A history of chronic morphine exposure during adolescence increases despair-like behaviour and strain-dependently promotes sociability in abstinent adult mice

Persistent effects of repeated adolescent and adult heroin vapor inhalation in female Wistar rats

Adolescent drug exposure has been associated with more severe mental health outcomes related to substance abuse and anxiety disorders. The aim of the present study was to contrast the long-term effects of repeated heroin vapor inhalation during adolescence with similar heroin exposure in adulthood. Groups of female Wistar rats underwent twice daily 30-minute sessions of heroin or propylene glycol (control) vapor inhalation from postnatal days (PND) 36-45 or PND 85-94, respectively. Nociception was assessed after vapor inhalation sessions and forty days later, for the Adolescent-Exposed and Adult-Exposed groups. Anxiety-like behavior was assessed with an elevated plus-maze (EPM) and spatial learning was assessed with a Barnes maze. Acute effects of naloxone (0.3 mg/kg, i.p.) and heroin (0.5 and 1.0 mg/kg, s.c.) on thermal nociception were determined on PND 140/189 and PND 149/198, respectively. Repeated heroin vapor inhalation produced anti-nociceptive tolerance across sessions in both adolescent and adult rats, with the adolescents exhibiting more complete tolerance. Heroin vapor inhalation produced anxiolytic effects, regardless of age of exposure. There were no effects of heroin on spatial learning. Naloxone produced acute hyperalgesia in all but the Adolescent-Exposed heroin group, and heroin anti-nociception was blunted in both heroin-exposed groups at the highest heroin dose. Repeated heroin vapor inhalation can produce lasting effects on nociception and anxiety-like behavior that persist for months after the exposure. Importantly, these findings suggest that adolescent exposure to heroin vapor produces specific effects on nociception that are not observed when exposure occurs in adulthood.

Acute adolescent morphine exposure improves dark avoidance memory and enhances long-term potentiation of ventral hippocampal CA1 during adulthood in rats

Adolescence represents a distinctive vulnerable period when exposure to stressful situations including opioid exposure can entail lasting effects on brain and can change neural mechanisms involved in memory formation for drug-associated cues, possibly increasing vulnerability of adolescents to addiction. Herein, the effects of acute adolescent morphine exposure (AAME, two injections of 2.5 mg/kg SC morphine on PND 31) were therefore investigated 6 weeks later (adulthood) on avoidance memory and hippocampal long-term potentiation (LTP) at Schaffer collateral-CA1 synapses in transvers slices from the ventral hippocampus in adult male rats using field recordings technique. Animal body weight was measured from PND 31 throughout PND 40 and also in four time points with 1 week intervals from adolescence to adulthood (PNDs 48, 55, 62 and 69) to evaluate the effect of AAME on the weight gain. We showed that there were no effects on body weight, anxiety-like behaviour and locomotor activity, even until adulthood. There was an improved dark avoidance memory during adulthood. Finally, AAME had no effects on baseline synaptic responses and resulted in a decrease in the mean values of the field excitatory postsynaptic potential slopes required to evoke the half-maximal population spike amplitude and an enhancement of LTP magnitude (%) in the ventral CA1 during adulthood. Briefly, our results suggest long-lasting effects of acute adolescent morphine exposure on the ventral hippocampus, which begin the enhancing of synaptic plasticity and the improving of emotional memory in adulthood.

Adolescent morphine exposure impairs dark avoidance memory and synaptic potentiation of ventral hippocampal CA1 during adulthood in rats

Adolescence is a neurobiological critical period for neurodevelopmental processes. Adolescent opioid exposure can affect cognitive abilities via regional-specific lasting changes in brain structure and function. The current study was therefore designed to assess the long-term effects of adolescent morphine exposure on dark avoidance memory and synaptic plasticity of the ventral hippocampal CA1. Adolescent Wistar rats received escalating doses of morphine for 10 days. Morphine injections were started with an incremental dose of 2.5 mg/kg to reach a dose of 25 mg/kg. 30 days after the last injection, inhibitory memory and in vitro field potential recording were evaluated. Also, the weight of the animals was measured during drug and post-drug exposure. We found that adolescent morphine exposure decreased weight gain during morphine and post-morphine exposure. Passive avoidance memory was impaired in the morphine group. Moreover, adolescent morphine exposure caused an increase in baseline synaptic responsiveness and failed long-term potentiation (LTP) in the ventral hippocampal CA1 during adulthood. In the morphine group, the mean values of the field excitatory postsynaptic potential (fEPSP) slopes required to elicit a half-maximal population spike (PS) amplitude were significantly greater than that of the saline group. Therefore, adolescent morphine exposure has a durable effect on memory functions, synaptic activity, and plasticity of ventral hippocampal CA1. Adults with adolescent morphine exposures may experience maladaptive behaviors and cognitive disabilities.

Impairment of spatial memory and dorsal hippocampal synaptic plasticity in adulthood due to adolescent morphine exposure

Opioid exposure during adolescence, a crucial period of neurodevelopment, has lasting neurological and behavioral consequences and affects the cognitive functions in adulthood. This study investigated the effects of adolescent morphine exposure in spatial learning and memory and synaptic plasticity of the CA1 area of the dorsal hippocampus. Adolescent Wistar rats received increasing doses of morphine for 1, 5, and 10 days. Acute morphine group was injected 2.5 mg/kg morphine for 1 day, subchronic morphine group for 5 days, with an increasing dose of 2.5 mg/kg and reached to the dose of 12.5 mg/kg and chronic morphine group for 10 days that began with an increasing dose of 2.5 mg/kg and reached to the dose of 25 mg/kg. Then after 25 days and reaching adulthood, spatial learning and memory were evaluated via the Morris water maze (MWM) test. Moreover, we test the electrophysiological properties of dorsal hippocampal plasticity in adult rats by in vitro field potential recordings. Subchronic and chronic adolescent morphine exposure impaired spatial learning and memory in the MWM test. Baseline synaptic responses in the chronic morphine group were increased and long-term potentiation (LTP) impaired in the CA1 area in subchronic and chronic morphine groups. In adulthood, the slope of the field excitatory postsynaptic potential (fEPSP) required to elicit a half-maximal population spike (PS) amplitude was significantly larger in subchronic and chronic adolescent morphine exposure compared to the saline group. Therefore, subchronic and chronic adolescent morphine exposure altered synaptic transmission and plasticity in addition to learning and memory. Long-term morphine exposure during adolescence can interfere with neurodevelopment, making a persistent impression on plasticity and cognitive capability in adulthood.

Adolescent opioid abuse: Role of glial and neuroimmune mechanisms

Opioids are widely prescribed for pain management, and prescription opioid misuse in adolescents has become a major epidemic in the United States and worldwide. Emerging data indicate that adolescence represents a critical period of brain development, and exposure to opioids during adolescence may increase the risk of addiction in adulthood. There is growing evidence that disruptions in brain glial function may be implicated in numerous chronic neuropathologies. Evidence suggests that glial mechanisms have an important role in the development and maintenance of opioid abuse and the risk for addiction. This review will describe glial and neuroimmune mechanisms involved in opioid use disorders during adolescence, which may increase substance use disorder liability later in life. Moreover, this review will identify some important neuro-glial targets, involved in opioid abuse and addiction, to develop future preventions and treatment strategies.

Review of addiction risk potential associated with adolescent opioid use

Adolescence is a critical period of development with robust behavioral, morphological, hormonal, and neurochemical changes including changes in brain regions implicated in the reinforcing effects of drugs such as opioids. Here we examine the preclinical and, where appropriate complementary clinical literature, for the behavioral and neurological changes induced by adolescent opioid exposure/use and their long-term consequences during adulthood. Adolescent opioid exposure results in a widened biphasic shift in reinforcement with increased impact of positive rewarding aspects during initial use and profound negative reinforcement during adulthood. Females may have enhanced vulnerability due to fast onset of antinociceptive tolerance and reduced severity of somatic withdrawal symptoms during adolescence. Overall, adolescent opioid exposure, be it legally prescribed protracted intake or illicit consumption, results in significant and prolonged consequences of increased opioid reward concomitant with reduced analgesic efficacy and exacerbated somatic withdrawal severity during opioid use/exposure in adulthood. These findings are highly relevant to physicians, parents, law makers, and the general public as adolescent opioid exposure/misuse results in heightened risk for substance use disorders.

Modelling decision-making under uncertainty: A direct comparison study between human and mouse gambling data

Decision-making is a conserved evolutionary process enabling us to choose one option among several alternatives, and relies on reward and cognitive control systems. The Iowa Gambling Task allows the assessment of human decision-making under uncertainty by presenting four card decks with various cost-benefit probabilities. Participants seek to maximise their monetary gain by developing long-term optimal-choice strategies. Animal versions have been adapted with nutritional rewards, but interspecies data comparisons are scarce. Our study directly compares the non-pathological decision-making performance between humans and wild-type C57BL/6 mice. Human participants completed an electronic Iowa Gambling Task version, while mice a maze-based adaptation with four arms baited in a probabilistic way. Our data shows closely matching performance between both species with similar patterns of choice behaviours. However, mice showed a faster learning rate than humans. Moreover, both populations were clustered into good, intermediate and poor decision-making categories with similar proportions. Remarkably, mice characterised as good decision-makers behaved the same as humans of the same category, but slight differences among species are evident for the other two subpopulations. Overall, our direct comparative study confirms the good face validity of the rodent gambling task. Extended behavioural characterisation and pathological animal models should help strengthen its construct validity and disentangle the determinants in animals and humans decision-making.

Adolescent drug exposure: A review of evidence for the development of persistent changes in brain function

Over the past decade, many studies have indicated that adolescence is a critical period of brain development and maturation. The refinement and maturation of the central nervous system over this prolonged period, however, makes the adolescent brain highly susceptible to perturbations from acute and chronic drug exposure. Here we review the preclinical literature addressing the long-term consequences of adolescent exposure to common recreational drugs and drugs-of-abuse. These studies on adolescent exposure to alcohol, nicotine, opioids, cannabinoids and psychostimulant drugs, such as cocaine and amphetamine, reveal a variety of long-lasting behavioral and neurobiological consequences. These agents can affect development of the prefrontal cortex and mesolimbic dopamine pathways and modify the reward systems, socio-emotional processing and cognition. Other consequences include disruption in working memory, anxiety disorders and an increased risk of subsequent drug abuse in adult life. Although preventive and control policies are a valuable approach to reduce the detrimental effects of drugs-of-abuse on the adolescent brain, a more profound understanding of their neurobiological impact can lead to improved strategies for the treatment and attenuation of the detrimental neuropsychiatric sequelae.

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.

Zebrafish models relevant to studying central opioid and endocannabinoid systems

The endocannabinoid and opioid systems are two interplaying neurotransmitter systems that modulate drug abuse, anxiety, pain, cognition, neurogenesis and immune activity. Although they are involved in such critical functions, our understanding of endocannabinoid and opioid physiology remains limited, necessitating further studies, novel models and new model organisms in this field. Zebrafish (Danio rerio) is rapidly emerging as one of the most effective translational models in neuroscience and biological psychiatry. Due to their high physiological and genetic homology to humans, zebrafish may be effectively used to study the endocannabinoid and opioid systems. Here, we discuss current models used to target the endocannabinoid and opioid systems in zebrafish, and their potential use in future translational research and high-throughput drug screening. Emphasizing the high degree of conservation of the endocannabinoid and opioid systems in zebrafish and mammals, we suggest zebrafish as an excellent model organism to study these systems and to search for the new drugs and therapies targeting their evolutionarily conserved mechanisms.

Differential behavioral and molecular alterations upon protracted abstinence from cocaine versus morphine, nicotine, THC and alcohol

Unified theories of addiction are challenged by differing drug-seeking behaviors and neurobiological adaptations across drug classes, particularly for narcotics and psychostimulants. We previously showed that protracted abstinence to opiates leads to despair behavior and social withdrawal in mice, and we identified a transcriptional signature in the extended amygdala that was also present in animals abstinent from nicotine, Δ9-tetrahydrocannabinol (THC) and alcohol. Here we examined whether protracted abstinence to these four drugs would also share common behavioral features, and eventually differ from abstinence to the prototypic psychostimulant cocaine. We found similar reduced social recognition, increased motor stereotypies and increased anxiety with relevant c-fos response alterations in morphine, nicotine, THC and alcohol abstinent mice. Protracted abstinence to cocaine, however, led to strikingly distinct, mostly opposing adaptations at all levels, including behavioral responses, neuronal activation and gene expression. Together, these data further document the existence of common hallmarks for protracted abstinence to opiates, nicotine, THC and alcohol that develop within motivation/emotion brain circuits. In our model, however, these do not apply to cocaine, supporting the notion of unique mechanisms in psychostimulant abuse.

Long-lasting effects of adolescent oxycodone exposure on reward-related behavior and gene expression in mice

RATIONALE

Prescription opioid abuse and transition to heroin use are growing problems in the USA. However, the long-term consequences of adolescent prescription opioid abuse on subsequent drug use and affective-like behavior are unknown.

OBJECTIVES

This study aims to determine if adolescent exposure to oxycodone alters the rewarding effects of morphine, anxiety-like behavior, and reward-related gene expression later in adulthood.

METHODS

Adolescent male C57Bl/6 mice were exposed to oxycodone (3 mg/kg/day) via osmotic minipumps for 28 days. Following a 28-day withdrawal period, mice were tested in morphine-conditioned place preference paradigm (CPP), morphine sensitization, open field, marble burying, and forced swim (FST) tests. To determine if effects were specific to adolescent exposure, adult mice were exposed to oxycodone for 28 days and underwent 28 days of withdrawal prior to the same behavioral testing schedule. Expression of reward-related genes including dopamine receptor 1 (D1) and dopamine transporter (DAT) in the nucleus accumbens (NAc) and ventral tegmental area (VTA) was examined.

RESULTS

Adolescent oxycodone exposure significantly increased (300 %) response to morphine CPP during adulthood and significantly reduced D1 expression (30 %) in the NAc and DAT expression (75 %) in the VTA. Adult oxycodone exposure did not affect subsequent responses to morphine CPP. Oxycodone exposure did not affect the development of morphine sensitization or affective-like behaviors. Corticosterone response to a stressor (FST) was significantly reduced (65 %) in mice exposed to oxycodone during adolescence but not adulthood.

CONCLUSIONS

Adolescent oxycodone exposure enhances rewarding effects of morphine in adulthood with no effect on other affective-like behaviors.

Substance use modulates stress reactivity: Behavioral and physiological outcomes

Drug addiction is a major public health concern in the United States costing taxpayers billions in health care costs, lost productivity and law enforcement. However, the availability of effective treatment options remains limited. The development of novel therapeutics will not be possible without a better understanding of the addicted brain. Studies in both clinical and preclinical models indicate that chronic drug use leads to alterations in the body and brain's response to stress. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis may shed light on the ability of stress to increase vulnerability to relapse. Further, within both the HPA axis and limbic brain regions, corticotropin-releasing factor (CRF) is critically involved in the brain's response to stress. Alterations in both central and peripheral CRF activity seen following chronic drug use provide a mechanism by which substance use can alter stress reactivity, thus mediating addictive phenotypes. While many reviews have focused on how stress alters drug-mediated changes in physiology and behavior, the goal of this review is to focus on how substance use alters responses to stress.

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).

Dissociation of heroin-induced emotional dysfunction from psychomotor activation and physical dependence among inbred mouse strains

RATIONALE

Opiate addiction is a brain disorder emerging through repeated intoxication and withdrawal episodes. Epidemiological studies also indicate that chronic exposure to opiates may lead in susceptible individuals to the emergence of depressive symptoms, strongly contributing to the severity and chronicity of addiction. We recently established a mouse model of heroin abstinence, characterized by the development of depressive-like behaviors following chronic heroin exposure.

OBJECTIVES

While genetic factors regulating immediate behavioral responses to opiates have been largely investigated, little is known about their contribution to long-term emotional regulation during abstinence. Here, we compared locomotor stimulation and physical dependence induced by heroin exposure, as well as emotional dysfunction following abstinence, across mice strains with distinct genetic backgrounds.

METHODS

Mice from three inbred strains (C57BL/6J, Balb/cByJ, and 129S2/SvPas) were exposed to an escalating chronic heroin regimen (10-50 mg/kg). Independent cohorts were used to assess drug-induced locomotor activity during chronic treatment, naloxone-precipitated withdrawal at the end of chronic treatment, and emotional-like responses after a 4-week abstinence period.

RESULTS

Distinct behavioral profiles were observed across strains during heroin treatment, with no physical dependence and low locomotor stimulation in 129S2/SvPas. In addition, different behavioral impairments developed during abstinence across the three strains, with increased despair-like behavior in 129S2/SvPas and Balb/cByJ, and low sociability in 129S2/SvPas and C57BL/6J.

CONCLUSIONS

Our results indicate that depressive-like behaviors emerge during heroin abstinence, whatever the severity of immediate behavioral responses to the drug. In addition, inbred mouse strains will allow studying several aspects of mood-related deficits associated with addiction.

Distinct mu, delta, and kappa opioid receptor mechanisms underlie low sociability and depressive-like behaviors during heroin abstinence

Addiction is a chronic disorder involving recurring intoxication, withdrawal, and craving episodes. Escaping this vicious cycle requires maintenance of abstinence for extended periods of time and is a true challenge for addicted individuals. The emergence of depressive symptoms, including social withdrawal, is considered a main cause for relapse, but underlying mechanisms are poorly understood. Here we establish a mouse model of protracted abstinence to heroin, a major abused opiate, where both emotional and working memory deficits unfold. We show that delta and kappa opioid receptor (DOR and KOR, respectively) knockout mice develop either stronger or reduced emotional disruption during heroin abstinence, establishing DOR and KOR activities as protective and vulnerability factors, respectively, that regulate the severity of abstinence. Further, we found that chronic treatment with the antidepressant drug fluoxetine prevents emergence of low sociability, with no impact on the working memory deficit, implicating serotonergic mechanisms predominantly in emotional aspects of abstinence symptoms. Finally, targeting the main serotonergic brain structure, we show that gene knockout of mu opioid receptors (MORs) in the dorsal raphe nucleus (DRN) before heroin exposure abolishes the development of social withdrawal. This is the first result demonstrating that intermittent chronic MOR activation at the level of DRN represents an essential mechanism contributing to low sociability during protracted heroin abstinence. Altogether, our findings reveal crucial and distinct roles for all three opioid receptors in the development of emotional alterations that follow a history of heroin exposure and open the way towards understanding opioid system-mediated serotonin homeostasis in heroin abuse.

The kappa opioid receptor: from addiction to depression, and back

Comorbidity is a major issue in psychiatry that notably associates with more severe symptoms, longer illness duration, and higher service utilization. Therefore, identifying key clusters of comorbidity and exploring the underlying pathophysiological mechanisms represent important steps toward improving mental health care. In the present review, we focus on the frequent association between addiction and depression. In particular, we summarize the large body of evidence from preclinical models indicating that the kappa opioid receptor (KOR), a member of the opioid neuromodulatory system, represents a central player in the regulation of both reward and mood processes. Current data suggest that the KOR modulates overlapping neuronal networks linking brainstem monoaminergic nuclei with forebrain limbic structures. Rewarding properties of both drugs of abuse and natural stimuli, as well as the neurobiological effects of stressful experiences, strongly interact at the level of KOR signaling. In addiction models, activity of the KOR is potentiated by stressors and critically controls drug-seeking and relapse. In depression paradigms, KOR signaling is responsive to a variety of stressors, and mediates despair-like responses. Altogether, the KOR represents a prototypical substrate of comorbidity, whereby life experiences converge upon common brain mechanisms to trigger behavioral dysregulation and increased risk for distinct but interacting psychopathologies.

The multiple facets of opioid receptor function: implications for addiction

Addiction is characterized by altered reward processing, disrupted emotional responses and poor decision-making. Beyond a central role in drug reward, increasing evidence indicate that opioid receptors are broadly involved in all these processes. Recent studies establish the mu opioid receptor as a main player in social reward, which attracts increasing attention in psychiatric research. There is growing interest in blocking the kappa opioid receptor to prevent relapse, and alleviate the negative affect of withdrawal. The delta opioid receptor emerges as a potent mood enhancer, whose involvement in addiction is less clear. All three opioid receptors are likely implicated in addiction-depression comorbidity, and understanding of their roles in cognitive deficits associated to drug abuse is only beginning.