University of California, Los Angeles researchers will recruit healthy participants (age 18-35) to participate in a study examining whether the administration of naltrexone, an opioid antagonist, eliminates the ability of social support figure reminders to enhance fear extinction--a process during which a threatening cue is learned to not predict a negative or threatening outcome (i.e., electric shock) by being repeatedly presented in the absence of that outcome.

After undergoing an email screening, a telephone screening, an in lab screening, and a health screening, 60 participants will be enrolled in the study. During the experiment, 30 participants will be administered naltrexone and 30 participants will be administered placebo (both participants and experimenters will be blind to condition) before undergoing a fear extinction procedure in which threatening cues--cues that predict electric shock--will be paired with either an image of a social support figure (provided by participants) or a second threatening cue. These pairings will be presented repeatedly in the absence of shock in order for fear extinction to occur. Participants will return for a follow-up test to determine if fear extinction was successful.

Inclusion Criteria:

• healthy adults 18 and 35
• fluent in English
• no history of mental illness (including anxiety, depression, phobia, or any other mental health related disorder diagnosed by a mental health professional)

Exclusion Criteria:

• pregnant or planning to become pregnant during the experiment period
• presence of chronic mental illness (as determined by the report of a past diagnosis of mental illness by a physician or psychologist and/or the prescription of medication related to mental health disorder; including anxiety, depression, phobia, or any other diagnosed psychological disorder)
• chronic physical illness (as determined by past diagnosis and/or treatment by a physician)
• history of liver disease or abnormal liver function
• current and regular use of prescription medications related to mental health disorders or liver function
• previous history of fainting during blood draws
• difficulty or discomfort swallowing pills
• history of substance abuse or addiction (especially alcohol or opiate abuse)

Other exclusion criteria may apply.