This study will test the reliability of positron emission tomography (PET) imaging using the radioactive substance [(11)C]MePPEP for measuring cannabinoid type 1 (CB 1) receptors. The CB 1 protein is the target for the active ingredient in marijuana. It can affect many functions, including memory, cognition, mood, sensory perception, appetite, pain, tremor and body temperature. Altered function of the CB 1 protein is seen in several neurological, psychiatric and eating disorders, including Parkinson disease, Huntington disease, Alzheimer disease, schizophrenia, anorexia nervosa and binge eating.

Healthy normal volunteers between 18 and 65 years of age may be eligible for this study. Candidates are screened with a medical history, physical examination and laboratory tests. Participants undergo the following procedures:

• PET scans: PET uses small amounts of a radioactive chemical called a tracer that labels active areas of the brain so the activity can be seen with a special camera. The tracer used in this study is [(11)C]MePPEP. Before starting the scan, a catheter (plastic tube) is placed in a vein in the arm to inject the tracer and another catheter is placed in an artery in the wrist to obtain blood samples during the scan. For the procedure, the subject lies on the scanner bed. A special mask is fitted to the head and attached to the bed to help keep the person's head still during the scan so the images will be clear. A brief scan is done just before the tracer is injected to provide measures of the brain that are helpful in calculating information from subsequent scans. After the tracer is injected, pictures are taken for about 2.5 hours, while the subject lies still on the scanner bed. A second scan is done the same day or within 8 weeks of the first.
• Magnetic resonance imaging (MRI): An MRI scan is done within 1 year (before or after) of the PET scan. This procedure uses a magnetic field and radio waves to produce images of the brain. The subject lies on a table that is moved into the scanner (a tube-like device), wearing earplugs to muffle the noise of the machine during the scanning process. The test takes about 1 hour.

MePPEP ((3R,5R)-5-(3-methoxy-phenyl)-3-((R)-1-phenyl-ethylamino)-1-(4-trifluoromethyl-phenyl)-pyrrolidin-2-one) was developed through a CRADA between Lilly at NIMH and is a ligand that is highly selective for the cannabinoid type 1 (CB1) receptor. This receptor is considered the most common G-coupled protein receptor in the brain (Pacher et al., 2006). The CB1 receptor is the site of action of Delta-9-tetrahydrocannbinol (THC), the active compound in marijuana, and is found primarily on the presynaptic terminals of dopaminergic, glutamatergic, GABAergic neurons (Howlett et al., 2002). The function of the CB1 receptor is not entirely clear; however it has been implicated in several neurological and psychiatric disorders, and a selective inverse agonist, rimonabant, is currently in use in Europe for the treatment of obesity (Van Gaal et al., 2005). [11C]MePPEP was developed to enhance our understanding of the in vivo characteristics of the CB1 receptor (e.g., receptor density and receptor occupancy with pharmaceuticals).

The purpose of this protocol is to establish an accurate method to measure CB1 receptor levels in brain by performing test/retest brain imaging studies. The results of this overall study are required to apply this PET ligand in various neurological and psychiatric disorders in the future.

• INCLUSION CRITERIA:

All subjects must be healthy and aged 18-65 years, with history/physical exam, ECG, and laboratory tests within one year of the PET scan. The volunteer must sign an informed consent form.

EXCLUSION CRITERIA:

• Current psychiatric illness, substance abuse including marijuana use, or severe systemic disease based on history and physical exam.
• Laboratory tests with clinically significant abnormalities or positive urine toxicology screen.
• Prior participation in other research protocols in the last year such that radiation exposure would exceed the annual limits.
• Pregnancy and breast feeding.
• Claustrophobia.
• Presence of ferromagnetic metal in the body or heart pacemaker.
• Positive HIV test.
• Employee of the investigative site or an immediate family member of an employee of the investigative site. Immediate family member is defined as a spouse, parent, child, or sibling, whether biological or legally adopted.
• Employee of Eli Lilly and Company.