beta-Funaltrexamine, a gauge for the recognition site of wildtype and mutant H297Q mu-opioid receptors

TRV130 partial agonism and capacity to induce anti-nociceptive tolerance revealed through reducing available μ-opioid receptor number

BACKGROUND AND PURPOSE

β-Arrestin2 recruitment to μ-receptors may contribute to the development of opioid side effects. This possibility led to the development of TRV130 and PZM21, opioids reportedly biased against β-arrestin2 recruitment in favour of G-protein signalling. However, low efficacy β-arrestin2 recruitment by TRV130 and PZM21 may simply reflect partial agonism overlooked due to overexpression of μ-receptors.

EXPERIMENTAL APPROACH

Efficacies and apparent potencies of DAMGO, morphine, PZM21 and TRV130 as stimulators of β-arrestin2 recruitment and inhibitors of cAMP accumulation were assessed in CHO cells stably expressing μ-receptors. Receptor availability was depleted through prior exposure of cells to the irreversible antagonist, β-FNA. We also examined whether μ-receptor availability influences TRV130 anti-nociception and/or tolerance using the tail withdrawal assay in wild-type C57BL/6 and μ+/- mice.

KEY RESULTS

Morphine, PZM21 and TRV130 were partial agonists in the β-arrestin2 recruitment assay. Only TRV130 exhibited partial agonism in the cAMP assay. Exposure to β-FNA to reduce μ-receptor availability further limited the efficacy of TRV130 and revealed morphine and PZM21 to be partial agonists. Despite having partial efficacy in vitro, TRV130 caused potent anti-nociception (ED50 : 0.33 mg·kg-1 ) in wild-type mice, without tolerance after daily administration for 10 days. TRV130 caused similar anti-nociception in μ+/- mice, with marked tolerance on day 4 of injections.

CONCLUSION AND IMPLICATIONS

Our findings emphasise the importance of receptor reserve when characterising μ-receptor agonists. Reduced receptor availability reveals that TRV130 is a partial agonist capable of tolerance, despite having limited efficacy for β-arrestin2 recruitment to the μ-receptor.

Homology modeling of opioid receptor-ligand complexes using experimental constraints

Opioid receptors interact with a variety of ligands, including endogenous peptides, opiates, and thousands of synthetic compounds with different structural scaffolds. In the absence of experimental structures of opioid receptors, theoretical modeling remains an important tool for structure-function analysis. The combination of experimental studies and modeling approaches allows development of realistic models of ligand-receptor complexes helpful for elucidation of the molecular determinants of ligand affinity and selectivity and for understanding mechanisms of functional agonism or antagonism. In this review we provide a brief critical assessment of the status of such theoretical modeling and describe some common problems and their possible solutions. Currently, there are no reliable theoretical methods to generate the models in a completely automatic fashion. Models of higher accuracy can be produced if homology modeling, based on the rhodopsin X-ray template, is supplemented by experimental structural constraints appropriate for the active or inactive receptor conformations, together with receptor-specific and ligand-specific interactions. The experimental constraints can be derived from mutagenesis and cross-linking studies, correlative replacements of ligand and receptor groups, and incorporation of metal binding sites between residues of receptors or receptors and ligands. This review focuses on the analysis of similarity and differences of the refined homology models of mu, delta, and kappa-opioid receptors in active and inactive states, emphasizing the molecular details of interaction of the receptors with some representative peptide and nonpeptide ligands, underlying the multiple modes of binding of small opiates, and the differences in binding modes of agonists and antagonists, and of peptides and alkaloids.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Relieving effects of electroacupuncture on mechanical allodynia in neuropathic pain model of inferior caudal trunk injury in rat: mediation by spinal opioid receptors

The relieving effects of electroacupuncture (EA) on mechanical allodynia and its mechanism related to the spinal opioid system were investigated in a rat model of neuropathic pain. To produce neuropathic pain in the tail, the right superior caudal trunk was resected between the S1 and S2 spinal nerves. Two weeks after the surgery, EA stimulation (2 or 100 Hz, 0.3 ms, 0.2-0.3 mA) was delivered to Zusanli (ST36) for 30 min. The degree of mechanical allodynia was evaluated quantitatively by touching the tail with von Frey hair (2.0 g) at 10 min intervals. These rats were then subjected to an i.t. injection with one of the three specific opioid agonists in successive ways: the mu agonist (DAMGO 25, 50 and 100 pmol), the delta agonist (DADELT II 0.5, 1 and 2 nmol), and the kappa agonist (U50488H 5, 10 and 20 nmol) separated by 10 min in cumulative doses. During 30 min of EA stimulation, specific opioid antagonists were subjected to i.t. injection: the mu antagonist (beta-FNA 5, 10 and 20 nmol), the delta antagonist (naltrindole 5, 10 and 20 nmol), and the kappa antagonist (nor-BNI 3, 6 and 12 nmol) separated by 10 min in cumulative doses. As a result, EA reduced the behavioral signs of mechanical allodynia. Two Hz EA induced a robust and longer lasting effect than 100 Hz. All three opioid agonists also showed relieving effects on mechanical allodynia. However, nor-BNI could not block the EA effects on mechanical allodynia, whereas beta-FNA or naltrindole significantly blocked EA effects. These results suggest that the mu and delta, but not kappa, opioid receptors in the spinal cord of the rat, play important roles in mediating relieving effects on mechanical allodynia induced by 2 Hz EA.

Kinetics of ?-funaltrexamine binding to wild-type and mutant ?-opioid receptors expressed in Chinese hamster ovary cells

The two-stage reaction whereby the antagonist beta-funaltrexamine (beta-FNA) binds covalently to micro opioid receptors makes it a highly discriminating probe into the tertiary structure of the receptor's recognition pocket. To obtain a quantitative measure of how well this pocket is preserved in a mutated form of the receptor, in which His-297 is substituted with glutamine, we employed [3H]-beta-FNA to evaluate the kinetic rate constants for both the reversible as well as the irreversible stages of its binding to wild-type and mutant H297Q micro receptors stably expressed in Chinese hamster ovary cells. The expression levels of the wild-type and mutant H297Q receptors were matched by exploiting the variation in receptor density as a function of plating day and by raising the expression level by pretreatment with naloxone. We found that all of the kinetic rate constants for [3H]-beta-FNA were diminished by about one-half at the mutant H297Q micro receptors with respect to wild-type receptors. By comparison, the association rate constant of [3H]-naloxone likewise decreased by one-half; however, the dissociation rate constant increased 5-fold at the mutant H297Q receptor. We conclude that the mutation has had only minor influence on the recognition site and that the function of position 297 is more likely as a link in the transduction chain.