Morphine activation of mu opioid receptors causes disinhibition of neurons in the ventral tegmental area mediated by β-arrestin2 and c-Src

Delta opioid receptors engage multiple signaling cascades to differentially modulate prefrontal GABA release with input and target specificity

Opioids regulate circuits associated with motivation and reward across the brain. Of the opioid receptor types, delta opioid receptors (DORs) appear to have a unique role in regulating the activity of circuits related to reward without a liability for abuse. In neocortex, DORs are expressed primarily in interneurons, including parvalbumin- and somatostatin-expressing interneurons that inhibit somatic and dendritic compartments of excitatory pyramidal cells, respectively. But how DORs regulate transmission from these key interneuron classes is unclear. We found that DORs regulate inhibition from these interneuron classes using different G-protein signaling pathways that both converge on presynaptic calcium channels, but regulate distinct aspects of calcium channel function. This imposes different temporal filtering effects, via short-term plasticity, that depend on how calcium channels are regulated. Thus, DORs engage differential signaling cascades to regulate inhibition depending on the postsynaptic target compartment, with different effects on synaptic information transfer in somatic and dendritic domains.

A Balancing Act: Learning from the Past to Build a Future-Focused Opioid Strategy

The harmful side effects of opioid drugs such as respiratory depression, tolerance, dependence, and abuse potential have limited the therapeutic utility of opioids for their entire clinical history. However, no previous attempt to develop effective pain drugs that substantially ameliorate these effects has succeeded, and the current opioid epidemic affirms that they are a greater hindrance to the field of pain management than ever. Recent attempts at new opioid development have sought to reduce these side effects by minimizing engagement of the regulatory protein arrestin-3 at the mu-opioid receptor, but there is significant controversy around this approach. Here, we discuss the ongoing effort to develop safer opioids and its relevant historical context. We propose a new model that reconciles results previously assumed to be in direct conflict to explain how different signaling profiles at the mu-opioid receptor contribute to opioid tolerance and dependence. Our goal is for this framework to inform the search for a new generation of lower liability opioid analgesics.

Shared Mechanisms of GABAergic and Opioidergic Transmission Regulate Corticolimbic Reward Systems and Cognitive Aspects of Motivational Behaviors

The functional interplay between the corticolimbic GABAergic and opioidergic systems plays a crucial role in regulating the reward system and cognitive aspects of motivational behaviors leading to the development of addictive behaviors and disorders. This review provides a summary of the shared mechanisms of GABAergic and opioidergic transmission, which modulate the activity of dopaminergic neurons located in the ventral tegmental area (VTA), the central hub of the reward mechanisms. This review comprehensively covers the neuroanatomical and neurobiological aspects of corticolimbic inhibitory neurons that express opioid receptors, which act as modulators of corticolimbic GABAergic transmission. The presence of opioid and GABA receptors on the same neurons allows for the modulation of the activity of dopaminergic neurons in the ventral tegmental area, which plays a key role in the reward mechanisms of the brain. This colocalization of receptors and their immunochemical markers can provide a comprehensive understanding for clinicians and researchers, revealing the neuronal circuits that contribute to the reward system. Moreover, this review highlights the importance of GABAergic transmission-induced neuroplasticity under the modulation of opioid receptors. It discusses their interactive role in reinforcement learning, network oscillation, aversive behaviors, and local feedback or feedforward inhibitions in reward mechanisms. Understanding the shared mechanisms of these systems may lead to the development of new therapeutic approaches for addiction, reward-related disorders, and drug-induced cognitive impairment.

Key differences in regulation of opioid receptors localized to presynaptic terminals compared to somas: Relevance for novel therapeutics

Opioid receptors are G protein-coupled receptors (GPCRs) that regulate activity within peripheral, subcortical and cortical circuits involved in pain, reward, and aversion processing. Opioid receptors are expressed in both presynaptic terminals where they inhibit neurotransmitter release and postsynaptic locations where they act to hyperpolarize neurons and reduce activity. Agonist activation of postsynaptic receptors at the plasma membrane signal via ion channels or cytoplasmic second messengers. Agonist binding initiates regulatory processes that include phosphorylation by G protein receptor kinases (GRKs) and recruitment of beta-arrestins that desensitize and internalize the receptors. Opioid receptors also couple to effectors from endosomes activating intracellular enzymes and kinases. In contrast to postsynaptic opioid receptors, receptors localized to presynaptic terminals are resistant to desensitization such that there is no loss of signaling in the continuous presence of opioids over the same time scale. Thus, the balance of opioid signaling in circuits expressing pre- and postsynaptic opioid receptors is shifted toward inhibition of presynaptic neurotransmitter release during continuous opioid exposure. The functional implication of this shift is not often acknowledged in behavioral studies. This review covers what is currently understood about regulation of opioid/nociceptin receptors, with an emphasis on opioid receptor signaling in pain and reward circuits. Importantly, the review covers regulation of presynaptic receptors and the critical gaps in understanding this area, as well as the opportunities to further understand opioid signaling in brain circuits. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Prolonged morphine exposure during adolescence alters the responses of lateral paragigantocellularis neurons to naloxone in adult morphine dependent rats

INTRODUCTION

Adolescence is a critical period in brain development, and it is characterized by persistent maturational alterations in the function of central nervous system. In this respect, many studies show the non-medical use of opioid drugs by adolescents. Although this issue has rather widely been addressed during the last decade, cellular mechanisms through which adolescent opioid exposure may induce long-lasting effects are not duly understood. The present study examined the effect of adolescent morphine exposure on neuronal responses of lateral paragigantocellularis nucleus to naloxone in adult morphine-dependent rats.

METHODS

Adolescent male Wistar rats (31 days old) received increasing doses of morphine (from 2.5 to 25 mg/kg, twice daily, s.c.) for 10 days. Control subjects were injected saline with the same protocol. After a drug-free interval (20 days), animals were rendered dependent on morphine during 10 days (10 mg/kg, s.c., twice daily). Then, extracellular single-unit recording was performed to investigate neural response of LPGi to naloxone in adult morphine-dependent rats.

RESULTS

Results indicated that adolescent morphine treatment increases the number of excitatory responses to naloxone, enhances the baseline activity and alters the pattern of firing in neurons with excitatory responses in adult morphine-dependent rats. Moreover, the intensity of excitatory responses is reduced following the early life drug intake.

CONCLUSION

It seems that prolonged opioid exposure during adolescence induces long-lasting neurobiological changes in LPGi responsiveness to future opioid withdrawal challenges.

Can Src protein tyrosine kinase inhibitors be combined with opioid analgesics? Src and opioid-induced tolerance, hyperalgesia and addiction

The clinical application of opioids may be accompanied by a series of adverse consequences, such as opioid tolerance, opioid-induced hyperalgesia, opioid dependence or addiction. In view of this issue, clinicians are faced with the dilemma of treating various types of pain with or without opioids. In this review, we discuss that Src protein tyrosine kinase plays an important role in these adverse consequences, and Src inhibitors can solve these problems well. Therefore, Src inhibitors have the potential to be used in combination with opioids to achieve synergy. How to combine them together to maximize the analgesic effect while avoiding unnecessary trouble provides a topic for follow-up research.

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.

Ventral tegmental area GABA, glutamate, and glutamate-GABA neurons are heterogeneous in their electrophysiological and pharmacological properties

The ventral tegmental area (VTA) contains dopamine neurons intermixed with GABA-releasing (expressing vesicular GABA transporter, VGaT), glutamate-releasing (expressing vesicular glutamate transporter 2, VGluT2), and glutamate-GABA co-releasing (co-expressing VGluT2 and VGaT) neurons. By delivering INTRSECT viral vectors into the VTA of double vglut2-Cre/vgat-Flp transgenic mice, we targeted specific VTA cell populations for ex vivo recordings. We found that VGluT2+ VGaT- and VGluT2+ VGaT+ neurons on average had relatively hyperpolarized resting membrane potential, greater rheobase, and lower spontaneous firing frequency compared to VGluT2- VGaT+ neurons, suggesting that VTA glutamate-releasing and glutamate-GABA co-releasing neurons require stronger excitatory drive to fire than GABA-releasing neurons. In addition, we detected expression of Oprm1mRNA (encoding µ opioid receptors, MOR) in VGluT2+ VGaT- and VGluT2- VGaT+ neurons, and that the MOR agonist DAMGO hyperpolarized neurons with these phenotypes. Collectively, we demonstrate the utility of the double transgenic mouse to access VTA glutamate, glutamate-GABA, and GABA neurons to determine their electrophysiological properties. SIGNIFICANT STATEMENT: Some physiological properties of VTA glutamate-releasing and glutamate-GABA co-releasing neurons are distinct from those of VTA GABA-releasing neurons. µ-opioid receptor activation hyperpolarizes some VTA glutamate-releasing and some GABA-releasing neurons.

Signaling mechanisms of μ-opioid receptor (MOR) in the hippocampus: disinhibition versus astrocytic glutamate regulation

μ-opioid receptor (MOR) is a class of opioid receptors that is critical for analgesia, reward, and euphoria. MOR is distributed in various brain regions, including the hippocampus, where traditionally, it is believed to be localized mainly at the presynaptic terminals of the GABAergic inhibitory interneurons to exert a strong disinhibitory effect on excitatory pyramidal neurons. However, recent intensive research has uncovered the existence of MOR in hippocampal astrocytes, shedding light on how astrocytic MOR participates in opioid signaling via glia-neuron interaction in the hippocampus. Activation of astrocytic MOR has shown to cause glutamate release from hippocampal astrocytes and increase the excitability of presynaptic axon fibers to enhance the release of glutamate at the Schaffer Collateral-CA1 synapses, thereby, intensifying the synaptic strength and plasticity. This novel mechanism involving astrocytic MOR has been shown to participate in hippocampus-dependent conditioned place preference. Furthermore, the signaling of hippocampal MOR, whose action is sexually dimorphic, is engaged in adult neurogenesis, seizure, and stress-induced memory impairment. In this review, we focus on the two profoundly different hippocampal opioid signaling pathways through either GABAergic interneuronal or astrocytic MOR. We further compare and contrast their molecular and cellular mechanisms and their possible roles in opioid-associated conditioned place preference and other hippocampus-dependent behaviors.

Activation of μ-opioid receptors by MT-45 (1-cyclohexyl-4-(1,2-diphenylethyl)piperazine) and its fluorinated derivatives

BACKGROUND AND PURPOSE

A fluorinated derivative (2F-MT-45) of the synthetic μ-opioid receptor agonist MT-45 (1-cyclohexyl-4-(1,2-diphenylethyl)piperazine) was recently identified in a seized illicit tablet. While MT-45 is a Class A drug, banned in a number of countries, nothing is known about the pharmacology of 2F-MT-45. This study compares the pharmacology of MT-45, its fluorinated derivatives and two of its metabolites.

EXPERIMENTAL APPROACH

We used a β-arrestin2 recruitment assay in CHO cells stably expressing μ receptors to quantify the apparent potencies and efficacies of known (MT-45, morphine, fentanyl and DAMGO) and potential agonists. In addition, the GloSensor protein was transiently expressed to quantify changes in cAMP levels. We measured Ca²⁺ to investigate whether MT-45 and its metabolites have effects on GluN1/N2A NMDA receptors stably expressed in Ltk- cells.

KEY RESULTS

The fluorinated MT-45 derivatives have higher apparent potencies (2F-MT-45: 42 nM) than MT-45 (1.3 μM) for inhibition of cAMP accumulation and β-arrestin2 recruitment (2F-MT-45: 196 nM; MT-45: 23.1 μM). While MT-45 and 2F-MT-45 are poor recruiters of β-arrestin2, they have similar efficacies for reducing cAMP levels as DAMGO. Two MT-45 metabolites displayed negligible potencies as μ receptor agonists, but one, 1,2-diphenylethylpiperazine, inhibited the NMDA receptor with an IC₅₀ of 29 μM.

CONCLUSION AND IMPLICATIONS

Fluorinated derivatives of MT-45 are potent μ receptor agonists and this may pose a danger to illicit opioid users. Inhibition of NMDA receptors by a metabolite of MT-45 may contribute to the reported dissociative effects.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Perioperative opioid analgesia-when is enough too much? A review of opioid-induced tolerance and hyperalgesia

Opioids are a mainstay of acute pain management but can have many adverse effects, contributing to problematic long-term use. Opioid tolerance (increased dose needed for analgesia) and opioid-induced hyperalgesia (paradoxical increase in pain with opioid administration) can contribute to both poorly controlled pain and dose escalation. Hyperalgesia is particularly problematic as further opioid prescribing is largely futile. The mechanisms of opioid tolerance and hyperalgesia are complex, involving μ opioid receptor signalling pathways that offer opportunities for novel analgesic alternatives. The intracellular scaffold protein β-arrestin-2 is implicated in tolerance, hyperalgesia, and other opioid side-effects. Development of agonists biased against recruitment of β-arrestin-2 could provide analgesic efficacy with fewer side-effects. Alternative approaches include inhibition of peripheral μ opioid receptors and blockade of downstream signalling mechanisms, such as the non-receptor tyrosine kinase Src or N-methyl-D-aspartate receptors. Furthermore, it is prudent to use multimodal analgesic regimens to reduce reliance on opioids during the perioperative period. In the third paper in this Series we focus on clinical and mechanism-based understanding of tolerance and opioid-induced hyperalgesia, and discuss current and future strategies for pain management.

Formation of a morphine-conditioned place preference does not change the size of evoked potentials in the ventral hippocampus-nucleus accumbens projection

In opioid addiction, cues and contexts associated with drug reward can be powerful triggers for drug craving and relapse. The synapses linking ventral hippocampal outputs to medium spiny neurons of the accumbens may be key sites for the formation and storage of associations between place or context and reward, both drug-related and natural. To assess this, we implanted rats with electrodes in the accumbens shell to record synaptic potentials evoked by electrical stimulation of the ventral hippocampus, as well as continuous local-field-potential activity. Rats then underwent morphine-induced (10 mg/kg) conditioned-place-preference training, followed by extinction. Morphine caused an acute increase in the slope and amplitude of accumbens evoked responses, but no long-term changes were evident after conditioning or extinction of the place preference, suggesting that the formation of this type of memory does not lead to a net change in synaptic strength in the ventral hippocampal output to the accumbens. However, analysis of the local field potential revealed a marked sensitization of theta- and high-gamma-frequency activity with repeated morphine administration. This phenomenon may be linked to the behavioral changes—such as psychomotor sensitization and the development of drug craving—that are associated with chronic use of addictive drugs.