Opioid receptor internalization contributes to dermorphin-mediated antinociception

Fluorescent opioid receptor ligands as tools to study opioid receptor function

Opioid receptors are divided into the three classical types: MOP(μ:mu), DOP(δ:delta) and KOP(κ:kappa) that are naloxone-sensitive and an additional naloxone-insensitive nociceptin/orphanin FQ(N/OFQ) peptide receptor(NOP). Studies to determine opioid receptor location and turnover variably rely on; (i) measuring receptor mRNA, (ii) genetically tagging receptors, (iii) labelling receptors with radioligands, (iv) use of antibodies in immunohistochemistry/Western Blotting or (v) measuring receptor function coupled with the use of selective antagonists. All have their drawbacks with significant issues relating to mRNA not necessarily predicting protein, poor antibody selectivity and utility of radiolabels in low expression systems. In this minireview we discuss use of fluorescently labelled opioid receptor ligands. To maintain the pharmacological properties of the corresponding parent ligand fluorescently labelled ligands must take into account fluorophore (brightness and propensity to bleach), linker length and chemistry, and site to which the linker (and hence probe) will be attached. Use of donor and acceptor fluorophores with spectral overlap facilitates use in FRET type assays to determine proximity of ligand or tagged receptor pairs. There is a wide range of probes of agonist and antagonist nature for all four opioid receptor types; caution is needed with agonist probes due to the possibility for internalization. We have produced two novel ATTO based probes; Dermorphin_ATTO488 (MOP) and N/OFQ_ATTO594 (NOP). These probes label MOP and NOP in a range of preparations and using N/OFQ_ATTO594 we demonstrate internalization and ligand-receptor interaction by FRET. Fluorescent opioid probes offer potential methodological advantages over more traditional use of antibodies and radiolabels.

Neurons derived from human-induced pluripotent stem cells express mu and kappa opioid receptors

Neuroprotection studies have shown that induced pluripotent stem (iPS) cells have the possibility to transform neuroprotection research. In the present study, iPS cells were generated from human renal epithelial cells and were then differentiated into neurons. Cells in the iPS-cell group were maintained in stem cell medium. In contrast, cells in the iPS-neuron group were first maintained in neural induction medium and expansion medium containing ROCK inhibitors, and then cultivated in neuronal differentiation medium and neuronal maturation medium to induce the neural stem cells to differentiate into neurons. The expression of relevant markers was compared at different stages of differentiation. Immunofluorescence staining revealed that cells in the iPS-neuron group expressed the neural stem cell markers SOX1 and nestin on day 11 of induction, and neuronal markers TUBB3 and NeuN on day 21 of induction. Polymerase chain reaction results demonstrated that, compared with the iPS-cell group, TUBB3 gene expression in the iPS-neuron group was increased 15.6-fold. Further research revealed that, compared with the iPS-cell group, the gene expression and immunoreactivity of mu opioid receptor in the iPS-neuron group were significantly increased (38.3-fold and 5.7-fold, respectively), but those of kappa opioid receptor had only a slight change (1.33-fold and 1.57-fold increases, respectively). Together, these data indicate that human iPS cells can be induced into mu opioid receptor- and kappa opioid receptor-expressing neurons, and that they may be useful to simulate human opioid receptor function in vitro and explore the underlying mechanisms of human conditions.

Regular physical activity reduces the percentage of spinally projecting neurons that express mu-opioid receptors from the rostral ventromedial medulla in mice

Introduction

Regular physical activity/exercise is an effective nonpharmacological treatment for individuals with chronic pain. Central inhibitory mechanisms, involving serotonin and opioids, are critical to analgesia produced by regular physical activity. The rostral ventromedial medulla (RVM) sends projections to the spinal cord to inhibit or facilitate nociceptive neurons and plays a key role in exercise-induced analgesia.

Objective

The goal of these studies was to examine if regular physical activity modifies RVM-spinal cord circuitry.

Methods

Male and female mice received Fluoro-Gold placed on the spinal cord to identify spinally projecting neurons from the RVM and the nucleus raphe obscurus/nucleus raphe pallidus, dermorphin-488 into caudal medulla to identify mu-opioid receptors, and were immunohistochemically stained for either phosphorylated-N-methyl-d-aspartate subunit NR1 (p-NR1) to identify excitatory neurons or tryptophan hydroxylase (TPH) to identify serotonin neurons. The percentage of dermorphin-488-positive cells that stained for p-NR1 (or TPH), and the percentage of dermorphin-488-positive cells that stained for p-NR1 (or TPH) and Fluoro-Gold was calculated. Physically active animals were provided running wheels in their cages for 8 weeks and compared to sedentary animals without running wheels. Animals with chronic muscle pain, induced by 2 intramuscular injections of pH 4.0, were compared to sham controls (pH 7.2).

Results

Physically active animals had less mu-opioid-expressing neurons projecting to the spinal cord when compared to sedentary animals in the RVM, but not the nucleus raphe obscurus/nucleus raphe pallidus. No changes were observed for TPH.

Conclusions

These data suggest that regular exercise alters central facilitation so that there is less descending facilitation to result in a net increase in inhibition.

Sex Differences in GABAA Signaling in the Periaqueductal Gray Induced by Persistent Inflammation

The ventrolateral periaqueductal gray (vlPAG) is a key structure in the descending pain modulatory circuit. Activation of the circuit occurs via disinhibition of GABAergic inputs onto vlPAG output neurons. In these studies, we tested the hypothesis that GABAergic inhibition is increased during persistent inflammation, dampening activation of the descending circuit from the vlPAG. Our results indicate that persistent inflammation induced by Complete Freund's adjuvant (CFA) modulates GABA signaling differently in male and female rats. CFA treatment results in increased presynaptic GABA release but decreased high-affinity tonic GABAA currents in female vlPAG neurons. These effects are not observed in males. The tonic currents in the vlPAG are dependent on GABA transporter activity and are modulated by agonists that activate GABAA receptors containing the δ subunit. The GABAA δ agonist THIP (gaboxadol) induced similar amplitude currents in naive and CFA-treated rats. In addition, a positive allosteric modulator of the GABAA δ subunit, DS2 (4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl]benzamide), increased tonic currents. These results indicate that GABAA δ receptors remain on the cell surface but are less active in CFA-treated female rats. In vivo behavior studies showed that morphine induced greater antinociception in CFA-treated females that was reversed with microinjections of DS2 directly into the vlPAG. DS2 did not affect morphine antinociception in naive or CFA-treated male rats. Together, these data indicate that sex-specific adaptations in GABAA receptor signaling modulate opioid analgesia in persistent inflammation. Antagonists of GABAA δ receptors may be a viable strategy for reducing pain associated with persistent inflammation, particularly in females.

SIGNIFICANCE STATEMENT

These studies demonstrate that GABA signaling is modulated in the ventrolateral periaqueductal gray by persistent inflammation differently in female and male rats. Our results indicate that antagonists or negative allosteric modulators of GABAA δ receptors may be an effective strategy to alleviate chronic inflammatory pain and promote opioid antinociception, especially in females.

Ligand-biased activation of extracellular signal-regulated kinase 1/2 leads to differences in opioid induced antinociception and tolerance

Opioids produce antinociception by activation of G protein signaling linked to the mu-opioid receptor (MOPr). However, opioid binding to the MOPr also activates β-arrestin signaling. Opioids such as DAMGO and fentanyl differ in their relative efficacy for activation of these signaling cascades, but the behavioral consequences of this differential signaling are not known. The purpose of this study was to evaluate the behavioral significance of G protein and internalization dependent signaling within ventrolateral periaqueductal gray (vlPAG). Antinociception induced by microinjecting DAMGO into the vlPAG was attenuated by blocking Gαi/o protein signaling with administration of pertussis toxin (PTX), preventing internalization with administration of dynamin dominant-negative inhibitory peptide (dyn-DN) or direct inhibition of ERK1/2 with administration of the MEK inhibitor, U0126. In contrast, the antinociceptive effect of microinjecting fentanyl into the vlPAG was not altered by administration of PTX or U0126, and was enhanced by administration of dyn-DN. Microinjection of DAMGO, but not fentanyl, into the vlPAG induced phosphorylation of ERK1/2, which was blocked by inhibiting receptor internalization with administration of dyn-DN, but not by inhibition of Gαi/o proteins. ERK1/2 inhibition also prevented the development and expression of tolerance to repeated DAMGO microinjections, but had no effect on fentanyl tolerance. These data reveal that ERK1/2 activation following MOPr internalization contributes to the antinociceptive effect of some (e.g., DAMGO), but not all opioids (e.g., fentanyl) despite the known similarities for these agonists to induce β-arrestin recruitment and internalization.

GABAergic transmission and enhanced modulation by opioids and endocannabinoids in adult rat rostral ventromedial medulla

KEY POINTS

Electrical stimulation of the rostral ventromedial medulla (RVM) facilitates pain behaviours in neonates but inhibits these behaviours in adults. The cellular mechanisms underlying these changes in RVM modulation of pain behaviours are not known. We optimized whole-cell patch-clamp recordings for RVM neurons in animals older than postnatal day 30 and compared the results to postnatal day 10-21 animals. Our results demonstrate that the γ-aminobutyric acid (GABA) release is lower and opioid effects are more evident in adult rats compared to early postnatal rats. A cannabinoid receptor antagonist significantly increased GABA release in mature but not in immature RVM neurons suggesting the presence of local endocannabinoid tone in mature RVM. Neurons in the rostral ventromedial medulla (RVM) play critical and complex roles in pain modulation. Recent studies have shown that electrical stimulation of the RVM produces pain facilitation in young animals (postnatal (PN) day < 21) but predominantly inhibits pain behaviours in adults. The cellular mechanisms underlying these changes in RVM modulation of pain behaviours are not known. This is in part because whole-cell patch-clamp studies in RVM to date have been in young (PN day < 18) animals because the organization and abundance of myelinated fibres in this region make the RVM a challenging area for whole-cell patch-clamp recording in adults. Several neurotransmitter systems, including GABAergic neurotransmission, undergo developmental changes that mature by PN day 21. Thus, we focused on optimizing whole-cell patch-clamp recordings for RVM neurons in animals older than PN day 30 and compared the results to animals at PN day 10-21. Our results demonstrate that the probability of GABA release is lower and that opioid and endocannabinoid effects are more evident in adult rats (mature) compared to early postnatal (immature) rats. Differences in these properties of RVM neurons may contribute to the developmental changes in descending control of pain from the RVM to the spinal cord.

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.

Cannabinoid 2 receptor- and beta Arrestin 2-dependent upregulation of serotonin 2A receptors

Recent evidence suggests that cannabinoid receptor agonists may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission in the brain, although no molecular mechanism has been identified. Here, we present experimental evidence that sustained treatment with a non-selective cannabinoid agonist (CP55,940) or selective CB2 receptor agonists (JWH133 or GP1a) upregulate 5-HT(2A) receptors in a neuronal cell line. Furthermore, this cannabinoid receptor agonist-induced upregulation of 5-HT(2A) receptors was prevented in cells stably transfected with either CB2 or β-Arrestin 2 shRNA lentiviral particles. Additionally, inhibition of clathrin-mediated endocytosis also prevented the cannabinoid receptor-induced upregulation of 5-HT(2A) receptors. Our results indicate that cannabinoid agonists might upregulate 5-HT(2A) receptors by a mechanism that requires CB2 receptors and β-Arrestin 2 in cells that express both CB2 and 5-HT(2A) receptors. 5-HT(2A) receptors have been associated with several physiological functions and neuropsychiatric disorders such as stress response, anxiety and depression, and schizophrenia. Therefore, these results might provide a molecular mechanism by which activation of cannabinoid receptors might be relevant to some cognitive and mood disorders in humans.

Cannabinoid agonists increase the interaction between β-Arrestin 2 and ERK1/2 and upregulate β-Arrestin 2 and 5-HT(2A) receptors

We have recently reported that selective cannabinoid 2 (CB(2)) receptor agonists upregulate 5-HT(2A) receptors by enhancing ERK1/2 signaling in prefrontal cortex (PFCx). Increased activity of cortical 5-HT(2A) receptors has been associated with several neuropsychiatric disorders such as anxiety and schizophrenia. Here we examine the mechanisms involved in this enhanced ERK1/2 activation in rat PFCx and in a neuronal cell model. Sprague-Dawley rats treated with a non-selective cannabinoid agonist (CP55940, 50μg/kg, 7 days, i.p.) showed enhanced co-immunoprecipitation of β-Arrestin 2 and ERK1/2, enhanced pERK protein levels, and enhanced expression of β-Arrestin 2 mRNA and protein levels in PFCx. In a neuronal cell line, we found that selective CB(2) receptor agonists upregulate β-Arrestin 2, an effect that was prevented by selective CB(2) receptor antagonist JTE-907 and CB(2) shRNA lentiviral particles. Additionally, inhibition of clathrin-mediated endocytosis, ERK1/2, and the AP-1 transcription factor also prevented the cannabinoid receptor-induced upregulation of β-Arrestin 2. Our results suggest that sustained activation of CB(2) receptors would enhance β-Arrestin 2 expression possibly contributing to its increased interaction with ERK1/2, thereby driving the upregulation of 5-HT(2A) receptors. The CB(2) receptor-mediated upregulation of β-Arrestin 2 would be mediated, at least in part, by an ERK1/2-dependent activation of AP-1. These data could provide the rationale for some of the adverse effects associated with repeated cannabinoid exposure and shed light on some CB(2) receptor agonists that could represent an alternative therapeutic because of their minimal effect on serotonergic neurotransmission.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 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 (Section 16); and immunological responses (Section 17).

Nonopioid effect of β-endorphin

This review presents the generalized literature data and the results of our own research of the nonopioid effect of β-endorphin, an opioid neuropeptide interacting not only with opioid but also with nonopioid (insensitive to the opioid antagonist naloxone) receptors. The roles of the hormone and its receptors in regulation of the immune, nervous, and endocrine systems are discussed. The effect of neuromediator on the immune system mediated by both opioid and nonopioid receptors is considered in detail. The data on distribution and function of the nonopioid β-endorphin receptor in human and animal organisms are presented. All available data on the characteristics of the nonopioid β-endorphin receptor obtained by means of radioligand analysis are given. The discussed information is supposed to extend our conceptions of the role of β-endorphin in mammals and to be of extensive use in medicine and pharmacology.