Tocris ScientificDopamine Review Receptors Series Tocri-lu-2945 Dopamine Receptors Philip G. Strange1 and Kim Neve2 History 1School of Pharmacy, University of Reading, Whiteknights, It was not until the late 1950s that dopamine was recognized Reading, RG6 6AJ, UK, Email: [email protected] as a neurotransmitter in its own right, but the demonstration of its non-uniform distribution in the brain, distinct from the 2 VA Medical Center and Department of Behavioral Neuroscience, distribution of noradrenaline, suggested a specific functional Oregon Health & Science University, Portland, OR 97239, USA, role for dopamine.1 Interest in dopamine was intensified by Email: [email protected] the realization that dopamine had an important role in the Professor Philip Strange has worked on the structure and pathogenesis or drug treatment of certain neurological disorders, function of G protein-coupled receptors for many years. A major e.g. Parkinson’s disease and schizophrenia.2,3 This led to much focus of his work has been the receptors for the neurotransmitter research on the sites of action of dopamine and the dopamine dopamine, with particular emphasis on their role as targets receptors (Box 1). One milestone was the suggestion by Cools for drugs and understanding the mechanisms of agonism and and van Rossum, based on anatomical, electrophysiological inverse agonism at these receptors. and pharmacological studies, that there might be more than one kind of receptor for dopamine in the brain.4 Biochemical Professor Kim Neve has studied dopamine receptors for most studies on dopamine receptors in the 1970s based on second of his scientific career, with an emphasis on relating structural messenger assays (e.g. stimulation of cAMP production and features of the receptors to specific functions and assessing how ligand binding assays) supported the idea, and it was given a firm receptor responsiveness is altered by denervation or prolonged foundation by Kebabian and Calne in their 1979 review.5 They treatment with agonists. extended an earlier suggestion by Spano et al,6 and proposed that there were two classes of dopamine receptor, D1 and D2, with different biochemical and pharmacological properties, mediating different physiological functions. The properties of these two subtypes are summarized in Table 1. Selective agonists and antagonists exist to define the two subtypes in functional assays and some of these are shown in Table 1. Both the D1 and D2 subtypes are G protein-coupled receptors (GPCRs), yet different G proteins and effectors are involved in their signaling pathways (Figure 1, Table 1). Although there were some indications of further heterogeneity of these dopamine receptor subtypes in biochemical studies, it was not until the late 1980s that the true extent of this was revealed with the application of gene cloning techniques. These studies have shown that there are at least five dopamine receptors (D1– D5) that may be divided into two subfamilies whose properties 7,8 resemble the original D1 and D2 receptors. The D1-like receptor family, which comprises D1 and D5, corresponds to the original D1 receptors whilst the D2-like receptor family (D2, D3 and D4 Contents receptors) corresponds to the original D2 receptors. A selection History ...................................................................................................... 1 of the key properties of the receptor subtypes are summarized in Tables 2 and 3. Properties of the Dopamine Receptor Subtypes ................................. 2 Future Directions .................................................................................... 6 In subsequent discussion we refer to receptor subtypes defined from cloned genes as D , D , D , D , D , and where only the References .............................................................................................. 8 1 2 3 4 5 subfamily of receptor has been defined pharmacologically we Dopamine Compounds .......................................................................... 9 use the D1-like and D2-like nomenclature. tocris.com | 1 Tocris Scientific Review Series Properties of the Dopamine Receptor the membrane to form the ligand binding site (Figure 2); some information is available on the residues that make contact with Subtypes ligands.11,12 There is an intracellular carboxyl terminus, probably bearing a palmitoyl group, which may form a further link to the Common receptor properties membrane. The D1-like receptors have short third intracellular loops and long carboxyl terminal tails, whereas the D -like Analysis of the amino acid sequences of the dopamine receptor 2 subtypes has shown that significant homologies exist among receptors have long third intracellular loops and short carboxyl the subtypes, with the greatest being found between members terminal tails. This provides a structural basis for the division of either subfamily.7,8 Each receptor has been shown to contain of the receptors into two subfamilies but is also likely to have seven stretches of amino acids that are hydrophobic and long a functional significance, possibly related to the specificity of enough to span the membrane. It seems therefore that each of receptor/G protein interaction. the dopamine receptors conforms to the general structural model The third intracellular loop, termed ‘I3’, is important for the for a GPCR,9–11 with an extracellular amino terminus and seven interaction of the receptor and G protein. For the D2-like putative membrane spanning α-helices linked by intracellular receptors, variants of the subtypes exist based on this loop. For and extracellular protein loops (Figure 2). One or more potential example, there are short and long splice variants of the D2 and D3 sites for glycosylation are found on the amino terminus and receptors with the long forms having an insertion (29 amino acids second extracellular loop. The helices are bundled together in 13,14 for the long D2, D2L) in this loop. Polymorphic variants of the D2 receptor have been described with single amino acid changes 15 in I3. The D4 receptor is highly polymorphic in the human Figure 1 | Regulation of adenylyl cyclase by D and D dopamine 1 2 population with variants containing different length insertions receptors 16,17 in I3. In some cases, these D2-like receptor variants may Dopamine Dopamine Figure 2 | Schematic representation of a G protein-coupled D1-like receptor D2-like receptor dopamine receptor AC Extracellular space H N βγ + – βγ 2 Gαs Gαi/o Disulfide bond ATP E1 cAMP E2 E3 3 4 2 1 The diagram shows the effects of dopamine to stimulate or inhibit 5 α 6 7 adenylyl cyclase (AC) via the D1-like receptor and G protein G s or the D2-like receptor and G protein Gai/o, respectively. BoxBox 1: Dopamine 1: Dopamine receptor receptor products products Box 1: BoxDopamine 1: Dopamine receptor receptor products products Box 1 | Dopamine synthesis and metabolism I1 Palmitoyl NO O NO2 2 O P I2 group NO2 NO2O O O N O N H P O N O H N HO NH H H I3 HO NH2 2 HO HO NH2 NH2 OH HO OH HO OH OH HOOC HO HO OH OH Cytoplasm OH OH P P DopamineDopamine (3548) (3548) NPEC-caged-dopamineNPEC-caged-dopamine (3992) (3992) DopamineEndogenousDopamineEndogenous (3548)Dopamine dopamine (3548) dopamine (3548) NPEC-caged-dopamineNPEC-caged-dopamineCagedNPEC-caged-dopamineCaged version version of dopamine (3992)of (3992)dopamine (3992) EndogenousreceptorEndogenousEndogenousreceptor dopamine agonist agonist dopamine dopamine CagedCaged versionCaged version of version dopamine of dopamine of dopamine receptor agonistreceptorreceptor agonist agonist The diagram shows the seven helices bundled together in the membrane and the intra- (I1, I2, I3) and extracellular (E1, E2, E3) SO Me SO2Me 2 loops. The ligand binding site is contained in the cavity formed SO2Me SO2Me between the helices. There may be an eighth helix formed in the HO CO H H HO CO2H 2 H carboxyl terminus parallel to the membrane (not shown). There is HO HO CO2H CO2H H H N N also a disulfide bond between E2 and the top of helix 3. The NH NH2 HO 2 N N HO NH2 NH2 helices have been drawn parallel to one another for clarity but in HO HO fact there are kinks in the helices and they are not fully parallel. L-DOPAL-DOPA (3788)L-DOPA (3788) (3788) OSUOSU OSU6162 6162 6162(2599) (2599) (2599) I3 and the carboxyl terminus contain multiple sites for L-DOPADopamine (3788)L-DOPADopamineDopamine precursor (3788) precursor precursor OSU Dopamine6162DopamineOSU Dopamine(2599) 6162 stabilizer stabilizer (2599) stabilizer phosphorylation that are involved in regulation of receptor DopamineDopamine precursor precursor DopamineDopamine stabilizer stabilizer responsiveness and interactions with adapter proteins. 2 | tocris.com Dopamine Receptors 18,19 have differential abilities to couple to or activate G proteins, D1-like antagonist that is useful because it is structurally distinct and may also exhibit slightly different pharmacological from the benzazepines. Thioxanthines such as flupentixol and properties.16,20,21 Lines of mice have been developed in which the phenothiazines such as fluphenazine also show high affinity I3 insertion that produces the long D2 receptor variant (D2L) is but are not selective for D1-like over D2-like receptors. The deleted, resulting in the expression of only the short variant (D2S). development of the first 1D -like receptor agonist, SKF 38393, was Characterization of these mice suggests that the splice variants important for differentiating between activation of D1-like and are not fully interchangeable; some