Long-Term Reduction of Kappa Opioid Receptor Function by the Biased Ligand, Norbinaltorphimine, Requires c-Jun N-Terminal Kinase Activity and New Protein Synthesis in Peripheral Sensory Neurons

Activation of G protein gated inwardly rectifying potassium (GIRK) channels in keratinocytes mediates peripheral kappa opioid receptor-mediated antinociception

Kappa opioid receptors (KOR) expressed by peripheral pain-sensing neurons (nociceptors) are a promising target for development of effective and safer analgesics for inflammatory pain that are devoid of central nervous system adverse effects. Here we sought to delineate the signaling pathways that underlie peripheral KOR-mediated antinociception in adult male and female Sprague-Dawley rats. In an inflammatory model of pain, local intraplantar (i.pl.) injection of pertussis toxin prevented antinociception induced by the KOR agonist, U50488, indicating that members of the Gi/o family mediate the antinociceptive response. Furthermore, i.pl. injection of the G protein-coupled inward-rectifying potassium (GIRK) channel blocker, TPNQ, as well as GIRK2 subunit-targeted siRNA abolished U50488-mediated antinociceptive behavioral responses in both male and female rats. Consistent with these data, i.pl. injection of ML297, a direct activator of GIRK1 subunit-containing channels, elicited peripheral antinociceptive behavior. It is well known that intraepidermal nerve fibers (IENF) that innervate the hindpaw propagate nociceptive signals to the spinal cord. However, recent studies suggest that keratinocytes, the major cell type in the epidermis, also play an active role in pain and sensory processing. Results from RT-qPCR, RNAscope and immunohistochemistry experiments confirmed that both KOR and GIRK are expressed in keratinocytes in the epidermal layer of the rat hindpaw. Knockdown of either KOR or GIRK2 subunits selectively in keratinocytes by i.pl. injection of shRNA plasmids, prevented the antinociceptive response to U50488. Taken together, these data suggest that KOR-mediated activation of GIRK channels in keratinocytes is required for peripherally-mediated antinociception.

Kappa opioid receptor in nucleus accumbens regulates depressive-like behaviors following prolonged morphine withdrawal in mice

Prolonged withdrawal from opioids leads to negative emotions. Kappa opioid receptor (KOR) plays an important role in opioid addiction and affective disorders. However, the underlying mechanism of KOR in withdrawal-related depression is still lacking. We found that escitalopram treatment had a limited effect in improving depression symptoms in heroin-dependent patients. In mice, we demonstrated prolonged (4 weeks) but not acute (24 h) withdrawal from morphine induced depressive-like behaviors. The number of c-Fos positive cells and the expression of KOR in the nucleus accumbens (NAc), were significantly increased in the prolonged morphine withdrawal mice. Conditional KOR knockdown in NAc significantly improved depressive-like behaviors. Repeated but not acute treatment with the KOR antagonist norBNI improved depressive-like behaviors and reversed PSD95, synaptophysin, p-ERK, p-CREB, and BDNF in NAc. This study demonstrated the important role of striatal KOR in morphine withdrawal-related depressive-like behaviors and offered therapeutic potential for the treatment of withdrawal-related depression.

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.

14-3-3γ mediates the long-term inhibition of peripheral kappa opioid receptor antinociceptive signaling by norbinaltorphimine

Long-term inhibition of kappa opioid receptor (KOR) signaling in peripheral pain-sensing neurons is a potential obstacle for development of peripherally-restricted KOR agonists that produce analgesia. Such a long-term inhibitory mechanism is invoked from activation of c-Jun N-terminal kinase (JNK) that follows a single injection of the KOR antagonist norbinaltorphimine (norBNI). This effect requires protein synthesis of an unknown mediator in peripheral pain-sensing neurons. Using 2D difference gel electrophoresis with tandem mass spectrometry, we have identified that the scaffolding protein 14-3-3γ is upregulated in peripheral sensory neurons following activation of JNK with norBNI. Knockdown of 14-3-3γ by siRNA eliminates the long-term reduction in KOR-mediated cAMP signaling by norBNI in peripheral sensory neurons in culture. Similarly, knockdown of 14-3-3γ in the rat hind paw abolished the norBNI-mediated long-term reduction in peripheral KOR-mediated antinociception. Further, overexpression of 14-3-3γ in KOR expressing CHO cells prevented KOR-mediated inhibition of cAMP signaling. These long-term effects are selective for KOR as heterologous regulation of other receptor systems was not observed. These data suggest that 14-3-3γ is both necessary and sufficient for the long-term inhibition of KOR by norBNI in peripheral sensory neurons.

Kappa Opioid Receptor Ligands and Pharmacology: Diphenethylamines, a Class of Structurally Distinct, Selective Kappa Opioid Ligands

The kappa opioid receptor (KOR), a G protein-coupled receptor, and its endogenous ligands, the dynorphins, are prominent members of the opioid neuromodulatory system. The endogenous kappa opioid system is expressed in the central and peripheral nervous systems, and has a key role in modulating pain in central and peripheral neuronal circuits and a wide array of physiological functions and neuropsychiatric behaviors (e.g., stress, reward, emotion, motivation, cognition, epileptic seizures, itch, and diuresis). We review the latest advances in pharmacology of the KOR, chemical developments on KOR ligands with advances and challenges, and therapeutic and potential applications of KOR ligands. Diverse discovery strategies of KOR ligands targeting natural, naturally derived, and synthetic compounds with different scaffolds, as small molecules or peptides, with short or long-acting pharmacokinetics, and central or peripheral site of action, are discussed. These research efforts led to ligands with distinct pharmacological properties, as agonists, partial agonists, biased agonists, and antagonists. Differential modulation of KOR signaling represents a promising strategy for developing pharmacotherapies for several human diseases, either by activating (treatment of pain, pruritus, and epilepsy) or blocking (treatment of depression, anxiety, and addiction) the receptor. We focus on the recent chemical and pharmacological advances on diphenethylamines, a new class of structurally distinct, selective KOR ligands. Design strategies and investigations to define structure-activity relationships together with in vivo pharmacology of diphenethylamines as agonists, biased agonists, and antagonists and their potential use as therapeutics are discussed.

Biased Ligands at the Kappa Opioid Receptor: Fine-Tuning Receptor Pharmacology

The kappa opioid receptor (KOR) is a G protein-coupled receptor (GPCR) that can signal through multiple signaling pathways. KOR agonists are known to relieve pain and itch, as well as induce dysphoria, sedation, hallucinations, and diuresis. As is the case with many other GPCRs, specific signaling pathways downstream of the KOR have been linked to certain physiological responses induced by the receptor. Those studies motivated the search and discovery of a number of KOR ligands that preferentially activate one signaling pathway over another. Such compounds are termed functionally selective or biased ligands, and may present a way of inducing desired receptor effects with reduced adverse reactions. In this chapter, I review the molecular intricacies of KOR signaling and discuss the studies that have used biased signaling through the KOR as a way to selectively modulate in vivo physiology.

Targeting opioid dysregulation in depression for the development of novel therapeutics

Since the serendipitous discovery of the first class of modern antidepressants in the 1950's, all pharmacotherapies approved by the Food and Drug Administration for major depressive disorder (MDD) have shared a common mechanism of action, increased monoaminergic neurotransmission. Despite the widespread availability of antidepressants, as many as 50% of depressed patients are resistant to these conventional therapies. The significant length of time required to produce meaningful symptom relief with these medications, 4-6 weeks, indicates that other mechanisms are likely involved in the pathophysiology of depression which may yield more viable targets for drug development. For decades, no viable candidate target with a different mechanism of action to that of conventional therapies proved successful in clinical studies. Now several exciting avenues for drug development are under intense investigation. One of these emerging targets is modulation of endogenous opioid tone. This review will evaluate preclinical and clinical evidence pertaining to opioid dysregulation in depression, focusing on the role of the endogenous ligands endorphin, enkephalin, dynorphin, and nociceptin/orphanin FQ (N/OFQ) and their respective receptors, mu (MOR), delta (DOR), kappa (KOR), and the N/OFQ receptor (NOP) in mediating behaviors relevant to depression and anxiety. Finally, putative opioid based antidepressants that are under investigation in clinical trials, ALKS5461, JNJ-67953964 (formerly LY2456302 and CERC-501) and BTRX-246040 (formerly LY-2940094) will be discussed. This review will illustrate the potential therapeutic value of targeting opioid dysregulation in developing novel therapies for MDD.

Making Sense of Pharmacology: Inverse Agonism and Functional Selectivity

Constitutive receptor activity/inverse agonism and functional selectivity/biased agonism are 2 concepts in contemporary pharmacology that have major implications for the use of drugs in medicine and research as well as for the processes of new drug development. Traditional receptor theory postulated that receptors in a population are quiescent unless activated by a ligand. Within this framework ligands could act as agonists with various degrees of intrinsic efficacy, or as antagonists with zero intrinsic efficacy. We now know that receptors can be active without an activating ligand and thus display "constitutive" activity. As a result, a new class of ligand was discovered that can reduce the constitutive activity of a receptor. These ligands produce the opposite effect of an agonist and are called inverse agonists. The second topic discussed is functional selectivity, also commonly referred to as biased agonism. Traditional receptor theory also posited that intrinsic efficacy is a single drug property independent of the system in which the drug acts. However, we now know that a drug, acting at a single receptor subtype, can have multiple intrinsic efficacies that differ depending on which of the multiple responses coupled to a receptor is measured. Thus, a drug can be simultaneously an agonist, an antagonist, and an inverse agonist acting at the same receptor. This means that drugs have an additional level of selectivity (signaling selectivity or "functional selectivity") beyond the traditional receptor selectivity. Both inverse agonism and functional selectivity need to be considered when drugs are used as medicines or as research tools.

Lateralized kappa opioid receptor signaling from the amygdala central nucleus promotes stress-induced functional pain

The response of diffuse noxious inhibitory controls (DNIC) is often decreased, or lost, in stress-related functional pain syndromes. Because the dynorphin/kappa opioid receptor (KOR) pathway is activated by stress, we determined its role in DNIC using a model of stress-induced functional pain. Male, Sprague-Dawley rats were primed for 7 days with systemic morphine resulting in opioid-induced hyperalgesia. Fourteen days after priming, when hyperalgesia was resolved, rats were exposed to environmental stress and DNIC was evaluated by measuring hind paw response threshold to noxious pressure (test stimulus) after capsaicin injection in the forepaw (conditioning stimulus). Morphine priming without stress did not alter DNIC. However, stress produced a loss of DNIC in morphine-primed rats in both hind paws that was abolished by systemic administration of the KOR antagonist, nor-binaltorphimine (nor-BNI). Microinjection of nor-BNI into the right, but not left, central nucleus of the amygdala (CeA) prevented the loss of DNIC in morphine-primed rats. Diffuse noxious inhibitory controls were not modulated by bilateral nor-BNI in the rostral ventromedial medulla. Stress increased dynorphin content in both the left and right CeA of primed rats, reaching significance only in the right CeA; no change was observed in the rostral ventromedial medulla or hypothalamus. Although morphine priming alone is not sufficient to influence DNIC, it establishes a state of latent sensitization that amplifies the consequences of stress. After priming, stress-induced dynorphin/KOR signaling from the right CeA inhibits DNIC in both hind paws, likely reflecting enhanced descending facilitation that masks descending inhibition. Kappa opioid receptor antagonists may provide a new therapeutic strategy for stress-related functional pain disorders.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 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, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.