Dopamine D1-like receptors in the thymus of aged rats: a radioligand binding and autoradiographic study.

Age-dependent changes in the density and pattern of dopamine D1-like receptors were studied in the thymus of young (3 months), adult (12 months) and aged (24 months) male Wistar rats using combined radioligand binding and autoradiographic techniques. [3H]SCH 23390, which was used as a ligand, was specifically bound to sections of the thymus in a manner consistent with the labelling of dopamine D5 receptor. The dissociation 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 reduced in the thymus of adult in comparison with young rats and further reduced in aged animals. Silver grains which correspond to [3H]SCH 23390 binding sites were revealed by light microscope autoradiography primarily in the cortex of the thymus and in lesser amounts within thymic corpuscles. 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 silver grains revealed with autoradiography is more moderate than the decrease in the density of binding sites shown by radioligand binding. Silver grains developed per single cells (probably lymphocytes) of the thymic cortex were reduced between young and adult rats and further decreased in old rats. The above findings suggest that the age-related decline in the density of dopamine D5 receptor assayed in the thymus is due in part to the reduced thymic mass with agin