 - SPECT: This nuclear medicine test produces a picture of the receptors in the brain. On the night before the scan, the day of the scan, and for 4 days after the scan, subjects take an oral dose of potassium iodide to protect the thyroid gland from the radioactive tracer used in the SPECT procedure. People allergic to potassium iodide will take potassium perchlorate instead. Before the scan, small radioactive markers containing 99Tc are glued to the subjects head. Two catheters thin, flexible tubes are placed in veins in the arms to inject the radioactive tracer 123I5-I-A-85380 and to draw blood samples. Another catheter is placed in an artery in the wrist to draw arterial blood samples. During the scan, the subject lies on a bed with his or her head held still with a head holder. The scans are taken over a 6-hour period after injection of 123I5-I-A-85380. An electrocardiogram, respiration, and blood pressure measures are taken before the tracer is injected, then 5 minutes after the injection, and again 30 to 60 minutes after the injection. Blood and urine samples are collected 5 to 6 hours after starting the scan. Participants are asked to urinate at least every 2 hours for 12 hours after injection of 123I5-I-A-85380 to decrease radiation exposure. 