Receptors for neurotransmitters and related substances

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.

Coupling of D2 and D3 dopamine receptors to G-proteins

Recombinant cell lines expressing D2 and D3 dopamine receptor isoforms have been used to study coupling of these receptors to G-proteins from the effects of GTP to reduce agonist binding affinities in agonist/[3H]spiperone competition experiments. D2(short) expressed in Ltk- cells, D2(long) expressed in Ltk- or CHO cells and D3 expressed in CHO cells all showed coupling to the endogenous G-proteins of the cells. The detailed agonist binding characteristics indicated that there were differences in receptor/G-protein coupling for the same receptor(D2(long)) expressed in two cell types or for different receptors (D2(short), D2(long)) expressed in the same cell type.

Studies on the structure of the ligand-binding site of the brain D1 dopamine receptor

A series of group-specific modifying reagents were tested for their effects on [3H]SCH23390 binding to brain D1 dopamine receptors in order to identify amino acid residues at the ligand binding site of the D1 dopamine receptor that are critical for ligand binding. The dependence of ligand binding on the pH of the incubation medium was also examined. The histidine-selective reagent, diethylpyrocarbonate did affect ligand binding but this is probably not due to an effect at the ligand binding site. Experiments with N-acetylimidazole and ethylacetimidate indicated that modification of tyrosine and amino residues did not exert major influences at the ligand binding site. The use of the thiol dithiothreitol indicated that breakage of a disulphide bond altered ligand binding, probably by affecting the receptor conformation, and the use of the sulphydryl reagent 5,5'-dithio-bis-nitrobenzoic acid showed that modification of a sulphydryl group on the receptor inhibited ligand binding. The carboxyl reagent N,N'-dicyclohexyl carbodiimide (DCCD) potently inhibited ligand binding and the effect could be prevented by occupancy of the receptor site by an agonist or antagonist so that there is an important carboxyl group at the receptor binding site. The total number of D1 receptors was reduced after the modification by DCCD and 70% of the residual receptors showed a reduced affinity for binding [3H]SCH23390, the remainder having the same affinity as untreated receptors. [3H]SCH23390 binding is also reduced by a decrease of pH and this effect seems to depend on the protonation of a group of pKa 6.9. Saturation analysis of [3H]SCH23390 binding performed at pH 7.5 shows a single class of high affinity sites whereas at pH 6.0, two classes of sites with higher and lower affinities are seen. These studies suggested a model whereby [3H]SCH23390 binding is to two receptor isoforms with different pH dependencies for [3H]SCH23390 binding.

Importance of thiol groups in ligand binding to D2 dopamine receptors from brain and anterior pituitary gland

The effects of the thiol group reagent 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) on D2 dopamine receptors have been examined in three brain regions (caudate nucleus, putamen and olfactory tubercle), and in the anterior and neurointermediate lobes of the pituitary gland. Whereas the receptors in brain were insensitive to DTNB, a dose-dependent inhibition of [3H]spiperone binding to D2 receptors in both lobes of the pituitary gland was observed. The effects in the pituitary could be reversed by subsequent treatment with dithiothreitol and could be prevented by prior occupancy of the receptor binding site with a ligand. The effect of DTNB was to decrease the number of ligand-binding sites without altering the affinity of those remaining. The results show that modification of a thiol group of the D2 dopamine receptor in the two lobes of the pituitary gland tested here significantly affects ligand binding. There is therefore a difference between the D2 dopamine receptor populations in brain and pituitary in their sensitivity to modification by DTNB, and this may imply the existence of different receptor isoforms in the two issues.