Dopamine D5 receptors in human peripheral blood lymphocytes: a radioligand binding study. 
In the present study we have investigated, using radioligand binding techniques and the dopamine receptor antagonist [3H]SCH 23390 as a ligand, the existence of specific dopamine D1-like receptors in human peripheral blood lymphocytes. [ 3H]SCH 23390 binding to human peripheral blood lymphocytes was time-, temperature-, concentration-dependent and of high affinity with a dissociation constant value (Kd) of 0.58 +/- 0.05 nM and a maximum binding density (Bmax) of 11.02 +/- 0.3 fmol/5 x 10(6) cells.  The binding was also reversible.  Pharmacological analysis displacement curves of [3H]SCH 23390 binding with dopamine competing with the radioligand in the submicromolar range suggests that peripheral blood lymphocytes express dopamine D5 receptors rather than dopamine D1 receptors.  These results, which are consistent with studies performed with molecular biology techniques, suggest that dopamine may modulate peripheral blood lymphocyte activity.  Radioligand binding techniques, applied to lymphocyte receptor studies for their feasibility and flexibility may be used to investigate the possible relationship between the immune and dopaminergic systems.  Moreover, they could be employed as a tool in Parkinson's disease, migraine, schizophrenia and hypertension research.