A Study of Aticaprant 10 mg as Adjunctive Therapy in Adult Participants With Major Depressive Disorder (MDD) With Moderate-to-severe Anhedonia and Inadequate Response to Current Antidepressant Therapy and Long-term Extension Treatment With Aticaprant

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 double blind (DB) baseline * Meet diagnostic and statistical manual of mental disorders-5th edition (DSM-5) diagnostic criteria for recurrent or single episode major depressive disorder (MDD), without psychotic features (DSM-5 296.22, 296.23, 296.32, or 296.33), based upon clinical assessment and confirmed by the structured clinical interview for 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 MDE module symptom Item 2) on the SCID-CT at screening * Have had an inadequate response to at least 1 and up to 5 (inclusive) oral antidepressant treatments, administered at an adequate dose * Is currently receiving and tolerating well any one of the following selective serotonin reuptake inhibitor or serotonin-norepinephrine reuptake inhibitor (SSRI or SNRI) antidepressants at screening, in any approved formulation and available in the participating country/territory: citalopram, duloxetine, escitalopram, fluvoxamine, fluoxetine, milnacipran, levomilnacipran, paroxetine, sertraline, venlafaxine, or desvenlafaxine at a stable dose (at or above the minimum therapeutic dose per Massachusetts general hospital antidepressant treatment response questionnaire (MGH ATRQ) for at least 6 weeks. The current antidepressant cannot be the first antidepressant treatment for the first lifetime episode of depression Exclusion Criteria: * Has had no response (treatment failure) to 2 or more consecutive antidepressant treatments administered at an adequate dose (at or above the minimum therapeutic dose) and duration (at least 6 weeks) in the current episode of depression including the current SSRI/SNRI (that is the one to be continued in the treatment phases) assessed using the MGH ATRQ * Has one or more of the following diagnoses: (1) a current or prior (lifetime) DSM-5 diagnosis of: (a) a psychotic disorder or MDD with psychotic features, (b) bipolar or related disorders, (c) intellectual disability, (d) autism spectrum disorder, (e) borderline personality disorder, (f) antisocial personality disorder, (g) histrionic personality disorder, (h) narcissistic personality disorders, (i) somatoform disorders; (2) A primary DSM-5 diagnosis (which has been the primary focus of psychiatric treatment within the past 2 years) of: (a) panic disorder, (b) generalized anxiety disorder, (c) social anxiety disorder, (d) specific phobia; (3) A current (in the past year) DSM-5 diagnosis of: (a) obsessive-compulsive disorder, (b) post-traumatic stress disorder, (c) anorexia nervosa, (d) bulimia nervosa * 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 in the current depressive episode had vagal nerve stimulation or deep brain stimulation device in the current episode or has had an inadequate response to an adequate course of intravenous or intranasal ketamine or esketamine (greater than \[\>\] 2 treatments), or electroconvulsive therapy (that is at least 7 treatments) * 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, per the investigator's clinical judgment or based on the columbia suicidality severity rating scale (C-SSRS), corresponding to a response of "Yes" on Item 4 (active suicidal ideation with some intent to act, without specific plan) or Item 5 (active suicidal ideation with specific plan and intent) for suicidal ideation on the C-SSRS, or a history of suicidal behavior within the past 6 months prior to the start of the screening phase. Participants reporting suicidal ideation with intent to act or suicidal behavior at DB baseline should be excluded