pH dependence of sulpiride binding to D2 dopamine receptors in bovine brain

Imaging the dopamine system with in vivo [11C]raclopride displacement studies: understanding the true mechanism

Measuring changes in dopamine (DA) levels in humans using radioligand-displacement studies and positron emission tomography (PET) has provided important empirical findings in diseases and normal neurophysiology. These studies are based on the assumption that DA exerts a competitive inhibition on D(2)-radioligand binding. However, the transfer of this hypothesis to a proven mechanism has not been fully achieved yet and an accumulating number of studies challenge it. In addition, new evidence suggests that DA exerts a noncompetitive inhibition on D(2)-radioligand binding under amphetamine conditions. This article reviews the theoretical basis for the DA competition hypothesis, the in vivo and in vitro evidences supporting a noncompetitive action of DA on D(2)-radioligand binding under amphetamine conditions, and discusses possible mechanisms underlying this noncompetitive interaction. Finally, we propose that such noncompetitive interactions may have important implications for how one interprets findings obtained from radioligand-displacement PET studies in neuropsychiatric diseases, especially in schizophrenia in which a dysregulation of the DA-promoted internalization of D(2) receptors was recently suggested.

Interaction of permanently charged metoclopramide analogs with D-2 dopamine receptors

1. The binding of permanently charged benzamides to the D-2 dopamine receptor of striatal membranes was compared with that of tertiary amine benzamides. 2. The permanently charged benzamides were able to inhibit the binding of [3H]-spiperone to striatal membranes but were less potent than the corresponding tertiary amines. 3. Removal of sodium or decreasing the pH from 7.8 to 6.2 decreased the binding of all benzamides tested, but the permanently charged analogs were affected less by these changes than the tertiary amines. 4. These results suggest that while the binding properties of the permanently charged benzamides are similar to those of the tertiary amine benzamides, there are differences in the manner in which these compounds interact with the D-2 receptor.

Structural studies on D2 dopamine receptors: mutation of a histidine residue specifically affects the binding of a subgroup of substituted benzamide drugs

A histidine residue (His394) that is likely to be located in the ligand-binding region of the D2 dopamine receptor has been mutated to a leucine (Leu394), and the properties of the mutant receptor have been determined. For a range of antagonists the mutation has only a minor effect on the affinity of the receptor for the antagonist. The mutation does, however, elicit a structurally specific effect on the affinity with which certain members of the substituted benzamide class of antagonist bind to the receptor. Some of these drugs, e.g., sulpiride, sultopride, and tiapride, bind with reduced affinity to the mutated receptor, whereas others, e.g., clebopride and metoclopramide, bind with increased affinity. However, the Na+/H+ sensitivity of the binding of sulpiride to the receptor is not reduced by the mutation. These findings have been interpreted in terms of the productive or unfavourable interaction of the His394 residue with these compounds.

New insights into dopamine receptors in the central nervous system

Whereas biochemical and pharmacological studies indicated that there were two subclasses of dopamine receptor (D1, D2) the application of molecular biology techniques has defined at least six dopamine receptor isoforms. These may be divided into D1-like (D1, D5) and D2-like (D2(short), D2(long), D3, D4) subfamilies on the basis of their structural and pharmacological properties. In this commentary the common properties of these dopamine receptor species are described, including the predicted structures of seven transmembrane alpha-helices, amino acid homologies and conserved amino acids that may play important structural and functional roles. The D1-like and D2-like receptor isoforms have individual properties and these are described in terms of their structures, pharmacological and biochemical properties and localizations in different brain regions. The existence of multiple dopamine receptor isoforms is important for understanding how certain drugs achieve their therapeutic effects and how unwanted side effects arise. This is considered for the anti-parkinsonian and anti-schizophrenic drugs. The localization of D1 and D2 dopamine receptors to particular cell types in the neostriatum allows new insights to be made into the normal mode of action of dopamine to control motor function and how this is disturbed in disease stages e.g. Parkinson's disease, Huntington's disease. The detailed mode of action of anti-parkinsonian drugs can also be better understood from this. The availability, from molecular biology studies, of the amino acid sequences of the receptor isoforms allows predictions to be made of the structures of these species. In particular it is possible to produce speculative models of the three dimensional structures of the ligand-binding sites of these receptors. These speculations can be complemented by chemical modification, pH dependency and mutagenesis studies which provide information on the amino acid residues at the ligand binding site that actually interact with the ligand. In time it should be possible to understand in some detail the mechanism of receptor-ligand interaction and this will be important for the design of drugs targeted at specific isoforms.