Differential effects of scopolamine on in vivo binding of dopamine transporter and vesicular monoamine transporter radioligands in rat brain.

The in vivo equilibrium specific binding of d-threo-[3H]methylphenidate, a radioligand for the dopamine transporter (DAT), and +-alpha-[3H]dihydrotetrabenazine, a radioligand for the vesicular monoamine transporter (VMAT2), were examined in rat brain with and without prior administration of 5 mg/kg scopolamine. Drug-treated animals exhibited a 30% increase in d-threo-[3H]methylphenidate binding to the DAT in the striatum relative to controls. No changes in specific binding of +-alpha-[3H]dihydrotetrabenazine were observed in any brain region following scopolamine pretreatment. Cholinergic drugs thus differentially affect in vivo specific binding of DAT and VMAT2 radioligands, suggesting this should be a consideration in selection of in vivo markers for imaging studies of dopaminergic terminals in the brain of animals and humans.