Increasing serotonin in the brain appears to increase people’s motivation by reducing the perceived cost of effort, according to research published in eLife.

The study of 58 healthy volunteers found those who were given the antidepressant drug escitalopram, which increases serotonin levels, put in more effort to get a reward. The volunteers in the experiment won more money the longer they squeezed a handgrip.

PsyPost interviewed the study’s corresponding author, Florent Meyniel of the Institut du Cerveau et de la Moelle épinière. Read his explanation of the research below:

PsyPost: Why were you interested in this topic?

Meyniel: Motivation is a key aspect of our daily life: why do we do what we do? I may engage in a particular action because I think the payoff is high and because I think it is easy to get it. This is an interpretation of motivation in terms of cost and benefit. We can think of more sophisticated frameworks, but this simple distinction, cost versus benefit, is already powerful and yet surprisingly under-explored. When motivation is dysfunctional, it is often not clear what the cause is. Depression provides a interesting example. It is a condition in which people disengage from their usual activities. This is an objective description of people’s behavior in depression. In principle, the reason could be that depressed people suffer from a loss of interest: they estimate that the benefit of their action is abnormally low.

Alternatively, they could suffer from an inflation of the perceived cost: the benefit could be unaffected, but the estimated cost to achieve it (e.g. how effortful the action is) may appear abnormally high. This distinction may be all the more important that different types of dysfunction, affecting either the cost or benefit, may appeal for different treatments. One half of the general population undergoes a major depressive episode during their lifetime, so that the cost/benefit distinction is important not only for basic science, but for also for the clinics and society.

What should the average person take away from your study?

We were interested in the role of serotonin in motivated behaviors. Serotonin is a neuromodulator, a particular class of molecules that is released naturally within the brain and impacts information processing by modulating the neural circuits they are released in. Serotonin has been is implicated in the regulation of mood, anxiety, impulsivity, learning. It is targeted by most prescribed medications for major depression, like selective serotonin reuptake inhibitors. Yet, the beneficial effect of treatment based on serotonin on depression is currently not well understood in term of the simple cost/benefit distinction.

We used a task in which physical effort was traded against monetary reward. Since sensitivity to cost and benefit can result in intricate effects on the behavior, we used a mathematical model that disentangles their contributions. Our results suggest that a selective serotonin reuptake inhibitor enhances performance by alleviating effort cost: subjects who received the treatment achieved longer efforts and hence higher payoffs, than those receiving a placebo. Together with previous findings, this suggests that serotonin could be implicated in the processing of cost in general, be it effort in our task, or delays in reward delivery and punishments in previous decision tasks. This general role is compatible with a broad range of effects of serotonin on behavioral activation.

Are there any major caveats? What questions still need to be addressed?

We choose to study the role of a selective serotonin reuptake inhibitor on cost and benefit in healthy people rather than in depressed patients. The reason is that in healthy people, it is easier to understand the effect of the treatment. By contrast in patients, the treatment interacts with a system that is already dysfunctional, which may obscure our understanding of the process. I think this is a strength of our approach. But it is at the same time a limitation that precludes extending our results directly to the clinics. It remains to the tested whether treatment for depression based on serotonin alleviates depression by impacting costs rather than benefits. We also know from previous studies that serotonin regulate behavior by interacting with other neuromodulators, in particular dopamine. It also remains to be tested what the precise contribution of each neuromodulator is.

The study, “A specific role for serotonin in overcoming effort cost“, was also co-authored by Guy M Goodwin, JF William Deakin, Corinna Klinge, Christine MacFadyen, Holly Milligan, Emma Mullings, Mathias Pessiglione and Raphaël Gaillard.