Background: PDE4D is a protein in the body that plays a role in thinking and depression. This protein may play a major role in disorders such as Alzheimer disease or major depressive disorder. To learn more about these disorders, researchers want to be able to detect levels of PDE4D in the brain. 11C-JMJ-129 is a new radiotracer (a radioactive substance that highlights parts of the body during imaging scans) that was developed to attach only to PDE4D.

Objective: To test the new radiotracer 11C-JMJ-129 during imaging scans in healthy volunteers.

Eligibility: Healthy people aged 18 years and older who had a screening assessment under protocol 01-M-0254.

Design: Participants will have 2 to 4 clinic visits. Participants will be screened. They will have a physical exam. They will have blood tests and a test of their heart function. Participants will undergo 1 or more of these scans: A positron emission tomography (PET) scan of the whole body. The radiotracer will be injected through a tube placed in a vein in the arm. Participants will lie on a table while a donut-shaped machine passes over them. Blood will be drawn from the arm during this scan. A magnetic resonance imaging (MRI) scan of the brain. Participants will lie on a table that slides into a tube. A PET scan of the brain. These participants will be injected with the radiotracer. They will lie on a table with their head in the scanner. Participants will be called within 3 days after each PET scan for a check on their health.