D2-type dopamine receptors are major recognition sites for antipsychotic drugs. There are two splice variants: D2S and D2L with an additional 29 amino acid sequence in the third intracellular loop. Only little comparative information is hitherto available about their pharmacological properties and none of these studies dealt with intact cell systems. This prompted us to investigate the binding properties of [3H]-raclopride, a hydrophilic benzamide, and [3H]-spiperone, a highly hydrophobic butyrophenone, to intact CHO cells expressing recombinant human D2L-receptors . Presently, we have repeated and extended this experimental approach to the human D2S-receptors in the same cell system. Except for a slower dissociation of [3H]-spiperone from D2S, the binding properties of these, and other antagonists were not significantly different for both isoforms (P > 0.05). The very slow dissociation of the atypical antipsychotic clozapine was surprising in light of its low affinity. Two experiments pointed at the existence of non-competitive interactions between raclopride and spiperone for D2S as well as D2L (Packeu et al., manuscript submitted). Alongside the different physicochemical properties of these ligands, this finding fits with a model  wherein the hydrophilic raclopride approaches the D2L-receptor from the aqueous phase while the hydrophobic spiperone approaches the receptor by lateral diffusion between the membrane lipids. These different modes of approach could imply the existence of topologically distinct ligand binding sites at D2-receptors.
|Number of pages||11|
|Journal||Fundamental and Clinical Pharmacology|
|Publication status||Published - Jun 2010|
- dopamine receptor
- intact cells
- radioligand binding