Results
Plasma concentration after intravenous MP challenge
Plasma levels of MP were undetectable before the injection of intravenous MP both in the treatment-naive and long-term treatment conditions. After intravenous MP injection, there were no differences in plasma MP concentrations (nanograms per milliliters) between the treatment-naive and long-term treatment conditions for any of the different time points: 15 min (132 ± 34 vs 134 ± 32, respectively), 30 min (96 ± 17 vs 93 ± 26), 45 min (69 ± 19 vs 74 ± 23), or 60 min (55 ± 14 vs 56 ± 13).
DA D2 receptor availability with placebo
The ROI analysis on the measures of D2/D3 receptor availability (BPND of [11C]raclopride) obtained with placebo (without intravenous MP) revealed higher D2/D3 receptor availability for the measures obtained before treatment initiation (treatment-naive) than those obtained after long-term treatment with oral MP, both for caudate (p < 0.005) and putamen (p < 0.006) but not for VS (Table 2). The SPM analysis showed the same trend (p < 0.05 uncorrected), although the effect was not significant when corrected for multiple comparisons with familywise error (pFWE-CORR > 0.05).
Table 2.
Measures of K1 and of D2/D3 receptor availability (BPND) in striatal regions and prefrontal cortex obtained with the ROI analysis for the treatment-naive and long-term treatment conditions
Effects of intravenous MP on K1 and D2/D3 receptor availability (BPND) with intravenous MP
Intravenous MP did not change K1 (transport constant of radiotracer from plasma to tissue) in striatum for neither the naive nor long-term treatment condition (Table 2). Conversely, intravenous MP significantly reduced D2/D3 receptor availability in striatal regions (caudate, putamen, VS) (Fig. 1) (Table 2). Comparison between conditions revealed that the reductions in BPND with intravenous MP were significantly higher for the treatment-naive than for the long-term treatment condition in putamen (26 ± 7 vs 18 ± 10%, p < 0.01, respectively) and showed a trend in VS (18 ± 15 vs 9 ± 15%, p < 0.07); in caudate, the difference was not statistically significant (10 ± 8 vs 5 ± 10%) (Table 2).
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Figure 1.
Averaged DA D2/D3 receptor availability (BPND) images for the [11C]raclopride scans done after intravenous placebo and after intravenous MP (iv-MP) for the treatment-naive and long-term treatment conditions. Intravenous MP reduced DA D2/D3 receptor availability in striatum in both conditions, reflecting the DA increases elicited by the drug.
The SPM analysis corroborated a significant decrease (pFWE-CORR < 0.05) in striatal D2/D3 receptor availability with intravenous MP for both the treatment-naive and long-term treatment conditions (Fig. 2, Table 3) and corroborated the significantly larger decreases in D2/D3 receptor availability elicited by the intravenous MP challenge before treatment (treatment-naive) than after long-term treatment in putamen and VS (Fig. 2, Table 3).
View larger version: Figure 2.
SPM results for the effects of intravenous MP on DA D2/D3 receptor availability (BPND) for the measures made for the treatment-naive and long-term treatment conditions and SPM results for the comparisons between treatment-naive and long-term treatment. MP reduced DA D2/D3 receptor availability in striatal regions and also in frontal and temporal cortices. The reductions in DA D2/D3 receptor availability induced by intravenous MP were significantly greater for the treatment-naive than for the long-term treatment condition in striatal regions but did not differ in cortical regions.
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Table 3.
SPM results showing the brain regions in which intravenous MP increased DA (decreased D2/D3 receptor availability) combined for treatment-naive and long-term treatments and SPM results showing the brain regions in which the DA increases induced by intravenous MP were larger for the naive than for the long-term treatment (Naive vs chronic)
In addition, the SPM analysis identified several extrastriatal regions in which the intravenous MP challenge significantly decreased D2/D3 receptor availability, including frontal and temporal cortical regions (Fig. 2, Table 3). The decreases in D2/D3 receptor availability in these cortical regions did not differ between the treatment-naive and long-term treatment conditions.
After the SPM findings, we extracted ROIs in prefrontal cortex and assessed the effects of intravenous MP on K1 and in D2/D3 receptor availability and showed that, although intravenous MP did not affect K1, it significantly decreased D2/D3 receptor availability (Table 2).
None of these findings was affected when we used as covariates the scores on the Hamilton's Depression and Anxiety scales.
Correlations between brain DA measures and clinical responses to MP
The prospective clinical treatment with MP was associated with a significant decrease in average ratings of ADHD symptoms. All scales of the CAARS were significantly higher before treatment initiation than after 1 year of treatment with the titrated regimen or oral MP (Table 1).
The ROI analysis revealed that the correlations between improvement in clinical symptoms with oral MP treatment (before treatment initiation vs after long-term treatment) and the DA increases elicited by intravenous MP (quantified as decreases in D2/D3 receptor availability) were significant for the measures obtained after long-term treatment but not with those obtained before treatment initiation (treatment-naive). Specifically, these correlations were significant for MP-induced DA increases in VS (but not for caudate or putamen) for the ratings of inattention (CAARS A, r = 0.70, p < 0.001) and showed a trend for ratings of hyperactivity (CAARS B, r = 0.50, p < 0.03) and total symptoms (CAARS G, r = 0.54, p < 0.02). Subjects with the larger DA increases in VS with intravenous MP had the greatest reduction in symptoms with long-term MP treatment.
The SPM results for the voxelwise analysis corroborated the significant correlation between DA increases elicited by intravenous MP in VS and the reduction in symptoms of inattention (CAARS A) after long-term treatment and also identified a significant correlation with DA increases in other striatal regions (caudate and putamen) and prefrontal (BA 8, BA 9, BA 24) and temporal (BA 22) cortices (Fig. 3, Table 4). However, the correlations with hyperactivity (CAARS B) or total symptoms (CAARS G) did not reach significance. View larger version: Figure 3. SPM results for the voxelwise correlation between intravenous MP-induced changes in DA D2/D3 receptor availability (BPND) (measures taken after long-term MP treatment) and the changes in scores of symptoms of inattention (CAARS A; Δtreatment-naive > long-term treatment) shown in an axial and a sagittal plane at the levels where the VS (A) and the prefrontal cortex (B) are located along with the respective regression slopes for the ROIs identified from SPM.