Change in functional selectivity of morphine with the development of antinociceptive tolerance

BACKGROUND AND PURPOSE

Opioids, such as morphine, are the most effective treatment for pain but their efficacy is diminished with the development of tolerance following repeated administration. Recently, we found that morphine activated ERK in opioid-tolerant but not in naïve rats, suggesting that morphine activation of μ-opioid receptors is altered following repeated morphine administration. Here, we have tested the hypothesis that μ-opioid receptor activation of ERK in the ventrolateral periaqueductal gray (vlPAG) is dependent on dynamin, a protein implicated in receptor endocytosis.

EXPERIMENTAL APPROACH

Rats were made tolerant to repeated microinjections of morphine into the vlPAG. The effects of dynamin on ERK activation and antinociception were assessed by microinjecting myristoylated dominant-negative dynamin peptide (Dyn-DN) or a scrambled control peptide into the vlPAG. Microinjection of a fluorescent dermorphin analogue (DERM-A594) into the vlPAG was used to monitor μ-opioid receptor internalization.

KEY RESULTS

Morphine did not activate ERK and Dyn-DN administration had no effect on morphine-induced antinociception in saline-pretreated rats. In contrast, morphine-induced ERK activation in morphine-pretreated rats that was blocked by Dyn-DN administration. Dyn-DN also inhibited morphine antinociception. Finally, morphine reduced DERM-A594 internalization only in morphine-tolerant rats indicating that μ-opioid receptors were internalized and unavailable to bind DERM-A594.

CONCLUSIONS AND IMPLICATIONS

Repeated morphine administration increased μ-opioid receptor activation of ERK signalling via a dynamin-dependent mechanism. These results demonstrate that the balance of agonist signalling to G-protein and dynamin-dependent pathways is altered, effectively changing the functional selectivity of the agonist-receptor complex.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Conditioned place preference and locomotor activity in response to methylphenidate, amphetamine and cocaine in mice lacking dopamine D4 receptors

Methylphenidate (MP) and amphetamine (AMPH) are the most frequently prescribed medications for the treatment of attention-deficit/hyperactivity disorder (ADHD). Both drugs are believed to derive their therapeutic benefit by virtue of their dopamine (DA)-enhancing effects, yet an explanation for the observation that some patients with ADHD respond well to one medication but not to the other remains elusive. The dopaminergic effects of MP and AMPH are also thought to underlie their reinforcing properties and ultimately their abuse. Polymorphisms in the human gene that codes for the DA D4 receptor (D4R) have been repeatedly associated with ADHD and may correlate with the therapeutic as well as the reinforcing effects of responses to these psychostimulant medications. Conditioned place preference (CPP) for MP, AMPH and cocaine were evaluated in wild-type (WT) mice and their genetically engineered littermates, congenic on the C57Bl/6J background, that completely lack D4Rs (knockout or KO). In addition, the locomotor activity in these mice during the conditioning phase of CPP was tested in the CPP chambers. D4 receptor KO and WT mice showed CPP and increased locomotor activity in response to each of the three psychostimulants tested. D4R differentially modulates the CPP responses to MP, AMPH and cocaine. While the D4R genotype affected CPP responses to MP (high dose only) and AMPH (low dose only) it had no effects on cocaine. Inasmuch as CPP is considered an indicator of sensitivity to reinforcing responses to drugs these data suggest a significant but limited role of D4Rs in modulating conditioning responses to MP and AMPH. In the locomotor test, D4 receptor KO mice displayed attenuated increases in AMPH-induced locomotor activity whereas responses to cocaine and MP did not differ. These results suggest distinct mechanisms for D4 receptor modulation of the reinforcing (perhaps via attenuating dopaminergic signalling) and locomotor properties of these stimulant drugs. Thus, individuals with D4 receptor polymorphisms might show enhanced reinforcing responses to MP and AMPH and attenuated locomotor response to AMPH.

Amphetamine, 3,4-methylenedioxymethamphetamine, lysergic acid diethylamide, and metabolites of the catecholamine neurotransmitters are agonists of a rat trace amine receptor

The trace amine para-tyramine is structurally and functionally related to the amphetamines and the biogenic amine neurotransmitters. It is currently thought that the biological activities elicited by trace amines such as p-tyramine and the psychostimulant amphetamines are manifestations of their ability to inhibit the clearance of extracellular transmitter and/or stimulate the efflux of transmitter from intracellular stores. Here we report the discovery and pharmacological characterization of a rat G protein-coupled receptor that stimulates the production of cAMP when exposed to the trace amines p-tyramine, beta-phenethylamine, tryptamine, and octopamine. An extensive pharmacological survey revealed that psychostimulant and hallucinogenic amphetamines, numerous ergoline derivatives, adrenergic ligands, and 3-methylated metabolites of the catecholamine neurotransmitters are also good agonists at the rat trace amine receptor 1 (rTAR1). These results suggest that the trace amines and catecholamine metabolites may serve as the endogenous ligands of a novel intercellular signaling system found widely throughout the vertebrate brain and periphery. Furthermore, the discovery that amphetamines, including 3,4-methylenedioxymethamphetamine (MDMA; "ecstasy"), are potent rTAR1 agonists suggests that the effects of these widely used drugs may be mediated in part by this receptor as well as their previously characterized targets, the neurotransmitter transporter proteins.

Evaluation of laboratory testing methods for Chlamydia trachomatis infection in the era of nucleic acid amplification

Diagnostic tests presently available for Chlamydia trachomatis have widely varying performance characteristics. To assess evolving laboratory testing practices since the introduction of nucleic acid amplification tests (NAAT), we surveyed laboratories in Washington State about their testing practices in 1998 and compared our findings to a similar survey conducted in 1995. Laboratory directors of 61 (87%) of 70 laboratories performing chlamydial tests in 1998 returned a survey. Between 1995 and 1998, 36 laboratories discontinued chlamydial testing, and the total number of laboratories performing tests in the state decreased from 92 to 70, a 24% decline. Of the 36 laboratories that discontinued testing, 25 (69%) had previously used rapid tests. While no laboratory routinely used NAAT in 1995, ligase chain reaction (LCR) was used in 23% of laboratories in 1998 and accounted for 113,624 (36%) of the 318,133 tests performed that year. Among the remaining 204,509 tests performed in 1998, other tests employed included DNA probe (29%), enzyme immunoassay (20%), culture (12%), direct fluorescent antibody assays (3%), and rapid tests (<1%). The majority (65%) of tests performed in 1998 using technologies other than LCR or culture were done in laboratories that did more than 10,000 tests. Cost and loss of revenue to laboratories were the most frequently cited reasons for not adopting NAAT. We conclude that in Washington State, NAAT have been rapidly adopted in larger laboratories, but most patients are still tested with much less sensitive technologies. Financial constraints represent the major barrier to more widespread use of DNA amplification tests.

Osteoprotegerin is an alpha vbeta 3-induced, NF-kappa B-dependent survival factor for endothelial cells

Osteopontin protects endothelial cells from apoptosis induced by growth factor withdrawal. This interaction is mediated by the alpha(v)beta(3) integrin and is NF-kappaB-dependent (Scatena, M., Almeida, M., Chaisson, M. L., Fausto, N., Nicosia, R. F., and Giachelli, C. M. (1998) J. Cell Biol. 141, 1083-1093). In the present study we used differential cloning to identify osteopontin-induced, NF-kappaB-dependent genes in endothelial cells. One of the genes identified in this screen was osteoprotegerin, a member of the tumor necrosis factor receptor superfamily. By Northern and Western blot analysis, osteoprotegerin mRNA and protein levels were very low in endothelial cells plated on the non-integrin cell attachment factor, poly-d-lysine. In contrast, osteoprotegerin mRNA and protein levels were induced 5-7-fold following alpha(v)beta(3) ligation by osteopontin. Osteoprotegerin induction by osteopontin was time-dependent and observed as early as 3 h following treatment. NF-kappaB inactivation achieved by over expression of an IkappaB super repressor in endothelial cells completely inhibited osteoprotegerin induction by osteopontin. Finally, purified osteoprotegerin protected endothelial cells with inactive NF-kappaB from apoptosis induced by growth factor deprivation. These data suggest that alpha(v)beta(3)-mediated endothelial survival depends on osteoprotegerin induction by NF-kappaB and indicate a new function for osteoprotegerin in endothelial cells.

Laboratory methods for detection of Chlamydia trachomatis: survey of laboratories in Washington State

The last decade has witnessed the development of a wide variety of diagnostic tests for Chlamydia trachomatis. In order to determine what laboratory methods are being used to detect C. trachomatis infections in Washington State and to identify factors influencing test selection, between April 1995 and October 1995 we conducted a mailed questionnaire survey of all 112 laboratories certified to do chlamydia testing. Of these, 20 had discontinued testing for C. trachomatis, and responses were obtained from 89 (97%) of the remaining 92 laboratories. Surprisingly, 38 (43%) of the 89 laboratories used rapid tests such as Clearview and Surecell, making such tests the most commonly used laboratory tests. Laboratories which used rapid tests had lower test volumes, less experience performing tests for C. trachomatis, less frequent attendance at professional meetings, and greater reliance on manufacturers for information compared with laboratories which used other methods. Confirmation of non-culture-positive results was provided by 28 (34%) of the 82 laboratories doing non-culture-based tests. Forty-one (47%) of 88 laboratories reported having compared their method with another method. Test volume was the strongest predictor of laboratories which confirmed positive non-culture-based test results and which had performed a laboratory comparison of methods. We conclude that rapid tests for C. trachomatis are often being used inappropriately and that efforts are needed to improve effective implementation and quality assurance of laboratory testing for C. trachomatis.