REDUCED STRIATAL DOPAMINE-RECEPTORS IN ALZHEIMERS-DISEASE - SINGLE-PHOTON EMISSION TOMOGRAPHY STUDY WITH THE D-2 TRACER [I-123] IBZM

Many patients with Alzheimer's disease (AD) develop parkinsonian sympt oms, suggesting an overlapping between AD and Parkinson's disease (PD) . However, pathologic and neurochemical studies indicate that the invo lvement of the dopamine system may be different in the two conditions. Using single photon emission tomography, we determined the relative s pecific striatal uptake (striatum to cerebellum ratio) of the D-2 rece ptor ligand [I-123]-IBZM in 15 AD patients without overt extrapyramida l symptoms (three subjects presented mild rigidity and bradykinesia) a nd nine age-matched controls. Mean specific activity in striatal regio ns of AD patients (1.35 +/- 0.09) was significantly reduced from contr ol mean (1.59 +/- 0.03). Because such changes were evident even in the absence of overt parkinsonian symptomatology, our data indicate that alterations of striatal D-2 receptors may be part of the pathologic ab normalities of AD. In addition, the mechanisms underlying extrapyramid al symptoms in AD (decline of postsynaptic striatal dopamine receptors ) appear different from the prevalent presynaptic nigrostriatal altera tions typical of PD.