Ascorbic acid inhibits 125I-SCH 23982 binding but increases the affinity of dopamine for D1 dopamine receptors

Effects of ascorbic acid (AA) on 125I-SCH 23982 binding to D1 dopaminergic receptors in membrane preparations from rat striatum were investigated. AA in the range of 0.03 microM-0.33 mM inhibited 75% of specific binding of 125I-SCH 23982 in a dose-dependent manner. At higher concentrations, this inhibition of binding activity by AA was less potent, and 3.3 mM AA inhibited only 30% of specific binding. Reduced glutathione did not alter the inhibition of binding by 0.33 mM AA, but reduced the inhibition by 3.3 mM AA to 8% of specific binding. The loss of specific binding by AA was rescued by 1 mM EDTA, an inhibitor of lipid peroxidation. In the absence of AA, competition experiments with the agonist, dopamine, revealed the presence of high-affinity (Kh = 224.9 +/- 48.9 nm) and low-affinity (Kl = 21,100 +/- 2,400 nM) binding sites. Although the maximum binding of 125I-SCH 23982 decreased to 40% without affecting the KD value in the presence of 1.67 mM AA, the value of the high-affinity site for dopamine was increased (Kh = 23.3 +/- 9.4 nM) and that of the low-affinity site was decreased (Kl = 136,800 +/- 40,900 nM). These results suggest that AA may affect D1 dopamine receptor function by lipid peroxidation, competition with dopamine for low-affinity sites, and reduced oxidation of dopamine.