Sodium-dependent isomerization of dopamine D-2 receptors characterized using [¹²⁵I]epidepride, a high-affinity substituted benzamide ligand

We have characterized the in vitro binding of a new ligand, [¹²⁵I]epidepride, and used this substituted benzamide to assess the sensitivity of dopamine D-2 receptors to sodium. Both direct and indirect binding studies with [¹²⁵I]epidepride and unlabeled epidepride, respectively, demonstrated that the affinity of D-2 receptors for the ligand was decreased from 20 to 30 pM in the presence of sodium to 350 to 500 pM in the absence of sodium. The density of binding sites for [¹²⁵I]epidepride was identical in the presence and absence of NaCl. The time courses for association of [¹²⁵I]epidepride to and dissociation from D-2 receptors in the presence of sodium were not consistent with simple bimolecular reactions, suggesting the possibility of a sodium-dependent ligand-induced receptor isomerization. Thus, dissociation of [¹²⁵I]epidepride was biphasic in the presence of sodium, but monophasic in the absence of sodium. The rank order of potency for inhibition of [¹²⁵I]epidepride binding by drugs was identical in rat striatum and cells expressing a D-2 receptor cDNA, and similar to the previously described pharmacological profile of D-2 receptors labeled by [³H]spiperone. [¹²⁵I]Epidepride bound to two classes of binding sites in rat medial prefrontal cortex. One class, present at a density of 10 fmol/mg of protein and with a K(d) value of approximately 40 pM, was pharmacologically indistinguishable from D-2 receptors in striatum and transfected cells. The pharmacological profile of the second class of sites was similar to that of alpha-2 adrenergic receptors, [¹²⁵I]Epidepride had 50- to 100-fold lower affinity (approximately 2 nM) for alpha-2 receptors than for D-2 receptors. These results are consistent with the hypothesis that the binding of [¹²⁵I]epidepride to D-2 receptors is associated with a sodium-dependent receptor isomerization that increases the affinity of the receptors for [¹²⁵I]epidepride and other substituted benzamide derivatives. [¹²⁵I]Epidepride is the best available radioligand for many purposes because of its selectivity, high affinity for D-2 receptors and high specific radioactivity.