Efficacy in drug receptor theory: outdated concept or under-valued tool?

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

Asthma and COPD: A Focus on β-Agonists - Past, Present and Future

Asthma has been recognised as a respiratory disorder for millennia and the focus of targeted drug development for the last 120 years. Asthma is one of the most common chronic non-communicable diseases worldwide. Chronic obstructive pulmonary disease (COPD), a leading cause of morbidity and mortality worldwide, is caused by exposure to tobacco smoke and other noxious particles and exerts a substantial economic and social burden. This chapter reviews the development of the treatments of asthma and COPD particularly focussing on the β-agonists, from the isolation of adrenaline, through the development of generations of short- and long-acting β-agonists. It reviews asthma death epidemics, considers the intrinsic efficacy of clinical compounds, and charts the improvement in selectivity and duration of action that has led to our current medications. Important β2-agonist compounds no longer used are considered, including some with additional properties, and how the different pharmacological properties of current β2-agonists underpin their different places in treatment guidelines. Finally, it concludes with a look forward to future developments that could improve the β-agonists still further, including extending their availability to areas of the world with less readily accessible healthcare.

Insights into the operational model of agonism of receptor dimers

INTRODUCTION

Accurate ranking of efficacies and potencies of agonists is essential in the discovery of new selective agonists. For the purpose of system-independent ranking of agonists, the operational model of agonism (OMA) has become a standard. Many receptors function as oligomers which makes functional responses more complex, requiring an extension of the original OMA.

AREAS COVERED

Explicit equations of the operational model of agonism of receptor dimers (OMARD) were derived. The OMARD can be applied to any receptor possessing two orthosteric sites. The behavior of OMARD was analyzed to demonstrate its complexity and relation to experimental data. Properties of OMARD and OMA equations were compared to demonstrate their pros and cons.

EXPERT OPINION

Extension of OMA by slope factors gives simple equations of functional response that are easy to fit experimental data but results may be inaccurate because of exponentiation of operational efficacy. Also, such equations cannot accommodate bell-shaped curves. Explicit equations of OMARD give accurate results but are complex and tedious to fit experimental data. All operational models use inter-dependent parameters that are a hurdle in the fitting. A good understanding of OMARD behavior helps to overcome such obstacles.

The signaling and selectivity of α-adrenoceptor agonists for the human α2A, α2B and α2C-adrenoceptors and comparison with human α1 and β-adrenoceptors

α2‐adrenoceptors, (α2A, α2B and α2C‐subtypes), are Gi‐coupled receptors. Central activation of brain α2A and α2C‐adrenoceptors is the main site for α2‐agonist mediated clinical responses in hypertension, ADHD, muscle spasm and ITU management of sedation, reduction in opiate requirements, nausea and delirium. However, despite having the same Gi‐potency in functional assays, some α2‐agonists also stimulate Gs‐responses whilst others do not. This was investigated. Agonist responses to 49 different α‐agonists were studied (CRE‐gene transcription, cAMP, ERK1/2‐phosphorylation and binding affinity) in CHO cells stably expressing the human α2A, α2B or α2C‐adrenoceptor, enabling ligand intrinsic efficacy to be determined (binding K_D/Gi‐IC₅₀). Ligands with high intrinsic efficacy (e.g., brimonidine and moxonidine at α2A) stimulated biphasic (Gi‐Gs) concentration responses, however for ligands with low intrinsic efficacy (e.g., naphazoline), responses were monophasic (Gi‐only). ERK1/2‐phosphorylation responses appeared to be Gi‐mediated. For Gs‐mediated responses to be observed, both a system with high receptor reserve and high agonist intrinsic efficacy were required. From the Gi‐mediated efficacy ratio, the degree of Gs‐coupling could be predicted. The clinical relevance and precise receptor conformational changes that occur, given the structural diversity of compounds with high intrinsic efficacy, remains to be determined. Comparison with α1 and β1/β2‐adrenoceptors demonstrated subclass affinity selectivity for some compounds (e.g., α2:dexmedetomidine, α1:A61603) whilst e.g., oxymetazoline had high affinity for both α2A and α1A‐subtypes, compared to all others. Some compounds had subclass selectivity due to selective intrinsic efficacy (e.g., α2:brimonidine, α1:methoxamine/etilefrine). A detailed knowledge of these agonist characteristics is vital for improving computer‐based deep‐learning and drug design.

The selectivity of α-adrenoceptor agonists for the human α1A, α1B, and α1D-adrenoceptors

Highly selective drugs offer a way to minimize side-effects. For agonist ligands, this could be through highly selective affinity or highly selective efficacy, but this requires careful measurements of intrinsic efficacy. The α1-adrenoceptors are important clinical targets, and α1-agonists are used to manage hypotension, sedation, attention deficit hypersensitivity disorder (ADHD), and nasal decongestion. With 100 years of drug development, there are many structurally different compounds with which to study agonist selectivity. This study examined 62 α-agonists at the three human α1-adrenoceptor (α1A, α1B, and α1D) stably expressed in CHO cells. Affinity was measured using whole-cell 3 H-prazosin binding, while functional responses were measured for calcium mobilization, ERK1/2-phosphorylation, and cAMP accumulation. Efficacy ratios were used to rank compounds in order of intrinsic efficacy. Adrenaline, noradrenaline, and phenylephrine were highly efficacious α1-agonists at all three receptor subtypes. A61603 was the most selective agonist and its very high α1A-selectivity was due to selective α1A-affinity (>660-fold). There was no evidence of Gq-calcium versus ERK-phosphorylation biased signaling at the α1A, α1B, or α1D-adrenoceptors. There was little evidence for α1A calcium versus cAMP biased signaling, although there were suggestions of calcium versus cAMP bias the α1B-adrenoceptor. Comparisons of the rank order of ligand intrinsic efficacy suggest little evidence for selective intrinsic efficacy between the compounds, with perhaps the exception of dobutamine which may have some α1D-selective efficacy. There seems plenty of scope to develop affinity selective and intrinsic efficacy selective drugs for the α1-adrenoceptors in future.

Conceptual and experimental issues in biased agonism

In this review, we discuss the theoretical and experimental foundations for assessing agonism in the context of signalling bias in GPCRs. We show that the formulation of efficacy in classical receptor theory and the definition of ligand-induced allosteric effect in chemical thermodynamics are coincident measures of agonism, only if we recognize that the classical model cannot be considered as a mechanistic description of the physicochemical events underlying ligand-receptor signalling. It represents instead a mathematical tool, fortuitously capable of extracting efficacy information from concentration-dependent functional data, where both ligand-dependent and ligand-independent information are present. We also assert that dissecting efficacy from affinity, as originally advocated in classical theory, is imperative for understanding the molecular property underlying agonism, and the biased agonism that leads to preferential formation of diverse GPCR-transducer complexes. Finally, we argue that beyond the assumed translational value of functional selectivity (i.e. signalling bias), the identification of ligands with true bias of efficacy is of fundamental importance for unravelling the conformational space that determines the complex functional chemistry of GPCRs.

Multiple active receptor conformation, agonist efficacy and maximum effect of the system: the conformation-based operational model of agonism

The operational model of agonism assumes that the maximum effect a particular receptor system can achieve (the Em parameter) is fixed. Em estimates are above but close to the asymptotic maximum effects of endogenous agonists. The concept of Em is contradicted by superagonists and those positive allosteric modulators that significantly increase the maximum effect of endogenous agonists. An extension of the operational model is proposed that assumes that the Em parameter does not necessarily have a single value for a receptor system but has multiple values associated to multiple active receptor conformations. The model provides a mechanistic link between active receptor conformation and agonist efficacy, which can be useful for the analysis of agonist response under different receptor scenarios.

Physiology and emerging biochemistry of the glucagon-like peptide-1 receptor

The glucagon-like peptide-1 (GLP-1) receptor is one of the best validated therapeutic targets for the treatment of type 2 diabetes mellitus (T2DM). Over several years, the accumulation of basic, translational, and clinical research helped define the physiologic roles of GLP-1 and its receptor in regulating glucose homeostasis and energy metabolism. These efforts provided much of the foundation for pharmaceutical development of the GLP-1 receptor peptide agonists, exenatide and liraglutide, as novel medicines for patients suffering from T2DM. Now, much attention is focused on better understanding the molecular mechanisms involved in ligand induced signaling of the GLP-1 receptor. For example, advancements in biophysical and structural biology techniques are being applied in attempts to more precisely determine ligand binding and receptor occupancy characteristics at the atomic level. These efforts should better inform three-dimensional modeling of the GLP-1 receptor that will help inspire more rational approaches to identify and optimize small molecule agonists or allosteric modulators targeting the GLP-1 receptor. This article reviews GLP-1 receptor physiology with an emphasis on GLP-1 induced signaling mechanisms in order to highlight new molecular strategies that help determine desired pharmacologic characteristics for guiding development of future nonpeptide GLP-1 receptor activators.

Deducing the temporal order of cofactor function in ligand-regulated gene transcription: theory and experimental verification

Cofactors are intimately involved in steroid-regulated gene expression. Two critical questions are (1) the steps at which cofactors exert their biological activities and (2) the nature of that activity. Here we show that a new mathematical theory of steroid hormone action can be used to deduce the kinetic properties and reaction sequence position for the functioning of any two cofactors relative to a concentration limiting step (CLS) and to each other. The predictions of the theory, which can be applied using graphical methods similar to those of enzyme kinetics, are validated by obtaining internally consistent data for pair-wise analyses of three cofactors (TIF2, sSMRT, and NCoR) in U2OS cells. The analysis of TIF2 and sSMRT actions on GR-induction of an endogenous gene gave results identical to those with an exogenous reporter. Thus new tools to determine previously unobtainable information about the nature and position of cofactor action in any process displaying first-order Hill plot kinetics are now available.

The pharmacological effects of the thermostabilising (m23) mutations and intra and extracellular (β36) deletions essential for crystallisation of the turkey β-adrenoceptor

The X-ray crystal structure of the turkey β-adrenoceptor has recently been determined. However, mutations were introduced into the native receptor that was essential for structure determination. These may cause alterations to the receptor pharmacology. It is therefore essential to understand the effects of these mutations on the pharmacological characteristics of the receptor. This study examined the pharmacological effects of both the m23 mutations and the β36 deletions, both alone and then in combination in the β36-m23 mutant used in the crystallisation and structure determination of the turkey β-adrenoceptor. Stable CHO-K1 cell lines were made of each of the receptor mutants and the affinity and efficacy of ligands assessed by (3)H-CGP 12177 whole cell ligand binding, (3)H-cAMP accumulation, and CRE-SPAP gene transcription assays. The m23 mutations reduced affinity for agonists, partial agonists and neutral antagonists by about tenfold whilst the β36 deletions alone had no effect on ligand affinity. Both sets of changes appeared to reduce the agonist activation of the receptor. Both the m23 and the β36 receptors retained two active agonist-induced receptor conformations similar to that of the original tβtrunc receptor. The combined β36-m23 receptor bound ligands with similar affinity to the m23 receptor; however, agonist activation was only observed with a few agonists including the catecholamines. Although the combination of mutations severely reduced the activation ability, the final crystallised receptor (β36-m23) was still a fully functional receptor capable of binding agonist and antagonist ligands and activating intracellular agonist responses.

Quantifying ligand bias at seven-transmembrane receptors

Seven transmembrane receptors (7TMRs), commonly referred to as G protein-coupled receptors, form a large part of the "druggable" genome. 7TMRs can signal through parallel pathways simultaneously, such as through heterotrimeric G proteins from different families, or, as more recently appreciated, through the multifunctional adapters, β-arrestins. Biased agonists, which signal with different efficacies to a receptor's multiple downstream pathways, are useful tools for deconvoluting this signaling complexity. These compounds may also be of therapeutic use because they have distinct functional and therapeutic profiles from "balanced agonists." Although some methods have been proposed to identify biased ligands, no comparison of these methods applied to the same set of data has been performed. Therefore, at this time, there are no generally accepted methods to quantify the relative bias of different ligands, making studies of biased signaling difficult. Here, we use complementary computational approaches for the quantification of ligand bias and demonstrate their application to two well known drug targets, the β2 adrenergic and angiotensin II type 1A receptors. The strategy outlined here allows a quantification of ligand bias and the identification of weakly biased compounds. This general method should aid in deciphering complex signaling pathways and may be useful for the development of novel biased therapeutic ligands as drugs.

The selectivity of beta-adrenoceptor agonists at human beta1-, beta2- and beta3-adrenoceptors

BACKGROUND AND PURPOSE

There are two important properties of receptor-ligand interactions: affinity (the ability of the ligand to bind to the receptor) and efficacy (the ability of the receptor-ligand complex to induce a response). Ligands are classified as agonists or antagonists depending on whether or not they have efficacy. In theory, it is possible to develop selective agonists based on selective affinity, selective intrinsic efficacy or both. This study examined the affinity and intrinsic efficacy of 31 beta-adrenoceptor agonists at the three human beta-adrenoceptors to determine whether the current agonists are subtype selective because of affinity or intrinsic efficacy.

EXPERIMENTAL APPROACH

Stable clonal CHO-K1 cell lines, transfected with either the human beta(1), beta(2) or beta(3)-adrenoceptor, were used, and whole-cell [(3)H]-CGP 12177 radioligand binding and [(3)H]-cAMP accumulation were measured.

KEY RESULTS

Several agonists were found to be highly subtype selective because of selective affinity (e.g. salmeterol and formoterol, for the beta(2)-adrenoceptor over the beta(1) or beta(3)), while others (e.g. isoprenaline) had little affinity-selectivity. However, the intrinsic efficacy of salmeterol, formoterol and isoprenaline was similar across all three receptor subtypes. Other ligands (e.g. denopamine for beta(1); clenbuterol, AZ 40140d, salbutamol for beta(2)) were found to have subtype-selective intrinsic efficacy. Several ligands appeared to activate two agonist conformations of the beta(1)- and beta(3)-adrenoceptors.

CONCLUSIONS AND IMPLICATIONS

There are agonists with subtype selectivity based upon both selective affinity and selective intrinsic efficacy. Therefore, there is scope to develop better selective agonists based upon both selective affinity and selective intrinsic efficacy.

New pansomatostatin ligands and their chelated versions: affinity profile, agonist activity, internalization, and tumor targeting

PURPOSE

Somatostatin receptor (sst) targeting is an established method to image and treat sst-positive tumors. Particularly, neuroendocrine tumors express the receptor subtype 2 in high density, but sst1, sst3, sst4, and sst5 are also expressed to some extent in different human tumors. Currently used targeting peptides mainly have sst2 affinity. We aimed at developing (radio)peptides that bind with high affinity to all receptor subtypes.

EXPERIMENTAL DESIGN

Carbocyclic octapeptides were coupled with macrocyclic chelators for radiometal labeling. Affinity, internalization, and agonist potencies were determined on sst1- to sst5-expressing cell lines. Biodistribution was determined on nude mice bearing HEK-sst2 or AR4-2J and HEK-sst3 tumors.

RESULTS

High affinity to all receptor subtypes was found. Y(III)-KE88 showed agonistic properties at all five sst receptor subtypes as it inhibits forskolin-stimulated cyclic AMP production. Surprisingly, very low or even absent sst2 receptor internalization was found compared with currently clinically established octapeptides, whereas the sst3 internalization was very efficient. Biodistribution studies of [(111)In]KE88 and [(67)Ga]KE88/[(68)Ga]KE88 reflected the in vitro data. In nude mice with s.c. implanted sst2 (HEK-sst2, AR4-2J)-expressing and sst3 (HEK-sst3)-expressing tumors, high and persistent uptake was found in sst3-expressing tumors, whereas the uptake in the sst2-expressing tumors was lower and showed fast washout. The kidney uptake was high but blockable by coinjection of lysine.

CONCLUSION

This peptide family shows pansomatostatin potency. As radiopeptides, they are the first to show a full pansomatostatin profile. Despite some drawback, they should be useful for imaging sst2-expressing tumors with short-lived radiometals, such as (68)Ga, at early time points and for sst3-expressing tumors at later time points with longer-lived radiometals, such as (64)Cu or (86)Y.

Thiopurine therapies: problems, complexities, and progress with monitoring thioguanine nucleotides

Metabolism of thiopurine drugs--azathioprine, 6-mercaptopurine, and 6-thioguanine--has provided a powerful pharmacogenetic model incorporating polymorphism of the enzyme thiopurine methyltransferase (TPMT) and the primary active metabolite, thioguanine nucleotide (TGN). However, a sense of uncertainty about the usefulness of TGNs and other thiopurine metabolites has appeared. This review critically appraises the basis of thiopurine metabolism and reveals the problems and complexities in TGN research. Erythrocyte TGN is used in transplantation medicine and in chronic inflammatory conditions such as Crohn's disease, as a "surrogate" pharmacokinetic parameter for TGN in the target cells: leukocytes or bone marrow. It is not generally appreciated that erythrocytes do not express the enzyme IMP dehydrogenase and cannot convert mercaptopurine to TGN, which explains some of the confusion in interpretation of erythrocyte TGN measurements. TGN routinely measured in erythrocytes derives from hepatic metabolism. Another concern is that TGN are not generally assayed directly: most methods assay the thiopurine bases. Ion-exchange HPLC and enzymatic conversion of TGNs to nucleosides have been used to overcome this, and may reveal undisclosed roles for an unusual cytotoxic nucleotide, thio-inosine triphosphate, and methylated thiopurines. There appear to be additional interactions between xanthine oxidase and TPMT, and folate and TPMT, that could predict leukopenia. Difficult questions remain to be answered, which may be assisted by technological advances. Prospective TGN studies, long overdue, are at last revealing clearer results.

Human adenosine A(3) receptor leads to intracellular Ca(2+) mobilization but is insufficient to activate the signaling pathway via phosphoinositide 3-kinase gamma in mice

Selective antagonists for the adenosine A(3) receptor (A3AR), a member of the G protein-coupled receptors, have been indicated as potential drugs for anti-asthma or anti-inflammation. However, potent antagonists for the rodent A3AR have not been identified. To evaluate the pharmacological effects of human A3AR antagonists in mice, we here generated A3AR-humanized mice, in which the mouse A3AR gene was replaced by its human counterpart. The expression levels of human A3AR in the A3AR-humanized mice were equivalent to those of mouse A3AR in wild-type mice. Elevation of the intracellular Ca(2+) concentration induced by an A3AR agonist was observed in bone marrow-derived mast cells from the A3AR-humanized mice and this Ca(2+) mobilization was completely antagonized by a human A3AR antagonist. However, antigen-dependent degranulation was not potentiated by the A3AR agonist in the mast cells from A3AR-humanized mice. The agonist-stimulated human A3AR did not lead to the phosphorylation of either extracellular signal-regulated kinase 1/2 or protein kinase B in A3AR-humanized mice. The rate of human A3AR internalization in the mast cells was also markedly decreased compared with that of mouse A3AR in the mast cells. These results demonstrate that the human A3AR is insufficient to activate phosphoinositide 3-kinase gamma-dependent signaling pathways in mice, probably due to the uncoupling of member(s) of the G proteins, which are capable of activating phosphoinositide 3-kinase gamma, to the human A3AR, despite the mouse G protein(s) responsible for the Ca(2+) elevation are coupled with the human A3AR.

The novel antipsychotic aripiprazole is a partial agonist at short and long isoforms of D2 receptors linked to the regulation of adenylyl cyclase activity and prolactin release

Aripiprazole is a novel antipsychotic with a unique mechanism of action, which differs from currently marketed typical and atypical antipsychotics. Aripiprazole has been shown to be a partial agonist at the D(2) family of dopamine (DA) receptors in biochemical and pharmacological studies. To demonstrate aripiprazole's action as a partial D(2) agonist in pituitary cells at the molecular level, we retrovirally transduced the short (D(2S)) and the long (D(2L)) form of the human DA D(2) receptor gene into a rat pituitary cell line, GH4C1. [(3)H]-raclopride saturation binding analyses revealed a B(max) value approximately four-fold higher at D(2S) receptor-expressing GH4C1 cells than at D(2L) receptor-expressing GH4C1 cells, while a K(d) value was similar. Aripiprazole inhibited forskolin-stimulated release of prolactin in both D(2S) and D(2L) receptor-expressing GH4C1 cells, whereas the maximal inhibition of prolactin release was less than that of DA. Similarly, aripiprazole partially inhibited forskolin-induced cAMP accumulation in both D(2) receptor-expressing cells. Aripiprazole antagonized the suppression attained by DA (10(-7) M) in both D(2) receptor-expressing cells and, at the maximal blockade of cAMP, yielded residual cAMP levels equal to those produced by aripiprazole alone. These results indicate that aripiprazole acts as a partial agonist at both D(2S) and D(2L) receptors expressed in GH4C1 cells. These data may explain, at least in part, the observations that aripiprazole shows a novel antipsychotic activity with minimal potential for adverse events including no significant increase of serum prolactin levels in clinical studies.

Pro7.33(303) of the human GnRH receptor regulates selective binding of mammalian GnRH

Mammalian gonadotropin releasing hormone (GnRH) receptors have a conserved acidic residue (Glu7.32(301) or Asp7.32(302)) in extracellular loop (ECL) three that confers selectivity for mammalian GnRH, which has Arg8. Comparison of mammalian and non-mammalian GnRH receptors suggested that the acidic residue is not the only determinant of ligand selectivity in mammalian receptors. The acidic residue is followed by a conserved Pro7.33 in mammalian GnRH receptors, but not non-mammalian receptors. Unique structural constraints imposed by Pro residues suggested that Pro7.33 determines selective binding of Arg8-containing GnRH, by stabilising the conformation of the third extracellular loop of the receptor. Substituting Pro7.33(303) or introducing Pro to position 7.31 decreased affinity for GnRH, but not analogs lacking Arg8. Substituting Pro7.33(303) changed the predicted alpha-helix content of the loop-helix interface. These results show that Pro7.33(303) of the human GnRH receptor is required for selective high affinity binding of mammalian GnRH and supports the hypothesis that Pro7.33(303) stabilises a loop conformation that is necessary for selective ligand binding.

Possible role of intramembrane receptor-receptor interactions in memory and learning via formation of long-lived heteromeric complexes: focus on motor learning in the basal ganglia

Learning in neuronal networks occurs by instructions to the neurons to change their synaptic weights (i.e., efficacies). According to the present model a molecular mechanism that can contribute to change synaptic weights may be represented by multiple interactions between membrane receptors forming aggregates (receptor mosaics) via oligomerization at both pre- and post-synaptic level. These assemblies of receptors together with inter alia single receptors, adapter proteins, G-proteins and ion channels form the membrane bound part of a complex three-dimensional (3D) molecular circuit, the cytoplasmic part of which consists especially of protein kinases, protein phosphatases and phosphoproteins. It is suggested that this molecular circuit has the capability to learn and store information. Thus, engram formation will depend on the resetting of 3D molecular circuits via the formation of new receptor mosaics capable of addressing the transduction of the chemical messages impinging on the cell membrane to certain sets of G-proteins. Short-term memory occurs by a transient stabilization of the receptor mosaics producing the appropriate change in the synaptic weight. Engram consolidation (long-term memory) may involve intracellular signals that translocate to the nucleus to cause the activation of immediate early genes and subsequent formation of postulated adapter proteins which stabilize the receptor mosaics with the formation of long-lived heteromeric receptor complexes. The receptor mosaic hypothesis of the engram formation has been formulated in agreement with the Hebbian rule and gives a novel molecular basis for it by postulating that the pre-synaptic activity change in transmitter and modulator release reorganizes the receptor mosaics at post-synaptic level and subsequently at pre-synaptic level with the formation of novel 3D molecular circuits leading to a different integration of chemical signals impinging on pre- and post-synaptic membranes hence leading to a new value of the synaptic weight. Engram retrieval is brought about by the scanning of the target networks by the highly divergent arousal systems. Hence, a continuous reverberating process occurs both at the level of the neural networks as well as at the level of the 3D molecular circuits within each neuron of the network until the appropriate tuning of the synaptic weights is obtained and, subsequently, the reappearance of the engram occurs. Learning and memory in the basal ganglia is discussed in the frame of the present hypothesis. It is proposed that formation of long-term memories (consolidated receptor mosaics) in the plasma membranes of the striosomal GABA neurons may play a major role in the motivational learning of motor skills of relevance for survival. In conclusion, long-lived heteromeric receptor complexes of high order may be crucial for learning, memory and retrieval processes, where extensive reciprocal feedback loops give rise to coherent synchronized neural activity (binding) essential for a sophisticated information handling by the central nervous system.

Changes in amino-terminal portion of human B2 receptor selectively increase efficacy of synthetic ligand HOE 140 but not of cognate ligand bradykinin

Recently, we have shown that a widely used antagonist of the human bradykinin B(2) receptor (B(2)R) HOE 140 acts as a full agonist of the chicken ornithokinin receptor (B(o)R). To understand the molecular mechanisms underlying differential efficacy of HOE 140 for the various kinin receptors, we have constructed chimeric kinin receptors (CKR) in which the amino-terminal portion including the first two transmembrane regions and the first extracellular loop (CKR-2) or only the second transmembrane region and the first extracellular loop (CKR-1) of B(2)R were substituted with the corresponding segments of B(o)R. Ligand efficacy of synthetic ligand HOE 140 decreased in the order B(o)R > CKR-2 > CKR-1 > B(2)R, whereas the efficacy of the endogenous kinin ligand was unchanged. Enhanced HOE 140 efficacy was not due to a structural change in the ligand binding site or to an enhanced receptor expression level. Rather, heterologous binding competition studies indicated that structural change(s) introduced into the engineered receptors caused a selective reduction in apparent affinity of HOE 140 for the uncoupled inactive receptor state R but not for the active G protein-coupled state R*, thereby increasing the ratio of R* over R for a given ligand concentration. Our results may help explain the unusually broad efficacy spectrum of HOE 140, which varies from inverse to full agonism, depending on kinin receptor subtype, tissue origin, or species.

Opioid agonists have different efficacy profiles for G protein activation, rapid desensitization, and endocytosis of mu-opioid receptors

The differential ability of various mu-opioid receptor (MOP) agonists to induce rapid receptor desensitization and endocytosis of MOP could arise simply from differences in their efficacy to activate G proteins or, alternatively, be due to differential capacity for activation of other signaling processes. We used AtT20 cells stably expressing a low density of FLAG-tagged MOP to compare the efficacies of a range of agonists to 1) activate G proteins using inhibition of calcium channel currents (ICa) as a reporter before and after inactivation of a fraction of receptors by beta-chlornaltrexamine, 2) produce rapid, homologous desensitization of ICa inhibition, and 3) internalize receptors. Relative efficacies determined for G protein coupling were [Tyr-D-Ala-Gly-MePhe-Glyol]enkephalin (DAMGO) (1) > or = methadone (0.98) > morphine (0.58) > pentazocine (0.15). The same rank order of efficacies for rapid desensitization of MOP was observed, but greater concentrations of agonist were required than for G protein activation. By contrast, relative efficacies for promoting endocytosis of MOP were DAMGO (1) > methadone (0.59) >> morphine (0.07) > or = pentazocine (0.03). These results indicate that the efficacy of opioids to produce activation of G proteins and rapid desensitization is distinct from their capacity to internalize mu-opioid receptors but that, contrary to some previous reports, morphine can produce rapid, homologous desensitization of MOP.

Characterization and channel coupling of the P2Y(12) nucleotide receptor of brain capillary endothelial cells

Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.

Aripiprazole, a novel antipsychotic, is a high-affinity partial agonist at human dopamine D2 receptors

Aripiprazole is the first next-generation atypical antipsychotic with a mechanism of action that differs from currently marketed typical and atypical antipsychotics. Aripiprazole displays properties of an agonist and antagonist in animal models of dopaminergic hypoactivity and hyperactivity, respectively. This study examined the interactions of aripiprazole with a single population of human D2 receptors to clarify further its pharmacologic properties. In membranes prepared from Chinese hamster ovary cells that express recombinant D2L receptors, aripiprazole bound with high affinity to both the G protein-coupled and uncoupled states of receptors. Aripiprazole potently activated D2 receptor-mediated inhibition of cAMP accumulation. Partial receptor inactivation using the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) significantly reduced the maximum effect of aripiprazole on inhibition of cAMP accumulation. This effect was seen with concentrations of EEDQ that did not alter the maximal inhibitory effect of dopamine. Consistent with the expected effects of a partial agonist, increasing concentrations of aripiprazole blocked the action of dopamine with maximal blockade equal to the agonist effect of aripiprazole alone. The efficacy of aripiprazole relative to that of dopamine varied from 25% in cells that lacked spare receptors for dopamine to 90% in cells with receptor reserve. These results, together with previous studies demonstrating partial agonist activity at serotonin 5-hydroxytryptamine (5-HT)1A receptors and antagonist activity at 5-HT2A receptors, support the identification of aripiprazole as a dopamine-serotonin system stabilizer. The receptor activity profile may underlie the unique activity of aripiprazole in animals and its antipsychotic activity in humans.

Differential antagonism of endomorphin-1 and endomorphin-2 supraspinal antinociception by naloxonazine and 3-methylnaltrexone

To determine if different subtypes of mu-opioid receptors were involved in antinociception induced by endomorphin-1 and endomorphin-2, the effect of pretreatment with various mu-opioid receptor antagonists beta-funaltrexamine, naloxonazine and 3-methylnaltrexone on the inhibition of the paw-withdrawal induced by endomorphin-1 and endomorphin-2 given intracerebroventricularly (i.c.v.) were studied in ddY male mice. The inhibition of the paw-withdrawal induced by i.c.v. administration of endomorphin-1, endomorphin-2 or DAMGO was completely blocked by the pretreatment with a selective mu-opioid receptor antagonist beta-funaltrexamine (40 mg/kg), indicating that the antinociception induced by all these peptides are mediated by the stimulation of mu-opioid receptors. However, naloxonazine, a mu1-opioid receptor antagonist pretreated s.c. for 24h was more effective in blocking the antinociception induced by endomorphin-2, than by endomorphin-1 or DAMGO given i.c.v. Pretreatment with a selective morphine-6 beta-glucuronide blocker 3-methylnaltrexone 0.25mg/kg given s.c. for 25 min or co-administration of 3-methylnaltrexone 2.5 ng given i.c.v. effectively attenuated the antinociception induced by endomorphin-2 given i.c.v. and co-administration of 3-methylnaltrexone shifted the dose-response curves for endomorphin-2 induced antinociception to the right by 4-fold. The administration of 3-methylnaltrexone did not affect the antinociception induced by endomorphin-1 or DAMGO given i.c.v. Our results indicate that the antinociception induced by endomorphin-2 is mediated by the stimulation of subtypes of mu-opioid receptor, which is different from that of mu-opioid receptor subtype stimulation by endomorphin-1 and DAMGO.

Drug efficacy at G protein-coupled receptors

Efficacy has been defined in receptor pharmacology as a proportionality factor denoting the amount of physiological response a given ligand imparts to a biological system for a given amount of receptor occupancy. While first defined in terms of response, the concept can be expanded to a wide variety of G protein-coupled receptor (GPCR) behaviors, which includes pleiotropic interaction with multiple G proteins, internalization, oligomerization, desensitization, and interaction with membrane auxilliary proteins. Thus, there can be numerous types of efficacy, and different ligands can have a range of efficacies for different receptor behaviors. This review discusses the use of the efficacy concept in GPCR models based on the thermodynamic linkage theory and also in terms of the protein ensemble theory, in which macroaffinity of ligands for an ensemble of receptor microstates produces a new ligand-bound ensemble. The pharmacological characteristics of the ligand emerge from the intersection of the ligand-bound ensemble with the various ensembles defining pharmacological receptor behaviors. Receptor behaviors discussed are activation of G proteins; ability to be phosphorylated, desensitized, and internalized; formation of dimers and oligomers; and the interaction with auxiliary membrane and cytosolic proteins. The concepts of ligand-specific receptor conformation and conditional efficacy are also discussed in the context of ligand control of physiological response.

Efficacy at G-protein-coupled receptors

At present, the drug-discovery process centres on ligands that either block or produce physiological responses. However, there are therapeutic uses for ligands that do neither of these things, but which still affect receptors in other ways. This review discusses the intimate relationship between the affinity of a ligand for its receptor, and the probability that the binding of the ligand will produce some change in the receptor, resulting in efficacy. This, in turn, argues that ligands that have affinity should be tested more broadly, for a wider range of efficacies, to detect hidden therapeutic activities.

Abolished tubuloglomerular feedback and increased plasma renin in adenosine A1 receptor-deficient mice

The hypothesis that adenosine acting on adenosine A1 receptors (A1R) regulates several renal functions and mediates tubuloglomerular feedback (TGF) was examined using A1R knockout mice. We anesthetized knockout, wild-type, and heterozygous mice and measured glomerular filtration rate, TGF response using the stop-flow pressure (P(sf)) technique, and plasma renin concentration. The A1R knockout mice had an increased blood pressure compared with wild-type and heterozygote mice. Glomerular filtration rate was similar in all genotypes. Proximal tubular P(sf) was decreased from 36.7 +/- 1.2 to 25.3 +/- 1.6 mmHg in the A1R+/+ mice and from 38.1 +/- 1.0 to 27.4 +/- 1.1 mmHg in A1R+/- mice in response to an increase in tubular flow rate from 0 to 35 nl/min. This response was abolished in the homozygous A1R-/- mice (from 39.1 +/- 4.1 to 39.2 +/- 4.5 mmHg). Plasma renin activity was significantly greater in the A1R knockout mice [74.2 +/- 14.3 milli-Goldblatt units (mGU)/ml] mice compared with the wild-type and A1R+/- mice (36.3 +/- 8.5 and 34.1 +/- 9.6 mGU/ml), respectively. The results demonstrate that adenosine acting on A1R is required for TGF and modulates renin release.

Factors affecting prostacyclin receptor agonist efficacy in different cell types

Octimibate and related nonprostanoid prostacyclin mimetics are partial agonists displaying highly tissue-specific responses. Octimibate demonstrated considerably greater efficacy for stimulation of adenylyl cyclase activity in Chinese hamster ovary cells transiently expressing mouse prostacyclin receptors (mIP-CHO cells) when compared to human SK-N-SH neuroblastoma cells, which endogenously express prostacyclin (IP) receptors. Pretreatment of both cell types with pertussis toxin (PTx) failed to influence IP agonist efficacy or potency, indicating a lack of involvement of an agonist-stimulated inhibitory G(i)-coupled pathway. Although stimulation of mIP-CHO cells with the full agonist cicaprost increased both [3H]cyclic AMP and [3H]inositol phosphate ([3H]IP) accumulation (pEC(50) values of 8.35 and 6.82, respectively), IP receptor signalling through G(q) in SK-N-SH cells was absent. Inhibition of protein kinase C (PKC) in mIP-CHO cells increased [3H]IP accumulation but had no effect on [3H]cyclic AMP accumulation. Therefore, the poor coupling of the IP receptor in SK-N-SH cells to G(q) is unlikely to explain the relatively low efficacy of octimibate for stimulating adenylyl cyclase in these cells. Furthermore, protein kinase A (PKA) inhibition appears to enhance IP receptor signalling through both G(s) and G(q) in mIP-CHO cells.

Type A allatostatins from Drosophila melanogaster and Diplotera puncata activate two Drosophila allatostatin receptors, DAR-1 and DAR-2, expressed in CHO cells

The type-A allatostatins A (AST-A) are a group of insect peptides with a common C-terminal motif Y/FXFGL-NH(2). The existence of at least four putative type A Drosophila melanogaster ASTs (called type A drostatins or DST-As) has been predicted from the sequence of a recently cloned DST-A preprohormone [C. Lenz et al. (2000) Biochem. Biophys. Res. Commun. 273, 126-1131]. SRPYSFGL-NH(2), (DST-3A), the only DST isolated from Drosophila so far, activated the first cloned DST-A GPCR (DAR-1) [N. Birgül et al. (1999) EMBO J. 18, 5892-5900]. A newly cloned orphan Dm GPCR, which shares 47% overall and 60% transmembrane region sequence identity with DAR-1, was classified as a second putative Dm DST-A receptor (DAR-2) [C. Lenz et al. (2000) Biochem. Biophys. Res. Commun. 273, 571-577]. Although activation of DAR-2 by DSTs has been postulated, no experimental evidence for that has been presented to date. In this study, we expressed both DAR-1 and DAR-2 in CHO cells and used a GTPgammaS and a Ca(2+) mobilization assay for pharmacological evaluation of the receptors. Synthetically prepared DST-As, as well as selected Diplotera punctata (cockroach) ASTs, activated DAR-1 and DAR-2 in both functional assays indicating ligand redundancy and cross species activity. Cell pretreatment with pertussis toxin led to some differences in the nature and magnitude of signaling pathways at the DAR-1 and DAR-2 receptors, suggesting possible differential coupling to cellular effector system(s) and distinct biological functions of each receptor in vivo.

Two active molecular phenotypes of the tachykinin NK1 receptor revealed by G-protein fusions and mutagenesis

The NK1 neurokinin receptor presents two non-ideal binding phenomena, two-component binding curves for all agonists and significant differences between agonist affinity determined by homologous versus heterologous competition binding. Whole cell binding with fusion proteins constructed between either Galpha(s) or Galpha(q) and the NK1 receptor with a truncated tail, which secured non-promiscuous G-protein interaction, demonstrated monocomponent agonist binding closely corresponding to either of the two affinity states found in the wild-type receptor. High affinity binding of both substance P and neurokinin A was observed in the tail-truncated Galpha(s) fusion construct, whereas the lower affinity component was displayed by the tail-truncated Galpha(q) fusion. The elusive difference between the affinity determined in heterologous versus homologous binding assays for substance P and especially for neurokinin A was eliminated in the G-protein fusions. An NK1 receptor mutant with a single substitution at the extracellular end of TM-III-(F111S), which totally uncoupled the receptor from Galpha(s) signaling, showed binding properties that were monocomponent and otherwise very similar to those observed in the tail-truncated Galpha(q) fusion construct. Thus, the heterogenous pharmacological phenotype displayed by the NK1 receptor is a reflection of the occurrence of two active conformations or molecular phenotypes representing complexes with the Galpha(s) and Galpha(q) species, respectively. We propose that these molecular forms do not interchange readily, conceivably because of the occurrence of microdomains or "signal-transductosomes" within the cell membrane.

Receptor constants for endomorphin-1 and endomorphin-1-ol indicate differences in efficacy and receptor occupancy

The opioid properties of endomorphin derivatives containing a C-terminal alcoholic(-ol) function were compared to the parent amidated compounds in isolated organs (longitudinal muscle strip of guinea-pig ileum and mouse vas deferens). Similar data were also generated for the mu-opioid receptor selective agonist synthetic peptide (D-Ala2, MePhe4, Gly5-ol)-enkephalin (DAMGO) and its Gly5-NH2 congener (DAMGA). Endomorphin-1-ol (Tyr-Pro-Trp-Phe-ol) had an IC50 of 80.6 nM in mouse vas deferens and 61.2 nM in guinea-pig ileum; the corresponding values for endomorphin-2-ol (Tyr-Pro-Phe-Phe-ol) were 49.6 and 48.2 nM, for DAMGO 59.8 and 29.2 nM, respectively. As it was indicated by the antagonism by naltrexone, the agonist actions were exerted exclusively at mu-opioid receptors in both organs. The -ol derivatives were slightly (2.3-4.3 times) less potent than the parent amides in the bioassays: all peptides had, apparently, full agonist properties in intact preparations. With the aim of revealing potential partial agonist properties among the investigated peptides, we partially inactivated the mu-opioid receptor pool in mouse vas deferens by 5x10(-7) M beta-funaltrexamine. The calculated receptor constants indicated a "high-affinity, low intrinsic efficacy" profile (i.e. a potential partial agonist property) for endomorphin-1, an intermediate character for endomorpin-1-ol and full agonism for DAMGA and DAMGO. Apparently, a higher receptor fraction remained accessible for endomorphin-1 (42.8%) than for the -ol congener (14.0%), DAMGO (20.2%) and DAMGA (14.1%) after partial inactivation.

Inverse, protean, and ligand-selective agonism: matters of receptor conformation

Concepts regarding the mechanisms by which drugs activate receptors to produce physiological response have progressed beyond considering the receptor as a simple on-off switch. Current evidence suggests that the idea that agonists produce only varying degrees of receptor activation is obsolete and must be reconciled with data to show that agonist efficacy has texture as well as magnitude. Thus, agonists can block system constitutive response (inverse agonists), behave as positive and inverse agonists on the same receptor (protean agonists), and differ in the stimulus pattern they produce in physiological systems (ligand-selective agonists). The molecular mechanism for this seemingly diverse array of activities is the same, namely, the selective microaffinity of ligands for different conformational states of the receptor. This paper reviews evidence for the existence of the various types of agonism and the potential therapeutic utility of different agonist types.-Kenakin, T. Inverse, protean, and ligand-selective agonism: matters of receptor conformation.

Insights into peptide and protein function: a convergent approach

From viruses to multicellular organisms, life is inseparable from the genetic instructions aimed at regulating its maintenance, division, multiplication, differentiation and death (apoptosis). Over the past 15 years, structural studies have begun to resolve the complex reactions involved in these fundamental processes in biology. The three-dimensional representations of the complexes formed with peptides and/or proteins have allowed interpretation of the biochemical data and formulation of novel hypotheses about the control and execution of these processes. Moreover, they have opened the way to rational approaches for designing compounds able to interfere with these crucial events in normal or pathological conditions. Various results obtained in our laboratory in these fields are briefly summarized in this review.

Nonlinear analysis of partial dopamine agonist effects on cAMP in C6 glioma cells

Most drugs have some efficacy so that improved methods to determine the relative intrinsic efficacy of partial agonists should be of benefit to preclinical and clinical investigators. We examined the effects of partial D(1) or partial D(2) dopamine agonists using a partial agonist interaction model. The dependent variable was the modulation of the dopamine-receptor-mediated cAMP response in C6 glioma cells selectively and stably expressing either D(1) or D(2) recombinant dopamine receptors. The dissociation constant (K(B)) and relative intrinsic efficacy (E(r)) for each partial agonist were calculated using a partial agonist interaction null model in which the effects of fixed concentrations of each partial agonist on the dopamine dose-response curve were evaluated. This model is an extension of the competitive antagonist null model to drugs with efficacy and assumes only that the log-dose--response curve is monotonic. Generally, the partial agonist interaction model fit the data, as well as fits of the independent logistic curves. Furthermore, the partial agonist K(B) values could be shared across partial agonist concentrations without worsening the model fit (by increasing the residual variance). K(B) values were also similar to drug affinities reported in the literature. The model was validated in three ways. First, we assumed a common tissue stimulus parameter (beta) and calculated the E(r) values. This provided a qualitative check on the interaction model results. Second, we calculated new relative efficacy values, E(r)(beta), using the beta estimate. Third, we calculated relative efficacy using relative maxima times midpoint shift ratios (J. Theor. Biol. 198 (1999) 347.). All three methods indicated that the present model yielded reasonable estimates of affinity and relative efficacy for the set of compounds studied. Our results provide a quick and convenient method of quantification of partial agonist efficacy. Special applications and limitations of the model are discussed. In addition, the present results are the first report of the relative intrinsic efficacy values for this set of D(2) ligands.

Morphine-6 beta-glucuronide has a higher efficacy than morphine as a mu-opioid receptor agonist in the rat locus coeruleus

1. The pharmacological properties of the active morphine metabolite, morphine-6 beta-D-glucuronide (M6G), and the parent compound were compared in rat locus coeruleus neurons by electrophysiological recording in brain slices. 2. M6G and morphine activated potassium currents in voltage clamped neurons, which were blocked by the opioid receptor antagonist naloxone. 3. Both M6G and morphine behaved as partial agonists that produced maximal responses smaller than the system maximum, which was measured using [Met(5)]-enkephalin. M6G produced a larger maximal response (78%) than morphine (62%), which we estimated was due to a 2 - 4 fold difference in the relative efficacy of the agonists. 4. 3-O-methoxynaltrexone, which has been reported to behave as a selective antagonist of a M6G preferring receptor, was equally effective at blocking currents produced by M6G and the selective mu-opioid receptor agonist DAMGO. 5. M6G currents were occluded by a prior application of morphine, and were reduced when mu-opioid receptors were desensitized by using [Met(5)]-enkephalin. 6. Morphine-3 beta-D-glucuronide did not affect action potential firing or membrane currents in locus coeruleus neurons and had no effect on currents produced by M6G. 7. These results show that the relative efficacy of M6G is higher than morphine in locus coeruleus neurons, contrary to what has been shown using mu-opioid receptors expressed in cell clones.

Alpha 1-adrenergic receptor regulation: basic science and clinical implications

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor family, which includes alpha 1ARs, alpha 2ARs, beta 1ARs, beta 2ARs, beta 3ARs, adenosine, muscarinic, angiotensin, endothelin receptors, and many others that are responsible for a large variety of physiologic effects through G-protein coupling. This review focuses on alpha 1ARs and their regulation at both the mRNA and protein levels. Currently, three alpha 1AR subtypes have been characterized both pharmacologically and at the gene level: alpha 1aAR, alpha 1bAR, and alpha 1dAR. These are expressed in a species- and tissue-dependent manner. Mutagenesis approaches have been extremely valuable in the identification of key residues that govern alpha 1AR ligand binding and signaling. These studies reveal that alpha 1ARs have evolved an exquisitely sensitive regulation of their activity in which any disruption of the native structure has profound effects on subsequent function and effector coupling. Significant advances have also been made in the elucidation of signaling pathway components, resulting in the identification of novel pathways that can lead to pathologic conditions. Specific topics include mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and G-protein-coupled receptor cross-talk pathways. Within this context, recent studies identifying underlying transcriptional mechanisms involved in the regulation of the alpha 1AR subtypes are also discussed. Finally, given the potentially important role of alpha 1ARs in the vasculature, as well as in the pathology of many diseases, such as myocardial hypertrophy and benign prostatic hyperplasia, the clinical relevance of alpha 1AR distribution, pharmacology, and therapeutic intervention is reviewed.

Pharmacological examination of contractile responses of the guinea-pig isolated ileum produced by mu-opioid receptor antagonists in the presence of, and following exposure to, morphine

We have assessed the potential of several mu-opioid receptor antagonists to elicit a response in the guinea-pig isolated ileum in the presence of, and following overnight exposure to, morphine. Naloxone, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), (-)-5, 9alpha-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomorphan (MR2266), but not D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), produced a transient inhibition of electrically-evoked contractions of the guinea-pig ileum. The effect of 1 microM CTOP, but not that to MR2266, was inhibited by 1 microM somatostatin. Naloxone (0.3 microM), CTOP (3 microM), CTAP (3 microM) and MR2266 (0.3 microM) antagonized the inhibitory effect of morphine on electrically-evoked contractions of the guinea-pig to a similar degree and, following 60 min exposure to morphine, produced non-sustained contractions. The response to 3 microM CTOP was significantly smaller than that to 3 microM CTAP. None of the antagonists produced a response in the absence of morphine. Following overnight exposure of the ileum to 0.3 microM morphine (4 degrees C), and repeated washing to remove the agonist, all four antagonists elicited non-sustained contractions. However, the responses to 3 microM CTOP and 0.3 microM MR2266 were significantly smaller than those elicited by 0.3 microM naloxone and 3 microM CTAP. Somatostatin (1 microM) significantly reduced naloxone-induced contractions, but not those to CTAP. While all four mu-opioid antagonists elicited contractions in the presence of, and following prolonged exposure to, morphine, differences between them were noted which may be a consequence of non-opioid actions.

Differential antagonism of endomorphin-1 and endomorphin-2 spinal antinociception by naloxonazine and 3-methoxynaltrexone

To determine the role of spinal mu-opioid receptor subtypes in antinociception induced by intrathecal (i.t.) injection of endomorphin-1 and -2, we assessed the effects of beta-funaltrexamine (a selective mu-opioid receptor antagonist) naloxonazine (a selective antagonist at the mu(1)-opioid receptor) and a novel receptor antagonist (3-methoxynaltrexone) using the paw-withdrawal test. Antinociception of i.t. endomorphins and [D-Ala(2), MePhe(4), Gly(ol)(5)]enkephalin (DAMGO) was completely reversed by pretreatment with beta-funaltrexamine (40 mg/kg s.c.). Pretreatment with a variety of doses of i.t. or s.c. naloxonazine 24 h before testing antagonized the antinociception of endomorphin-1, -2 and DAMGO. Judging from the ID(50) values of naloxonazine, the antinociceptive effect of endomorphin-2 was more sensitive to naloxonazine than that of endomorphin-1 or DAMGO. The selective morphine-6beta-glucuronide antagonist, 3-methoxynaltrexone, which blocked endomorphin-2-induced antinociception at each dose (0.25 mg/kg s.c. or 2.5 ng i.t.) that was inactive against DAMGO, did not affect endomorphin-1-induced antinociception but shifted the dose-response curve of endomorphin-2 3-fold to the right. These findings may be interpreted as indicative of the existence of a novel mu-opioid receptor subtype in spinal sites, where antinociception of morphine-6beta-glucuronide and endomorphin-2 are antagonized by 3-methoxynaltrexone. The present results suggest that endomorphin-1 and endomorphin-2 may produce antinociception through different subtypes of mu-opioid receptor.

Nerve growth factor (NGF) loop 4 dimeric mimetics activate ERK and AKT and promote NGF-like neurotrophic effects

Previous work indicating that nerve growth factor (NGF) protein loops 2 and 4 interact with TrkA receptors raise the possibility that small molecule mimetics corresponding to TrkA-interacting domains that have NGF agonist activity can be developed. We applied our previously developed strategy of dimeric peptidomimetics to address the hypothesis that loop 4 small molecule dimeric mimetics would activate TrkA-related signal transduction and mimic NGF neurotrophic effects in a structure-specific manner. A loop 4 cyclized peptide dimer demonstrated NGF-like neurotrophic activity, whereas peptides with scrambled sequence, added or substituted residues, or cyclized in monomeric form were inactive. Activity was blocked by the TrkA inhibitors K252a and AG879 but not by NGF p75 receptor blocking antibody. Dimeric, but not monomeric, peptides partially blocked NGF activity. This profile was consistent with that of a NGF partial agonist. ERK and AKT phosphorylation was stimulated only by biologically active peptides and was blocked by K252a. The ERK inhibitor U0126 blocked the neurite- but not the survival-promoting activity of both NGF and active peptide. These studies support the proof of concept that small molecule NGF loop 4 mimetics can activate NGF signaling pathways and can mimic death-preventing and neurite-promoting effects of NGF. This finding will guide the rational design of NGF single-domain mimetics and contribute to elucidating NGF signal transduction mechanisms.

CCK-B/Gastrin receptor transmembrane domain mutations selectively alter synthetic agonist efficacy without affecting the activity of endogenous peptides

Recent efforts have focused on identifying small nonpeptide molecules that can mimic the activity of endogenous peptide hormones. Understanding the molecular basis of ligand-induced receptor activation by these divergent classes of ligands should expedite the process of drug development. Using the cholecystokinin-B/gastrin receptor (CCK-BR) as a model system, we have recently shown that both affinity and efficacy of nonpeptide ligands are markedly affected by amino acid alterations within a putative transmembrane domain (TMD) ligand pocket. In this report, we examine whether residues projecting into the TMD pocket determine the pharmacologic properties of structurally diverse CCK-BR ligands, including peptides and synthetic peptide-derived partial agonists (peptoids). Nineteen mutant human CCK-BRs, each including a single TMD amino acid substitution, were transiently expressed in COS-7 cells and characterized. Binding affinities as well as ligand-induced inositol phosphate production at the mutant CCK-BRs were assessed for peptides (CCK-8 and CCK-4) and for peptoids (PD-135,158 and PD-136, 450). Distinct as well as overlapping determinants of peptide and peptoid binding affinity were identified, supporting that both classes of ligands, at least in part, interact with the CCK-BR TMD ligand pocket. Eight point mutations resulted in marked increases or decreases in the functional activity of the synthetic peptoid ligands. In contrast, the functional activity of both peptides, CCK-8 and CCK-4, was not affected by any of the CCK-BR mutations. These findings suggest that the mechanisms underlying activation of G-protein-coupled receptors by endogenous peptide hormones versus synthetic ligands may markedly differ.

Non-prostanoid prostacyclin mimetics as neuronal stimulants in the rat: comparison of vagus nerve and NANC innervation of the colon

The spontaneous activity of the rat isolated colon is suppressed by prostacyclin analogues such as cicaprost (IC(50)=4.0 nM). Activation of prostanoid IP(1)-receptors located on NANC inhibitory neurones is involved. However, several non-prostanoids, which show medium to high IP(1) agonist potency on platelet and vascular preparations, exhibit very weak inhibitory activity on the colon. The aim of the study was to investigate this discrepancy. Firstly, we have demonstrated the very high depolarizing potency of cicaprost on the rat isolated vagus nerve (EC(50)=0.23 nM). Iloprost, taprostene and carbacyclin were 7.9, 66, and 81 fold less potent than cicaprost, indicating the presence of IP(1) as opposed to IP(2)-receptors. Three non-prostanoid prostacyclin mimetics, BMY 45778, BMY 42393 and ONO-1301, although much less potent than cicaprost (195, 990 and 1660 fold respectively), behaved as full agonists on the vagus nerve. On re-investigating the rat colon, we found that BMY 45778 (0.1 - 3 microM), BMY 42393 (3 microM) and ONO-1301 (3 microM) behaved as specific IP(1) partial agonists, but their actions required 30 - 60 min to reach steady-state and only slowly reversed on washing. This profile contrasted sharply with the rapid and readily reversible contractions elicited by a related non-prostanoid ONO-AP-324, which is an EP(3)-receptor agonist. The full versus partial agonism of the non-prostanoid prostacyclin mimetics may be explained by the markedly different IP(1) agonist sensitivities of the two rat neuronal preparations. However, the slow kinetics of the non-prostanoids on the NANC system of the colon remain unexplained, and must be taken into account when characterizing neuronal IP-receptors.