EXPRESSION OF THE BOVINE STRIATAL D-2 RECEPTOR, BUT NOT THE D-1 RECEPTOR, IN BOVINE ADRENAL-MEDULLA

At 37 degrees, the specific binding of [H-3]SCH23390 to purified adren al medullary plasma membranes accounted for only 20% of total binding. At 4 degrees, the binding did not saturate; therefore, equilibrium bi nding constants could not be estimated. Similar results were obtained with I-125-SCH23982, a ligand that exhibits 25-fold higher specific ac tivity, compared with [H-3]SCH23390. Of 11 dopamine receptor ligands u sed, only (+)-SCH23390 and (+/-)-SKF83566 inhibited the binding of [H- 3]SCH23390, but with very low affinities (IC50 values of 446 and 635 n M, respectively). In striatal membranes, binding of [H-3]SCH23390 and of I-125-SCH23982 followed saturation isotherms. [3H]SCH23390 exhibite d a K-d Of 383 PM and a B-max of 479 fmol/mg of protein, and I-125-SCH 23982 exhibited a K-d of 664 pM and a B-max of 453 fmol/mg of protein. The radioligand was displaced by the D-1-selective compounds (+)-SCH2 3390 (IC50 of 3 nM), (+/-)-SKF83566 (IC50 of 5 nM), and (+)-SKF38393 ( IC50 of 17 nM); spiperone and quinpirole were ineffective. [H-3]Spiper one binding to bovine striatal and adrenal medullary plasma membranes exhibited similar characteristics, compatible with a typical D-2 recep tor. Northern blot analysis revealed the presence of D-1 receptor mRNA in poly(A)(+) RNA preparations from bovine brain striatum. When North ern blots containing poly(A)(+) from bovine adrenal medulla were probe d, no specific hybridization band for D-1 receptors was observed; in c ontrast, a band of the expected size for D-2 receptors was obtained. S imilar results were obtained with in situ hybridization techniques and with more sensitive reverse transcription-polymerase chain reaction m ethods. The data support the idea that the peripheral D-2 receptor pre sent in bovine adrenal medulla is similar to striatal D-2 receptors; i n contrast, striatal D-1 receptors do not seem to have a counterpart i n bovine adrenal medullary tissues.