This is a randomized, double-blind, placebo-controlled study of single and multiple ascending dosage levels of AZD6423 in healthy volunteers. The study includes an up to 28-day screening period, an in-house period during which AZD6423 or placebo will be administered IV, and a 4- to 10-day follow-up period after discharge.

In Part 1 of the study (single ascending dose portion), approximately 40 subjects will be randomly assigned. Eight (8) subjects will be randomized in each of 5 dosage-level cohorts (AZD6423 or placebo). Within each cohort, 6 subjects will be randomized to receive AZD6423 and 2 subjects will be randomized to receive placebo. Each subject will receive only one dose of either AZD6423 or placebo on Day 1. In Part 2 of the study (multiple ascending dose portion), approximately 24 subjects will be randomly assigned. Eight (8) subjects will be randomized in each of 3 dosage-level cohorts. Within each cohort, 6 subjects will be randomized to receive AZD6423 and 2 subjects will be randomized to receive placebo. Each subject will receive three doses of either AZD6423 or placebo.

Inclusion Criteria:

1. Subjects must understand the nature of the study and must provide signed and dated written informed consent in accordance with local regulations before the conduct of any study-specific procedures.
2. All male subjects who are biologically capable of having children must agree and commit to the use of a reliable method of birth control for the duration of the study and for 15 days after dose administration. A male subject is considered biologically capable of having children even if his sexual partner is sterile or using contraceptives.
3. Subjects must have a body mass index (BMI) between 19 and 30, inclusive and weigh between 50kg and 100kg inclusive.
4. Subjects must be fluent in English.

Exclusion Criteria:

1. A history or presence of a clinically significant hepatic, renal, gastrointestinal, cardiovascular, endocrine, respiratory, immunologic, hematologic, dermatologic, or neurologic abnormality.
2. A clinically significant abnormality on physical examination, neurological examination, EEG, ECG, vital signs or laboratory evaluations at screen or between screen and dose administration.
3. A history of seizure.
4. A history of head trauma, including closed head injury with loss of consciousness.
5. Any history of suicide attempt or suicidal behavior, or, in the opinion of the investigator, clinically significant risk of suicide or violent behavior.
6. Urine drug screen positive for a drug of abuse
7. A family history of schizophrenia, schizoaffective disorder, or psychosis in first degree relatives.