Monetary discounting and ventral striatal dopamine receptor availability in nontreatment-seeking alcoholics and social drinkers

Abstract

Rationale: Dopamine (DA) in the ventral striatum (VST) has long been implicated in addiction pathologies, yet its role in temporal decision-making is not well-understood.

Objectives: To determine if VST DA D2 receptor availability corresponds with greater impulsive choice in both nontreatment-seeking alcoholics (NTS) and social drinkers (SD).

Methods: NTS subjects (n = 10) and SD (n = 13) received PET scans at baseline with the D2/D3 radioligand [(11)C]raclopride (RAC). Outside the scanner, subjects performed a delay discounting procedure with monetary rewards. RAC binding potential (BPND) was estimated voxelwise, and correlations were performed to test for relationships between VST BPND and delay discounting performance. Self-reported impulsivity was also tested for correlations with BPND.

Results: Across all subjects, greater impulsive choice for $20 correlated with lower BPND in the right VST. NTS showed greater impulsive choice than SD and were more impulsive by self-report. Across all subjects, the capacity of larger rewards to reduce impulsive choice (the magnitude effect) correlated negatively (p = 0.028) with problematic alcohol use (AUDIT) scores. Self-reported impulsivity did not correlate with BPND in VST.

Conclusions: Preference for immediate reinforcement may reflect greater endogenous striatal DA or lower D2 number, or both. Alcoholic status did not mediate significant effects on VST BPND, suggesting minimal effects from alcohol exposure. The apparent lack of BPND correlation with self-reported impulsivity highlights the need for objective behavioral assays in the study of the neurochemical substrates of behavior. Finally, our results suggest that the magnitude effect may be more sensitive to alcohol-induced problems than single discounting measures.

Fig. 1

Delay discounting. (a) Social drinkers (n = 13, open circles) and nontreatment-seeking alcoholics (NTS; n = 10, filled triangles) discounted $20 or (b) $60 across a range of delays (2 day delay unlabeled). Group × Delay interaction demonstrated NTS’ preference for immediate monetary reward. Subjective value = mean indifference points ± SEM by Group.

Fig. 2

Magnitude effects. Shading illustrates the difference in area under the curve between amounts. (a) Magnitude effects were detected in social drinkers (n = 13). (b) No magnitude effects were detected in non treatment seeking alcoholics (NTS; n = 10). y-axis: subjective value is shown as a percentage of the larger delayed amount. Delays are as Fig. 1.

Fig. 3

Histogram (a) Alcohol Use Disorders Identification Test (AUDIT) score distribution in all subjects (n = 23) spans virtually the entire range (0-40 possible). Correlation (b) The magnitude effect (illustrated in Fig. 2) negatively correlates with AUDIT; i.e. larger differences in discounting by amount corresponds with less alcohol-related problems.

Fig. 4

(a) [¹¹C]raclopride baseline scan. Right ventral striatal (R VST) DA D₂ binding potential (BP_ND) positively correlates with area under the curve (AUC) in choice behavior for $20 across delays in all subjects with scans (n = 21; p_FWE < 0.05 corrected for R VST anatomical volume). The boundaries of the striatal mask are shown in green. The color bar indicates the voxel-wise t statistic; display threshold at p < 0.01, uncorrected. (b) The nature of the correlation is illustrated by mean BP_ND extracted values (threshold p < 0.01), indicating that increased binding potential corresponds with increased preference for delayed monetary rewards. NTS (n = 10, closed triangles), Social Drinkers (n = 11, open circles).