A Study of Aticaprant Plus an Antidepressant to Prevent Return of Depression Symptoms in Participants With Major Depressive Disorder Who Experience a Loss of Interest and Pleasure

Inclusion Criteria: * Be medically stable based on physical examination (including a brief neurologic examination), medical history, vital signs (including blood pressure), and 12-lead electrocardiogram (ECG) performed at screening and OL baseline * Be medically stable based on clinical laboratory tests performed at screening * Meet DSM-5 diagnostic criteria for recurrent or single episode MDD, without psychotic features upon clinical assessment and confirmed by the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders-5th Edition (DSM-5) Axis I Disorders-Clinical Trials version (SCID-CT) * Have symptoms of anhedonia based on clinical assessment and confirmed by presence of anhedonia (positive response to major depressive episode (MDE) module symptom Item 2) on the SCID-CT at screening Exclusion Criteria: * Has had no response to 2 or more consecutive antidepressant treatments administered at adequate dose and duration in the current episode of depression including the current selective SSRI/SNRI assessed using the Massachusetts General Hospital Antidepressant Treatment Response Questionnaire (MGH ATRQ) * Has a history or evidence of clinically meaningful noncompliance with current antidepressant therapy * Has a history of moderate-to-severe substance use disorder including alcohol use disorder according to DSM-5 criteria within 6 months before screening * Has homicidal ideation/intent, per the investigator's clinical judgment, or has suicidal ideation with some intent to act within 3 months prior to the start of the screening phase * Has cognitive impairment per investigator judgment that would render the informed consent invalid or limit the ability of the participant to comply with the study requirements