Corticotropin-releasing factor, but not corticosterone, is involved in stress-induced relapse to heroin-seeking in rats

Poststress block of kappa opioid receptors rescues long-term potentiation of inhibitory synapses and prevents reinstatement of cocaine seeking

BACKGROUND

Dopaminergic neurons in the ventral tegmental area of the brain are an important site of convergence of drugs and stress. We previously identified a form of long-term potentiation of gamma-aminobutyric acid (GABA)ergic synapses on these neurons (LTPGABA). Our studies have shown that exposure to acute stress blocks this LTP and that reversal of the block of LTPGABA is correlated with prevention of stress-induced reinstatement of cocaine-seeking behavior.

METHODS

Sprague-Dawley rats were subjected to cold-water swim stress. Midbrain slices were prepared following stress, and whole-cell patch clamp recordings of inhibitory postsynaptic currents were performed from ventral tegmental area dopamine neurons. Antagonists of glucocorticoid receptors and kappa opioid receptors (κORs) were administered at varying time points after stress. Additionally, the ability of a kappa antagonist administered following stress to block forced swim stress-induced reinstatement of cocaine self-administration was tested.

RESULTS

We found that an acute stressor blocks LTPGABA for 5 days after stress through a transient activation of glucocorticoid receptors and more lasting contribution of κORs. Even pharmacological block of κORs beginning 4 days after stress has occurred reversed the block of LTPGABA. Administration of a κORs antagonist following stress prevents reinstatement of cocaine-seeking behavior.

CONCLUSIONS

A brief stressor produces changes in the reward circuitry lasting several days. Our findings reveal roles for glucocorticoid receptors and κORs as mediators of the lasting effects of stress on synaptic plasticity. κORs antagonists reverse the neuroadaptations underlying stress-induced drug-seeking behavior and may be useful in the treatment of cocaine addiction.

Beta-2 adrenergic receptors mediate stress-evoked reinstatement of cocaine-induced conditioned place preference and increases in CRF mRNA in the bed nucleus of the stria terminalis in mice

RATIONALE

Understanding the mechanisms responsible for stress-induced relapse is important for guiding treatment strategies aimed at minimizing the contribution of stress to addiction. Evidence suggests that these mechanisms involve interactions between noradrenergic systems and the neuropeptide corticotropin-releasing factor (CRF).

OBJECTIVES

The interaction between β-adrenergic receptors (ARs) and CRF as it relates to the reinstatement of cocaine-conditioned reward in response to a stressor was examined in mice. We hypothesized that β2-ARs are required for stress-induced activation of CRF pathways responsible for reinstatement.

METHODS

Stress-induced relapse was examined based on the re-establishment of cocaine-induced conditioned place preference (CPP; 4 × 15 mg/kg cocaine, i.p.) after extinction using forced swim (6 min at 22 °C) or an injection of the β2-AR agonist, clenbuterol (4 mg/kg, i.p.). The CRF-R1 antagonist antalarmin (10 mg/kg, i.p.) or the β2-AR antagonist ICI-118,551 (1 mg/kg, i.p.) were given 30 min prior to reinstating stimuli. Quantitative PCR was conducted in dissected bed nucleus of the stria terminalis (BNST) and amygdala, putative sources of CRF that contribute to reinstatement, to examine the effects of ICI-118,551 on swim-induced increases in CRF messenger RNA (mRNA) in mice with a cocaine history.

RESULTS

Pretreatment with ICI-118,551 or antalarmin blocked swim-induced reinstatement of CPP. Reinstatement by clenbuterol was also blocked by antalarmin. ICI-118,551 pretreatment prevented swim-induced increases in CRF mRNA in the BNST. Effects in the amygdala were not observed.

CONCLUSIONS

These findings indicate that, during stress, norepinephrine, via β2-ARs, either directly or indirectly activates CRF-releasing neurons in the BNST that interface with motivational neurocircuitry to induce reinstatement of cocaine-conditioned reward.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.

Psychological and physiological stress negatively impacts early engagement and retention of opioid-dependent individuals on methadone maintenance

The present study investigated whether psychological and/or physiological measures of stress would impede induction onto methadone maintenance and predict early (<6 months) discontinuation. Compared with controls, opioid-dependent subjects displayed increased distress on the perceived stress scale (PSS) and post-traumatic stress disorder checklist (PCLC); 60% exhibited abnormal cortisol. Addiction severity index (ASI), drug-use, and stress indices explained between 17 and 37% of the variance in engagement including attendance, opioid abstinence, and methadone stabilization. Participants who discontinued treatment displayed poor engagement, abnormal cortisol, elevated withdrawal symptoms, higher distress, and increased ongoing opioid use versus compliant individuals. Discontinuation was initially related to drug-use severity; however, by 6 months, retention depended primarily upon cortisol abnormalities, which increased an individual's discontinuation risk by 7.7-fold. These findings support admission screening with the ASI/cortisol for drop out, and stress/drug-use indices for engagement that together may enable clinically-relevant early recognition and interventions for prevention of stress-induced relapse in opioid-dependent populations.

Amplified reacquisition of nicotine self-administration in rats by repeated stress during abstinence

RATIONALE

Quitting smoking is often very challenging, leading to frequent relapse. Exposure to acute and chronic stress during abstinence increases the likelihood of relapse to smoking. In rodents, stress acutely reinstates nicotine seeking after extinction of nicotine self-administration (SA). However, whether reacquisition of nicotine taking is amplified by chronic stress during abstinence from nicotine SA has not been determined in animals.

OBJECTIVES

We sought to determine effects of repeated restraint stress during abstinence on reacquisition of nicotine SA.

METHODS

Rats acquired nicotine SA (23 h/day) under a fixed-ratio (FR) 5 schedule of reinforcement, which was followed by an abstinence phase. Restraint (0, 2, and 4 times) was administered during abstinence. Animals reacquired nicotine SA, first under a progressive ratio (PR) schedule, beginning immediately after the final stress, followed by an FR5 schedule. In another experiment, reacquisition (FR5) began 24 h after the final stress. No PR testing was conducted.

RESULTS

Four restraint stress exposures during abstinence, but not only two, enhanced reacquisition of nicotine SA by increasing nicotine injections under a PR schedule beginning immediately after the final stress (p < 0.05) followed by increasing nicotine intake under an FR5 schedule (p < 0.05). This was observed even when the final stress and reacquisition trial were separated by 24 h. Moreover, repeated stress-induced nicotine taking during the behaviorally inactive phase (i.e., lights on) of the 24-h diurnal cycle.

CONCLUSIONS

Chronic (i.e., repeated) stress during abstinence promotes reacquisition of nicotine SA and affects diurnal pattern of nicotine intake.

Corticotropin releasing factor: a key role in the neurobiology of addiction

Drug addiction is a chronically relapsing disorder characterized by loss of control over intake and dysregulation of stress-related brain emotional systems. Since the discovery by Wylie Vale and his colleagues of corticotropin-releasing factor (CRF) and the structurally-related urocortins, CRF systems have emerged as mediators of the body's response to stress. Relatedly, CRF systems have a prominent role in driving addiction via actions in the central extended amygdala, producing anxiety-like behavior, reward deficits, excessive, compulsive-like drug self-administration and stress-induced reinstatement of drug seeking. CRF neuron activation in the medial prefrontal cortex may also contribute to the loss of control. Polymorphisms in CRF system molecules are associated with drug use phenotypes in humans, often in interaction with stress history. Drug discovery efforts have yielded brain-penetrant CRF1 antagonists with activity in preclinical models of addiction. The results support the hypothesis that brain CRF-CRF1 systems contribute to the etiology and maintenance of addiction.

Pharmacological treatment of comorbid PTSD and substance use disorder: recent progress

Previous research has identified a strong association between posttraumatic stress disorder (PTSD) and substance use disorder (SUD), necessitating the development of treatments that address both conditions. Some pharmacotherapies are effective for the treatment of PTSD and SUD alone, however; no medications have been proven to be effective for the combination of these conditions. We review the recent advances in pharmacological treatment of comorbid PTSD and SUD. A randomized clinical trial of sertraline, a serotonin reuptake inhibitor (SSRI), did not show overall efficacy for comorbid PTSD and alcohol dependence (AD), although it may have efficacy among light drinkers. Another clinical trial demonstrated the efficacy of both disulfiram and naltrexone for the treatment of AD in individuals with PTSD. A more recent clinical trial suggested that norepinephrine uptake inhibitors may also have efficacy for the treatment of comorbid PTSD and AD. In animal and preliminary human studies, brain norepinephrine and glutamate/GABA have emerged as potential treatment targets for comorbid PTSD and SUD. Noradrenergic medications that are promising for comorbid PTSD and SUD include prazosin, guanfacine, and atomoxetine. Promising glutamate/GABA medications include topiramate, memantine, acamprosate, N-acetylcysteine (NAC), and ketamine. The safety and efficacy of these medications for the treatment of PTSD and SUD need to be tested in controlled clinical trials.

Neurobiological mechanisms underlying relapse to cocaine use: contributions of CRF and noradrenergic systems and regulation by glucocorticoids

Considering its pervasive and uncontrollable influence in drug addicts, understanding the neurobiological processes through which stress contributes to drug use is a critical goal for addiction researchers and will likely be important for the development of effective medications aimed at relapse prevention. In this paper, we review work from our laboratory and others focused on determining the neurobiological mechanisms that underlie and contribute to stress-induced relapse of cocaine use with an emphasis on the actions of corticotropin-releasing factor in the ventral tegmental area (VTA) and a key pathway from the bed nucleus of the stria terminalis to the VTA that is regulated by norepinephrine and beta adrenergic receptors. Additionally, we discuss work suggesting that the influence of stress in cocaine addiction changes and intensifies with repeated cocaine use in an intake-dependent manner and examine the potential role of glucocorticoid hormones in the underlying drug-induced neuroadaptations. It is our hope that research in this area will inform clinical practice and medication development aimed at minimizing the contribution of stress to the addiction cycle, thereby improving treatment outcomes and reducing the societal costs of addiction.

Heart rate variability biofeedback in adolescent substance abuse treatment

Strategies are needed to improve adolescent substance abuse treatment outcomes. For example, during outpatient substance abuse treatment, up to 80% of adolescents continue to use.¹,² Following residential substance abuse treatment, 88% of adolescents relapse within 6 months.³

Role of GABAA receptors in dorsal raphe nucleus in stress-induced reinstatement of morphine-conditioned place preference in rats

RATIONALE

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Our data indicate that stress inhibits the dorsal raphe nucleus (DRN)-5-HT system via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor and, more recently, that morphine history sensitizes DRN-5-HT neurons to GABAergic inhibitory effects of stress.

OBJECTIVES

We tested the hypothesis that DRN GABAA receptors contribute to stress-induced reinstatement of morphine-conditioned place preference (CPP).

METHODS

First, we tested if activation of GABAA receptors in the DRN would reinstate morphine CPP. Second, we tested if blockade of GABAA receptors in the DRN would attenuate swim stress-induced reinstatement of morphine CPP. CPP was induced by morphine (5 mg/kg) in a 4-day conditioning phase followed by a conditioning test. Upon acquiring conditioning criteria, subjects underwent 4 days of extinction training followed by an extinction test. Upon acquiring extinction criteria, animals underwent a reinstatement test. For the first experiment, the GABAA receptor agonist muscimol (50 ng) or vehicle was injected into the DRN prior to the reinstatement test. For the second experiment, the GABAA receptor antagonist bicuculline (75 ng) or vehicle was injected into the DRN prior to a forced swim stress, and then, animals were tested for reinstatement of CPP.

RESULTS

Intraraphe injection of muscimol reinstated morphine CPP, while intraraphe injection of bicuculline attenuated swim stress-induced reinstatement.

CONCLUSIONS

These data provide evidence that GABAA receptor-mediated inhibition of the serotonergic DRN contributes to stress-induced reinstatement of morphine CPP.

The reinstatement model of drug relapse: recent neurobiological findings, emerging research topics, and translational research

BACKGROUND AND RATIONALE

Results from many clinical studies suggest that drug relapse and craving are often provoked by acute exposure to the self-administered drug or related drugs, drug-associated cues or contexts, or certain stressors. During the last two decades, this clinical scenario has been studied in laboratory animals by using the reinstatement model. In this model, reinstatement of drug seeking by drug priming, drug cues or contexts, or certain stressors is assessed following drug self-administration training and subsequent extinction of the drug-reinforced responding.

OBJECTIVE

In this review, we first summarize recent (2009-present) neurobiological findings from studies using the reinstatement model. We then discuss emerging research topics, including the impact of interfering with putative reconsolidation processes on cue- and context-induced reinstatement of drug seeking, and similarities and differences in mechanisms of reinstatement across drug classes. We conclude by discussing results from recent human studies that were inspired by results from rat studies using the reinstatement model.

CONCLUSIONS

Main conclusions from the studies reviewed highlight: (1) the ventral subiculum and lateral hypothalamus as emerging brain areas important for reinstatement of drug seeking, (2) the existence of differences in brain mechanisms controlling reinstatement of drug seeking across drug classes, (3) the utility of the reinstatement model for assessing the effect of reconsolidation-related manipulations on cue-induced drug seeking, and (4) the encouraging pharmacological concordance between results from rat studies using the reinstatement model and human laboratory studies on cue- and stress-induced drug craving.

Inactivation of the lateral habenula reduces anxiogenic behavior and cocaine seeking under conditions of heightened stress

Recent anatomical and functional studies have renewed interest in the lateral habenula (LHb), a critical brain region that works in an opponent manner to modulate aversive and appetitive processes. In particular, increased LHb activation is believed to drive anxiogenic states during stressful conditions. Here, we reversibly inactivated the LHb with GABA receptor agonists (baclofen/muscimol) in rats prior to testing in an open field, elevated plus maze, and defensive burying task in the presence or absence of yohimbine, a noradrenergic α2-receptor antagonist that acts as an anxiogenic stressor. In a second set of experiments using a cocaine self-administration and reinstatement model, we inactivated the LHb during extinction responding and cue-induced reinstatement of cocaine seeking in the presence or absence of yohimbine pretreatment. Inactivation of the LHb after yohimbine treatment attenuated anxiogenic behavior by increasing time spent in the open arms and reducing the time spent burying. Inactivation of the LHb also reduced cocaine seeking when cue-induced reinstatement occurred in the presence of yohimbine, but did not affect extinction responding or cue-induced reinstatement by itself. These data demonstrate that the LHb critically regulates states of heightened anxiety during both unconditioned behavior and conditioned appetitive processes.

Neurobiological mechanisms that contribute to stress-related cocaine use

The ability of stressful life events to trigger drug use is particularly problematic for the management of cocaine addiction due to the unpredictable and often uncontrollable nature of stress. For this reason, understanding the neurobiological processes that contribute to stress-related drug use is important for the development of new and more effective treatment strategies aimed at minimizing the role of stress in the addiction cycle. In this review we discuss the neurocircuitry that has been implicated in stress-induced drug use with an emphasis on corticotropin releasing factor actions in the ventral tegmental area (VTA) and an important pathway from the bed nucleus of the stria terminalis to the VTA that is regulated by norepinephrine via actions at beta adrenergic receptors. In addition to the neurobiological mechanisms that underlie stress-induced cocaine seeking, we review findings suggesting that the ability of stressful stimuli to trigger cocaine use emerges and intensifies in an intake-dependent manner with repeated cocaine self-administration. Further, we discuss evidence that the drug-induced neuroadaptations that are necessary for heightened susceptibility to stress-induced drug use are reliant on elevated levels of glucocorticoid hormones at the time of cocaine use. Finally, the potential ability of stress to function as a "stage setter" for drug use - increasing sensitivity to cocaine and drug-associated cues - under conditions where it does not directly trigger cocaine seeking is discussed. As our understanding of the mechanisms through which stress promotes drug use advances, the hope is that so too will the available tools for effectively managing addiction, particularly in cocaine addicts whose drug use is stress-driven. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Behavioral, biological, and chemical perspectives on targeting CRF(1) receptor antagonists to treat alcoholism

BACKGROUND

Alcohol use disorders are chronic disabling conditions for which existing pharmacotherapies have only modest efficacy. In the present review, derived from the 2012 Behavior, Biology and Chemistry "Translational Research in Addiction" symposium, we summarize the anti-relapse potential of corticotropin-releasing factor type 1 (CRF(1)) receptor antagonists to reduce negative emotional symptoms of acute and protracted alcohol withdrawal and stress-induced relapse to alcohol seeking.

METHODS

We review the biology of CRF(1) systems, the activity of CRF(1) receptor antagonists in animal models of anxiolytic and antidepressant activity, and experimental findings in alcohol addiction models. We also update the clinical trial status of CRF(1) receptor antagonists, including pexacerfont (BMS-562086), emicerfont (GW876008), verucerfont (GSK561679), CP316311, SSR125543A, R121919/NBI30775, R317573/19567470/CRA5626, and ONO-2333Ms. Finally, we discuss the potential heterogeneity and pharmacogenomics of CRF(1) receptor pharmacotherapy for alcohol dependence.

RESULTS

The evidence suggests that brain penetrant-CRF(1) receptor antagonists have therapeutic potential for alcohol dependence. Lead compounds with clinically desirable pharmacokinetic properties now exist, and longer receptor residence rates (i.e., slow dissociation) may predict greater CRF(1) receptor antagonist efficacy. Functional variants in genes that encode CRF system molecules, including polymorphisms in Crhr1 (rs110402, rs1876831, rs242938) and Crhbp genes (rs10055255, rs3811939) may promote alcohol seeking and consumption by altering basal or stress-induced CRF system activation.

CONCLUSIONS

Ongoing clinical trials with pexacerfont and verucerfont in moderately to highly severe dependent anxious alcoholics may yield insight as to the role of CRF(1) receptor antagonists in a personalized medicine approach to treat drug or alcohol dependence.

Morphine history sensitizes postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Previous work has shown that the dorsal raphe nucleus (DR)-5-HT system is inhibited by swim stress via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor (CRF). Additionally, the DR 5-HT system is regulated by opioids. The present study tests the hypothesis that the DR 5-HT system regulates stress-induced opioid relapse. In the first experiment, electrophysiological recordings of GABA synaptic activity in 5-HT DR neurons were conducted in brain slices from Sprague-Dawley rats that were exposed to swim stress-induced reinstatement of previously extinguished morphine conditioned place preference (CPP). Behavioral data indicate that swim stress triggers reinstatement of morphine CPP. Electrophysiology data indicate that 5-HT neurons in the morphine-conditioned group exposed to stress had increased amplitude of inhibitory postsynaptic currents (IPSCs), which would indicate greater postsynaptic GABA receptor density and/or sensitivity, compared to saline controls exposed to stress. In the second experiment, rats were exposed to either morphine or saline CPP and extinction, and then 5-HT DR neurons from both groups were examined for sensitivity to CRF in vitro. CRF induced a greater inward current in 5-HT neurons from morphine-conditioned subjects compared to saline-conditioned subjects. These data indicate that morphine history sensitizes 5-HT DR neurons to the GABAergic inhibitory effects of stress as well as to some of the effects of CRF. These mechanisms may sensitize subjects with a morphine history to the dysphoric effects of stressors and ultimately confer an enhanced vulnerability to stress-induced opioid relapse.

Mifepristone in the central nucleus of the amygdala reduces yohimbine stress-induced reinstatement of ethanol-seeking

Chronic ethanol exposure leads to dysregulation of the hypothalamic-pituitary-adrenal axis, leading to changes in glucocorticoid release and function that have been proposed to maintain pathological alcohol consumption and increase vulnerability to relapse during abstinence. The objective of this study was to determine whether mifepristone, a glucocorticoid receptor antagonist, plays a role in ethanol self-administration and reinstatement. Male, Long-Evans rats were trained to self-administer either ethanol or sucrose in daily 30 min operant self-administration sessions using a fixed ratio 3 schedule of reinforcement. Following establishment of stable baseline responding, we examined the effects of mifepristone on maintained responding and yohimbine-induced increases in responding for ethanol and sucrose. Lever responding was extinguished in separate groups of rats and animals were tested for yohimbine-induced reinstatement and corticosterone release. We also investigated the effects of local mifepristone infusions into the central amygdala (CeA) on yohimbine-induced reinstatement of ethanol- and sucrose-seeking. In addition, we infused mifepristone into the basolateral amygdala (BLA) in ethanol-seeking animals as an anatomical control. We show that both systemic and intra-CeA (but not BLA) mifepristone administration suppressed yohimbine-induced reinstatement of ethanol-seeking, while only systemic injections attenuated sucrose-seeking. In contrast, baseline consumption, yohimbine-induced increases in responding, and circulating CORT levels were unaffected. The data indicate that the CeA plays an important role in the effects of mifepristone on yohimbine-induced reinstatement of ethanol-seeking. Mifepristone may be a valuable pharmacotherapeutic strategy for preventing relapse to alcohol use disorders and, as it is FDA approved, may be a candidate for clinical trials in the near future.

Increases of CRF in the amygdala are responsible for reinstatement of methamphetamine-seeking behavior induced by footshock

Recent evidence suggests the involvement of corticotropin-releasing factor (CRF) in drug abuse. Here, we evaluated whether CRF modulates the reinstatement of methamphetamine (METH)-seeking behavior induced by stress using a drug-self administration paradigm in rats. Rats were trained to lever-press for intravenous METH (0.02 mg/infusion) accompanied by light and tone (drug-associated cues) and then underwent extinction training (saline substituted for METH without cues). Under the extinction condition, the inhibitory effects of a CRF receptor antagonist on the stress-induced reinstatement of METH-seeking behavior were assessed. Anxiety-like behaviors during METH withdrawal in METH self-administered rats were also evaluated. The non-selective CRF receptor antagonist α-helical CRF(9-41) attenuated METH-seeking behavior induced by footshock stress. CRF levels both in the amygdala and in plasma were significantly increased on day 10 of withdrawal after METH self-administration. However, plasma corticosterone concentrations were unchanged during the withdrawal. In addition, METH-seeking behavior was not affected by an inhibitor of corticosterone synthesis, metyrapone. In the elevated plus maze test, METH self-administered rats showed a decrease in the duration of time spent in the open arms on day 10 of withdrawal. The increased CRF levels in the amygdala may, at least in part, contribute to the footshock-induced reinstatement of METH-seeking behavior and the increase in anxiety-like behavior. The present findings indicate that CRF receptor antagonists would be useful as a therapeutic agent for METH-dependence.

Maintenance medication for opiate addiction: the foundation of recovery

Illicit use of opiates is the fastest growing substance use problem in the United States, and the main reason for seeking addiction treatment services for illicit drug use throughout the world. It is associated with significant morbidity and mortality related to human immunodeficiency virus, hepatitis C, and overdose. Treatment for opiate addiction requires long-term management. Behavioral interventions alone have extremely poor outcomes, with more than 80% of patients returning to drug use. Similarly poor results are seen with medication-assisted detoxification. This article provides a topical review of the three medications approved by the Food and Drug Administration for long-term treatment of opiate dependence: the opioid-agonist methadone, the partial opioid-agonist buprenorphine, and the opioid-antagonist naltrexone. Basic mechanisms of action and treatment outcomes are described for each medication. Results indicate that maintenance medication provides the best opportunity for patients to achieve recovery from opiate addiction. Extensive literature and systematic reviews show that maintenance treatment with either methadone or buprenorphine is associated with retention in treatment, reduction in illicit opiate use, decreased craving, and improved social function. Oral naltrexone is ineffective in treating opiate addiction, but recent studies using extended-release naltrexone injections have shown promise. Although no direct comparisons between extended-release naltrexone injections and either methadone or buprenorphine exist, indirect comparison of retention shows inferior outcome compared with methadone and buprenorphine. Further work is needed to directly compare each medication and determine individual factors that can assist in medication selection. Until such time, selection of medication should be based on informed choice following a discussion of outcomes, risks, and benefits of each medication.

Induction of multiple reinstatements of ethanol- and sucrose-seeking behavior in Long-Evans rats by the α-2 adrenoreceptor antagonist yohimbine

RATIONALE

Developing models to efficiently explore the mechanisms by which stress can mediate reinstatement of drug-seeking behavior is crucial to the development of new pharmacotherapies for alcohol use disorders.

OBJECTIVES

We examined the effects of multiple reinstatement sessions using the pharmacological stressor, yohimbine, in ethanol- and sucrose-seeking rats in order to develop a more efficient model of stress-induced reinstatement.

METHODS

Long-Evans rats were trained to self-administer 10% ethanol with a sucrose-fading procedure, 20% ethanol without a sucrose-fading procedure, or 5% sucrose in 30-min operant self-administration sessions, followed by extinction training. After reaching extinction criteria, the animals were tested once per week with yohimbine vehicle and yohimbine (2 mg/kg), respectively, 30 min prior to the reinstatement sessions or blood collection. Levels of reinstatement and plasma corticosterone (CORT) were determined each week for four consecutive weeks.

RESULTS

Yohimbine induced reinstatement of ethanol- and sucrose-seeking in each of the 4 weeks. Interestingly, the magnitude of the reinstatement decreased for the 10% ethanol group after the first reinstatement session but remained stable for the 20% ethanol group trained without sucrose. Plasma CORT levels in response to injection of both vehicle and yohimbine were significantly higher in the ethanol-trained animals compared to sucrose controls.

CONCLUSIONS

The stable reinstatement in the 20% ethanol group supports the use of this training procedure in studies using within-subject designs with multiple yohimbine reinstatement test sessions. Additionally, these results indicate that the hormonal response to stressors can be altered following extinction from self-administration of relatively modest amounts of ethanol.

Translational and reverse translational research on the role of stress in drug craving and relapse

RATIONALE AND BACKGROUND

High relapse rates during abstinence are a pervasive problem in drug addiction treatment. Relapse is often associated with stress exposure, which can provoke a subjective state of drug craving that can also be demonstrated under controlled laboratory conditions. Stress-induced relapse and craving in humans can be modeled in mice, rats, and monkeys using a reinstatement model in which drug-taking behaviors are extinguished and then reinstated by acute exposure to certain stressors. Studies using the reinstatement model in rats have identified the role of several neurotransmitters and brain sites in stress-induced reinstatement of drug seeking, but the degree to which these preclinical findings are relevant to the human condition is largely unknown.

OBJECTIVES AND HIGHLIGHTS

Here, we address this topic by discussing recent results on the effect of alpha-2 adrenoceptors and substance P-NK1 receptor antagonists on stress-induced reinstatement in mice and rats and stress-induced craving and potentially stress-induced relapse in humans. We also discuss brain sites and circuits involved in stress-induced reinstatement of drug seeking in rats and those activated during stress-induced craving in humans.

CONCLUSIONS

There is evidence that alpha-2 adrenoceptor agonists and NK1 receptor antagonists decrease stress-induced drug seeking in rats and stress-induced craving in humans. Whether these drugs would also prevent stress-induced drug relapse in humans and whether similar or different brain mechanisms are involved in stress-induced reinstatement in non-humans and stress-induced drug craving and relapse in humans are subjects for future research.

Prevention of social stress-escalated cocaine self-administration by CRF-R1 antagonist in the rat VTA

RATIONALE

Intermittent exposure to social defeat stress can induce long-term neural plasticity that may influence escalated cocaine-taking behavior. Stressful encounters can lead to activation of dopamine neurons in the ventral tegmental area (VTA), which are modulated by corticotropin releasing factor (CRF) neurons.

OBJECTIVE

The study aims to prevent the effects of intermittently scheduled, brief social defeat stress on subsequent intravenous (IV) cocaine self-administration by pretreatment with a CRF receptor subtype 1 (CRF-R1) antagonist.

MATERIALS AND METHODS

Long-Evans rats were submitted to four intermittent social defeat experiences separated by 72 h over 10 days. Two experiments examined systemic or intra-VTA antagonism of CRF-R1 subtype during stress on the later expression of locomotor sensitization and cocaine self-administration during fixed (0.75 mg/kg/infusion) and progressive ratio schedules of reinforcement (0.3 mg/kg/infusion), including a continuous 24-h "binge" (0.3 mg/kg/infusion).

RESULTS

Pretreatment with a CRF-R1 antagonist, CP 154,526, (20 mg/kg i.p.) prior to each social defeat episode prevented the development of stress-induced locomotor sensitization to a cocaine challenge and prevented escalated cocaine self-administration during a 24-h "binge". In addition, pretreatment with a CRF-R1 antagonist (0.3 μg/0.5 μl/side) into the VTA prior to each social defeat episode prevented stress-induced locomotor sensitization to a cocaine challenge and prevented escalated cocaine self-administration during a 24-h "binge".

CONCLUSIONS

The current results suggest that CRF-R1 subtype in the VTA is critically involved in the development of stress-induced locomotor sensitization which may contribute to escalated cocaine self-administration during continuous access in a 24-h "binge".

Cue-induced reactivity, cortisol response and substance use outcome in treated heroin dependent individuals

BACKGROUND

Environmental stimuli associated with drug taking have been known to elicit drug craving and physiologic arousal, as well as hypothalamic-pituitary-adrenal axis activation. However, the relationship between these responses and substance use outcomes in heroin-dependent subjects has not been previously studied. We investigated the relationship among subjective and physiologic reactivity, biological stress response evoked in the laboratory, and relapse to substance use in treated opiate-dependent individuals.

METHODS

Eighteen opiate-abstinent methadone- or buprenorphine-treated patients and 13 control subjects were exposed to neutral- and drug-cue exposure laboratory sessions with a 3-month follow-up period. Exposure to cues involved both videotapes and handling during a 100-min session. Subjective craving, agonistic effects, withdrawal feelings, galvanic skin resistance, and salivary cortisol were assessed. Substance use outcome among patients was examined during the follow-up phase. Differences between relapsers, nonrelapsers, and controls were analyzed with respect to the data on drug-cue responsivity and on cortisol responses using repeated-measures analysis of variance. The association with substance use outcome was assessed using a nominal logistic model.

RESULTS

Relapsers experienced greater drug-cue induced subjective responses and an increased cortisol response compared with both nonrelapsers and control subjects. After adjusting on covariates, cue-induced cortisol response was associated with the relapser group and was highly correlated with self-reports of "high."

CONCLUSIONS

Subjects defined as relapsers presented a higher cue-induced reactivity during the drug-cue exposure as well as an increased cortisol response to drug cues. Higher cortisol response to drug cues may increase relapse vulnerability in stable-dose buprenorphine or methadone-maintained subjects.

α(2) Adrenergic and imidazoline receptor agonists prevent cue-induced cocaine seeking

BACKGROUND

Drug-associated cues can elicit stress-like responses in addicted individuals, indicating that cue- and stress-induced drug relapse may share some neural mechanisms. It is unknown whether α(2) adrenergic receptor agonists, which are known to attenuate stress-induced reinstatement of drug seeking in rats, also reduce cue-induced reinstatement.

METHODS

Rats were tested for reinstatement of drug seeking following cocaine self-administration and extinction. We first evaluated the effects of clonidine, an agonist at α(2) and imidazoline-1 (I(1)) receptors, on relapse to cocaine seeking. To explore possible mechanisms of clonidine's effects, we then tested more specific α(2) or I(1) agonists, postsynaptic adrenergic receptor (α(1) and β) antagonists, and corticotropin-releasing factor receptor-1 antagonists.

RESULTS

We found that clonidine, and the more selective α(2) agonists UK-14,304 and guanfacine, decreased cue-induced reinstatement of cocaine seeking. The specific I(1) receptor agonist moxonidine reduced cue-induced as well as cocaine-induced reinstatement. Clonidine or moxonidine effects on cue-induced reinstatement were reversed by the selective α(2) receptor antagonist RS-79948, indicating a role for α(2) receptors. Prazosin and propranolol, antagonists at the α(1) and β receptor, respectively, reduced cue-induced reinstatement only when administered in combination. Finally, the corticotropin-releasing factor receptor-1 antagonist CP-154,526 reduced cue-induced reinstatement, as previously observed for stress-induced reinstatement, indicating possible overlap between stress and cue mechanisms.

CONCLUSIONS

These results indicate that α(2) and I(1) receptor agonists are novel therapeutic options for prevention of cue-induced cocaine relapse. Given that α(2) receptor stimulation is associated with sedation in humans, the I(1) agonist moxonidine seems to have substantial potential for treating addictive disorders.

Corticotrophin releasing factor (CRF) induced reinstatement of cocaine seeking in male and female rats

Significant sex differences have been demonstrated in clinical and preclinical studies of cocaine addiction, with some of the most consistent differences noted in regard to the role of stress and craving. The current study examined stress-induced reinstatement of cocaine seeking in male and female rats in an animal model of relapse using corticotropin-releasing factor (CRF) administration. Both male and female rats demonstrated increased cocaine seeking in response to CRF. CRF-induced reinstatement was highly variable across both male and female rats, and further analysis revealed a subpopulation that was particularly sensitive to CRF (high responders). Female high responders displayed significantly increased responding to CRF compared to males. Individual differences in stress responsivity could thus contribute to the likelihood of relapse, with females showing greater heterogeneity to stress-induced relapse.

Stress modulation of drug self-administration: implications for addiction comorbidity with post-traumatic stress disorder

Drug abuse and dependence present significant health burdens for our society, affecting roughly 10% of the population. Stress likely contributes to the development and persistence of drug use; for example, rates of substance dependence are elevated among individuals diagnosed with post-traumatic stress disorder (PTSD). Thus, understanding the interaction between stress and drug use, and associated neuroadaptations, is key for developing therapies to combat substance use disorders. For this purpose, many rodent models of the effects of stress exposure on substance use have been developed; the models can be classified according to three categories of stress exposure: developmental, adult nonsocial, and adult social. The present review addresses preclinical findings on the effect of each type of trauma on responses to and self-administration of drugs of abuse by focusing on a key exemplar for each category. In addition, the potential efficacy of targeting neuropeptide systems that have been implicated in stress responses and stress system neuroadaptation in order to treat comorbid PTSD and substance abuse will be discussed. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Adrenal activity during repeated long-access cocaine self-administration is required for later CRF-Induced and CRF-dependent stressor-induced reinstatement in rats

Understanding the neurobiological processes that contribute to the establishment and expression of stress-induced regulation of cocaine use in addicted individuals is important for the development of new and better treatment approaches. It has been previously shown that rats self-administering cocaine under long-access conditions (6 h daily) display heightened susceptibility to the reinstatement of extinguished cocaine seeking by a stressor, electric footshock, or i.c.v. administration of the stressor-responsive neuropeptide, corticotropin-releasing factor (CRF). This study tested the hypothesis that adrenal responsiveness during earlier long-access cocaine self-administration (SA) is necessary for the establishment of later CRF-dependent stress-induced reinstatement. Reinstatement by footshock, but not a cocaine challenge (10 mg/kg, i.p.) following long-access SA, was blocked by i.c.v. administration of the CRF receptor antagonist, α-helical CRF(9-41) (10 μg). Elimination of SA-induced adrenal responses through surgical adrenalectomy and diurnal corticosterone replacement (ADX/C) before 14 days of SA under long-access conditions had minimal impact on cocaine SA, but blocked later footshock-induced reinstatement. By contrast, ADX/C after SA, but before extinction and reinstatement testing, failed to reduce footshock-induced reinstatement. Likewise, ADX/C before 14 days long-access SA prevented later reinstatement by i.c.v. CRF (0.5 or 1.0 μg). However, significant CRF-induced reinstatement was observed when rats underwent ADX/C following SA, but before extinction and reinstatement testing, although a modest but statistically nonsignificant reduction in sensitivity to CRF's reinstating effects was observed. Taken together, these findings suggest that adrenal-dependent neuroadaptations in CRF responsiveness underlie the increased susceptibility to stress-induced relapse that emerges with repeated cocaine use.

β-Adrenergic receptors enhance excitatory transmission in the bed nucleus of the stria terminalis through a corticotrophin-releasing factor receptor-dependent and cocaine-regulated mechanism

BACKGROUND

Evidence suggests that the noradrenergic and corticotrophin-releasing factor (CRF) systems play critical roles in relapse and stress-related behaviors. In particular, behavioral studies point to a serial signaling process initiated by β-adrenergic receptors that requires CRF receptor (CRFR)-dependent signaling in the bed nucleus of the stria terminalis (BNST) to produce stress-induced relapse to cocaine seeking.

METHODS

We used whole cell patch clamp recordings from acutely prepared mouse brain slices to examine the actions of β-adrenergic receptors and CRFR1 on excitatory transmission in BNST. We examined the effects of agonists of these receptors in slices prepared from naive, sham, and cocaine-conditioned mice.

RESULTS

β(1)-adrenergic receptor activation within the BNST produces an enhancement of excitatory synaptic transmission that requires CRFR1-dependent signaling. We show that chronic cocaine administration transiently disrupts β(1)-adrenergic- and CRFR1-dependent enhancement of glutamatergic transmission, that this disruption wanes with time, and that it can be reintroduced with a cocaine challenge.

CONCLUSIONS

In total, these studies identify a circuit mechanism within the BNST that may play an important role in CRF- and norepinephrine-regulated behaviors.

Review of biological mechanisms and pharmacological treatments of comorbid PTSD and substance use disorder

Posttraumatic stress disorder (PTSD) and alcohol/substance use disorder (A/SUD) are frequently comorbid. Comorbidity is associated with poorer psychological, functional, and treatment outcomes than either disorder alone. This review outlines biological mechanisms that are potentially involved in the development and maintenance of comorbid PTSD and A/SUD including neurotransmitter and hypothalamic-pituitary-adrenal dysregulation, structural differences in the brain, and shared genetic risk factors. The literature regarding pharmacological treatments that have been investigated for comorbid PTSD and A/SUD is also reviewed. Empirical data for each proposed mechanism and pharmacological approach is reviewed with the goal of making recommendations for future research. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Delta opioid receptors colocalize with corticotropin releasing factor in hippocampal interneurons

The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect, likely playing a critical role in the interaction between stress and drug addiction. Prior study findings suggest that the stress-related neuropeptide corticotropin releasing factor (CRF) and the delta opioid receptor (DOR) may localize to similar neuronal populations within HF lamina. Here, hippocampal sections of male and cycling female adult Sprague-Dawley rats were processed for immunolabeling using antisera directed against the DOR and CRF peptide, as well as interneuron subtype markers somatostatin or parvalbumin, and analyzed by fluorescence and electron microscopy. Both DOR- and CRF-labeling was observed in interneurons in the CA1, CA3, and dentate hilus. Males and normal cycling females displayed a similar number of CRF immunoreactive neurons co-labeled with DOR and a similar average number of CRF-labeled neurons in the dentate hilus and stratum oriens of CA1 and CA3. In addition, 70% of DOR/CRF dual-labeled neurons in the hilar region co-labeled with somatostatin, suggesting a role for these interneurons in regulating perforant path input to dentate granule cells. Ultrastructural analysis of CRF-labeled axon terminals within the hilar region revealed that proestrus females have a similar number of CRF-labeled axon terminals that contain DORs compared to males but an increased number of CRF-labeled axon terminals without DORs. Taken together, these findings suggest that while DORs are anatomically positioned to modulate CRF immunoreactive interneuron activity and CRF peptide release, their ability to exert such regulatory activity may be compromised in females when estrogen levels are high.

Role of corticotropin-releasing factor in drug addiction: potential for pharmacological intervention

Drug dependence is a chronically relapsing disorder that places an enormous strain on healthcare systems. For treatments to have long-term clinical value, they must address the causes of relapse. Corticotropin-releasing factor (CRF), a neuropeptide central to the stress response, may be one key to solving the relapse cycle. CRF is hypothesized to mediate the elevated anxiety and negative emotional states experienced during the development of dependence. This review summarizes existing data on changes in the CRF system produced by drugs of abuse and the function of CRF receptors in regulating behavioural responses to drugs of abuse, with an emphasis on drug dependence. Drug-induced changes in neuronal excitability throughout the limbic system, as well as the reversal of these neuroadaptations by CRF receptor antagonists, are also addressed. CRF receptor antagonists, by reducing the motivational effects of drug withdrawal and protracted abstinence, are proposed to be novel therapeutic targets for drug abuse and addiction.

Enhanced dendritic availability of μ-opioid receptors in inhibitory neurons of the extended amygdala in mice deficient in the corticotropin-releasing factor-1 receptor

Activation of the corticotropin-releasing factor-1 (CRF-1) receptor in the anterolateral BNST (BSTal), a key subdivision of the extended amygdala, elicits opiate-seeking behavior exacerbated by stress. However, it is unknown whether the presence of CRF-1 affects expression of the μ-opioid receptor (μ-OR) in the many GABAergic BSTal neurons implicated in the stress response. We hypothesized that deletion of the CRF-1 receptor gene would alter the density and/or subcellular distribution of μ-ORs in GABAergic neurons of the BSTal. We used electron microscopy to quantitatively examine μ-OR immunogold and γ-aminobutyric acid (GABA) immunoperoxidase labeling in the BSTal of CRFr-1 knockout (KO) compared to wild-type (WT) mice. To assess regional specificity, we examined μ-OR distribution in dorsal striatum. The μ-ORs in each region were predominantly localized in dendrites, many of which were GABA-immunoreactive. Significantly, more cytoplasmic μ-OR gold particles per dendritic area were observed selectively in GABA-containing dendrites of the BSTal, but not of the dorsal striatum, in KO compared to WT mice. In both regions, however, significantly fewer GABA-immunoreactive axon terminals were present in KO compared to WT mice. Our results suggest that the absence of CRF-1 results in enhanced expression and/or dendritic trafficking of μ-ORs in inhibitory BSTal neurons. They also suggest that the expression of CRF-1 is a critical determinant of the availability of GABA in functionally diverse brain regions. These findings underscore the complex interplay between CRF, opioid, and GABA systems in limbic and striatal regions and have implications for the role of CRF-1 in influencing the pharmacological effects of opiates active at μ-ORs.

Comparison of self-administration behavior and responsiveness to drug-paired cues in rats running an alley for intravenous heroin and cocaine

RATIONALE

Evidence suggests that responsiveness to a drug-paired cue is predicted by the reinforcing magnitude of the drug during prior self-administration. It remains unclear, however, if this principle holds true when comparisons are made across drug reinforcers.

OBJECTIVE

The current study was therefore devised to test the hypothesis that differences in the animals' responsiveness to a cocaine- or heroin-paired cue presented during extinction would reflect differences in the patterns of prior cocaine and heroin runway self-administration.

METHODS

Rats ran a straight alley for single intravenous injections of either heroin (0.1 mg/kg/inj) or cocaine (1.0 mg/kg/inj) each paired with a distinct olfactory cue. Animals experienced 15 trials with each drug reinforcer in a counterbalanced manner. Start latencies, run times, and retreat behaviors (a form of approach-avoidance conflict) provided behavioral indices of the subjects' motivation to seek the reinforcer on each trial. Responsiveness to each drug-paired cue was assessed after 7, 14, or 21 days of non-reinforced extinction trials. Other animals underwent conditioned place preference (CPP) testing to ensure that the two drug reinforcers were capable of producing drug-cue associations.

RESULTS

While both drugs produced comparable CPPs, heroin served as a stronger incentive stimulus in the runway as evidenced by faster start and run times and fewer retreats. In contrast, cocaine- but not heroin-paired cues produced increases in drug-seeking behavior during subsequent extinction trials.

CONCLUSIONS

The subjects' responsiveness to drug-paired cues during extinction was not predicted by differences in the motivation to seek heroin versus cocaine during prior drug self-administration.

Role of dorsal medial prefrontal cortex dopamine D1-family receptors in relapse to high-fat food seeking induced by the anxiogenic drug yohimbine

In humans, relapse to maladaptive eating habits during dieting is often provoked by stress. In rats, the anxiogenic drug yohimbine, which causes stress-like responses in both humans and nonhumans, reinstates food seeking in a relapse model. In this study, we examined the role of medial prefrontal cortex (mPFC) dopamine D1-family receptors, previously implicated in stress-induced reinstatement of drug seeking, in yohimbine-induced reinstatement of food seeking. We trained food-restricted rats to lever press for 35% high-fat pellets every other day (9-15 sessions, 3 h each); pellet delivery was accompanied by a discrete tone-light cue. We then extinguished operant responding for 10-16 days by removing the pellets. Subsequently, we examined the effect of yohimbine (2 mg/kg, i.p.) on reinstatement of food seeking and Fos (a neuronal activity marker) induction in mPFC. We then examined the effect of systemic injections of the D1-family receptor antagonist SCH23390 (10 μg/kg, s.c.) on yohimbine-induced reinstatement and Fos induction, and that of mPFC SCH23390 (0.5 and 1.0 μg/side) injections on this reinstatement. Yohimbine-induced reinstatement was associated with strong Fos induction in the dorsal mPFC and with weaker Fos induction in the ventral mPFC. Systemic SCH23390 injections blocked both yohimbine-induced reinstatement and mPFC Fos induction. Dorsal, but not ventral, mPFC injections of SCH23390 decreased yohimbine-induced reinstatement of food seeking. In addition, dorsal mPFC SCH23390 injections decreased pellet-priming-induced reinstatement, but had no effect on ongoing high-fat pellet self-administration or discrete-cue-induced reinstatement. Results indicate a critical role of dorsal mPFC dopamine D1-family receptors in stress-induced relapse to palatable food seeking, as well as relapse induced by acute re-exposure to food taste, texture, and smell.

Pharmacologically-induced stress: a cross-species probe for translational research in drug addiction and relapse

Stress plays a major role in the process of drug addiction and various stressors are known to increase measures of craving in drug dependent human laboratory subjects. Animal models of stress-induced reinstatement of drug-seeking have also been developed in order to determine the neuropharmacological and neurobiological features of stress-induced relapse. Here, we review experimental approaches that use various pharmacological agents to induce a stress response and subsequent craving or drug-seeking for drugs of abuse. The advantages of such an approach are that the exact same stressor can be used in different species, pharmacological stress activation works on identifiable pathways, and stress levels can be varied via dose dependent manipulations. To date, successful use of such probes in both humans and experimental animals have been achieved with noradrenergic compounds and corticotrophin-releasing hormone (CRH). Other possible approaches, such as neuroactive peptides related to central stress responses (e.g., vasopressin and substance P) and inverse benzodiazepine agonists show some promise, and we discuss recent experiments using these compounds. Future development and application of pharmacological stressors across species will be useful in assessing stress-induced craving and relapse in both human drug addiction and animal models of relapse. Through this translational approach, novel treatment interventions for addiction may be designed and tested.

Brain neuropeptide Y and corticotropin-releasing hormone in mediating stress and anxiety

Neuropeptides such as neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH) have been implicated not only in acute regulation of stress/anxiety-related behaviors, but adaptations and changes in these neuropeptide systems may also participate in the regulation of behavior and endocrine responses during chronic stress. NPY is an endogenous anxiolytic neuropeptide, while CRH has anxiogenic properties upon central administration. Changes in these neuropeptide systems may contribute to disease states and give us indications for putative treatment targets for stress/anxiety disorders as well as alcohol/drug dependence. In this review, we briefly present these two systems and review their involvement in mediating the responses to acute and chronic stressors, as well as their possible roles in the development and progression of stress/anxiety disorders. We suggest that neuropeptides may be attractive in treatment development for stress/anxiety disorders, as well as for alcohol/drug dependence, based on their specificity and activity following exposure to external challenges, i.e. stressors, and their differential adaptations during transition from an acute to a chronic stress exposure state.

Ovarian hormones and propensity to drug relapse: a review

Sex differences have been reported in various phases of substance abuse, including relapse. In general, women show greater propensity to drug relapse than men, owing perhaps to divergent withdrawal experiences and increased reactivity to internal (emotional) and external (drug-associated) cues. Animal research tends to parallel human findings, revealing enhanced reinstatement of drug administration in females than males. Moreover, differences in vulnerability to relapse/reinstatement have been documented in women and female rodents across the ovarian cycles. Thus ovarian hormones seem to play an important role in determining susceptibility to relapse. Indeed, ovarian hormones interact with many of the neural circuits implicated in drug-primed, cue-instigated, and stress-induced relapse. By understanding the effects of ovarian hormones on the neural and behavioral mechanisms of drug relapse, sex differences and cyclical variations in relapse susceptibility can be elucidated and more effective treatment strategies can be explored.

CRF-1 antagonist and CRF-2 agonist decrease binge-like ethanol drinking in C57BL/6J mice independent of the HPA axis

Recent evidence suggests that corticotropin-releasing factor (CRF) receptor (CRFR) signaling is involved in modulating binge-like ethanol consumption in C57BL/6J mice. In this report, a series of experiments were performed to further characterize the role of CRFR signaling in binge-like ethanol consumption. The role of central CRFR signaling was assessed with intracerebroventricular (i.c.v.) infusion of the nonselective CRFR antagonist, alpha-helical CRF(9-41) (0, 1, 5, 10 microg/1 microl). The contribution of central CRF type 2 receptor (CRF(2)R) signaling was assessed with i.c.v. infusion of the selective CRF(2)R agonist, urocortin (Ucn) 3 (0, 0.05, 0.1, or 0.5 microg/1 microl). The role of the hypothalamic-pituitary-adrenal (HPA) axis was assessed by pretreating mice with intraperitoneal (i.p.) injection of (1) the corticosterone synthesis inhibitor, metyrapone (0, 50, 100, 150 mg/kg) or (2) the glucocorticoid receptor antagonist, mifepristone (0, 25, 50 mg/kg), and (3) by using radioimmunoassay to determine whether binge-like ethanol intake influenced plasma corticosterone levels. Finally, we determined whether the ability of the CRF(1)R antagonist, CP-154,526 (CP; 0, 10, 15 mg/kg, i.p.), to blunt binge-like drinking required normal HPA axis signaling by comparing the effectiveness of CP in adrenalectomized (ADX) and normal mice. Results showed that i.c.v. infusion of a 1 microg dose of alpha-helical CRF(9-41) significantly attenuated binge-like ethanol consumption relative to vehicle treatment, and i.c.v. infusion of Ucn 3 dose-dependently blunted binge-like drinking. On the other hand, metyrapone nonselectively reduced both ethanol and sucrose consumption, mifepristone did not alter ethanol drinking, and binge-like drinking did not correlate with plasma corticosterone levels. Finally, i.p. injection of CP significantly attenuated binge-like ethanol intake in both ADX and normal mice. Together, these results suggest that binge-like ethanol intake in C57BL/6J mice is modulated by CRF(1)R and CRF(2)R signaling, such that blockade of CRF(1)R or activation of CRF(2)R effectively reduces excessive ethanol intake. Furthermore, normal HPA axis signaling is not necessary to achieve binge-like drinking behavior.

The role of CRF and CRF-related peptides in the dark side of addiction

Drug addiction is a chronically relapsing disorder characterized by a compulsion to seek and take drugs, the development of dependence, and the manifestation of a negative emotional state when the drug is removed. Activation of brain stress systems is hypothesized to be a key element of the negative emotional state produced by dependence that drives drug-seeking through negative reinforcement mechanisms, defined as the "dark side" of addiction. The focus of the present review is on the role of corticotropin-releasing factor (CRF) and CRF-related peptides in the dark side of addiction. CRF is a key mediator of the hormonal, autonomic, and behavior responses to stressors. Emphasis is placed on the role of CRF in extrahypothalamic systems in the extended amygdala, including the central nucleus of the amygdala, bed nucleus of the stria terminalis, and a transition area in the shell of the nucleus accumbens, in the dark side of addiction. The urocortin/CRF(2) systems have been less explored, but results suggest their role in the neuroadaptation associated with chronic drug use, sometimes in opposition to the effects produced by the CRF(1) receptor. Compelling evidence argues that the CRF stress system, including its activation of the hypothalamic-pituitary-adrenal axis, plays a key role in engaging the transition to dependence and maintaining dependence once it is initiated. Understanding the role of the CRF systems in addiction not only provides insight into the neurobiology of the dark side of addiction, but also provides novel targets for identifying vulnerability to addiction and the treatment of addiction.

The locus coeruleus: A key nucleus where stress and opioids intersect to mediate vulnerability to opiate abuse

The interaction between the stress axis and endogenous opioid systems has gained substantial clinical attention as it is increasingly recognized that stress predisposes to opiate abuse. For example, stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Numerous reports indicating that stress alters individual sensitivity to opiates suggest that prior stress can influence the pharmacodynamics of opiates that are used in clinical settings. Conversely, the effects of opiates on different components of the stress axis can impact on individual responsivity to stressors and potentially predispose individuals to stress-related psychiatric disorders. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC), the major brain norepinephrine (NE)-containing nucleus. This review summarizes our current knowledge regarding the anatomical and neurochemical afferent regulation of the LC. It then presents physiological studies demonstrating opposing interactions between opioids and stress-related neuropeptides in the LC and summarizes results showing that chronic morphine exposure sensitizes the LC-NE system to corticotropin releasing factor and stress. Finally, new evidence for novel presynaptic actions of kappa-opioids on LC afferents is provided that adds another dimension to our model of how this central NE system is co-regulated by opioids and stress-related peptides.

Role of CRF and other neuropeptides in stress-induced reinstatement of drug seeking

A central problem in the treatment of drug addiction is high rates of relapse to drug use after periods of forced or self-imposed abstinence. This relapse is often provoked by exposure to stress. Stress-induced relapse to drug seeking can be modeled in laboratory animals using a reinstatement procedure. In this procedure, drug-taking behaviors are extinguished and then reinstated by acute exposure to stressors like intermittent unpredictable footshock, restraint, food deprivation, and systemic injections of yohimbine, an alpha-2 adrenoceptor antagonist that induces stress-like responses in humans and nonhumans. For this special issue entitled "The role of neuropeptides in stress and addiction", we review results from studies on the role of corticotropin-releasing factor (CRF) and several other peptides in stress-induced reinstatement of drug seeking in laboratory animals. The results of the studies reviewed indicate that extrahypothalamic CRF plays a critical role in stress-induced reinstatement of drug seeking; this role is largely independent of drug class, experimental procedure, and type of stressor. There is also limited evidence for the role of dynorphins, hypocretins (orexins), nociceptin (orphanin FQ), and leptin in stress-induced reinstatement of drug seeking.

Neurobiological mechanisms of addiction: focus on corticotropin-releasing factor

Drug addiction is a chronically relapsing disorder that is characterized by a compulsion to take drugs and loss of control in limiting intake. Medications that are on the market for the treatment of drug addiction target either the direct reinforcing effects of abuse (eg, naltrexone) or the consequent protracted abstinence syndrome (eg, acamprosate). Both conceptual and neurobiological advances in research have suggested that brain stress systems contribute to the withdrawal/negative affect and preoccupation/anticipation stages of the addiction cycle that promote the compulsivity of drug-taking in addiction. Validated animal models of the stress component of addiction and improved understanding of the neurocircuitry and neuropharmacological mechanisms involved in perturbations of this component suggest that corticotropin-releasing factor systems are a viable target for the development of future medications for drug addiction.

CRF1-R activation of the dynorphin/kappa opioid system in the mouse basolateral amygdala mediates anxiety-like behavior

Stress is a complex human experience and having both rewarding and aversive motivational properties. The adverse effects of stress are well documented, yet many of underlying mechanisms remain unclear and controversial. Here we report that the anxiogenic properties of stress are encoded by the endogenous opioid peptide dynorphin acting in the basolateral amygdala. Using pharmacological and genetic approaches, we found that the anxiogenic-like effects of Corticotropin Releasing Factor (CRF) were triggered by CRF₁-R activation of the dynorphin/kappa opioid receptor (KOR) system. Central CRF administration significantly reduced the percent open-arm time in the elevated plus maze (EPM). The reduction in open-arm time was blocked by pretreatment with the KOR antagonist norbinaltorphimine (norBNI), and was not evident in mice lacking the endogenous KOR ligand dynorphin. The CRF₁-R agonist stressin 1 also significantly reduced open-arm time in the EPM, and this decrease was blocked by norBNI. In contrast, the selective CRF₂-R agonist urocortin III did not affect open arm time, and mice lacking CRF₂-R still showed an increase in anxiety-like behavior in response to CRF injection. However, CRF₂-R knockout animals did not develop CRF conditioned place aversion, suggesting that CRF₁-R activation may mediate anxiety and CRF₂-R may encode aversion. Using a phosphoselective antibody (KORp) to identify sites of dynorphin action, we found that CRF increased KORp-immunoreactivity in the basolateral amygdala (BLA) of wildtype, but not in mice pretreated with the selective CRF₁-R antagonist, antalarmin. Consistent with the concept that acute stress or CRF injection-induced anxiety was mediated by dynorphin release in the BLA, local injection of norBNI blocked the stress or CRF-induced increase in anxiety-like behavior; whereas norBNI injection in a nearby thalamic nucleus did not. The intersection of stress-induced CRF and the dynorphin/KOR system in the BLA was surprising, and these results suggest that CRF and dynorphin/KOR systems may coordinate stress-induced anxiety behaviors and aversive behaviors via different mechanisms.

Corticotropin releasing factor and neuroplasticity in cocaine addiction

Corticotropin releasing factor (CRF), one of the major effectors of stress, plays a major role in the natural course of drug addiction by accelerating the acquisition of psychostimulant self-administration and increasing incentive motivation for the drug itself and for drug-associated stimuli. Stress-induced CRF is also considered a predictor of relapse and is responsible for feelings of anxiety and distress during cocaine withdrawal. Despite this knowledge, the role of CRF has not been explored in the context of recent research on reward-related learning, built on the hypothesis that neuroplastic changes in the mesocorticolimbic circuitry underlie addiction. The present review explores the effects of stress on the pattern of interaction between CRF, dopamine and glutamate in distinct structures of the mesocorticolimbic circuitry, including the ventral tegmental area (VTA), amygdala, bed nucleus of stria terminalis (BNST) and the prefrontal cortex (PFC), after acute and chronic cocaine consumption as well as in early withdrawal and protracted abstinence. A better knowledge of the neurochemical and cellular mechanisms involved in these interactions would be useful to elucidate the role of CRF in cocaine-induced neuronal plasticity, which could be useful in developing new pharmacological strategies for the treatment of cocaine addiction.

Behavioral and hormonal regulation of expression of the clock protein, PER2, in the central extended amygdala

PER2, a key molecular component of the mammalian circadian clock, is expressed rhythmically in many brain areas and peripheral tissues in mammals. Here we review findings from our work on the nature and regulation of rhythms of expression of PER2 in two anatomically and neurochemically defined subregions of the central extended amygdala, the oval nucleus of the bed nucleus of the stria terminalis (BNSTov) and the central nucleus of the amygdala (CEA). Daily rhythms in the expression of PER2 in these regions are coupled to those of the master circadian pacemaker, the suprachiasmatic nucleus (SCN) but, importantly, they are sensitive to homeostatic perturbations and to hormonal states that directly influence motivated behavior.

Molecular and genetic substrates linking stress and addiction

Drug addiction is one of the top three health concerns in the United States in terms of economic and health care costs. Despite this, there are very few effective treatment options available. Therefore, understanding the causes and molecular mechanisms underlying the transition from casual drug use to compulsive drug addiction could aid in the development of treatment options. Studies in humans and animal models indicate that stress can lead to both vulnerability to develop addiction, and increased drug taking and relapse in addicted individuals. Exposure to stress or drugs of abuse results in long-term adaptations in the brain that are likely to involve persistent alterations in gene expression or activation of transcription factors, such as the cAMP Response Element Binding (CREB) protein. The signaling pathways controlled by CREB have been strongly implicated in drug addiction and stress. Many potential CREB target genes have been identified based on the presence of a CRE element in promoter DNA sequences. These include, but are not limited to CRF, BDNF, and dynorphin. These genes have been associated with initiation or reinstatement of drug reward and are altered in one direction or the other following stress. While many reviews have examined the interactions between stress and addiction, the goal of this review was to focus on specific molecules that play key roles in both stress and addiction and are therefore posed to mediate the interaction between the two. Focus on these molecules could provide us with new targets for pharmacological treatments for addiction.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.