Modulation of frontostriatal interaction aligns with reduced primary reward processing under serotonergic drugs

Abstract

Recently, functional interactions between anteroventral prefrontal cortex and nucleus accumbens (NAcc) have been shown to relate to behavior counteracting reward-desiring (Diekhof and Gruber, 2010). Downregulation of the reward system by serotonin has also been suggested as the mode of action accounting for unsatisfactory effects of serotonin reuptake inhibitors (SSRIs) such as insufficient alleviation or even increase of anhedonia, and loss of interest. However, understanding of the in vivo mechanisms of SSRI-related alteration of the human reward system is still incomplete. Using functional magnetic resonance imaging (fMRI) within a double-blind cross-over within-subjects study design and administering the SSRI paroxetine, the dopamine/norepinephrine reuptake inhibitor bupropione, and placebo for 7 d each, we investigated a group of 18 healthy male subjects. Under paroxetine, subjects showed significantly decreased activation of the bilateral NAcc during processing of primary rewards (erotic videos), but not under bupropion. Similar to the previous study, analysis of psychophysiological interactions revealed that this downregulation relied on negative interactions between left and right NAcc fMRI signals and the bilateral anteroventral prefrontal cortex that now were significantly enhanced under paroxetine and reduced under bupropion. Individual drug-dependent modulations of interacting brain regions were significantly associated with individual expressions of impulsivity as a personality trait. Our results corroborate and extend previous insights on interregional crosstalk from secondary to primary rewards and demonstrate parallels between active inhibitory control of and serotonergic effects on the dopaminergic reward system's activity.

Figure 1.

A, Task-related fMRI activation (differences in parameter estimates of modeled effects) within the left NAcc ROI for the contrast erotic minus nonerotic video clips with significantly reduced activation under the selective serotonin reuptake inhibitor paroxetine compared with the dopamine/norepinephrine reuptake inhibitor bupropion and placebo. B, ROIs for NAcc, defined according to the Harvard-Oxford cortical and subcortical structural atlases, for the avPFC defined according to Diekhof and Gruber (2010) and for the raphé nuclei defined according to Walter et al. (2007). C, Parameter estimates of negative PPI strengths between left NAcc and right avPFC with significantly greater negative interaction under paroxetine than under placebo and bupropion and less negative interaction under bupropion compared with placebo.

Figure 2.

A, B, Whole-brain results of psychophysiological interactions of right (A) and left (B) NAcc with the bilateral avPFC and periaqueductal gray in the area of the raphé nuclei significant under paroxetine (threshold of p < 0.001, uncorrected, minimum cluster size of 10 continuously significant voxels), illustrating the specificity of the region-of-interest results.

Figure 3.

Correlational results of the differential height of negative PPIs between left NAcc and right avPFC with trait impulsivity, as assessed with the TAS subscale of the sensation seeking scale SSS-V. A, More negative PPIs under paroxetine versus placebo are negatively correlated with TAS scores. B, Less negative PPIs under bupropion versus placebo are positively correlated with TAS scores.