Emotional Blunting Among Antidepressant Users

Scientists have worked out why typical antidepressants cause approximately half of the users to feel emotionally 'blunted'. In a study published today, they reveal that medications alter reinforcement learning, a crucial behavioral process that allows us to learn from our surroundings (1^✔ ✔Trusted Source
Chronic escitalopram in healthy volunteers has specific effects on reinforcement sensitivity: a double-blind, placebo-controlled semi-randomised study

Go to source). In 2021–2022, more than 8.3 million patients in England received an antidepressant prescription, according to the NHS. Selective serotonin reuptake inhibitors (SSRIs) are a popular type of antidepressant, particularly for chronic or severe depression. These medications target serotonin, a neurotransmitter that transports messages between nerve cells in the brain and has been termed the 'pleasure chemical'.

One of the most commonly reported negative effects of SSRIs is 'blunting,' in which patients report feeling emotionally dull and finding things less pleasant than before. This side effect is thought to affect 40–60% of patients using SSRIs.

Most SSRI studies to date have only looked at their short-term usage; however, for clinical use in depression, these medications are taken consistently for a longer period. A team led by researchers at the University of Cambridge, in collaboration with the University of Copenhagen, attempted to address this by recruiting healthy volunteers and administering escitalopram. It is an SSRI that is known to be one of the best tolerated over several weeks of assessing the drug's impact on their performance on a suite of cognitive tests.

The experiment included 66 volunteers, 32 of whom were given escitalopram, and the remaining 34 were given a placebo. Volunteers completed a thorough set of self-report questionnaires and were given a series of tests to examine cognitive skills such as learning, inhibition, executive function, reinforcement behavior, and decision-making for at least 21 days.

Cold Cognition

When it comes to 'cold' cognition, such as attention and memory, the researchers discovered no significant group differences. Most tests of 'hot' cognition—cognitive functions that engage our emotions—revealed no changes.

The key novel discovery, however, was that the escitalopram group had lower reinforcement sensitivity on two tasks when compared to the placebo group. Reinforcement learning is the process through which we learn via feedback from our actions and surroundings.

Probabilistic Reversal Test

The researchers employed a 'probabilistic reversal test' to determine reinforcement sensitivity. A subject in this activity would normally be offered two stimuli, A and B. If they chose A, they would receive a prize four times out of five; if they chose B, they would only receive a reward once out of five. Volunteers would not be taught this rule but would have to figure it out for themselves, and at some point during the experiment, the odds would change and participants would have to figure it out themselves.

When it comes to 'cold' cognition, such as attention and memory, the researchers discovered no significant group differences. Most tests of 'hot' cognition—cognitive functions including our emotions—revealed no changes.

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The key novel discovery, however, was that the escitalopram group had lower reinforcement sensitivity on two tasks when compared to the placebo group. Reinforcement learning is the process through which we learn by receiving feedback from our actions and surroundings.

To measure reinforcement sensitivity, the researchers used a 'probabilistic reversal test'. In this exercise, a subject would generally be presented with two stimuli, A and B. They would receive a prize four times out of five if they chose A; if they chose B, they would only receive a reward once out of five. Volunteers would not be taught this rule but would have to find it out for themselves, and the odds would alter at some point throughout the experiment, forcing volunteers to figure it out for themselves.

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The researchers discovered that escitalopram users were less likely than placebo users to use positive and negative feedback to influence their task learning. This implies that the medicine altered their sensitivity to incentives and their capacity to respond appropriately.

The discovery may also explain the one difference the researchers saw in the self-reported questionnaires: participants using escitalopram had more difficulty reaching orgasm when having sex, a common side effect described by patients.

Professor Barbara Sahakian, a senior author from the Department of Psychiatry at the University of Cambridge and a Fellow at Clare Hall, said: "Emotional blunting is a common side effect of SSRI antidepressants. In a way, this may be in part how they work—they take away some of the emotional pain that people who experience depression feel, but, unfortunately, it seems that they also take away some of the enjoyment. From our study, we can now see that this is because they become less sensitive to rewards, which provide important feedback.

Dr. Christelle Langley, also a joint first author from the Department of Psychiatry, added: "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."

Reference

1. Chronic escitalopram in healthy volunteers has specific effects on reinforcement sensitivity: a double-blind, placebo-controlled semi-randomised study - (https://www.nature.com/articles/s41386-022-01523-x)

Source-Medindia