Antidepressant medications known as selective serotonin reuptake inhibitors (SSRIs) are most commonly prescribed to treat depression and anxiety. How antidepressants work on the brain to alter mood and behaviour is not well understood. This study will use a brain scanning technique (functional magnetic resonance imaging) to examine how aging impacts brain activation during emotional tasks after the administration of intravenous (IV) citalopram (this is the only SSRI available in this form, and is well tolerated and safe in young and old adults). The investigators will further determine what role genetic differences play in this relationship. The investigators expect to see an increase in brain signal as the concentration of IV citalopram increases. However, the investigators propose that the brain signal in older adults will not be as strong as in younger adults. Furthermore, the investigators expect that participants genetically predisposed to have fewer serotonin transporters (the site of action of SSRIs) will show greater decreases in brain activation with citalopram.