The effects of apolipoprotein E deficiency on brain cholinergic neurons

Previous studies utilizing apolipoprotein E (apoE)-deficient mice revealed distinct decreases in the levels of cholinergic synaptic markers of projecting basal forebrain cholinergic neurons and no such alterations in other brain cholinergic systems. In order to investigate the mechanisms underlying these neuron-specific cholinergic effects, primary neuronal cultures from apoE-deficient and control mice were prepared and characterized. These include basal forebrain cultures, which are enriched in projecting cholinergic neurons, and cortical cultures, which contain cholinergic interneurons. The levels of cholinergic nerve terminals in these cultures were assessed by ligand binding measurements of the levels of the vesicular acetylcholine transporter (VAChT). This revealed that basal forebrain cultures of apoE-deficient mice contain markedly lower VAChT levels (~50%) than do control cultures, but that VAChT levels of the corresponding cortical cultures of the apoE-deficient and control mice were the same. Time course studies revealed that VAChT levels of the basal forebrain cultures increased with culture age, but that the relative reduction in VAChT levels of the apoE-deficient cholinergic neurons was unaltered and was the same for freshly prepared and for 96 h old cultures. These in vitro observations are in accordance with the in vivo findings and suggest that projecting basal forebrain cholinergic neurons, but not cholinergic interneurons, are markedly dependent on apoE and that similar mechanisms mediate the in vivo and in vitro effects of apoE deficiency on cholinergic function.