The goal of this experimental medicine study is to examine the effect of increasing serotonin on reward processing in healthy volunteers. The main questions it aims to answer are:

1. Does a subacute increase in serotonin influence the activation regions during reward learning
2. Does a subacute increase in serotonin influence behavioural markers of reward valuation (effort task), responsiveness (taste task) and learning (learning task)

Participants will be:

given a 7-day course of the selective serotonin reuptake inhibitor, citalopram. undergo behavioural testing complete a reward learning task whilst undergoing fMRI

Researchers will compare results against a placebo group.

Inclusion Criteria:

• Participant is willing and able to give informed consent for participation in the research
• Aged between 18 to 65 years
• Sufficient knowledge of English language to understand and complete study tasks

Exclusion Criteria:

• Current or past probable diagnosis of psychiatric illness, according to DSM-5 criteria, requiring intervention by a healthcare professional, including but not limited to psychosis, bipolar disorder, major depression, OCD, PTSD, substance abuse disorder or any eating disorder
• Current or past diagnosis of any significant personality disorder (e.g. borderline personality disorder) according to self-report
• Diagnosis of attention deficit hyperactive disorder or autistic spectrum disorder that impairs daily functioning, requires pharmacotherapy or in the opinion of the study medic would affect the scientific integrity of the study
• Current use of medication that might interact with the effects of citalopram or affect the scientific integrity of the study
• Previous suicide attempt or previous prolonged period (e.g. > 5 days) of thoughts to end life
• Known contraindication to citalopram including: past allergic reaction to citalopram or any other medicines, diagnosis of a cardiovascular condition, glaucoma, type 1 or type 2 diabetes, diagnosis of epilepsy, previous diagnosis of angle-closure glaucoma, or current use of any other medication whose use interacts with citalopram (according to BNF guidance) e.g. associated with prolonged QT-interval
• Any other current or past medical conditions which in the opinion of the study medic may interfere with the safety of the participant or the scientific integrity of the study including epilepsy/seizures, brain injury, hepatic or renal disease, diabetes, severe gastro-intestinal problems, Central Nervous System (CNS) tumours, neurological conditions
• First-degree relative with a diagnosis of schizophrenia-spectrum or other psychotic disorder, or bipolar disorder
• Severely underweight (BMI<17) or very obese (BMI>40) in a manner that renders them unsuitable for the study in the opinion of the study medic
• Heavy use of cigarettes (smoke > 20 cigarettes per day)
• Heavy use of caffeine (drink > 4 250ml cups/cans of coffee/energy drinks per day)
• Lactose intolerance (due to the study involving administration of a lactose placebo tablet)
• Known allergy to citric acid, sodium chloride, sucrose or quinine
• Pregnancy (as determined by urine pregnancy test taken during the Part 2 screening visit), breast feeding or plans to become pregnant
• past history of dependence on illicit substances or regular illicit substance use within previous three months
• Evidence of current or past harmful use of alcohol
• previous participation in a study involving the tasks used in this study or involving use of citalopram in the last year
• physical (including visual and auditory) or language impairment that would make complying with the study protocol challenging
• ongoing deficit in sense of smell or taste e.g. following Covid-19 infection
• Participant is unlikely to comply with the clinical study protocol or is unsuitable for any other reason, in the opinion of the Investigator
• Not suitable for MRI neuroimaging e.g. claustrophobia, difficulty remaining still for duration of scan
• Any MRI contraindications outlined in FMRIB 3 Tesla scanning safety form

Research data will be anonymised and then processed for analysis. Anonymised data will be made available post-publication on Open Science Framework (OSF).

defaced neural images will be shared via governed access through Zenodo with data sharing agreements in place