Dopamine depletion increases in vivo [11C]DTBZ binding in awake rat brain

Influence of a low dose of amphetamine on vesicular monoamine transporter binding: a PET (+)[11C]DTBZ study in humans

We previously reported increased binding of (+)[11C]DTBZ (dihydrotetrabenazine), the vesicular monoamine transporter (VMAT2) positron emission tomography (PET) radioligand, in striatum of some methamphetamine users. This finding might be explained by stimulant-induced vesicular DA depletion resulting in decreased DA (+)[11C]DTBZ competition at VMAT2. In a prospective PET study, we now find that administration of an acute oral dose of amphetamine (0.4 mg/kg) to humans does not cause increased striatal (+)[11C]DTBZ binding but a slight 5% decrease. Our data suggest that a low amphetamine dose is unlikely to cause sufficient DA depletion to detect increased (+)[11C]DTBZ binding and that a higher dose might be required.

In vivo [11C]dihydrotetrabenazine binding in rat striatum: sensitivity to dopamine concentrations

INTRODUCTION

The sensitivity of the in vivo binding of [(11)C]dihydrotetrabenazine ([(11)C]DTBZ) and [(11)C]methylphenidate ([(11)C]MPH) to their respective targets - vesicular monoamine transporter type 2 (VMAT2) and neuronal membrane dopamine transporter - after alterations in endogenous levels of dopamine was examined in the rat brain.

METHODS

In vivo binding of [(11)C]DTBZ and [(11)C]MPH was determined using a bolus+infusion protocol. The in vitro number of VMAT2 binding sites was determined by autoradiography.

RESULTS

Repeated dosing with alpha-methyl-p-tyrosine (AMPT) at doses that significantly (-75%) depleted brain tissue dopamine levels resulted in increased (+36%) in vivo [(11)C]DTBZ binding to VMAT2 in the striatum. The increase in binding could be completely reversed via treatment with L-DOPA/benserazide to restore dopamine levels. There were no changes in the total number of VMAT2 binding sites, as measured using in vitro autoradiography. No changes were observed for in vivo [(11)C]MPH binding to the dopamine transporter in the striatum following AMPT pretreatment.

CONCLUSION

These results indicate that large reductions in dopamine concentrations in the rat brain can produce modest but significant changes in the binding of radioligands to VMAT2, which can be reversed by replenishment of dopamine using exogenous L-DOPA.