Comparison of the kappa-opioid receptor antagonist DIPPA in tests of anxiety-like behavior between Wistar Kyoto and Sprague Dawley rats

A review of the kappa opioid receptor system in opioid use

The kappa opioid receptor (KOR) system is implicated in dysphoria and as an "anti-reward system" during withdrawal from opioids. However, no clear consensus has been made in the field, as mixed findings have been reported regarding the relationship between the KOR system and opioid use. This review summarizes the studies to date on the KOR system and opioids. A systematic scoping review was reported following PRISMA guidelines and conducted based on the published protocol. Comprehensive searches of several databases were done in the following databases: MEDLINE, Embase, PsycINFO, Web of Science, Scopus, and Cochrane. We included preclinical and clinical studies that tested the administration of KOR agonists/antagonists or dynorphin and/or measured dynorphin levels or KOR expression during opioid intoxication or withdrawal from opioids. One hundred studies were included in the final analysis. Preclinical administration of KOR agonists decreased drug-seeking/taking behaviors and opioid withdrawal symptoms. KOR antagonists showed mixed findings, depending on the agent and/or type of withdrawal symptom. Administration of dynorphins attenuated opioid withdrawal symptoms both in preclinical and clinical studies. In the limited number of available studies, dynorphin levels were found to increase in cerebrospinal fluid (CSF) and peripheral blood lymphocytes (PBL) of opioid use disorder subjects (OUD). In animals, dynorphin levels and/or KOR expression showed mixed findings during opioid use. The KOR/dynorphin system appears to have a multifaceted and complex nature rather than simply functioning as an anti-reward system. Future research in well-controlled study settings is necessary to better understand the clinical role of the KOR system in opioid use.

Co-targeting the kappa opioid receptor and dopamine transporter reduces motivation to self-administer cocaine and partially reverses dopamine system dysregulation

Cocaine disrupts dopamine (DA) and kappa opioid receptor (KOR) system activity, with long-term exposure reducing inhibiton of DA uptake by cocaine and increasing KOR system function. Single treatment therapies have not been successful for cocaine use disorder; therefore, this study focuses on a combination therapy targeting the dopamine transporter (DAT) and KOR. Sprague Dawley rats self-administered 5 days of cocaine (1.5 mg/kg/inf, max 40 inf/day, FR1), followed by 14 days on a progressive ratio (PR) schedule (0.19 mg/kg/infusion). Behavioral effects of individual and combined administration of phenmetrazine and nBNI were then examined using PR. Additionally, ex vivo fast scan cyclic voltammetry was then used to assess alterations in DA and KOR system activity in the nucleus accumbens before and after treatments. Chronic administration of phenmetrazine as well as the combination of phenmetrazine and nBNI-but not nBNI alone-significantly reduced PR breakpoints. In addition, the combination of phenmetrazine and nBNI partially reversed cocaine-induced neurodysregulations of the KOR and DA systems, indicating therapeutic benefits of targeting the DA and KOR systems in tandem. These data highlight the potential benefits of the DAT and KOR as dual-cellular targets to reduce motivation to administer cocaine and reverse cocaine-induced alterations of the DA system.

The Antidepressant Effect of Magnolol on Depression-Like Behavior of CORT-Treated Mice

Although the antidepressant-like effect of magnolol has been revealed in previous reports, the mechanism remains unclear. In this study, the antidepressant-like effect of magnolol on corticosterone-induced (CORT-induced) mice was investigated in vivo. After 21 days of CORT induction, the mice showed marked depressive-like behaviors, with a decrease in sucrose preference score and an increase in immobility time in the tail suspension test (TST) and forced swimming test (FST). Pretreatment with either magnolol (50 mg/kg, i.p.) or the kappa opioid receptor (KOR) antagonist nor-BNI (10 mg/kg, i.p.) prevented CORT-induced depression-like behavior and reduced CORT-induced dynorphin (DYN A) elevation in the hippocampal ventral DG. However, no depression-like behavior was observed in mice with KOR downregulation in the ventral DG. We further found that upregulation of DYN A in the DG caused depression-like behavior, which was blocked by intraperitoneal injection of nor-BNI and modulated by magnolol. The present study demonstrated that magnolol could ameliorate CORT-induced depression-like behaviors, by modulating the DYN A/KOR system in the ventral DG of the hippocampus.

Dysfunctional Heteroreceptor Complexes as Novel Targets for the Treatment of Major Depressive and Anxiety Disorders

Among mental diseases, major depressive disorder (MDD) and anxiety deserve a special place due to their high prevalence and their negative impact both on society and patients suffering from these disorders. Consequently, the development of novel strategies designed to treat them quickly and efficiently, without or at least having limited side effects, is considered a highly important goal. Growing evidence indicates that emerging properties are developed on recognition, trafficking, and signaling of G-protein coupled receptors (GPCRs) upon their heteromerization with other types of GPCRs, receptor tyrosine kinases, and ionotropic receptors such as N-methyl-D-aspartate (NMDA) receptors. Therefore, to develop new treatments for MDD and anxiety, it will be important to identify the most vulnerable heteroreceptor complexes involved in MDD and anxiety. This review focuses on how GPCRs, especially serotonin, dopamine, galanin, and opioid heteroreceptor complexes, modulate synaptic and volume transmission in the limbic networks of the brain. We attempt to provide information showing how these emerging concepts can contribute to finding new ways to treat both MDD and anxiety disorders.

KOR Control over Addiction Processing: An Exploration of the Mesolimbic Dopamine Pathway

Drug addiction is a complex, persistent, and chronically relapsing neurological disorder exacerbated by acute and chronic stress. It is well known that the dynorphin/kappa opioid receptor (KOR) system regulates stress perception and responsivity, while the mesolimbic dopamine system plays a role in reward and reinforcement associated with alcohol and substance use disorders. Interestingly, the dopamine and dynorphin/KOR systems are highly integrated in mesolimbic areas, with KOR activation leading to inhibition of dopamine release, further altering the perception of reinforcing and aversive stimuli. Chronic or repeated exposure to stress or drugs potentiates KOR function ultimately contributing to a hypodopaminergic state. This hypodopaminergic state is one of the hallmarks of hyperkatifeia, defined as the hypersensitivity to emotional distress that is exacerbated during drug withdrawal and abstinence. The relationship between stress and drug addiction is bidirectional; repeated/chronic stress promotes pro-addictive behaviors, and repeated cycles of drug exposure and withdrawal, across various drug classes, produces stress. Neuroadaptations driven by this bidirectional relationship ultimately influence the perception of the reinforcing value of rewarding stimuli. In this chapter, we address the involvement of the dopamine and dynorphin/KOR systems and their interactions in shaping reinforcement value processing after drug and stress exposure, as well as a combinatorial impact of both drugs and stress.

Corticotropin releasing factor and norepinephrine related circuitry changes in the bed nucleus of the stria terminalis in stress and alcohol and substance use disorders

Alcohol Use Disorder (AUD) affects around 14.5 million individuals in the United States, with Substance Use Disorder (SUD) affecting an additional 8.3 million individuals. Relapse is a major barrier to effective long-term treatment of this illness with stress often described as a key trigger for a person with AUD or SUD to relapse during a period of abstinence. Two signaling molecules, norepinephrine (NE) and corticotropin releasing factor (CRF), are released during the stress response, and also play important roles in reward behaviors and the addiction process. Within the addiction literature, one brain region in which there has been increasing research focus in recent years is the bed nucleus of the stria terminalis (BNST). The BNST is a limbic structure with numerous cytoarchitecturally and functionally different subregions that has been implicated in drug-seeking behaviors and stress responses. This review focuses on drug and stress-related neurocircuitry changes in the BNST, particularly within the CRF and NE systems, with an emphasis on differences and similarities between the major dorsal and ventral BNST subregions.

The Role of Dynorphin and the Kappa Opioid Receptor in Schizophrenia and Major Depressive Disorder: A Translational Approach

The kappa opioid receptor (KOR) and its endogenous ligands dynorphins (DYN) have been implicated in the development or symptomatology of a variety of neuropsychiatric disorders. This review covers a brief history of the development of KOR agonists and antagonists, their effects in healthy volunteers, and the potential role of DYN/KOR dysfunction in schizophrenia and major depressive disorder from a translational perspective. The potential role of DYN/KOR dysfunction in schizophrenia is based on several lines of evidence. Selective KOR agonists induce affective states in healthy volunteers with similarities to the symptoms of schizophrenia. Studies have shown increased DYN in patients with schizophrenia, although the data have been mixed. Finally, meta-analytic data have shown that opioid antagonists are associated with reductions in the symptoms of schizophrenia. The potential role of DYN/KOR dysfunction in major depressive disorder is also based on a combination of preclinical and clinical data. Selective KOR agonists have shown pro-depressive effects in human volunteers, while selective KOR antagonists have shown robust efficacy in several preclinical models of antidepressant activity. Small studies have shown that nonselective KOR antagonists may have efficacy in treatment-resistant depression. Additionally, recent clinical data have shown that the KOR may be an effective target for treating anhedonia, a finding relevant to both schizophrenia and depression. Finally, recommendations are provided for translating preclinical models for schizophrenia and major depressive disorder into the clinic.

Dynorphin/Kappa-Opioid Receptor System Modulation of Cortical Circuitry

Cortical circuits control a plethora of behaviors, from sensation to cognition. The cortex is enriched with neuropeptides and receptors that play a role in information processing, including opioid peptides and their cognate receptors. The dynorphin (DYN)/kappa-opioid receptor (KOR) system has been implicated in the processing of sensory and motivationally-charged emotional information and is highly expressed in cortical circuits. This is important as dysregulation of DYN/KOR signaling in limbic and cortical circuits has been implicated in promoting negative affect and cognitive deficits in various neuropsychiatric disorders. However, research investigating the role of this system in controlling cortical circuits and computations therein is limited. Here, we review the (1) basic anatomy of cortical circuits, (2) anatomical architecture of the cortical DYN/KOR system, (3) functional regulation of cortical synaptic transmission and microcircuit function by the DYN/KOR system, (4) regulation of behavior by the cortical DYN/KOR system, (5) implications for the DYN/KOR system for human health and disease, and (6) future directions and unanswered questions for the field. Further work elucidating the role of the DYN/KOR system in controlling cortical information processing and associated behaviors will be of importance to increasing our understanding of principles underlying neuropeptide modulation of cortical circuits, mechanisms underlying sensation and perception, motivated and emotional behavior, and cognition. Increased emphasis in this area of study will also aid in the identification of novel ways to target the DYN/KOR system to treat neuropsychiatric disorders.

Kappa Opioid Receptor-mediated Modulation of Social Responding in Adolescent Rats and in Rats Prenatally Exposed to Valproic Acid

The kappa opioid receptor (KOP) system modulates social play responding, however a paucity of studies have examined effects on social motivation and cognition in the absence of play. Prenatal exposure to the anti-epileptic and mood stabiliser valproic acid (VPA) is associated with impaired social responding and altered gene expression of KOP (oprk1) and dynorphin (pdyn) in several brain regions. The present study examined if pharmacological modulation of KOP altered social motivation and cognition, immediate early gene (IEG) and oprk1-pdyn expression in adolescent male rats and rats prenatally exposed to VPA. In control rats, the KOP antagonist DIPPA enhanced sociability, while both DIPPA and the KOP agonist U50488 decreased social novelty preference. In rats exposed prenatally to VPA, neither U50488 nor DIPPA altered sociability or social novelty preference. Analysis of IEG expression revealed that DIPPA reduced expression of egr-1 expression in the prefrontal cortex of control rats and U50488 increased junb expression in the PFC of both control and VPA-exposed rats. VPA-exposed rats exhibited increased expression of oprk1 and pdyn in the prefrontal cortex and amygdala compared with control rats. DIPPA and U50488 increased oprk1 expression in the amygdala of control rats and decreased oprk1 expression in the prefrontal cortex of VPA-exposed rats. Taken together, these data demonstrate that pharmacological modulation of the KOP system alters social motivation and cognition in control rats, an effect not observed in rats prenatally exposed to VPA. These data provide support that prenatal exposure to VPA is associated with alterations in the expression and functionality of KOP system.

Kappa opioid receptors in the bed nucleus of the stria terminalis regulate binge-like alcohol consumption in male and female mice

Binge drinking is the most common pattern of excessive alcohol consumption and is a significant contributor to the development of Alcohol Use Disorder and dependence. Previous studies demonstrated involvement of kappa opioid receptors (KOR) in binge-like drinking in mice using the Drinking-in-the-Dark model. The current studies examined the role of KOR specifically in the bed nucleus of the stria terminals (BNST) in binge-like alcohol consumption in male and female mice. Direct administration of the long lasting KOR antagonist, nor-BNI, into the BNST decreased binge-like alcohol consumption and blood alcohol concentrations in male and female C57BL/6J mice. Similarly, direct nor-BNI administration into the BNST modestly reduced sucrose consumption and the suppression of fluid intake was not related to reduced locomotor activity. To further determine the role of KOR within the BNST on binge-like alcohol consumption, the KOR agonist U50,488 was administered systemically which resulted in a robust increase in alcohol intake. Microinjection of nor-BNI into the BNST blocked the high level of alcohol intake after systemic U50,488 challenge reducing intake and resultant blood alcohol concentrations. Together, these data suggest that KOR activity in the BNST contributes to binge-like alcohol consumption in both male and female mice. This article is part of the special issue on 'Neuropeptides'.

Behavioral effects of the kappa opioid receptor partial agonist nalmefene in tests relevant to depression

Compounds with high affinity at kappa and mu opioid receptors may have clinical utility in treating major depressive disorder. Nalmefene (NMF) is a partial kappa opioid receptor agonist and potent mu opioid receptor antagonist, but there has been no preclinical evaluation of NMF in rodent tests relevant to depression and anxiety. To address this, the effects of NMF on neurochemical and behavioral endpoints in C57BL/6J mice were examined and contrasted with a structurally related analog, naltrexone (NTX). NMF exhibited kappa opioid receptor agonist activity, measured as a reduction in extracellular dopamine release in the nucleus accumbens using in vivo microdialysis following acute but not chronic administration. In the mouse forced swim test, female mice were more responsive to higher doses of NMF and NTX compared to male mice. The behavioral effects of NMF in the forced swim test were blocked in Oprk1^-/- and Oprm1^-/- mice. Conversely, the effects of NTX were blocked only in Oprm1^-/- mice. These results indicate that both kappa and mu opioid receptors mediate the behavioral effects of NMF, but the effects of NTX in this test were modified only by mu opioid receptor engagement. Unlike NTX, NMF did not produce conditioned place aversion in either sex. Finally, NMF's activity in the marble burying test and forced swim test were retained following chronic administration. The sustained effects exerted by NMF on tests that are sensitive to antidepressant and anxiolytic compounds support further investigation of NMF as a potential therapeutic for depression.

Role of deep breaths in ultrasonic vocal production of Sprague-Dawley rats

Deep breaths are one of three breathing patterns in rodents characterized by an increased tidal volume. While humans incorporate deep breaths into vocal behavior, it was unknown whether nonhuman mammals use deep breaths for vocal production. We have utilized subglottal pressure recordings in awake, spontaneously behaving male Sprague-Dawley rats in five contexts: sleep, rest, noxious stimulation, exposure to a female in estrus, and exposure to an unknown male. Deep breaths were produced at rates ranging between 17.5 and 90.3 deep breaths per hour. While overall breathing and vocal rates were higher in social and noxious contexts, the rate of deep breaths was only increased during the male's interaction with a female. Results also inform our understanding of vocal-respiratory integration in rats. The rate of deep breaths that were associated with a vocalization during the exhalation phase increased with vocal activity. The proportion of deep breaths that were associated with a vocalization (on average 22%) was similar to the proportion of sniffing or eupnea breaths that contain a vocalization. Therefore, vocal motor patterns appear to be entrained to the prevailing breathing rhythm, i.e., vocalization uses the available breathing pattern rather than recruiting a specific breathing pattern. Furthermore, the pattern of a deep breath was different when it was associated with a vocalization, suggesting that motor planning occurs. Finally, deep breaths are a source for acoustic variation; for example, call duration and fundamental frequency modulation were both larger in 22-kHz calls produced following a deep inhalation.NEW & NOTEWORTHY The emission of a long, deep, audible breath can express various emotions. The investigation of deep breaths, also known as sighing, in a nonhuman mammal demonstrated the occasional use of deep breaths for vocal production. Similar to the human equivalent, acoustic features of a deep breath vocalization are characteristic.

Kappa Opioid Receptor Antagonists as Potential Therapeutics for Stress-Related Disorders

Exposure to stressful stimuli activates kappa opioid receptor (KOR) signaling, a process known to produce aversion and dysphoria in humans and other species. This endogenous opioid system is dysregulated in stress-related disorders, specifically in major depressive disorder (MDD). These findings serve as the foundation for a growing interest in the therapeutic potential of KOR antagonists as novel antidepressants. In this review, data supporting the hypothesis of dysregulated KOR function in MDD are considered. The clinical data demonstrating the therapeutic efficacy and safety of selective and mixed opioid antagonists are then presented. Finally, the preclinical evidence illustrating the induction of behaviors relevant to the endophenotypes of MDD and KOR antagonist activity in stress-naïve and stress-exposed animals is evaluated. Overall, this review highlights the emergent literature supporting the pursuit of KOR antagonists as novel therapeutics for MDD and other stress-related disorders.

Activation of Kappa Opioid Receptor Regulates the Hypothermic Response to Calorie Restriction and Limits Body Weight Loss

Mammals maintain a nearly constant core body temperature (T_b) by balancing heat production and heat dissipation. This comes at a high metabolic cost that is sustainable if adequate calorie intake is maintained. When nutrients are scarce or experimentally reduced such as during calorie restriction (CR), endotherms can reduce energy expenditure by lowering T_b [1-6]. This adaptive response conserves energy, limiting the loss of body weight due to low calorie intake [7-10]. Here we show that this response is regulated by the kappa opioid receptor (KOR). CR is associated with increased hypothalamic levels of the endogenous opioid Leu-enkephalin, which is derived from the KOR agonist precursor dynorphin [11]. Pharmacological inhibition of KOR, but not of the delta or the mu opioid receptor subtypes, fully blocked CR-induced hypothermia and increased weight loss during CR independent of calorie intake. Similar results were seen with DIO mice subjected to CR. In contrast, inhibiting KOR did not change T_b in animals fed ad libitum (AL). Chemogenetic inhibition of KOR neurons in the hypothalamic preoptic area reduced the CR-induced hypothermia, whereas chemogenetic activation of prodynorphin-expressing neurons in the arcuate or the parabrachial nucleus lowered T_b. These data indicate that KOR signaling is a pivotal regulator of energy homeostasis and can affect body weight during dieting by modulating T_b and energy expenditure.

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.

Contribution of Dynorphin and Orexin Neuropeptide Systems to the Motivational Effects of Alcohol

Understanding the neural systems that drive alcohol motivation and are disrupted in alcohol use disorders is of critical importance in developing novel treatments. The dynorphin and orexin/hypocretin neuropeptide systems are particularly relevant with respect to alcohol use and misuse. Both systems are strongly associated with alcohol-seeking behaviors, particularly in cases of high levels of alcohol use as seen in dependence. Furthermore, both systems also play a role in stress and anxiety, indicating that disruption of these systems may underlie long-term homeostatic dysregulation seen in alcohol use disorders. These systems are also closely interrelated with one another - dynorphin/kappa opioid receptors and orexin/hypocretin receptors are found in similar regions and hypocretin/orexin neurons also express dynorphin - suggesting that these two systems may work together in the regulation of alcohol seeking and may be mutually disrupted in alcohol use disorders. This chapter reviews studies demonstrating a role for each of these systems in motivated behavior, with a focus on their roles in regulating alcohol-seeking and self-administration behaviors. Consideration is also given to evidence indicating that these neuropeptide systems may be viable targets for the development of potential treatments for alcohol use disorders.

The Effect of Methylphenidate on the Microstructure of Schedule-Induced Polydipsia in an animal model of ADHD

Schedule-induced polydipsia (SIP) was established in spontaneously hypertensive rats (SHR), Wistar Kyoto rats (WKY), and Wistar rats, using a multiple fixed-time (FT) schedule of food delivery, with 30- and 90-s components. Thereafter, animals were exposed to methylphenidate (MPH; 2.5mg/kg/d) for six consecutive SIP sessions. A test to assess possible sensitization effects was also conducted four days after termination of the drug treatment. At baseline, FT 90-s produced longer and more frequent drinking episodes in SHR than in WKY. An analysis of the distribution of inter-lick intervals revealed that drinking was organized in bouts, which were shorter in SHR than in WKY. Across strains and schedules, MPH shifted drinking episodes towards the beginning of inter-food intervals, which may reflect a stimulant effect on SIP. MPH transiently reduced the frequency of drinking episodes in WKY in FT 30-s, and more permanently reduced the frequency of licking bouts in Wistar rats. MPH also increased the length of licking bouts in Wistar rats. Overall, SHR displayed a hyperactive-like pattern of drinking (frequent but short bouts), which 2.5mg/kg MPH appears to reduce in WKY and Wistar but not in SHR rats. It appears that therapeutic effects of MPH on hyperactive-like SIP require higher doses in SHR relative to control strains.

Genetic variation in the behavioral effects of buprenorphine in female mice derived from a murine model of the OPRM1 A118G polymorphism

Pharmacogenetic studies have identified the non-synonymous single nucleotide polymorphism (A118G) in the human mu opioid receptor (MOR) gene (OPRM1) as a critical genetic variant capable of altering the efficacy of opioid therapeutics. To date few studies have explored the potential impact of the OPRM1 A118G polymorphism on the pharmacological effects of buprenorphine (BPN), a potent MOR partial agonist and kappa opioid receptor antagonist, which is approved by the FDA for the treatment of opioid addiction and chronic pain. The goal of these studies was to determine whether the MOR-mediated behavioral effects of BPN were altered in the Oprm1 A112G mouse model of the human OPRM1 A118G SNP. All studies were conducted in female, AA, AG and GG mice. BPN's maximal analgesic effect in the hot plate test was significantly blunted in AG and GG mice compared to wild type AA mice. Similarly, the BPN-induced reduction of latency to consume food in the novelty induced hypophagia test was blocked entirely in AG and GG mice compared to their AA littermates. In addition, GG mice exhibited marked reductions in psychostimulant hyperlocomotor activity compared to the AA group. In contrast, reduced immobility in the forced swim test, an effect of BPN mediated by kappa opioid receptors, was not affected by genotype. These studies demonstrate the ability of the Oprm1 A112G SNP to attenuate the analgesic, anxiolytic and hyperlocomotor effects of BPN. Overall, these data suggest that the OPRM1 A118G SNP will significantly impact the clinical efficacy of BPN in its therapeutic applications.

Kappa Opioids, Salvinorin A and Major Depressive Disorder

Opioids are traditionally associated with pain, analgesia and drug abuse. It is now clear, however, that the opioids are central players in mood. The implications for mood disorders, particularly clinical depression, suggest a paradigm shift from the monoamine neurotransmitters to the opioids either alone or in interaction with monoamine neurons. We have a special interest in dynorphin, the last of the major endogenous opioids to be isolated and identified. Dynorphin is derived from the Greek word for power, dynamis, which hints at the expectation that the neuropeptide held for its discoverers. Yet, dynorphin and its opioid receptor subtype, kappa, has always taken a backseat to the endogenous b-endorphin and the exogenous morphine that both bind the mu opioid receptor subtype. That may be changing as the dynorphin/ kappa system has been shown to have different, often opposite, neurophysiological and behavioral influences. This includes major depressive disorder (MDD). Here, we have undertaken a review of dynorphin/ kappa neurobiology as related to behaviors, especially MDD. Highlights include the unique features of dynorphin and kappa receptors and the special relation of a plant-based agonist of the kappa receptor salvinorin A. In addition to acting as a kappa opioid agonist, we conclude that salvinorin A has a complex pharmacologic profile, with potential additional mechanisms of action. Its unique neurophysiological effects make Salvinorina A an ideal candidate for MDD treatment research.

Kappa-Opioid Antagonists for Psychiatric Disorders: From Bench to Clinical Trials

Kappa-opioid receptor (KOR) antagonists are currently being considered for the treatment of a variety of neuropsychiatric conditions, including depressive, anxiety, and substance abuse disorders. A general ability to mitigate the effects of stress, which can trigger or exacerbate these conditions, may explain their putative efficacy across such a broad array of conditions. The discovery of their potentially therapeutic effects evolved from preclinical research designed to characterize the molecular mechanisms by which experience causes neuroadaptations in the nucleus accumbens (NAc), a key element of brain reward circuitry. This research established that exposure to drugs of abuse or stress increases the activity of the transcription factor CREB (cAMP response element binding protein) in the NAc, which leads to elevated expression of the opioid peptide dynorphin that in turn causes core signs of depressive- and anxiety-related disorders. Disruption of KORs-the endogenous receptors for dynorphin-produces antidepressant- and anxiolytic-like actions in screening procedures that identify standard drugs of these classes, and reduces stress effects in tests used to study addiction and stress-related disorders. Although interest in this target is high, prototypical KOR antagonists have extraordinarily persistent pharmacodynamic effects that complicate clinical trials. The development of shorter acting KOR antagonists together with more rapid designs for clinical trials may soon provide insight on whether these drugs are efficacious as would be predicted by preclinical work. If successful, KOR antagonists would represent a unique example in psychiatry where the therapeutic mechanism of a drug class is understood before it is shown to be efficacious in humans.

Effects of dopamine agents on a schedule-induced polydipsia procedure in the spontaneously hypertensive rat and in Wistar control rats

The spontaneously hypertensive rat (SHR) has been proposed as an animal model for attention deficit hyperactivity disorder (ADHD), and typically develops excessive patterns of response under most behavioural protocols. Schedule-induced polydipsia (SIP) is the excessive water consumption that occurs as a schedule effect when food is intermittently delivered and animals are partially food- but not water-deprived. SIP has been used as a model of excessive behaviour, and considerable evidence has involved the dopaminergic system in its development and maintenance. The aim of this study was to evaluate the effects of the most common psychostimulants used in ADHD treatment on SIP, comparing their effects in SHRs with rats from control populations. SHR, Wistar Kyoto (WKY) and Wistar rats were submitted to a multiple fixed time (FT) food schedule with two components: 30 s and 90 s. The acute effects of different dopaminergic compounds were evaluated after 40 sessions of SIP acquisition. All animals showed higher adjunctive drinking under FT 30 s than FT 90 s, and SHRs displayed higher asymptotic SIP levels in FT 90 s compared to WKY and Wistar rats. SHRs were less sensitive to dopaminergic agents than control rats in terms of affecting rates of adjunctive drinking. These differences point to an altered dopaminergic system in the SHR and provide new insights into the neurobiological basis of ADHD pharmacological treatments.

The anxiolytic- and antidepressant-like effects of ATPM-ET, a novel κ agonist and μ partial agonist, in mice

RATIONALE

Opioid receptors are implicated in the regulation of motivation and emotion. However, animal studies show that activation of κ opioid receptor produces contrasting mood-altering effects in models of anxiety-like and depressive-like behaviors, and consequently, the role of κ receptor in mood control remains unsettled. The effect of κ/μ opioid combination in emotion regulation was unexplored.

OBJECTIVES

The aim of the study was to investigate the effects of (-)-3-N-ethylaminothiazolo [5,4-b]-N-cyclopropylmethylmorphinan hydrochloride (ATPM-ET), a novel κ agonist and μ partial agonist, in regulating emotional responses.

METHODS

The emotional responses of ATPM-ET were detected in the elevated plus maze (EPM), open field test (OFT), forced swim test (FST), and tail suspension test (TST). Selective κ antagonist nor-binaltorphimine (nor-BNI) and μ antagonist β-funaltrexamine (β-FNA) were applied to determine the type of receptor involved. The conditioned place aversion model was used to evaluate the effects on aversive emotion.

RESULTS

In the EPM and OFT, ATPM-ET (1 and 2 mg/kg, s.c.) significantly increased the time spent in the open arm and in the central area, respectively. In the FST and TST, ATPM-ET (0.5 and 1 mg/kg, s.c.) significantly reduced the duration of immobility. These effects were prevented by nor-BNI (10 mg/kg, i.p., -24 h), but not by β-FNA (10 and20 mg/kg, i.p., -24 h) pretreatment. At the dose of 2 mg/kg, ATPM-ET did not induce conditioned place aversion.

CONCLUSIONS

ATPM-ET, at doses from 0.5 to 2 mg/kg, produced anxiolytic- and antidepressant-like effects without inducing aversive emotion. These effects were more closely mediated by activation of κ receptor than μ receptor.

κ Opioid receptor activation in different brain regions differentially modulates anxiety-related behaviors in mice

κ Opioid receptor system is widely implicated in the regulation of emotion. However, the findings about the role on anxiety in rodents are highly controversial, since both anxiogenic- and anxiolytic-like effects have been reported with κ opioid receptor activation. The mechanism and the underlying neuroanatomical substrates are unexplored. In the present study, we first investigated the effects of κ agonist U50,488H on anxiety-related behaviors over a wide range of doses, and we found that U50,488H produced dual effects in anxiety, with low dose being anxiogenic and high dose being anxiolytic. To assess the potential neuroanatomical substrates, we used phosphorylation of extracellular signal-related kinase1/2 (pERK1/2) to map the underlying neural circuits. We found that the anxiogenic effect of U50,488H was paralleled by an increase of pERK1/2 in the nucleus accumbens, whereas the anxiolytic effect was paralleled by an increase of pERK1/2 in the lateral septal nucleus. We then examined the behavioral consequences with locally microinjection of U50,488H, and we found that microinjection of U50,488H into the nucleus accumbens exerted anxiogenic-like effects, whereas microinjection of U50,488H into the lateral septal nucleus. Both effects can be abolished by κ antagonist nor-BNI pretreatment. To the best of our knowledge, the present work firstly provides the neuroanatomical sites that mediating the dual anxiogenic- and anxiolytic-like effects of U50,488H in mice. This study may help to explain current controversial role of κ receptor activation in anxiety-related behaviors in rodents, and may open new perspectives in the areas of anxiety disorders and κ receptor function.

Naltrexone alters the processing of social and emotional stimuli in healthy adults

Endogenous opioids have complex social effects that may depend on specific receptor actions and vary depending on the "stage" of social behavior (e.g., seeking vs. responding to social stimuli). We tested the effects of a nonspecific opioid antagonist, naltrexone (NTX), on social processing in humans. NTX is used to treat alcohol and opiate dependence, and may affect both mu and kappa-opioid systems. We assessed attention ("seeking"), and subjective and psychophysiological responses ("responding") to positive and negative social stimuli. Based on literature suggesting mu-opioid blockade impairs positive social responses, we hypothesized that NTX would decrease responses to positive social stimuli. We also tested responses to negative stimuli, which might be either increased by NTX's mu-opioid effects or decreased by its kappa-opioid effects. Thirty-four healthy volunteers received placebo, 25 mg, or 50 mg NTX across three sessions under double-blind conditions. At each session, participants completed measures of attention, identification, and emotional responses for emotional faces and scenes. NTX increased attention to emotional expressions, slowed identification of sadness and fear, and decreased ratings of arousal for social and nonsocial emotional scenes. These findings are more consistent with anxiolytic kappa-antagonist than mu-blocking effects, suggesting effects on kappa receptors may contribute to the clinical effects of NTX.

The role of the dynorphin/κ opioid receptor system in anxiety

Anxiety disorders are the most common and prevalent forms of psychiatric disease, although the biological basis of anxiety is not well understood. The dynorphin/κ opioid receptor system is widely distributed in the central nervous system and has been shown to play a critical role in modulating mood and emotional behaviors. In the present review, we summarize current literature relating to the role played by the dynorphin/κ opioid receptor system in anxiety and κ opioid receptor antagonists as potential therapeutic agents for the treatment of anxiety disorders.

Stress and Drug Dependence Differentially Modulate Norepinephrine Signaling in Animals with Varied HPA Axis Function

Previous work has demonstrated the importance of genetic factors and stress-sensitive circuits in the development of affective disorders. Anxiety and numerous psychological disorders are comorbid with substance abuse, and noradrenergic signaling in the bed nucleus of the stria terminalis (BNST) is thought to be a source of this convergence. Here, we examined the effects of different stressors on behavior and norepinephrine dynamics in the BNST of rat strains known to differ in their HPA-axis function. We compared the effects of acute morphine dependence and social isolation in non-anxious Sprague Dawley (SD) rats, and a depression model, Wistar-Kyoto (WKY) rats. We found a shared phenotype in drug-dependent and singly housed SD rats, characterized by slowed norepinephrine clearance, decreased autoreceptor function, and elevated anxiety. WKY rats exhibited changes in anxiety and autoreceptor function only following morphine dependence. To ascertain the influence of LC inhibition on this plasticity, we administered the LC-terminal-selective toxin DSP-4 to SD and WKY rats. DSP-4-treated SD rats demonstrated a dependence-like phenotype, whereas WKY rats were unchanged. Overall, our findings suggest that individuals with varying stress susceptibilities have different noradrenergic signaling changes in response to stress. These changes may establish conditions that favor stress-induced reinstatement and increase the risk for addiction.

Repeated stress exposure causes strain-dependent shifts in the behavioral economics of cocaine in rats

Cocaine-experienced Wistar and Wistar Kyoto (WKY) rats received four daily repeated forced swim stress sessions (R-FSS), each of which preceded 4-hour cocaine self-administration sessions. Twenty-four hours after the last swim stress, cocaine valuation was assessed during a single-session threshold procedure. Prior exposure to R-FSS significantly altered cocaine responding in Wistar, but not WKY, rats. Behavioral economic analysis of responding revealed that the Wistar rats that had received R-FSS exhibited an increase in the maximum price that they were willing to pay for cocaine (Pmax ). Pre-treatment with the long-lasting kappa opioid receptor (KOR) antagonist norbinaltorphimine prevented the stress-induced increase in Pmax . Thus, R-FSS exposure had strain-dependent effects on cocaine responding during the threshold procedure, and the stress effects on cocaine valuation exhibited by Wistar, but not WKY, required intact KOR signaling.

Effects of buprenorphine on behavioral tests for antidepressant and anxiolytic drugs in mice

RATIONALE

Buprenorphine (BPN) has been shown to rapidly improve mood in treatment-resistant depressed patients in small clinical studies. However, BPN's effects in preclinical tests for mood and antidepressant efficacy are largely unexplored.

OBJECTIVE

The current study examined the effects of BPN in the forced swim test (FST) and novelty-induced hypophagia (NIH) test as measures of antidepressant and anxiolytic-like effects in C57BL/6 J mice. Microdialysis was used to measure whether BPN engaged kappa-opioid receptor (KORs) in the nucleus accumbens shell (NAcSh) at a behaviorally active dose (0.25 mg/kg).

METHODS

BPN was tested in the FST at both 30 min and 24 h post-administration. Also measured in the FST at 24 h post-administration were the KOR antagonist norbinaltorphimine (nor-BNI), the MOR agonist morphine and the reference antidepressant desipramine. The anxiolytic effects of BPN were examined in the NIH test 24 h after treatment. The effects of acute injection of BPN and the KOR agonist U50,488 were measured on extracellular dopamine (DA) levels in the NAcSh.

RESULTS

BPN produced significant reductions in FST immobility without changing locomotor activity and reduced approach latencies in the novel environment of the NIH test when tested 24 h after treatment. Repeated daily BPN injections for 6 days did not produce tolerance to these behavioral effects. nor-BNI produced a similar antidepressant-like response in the FST 24 h post-injection but morphine and desipramine were ineffective. BPN (0.25 mg/kg) did not alter DA levels when given alone but prevented the KOR agonist U50,488 from reducing DA levels.

CONCLUSIONS

Acute and subchronic treatment with BPN produced antidepressant and anxiolytic-like responses in mice at doses that engage KORs. These studies support the clinical evidence that BPN may be a novel rapid-acting antidepressant medication and provides rodent models for investigating associated neurochemical mechanisms.

Determining pharmacological selectivity of the kappa opioid receptor antagonist LY2456302 using pupillometry as a translational biomarker in rat and human

BACKGROUND

Selective kappa opioid receptor antagonism is a promising experimental strategy for the treatment of depression. The kappa opioid receptor antagonist, LY2456302, exhibits ~30-fold higher affinity for kappa opioid receptors over mu opioid receptors, which is the next closest identified pharmacology.

METHODS

Here, we determined kappa opioid receptor pharmacological selectivity of LY2456302 by assessing mu opioid receptor antagonism using translational pupillometry in rats and humans.

RESULTS

In rats, morphine-induced mydriasis was completely blocked by the nonselective opioid receptor antagonist naloxone (3mg/kg, which produced 90% mu opioid receptor occupancy), while 100 and 300 mg/kg LY2456302 (which produced 56% and 87% mu opioid receptor occupancy, respectively) only partially blocked morphine-induced mydriasis. In humans, fentanyl-induced miosis was completely blocked by 50mg naltrexone, and LY2456302 dose-dependently blocked miosis at 25 and 60 mg (minimal-to-no blockade at 4-10mg).

CONCLUSIONS

We demonstrate, for the first time, the use of translational pupillometry in the context of receptor occupancy to identify a clinical dose of LY2456302 achieving maximal kappa opioid receptor occupancy without evidence of significant mu receptor antagonism.

Stress and opioids: role of opioids in modulating stress-related behavior and effect of stress on morphine conditioned place preference

Research studies have defined the important role of endogenous opioids in modulating stress-associated behavior. The release of β-endorphins in the amygdala in response to stress helps to cope with a stressor by inhibiting the over-activation of HPA axis. Administration of mu opioid agonists reduces the risk of developing post-traumatic stress disorder (PTSD) following a traumatic event by inhibiting fear-related memory consolidation. Similarly, the release of endogenous enkephalin and nociceptin in the basolateral amygdala and the nucleus accumbens tends to produce the anti-stress effects. An increase in dynorphin levels during prolonged exposure to stress may produce learned helplessness, dysphoria and depression. Stress also influences morphine-induced conditioned place preference (CPP) depending upon the intensity and duration of the stressor. Acute stress inhibits morphine CPP, while chronic stress potentiates CPP. The development of dysphoria due to increased dynorphin levels may contribute to chronic stress-induced potentiation of morphine CPP. The activation of ERK/cyclic AMP responsive element-binding (CREB) signaling in the mesocorticolimbic area, glucocorticoid receptors in the basolateral amygdala, and norepinephrine and galanin system in the nucleus accumbens may decrease the acute stress-induced inhibition of morphine CPP. The increase in dopamine levels in the nucleus accumbens and augmentation of GABAergic transmission in the median prefrontal cortex may contribute in potentiating morphine CPP. Stress exposure reinstates the extinct morphine CPP by activating the orexin receptors in the nucleus accumbens, decreasing the oxytocin levels in the lateral septum and amygdala, and altering the GABAergic transmission (activation of GABAA and inactivation of GABAB receptors). The present review describes these varied interactions between opioids and stress along with the possible mechanism.

Buprenorphine, methadone, and morphine treatment during pregnancy: behavioral effects on the offspring in rats

Methadone and buprenorphine are widely used for treating people with opioid dependence, including pregnant women. Prenatal exposure to opioids has devastating effects on the development of human fetuses and may induce long-term physical and neurobehavioral changes during postnatal maturation. This study aimed at comparing the behavioral outcomes of young rats prenatally exposed to buprenorphine, methadone, and morphine. Pregnant Sprague-Dawley rats were administered saline, morphine, methadone, and buprenorphine during embryonic days 3-20. The cognitive function, social interaction, anxiety-like behaviors, and locomotor activity of offsprings were examined by novel object recognition test, social interaction test, light-dark transition test, elevated plus-maze, and open-field test between 6 weeks and 10 weeks of age. Prenatal exposure to methadone and buprenorphine did not affect locomotor activity, but significantly impaired novel object recognition and social interaction in both male and female offsprings in the same manner as morphine. Although prenatal exposure to methadone or buprenorphine increased anxiety-like behaviors in the light-dark transition in both male and female offsprings, the effects were less pronounced as compared to that of morphine. Methadone affected elevated plus-maze in both sex, but buprenorphine only affected the female offsprings. These findings suggest that buprenorphine and methadone maintenance therapy for pregnant women, like morphine, produced detrimental effects on cognitive function and social behaviors, whereas the offsprings of such women might have a lower risk of developing anxiety disorders.

Association of in vivo κ-opioid receptor availability and the transdiagnostic dimensional expression of trauma-related psychopathology

IMPORTANCE

Exposure to trauma increases the risk for developing threat (ie, fear) symptoms, such as reexperiencing and hyperarousal symptoms, and loss (ie, dysphoria) symptoms, such as emotional numbing and depressive symptoms. While preclinical data have implicated the activated dynorphin/κ-opioid receptor (KOR) system in relation to these symptoms, the role of the KOR system in mediating these phenotypes in humans is unknown. Elucidation of molecular targets implicated in threat and loss symptoms is important because it can help inform the development of novel, mechanism-based treatments for trauma-related psychopathology.

OBJECTIVE

To use the newly developed [11C]LY2795050 radiotracer and high-resolution positron emission tomography to evaluate the relation between in vivo KOR availability in an amygdala-anterior cingulate cortex-ventral striatal neural circuit and the severity of threat and loss symptoms. We additionally evaluated the role of 24-hour urinary cortisol levels in mediating this association.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional positron emission tomography study under resting conditions was conducted at an academic medical center. Thirty-five individuals representing a broad transdiagnostic and dimensional spectrum of trauma-related psychopathology, ranging from nontrauma-exposed psychiatrically healthy adults to trauma-exposed adults with severe trauma-related psychopathology (ie, posttraumatic stress disorder, major depressive disorder, and/or generalized anxiety disorder).

MAIN OUTCOMES AND MEASURES

[11C]LY2795050 volume of distribution values in amygdala-anterior cingulate cortex-ventral striatal neural circuit; composite measures of threat (ie, reexperiencing, avoidance, and hyperarousal symptoms) and loss (ie, emotional numbing, major depressive disorder, and generalized anxiety disorder symptoms) symptoms as assessed using the Clinician-Administered PTSD Scale, Hamilton Depression Rating Scale, and Hamilton Rating Scale for Anxiety; and 24-hour urinary cortisol levels.

RESULTS

[11C]LY2795050 volume of distribution values in an amygdala-anterior cingulate cortex-ventral striatal neural circuit were negatively associated with severity of loss (r = -0.39; 95% CI, -0.08 to -0.66), but not threat (r = -0.03; 95% CI, -0.30 to 0.27), symptoms; this association was most pronounced for dysphoria symptoms (r = -0.45; 95% CI, -0.10 to -0.70). Path analysis revealed that lower [11C]LY2795050 volume of distribution values in this circuit was directly associated with greater severity of loss symptoms and indirectly mediated by 24-hour urinary cortisol levels.

CONCLUSIONS AND RELEVANCE

Results of this study suggest that KOR availability in an amygdala-anterior cingulate cortex-ventral striatal neural circuit mediates the phenotypic expression of trauma-related loss (ie, dysphoria) symptoms. They further suggest that an activated corticotropin-releasing factor/hypothalamic-pituitary-adrenal axis system, as assessed by 24-hour urinary cortisol levels, may indirectly mediate this association. These results may help inform the development of more targeted, mechanism-based transdiagnostic treatments for loss (ie, dysphoric) symptoms.

Antidepressant-like effects of buprenorphine in rats are strain dependent

The prevalence of major depressive disorder and the limited efficacy of conventional drug treatments provide significant impetus to develop novel and more rapidly acting antidepressants for individuals with treatment resistant forms of depression. The primary goal of these studies was to ascertain whether buprenorphine (BPN), a medically available drug with mixed effects at opioid receptors, was effective in behavioral tests using the Wistar Kyoto (WKY) rat strain, a rodent model of exaggerated depressive and anxiety behaviors that demonstrates resistance to certain antidepressants. As WKY rats are maintained by different sources, we assessed the behavioral effects of BPN using the modified rat forced swim test (FST) and the emergence test in WKY rat colonies obtained from different vendors. BPN dose-dependently reduced immobility and increased swimming behavior in the FST and reduced emergence latencies in two WKY lines (Charles River (WKY/NCrl) and Harlan laboratories (WKY/NHsd)) that also showed high baseline immobility in the FST. WKY rats from Taconic (WKY/NTac) did not show high baseline immobility in the FST or anxiety as had been previously reported, suggesting a drift in the phenotype of rats from this supplier. Furthermore, BPN did not reduce immobility in the FST or reduce latencies in the emergence test in WKY rats from Taconic. BPN also failed to produce antidepressant-like effects in Wistar and Sprague-Dawley rats. These results indicate a striking strain-selectivity for the effects of BPN, producing antidepressant and anxiolytic-like responses in WKY/NCrl and WKY/NHsd lines but not in the normosensitive control Wistar and Sprague-Dawley strains.

Potentiation of GluN2C/D NMDA receptor subtypes in the amygdala facilitates the retention of fear and extinction learning in mice

NMDA receptors are glutamate receptor ion channels that contribute to synaptic plasticity and are important for many forms of learning and memory. In the amygdala, NMDA receptors are critical for the acquisition, retention, and extinction of classically conditioned fear responses. Although the GluN2B subunit has been implicated in both the acquisition and extinction of conditioned fear, GluN2C-knockout mice show reduced conditioned fear responses. Moreover, D-cycloserine (DCS), which facilitates fear extinction, selectively enhances the activity of GluN2C-containing NMDA receptors. To further define the contribution of GluN2C receptors to fear learning, we infused the GluN2C/GluN2D-selective potentiator CIQ bilaterally into the basolateral amygdala (3, 10, or 30 μg/side) following either fear conditioning or fear extinction training. CIQ both increased the expression of conditioned fear 24 h later and enhanced the extinction of the previously conditioned fear response. These results support a critical role for GluN2C receptors in the amygdala in the consolidation of learned fear responses and suggest that increased activity of GluN2C receptors may underlie the therapeutic actions of DCS.

Role of kappa-opioid receptors in stress and anxiety-related behavior

RATIONALE

Accumulating evidence indicates that brain kappa-opioid receptors (KORs) and dynorphin, the endogenous ligand that binds at these receptors, are involved in regulating states of motivation and emotion. These findings have stimulated interest in the development of KOR-targeted ligands as therapeutic agents. As one example, it has been suggested that KOR antagonists might have a wide range of indications, including the treatment of depressive, anxiety, and addictive disorders, as well as conditions characterized by co-morbidity of these disorders (e.g., post-traumatic stress disorder) A general effect of reducing the impact of stress may explain how KOR antagonists can have efficacy in such a variety of animal models that would appear to represent different disease states.

OBJECTIVE

Here, we review evidence that disruption of KOR function attenuates prominent effects of stress. We will describe behavioral and molecular endpoints including those from studies that characterize the effects of KOR antagonists and KOR ablation on the effects of stress itself, as well as on the effects of exogenously delivered corticotropin-releasing factor, a brain peptide that mediates key effects of stress.

CONCLUSION

Collectively, available data suggest that KOR disruption produces anti-stress effects and under some conditions can prevent the development of stress-induced adaptations. As such, KOR antagonists may have unique potential as therapeutic agents for the treatment and even prevention of stress-related psychiatric illness, a therapeutic niche that is currently unfilled.

Ablation of kappa-opioid receptors from brain dopamine neurons has anxiolytic-like effects and enhances cocaine-induced plasticity

Brain kappa-opioid receptors (KORs) are implicated in states of motivation and emotion. Activation of KORs negatively regulates mesolimbic dopamine (DA) neurons, and KOR agonists produce depressive-like behavioral effects. To further evaluate how KOR function affects behavior, we developed mutant mice in which exon 3 of the KOR gene (Oprk1) was flanked with Cre-lox recombination (loxP) sites. By breeding these mice with lines that express Cre-recombinase (Cre) in early embryogenesis (EIIa-Cre) or only in DA neurons (dopamine transporter (DAT)-Cre), we developed constitutive KOR knockouts (KOR(-/-)) and conditional knockouts that lack KORs in DA-containing neurons (DAT-KOR(lox/lox)). Autoradiography demonstrated complete ablation of KOR binding in the KOR(-/-) mutants, and reduced binding in the DAT-KOR(lox/lox) mutants. Quantitative reverse transcription PCR (qPCR) studies confirmed that KOR mRNA is undetectable in the constitutive mutants and reduced in the midbrain DA systems of the conditional mutants. Behavioral characterization demonstrated that these mutant lines do not differ from controls in metrics, including hearing, vision, weight, and locomotor activity. Whereas KOR(-/-) mice appeared normal in the open field and light/dark box tests, DAT-KOR(lox/lox) mice showed reduced anxiety-like behavior, an effect that is broadly consistent with previously reported effects of KOR antagonists. Sensitization to the locomotor-stimulating effects of cocaine appeared normal in KOR(-/-) mutants, but was exaggerated in DAT-KOR(lox/lox) mutants. Increased sensitivity to cocaine in the DAT-KOR(lox/lox) mutants is consistent with a role for KORs in negative regulation of DA function, whereas the lack of differences in the KOR(-/-) mutants suggests compensatory adaptations after constitutive receptor ablation. These mouse lines may be useful in future studies of KOR function.

Development of κ opioid receptor antagonists

κ opioid receptors (KORs) belong to the G-protein-coupled class of receptors (GPCRs). They are activated by the endogenous opioid peptide dynorphin (DYN) and expressed at particularly high levels within brain areas implicated in modulation of motivation, emotion, and cognitive function. Chronic activation of KORs in animal models has maladaptive effects including increases in behaviors that reflect depression, the propensity to engage in drug-seeking behavior, and drug craving. The fact that KOR activation has such a profound influence on behaviors often triggered by stress has led to interest in selective KOR antagonists as potential therapeutic agents. This Perspective provides a description of preclinical research conducted in the development of several different classes of selective KOR antagonists, a summary of the clinical studies conducted thus far, and recommendations for the type of work needed in the future to determine if these agents would be useful as pharmacotherapies for neuropsychiatric illness.

Effects of a post-shock injection of the kappa opioid receptor antagonist norbinaltorphimine (norBNI) on fear and anxiety in rats

Exposure of rats to footshocks leads to an enduring behavioral state involving generalized fear responses and avoidance. Recent evidence suggests that the expression of negative emotional behaviors produced by a stressor is in part mediated by dynorphin and its main receptor, the kappa opioid receptor (KOR). The purpose of this study was to determine if a subcutaneous injection of the long-acting KOR antagonist norbinaltorphimine (norBNI; 15.0 and 30.0 mg/kg) given 2 days after an acute exposure of rats to footshooks (5×2 s episodes of 1.5 mA delivered over 5 min) attenuates the expression of lasting fear and anxiety. We report that exposure of rats to acute footshock produced long-lasting (>4 weeks) fear (freezing) and anxiety (avoidance of an open area in the defensive withdrawal test). The 30 mg dose of norBNI attenuated the fear expressed when shock rats were placed in the shock context at Day 9 but not Day 27 post-shock. The same dose of norBNI had no effect on the expression of generalized fear produced when shock rats were placed in a novel chamber at Days 8 and 24. In contrast, the 30 mg dose of norBNI produced consistent anxiolytic effects in shock and nonshock rats. First, the 30 mg dose was found to decrease the latency to enter the open field in the defensive withdrawal test done 30 days after the shock exposure. Second, the same high dose also had anxiolytic effects in both nonshock and shock rats as evidence by a decrease in the mean time spent in the withdrawal box. The present study shows that systemic injection of the KOR antagonist norBNI had mixed effect on fear. In contrast, norBNI had an anxiolytic effect which included the attenuation of the enhanced avoidance of a novel area produced by a prior shock experience.

Sex differences in U50,488H-induced phosphorylation of p44/42 mitogen-activated protein kinase in the guinea pig brain

Recently there has been a widespread interest in the development of kappa opioid receptor (KOPR) ligands for treatment of pain, depression and anxiety, and prevention of stress-induced drug relapse. However, most of these preclinical studies have been conducted using male experimental animals. In the present study we examined if sex differences exist in neural activity induced by the KOPR agonist trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl) benzeneacetamide methanesulfonate (U50,488H). Here, we used immunohistochemistry to detect activation (phosphorylation) of p44/42 mitogen-activated protein kinase (MAPK) as an indicator of neural activity. Following habituation to injection for 3 days, adult guinea pigs received a single injection of U50,488H (5mg/kg, s.c.) and perfused 30-45 min later. U50,488H-induced an increase in the number of cells immuno-positive for phosphorylated p44/42 MAPK in subregions of the amygdala, thalamus, paraventricular nucleus of the hypothalamus, periaqueductal gray, and dorsal raphe nuclei. In contrast, U50,488H-induced a decrease in immuno-positive cells in the ventrolateral and lateral orbital cortex. Pretreatment with the KOPR antagonist norbinaltorphimine (10mg/kg, i.p.) 18 h prior to U50,488H significantly reversed the effects of U50,488H in most regions. In addition, we observed a notable sex difference in the basolateral amygdala; in males, U50,488H induced an increase in immuno-positive cell numbers but a decrease in females. However, across other brain regions males were generally more sensitive to U50,488H-induced alterations than females. These results suggest the need to include female subjects in studies examining emotional responses to KOPR ligands.

Stress-induced epigenetic regulation of κ-opioid receptor gene involves transcription factor c-Myc

Exposure to stress is associated with adverse emotional and behavioral responses. Whereas the κ-opioid receptor (KOR) system is known to mediate some of the effects, it is unclear whether and how stress affects epigenetic regulation of this gene. Because the KOR gene can use two promoters (Pr1 and Pr2) and two polyadenylation signals (PA1 and PA2), it is also interesting whether and how these distinct regulatory mechanisms are differentially modulated by stress. The current study examined the effects of stress on these different regulatory mechanisms of the KOR gene. Results showed that stress selectively increased the expression of KOR mRNA isoforms controlled by Pr1 and terminated at PA1 in specific brain areas including the medial-prefrontal cortex, hippocampus, brainstem, and sensorimotor cortex, but not in the amygdala or hypothalamus. These effects correlated with altered epigenetic state of KOR Pr1 chromatin, as well as elevation and increased recruitment of the principal transcription factor c-Myc, which could activate Pr1. Stress-induced modulation of Pr1 was further validated using glutamate-sensitive murine hippocampal cell line, HT22. The results revealed a common molecular mechanism underlying the effect of stress on selected chromatin regions of this gene at the cellular level and in the context of whole animal and identified a critical role for c-Myc in stress-triggered epigenetic regulation of the KOR gene locus. This study sheds light on the mechanisms of stress-induced epigenetic regulation that targets specific chromatin segments and suggests certain KOR transcripts and its principal transcription factor c-Myc as potential targets for brain-area-specific intervention.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

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).

Kappa opioid receptor signaling in the basolateral amygdala regulates conditioned fear and anxiety in rats

BACKGROUND

The kappa opioid receptor (KOR) system contributes to the prodepressive and aversive consequences of stress and is implicated in the facilitation of conditioned fear and anxiety in rodents. Here, we sought to identify neural circuits that mediate KOR system effects on fear and anxiety in rats.

METHODS

We assessed whether fear conditioning induces plasticity in KOR or dynorphin (the endogenous KOR ligand) messenger RNA (mRNA) expression in the basolateral (BLA) and central (CeA) nuclei of the amygdala, hippocampus, or striatum. We then assessed whether microinfusions of the KOR antagonist JDTic (0-10 μg/side) into the BLA or CeA affect the expression of conditioned fear or anxiety. Finally, we examined whether fear extinction induces plasticity in KOR mRNA expression that relates to the quality of fear extinction.

RESULTS

Fear conditioning upregulated KOR mRNA in the BLA by 65% and downregulated it in the striatum by 22%, without affecting KOR levels in the CeA or hippocampus, or dynorphin levels in any region. KOR antagonism in either the BLA or CeA decreased conditioned fear in the fear-potentiated startle paradigm, whereas KOR antagonism in the BLA, but not the CeA, produced anxiolytic-like effects in the elevated plus maze. Effective fear extinction was associated with a 67% reduction in KOR mRNA in the BLA.

CONCLUSIONS

These findings suggest that fear conditioning and extinction dynamically regulate KOR expression in the BLA and provide evidence that the BLA and CeA are important neural substrates mediating the anxiolytic-like effects of KOR antagonists in models of fear and anxiety.

Sex difference in κ-opioid receptor (KOPR)-mediated behaviors, brain region KOPR level and KOPR-mediated guanosine 5'-O-(3-[35S]thiotriphosphate) binding in the guinea pig

We examined whether sex differences in κ-opioid receptor (KOPR) pharmacology exist in guinea pigs, which are more similar to humans in the expression level and distribution of KOPR in the brain than rats and mice. The KOPR agonist trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)benzeneacetamide methanesulfonate (U50,488H) produced a dose-dependent increase in abnormal postures and immobility with more effects in males than females. Males also showed more U50,488H-induced antinociception in the paw pressure test than females. Pretreatment with the KOPR antagonist norbinaltorphimine blocked U50,488H-induced abnormal body postures and antinociception. In contrast, inhibition of cocaine-induced hyperambulation by U50,488H was more effective in females than males. Thus, sex differences in the effects of U50,488H are endpoint-dependent. We then examined whether sex differences in KOPR levels and KOPR-mediated G protein activation in brain regions may contribute to the observed differences using quantitative in vitro autoradiography of [(3)H](5a,7a,8b)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)1-oxaspiro(4,5)dec-8-yl)benzeacetamide ([(3)H]U69,593) binding to the KOPR and U50,488H-stimulated guanosine 5'-O-(3-[(35)S]thiotriphosphate ([(35)S]GTPγS) binding. Compared with females, males exhibited more [(3)H]U69,593 binding in the deep layers of somatosensory and insular cortices, claustrum, endopiriform nucleus, periaqueductal gray, and substantial nigra. Concomitantly, U50,488H-stimulated [(35)S]GTPγS binding was greater in males than females in the superficial and deep layers of somatosensory and insular cortices, caudate putamen, claustrum, medial geniculate nucleus, and cerebellum. In contrast, compared with males, females showed more U50,488H-stimulated [(35)S]GTPγS binding in the dentate gyrus and a trend of higher [(35)S]GTPγS binding in the hypothalamus. These data demonstrate that males and females differ in KOPR expression and KOPR-mediated G protein activation in distinct brain regions, which may contribute to the observed sex differences in KOPR-mediated pharmacology.

Extending pharmacological spectrum of opioids beyond analgesia: multifunctional aspects in different pathophysiological states

Opioids are well known to exert potent central analgesic actions. In recent years, the numerous studies have unfolded the critical role of opioids in the pathophysiology of various diseases as well as in biological phenomenon of therapeutic interest. The endogenous ligands of opioid receptors are derived from three independent genes and their appropriate processing yields the major representative opioid peptides beta-endorphin, met-enkephalin, leu-enkephalin and dynorphin, respectively. These peptides and their derivatives exhibit different affinity and selectivity for the mu-, delta- and kappa-receptors located on the central and the peripheral neurons, neuroendocrine, immune, and mucosal cells and on many other organ systems. The present review article highlights the role of these peptides in central nervous system disorders such as depression, anxiety, epilepsy, and stress; gastrointestinal disorders such as diarrhea, postoperative ileus, ulceration, and irritable bowel syndrome; immune system and related inflammatory disorders such as osteoarthritis and rheumatoid arthritis; and others including respiratory, alcoholism and obesity/binge eating. Furthermore, the key role of opioids in different forms of pre- and post-conditioning including ischemic and pharmacological along with in remote preconditioning has also been described.

Sex differences in kappa opioid pharmacology

In recent years it has become apparent that sex is a major factor involved in modulating the pharmacological effects of exogenous opioids. The kappa opioid receptor (KOPR) system is a potential therapeutic target for pain, mood disorders and addiction. In humans mixed KOPR/MOPR ligands have been found to produce greater analgesia in women than men. In contrast, in animals, selective KOPR agonists have been found to produce greater antinociceptive effects in males than females. Collectively, the studies indicate that the direction and magnitude of sex differences of KOPR-mediated antinociception/analgesia are dependent on species, strain, ligand and pain model examined. Of interest, and less studied, is whether sex differences in other KOPR-mediated effects exist. In the studies conducted thus far, greater effects of KOPR agonists in males have been found in neuroprotection against stroke and suppression of food intake behavior. On the other hand, greater effects of KOPR agonists were found in females in mediation of prolactin release. In modulation of drugs of abuse, sex differences in KOPR effects were observed but appear to be dependent on the drug examined. The mechanism(s) underlying sex differences in KOPR-mediated effects may be mediated by sex chromosomes, gonadal hormonal influence on organization (circuitry) and/or acute hormonal influence on KOPR expression, distribution and localization. In light of the diverse pharmacology of KOPR we discuss the need for future studies characterizing the sexual dimorphism of KOPR neural circuitry and in examining other behaviors and processes that are modulated by the KOPR.