PHARMACOLOGICAL CHARACTERIZATION AND AUTORADIOGRAPHIC LOCALIZATION OFDOPAMINE-RECEPTORS IN THE RAT ADRENAL-MEDULLA

The pharmacological profile and the anatomical localization of dopamin e D-1-like and D-2-like receptors were studied in sections of rat adre nal medulla, with radioligand binding and autoradiographic techniques, respectively. [H-3]([R]-(+)-chloro-2,3,4,5-tetrahydro-5-phenyl-l H-3b enzazepin-al hemimaleate) (SCH 23390) was used as a ligand for dopamin e D-1-like receptors and [H-3]spiperone was used as a ligand for dopam ine D-2-like receptors. Radioligand binding and light microscope autor adiography did not show specific [3H]SCH 23390 binding in sections of rat adrenal medulla. This suggests that rat adrenal medulla does not e xpress dopamine D-1-like receptors, [3H]Spiperone was specifically bou nd to sections of rat adrenal medulla. The binding was time-, temperat ure- and concentration-dependent, with a dissociation constant (K-d) o f 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 bind ing to rat adrenal medulla was similar to that displayed by neostriatu m, which is known to express dopamine D-2 receptors. Light microscope autoradiography showed the accumulation of specifically bound [3H]spip erone as silver grains within sections of adrenal medulla. Silver grai ns were found primarily over the cellular membrane of chromaffin cells . The above data indicate that chromaffin cells of the rat adrenal med ulla express dopamine receptors belonging to the dopamine D-2 receptor subtype. These receptors are probably involved in the modulation of c atecholamine release from chromaffin cells, as documented by functiona l studies.