The influence of cholesterol on synaptic fluidity, dopamine D1 binding and dopamine-stimulated adenylate cyclase

The present study examined the influence of the synaptic cholesterol/phospholipid ratio on fluorescence polarization, the binding of SCH23390 to dopaminergic D1 binding sites and dopamine-stimulated adenylate cyclase activity. The cholesterol/phospholipid ratio of synaptic membranes from bovine caudate was modified by incubating the membranes with a lipid transfer protein and liposomes which were either loaded with or lacking cholesterol. The results of this study demonstrated that the number of binding sites (Bmax) for SCH23390 was insensitive to alterations in the synaptic cholesterol/phospholipid ratio and membrane order. However, when the cholesterol/phospholipid ratio was decreased by 30%, membrane order and binding affinity (Kd) were decreased. Despite the lack of change in the number of D1 binding sites, the activity of dopamine-stimulated adenylate cyclase was markedly inhibited by an elevated cholesterol/phospholipid ratio. The results of these studies are discussed in terms of their potential relevance to aging.