Pharmacological characterization and autoradiographic localization of dopamine receptors in the rat adrenal medulla

The pharmacological profile and the anatomical localization of dopamine D-1-like and D-2-like receptors were studied in sections of rat adrenal medulla, with radioligand binding and autoradiographic techniques, respectively. [H-3]([R]-(+)-chloro-2,3,4,5-tetrahydro-5-phenyl-l H-3benzazepin-al hemimaleate) (SCH 23390) was used as a ligand for dopamine D-1-like receptors and [H-3]spiperone was used as a ligand for dopamine D-2-like receptors. Radioligand binding and light microscope autoradiography did not show specific [3H]SCH 23390 binding in sections of rat adrenal medulla. This suggests that rat adrenal medulla does not express dopamine D-1-like receptors, [3H]Spiperone was specifically bound to sections of rat adrenal medulla. The binding was time-, temperature- and concentration-dependent, with a dissociation constant (K-d) of 1.05 nM and a maximum density of binding sites (B-max) of 100.2 +/- 3.8 fmol/mg tissue. The pharmacological profile of [H-3]spiperone binding to rat adrenal medulla was similar to that displayed by neostriatum, which is known to express dopamine D-2 receptors. Light microscope autoradiography showed the accumulation of specifically bound [3H]spiperone as silver grains within sections of adrenal medulla. Silver grains were found primarily over the cellular membrane of chromaffin cells. The above data indicate that chromaffin cells of the rat adrenal medulla express dopamine receptors belonging to the dopamine D-2 receptor subtype. These receptors are probably involved in the modulation of catecholamine release from chromaffin cells, as documented by functional studies.