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C Scaramellini

Publications and source records attributed to C Scaramellini.

5 recordsLinked to original sources
PubMed1998

A three-state receptor model: predictions of multiple agonist pharmacology for the same receptor type.

Recent studies have demonstrated that activation of the same G-protein coupled receptor can generate different agonist pharmacology depending on the signaling pathway(s) to which it couples. Two types of behavior have been exemplified; differences in affinity order, and differences in efficacy order with the same affinity order. The two-state model of receptor activation cannot explain these data, since a single active receptor state cannot couple differently to the two response pathways for different ligands. We have therefore extended the two-state model to a three-state model in which receptors exist in three states: an inactive state, R, and two different active states, R* and R**. The model has two modes, the 'intact mode', in which all the equilibria are linked; and the 'isolated mode' in which the two response pathways are isolated from each other, giving effectively two separate two-state systems. In the 'intact mode' the same agonist affinity order is predicted for both response pathways, but a different efficacy order. In the 'isolated mode', since the equilibria are no longer linked, the model predicts that a different affinity order may be obtained for the two pathways. Owing to the linkage of all the equilibria in the intact three-state model the level of constitutive activity through one pathway can affect the direction of agonism through the other pathway, resulting in the conversion of an inverse agonist into a positive agonist. This change in the direction of agonism is also predicted to occur when the two response pathways are isolated. The three-state model therefore predicts that agonists, acting at the same receptor, may show different affinity orders and different efficacy orders depending upon which response is measured and the assay system used, and also predicts that inverse agonism may be system dependent.

Animals↗
PubMed1998

Effector pathway-dependent relative efficacy at serotonin type 2A and 2C receptors: evidence for agonist-directed trafficking of receptor stimulus.

There are many examples of a single receptor coupling directly to more than one cellular signal transduction pathway. Although traditional receptor theory allows for activation of multiple cellular effectors by agonists, it predicts that the relative degree of activation of each effector pathway by an agonist (relative efficacy) must be the same. In the current experiments, we demonstrate that agonists at the human serotonin2A (5-HT2A) and 5-HT2C receptors activate differentially two signal transduction pathways independently coupled to the receptors [phospholipase C (PLC)-mediated inositol phosphate (IP) accumulation and phospholipase A2 (PLA2)-mediated arachidonic acid (AA) release]. The relative efficacies of agonists differed depending on which signal transduction pathway was measured. Moreover, relative to 5-HT, some 5-HT2C agonists (e.g., 3-trifluoromethylphenyl-piperazine) preferentially activated the PLC-IP pathway, whereas others (e.g., lysergic acid diethylamide) favored the PLA2-AA pathway. In contrast, when two dependent responses were measured (IP accumulation and calcium mobilization), agonist relative efficacies were not different. These data strongly support the hypothesis termed "agonist-directed trafficking of receptor stimulus" recently proposed by Kenakin [Trends Pharmacol Sci 16:232-238 (1995)]. Concentration-response curves to 5-HT2C agonists were fit well by a three-state model of receptor activation, suggesting that two active receptor states may be sufficient to explain pathway-dependent agonist efficacy. Rational drug design that optimizes preferential effector activity within a group of receptor-selective drugs holds the promise of increased selectivity in clinically useful agents.

Animals↗
PubMed1997

A three-state receptor model of agonist action.

The concept that receptors can exist in multiple conformational states is becoming a physical reality. A fundamental question is how many active states need to be proposed in order to account for pharmacological observations, in particular, the finding that the same receptor type can exhibit a different agonist pharmacology when coupled to different effector pathways. In this article, Paul Leff, Clare Scaramellini, Clare Law and Ken McKechnie propose and develop a three-state receptor model in which two active conformations are assumed to exist. They show that this relatively simple theoretical model provides a basis for predicting variable agonist and inverse agonist behaviour in different systems containing the same receptor, and that it is able to account for emerging data obtained on promiscuously coupled receptors. It is argued that, while these new theoretical considerations challenge the fundamental assumptions and concepts of traditional receptor theory, the general principles of pharmacological receptor classification are largely preserved.

Animals↗
PubMed1997

Analysis of agonist-agonist interactions: the crucial influence of curve shape.

The two-receptor:one-transducer model (Leff, 1987) is here extended to analyze interactions between agonists displaying E[A] curves of different shapes, by incorporating slope factors into the separate and common parts of the transduction pathway. Interactions were modelled as the effect of one agonist, at fixed concentration, on the curve to the other. A variety of patterns of position and slope changes are predicted. These do not depend on the shape of the control curve, rather, they depend on the slope factors in the separate and common pathways. The following specific predictions are made: (1) when the common pathway is steep, curves undergo potentiation and flattening; (2) when the common pathway is flat, curves undergo right-shift and steepening; (3) when the common pathway is hyperbolic, curves undergo right-shift, with no slope change; (4) when the slope depends on the separate pathways, curves only undergo right-shift with no change in slope. The model provides a sound basis for classifying agonist interactions and for detecting additional, synergistic or antagonistic properties. This analysis indicates that methods based on dose-additivity or independence are less reliable for these purposes. The model provides a practical test, based on slope changes, to detect and quantify additional properties.

Dose-Response Relationship, Drug↗