For nearly half of patients, common antidepressants cause feelings of emotional dullness. Researchers explain this by showing that these serotonin reuptake drugs affect reinforcement learning, a behavioral process that allows us to learn from our environment.
To treat major depressive disorder, anxiety or obsessive compulsive disorder, drugs that target the transmission of serotonin – a neurotransmitter involved in several brain functions – remain the first choice of healthcare professionals. Called “selective serotonin reuptake inhibitors” (SSRIs), however, they have side effects such as emotional “dulling,” which can rob patients of positive emotions. Between 40 and 60% of them would experience this side effect.
The modulating role of serotonin
The scientists note that it is particularly important to know what is the chemical origin of this condition and thus to understand the modulating role of serotonin on cognition and reinforcement learning, a process that allows us to learn from our environment. According to previous research, a wide range of cognitive functions are affected by the modulation of serotonin, even in healthy volunteers.
A team led by researchers from the University of Cambridge, in collaboration with the University of Copenhagen, wanted to know the chronic effects attributed to a well-tolerated SSRI (escitalopram) on 32 “healthy” volunteers. Thirty-four other people were given a placebo to compare the effects. The volunteers took the antidepressant (20 mg) or the placebo for an average of 26 days and completed questionnaires. They also took tests that measure ‘cold’ cognition (eg attention, cognitive flexibility and memory) and ‘warm’ cognition, which involves our emotions (eg decision making and learning by reinforcement).
The drug affects sensitivity to rewards after three weeks
Result: the researchers found no other significant difference between the two groups on “cold” or “hot” cognition, except sensitivity to reinforcement. Reinforcement allows us to learn from the feedback of our actions and our environment. Volunteers taking escitalopram used positive and negative feedback significantly less to guide them when learning a task, compared to participants taking placebo. The drug would therefore have affected their sensitivity to rewards and their ability to react accordingly.
« Emotional blunting is a common side effect of SSRI antidepressantssaid Professor Barbara Sahakian, from the Department of Psychiatry at the University of Cambridge. In a way, that may be their mode of action: they dull some of the emotional pain that people with depression feel, but unfortunately it seems they also take away some of the fun. Through our study, we can now see that this is because they become less responsive to rewards, which provide important feedback. »
