-aspartate (NMDA) receptor channel antagonist MK-801 (10 microM) abolished acute neuronal swelling and rescued from late degeneration both cholinergic and noncholinergic cells when concentrations of glutamate up to 500 microM were added to the cultures. Protective effects declined progressively with increasing concentrations of the amino acid, even when MK-801 was raised to its highest nontoxic levels, e.g., 50 microM. the kainate/quisqualate receptor antagonist CNQX provided no protection alone or in combination with MK-801. Nerve growth factor (NGF), used in standard culture conditions to stimulate the expression of the cholinergic phenotype, was shown not to influence excitotoxic neurodegenerative changes. Several observations suggested that nitric oxide (NO) may act as an intercellular messenger of NMDA-mediated cell death in septal cultures: 1) Most of the cholinergic neurons contained the NO synthase enzyme as characterized by NADPH-diaphorase (NADPH-d) staining; 2) sodium nitroprusside (SNP) [a chemical with the ability of generating NO] was capable of mimicking some of the aspects of the glutamate-induced degenerative process; 3) the rise in cyclic GMP which was observed in the presence of toxic levels of glutamate and which is usually taken as an index of NO production, was antagonized by MK-801 and by the inhibitor of the NO synthase enzyme, L-NOARG. Yet, the fact that L-NOARG and its congener, L-NAME, were ineffective in preventing glutamate-induced neurodegenerative changes in our culture system did not substantiate our working hypothesis. Altogether these results suggest that glutamate-induced cholinergic neuronal death is the consequence of an overstimulation of NMDA receptors and that neither NGF nor NO plays a key role in the degenerative process.