Combined Antidepressant and Behavioural Intervention

Major Depressive Disorder (MDD) is a common psychiatric condition characterised by pervasive low mood as well as changes in sleep, appetite and cognition. In the UK, it is estimated that 3 in 100 people meet criteria for MDD every week, costing the economy £20.2-23.8 billion every year in health service, lost earnings and lower productivity.

A variety of pharmacological and psychological interventions for MDD are available. Selective serotonin reuptake inhibitors (SSRIs) are the first-choice medication for depression in the UK, as recommended by National Institute of Health and Care Excellence (NICE) guidelines. Whilst SSRIs have been demonstrated to be more effective than placebo, only about 50% of patients respond to their first-prescribed antidepressant. Understanding the mechanism through which SSRIs exert their antidepressant effect could explain why their effectiveness is not universal and allow for personalised treatments.

Experimental medicine studies conducted over the last 15-20 years have shown that SSRIs have early effects on emotional cognition, and that these precede clinically significant changes in depressive symptomatology. For example, one week of treatment with citalopram was found to decrease recognition of negative facial expressions and increase the relative recall of positive vs negative words. The switch from implicit changes in cognitive bias to explicit changes in mood is thought to occur through interaction with the environment and re-learning of positive associations. Indeed, rodent studies suggest that SSRIs only induce a positive bias in memory when administered before learning and that SSRI-induced neuroplasticity only occurs in the presence of an positive/enriched environment.

These findings imply that an enriched environment from which patients can derive positive and meaningful reinforcement may be important for antidepressant function. This hypothesis is indirectly supported by multiple reports of a positive correlation between antidepressant treatment outcome and patients' socioeconomic status as a proxy for environmental enrichment, but to our knowledge this has not been experimentally tested in humans.

This experimental medicine study will explore whether the early cognitive effects of the SSRI citalopram are modulated by different levels of environmental enrichment. The presence/absence of environmental enrichment will be modulated through provision of two weeks of behavioural activation (BA) training to a third of our sample; BA is a psychological intervention aimed to increase levels of environmental reinforcement by monitoring and adjusting an individual's daily activities. By comparing the combined effects of citalopram and BA to those of citalopram alone and placebo alone on performance of emotional cognition tasks (Emotional Test Battery (ETB)), the investigators will test whether the early cognitive changes induced by antidepressants are increased in the context of a more rewarding environment. The investigators hypothesise that the participant group receiving citalopram and BA will show greater changes in emotional cognition in the expected direction (decrease in negative bias/increase in positive bias) compared to the citalopram alone and placebo alone groups after the two-week intervention.