M2 Receptor Measurements in Aging and in Alzheimer's Disease

The Geriatric Psychiatry Branch (GPB) of the National Institute of Mental Health (NIMH) proposes to study the effect of aging and Alzheimer's Disease (AD) on muscarinic type 2 (M2) receptor density and function using the ligand [F-18] FP-TZTP. M2 receptors are primarily autoreceptors found on acetylcholine (ACh) neurons, and are lost along with ACh neurons in aging and AD, particularly in the cerebral cortex. As we near completion of the study of the young versus the old, we would like to add a few more healthy subjects to achieve better gender balance, continue the accrual of Alzheimer's patients and expand the Protocol to include its natural scientific progression. Quantification of M2 receptors with a single FP-TZTP scan as we are currently performing with 99-M-0073 provides a measure of the number of cholinergic receptors in the brain, but not their capacity to release acetylcholine into the synapse. To test this capacity requires a second TZTP scan and the use of an agent that alters acetylcholine concentrations in the synapse. Non-human primate studies performed here (1) have found PET scans with FP-TZTP to be sensitive to changes in acetylcholine synapse concentrations, as a result of competition, in response to the adminstration of the acetylinesterase inhibitor physostigmine. We would like to use physostigmine in all groups; young, older normal, and Alzheimer's patients to infer differences in capacity to release acetylcholine.