Pharmacological analysis of dopamine stimulation of [35S]-GTP gamma S binding via human D2short and D2long dopamine receptors expressed in recombinant cells

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Investigating the ligand agonism and antagonism at the D2long receptor by dynamic mass redistribution

The signalling of the D₂ receptor (D₂R), a G protein-coupled receptor (GPCR), is a complex process consisting of various components. For the screening of D₂R ligands, methods quantifying distinct second messengers such as cAMP or the interaction of the receptor with β-arrestin, are commonly employed. In contrast, a label-free biosensor technology like dynamic mass redistribution (DMR), where it is mostly unknown how the individual signalling pathways contribute to the DMR signal, provides a holistic readout of the complex cellular response. In this study, we report the successful application of the DMR technology to CHO-K1 cells stably expressing the human dopamine D_2long receptor. In real-time kinetic experiments, studies of D₂R reference compounds yielded results for agonists and antagonists that were consistent with those obtained by conventional methods and also allowed a discrimination between partial and full agonists. Furthermore, investigations on the signalling pathway in CHO-K1 hD_2longR cells identified the Gα_i/o protein as the main proximal trigger of the observed DMR response. The present study has shown that the DMR technology is a valuable method for the characterisation of putative new ligands and, due to its label-free nature, suggests its use for deorphanisation studies of GPCRs.

Diabetes and Cognitive Impairment: A Role for Glucotoxicity and Dopaminergic Dysfunction

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by hyperglycemia, responsible for the onset of several long-term complications. Recent evidence suggests that cognitive dysfunction represents an emerging complication of DM, but the underlying molecular mechanisms are still obscure. Dopamine (DA), a neurotransmitter essentially known for its relevance in the regulation of behavior and movement, modulates cognitive function, too. Interestingly, alterations of the dopaminergic system have been observed in DM. This review aims to offer a comprehensive overview of the most relevant experimental results assessing DA’s role in cognitive function, highlighting the presence of dopaminergic dysfunction in DM and supporting a role for glucotoxicity in DM-associated dopaminergic dysfunction and cognitive impairment. Several studies confirm a role for DA in cognition both in animal models and in humans. Similarly, significant alterations of the dopaminergic system have been observed in animal models of experimental diabetes and in diabetic patients, too. Evidence is accumulating that advanced glycation end products (AGEs) and their precursor methylglyoxal (MGO) are associated with cognitive impairment and alterations of the dopaminergic system. Further research is needed to clarify the molecular mechanisms linking DM-associated dopaminergic dysfunction and cognitive impairment and to assess the deleterious impact of glucotoxicity.

Quantification of kappa opioid receptor ligand potency, efficacy and desensitization using a real-time membrane potential assay

We explored the utility of the real-time FLIPR Membrane Potential (FMP) assay as a method to assess kappa opioid receptor (KOR)-induced hyperpolarization. The FMP Blue dye was used to measure fluorescent signals reflecting changes in membrane potential in KOR expressing CHO (CHO-KOR) cells. Treatment of CHO-KOR cells with kappa agonists U50,488 or dynorphin [Dyn (1-13)NH₂] produced rapid and concentration-dependent decreases in FMP Blue fluorescence reflecting membrane hyperpolarization. Both the nonselective opioid antagonist naloxone and the κ-selective antagonists nor-binaltorphimine (nor-BNI) and zyklophin produced rightward shifts in the U50,488 concentration-response curves, consistent with competitive antagonism of the KOR mediated response. The decrease in fluorescent emission produced by U50,488 was blocked by overnight pertussis toxin pretreatment, indicating the requirement for PTX-sensitive G proteins in the KOR mediated response. We directly compared the potency of U50,488 and Dyn (1-13)NH₂ in the FMP and [³⁵S]GTPγS binding assays, and found that both were approximately 10 times more potent in the cellular fluorescence assay. The maximum responses of both U50,488 and Dyn (1-13)NH₂ declined following repeated additions, reflecting receptor desensitization. We assessed the efficacy and potency of structurally distinct KOR small molecule and peptide ligands. The FMP assay reliably detected both partial agonists and stereoselectivity. Using KOR-selective peptides with varying efficacies, we found that the FMP assay allowed high throughput quantification of peptide efficacy. These data demonstrate that the FMP assay is a sensitive method for assessing κ-opioid receptor induced hyperpolarization, and represents a useful approach for quantification of potency, efficacy and desensitization of KOR ligands.

Increasing the throughput of label-free cell assays to study the activation of G-protein-coupled receptors by using a serial agonist exposure protocol

Label-free, holistic assays, monitoring, for example, the impedance of cells on electrodes, are gaining increasing popularity in the evaluation of G-protein-coupled receptor (GPCR) ligands. It is the strength of these approaches to provide the integrated cellular response non-invasively, highly automated and with a device-dependent time resolution down to several milliseconds. With an increasing number of samples to be studied in parallel, the available time resolution is, however, reduced and the cost for the disposable sensor arrays may become limiting. Inspired by protocols from organ pharmacology, we investigated a simple serial agonist addition assay that circumvents these limitations in impedance-based cellular assays. Using a serial addition of increasing concentrations of a GPCR agonist while continuously monitoring the sample's impedance, we were able to establish a full concentration-response curve for the endogenous agonist histamine on a single layer of U-373 MG cells endogenously expressing the histamine 1 receptor (H1R). This approach is validated with respect to conventional, parallel agonist addition protocols and studies using H1R antagonists such as mepyramine. Applicability of the serial agonist addition assay was shown for other GPCRs known for their signaling via one of the canonical G-protein pathways, Gq, Gi/0 or Gs as well. The serial agonist addition protocol has the potential to further strengthen the output of label-free analysis of GPCR activation.

Dopamine: Functions, Signaling, and Association with Neurological Diseases

The dopaminergic system plays important roles in neuromodulation, such as motor control, motivation, reward, cognitive function, maternal, and reproductive behaviors. Dopamine is a neurotransmitter, synthesized in both central nervous system and the periphery, that exerts its actions upon binding to G protein-coupled receptors. Dopamine receptors are widely expressed in the body and function in both the peripheral and the central nervous systems. Dopaminergic signaling pathways are crucial to the maintenance of physiological processes and an unbalanced activity may lead to dysfunctions that are related to neurodegenerative diseases. Unveiling the neurobiology and the molecular mechanisms that underlie these illnesses may contribute to the development of new therapies that could promote a better quality of life for patients worldwide. In this review, we summarize the aspects of dopamine as a catecholaminergic neurotransmitter and discuss dopamine signaling pathways elicited through dopamine receptor activation in normal brain function. Furthermore, we describe the potential involvement of these signaling pathways in evoking the onset and progression of some diseases in the nervous system, such as Parkinson's, Schizophrenia, Huntington's, Attention Deficit and Hyperactivity Disorder, and Addiction. A brief description of new dopaminergic drugs recently approved and under development treatments for these ailments is also provided.

Differential expression and signaling of the human histamine H3 receptor isoforms of 445 and 365 amino acids expressed in human neuroblastoma SH-SY5Y cells

In stably-transfected human neuroblastoma SH-SY5Y cells, we have compared the effect of activating two isoforms of 445 and 365 amino acids of the human histamine H₃ receptor (hH₃R₄₄₅ and hH₃R₃₆₅) on [³⁵S]-GTPγS binding, forskolin-induced cAMP formation, depolarization-induced increase in the intracellular concentration of Ca²⁺ ions ([Ca²⁺]i) and depolarization-evoked [³H]-dopamine release. Maximal specific binding (B_max) of [³H]-N-methyl-histamine to cell membranes was 953 ± 204 and 555 ± 140 fmol/mg protein for SH-SY5Y-hH₃R₄₄₅ and SH-SY5Y-hH₃R₃₆₅ cells, respectively, with similar dissociation constants (K_d, 0.86 nM and 0.81 nM). The mRNA of the hH₃R₃₆₅ isoform was 40.9 ± 7.9% of the hH₃R₄₄₅ isoform. No differences in receptor affinity were found for the H₃R ligands histamine, immepip, (R)(-)-α-methylhistamine (RAMH), A-331440, clobenpropit and ciproxifan. Both the stimulation of [³⁵S]-GTPγS binding and the inhibition of forskolin-stimulated cAMP accumulation by the agonist RAMH were significantly larger in SH-SY5Y-hH₃R₄₄₅ cells ([³⁵S]-GTPγS binding, 158.1 ± 7.5% versus 136.5 ± 3.6% for SH-SY5Y-hH₃R₃₆₅ cells; cAMP accumulation, -74.0 ± 4.9% versus -43.5 ± 5.3%), with no significant effect on agonist potency. In contrast, there were no differences in the efficacy and potency of RAMH to inhibit [³H]-dopamine release evoked by 100 mM K⁺ (-18.9 ± 3.0% and -20.5 ± 3.3%, for SH-SY5Y-hH₃R₄₄₅ and SH-SY5Y-hH₃R₃₆₅ cells), or the inhibition of depolarization-induced increase in [Ca²⁺]i (S2/S1 ratios: parental cells 0.967 ± 0.069, SH-SY5Y-hH₃R₄₄₅ cells 0.639 ± 0.049, SH-SY5Y-hH₃R₃₆₅ cells 0.737 ± 0.045). These results indicate that in SH-SY5Y cells, hH₃R₄₄₅ and hH₃R₃₆₅ isoforms regulate in a differential manner the signaling pathways triggered by receptor activation.

Advances and challenges in the search for D2 and D3 dopamine receptor-selective compounds

Compounds that target D2-like dopamine receptors (DRs) are currently used as therapeutics for several neuropsychiatric disorders including schizophrenia (antagonists) and Parkinson's disease (agonists). However, as the D₂R and D₃R subtypes are highly homologous, creating compounds with sufficient subtype-selectivity as well as drug-like properties for therapeutic use has proved challenging. This review summarizes the progress that has been made in developing D₂R- or D₃R-selective antagonists and agonists, and also describes the experimental conditions that need to be considered when determining the selectivity of a given compound, as apparent selectivity can vary widely depending on assay conditions. Future advances in this field may take advantage of currently available structural data to target alternative secondary binding sites through creating bivalent or bitopic chemical structures. Alternatively, the use of high-throughput screening techniques to identify novel scaffolds that might bind to the D₂R or D₃R in areas other than the highly conserved orthosteric site, such as allosteric sites, followed by iterative medicinal chemistry will likely lead to exceptionally selective compounds in the future. More selective compounds will provide a better understanding of the normal and pathological functioning of each receptor subtype, as well as offer the potential for improved therapeutics.

Evidence for Noncanonical Neurotransmitter Activation: Norepinephrine as a Dopamine D2-Like Receptor Agonist

The Gαi/o-coupled dopamine D2-like receptor family comprises three subtypes: the D2 receptor (D2R), with short and long isoform variants (D2SR and D2LR), D3 receptor (D3R), and D4 receptor (D4R), with several polymorphic variants. The common overlap of norepinephrine innervation and D2-like receptor expression patterns prompts the question of a possible noncanonical action by norepinephrine. In fact, previous studies have suggested that norepinephrine can functionally interact with D4R. To our knowledge, significant interactions between norepinephrine and D2R or D3R receptors have not been demonstrated. By using radioligand binding and bioluminescent resonance energy transfer (BRET) assays in transfected cells, the present study attempted a careful comparison between dopamine and norepinephrine in their possible activation of all D2-like receptors, including the two D2R isoforms and the most common D4R polymorphic variants. Functional BRET assays included activation of G proteins with all Gαi/o subunits, adenylyl cyclase inhibition, and β arrestin recruitment. Norepinephrine acted as a potent agonist for all D2-like receptor subtypes, with the general rank order of potency of D3R > D4R ≥ D2SR ≥ D2L. However, for both dopamine and norepinephrine, differences depended on the Gαi/o protein subunit involved. The most striking differences were observed with Gαi2, where the rank order of potencies for both dopamine and norepinephrine were D4R > D2SR = D2LR >> D3R. Furthermore the results do not support the existence of differences in the ability of dopamine and norepinephrine to activate different human D4R variants. The potency of norepinephrine for adrenergic α2A receptor was only about 20-fold higher compared with D3R and D4R across the three functional assays.

A single-point mutation (Ala280Val) in the third intracellular loop alters the signalling properties of the human histamine H₃ receptor stably expressed in CHO-K1 cells

BACKGROUND AND PURPOSE

An alanine to valine exchange at amino acid position 280 (A280V) in the third intracellular loop of the human histamine H₃ receptor was first identified in a patient suffering from Shy-Drager syndrome and later reported as a risk factor for migraine. Here, we have compared the pharmacological and signalling properties of wild-type (hH₃ R(WT)) and A280V mutant (hH₃ R(A280V)) receptors stably expressed in CHO-K1 cells.

EXPERIMENTAL APPROACH

The hH₃ R(A280V) cDNA was created by overlapping extension PCR amplification. Receptor expression and affinity were assessed by radioligand (N-α-[methyl-³H]-histamine) binding to cell membranes, and receptor function by the inhibition of forskolin-induced cAMP accumulation and stimulation of ERK1/2 phosphorylation in intact cells, as well as stimulation of [³⁵S]-GTPγS binding to cell membranes.

KEY RESULTS

Both receptors were expressed at similar levels with no significant differences in their affinities for H₃ receptor ligands. Upon activation the hH₃ RWT was significantly more efficacious to inhibit forskolin-induced cAMP accumulation and to stimulate [³⁵S]-GTPγS binding, with no difference in pEC50 estimates. The hH₃ RWT was also more efficacious to stimulate ERK1/2 phosphorylation, but this difference was not significant. The inverse agonist ciproxifan was more efficacious at hH3 RWT to reduce [³⁵S]-GTPγS binding but, for both receptors, failed to enhance forskolin-induced cAMP accumulation.

CONCLUSIONS AND IMPLICATIONS

The A280V mutation reduces the signalling efficacy of the human H₃ receptor. This effect may be relevant to the pathophysiology of disorders associated with the mutation.

Characterisation of AmphiAmR11, an amphioxus (Branchiostoma floridae) D2-dopamine-like G protein-coupled receptor

The evolution of the biogenic amine signalling system in vertebrates is unclear. However, insights can be obtained from studying the structures and signalling properties of biogenic amine receptors from the protochordate, amphioxus, which is an invertebrate species that exists at the base of the chordate lineage. Here we describe the signalling properties of AmphiAmR11, an amphioxus (Branchiostoma floridae) G protein-coupled receptor which has structural similarities to vertebrate α₂-adrenergic receptors but which functionally acts as a D₂ dopamine-like receptor when expressed in Chinese hamster ovary -K1 cells. AmphiAmR11 inhibits forskolin-stimulated cyclic AMP levels with tyramine, phenylethylamine and dopamine being the most potent agonists. AmphiAmR11 also increases mitogen-activated protein kinase activity and calcium mobilisation, and in both pathways, dopamine was found to be more potent than tyramine. Thus, differences in the relative effectiveness of various agonists in the different second messenger assay systems suggest that the receptor displays agonist-specific coupling (biased agonism) whereby different agonists stabilize different conformations of the receptor which lead to the enhancement of one signalling pathway over another. The present study provides insights into the evolution of α₂-adrenergic receptor signalling and support the hypothesis that α₂-adrenergic receptors evolved from D₂-dopamine receptors. The AmphiAmR11 receptor may represent a transition state between D₂-dopamine receptors and α₂-adrenergic receptors.

Homologous desensitization of human histamine H₃ receptors expressed in CHO-K1 cells

Histamine H₃ receptors (H₃Rs) modulate the function of the nervous system at the pre- and post-synaptic levels. In this work we aimed to determine whether, as other G protein-coupled receptors (GPCRs), H₃Rs desensitize in response to agonist exposure. By using CHO-K1 cells stably transfected with the human H₃R (hH3R) we show that functional responses (inhibition of forskolin-induced cAMP accumulation in intact cells and stimulation of [(35)S]-GTPγS binding to cell membranes) were markedly reduced after agonist exposure. For cAMP accumulation assays the effect was significant at 60 min with a maximum at 90 min. Agonist exposure resulted in decreased binding sites for the radioligand [(3)H]-N-methyl-histamine ([(3)H]-NMHA) to intact cells and modified the sub-cellular distribution of H₃Rs, as detected by sucrose density gradients and [(3)H]-NMHA binding to cell membranes, suggesting receptor internalization. The reduction in the inhibition of forskolin-stimulated cAMP formation observed after agonist pre-incubation was prevented by incubation in hypertonic medium or in ice-cold medium. Agonist-induced loss in binding sites was also prevented by hypertonic medium or incubation at 4 °C, but not by filipin III, indicating clathrin-dependent endocytosis. Immunodetection showed that CHO-K1 cells express GPCR kinases (GRKs) 2/3, and both the GRK general inhibitor ZnCl₂ and a small interfering RNA against GRK-2 reduced receptor desensitization. Taken together these results indicate that hH₃Rs experience homologous desensitization upon prolonged exposure to agonists, and that this process involves the action of GRK-2 and internalization via clathrin-coated vesicles.

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Increasing the density of the D2L receptor and manipulating the receptor environment are required to evidence the partial agonist properties of aripiprazole

The clinical efficacy of aripiprazole in the treatment of psychosis relies on a partial agonism at D2 receptors. As the expression of this receptor differs physiologically between pre- and post-synaptic sites and is affected by pathological conditions or pharmacological treatments, it appears difficult to predict the clinical response to partial agonists. In addition, the response to this novel antipsychotic was shown to depend on the cell-line and the pathway analyzed, suggesting a functional selective profile at the D2 receptor. This study aims at examining the influence of receptor density and ionic environment on the pharmacological properties of aripiprazole. A cell line was developed in which the expression of the recombinant D2 receptor can be tightly manipulated using doxycycline and sodium butyrate. The potency and efficacy of aripiprazole and other reference D2 receptor ligands were examined in [35S]GTPγS binding assays, in buffers containing either NaCl or N-methyl-D-glucamine (NMDG) which is proposed to enhance G protein coupling. Increasing the density of D2 receptors considerably enhanced the [35S]GTPγS binding induced by dopamine and the full agonist NPA. In maximally induced cells, the agonist properties of the partial agonist (-)-3-PPP was revealed in a buffer containing NaCl, whereas the response to aripiprazole was not evidenced. Substituting NMDG for NaCl promoted the response to dopamine and (-)3-PPP and was proven efficient to reveal the partial agonist profile of aripiprazole. While NMDG substitution for NaCl strongly enhanced receptor-G protein coupling, these ionic manipulations are likely to influence receptor conformations, thereby modulating the activation of signaling pathways. Our data obtained with partial agonists acting at the D2 receptor suggest that these changes in the experimental conditions could contribute to reveal the functional selective profile of GPCR ligands. They also emphasize that the properties of functional selective ligands do not only depend on receptor density but also on the surrounding environment which likely differs between brain structures.

Use of the GTPγS ([35S]GTPγS and Eu-GTPγS) binding assay for analysis of ligand potency and efficacy at G protein-coupled receptors

In this review I consider assays for G protein-coupled receptor (GPCR) activity based on the binding of labelled analogues of GTPγS ([(35) S]GTPγS or Eu-GTPγS) to G proteins in tissues (GTPγS binding assays). Such assays provide convenient measures of GPCR activity close to the receptor in the signalling cascade. In order to set up a GTPγS binding assay, the requirements of the assay must be considered. These are tissue source, GTPγS analogue, G protein, GDP, Mg(2+) /Na(+) ions, saponin, incubation time. The assay, once optimized, can be used to generate concentration/response curves for GPCRs signalling via G(i/o) proteins (or to other G proteins with a modified assay) and actions of agonists, inverse agonists and antagonists may, in principle, be assessed. For agonists and inverse agonists, data for the maximal agonist effect, the concentration of ligand giving a half-maximal response and the Hill coefficient may be derived. For antagonists, data for the equilibrium dissociation constant can be obtained. The mechanistic basis of the assay is considered. Although the assay can be used to profile ligands, under the conditions it is used, it may not be measuring the same event that determines GPCR action in cells.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6

Constitutive phosphorylation by protein kinase C regulates D1 dopamine receptor signaling

The D(1) dopamine receptor (D(1) DAR) is robustly phosphorylated by multiple protein kinases, yet the phosphorylation sites and functional consequences of these modifications are not fully understood. Here, we report that the D(1) DAR is phosphorylated by protein kinase C (PKC) in the absence of agonist stimulation. Phosphorylation of the D(1) DAR by PKC is constitutive in nature, can be induced by phorbol ester treatment or through activation of Gq-mediated signal transduction pathways, and is abolished by PKC inhibitors. We demonstrate that most, but not all, isoforms of PKC are capable of phosphorylating the receptor. To directly assess the functional role of PKC phosphorylation of the D(1) DAR, a site-directed mutagenesis approach was used to identify the PKC sites within the receptor. Five serine residues were found to mediate the PKC phosphorylation. Replacement of these residues had no effect on D(1) DAR expression or agonist-induced desensitization; however, G protein coupling and cAMP accumulation were significantly enhanced in PKC-null D(1) DAR. Thus, constitutive or heterologous PKC phosphorylation of the D(1) DAR dampens dopamine activation of the receptor, most likely occurring in a context-specific manner, mediated by the repertoire of PKC isozymes within the cell.

Concentration of receptor and ligand revisited in a modified receptor binding protocol for high-affinity radioligands: [3H]Spiperone binding to D2 and D3 dopamine receptors

In receptor binding assays with ultra-high-affinity radioligands, it is difficult, in practice, to adhere the golden rule that the receptor concentration in the assay should be substantially (at least 10-fold) lower than the dissociation constant (K(d)) of the radioligand and inhibition constant (K(i)) of compound. Especially for low specific activity radioligands (usually tritiated ligands of a couple of TBq/mmol), routinely applied in concentrations at around or below the K(d), the use of extremely small amounts of receptor protein per assay will result in low levels of bound radioactivity; the alternative use of larger assay volumes will make it difficult to apply 96-well filtration devices. For assessing the inhibition constant (K(i)) of competitive inhibitors under conditions violating the above golden rule, equations are available incorporating both [receptor] and [ligand] versus K(d); however, their application requires precise knowledge of [receptor] or initial bound/free [radioligand] ratio. In this study, we present the theoretical basis for determining the K(i) for a competitive inhibitor in a new protocol at high [protein] and high [radioligand] with the simple Cheng-Prusoff correction without the need to correct for [receptor] or initial bound/free [radioligand] ratio. In addition, we present results on the binding of the ultra-high-affinity ligand [(3)H]spiperone to dopamine D(2) and D(3) receptors validating the K(i) values calculated with the new protocol for competitive inhibitors as compared with those calculated with the most comprehensive equation available to date, that of Munson and Rodbard (1988). Binding was measured at varying [radioligand] and [receptor], test compounds (including (-)5-OH-DPAT, (+/-)7-OH-DPAT, and ropinirole) were run with varying [receptor], and simulations were done at vastly varying [radioligand] for inhibitors with vastly different K(i)s. The modified high [radioligand] protocol presented here removes a major hindrance in the proper execution of binding assays with ultra-high-affinity tritiated ligands with K(d) values in the sub-nanomolar range, allowing the use of 96-well plates with small volumes of 100-200 microl per binding assay.

G protein-coupled receptor kinase-2 constitutively regulates D2 dopamine receptor expression and signaling independently of receptor phosphorylation

We investigated the regulatory effects of GRK2 on D(2) dopamine receptor signaling and found that this kinase inhibits both receptor expression and functional signaling in a phosphorylation-independent manner, apparently through different mechanisms. Overexpression of GRK2 was found to suppress receptor expression at the cell surface and enhance agonist-induced internalization, whereas short interfering RNA knockdown of endogenous GRK2 led to an increase in cell surface receptor expression and decreased agonist-mediated endocytosis. These effects were not due to GRK2-mediated phosphorylation of the D(2) receptor as a phosphorylation-null receptor mutant was regulated similarly, and overexpression of a catalytically inactive mutant of GRK2 produced the same effects. The suppression of receptor expression is correlated with constitutive association of GRK2 with the receptor complex as we found that GRK2 and several of its mutants were able to co-immunoprecipitate with the D(2) receptor. Agonist pretreatment did not enhance the ability of GRK2 to co-immunoprecipitate with the receptor. We also found that overexpression of GRK2 attenuated the functional coupling of the D(2) receptor and that this activity required the kinase activity of GRK2 but did not involve receptor phosphorylation, thus suggesting the involvement of an additional GRK2 substrate. Interestingly, we found that the suppression of functional signaling also required the G betagamma binding activity of GRK2 but did not involve the GRK2 N-terminal RH domain. Our results suggest a novel mechanism by which GRK2 negatively regulates G protein-coupled receptor signaling in a manner that is independent of receptor phosphorylation.

G protein-coupled receptor kinase-mediated phosphorylation regulates post-endocytic trafficking of the D2 dopamine receptor

We investigated the role of G protein-coupled receptor kinase (GRK)-mediated phosphorylation in agonist-induced desensitization, arrestin association, endocytosis, and intracellular trafficking of the D(2) dopamine receptor (DAR). Agonist activation of D(2) DARs results in rapid and sustained receptor phosphorylation that is solely mediated by GRKs. A survey of GRKs revealed that only GRK2 or GRK3 promotes D(2) DAR phosphorylation. Mutational analyses resulted in the identification of eight serine/threonine residues within the third cytoplasmic loop of the receptor that are phosphorylated by GRK2/3. Simultaneous mutation of these eight residues results in a receptor construct, GRK(-), that is completely devoid of agonist-promoted GRK-mediated receptor phosphorylation. We found that both wild-type (WT) and GRK(-) receptors underwent a similar degree of agonist-induced desensitization as assessed using [(35)S]GTPgammaS binding assays. Similarly, both receptor constructs internalized to the same extent in response to agonist treatment. Furthermore, using bioluminescence resonance energy transfer assays to directly assess receptor association with arrestin3, we found no differences between the WT and GRK(-) receptors. Thus, phosphorylation is not required for arrestin-receptor association or agonist-induced desensitization or internalization. In contrast, when we examined recycling of the D(2) DARs to the cell surface, subsequent to agonist-induced endocytosis, the GRK(-) construct exhibited less recycling in comparison with the WT receptor. This impairment appears to be due to a greater propensity of the GRK(-) receptors to down-regulate once internalized. In contrast, if the receptor is highly phosphorylated, then receptor recycling is promoted. These results reveal a novel role for GRK-mediated phosphorylation in regulating the post-endocytic trafficking of a G protein-coupled receptor.

Mechanisms underlying agonist efficacy

Agonist efficacy is a measure of how well an agonist can stimulate a response system linked to a receptor. Efficacy can be assessed in functional assays and various parameters (E(max), K(A)/EC(50), E(max).K(A)/EC(50)) determined. The E(max).K(A)/EC(50) parameter provides a good estimate of efficacy across the full range of efficacy. A convenient assay for the efficacy of agonists for some receptors is provided by the [(35)S]GTP[S] (guanosine 5'-[gamma-[(35)S]thio]triphosphate)-binding assay. In this assay, the normal GTP-binding event in GPCR (G-protein-coupled receptor) activation is replaced by the binding of the non-hydrolysable analogue [(35)S]GTP[S]. This assay may be used to profile ligands for their efficacy, and an example here is the D(2) dopamine receptor where an efficacy scale has been set up using this assay. The mechanisms underlying the assay have been probed. The time course of [(35)S]GTP[S] binding follows a pseudo-first-order reaction with [(35)S]GTP[S] binding reaching equilibrium after approx. 3 h. The [(35)S]GTP[S]-binding event is the rate-determining step in the assay. Agonists regulate the maximal level of [(35)S]GTP[S] bound, rather than the rate constant for binding. The [(35)S]GTP[S]-binding assay therefore determines agonist efficacy on the basis of the amount of [(35)S]GTP[S] bound rather than the rate of binding.

2-[(4-phenylpiperazin-1-yl)methyl]imidazo(di)azines as selective D4-ligands. Induction of penile erection by 2-[4-(2-methoxyphenyl)piperazin-1-ylmethyl]imidazo[1,2-a]pyridine (PIP3EA), a potent and selective D4 partial agonist

A series of novel 2-[(4-phenylpiperazin-1-yl)methyl]imidazoazines and aza-analogues were prepared and screened at selected dopamine, serotonin, and adrenergic receptor subtypes. 2-Substituted imidazopyridines and pyridazines presented high affinities and selectivities for D4 dopamine receptors. Whereas functional experiments indicated neutral antagonists or weak partial agonist effects for most of the target compounds, the 2-methoxyphenyl substituted 2-piperazinylmethylimidazopyridine 3c (PIP3EA) displayed substantial agonist efficacy in mitogenesis experiments and GTPgammaS binding tests, resulting in EC50 values of 3.0 (46%) and 4.5 nM (57%), respectively. Our D4 agonist 3c induced penile erection in vivo when administered to rats. This effect was inhibited by L-745,870 a D4 selective antagonist, confirming the mechanistic pathway.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

The role of dopamine in reward and pleasure behaviour--review of data from preclinical research

OBJECTIVE

The purpose of this article is to review some of the basic aspects of the dopaminergic system and its role in reward and pleasure behaviour. We also discuss the association between dopamine and unpleasant symptoms that are commonly found in neuropsychiatric disorders and may also be side-effects of neuroleptic drugs.

METHOD

A computer-based search of the literature, augmented by extensive bibliography-guided article reviews, were used to find basic information on the dopamine and the reward systems, and symptoms such as dysphoria, anhedonia and depression.

RESULTS

Central dopaminergic neurotransmission is complex, having multiple actions at each level of the mesocorticolimbic reward pathway. The role of dopamine in the reward process was classically associated with the ability to experience pleasure; recent data suggest a more motivational role. Dysfunction of the dopamine transmission in the reward circuit is associated with symptoms such as anhedonia, apathy and dysphoria found in several neuropsychiatric disorders, including Parkinson's disease, depression, drug addiction, and neuroleptic-induced dysphoria.

CONCLUSION

Viewing the dysfunctions of the reward pathways within a broader spectrum and exploring its complex relations with the dopaminergic transmission may help understand the pathophysiology of these neuropsychiatric disorders and lead to a rational development of novel treatments.

Fancy bioisosteres: metallocene-derived G-protein-coupled receptor ligands with subnanomolar binding affinity and novel selectivity profiles

Metallocene-derived bioisosteres lead to exceptionally strong binding G-protein-coupled receptor ligands, indicating substantial plasticity of the receptor excluded volume. Novel binding profiles of ferrocenylcarboxamides combining subnanomolar Ki values for the dopamine D4 receptor (1a, 0.52 nM; 1b, 0.63 nM) with superpotent serotonin 5-hydroxytryptamine1A (1a, 0.50 nM) and dopamine D3 receptor binding (1b, 0.64 nM) and selective D4 agonist properties of the ruthenocene 1c may be a starting point for highly beneficial central nervous system active drugs.

CCL3, acting via the chemokine receptor CCR5, leads to independent activation of Janus kinase 2 (JAK2) and Gi proteins

The interaction of the chemokine receptor, CCR5, expressed in recombinant cells, with different G proteins was investigated and CCR5 was found to interact with Gi, Go and Gq species. Interaction with Gi leads to G protein activation, whereas Gq does not seem to be activated. Additionally, CCR5 activation also leads to phosphorylation of Janus kinase 2 (JAK2). Activation of JAK2 is independent of Gi or Gq activation. Gi protein activation was not prevented by inhibition of JAK, showing that heterotrimeric G protein activation and activation of the JAK/signal transducer and activator of transcription (STAT) pathway are independent of each other.

Mechanisms of agonist action at D2 dopamine receptors

In this study, we investigated the biochemical mechanisms of agonist action at the G protein-coupled D2 dopamine receptor expressed in Chinese hamster ovary cells. Stimulation of guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding by full and partial agonists was determined at different concentrations of [35S]GTPgammaS (0.1 and 10 nM) and in the presence of different concentrations of GDP. At both concentrations of [35S]GTPgammaS, increasing GDP decreased the [35S]GTPgammaS binding observed with maximally stimulating concentrations of agonist, with partial agonists exhibiting greater sensitivity to the effects of GDP than full agonists. The relative efficacy of partial agonists was greater at the lower GDP concentrations. Concentration-response experiments were performed for a range of agonists at the two [35S]GTPgammaS concentrations and with different concentrations of GDP. At 0.1 nM [35S]GTPgammaS, the potency of both full and partial agonists was dependent on the GDP concentration in the assays. At 10 nM [35S]GTPgammaS, the potency of full agonists exhibited a greater dependence on the GDP concentration, whereas the potency of partial agonists was virtually independent of GDP. We concluded that at the lower [35S]GTPgammaS concentration, the rate-determining step in G protein activation is the binding of [35S]GTPgammaS to the G protein. At the higher [35S]GTPgammaS concentration, for full agonists, [35S]GTPgammaS binding remains the slowest step, whereas for partial agonists, another (GDP-independent) step, probably ternary complex breakdown, becomes rate-determining.

Investigation of the mechanism of agonist and inverse agonist action at D2 dopamine receptors

This study investigated, for the D2 dopamine receptor, the relation between the ability of agonists and inverse agonists to stabilise different states of the receptor and their relative efficacies. Ki values for agonists were determined in competition versus the binding of the antagonist [3H]spiperone. Competition data were fitted best by a two-binding site model (with the exception of bromocriptine, for which a one-binding site model provided the best fit) and agonist affinities for the higher (Kh) (G protein-coupled) and lower affinity (Kl) (G protein-uncoupled) sites determined. Ki values for agonists were also determined in competition versus the binding of the agonist [3H]N-propylnorapomorphine (NPA) to provide a second estimate of Kh. Maximal agonist effects (Emax) and their potencies (EC50) were determined from concentration-response curves for agonist stimulation of guanosine-5'-O-(3-[32S]thiotriphosphate) ([35S]GTPgammaS) binding. The ability of agonists to stabilise the G protein-coupled state of the receptor (Kl/Kh determined from ligand-binding assays) did not correlate with either of two measures of relative efficacy (relative Emax, Kl/EC50) of agonists determined in [35S]GTPgammaS-binding assays, when the data for all of the compounds tested were analysed. For a subset of compounds, however, there was a relation between Kl/Kh and Emax. Competition-binding data versus [3H]spiperone and [3H]NPA for a range of inverse agonists were fitted best by a one-binding site model. Ki values for the inverse agonists tested were slightly lower in competition versus [3H]NPA compared to [3H]spiperone. These data do not provide support for the idea that inverse agonists act by binding preferentially to the ground state of the receptor.

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: I. Cellular, electrophysiological, and neurochemical profile in comparison with ropinirole

S32504 [(+)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] displayed marked affinity for cloned, human (h)D(3) receptors (pK(i), 8.1) at which, in total G-protein ([(35)S]GTPgammaS binding, guanosine-5'-O-(3-[(35)S]thio)-triphosphate), Galpha(i3) (antibody capture/scintillation proximity), and mitogen-activated protein kinase (immunoblot) activation procedures, it behaved as an agonist: pEC(50) values, 8.7, 8.6, and 8.5, respectively. These actions were blocked by haloperidol and the selective D(3) receptor antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl) benzamide)]. S32504 showed lower potency at hD(2S) and hD(2L) receptors in [(35)S]GTPgammaS binding (pEC(50) values, 6.4 and 6.7) and antibody capture/scintillation proximity (hD(2L), pEC(50), 6.6) procedures. However, reflecting signal amplification, it potently stimulated hD(2L) receptor-coupled mitogen-activated protein kinase (pEC(50), 8.6). These actions were blocked by haloperidol and the selective D(2) receptor antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol]. The affinity of S32504 for hD(4) receptors was low (5.3) and negligible for hD(1) and hD(5) receptors (pK(i), <5.0). S32504 showed weak agonist properties at serotonin(1A) ([(35)S]GTPgammaS binding, pEC(50), 5.0) and serotonin(2A) (G(q), pEC(50), 5.2) receptors and low affinity for other (>50) sites. In anesthetized rats, S32504 (0.0025-0.01 mg/kg, i.v.) suppressed electrical activity of ventrotegmental dopaminergic neurons. Correspondingly, S32504 (0.0025-0.63 mg/kg, s.c.) potently reduced dialysis levels (and synthesis) of dopamine in striatum, nucleus accumbens, and frontal cortex of freely moving rats, actions blocked by haloperidol and L741,626 but not by S33084. In contrast, S32504 only weakly inhibited serotonergic transmission and failed to affect noradrenergic transmission. Actions of S32504 were expressed stereospecifically versus its less active enantiomer S32601 [(-)-trans-3,4,4a,5,6,10b-hexahydro-9-carbomoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine]. Although the D(3)/D(2) agonist and antiparkinsonian agent ropinirole mimicked the profile of S32504, it was less potent. In conclusion, S32504 is a potent and selective agonist at dopamine D(3) and D(2) receptors.

Anesthetics inhibit high-affinity states of dopamine D2 and other G-linked receptors

The high-affinity states of dopamine D2 and D3 receptors, serotonin 5HT-2A receptors, beta-2-adrenoceptors, alpha-1 and alpha-2 adrenoceptors, opiate receptors, and muscarinic receptors were inhibited by clinical concentrations of anesthetics, including isoflurane, halothane, chloral hydrate, ketamine, and ethanol. The inhibition occurred not only in vitro, but also in vivo in rats anesthetized with isoflurane, with the high-affinity states recovering at different rates. Because the high-affinity states of G-protein-linked receptors are physiologically functional, their general inhibition by clinical concentrations of anesthetics may underlie general anesthesia and may explain some of the side effects of anesthetics. Subanesthetic concentrations of the anesthetics, including ketamine, stimulated the incorporation of GTP into the cloned dopamine D2 receptors. It is possible that the classical stage 2 excitement phase which occurs with subanesthetic concentrations of general anesthetics and ketamine may be associated with this general stimulation of a variety of G-protein-linked receptors, as found in the present study, while the stage 3 level of surgical anesthesia may be associated with the inhibition of the high-affinity states of several receptors.

Pharmacological analysis of a dopamine D(2Short):G(alphao) fusion protein expressed in Sf9 cells

A dopamine D(2Short) receptor:G(alphao) fusion protein was expressed in Sf9 cells using the baculovirus expression system. [(3)H]Spiperone bound to D(2Short):G(alphao) with a pK(d) approximately 10. Dopamine stimulated the binding of [(35)S]guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) to D(2Short):G(alphao) expressed with Gbeta(1)gamma(2) (E(max)>460%; pEC(50) 5.43+/-0.06). Most of the putative D(2) antagonists behaved as inverse agonists (suppressing basal [(35)S]GTPgammaS binding) at D(2Short):G(alphao)/Gbeta(1)gamma(2) although (-)-sulpiride and ziprasidone were neutral antagonists. Competition of [(3)H]spiperone binding by dopamine and 10,11-dihydroxy-N-n-propylnorapomorphine revealed two binding sites of different affinities, even in the presence of GTP (100 micro M). The D(2Short):G(alphao) fusion protein is therefore a good model for characterising D(2) receptors.

Analysis of molecular determinants of affinity and relative efficacy of a series of R- and S-2-(dipropylamino)tetralins at the 5-HT1A serotonin receptor

1. Factors influencing agonist affinity and relative efficacy have been studied for the 5-HT(1A) serotonin receptor using membranes of CHO cells expressing the human form of the receptor and a series of R-and S-2-(dipropylamino)tetralins (nonhydroxylated and monohydroxylated (5-OH, 6-OH, 7-OH, 8-OH) species). 2. Ligand binding studies were used to determine dissociation constants for agonist binding to the 5-HT(1A) receptor: (a) K(i) values for agonists were determined in competition versus the binding of the agonist [(3)H]-8-OH DPAT. Competition data were all fitted best by a one-binding site model. (b) K(i) values for agonists were also determined in competition versus the binding of the antagonist [(3)H]-NAD-199. Competition data were all fitted best by a two-binding site model, and agonist affinities for the higher (K(h)) and lower affinity (K(l)) sites were determined. 3. The ability of the agonists to activate the 5-HT(1A) receptor was determined using stimulation of [(35)S]-GTPgammaS binding. Maximal effects of agonists (E(max)) and their potencies (EC(50)) were determined from concentration/response curves for stimulation of [(35)S]-GTPgammaS binding. 4. K(l)/K(h) determined from ligand binding assays correlated with the relative efficacy (relative E(max)) of agonists determined in [(35)S]-GTPgammaS binding assays. There was also a correlation between K(l)/K(h) and K(l)/EC(50) for agonists determined from ligand binding and [(35)S]-GTPgammaS binding assays. 5. Simulations of agonist binding and effect data were performed using the Ternary Complex Model in order to assess the use of K(l)/K(h) for predicting the relative efficacy of agonists.

Functional coupling of the human dopamine D2 receptor with G alpha i1, G alpha i2, G alpha i3 and G alpha o G proteins: evidence for agonist regulation of G protein selectivity

(1) The human dopamine D(2long) (D(2L)) receptor was expressed with four different G proteins in Sf9 cells using the baculovirus expression system. When co-expressed with G(i)/G(o) G proteins (G(i1)alpha, G(i2)alpha, G(i3)alpha, or G(o)alpha, plus Gbeta(1) and Ggamma(2)), the receptor displayed a high-affinity binding site for the agonists (dopamine and NPA), which was sensitive to GTP (100 micro M), demonstrating interaction between the receptor and the different G proteins. (2) The receptor to G protein ratio (R : G ratio) was evaluated using [(3)H]-spiperone saturation binding (R) and [(35)S]-GTPgammaS saturation binding (G). R : G ratios of 1 : 12, 1 : 3, 1 : 14 and 1 : 5 were found for G(i1), G(i2), G(i3), and G(o) preparations, respectively. However, when R : G ratios of 1 : 2 and 1 : 12 were compared for G(i2) and G(o), no difference was found for the stimulation of [(35)S]-GTPgammaS binding. (3) Several agonists were tested for their ability to stimulate [(35)S]-GTPgammaS binding to membranes co-expressing the receptor and various G proteins. All the compounds tested showed agonist activity in preparations expressing G(i3) and G(o). However, for G(i2) and G(i1) preparations, compounds such as S-(-)-3-PPP and p-tyramine were unable to stimulate [(35)S]-GTPgammaS binding. (4) Most of the compounds showed higher relative efficacies (compared to dopamine) and higher potencies in the preparation expressing G(o). Comparison of the effects of different agonists in the different preparations showed that each agonist differentially activates the four G proteins. (5) We conclude that the degree of selectivity of G protein activation by the D(2L) receptor can depend on the conformation of the receptor stabilised by an agonist.

Mechanisms of ligand binding and efficacy at the human D2(short) dopamine receptor

Mechanisms of ligand binding and receptor activation for the human D2(short) dopamine receptor have been probed using two homologous series of monohydroxylated and dihydroxylated agonists (phenylethylamines and 2-dipropylaminotetralins). In ligand binding studies, the majority of compounds exhibited competition curves versus [3H]spiperone that were best fitted using a two site binding model. The compounds had different abilities (potencies and maximal effects) to stimulate [35S]GTPgammaS binding and to inhibit forskolin-stimulated cAMP accumulation. From the data it can be concluded that: (i) the ability of an agonist to stabilize receptor/G protein coupling can be used to predict agonist efficacy for some groups of compounds (2-dipropylaminotetralins) but not for others (phenylethylamines); (ii) the receptor may be activated by unhydroxylated compounds; (iii) single hydroxyl groups or pairs of hydroxyl groups on the agonist may contribute to binding affinity, potency and efficacy; and (iv) for the 2-dipropylaminotetralin series two modes of agonist/receptor interaction have been identified associated with different relative efficacy.

Pathways for internalization and recycling of the chemokine receptor CCR5

M-tropic human immunodeficiency virus (HIV-1) strains enter the cell after interaction with their receptors, CD4 and the G-protein-coupled chemokine receptor CCR5. The number of cell surface CCR5 molecules is thought to be important in determining the infection rate for HIV. Cell surface CCR5 is dependent on the rate of receptor internalization and recycling. Internalization of G-protein-coupled receptors after agonist activation is thought to occur either through clathrin-coated pits or through caveolae. In this study, the role of these different pathways was investigated in Chinese hamster ovary cells expressing CCR5 using specific inhibitors. Internalization of CCR5 after chemokine treatment was inhibited by sucrose, indicating a role for the clathrin-coated pit pathway. Activation of CCR5 leads to arrestin-2 movement in the cells, providing further evidence for the involvement of clathrin-coated pits. Nystatin and filipin also affected the rate of internalization of CCR5, indicating a role for caveolae. Using inhibitors of vesicle transport in the cell, it was found that the CCR5 recycling pathway is independent of the Golgi apparatus and late endosomes. Protein synthesis is not involved in receptor recovery. It seems likely that after internalization, CCR5 is directed to early endosomes and subsequently recycled to the cell surface.

Pharmacological characterization of the chemokine receptor, CCR5

1. We investigated the effects of a number of naturally occurring chemokines (MIP-1alpha, MIP-1beta, RANTES, MCP-2, MCP-3, MCP-4) on different processes linked to the chemokine receptor CCR5 in recombinant CHO cells expressing the receptor at different levels. 2. Internalization of CCR5 following chemokine treatment was studied and MIP-1alpha, MIP-1beta and RANTES (50 nM) were able to induce internalization (similar50%) of the receptor. Internalization due to MCP-2, MCP-3 and MCP-4 was less (similar20%). 3. Phosphorylation of CCR5 following chemokine treatment was studied and MIP-1alpha, MIP-1beta and RANTES (50 nM) were able to induce phosphorylation of CCR5 whereas the other chemokines did not induce CCR5 phosphorylation. 4. MIP-1alpha, MIP-1beta, RANTES and MCP-2 were able to stimulate [(35)S]-GTPgammaS binding, an index of receptor/G protein activation, whereas MCP-3 and MCP-4 had no effect in this assay. MCP-2 was a partial agonist (similar80%) compared to MIP-1alpha, MIP-1beta and RANTES, which gave similar maximal stimulations in this assay. 5. MIP-1alpha, MIP-1beta, RANTES, MCP-2 and MCP-4 were able to stimulate increases in intracellular calcium ions via activation of CCR5 whereas MCP-3 was without effect. 6. It is concluded that different chemokines interacting with CCR5 mediate different patterns of cellular responses.

Structure-activity studies on nociceptin analogues: ORL1 receptor binding and biological activity of cyclic disulfide-containing analogues of nociceptin peptides

Nociceptin/Orphanin FQ is an endogenous peptide ligand for the opioid receptor-like 1 (ORL1) receptor. To investigate the structural and conformational requirements of the nociceptin (NC)-receptor interaction, six cyclic analogues containing Cys disulfide linkages were designed and synthesized. Analogues cyclized at the N-terminal part, cyclo[Cys(0), Cys(7)]NC(1-13)-NH(2) (2) and cyclo[Cys(0), Cys(11)]NC(1-13)-NH(2) (4), and their corresponding linear peptides had very low activities in both the receptor binding and the GTP gamma S functional assays using human ORL1 transfected cell membranes. On the contrary, analogues cyclized at the C-terminal parts by the disulfide linkages at positions 6-10, 7-11, 7-14, and 10-14 sustained relatively high potencies in both assays. Notably, cyclo[Cys(10), Cys(14)]NC(1-14)-NH(2) (12) was found to be a potent NC agonist nearly as active as the parent peptide or NC. The maximum efficacy (Emax) of the C-terminally cyclized analogues and their linear counterparts in the GTP gamma S functional assay showed more than 94% (vs NC as 100%), suggesting that these analogues are full agonists. Analogue 12 is the first conformationally constrained NC analogue with almost full activity, and thus may serve to analyze the bioactive conformations of NC at the receptor site as well as serving as a template for more potent NC agonists.

Intracellular retention of the two isoforms of the D2 dopamine receptor promotes endoplasmic reticulum disruption

The dopamine D2 receptor exists as a long (D2a) and a short (D2b) isoform generated by alternative splicing of the corresponding transcript, which modifies the length of the third cytoplasmic loop implicated in heterotrimeric G-protein-coupling. Anatomical data suggested that this segment regulates the intracellular traffic and localization of the receptor. To directly address this question we used a combination of tagging procedures and immunocytochemical techniques to detect each of the two D2 receptor isoforms. Surprisingly, most of the newly synthesized receptors accumulate in large intracellular compartments, the plasma membrane being only weakly labeled, without significant difference between the two receptor isoforms. Double labeling experiments showed that this localization corresponded neither to endosomal compartments nor to the Golgi apparatus. The D2 receptor is mostly retained in the endoplasmic reticulum (ER), the long isoform more efficiently than the short one. It is accompanied by a striking vacuolization of the ER, roughly proportional to the expression levels of the two receptor isoforms. This phenomenon is partly overcome by treatment with pertussis toxin. In addition, an intrinsic activity of the D2 receptor isoforms is revealed by [35S]-GTPγS binding and cAMP assay, which suggested that expression of weakly but constitutively active D2 receptors promotes activation of heterotrimeric G protein inside the secretory pathway. This mechanism may participate in the regulation of the cellular traffic of the D2 receptors isoforms.

Agonist regulation of D(2) dopamine receptor/G protein interaction. Evidence for agonist selection of G protein subtype

The D(2) dopamine receptor has been expressed in Sf21 insect cells together with the G proteins G(o) and G(i2), using the baculovirus system. Expression levels of receptor and G protein (alpha, beta, and gamma subunits) in the two preparations were similar as shown by binding of [(3)H]spiperone and quantitative Western blot, respectively. For several agonists, binding data were fitted best by a two-binding site model in either preparation, showing interaction of expressed receptor and G protein. For some agonists, binding to the higher affinity site was of higher affinity in D(2)/G(o) than in the D(2)/G(i2) preparation. Some agonists exhibited binding data that were best fitted by a two-binding site model in D(2)/G(o) and a one-binding site model in D(2)/G(i2). Therefore, receptor/G protein interaction seemed to be stronger in the D(2)/G(o) preparation. Agonist stimulation of [(35)S]GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) binding in the two preparations also gave evidence for higher affinity D(2)/G(o) interaction. In the D(2)/G(o) preparation, agonist stimulation of [(35)S]GTP gamma S binding occurred at higher potency for several agonists, and a higher stimulation (relative to dopamine) was achieved in D(2)/G(o) compared with D(2)/G(i2). Some agonists were able to stimulate [(35)S]GTP gamma S binding in the D(2)/G(o) preparation but not in D(2)/G(i2). The extent of D(2) receptor selectivity for G(o) over G(i2) is therefore dependent on the agonist used, and thus agonists may stabilize different conformations of the receptor with different abilities to couple to and activate G proteins.

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Constitutive activity at serotonin 5-HT(1D) receptors: detection by homologous GTPgammaS versus [(35)S]-GTPgammaS binding isotherms

Although many G-protein-coupled receptors (GPCRs) may display constitutive activity, their detection has, to date, depended on the use of inverse agonists. The present study exploited a novel procedure to investigate constitutive activity at recombinant human (h) serotonin (5-HT) 5-HT(1D) receptors stably expressed in Chinese hamster ovary (CHO) cells. 5-HT modestly stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]-GTPgammaS) binding to CHO-h5-HT(1D) membranes whereas methiothepin and the 5-HT(1B/1D)-selective ligand, SB224,289, exerted robust inhibition of basal [(35)S]-GTPgammaS binding (inverse agonism). These actions were specific inasmuch as they were reversed by the novel, selective 5-HT(1B/1D) ligand, S18127. Constitutive activity was investigated by homologous inhibition of [(35)S]-GTPgammaS binding to CHO-h5-HT(1D) membranes with unlabelled GTPgammaS. Under 'basal' conditions (absence of receptor ligand), biphasic isotherms were observed. Most (80%) [(35)S]-GTPgammaS binding sites were in the high affinity (HA) versus low affinity (LA) component of the isotherms. HA binding was augmented by 5-HT (to 155%; relative to basal values=100%), but decreased by methiothepin (to 23%) and by SB224,289 (to 67%). In contrast, LA binding was not altered. Further, membranes of untransfected CHO cells exhibited only LA binding sites, indicating that the latter are not related to h5-HT(1D) receptor-G-protein coupling. Thus, at 5-HT(1D) receptors expressed in this CHO cell line, HA binding detected in homologous inhibition experiments (GTPgammaS versus [(35)S]-GTPgammaS) under basal conditions provides a measure of constitutive G-protein activation. Thus, it is suggested that for h5-HT(1D) receptors and, possibly, other GPCRs, inverse agonists will be detectable by [(35)S]-GTPgammaS binding if a HA component is present under basal conditions.

Autoradiographic analysis of dopamine receptor-stimulated [(35)S]GTPgammaS binding in rat striatum

Autoradiographic analysis of [(35)S]GTPgammaS binding was used to investigate functional activation of dopamine receptors in rat striatum. Dopamine-stimulated [(35)S]GTPgammaS binding was observed with a maximal increase of 38% over basal activity. A similar stimulatory response was obtained with the D(2) agonist quinpirole, but not SKF-238393, a D(1) agonist. The effect of dopamine was blocked by the D(2) antagonist raclopride, but was unaffected by SCH-23990, a D(1) antagonist. There appeared to be a differential distribution of dopamine-stimulated [(35)S]GTPgammaS binding, with the lowest activity obtained in the medial portion of the caudal striatum. These results demonstrate, using an autoradiographic approach, (i) that dopamine stimulated [(35)S]GTPgammaS binding in the rat striatum occurs through activation of D(2) receptors, and (ii) that the effects of dopamine activation vary in different areas of the rat striatum.

Highly potent nociceptin analog containing the Arg-Lys triple repeat

One of the structural characteristics of a neuropeptide nociceptin is the existence of Arg-Lys (RK) residues at positions 8-9 and 12-13; both RKs have been suggested to bind to the acidic amino acid cluster in the second extracellular loop of the seven transmembrane domain receptor ORL1. With a design strategy of attempting to obtain an analog that binds more strongly to the receptor's acidic cluster, we synthesized a series of nociceptin analogs in which the RK dipeptide unit was placed at positions 6-7, 10-11, or 14-15 adjacent to the parent RKs. Among these nociceptin analogs containing the RK triple repeat, [Arg-Lys(6-7)]- and [Arg-Lys(10-11)]nociceptins exhibited weak activities (6-9 and 60-90% of nociceptin, respectively) both in the receptor binding assay and in the [(35)S]GTPgammaS binding functional assay. In contrast, [Arg-Lys(14-15)]nociceptin was found to be very potent in both assays (3-fold in binding and 17-fold in GTPgammaS functional assay). [Arg-Lys(14-15)]nociceptin was the first peptide analog found to be stronger than the parent nociceptin, and structure-activity studies have suggested that the incorporated Arg-Lys(14-15) interacts with either the receptor acidic amino acid cluster or the receptor aromatic amino acid residues.

Advantages of heterologous expression of human D2long dopamine receptors in human neuroblastoma SH-SY5Y over human embryonic kidney 293 cells

The human D2long dopamine receptor when expressed heterologously in a human neuronal cell line, SH-SY5Y, produced more robust functional signals than when expressed in a human embryonic kidney cell line, HEK293. Quinpirole (agonist)-induced GTPgamma(35)S binding and high affinity sites were 3 - 4 fold greater in SH-SY5Y than in HEK293 cells. N-type Ca(2+) channel currents present in SH-SY5Y cells, but not HEK293 cells, were inhibited potently by quinpirole with a half-maximal inhibitory concentration of 0.15+/-0.03 nM. Inhibition of adenylyl cyclases by agonists, on the other hand, was of similar potency and efficacy in the two cell lines. GTPgamma(35)S-Bound Galpha subunits from quinpirole-activated and solubilized membranes were monitored upon immobilization with various Galpha-specific antibodies. Galpha(i) and Galpha(o) subunits were highly labelled with GTPgamma(35)S in SH-SY5Y cells, but only Galpha(i) subunits were labelled in HEK293 cells. The additional G(o) coupling in SH-SY5Y cells could arise, at least in part, from the presence of G(o) coupled-effectors, such as the N-type Ca(2+) channel, and may contribute to robust agonist-induced GTPgamma(35)S binding, which is a reliable means for measuring ligand intrinsic efficacy. It appears that expression of neuronal G protein-coupled receptors in neuronal environments could reveal additional functional characteristics that are absent in non-neuronal cell lines. This appears to be due to, at least in part, to the presence of neuron-specific effectors. These findings underscore the importance of the cellular environment in which drug actions are examined, particularly in the face of intensive efforts to develop drugs for G protein-coupled receptors of various origins.

Activation of dopamine D(3) receptors induces c-fos expression in primary cultures of rat striatal neurons

A modulation of the expression of immediate-early genes (IEGs) such as c-fos is likely involved in the long-term influence of dopaminergic ligands on the activity of basal ganglia neurons. The roles of individual dopamine receptor types in this regard remain unclear, and the present study employed primary cultures of rat striatal neurons to examine the potential modulation of c-fos expression by D(3) receptors. Neurons were treated with the preferential D(3) receptor agonists, (+/-)-7-OH-DPAT and PD 128,907, which showed, respectively, 16-fold and 56-fold selectivity for recombinant rat D(3) vs. D(2) receptors (K(i) values, rD(2)/rD(3) = 59.5/3.7 nM and 251/4.5 nM, respectively). At concentrations of 3 and 30 nM, respectively, (+/-)-7-OH-DPAT and PD 128,907 significantly increased the expression of c-fos mRNA. The action of (+/-)-7-OH-DPAT was expressed stereospecifically; its (+)-isomer (K(i) values, D(3)/D(2) = 1.6/56.7 elicited a 26% +/- 7.6% increase in c-fos expression whereas its (-)-isomer (K(i) values, D(3)/D(2) = 59/1,060 nM) was ineffective. Further, stimulation of c-fos mRNA expression by PD 128,907 (20 nM) was markedly and significantly (P < 0.05) attenuated (-91.8% +/- 5.3%) by 30 nM of the selective D(3) receptor antagonist, (+)-S 14297 (K(i) values, D(3)/D(2) = 11/401 nM). In contrast, the action of PD 128,907 was significantly less affected (-24.5% +/- 7.8%) by 30 nM of its less active distomer, (-)-S 17777 (K(i) values, D(3)/D(2) = 294/3,191 nM). An examination of the concentration-response relationship revealed that (+/-)-7-OH-DPAT and PD 128,907 both produced bell-shaped dose-response curves for c-fos induction. The sequential activation of D(2) receptors-which inhibit striatal c-fos expression (Simpson and Morris [1995] Neuroscience 68:97-106)-by higher concentrations of (+/-)-7-OH-DPAT and PD 128,907 is presumably involved in the inflexion at higher doses. In conclusion, the present data demonstrate that activation of D(3) receptors results in a selective induction of c-fos mRNA expression in cultured striatal neurons. These data show that neuronal D(3) receptors control the expression of IEGs and suggest that D(3) receptors may mediate long-term adapative changes in the activity of neurons in the basal ganglia.

Modulation of [(35)S]GTPgammaS binding to chinese hamster ovary cell membranes by D(2(short)) dopamine receptors

Rat dopamine D(2short) expressed in Chinese hamster ovary (CHO) cells were characterized by means of activation of [(35)S]-guanosine 5'-O-(gamma-thiotriphosphate) ([(35)S]GTPgammaS) binding and inhibition of [(3)H]raclopride binding. Among 18 dopaminergic ligands studied dopamine, NPA, apomorphine and quinpirole were full agonists in activation of [(35)S]GTPgammaS binding, while seven ligands were partial agonists with efficacies from 16 to 69% of the effect of dopamine and seven ligands were antagonists having no effect on the basal level of [(35)S]GTPgammaS binding, but inhibited dopamine-dependent activation in a dose-response manner. Despite the different efficacies, the potencies of all 18 ligands to modulate [(35)S]GTPgammaS binding revealed a good correlation with their potencies to inhibit [(3)H]raclopride binding in the CHO cell membranes. This indicates that the binding of the ligand to the receptor determines its potency, but has no direct correlation with its intrinsic activity.

Mechanisms of agonism and inverse agonism at serotonin 5-HT1A receptors

Mechanisms of agonist and inverse agonist action at the serotonin 5-HT1A receptor have been studied using the modulation of guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) binding in membranes of Chinese hamster ovary (CHO) cells expressing the receptor (CHO-5-HTA1A cells). A range of agonists increased [35S]GTPgammaS binding with different potencies and to different maximal extents, whereas two compounds, methiothepin and spiperone, inhibited both agonist-stimulated and basal [5S]GTPgammaS binding, thus exhibiting inverse agonism. Potencies of agonists to stimulate [35S]GTPgammaS binding in membranes from CHO-5-HT1A cells were reduced by adding increasing concentrations of GDP to assays, whereas changes in sodium ion concentration did not affect agonist potency. The maximal effect of the agonists was increased by increasing sodium ion concentrations. The affinities of agonists in ligand binding assays were unaffected by changes in sodium ion concentration. Increasing GDP in the assays of the inverse agonists increased potency for spiperone to inhibit [35S]GTPgammaS binding and had no effect for methiothepin, in agreement with the sensitivity of these compounds to guanine nucleotides in ligand binding assays. Potencies for these inverse agonists were unaffected by changes in sodium ion concentration. These data were simulated using the extended ternary complex model. These simulations showed that the data obtained with agonists were consistent with these compounds achieving agonism by stabilising the ternary complex. For inverse agonists, the simulations showed that the mechanism for spiperone may be to stabilise forms of the receptor uncoupled from G proteins. Methiothepin, however, probably does not alter the equilibrium distribution of different receptor species; rather, this inverse agonist may stabilise an inactive form of the receptor that can still couple to G protein.