Influence of motor activity on striatal dopamine release: A study using iodobenzamide and SPECT

Pharmacologically induced dopamine release can influence the postsynaptic r eceptor binding of dopaminergic radioligands. This effect has recently been described using in vivo imaging methods and has been attributed to competi tion of radiotracers with the endogenous ligand. The present study examines the effect of a motor activation task on dopamine release and the conseque nces of this release on the binding of the selective D-2 receptor ligand I- 123-iodobenzamide (IBZM) to striatal dopamine D-2 receptors. Eight subjects were asked to write a text beginning immediately before IBZM injection and continuing for 30 min thereafter. Eighteen other subjects remained in a su pine resting state during this period and served as a control group. All su bjects were right handed. We hypothesized that the writing task would lead to an increase of dopamine release. The increased competition of the endoge nous ligand with IBZM should lead to a decreased postsynaptic IBZM binding in the experimental group. Images were acquired and reconstructed identical ly and anatomically normalized to a computerized brain atlas. Regions of in terest were drawn covering the striatum and three different reference regio ns. Ratios of striatal-to-reference-tissue radioactivity accumulation were calculated as semi-quantitative estimates of D-2 receptor binding potential . This decreased bilaterally, although right-sided significantly more than left, regardless of the choice of reference region. These data show that wr iting with the right hand compared to a supine resting state leads to a dec rease of striatal IBZM accumulation. According to our primary hypothesis th is reflects dopamine release. (C) 1999 Academic Press.