Characterisation of recombinant serotonin 5-HT1A receptors expressed in Chinese hamster ovary cells: the agonist [3H]lisuride labels free receptor and receptor coupled to G protein

Serotonin 5-HT1A receptors expressed stably in recombinant Chinese hamster ovary cells have been studied using radioligand binding with the radiolabelled agonist [3H]lisuride. Competition studies with a range of antagonists versus [3H]lisuride confirmed that all of the specific [3H]lisuride binding was to 5-HT1A receptors on the cells. Competition studies with the antagonist spiperone and several agonists gave data that fitted best to two-binding-site models. The affinities of these competing ligands at the two classes of sites were generally in agreement with their corresponding affinities determined in previous work with either 8-[3H]hydroxydipropylaminotetralin ([3H]8-OH-DPAT; labels receptor coupled to G protein) or [3H]spiperone (labels free receptor). Saturation analyses with [3H]lisuride showed that this radioligand labels a single class of binding sites, but the level of radioligand binding was approximately twice that seen when either [3H]8-OH-DPAT or [3H]spiperone was used. [3H]Lisuride binding was partially inhibited by addition of guanine nucleotides, and the extent of inhibition decreased as the [3H]lisuride concentration was increased. This inhibition was due to the effect of guanine nucleotide to decrease slightly the affinity of [3H]lisuride for binding to the 5-HT1A receptors on the cells. It is concluded that [3H]lisuride can label both the free receptor and the receptor coupled to G proteins but with slightly different affinities and that these two states of the receptor exist in roughly equal amounts in the cells. Agonists generally have a higher affinity for the receptor coupled to G protein, whereas antagonists, with the exception of spiperone (which has a higher affinity for the free receptor), have roughly equal affinities for the free receptor and the receptor coupled to G proteins.

pH dependence of ligand binding to D2 dopamine receptors

The binding of a range of ligands to D2 dopamine receptors in bovine caudate nucleus and recombinant CHO cells expressing the receptor has been determined at different pH values between 4.5 and 8.5. The maximum number of D2 dopamine receptor binding sites in each tissue was not affected by the change in pH, but the affinity of ligands for binding to the receptors was decreased as the pH was decreased. For classical dopamine antagonists, e.g. spiperone and haloperidol, the data on pH dependence of the dissociation constant for receptor binding indicated that the protonation of a single ionizing group on the receptor (pKa approximately 6) influenced the binding process. For antagonists of the substituted benzamide class, the data indicated that the protonation of two ionizing groups (pKa between 6 and 7) influenced the ligand binding process. These ionizing residues may correspond to Asp 114 for the classical antagonists and Asp 114 and Asp 80 for the substituted benzamide antagonists. Further evidence for the participation of carboxyl residues in the ligand binding process was obtained from the inhibition by N,N'-dicyclohexylcarbodiimide of the binding of [3H]spiperone and [3H]YM 09151-2 to D2 receptors in the recombinant CHO cells.

[3H]nemonapride and [3H]spiperone label equivalent numbers of D2 and D3 dopamine receptors in a range of tissues and under different conditions

[3H]Nemonapride and [3H]spiperone are very widely used to study dopaminergic systems in vitro and in vivo, but it has been reported that [3H]nemonapride and [3H]spiperone give markedly different Bmax values for preparations of D2 dopamine receptors from recombinant cell lines or animal tissues. We have used the two radioligands in parallel to study a range of dopamine receptors [D2(short), D2(long), and D3] in different buffers. Bmax values derived using either radioligand differ by an average of < 20%, independent of receptor type or buffer conditions. All competition experiments show that the two ligands compete at a single site. It seems that [3H]spiperone and [3H]nemonapride do not differentiate between different forms or populations of D2-like receptors.

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.

Dopamine receptors in the basal ganglia: relevance to Parkinson's disease

At least five receptors for dopamine (D1-D5) have been recognised from molecular biological studies, and their pharmacological properties and brain localisations have been determined. The D1 and D2 subtypes are the principal subtypes in brain, and their cellular localisations in the caudate nucleus and putamen have been determined. With recent advances in the understanding of basal ganglia neuronal function, these localisation data enable insights into the mode of action of drugs used at present and in the future to treat Parkinson's disease.

Characterization of recombinant human serotonin 5HT1A receptors expressed in Chinese hamster ovary cells. [3H]spiperone discriminates between the G-protein-coupled and -uncoupled forms

5HT1A serotonin (5-hydroxytryptamine) receptors have been characterized by ligand binding in a recombinant Chinese Hamster Ovary cell line expressing the human receptor gene. The agonist ligand [3H]2-(N,N-dipropylamino)-8-hydroxy-1,2,3,4-tetrahydronaphthalene ([3H]8-OH-DPAT) and the antagonist [3H]spiperone were used. For both radioligands the binding sites labelled have the properties of 5HT1A receptors and most antagonists show roughly equal affinities for the receptors labelled by either [3H]8-OH-DPAT or [3H]spiperone. Agonists, however, show higher affinities for the sites labelled by [3H]8-OH-DPAT and the antagonist spiperone conversely shows a higher affinity for the sites labelled by [3H]spiperone. Whereas [3H]8-OH-DPAT binding is inhibited by guanosine triphosphate (GTP) the binding of [3H]spiperone is increased by GTP. A model is proposed for the results whereby [3H]8-OH-DPAT labels a form of the receptor coupled to a G-protein and [3H]spiperone labels a form of the receptor uncoupled from G-proteins (or possibly coupled to a different G-protein).

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.

Antisera specific for D2 dopamine receptors

Antisera have been raised against two peptides from the sequence of D2 dopamine receptors: peptide 1 from the predicted second extracellular loop and peptide 2 from the predicted third intracellular loop. The antisera recognize specifically a 95 kDa band in Western blots of several bovine brain regions, which corresponds to the denatured D2 dopamine receptor, whereas in recombinant CHO cells expressing D2 dopamine receptors a 80 kDa band is seen. The antisera immunoprecipitate 10-20% of the D2 dopamine receptors from soluble preparations of bovine brain. The antisera recognize D2 dopamine receptors in immunofluorescence analyses of recombinant CHO cells bearing the receptor gene. The antisera directed against the third intracellular loop, but not those against the second extracellular loop, will interfere with the coupling of D2 dopamine receptors and G-proteins in bovine brain preparations.