The purpose of this study is to assess the effect of cytochrome P450 isoenzyme 2D6 (CYP2D6) inhibition by quinidine on the pharmacokinetic (PK) parameters of a single dose SEP-380135 in healthy adult participants.

Inclusion Criteria:

1. Male or female participants between 18 and 55 years of age (inclusive) at time of consent

2. Body mass index from 18.0 to 32.0 kilograms per square meter (kg/m^2) (inclusive).

3. In good health as determined by:

   1. Medical history
   2. Physical examination
   3. Vital signs
   4. Neurological examination
   5. Electrocardiogram
   6. Serum chemistry, lipid panel, thyroid panel, coagulation panel, urinalysis, hematology, and serology tests (from screening visit).

4. Willing to stay in the clinic for the required period (i.e., to be hospitalized for 10 days) and willing to be contacted for the safety follow-up telephone call.

5. Ability to provide written, informed consent prior to initiation of any trial-related procedures, and ability, in the opinion of the Principal Investigator, to comply with all the requirements of the trial.

Exclusion Criteria:

1. Have taken an investigational drug within 30 days or 5 half-lives, if known, (whichever is longer) prior to screening.
2. Have had previous exposure to SEP-380135.
3. Are currently participating in another clinical trial.
4. Attempted suicide within 12 months prior to screening.
5. A history of sick sinus syndrome, first-, second-, or third-degree atrioventricular block, myocardial infarction, New York Heart Association Class II-IV heart failure, cardiomyopathy, pulmonary congestion, cardiac arrhythmia, prolonged QT interval, congenital long QT syndrome, family history of long QT, or moderate to severe hypokalemia. A participant with non-clinically significant ECG abnormalities at screening and check-in requires approval from the medical monitor and the sponsor's physician.
6. In the opinion of the investigator, should not participate in the trial.

Note: Other protocol-specified inclusion/exclusion criteria may apply.

Anonymized individual participant data (IPD) that underlie the results of this study will be shared with researchers to achieve aims pre-specified in a methodologically sound research proposal. Small studies with less than 25 participants are excluded from data sharing.