A role for the CRF-containing pathway from central nucleus of the amygdala to bed nucleus of the stria terminalis in the stress-induced reinstatement of cocaine seeking in rats

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

Adaptation of the nervous system to different chemical and physiologic conditions is important for the homeostasis of brain processes and for learning and remembering appropriate responses to challenges. Although processes such as tolerance and dependence to various drugs of abuse have been known for a long time, it was recently discovered that even a single pharmacologically relevant dose of various drugs of abuse induces neuroplasticity in selected neuronal populations, such as the dopamine neurons of the ventral tegmental area, which persist long after the drug has been excreted. Prolonged (self-) administration of drugs induces gene expression, neurochemical, neurophysiological, and structural changes in many brain cell populations. These region-specific changes correlate with addiction, drug intake, and conditioned drugs effects, such as cue- or stress-induced reinstatement of drug seeking. In rodents, adolescent drug exposure often causes significantly more behavioral changes later in adulthood than a corresponding exposure in adults. Clinically the most impairing and devastating effects on the brain are produced by alcohol during fetal development. In adult recreational drug users or in medicated patients, it has been difficult to find persistent functional or behavioral changes, suggesting that heavy exposure to drugs of abuse is needed for neurotoxicity and for persistent emotional and cognitive alterations. This review describes recent advances in this important area of research, which harbors the aim of translating this knowledge to better treatments for addictions and related neuropsychiatric illnesses.

Ethanol and corticotropin releasing factor receptor modulation of central amygdala neurocircuitry: An update and future directions

The central amygdala is a critical brain region for many aspects of alcohol dependence. Much of the work examining the mechanisms by which the central amygdala mediates the development of alcohol dependence has focused on the interaction of acute and chronic ethanol with central amygdala corticotropin releasing factor signaling. This work has led to a great deal of success in furthering the general understanding of central amygdala neurocircuitry and its role in alcohol dependence. Much of this work has primarily focused on the hypothesis that ethanol utilizes endogenous corticotropin releasing factor signaling to upregulate inhibitory GABAergic transmission in the central amygdala. Work that is more recent suggests that corticotropin releasing factor also plays an important role in mediating anxiety-like behaviors via the enhancement of central amygdala glutamatergic transmission, implying that ethanol/corticotropin releasing factor interactions may modulate excitatory neurotransmission in this brain region. In addition, a number of studies utilizing optogenetic strategies or transgenic mouse lines have begun to examine specific central amygdala neurocircuit dynamics and neuronal subpopulations to better understand overall central amygdala neurocircuitry and the role of neuronal subtypes in mediating anxiety-like behaviors. This review will provide a brief update on this literature and describe some potential future directions that may be important for the development of better treatments for alcohol addiction.

Stress, sex, and addiction: potential roles of corticotropin-releasing factor, oxytocin, and arginine-vasopressin

Stress sensitivity and sex are predictive factors for the development of neuropsychiatric disorders. Life stresses are not only risk factors for the development of addiction but also are triggers for relapse to drug use. Therefore, it is imperative to elucidate the molecular mechanisms underlying the interactions between stress and drug abuse, as an understanding of this may help in the development of novel and more effective therapeutic approaches to block the clinical manifestations of drug addiction. The development and clinical course of addiction-related disorders do appear to involve neuroadaptations within neurocircuitries that modulate stress responses and are influenced by several neuropeptides. These include corticotropin-releasing factor, the prototypic member of this class, as well as oxytocin and arginine-vasopressin that play important roles in affiliative behaviors. Interestingly, these peptides function to balance emotional behavior, with sexual dimorphism in the oxytocin/arginine-vasopressin systems, a fact that might play an important role in the differential responses of women and men to stressful stimuli and the specific sex-based prevalence of certain addictive disorders. Thus, this review aims to summarize (i) the contribution of sex differences to the function of dopamine systems, and (ii) the behavioral, neurochemical, and anatomical changes in brain stress systems.

Neuropeptide regulation of signaling and behavior in the BNST

Recent technical developments have transformed how neuroscientists can probe brain function. What was once thought to be difficult and perhaps impossible, stimulating a single set of long range inputs among many, is now relatively straight-forward using optogenetic approaches. This has provided an avalanche of data demonstrating causal roles for circuits in a variety of behaviors. However, despite the critical role that neuropeptide signaling plays in the regulation of behavior and physiology of the brain, there have been remarkably few studies demonstrating how peptide release is causally linked to behaviors. This is likely due to both the different time scale by which peptides act on and the modulatory nature of their actions. For example, while glutamate release can effectively transmit information between synapses in milliseconds, peptide release is potentially slower [See the excellent review by Van Den Pol on the time scales and mechanisms of release (van den Pol, 2012)] and it can only tune the existing signals via modulation. And while there have been some studies exploring mechanisms of release, it is still not as clearly known what is required for efficient peptide release. Furthermore, this analysis could be complicated by the fact that there are multiple peptides released, some of which may act in contrast. Despite these limitations, there are a number of groups making progress in this area. The goal of this review is to explore the role of peptide signaling in one specific structure, the bed nucleus of the stria terminalis, that has proven to be a fertile ground for peptide action.

GluN2B-containing NMDA receptors blockade rescues bidirectional synaptic plasticity in the bed nucleus of the stria terminalis of cocaine self-administering rats

Drugs of abuse have detrimental effects on homeostatic synaptic plasticity in the motivational brain network. Bidirectional plasticity at excitatory synapses helps keep neural circuits within a functional range to allow for behavioral flexibility. Therefore, impaired bidirectional plasticity of excitatory synapses may contribute to the behavioral hallmarks of addiction, yet this relationship remains unclear. Here we tracked excitatory synaptic strength in the oval bed nucleus of the stria terminalis (ovBNST) using whole-cell voltage-clamp recordings in brain slices from rats self-administering sucrose or cocaine. In the cocaine group, we measured both a persistent increase in AMPA to NMDA ratio (A:N) and slow decay time of NMDA currents throughout the self-administration period and after withdrawal from cocaine. In contrast, the sucrose group exhibited an early increase in A:N ratios (acquisition) that returned toward baseline values with continued self-administration (maintenance) and after withdrawal. The sucrose rats also displayed a decrease in NMDA current decay time with continued self-administration (maintenance), which normalized after withdrawal. Cocaine self-administering rats exhibited impairment in NMDA-dependent long-term depression (LTD) that could be rescued by GluN2B-containing NMDA receptor blockade. Sucrose self-administering rats demonstrated no impairment in NMDA-dependent LTD. During the maintenance period of self-administration, in vivo (daily intraperitoneally for 5 days) pharmacologic blockade of GluN2B-containing NMDA receptors did not reduce lever pressing for cocaine. However, in vivo GluN2B blockade did normalize A:N ratios in cocaine self-administrating rats, and dissociated the magnitude of ovBNST A:N ratios from drug-seeking behavior after protracted withdrawal. Altogether, our data demonstrate when and how bidirectional plasticity at ovBNST excitatory synapses becomes dysfunctional with cocaine self-administration and that NMDA-mediated potentiation of AMPA receptors in this region may be part of the neural circuits of drug relapse.

Role of bed nucleus of the stria terminalis corticotrophin-releasing factor receptors in frustration stress-induced binge-like palatable food consumption in female rats with a history of food restriction

We developed recently a binge-eating model in which female rats with a history of intermittent food restriction show binge-like palatable food consumption after 15 min exposure to the sight of the palatable food. This "frustration stress" manipulation also activates the hypothalamic-pituitary-adrenal stress axis. Here, we determined the role of the stress neurohormone corticotropin-releasing factor (CRF) in stress-induced binge eating in our model. We also assessed the role of CRF receptors in the bed nucleus of the stria terminalis (BNST), a brain region implicated in stress responses and stress-induced drug seeking, in stress-induced binge eating. We used four groups that were first exposed or not exposed to repeated intermittent cycles of regular chow food restriction during which they were also given intermittent access to high-caloric palatable food. On the test day, we either exposed or did not expose the rats to the sight of the palatable food for 15 min (frustration stress) before assessing food consumption for 2 h. We found that systemic injections of the CRF1 receptor antagonist R121919 (2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7 dipropylamino pyrazolo[1,5-a]pyrimidine) (10-20 mg/kg) and BNST (25-50 ng/side) or ventricular (1000 ng) injections of the nonselective CRF receptor antagonist D-Phe-CRF(12-41) decreased frustration stress-induced binge eating in rats with a history of food restriction. Frustration stress also increased Fos (a neuronal activity marker) expression in ventral and dorsal BNST. Results demonstrate a critical role of CRF receptors in BNST in stress-induced binge eating in our rat model. CRF1 receptor antagonists may represent a novel pharmacological treatment for bingeing-related eating disorders.

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.

Stress-induced cocaine seeking requires a beta-2 adrenergic receptor-regulated pathway from the ventral bed nucleus of the stria terminalis that regulates CRF actions in the ventral tegmental area

The ventral bed nucleus of the stria terminalis (vBNST) has been implicated in stress-induced cocaine use. Here we demonstrate that, in the vBNST, corticotropin releasing factor (CRF) is expressed in neurons that innervate the ventral tegmental area (VTA), a site where the CRF receptor antagonist antalarmin prevents the reinstatement of cocaine seeking by a stressor, intermittent footshock, following intravenous self-administration in rats. The vBNST receives dense noradrenergic innervation and expresses β adrenergic receptors (ARs). Footshock-induced reinstatement was prevented by bilateral intra-vBNST injection of the β-2 AR antagonist, ICI-118,551, but not the β-1 AR antagonist, betaxolol. Moreover, bilateral intra-vBNST injection of the β-2 AR agonist, clenbuterol, but not the β-1 agonist, dobutamine, reinstated cocaine seeking, suggesting that activation of vBNST β-2 AR is both necessary for stress-induced reinstatement and sufficient to induce cocaine seeking. The contribution of a β-2 AR-regulated vBNST-to-VTA pathway that releases CRF was investigated using a disconnection approach. Injection of ICI-118,551 into the vBNST in one hemisphere and antalarmin into the VTA of the contralateral hemisphere prevented footshock-induced reinstatement, whereas ipsilateral manipulations failed to attenuate stress-induced cocaine seeking, suggesting that β-2 AR regulate vBNST efferents that release CRF into the VTA, activating CRF receptors, and promoting cocaine use. Last, reinstatement by clenbuterol delivered bilaterally into the vBNST was prevented by bilateral vBNST pretreatment with antalarmin, indicating that β-2 AR-mediated actions in the vBNST also require local CRF receptor activation. Understanding the processes through which stress induces cocaine seeking should guide the development of new treatments for addiction.

Pituitary adenylate cyclase-activating polypeptide (PACAP) in the bed nucleus of the stria terminalis (BNST) increases corticosterone in male and female rats

Single nucleotide polymorphisms (SNP) in the genes for pituitary adenylate cyclase-activating polypeptide (PACAP) and the PAC1 receptor have been associated with several psychiatric disorders whose etiology has been associated with stressor exposure and/or dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. In rats, exposure to repeated variate stress has been shown to increase PACAP and its cognate PAC1 receptor expression in the bed nucleus of the stria terminalis (BNST), a brain region implicated in anxiety and depression-related behaviors as well as the regulation of HPA axis activity. We have argued that changes in BNST PACAP signaling may mediate the changes in emotional behavior and dysregulation of the HPA axis associated with anxiety and mood disorders. The current set of studies was designed to determine whether BNST PACAP infusion leads to activation of the HPA axis as determined by increases in plasma corticosterone. We observed an increase in plasma corticosterone levels 30min following BNST PACAP38 infusion in male and female rats, which was independent of estradiol (E2) treatment in females, and we found that plasma corticosterone levels were increased at both 30min and 60min, but returned to baseline levels 4h following the highest dose. PACAP38 infusion into the lateral ventricles immediately above the BNST did not alter plasma corticosterone level, and the increased plasma corticosterone following BNST PACAP was not blocked by BNST corticotropin releasing hormone (CRH) receptor antagonism. These results support others suggesting that BNST PACAP plays a key role in regulating stress responses.

Repeated intravenous administrations of teneurin-C terminal associated peptide (TCAP)-1 attenuates reinstatement of cocaine seeking by corticotropin-releasing factor (CRF) in rats

The teneurin c-terminal associated peptides (TCAP) have been implicated in the regulation of the stress response, possibly via a corticotropin-releasing factor (CRF)-related mechanism. We have previously shown that repeated intracerebroventricular (ICV) injections of TCAP-1 attenuate the reinstatement of cocaine seeking by CRF in rats. Here, we determined whether intravenous (IV) administrations of TCAP-1 would likewise attenuate CRF-induced reinstatement, and whether this effect would vary depending on the rat's history of cocaine self administration. Rats were trained to self-administer cocaine for 10 days, during once daily sessions that were either 3h ("short access"; ShA) or 6h ("long access"; LgA). Rats were then given five daily injections of TCAP-1 (0, 300, or 3,000 pmol, IV) in their home cage. Subsequently, they were returned to the self-administration chambers where extinction of cocaine seeking and testing for CRF-induced reinstatement of cocaine seeking was carried out. Repeated IV administrations of TCAP-1 were efficacious in attenuating CRF-induced reinstatement of cocaine seeking, but at different doses in ShA and LgA rats. Taken together, the findings extend previous work showing a consistent effect of repeated ICV TCAP-1 on CRF-induced reinstatement of cocaine seeking, and point to a potential therapeutic benefit of TCAP-1 in attenuating cocaine seeking behaviors.

The oxytocin analogue carbetocin prevents emotional impairment and stress-induced reinstatement of opioid-seeking in morphine-abstinent mice

The main challenge in treating opioid addicts is to maintain abstinence due to the affective consequences associated with withdrawal which may trigger relapse. Emerging evidence suggests a role of the neurohypophysial peptide oxytocin (OT) in the modulation of mood disorders as well as drug addiction. However, its involvement in the emotional consequences of drug abstinence remains unclear. We investigated the effect of 7-day opioid abstinence on the oxytocinergic system and assessed the effect of the OT analogue carbetocin (CBT) on the emotional consequences of opioid abstinence, as well as relapse. Male C57BL/6J mice were treated with a chronic escalating-dose morphine regimen (20-100 mg/kg/day, i.p.). Seven days withdrawal from this administration paradigm induced a decrease of hypothalamic OT levels and a concomitant increase of oxytocin receptor (OTR) binding in the lateral septum and amygdala. Although no physical withdrawal symptoms or alterations in the plasma corticosterone levels were observed after 7 days of abstinence, mice exhibited increased anxiety-like and depressive-like behaviors and impaired sociability. CBT (6.4 mg/kg, i.p.) attenuated the observed negative emotional consequences of opioid withdrawal. Furthermore, in the conditioned place preference paradigm with 10 mg/kg morphine conditioning, CBT (6.4 mg/kg, i.p.) was able to prevent the stress-induced reinstatement to morphine-seeking following extinction. Overall, our results suggest that alterations of the oxytocinergic system contribute to the mechanisms underlying anxiety, depression, and social deficits observed during opioid abstinence. This study also highlights the oxytocinergic system as a target for developing pharmacotherapy for the treatment of emotional impairment associated with abstinence and thereby prevention of relapse.

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.

Direct targeting of peptidergic amygdalar neurons by noradrenergic afferents: linking stress-integrative circuitry

Amygdalar norepinephrine (NE) plays a key role in regulating neural responses to emotionally arousing stimuli and is involved in memory consolidation of emotionally charged events. Corticotropin-releasing factor (CRF) and dynorphin (DYN), two neuropeptides that mediate the physiological and behavioral responses to stress, are abundant in the central nucleus of the amygdala (CeA), and directly innervate brainstem noradrenergic locus coeruleus (LC) neurons. Whether the CRF- and DYN-containing amygdalar neurons receive direct noradrenergic innervation has not yet been elucidated. The present study sought to define cellular substrates underlying noradrenergic modulation of CRF- and DYN-containing neurons in the CeA using immunohistochemistry and electron microscopy. Ultrastructural analysis revealed that NE-labeled axon terminals form synapses with CRF- and DYN-containing neurons in the CeA. Semi-quantitative analysis showed that approximately 31 % of NET-labeled axon terminals targeted CeA neurons that co-expressed DYN and CRF. As a major source of CRF innervation to the LC, it is also not known whether CRF-containing CeA neurons are directly targeted by noradrenergic afferents. To test this, retrograde tract tracing using FluoroGold from the LC was combined with immunocytochemical detection of CRF and NET in the CeA. Our results revealed a population of LC-projecting CRF-containing CeA neurons that are directly innervated by NE afferents. Analysis showed that approximately 34 % of NET-labeled axon terminals targeted LC-projecting CeA neurons that contain CRF. Taken together, these results indicate significant interactions between NE, CRF and DYN in this critical limbic region and reveal direct synaptic interactions of NE with amygdalar CRF that influence the LC-NE arousal system.

Developmental changes in the acute ethanol sensitivity of glutamatergic and GABAergic transmission in the BNST

Glutamatergic and GABAergic transmission undergo significant changes during adolescence. Receptors for both of these transmitters (NMDAR, and GABAA) are known to be key targets for the acute effects of ethanol in adults. The current study set out to investigate the acute effects of ethanol on both NMDAR-mediated excitatory transmission and GABAergic inhibitory transmission within the bed nucleus of the stria terminalis (BNST) across age. The BNST is an area of the brain implicated in the negative reinforcing properties associated with alcohol dependence, and the BNST plays a critical role in stress-induced relapse. Therefore, assessing the developmental regulation of ethanol sensitivity in this key brain region is important to understanding the progression of ethanol dependence. To do this, whole-cell recordings of isolated NMDAR-evoked excitatory postsynaptic currents (eEPSCs) or evoked GABAergic inhibitory postsynaptic currents (eIPSCs) were performed on BNST neurons in slices from 4- or 8-week-old male C57BL/6J mice. Ethanol (50 mm) produced greater inhibition of NMDAR-eEPSCs in adolescent mice than in adult mice. This enhanced sensitivity in adolescence was not a result of shifts in function of the GluN2B subunit of the NMDAR, measured by Ro25-6981 inhibition and decay kinetics measured across age. Adolescent mice also exhibited greater ethanol sensitivity of GABAergic transmission, as ethanol (50 mm) enhanced eIPSCs in the BNST of adolescent but not adult mice. Collectively, this work illustrates that a moderate dose of ethanol produces greater inhibition of transmission in the BNST (through greater excitatory inhibition and enhancement of inhibitory transmission) in adolescents compared to adults. Given the role of the BNST in alcohol dependence, these developmental changes in acute ethanol sensitivity could accelerate neuroadaptations that result from chronic ethanol use during the critical period of adolescence.

Inhibition of corticotropin releasing factor expression in the central nucleus of the amygdala attenuates stress-induced behavioral and endocrine responses

Corticotropin releasing factor (CRF) is a primary mediator of endocrine, autonomic and behavioral stress responses. Studies in both humans and animal models have implicated CRF in a wide-variety of psychiatric conditions including anxiety disorders such as post-traumatic stress disorder (PTSD), depression, sleep disorders and addiction among others. The central nucleus of the amygdala (CeA), a key limbic structure with one of the highest concentrations of CRF-producing cells outside of the hypothalamus, has been implicated in anxiety-like behavior and a number of stress-induced disorders. This study investigated the specific role of CRF in the CeA on both endocrine and behavioral responses to stress. We used RNA Interference (RNAi) techniques to locally and specifically knockdown CRF expression in CeA. Behavior was assessed using the elevated plus maze (EPM) and open field test (OF). Knocking down CRF expression in the CeA had no significant effect on measures of anxiety-like behavior in these tests. However, it did have an effect on grooming behavior, a CRF-induced behavior. Prior exposure to a stressor sensitized an amygdalar CRF effect on stress-induced HPA activation. In these stress-challenged animals silencing CRF in the CeA significantly attenuated corticosterone responses to a subsequent behavioral stressor. Thus, it appears that while CRF projecting from the CeA does not play a significant role in the expression stress-induced anxiety-like behaviors on the EPM and OF it does play a critical role in stress-induced HPA activation.

Corticotropin-releasing factor receptor type-2 is involved in the cocaine-primed reinstatement of cocaine conditioned place preference in rats

Here we explored the in vivo role of brain corticotropin-releasing factor receptor type-2 (CRFR2) in cocaine-primed reinstatement of drug seeking. Conditioned place preference (CPP) procedure was used to assess the acquisition, extinction and reinstatement of cocaine-seeking behavior in rats. First, expressions of CRFR2 were shown to be affected in a brain region-specific manner within cocaine-induced CPP and cocaine-extinct CPP models. Bilateral blockade of CRFR2 in the dorsal portion of the medial prefrontal cortex (mPFC), or hippocampus (HP) was partially inhibited, but in the dorsal striatum (DS) did not affect, the cocaine-primed reinstatement of cocaine CPP.

Optogenetic strategies to investigate neural circuitry engaged by stress

Optogenetic techniques have given researchers unprecedented access to the function of discrete neural circuit elements and have been instrumental in the identification of novel brain pathways that become dysregulated in neuropsychiatric diseases. For example, stress is integrally linked to the manifestation and pathophysiology of neuropsychiatric illness, including anxiety, addiction and depression. Due to the heterogeneous populations of genetically and neurochemically distinct neurons in areas such as the bed nucleus of the stria terminalis (BNST), as well as their substantial number of projections, our understanding of how neural circuits become disturbed after stress has been limited. Using optogenetic tools, we are now able to selectively isolate distinct neural circuits that contribute to these disorders and perturb these circuits in vivo, which in turn may lead to the normalization of maladaptive behavior. This review will focus on current optogenetic strategies to identify, manipulate, and record from discrete neural circuit elements in vivo as well as highlight recent optogenetic studies that have been utilized to parcel out BNST function.

Comprehensive mapping of regional expression of the clock protein PERIOD2 in rat forebrain across the 24-h day

In mammals, a light-entrainable clock located in the suprachiasmatic nucleus (SCN) regulates circadian rhythms by synchronizing oscillators throughout the brain and body. Notably, the nature of the relation between the SCN clock and subordinate oscillators in the rest of the brain is not well defined. We performed a high temporal resolution analysis of the expression of the circadian clock protein PERIOD2 (PER2) in the rat forebrain to characterize the distribution, amplitude and phase of PER2 rhythms across different regions. Eighty-four LEW/Crl male rats were entrained to a 12-h: 12-h light/dark cycle, and subsequently perfused every 30 min across the 24-h day for a total of 48 time-points. PER2 expression was assessed with immunohistochemistry and analyzed using automated cell counts. We report the presence of PER2 expression in 20 forebrain areas important for a wide range of motivated and appetitive behaviors including the SCN, bed nucleus, and several regions of the amygdala, hippocampus, striatum, and cortex. Eighteen areas displayed significant PER2 rhythms, which peaked at different times of day. Our data demonstrate a previously uncharacterized regional distribution of rhythms of a clock protein expression in the brain that provides a sound basis for future studies of circadian clock function in animal models of disease.

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'.

Amygdala and bed nucleus of the stria terminalis circuitry: Implications for addiction-related behaviors

Complex motivated behavioral processes, such as those that can go awry following substance abuse and other neuropsychiatric disorders, are mediated by a distributive network of neurons that reside throughout the brain. Neural circuits within the amygdala regions, such as the basolateral amygdala (BLA), and downstream targets such as the bed nucleus of the stria terminalis (BNST), are critical neuroanatomical structures for orchestrating emotional behavioral responses that may influence motivated actions such as the reinstatement of drug seeking behavior. Here, we review the functional neurocircuitry of the BLA and the BNST, and discuss how these circuits may guide maladaptive behavioral processes such as those seen in addiction. Thus, further study of the functional connectivity within these brain regions and others may provide insight for the development of new treatment strategies for substance use disorders. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Systems level neuroplasticity in drug addiction

Drug addiction is a chronic relapsing disorder for which research has been dedicated to understand the various factors that contribute to development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide a broad overview of various theories of addiction, drugs of abuse, and the neurobiology involved across the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the involvement of these pathways and associated circuits in mediating conditioned responses, drug craving, and loss of behavioral control thought to underlie withdrawal and relapse. With a better understanding of the neurobiological factors that underlie drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Kappa opioid receptors regulate stress-induced cocaine seeking and synaptic plasticity

Stress facilitates reinstatement of addictive drug seeking in animals and promotes relapse in humans. Acute stress has marked and long-lasting effects on plasticity at both inhibitory and excitatory synapses on dopamine neurons in the ventral tegmental area (VTA), a key region necessary for drug reinforcement. Stress blocks long-term potentiation at GABAergic synapses on dopamine neurons in the VTA (LTPGABA), potentially removing a normal brake on activity. Here we show that blocking kappa opioid receptors (KORs) prior to forced-swim stress rescues LTPGABA. In contrast, blocking KORs does not prevent stress-induced potentiation of excitatory synapses nor morphine-induced block of LTPGABA. Using a kappa receptor antagonist as a selective tool to test the role of LTPGABA in vivo, we find that blocking KORs within the VTA prior to forced-swim stress prevents reinstatement of cocaine seeking. These results suggest that KORs may represent a useful therapeutic target for treatment of stress-triggered relapse in substance abuse.

A corticotropin releasing factor pathway for ethanol regulation of the ventral tegmental area in the bed nucleus of the stria terminalis

A growing literature suggests that catecholamines and corticotropin-releasing factor (CRF) interact in a serial manner to activate the bed nucleus of the stria terminalis (BNST) to drive stress- or cue-induced drug- and alcohol-seeking behaviors. Data suggest that these behaviors are driven in part by BNST projections to the ventral tegmental area (VTA). Together, these findings suggest the existence of a CRF-signaling pathway within the BNST that is engaged by catecholamines and regulates the activity of BNST neurons projecting to the VTA. Here we test three aspects of this model to determine: (1) whether catecholamines modify CRF neuron activity in the BNST; (2) whether CRF regulates excitatory drive onto VTA-projecting BNST neurons; and (3) whether this system is altered by ethanol exposure and withdrawal. A CRF neuron fluorescent reporter strategy was used to identify BNST CRF neurons for whole-cell patch-clamp analysis in acutely prepared slices. Using this approach, we found that both dopamine and isoproterenol significantly depolarized BNST CRF neurons. Furthermore, using a fluorescent microsphere-based identification strategy we found that CRF enhances the frequency of spontaneous EPSCs onto VTA-projecting BNST neurons in naive mice. This action of CRF was occluded during acute withdrawal from chronic intermittent ethanol exposure. These findings suggest that dopamine and isoproterenol may enhance CRF release from local BNST sources, leading to enhancement of excitatory neurotransmission on VTA-projecting neurons, and that this pathway is engaged by patterns of alcohol exposure and withdrawal known to drive excessive alcohol intake.

Corticotropin releasing factor and catecholamines enhance glutamatergic neurotransmission in the lateral subdivision of the central amygdala

Glutamatergic neurotransmission in the central nucleus of the amygdala (CeA) plays an important role in many behaviors including anxiety, memory consolidation and cardiovascular responses. While these behaviors can be modulated by corticotropin releasing factor (CRF) and catecholamine signaling, the mechanism(s) by which these signals modify CeA glutamatergic neurotransmission remains unclear. Utilizing whole-cell patch-clamp electrophysiology recordings from neurons in the lateral subdivision of the CeA (CeAL), we show that CRF, dopamine (DA) and the β-adrenergic receptor agonist isoproterenol (ISO) all enhance the frequency of spontaneous excitatory postsynaptic currents (sEPSC) without altering sEPSC kinetics, suggesting they increase presynaptic glutamate release. The effect of CRF on sEPSCs was mediated by a combination of CRFR1 and CRFR2 receptors. While previous work from our lab suggests that CRFRs mediate the effect of catecholamines on excitatory transmission in other subregions of the extended amygdala, blockade of CRFRs in the CeAL failed to significantly alter effects of DA and ISO on glutamatergic transmission. These findings suggest that catecholamine and CRF enhancement of glutamatergic transmission onto CeAL neurons occurs via distinct mechanisms. While CRF increased spontaneous glutamate release in the CeAL, CRF caused no significant changes to optogenetically evoked glutamate release in this region. The dissociable effects of CRF on different types of glutamatergic neurotransmission suggest that CRF may specifically regulate spontaneous excitatory transmission.

Not all stress is equal: CREB is not necessary for restraint stress reinstatement of cocaine-conditioned reward

Stress elicits relapse to cocaine seeking in humans and in animal models. Cyclic AMP response element binding protein (CREB) is required for swim stress-induced reinstatement of cocaine conditioned place preference. However, the role of CREB in other stress-induced reinstatement models has not been examined. To determine whether CREB is required across different stressors we examined the ability of restraint to elicit reinstatement of cocaine-conditioned place preference in wild-type and CREBαΔ mutant mice. In contrast to previously published differences in swim stress-induced reinstatement, both wild-type and CREBαΔ mutant mice demonstrated restraint stress elicited reinstatement of cocaine-conditioned reward. While CREB is necessary for swim stress-elicited zif268 expression within the nucleus accubmens (NAc) shell and prelimbic cortex (PrL), restraint-stress-elicited comparable increases in zif268 expression within these regions in both wild-type and CREBαΔ mutant mice. Our findings suggest that not all stressors engage the same circuits or molecular mechanisms to elicit reinstatement behavior.

Cannabinoid CB1 receptors mediate the effects of corticotropin-releasing factor on the reinstatement of cocaine seeking and expression of cocaine-induced behavioural sensitization

BACKGROUND AND PURPOSE

The endocannabinoid and corticotropin-releasing factor (CRF) systems have been implicated in several long-lasting behavioural effects of prior cocaine experience. The present experiments were designed to probe functional interactions between endocannabinoids and CRF by testing the role of cannabinoid CB(1) receptors in cocaine-related behaviours induced or mediated by CRF.

EXPERIMENTAL APPROACH

In Experiment 1, rats trained to self-administer cocaine were pretreated with the CB(1) receptor antagonist, AM251 (0, 10, 100 or 200 µg, i.c.v.), before tests for reinstatement in response to CRF (0, 0.5 µg, i.c.v.), intermittent footshock stress (0, 0.9 mA) or cocaine (0, 10 mg·kg(-1) , i.p.). In Experiment 2, rats pre-exposed to cocaine (15-30 mg·kg(-1) , i.p.) or saline for 7 days were pretreated with AM251 (0, 10 or 100 µg, i.c.v.) before tests for locomotion in response to CRF (0.5 µg, i.c.v.), cocaine (15 mg·kg(-1) , i.p.) or saline (i.c.v.).

KEY RESULTS

Pretreatment with AM251 selectively interfered with CRF-, but not footshock- or cocaine-induced reinstatement. AM251 blocked the expression of behavioural sensitization induced by challenge injections of both CRF and cocaine.

CONCLUSIONS AND IMPLICATIONS

These findings reveal a mediating role for CB(1) receptor transmission in the effects of CRF on cocaine-related behaviours.

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.

Neurocircuitry for modeling drug effects

The identification and functional understanding of the neurocircuitry that mediates alcohol and drug effects that are relevant for the development of addictive behavior is a fundamental challenge in addiction research. Here we introduce an assumption-free construction of a neurocircuitry that mediates acute and chronic drug effects on neurotransmitter dynamics that is solely based on rodent neuroanatomy. Two types of data were considered for constructing the neurocircuitry: (1) information on the cytoarchitecture and neurochemical connectivity of each brain region of interest obtained from different neuroanatomical techniques; (2) information on the functional relevance of each region of interest with respect to alcohol and drug effects. We used mathematical data mining and hierarchical clustering methods to achieve the highest standards in the preprocessing of these data. Using this approach, a dynamical network of high molecular and spatial resolution containing 19 brain regions and seven neurotransmitter systems was obtained. Further graph theoretical analysis suggests that the neurocircuitry is connected and cannot be separated into further components. Our analysis also reveals the existence of a principal core subcircuit comprised of nine brain regions: the prefrontal cortex, insular cortex, nucleus accumbens, hypothalamus, amygdala, thalamus, substantia nigra, ventral tegmental area and raphe nuclei. Finally, by means of algebraic criteria for synchronizability of the neurocircuitry, the suitability for in silico modeling of acute and chronic drug effects is indicated. Indeed, we introduced as an example a dynamical system for modeling the effects of acute ethanol administration in rats and obtained an increase in dopamine release in the nucleus accumbens-a hallmark of drug reinforcement-to an extent similar to that seen in numerous microdialysis studies. We conclude that the present neurocircuitry provides a structural and dynamical framework for large-scale mathematical models and will help to predict chronic drug effects on brain function.

Yohimbine depresses excitatory transmission in BNST and impairs extinction of cocaine place preference through orexin-dependent, norepinephrine-independent processes

The alpha2 adrenergic receptor (α(2)-AR) antagonist yohimbine is a widely used tool for the study of anxiogenesis and stress-induced drug-seeking behavior. We previously demonstrated that yohimbine paradoxically depresses excitatory transmission in the bed nucleus of the stria terminalis (BNST), a region critical to the integration of stress and reward pathways, and produces an impairment of extinction of cocaine-conditioned place preference (cocaine-CPP) independent of α(2)-AR signaling. Recent studies show yohimbine-induced drug-seeking behavior is attenuated by orexin receptor 1 (OX(1)R) antagonists. Moreover, yohimbine-induced cocaine-seeking behavior is BNST-dependent. Here, we investigated yohimbine-orexin interactions. Our results demonstrate yohimbine-induced depression of excitatory transmission in the BNST is unaffected by alpha1-AR and corticotropin-releasing factor receptor-1 (CRFR(1)) antagonists, but is (1) blocked by OxR antagonists and (2) absent in brain slices from orexin knockout mice. Although the actions of yohimbine were not mimicked by the norepinephrine transporter blocker reboxetine, they were by exogenously applied orexin A. We find that, as with yohimbine, orexin A depression of excitatory transmission in BNST is OX(1)R-dependent. Finally, we find these ex vivo effects are paralleled in vivo, as yohimbine-induced impairment of cocaine-CPP extinction is blocked by a systemically administered OX(1)R antagonist. These data highlight a new mechanism for orexin on excitatory anxiety circuits and demonstrate that some of the actions of yohimbine may be directly dependent upon orexin signaling and independent of norepinephrine and CRF in the BNST.

Yohimbine anxiogenesis in the elevated plus maze is disrupted by bilaterally disconnecting the bed nucleus of the stria terminalis from the central nucleus of the amygdala

The α2 adrenergic receptor antagonist yohimbine (YO) is a sympathomimetic drug that crosses the blood-brain barrier after systemic administration. YO promotes increased transmitter release from noradrenergic (NA) axon terminals in the central nucleus of the amygdala (CEA), bed nucleus of the stria terminalis (BST), hypothalamus, and other brain regions implicated in physiological and behavioral responses to stressful and threatening stimuli. YO is potently anxiogenic in humans and experimental animals, including rats. To determine whether direct connections between the CEA and anterolateral group of BST nuclei (algBST) are necessary for YO anxiogenesis in rats, neurotoxic ibotenate lesions of the CEA in one hemisphere and the ipsi- or contralateral algBST were conducted to disrupt CEA-algBST communication uni- or bilaterally. Sham-lesioned controls received microinjections of vehicle into the CEA and algBST. Two weeks later, behavior was assessed in the elevated plus maze (EPMZ) in rats after i.p. saline or YO (1.0mg/kg). Central ibotenate lesion placement and extent was assessed post-mortem in NeuN-immunolabeled tissue sections. The ability of YO to increase anxiety-like behavior in the EPMZ was similarly robust in rats with sham lesions or ipsilateral CEA-algBST lesions. Conversely, YO anxiogenesis in the EPMZ was disrupted in rats with asymmetric lesions designed to bilaterally disconnect the CEA and algBST, whereas neither unilateral nor bilateral disconnecting lesions altered EPMZ behavior in rats after i.p. saline. We conclude that the anxiogenic effects of increased NA signaling in rats after YO require direct CEA-algBST interactions that do not shape EPMZ behavior under baseline conditions.

Effects of acupuncture on stress-induced relapse to cocaine-seeking in rats

RATIONALE

Cocaine addiction is associated with high rates of relapse, and stress has been identified as a major risk factor. We have previously demonstrated that acupuncture reduces drug self-administration and dopamine release in the nucleus accumbens (NAc), a brain structure implicated in stress-induced reinstatement of drug-seeking behavior.

OBJECTIVE

This study was conducted to investigate the effects of acupuncture on footshock-induced reinstatement of cocaine-seeking and the expression of c-Fos and the transcription factor cAMP response element-binding protein (CREB) in the NAc, used as markers of neuronal activation in conditions of stress-induced reinstatement to cocaine.

METHODS

Male Sprague-Dawley rats were trained to self-administer cocaine (1.0 mg/kg) for 14 days, followed by extinction and then footshock stress. Acupuncture was applied at bilateral Shenmen (HT7) points for 1 min after footshock stress.

RESULTS AND CONCLUSIONS

Acute footshock stress reinstated cocaine-seeking behavior and enhanced c-Fos expression and phosphorylated CREB (pCREB) activation in the NAc shell in cocaine pre-exposed rats. On the other hand, acupuncture at HT7, but not at control point (LI5), markedly reduced reinstatement of cocaine-seeking (86.5 % inhibition vs. control value), c-Fos expression (81.7% inhibition), and pCREB activation (79.3% inhibition) in the NAc shell. These results suggest that acupuncture attenuates stress-induced relapse by regulating neuronal activation in the NAc shell.

Corticotropin-releasing factor modulation of forebrain GABAergic transmission has a pivotal role in the expression of anabolic steroid-induced anxiety in the female mouse

Increased anxiety is commonly observed in individuals who illicitly administer anabolic androgenic steroids (AAS). Behavioral effects of steroid abuse have become an increasing concern in adults and adolescents of both sexes. The dorsolateral bed nucleus of the stria terminalis (dlBnST) has a critical role in the expression of diffuse anxiety and is a key site of action for the anxiogenic neuromodulator, corticotropin releasing factor (CRF). Here we demonstrate that chronic, but not acute, exposure of female mice during adolescence to AAS augments anxiety-like behaviors; effects that were blocked by central infusion of the CRF receptor type 1 antagonist, antalarmin. AAS treatment selectively increased action potential (AP) firing in neurons of the central amygdala (CeA) that project to the dlBnST, increased the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in dlBnST target neurons, and decreased both c-FOS immunoreactivity (IR) and AP frequency in these postsynaptic cells. Acute application of antalarmin abrogated the enhancement of GABAergic inhibition induced by chronic AAS exposure whereas application of CRF to brain slices of naïve mice mimicked the actions of this treatment. These results, in concert with previous data demonstrating that chronic AAS treatment results in enhanced levels of CRF mRNA in the CeA and increased CRF-IR in the dlBnST neuropil, are consistent with a mechanism in which the enhanced anxiety elicited by chronic AAS exposure involves augmented inhibitory activity of CeA afferents to the dlBnST and CRF-dependent enhancement of GABAergic inhibition in this brain region.

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

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

Pain, nicotine, and smoking: research findings and mechanistic considerations

Tobacco addiction and chronic pain represent 2 highly prevalent and comorbid conditions that engender substantial burdens upon individuals and systems. Interrelations between pain and smoking have been of clinical and empirical interest for decades, and research in this area has increased dramatically over the past 5 years. We conceptualize the interaction of pain and smoking as a prototypical example of the biopsychosocial model. Accordingly, we extrapolated from behavioral, cognitive, affective, biomedical, and social perspectives to propose causal mechanisms that may contribute to the observed comorbidity between these 2 conditions. The extant literature was 1st dichotomized into investigations of either effects of smoking on pain or effects of pain on smoking. We then integrated these findings to present a reciprocal model of pain and smoking that is hypothesized to interact in the manner of a positive feedback loop, resulting in greater pain and increased smoking. Finally, we proposed directions for future research and discussed clinical implications for smokers with comorbid pain disorders. We observed modest evidence that smoking may be a risk factor in the multifactorial etiology of some chronically painful conditions and that pain may come to serve as a potent motivator of smoking. We also found that whereas animal studies yielded consistent support for direct pain-inhibitory effects of nicotine and tobacco, results from human studies were much less consistent. Future research in the emerging area of pain and smoking has the potential to inform theoretical and clinical applications with respect to tobacco smoking, chronic pain, and their comorbid presentation. (PsycINFO Database Record (c) 2011 APA, all rights reserved).

Dissociation of corticotropin-releasing factor receptor subtype involvement in sensitivity to locomotor effects of methamphetamine and cocaine

RATIONALE

Enhanced sensitivity to the euphoric and locomotor-activating effects of psychostimulants may influence an individual's predisposition to drug abuse and addiction. While drug-induced behaviors are mediated by the actions of several neurotransmitter systems, past research revealed that the corticotropin-releasing factor (CRF) system is important in driving the acute locomotor response to psychostimulants.

OBJECTIVES

We previously reported that genetic deletion of the CRF type-2 receptor (CRF-R2), but not the CRF type-1 receptor (CRF-R1) dampened the acute locomotor stimulant response to methamphetamine (1 mg/kg). These results contrasted with previous studies implicating CRF-R1 in the locomotor effects of psychostimulants. Since the majority of previous studies focused on cocaine, rather than methamphetamine, we set out to test the hypothesis that these drugs differentially engage CRF-R1 and CRF-R2.

METHODS

We expanded our earlier findings by first replicating our previous experiments at a higher dose of methamphetamine (2 mg/kg), and by assessing the effects of the CRF-R1-selective antagonist CP-376,395 (10 mg/kg) on methamphetamine-induced locomotor activity. Next, we used both genetic and pharmacological tools to examine the specific components of the CRF system underlying the acute locomotor response to cocaine (5-10 mg/kg).

RESULTS

While genetic deletion of CRF-R2 dampened the locomotor response to methamphetamine (but not cocaine), genetic deletion and pharmacological blockade of CRF-R1 dampened the locomotor response to cocaine (but not methamphetamine).

CONCLUSIONS

These findings highlight the differential involvement of CRF receptors in acute sensitivity to two different stimulant drugs of abuse, providing an intriguing basis for the development of more targeted therapeutics for psychostimulant addiction.

GluN2B subunit deletion reveals key role in acute and chronic ethanol sensitivity of glutamate synapses in bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a critical region for alcohol/drug-induced negative affect and stress-induced reinstatement. NMDA receptor (NMDAR)-dependent plasticity, such as long-term potentiation (LTP), has been postulated to play key roles in alcohol and drug addiction; yet, to date, little is understood regarding the mechanisms underlying LTP of the BNST, or its regulation by ethanol. Acute and chronic exposure to ethanol modulates glutamate transmission via actions on NMDARs. Despite intense investigation, tests of subunit specificity of ethanol actions on NMDARs using pharmacological approaches have produced mixed results. Thus, we use a conditional GluN2B KO mouse line to assess both basal and ethanol-dependent function of this subunit at glutamate synapses in the BNST. Deletion of GluN2B eliminated LTP, as well as actions of ethanol on NMDAR function. Further, we show that chronic ethanol exposure enhances LTP formation in the BNST. Using KO-validated pharmacological approaches with Ro25-6981 and memantine, we provide evidence suggesting that chronic ethanol exposure enhances LTP in the BNST via paradoxical extrasynaptic NMDAR involvement. These findings demonstrate that GluN2B is a key point of regulation for ethanol's actions and suggest a unique role of extrasynaptic GluN2B-containing receptors in facilitating LTP.

The persistence of maladaptive memory: addiction, drug memories and anti-relapse treatments

Addiction is a chronic, relapsing disorder, characterised by the long-term propensity of addicted individuals to relapse. A major factor that obstructs the attainment of abstinence is the persistence of maladaptive drug-associated memories, which can maintain drug-seeking and taking behaviour and promote unconscious relapse of these habits. Thus, addiction can be conceptualised as a disorder of aberrant learning of the formation of strong instrumental memories linking actions to drug-seeking and taking outcomes that ultimately are expressed as persistent stimulus-response habits; of previously neutral environmental stimuli that become associated with drug highs (and/or withdrawal states) through pavlovian conditioning, and of the subsequent interactions between pavlovian and instrumental memories to influence relapse behaviour. Understanding the psychological, neurobiological and molecular basis of these drug memories may produce new methods of pro-abstinence, anti-relapse treatments for addiction.

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.

Addiction circuitry in the human brain

A major challenge in understanding substance-use disorders lies in uncovering why some individuals become addicted when exposed to drugs, whereas others do not. Although genetic, developmental, and environmental factors are recognized as major contributors to a person's risk of becoming addicted, the neurobiological processes that underlie this vulnerability are still poorly understood. Imaging studies suggest that individual variations in key dopamine-modulated brain circuits, including circuits involved in reward, memory, executive function, and motivation, contribute to some of the differences in addiction vulnerability. A better understanding of the main circuits affected by chronic drug use and the influence of social stressors, developmental trajectories, and genetic background on these circuits is bound to lead to a better understanding of addiction and to more effective strategies for the prevention and treatment of substance-use disorders.

Effects of lidocaine-induced inactivation of the bed nucleus of the stria terminalis, the central or the basolateral nucleus of the amygdala on the opponent-process actions of self-administered cocaine in rats

RATIONALE

In addition to its rewarding actions, cocaine has profound negative effects that are unmasked as the rewarding impact of the drug fades. While much is known about the neurobiology of cocaine reward, the mechanisms underlying the negative actions of the drug remain unclear.

OBJECTIVES

The current study investigates the role of three brain regions each implicated in the modulation of negative affective states-the bed nucleus of the stria terminalis (BNST), the central (CeA), and the basolateral (BLA) nucleus of the amygdala.

METHODS

The dual actions of cocaine were assessed using a runway self-administration procedure in which rats exhibit both approach to and avoidance of a goal box associated with cocaine administration (retreat behaviors). Here, rats ran a straight alley once/day for i.v. cocaine (1.0 mg/kg/injection) over 14 days during which the BNST, CeA, or BLA was inactivated via bilateral intracranial infusions of lidocaine (0 or 20 μg/0.5 μl/side) administered 15 min prior to testing. The impact of lidocaine on spontaneous locomotor activity was also assessed to rule out nonspecific actions of the treatments.

RESULTS

Control animals running for cocaine developed the expected pattern of approach-avoidance retreat behavior. Inactivation of the BNST attenuated such behavior, BLA inactivation had no appreciable effects, and CeA inactivation produced intermediate and more variable results. Locomotor activity was unaffected by any of the treatments.

CONCLUSIONS

These data suggest that the BNST and to a lesser extent the CeA, but not the BLA, play a role in mediating the opponent-process actions of self-administered cocaine.

CREB-mediated alterations in the amygdala transcriptome: coordinated regulation of immune response genes following cocaine

The neuronal circuitry underlying stress- and drug-induced reinstatement of cocaine-seeking has been relatively well characterized; however, less is known regarding the long-term molecular changes following cocaine administration that may promote future reinstatement. The transcription factor cAMP response element-binding protein (CREB) is necessary for stress- but not cocaine-induced reinstatement of conditioned reward, suggesting that different molecular mechanisms may underlie these two types of reinstatement. To explore the relationship between this transcription factor and reinstatement, we utilized the place-conditioning paradigm to examine alterations in gene expression in the amygdala, a neural substrate critically involved in stress-induced reinstatement, following the development of cocaine reward and subsequent extinction. Our findings demonstrate that the amygdala transcriptome was altered by CREB deficiency more than by previous cocaine experience, with an over-representation of genes involved in the immune response. However, a subset of genes involved in stress and immune response demonstrated a drug×genotype interaction, indicating that cocaine produces different long-term alterations in gene expression depending on the presence or absence of CREB. This profile of gene expression in the context of addiction enhances our understanding of the long-term molecular changes that occur throughout the addiction cycle and identifies novel genes and pathways that might lead to the creation of better therapeutic agents.

Chronic cocaine self-administration attenuates the anxiogenic-like and stress potentiating effects of the benzodiazepine inverse agonist, FG 7142

Stress is a well-known risk factor in relapse to drug abuse. Several forms of stress in animals have been used with varied degrees of success to elicit reinstatement of drug-seeking after chronic drug self-administration. Here, we tested the ability of the benzodiazepine (BZ) inverse agonist, FG 7142, to elicit anxiety-like behavior and potentiate stress responses in rats as measured by standard behavioral and hormonal indices and for its ability to affect reinstatement of cocaine-seeking in rats with a prior history of cocaine self-administration. FG 7142 elicited anxiety-like behavior on the elevated plus maze (EPM) in cocaine-naïve rats, and cocaine-naïve rats injected with FG 7142 exhibited increased plasma corticosterone levels following EPM exposure. However, in animals with a history of cocaine self-administration, FG 7142 failed to affect elevated plus maze performance and did not affect plasma corticosterone response to the EPM. Furthermore, FG 7142 failed to reinstate cocaine-seeking, nor did it alter conditioned cue-induced reinstatement. These data indicate that the anxiety-related and stress potentiating qualities of BZ inverse agonism are attenuated in cocaine-experienced animals and do not lead to reinstatement of cocaine-seeking.

Augmented cocaine seeking in response to stress or CRF delivered into the ventral tegmental area following long-access self-administration is mediated by CRF receptor type 1 but not CRF receptor type 2

Stressful events are determinants of relapse in recovering cocaine addicts. Excessive cocaine use may increase susceptibility to stressor-induced relapse through alterations in brain corticotropin-releasing factor (CRF) regulation of neurocircuitry involved in drug seeking. We previously reported that the reinstatement of cocaine seeking by a stressor (footshock) is CRF dependent and is augmented in rats that self-administered cocaine under long-access (LgA; 6 h daily) conditions for 14 d when compared with rats provided shorter daily cocaine access [short access (ShA) rats; 2 h daily]. Further, we have demonstrated that reinstatement in response to intracerebroventricular CRF administration is heightened in LgA rats. This study examined the role of altered ventral tegmental area (VTA) responsiveness to CRF in intake-dependent increases in CRF- and stress-induced cocaine seeking. Bilateral intra-VTA administration of CRF (250 or 500 ng/side) produced reinstatement in LgA but not ShA rats. In LgA rats, intra-VTA CRF-induced reinstatement was blocked by administration of the CRF-receptor type 1 (CRF-R1) antagonist antalarmin (500 ng/side) or CP-376395 (500 ng/side), but not the CRF-R2 antagonist astressin-2B (500 ng or 1 μg/side) or antisauvagine-30 (ASV-30; 500 ng/side) into the VTA. Likewise, intra-VTA antalarmin, but not astressin-2B, blocked footshock-induced reinstatement in LgA rats. By contrast, neither intra-VTA antalarmin nor CP-376395 altered food-reinforced lever pressing. Intra-VTA injection of the CRF-R1-selective agonist cortagine (100 ng/side) but not the CRF-R2-selective agonist rat urocortin II (rUCN II; 250 ng/side) produced reinstatement. These findings reveal that excessive cocaine use increases susceptibility to stressor-induced relapse in part by augmenting CRF-R1-dependent regulation of addiction-related neurocircuitry in the VTA.

Teneurin C-terminal associated peptide-1 blocks the effects of corticotropin-releasing factor on reinstatement of cocaine seeking and on cocaine-induced behavioural sensitization

BACKGROUND AND PURPOSE

The stress-related neuropeptide, corticotropin-releasing factor (CRF), has become an important focus of studies of cocaine addiction, and in particular, the effects of stress on cocaine-related behaviours. A recently discovered peptide system, the teneurin C-terminal associated peptides (TCAP), has been implicated in the regulation of the stress response, via a CRF-related mechanism. Here we have determined whether treatment with TCAP-1, a synthetic analogue of TCAP, modulated two cocaine-related behaviours induced by CRF: reinstatement of cocaine seeking, and expression of cocaine-induced behavioural sensitization.

EXPERIMENTAL APPROACH

In Experiment 1, rats trained to self-administer cocaine were given acute or repeated (once daily for 5 days) i.c.v. injections of TCAP-1 before tests for reinstatement in response to CRF (105 pmol, i.c.v.), intermittent footshock stress (0.9 mA), or cocaine (15 mg·kg⁻¹, i.p.). In Experiment 2, rats pre-exposed to cocaine (15-30 mg·kg⁻¹, i.p.) or saline for 7 days were treated with TCAP-1 (once daily for 5 days; i.c.v.) and subsequently tested for locomotor responses to CRF (105 pmol, i.c.v.) or cocaine (15 mg·kg⁻¹, i.p.).

KEY RESULTS

Five day pre-exposure with TCAP-1 blocked CRF-, but not footshock- or cocaine-induced reinstatement of cocaine seeking; acute pretreatment with TCAP-1 was without effect in all test conditions. Similarly, repeated TCAP-1 pre-exposure blocked the cocaine-sensitized locomotor response to CRF, but not to cocaine.

CONCLUSIONS AND IMPLICATIONS

Repeated TCAP-1 exposure induced robust and selective inhibition of cocaine-related behavioural responses to CRF, suggesting that TCAP-1 may normalize signalling within CRF systems dysregulated by cocaine exposure.

A switch in the neuromodulatory effects of dopamine in the oval bed nucleus of the stria terminalis associated with cocaine self-administration in rats

Chronic exposure to drugs of abuse alters brain reward circuits and produces functional changes in the dopamine (DA) system. However, it is not known whether these changes are directly related to drug-driven behaviors or whether they simply are adaptive responses to long-term drug exposure. Here, we combined the rat model of cocaine self-administration with brain slice electrophysiology to identify drug-use related alterations in the neuromodulatory effects of DA in the oval bed nucleus of the stria terminalis (ovBST), a robust DA terminal field. Long-Evans rats self-administered cocaine intravenously (0.75 mg/kg/injection) for an average of 15 d, on reward-lean or -rich schedules of reinforcement. Brain slice recordings conducted 20 h after the last self-administration session revealed a reversal of the neuromodulatory effect of DA on GABA(A)-IPSCs. Specifically, the effect of DA switched from a D2-mediated decrease in drug-naive rats to a D1-receptor-mediated increase in GABA(A)-IPSC in cocaine self-administering rats. Furthermore, the switch in DA modulation of GABA(A)-IPSC remained after a 30 d withdrawal period. In contrast, this switch was not observed after the acquisition phase of cocaine self-administration, when rats received cocaine passively, or in rats maintaining sucrose self-administration. Therefore, our study reveals a reversal in the effects of DA on inhibitory transmission, from reduction to enhancement, in the ovBST of cocaine self-administering rats. This change was unique to voluntary intake of cocaine and maintained after a withdrawal period, suggesting a mechanism underlying the maintenance of cocaine self-administration and perhaps craving during drug-free periods.

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.