In this study, we evaluated VMAT2 availability with [11
C]DTBZ and PET in a group of healthy young adults before and after six months of supplementation of a FDA approved formulation of n–3 PUFA (Lovaza, 2 g/day). Despite the fact that the formulation used in this study led to significant elevations in RBC DHA (1.75-fold) and EPA (4.5-fold) levels relative to pre-supplementation values, we failed to detect an effect for it on striatal VMAT2 availability. The mean change in [11
in the striatal subdivisions (range −1 to −4%) after n–3 PUFA supplementation was well within the reported test-retest variability (4 to 7%) for this radioligand 
. This observation in humans is somewhat inconsistent with rodent studies that suggest n–3 PUFA deficient animals relative to controls have 25 to 60% less VMAT2 binding in the ventral striatum 
. An important difference that led to the inability to detect an effect on [11
C]DTBZ binding might be related to the fact that healthy humans were supplemented with n–3 PUFA in this study, as opposed to the rodent studies in which a group of animals were developmentally deprived of n–3 PUFA and compared to controls.
Thus, the possibility of dietary depletion of n–3 PUFA leading to a reduction in striatal VMAT2 availability in humans cannot be excluded based on the six-month supplementation data. Because individuals with diets deficient in n–3 PUFA are likely to have less RBC DHA/EPA, we evaluated whether lower RBC DHA/EPA levels are associated with lower striatal VMAT2 availability in subjects before supplementation. Contrary to this hypothesis, we found no relationship between the RBC DHA/EPA levels and striatal [11
. Taken together these data do not support an effect for n–3 PUFA on striatal VMAT2 in healthy adults.
Two interesting observations are reported in this study. The first is that in this group of young adults superior working memory performance in the 3-back condition prior to supplementation was correlated with higher RBC DHA. This finding is consistent with a previous report in which higher serum DHA was related to superior performance on tests of non verbal reasoning and working memory in a relatively large cohort of middle aged adults 
. Second, there was an improvement in working memory performance in the 3-back condition after six months of n–3 PUFA supplementation. Although, practice-effects cannot be ruled out as the reason for this observation in this cohort, this result is consistent with some clinical trials suggesting that n–3 PUFA (fish oil) supplementation improves cognitive functioning in elderly adults with mild to no cognitive impairment 
. Surprisingly, 3-back performance improvement was significant despite the fact that there was no correlation between changes in AHR and RBC DHA/EPA levels following supplementation with n–3 PUFA. But, when individuals were stratified into two groups based on their pre-supplementation DHA levels (i.e., less than or greater than 3% mol of total fatty acid pool) we found that the mean change in AHR 3-back was 0.29±0.18 in the low DHA group (n
6 subjects) and −0.01±0.14 in the high DHA group (n
5 subjects). This suggests that the individuals with low pre-supplementation DHA levels benefitted the most by the n–3 PUFA. Further investigation in larger samples is needed to understand this relationship.
The fact that working memory performance was enhanced by n–3 PUFA supplementation regardless of an effect on striatal VMAT2 suggests that its potential pro-cognitive effects, are mediated via extrastriatal dopamine or other non-dopaminergic mechanisms such as effects on inflammation, cellular signaling and trafficking etc. Alternatively other mechanisms that govern the release and storage of dopamine such as afferent regulation of dopamine cell activity or dopamine synthesis may play a role. Future studies are needed to investigate the role of n–3 PUFA on dopamine release mechanisms as well as indices of prefrontal cortical dopamine function. The latter studies are especially critical because prefrontal cortical dopamine is linked to working memory performance 
. Since the concentration of dopamine in the prefrontal cortex is 10 to 35-fold lower than in the striatum (dopamine concentration: cortex 0.5 nM vs striatum 5–18 nM) it is likely that a relatively small increase in dopamine following n–3 PUFA supplementation has a greater impact in the cortex and translates to pro-cognitive effects 
. In addition, the likelihood to detect relatively small changes in dopamine concentration is better in the prefrontal cortex than in the striatum because of the low baseline dopamine levels in this region. Future studies with D1
receptor PET radiotracers to evaluate the effects of n–3 PUFA on prefrontal cortical dopamine and its relationship with working memory are necessary to address these issues.
The current investigation was designed as a proof of concept study to clarify whether n–3 PUFA supplementation leads to increased VMAT2 availability in the human striatum. This question arose based on a recent PET imaging study in which we showed that cocaine addicts have lower vesicular monoamine transporter type 2 in the striatum relative to healthy controls 
. This reduction in VMAT2, which suggests fewer dopamine storage vesicles in the terminals, is one of the mechanisms that lead to the blunted (or less) dopamine release in the striatum after a psychostimulant challenge in cocaine addicts compared to controls 
. In addition, more recent data links this blunted dopamine release in the striatum to relapse and treatment failure in cocaine addicts 
. Since preclinical studies in rodents signaled that diets deficient in n–3 PUFAs lead to lower striatal VMAT2 density by 25 to 60% and reduce stimulant-induced DA release 
we were interested in evaluating the potential of n–3 PUFA as means to increase VMAT2 availability, enhance DA storage and release, and prevent relapse in cocaine addicts. The result of this human imaging study suggests that n–3 PUFA supplementation is unlikely to enhance striatal DA transmission in cocaine addicts and promote abstinence.
In summary, we found no effect for n–3 PUFA supplementation on striatal VMAT2 availability in healthy humans using [11C]DTBZ and PET. Higher RBC DHA levels were associated with better working memory performance in this cohort of young adults, which is consistent with that previously shown in middle-aged adults. Also, n–3 PUFA supplementation improved working memory performance, which is consistent with some but not all clinical trials that have evaluated the pro-cognitive effects of n–3 PUFA in humans. Further research is warranted to elucidate the mechanisms by which n–3 PUFA enhances cognitive performance in healthy individuals.