Neural correlates of the motivational and somatic components of naloxone-precipitated morphine withdrawal

Model of negative affect induced by withdrawal from acute and chronic morphine administration in male mice

Opioid use disorder (OUD) is a chronic relapsing disorder that is a major burden for the lives of affected individuals, and society as a whole. Opioid withdrawal is characterized by strong physical symptoms, along with signs of negative affect. Negative affect due to opioid withdrawal is a major obstacle to recovery and relapse prevention. The mechanisms behind negative affect due to either spontaneous or antagonist-precipitated opioid withdrawal are not well known, and more animal models need be developed. Here, we present behavioral models of negative affect upon naloxone-precipitated morphine withdrawal in adult male mice. Social, anxiety, and despair-like deficits were investigated following naloxone administration in mice receiving morphine under three dosing regimens; acute, chronic constant dose and chronic escalating doses. Social behaviour in the three-chamber social preference test was decreased following withdrawal from chronic and escalating but not acute morphine. Anxiety-like behaviour in the open field was increased for all three treatments. Despair-like behaviour was increased following withdrawal from chronic and escalating but not acute morphine. Altogether, these animal models will contribute to study behavioural and neuronal circuitries involved in the several negative affective signs characterizing OUD.

Distinctive roles of L-type calcium channels subtypes within the dorsal hippocampus in formation of morphine withdrawal-induced aversion in rats

Although the negative effects coming along with opiate withdrawal are in part modulated by L-type calcium channels (LTCCs), the distinctive physiological properties and functions of LTCCs subtypes suggest differential roles of subtypes during withdrawal. The present study aimed to examine the contributions of LTCC subtypes, Cav1.2 and Cav1.3, within the dorsal hippocampus (DH) in naloxone-precipitated morphine withdrawal using the conditioned place aversion (CPA) paradigm. Firstly, we injected the non-specific LTCCs antagonist verapamil into the DH of morphine-dependent rats before conditioning an environment with naloxone-precipitated withdrawal. Our results showed that verapamil blocked the acquisition of CPA. Then, to explore the molecular mechanisms of LTCCs subtypes during withdrawal, we measured the protein expression of Cav1.2 and Cav1.3 in morphine-dependent rats under different conditions. In morphine-dependent rats, conditioning with withdrawal increased Cav1.2 expression in the membrane, while only acute naloxone injection increased the membrane expression of Cav1.3. To further determine the causal roles of LTCCs subtypes in the withdrawal process, we used Cav1.2 siRNA or Cav1.3 shRNA to knock down the expression of subtypes and detected the effects on CPA and somatic withdrawal signs in morphine-dependent rats. Cav1.2 siRNA, but not Cav1.3 shRNA, inhibited the acquirement of CPA and relieved somatic withdrawal symptoms. Together, our findings reveal that Cav1.2, but not Cav1.3 plays an important role in mediating morphine withdrawal, suggesting this subtype may serve as a potential therapeutic target for the treatment of negative effects in opiate dependence.

Neuroplasticity of the extended amygdala in opioid withdrawal and prolonged opioid abstinence

Opioid use disorder is characterized by excessive use of opioids, inability to control its use, a withdrawal syndrome upon discontinuation of opioids, and long-term likelihood of relapse. The behavioral stages of opioid addiction correspond with affective experiences that characterize the opponent process view of motivation. In this framework, active involvement is accompanied by positive affective experiences which gives rise to "reward craving," whereas the opponent process, abstinence, is associated with the negative affective experiences that produce "relief craving." Relief craving develops along with a hypersensitization to the negatively reinforcing aspects of withdrawal during abstinence from opioids. These negative affective experiences are hypothesized to stem from neuroadaptations to a network of affective processing called the "extended amygdala." This negative valence network includes the three core structures of the central nucleus of the amygdala (CeA), the bed nucleus of the stria terminalis (BNST), and the nucleus accumbens shell (NAc shell), in addition to major inputs from the basolateral amygdala (BLA). To better understand the major components of this system, we have reviewed their functions, inputs and outputs, along with the associated neural plasticity in animal models of opioid withdrawal. These models demonstrate the somatic, motivational, affective, and learning related models of opioid withdrawal and abstinence. Neuroadaptations in these stress and motivational systems are accompanied by negative affective and aversive experiences that commonly give rise to relapse. CeA neuroplasticity accounts for many of the aversive and fear-related effects of opioid withdrawal via glutamatergic plasticity and changes to corticotrophin-releasing factor (CRF)-containing neurons. Neuroadaptations in BNST pre-and post-synaptic GABA-containing neurons, as well as their noradrenergic modulation, may be responsible for a variety of aversive affective experiences and maladaptive behaviors. Opioid withdrawal yields a hypodopaminergic and amotivational state and results in neuroadaptive increases in excitability of the NAc shell, both of which are associated with increased vulnerability to relapse. Finally, BLA transmission to hippocampal and cortical regions impacts the perception of conditioned aversive effects of opioid withdrawal by higher executive systems. The prevention or reversal of these varied neuroadaptations in the extended amygdala during opioid withdrawal could lead to promising new interventions for this life-threatening condition.

Opioid withdrawal: role in addiction and neural mechanisms

Withdrawal from opioids involves a negative affective state that promotes maintenance of drug-seeking behavior and relapse. As such, understanding the neurobiological mechanisms underlying withdrawal from opioid drugs is critical as scientists and clinicians seek to develop new treatments and therapies. In this review, we focus on the neural systems known to mediate the affective and somatic signs and symptoms of opioid withdrawal, including the mesolimbic dopaminergic system, basolateral amygdala, extended amygdala, and brain and hormonal stress systems. Evidence from preclinical studies suggests that these systems are altered following opioid exposure and that these changes mediate behavioral signs of negative affect such as aversion and anxiety during withdrawal. Adaptations in these systems also parallel the behavioral and psychological features of opioid use disorder (OUD), highlighting the important role of withdrawal in the development of addictive behavior. Implications for relapse and treatment are discussed as well as promising avenues for future research, with the hope of promoting continued progress toward characterizing neural contributors to opioid withdrawal and compulsive opioid use.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Opioid Withdrawal Abruptly Disrupts Amygdala Circuit Function by Reducing Peptide Actions

While the physical signs of opioid withdrawal are most readily observable, withdrawal insidiously drives relapse and contributes to compulsive drug use, by disrupting emotional learning circuits. How these circuits become disrupted during withdrawal is poorly understood. Because amygdala neurons mediate relapse, and are highly opioid sensitive, we hypothesized that opioid withdrawal would induce adaptations in these neurons, opening a window of disrupted emotional learning circuit function. Under normal physiological conditions, synaptic transmission between the basolateral amygdala (BLA) and the neighboring main island (Im) of GABAergic intercalated cells (ITCs) is strongly inhibited by endogenous opioids. Using patch-clamp electrophysiology in brain slices prepared from male rats, we reveal that opioid withdrawal abruptly reduces the ability of these peptides to inhibit neurotransmission, a direct consequence of a protein kinase A (PKA)-driven increase in the synaptic activity of peptidases. Reduced peptide control of neurotransmission in the amygdala shifts the excitatory/inhibitory balance of inputs onto accumbens-projecting amygdala cells involved in relapse. These findings provide novel insights into how peptidases control synaptic activity within the amygdala and presents restoration of endogenous peptide activity during withdrawal as a viable option to mitigate withdrawal-induced disruptions in emotional learning circuits and rescue the relapse behaviors exhibited during opioid withdrawal and beyond into abstinence.SIGNIFICANCE STATEMENT We find that opioid withdrawal dials down inhibitory neuropeptide activity in the amygdala. This disrupts both GABAergic and glutamatergic transmission through amygdala circuits, including reward-related outputs to the nucleus accumbens. This likely disrupts peptide-dependent emotional learning processes in the amygdala during withdrawal and may direct behavior toward compulsive drug use.

Male and female rats show opiate withdrawal-induced place aversion and extinction in a Y-maze paradigm

Gender differences have been observed in the vulnerability to drug abuse and in the different stages of the addictive process. In opiate dependence, differences between sexes have been shown in humans and laboratory animals in various phases of opiate addiction, especially in withdrawal-associated negative affective states. Using a Y-maze conditioned place aversion paradigm, we investigated potential sex differences in the expression and extinction of the aversive memory of precipitated opiate withdrawal state in morphine-dependent rats. No significant difference between sexes was observed in the occurrence of withdrawal signs following naloxone injection during conditioning. Moreover, opiate withdrawal memory expression and extinction following repeated testing was demonstrated in both male and female rats, with no significant differences between sexes. Finally, we report spontaneous recovery following extinction of opiate withdrawal memory. Altogether these data provide further evidence that persistent withdrawal-related memories may be strong drivers of opiate dependence, and demonstrate that both males and females can be used in experimental rodent cohorts to better understand opiate-related effects, reward, aversive state of withdrawal, abstinence and relapse.

Rapastinel accelerates loss of withdrawal signs after repeated morphine and blunts relapse to conditioned place preference

The purpose of the present study was to evaluate the efficacy of rapastinel, an allosteric modulator of NMDA receptor function, to accelerate the loss of opioid withdrawal symptoms and blunt or prevent relapse to morphine conditioned place preference (CPP) in rats. Two studies were conducted. In study 1, adult and adolescent male and female rats were treated with increasing doses of morphine (5 mg/kg, bid to 25 mg/kg bid) for 5 days. On day 6 animals were treated with naloxone (1 mg/kg) and withdrawal was assessed. They were then treated with saline or rapastinel (5 mg/kg) on days 6 and 8, and withdrawal was assessed on day 9. Rapastinel treated animals exhibited significantly lower levels of withdrawal signs on day 9. No sex or age differences were observed. In Study 2, CPP for morphine was established in adult rats (males and females) by 4 daily pairings with saline and morphine (am/pm alternation). They were tested for CPP on day 5, and then treated with rapastinel (5 mg/kg) or saline daily on days 6-10 of extinction. On day 11 they received a final dose of rapastinel or saline followed by extinction trial. On day 12, animals received 1 mg/kg of morphine and were tested for relapse. Rapastinel did not affect extinction of CPP, but rapastinel-treated animals spent significantly less time in the previously morphine-paired side than saline-treated animals during the relapse trial. These findings of accelerated loss of withdrawal signs and blunted relapse to CPP suggest that rapastinel could provide an adjunctive therapy for opioid dependence during initiation of pharmacotherapy for opioid dependence.

NEURONAL CORRELATES OF HYPERALGESIA AND SOMATIC SIGNS OF HEROIN WITHDRAWAL IN MALE AND FEMALE MICE

Opioid withdrawal involves the manifestation of motivational and somatic symptoms. However, the brain structures that are involved in the expression of different opioid withdrawal signs remain unclear. We induced opioid dependence by repeatedly injecting escalating heroin doses in male and female C57BL/6J mice. We assessed hyperalgesia during spontaneous heroin withdrawal and somatic signs of withdrawal that was precipitated by the preferential µ-opioid receptor antagonist naloxone. Heroin-treated mice exhibited significantly higher hyperalgesia and somatic signs than saline-treated mice. Following behavioral assessment, we measured regional changes in brain activity by automated the counting of c-Fos expression (a marker of cellular activity). Using Principal Component Analysis, we determined the association between behavior (hyperalgesia and somatic signs of withdrawal) and c-Fos expression in different brain regions. Hyperalgesia was associated with c-Fos expression in the lateral hypothalamus, central nucleus of the amygdala, ventral tegmental area, parabrachial nucleus, dorsal raphe, and locus coeruleus. Somatic withdrawal was associated with c-Fos expression in the paraventricular nucleus of the thalamus, lateral habenula, dorsal raphe, and locus coeruleus. Thus, hyperalgesia and somatic withdrawal signs were each associated with c-Fos expression in unique sets of brain areas. The expression of c-Fos in the dorsal raphe and locus coeruleus was associated with both hyperalgesia and somatic withdrawal. Understanding common neurobiological mechanisms of acute and protracted opioid withdrawal may help identify new targets for treating this salient aspect of opioid use disorder.SIGNIFICANCE STATEMENTThe public impact of the opioid crisis has prompted an effort to understand the neurobiological mechanisms of opioid use disorder (OUD). The need to avoid withdrawal symptoms is hypothesized to drive compulsive drug-taking and -seeking in OUD. Thus, understanding the mechanisms of acute and protracted opioid withdrawal may help identify new targets for treating this salient aspect of OUD. We reported brain structures that are associated with the expression of hyperalgesia and somatic signs of opioid withdrawal in male and female heroin-dependent mice. Hyperalgesia during spontaneous opioid withdrawal and somatic withdrawal resulted in c-Fos expression in autonomic and limbic brain regions. The expression of c-Fos in the dorsal raphe and locus coeruleus were associated with both hyperalgesia and somatic withdrawal.

Contributions of the GABAergic system of the prelimbic cortex and basolateral amygdala to morphine withdrawal-induced contextual fear

Morphine withdrawal can trigger disruptions in neuronal pathways involved in the modulation and expression of anxiety and fear-related behaviors, particularly those involved in associative learning. When it comes to contextual fear, specific subdivisions of the medial prefrontal cortex (mPFC) regulate the expression of defensive behaviors through projections to specific amygdala (AM) nuclei, such as the prelimbic cortex (PrL). The basolateral nucleus (BLA) of the AM has been shown to be involved in the modulation and expression of associative memories of fear, including those associated with opiate withdrawal-related aversive events. The purpose of this study is to determine the role of GABA mechanisms in the PrL and BLA in startle potentiation and freezing behavior caused by morphine-precipitated withdrawal. Our findings show that morphine withdrawal promotes the emergence of contextual conditioned fear in animals when they are exposed to the same environment where the withdrawal sessions were performed. This suggests that the neural circuits underlying the organism's response to conditioned stressors and the circuits modulating the negative affective states induced by drug withdrawal may overlap. The pharmacological manipulation of GABAergic neurotransmission in the PrL and BLA can reverse contextual fear in morphine-withdrawn rats, an effect that appears to be mediated, at least in part, by GABA_A receptors.

The role of hippocampus in memory reactivation: an implication for a therapeutic target against opioid use disorder

Purpose of the review

The abuse of opioids induces many terrible problems in human health and social stability. For opioid-dependent individuals, withdrawal memory can be reactivated by context, which is then associated with extremely unpleasant physical and emotional feelings during opioid withdrawal. The reactivation of withdrawal memory is considered one of the most important reasons for opioid relapse, and it also allows for memory modulation based on the reconsolidation phenomenon. However, studies exploring withdrawal memory modulation during the reconsolidation window are lacking. By summarizing the previous findings about the reactivation of negative emotional memories, we are going to suggest potential neural regions and systems for modulating opioid withdrawal memory.

Recent findings

Here, we first present the role of memory reactivation in its modification, discuss how the hippocampus participates in memory reactivation, and discuss the importance of noradrenergic signaling in the hippocampus for memory reactivation. Then, we review the engagement of other limbic regions receiving noradrenergic signaling in memory reactivation. We suggest that noradrenergic signaling targeting hippocampus neurons might play a potential role in strengthening the disruptive effect of withdrawal memory extinction by facilitating the degree of memory reactivation.

Summary

This review will contribute to a better understanding of the mechanisms underlying reactivation-dependent memory malleability and will provide new therapeutic avenues for treating opioid use disorders.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Cues conditioned to withdrawal and negative reinforcement: Neglected but key motivational elements driving opioid addiction

Opioid use disorder (OUD) is a debilitating disorder that affects millions of people. Neutral cues can acquire motivational properties when paired with the positive emotional effects of drug intoxication to stimulate relapse. However, much less research has been devoted to cues that become conditioned to the aversive effects of opioid withdrawal. We argue that environmental stimuli promote motivation for opioids when cues are paired with withdrawal (conditioned withdrawal) and generate opioid consumption to terminate conditioned withdrawal (conditioned negative reinforcement). We review evidence that cues associated with pain drive opioid consumption, as patients with chronic pain may misuse opioids to escape physical and emotional pain. We highlight sex differences in withdrawal-induced stress reactivity and withdrawal cue processing and discuss neurocircuitry that may underlie withdrawal cue processing in dependent individuals. These studies highlight the importance of studying cues associated with withdrawal in dependent individuals and point to areas for exploration in OUD research.

Paternal exposure to morphine during adolescence potentiates morphine withdrawal in male offspring: Involvement of the lateral paragigantocellularis nucleus

BACKGROUND

Opiate exposure during adolescence perturbs the brain's maturation process and potentially confers long-term adverse consequences, not only in exposed individuals but also in their posterity. Here, we investigate the outcomes of adolescent paternal morphine exposure on morphine withdrawal profile in male offspring.

METHODS

Male Wistar rats were chronically subjected to 10 days of an escalating regimen of morphine during adolescence. After a 20-day washout period, adult males were allowed to copulate with naïve females. The adult male offspring were tested for somatic and affective components of naloxone-precipitated morphine withdrawal using conditioned place aversion. Moreover, electrical activity of the lateral paragigantocellularis (LPGi) nucleus, which is involved in development of opiate dependence, was recorded in response to a challenge dose of morphine via extracellular single-unit recordings.

RESULTS

Morphine-sired offspring exhibited augmented expression of naloxone-induced somatic and affective signs of opiate withdrawal compared to the control saline-sired counterparts. In vivo recording revealed that LPGi neurons displayed heterogeneous responses (inhibitory, excitatory, and no change) to acute morphine administration in both morphine- and saline-sired animals. The morphine-induced discharge inhibition was potentiated in morphine-sired offspring. However, the extent of discharge excitation in response to morphine did not reach significance in these subjects. Moreover, the lack of alteration in maternal behavior toward morphine-sired offspring indicates that this is due to germline-dependent transmission of epigenetic traits across generations.

CONCLUSIONS

Preconception paternal exposure to morphine during adolescence potentiates opiate withdrawal signs in male offspring which is mediated, at least in part, by epigenetic alteration of LPGi-related brain circuitry.

Opioid withdrawal and memory consolidation

It is well established that learning and memory are central to substance dependence. This paper specifically reviews the effect of opioid withdrawal on memory consolidation. Although there is evidence that opioid withdrawal can interfere with initial acquisition and retrieval of older memories, there are several reasons to postulate a facilitatory action on the consolidation of newly acquired memories. In fact, there is substantial evidence that memory consolidation is facilitated by the release of stress hormones, that it requires the activation of the amygdala, of central noradrenergic and cholinergic pathways, and that it involves long-term potentiation. This review highlights evidence that very similar neurobiological processes are involved in opioid withdrawal, and summarizes recent results indicating that naltrexone-precipitated withdrawal enhanced consolidation in rats. From this neurocognitive perspective, therefore, opioid use may escalate during the addiction cycle in part because memories of stimuli and actions experienced during withdrawal are strengthened.

Role of glucocorticoids on noradrenergic and dopaminergic neurotransmission within the basolateral amygdala and dentate gyrus during morphine withdrawal place aversion

Aversive memories related to drug withdrawal can generate a motivational state leading to compulsive drug taking. However, the mechanisms underlying the generation of these withdrawal memories remain unclear. Limbic structures, such as the basolateral amygdala (BLA) and the dentate gyrus (DG) of the hippocampus, play a crucial role in the negative affective component of morphine withdrawal. Given the prominent role of glucocorticoids (GCs), noradrenaline (NA), and dopamine (DA) in memory-related processes, in the present study, we employed the conditioned place aversion (CPA) paradigm to uncover the role of GCs on NA and DA neurotransmission within the BLA and NA neurotransmission within the DG during opiate-withdrawal conditioning (memory formation consolidation), and after reexposure to the conditioned environment (memory retrieval). We observed that adrenalectomy impaired naloxone-induced CPA. Memory retrieval was associated with an increase in dihydroxyphenylacetic acid (DOPAC) levels in the BLA in morphine-addicted animals in a GC-independent manner. Importantly, NA turnover was related with the expression of withdrawal physical signs during the conditioning phase and with locomotor activity during the test phase. On the other hand, reduced DA concentration in the BLA was correlated with the CPA score. Our results indicate that while noradrenergic system is more associated with the somatic consequences of withdrawal, dopaminergic neurotransmission modulates the affective state. Nevertheless, it seems necessary that both systems work together with GCs to enable aversive-memory formation and recall.

Mu opioid receptors in the medial habenula contribute to naloxone aversion

The medial habenula (MHb) is considered a brain center regulating aversive states. The mu opioid receptor (MOR) has been traditionally studied at the level of nociceptive and mesolimbic circuits, for key roles in pain relief and reward processing. MOR is also densely expressed in MHb, however, MOR function at this brain site is virtually unknown. Here we tested the hypothesis that MOR in the MHb (MHb-MOR) also regulates aversion processing. We used chnrb4-Cre driver mice to delete the Oprm1 gene in chnrb4-neurons, predominantly expressed in the MHb. Conditional mutant (B4MOR) mice showed habenula-specific reduction of MOR expression, restricted to chnrb4-neurons (50% MHb-MORs). We tested B4MOR mice in behavioral assays to evaluate effects of MOR activation by morphine, and MOR blockade by naloxone. Locomotor, analgesic, rewarding, and motivational effects of morphine were preserved in conditional mutants. In contrast, conditioned place aversion (CPA) elicited by naloxone was reduced in both naïve (high dose) and morphine-dependent (low dose) B4MOR mice. Further, physical signs of withdrawal precipitated by either MOR (naloxone) or nicotinic receptor (mecamylamine) blockade were attenuated. These data suggest that MORs expressed in MHb B4-neurons contribute to aversive effects of naloxone, including negative effect and aversive effects of opioid withdrawal. MORs are inhibitory receptors, therefore we propose that endogenous MOR signaling normally inhibits chnrb4-neurons of the MHb and moderates their known aversive activity, which is unmasked upon receptor blockade. Thus, in addition to facilitating reward at several brain sites, tonic MOR activity may also limit aversion within the MHb circuitry.

Liposome-Encapsulated Morphine Affords a Prolonged Analgesia While Facilitating Extinction of Reward and Aversive Memories

Morphine is thoroughly used for pain control; however, it has a high addictive potential. Opioid liposome formulations produce controlled drug release and have been thoroughly tested for pain treatment although their role in addiction is still unknown. This study investigated the effects of free morphine and morphine encapsulated in unilamellar and multilamellar liposomes on antinociception and on the expression and extinction of the positive and negative memories associated with environmental cues. The hot plate test was used to measure central pain. The rewarding effects of morphine were analyzed by the conditioned-place preference (CPP) test, and the aversive aspects of naloxone-precipitated morphine withdrawal were evaluated by the conditioned-place aversion (CPA) paradigm. Our results show that encapsulated morphine yields prolonged antinociceptive effects compared with the free form, and that CPP and CPA expression were similar in the free- or encapsulated-morphine groups. However, we demonstrate, for the first time, that morphine encapsulation reduces the duration of reward and aversive memories, suggesting that this technological process could transform morphine into a potentially less addictive drug. Morphine encapsulation in liposomes could represent a pharmacological approach for enhancing extinction, which might lead to effective clinical treatments in drug addiction with fewer side effects.

Neuroligin-2 Determines Inhibitory Synaptic Transmission in the Lateral Septum to Optimize Stress-Induced Neuronal Activation and Avoidance Behavior

BACKGROUND

Investigations in the neocortex have revealed that the balance of excitatory and inhibitory synaptic transmission (E/I ratio) is important for proper information processing. The disturbance of this balance underlies many neuropsychiatric illnesses, including autism spectrum disorder and schizophrenia. However, little is known about the contribution of E/I balance to the functioning of subcortical brain regions, such as the lateral septum (LS), a structure that plays important roles in regulating anxiety-related behavior.

METHODS

We manipulated E/I balance in the mouse LS by localized conditional deletion of neuroligin-2, a postsynaptic cell adhesion protein located at gamma-aminobutyric acidergic synapses and important for inhibitory synaptic transmission. We then performed analyses of synaptic transmission in the LS, stress-induced expression of immediate early gene c-fos, and anxiety-related and depression-related behavior.

RESULTS

The absence of neuroligin-2 in the LS in the mature mouse brain resulted in postsynaptic impairment of inhibitory synaptic transmission. Importantly, the reduced inhibition and resulting E/I imbalance decreased the responsiveness of LS neurons to stress. Furthermore, this E/I imbalance in the LS was associated with impaired stress-induced activation of downstream hypothalamic nuclei and reduced avoidance behavior of the animals in the elevated plus maze.

CONCLUSIONS

Our results described the synaptic function of neuroligin-2 in the LS, uncovered a positive association between c-Fos-expressing neurons in the LS and downstream hypothalamic areas and avoidance behavior, and demonstrated that intact inhibitory synaptic transmission and proper E/I balance are required for the optimal functioning of this subcortical circuit.

GABAB receptors modulate morphine antinociception: Pharmacological and genetic approaches

Previous studies in our laboratory showed an interaction between the GABAergic and opioid systems involved in the analgesic effect of baclofen (BAC). Furthermore, it is known that sex differences exist regarding various pharmacological responses of morphine (MOR) and they are related to an increased sensitivity to MOR effects in males. The aims of the present study were to evaluate the possible involvement of the GABA_B receptors in the antinociceptive responses induced by MOR (1, 3 and 9 mg/kg, s.c.) administration using both pharmacological (BAC 2 mg/kg, i.p.; and 2-OH-saclofen, SAC 0.3 mg/kg, intra cisterna magna) and genetic approaches (GABA_B1 knockout mice; GABA_B1 KO) in mice of both sexes. In addition, we explored the alterations in c-Fos expression of different brain areas involved in the antinociceptive effect of MOR using both approaches. The pharmacological approach showed a higher dose-dependent antinociceptive effect of MOR in male mice compared to female mice. BAC and SAC pretreatment potentiated and attenuated the antinociceptive effect of MOR, respectively, in both sexes. The genetic approach revealed a dose-dependent antinociceptive effect of MOR in the wild type mice, but not in the GABA_B1 KO mice and no sex differences were observed. Additionally, BAC and SAC pretreatment and the lack of GABA_B1 subunit of the GABA_B receptor prevented the changes observed in c-Fos expression in the cingulate cortex and nucleus accumbens of male mice. Our results suggest that the GABA_B receptors are involved in the MOR antinociceptive effect of both male and female mice.

Morphine-dependent and abstinent mice are characterized by a broader distribution of the neurons co-expressing mu and delta opioid receptors

Opiate addiction develops as a chronic relapsing disorder upon drug recreational use or following misuse of analgesic prescription. Mu opioid (MOP) receptors are the primary molecular target of opiates but increasing evidence support in vivo functional heteromerization with the delta opioid (DOP) receptor, which may be part of the neurobiological processes underlying opiate addiction. Here, we used double knock-in mice co-expressing fluorescent versions of the MOP and DOP receptors to examine the impact of chronic morphine administration on the distribution of neurons co-expressing the two receptors. Our data show that MOP/DOP neuronal co-expression is broader in morphine-dependent mice and is detected in novel brain areas located in circuits related to drug reward, motor activity, visceral control and emotional processing underlying withdrawal. After four weeks of abstinence, MOP/DOP neuronal co-expression is still detectable in a large number of these brain areas except in the motor circuit. Importantly, chronic morphine administration increased the proportion of MOP/DOP neurons in the brainstem of morphine-dependent and abstinent mice. These findings establish persistent changes in the abstinent state that may modulate relapse and opiate-induced hyperalgesia and also point to the therapeutic potential of MOP/DOP targeting. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Arc reactivity in accumbens nucleus, amygdala and hippocampus differentiates cue over context responses during reactivation of opiate withdrawal memory

Opiate withdrawal induces an early aversive state which can be associated to contexts and/or cues, and re-exposure to either these contexts or cues may participate in craving and relapse. Nucleus accumbens (NAC), hippocampus (HPC) and basolateral amygdala (BLA) are crucial substrates for acute opiate withdrawal, and for withdrawal memory retrieval. Also HPC and BLA interacting with the NAC are suggested to respectively mediate the processing of context and cue representations of drug-related memories. Here we used a paradigm of conditioned suppression of operant food seeking, allowing to differentiate context and cue related responses, to study the influence of withdrawal memories on operant behavior and the underlying neural substrates. catFISH for Arc mRNA expression was used to discriminate cellular responses during context and cue (flashing light) periods in this paradigm. We show that reactivation of the memory of the negative affective state of withdrawal suppresses active lever pressing for food, and this conditioned suppression is generalized to the context. Interestingly the behavioral responses during the context and cue light periods are associated with differential Arc mRNA activations within the NAC, BLA, and HPC. Indeed both periods led to NAC shell activation whereas the NAC core was responsive only following the cue light period. Moreover, BLA and HPC were more responsive during cue-light and context period respectively. These data further support the already reported differential role of these brain structures on cue vs context-induced reinstatement of operant behaviors, and highlight the existence of common mechanisms for the processing of positive and aversive emotional memories.

Distinct regulation pattern of Egr-1, BDNF and Arc during morphine-withdrawal conditioned place aversion paradigm: Role of glucocorticoids

Negative affective aspects of opiate abstinence contribute to the persistence of substance abuse. Importantly, interconnected brain areas involved in aversive motivational processes, such as the ventral tegmental area (VTA) and medial prefrontal cortex (mPFC), become activated when animals are confined to withdrawal-paired environments. In the present study, place aversion was elicited in sham and adrenalectomized (ADX) animals by conditioned naloxone-precipitated drug withdrawal following exposure to chronic morphine. qPCR was employed to detect the expression of brain derived neurotrophic factor (Bdnf) and the immediate early genes (IEG) early growth response 1 (Egr-1) and activity-regulated cytoskeletal-associated protein (Arc) mRNAs in the VTA and mPFC at different time points of the conditioned place aversion (CPA) paradigm: after the conditioning phase and after the test phase. Sham + morphine rats exhibited robust CPA, which was impaired in ADX + morphine animals. Egr-1 and Arc were induced in the VTA and mPFC after morphine-withdrawal conditioning phase. Furthermore, Bdnf expression was enhanced in the VTA during the test phase. Bdnf induction seemed to be glucocorticoid-dependent, given that was correlated with HPA axis function and was not observed in morphine-dependent ADX animals. In addition, BDNF regulation and function was opposite in the VTA and mPFC during aversive-withdrawal memory retrieval. Our results suggest that IEGs and BDNF in these brain regions may play key roles in mediating the negative motivational component of opiate withdrawal.

Mu Opioid Receptors in Gamma-Aminobutyric Acidergic Forebrain Neurons Moderate Motivation for Heroin and Palatable Food

BACKGROUND

Mu opioid receptors (MORs) are central to pain control, drug reward, and addictive behaviors, but underlying circuit mechanisms have been poorly explored by genetic approaches. Here we investigate the contribution of MORs expressed in gamma-aminobutyric acidergic forebrain neurons to major biological effects of opiates, and also challenge the canonical disinhibition model of opiate reward.

METHODS

We used Dlx5/6-mediated recombination to create conditional Oprm1 mice in gamma-aminobutyric acidergic forebrain neurons. We characterized the genetic deletion by histology, electrophysiology, and microdialysis; probed neuronal activation by c-Fos immunohistochemistry and resting-state functional magnetic resonance imaging; and investigated main behavioral responses to opiates, including motivation to obtain heroin and palatable food.

RESULTS

Mutant mice showed MOR transcript deletion mainly in the striatum. In the ventral tegmental area, local MOR activity was intact, and reduced activity was only observed at the level of striatonigral afferents. Heroin-induced neuronal activation was modified at both sites, and whole-brain functional networks were altered in live animals. Morphine analgesia was not altered, and neither was physical dependence to chronic morphine. In contrast, locomotor effects of heroin were abolished, and heroin-induced catalepsy was increased. Place preference to heroin was not modified, but remarkably, motivation to obtain heroin and palatable food was enhanced in operant self-administration procedures.

CONCLUSIONS

Our study reveals dissociable MOR functions across mesocorticolimbic networks. Thus, beyond a well-established role in reward processing, operating at the level of local ventral tegmental area neurons, MORs also moderate motivation for appetitive stimuli within forebrain circuits that drive motivated behaviors.

Memories of Opiate Withdrawal Emotional States Correlate with Specific Gamma Oscillations in the Nucleus Accumbens

Affective memories associated with the negative emotional state experienced during opiate withdrawal are central in maintaining drug taking, seeking, and relapse. Nucleus accumbens (NAC) is a key structure for both acute withdrawal and withdrawal memories reactivation, but the NAC neuron coding properties underpinning the expression of these memories remain largely unknown. Here we aimed at deciphering the role of NAC neurons in the encoding and retrieval of opiate withdrawal memory. Chronic single neuron and local field potentials recordings were performed in morphine-dependent rats and placebo controls. Animals were subjected to an unbiased conditioned placed aversion protocol with one compartment (CS+) paired with naloxone-precipitated withdrawal, a second compartment with saline injection (CS-), and a third being neutral (no pairing). After conditioning, animals displayed a typical place aversion for CS+ and developed a preference for CS- characteristic of safety learning. We found that distinct NAC neurons code for CS+ or CS-. Both populations also displayed highly specific oscillatory dynamics, CS+ and CS- neurons, respectively, following 80 Hz (G80) and 60 Hz (G60) local field potential gamma rhythms. Finally, we found that the balance between G60 and G80 rhythms strongly correlated both with the ongoing behavior of the animal and the strength of the conditioning. We demonstrate here that the aversive and preferred environments are underpinned by distinct groups of NAC neurons as well as specific oscillatory dynamics. This suggest that G60/G80 interplay-established through the conditioning process-serves as a robust and versatile mechanism for a fine coding of the environment emotional weight.

Glucocorticoid receptor but not mineralocorticoid receptor mediates the activation of ERK pathway and CREB during morphine withdrawal

Recent research suggests that glucocorticoids are involved in the development of addiction to drugs of abuse. They share this role with dopamine (DA), and with different signalling pathways and/or transcription factors such as extracellular-signal regulated kinases (ERK) and cAMP response element binding protein (CREB). However, the relation between them is not completely elucidated. In this report, we further characterize the role of glucocorticoid and mineralocorticoid receptor (GR and MR) signalling in DA turnover at the Nacc, and in opiate withdrawal-induced tyrosine hydroxylase (TH) expression, ERK and CREB phosphorylation (activation) in the nucleus of tractus solitarius (NTS-A₂ ). The role of GR and MR signalling was assessed with the selective GR antagonist, mifepristone or the MR antagonist, spironolactone (i.p.). Rats were implanted two morphine (or placebo) pellets. Six days later rats were pretreated with mifepristone, spironolactone or vehicle 30 min before naloxone, and DA turnover, TH expression, ERK and CREB phosphorylation, were measured using HPLC and immunoblotting. Glucocorticoid receptor blockade attenuated ERK and CREB phosphorylation and the TH expression induced by morphine withdrawal. In contrast, no changes were seen after MR blockade. Finally, GR and MR blockade did not alter the morphine withdrawal-induced increase seen both in DA turnover and DA metabolite production, in the NAcc. These results show that not only ERK and CREB phosphorylation but also TH expression in the NTS is modulated by GR signalling. The present results suggest that GR is a therapeutic target to improve aversive events associated with opiate withdrawal.

Different contribution of glucocorticoids in the basolateral amygdala to the formation and expression of opiate withdrawal-associated memories

Drug-withdrawal aversive memories generate a motivational state leading to compulsive drug taking, with plasticity changes in the basolateral amygdala (BLA) being essential in aversive motivational learning. The conditioned-place aversion (CPA) paradigm allows for measuring the negative affective component of drug withdrawal. First, CPA triggers association between negative affective consequences of withdrawal with context (memory consolidation). Afterwards, when the animals are re-exposed to the paired environment, they avoid it due to the association between the context and aversive memories (memory retrieval). We examined the influence of glucocorticoids (GCs) for a morphine-withdrawal CPA paradigm, along with plasticity changes in the BLA, in sham-operated and adrenalectomized (ADX) animals. We demonstrated that sham+morphine animals robustly displayed CPA, whereas ADX-dependent animals lacked the affective-like signs of opiate withdrawal but displayed increased somatic signs of withdrawal. Glucocorticoid receptor (GR) actions promote memory consolidation but highly depend on increases in GC levels. Interestingly, we observed that GCs were only increased in sham-dependent rodents during aversive-withdrawal memory consolidation, and that GR expression correlated with phosphorylated cAMP response element binding (pCREB) protein, early growth response 1 (Egr-1) and activity-regulated cytoskeletal-associated (Arc) mRNA induction in this experimental group. In contrast, ADX-animals displayed reduced (pCREB). GCs are also known to impair memory retrieval. Accordingly, we showed that GCs levels remained at basal levels in all experimental groups following memory retrieval, and consequently GRs no longer acted as transcriptional regulators. Importantly, memory retrieval elicited increased pCREB levels in sham+morphine animals (not in ADX+morphine group), which were directly correlated with enhanced Arc mRNA/protein expression mainly in glutamatergic neurons. In conclusion, context-withdrawal associations are accompanied plasticity changes in the BLA, which are, in part, regulated by GR signaling. Moreover, dysregulation of CREB signaling, in part through Arc expression, may enhance reconsolidation, resulting in the maintenance of excessive aversive states. These findings might have important implications for drug-seeking behavior.

Glucocorticoid Homeostasis in the Dentate Gyrus Is Essential for Opiate Withdrawal-Associated Memories

Drug-withdrawal-associated aversive memories might trigger relapse to drug-seeking behavior. However, changes in structural and synaptic plasticity, as well as epigenetic mechanisms, which may be critical for long-term aversive memory, have yet to be elucidated. We used male Wistar rats and performed conditioned-place aversion (CPA) paradigm to uncover the role of glucocorticoids (GCs) on plasticity-related processes that occur within the dentate gyrus (DG) during opiate-withdrawal conditioning (memory formation-consolidation) and after reactivation by re-exposure to the conditioned environment (memory retrieval). Rats subjected to conditioned morphine-withdrawal robustly expressed CPA, while adrenalectomy impaired naloxone-induced CPA. Importantly, while activity-regulated cytoskeletal-associated protein (Arc) expression was induced in sham- and ADX-dependent animals during the conditioning phase, Arc and early growth response 1 (Egr-1) induction was restricted to sham-dependent rats following memory retrieval. Moreover, we found a correlation between Arc induction and CPA score, and Arc was selectively expressed in the granular zone of the DG in dopaminoceptive, glutamatergic and GABAergic neurons. We further found that brain-derived neurotrophic factor was regulated in the opposite way during the test phase. Our results also suggest a role for epigenetic regulation on the expression of glucocorticoid receptors and Arc following memory retrieval. Our data provide the first evidence that GC homeostasis is important for the expression of long-term morphine-withdrawal memories. Moreover, our results support the idea that targeting Arc and Egr-1 in the DG may provide important insights into the role of these signaling cascades in withdrawal-context memory re-consolidation. Together, disrupting these processes in the DG might lead to effective treatments in drug addiction thereby rapidly and persistently reducing invasive memories and subsequent drug seeking.

Pharmacologic and Behavioral Withdrawal From Addictive Drugs

Recent theories suggest that drug withdrawal does not motivate drug use and relapse. However, data now show that withdrawal produces complex changes over time in at least two symptoms (i.e., negative affect and urges) that are highly predictive of relapse. Evidence suggests that falling levels of the drug in the blood and interruption of the drug self-administration ritual both affect these symptoms. Both of these forms of withdrawal motivate renewed drug use in addicted individuals.

Double Dissociation of Monoacylglycerol Lipase Inhibition and CB1 Antagonism in the Central Amygdala, Basolateral Amygdala, and the Interoceptive Insular Cortex on the Affective Properties of Acute Naloxone-Precipitated Morphine Withdrawal in Rats

Both CB1 receptor antagonism and agonism, in particular by 2-arachidonyl glycerol (2-AG), have been shown to reduce somatic symptoms of morphine withdrawal (MWD). Here we evaluated the effects of both systemic pretreatment with the monoacylglycerol lipase (MAGL) inhibitor MJN110 (which selectively elevates 2-AG) and central administration of both MJN110 and the CB1 antagonist (AM251) on the affective properties of MWD. Acute MWD induced place aversion occurs when naloxone is administered 24 h following a single exposure to a high dose of morphine. Systemic pretreatment with the MAGL inhibitor, MJN110, prevented the aversive effects of acute MWD by a CB1 receptor-dependent mechanism. Furthermore, in a double dissociation, AM251 infusions into the central amygdala, but MJN110 infusions into the basolateral amygdala, interfered with the naloxone-precipitated MWD induced place aversion. As well, MJN110, but not AM251, infusions into the interoceptive insular cortex (a region known to be activated in acute MWD) also prevented the establishment of the place aversion by a CB1 mechanism of action. These findings reveal the respective sites of action of systemically administered MJN110 and AM251 in regulating the aversive effects of MWD.

Toward a systems-oriented approach to the role of the extended amygdala in adaptive responding

Research into the structure and function of the basal forebrain macrostructure called the extended amygdala (EA) has recently seen considerable growth. This paper reviews that work, with the objectives of identifying underlying themes and developing a common goal towards which investigators of EA function might work. The paper begins with a brief review of the structure and the ontological and phylogenetic origins of the EA. It continues with a review of research into the role of the EA in both aversive and appetitive states, noting that these two seemingly disparate avenues of research converge on the concept of reinforcement - either negative or positive - of adaptive responding. These reviews lead to a proposal as to where the EA may fit in the organization of the basal forebrain, and an invitation to investigators to place their findings in a unifying conceptual framework of the EA as a collection of neural ensembles that mediate adaptive responding.

Regulation of dopaminergic markers expression in response to acute and chronic morphine and to morphine withdrawal

Dopamine (DA) is thought to represent a teaching signal and has been implicated in the induction of addictive behaviours. Dysfunction of DA homeostasis leading to high or low DA levels is causally linked to addiction. Previously, it has been proposed that the transcription factors Nurr1 and Pitx3, which are critical for transcription of a set of genes involved in DA metabolism in the mesolimbic pathway, are associated with addiction pathology. Using quantitative real-time polymerase chain reaction, immunofluorescence and Western blotting, we studied the effects of single morphine administration, morphine dependence and withdrawal on the DA markers DA transporters (DAT), vesicular monoamine transporters (VMAT2) and DA 2 receptor subtype (DRD2), DA 1 receptor subtype as well as tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) and/or nucleus accumbens (NAc). In addition, Nurr1 and Pitx3 expression was also measured. Present data showed a high degree of colocalization of Nurr1 and Pitx3 with TH(+) neurons in the VTA. We found that the increased Nurr1 and/or Pitx3 levels during morphine dependence and in morphine-withdrawn rats were associated to an increase of DAT, VMAT2 and DRD2. Altogether, present data indicate that morphine dependence and withdrawal induced consistent alterations of most of the DA markers, which was correlated with transcription factors involved in the maintenance of DA neurons in drug-reward pathways, suggesting that Nurr1 and Pitx3 regulation might be associated with controlling adaptation to chronic morphine and to morphine withdrawal-induced alterations of DA neurons activity in the mesolimbic pathway.

Expression of heat shock protein 27 and troponin T and troponin I after naloxone-precipitated morphine withdrawal

Heat shock protein (Hsp27) renders cardioprotection from stress situations but little is known about its role in myofilaments. In this study we have evaluated the relationship between Hsp27 and troponin response after naloxone-induced morphine withdrawal. Rats were treated with two morphine (75 mg) pellets during six days. Precipitated withdrawal was induced by naloxone on day seven. Hsp27 expression, Hsp27 phosphorylated at serine 82 (Ser82), cardiac troponin T (cTnT), cardiac troponin I (cTnI) and µ-calpain were evaluated by immunoblotting in left ventricle. Hsp, cTnT and cTnI was also evaluated by immunofluorescence procedure. Our results show that enhancement in Hsp27 expression and phosphorylation induced by naloxone-precipitated morphine withdrawal occurs with concomitant increases of cTnT and µ-calpain expression, whereas cTnI was decreased. We also observed co-localization of Hsp27 with cTnT in cardiac tissues. These findings provide new information into the possible role of Hsp27 in the protection of cTnT degradation by µ-calpain (a protease mediating proteolysis of cTnT and cTnI) after morphine withdrawal.

Persistent Adaptations in Afferents to Ventral Tegmental Dopamine Neurons after Opiate Withdrawal

Protracted opiate withdrawal is accompanied by altered responsiveness of midbrain dopaminergic (DA) neurons, including a loss of DA cell response to morphine, and by behavioral alterations, including affective disorders. GABAergic neurons in the tail of the ventral tegmental area (tVTA), also called the rostromedial tegmental nucleus, are important for behavioral responses to opiates. We investigated the tVTA-VTA circuit in rats after chronic morphine exposure to determine whether tVTA neurons participate in the loss of opiate-induced disinhibition of VTA DA neurons observed during protracted withdrawal. In vivo recording revealed that VTA DA neurons, but not tVTA GABAergic neurons, are tolerant to morphine after 2 weeks of withdrawal. Optogenetic stimulation of tVTA neurons inhibited VTA DA neurons similarly in opiate-naive and long-term withdrawn rats. However, tVTA inactivation increased VTA DA activity in opiate-naive rats, but not in withdrawn rats, resembling the opiate tolerance effect in DA cells. Thus, although inhibitory control of DA neurons by tVTA is maintained during protracted withdrawal, the capacity for disinhibitory control is impaired. In addition, morphine withdrawal reduced both tVTA neural activity and tonic glutamatergic input to VTA DA neurons. We propose that these changes in glutamate and GABA inputs underlie the apparent tolerance of VTA DA neurons to opiates after chronic exposure. These alterations in the tVTA-VTA DA circuit could be an important factor in opiate tolerance and addiction. Moreover, the capacity of the tVTA to inhibit, but not disinhibit, DA cells after chronic opiate exposure may contribute to long-term negative affective states during withdrawal.

SIGNIFICANCE STATEMENT

Dopaminergic (DA) cells of the ventral tegmental area (VTA) are the origin of a brain reward system and are critically involved in drug abuse. Morphine has long been known to affect VTA DA cells via GABAergic interneurons. Recently, GABAergic neurons caudal to the VTA were discovered and named the tail of VTA (tVTA). Here, we show that tVTA GABA neurons lose their capacity to disinhibit, but not to inhibit, VTA DA cells after chronic opiate exposure. The failure of disinhibition was associated with a loss of glutamatergic input to DA neurons after chronic morphine. These findings reveal mechanisms by which the tVTA may play a key role in long-term negative affective states during opiate withdrawal.

Morphine administration modulates expression of Argonaute 2 and dopamine-related transcription factors involved in midbrain dopaminergic neurons function

BACKGROUND AND PURPOSE

Alterations in transcription factors that regulate the development and maintenance of dopamine (DA) neurons (such as Nurr1 and Pitx3) play an important role in the pathogenesis of addiction diseases. We have examined the effects of acute and chronic morphine and morphine withdrawal on TH expression and activity as well as expression of Nurr1, Pitx3 and Ago2 in the ventral tegmental area (VTA) and nucleus accumbens (NAc) of the rat.

EXPERIMENTAL APPROACH

Rats were injected acutely with morphine and decapitated 1 or 2 h later. Another set of rats were made dependent on morphine by implantation of two morphine pellets. Precipitated withdrawal was induced by injection of naloxone. Ago2, Pitx3, Nurr1, total TH (tTH), TH phosphorylated at Ser31 and at Ser40, and 3,4-Dihydroxyphenylacetic acid, and DA determination in the VTA and/or NAc were measured using immunoblotting, HPLC and immunofluorescence.

KEY RESULTS

Acute morphine produced a marked increase in TH activity and DA turnover in the NAc, concomitantly with increased Nurr1 and Pitx3 expression in the VTA. In contrast, precipitated morphine withdrawal decreased TH activation, TH expression and did not increase DA turnover in the NAc. These effects paralleled decreases in Ago2 expression, which was accompanied by increased Nurr1 and Pitx3, TH activity and normalized TH protein levels in the VTA.

CONCLUSIONS AND IMPLICATIONS

The combined decrease in Ago2 and increases in Nurr1 and Pitx3 might represent some of the mechanisms that served to protect against accumbal TH regulation observed in morphine withdrawn rats, which may be critical for DA bioavailability to influence behaviour.

Regulation of Pleiotrophin, Midkine, Receptor Protein Tyrosine Phosphatase β/ζ, and Their Intracellular Signaling Cascades in the Nucleus Accumbens During Opiate Administration

BACKGROUND

Most classes of addictive substances alter the function and structural plasticity of the brain reward circuitry. Midkine (MK) and pleiotrophin (PTN) are growth/differentiation cytokines which, similarly to neurotrophins, play an important role in repair, neurite outgrowth, and cell differentiation. PTN or MK signaling through receptor protein tyrosine phosphatase β/ζ (RPTPβ/ζ), leads to the activation of extracellular signal-regulated kinases and thymoma viral proto-oncogene. This activation induces morphological changes and modulates addictive behaviors. Besides, there is increasing evidence that during the development of drug addiction, astrocytes contribute to the synaptic plasticity by synthesizing and releasing substances such as cytokines.

METHODS

In the present work we studied the effect of acute morphine administration, chronic morphine administration, and morphine withdrawal on PTN, MK, and RPTPβ/ζ expression and on their signaling pathways in the nucleus accumbens.

RESULTS

Present results indicated that PTN, MK, and RPTPβ/ζ levels increased after acute morphine injection, returned to basal levels during chronic opioid treatment, and were up-regulated again during morphine withdrawal. We also observed an activation of astrocytes after acute morphine injection and during opiate dependence and withdrawal. In addition, immunofluorescence analysis revealed that PTN, but not MK, was overexpressed in astrocytes and that dopaminoceptive neurons expressed RPTPβ/ζ.

CONCLUSIONS

All these observations suggest that the neurotrophic and behavioral adaptations that occur during opiate addiction could be, at least partly, mediated by cytokines.

Differential effects of endocannabinoid catabolic inhibitors on morphine withdrawal in mice

BACKGROUND

Inhibition of endocannabinoid catabolic enzymes fatty acid amide hydrolase (FAAH) and/or monoacylglycerol lipase (MAGL) reduces somatic morphine withdrawal signs, but its effects on aversive aspects of withdrawal are unknown. The present study investigated whether Δ(9)-tetrahydrocannabinol (THC), the MAGL inhibitor JZL184, the FAAH inhibitor PF-3845, or the dual FAAH/MAGL inhibitor SA-57 would reduce acquisition of morphine withdrawal-induced conditioned place avoidance (CPA) and jumping.

METHODS

Mice were implanted with placebo or 75 mg morphine pellets, 48 h later injected with naloxone or saline and placed in the conditioning apparatus, and assessed for CPA at 72 h. Subjects were also observed for jumping behavior following naloxone challenge.

RESULTS

Naloxone (0.056 mg/kg) produced robust CPA in morphine-pelleted, but not placebo-pelleted, mice. Morphine pretreatment prevented the occurrence of withdrawal CPA and withdrawal jumping, while clonidine (an α2 adrenergic receptor agonist) only blocked withdrawal CPA. THC, JZL184, and SA-57 significantly reduced the percentage of mice that jumped during the conditioning session, but did not affect acquisition of withdrawal CPA. PF-3845 did not reduce morphine withdrawal CPA or jumping. Finally, neither THC nor the endocannabinoid catabolic enzyme inhibitors in non-dependent mice elicited a conditioned place preference or aversion.

CONCLUSIONS

These findings suggest that inhibiting endocannabinoid catabolic enzymes reduces somatic morphine withdrawal signs, but not aversive aspects as inferred in the CPA paradigm. The observation that non-dependent mice administered inhibitors of endocannabinoid degradation did not display place preferences is consistent with the idea that that endocannabinoid catabolic enzymes might be targeted therapeutically, with reduced risk of abuse.

Morphine regulates Argonaute 2 and TH expression and activity but not miR-133b in midbrain dopaminergic neurons

Epigenetic changes such as microRNAs (miRs)/Ago2-induced gene silencing represent complex molecular signature that regulate cellular plasticity. Recent studies showed involvement of miRs and Ago2 in drug addiction. In this study, we show that changes in gene expression induced by morphine and morphine withdrawal occur with concomitant epigenetic modifications in the mesolimbic dopaminergic (DA) pathway [ventral tegmental area (VTA)/nucleus accumbens (NAc) shell], which is critically involved in drug-induced dependence. We found that acute or chronic morphine administration as well as morphine withdrawal did not modify miR-133b messenger RNA (mRNA) expression in the VTA, whereas Ago2 protein levels were decreased and increased in morphine-dependent rats and after morphine withdrawal, respectively. These changes were paralleled with enhanced and decreased NAc tyrosine hydroxylase (TH) protein (an early DA marker) in morphine-dependent rats and after withdrawal, respectively. We also observed changes in TH mRNA expression in the VTA that could be related to Ago2-induced translational repression of TH mRNA during morphine withdrawal. However, the VTA number of TH-positive neurons suffered no alterations after the different treatment. Acute morphine administration produced a marked increase in TH activity and DA turnover in the NAc (shell). In contrast, precipitated morphine withdrawal decreased TH activation and did not change DA turnover. These findings provide new information into the possible correlation between Ago2/miRs complex regulation and DA neurons plasticity during opiate addiction.

CB1 antagonism: interference with affective properties of acute naloxone-precipitated morphine withdrawal in rats

RATIONALE

Modulation of the endocannabinoid system has been found to interfere with opiate withdrawal. The potential of activation and blockade of the endocannabinoid system to prevent the aversive-affective state of naloxone-precipitated morphine withdrawal (MWD) was investigated in a one-trial conditioned place aversion (CPA) paradigm.

OBJECTIVE

CPA provides a sensitive measure of the motivational effects of acute MWD. The potential of the fatty acid amide hydrolase (FAAH) inhibitors, URB597 and PF-3845, the CB1 antagonist/inverse agonist, AM251, and the neutral CB1 antagonists, AM4113 and AM6527 (oral), to interfere with establishment of a MWD-induced CPA was investigated. As well, the potential of AM251 and AM4113 to interfere with reinstatement of a previously established MWD-induced CPA was investigated.

MATERIALS AND METHODS

Using a one-trial place conditioning paradigm, rats were administered naloxone (1 mg/kg, subcutaneous (s.c.)) 24 h after receiving a high dose of morphine (20 mg/kg, s.c.) and were placed on the conditioning floor. To determine the effect of each pretreatment drug on the establishment of the MWD-induced CPA, URB597 (0.3 mg/kg, intraperitoneally (i.p.)), PF-3845 (10 mg/kg, i.p.), AM251 (1 or 2.5 mg/kg, i.p.), AM4113 (1 or 2.5 mg/kg, i.p.), and AM6527 (5 mg/kg, oral) were administered prior to conditioning.

RESULTS

AM251 (2.5, but not 1 mg/k), AM4113, and AM6527, but not URB597 or PF-3845, interfered with the establishment of the MWD-induced CPA. AM251 and AM4113 did not prevent reinstatement of the CPA.

CONCLUSIONS

Neutral antagonism of the CB1 receptor reduces the aversive affective properties of morphine withdrawal.

Acute morphine alters GABAergic transmission in the central amygdala during naloxone-precipitated morphine withdrawal: role of cyclic AMP

The central amygdala (CeA) plays an important role in opioid addiction. Therefore, we examined the effects of naloxone-precipitated morphine withdrawal (WD) on GABAergic transmission in rat CeA neurons using whole-cell recordings with naloxone in the bath. The basal frequency of miniature inhibitory postsynaptic currents (mIPSCs) increased in CeA neurons from WD compared to placebo rats. Acute morphine (10 μ M) had mixed effects (≥20% change from baseline) on mIPSCs in placebo and WD rats. In most CeA neurons (64%) from placebo rats, morphine significantly decreased mIPSC frequency and amplitude. In 32% of placebo neurons, morphine significantly increased mIPSC amplitudes but had no effect on mIPSC frequency. In WD rats, acute morphine significantly increased mIPSC frequency but had no effect on mIPSC amplitude in 41% of CeA neurons. In 45% of cells, acute morphine significantly decreased mIPSC frequency and amplitude. Pre-treatment with the cyclic AMP inhibitor (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium (RP), prevented acute morphine-induced potentiation of mIPSCs. Pre-treatment of slices with the Gi/o G-protein subunit inhibitor pertussis toxin (PTX) did not prevent the acute morphine-induced enhancement or inhibition of mIPSCs. PTX and RP decreased basal mIPSC frequencies and amplitudes only in WD rats. The results suggest that inhibition of GABAergic transmission in the CeA by acute morphine is mediated by PTX-insensitive mechanisms, although PTX-sensitive mechanisms cannot be ruled out for non-morphine responsive cells; by contrast, potentiation of GABAergic transmission is mediated by activated cAMP signaling that also mediates the increased basal GABAergic transmission in WD rats. Our data indicate that during the acute phase of WD, the CeA opioid and GABAergic systems undergo neuroadaptative changes conditioned by a previous chronic morphine exposure and dependence.

Dysregulation of dopaminergic regulatory mechanisms in the mesolimbic pathway induced by morphine and morphine withdrawal

Dopamine (DA) is thought to represent a teaching signal and has been implicated in the induction of addictive behaviours. Previously, it has been proposed that the transcription factors Nurr1 and Pitx3, which are critical for transcription of a set of genes involved in DA metabolism in the mesolimbic pathway, are associated with addiction pathology. The aim of our study was to investigate abnormalities in the mesolimbic pathway associated with morphine dependence and withdrawal. Using quantitative real-time PCR, immunofluorescence, HPLC and Western blotting, here we studied the effects of single morphine administration, morphine dependence and morphine withdrawal on Nurr1 and Pitx3 expression as well as on the DA marker tyrosine hydroxylase (TH) and the turnover of DA in the ventral tegmental area (VTA) and/or nucleus accumbens. We showed that the three experimental conditions caused induction of Nurr1 and Pitx3 in the VTA, which correlated with changes in TH expression during chronic morphine administration. Present data also confirmed the colocalization of Nurr1 and Pitx3 with TH-positive neurons in the posterior VTA. Furthermore, during morphine dependence, Nurr1 was detected in the nucleus compartment of VTA TH-positive neurons, whereas Pitx3 was strongly detected in the nucleus of TH-positive neurons after single morphine administration and during morphine withdrawal. The number of TH neurons, number of Nurr1 or Pitx3-positive cells, and the number of TH neurons expressing Nurr1 or Pitx3 were not modified in the subpopulations of DA neurons. Present data provide novel insight into the potential correlation between Nurr1 and Pitx3 and DA neurons plasticity during opiate addiction in the mesolimbic pathway.

Acupuncture Stimulation Attenuates Impaired Emotional-Like Behaviors and Activation of the Noradrenergic System during Protracted Abstinence following Chronic Morphine Exposure in Rats

The purpose of this study was to evaluate whether acupuncture stimulation attenuates withdrawal-induced behaviors in the rats during protracted abstinence following chronic morphine exposure. To do this, male rats were first exposed to morphine gradually from 20 to 100 mg/kg for 5 days, and subsequently naloxone was injected once to extend despair-related withdrawal behaviors for 4 weeks. Acupuncture stimulation was performed once at the SP6 (Sanyinjiao) acupoint on rat's; hind leg for 5 min during protracted abstinence from morphine. The acupuncture stimulation significantly decreased despair-like behavior deficits in the forced swimming test and low sociability in the open-field test as well as increased open-arm exploration in the elevated plus maze test in the last week of 4-week withdrawal period. Also the acupuncture stimulation significantly suppressed the increase in the hypothalamic corticotropin-releasing factor (CRF) expression, the decrease in the tyrosine hydroxylase expression in the locus coeruleus, and the decrease in the hippocampal brain-derived neurotrophic factor mRNA expression, induced by repeated injection of morphine. Taken together, these findings demonstrate that the acupuncture stimulation of SP6 significantly reduces withdrawal-induced behaviors, induced by repeated administration of morphine in rats, possibly through the modulation of hypothalamic CRF and the central noradrenergic system.

Crosstalk between G protein-coupled receptors (GPCRs) and tyrosine kinase receptor (TXR) in the heart after morphine withdrawal

G protein-coupled receptors (GPCRs) comprise a large family of membrane receptors involved in signal transduction. These receptors are linked to a variety of physiological and biological processes such as regulation of neurotransmission, growth, and cell differentiation among others. Some of the effects of GPCRs are known to be mediated by the activation of mitogen-activated extracellular kinase (MAPK) pathways. Cross-talk among various signal pathways plays an important role in activation of intracellular and intranuclear signal transduction cascades. Naloxone-induced morphine withdrawal leads to an up-regulation of adenyl cyclase-mediated signaling, resulting in high expression of protein kinase (PK) A. In addition, there is also an increased expression of extracellular signal regulated kinase (ERK), one member of MAPK. For this reason, the crosstalk between these GPCRs and receptors with tyrosine kinase activity (TKR) can be considered a possible mechanism for adaptive changes that occurs after morphine withdrawal. Morphine withdrawal activates ERK1/2 and phosphorylated tyrosine hydroxylase (TH) at Ser31 in the right and left ventricle. When N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004), a PKA inhibitor was infused, the ability of morphine withdrawal to activate ERK, which phosphorylates TH at Ser31, was reduced. The present finding demonstrated that the enhancement of ERK1/2 expression and the phosphorylation state of TH at Ser31 during morphine withdrawal are dependent on PKA and suggest cross-talk between PKA and ERK1/2 transduction pathway mediating morphine withdrawal-induced activation of TH. Increasing understanding of the mechanisms that interconnect the two pathway regulated by GPCRs and TKRs may facilitate the design of new therapeutic strategies.

Opiate dependence induces network state shifts in the limbic system

Among current theories of addiction, hedonic homeostasis dysregulation predicts that the brain reward systems, particularly the mesolimbic dopamine system, switch from a physiological state to a new "set point." In opiate addiction, evidence show that the dopamine system principal targets, prefrontal cortex (PFC), nucleus accumbens (NAC) and basolateral amygdala complex (BLA) also adapt to repeated drug stimulation. Here we investigated the impact of chronic morphine on the dynamics of the network of these three interconnected structures. For that purpose we performed simultaneous electrophysiological recordings in freely-moving rats subcutaneously implanted with continuous-release morphine pellets. Chronic morphine produced a shift in the network state underpinned by changes in Delta and Gamma oscillations in the LFP of PFC, NAC and BLA, in correlation to behavioral changes. However despite continuous stimulation by the drug, an apparent normalization of the network activity and state occurred after 2 days indicating large scale adaptations. Blockade of μ opioid receptors was nonetheless sufficient to disrupt this acquired new stability in morphine-dependent animals. In line with the homeostatic dysregulation theory of addiction, our study provides original direct evidence that the PFC-NAC-BLA network of the dependent brain is characterized by a de novo balance for which the drug of abuse becomes the main contributor.