SSRI antidepressants reduce the ability to reinforcement learning

Published: 06-10-2023

Approximately 40-60% of patients taking SSRIs experience the so-called emotional blunting, i.e. they feel both negative and positive emotions less strongly. The latest research indicates that the reason may be a weakening of sensitivity to feedback, including rewards.

SSRIs, i.e. selective serotonin reuptake inhibitors, are a popular group of antidepressants. They increase the concentration of serotonin in the synaptic gap between nerve cells, which causes more intense transmission of nerve signals between neurons that use this neurotransmitter. Drugs from this group include: fluoxetine, citalopram, dapoxetine, escitalopram, fluvoxamine, paroxetine, sertraline.

SSRI antidepressants reduce the feeling of negative emotions in many patients, which is desirable, but at the same time they also often weaken the feeling of positive emotions. Patients become emotionally “blunted” and it has not yet been known why this happens. The latest study conducted by Barbara Sahakian and her colleagues shows what mechanism may be responsible for such a condition.

66 healthy people took part in the study. One group of participants took escitalopram and the other group took a placebo. The substances were taken for at least 21 days, and after that time, each study participant completed a set of questionnaires and took part in various cognitive tests.

Everyone performed similarly in most tests, but there was one interesting difference. It turned out that the group taking an SSRI performed worse on a task requiring reinforcement learning than the group taking a placebo. Reinforcement learning involves learning based on feedback we receive from our actions or from the environment.

In the task testing this ability, participants were presented with two stimuli: A and B, and they had to choose one of them. By choosing A, they received a reward 4 times out of 5, while B gave a reward only 1 time out of 5. This rule was not presented to the participants. They had to figure out which stimulus was better to choose. To make things more difficult, at some point the reward proportions were reversed and participants had to discover a new rule.

The task therefore required the ability to learn from the feedback they received each time they chose A or B (reward or no reward). People who took escitalopram performed worse on this task than those who took a placebo. This suggests that SSRI antidepressants may reduce sensitivity to feedback, including positive information in the form of reward. And this may result in a weaker experience of both negative and positive emotions in everyday life.

Researchers are further exploring this mechanism with a current study using brain neuroimaging. They hope this will help better understand how SSRI antidepressants affect the brain during reward-based learning.