Expression of the bovine striatal D2 receptor, but not the D1 receptor, in bovine adrenal medulla

R Maroto, MG Lopez, M del Valle, JR Naranjo, B Mellstrom and AG Garcia

Departamento de Fisiologia y Farmacologia, Facultad de Medicina, Universidad de Salamanca, Spain.

At 37 degrees, the specific binding of [3H]SCH23390 to purified adrenal medullary plasma membranes accounted for only 20% of total binding. At 4 degrees, the binding did not saturate; therefore, equilibrium binding constants could not be estimated. Similar results were obtained with 125I-SCH23982, a ligand that exhibits 25-fold higher specific activity, compared with [3H]SCH23390. Of 11 dopamine receptor ligands used, only (+)-SCH23390 and (+/-)-SKF83566 inhibited the binding of [3H]SCH23390, but with very low affinities (IC50 values of 446 and 635 nM, respectively). In striatal membranes, binding of [3H]SCH23390 and of 125I-SCH23982 followed saturation isotherms. [3H]SCH23390 exhibited a Kd of 383 pM and a Bmax of 479 fmol/mg of protein, and 125I-SCH23982 exhibited a Kd of 664 pM and a Bmax of 453 fmol/mg of protein. The radioligand was displaced by the D1-selective compounds (+)-SCH23390 (IC50 of 3 nM), (+/-)-SKF83556 (IC50 of 5 nM), and (+)-SKF38393 (IC50 of 17 nM); spiperone and quinpirole were ineffective. [3H]Spiperone binding to bovine striatal and adrenal medullary plasma membranes exhibited similar characteristics, compatible with a typical D2 receptor. Northern blot analysis revealed the presence of D1 receptor mRNA in poly(A)+ RNA preparations from bovine brain striatum. When Northern blots containing poly(A)+ from bovine adrenal medulla were probed, no specific hybridization band for D1 receptors was observed; in contrast, a band of the expected size for D2 receptors was obtained. Similar results were obtained with in situ hybridization techniques and with more sensitive reverse transcription-polymerase chain reaction methods. The data support the idea that the peripheral D2 receptor present in bovine adrenal medulla is similar to striatal D2 receptors; in contrast, striatal D1 receptors do not seem to have a counterpart in bovine adrenal medullary tissues.

Volume 47, Issue 1, pp. 40-50, 01/01/1995
Copyright © 1995 by American Society for Pharmacology and Experimental Therapeutics

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