Beyond Monoamines: The Role of the Nociceptin/Orphanin FQ Receptor in Major Depression

Inclusion Criteria for all participants: * All genders, races, and ethnic origins, aged between 18 and 45 * Capable of providing written informed consent, and fluent in English * Right-handed * Absence of any psychotropic medications for at least 2 weeks * Has a smartphone (iPhone or Android) (needed for Ecological Momentary Assessment) Inclusion Criteria for "Remitted MDD" group: * Meets inclusion criteria for all subjects, plus: * History of MDD as defined by DSM-5 * Absence of anxiety disorder for the past two months Inclusion Criteria for "Current MDD" group: * Meets inclusion criteria for all subjects, plus: * Presence of MDD as defined by DSM-5 * Absence of anxiety disorder for the past two months Exclusion Criteria for all participants: * Subjects with suicidal ideation where outpatient treatment is determined unsafe by the study clinician. These patients will be immediately referred to appropriate clinical treatment * Pregnant women or women of childbearing potential who are not using a medically accepted means of contraception (defined as oral contraceptive pill or implant, condom, diaphragm, spermicide, IUD, s/p tubal ligation, or partner with vasectomy) * Serious or unstable medical illness, including cardiovascular, hepatic, renal, respiratory, endocrine, neurologic or hematologic disease * History of seizure disorder * History of psychiatric illnesses, other than depression or anxiety disorders among the Current MDD and Remitted MDD groups * History of substance use disorder or alcohol use disorder (as these terms are defined by DSM-5); except depressed subjects may have a history of 'Mild' substance/alcohol use disorder only if it ended as least 12 months ago * History of cocaine or stimulant use or dopaminergic drugs * History or current diagnosis of dementia, or a score of \< 26 on the Mini Mental State Examination at the screening visit; * Patients with mood congruent or mood incongruent psychotic features * Current use of other psychotropic drugs * Clinical or laboratory evidence of hypothyroidism * Patients with a lifetime history of electroconvulsive therapy (ECT) * Failure to meet standard MRI safety requirements * Abnormal ECG and lab results * History of seizure disorder * Contraindications for arterial line (e.g., abnormal result on Allen test, Raynaud's syndrome, history of anemia or bleeding disorder, history of fainting from blood draws).