Cognitive dysfunction in Alzheimer's disease (AD) patients are thought to arise in part from underlying losses of cholinergic input to cerebral cortex and hippocampus. Consistent marked decreases of alpha4beta2 nicotinic acetylcholine receptors (nAChRs) are found in postmortem AD brains. However, knowledge of the in vivo status of this receptor system is limited due to the lack of suitable tracers for PET/SPECT imaging. We plan to use a new SPECT tracer, [123I]5-I-A-85380, which appears suitable for imaging the alpha4beta2 subtype of nAChRs, in order to delineate abnormalities of this receptor system in a "subgroup" of 20 mild to moderately demented AD patients against 20 age matched healthy controls, the "subgroup" being characterized by carrying the AD susceptibility Apolipoprotein E epsilon4 allele. In addition, we plan to examine the relationship between cognitive dysfunction and alpha4beta2 nAChr deficits in the cerebral cortex and hippocampus of these AD patients. This study represents our initial effort toward our long-term goal of understanding the role this important receptor system plays in the pathophysiological mechanisms and drug manipulation of AD.