Pharmacological activation of kappa opioid receptors: aversive effects in adolescent and adult male rats

Dynorphinergic lateral hypothalamus to posterior ventral tegmental area pathway matures after adolescence in male rats

The highly plastic nature of the adolescent brain is well-known, and is thought to contribute to the unique susceptibility of adolescents to drugs of abuse. However, much investigation of adolescent plasticity has been focused on synaptic plasticity, as synapses are strengthened and pruned. Here, we show that dynorphin+ neurons in the lateral hypothalamus of adolescent male rats do not respond to low doses of intravenous combined nicotine + ethanol, while male adult lateral hypothalamus dynorphin+ neurons do. We also provide evidence that the dynorphinergic projection from the lateral hypothalamus to the posterior ventral tegmental area is not present in adolescent males, suggesting that axons are still extending during this time. Together, these results suggest a mechanism for the increased susceptibility of adolescent male rats to drug reward.

Opioid receptor signaling throughout ontogeny: Shaping neural and behavioral trajectories

Due to the recent and ongoing opioid crisis in the United States, exposure to opioid drugs in utero is becoming more common, including during medication-assisted therapy used to treat opioid use disorder. As such, careful consideration of opioidergic signaling in utero and beyond, as well as alterations to this signaling via introduction of exogenous opioids, is warranted. This review explores the ontogeny and function of the Mu, Kappa and Delta opioid receptor systems throughout the lifespan, highlighting their importance in guiding neurobehavioral development. We argue for a paradigm shift in conceptualization of opioids as not only contributors within their own system, but also vital regulators of a multitude of downstream neurodevelopmental processes.

Aticaprant: (a κ-opioid receptor antagonist) for major depressive disorder

INTRODUCTION

Major depression is a common, disabling mental health condition associated with the highest disease burden for any neuropsychiatric disorder worldwide, according to the WHO. Due to the imperfect efficacy and tolerability profiles of existing treatments, investigational compounds in novel treatment classes are needed. Opioid-receptor antagonists are a potential new class of treatments currently under investigation.

AREAS COVERED

Major depressive disorder is first overviewed. Existing treatments, both their mechanisms of action and their place within the antidepressant space, are discussed herein. Then, the profile of Aticaprant and the wider context of kappa-opioid antagonism for depression are discussed in focus.

EXPERT OPINION

Early evidence indicates that Aticaprant may possess desirable pharmacodynamic and pharmacokinetic properties. A lack of convincing efficacy data at the time of writing precludes any definitive statement on its potential as an antidepressant.

Low doses of acetaminophen produce antidepressive-like effects through the opioid system in mice

Acetaminophen (paracetamol) is one of the most popular analgesics for the management of fever and pain but few reports have investigated its antidepressant-like effect. Moreover, the role of the opioidergic pathway has been indicated in depression pathophysiology. This study aimed to examine the involvement of the opioid receptors in the antidepressant-like effect of acetaminophen after acute and sub-chronic administration using mice forced swimming test (FST). Our finding showed that administration of acetaminophen (50 and 100 mg/kg, i.p.) 30 min before the FST produced an antidepressant effect which was reduced by naloxone (1 mg/kg, i.p., a nonselective opioid receptor antagonist). Moreover, we observed that acetaminophen in higher doses (200 and 400 mg/kg) was ineffective. Also, the response of the non-effective dose of acetaminophen (25 mg/kg) was potentiated by the non-effective dose of morphine (0.1 mg/kg) in the FST that was antagonized by naloxone. Also, in contrast to morphine (10 mg/kg), acetaminophen (100 mg/kg, i.p.) induced neither tolerance to the anti-immobility behavior nor withdrawal syndrome after repeated administration. In addition, RT-PCR showed that hippocampal mu- and kappa-opioid receptor mRNA expression increased in mice after repeated administration of acetaminophen; however, morphine therapy for 6 days did not affect kappa-opioid receptor expression. Our findings demonstrated that acetaminophen in lower doses but not high doses revealed an antidepressant-like activity without inducing tolerance and withdrawal syndromes. Moreover, the observed effect of acetaminophen may be via altering the opioid system, particularly hippocampal mu- and kappa-receptors.

Opioid Mechanisms and the Treatment of Depression

Opioid receptors are widely expressed in the brain, and the opioid system has a key role in modulating mood, reward processing and stress responsivity. There is mounting evidence that the endogenous opioid system may be dysregulated in depression and that drug treatments targeting mu, delta and kappa opioid receptors may show antidepressant potential. The mechanisms underlying the therapeutic effects of opioid system engagement are complex and likely multi-factorial. This chapter explores various pathways through which the modulation of the opioid system may influence depression. These include impacts on monoaminergic systems, the regulation of stress and the hypothalamic-pituitary-adrenal axis, the immune system and inflammation, brain-derived neurotrophic factors, neurogenesis and neuroplasticity, social pain and social reward, as well as expectancy and placebo effects. A greater understanding of the diverse mechanisms through which opioid system modulation may improve depressive symptoms could ultimately aid in the development of safe and effective alternative treatments for individuals with difficult-to-treat depression.

Ethanol-induced conditioned taste aversion and associated neural activation in male rats: Impact of age and adolescent intermittent ethanol exposure

Individuals that initiate alcohol use at younger ages and binge drink during adolescence are more susceptible to developing alcohol use disorder. Adolescents are relatively insensitive to the aversive effects of alcohol and tend to consume significantly more alcohol per occasion than adults, an effect that is conserved in rodent models. Adolescent typical insensitivity to the aversive effects of alcohol may promote greater alcohol intake by attenuating internal cues that curb its consumption. Attenuated sensitivity to the aversive effects of alcohol is also retained into adulthood following protracted abstinence from adolescent intermittent ethanol (AIE) exposure. Despite these effects, much remains unknown regarding the neural contributors. In the present study, we used a conditioned taste aversion (CTA) paradigm to investigate neuronal activation in late-developing forebrain structures of male adolescents and adult cFos-LacZ transgenic rats as well as in AIE adults following consumption of 0.9% sodium chloride previously paired with an intraperitoneal injection of 0, 1.5 or 2.5 g/kg of ethanol. Adults that were non-manipulated or received water exposure during adolescence showed CTA to both ethanol doses, whereas adolescents displayed CTA only to the 2.5 g/kg ethanol dose. Adults who experienced AIE did not show CTA. Adults displayed increased neuronal activation indexed via number of β-galactosidase positive (β-gal+) cells in the prefrontal and insular cortex that was absent in adolescents, whereas adolescents but not adults had a reduced number of β-gal+ cells in the central amygdala. Adults also displayed greater cortical-insular functional connectivity than adolescents as well as insular-amygdalar and prefrontal cortex-accumbens core functional connectivity. Like adolescents, adults previously exposed to AIE displayed reduced prefrontal-insular cortex and prefrontal-accumbal core functional connectivity. Taken together, these results suggest that attenuated sensitivity to the aversive effects of ethanol is related to a loss of an insular-prefrontal cortex-accumbens core circuit.

The Opioid System in Depression

Opioid receptors are widely distributed throughout the brain and play an essential role in modulating aspects of human mood, reward, and well-being. Accumulating evidence indicates the endogenous opioid system is dysregulated in depression and that pharmacological modulators of mu, delta, and kappa opioid receptors hold potential for the treatment of depression. Here we review animal and clinical data, highlighting evidence to support: dysregulation of the opioid system in depression, evidence for opioidergic modulation of behavioural processes and brain regions associated with depression, and evidence for opioidergic modulation in antidepressant responses. We evaluate clinical trials that have examined the safety and efficacy of opioidergic agents in depression and consider how the opioid system may be involved in the effects of other treatments, including ketamine, that are currently understood to exert antidepressant effects through non-opioidergic actions. Finally, we explore key neurochemical and molecular mechanisms underlying the potential therapeutic effects of opioid system engagement, that together provides a rationale for further investigation into this relevant target in the treatment of depression.

Morphine exposure alters Fos expression in a sex-, age-, and brain region-specific manner during adolescence

Data in both humans and preclinical animal models clearly indicate drug exposure during adolescence, when the "reward" circuitry of the brain develops, increases the risk of substance use and other mental health disorders later in life. Human data indicate that different neural and behavioral sequelae can be observed in early versus late adolescence. However, most studies with rodent models examine a single adolescent age compared to a mature adult age, and often only in males. Herein, we sought to determine whether the acute response to the opioid morphine would also differ across adolescence, and by sex. By quantifying Fos positive cells, a proxy for neural activity, at different stages during adolescence (pre-, early, mid-, and late adolescence) and in multiple reward regions (prefrontal cortex, nucleus accumbens, caudate/putamen), we determined that the neural response to acute morphine is highly dependent on adolescent age, sex, and brain region. These data suggest that heterogeneity in the consequences of adolescent opioid exposure may be due to age- and sex-specific developmental profiles in individual reward processing regions. In future studies, it will be important to add age within adolescence as an independent variable for a holistic view of healthy or abnormal reward-related neural development.

Adolescent forced swim stress increases social anxiety-like behaviors and alters kappa opioid receptor function in the basolateral amygdala of male rats

Adolescence is a developmental period marked by robust neural alterations and heightened vulnerability to stress, a factor that is highly associated with increased risk for emotional processing deficits, such as anxiety. Stress-induced upregulation of the dynorphin/kappa opioid receptor (DYN/KOP) system is thought to, in part, underlie the negative affect associated with stress. The basolateral amygdala (BLA) is a key structure involved in anxiety, and neuromodulatory systems, such as the DYN/KOP system, can 1) regulate BLA neural activity in an age-dependent manner in stress-naïve animals and 2) underlie stress-induced anxiety in adults. However, the role of the DYN/KOP system in modulating stress-induced anxiety in adolescents is unknown. To test this, we examined the impact of an acute, 2-day forced swim stress (FSS - 10 min each day) on adolescent (~postnatal day (P) 35) and adult Sprague-Dawley rats (~P70), followed by behavioral, molecular and electrophysiological assessment 24 h following FSS. Adolescent males, but not adult males or females of either age, demonstrated social anxiety-like behavioral alterations indexed via significantly reduced social investigation and preference when tested 24 h following FSS. Conversely, adult males exhibited increased social preference. While there were no FSS-induced changes in expression of genes related to the DYN/KOP system in the BLA, these behavioral alterations were associated with alterations in BLA KOP function. Specifically, while GABA transmission in BLA pyramidal neurons from non-stressed adolescent males responded variably (potentiated, suppressed, or was unchanged) to the KOP agonist, U69593, U69593 significantly inhibited BLA GABA transmission in the majority of neurons from stressed adolescent males, consistent with the observed anxiogenic phenotype in stressed adolescent males. This is the first study to demonstrate stress-induced alterations in BLA KOP function that may contribute to stress-induced social anxiety in adolescent males. Importantly, these findings provide evidence for potential KOP-dependent mechanisms that may contribute to pathophysiological interactions with subsequent stress challenges.

Age and sex regulate kappa opioid receptor-mediated anxiety-like behavior in rats

Anxiety occurs across ontogeny, but there is evidence that its etiology may vary across the lifespan. The kappa opioid receptor (KOR) system mediates some of the anxiogenic effects of stress and drug exposure, and is involved in aversive responses to environmental stimuli. However, much of this work has been conducted in adult males. Work assessing the effects of KOR activation in younger males has demonstrated that this system produces an anxiolytic/no response, indicating that that this system may be developmentally regulated. Despite these discrepancies, a direct comparison of KOR-induced anxiety in stress-naïve adolescents and adults has not been done. Additionally, the effects of KOR activation in females are poorly understood. Therefore, we assessed the impact of KOR activation on anxiety-like behavior in adolescent and adult male and female Sprague-Dawley rats. Animals were given an i.p. injection of the KOR agonist U69593 (0.01, 0.1, 1.0 mg/kg or vehicle) and were tested using the elevated plus maze. U69593 decreased open arm time in adult males, indicating increased anxiety-like behavior. Adolescents exhibited decreased stretch attend postures when collapsed across sex, suggesting reduced anxiety-like behavior. Adult females were not affected by U69593 administration. These data support studies that have identified age-dependent changes in the KOR system in males, and provide novel evidence that females may not exhibit this ontogenetic change. Given the prevalence of stress and drug exposure during the adolescent period, differences in how the KOR system may mediate the effects of these exposures across age and sex should be explored.

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.

Nicotine Gateway Effects on Adolescent Substance Use

Given the rise in teenage use of electronic nicotine delivery systems ("vaping") in congruence with the increasing numbers of drug-related emergencies, it is critical to expand the knowledge of the physical and behavioral risks associated with developmental nicotine exposure. A further understanding of the molecular and neurochemical underpinnings of nicotine's gateway effects allows emergency clinicians to advise patients and families and adjust treatment accordingly, which may minimize the use of tobacco, nicotine, and future substances. Currently, the growing use of tobacco products and electronic cigarettes among teenagers represents a major public health concern. Adolescent exposure to tobacco or nicotine can lead to subsequent abuse of nicotine and other substances, which is known as the gateway hypothesis. Adolescence is a developmentally sensitive time period when risk-taking behaviors, such as sensation seeking and drug experimentation, often begin. These hallmark behaviors of adolescence are largely due to maturational changes in the brain. The developing brain is particularly vulnerable to the harmful effects of drugs of abuse, including tobacco and nicotine products, which activate nicotinic acetylcholine receptors (nAChRs). Disruption of nAChR development with early nicotine use may influence the function and pharmacology of the receptor subunits and alter the release of reward-related neurotransmitters, including acetylcholine, dopamine, GABA, serotonin, and glutamate. In this review, we emphasize that the effects of nicotine are highly dependent on timing of exposure, with a dynamic interaction of nAChRs with dopaminergic, endocannabinoid, and opioidergic systems to enhance general drug reward and reinforcement. We analyzed available literature regarding adolescent substance use and nicotine's impact on the developing brain and behavior using the electronic databases of PubMed and Google Scholar for articles published in English between January 1968 and November 2018. We present a large collection of clinical and preclinical evidence that adolescent nicotine exposure influences long-term molecular, biochemical, and functional changes in the brain that encourage subsequent drug abuse.

Age as a factor in stress and alcohol interactions: A critical role for the kappa opioid system

The endogenous kappa opioid system has primarily been shown to be involved with a state of dysphoria and aversion. Stress and exposure to drugs of abuse, particularly alcohol, can produce similar states of unease and anxiety, implicating the kappa opioid system as a target of stress and alcohol. Numerous behavioral studies have demonstrated reduced sensitivity to manipulations of the kappa opioid system in early life relative to adulthood, and recent reports have shown that the kappa opioid system is functionally different across ontogeny. Given the global rise in early-life stress and alcohol consumption, understanding how the kappa opioid system responds and adapts to stress and/or alcohol exposure differently in early life and adulthood is imperative. Therefore, the objective of this review is to highlight and discuss studies examining the impact of early-life stress and/or alcohol on the kappa opioid system, with focus on the documented neuroadaptations that may contribute to future vulnerability to stress and/or increase the risk of relapse. We first provide a brief summary of the importance of studying the effects of stress and alcohol during early life (prenatal, neonatal/juvenile, and adolescence). We then discuss the literature on the effects of stress or alcohol during early life and adulthood on the kappa opioid system. Finally, we discuss the few studies that have shown interactions between stress and alcohol on the kappa opioid system and provide some discussion about the need for studies investigating the development of the kappa opioid system.

Stress alters social behavior and sensitivity to pharmacological activation of kappa opioid receptors in an age-specific manner in Sprague Dawley rats

The dynorphin/kappa opioid receptor (DYN/KOR) system has been identified as a primary target of stress due to behavioral effects, such as dysphoria, aversion, and anxiety-like alterations that result from activation of this system. Numerous adaptations in the DYN/KOR system have also been identified in response to stress. However, whereas most studies examining the function of the DYN/KOR system have been conducted in adult rodents, there is growing evidence suggesting that this system is ontogenetically regulated. Likewise, the outcome of exposure to stress also differs across ontogeny. Based on these developmental similarities, the objective of this study was to systematically test effects of a selective KOR agonist, U-62066, on various aspects of social behavior across ontogeny in non-stressed male and female rats as well as in males and females with a prior history of repeated exposure to restraint (90 min/day, 5 exposures). We found that the social consequences of repeated restraint differed as a function of age: juvenile stress produced substantial increases in play fighting, whereas adolescent and adult stress resulted in decreases in social investigation and social preference. The KOR agonist U-62066 dose-dependently reduced social behaviors in non-stressed adults, producing social avoidance at the highest dose tested, while younger animals displayed reduced sensitivity to this socially suppressing effect of U-62066. Interestingly, in stressed animals, the socially suppressing effects of the KOR agonist were blunted at all ages, with juveniles and adolescents exhibiting increased social preference in response to certain doses of U-62066. Taken together, these findings support the hypothesis that the DYN/KOR system changes with age and differentially responds and adapts to stress across development.

Adolescent rats fail to demonstrate a LiCl-induced pre-exposure effect: Implications for the balance of drug reward and aversion in adolescence

Adolescents display weaker taste avoidance induced by both abused and non-abused drugs than adults. Drug history attenuates avoidance learning in adults (the drug pre-exposure effect), but little is known about this phenomenon in adolescents. Given that the weaker taste avoidance in adolescence is thought to be a function of their relative insensitivity to the drug's aversive effects, it might be expected that the drug pre-exposure effect would be weaker in adolescents given that for some drugs this effect is mediated by associative blocking that depends on the association of environmental cues with the drug's aversive effects. To address this, in the present studies male adolescent (Experiment 1) and adult (Experiment 2) rats were given five spaced injections of LiCl prior to subsequent taste avoidance conditioning with LiCl. Consistent with past reports, adolescents displayed weaker taste avoidance than adults. While adults displayed attenuated LiCl-induced taste avoidance following LiCl pre-exposure, adolescents showed no evidence of this pre-exposure. This work is consistent with the view that adolescents are relatively insensitive to the aversive effects of drugs, an insensitivity potentially important in subsequent intake of drugs of abuse given that such intake is a function of the balance of their rewarding and aversive effects.

Nicotine Increases Alcohol Intake in Adolescent Male Rats

Background: Use of alcohol and tobacco, the two most concurrently abused drugs, typically first occurs during adolescence. Yet, there have been no systematic analyses of ethanol (EtOH) and nicotine (Nic) interactions during adolescence. Recent animal studies report that kappa-opioid (KOR) receptor activation mediates age differences in drug reinforcement. Our hypothesis is that concurrent self-administration of EtOH and Nic will be greater in adolescent rats because of age differences in KOR function. Furthermore, exposure to alcohol and nicotine during adolescence has been reported to increase EtOH intake in adulthood. We performed a longitudinal animal study and hypothesized adolescent rats allowed to self-administer nicotine would drink more alcohol as adults. Methods: Adolescent, postnatal day (P)32, and adult (P90) male and female Sprague-Dawley rats were allowed to self-administer EtOH, Nic, or a combination of both, EtOH+Nic, in an intravenous self-administration paradigm. The role of KOR was pharmacologically evaluated with the KOR antagonist, norbinaltorphamine (norBNI) and with the KOR agonist, U50,488H. Alcohol drinking was subsequently evaluated with male rats in a drinking in the dark (DID), 2-bottle choice test. Results: Concurrent Nic increased EtOH intake in adolescent males, but not in adults or females. Pharmacological blockade of KOR with norBNI robustly increased EtOH+Nic self-administration in adult male rats, but had no effect with female rats. Lastly, in our longitudinal study with male rats, we found prior self-administration of Nic or EtOH+Nic during adolescence increased subsequent oral EtOH intake, whereas prior self-administration of EtOH alone in adults increased subsequent EtOH drinking. Conclusions: There are major age- and sex-differences in the reinforcing effects of EtOH+Nic. Adolescent males are sensitive to the reinforcing interactions of the two drugs, whereas this effect is inhibited by KOR activation in male adults. Nicotine self-administration in adolescent males also increased subsequent oral EtOH intake. These findings suggest that brain mechanisms underlying the reinforcing effects of EtOH and nicotine are both age- and sex-dependent, and that tobacco or e-cigarette use may increase the vulnerability of teenage boys to alcohol abuse.

Age-dependent regulation of GABA transmission by kappa opioid receptors in the basolateral amygdala of Sprague-Dawley rats

Anxiety disorders are one of the most common and debilitating mental illnesses worldwide. Growing evidence indicates an age-dependent rise in the incidence of anxiety disorders from adolescence through adulthood, suggestive of underlying neurodevelopmental mechanisms. Kappa opioid receptors (KORs) are known to contribute to the development and expression of anxiety; however, the functional role of KORs in the basolateral amygdala (BLA), a brain structure critical in mediating anxiety, particularly across ontogeny, are unknown. Using whole-cell patch-clamp electrophysiology in acute brain slices from adolescent (postnatal day (P) 30-45) and adult (P60+) male Sprague-Dawley rats, we found that the KOR agonist, U69593, increased the frequency of GABA_A-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in the adolescent BLA, without an effect in the adult BLA or on sIPSC amplitude at either age. The KOR effect was blocked by the KOR antagonist, nor-BNI, which alone did not alter GABA transmission at either age, and the effect of the KOR agonist was TTX-sensitive. Additionally, KOR activation did not alter glutamatergic transmission in the BLA at either age. In contrast, U69593 inhibited sIPSC frequency in the central amygdala (CeA) at both ages, without altering sIPSC amplitude. Western blot analysis of KOR expression indicated that KOR levels were not different between the two ages in either the BLA or CeA. This is the first study to provide compelling evidence for a novel and unique neuromodulatory switch in one of the primary brain regions involved in initiating and mediating anxiety that may contribute to the ontogenic rise in anxiety disorders.

Cebranopadol, a Mixed Opioid Agonist, Reduces Cocaine Self-administration through Nociceptin Opioid and Mu Opioid Receptors

Cocaine addiction is a widespread psychiatric condition still waiting for approved efficacious medications. Previous studies suggested that simultaneous activation of nociceptin opioid (NOP) and mu opioid (MOP) receptors could be a successful strategy to treat cocaine addiction, but the paucity of molecules co-activating both receptors with comparable potency has hampered this line of research. Cebranopadol is a non-selective opioid agonist that at nanomolar concentration activates both NOP and MOP receptors and that recently reached phase-III clinical trials for cancer pain treatment. Here, we tested the effect of cebranopadol on cocaine self-administration (SA) in the rat. We found that under a fixed-ratio-5 schedule of reinforcement, cebranopadol (25 and 50 µg/kg) decreased cocaine but not saccharin SA, indicating a specific inhibition of psychostimulant consumption. In addition, cebranopadol (50 µg/kg) decreased the motivation for cocaine as detected by reduction of the break point measured in a progressive-ratio paradigm. Next, we found that cebranopadol retains its effect on cocaine consumption throughout a 7-day chronic treatment, suggesting a lack of tolerance development toward its effect. Finally, we found that only simultaneous blockade of NOP and MOP receptors by concomitant administration of the NOP antagonist SB-612111 (30 mg/kg) and naltrexone (2.5 mg/kg) reversed cebranopadol-induced decrease of cocaine SA, demonstrating that cebranopadol activates both NOP and classical opioid receptors to exert its effect. Our data, together with the fairly advanced clinical development of cebranopadol and its good tolerability profile in humans, indicate that cebranopadol is an appealing candidate for cocaine addiction treatment.

Reward-centricity and attenuated aversions: An adolescent phenotype emerging from studies in laboratory animals

Adolescence is an evolutionarily conserved developmental period, with neural circuits and behaviors contributing to the detection, procurement, and receipt of rewards bearing similarity across species. Studies with laboratory animals suggest that adolescence is typified by a "reward-centric" phenotype-an increased sensitivity to rewards relative to adults. In contrast, adolescent rodents are reportedly less sensitive to the aversive properties of many drugs and naturally aversive stimuli. Alterations within the mesocorticolimbic dopamine and endocannabinoid systems likely contribute to an adolescent reward-sensitive, yet aversion-resistant, phenotype. Although early hypotheses postulated that developmental changes in dopaminergic circuitry would result in a "reward deficiency" syndrome, evidence now suggests the opposite: that adolescents are uniquely poised to seek out hedonic stimuli, experience greater "pleasure" from rewards, and consume rewarding stimuli in excess. Future studies that more clearly define the role of specific brain regions and neurotransmitter systems in the expression of behaviors toward reward- and aversive-related cues and stimuli are necessary to more fully understand an adolescent-proclivity for and vulnerability to rewards and drugs of potential abuse.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Maturational alterations in constitutive activity of medial prefrontal cortex kappa-opioid receptors in Wistar rats

Opioid receptors can display spontaneous agonist-independent G-protein signaling (basal signaling/constitutive activity). While constitutive κ-opioid receptor (KOR) activity has been documented in vitro, it remains unknown if KORs are constitutively active in native systems. Using [(35) S] guanosine 5'-O-[gamma-thio] triphosphate coupling assay that measures receptor functional state, we identified the presence of medial prefrontal cortex KOR constitutive activity in young rats that declined with age. Furthermore, basal signaling showed an age-related decline and was insensitive to neutral opioid antagonist challenge. Collectively, the present data are first to demonstrate age-dependent alterations in the medial prefrontal cortex KOR constitutive activity in rats and changes in the constitutive activity of KORs can differentially impact KOR ligand efficacy. These data provide novel insights into the functional properties of the KOR system and warrant further consideration of KOR constitutive activity in normal and pathophysiological behavior. Opioid receptors exhibit agonist-independent constitutive activity; however, kappa-opioid receptor (KOR) constitutive activity has not been demonstrated in native systems. Our results confirm KOR constitutive activity in the medial prefrontal cortex (mPFC) that declines with age. With the ability to presynaptically inhibit multiple neurotransmitter systems in the mPFC, maturational or patho-logical alterations in constitutive activity could disrupt corticofugal glutamatergic pyramidal projection neurons mediating executive function. Regulation of KOR constitutive activity could serve as a therapeutic target to treat compromised executive function.

Effect of norbinaltorphimine on ∆⁹-tetrahydrocannabinol (THC)-induced taste avoidance in adolescent and adult Sprague-Dawley rats

RATIONALE

The aversive effects of ∆(9)-tetrahydrocannabinol (THC) are mediated by activity at the kappa opioid receptor (KOR) as assessed in adult animals; however, no studies have assessed KOR involvement in the aversive effects of THC in adolescents. Given that adolescents have been reported to be insensitive to the aversive effects induced by KOR agonists, a different mechanism might mediate the aversive effects of THC in this age group.

OBJECTIVES

The present study was designed to assess the impact of KOR antagonism on the aversive effects of THC in adolescent and adult rats using the conditioned taste avoidance (CTA) procedure.

METHODS

Following a single pretreatment injection of norbinaltorphimine (norBNI; 15 mg/kg), CTAs induced by THC (0, 0.56, 1.0, 1.8, and 3.2 mg/kg) were assessed in adolescent (n = 84) and adult (n = 83) Sprague-Dawley rats.

RESULTS

The KOR antagonist, norBNI, had weak and inconsistent effects on THC-induced taste avoidance in adolescent rats in that norBNI both attenuated and strengthened taste avoidance dependent on dose and trial. norBNI had limited impact on the final one-bottle avoidance and no effects on the two-bottle preference test. Interestingly, norBNI had no effect on THC-induced taste avoidance in adult rats as well.

CONCLUSIONS

That norBNI had no significant effect on THC-induced avoidance in adults, and a minor and inconsistent effect in adolescents demonstrates that the aversive effects of THC are not mediated by KOR activity as assessed by the CTA design in Sprague-Dawley rats.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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; alcohol and drugs of abuse; 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.

New evidence of ethanol's anxiolytic properties in the infant rat

Ethanol induces appetitive, aversive, and anxiolytic effects that are involved in the development of ethanol use and dependence. Because early ethanol exposure produces later increased responsiveness to ethanol, considerable effort has been devoted to analysis of ethanol's appetitive and aversive properties during early ontogeny. Yet, there is a relative scarcity of research related to the anxiolytic effects of ethanol during early infancy, perhaps explained by a lack of age-appropriate tests. The main aim of this study was to validate a model for the assessment of ethanol's anxiolytic effects in the infant rat (postnatal days 13-16). The potentially anxiolytic effects of ethanol tested included: i) amelioration of conditioned place aversion, ii) ethanol intake in the presence of an aversive conditioned stimulus, iii) the inhibitory behavioral effect in an anxiogenic environment, and iv) innate aversion to a brightly illuminated area in a modified light/dark paradigm. Ethanol doses employed across experiments were 0.0, 0.5, and 2.0 g/kg. Results indicated that a low ethanol dose (0.5 g/kg) was effective in attenuating expression of a conditioned aversion. Ethanol intake, however, was unaffected by simultaneous exposure to an aversive stimulus. An anxiogenic environment diminished ethanol-induced locomotor stimulation. Finally, animals given 0.5 g/kg ethanol and evaluated in a light/dark box showed increased time spent in the illuminated area and increased latency to escape from the brightly lit compartment than rats treated with a higher dose of ethanol or vehicle. These new results suggest that ethanol doses as low as 0.5 g/kg are effective in ameliorating an aversive and/or anxiogenic state in preweanling rats. These behavioral preparations can be used to assess ethanol's anxiolytic properties during early development.

Adolescents and alcohol: acute sensitivities, enhanced intake, and later consequences

Adolescence is an evolutionarily conserved developmental period characterized by notable maturational changes in the brain along with various age-related behavioral characteristics, including the propensity to initiate alcohol and other drug use and consume more alcohol per occasion than adults. After a brief review of adolescent neurobehavioral function from an evolutionary perspective, the paper will turn to assessment of adolescent alcohol sensitivity and consequences, with a focus on work from our laboratory. After summarizing evidence showing that adolescents differ considerably from adults in their sensitivity to various effects of alcohol, potential contributors to these age-typical sensitivities will be discussed, and the degree to which these findings are generalizable to other drugs and to human adolescents will be considered. Recent studies are then reviewed to illustrate that repeated alcohol exposure during adolescence induces behavioral, cognitive, and neural alterations that are highly specific, replicable, persistent and dependent on the timing of the exposure. Research in this area is in its early stages, however, and more work will be necessary to characterize the extent of these neurobehavioral alterations and further determine the degree to which observed effects are specific to alcohol exposure during adolescence.

Fischer rats are more sensitive than Lewis rats to the suppressive effects of morphine and the aversive kappa-opioid agonist spiradoline

Data have suggested that rats avoid intake of an otherwise palatable saccharin cue when paired with a drug of abuse, at least, in part, because the value of the taste cue pales in anticipation of the availability of the highly rewarding drug. Earlier support for this hypothesis was provided by the finding that, relative to the less sensitive Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when paired with a rewarding sucrose or cocaine unconditioned stimulus (US), but not when paired with the aversive agent, lithium chloride. More recent data, however, have shown that Fischer rats actually exhibit greater, not less, avoidance of the same saccharin cue when morphine serves as the US. Therefore, Experiment 1 evaluated morphine-induced suppression of intake of the taste cue in Lewis and Fischer rats when the morphine US was administered subcutaneously, rather than intraperitoneally. Experiment 2 examined the effect of strain on the suppression of intake of the saccharin cue when paired with spiradoline, a selective kappa-opioid receptor agonist. The results confirmed that Fischer rats are more responsive to the suppressive effects of morphine than Lewis rats, and that Fischer rats also exhibit greater avoidance of the saccharin cue when paired with spiradoline, despite the fact that spiradoline is devoid of reinforcing properties. Taken together, the data suggest that the facilitated morphine-induced suppression observed in Fischer rats, compared with Lewis rats, may reflect an increased sensitivity to the aversive, kappa-mediated properties of opiates.

Stress, κ manipulations, and aversive effects of ethanol in adolescent and adult male rats

Elevated ethanol use during adolescence, a potentially stressful developmental period, is accompanied by insensitivity to many aversive effects of ethanol relative to adults. Given evidence that supports a role for stress and the kappa opioid receptor (KOR) system in mediating aversive properties of ethanol and other drugs, the present study assessed the role of KOR antagonism by nor-binaltorphimine (nor-BNI) on ethanol-induced conditioned taste aversion (CTA) in stressed (exposed to repeated restraint) and non-stressed male rats (Experiment 1), with half of the rats pretreated with nor-BNI before stressor exposure. In Experiment 2, CTA induced by the kappa agonist U62,066 was also compared in stressed and non-stressed adolescents and adults. A highly palatable solution (chocolate Boost) was used as the conditioned stimulus (CS), thereby avoiding the need for water deprivation to motivate consumption of the CS during conditioning. No effects of stress on ethanol-induced CTA were found, with all doses eliciting aversions in adolescents and adults in both stress conditions. However, among stressed subjects, adults given nor-BNI before the repeated stressor displayed blunted ethanol aversion relative to adults given saline at that time. This effect of nor-BNI was not seen in adolescents, findings that support a differential role for the KOR involvement in ethanol CTA in stressed adolescents and adults. Results from Experiment 2 revealed that all doses of U62,066 elicited aversions in non-stressed animals of both ages that were attenuated in stressed animals, findings that support a modulatory role for stress in aversive effects of KOR activation. Collectively, these results suggest that although KOR sensitivity appears to be reduced in stressed subjects, this receptor system does not appear to contribute to age differences in ethanol-induced CTA under the present test circumstances.