A particularly common group of antidepressants are SSRI (selective serotonin reuptake inhibitors). These drugs ensure that the concentration of the neurotransmitter serotonin in the brain increases and influences the mood there. While they are effective against depression, they can also lead to emotional numbing – one of the most commonly reported side effects of SSRIs.
Those affected report that they only perceive feelings dullly, including the positive ones. This side effect occurs in 40 to 60 percent of patients who take SSRIs. A research team from the University of Cambridge, in collaboration with the University of Copenhagen, has now investigated why this happens.
Also frequently prescribed in Austria
The drug belongs to the SSRI group Escitalopram, which is considered to be well tolerated and is also frequently prescribed in Austria. This antidepressant was the focus of the Study, which was published today in the journal Neuropsychopharmacology. The team around the study authors Barbara Sahakian and Christelle Langley from the University of Cambridge’s Department of Psychiatry examined the effects of taking Escitalopram over several weeks on healthy volunteers.
A total of 66 people took part in the study: 32 of them received the antidepressant, while the other 34 received a placebo. During the three-week experiment, subjects completed self-assessment questionnaires. In addition, a series of tests were used to assess their cognitive abilities, i.e. mental abilities that humans need to cope with a wide variety of tasks. These include attention, learning, memory, control and decision-making.
Less attention to feedback
The research team did not find any significant differences between the two test groups, neither in cognitive abilities such as attention and memory, nor in cognitive abilities related to feelings. Only one ability showed a clear difference: learning through reinforcement. This means that people learn through reactions to their behavior. In that group who Escitalopram this sensitivity to environmental feedback was less than in the placebo group.
This was examined using a task in which the subjects were presented with two different incentives: A and B. If they chose A, they received a reward four out of five times. If they chose B, they received a reward only one out of five times. At some point during the experiment, the probabilities were reversed and subjects received a reward only one time in five for stimulus A, but four times for stimulus B.
Indifferent to rewards
Those participants who took the antidepressant did not use either the positive or negative feedback to learn the rules. In complete contrast to the placebo group: Here the rules were quickly internalized – with the aim of receiving more rewards. This suggests, the study says, that the antidepressant impairs sensitivity to rewards and the ability to respond accordingly.
This result could also explain the only difference between the two experimental groups that the team found in the self-reported questionnaires: the subjects who Escitalopram were more difficult to climax during sex – a side effect that patients often report and that is also listed in the drug’s package insert.
“SSRI take part in the joy”
“In a way, perhaps emotional deadening is part of how SSRIs work – they take away some of the emotional pain in people who are depressed, but unfortunately it appears they also take away some of the joy,” the study author said Barbara Sahakian. Rewards are important feedback, and the study shows that patients who take SSRIs are less responsive to them. And that leads to emotional numbness.
In any case, the study provides “important clues about the role of serotonin in learning through reinforcement,” according to study author Christelle Langley. In a further study, the researchers want to use neuroimaging – a process that is used to visualize the brain – to investigate in detail how Escitalopram the brain is influenced during learning by reinforcement from the environment.