[3H]Spiroperidol Identifies a D-2 Dopamine Receptor Inhibiting Adenylate Cyclase Activity in the Intermediate Lobe of the Rat Pituitary Gland

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Abstract
[3H]Spiroperidol ([3H]SPIRO) binds with high affinity (Kd = 0.3 nM) to cell-free homogenates of the neurointermediate lobe of the rat pituitary gland. The neurointermediate lobe contains 19.2 fmol of specific binding sites, 86% of which occur in the intermediate lobe (IL). Compounds active upon the D-2 dopamine receptor in the IL compete with [3H] SPIRO for occupancy of the specific binding site. Guanosine 5′- triphosphate decreases the affinity of agonists, but not antagonists, for the specific binding site. For each drug tested, methods derived from competitive enzyme kinetics were used to calculate the apparent affinity constants of the drug for the binding site and for the receptor regulating adenylate cyclase activity. The pharmacological properties of the specific [3H]SPIRO binding site were compared to the pharmacological properties of the D- 2 dopamine receptor inhibiting adenylate cyclase activity in the IL. The similarity between the affinities determined from the binding and enzyme assays suggests that some or all of the specific [3H]SPIRO binding sites in the IL are D-2 dopamine receptors inhibiting adenylate cyclase activity.