- Physostigmine challenge Physostigmine will be administered as follows. Glycopyrrolate, a cholinergic antagonist that does not have central side-effects, will be administered prior to physostigmine challenge to block physostigmine peripheral side-effects (e.g. nausea). Subjects will receive 200 mcg /ml x 1 ml of glycopyrrolate through an IV. Physostigmine administered i.v. has a short half life of 20 min with peak plasma levels 20-30 min post administration. The same dose of physostigmine will be administered as in the preliminary data: 1.5 mg/hr for 1 hr. Vital signs, including systolic and diastolic blood pressure, heart rate and respiration rate, will be monitored before physostigmine and then at 10, 20, 30, 60 min after the beginning of the infusion and then hourly until the end of the study day. Subjects will be questioned before, during, and after physostigmine challenge about potential adverse reactions typical to this medication (nausea, upset stomach, etc., as in Risks section). [If there is a significant and persistent drop in subjects' heart rate (>15% for at least 1 min), the IV physostigmine infusion will be stopped but the subject will continue to be monitored and will be discharged at the discretion of the study doctor. In emergency situation, PET center protocol will be followed accordingly (on file with HIC).