Limitations
Changes in [11C]raclopride binding are related to extracellular DA, but the relationship with synaptic DA is not completely understood (Gjedde et al., 2005). The [11C]raclopride method is best suited for regions with high D2/D3 receptor density, such as striatum, and much less sensitive to cortical regions, which renders our cortical findings as preliminary. ROIs were obtained directly from the [11C]raclopride images and not from MRIs but, because measures of D2/D3 receptor availability obtained using ROIs selected directly from [11C]raclopride images do not differ from those selected from the subject's MRIs (Wang et al., 1996), this is unlikely to have influenced our findings. We show dampening of the DA-enhancing effects of MP after long-term treatment, but we cannot determine whether this is associated with tolerance to clinical efficacy, whether it is long lasting or dissipates promptly. Because of ethical reasons, we did not use a group of untreated ADHD controls to rule out the possibility that the attenuation reflects an age-related decline. To assess clinical efficacy, we did not use a measure of cognitive performance because we were interested in assessing the relationship between enhancement of DA signaling by MP and symptom improvement, which is why we selected the CAARS, an instrument with extensive validity for its use to monitor treatment response in ADHD (Rösler et al., 2010). In our study, we did not obtain measures of motivation, which would have allowed us to test whether DA increases in VS were associated with increased motivation while on clinical treatment. Finally, this study focused on DA, and future studies are necessary to evaluate the noradrenergic effects of MP.
In summary, we show an association between the DA increases elicited by a challenge with intravenous MP in the VS and the improvement in ratings of inattention with long-term oral MP treatment, which provides evidence that the enhancement of DA signaling by MP in VS (a key brain region involved with reward and motivation) is involved in its therapeutic benefits in ADHD. We also give preliminary evidence that MP increases DA in prefrontal and temporal cortices, which may be therapeutically relevant, and that chronic treatment with oral MP may result in adaptations in striatal DA signaling.
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Footnotes
Received August 31, 2011.
Revision received October 14, 2011.
Accepted November 7, 2011.
This research was supported in part by the Intramural Research Program of the National Institutes of Health (National Institute on Alcoholism and Alcohol Abuse), National Institute of Mental Health Grant R01MH66961 perations; Donald Warner, David Alexoff, and Paul Vaska for PET operations; Richard Ferrieri, Colleen Shea, Youwen Xu, Lisa Muench, and Payton King for radiotracer preparation and analysis, Karen Apelskog-Torres for study protocol preparation; Millard Jayne, Joseph English, Allan Chrisman, Barbara Hubbard, and Pauline Carter for patient care; and Ruben Baler and Denise Pintello for editorial assistance. This paper was presented at the Society of Nuclear Medicine in 2011. (G.-J.W.), with infrastructure support from U.S. Department of Energy/Office of Biological and Environmental Research Grant DE-ACO2-76CH00016. We thank the following: David Schlyer and Michael Schueller for cyclotron oCorrespondence should be addressed to Dr. Nora D. Volkow, National Institute on Drug Abuse, 6001 Executive Boulevard, Room 5274, Bethesda, MD 20892. Copyright © 2012 the authors 0270-6474/12/320841-09$15.00/0