Pharmacological characterization and autoradiographic localization of dopamine D1-like receptors in the thymus

The present study was designed to identify the pharmacological profile and the anatomical localisation of dopamine D1-like receptor sites in the rat thymus using [3H]SCH 23390 as a ligand. [3H]SCH 23390 was specifically bound to sections of the thymus. Binding was time, temperature and concentration-dependent belonging in the range of concentrations of radioligand used to a single class of high affinity sites. The dissociation constant was 1.6 nM and the maximal density of binding sites averaged to 170 fmol/mg tissue. The pharmacological profile of [3H]SCH 23390 binding to sections of the rat thymus is consistent with the labelling of dopamine D1-like sites. Dopamine was able to compete with [3H]SCH 23390 binding to sections of rat thymus in the range of nanomolar concentrations. This suggests the labelling of dopamine D5 receptor sites. Light microscope autoradiography revealed the localisation of [3H]SCH 23390 binding sites primarily in the cortex of the thymus and in lesser amounts at the level of thymic corpuscles. The possible functional significance of dopamine D1-like receptors in the rat thymus is discussed.