Identification of dopamine_1A Receptors in the Rat Small Intestine

Dopamine receptors have been localized to several tissues outside the central nervous system including the kidney and mesenteric vessels. To determine if there are dopamine₁ receptors within the small intestine, homogenates of the antimesenteric halves of the entire jejunum and ileum of adult rats were prepared and competitive inhibition studies and Scatchard analysis were performed at room temperature using ¹²⁵I-SCH 23982 and SCH 23390. The specific binding of ¹²⁵I-SCH 23982 to the intestinal tissue homogenates was rapid, saturable with ligand concentration, and reversible. Analysis of the Scatchard plots revealed a single class of receptors with an apparent dissociation constant of 10.77 ± 2.32 nM and maximum receptor density of 1.37 ± 0.34 fmole/mg protein. Emulsion autoradiography performed using ¹²⁵I-SCH 23982 on antimesenteric sections of the rat small intestine revealed that the dopamine₁ receptors are located on cells at the base of the intestinal crypts. Two dopamine₁ subtypes (D_1A and D_1B) have been identified by molecular biological techniques. Using a ribonuclease protection assay we found expression of the D_1A receptor gene in the small intestinal tissue. These studies are the first to identify, characterize, and localize receptors for the endogenous catecholamine, dopamine, within the rat small intestine and to confirm the expression of the D_1A receptor gene.