A new study has revealed why anti-depressants often cause emotional blunting.
A team led by researchers from the University of Cambridge (UK) has identified the behavioral process that’s affected in individuals taking anti-depressants, which could be contributing to the common side effect known as emotional blunting. They demonstrated that reinforcement learning, the process that allows reward to inform future actions, was disrupted by anti-depressants, contributing to emotional blunting.
It was estimated that 8.3 million people were prescribed anti-depressant drugs in England in 2022. One of the most common classes of anti-depressants is selective serotonin reuptake inhibitors (SSRIs), which target serotonin. However, SSRIs are closely linked to a side effect called emotional blunting, a byproduct of the medication that causes patients to feel emotionally dull. Approximately 50% of people taking SSRIs are thought to have experienced emotional blunting.
Previous studies have investigated the effects of SSRI medication over a short time period; however, these drugs are commonly prescribed for longer durations. The current research team wanted to dig deeper to understand how the medication, commonly administered for chronic depression, affects one’s emotions over several weeks and why.
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The team gathered 66 healthy volunteers and administered escitalopram, one of the best-tolerated SSRIs, to 32 individuals and placebos to the remaining 34. Both groups took their prescribed pills for at least 21 days, completing a number of self-reporting questionnaires and tests assessing cognitive functions.
They found that there was no significant difference between the groups in terms of attention, memory or emotion. However, they did find that individuals who had taken the SSRI had lower sensitivity to reinforcement learning, also referred to as reward learning. When faced with a ‘probabilistic reversal test’, a learning task that requires participants to choose one of two stimuli that have different probabilities of the participant receiving a reward, the SSRI group was less likely to use the test’s feedback to guide their learning of the task than the placebo group. Halfway through the task, the probability of receiving a reward for each stimulus changed, requiring the participants to learn new rules. Those in the SSRI group struggled to do this.
The significant difference between these two groups in the probabilistic reversal test demonstrates that the drug may affect the sensitivity of individuals to reward and their ability to respond accordingly. This inability to recognize reward, and therefore seek it out, could explain why pleasure is dampened for those taking SSRIs.
Additionally, these findings may shed light on why many individuals in the SSRI group had a harder time reaching orgasm during sex, a common complaint of individuals taking SSRIs.
Co-first author Christelle Langley concluded, “our findings provide important evidence for the role of serotonin in reinforcement learning. We are following this work up with a study examining neuroimaging data to understand how escitalopram affects the brain during reward learning.”
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