Extended access cocaine self-administration differentially activates dorsal raphe and amygdala corticotropin-releasing factor systems in rats

Corticotropin releasing factor and drug seeking in substance use disorders: Preclinical evidence and translational limitations

The neuropeptide, corticotropin releasing factor (CRF), has been an enigmatic target for the development of medications aimed at treating stress-related disorders. Despite a large body of evidence from preclinical studies in rodents demonstrating that CRF receptor antagonists prevent stressor-induced drug seeking, medications targeting the CRF-R1 have failed in clinical trials. Here, we provide an overview of the abundant findings from preclinical rodent studies suggesting that CRF signaling is involved in stressor-induced relapse. The scientific literature that has defined the receptors, mechanisms and neurocircuits through which CRF contributes to stressor-induced reinstatement of drug seeking following self-administration and conditioned place preference in rodents is reviewed. Evidence that CRF signaling is recruited with repeated drug use in a manner that heightens susceptibility to stressor-induced drug seeking in rodents is presented. Factors that may determine the influence of CRF signaling in substance use disorders, including developmental windows, biological sex, and genetics are examined. Finally, we discuss the translational failure of medications targeting CRF signaling as interventions for substance use disorders and other stress-related conditions. We conclude that new perspectives and research directions are needed to unravel the mysterious role of CRF in substance use disorders.

Nalmefene, a mu opioid receptor antagonist/kappa opioid receptor partial agonist, potentiates cocaine motivation but not intake with extended access self-administration in adult male mice

The mu opioid receptor antagonist/kappa opioid receptor (KOR) partial agonist nalmefene (NMF), a close structural analog of naltrexone (NTX), has been shown to reduce cocaine reward in preclinical models. Given the greater KOR potency and improved bioavailability compared to NTX, NMF may be a promising pharmacotherapeutic for cocaine use disorder (CUD). Here we examine the effects of NMF pretreatment on chronic daily extended access (4h) cocaine intravenous self-administration (IVSA) in adult male C57Bl/6J mice.

METHODS

separate groups of mice had daily 4h cocaine IVSA sessions (0.25 or 0.5 mg/kg/inf, FR1) for 14 days. Starting on day 8, mice were pretreated with NMF (0, 1, or 10 mg/kg) 30m before each session. A separate group of mice acquired cocaine IVSA [seven days FR1 then four FR3 of 4h daily sessions (0.5 mg/kg/inf)] prior to a single progressive ratio 3 session to examine the effect of 1 mg/kg NMF on cocaine motivation.

RESULTS

No significant effect of NMF pretreatment on cocaine intake was observed. Acute pretreatment of 1 mg/kg NMF significantly potentiated cocaine motivation as measured by progressive ratio breakpoint.

CONCLUSIONS

NMF did not significantly attenuate cocaine intake and increased motivation for cocaine suggesting that NMF may not be suitable for non-abstinent CUD patients. Further research is needed with KOR selective partial or full agonists to determine their effect on cocaine reinforcement.

Shifts in the neurobiological mechanisms motivating cocaine use with the development of an addiction-like phenotype in male rats

RATIONALE

The development of addiction is accompanied by a shift in the mechanisms motivating cocaine use from nucleus accumbens (NAc) dopamine D₁ receptor (D₁R) signaling to glutamate AMPA-kainate receptor (AMPA-R) signaling.

OBJECTIVE

Here, we determined whether similar shifts occur for NAc-D₂R signaling and following systemic manipulation of D₁R, D₂R, and AMPA-R signaling.

METHODS

Male rats were given short-access (20 infusions/day) or extended-access to cocaine (24 h/day, 96 infusions/day, 10 days). Motivation for cocaine was assessed following 14 days of abstinence using a progressive-ratio schedule. Once responding stabilized, the effects of NAc-D₂R antagonism (eticlopride; 0-10.0 μg/side) and systemic D₁R (SCH-23390; 0-1.0 mg/kg), D₂R (eticlopride; 0-0.1 mg/kg), and AMPA-R (CNQX; 0-1.5 mg/kg) antagonism, and NAc-dopamine-R gene expression (Drd1/2/3) were examined.

RESULTS

Motivation for cocaine was markedly higher in the extended- versus short-access group confirming the development of an addiction-like phenotype in the extended-access group. NAc-infused eticlopride decreased motivation for cocaine in both the short- and extended-access groups although low doses (0.1-0.3 μg) were more effective in the short-access group and high doses (3-10 μg/side) tended to be more effective in the extended-access group. Systemic administration of eticlopride (0.1 mg/kg) was more effective in the extended-access group, and systemic administration of CNQX was effective in the extended- but not short-access group. NAc-Drd2 expression was decreased in both the short- and extended-access groups.

CONCLUSION

These findings indicate that in contrast to NAc-D₁R, D₂R remain critical for motivating cocaine use with the development of an addiction-like phenotype. These findings also indicate that shifts in the mechanisms motivating cocaine use impact the response to both site-specific and systemic pharmacological treatment.

Limited versus extended cocaine intravenous self-administration: Behavioral effects and electrophysiological changes in insular cortex

Aims

Limited vs extended drug exposure has been proposed as one of the key factors in determining the risk of relapse, which is the primary characteristic of addiction behaviors. The current studies were designed to explore the related behavioral effects and neuronal alterations in the insular cortex (IC), an important brain region involved in addiction.

Methods

Experiments started with rats at the age of 35 days, a typical adolescent stage when initial drug exposure occurs often in humans. The drug‐seeking/taking behaviors, and membrane properties and intrinsic excitability of IC pyramidal neurons were measured on withdrawal day (WD) 1 and WD 45‐48 after limited vs extended cocaine intravenous self‐administration (IVSA).

Results

We found higher cocaine‐taking behaviors at the late withdrawal period after limited vs extended cocaine IVSA. We also found minor but significant effects of limited but not extended cocaine exposure on the kinetics and amplitude of action potentials on WD 45, in IC pyramidal neurons.

Conclusion

Our results indicate potential high risks of relapse in young rats with limited but not extended drug exposure, although the adaptations detected in the IC may not be sufficient to explain the neural changes of higher drug‐taking behaviors induced by limited cocaine IVSA.

Hold-down as an alternative to unit dose in cocaine self-administration experiments: Characterization using a progressive ratio schedule

RATIONALE

Virtually all cocaine self-administration studies have used a "unit dose" as a reinforcing stimulus; the subject is a passive recipient of an experimenter-selected dose.

OBJECTIVES

The present experiments examined the consequence of requiring the subject to actively determine the dose and speed of each injection.

METHODS

A two-lever procedure was used in which responding on a progressive ratio (PR) schedule provided access to cocaine on a hold down (HD) schedule. With HD, the pump is turned on for the duration that the lever is held down, thus the dose and speed of injection is determined by the behavior of the subject. The procedure allows for the evaluation of both drug taking and drug seeking responses.

RESULTS

The results were qualitatively different from PR self-administration studies using unit dose. The self-administered HD dose varied across the session; the self-administered dose was found to inversely correlate with drug levels at the time of access. Importantly, the 2 L-PR-HD procedure identified a subpopulation of subjects that showed extremes in both drug seeking and drug taking. Subjects at the top end of the distribution displayed unprecedented final ratios (> 900) and rapidly self-administered very large doses (> 1.4 mg; ~ 4.2 mg/kg). Manipulation of drug-taking variables (HD access duration and concentration of drug in the pump) showed that the immediacy of a cocaine bolus, not the duration of access, is the major determinant of drug seeking.

CONCLUSIONS

Incorporating a consummatory response into a PR procedure provides a unique perspective on the interactions of drug-seeking and drug-taking.

The duration of intermittent access to preferred sucrose-rich food affects binge-like intake, fat accumulation, and fasting glucose in male rats

Many people restrict their palatable food intake. In animal models, time-limiting access to palatable foods increases their intake while decreasing intake of less preferred alternatives; negative emotional withdrawal-like behavior is sometimes reported. In drug addiction models, intermittent extended access drives greater changes in use than brief access. When it comes to palatable food, the impact of briefer vs. longer access durations within intermittent access conditions remains unclear. Here, we provided male rats with chow or with weekday access to a preferred, sucrose-rich diet (PREF) (2, 4, or 8 h daily) with chow otherwise available. Despite normal energy intake, all restricted access conditions increased weight gain by 6 weeks and shifted diet acceptance within 1 week. They increased daily and 2-h intake of PREF with individual vulnerability and decreased chow intake. Rats with the briefest access had the greatest binge-like (2-h) intake, did not lose weight on weekends despite undereating chow, and were fattier by 12 weeks. Extended access rats (8 h) showed the greatest daily intake of preferred food and corresponding undereating of chow, slower weight gain when PREF was unavailable, and more variable daily energy intake from week to week. Increased fasting glucose was seen in 2-h and 8-h access rats. During acute withdrawal from PREF to chow diet, restricted access rats showed increased locomotor activity. Thus, intermittent access broadly promoted weight gain, fasting hyperglycemia and psychomotor arousal during early withdrawal. More restricted access promoted greater binge-like intake and fat accumulation, whereas longer access promoted evidence of greater food reward tolerance.

Knockdown of hypocretin attenuates extended access of cocaine self-administration in rats

The hypocretin/orexin (HCRT) neuropeptide system regulates feeding, arousal state, stress responses, and reward, especially under conditions of enhanced motivational relevance. In particular, HCRT neurotransmission facilitates drug-seeking behavior in circumstances that demand increased effort and/or motivation to take the drug. The present study used a shRNA-encoding adeno-associated viral vector to knockdown Hcrt expression throughout the dorsal hypothalamus in adult rats and determine the role of HCRT in cocaine self-administration. Chronic Hcrt silencing did not impact cocaine self-administration under short-access conditions, but robustly attenuated cocaine intake under extended access conditions, a model that mimics key features of compulsive cocaine taking. In addition, Hcrt silencing decreased motivation for both cocaine and a highly palatable food reward (i.e., sweetened condensed milk; SCM) under a progressive ratio schedule of reinforcement, but did not alter responding for SCM under a fixed ratio schedule. Importantly, Hcrt silencing did not affect food or water consumption, and had no consequence for general measures of arousal and stress reactivity. At the molecular level, chronic Hcrt knockdown reduced the number of neurons expressing dynorphin (DYN), and to a smaller extent melanin-concentrating hormone (MCH), in the dorsal hypothalamus. These original findings support the hypothesis that HCRT neurotransmission promotes operant responding for both drug and non-drug rewards, preferentially under conditions requiring a high degree of motivation. Furthermore, the current study provides compelling evidence for the involvement of the HCRT system in cocaine self-administration also under low-effort conditions in rats allowed extended access, possibly via functional interactions with DYN and MCH signaling.

Escalated cocaine "binges" in rats: enduring effects of social defeat stress or intra-VTA CRF

RATIONALE

Exposure to intermittent social defeat stress elicits corticotropin releasing factor (CRF) release into the VTA and induces long-term modulation of mesocorticolimbic dopamine activity in rats. These adaptations are associated with an intense cocaine-taking phenotype, which is prevented by CRF receptor antagonists.

OBJECTIVE

The present studies examine whether infusion of CRF into the VTA is sufficient to escalate cocaine-taking behavior, in the absence of social defeat experience. Additionally, we aimed to characterize changes in cocaine valuation that may promote binge-like cocaine intake.

METHODS

Male Long-Evans rats were microinjected into the VTA with CRF (50 or 500 ng/side), vehicle, or subjected to social defeat stress, intermittently over 10 days. Animals were then trained to self-administer IV cocaine (FR5). Economic demand for cocaine was evaluated using a within-session behavioral-economics threshold procedure, which was followed by a 24-h extended access "binge."

RESULTS

Rats that experienced social defeat or received intra-VTA CRF microinfusions (50 ng) both took significantly more cocaine than controls over the 24-h binge but showed distinct patterns of intake. Behavioral economic analysis revealed that individual demand for cocaine strongly predicts binge-like consumption, and demand elasticity (i.e. α) is augmented by intra-VTA CRF, but not by social defeat. The effects of CRF on cocaine-taking were also prevented by intra-VTA pretreatment with CP376395, but not Astressin-2B.

CONCLUSIONS

Repeated infusion of CRF into the VTA persistently alters cocaine valuation and intensifies binge-like drug intake in a CRF-R1-dependent manner. Conversely, the persistent pattern of cocaine bingeing induced by social defeat stress may suggest impaired inhibitory control, independent of reward valuation.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

Hypocretin Neurotransmission Within the Central Amygdala Mediates Escalated Cocaine Self-administration and Stress-Induced Reinstatement in Rats

BACKGROUND

Cocaine addiction is characterized by patterns of compulsive drug-taking, including preoccupation with obtaining cocaine and loss of control over drug intake. The lateral hypothalamic hypocretin/orexin (HCRT) system has been implicated in drug-taking and the reinstatement of drug-seeking. Evidence suggests that HCRT may drive drug-seeking through activation of specific brain regions implicated in stress system dysfunction, including the central amygdala (CeA). The role of HCRT in the persistence of compulsive-like cocaine-taking has yet to be fully elucidated.

METHODS

Systemic and intra-CeA microinfusions of the HCRT-receptor 1 antagonist, SB-334867, were administered to rats allowed either short (1 hour; ShA) or long (6 hours; LgA) access to cocaine self-administration. Animals were tested for fixed and progressive ratio responding for cocaine and stress-induced reinstatement of drug-seeking. In addition, using electrophysiological techniques on in vitro slices, we investigated gamma-aminobutyric acidergic (GABAergic) neurotransmission in the medial CeA and the sensitivity of GABAergic synapses to modulation of the HCRT system in ShA or LgA rats.

RESULTS

We found systemic administration of SB-334867 (0, 7.5, 15, 30 mg/kg) dose dependently decreased cocaine intake specifically in LgA rats but not in ShA rats. Microinjections of SB-334867 (20 nmol) bilaterally into the CeA significantly reduced cocaine intake in LgA rats. We also observed a significant attenuation of yohimbine-induced reinstatement of cocaine-seeking after intra-CeA SB-334867 (10 nmol) administration. Finally, electrophysiological data indicated enhanced GABAergic neurotransmission within the medial CeA in LgA rats, which was blocked with SB-334867 (10 μmol/L).

CONCLUSIONS

These findings suggest that HCRT neurotransmission within the CeA is implicated in compulsive-like cocaine-seeking.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3 mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Knockdown of CRF1 receptors in the ventral tegmental area attenuates cue- and acute food deprivation stress-induced cocaine seeking in mice

Corticotrophin-releasing factor (CRF) modulates the influence of stress on cocaine reward and reward seeking acting at multiple sites, including the ventral tegmental area (VTA). There is controversy, however, concerning the contribution of CRF receptor type 1 (CRFR1) to this effect and whether CRF within the VTA is involved in other aspects of reward seeking independent of acute stress. Here we examine the role of CRFR1 within the VTA in relation to cocaine and natural reward using viral delivery of short hairpin RNAs (lenti-shCRFR1) and investigate the effect on operant self-administration and motivation to self-administer, as well as stress- and cue-induced reward seeking in mice. While knockdown of CRFR1 in the VTA had no effect on self-administration behavior for either cocaine or sucrose, it effectively blocked acute food deprivation stress-induced reinstatement of cocaine seeking. We also observed reduced cue-induced cocaine seeking assessed in a single extinction session after extended abstinence, but cue-induced sucrose seeking was unaffected, suggesting dissociation between the contribution of CRFR1 in the VTA in cocaine reward and sucrose and cocaine seeking. Further, our data indicate a role for VTA CRFR1 signaling in cocaine seeking associated with, and independent of, stress potentially involving conditioning and/or salience attribution of cocaine reward-related cues. CRFR1 signaling in the VTA therefore presents a target for convergent effects of both cue- and stress-induced cocaine-seeking pathways.

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.

Sex differences in the neurobiology of drug addiction

Epidemiological data demonstrate that while women report lower rates of drug use than men, the number of current drug users and abusers who are women continues to increase. In addition women progress through the phases of addiction differently than men; women transition from casual drug use to addiction faster, are more reactive to stimuli that trigger relapse, and have higher rates of relapse then men. Sex differences in physiological and psychological responses to drugs of abuse are well documented and it is well established that estrogen effects on dopamine (DA) systems are largely responsible for these sex differences. However, the downstream mechanisms that result from interactions between estrogen and the effects of drugs of abuse on the DA system are just beginning to be explored. Here we review the basic neurocircuitry which underlies reward and addiction; highlighting the neuroadaptive changes that occur in the mesolimbic dopamine reward and anti-reward/stress pathways. We propose that sex differences in addiction are due to sex differences in the neural systems which mediate positive and negative reinforcement and that these differences are modulated by ovarian hormones. This forms a neurobehavioral basis for the search for the molecular and cellular underpinnings that uniquely guide motivational behaviors and make women more vulnerable to developing and sustaining addiction than men.

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.

Cocaine-induced membrane adaptation in the central nucleus of amygdala

Exposure to drugs of abuse lead to both rewarding effects and the subsequent development of negative affects. The progressive dysregulation of both processes is thought to critically contribute to the addictive state. Whereas cocaine-induced maladaptations in reward circuitry have been extensively examined, the cellular substrates underlying negative affect remain poorly understood. This study focuses on the central nucleus of the amygdala (CeA), a brain region that has been implicated in negative affective states upon withdrawal from chronic cocaine use. We observed that the two major types of CeA neurons, low-threshold bursting (LTB) neurons and regular spiking (RS) neurons, exhibited different sensitivity to corticotrophin-releasing factor (CRF), a stress hormone that has been implicated in negative affect during drug withdrawal. Furthermore, LTB and RS neurons developed opposite membrane adaptations following short-term (5 day) cocaine self-administration; the membrane excitability was increased in LTB neurons but decreased in RS neurons. These short-term exposure-induced effects were transient as they were present on withdrawal day 1 but disappeared on withdrawal day 21. However, extended exposure (21 day) led to sustained increase in the membrane excitability of LTB neurons such that it lasted over 21 days into the withdrawal period. These results suggest that CeA neurons can be a cellular target for cocaine to reshape the circuitry mediating negative affects during withdrawal, and that the long-lasting cellular alterations in selective subpopulations of CeA neurons may lead to unbalanced CeA processing, thus contributing to the progressive aggravation of negative affective states during withdrawal from chronic cocaine exposure.

Pregabalin reduces cocaine self-administration and relapse to cocaine seeking in the rat

Pregabalin (Lyrica™) is a structural analog of γ-aminobutyric acid (GABA) and is approved by the FDA for partial epilepsy, neuropathic pain and generalized anxiety disorders. Pregabalin also reduces excitatory neurotransmitter release and post-synaptic excitability. Recently, we demonstrated that pregabalin reduced alcohol intake and prevented relapse to the alcohol seeking elicited by stress or environmental stimuli associated with alcohol availability. Here, we sought to extend these findings by examining the effect of pregabalin on cocaine self-administration (0.25 mg/infusion) and on cocaine seeking elicited by both conditioned stimuli and stress, as generated by administration of yohimbine (1.25 mg/kg). The results showed that oral administration of pregabalin (0, 10 or 30 mg/kg) reduced self-administration of cocaine over an extended period (6 hours), whereas it did not modify self-administration of food. In cocaine reinstatement studies, pregabalin (10 and 30 mg/kg) abolished the cocaine seeking elicited by both the pharmacological stressor yohimbine and the cues predictive of cocaine availability. Overall, these results demonstrate that pregabalin may have potential in the treatment of some aspects of cocaine addiction.

Coordinate regulation of noradrenergic and serotonergic brain regions by amygdalar neurons

Based on the importance of the locus coeruleus-norepinephrine (LC-NE) system and the dorsal raphe nucleus-serotonergic (DRN-5-HT) system in stress-related pathologies, additional understanding of brain regions coordinating their activity is of particular interest. One such candidate is the amygdalar complex, and specifically, the central nucleus (CeA), which has been implicated in emotional arousal and is known to send monosynaptic afferent projections to both these regions. Our present data using dual retrograde tract tracing is the first to demonstrate a population of amygdalar neurons that project in a collateralized manner to the LC and DRN, indicating that amygdalar neurons are positioned to coordinately regulate the LC and DRN, and links these brain regions by virtue of a common set of afferents. Further, we have also characterized the phenotype of a population of these collaterally projecting neurons from the amygdala as containing corticotropin releasing factor or dynorphin, two peptides heavily implicated in the stress response. Understanding the co-regulatory influences of this limbic region on 5HT and NE regions may help fill a gap in our knowledge regarding neural circuits impacting these systems and their adaptations in stress.

The neurocircuitry of illicit psychostimulant addiction: acute and chronic effects in humans

Illicit psychostimulant addiction remains a significant problem worldwide, despite decades of research into the neural underpinnings and various treatment approaches. The purpose of this review is to provide a succinct overview of the neurocircuitry involved in drug addiction, as well as the acute and chronic effects of cocaine and amphetamines within this circuitry in humans. Investigational pharmacological treatments for illicit psychostimulant addiction are also reviewed. Our current knowledge base clearly demonstrates that illicit psychostimulants produce lasting adaptive neural and behavioral changes that contribute to the progression and maintenance of addiction. However, attempts at generating pharmacological treatments for psychostimulant addiction have historically focused on intervening at the level of the acute effects of these drugs. The lack of approved pharmacological treatments for psychostimulant addiction highlights the need for new treatment strategies, especially those that prevent or ameliorate the adaptive neural, cognitive, and behavioral changes caused by chronic use of this class of illicit drugs.

A dysphoric-like state during early withdrawal from extended access to methamphetamine self-administration in rats

RATIONALE

Negative emotional states during drug withdrawal may contribute to compulsive drug intake and seeking in humans. Studies suggest that extended access to methamphetamine induces compulsive drug intake in rats.

OBJECTIVE

The present study tested the hypothesis that compulsive methamphetamine intake in rats with extended access is associated with negative emotional states during drug withdrawal.

METHODS

Rats with short (1 h, ShA) and extended access (6 h, LgA) to methamphetamine self-administration (0.05 mg/kg/infusion) were tested for reward thresholds using intracranial self-stimulation (ICSS). Different groups of ShA and LgA rats were examined for depression-like and anxiety-like states in the novelty-suppressed feeding, open field, defensive burying, and forced swim tests.

RESULTS

With extended access, ICSS thresholds gradually increased, which was correlated with the increase of drug intake. During drug withdrawal, the increased ICSS thresholds returned to levels observed before exposure to extended access to methamphetamine. Upon re-exposure to extended access to methamphetamine, ICSS thresholds showed a more rapid escalation than during the initial exposure. LgA rats showed a longer latency to approach chow in the center of a novel field and remained immobile longer in the forced swim test than ShA rats did during early withdrawal. In contrast, ShA rats actively buried an aversive shock probe whereas LgA rats remained immobile in the defensive burying test.

CONCLUSION

The data suggest that extended access to methamphetamine produces a more depressive-like state than anxiety-like state in rats during early withdrawal.

The neurocircuitry of illicit psychostimulant addiction: acute and chronic effects in humans

Illicit psychostimulant addiction remains a significant problem worldwide, despite decades of research into the neural underpinnings and various treatment approaches. The purpose of this review is to provide a succinct overview of the neurocircuitry involved in drug addiction, as well as the acute and chronic effects of cocaine and amphetamines within this circuitry in humans. Investigational pharmacological treatments for illicit psychostimulant addiction are also reviewed. Our current knowledge base clearly demonstrates that illicit psychostimulants produce lasting adaptive neural and behavioral changes that contribute to the progression and maintenance of addiction. However, attempts at generating pharmacological treatments for psychostimulant addiction have historically focused on intervening at the level of the acute effects of these drugs. The lack of approved pharmacological treatments for psychostimulant addiction highlights the need for new treatment strategies, especially those that prevent or ameliorate the adaptive neural, cognitive, and behavioral changes caused by chronic use of this class of illicit drugs.

Emerging role for corticotropin releasing factor signaling in the bed nucleus of the stria terminalis at the intersection of stress and reward

Stress and anxiety play an important role in the development and maintenance of drug and alcohol addiction. The bed nucleus of the stria terminalis (BNST), a brain region involved in the production of long-term stress-related behaviors, plays an important role in animal models of relapse, such as reinstatement to previously extinguished drug-seeking behaviors. While a number of neurotransmitter systems have been suggested to play a role in these behaviors, recent evidence points to the neuropeptide corticotropin releasing factor (CRF) as being critically important in BNST-mediated reinstatement behaviors. Although numerous studies indicate that the BNST is a complex brain region with multiple afferent and efferent systems and a variety of cell types, there has only been limited work to determine how CRF modulates this complex neuronal system at the circuit level. Recent work from our lab and others have begun to unravel these BNST neurocircuits and explore their roles in CRF-related reinstatement behaviors. This review will examine the role of CRF signaling in drug addiction and reinstatement with an emphasis on critical neurocircuitry within the BNST that may offer new insights into treatments for addiction.

Corticotropin-releasing factor in the mouse central nucleus of the amygdala: ultrastructural distribution in NMDA-NR1 receptor subunit expressing neurons as well as projection neurons to the bed nucleus of the stria terminalis

Corticotropin-releasing factor (CRF) and glutamate are critical signaling molecules in the central nucleus of the amygdala (CeA). Central amygdala CRF, acting via the CRF type 1 receptor (CRF-R1), plays an integral role in stress responses and emotional learning, processes that are generally known to involve functional NMDA-type glutamate receptors. There is also evidence that CRF expressing CeA projection neurons to the bed nucleus of the stria terminalis (BNST) play an important role in stress related behaviors. Despite the potentially significant interactions between CRF and NMDA receptors in the CeA, the synaptic organization of these systems is largely unknown. Using dual labeling high resolution immunocytochemical electron microscopy, it was found that individual somata and dendrites displayed immunoreactivity for CRF and the NMDA-NR1 (NR1) subunit in the mouse CeA. In addition, CRF-containing axon terminals contacted postsynaptic targets in the CeA, some of which also expressed NR1. Neuronal profiles expressing the CRF type 1 receptor (CRF-R1), identified by the expression of green fluorescent protein (GFP) in bacterial artificial chromosome (BAC) transgenic mice, also contained NR1, and GFP immunoreactive terminals formed synapses with NR1 containing dendrites. Although CRF and GFP were only occasionally co-expressed in individual somata and dendritic profiles, contacts between labeled axon terminals and dendrites were frequently observed. A combination of tract tracing and immunocytochemistry revealed that a population of CeA CRF neurons projected to the BNST. It was also found that CRF, or GFP expressing terminals directly contacted CeA-BNST projection neurons. These results indicate that the NMDA receptor is positioned for the postsynaptic regulation of CRF expressing CeA neurons and the modulation of signals conveyed by CRF inputs. Interactions between CRF and NMDA receptor mediated signaling in CeA neurons, including those projecting to the BNST, may provide the synaptic basis for integrating the experience of stress and relevant environmental stimuli with behaviors that may be of particular relevance to stress-related learning and the emergence of psychiatric disorders, including drug addiction.