DOPAMINE D-1-LIKE RECEPTORS IN THE THYMUS OF AGED RATS - A RADIOLIGAND BINDING AND AUTORADIOGRAPHIC STUDY

Age-dependent changes in the density and pattern of dopamine D-1-like receptors were studied in the thymus of young (3 months), adult (12 mo nths) and aged (24 months) male Wistar rats using combined radioligand binding and autoradiographic techniques. [H-3]SCH 23390, which was us ed as a ligand, was specifically bound to sections of the thymus in a manner consistent with the labelling of dopamine D-5 receptor. The dis sociation constant value was similar in the thymus of the three animal groups examined. The maximal density of binding sites, evaluated with conventional radioligand binding techniques, was significantly reduce d in the thymus of adult in comparison with young rats and further red uced in aged animals. Silver grains which correspond to [3H]SCH 23390 binding sites were revealed by light microscope autoradiography primar ily in the cortex of the thymus and in lesser amounts within thymic co rpuscles. A progressive decrease in the density of silver grains more pronounced in the cortex than in thymic corpuscles was observed in the thymus of adult and old in comparison with young rats. The loss of si lver grains revealed with autoradiography is more moderate than the de crease in the density of binding sites shown by radioligand binding. S ilver grains developed per single cells (probably lymphocytes) of the thymic cortex were reduced between young and adult rats and further de creased in old rats. The above findings suggest that the age-related d ecline in the density of dopamine D-5 receptor assayed in the thymus i s due in part to the reduced thymic mass with aging. The observation o f a decreased expression of dopamine D-5 receptor in cells of the thym ic cortex as a function of age suggests that this reduction cannot be attributed simply to loss of thymic lymphocytes.