Individual Differences in Extrastriatal DA Release
Abstract: Both personality and genetic factors are reliably associated with vulnerability for developing psychostimulant drug abuse. Some of this vulnerability is likely expressed as individual differences in the cognitive and subjective response to psychostimulants. PET and SPECT imaging studies of striatal dopamine (DA) binding indicate the presence of significant individual differences in the amount of DA released by psychostimulants. these differences in DA release are associated with the subjective effects of these agents. Studies also suggest that baseline variables, such as basal D2/D3 binding in the striatum, and novelty/sensation seeking personality traits may predict responsiveness to psychostimulants. Unfortunately, this literature has been limited to measurement of striatal DA functioning due to difficulties imaging extrastriatal DA with most available DA receptor ligands. However, substantial evidence indicates that extrastriatal regions are also involved in processes related to drug abuse. We propose to examine the relationship between extrastriatal DA functions, personality and responsiveness to oral amphetamine (d- AMPH) using [18F]Fallypride in 54 healthy human subjects. [18F]Fallypride is a high affinity D2/D3 ligand that labels both striatal and extrastriatal receptors and is sensitive to endogenous DA levels allowing it to index the amount of endogenous DA released by psychostimulants. Preliminary data using [18F]Fallypride PET in healthy subjects indicate that d-AMPH produces significant DA release in the striatum. The amount of DA released in both the striatum and extrastriatal regions was associated with objective psychomotor improvements on d-AMPH. However, significant associations between DA release and subjective effects of d-AMPH localized to extrastriatal regions, especially portions of the cingulate. Consistent with animal models, the data indicate that individuals high on sensation seeking have lower basal D2/D3 binding levels in both the striatum and midbrain DA producing regions (likely reflecting reduced autoreceptor density). The present proposal aims to confirm and extend these findings in a large sample of subjects. We additionally aim to test hypothesis that the catechol-o-methyltransferase val158met polymorphism effects DA release. This will be assessed by specifically recruiting equal numbers of met/met, val/met and val/val subjects. We will additionally provide an initial exploration of whether several additional candidate genes that are related to risk for drug abuse have a measurable effect on either baseline D2/D3 binding and/or DA release. Finally, in order to better understand the regional regulation of DA, we will examine the inter-relations between DA functioning in the midbrain, striatum, thalamus and cortex. Taken together, the study will provide unique information on how individual differences in the organization of the human DA system are associated with personality, genetics and the subjective and cognitive effects of psychostimulants.