PET Imaging of Dopamine D2 Receptors and Extracellular Dopamine With (18F)Fallypride, D-amphetamine, and Alpha-Methyl-Para-Tyrosine in Healthy Subjects

Abnormalities of dopaminergic function have been implicated in a number of neurological and psychiatric illnesses, including Parkinson's disease, schizophrenia, and psychostimulant dependence syndromes. Functional imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) have demonstrated the feasibility of in vivo measurement of the distribution and the density of dopamine (DA) D1 and D2 receptors in humans. Besides simple measurement of receptor density, it has been shown that the competition between endogenous neurotransmitters and radiolabeled tracers might provide a tool to estimate extracellular levels of neurotransmitters. However, most of those studies have been confined to the striatum. In this protocol using a PET tracer (18F)fallypride, we will estimate both stimulant-induced DA release and baseline DA levels in the striatum and extrastriatal regions by comparing baseline scans and those after d-amphetamine or alpha-methyl-para-tryosine (AMPT) adminstration. In addition, to explore genetic factors that determine synaptic DA levels, allelic variations of two genes that regulate DA levels, catechol-O-methyltransferase and dopamine transporter, will be studied.

A recent study showed that oral administration of d-amphetamine induced displacement of (11C) raclopride in a similar way as the commonly used method of i.v. administration. The current protocol will be performed in two steps. First, the method of d-amphetamine administration will be determined by studying effects of oral d-amphetamine on the binding of (18F) fallypride binding. If oral administration effectively displaces the radioligand binding, this method will be applied in the subsequent study of examining effects of each of d-amphetamine and AMPT in individual subjects.

If this study successfully detects the influence of DA levels on (18F)fallypride binding, the same design will be applied to the studies of patients with psychiatric and neurological disorders.