Enhanced morphine preference following prolonged abstinence: association with increased Fos expression in the extended amygdala

Analysis and study of the mechanism of narcotic addiction and withdrawal

Narcotic drugs refer to drugs that have anesthetic effects on the central nervous system, and they easily produce physical dependence and mental dependence and can be addictive due to continuous use, abuse or unreasonable use. In this paper, bioinformatics and data analysis and mining techniques were used to analyze the methylation differences in transcriptional and clinical data of narcotic addiction in public databases, to explore the mechanism of narcotic addiction, and to mine some norepinephrine drugs. This study confirmed the possibility of using norepinephrine as an auxiliary drug for drug addiction rehabilitation. In addition, we also conducted a similar analysis on the addiction of three drugs. The results showed that the differences in the body caused by the ingestion of opiates and cocaine were significantly greater than those caused by the ingestion of methamphetamine.

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.

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".

Ethanol modulation of cortico-basolateral amygdala circuits: Neurophysiology and behavior

This review highlights literature relating the anatomy, physiology, and behavioral contributions by projections between rodent prefrontal cortical areas and the basolateral amygdala. These projections are robustly modulated by both environmental experience and exposure to drugs of abuse including ethanol. Recent literature relating optogenetic and chemogenetic dissection of these circuits within behavior both compliments and occasionally challenges roles defined by more traditional pharmacological or lesion-based approaches. In particular, cortico-amygdala circuits help control both aversive and reward-seeking. Exposure to pathology-producing environments or abused drugs dysregulates the relative 'balance' of these outcomes. Modern circuit-based approaches have also shown that overlapping populations of neurons within a given brain region frequently govern both aversion and reward-seeking. In addition, these circuits often dramatically influence 'local' cortical or basolateral amygdala excitatory or inhibitory circuits. Our understanding of these neurobiological processes, particularly in relation to ethanol research, has just begun and represents a significant opportunity.

Anxiety during abstinence from alcohol: A systematic review of rodent and human evidence for the anterior insula's role in the abstinence network

Alcohol Use Disorder (AUD) is a chronic, relapsing disease that impacts almost a third of Americans. Despite effective treatments for attaining sobriety, the majority of patients relapse within a year, making relapse a substantial barrier to long-term treatment success. A major factor contributing to relapse is heightened negative affect that results from the combination of abstinence-related increases in stress-reactivity and decreases in reward sensitivity. Substantial research has contributed to the understanding of reward-related changes in AUD. However, less is known about anxiety during abstinence, a critical component of understanding addiction as anxiety during abstinence can trigger relapse. Most of what we know about abstinence-related negative affect comes from rodent studies which have identified key brain regions responsible for abstinence-related behaviors. This abstinence network is composed of brain regions that make up the extended amygdala: the nucleus accumbens (NAcc), the central nucleus of the amygdala (CeA), and the bed nucleus of the stria terminalis (BNST). More recently, emerging evidence from rodent and human studies suggests a fourth brain region, the anterior insula, might be part of the abstinence network. Here, we review current rodent and human literature on the extended amygdala's role in alcohol abstinence and anxiety, present evidence for the anterior insula's role in the abstinence network, and provide future directions for research to further elucidate the neural underpinnings of abstinence in humans. A better understanding of the abstinence network is critical toward understanding and possibly preventing relapse in AUD.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Gene coexpression patterns predict opiate-induced brain-state transitions

Opioid addiction is a chronic, relapsing disorder associated with persistent changes in brain plasticity. Reconfiguration of neuronal connectivity may explain heightened abuse liability in individuals with a history of chronic drug exposure. To characterize network-level changes in neuronal activity induced by chronic opiate exposure, we compared FOS expression in mice that are morphine-naïve, morphine-dependent, or have undergone 4 wk of withdrawal from chronic morphine exposure, relative to saline-exposed controls. Pairwise interregional correlations in FOS expression data were used to construct network models that reveal a persistent reduction in connectivity strength following opiate dependence. Further, we demonstrate that basal gene expression patterns are predictive of changes in FOS correlation networks in the morphine-dependent state. Finally, we determine that regions of the hippocampus, striatum, and midbrain are most influential in driving transitions between opiate-naïve and opiate-dependent brain states using a control theoretic approach. This study provides a framework for predicting the influence of specific therapeutic interventions on the state of the opiate-dependent brain.

The Negative Affect of Protracted Opioid Abstinence: Progress and Perspectives From Rodent Models

Opioid use disorder (OUD) is characterized by the development of a negative emotional state that develops after a history of long-term exposure to opioids. OUD represents a true challenge for treatment and relapse prevention. Human research has amply documented emotional disruption in individuals with an opioid substance use disorder, at both behavioral and brain activity levels; however, brain mechanisms underlying this particular facet of OUD are only partially understood. Animal research has been instrumental in elucidating genes and circuits that adapt to long-term opioid use or are modified by acute withdrawal, but research on long-term consequences of opioid exposure and their relevance to the negative affect of OUD remains scarce. In this article, we review the literature with a focus on two questions: 1) Do we have behavioral models in rodents, and what do they tell us? and 2) What do we know about the neuronal populations involved? Behavioral rodent models have successfully recapitulated behavioral signs of the OUD-related negative affect, and several neurotransmitter systems were identified (i.e., serotonin, dynorphin, corticotropin-releasing factor, oxytocin). Circuit mechanisms driving the negative mood of prolonged abstinence likely involve the 5 main reward-aversion brain centers (i.e., nucleus accumbens, bed nucleus of the stria terminalis, amygdala, habenula, and raphe nucleus), all of which express mu opioid receptors and directly respond to opioids. Future work will identify the nature of these mu opioid receptor-expressing neurons throughout reward-aversion networks, characterize their adapted phenotype in opioid abstinent animals, and hopefully position these primary events in the broader picture of mu opioid receptor-associated brain aversion networks.

Endocannabinoid control of the insular-bed nucleus of the stria terminalis circuit regulates negative affective behavior associated with alcohol abstinence

Negative affect is a core symptom domain associated with an array of neurological and psychiatric disorders and is only partially targeted by current therapies, highlighting the need for better, more targeted treatment options. This study focuses on negative affective symptoms associated with prolonged alcohol abstinence, one of the leading causes of relapse. Using a mouse model of chronic alcohol consumption followed by forced abstinence (CDFA), prolonged alcohol abstinence increased c-fos expression and spontaneous glutamatergic neurotransmission in the dorsal bed nucleus of the stria terminalis (dBNST), a region heavily implicated in negative affect in both humans and rodents. Further, pharmacologically enhancing endogenous cannabinoids (eCB) with JZL184 prevents abstinence-induced increases in dBNST neuronal activity, underscoring the therapeutic potential of drugs targeting the brain's eCB system. Next, we used a channelrhodopsin-assisted mapping strategy to identify excitatory inputs to the dBNST that could contribute to CDFA-induced negative affect. We identified the insular cortex (insula), a region involved in regulating interoception, as a dense, functional, eCB-sensitive input to the dBNST. Using a chemogenetic strategy to locally mimic eCB signaling, we demonstrate that the insula strongly influences the CDFA behavioral phenotype and dBNST neuronal activity. Lastly, we used an anterograde strategy for transynaptic targeting of Cre expression in combination with a Gq-DREADD to selectively recruit dBNST neurons receiving insula projections. Chemogenetic recruitment of these neurons mimicked behavioral and c-fos responses observed in CDFA. Collectively, this study supports a role for the insula-BNST neural circuit in negative affective disturbances and highlights the therapeutic potential of the eCB system for treating negative affective disorders.

Association of contextual cues with morphine reward increases neural and synaptic plasticity in the ventral hippocampus of rats

Drug addiction is associated with aberrant memory and permanent functional changes in neural circuits. It is known that exposure to drugs like morphine is associated with positive emotional states and reward-related memory. However, the underlying mechanisms in terms of neural plasticity in the ventral hippocampus, a region involved in associative memory and emotional behaviors, are not fully understood. Therefore, we measured adult neurogenesis, dendritic spine density and brain-derived neurotrophic factor (BDNF) and TrkB mRNA expression as parameters for synaptic plasticity in the ventral hippocampus. Male Sprague Dawley rats were subjected to the CPP (conditioned place preference) paradigm and received 10 mg/kg morphine. Half of the rats were used to evaluate neurogenesis by immunohistochemical markers Ki67 and doublecortin (DCX). The other half was used for Golgi staining to measure spine density and real-time quantitative reverse transcription-polymerase chain reaction to assess BDNF/TrkB expression levels. We found that morphine-treated rats exhibited more place conditioning as compared with saline-treated rats and animals that were exposed to the CPP without any injections. Locomotor activity did not change significantly. Morphine-induced CPP significantly increased the number of Ki67 and DCX-labeled cells in the ventral dentate gyrus. Additionally, we found increased dendritic spine density in both CA1 and dentate gyrus and an enhancement of BDNF/TrkB mRNA levels in the whole ventral hippocampus. Ki67, DCX and spine density were significantly correlated with CPP scores. In conclusion, we show that morphine-induced reward-related memory is associated with neural and synaptic plasticity changes in the ventral hippocampus. Such neural changes could underlie context-induced drug relapse.

Behavioral and transcriptional patterns of protracted opioid self-administration in mice

Chronic exposure to opioids induces adaptations in brain function that lead to the formation of the behavioral and physiological symptoms of drug dependence and addiction. Animal models commonly used to test these symptoms typically last less than two weeks, which is presumably too short to observe the alterations in the brain that accompany drug addiction. Here, we analyzed the phenotypic and molecular effects of nearly lifelong morphine or saccharin intake in C57BL/6J mice. We used multiple paradigms to evaluate the symptoms of compulsive drug intake: a progressive ratio schedule, intermittent access and a schedule involving a risk of punishment were programmed into an automated IntelliCage system. Gene expression profiles were evaluated in the striatum using whole-genome microarrays and further validated using quantitative polymerase chain reaction in the striatum and the prefrontal cortex. Mice voluntary self-administering morphine showed addiction-related behavioral pattern that included: higher motivation to work for a drug reward, increased reward seeking and increased craving. The analysis of molecular changes revealed a tolerance effect in the transcriptional response to morphine injection (20 mg/kg, ip), as well as some long-lasting alterations in gene expression profiles between the analyzed groups of animals. Interestingly, among the morphine-drinking animals, certain transcriptional profiles were found to be associated with alterations in behavior. In conclusion, our model represents a novel approach for investigating the behavioral and molecular mechanisms underlying opioid addiction. Prolonged morphine intake caused adaptive processes in the brain that manifested as altered behavior and transcriptional sensitivity to opioids.

The paraventricular nucleus of the thalamus is differentially recruited by stimuli conditioned to the availability of cocaine versus palatable food

The paraventricular nucleus of the thalamus (PVT) is not traditionally considered part of the brain addiction neurocircuitry but has received growing attention with regard to a role in the modulation of drug-seeking behavior. This study sought to establish the pattern of neural activation induced by a response-reinstating discriminative stimulus (SD ) conditioned to either cocaine (COC) or a conventional reinforcer using a palatable food substance, sweetened condensed milk (SCM). Male Wistar rats were trained to associate one SD (S+ ; COC or SCM availability) and a distinctly different SD (S- ; non-reward; i.e. the availability of saline or the absence of SCM). Following extinction of COC- and SCM-reinforced responding, rats were presented with the respective S+ or S- alone and tested for the reinstatement of reward seeking. The COC S+ and SCM S+ elicited identical reinstatement, whereas the non-reward S- was behaviorally ineffective. PVT sections were obtained following completion of the reinstatement tests and labeled for Fos. The number of Fos+ neurons was compared among rats that were presented with the COC S+ , SCM S+ or S- . Rats that were presented with the COC S+ exhibited a significant increase in Fos expression compared with rats that were presented with the S- . Moreover, Fos expression was significantly correlated with the number of reinstatement responses that were induced by the COC S+ . In contrast, the SCM S+ and S- produced identical increases in Fos expression, without behaviorally relevant correlations. The findings implicate the PVT as an important site that is selectively recruited during COC-seeking behavior.

Involvement of ventral tegmental area ionotropic glutamate receptors in the expression of ethanol-induced conditioned place preference

The ventral tegmental area (VTA) is a well-established neural substrate of reward-related processes. Activity within this structure is increased by the primary and conditioned rewarding effects of abused drugs and its engagement is heavily reliant on excitatory input from structures upstream. In the case of drug seeking, it is thought that exposure to drug-associated cues engages glutamatergic VTA afferents that signal directly to dopamine cells, thereby triggering this behavior. It is unclear, however, whether glutamate input to VTA is directly involved in ethanol-associated cue seeking. Here, the role of intra-VTA ionotropic glutamate receptor (iGluR) signaling in ethanol-cue seeking was evaluated in DBA/2J mice using an ethanol conditioned place preference (CPP) procedure. Intra-VTA iGluRs α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR)/kainate and N-methyl-d-aspartate (NMDAR) were blocked during ethanol CPP expression by co-infusion of antagonist drugs 6,7-dinitroquinoxaline-2,3-dione (DNQX; AMPA/kainate) and d-(-)-2-Amino-5-phosphonopentanoic acid (AP5; NMDA). Compared to aCSF, bilateral infusion of low (1 DNQX+100 AP5ng/side) and high (5 DNQX+500 AP5ng/side) doses of the AMPAR and NMDAR antagonist cocktail into VTA blocked ethanol CPP expression. This effect was site specific, as DNQX/AP5 infusion proximal to VTA did not significantly impact CPP expression. An increase in activity was found at the high but not low dose of DNQX/AP5. These findings demonstrate that activation of iGluRs within the VTA is necessary for ethanol-associated cue seeking, as measured by CPP.

Female Japanese quail with high levels of estradiol demonstrate cocaine-induced conditioned place preference

Preclinical research has indicated that females may be more sensitive to the rewarding properties of cocaine. However, the majority of this research has been done in rodent species. Environmental cues associated with human drug-taking behavior tend to be visual. Because rodents do not rely on the visual system as their primary sense modality, the use of a visually oriented species may add to our understanding of cue-elicited drug cravings and relapse. The present study examined the potential role of the steroid hormone, estradiol, in the rewarding properties of cocaine in female Japanese quail using a conditioned place preference (CPP) procedure. In the current experiment, female quail were housed on either an 8L:16D (light:dark) or 16L:8D (light:dark) cycle for 21 days to induce photoregression or photostimulation, respectively. They then received 10, 20, or 30 mg/kg cocaine, or saline during conditioning. Conditioning trials were carried out for 8 days, once per day for 30 min, for a total of 4 cocaine and 4 saline alternating conditioning trials. Results indicated that female quail housed in long-light conditions (16L:8D) had significantly higher levels of estradiol than short-cycle females. Additionally, photostimulated female quail developed a CPP to 10 and 20 mg/kg cocaine. Short-cycle females did not show cocaine-induced CPP to any dose tested. Results indicate that cocaine is dose-dependently rewarding to photostimulated female Japanese quail. Furthermore, the current findings suggest that estradiol may enhance the rewarding properties of cocaine in female quail. (PsycINFO Database Record

Reversal of morphine-induced cell-type-specific synaptic plasticity in the nucleus accumbens shell blocks reinstatement

Drug-evoked plasticity at excitatory synapses on medium spiny neurons (MSNs) of the nucleus accumbens (NAc) drives behavioral adaptations in addiction. MSNs expressing dopamine D1 (D1R-MSN) vs. D2 receptors (D2R-MSN) can exert antagonistic effects in drug-related behaviors, and display distinct alterations in glutamate signaling following repeated exposure to psychostimulants; however, little is known of cell-type-specific plasticity induced by opiates. Here, we find that repeated morphine potentiates excitatory transmission and increases GluA2-lacking AMPA receptor expression in D1R-MSNs, while reducing signaling in D2-MSNs following 10-14 d of forced abstinence. In vivo reversal of this pathophysiology with optogenetic stimulation of infralimbic cortex-accumbens shell (ILC-NAc shell) inputs or treatment with the antibiotic, ceftriaxone, blocked reinstatement of morphine-evoked conditioned place preference. These findings confirm the presence of overlapping and distinct plasticity produced by classes of abused drugs within subpopulations of MSNs that may provide targetable molecular mechanisms for future pharmacotherapies.

Nicotine self-administration induces CB1-dependent LTP in the bed nucleus of the stria terminalis

Nicotine addiction is characterized by repetitive drug taking and drug seeking, both tightly controlled by cannabinoid CB1 receptors. The responsiveness of neurons of the bed nucleus of the stria terminalis (BNST) to infralimbic cortex (ILCx) excitatory inputs is increased in rats with active, but not passive, nicotine taking. Therefore, we hypothesize that acquisition of the learned association between nicotine infusion and a paired cue light permits the strengthening of the ILCx-BNST synapses after ILCx tetanic stimulation. We exposed rats to intravenous nicotine self-administration for 2 months. Using a combination of in vivo protocols (electrical stimulations, extracellular recordings, and pharmacological manipulations), we characterized the effects of 10 Hz stimulation of the ILCx on BNST excitatory responses, under different conditions of exposure to nicotine. In addition, we tested whether the effects of the stimulation were CB1 receptor-dependent. The results show that nicotine self-administration supports the induction of evoked spike potentiation in the BNST in response to 10 Hz stimulation of ILCx afferents. Although not altered by nicotine abstinence, this cellular adaptation was blocked by CB1 receptor antagonism. Moreover, blockade of BNST CB1 receptors prevented increases in time-out responding subsequent to ILCx stimulation and decreased cue-induced reinstatement. Thus, the synaptic potentiation within the BNST in response to ILCx stimulation seems to contribute to the cue-elicited responding associated with nicotine self-administration and is tightly controlled by CB1 receptors.

Incentive learning for morphine-associated stimuli during protracted abstinence increases conditioned drug preference

Previous studies from our laboratory found that rats express increased preference for drug-paired stimuli following 2 or 5 weeks of protracted abstinence from chronic drug exposure as compared with naive animals. Here, we show that this increased morphine place preference depends upon experiencing drug-stimulus pairings specifically in the abstinent state, indicating a critical role for incentive learning. Male Sprague Dawley rats were initially conditioned for morphine place preference (8 mg/kg) and then made dependent on morphine (by subcutaneous morphine pellets) and subjected to forced abstinence. Place preference was tested every 1-2 weeks with no additional drug-cue conditioning. In this paradigm, there was no difference between morphine-pelleted (dependent) and placebo-pelleted (non-dependent) rats in place preference at any time during abstinence (up to 6 weeks). However, these same morphine-pelleted rats expressed significantly increased preference when they were subsequently re-conditioned for morphine place preference during protracted abstinence. Placebo-pelleted rats did not show enhanced preference after re-conditioning. These findings reveal that incentive learning has a key role in increased morphine place preference when drug is experienced during protracted abstinence. This indicates that incentive learning is involved not only in instrumental responding (as previously reported), but also in updating Pavlovian-conditioned responses to morphine-associated stimuli. Therefore, enhanced morphine preference is not a direct consequence of the negative affective state of abstinence, but instead reflects increased acquisition of morphine-stimulus associations during abstinence. These results indicate that, during the development of addiction in humans, drug-associated stimuli acquire increasingly stronger incentive properties each time they are re-experienced.

Role of dorsal hippocampal orexin-1 receptors in associating morphine reward with contextual stimuli

In this study, we evaluated the role of orexin receptors in the dorsal hippocampus (dHPC) in the development of morphine-induced conditioned place preference (CPP) and modification of hippocampal c-Fos and cyclic AMP response element-binding protein (CREB) levels. Orexin-A (0.5, 5, and 50 pmol) and the orexin-1 receptor antagonist, SB334867 (10, 20, and 40 nmol), were bilaterally infused into the dHPC immediately before conditioning with morphine (0.5 or 7.5 mg/kg) using the CPP task. Western blotting was then used to measure the protein levels of c-Fos, total CREB, and phosphorylated CREB (pCREB) in the hippocampus. Orexin did not enhance the rewarding efficacy of morphine (0.5 mg/kg), but caused a reduction in hippocampal c-Fos. Successful conditioning with morphine (7.5 mg/kg) was associated with increased levels of hippocampal c-Fos and CREB, but with decreased CREB phosphorylation. Intrahippocampal administration of SB334867 before conditioning sessions disrupted the rewarding effect of morphine (7.5 mg/kg) and blocked morphine-induced increases in hippocampal CREB protein levels. The results suggest that orexin signaling within the dHPC is necessary for the development of morphine CPP. Morphine reward is related to altered levels of hippocampal c-Fos and CREB. Inhibition of morphine-induced increases in CREB levels might be the underlying mechanism for the disruption of morphine CPP.

The role of the aversive effects of drugs in self-administration: assessing the balance of reward and aversion in drug-taking behavior

Since the first experimental demonstration that a drug of abuse supports instrumental behavior, drugs have been discussed in the context of their rewarding effects, which are assumed to drive and maintain drug-taking behavior. Indeed, drug reward has been fundamental in the formulation of most models of drug use, abuse, and addiction. Over the last several decades, however, drugs of abuse have been increasingly recognized as complex pharmacological compounds producing multiple stimulus effects, not all of which are rewarding. The aversive effects of such drugs, for example, have been described by a number of researchers working in the field, although few attempts have been made to investigate the role of these aversive effects in drug taking. The present paper offers a historical perspective on the view that drugs of abuse are complex pharmacological compounds with multiple stimulus effects. In doing so, we argue that the discussion of drug reward only may be insufficient in accounting for drug taking and we present evidence for the theoretical position that both the rewarding and the aversive effects of drugs should be taken into consideration in ongoing attempts to model drug-taking behavior. The present review summarizes several decades of research characterizing the aversive effects of major drugs of abuse, as well as more recent studies seeking to assess directly the role of drug aversion in drug taking.

Partial extinction of a conditioned context enhances preference for elements previously associated with cocaine but not with chocolate

Drug-associated stimuli are crucial to reinstatement of drug-seeking after periods of abstinence, representing a central problem in treatment of addiction. The present study investigated the influence of partial extinction of the conditioned context on the expression of conditioned place preference (CPP). Mice of the inbred DBA/2J strain were conditioned with cocaine or chocolate in a context identified by multiple elements (A+B) and subsequently CPP expression was evaluated in a context containing only one element (A or B) or both (A+B). Cocaine- and chocolate-conditioned mice showed CPP in presence of the original compound stimulus. However, cocaine-conditioned mice did not show CPP when tested in A or B context, while chocolate-conditioned mice did show CPP to single element context. After conditioning mice were exposed to extinction training of the context A or B and then tested for CPP 1 and 9 days after the end of the extinction (days 9 and 18). Cocaine-conditioned mice showed CPP 9 days after extinction while chocolate-conditioned mice were relatively insensitive to the extinction procedure on day 1 after extinction, but they did not show CPP for the partial or the original compound 9 days after extinction. Cocaine-conditioned mice not submitted to the extinction training (simple passage of time) or submitted to a Sham-extinction procedure (saline injections and confinement in a new environment) did not show CPP on day 9 or 18. Cocaine-conditioned mice exposed to extinction training showed increased c-Fos expression in several brain areas in comparison to mice exposed to Sham-extinction. The extinction procedure did not specifically reduce behavioral sensitization. The results suggest that extinction training involving only elements of a drug-associated context can result in increased associative strength of those elements.

Roles of dopaminergic innervation of nucleus accumbens shell and dorsolateral caudate-putamen in cue-induced morphine seeking after prolonged abstinence and the underlying D1- and D2-like receptor mechanisms in rats

Drug-associated cues can elicit relapse to drug seeking after abstinence. Studies with extinction-reinstatement models implicate dopamine (DA) in the nucleus accumbens shell (NAshell) and dorsolateral caudate-putamen (dlCPu) in cocaine seeking. However, less is known about their roles in cue-induced opiate seeking after prolonged abstinence. Using a morphine self-administration and abstinence-relapse model, we explored the roles of NAshell and dlCPu DA and the D1/D2-like receptor mechanisms underlying morphine rewarding and/or seeking. Acquisition of morphine self-administration was examined following 6-Hydroxydopamine hydrobromide (6-OHDA) lesions of the NAshell and dlCPu. For morphine seeking, rats underwent 3 weeks' morphine self-administration followed by 3 weeks' abstinence from morphine and the training environment. Prior to testing, 6-OHDA, D1 antagonist SCH23390, or D2 antagonist eticlopride was locally injected; then rats were exposed to morphine-associated contextual and discrete cues. Results show that acquisition of morphine self-administration was inhibited by NAshell (not dlCPu) lesions, while morphine seeking was attenuated by lesions of either region, by D1 (not D2) receptor blockade in NAshell, or by blockade of either D1 or D2 receptors in dlCPu. These data indicate a critical role of dopaminergic transmission in the NAshell (via D1-like receptors) and dlCPu (via D1- and D2-like receptors) in morphine seeking after prolonged abstinence.

Discriminative inhibitory control of cocaine seeking involves the prelimbic prefrontal cortex

BACKGROUND

Recent neuroimaging studies have shown that people with cocaine addiction retain some degree of control over drug craving that correlates with neural activity in the lateral prefrontal cortex (PFC). Here, we report similar findings in a rat model of inhibitory control of cocaine seeking.

METHODS

Rats actively responding for cocaine were trained to stop responding when presented with a discriminative stimulus that signaled lack of reinforcement. Rats were then tested for inhibitory control of cocaine seeking in novel behavioral contexts and in circumstances when cocaine seeking is particularly intense (e.g., following drug priming). The role of neuronal activity in different subregions of the PFC was assessed using local pharmacologic inactivation and c-Fos immunohistochemistry.

RESULTS

Rats progressively acquired the ability to stop cocaine seeking, even during drug intoxication and after a long history of cocaine self-administration. Inhibitory control of cocaine seeking was flexible, sufficiently strong to block cocaine-primed reinstatement, and selectively depended on increased neuronal activity within the prelimbic PFC, which is considered the rodent functional homolog of the human lateral PFC.

CONCLUSIONS

Parallel evidence in both animal models and humans indicate that recruitment of prefrontal inhibitory control of drug seeking is still functional after prolonged cocaine use. Preclinical investigation of the mechanisms underlying this capacity may contribute to designing new behavioral and/or pharmacologic strategies to promote its use for the prevention of relapse in addiction.

Repeated episodes of heroin cause enduring alterations of circadian activity in protracted abstinence

Opiate withdrawal is followed by a protracted abstinence syndrome consisting of craving and physiological changes. However, few studies have been dedicated to both the characterization and understanding of these long-term alterations in post-dependent subjects. The aim of the present study was to develop an opiate dependence model, which induces long-lasting behavioral changes in abstinent rats. Here, we first compared the effects of several protocols for the induction of opiate dependence (morphine pellets, repeated morphine or heroin injections) on the subsequent response to heroin challenges (0.25 mg/kg) at different time points during abstinence (3, 6, 9 and 18 weeks). In a second set of experiments, rats were exposed to increasing doses of heroin and subsequently monitored for general circadian activity up to 20 weeks of abstinence. Results show that heroin injections rather than the other methods of opiate administration have long-term consequences on rats’ sensitivity to heroin with its psychostimulant effects persisting up to 18 weeks of abstinence. Moreover, intermittent episodes of heroin dependence rather than a single exposure produce enduring alteration of the basal circadian activity both upon heroin cessation and protracted abstinence. Altogether, these findings suggest that the induction of heroin dependence through intermittent increasing heroin injections is the optimal method to model long-term behavioral alterations during protracted abstinence in rats. This animal model would be useful in further characterizing long-lasting changes in post-dependent subjects to help understand the prolonged vulnerability to relapse.

The effect of intermittent alcohol vapor or pulsatile heroin on somatic and negative affective indices during spontaneous withdrawal in Wistar rats

RATIONALE

Once dependent on alcohol or opioids, negative affect may accompany withdrawal. Dependent individuals are hypothesized to "self-medicate" in order to cope with withdrawal, which promotes escalated alcohol and drug use.

OBJECTIVES

The current study aimed to develop a reliable animal model to assess symptoms that occur during spontaneous alcohol and opioid withdrawal.

METHODS

Dependence was induced using intermittent alcohol exposure or pulsatile heroin delivery and assessed for the presence of withdrawal symptoms during acute withdrawal by measuring somatic signs, behavior in the forced swim test (FST), and air-puff-induced 22-kHz ultrasonic vocalizations (USVs). Additional animals subjected to 8 weeks of alcohol vapor exposure were evaluated for altered somatic signs, operant alcohol self-administration, and 22-kHz USV production, as well as performance in the elevated plus maze (EPM).

RESULTS

During spontaneous withdrawal from pulsatile heroin or intermittent alcohol vapor, animals displayed increased somatic withdrawal signs, FST immobility, and 22-kHz USV production but did not show any behavioral change in the EPM unless the duration of alcohol exposure was extended to 4 weeks. Following 8 weeks of alcohol vapor exposure, animals displayed somatic withdrawal signs, escalated alcohol self-administration, and increased 22-kHz USVs.

CONCLUSIONS

These paradigms provide consistent methods to evaluate the behavioral ramifications, and neurobiological substrates, of alcohol and opioid dependence during spontaneous withdrawal. As immobility in the FST and percent open-arm time in the EPM were dissociable, with 22-kHz USVs paralleling immobility in the FST, assessment of air-puff-induced 22-kHz USVs could provide an ethologically valid alternative to the FST.

Glutamate mechanisms underlying opiate memories

As the major excitatory neurotransmitter in the brain, glutamate plays an undisputable integral role in opiate addiction. This relates, in part, to the fact that addiction is a disorder of learning and memory, and glutamate is required for most types of memory formation. As opiate addiction develops, the addict becomes conditioned to engage in addictive behaviors, and these behaviors can be triggered by opiate-associated cues during abstinence, resulting in relapse. Some medications for opiate addiction exert their therapeutic effects at glutamate receptors, especially the NMDA receptor. Understanding the neural circuits controlling opiate addiction, and the locus of glutamate's actions within these circuits, will help guide the development of targeted pharmacotherapeutics for relapse.

Essential role of NR2B-containing NMDA receptor-ERK pathway in nucleus accumbens shell in morphine-associated contextual memory

Learned associations between the rewarding effect of addictive drugs and drug-paired contexts resist extinction and contribute to the high rate of relapse observed in drug addicts. Although it has been shown that extracellular signal-regulated kinase 1/2 (ERK1/2) activity in the nucleus accumbens (NAc) is modulated by the primary rewarding effect of opiates, little is known as to its role in the morphine-associated contextual memory. In the present study, we investigated the ERK1/2 activity indicated by phosphorylated ERK1/2 (pERK1/2) levels in rats using a morphine-induced conditioned place preference (CPP) procedure. Our results showed that, in rats that had undergone morphine conditioning, after testing (expression phase) pERK1/2 in the NAc shell but not the NAc core or the adjacent caudate putamen was specifically increased. pERK1/2 levels in several other parts of the brain involved in drug-seeking, such as the medial prefrontal cortex, dorsal hippocampus, and basolateral amygdala, showed no significant changes. A significant positive correlation was observed between the elevated pERK1/2 level in the NAc shell and the degree of conditioned preference for morphine-associated contexts. Bilateral injection of an inhibitor of ERK activation into the NAc shell attenuated ERK1/2 phosphorylation and prevented the expression of morphine CPP, but injections into the core did not. Selective inhibition of NR2B-containing NMDA receptor in the NAc shell by ifenprodil prevented CPP expression and down-regulated local ERK1/2 phosphorylation. These findings collectively suggest that recall of morphine-associated contextual memory depends specifically upon ERK1/2 activation in the NAc shell and that ERK1/2 phosphorylation is regulated by the upstream NR2B-containing NMDA receptor.

Lateral hypothalamic orexin/hypocretin neurons that project to ventral tegmental area are differentially activated with morphine preference

Orexin (or hypocretin) is synthesized exclusively in dorsomedial, perifornical, and lateral hypothalamus (LH). These neurons are implicated in several functions, including reward processing. We examined the ventral tegmental area (VTA) as a possible site of orexin action for drug preference during protracted morphine abstinence, and studied functional topography of orexin projections to VTA. Male Sprague Dawley rats were used to investigate whether orexin cells that project to VTA exhibit Fos activation with morphine conditioned place preference (CPP), and whether these cells exhibit increased Fos with morphine CPP during protracted abstinence. Unilateral injections of a retrograde tracer (WGA-Au, 350-400 nl) were made into the VTA or a nonreward area, locus ceruleus, and morphine or placebo pellets were implanted for 14 d. Approximately 2 weeks after pellet removal (post dependence), CPP conditioning and testing were conducted. Triple labeling for WGA-Au, Fos, and orexin revealed that the percentage of VTA-projecting orexin neurons Fos activated on the CPP test day significantly increased in post-dependent (vs nondependent) rats, and was exclusive to LH orexin neurons (not dorsomedial or perifornical). Post-dependent animals showed a positive correlation between CPP scores and percentages of Fos-activated, caudal VTA-projecting LH orexin cells. Unlike afferents to caudal VTA, percentages of rostral VTA-projecting, LH orexin cells that were Fos activated showed a positive correlation with CPP only in nondependent animals. Fos in LC-projecting orexin cells was not correlated with CPP in any group. These results indicate that VTA is a heterogeneous and functionally significant target of orexin neurons for morphine reward during protracted abstinence.

Investigation of the neuroanatomical substrates of reward seeking following protracted abstinence in mice

Persistent vulnerability to relapse represents a major challenge in the treatment of drug addiction. The brain circuitry that underlies relapse-like behaviour can be investigated using animal models of drug seeking. As yet there have been no comprehensive brain mapping studies that have specifically examined the neuroanatomical substrates of cue-induced opiate seeking following abstinence in a mouse operant paradigm. The aim of this study was to compare the brain regions involved in sucrose vs. morphine seeking following protracted abstinence in mice. Male CD1 mice were trained to respond for either sucrose (10% w/v) or intravenous morphine (0.1 mg kg(-1) per infusion) in an operant paradigm in the presence of a discrete cue. Once stable responding was established, mice were subjected to abstinence in their home cages for 3 weeks and then perfused for tissue collection, or returned to the operant chambers to assess cue-induced reward seeking before being perfused for tissue collection. Brain tissue was processed for Fos immunohistochemistry and Fos expression was quantified in a range of brain nuclei. We identified unique patterns of neuronal activation for sucrose and morphine seeking mice as well as some overlap. Structures activated in both ‘relapse' groups included the anterior cingulate and orbitofrontal cortex, nucleus accumbens shell, bed nucleus of the stria terminalis, substantia nigra pars compacta, ventral tegmental area, hippocampus, periaqueductal grey, locus coeruleus and lateral habenula. Structures that were more activated in morphine seeking mice included the nucleus accumbens core, basolateral amygdala, substantia nigra pars reticulata, and the central nucleus of the amygdala. The dorsal raphe was the only structure examined that was specifically activated in sucrose seeking mice. Overall our findings support a cortico-striatal limbic circuit driving opiate seeking, and we have identified some additional circuitry potentially relevant to reward seeking following abstinence.

Electroacupuncture suppresses discrete cue-evoked heroin-seeking and fos protein expression in the nucleus accumbens core in rats

Relapse to drug seeking was studied using a rodent model of reinstatement induced by exposure to drug-related cues. Here, we used intravenous drug self-administration procedures in rats to further investigate the beneficial effects of electroacupuncture (EA) on heroin-seeking behavior in a reinstatement model of relapse. We trained Sprague-Dawley rats to nose-poke for i.v. heroin either daily for 4 h or 25 infusions for 14 consecutive days. Then the rats were abstinent from heroin for two weeks. 2 Hz EA stimulation was conducted once daily for 14 days during heroin abstinence. We tested these animals for contextual and discrete cue-induced reinstatement of active responses. We also applied immunohistochemistry to detect Fos-positive nuclei in the nucleus accumbens (NACc) core and shell after reinstatement test. We found that active responses elicited by both contextual cues and discrete cues were high in the rats trained with heroin than in saline controls. EA treatment significantly reduced active responses elicited by discrete cues. EA stimulation attenuated Fos expression in the core but not the shell of the NACc. Altogether, these results highlight the therapeutic benefit of EA in preventing relapse to drug addiction.

Inactivation of PKMζ in the NAc shell abolished cocaine-conditioned reward

Preventing relapse to drug use is a major challenge for the treatment of drug addiction. Environmental cues are among the major determinants of relapse in abstinent cocaine users. The protein kinase M ζ (PKMζ) is involved in the generation and maintenance of long-term potentiation and is critical in memory storage. Here we show that inhibition of PKMζ in the nucleus accumbens (NAc) shell, a major component of the reward system that plays an important role in mediating drug craving and relapse, by a selective inhibitor ζ inhibitory peptide (ZIP), abolished cocaine-induced conditioned place preference (CPP). However, the injection of ZIP into the NAc core resulted in earlier onset of CPP extinction. Finally, we found that the levels of PKMζ and GluR2 in the NAc remained unchanged, while the GluR1 levels were elevated following CPP and fully reversed by ZIP injection. Together, our results suggest that inactivation of PKMζ in the NAc may result in the dissociation between the rewarding properties of the drug and the drug-related environment and may serve as a novel target for the treatment of drug relapse.

Intra-ventral tegmental area or intracerebroventricular orexin-A increases the intra-cranial self-stimulation threshold via activation of the corticotropin-releasing factor system in rats

Although orexin-A peptide was recently found to inhibit the brain reward system, the exact neural substrates for this phenomenon remain unclear. The aim of the present study was to investigate the role of orexin neurons in intra-cranial self-stimulation behavior and to clarify the pathways through which orexin-A inhibits the brain reward system. Immunohistochemical examination using Fos, a neuronal activation marker, revealed that the percentage of activated orexin cells was very low in the lateral hypothalamus even in the hemisphere ipsilateral to self-stimulation, suggesting that orexin neurons play only a small part, if any, in performing intra-cranial self-stimulation behavior. Intra-ventral tegmental area administration of orexin-A (1.0 nmol) significantly increased the intra-cranial self-stimulation threshold. Furthermore, the threshold-increasing effects of intra-ventral tegmental area or intracerebroventricular orexin-A were inhibited by administration of the nonspecific corticotropin-releasing factor receptor antagonist, d-Phe-CRF(12-41) (20 μg). Following intra-ventral tegmental area infusion of orexin-A, the percentage of cells double-labeled with corticotropin-releasing factor and Fos antibodies increased in the central nucleus of the amygdala but not in the bed nucleus of the stria terminalis, and brain microdialysis analyses indicated that dopamine efflux in both the central nucleus of the amygdala and bed nucleus of the stria terminalis were enhanced. Taken together, the present findings suggest that intra-ventral tegmental area or intracerebroventricular administration of orexin-A exerts its threshold-increasing effect via subsequent activation of the corticotropin-releasing factor system.

The effect of propranolol and midazolam on the reconsolidation of a morphine place preference in chronically treated rats

A stable memory can be disrupted if amnestic treatment is applied in conjunction with memory reactivation. Recent findings in the conditioned place preference (CPP) model suggest that blocking reconsolidation attenuates the ability of environmental cues to induce craving and relapse in drug addicts, but the impact of prior physical dependence has not been described. We examined the effect of post-reactivation amnestic treatment on reconsolidation of a CPP for morphine, in animals naïve to morphine, under chronic morphine experience or abstinent. Chronic morphine experience was induced by escalating doses of morphine from 10 mg/kg/day (s.c.), and maintained on 30 mg/kg/day during the course of conditioning and reactivation procedures, or conditioning alone. Naïve and morphine-experienced animals were trained in a three-compartment apparatus by four morphine (5 mg/kg, s.c.) and four saline experiences paired with either of two large conditioning compartments. The memory was then reactivated by a CPP test, and immediately afterward animals received an injection of the beta-adrenergic antagonist propranolol (10 mg/kg, s.c.), the GABAa agonist midazolam (1 mg/kg, i.p.), or saline. Morphine-naïve rats received only a single reconsolidation-blocking treatment (Experiment 1), while chronic morphine rats were given eight reactivation sessions each followed by amnestic treatment, either before (Experiment 2) or after 10 days of withdrawal (Experiment 3). Propranolol and midazolam disrupted reconsolidation in morphine-naïve rats, but failed to disrupt the CPP when rats were trained under chronic morphine treatment, even if they were recovered from chronic opiate exposure before reactivation. In fact, propranolol increased the preference for the drug-paired context in animals trained while maintained on chronic morphine. Midazolam had little effect. Morphine experience may produce neurochemical changes which alter memory storage processes and reduce the impact of amnestic treatments on reconsolidation.

Blockade of opioid receptors in anterior cingulate cortex disrupts ethanol-seeking behavior in mice

The anterior cingulate cortex (ACC) and opioid receptors have been suggested to play a role in attributing incentive motivational properties to drug-related cues. We examined whether blockade of ACC opioid receptors would reduce cue-induced ethanol-seeking behavior in mice. We show that intra-ACC opioid receptor blockade disrupted expression of an ethanol-induced conditioned place preference, suggesting that endogenous opioid modulation in the ACC may be critical for maintaining the cue's conditioned rewarding effects.

Expression of activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) in the nucleus accumbens is critical for the acquisition, expression and reinstatement of morphine-induced conditioned place preference

Activity-regulated cytoskeleton-associated protein (Arc), also known as activity-regulated gene 3.1 (Arg3.1), is an immediate early gene whose mRNA is selectively targeted to recently activated synaptic sites, where it is translated and enriched. This unique feature suggests a role for Arc/Arg3.1 in coupling synaptic activity to protein synthesis, leading to synaptic plasticity. Although the Arc/Arg3.1 gene has been shown to be induced by a variety of abused drugs and its protein has been implicated in diverse forms of long-term memory, relatively little is known about its role in drug-induced reward memory. In this study, we investigated the potential role of Arc/Arg3.1 protein expression in reward-related associative learning and memory using morphine-induced conditioned place preference (CPP) in rats. We found that (1) intraperitoneal (i.p.) injection of morphine (10mg/kg) increased Arc/Arg3.1 protein levels after 2h in the NAc core but not in the NAc shell. (2) In CPP experiments, Arc/Arg3.1 protein was increased in the NAc shell of rats following both morphine conditioning and the CPP expression test compared to rats that received the conditioning without the test or those that did not receive morphine conditioning. (3) Microinjection of Arc/Arg3.1 antisense oligodeoxynucleotide (AS) into the NAc core inhibited the acquisition, expression and reinstatement of morphine CPP; however, intra-NAc shell infusions of the AS only blocked the expression of CPP. These findings suggest that expression of the Arc/Arg3.1 protein in the NAc core is required for the acquisition, context-induced retrieval and reinstatement of morphine-associated reward memory, whereas Arc/Arg3.1 protein expression in the NAc shell is only critical for the context-induced retrieval of memory. As a result, Arc/Arg3.1 may be a potential therapeutic target for the prevention of drug abuse or the relapse of drug use.

Variations in daily expression of the circadian clock protein, PER2, in the rat limbic forebrain during stable entrainment to a long light cycle

The circadian clock in the mammalian suprachiasmatic nucleus (SCN) can be entrained by light cycles longer than the normal 24-h light/dark (LD) cycle, but little is known about the effect of such cycles on circadian clocks outside the SCN. Here we examined the effect of exposure to a 26-h T cycle (T26, 1 h:25 h LD) on patterns of expression of the clock protein, PERIOD2 (PER2), in the SCN and in four regions of the limbic forebrain known to exhibit robust circadian oscillations in PER2: the oval nucleus of the bed nucleus of the stria terminalis (BNSTov), central nucleus of the amygdala (CEA), basolateral amygdala (BLA), and dentate gyrus (DG). All rats showed stable entrainment of running wheel activity rhythms to the T26 cycle. As previously shown, PER2 expression in the SCN was stably entrained, peaking around the onset of locomotor activity. In contrast, exposure to the T26 cycle uncoupled the rhythms of PER2 expression in the BNSTov and CEA from that of the SCN, whereas PER2 rhythms in the BLA and DG were unaffected. These results show that exposure to long light cycles can uncouple circadian oscillators in select nuclei of the limbic forebrain from the SCN clock and suggest that such cycles may be used to study the functional consequences of coupling and uncoupling of brain circadian oscillators.

The role of progestins in the behavioral effects of cocaine and other drugs of abuse: human and animal research

This review summarizes findings from human and animal research investigating the influence of progesterone and its metabolites allopreganolone and pregnanolone (progestins) on the effects of cocaine and other drugs of abuse. Since a majority of these studies have used cocaine, this will be the primary focus; however, the influence of progestins on other drugs of abuse will also be discussed. Collectively, findings from these studies support a role for progestins in (1) attenuating the subjective and physiological effects of cocaine in humans, (2) blocking the reinforcing and other behavioral effects of cocaine in animal models of drug abuse, and (3) influencing behavioral responses to other drugs of abuse such as alcohol and nicotine in animals. Administration of several drugs of abuse in both human and nonhuman animals significantly increased progestin levels, and this is explained in terms of progestins acting as homeostatic regulators that decrease and normalize heightened stress and reward responses which lead to increased drug craving and relapse. The findings discussed here highlight the complexity of progestin-drug interactions, and they suggest a possible use for these agents in understanding the etiology of and developing treatments for drug abuse.

Orexins in the midline thalamus are involved in the expression of conditioned place aversion to morphine withdrawal

Previous studies have implicated the bed nucleus of the stria terminalis, central nucleus of the amygdala and the shell of the nucleus accumbens (collectively called the extended amygdala) as playing an important role in mediating the aversive emotion associated with opioid withdrawal. The paraventricular nucleus of the thalamus (PVT) provides a very dense input to the extended amygdala, and the PVT is densely innervated by orexin neurons, which appear to be involved in producing some of the physical and emotional effects associated with morphine withdrawal. In the present study, we confirm that the PVT is densely innervated by orexin fibers, whereas the regions of the extended amygdala associated with the effects of morphine withdrawal are poorly innervated. Microinjections of the orexin-1 receptor (OX1R) antagonist SB334867 or the orexin-2 receptor (OX2R) antagonist TCSOX229 at doses of 5.0 or 15.0 microg into the PVT region did not affect the acquisition of the conditioned place aversion (CPA) nor the physical effects produced by naloxone-precipitated morphine withdrawal. In contrast, microinjections of TCSOX229 (15.0 microg) in the PVT region significantly attenuated the expression of naloxone-induced CPA while microinjections of SB334867 at the same dose had no effect. The results from these experiments indicate a role for OX2R in the PVT on the expression of CPA associated with morphine withdrawal. Orexins may mediate the aversive effects of morphine withdrawal by engaging the extended amygdala indirectly through the action of orexins on the PVT.

Modafinil blocks reinstatement of extinguished opiate-seeking in rats: mediation by a glutamate mechanism

Opiate addiction is characterized by high rates of relapse even after long periods of abstinence, requiring new relapse-prevention treatments that do not have abuse potential. Recently, clinical studies suggested that the wake-promoting drug modafinil might decrease relapse in cocaine addicts. In addition, group II metabotropic glutamate receptors (mGlu2/3R) have been suggested as a new therapeutic target for drug addiction. Here, we investigated the ability of modafinil to prevent the acute morphine to promote reinstatement of extinguished preference for morphine, and the involvement of mGlu2/3Rs in this effect. Conditioned place preference (CPP) for morphine was induced in Sprague-Dawley rats, followed by extinction training. Preference for the morphine-paired side was reinstated following extinction by a morphine-priming injection. The results of our study showed that modafinil (300 mg/kg, i.p., but not 100 mg/kg) 30 min before the morphine-priming injection blocked reinstatement of extinguished CPP. The anti-reinstatement effect of modafinil was completely prevented by pretreatment with the selective mGlu2/3 antagonist LY341495. Additional experiments indicated that modafinil alone did not produce a preference, and that modafinil did not alter the expression of morphine CPP or the cueing properties of morphine either 1 or 14 days after morphine CPP conditioning. These data reveal a novel mechanism for modafinil actions, a role for mGlu2/3 receptors in reinstatement of opiate-seeking, and a new therapeutic option to treat relapse in opiate addiction.

Role of orexin/hypocretin in reward-seeking and addiction: implications for obesity

Orexins (also named hypocretins) are recently discovered neuropeptides made exclusively in the hypothalamus. Recent studies have shown that orexin cells located specifically in lateral hypothalamus (LH) are involved in motivated behavior for drugs of abuse as well as natural rewards. Administration of orexin has been shown to stimulate food consumption, and orexin signaling in VTA has been implicated in intake of high-fat food. In self-administration studies, the orexin 1 receptor antagonist SB-334867 (SB) attenuated operant responding for high-fat pellets, sucrose pellets and ethanol, but not cocaine, demonstrating that signaling at orexin receptors is necessary for reinforcement of specific rewards. The orexin system is also implicated in associations between rewards and relevant stimuli. For example, Fos expression in LH orexin neurons varied in proportion to conditioned place preference (CPP) for food, morphine, or cocaine. This Fos expression was altered accordingly for CPP administered during protracted abstinence from morphine or cocaine, when preference for natural rewards was decreased and drug preference was increased. Additionally, orexin has been shown to be involved in reward-stimulus associations in the self-administration paradigm, where SB attenuated cue-induced reinstatement of extinguished sucrose- or cocaine-seeking. Although the specific circuitry mediating the effects of orexin on food reward remains unknown, VTA seems likely to be a critical target for at least some of these orexin actions. Thus, recent studies have established a role for orexin in reward-based feeding, and further investigation is warranted for determining whether function/dysfunction of the orexin system may contribute to the overeating associated with obesity. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Sensitization processes in drug addiction

In 1993, Robinson and Berridge published their first review that laid out the incentive sensitization theory of addiction (Robinson and Berridge 1993 Brain Res Rev 18:247). Its basic point is that repeated exposure to drugs of abuse causes hypersensitivity to drugs and drug-associated stimuli of the neural circuits mediating incentive salience, an important way in which motivational stimuli influence behavior. In laymen's terms, it states that this drug-induced hypersensitivity of motivational circuitry would mediate an increase in drug "wanting," thus being responsible for the dramatically exaggerated motivation for drugs displayed by addicts. This theory has been exceptionally influential, as evidenced by the fact that the original review paper about this theory (Robinson and Berridge 1993 Brain Res Rev 18:247) has been cited 2,277 times so far, and subsequent updates of this view (Robinson and Berridge 2000 Addiction 95(Suppl 2):S91; Robinson and Berridge 2001 Addiction 96:103; Robinson and Berridge 2003 Ann Rev Psychol 54:25) have been cited 274, 297, and 365 times, respectively, adding up to more than 3,200 citations within 15 years. The present chapter aims to delineate the merits and limitations of the incentive sensitization view of addiction, and whether incentive sensitization occurs in humans. We conclude that since incentive sensitization most prominently occurs after the first few drug exposures, it may represent an important initial step in the addiction process. During the expression of full-blown addiction, characterized by loss of control over drug intake and use of large quantities of drugs, the expression of incentive sensitization may be transiently suppressed. However, detoxification and the gradual disappearance of tolerance and withdrawal symptoms may unmask sensitization, which could then play an important role in the high risk of relapse.

Basolateral amygdala and morphine-induced taste avoidance in the rat

The present experiment examined the influence of excitotoxic lesions of the basolateral amygdala (BLA) on morphine-induced saccharin avoidance. Neurologically intact subjects rapidly learned to avoid drinking the taste conditioned stimulus (CS), an effect that was sustained throughout the experiment. Although the BLA-lesioned (BLAX) rats showed CS avoidance over the first few trials, the effect was not sustained. That is, by the end of the experiment, the BLAX rats were drinking the same amount of saccharin after seven saccharin-morphine trials as they did on the first trial (i.e., prior to the morphine injections). Potential interpretations of the results are discussed including a disruption of the mechanism that governs drug-induced taste avoidance in normal subjects and the more rapid development of tolerance in BLAX rats.

Role of the bed nucleus of the stria terminalis in the control of ventral tegmental area dopamine neurons

Projections from neurons of the bed nucleus of the stria terminalis (BST) to the ventral tegmental area (VTA) are crucial to behaviors related to reward and motivation. Over the past few years, we have undertaken a series of studies to understand: 1) how excitatory inputs regulate in vivo excitable properties of BST neurons, and 2) how BST inputs in turn modulate neuronal activity of dopamine neurons in VTA. Using in vivo extracellular recording techniques in anesthetized rats and tract-tracing approaches, we have demonstrated that inputs from the infralimbic cortex and the ventral subiculum exert a strong excitatory influence on BST neurons projecting to the VTA. Thus, the BST is uniquely positioned to receive emotional and learning-associated informations and to integrate these into the reward/motivation circuitry. We will discuss how changes in the activity of BST neurons projecting to the VTA could participate in the development or exacerbation of psychiatric conditions such as drug addiction.

Lateral hypothalamic orexin/hypocretin neurons: A role in reward-seeking and addiction

Orexins (synonymous with hypocretins) are recently discovered neuropeptides made exclusively in hypothalamus. Behavioral, anatomical, and neurophysiological studies show that a subset of these cells, specifically those in lateral hypothalamus (LH), are involved in reward processing and addictive behaviors. Fos expression in LH orexin neurons varied in proportion to conditioned place preference (CPP) for morphine, cocaine, or food. This relationship occurred both in drug-naïve rats and in animals during protracted morphine withdrawal, when drug preference was elevated but food preference was decreased. Inputs to the LH orexin cell field from lateral septum and bed nucleus of the stria terminalis were Fos-activated during cocaine CPP in proportion to the preference expressed in each animal. This implies that these inputs may be involved in driving the conditioned responses in LH orexin neurons. Related studies showed that LH orexin neurons that project to ventral tegmental area (VTA) had greater Fos induction in association with elevated morphine preference during protracted withdrawal than non-VTA-projecting orexin neurons, indicating that the VTA is an important site of action for orexin's role in reward processing. In addition, stimulation of LH orexin neurons, or microinjection of orexin into VTA, reinstated an extinguished morphine preference. In self-administration studies, the orexin 1 receptor antagonist SB-334867 (SB) blocked cocaine-seeking induced by discrete or contextual cues previously associated with cocaine, but not by a priming injection of cocaine. There was no effect of SB on cocaine self-administration itself, indicating that it did not interfere with the drug's reinforcing properties. Neurophysiological studies revealed that locally applied orexin often augmented responses of VTA dopamine (DA) neurons to activation of the medial prefrontal cortex (mPFC), consistent with the view that orexin facilitates activation of VTA DA neurons by stimulus-reward associations. This LH-to-VTA orexin pathway was found to be necessary for learning a morphine place preference. These findings are consistent with results showing that orexin facilitates glutamate-mediated responses, and is necessary for glutamate-dependent long-term potentiation in VTA DA neurons. We surmise from these studies that LH orexin neurons play an important role in reward processing and addiction and that LH orexin cells are an important input to VTA for behavioral effects associated with reward-paired stimuli.

Enhancing effect of heroin on social recognition learning in male Sprague-Dawley rats: modulation by heroin pre-exposure

RATIONALE

There is evidence that pre-exposure to drugs of abuse can induce sensitization to several of their effects.

OBJECTIVE

Four experiments were conducted to investigate the effect of heroin pre-exposure on modulation of memory consolidation as indexed by heroin's action on rate of learning.

MATERIALS AND METHODS

Male Sprague-Dawley rats were tested on a social recognition learning task which assesses changes in investigation during repeated exposure to the same rat (habituation training: four sessions) and during exposure to a novel rat (dishabituation test). In the first experiment, rats received 0, 0.3, or 1 mg/kg heroin s.c. immediately following each training session, or 1 mg/kg heroin 2 h post-training. In experiments 2 and 3, rats received 1 mg/kg heroin post-training after a 7-day drug-free period from heroin pre-exposure achieved through conditioned place preference (1 mg/kg s.c., 1 injection/day x 4 days) or intravenous self-administration (0.05 mg/kg/infusion i.v., 3 h/day x 9 days) training. In experiment 4, rats received 0, 0.03, 0.3, or 3 mg/kg heroin post-training after a 7-day drug-free period from a regimen of heroin administration (i.e., 1 mg/kg heroin/day s.c. x 7 days) that induced locomotor sensitization.

RESULTS

Post-training administration of heroin enhanced social recognition learning in a dose- and time-dependent manner. Importantly, no regimen of heroin pre-exposure significantly altered this effect of heroin.

CONCLUSIONS

These results do not support the hypothesis that heroin pre-exposure leads to sensitization to its effect on memory consolidation of non-drug-related learning. However, this requires further testing using alternative heroin pre-exposure regimens, a wider range of post-training heroin doses, as well as other types of learning tasks.

Conditioned effects of heroin on proinflammatory mediators require the basolateral amygdala

Heroin administration alters the induction of nitric oxide, a molecule known to play a critical role in immune function. Previous research has shown that these alterations can be conditioned to environmental stimuli that have been associated with drug administration. Little is known about the brain areas that mediate these effects; however, the basolateral amygdala (BLA) has been implicated in the formation of stimulus-reward associations within models of drug abuse. The present study sought to determine whether inactivation of the BLA would alter heroin's conditioned effects on the expression of inducible nitric oxide synthase (iNOS) and the proinflammatory cytokines TNF-alpha and IL-1beta in the rat. The conditioning procedure involved repeated pairing of heroin with placement into a standard conditioning chamber. To test the conditioned response, animals were returned to the previously drug-paired environment 6 days after the final conditioning session. Prior to testing, animals received intra-BLA microinfusions of a mixture of the GABA agonists muscimol and baclofen. Following removal from the chambers on test day, all animals received subcutaneous lipopolysaccharide to induce systemic expression of iNOS, TNF-alpha and IL-1beta. Analyses using real-time RT-PCR indicated that inactivation of the BLA blocked the suppressive effect of heroin-associated environmental stimuli on iNOS induction and on the expression of the proinflammatory cytokines TNF-alpha and IL-1beta in spleen and liver tissue. This study is important because it is the first to demonstrate that heroin's conditioned effects on proinflammatory mediators require the BLA. These findings may have significant implications for the treatment of heroin users.