Membrane cholesterol removal and replenishment affect rat and monkey brain monoamine transporters

The dopamine transporter (DAT) is abundantly expressed in the striatum where it removes extracellular dopamine into the cytosol of presynaptic nerve terminals. It is the target of drugs of abuse and antidepressants. There is a loss of the DAT in Parkinson's disease affecting release of levodopa implicated in levodopa-induced dyskinesias. This study investigated the effect of cholesterol on DAT, serotonin transporter (SERT) and vesicular monoamine transporter 2 (VMAT2) in monkey and rat brains in vitro. DAT protein levels measured by Western blot remained unchanged with in vitro methyl-β-cyclodextrin (MCD) incubations to remove membrane cholesterol or with incubations to increase membrane cholesterol content. By contrast, striatal DAT specific binding labelled with [¹²⁵I]RTI-121 or with [¹²⁵I]RTI-55 decreased with increasing concentrations of MCD and increased with cholesterol loading. Moreover, [¹²⁵I]RTI-121 specific binding of striatal membranes depleted of cholesterol with MCD was restored to initial DAT content with addition of cholesterol showing its rapid and reversible effect. By contrast, striatal VMAT2 and SERT specific binding showed no or limited changes by cholesterol manipulations. Similar results were obtained for monkey caudate nucleus, putamen and nucleus accumbens. Membrane microviscosity was assessed by fluorescence polarization spectroscopy, using the probe 1,6-diphenyl-1,3,5-hexatriene. DAT changes positively correlated with changes of membrane microviscosity in rat and monkey brain regions investigated and with membrane cholesterol contents. Similar findings were observed with desmosterol but to a lower extent than with cholesterol. These results show an important effect of cholesterol on the DAT associated with microviscosity changes that should be considered in drug therapies.