The toxicity of tumor necrosis factor-alpha upon cholinergic neurons within the nucleus basalis and the role of norepinephrine in the regulation of inflammation: implications for Alzheimer's disease.

Inflammation and reduced forebrain norepinephrine are features of Alzheimer's disease that may interact to contribute to the degeneration of specific neural systems. We reproduced these conditions within the basal forebrain cholinergic system, a region that is vulnerable to degeneration in Alzheimer's disease. Tumor necrosis factor-alpha was infused into the basal forebrain of young mice pretreated with a norepinephrine neuronal toxin, N-(2-chloroethyl)-N-ethyl-2 bromobenzylamine (DSP4), with the expectation that the loss of noradrenergic input would enhance the loss of cholinergic neurons. The results indicate that chronic infusion of tumor necrosis factor-alpha alone significantly decreased cortical choline acetyltransferase activity and increased the number of activated microglia and astrocytes within the basal forebrain. The loss of forebrain norepinephrine following systemic treatment with DSP4 did not alter the level of cortical choline acetyltransferase activity or activate microglia but significantly activated astrocytes within the basal forebrain. Infusion of tumor necrosis factor-alpha into DSP4-pretreated mice also reduced cortical choline acetyltransferase activity on the side of the infusion; however, the decline was not significantly greater than that produced by the infusion of tumor necrosis factor-alpha alone. The neurodegeneration seen may be indirect since a double-immunofluorescence investigation did not find evidence for the co-existence of tumor necrosis factor-alpha type I receptors on choline acetyltransferase-positive cells in the basal forebrain. The results suggest that noradrenergic cell loss in Alzheimer's disease does not augment the consequences of the chronic neuroinflammation and does not enhance neurodegeneration of forebrain cholinergic neurons.