We did not find evidence for beneficial effects of L-dopa supplementation on cognitive performance and learning in healthy older adults. On the contrary, subjects receiving L-dopa improved less on visuospatial fluid intelligence, a primary outcome of the training intervention, after four weeks of working memory training compared to those who received placebo treatment. Subjects receiving L-dopa also progressed worse during training when compared to placebo subjects. The observed between-group differences in visuospatial fluid intelligence were still statistically significant 6 months after the intervention. The groups also demonstrated opposite direction of structural changes in a midbrain region, overlapping with the template location of substantia nigra, which is a key regulatory region rich in dopamine neurons involved in learning and plasticity30,31. Specifically, the control group exhibited increases of grey matter volume in this region whereas the group that received L-dopa showed reductions.

Our study rationale was based on animal studies demonstrating effects of L-dopa on electrophysiological markers of synaptic plasticity12, human studies showing effects of the drug on (nigrostriatal) dopamine release8,20, and especially on a few previous studies showing positive effects of L-dopa on learning in younger adults7,21. Our negative results are strikingly different from these previous studies. Although post-hoc and speculative, we interpret our results to suggest that the exogenous administration of the dopamine precursor may have perturbed a balanced dopaminergic system that as previous studies show, is to be involved in training-related effects on performance17. This was also supported by negative correlation between training-related improvements in performance and plasma concentrations of the drug observed in the active group. The differential volumetric changes further support this interpretation. Thus, whilst our results are in line with basic science that clearly implicates dopamine in learning and plasticity10,12,13, they indicate that exogenous administration the dopamine precursor is rather negative than beneficial for the healthy aging brain. Here we however hasten to clarify that exact interpretations of the study remain speculative and limited because we have no direct measure of dopaminergic neurotransmission. That is, we can link our results to L-dopa supplementation, but the biological pathways of the observed effects remain unknown, and it is possible that the findings are mediated by effects on other neurotransmitters or on glia cells.

In this context, it is also worth mentioning that the vast majority, if not all, of the studies showing plasticity- and learning-enhancing properties of L-dopa and its impact on dopamine levels consistently report larger effects in cases when dopaminergic structures are either lesioned or degenerated8,22,32. This may also point to the possibility that the negative effects observed in our study of healthy adults could be mediated by biological effects independent of dopaminergic neurotransmission. On the other hand, with this background, it is difficult to argue that the fact that we examined older adults explains why our results are in conflict with the previous ones showing positive effects of L-dopa on younger adults’ learning7,21. That is, if anything, one would expect even more positive effects of L-dopa supplementation on learning in older populations compared to the ones reported in healthy younger individuals, since age-related degeneration of dopaminergic circuits is a very well established observation16,17,33. An explanation for the mixed results could instead be that L-dopa may exhibit differential effects on learning in different cognitive domains. In the previous two longitudinal studies of repeated L-dopa administration the positive effects were demonstrated on world learning, which rather pertains to episodic memory domain, whereas the main domains studied in the present study was working memory and executive functions. This interpretation may also be in line with the findings from a recent study showing detrimental effects of L-dopa on reward reversal learning in younger and older healthy adults, which were of the same magnitude irrespective of initial cognitive performance and expected baseline dopamine levels34.

Further studies are needed to clarify under what circumstances and for which cognitive domains pro-dopaminergic pharmacological interventions may have differential effects, especially in the light of recent findings showing clear regional heterogeneity of age-related decline in dopamine receptor availability35. The exact biological mechanisms behind these effects also need to be traced down. We here speculate that the negative response to pro-dopaminergic medication observed in the present and some past studies already suggests that cognitive decline in healthy aging is a complex and multifactorial process, a process that appears to result in balanced physiological states, which, whilst being associated with reduced overall self-regulatory capacity36, still preserve mechanisms of homeostatic plasticity37,38,39, which implies that activity within the dopaminergic circuit is regulated by intrinsic feedback loops to maintain optimally balanced levels of the neurotransmitter thereby preventing excessive increases in excitatory activity. The negative effect observed in the present study appears to be the opposite of what is known as “dopamine supersensitivity” typically present in dopamine-deficient conditions40,41,42. Translating this finding further to clinical and cognitive neuroscience, it is worth noting that although the dopamine system is affected in several ways in aging17,23,35,43, acute pro-dopaminergic supplementation appears to perturb a balanced system, with negative behavioural consequences. This is fully in line with the results from positron emission tomography studies showing that despite apparent negative effects of age on dopamine transporters and receptor density in the healthy individuals, its synthesis capacity remains relatively unaffected43. It is also consistent with recent evidence indicating that the balance between dopamine release and receptor density is critical for cognitive performance44. In age-related neurodegenerative diseases, on the other hand, disruption of the homeostasis has reliably been observed and may even be the core pathophysiological abnormality triggering development and deterioration of cognitive functions45. Thus, it appears clear that aging should not be approached as a disease per se, but rather as a physiological process associated with a gradual decline in many interconnected biological and behavioural capacities. Maintenance of the aging organism may therefore be best achieved by early investments in healthy lifestyle and multi-modal interventions affecting it1,2.

Our results warrant more studies about the effects of L-dopa on brain and cognition in clinical populations, which often involve substantially higher dosages and longer time periods. Indeed, even though dopamine replacement therapy has been shown to be successful for counteracting motor impairment associated with Parkinson’s disease, studies that explore its effects on cognition have yielded mixed results22,46. The negative effects of L-dopa supplementation on learning and midbrain structures observed in our study present an urgent need to carefully investigate the longitudinal course of brain changes in de-novo Parkinson’s patients, for which early dopamine replacement therapy is currently a subject of debates47,48.

Some important study limitations need to be addressed. The most important one is the absence of additional control groups not receiving any cognitive training interventions. This limits interpretation of the results, as with only two groups we cannot unambiguously infer that the between-group differences are driven by the interaction of L-dopa with cognitive training and not by direct and prolonged effects of the drug on cognitive abilities or an interaction with re-test effects (which would still entail effects on learning but of a different kind). However, elimination of L-dopa is fast (blood concentrations were expected to be negligible when the subjects were leaving the training facility), and all subjects had minimum 24-hour washout period and were medication-free on pre- and post-testing days. This, however, does not completely rule out a possibility of a cumulative effect on the brain concentrations, which, in turn, may drive the observed detrimental effects on performance. In this context, it is also worth mentioning that L-dopa concentrations were significantly higher in the active group at the last compared to the first intake. We interpret this finding as due to accelerated gut absorption of the drug previously reported in animal studies with repeated L-dopa administration49. Nevertheless, similar to the main results, the placebo group reached higher difficulty levels over the course of training in all tasks. We think that these learning-related effects are the most likely explanation of the performance differences at post test, which may also explain why they were still present at long-term follow-up conducted 6 months after the intervention (at a point when differential L-dopa concentration between the groups are unlikely). Some caution in interpreting the results is also warranted because not all of the cognitive measures showed statistically significant effects. It is however worth noting that the observed direction of effect (i.e. placebo group improving more compared to L-dopa group) was generally consistent across all tasks. There may also be several reasons for why statistical significance is reached only for the primary outcome “spatial reasoning”. For example, the reasoning tasks have higher reliability as compared to the other tests, which, in turn, increases precision when estimating difference in change (i.e. larger signal-to-noise ratio of the measured domain). Another explanation may be that the primary outcomes were always the first to be collected during extensive evaluation weeks of psychometric testing. One such week of testing can be considered as a cognitive training activity in itself and measures collected later in the weeks may therefore show less room for improvements. We of course also selected spatial reasoning as a primary outcome because it is centrally important in the context of this study. The matrix measures of spatial intelligence have a very substantial working memory component26,27,28 and may therefore pick up task-independent training effects on working memory. In turn, a large literature of both human and animal studies links dopamine to several aspects of working memory.

It is also important to acknowledge limitations of the MRI-derived measures of grey matter probability. The differential changes in the midbrain were observed in measures that are derived from T1-weighted images, which are known to be highly sensitive to pharmacological manipulations50 and levels of functional activity51. Thus, we cannot determine whether the observed changes are due to true volumetric changes or whether they are a result of relatively transient changes in for example blood flow52. At the same time, it must be noted that the increases observed for the control group in the present study are consistent with previous reports of changes in the dopaminergic system induced by cognitive training10. In line with this background, we also hypothesise that the observed effects in the L-dopa group may reflect reactive changes in the dopamine system in response to repeated administration of an exogenous precursor of the neurotransmitter. It is, however, possible that these effects on brain structure index other processes than changes in dopaminergic neurons, such as for example effects of the drug on the glia cells in the midbrain. These changes may also be independent of the effect observed on the cognitive measures. Indeed, correlations between these changes were of small magnitudes and not statistically significant in the group receiving L-dopa.

In addition, even though our literature review indicated that the selected dosage was appropriate to induce the effects of interest with minimal side-effects, we cannot completely rule out the possibility that other drug amounts may lead to different results. However, a negative and linear correlation between plasma concentrations of the drug and improvements in visuospatial reasoning observed in the active group, as well as absence of any moderating effects of the body-mass index on the aforementioned improvements make this possibility unlikely. Finally, despite the fact that our study is well-powered and the largest of its kind7,21 an additional caution must be advised for making direct generalisations to broader populations, especially to patient groups discussed above.

We conclude that daily L-dopa supplementation does not enhance cognitive performance and learning during cognitive training in healthy older adults and may in fact have disadvantageous effects. Our findings raise serious concerns about usefulness of novel L-dopa-containing supplements that claim to have neuroprotective and learning-enhancing properties and suggest that caution is needed with regard to early dopamine replacement treatment interventions in neurological disorders, encouraging more rigorous evaluation of their effects on the brain and cognition in populations that often receive the drug in larger doses over long periods.