Dopamine D-2-like receptors on human peripheral blood lymphocytes: a radioligand binding assay and immunocytochemical study

1 Peripheral blood lymphocytes express dopamine D-1-like and D-2-like recep tors which were investigated using radioligand binding assay and molecular biology techniques. Analysis of dopamine D-2-like receptors expressed by hu man peripheral blood lymphocytes with radioligand binding assay may offer a rapid technique for assessing receptor changes in disorders characterized by involvement of the dopaminergic system. However, the suitability of radi oligand binding assay techniques to measure dopamine D-2-like receptors is questioned. 2 In view of the discrepancy between data of dopamine D-2-like receptor det ermination with molecular biology and radioligand binding assay techniques, we have assayed dopamine D-2-like receptors expressed by human peripheral blood lymphocytes using as radioligands the dopamine receptor agonist 7-[H- 3]-hydroxy-N,N-di-n-propyl-2-aminotetraline ([H-3]-7-OH-DPAT) and two antag onists ([H-3]-spiperone and [H-3]-nemonapride). 3 Analysis of saturation curves revealed a concentration-dependent binding of all compounds to human peripheral blood lymphocytes. Dissociation consta nt (K-d) values averaged between 0.15 and 0.40 nM for different radioligand s. The maximum density of binding sites (B-max) was low, ranging from 4.15 +/- 0.05 fmol/10(6) cells with [H-3]-spiperone and 8.66 +/- 0.04 fmol/10(6) cells with [H-3]-7-OH-DPAT. 4 Displacement curves of [H-3]-7-OH-DPAT, [H-3]-spiperone and [H-3]-nemonap ride binding to human peripheral blood lymphocytes revealed, using radiolig and concentrations giving the highest specific:non-specific binding ratio, a pharmacological profile consistent with the labelling of dopamine D-2-lik e receptors. The use of higher radioligand concentrations resulted in a poo rly displaceable and characterizable binding. 5 Detection of dopamine D-2, D-3 and D-4 receptor immunoreactivity in cytos pin centrifuged peripheral blood lymphocytes revealed dopamine D-3 and D-4 but not D-2 receptor immunostaining. 6 The above findings indicate in agreement with molecular biology studies, that dopamine D-2-like receptors expressed by human peripheral blood lympho cytes belong to the D-3 and D-4 receptor subtypes. These receptors are dete ctable using either dopamine D-2-like receptor agonists and antagonists as radioligands if controlled experimental conditions are followed. The standa rdisation of immunocytochemical techniques for detecting human peripheral b lood lymphocyte dopamine receptors may contribute to clarify their role in lymphocyte function or as a peripheral marker of the status of the dopamine rgic system.