Reduced density of dopamine D2-like receptors on peripheral blood lymphocytes in Alzheimer's disease

Clinical and pathological evidence points to an involvement of dopamine in Alzheimer's disease (AD). The present study was designed to assay dopamine D1-like and D2-like receptors on peripheral blood lymphocytes (PBL) in 20 patients with AD and in 25 healthy controls by radioligand binding assay techniques with ³H]R]-(+)-(?)chloro-2,3,4,5 tetrahydro-5-phenyl-1H-3-benzazepin-al-hemimaleate (SCH 23390) and ³H]7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7OH-DPAT) as radioligands. The density of dopamine D1-like receptors and the affinity of ³H]SCH 23390 and ³H]7OH-DPAT binding to PBL were similar in both groups investigated. AD patients revealed a lower density of dopamine D2-like receptors on PBL than controls (P=0.0016). The pharmacological profile of ³H]SCH 23390 and ³H]7OH-DPAT binding to PBL was consistent with the labeling of dopamine D5 and D3 receptor subtypes, respectively. The reduced density of dopamine D2-like receptors on PBL is consistent with the observation of changes in the expression of D2-like receptors in dopaminergic brain areas in AD. Our findings support the hypothesis of an involvement of dopamine in AD, even in those patients with no evidence of Parkinsonism, behavioral abnormalities or psychosis.