Selective activation of inhibitory G-protein alpha-subunits by partial agonists of the human 5-HT1A receptor

Exploring Novel Antidepressants Targeting G Protein-Coupled Receptors and Key Membrane Receptors Based on Molecular Structures

Major Depressive Disorder (MDD) is a complex mental disorder that involves alterations in signal transmission across multiple scales and structural abnormalities. The development of effective antidepressants (ADs) has been hindered by the dominance of monoamine hypothesis, resulting in slow progress. Traditional ADs have undesirable traits like delayed onset of action, limited efficacy, and severe side effects. Recently, two categories of fast-acting antidepressant compounds have surfaced, dissociative anesthetics S-ketamine and its metabolites, as well as psychedelics such as lysergic acid diethylamide (LSD). This has led to structural research and drug development of the receptors that they target. This review provides breakthroughs and achievements in the structure of depression-related receptors and novel ADs based on these. Cryo-electron microscopy (cryo-EM) has enabled researchers to identify the structures of membrane receptors, including the N-methyl-D-aspartate receptor (NMDAR) and the 5-hydroxytryptamine 2A (5-HT2A) receptor. These high-resolution structures can be used for the development of novel ADs using virtual drug screening (VDS). Moreover, the unique antidepressant effects of 5-HT1A receptors in various brain regions, and the pivotal roles of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and tyrosine kinase receptor 2 (TrkB) in regulating synaptic plasticity, emphasize their potential as therapeutic targets. Using structural information, a series of highly selective ADs were designed based on the different role of receptors in MDD. These molecules have the favorable characteristics of rapid onset and low adverse drug reactions. This review offers researchers guidance and a methodological framework for the structure-based design of ADs.

Update on GPCR-based targets for the development of novel antidepressants

Traditional antidepressants largely interfere with monoaminergic transport or degradation systems, taking several weeks to have their therapeutic actions. Moreover, a large proportion of depressed patients are resistant to these therapies. Several atypical antidepressants have been developed which interact with G protein coupled receptors (GPCRs) instead, as direct targeting of receptors may achieve more efficacious and faster antidepressant actions. The focus of this review is to provide an update on how distinct GPCRs mediate antidepressant actions and discuss recent insights into how GPCRs regulate the pathophysiology of Major Depressive Disorder (MDD). We also discuss the therapeutic potential of novel GPCR targets, which are appealing due to their ligand selectivity, expression pattern, or pharmacological profiles. Finally, we highlight recent advances in understanding GPCR pharmacology and structure, and how they may provide new avenues for drug development.

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

From Receptor Selectivity to Functional Selectivity: The Rise of Biased Agonism in 5-HT1A Receptor Drug Discovery

Despite extensive efforts to design serotonin 5-HT1A receptor compounds, there are currently no clinically available selective agonists to explore the therapeutic potential of activating this receptor. Commonly used drugs targeting 5-HT1A receptors, such as buspirone or other azapirone compounds, possess only limited selectivity over cross-reacting sites, act as partial agonists for 5-HT1A receptor activation, and are metabolically labile, generating active metabolites. In addition, drug discovery has been hampered by the multiplicity of 5-HT1A receptor subpopulations, expressed in different brain regions, that are coupled to distinct molecular signaling mechanisms and mediate a wide variety of physiological responses, both desired and undesired. In this context, advances in 5-HT1A receptor drug discovery have attracted attention of novel 'biased agonists' that are selective, efficacious and preferentially target the brain regions that mediate therapeutic activity without triggering side effects. The prototypical first-in-class compound NLX-101 (a.k.a. F15599; 3-chloro-4-fluorophenyl-[4-fluoro-4-[[(5-methylpyrimidin-2-ylmethyl)amino]methyl]piperidin- 1-yl]methanone), preferentially activates 5-HT1A receptors in cortical regions and exhibits potent, rapidacting and sustained antidepressant-like and procognitive properties in animal models. Here the background has been reviewed that led to the discovery of the class of 1-(1-benzoylpiperidin-4- yl)methanamine derivatives, including NLX-101, as well as recent advances in discovery of novel 5-HT1A receptor biased agonists, notably aryloxyethyl derivatives of 1‑(1-benzoylpiperidin-4yl)methanamine which show promising pharmacological activity both in vitro and in vivo. Overall, the results suggest that opportunities exist for innovative drug discovery of selective 5-HT1A receptor biased agonists that may open new avenues for the treatment of CNS disorders involving dysfunction of serotonergic neurotransmission.

Distinctive in vitro signal transduction profile of NLX-112, a potent and efficacious serotonin 5-HT1A receptor agonist

Objectives

NLX-112 (befiradol, F13640) is a selective serotonin 5-HT1A receptor agonist. Although it has been tested in vivo, little has been reported on its in vitro signal transduction profile.

Methods

NLX-112 was tested on G-protein activation, inhibition of adenylyl cyclase, ERK1/2 phosphorylation (pERK) and receptor internalization in recombinant cell lines. NLX-112 was also tested on G-protein activation in rat hippocampal membranes. Gα subunit mRNA expression in cell lines and rat brain tissue was quantified by quantitative PCR.

Key findings

For all signalling measures, NLX-112 exhibited agonist efficacy greater than for reference compounds ((±)8-OH-DPAT or buspirone), but similar to the endogenous agonist, serotonin, and was more potent for pERK than other responses. In rat hippocampal membranes, NLX-112 stimulated ‘total G-proteins' but, unlike (±)8-OH-DPAT and buspirone, was more potent for Gαo activation. Cell lines predominantly expressed Gαi1 and Gαi2 mRNA, with low levels of Gαo, whereas in rat brain Gαo subunits showed highest mRNA expression.

Conclusions

Unlike reference compounds, NLX-112 was a highly efficacious agonist in vitro, preferentially activating pERK in cell lines and Gαo proteins in rat hippocampal membranes. However, Gα subunit mRNA levels differ markedly between rat brain and cell lines, warranting caution when extrapolating from recombinant systems to native tissues.

How much do we know about the coupling of G-proteins to serotonin receptors?

Serotonin receptors are G-protein-coupled receptors (GPCRs) involved in a variety of psychiatric disorders. G-proteins, heterotrimeric complexes that couple to multiple receptors, are activated when their receptor is bound by the appropriate ligand. Activation triggers a cascade of further signalling events that ultimately result in cell function changes. Each of the several known G-protein types can activate multiple pathways. Interestingly, since several G-proteins can couple to the same serotonin receptor type, receptor activation can result in induction of different pathways. To reach a better understanding of the role, interactions and expression of G-proteins a literature search was performed in order to list all the known heterotrimeric combinations and serotonin receptor complexes. Public databases were analysed to collect transcript and protein expression data relating to G-proteins in neural tissues. Only a very small number of heterotrimeric combinations and G-protein-receptor complexes out of the possible thousands suggested by expression data analysis have been examined experimentally. In addition this has mostly been obtained using insect, hamster, rat and, to a lesser extent, human cell lines. Besides highlighting which interactions have not been explored, our findings suggest additional possible interactions that should be examined based on our expression data analysis.

Therapeutic potential of β-arrestin- and G protein-biased agonists

Members of the seven-transmembrane receptor (7TMR), or G protein-coupled receptor (GPCR), superfamily represent some of the most successful targets of modern drug therapy, with proven efficacy in the treatment of a broad range of human conditions and disease processes. It is now appreciated that β-arrestins, once viewed simply as negative regulators of traditional 7TMR-stimulated G protein signaling, act as multifunctional adapter proteins that regulate 7TMR desensitization and trafficking and promote distinct intracellular signals in their own right. Moreover, several 7TMR biased agonists, which selectively activate these divergent signaling pathways, have been identified. Here we highlight the diversity of G protein- and β-arrestin-mediated functions and the therapeutic potential of selective targeting of these in disease states.

Agonist-dependent modulation of G-protein coupling and transduction of 5-HT1A receptors in rat dorsal raphe nucleus

5-HT1A receptors couple to different Go/Gi proteins in order to mediate a wide range of physiological actions. While activation of post-synaptic 5-HT1A receptors is mainly related to inhibition of adenylyl cyclase activity, functionality of autoreceptors located in raphe nuclei has been classically ascribed to modifications of the activity of potassium and calcium channels. In order to evaluate the possible existence of agonist-directed trafficking for 5-HT1A autoreceptors in the rat dorsal raphe nucleus, we studied their activation by two agonists with a different profile of efficacy [(+)8-OH-DPAT and buspirone], addressing simultaneously the identification of the specific Galpha subtypes ([35S]GTPgammaS labelling and immunoprecipitation) involved and the subsequent changes in cAMP formation. A significant increase (32%, p<0.05) in (+)8-OH-DPAT-induced [35S]GTPgammaS labelling of immunoprecipitates was obtained with anti-Galphai3 antibodies but not with anti-Galphao, anti-Galphai1, anti-Galphai2, anti-Galphaz or anti-Galphas antibodies. In contrast, in the presence of buspirone, significant [35S]GTPgammaS labelling of immunoprecipitates was obtained with anti-Galphai3 (50%, p<0.01), anti-Galphao (32%, p<0.01) and anti-Galphai2 (29%, p<0.05) antibodies, without any labelling with anti-Galphai1, anti-Galphaz or anti-Galphas. The selective 5-HT1A antagonist WAY 100635 blocked the labelling induced by both agonists. Furthermore, (+)8-OH-DPAT failed to modify forskolin-stimulated cAMP accumulation, while buspirone induced a dose-dependent, WAY 100635-sensitive, inhibition of this response (Imax 30.8+/-4.9, pIC50 5.95+/-0.46). These results demonstrate the existence of an agonist-dependency pattern of G-protein coupling and transduction for 5-HT1A autoreceptors in native brain tissue. These data also open new perspectives for the understanding of the differential profiles of agonist efficacy in pre- vs. post-synaptic 5-HT1A receptor-associated responses.

Development of Generic Calcium Imaging Assay for Monitoring Gi-Coupled Receptors and G-Protein Interaction

G-protein-coupled receptors (GPCRs) are important therapeutic targets for many areas of drug research and development. Although chimeric Gα16 proteins are valuable tools for detecting the activation of Gαi/o-coupled receptors, the details of the activation process remain unclear. The authors introduce a series of chimeras that combine both Gα16 and Gα i/o (Gα16/o, Gα16/i2, and Gα 16/i3) into a well-established transient expression system to examine the ability of these chimeras to interact with D2 long-form (D 2L) dopamine and 5-HT1A serotonin receptors. The pEC 50 data obtained for known agonists were similar to results from previous studies that used other cell-based assays, thus indicating sufficient sensitivity for the assay. Moreover, quinpirole exhibited similar intrinsic activity to dopamine at the D2L receptor, whereas S-(—)-3-PPP displayed partial activity of dopamine and quinpirole in the presence of the Gα16/o chimera. The potency of dopamine for D2L receptors was similar among Gα16/o, Gα16/i2, and Gα 16/i3. In contrast, the 5-HT1A receptor exhibited a significantly preferential coupling for Gα16/i3 compared with Gα 16/i2 when serotonin was used as a ligand. This finding was in close agreement with the results of previous reports. The present system could therefore be used as a rapid functional assay for high-throughput screening and deorphanization. ( Journal of Biomolecular Screening 2009:781-788)

Signal transduction and functional selectivity of F15599, a preferential post-synaptic 5-HT1A receptor agonist

BACKGROUND AND PURPOSE

Activation of post-synaptic 5-HT(1A) receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT(1A) receptor agonist.

EXPERIMENTAL APPROACH

F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo.

KEY RESULTS

F15599 was highly selective for 5-HT(1A) receptors in binding experiments and in [(35)S]-GTPgammaS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT(1A) receptors. In cell lines expressing h5-HT(1A) receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT(1A) receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [(35)S]-GTPgammaS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated G(alphai) than G(alphao) activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT(1A) receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT(1A) receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT(1A) receptors in vivo almost as potently as F13714.

CONCLUSIONS AND IMPLICATIONS

F15599 showed a distinctive activation profiles for 5-HT(1A) receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT(1A) receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.

Activator of G protein signaling 3 null mice: I. Unexpected alterations in metabolic and cardiovascular function

Activator of G protein signaling (AGS)-3 plays functional roles in cell division, synaptic plasticity, addictive behavior, and neuronal development. As part of a broad effort to define the extent of functional diversity of AGS3-regulated-events in vivo, we generated AGS3 null mice. Surprisingly, AGS3 null adult mice exhibited unexpected alterations in cardiovascular and metabolic functions without any obvious changes in motor skills, basic behavioral traits, and brain morphology. AGS3 null mice exhibited a lean phenotype, reduced fat mass, and increased nocturnal energy expenditure. AGS3 null mice also exhibited altered blood pressure control mechanisms. These studies expand the functional repertoire for AGS3 and other G protein regulatory proteins providing unexpected mechanisms by which G protein systems may be targeted to influence obesity and cardiovascular function.

PGE2 amplifies the effects of IL-1beta on vascular smooth muscle cell de-differentiation: a consequence of the versatility of PGE2 receptors 3 due to the emerging expression of adenylyl cyclase 8

Transition of vascular smooth muscle cells from a contractile/quiescent to a secretory/proliferative phenotype is one of the critical steps in atherosclerosis and is instigated by pro-inflammatory cytokines released from macrophages that have infiltrated into the vascular wall. In most inflammatory diseases, cell activation induced by these compounds leads to a massive production of type E2 prostaglandin (PGE2) which often takes over and even potentiates the pro-inflammatory cytokine-related effects. To evaluate PGE2 incidence on atheroma plaque development, we investigated whether and how this compound could enhance the dedifferentiation of smooth muscle cells initially induced by interleukin-1beta (IL-1beta). To address this issue, we took advantage of vascular smooth muscle cells in primary culture and tracked two markers: PLA2 secretion and alpha-actin filament disorganization. In such a context, we found that PGE2 synergizes with IL-1beta to further enhance the phenotype transition of smooth muscle cells, through cAMP-protein kinase A. As indicated by pharmacological studies, the full PGE2-dependent potentiation of IL-1beta induced PLA2 secretion is associated with a change of regulation exerted by the subtypes 3 G(i)-coupled PGE2 receptors toward adenylyl cyclase(s) activated by the subtype 4 G(s)-linked PGE2 receptor. Whereas on contractile cells, stimulated subtypes 3 inhibit type 4-dependent PLA2 secretion, this negative regulation is switched to positive on IL-1beta-treated cells. Using real time PCR, pharmacological tools and small interfering RNA (siRNA), we demonstrated that the different integration of PGE2 signals depends on the upregulation of calcium/calmodulin stimulable adenylyl cyclase 8.

Differential coupling of beta3A- and beta3B-adrenergic receptors to endogenous and chimeric Galphas and Galphai

Chimeric G proteins made by replacing the COOH-terminal heptapeptide of G(alpha)q with the COOH-terminal heptapeptide of G(alpha)s or G(alpha)i were used to assess the relative coupling of beta(3)-adrenergic receptor (beta(3)-AR) splice variants (beta(3A) and beta(3B)) to G(alpha)s and G(alpha)i. The G(alpha)q/s and G(alpha)q/i chimeras transformed the response to receptor activation from regulation of adenylyl cyclase to mobilization of intracellular calcium (Ca(2+)(i)). Complementary high-throughput and single-cell approaches were used to evaluate agonist-induced coupling of the receptor to the G protein chimeras. In cells stably transformed with rat beta(3)-AR, transfected with the G protein chimeras, and evaluated using a scanning fluorometer, beta(3)-AR-induced coupling to G(alpha)q/s produced a rapid eightfold increase in Ca(2+)(i) followed by a slow decay to levels 25% above baseline. G(alpha)q/i also linked rat beta(3)-AR to mobilization of Ca(2+)(i) in a similar time- and agonist-dependent manner, but the net 2.5-fold increase in Ca(2+)(i) was only 30% of the response obtained with G(alpha)q/s. Activation of the rat beta(3)-AR also increased GTP binding to endogenous G(alpha)i threefold in membranes from CHO cells stably transformed with the receptor. A complementary single-cell imaging approach was used to assess the relative coupling of mouse beta(3A)- and beta(3B)-AR to G(alpha)i under conditions established to produce equivalent agonist-dependent coupling of the receptor splice variants to G(alpha)q/s and to increases in intracellular cAMP through endogenous G(alpha)s. The beta(3A)- and beta(3B)-AR coupled equivalently to G(alpha)q/i, but the temporal patterns of Ca(2+)(i) mobilization indicated that coupling was significantly less efficient than coupling to G(alpha)q/s. Collectively, these findings indicate less efficient but equivalent coupling of beta(3A)- and beta(3B)-AR to G(alpha)i vs. G(alpha)s and suggest that differential expression of the splice variants would not produce local differences in signaling networks linked to beta(3)-AR activation.

The G-protein regulatory (GPR) motif-containing Leu-Gly-Asn-enriched protein (LGN) and Gialpha3 influence cortical positioning of the mitotic spindle poles at metaphase in symmetrically dividing mammalian cells

We addressed the role of the G-protein regulatory (GPR) motif-containing Leu-Gly-Asn-enriched protein (LGN) and G-proteins (Gialpha3) in the positioning of the spindle pole during mammalian cell division. Immunocytochemistry indicated that both LGN and Gialpha3 co-localized at the spindle pole and at the midbody and the cell cortex during the different phases of mitosis. In marked contrast to the positioning of the spindle pole at metaphase midway between the cell cortex and the metaphase plate, the spindle pole was juxtaposed with the cell cortex at metaphase following increased expression of Gialpha3 and LGN. This repositioning of the spindle pole required the interaction of LGN with Gialpha. The influence of LGN and Gialpha3 on the cortical positioning of the spindle pole likely reflects either stronger pulling forces on the spindle pole exerted from the cell cortex or increased pushing forces exerted on the spindle pole from the mitotic spindle indicating that these events are regulated by GPR motif-containing proteins and G-proteins independent of asymmetry.

Activation of sPLA2-IIA and PGE2 production by high mobility group protein B1 in vascular smooth muscle cells sensitized by IL-1beta

Lipid mediators such as prostaglandin E2 (PGE2) play a central role during atherogenesis as a consequence of inflammation. PGE2 is produced from phospholipids by a cascade of enzymatic reactions involving phospholipase A2 (PLA2), cyclooxygenase (COX), and prostaglandin E synthase (PGES). It is released by several cell types, including vascular smooth muscle cells (VSMCs). Recent work has shown that the secretory PLA2-IIA (sPLA2-IIA), the most abundant isoform of secreted PLA2 in VSMCs, acts as a potent cytokine and activates VSMCs through a positive feedback loop. High mobility group protein 1 (HMGB1), also known as amphoterin, is a ubiquitous protein that plays various roles in the nucleus. HMGB1 is released by necrotic cells and by immune cells in response to various inflammatory mediators and acts as a potent proinflammatory cytokine. The present study investigates the role of HMGB1 in the activation of sPLA2-IIA expression and PGE2 production in VSMCs. Recombinant HMGB1 slightly activated the sPLA2-IIA, COX-2, and mPGES-1 genes but dramatically stimulated these genes in VSMCs that had been incubated with the proinflammatory cytokine IL-1beta for 24 h. This effect was accompanied by significantly increased PGE2 release. Induction of the three known receptors of HMGB1, namely RAGE, TLR-2, and TLR-4, by IL-1beta suggests that proinflammatory cytokines sensitize VSMCs to HMGB1. This provides new insights into the role of HMGB1 in VSMCs, suggesting it may be essential for the progression of atherosclerosis.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

5-HT receptors couple to activation of Akt, but not extracellular-regulated kinase (ERK), in cultured hippocampal neurons

5-HT(1A) receptors have been hypothesized to mediate some of the neuronal plasticity and behavioral responses stimulated by serotonin selective reuptake inhibitors. Although the cellular signaling pathways required for inducing these actions have not yet been determined, roles for the neuroprotective extracellular-regulated kinase (ERK) mitogen-activated protein (MAP) kinase and Akt pathways have been suggested. In the current studies we have utilized primary cultures to directly determine whether hippocampal 5-HT(1A) receptors couple to activation of Akt and ERK. We found that E18 hippocampal neurons exhibit a twofold activation of Akt when exposed to nanomolar concentrations of 5-HT. The 5-HT(1/7) receptor-selective agonist 5-carboxamidotryptamine maleate (5-CT) and the 5-HT(1A/7) receptor-selective agonist 8-hydroxy-N,N-dipropyl-aminotetralin (8-OH-DPAT) maleate were found to activate Akt with equal efficacy, and similar potency, to 5-HT. p-MPPI and WAY-100635, antagonists selective for 5-HT(1A) receptors, completely inhibited 5-CT- stimulated Akt activation. Activation of Akt was also inhibited by pretreatment with pertussis toxin as well as the phosphatidylinositol 3-kinase inhibitors, wortmannin and LY294002. In contrast, the 5-HT selective antagonist, SB269970, caused no inhibition. Although the density of 5-HT(1A) receptors expressed by cultured neurons was sufficient to activate Akt, no activation of ERK was observed. These findings suggest that Akt, and not ERK, may be relevant to previous reports of hippocampal 5-HT(1A) receptors mediating neurotrophic responses.

Differential ion current activation by human 5-HT(1A) receptors in Xenopus oocytes: evidence for agonist-directed trafficking of receptor signalling

The subject of the present study was the functional and pharmacological characterization of human 5-HT(1A) receptor regulation of ion channels in Xenopus oocytes. Activation of the heterologously expressed human 5-HT(1A) receptor induced two distinct currents in Xenopus oocytes, consisting of a smooth inward current (I(smooth)) and an oscillatory calcium-activated chloride current, I(Cl(Ca)). 5-HT(1A) receptor coupling to both ionic responses as well as to co-expressed inward rectifier potassium (GIRK) channels was pharmacologically characterized using 5-HT(1A) receptor agonists. The relative order of efficacy for activation of GIRK current was 5-HT approximately F 13714 approximately L 694,247 approximately LY 228,729>flesinoxan approximately (+/-)8-OH-DPAT. In contrast, flesinoxan and (+/-)8-OH-DPAT typically failed to activate I(Cl(Ca)). The other ligands behaved as full or partial agonists, exhibiting an efficacy rank order of 5-HT approximately L 694,247>F 13714 approximately LY 228,729. The pharmacological profile of I(smooth) activation was completely distinct: flesinoxan and F 13714 were inactive and rather exhibited an inhibition of this current. I(smooth) was activated by the other agonists with an efficacy order of L 694,247>5-HT approximately LY 228,729>(+/-)8-OH-DPAT. Moreover, activation of I(smooth) was not affected by application of pertussis toxin or the non-hydrolyzable GDP-analogue, guanosine-5'-O-(2-thio)-diphosphate (GDP betaS), suggesting a GTP binding protein-independent pathway. Together, these results suggest the existence of distinct and agonist-specific signalling states of this receptor.

AGS3 and Signal Integration by Gαs- and Gαi-coupled Receptors

AGS3-LONG and AGS3-SHORT contain G-protein regulatory motifs that interact with and stabilize the GDP-bound conformation of Galpha(i) > Galpha(o). AGS3 and related proteins may influence signal strength or duration as well as the adaptation of the signaling system associated with sustained stimulation. To address these issues, we determined the effect of AGS3 on the integration of stimulatory (Galpha(s)-mediated vasoactive intestinal peptide receptor) and inhibitory (Galpha(i)-mediated alpha(2)-adrenergic receptor (alpha(2)-AR)) signals to adenylyl cyclase in Chinese hamster ovary cells. AGS3-SHORT and AGS3-LONG did not alter the VIP-induced increase in cAMP or the inhibitory effect of alpha(2)-AR activation. System adaptation was addressed by determining the influence of AGS3 on the sensitization of adenylyl cyclase that occurs following prolonged activation of a Galpha(i)-coupled receptor. Incubation of cells with the alpha(2)-AR agonist UK14304 (1 microm) for 18 h resulted in a approximately 1.8-fold increase in the vasoactive intestinal peptide-induced activation of adenylyl cyclase, and this was associated with a decrease in membrane-associated Galpha(i3). Both effects were blocked by AGS3-SHORT. AGS3-SHORT also decreased the rate of Galpha(i3) decay. A mutant AGS3-SHORT incapable of binding G-protein was inactive. These data suggest that AGS3 and perhaps other G-protein regulatory motif-containing proteins increase the stability of Galpha(i) in the membrane, which influences the adaptation of the cell to prolonged activation of Galpha(i)-coupled receptors.

Ca2+ responses in Chinese hamster ovary-K1 cells demonstrate an atypical pattern of ligand-induced 5-HT1A receptor activation

Little experimental evidence has been reported for diverse signaling via 5-hydroxytryptamine (5-HT)1A receptors despite the fact that agonists seem to be more efficacious at dorsal raphe somatodendritic 5-HT1A autoreceptors than at postsynaptic 5-HT1A receptors. The present study investigated Ca2+ responses in Chinese hamster ovary (CHO)-K1 cells expressing a human 5-HT1A receptor by 5-HT, prototypical 5-HT1A agonists, N-(3-chloro-4-fluorobenzoyl)-4-fluoro-4-[(5-methyl-6-; methylaminopyridin-2-yl)-methylaminomethyl]-piperidine (F 14679), and especially N-(3-chloro-4-fluorobenzoyl)-4-fluoro-4-[(5-methylpyridin-2-yl)-; methylaminomethyl]piperidine (F 13640) as representative ligands of a new chemical class (methylamino-pyridine) that combines both high efficacy and selectivity for 5-HT1A receptors. 5-HT (pEC50 = 6.70 +/- 0.02) induced a pertussis toxin-sensitive, transient high-magnitude Ca2+ response. High-magnitude Ca2+ responses (Emax, percentage versus 5-HT) were also found with F 13640 (107 +/- 4), 5-carboxamidotryptamine (100 +/- 3), and F 14679 (87 +/- 3). In contrast, the prototypical 5-HT1A receptor agonists buspirone, ipsapirone, and 8-(hydroxy-2-(di-n-propylamino)tetralin, and also flesinoxan and eptapirone, were virtually inactive (< or =5). This atypical pattern of 5-HT1A receptor activation contrasts with the broad spectrum of the ligands' partial agonist properties as observed by measuring guanosine 5'-O-(3-[35 S]thio)triphosphate ([35S]GTPgammaS) binding responses with membranes of either CHO-K1 or C6-glial cells stably expressing a human 5-HT1A receptor. Remarkably, differences between ligands that seem small in the [35S]GTPgammaS binding assay translate into huge differences in the magnitude of Ca2+ responses. Therefore, some of these 5-HT1A ligands (i.e., F 13640) may in a selective way induce responses that may be not at all be achieved with other ligands (i.e., buspirone). In conclusion, the pharmacology of 5-HT1A receptor ligands seems to be codetermined by the effector pathway.

Differential activation of Gq/11 and Gi(3) proteins at 5-hydroxytryptamine(2C) receptors revealed by antibody capture assays: influence of receptor reserve and relationship to agonist-directed trafficking

As determined by a guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assay, which does not distinguish G protein subtypes, 5-hydroxytryptamine (5-HT) and 2(S)- 1-(6-chloro-5-fluoro-1H-indol-1-yl)-2-propanamine fumarate (Ro600175) behaved as full agonists at human 5-HT(2C) (h5-HT(2C)) receptors (VSV isoform) stably expressed in Chinese hamster ovary (CHO) cells, whereas 1-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), d-lysergic acid diethylamide (LSD), and lisuride exhibited partial agonist properties. After treatment with pertussis toxin to uncouple 5-HT(2C) receptors from Gi/Go but not Gq/11, DOI and LSD were as efficacious as 5-HT and Ro600175 in stimulating [(35)S]GTPgammaS binding, whereas lisuride still exhibited low efficacy (40%). Correspondingly, in a scintillation proximity assay employing specific antibodies against Gq/11, 5-HT, Ro600175, DOI, and LSD behaved as high-efficacy agonists, whereas lisuride showed efficacy of 36%. In contrast, when employing a specific antibody recognizing Gi(3), DOI and LSD were less efficacious (80 and 30%, respectively) than 5-HT and Ro600175, and lisuride was inactive. Agonist actions were specifically mediated by h5-HT(2C) receptors inasmuch as the selective 5-HT(2C) antagonist SB242,084 blocked [(35)S]GTPgammaS binding at both Gq/11 and Gi(3). Agonist potency for stimulation of Gi(3) was ~6- to 8-fold less than for Gq/11, indicating that the latter was preferentially engaged by h5-HT(2C) receptors. Inactivation of h5-HT(2C) receptors with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline did not modify the efficacy of 5-HT, Ro600175, and DOI at Gq/11, whereas their efficacies were substantially reduced at Gi(3), indicating a greater receptor reserve for the former. Finally, the preferential activation of Gq/11 versus Gi(3) by DOI, LSD, and lisuride was diminished in the presence of lower receptor number. In conclusion, h5-HT(2C) receptors couple to both Gq/11 and Gi(3) in CHO cells, and efficacy for G protein subtype activation is both ligand- and receptor reserve-dependent.

Antibody capture assay reveals bell-shaped concentration-response isotherms for h5-HT(1A) receptor-mediated Galpha(i3) activation: conformational selection by high-efficacy agonists, and relationship to trafficking of receptor signaling

Although serotonin 5-HT(1A) receptors couple to several Gi/o G-protein subtypes, little is known concerning their differential activation patterns. In this study, in membranes of Chinese hamster ovary cells expressing h5-hydroxytryptamine(1A) receptors (CHO-h5-HT(1A)), isotherms of 5-HT-stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding were biphasic, suggesting coupling to multiple G-protein subtypes. The high potency component was abolished by preincubation with an antibody recognizing Galpha(i3) subunits and was resistant to induction of [(35)S]GTPgammaS dissociation by unlabeled GTPgammaS, thus yielding a bell-shaped concentration-response isotherm. To directly investigate Galpha(i3) activation, we adopted an antibody-capture/scintillation proximity assay. 5-HT and other high-efficacy agonists yielded bell-shaped [(35)S]GTPgammaS binding isotherms, with peaks at nanomolar concentrations. As drug concentrations increased, Galpha(i3) stimulation progressively returned to basal values. In contrast, the partial agonists (-)-pindolol and 4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) displayed sigmoidal stimulation isotherms, whereas spiperone and other inverse agonists sigmoidally inhibited [(35)S]GTPgammaS binding. Agonist-induced stimulation and inverse agonist-induced inhibition of Galpha(i3) activation were i) abolished by pretreatment of CHO-h5-HT(1A) cells with pertussis toxin; ii) reversed by the selective 5-HT(1A) antagonist (N-[2-[4-(2-methoxy-phenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclohexane-carboxamide) fumarate (WAY100,635), and iii) absent in nontransfected CHO cell membranes. 5-HT isotherms could be modified by altering sodium concentration; only stimulatory actions were observed at 300mM NaCl, whereas only inhibitory actions were seen at 10 mM NaCl. Furthermore, bell-shaped isotherms were not detected at short incubation times, suggesting time-dependent changes in receptor/Galpha(i3) coupling. Taken together, these data show that low but not high concentrations of high-efficacy 5-HT(1A) agonists direct receptor signaling to Galpha(i3). In contrast, partial agonists favor h5-HT(1A) receptor signaling to Galpha(i3) over a wide concentration range, whereas inverse agonists inhibit constitutive Galpha(i3) activation.

Differential coupling of 5-HT(1) receptors to G proteins of the G(i) family

1: Since all 5-HT(1) receptors couple to G(i)-type G proteins and inhibit adenylyl cyclase, the functional significance of five distinct subtypes of 5-HT(1) receptors has been unclear. 2: In previous studies we have used transfected cells to demonstrate that 5-HT(1B) receptors can couple more efficiently than 5-HT(1A) receptors to activation of extracellular signal-regulated kinase (ERK) and to inhibition of adenylyl cyclase. These findings suggested the possibility that individual 5-HT(1) receptors differentially couple to isoforms of G(ialpha). 3: In the present study we utilized a model system in which pertussis toxin resistant forms of human G(ialpha1), G(ialpha2), and G(ialpha3) were used to directly compare the coupling of human 5-HT(1A), 5-HT(1B), and 5-HT(1D) receptors to each G(ialpha) in transfected human HeLa cells. 4: 5-HT(1A) receptors displayed a preference for G(ialpha1) and G(ialpha2), relative to G(ialpha3). Pertussis toxin resistant forms of G(ialpha1), G(ialpha2), and G(ialpha3) rescued 73%, 76%, and 44%, respectively, of the ERK activation stimulated by 5-HT in the absence of pertussis toxin. 5: In contrast, pertussis toxin resistant forms of G(ialpha1), G(ialpha2), and G(ialpha3) rescued 32%, 118%, and 35% of 5-HT(1B) receptor-stimulated activity, respectively, indicating that 5-HT(1B) receptors coupled primarily through G(ialpha2). A similar preference for G(ialpha2) was found in studies of the 5-HT(1D) receptor, where toxin resistant G(ialpha1), G(ialpha2), and G(ialpha3) rescued 30%, 70%, and 40% of activity, respectively. 6: In conclusion, the observed differential coupling of 5-HT(1) receptors to isoforms of G(ialpha), provides additional evidence for our previous findings that the subtypes of 5-HT(1) receptors exhibit similar, but distinct, functions.

Functional selectivity of dopamine receptor agonists. II. Actions of dihydrexidine in D2L receptor-transfected MN9D cells and pituitary lactotrophs

D(2)-like dopamine receptors mediate functional changes via activation of inhibitory G proteins, including those that affect adenylate cyclase activity, and potassium and calcium channels. Although it is assumed that the binding of a drug to a single isoform of a D(2)-like receptor will cause similar changes in all receptor-mediated functions, it has been demonstrated in brain that the dopamine agonists dihydrexidine (DHX) and N-n-propyl-DHX are "functionally selective". The current study explores the underlying mechanism using transfected MN9D cells and D(2)-producing anterior pituitary lactotrophs. Both dopamine and DHX inhibited adenylate cyclase activity in a concentration-dependent manner in both systems, effects blocked by D(2), but not D(1), antagonists. In the MN9D cells, quinpirole and R-(-)-N-propylnorapomorphine (NPA) also inhibited the K(+)-stimulated release of [(3)H]dopamine in a concentration-responsive, antagonist-reversible manner. Conversely, neither DHX, nor its analogs, inhibited K(+)-stimulated [(3)H]dopamine release, although they antagonized the effects of quinpirole. S-(+)-NPA actually had the reverse functional selectivity profile from DHX (i.e., it was a full agonist at D(2L) receptors coupled to inhibition of dopamine release, but a weak partial agonist at D(2L) receptor-mediated inhibition of adenylate cyclase). In lactotrophs, DHX had little intrinsic activity at D(2) receptors coupled to G protein-coupled inwardly rectifying potassium channels, and actually antagonized the effects of dopamine at these D(2) receptors. Together, these findings provide compelling evidence for agonist-induced functional selectivity with the D(2L) receptor. Although the underlying molecular mechanism is controversial (e.g., "conformational induction" versus "drug-active state selection"), such data are irreconcilable with the widely held view that drugs have "intrinsic efficacy".

Functional selectivity of dopamine receptor agonists. I. Selective activation of postsynaptic dopamine D2 receptors linked to adenylate cyclase

Dihydrexidine (DHX), the first high-affinity D(1) dopamine receptor full agonist, is only 10-fold selective for D(1) versus D(2) receptors, having D(2) affinity similar to the prototypical agonist quinpirole. The D(2) functional properties of DHX and its more D(2) selective analog N-n-propyl-dihydrexidine (PrDHX) were explored in rat brain and pituitary. DHX and PrDHX had binding characteristics to D(2) receptors in rat striatum typical of D(2) agonists, binding to both high- and low-affinity sites and being sensitive to guanine-nucleotides. Consistent with these binding data, both DHX and PrDHX inhibited forskolin-stimulated cAMP synthesis in striatum with a potency and intrinsic activity equivalent to that of quinpirole. Unexpectedly, however, DHX and PrDHX had little functional effect at D(2) receptors expressed on dopaminergic neurons that mediate inhibition of cell firing, dopamine release, or dopamine synthesis. Quantitative receptor competition autoradiography demonstrated that DHX bound to D(2) receptors in striatum (predominantly postsynaptic receptor sites) with equal affinity as D(2) sites in the substantia nigra (autoreceptor sites). The data from these experiments, coupled with what is known about the location of specific dopamine receptor isoforms, lead to the hypothesis that DHX, after binding to D(2L) and D(2S) receptors, causes agonist-typical functional changes only at some of these receptors. This phenomenon (herein termed "functional selectivity") suggests that drugs may be targeted not only at specific receptor isoforms but also at separate functions mediated by a single isoform, yielding novel approaches to drug discovery.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

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.

Identification of a truncated form of the G-protein regulator AGS3 in heart that lacks the tetratricopeptide repeat domains

AGS3, a 650-amino acid protein encoded by an approximately 4-kilobase (kb) mRNA enriched in rat brain, is a Galpha(i)/Galpha(t)-binding protein that competes with Gbetagamma for interaction with Galpha(GDP) and acts as a guanine nucleotide dissociation inhibitor for heterotrimeric G-proteins. An approximately 2-kb AGS3 mRNA (AGS3-SHORT) is enriched in rat and human heart. We characterized the heart-enriched mRNA, identified the encoded protein, and determined its ability to interact with and regulate the guanine nucleotide-binding properties of G-proteins. Screening of a rat heart cDNA library, 5'-rapid amplification of cDNA ends, and RNase protection assays identified two populations of cDNAs (1979 and 2134 nucleotides plus the polyadenylation site) that diverged from the larger 4-kb mRNA (AGS3-LONG) in the middle of the protein coding region. Transfection of COS-7 cells with AGS3-SHORT cDNAs resulted in the expression of a major immunoreactive AGS3 polypeptide (M(r) approximately 23,000) with a translational start site at Met(495) of AGS3-LONG. Immunoblots indicated the expression of the M(r) approximately 23,000 polypeptide in rat heart. Glutathione S-transferase-AGS3-SHORT selectively interacted with the GDP-bound versus guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-bound conformation of Galpha(i2) and inhibited GTPgammaS binding to Galpha(i2). Protein interaction assays with glutathione S-transferase-AGS3-SHORT and heart lysates indicated interaction of AGS3-SHORT with Galpha(i1/2) and Galpha(i3), but not Galpha(s) or Galpha(q). Immunofluorescent imaging and subcellular fractionation following transient expression of AGS3-SHORT and AGS3-LONG in COS-7 and Chinese hamster ovary cells indicated distinct subcellular distributions of the two forms of AGS3. Thus, AGS3 exists as a short and long form, both of which apparently stabilize the GDP-bound conformation of Galpha(i), but which differ in their tissue distribution and trafficking within the cell.

Ligand induced conformational states of the 5-HT(1A) receptor

It has been shown that G-protein coupled receptors have seven transmembrane alpha-helices, but the structural changes occurring in a G-protein coupled receptor as a response on agonist stimulus and the molecular events leading to blockade of the signal transduction by antagonists are not well understood. In the present study, the AMBER 5.0 force field was used for comparative molecular dynamics simulations of a 5-HT(1A) receptor model in the absence of ligand, in complex with a 5-HT(1A) receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT], in complex with a selective 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl ]-2-phenylpropanamide [(S)-WAY100135], and in complex with the partial agonist, buspirone. In the simulations, the agonist induced larger conformational changes into transmembrane helix 3 and 6 than into the other helices, while the main conformational differences between the agonist bound receptor and the antagonist bound receptor were in transmembrane helix 5 and 6. During the simulations, all the three ligands constrained the helical movements compared to those observed in the receptor without any ligand.

S 14506: novel receptor coupling at 5-HT(1A) receptors

S 14506 is chemically related to the inverse agonist at 5-HT(1A) receptors, spiperone, but S 14506 behaves as one of the most potent agonists known at these receptors, both in vitro and in vivo. In hippocampal membranes, the specific binding of [(3)H]-S 14506 (K(d)=0.79+/-0.2 nM; B(max)=400+/-32 fmol/mg protein) to 5-HT(1A) receptors resembled that of an antagonist in that it was increased by GppNHp, whereas GppNHp reduced the binding of the classic agonist [(3)H]-8-OH-DPAT (K(d)=1.5+/-0.5 nM; B(max)=303+/-20 fmol/mg protein). Manganese, magnesium and calcium reduced the binding of [(3)H]-S 14506 to 5-HT(1A) receptors whereas the binding of [(3)H]-8-OH-DPAT was increased. Further, sodium markedly reduced the binding of [(3)H]-8-OH-DPAT, without affecting the binding of [(3)H]-S 14506. [(3)H]-S 14506 also bound with high affinity to h 5-HT(1A) receptors stably expressed in membranes of CHO cells (K(d)=0.13+/-0.05 nM; B(max)=2.99+/-0.60 pmol/mg protein): the B(max) was double that of [(3)H]-8-OH-DPAT. GppNHp strongly decreased [(3)H]-8-OH-DPAT binding but scarcely changed [(3)H]-S 14506 binding; calcium, magnesium and manganese had little effect on [(3)H]-S 14506 binding in CHO cells. Antagonists (WAY 100635, WAY 100135) and inverse agonists (spiperone and metitepine) displaced [(3)H]-S 14506 binding with high affinity and Hill slopes close to unity, whereas agonists (5-HT and 5-CT) displayed low affinity with low Hill slopes: partial agonists (buspirone, ipsapirone) showed intermediate properties. In fusion proteins of h 5-HT(1A) receptors with G(ialpha1) the compound potently increased high-affinity GTPase, with a steeper Hill slope than for 5-HT, which may indicate positive cooperativity. The maximum response for S 14506 in these assays was equivalent to 5-HT, indicating it to be a full agonist.In molecular modelling studies, using a three-site model of the 5-HT(1A) receptor, S 14506 spanned between the 5-HT recognition site and the "arginine switch" (DRY microdomain) postulated to activate the interaction of the receptor with the G protein. Thus it is possible to synthesise ligands at G-protein-coupled receptors which are highly potent agonists, but which are structurally related to inverse agonists and show some features of antagonist/inverse agonist binding.

Modulation of 5-HT(1A) receptor activation by its interaction with wild-type and mutant g(alphai3) proteins

Constitutive and agonist-dependent activation of the recombinant human 5-HT(1A) receptor (RC: 2.1.5HT.01A) was investigated by co-expression with a rat G(alphai3) protein in Cos-7 cells. The interaction between the 5-HT(1A) receptor and rat G(alphai3) protein was modulated by substitution of the G(alphai3) protein site for pertussis toxin-catalysed ADP-ribosylation (cysteine(351)) by each of the natural amino acids. Enhanced basal [(35)S]GTPgammaS binding responses (+24 to +189%) were observed with the mutant G(alphai3) proteins containing at position 351 either a histidine, glutamine, serine, tyrosine or a nonpolar amino acid with the exception of a proline. With each of these mutant G(alphai3) proteins, spiperone (10 microM), but not WAY 100635 (10 microM), reduced (-22 to -60%, p<0.05) the enhanced basal [(35)S]GTPgammaS binding response. 5-HT (10 microM)-mediated [(35)S]GTPgammaS binding responses attained for some of the mutant G(alphai3)Cys(351) proteins (Phe, Met, Val and Ala) more than 300% of that obtained with the wt G(alphai3) protein. Similar results were also obtained with the prototypical 5-HT(1A) agonist 8-OH-DPAT and the partial agonist (-)-pindolol. Fusion proteins assembled from the 5-HT(1A) receptor and either the wt G(alphai3)Cys(351), mutant G(alphai3)Cys(351)Gly or G(alphai3)Cys(351)Ile protein displayed similar observations for these ligands as obtained by co-expression of the 5-HT(1A) receptor with each of these G(alphai3) proteins. Both the degree of 5-HT(1A) receptor activation by 8-OH-DPAT and (-)-pindolol, and its inhibition by spiperone, strongly correlate (r(2): 0.78-0.81) with the octanol/water partition coefficients of the mutated amino acid at position 351 of the G(alphai3) protein. The present data also suggest the wt G(alphai3) protein does not result in maximal activation of the 5-HT(1A) receptor by the agonists being investigated.

Partial agonist clonidine mediates alpha(2)-AR subtypes specific regulation of cAMP accumulation in adenylyl cyclase II transfected DDT1-MF2 cells

alpha2-Adrenergic receptor (alpha(2)-AR) activation in the pregnant rat myometrium at midterm potentiates beta(2)-AR stimulation of adenylyl cyclase (AC) via Gbetagamma regulation of the type II isoform of adenylyl cyclase. However, at term, alpha(2)-AR activation inhibits beta(2)-AR stimulation of AC. This phenomenon is associated with changes in alpha(2)-AR subtype expression (midterm alpha(2A/D)-AR >> alpha(2B)-AR; term alpha(2B) >or =alpha(2A/D)-AR), without any change in ACII mRNA, suggesting that alpha(2A/D)- and alpha(2B)-AR differentially regulate beta(2)-cAMP production. To address this issue, we have stably expressed the same density of alpha(2A/D)- or alpha(2B)-AR with AC II in DDT1-MF2 cells. Clonidine (partial agonist) increased beta(2)-AR-stimulated cAMP production in alpha(2A/D)-AR-ACII transfectants but inhibited it in alpha(2B)-AR-ACII transfectants. In contrast, epinephrine (full agonist) enhanced beta(2)-stimulated ACII in both alpha(2A)- and alpha(2B)-ACII clonal cell lines. 4-Azidoanilido-[alpha-(32)P]GTP-labeling of activated G proteins indicated that, in alpha(2B)-AR transfectants, clonidine activated only Gi(2), whereas epinephrine, the full agonist, effectively coupled to Gi(2) and Gi(3). Thus, partial and full agonists selectively activate G proteins that lead to drug specific effects on effectors. Moreover, these data indicate that Gi(3) activation is required for potentiation of beta(2)-AR stimulation of AC by alpha(2A/D) and alpha(2B)-AR in DDT1-MF2 cells. This may reflect an issue of the amount of Gbetagamma released upon receptor activation and/or betagamma composition of Gi(3) versus Gi(2).

Pasteurella (Mannheimia) haemolytica leukotoxin-induced cytolysis of bovine leukocytes: role of arachidonic acid and its regulation

Pasteurella (Mannheimia) haemolytica leukotoxin (Lkt) is the major factor that contributes to lung injury in bovine pneumonic pasteurellosis. Lkt is a pore-forming exotoxin that has the unique property of inducing cytolysis only in ruminant leukocytes and platelets. Cytolysis of many cell types is mediated by arachidonic acid (AA) and its generation by phospholipases is regulated by G-protein-coupled receptors. However, the contribution of Lkt-induced AA generation to cytolysis and the signalling cascade underlying AA generation in bovine leukocytes have not been determined. We have determined whether AA mediates Lkt-induced cytolysis and delineated the signalling mechanisms underlying AA generation in bovine leukocytes. Bovine lymphoma cells were used as an experimental system to investigate the Lkt-induced [(3)H] AA release, an index of AA generation and lactate dehydrogenase release, an index of cytolysis. The results indicate that Lkt induces AA release and cytolysis in a concentration- and time-dependent fashion. The AA analog, 5,8,11,14-eicosatetraynoic acid inhibited Lkt-induced cytolysis, but not AA release. Lkt-induced AA release and cytolysis were inhibited by pertussis toxin, inhibitors of cytosolic phospholipase A(2)(cPLA(2)), phospholipase C and protein kinase C (PKC), and by chelation of intracellular calcium. Furthermore, Western blot analysis revealed the presence of G(i), G(s)and G(q)type G-proteins. These results demonstrate that AA metabolites from cPLA(2)activation contribute to Lkt-induced cytolysis and G(i)type G-proteins, Ca(2+)and PKC, regulate the cPLA(2)activity.

Selective interaction of AGS3 with G-proteins and the influence of AGS3 on the activation state of G-proteins

AGS3 (activator of G-protein signaling 3) was isolated in a yeast-based functional screen for receptor-independent activators of heterotrimeric G-proteins. As an initial approach to define the role of AGS3 in mammalian signal processing, we defined the AGS3 subdomains involved in G-protein interaction, its selectivity for G-proteins, and its influence on the activation state of G-protein. Immunoblot analysis with AGS3 antisera indicated expression in rat brain, the neuronal-like cell lines PC12 and NG108-15, as well as the smooth muscle cell line DDT(1)-MF2. Immunofluorescence studies and confocal imaging indicated that AGS3 was predominantly cytoplasmic and enriched in microdomains of the cell. AGS3 coimmunoprecipitated with Galpha(i3) from cell and tissue lysates, indicating that a subpopulation of AGS3 and Galpha(i) exist as a complex in the cell. The coimmunoprecipitation of AGS3 and Galpha(i) was dependent upon the conformation of Galpha(i3) (GDP GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)). The regions of AGS3 that bound Galpha(i) were localized to four amino acid repeats (G-protein regulatory motif (GPR)) in the carboxyl terminus (Pro(463)-Ser(650)), each of which were capable of binding Galpha(i). AGS3-GPR domains selectively interacted with Galpha(i) in tissue and cell lysates and with purified Galpha(i)/Galpha(t). Subsequent experiments with purified Galpha(i2) and Galpha(i3) indicated that the carboxyl-terminal region containing the four GPR motifs actually bound more than one Galpha(i) subunit at the same time. The AGS3-GPR domains effectively competed with Gbetagamma for binding to Galpha(t(GDP)) and blocked GTPgammaS binding to Galpha(i1). AGS3 and related proteins provide unexpected mechanisms for coordination of G-protein signaling pathways.

Memory of an operant response and of depressed mood retained in activation states of 5-HT(1A) receptors: evidence from rodent models

Three series of studies were conducted to specify the role of 5-HT(1A) receptors in memory; using selective ligands that differentially activate 5-HT(1A) receptors, it was determined whether a change in the activation state of these receptors can lead to deficient retrieval, and whether a so-produced deficit can occur in an animal model of depression. First, in vitro studies of [35S]GTPgammaS binding responses identified ligands that differentially activate 5-HT(1A) receptors in rat hippocampus. WAY 100635, 8-OH-DPAT and flesinoxan induced 5-HT(1A) receptor activation that amounted to -2, +50 and +63%, respectively, of that produced by 5-HT. Second, we determined whether changes in the activation state of 5-HT(1A) receptors could impair the retrieval of an operant response in vivo. Rats treated with either a 5-HT(1A) receptor ligand or saline were trained to lever press for milk reward, and were then tested for retrieval with either the same or another treatment. Animals trained with 8-OH-DPAT retrieved the response when tested in the same state, but not when tested in the saline state, and vice versa. Rats trained with 0.16 mg/kg of 8-OH-DPAT also retrieved the response when tested with the other intermediate-efficacy ligand flesinoxan (0.63 mg/kg), but not when tested in a state of lower-magnitude activation (i.e. with 0.16 mg/kg of WAY 100635). Animals trained with 0.16 mg/kg of WAY 100635 retrieved the response when tested in this same state or with saline, but not when tested in a state of intermediate-magnitude activation (i.e. with 0.16 mg/kg of 8-OH-DPAT). Finally, studies using the forced swimming paradigm indicated that the retrieval of learned immobility was similarly dependent upon the activation state of 5-HT(1A) receptors. The findings indicate that changes in activation states of 5-HT(1A) receptors can impair the retrieval of learned responses. It is suggested that depression may in part be acquired in the course of ontogeny and may be available for retrieval in the same but not in other states; various biological rhythms conceivably define such states.

Diverse signalling by 5-hydroxytryptamine (5-HT) receptors

Fourteen different receptor subtypes might be regarded as a diversity that is sufficient to accommodate the wide-ranging physiological roles of 5-hydroxytryptamine (5-HT). However, it is becoming clear that, for 5-HT as for other neurotransmitters, the concept of a receptor as a gatekeeper for a specific cellular process or event is too restrictive. Multiple receptor-mediated biochemical cascades can be activated in cells in response to an agonist by a number of mechanisms. Whereas it is well established that different agonists do not necessarily elicit the same magnitude of response, they probably also select between various possible signal transduction pathways. Receptor signalling may be diverse via a single receptor subtype as a consequence of specific agonist-receptor-G protein interactions. 5-HT receptors are even more heterogeneous when one considers that the amino acid sequence of these receptor subtypes may vary from individual to individual, and that there is an increasing number of receptor isoforms due to alternative splicing and RNA editing of 5-HT receptor transcripts. Activation, in particular constitutive, agonist-independent activation, of some of these receptor isoforms has been reported to be altered. This implies that ligands with similar binding affinities may display different pharmacological properties (partial agonist, antagonist, or inverse agonist) versus these receptor isoforms, depending on their activation state. Therefore, intervention with receptor ligands to modify hampered neurotransmission pathways is a difficult task, and one needs to consider the growing evidence of diversity in G protein-coupled receptor signalling.

Selective breeding of 5-HT(1A) receptor-mediated responses: application to emotion and receptor action

Rat lines that were selectively bred for high (high DPAT-sensitive, HDS) or low (low DPAT-sensitive, LDS) hypothermic responses to the specific 5-HT(1A) receptor agonist, 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT), differ in receptor binding and certain behaviors related to anxiety and depression. After reviewing this literature, the present communication summarizes new experiments designed to clarify and extend the nature of the pharmacological and biochemical differences between the lines. A challenge with the 5-HT(2) receptor agonist, DOI, produced similar degrees of head shakes and skin crawls in the HDS and LDS rats, suggesting similar sensitivity of 5-HT(2A) and 5-HT(2C) receptors. In contrast, DOI-induced flat body posture (FBP), which has been linked to 5-HT(1A) receptor stimulation, was observed more readily in the HDS rats. The HDS and LDS rats exhibited similar degrees of increase in 8-OH-DPAT-stimulated [35S]GTPgammaS binding in several brain regions. This result suggests that the dramatic differences in hypothermia in HDS and LDS rats cannot be related to 5-HT(1A) receptor-mediated action on G proteins. Overall, these findings indicate that the selective breeding for 5-HT(1A)-mediated hypothermia has been fairly selective, and that differences in emotionally relevant behaviors between these two rat lines can strongly be associated with an unidentified component of the 5-HT(1A) receptor signaling pathway.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

Molecular analysis of beta(2)-adrenoceptor coupling to G(s)-, G(i)-, and G(q)-proteins

The beta(2)-adrenoceptor (beta(2)AR) couples to the G-protein G(s) to activate adenylyl cyclase. Intriguingly, several studies have demonstrated that the beta(2)AR can also interact with G-proteins of the G(i)- and G(q)-family. To assess the efficiency of beta(2)AR interaction with various G-protein alpha-subunits (G(xalpha)), we expressed fusion proteins of the beta(2)AR with the long (G(salphaL)) and short (G(salphaS)) splice variants of G(salpha), the G(i)-proteins G(ialpha2) and G(ialpha3), and the G(q)-proteins G(qalpha) and G(16alpha) in Sf9 cells. Fusion proteins provide a rigorous approach for comparing the coupling of a given receptor to G(xalpha) because of the defined 1:1 stoichiometry of receptor and G-protein and the efficient coupling. Here, we show that the beta(2)AR couples to G(s)-, G(i)-, and G(q)-proteins as assessed by ternary complex formation and ligand-regulated guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding. The combined analysis of ternary complex formation, GTPgammaS binding, agonist efficacies, and agonist potencies revealed substantial differences in the interaction of the beta(2)AR with the various classes of G-proteins. Comparison of the coupling of the beta(2)AR and formyl peptide receptor to G(ialpha2) revealed receptor-specific differences in the kinetics of GTPgammaS binding. We also detected highly efficient stimulation of GTPgammaS dissociation from G(salphaL), but not from G(qalpha) and G(16alpha), by a beta(2)AR agonist. Moreover, we show that the 1:1 stoichiometry of receptor to G-protein in fusion proteins reflects the in vivo stoichiometry of receptor/G-protein coupling more closely than was previously assumed. Collectively, our data show 1) that the beta(2)AR couples differentially to G(s)-, G(i)-, and G(q)-proteins, 2) that there is ligand-specific coupling of the beta(2)AR to G-proteins, 3) that receptor-specific G-protein conformational states may exist, and 4) that nucleotide dissociation is an important mechanism for G-protein deactivation.

Dopamine receptors: from structure to behavior

The responses obtained with drugs that act at dopamine receptors depend on the spectrum of receptors stimulated, the pattern of stimulation and the neuronal signal-transduction pathways that are activated. In the absence of drugs that reliably discriminate between the various cloned receptors, elucidating the role of these receptors has largely relied on molecular genetic approaches that include expression of genes for receptors in cell lines and manipulation of this expression in animal models. Connecting molecular events that occur consequent to receptor stimulation with the resulting physiological effects entails bridging a complex network of interactions. This article reviews the current understanding of the molecular, cellular and systemic consequences of activation of the different dopamine receptors.