Attenuated Tonic and Enhanced Phasic Release of Dopamine in Attention Deficit Hyperactivity Disorder.

It is unclear whether attention deficit hyperactive disorder (ADHD) is a hypodopaminergic or hyperdopaminergic condition. Different sets of data suggest either hyperactive or hypoactive dopamine system. Since indirect methods used in earlier studies have arrived at contradictory conclusions, we directly measured the tonic and phasic release of dopamine in ADHD volunteers. The tonic release in ADHD and healthy control volunteers was measured and compared using dynamic molecular imaging technique. The phasic release during performance of Eriksen's flanker task was measured in the two groups using single scan dynamic molecular imaging technique. In these experiments volunteers were positioned in a positron emission tomography (PET) camera and administered a dopamine receptor ligand (11)C-raclopride intravenously. After the injection PET data were acquired dynamically while volunteers either stayed still (tonic release experiments) or performed the flanker task (phasic release experiments). PET data were analyzed to measure dynamic changes in ligand binding potential (BP) and other receptor kinetic parameters. The analysis revealed that at rest the ligand BP was significantly higher in the right caudate of ADHD volunteers suggesting reduced tonic release. During task performance significantly lower ligand BP was observed in the same area, indicating increased phasic release. In ADHD tonic release of dopamine is attenuated and the phasic release is enhanced in the right caudate. By characterizing the nature of dysregulated dopamine neurotransmission in ADHD, the results explain earlier findings of reduced or increased dopaminergic activity

Increased ligand binding potential (BP) during task performance.

<p>In ADHD volunteers the ligand BP increased significantly in the caudate and putamen bilaterally in Incongruent condition which required inhibition of unwanted responses.</p

The ligand BP in ADHD and healthy control volunteers in Incongruent condition.

<p>ADHD volunteers had lower BP in all striatal areas (suggesting higher amount of dopamine release) but the difference was significant (p = 0.004) only in the right caudate. RC = right caudate, LC = left caudate, RP = right putamen, LP = left putamen.</p

The values of k₂ (the rate constant for the ligand transfer from free to the plasma compartment) were higher during task performance (Incongruent condition) in the ADHD group but the difference was not significant statistically.

<p>The values of k₂ (the rate constant for the ligand transfer from free to the plasma compartment) were higher during task performance (Incongruent condition) in the ADHD group but the difference was not significant statistically.</p

The rate of ligand displacement in the caudate in the Congruent and Incongruent conditions: Pictures show the location of right and left caudate where the rate of ligand displacement in the Incongruent condition was significantly greater than the rate in the Congruent condition, suggesting release of endogenous dopamine during task performance.

<p>The stereotactic coordinates (MNI) of the maxima were: 12, 14, 4 and -12, 14, 4. The curves show changes in the rate of ligand displacement over time during performance of the task under Congruent (control) and Incongruent conditions. The upper curves show the PET count (open circles) and the model fit (solid line). The lower curves (filled circle) depict the PET count in the reference region (cerebellum). Because of the lack of dopamine receptors in the cerebellum, the lower curve represents changes in nonspecific binding. Significant change after initiation of the Incongruent condition (red line) was observed in the striatum but not in the cerebellum. The ligand concentration is shown as x10, 000 mBq.</p

Comparison of the mean ligand binding potentials (BP) estimated at rest in ADHD and healthy control volunteers (ADHD>control).

<p>The BP was significantly greater in ADHD volunteers in the right caudate suggesting reduced tonic release of dopamine in this area.</p

The rate of ligand displacement in the putamen in Congruent and Incongruent condition: Pictures show the location of right and left putamen where the rate of ligand displacement in the Incongruent condition was significantly greater than the rate in the Congruent (control) condition, suggesting endogenous dopamine release during task performance.

<p>The curves represent changes in the ligand concentration in the putamen and the reference region as explained in the legend for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0137326#pone.0137326.g002" target="_blank">Fig 2</a>.</p

MNI coordinates of the maximum values of the ligand BP measured at rest in ADHD and healthy control volunteers in the caudate and putamen.

<p>MNI coordinates of the maximum values of the ligand BP measured at rest in ADHD and healthy control volunteers in the caudate and putamen.</p

MNI coordinates the striatal areas where significant changes in the rate of ligand displacement were observed in the Incongruent condition.

<p>The table also shows the t-values of the difference in displacement rates observed in the Congruent and Incongruent condition. The data suggests that the locations were almost identical in the ADHD and healthy control volunteers (in the putamen and left caudate) but the changes (t-values) were greater in ADHD.</p

Comparison of the ligand BP in ADHD and healthy control volunteers in the Incongruent condition (healthy control >ADHD).

<p>The comparison revealed significantly higher BP in the right caudate of ADHD volunteers. The MNI coordinates of the maxima were, x,y,z = 12,16,6).</p