Dopamine receptors in human lymphocytes: Radioligand binding and quantitative RT-PCR assays

Analysis of dopamine receptors (DR) in lymphocytes of the human peripheral blood mononuclear cell (PBMC) fraction is an attractive tool for evaluation of functional properties of dopaminergic function underlying variation in complex psychological/psychopathological traits. Receptor binding assays (RBAs) with selective radioligands, which are widely used in CNS studies, have not produced consistent results when applied to isolated PBMC. We tested the assay conditions that could be essential for detection of DR in human PBMC and their membrane preparations. Using [3H]SCH23390, a dopamine D1-like receptor antagonist, we demonstrated the presence of two binding sites in PBMC-derived membrane fraction. One of them is characterized by the Kd value consistent with that reported for D5 dopamine receptors in human lymphocytes, whereas the other Kd value possibly corresponds to serotonin receptor(s). Although D5 receptor binding sites in PBMC membranes could be characterized by binding assays, the low protein expression and the large volume of blood needed for membrane preparation render the binding method impracticable for individual phenotyping. In contrast, real-time RT-PCR may be used for this purpose, contingent on the relationship between DR expression in the brain and in lymphocytes. The expression of the DRD2-DRD5 genes, as detected by this method, varied widely among samples, whereas the DRD1 expression was not detected. The expression levels were comparable with those in the brain for DRD3 and DRD4, and were significantly lower for DRD2 and DRD5.