Prior, repeated exposure to cocaine potentiates locomotor responsivity to central injections of corticotropin-releasing factor (CRF) in rats

Social interaction reward in rats has anti-stress effects

Social interaction in an alternative context can be beneficial against drugs of abuse. Stress is known to be a risk factor that can exacerbate the effects of addictive drugs. In this study, we investigated whether the positive effects of social interaction are mediated through a decrease in stress levels. For that purpose, rats were trained to express cocaine or social interaction conditioned place preference (CPP). Behavioural, hormonal, and molecular stress markers were evaluated. We found that social CPP decreased the percentage of incorrect transitions of grooming and corticosterone to the level of naïve untreated rats. In addition, corticotropin-releasing factor (CRF) was increased in the bed nucleus of stria terminalis after cocaine CPP. In order to study the modulation of social CPP by the CRF system, rats received intracerebroventricular CRF or alpha-helical CRF, a nonselective antagonist of CRF receptors. The subsequent effects on CPP to cocaine or social interaction were observed. CRF injections increased cocaine CPP, whereas alpha-helical CRF injections decreased cocaine CPP. However, alpha-helical CRF injections potentiated social CPP. When social interaction was made available in an alternative context, CRF-induced increase of cocaine preference was reversed completely to the level of rats receiving cocaine paired with alpha-helical CRF. This reversal of cocaine preference was also paralleled by a reversal in CRF-induced increase of p38 MAPK expression in the nucleus accumbens shell. These findings suggest that social interaction could contribute as a valuable component in treatment of substance use disorders by reducing stress levels.

Intramuscular Route of Administration Increases Potency in Eliciting Cocaine-Induced Behavioral Sensitization

Background

Cocaine is the number one abused psychostimulant drug that reaches addiction criterion in the US. In animals, repeated administration of cocaine results in behavioral sensitization which is thought to represent adaptations in the mesolimbic dopamine neural circuitry, the reward pathway. Cocaine-induced behavioral sensitization is evident in rodents and quail when cocaine is administered intraperitoneally (IP).

Objectives

The purpose of the current study was to investigate dose-dependent and temporal effects of acute and chronic intramuscular (IM) administration of cocaine in male quail.

Methods

After habituation to the test chambers, male quail received an IM injection of saline, 3 or 10 mg/kg cocaine and were immediately placed in the chambers. Distance traveled (in meters) was recorded in 5 min time bins for 30 min. Testing was conducted once per day for ten days with each subject receiving the same treatment throughout the experiment. Other behaviors including pecking, preening, and feather fluffing were measured.

Results

Cocaine-induced behavioral sensitization and tolerance were evident at relatively low doses of IM cocaine. Dose-dependent effects were evident. IM cocaine also reduced feather fluffing, a behavior that typically occurs during hypothermia.

Conclusions

The findings replicated and extended previous research with pigeons and suggested that IM administration of cocaine may be a relatively potent route of administration. Potency of drugs of abuse may be related to the bioavailability of a drug and its addictive properties. Thus, studying drugs of abuse using an IM route of administration may be useful in drug addiction research.

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.

Cocaine exposure prior to pregnancy alters the psychomotor response to cocaine and transcriptional regulation of the dopamine D1 receptor in adult male offspring

There is evidence that maternal experience prior to pregnancy can play an important role in behavioral, physiological, and genetic programming of offspring. Likewise, exposure to cocaine in utero can result in marked changes in central nervous system function of offspring. In this study, we examined whether exposure of rat dams to cocaine prior to pregnancy subsequently alters indices of behavior, physiology, and gene expression in offspring. Multiple outcome measures were examined in adult male offspring: (1) behavioral expression of cocaine-induced psychomotor activation; (2) levels of corticosterone in response to immobilization stress; and (3) expression of multiple genes, including dopamine receptor D1 (DRD1) and D2 (DRD2), glucocorticoid receptor (GR), and corticotropin-releasing factor (CRF), in functionally relevant brain regions. Adult Sprague-Dawley females were exposed to cocaine (15-30 mg/kg, i.p.) or saline for 10 days, and were then mated to drug naïve males of the same strain. Separate groups of adult male offspring were tested for their acute psychomotor response to cocaine (0, 15, 30 mg/kg, i.p.), corticosterone responsivity to 20 min of immobilization stress, and expression of multiple genes using quantitative PCR. Offspring of dams exposed to cocaine prior to conception exhibited increased psychomotor sensitivity to cocaine, and upregulated gene expression of DRD1 in the medial prefrontal cortex (mPFC). Neither stress-induced corticosterone levels nor gene expression of GR or CRF genes were altered. These data suggest that cocaine exposure before pregnancy can serve to enhance psychomotor sensitivity to cocaine in offspring, possibly via alterations in dopamine function that include upregulation of the DRD1.

The anti-nausea effects of CB1 agonists are mediated by an action at the visceral insular cortex

BACKGROUND AND PURPOSE

Conditioned gaping reactions reflect nausea-induced behaviour in rats. Cannabinoid 1 receptor (CB(1) ) agonists interfere with the establishment of nausea-induced conditioned gaping; however, it is not known if their effects are mediated by an action at peripheral or central CB(1) receptors.

EXPERIMENTAL APPROACH

We utilized the conditioned gaping model of nausea to evaluate the effect of peripheral and central administration of the peripherally restricted CB(1) agonist, CB13, on the establishment of LiCl-induced gaping in rats. We further evaluated the ability of HU-210 administered to the gustatory insular cortex (GIC) or visceral insular cortex (VIC) to interfere with LiCl-induced conditioned gaping and determined if this effect was mediated by CB(1) receptors.

KEY RESULTS

Central, but not peripheral, CB13 suppressed LiCl-induced conditioned gaping. Central administration of the potent CB(1) agonist, HU-210, delivered to the VIC, but not the GIC, suppressed the establishment of LiCl-induced gaping reactions, but not LiCl-induced suppression of hedonic reactions or conditioned taste avoidance. This pattern of results suggests that HU-210 delivered to the VIC prevented LiCl-induced nausea, but not learning per se. The suppression of LiCl-induced conditioned gaping by HU-210 was mediated by CB(1) receptors because it was prevented by co-administration of CB(1) antagonist/inverse agonist, AM-251, into the VIC. A high dose of AM-251 (20 µg) administered alone into the VIC did not produce conditioned gaping reactions.

CONCLUSIONS AND IMPLICATIONS

The nausea-relieving effects of CB(1) agonists, but not the nausea-inducing effects of CB(1) inverse agonists, are mediated, at least in part, by their action at the VIC in rats.

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.

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

Cocaine-induced neuroadaptation of stress-related circuitry and increased access to cocaine each putatively contribute to the transition from cocaine use to cocaine dependence. The present study tested the hypothesis that rats receiving extended versus brief daily access to cocaine would exhibit regional differences in levels of the stress-regulatory neuropeptide corticotropin-releasing factor (CRF). A secondary goal was to explore how CRF levels change in relation to the time since cocaine self-administration. Male Wistar rats acquired operant self-administration of cocaine and were assigned to receive daily long access (6 hours/day, LgA, n=20) or short access (1 hour/day, ShA, n=18) to intravenous cocaine self-administration (fixed ratio 1, ~0.50 mg/kg/infusion). After at least 3 weeks, tissue CRF immunoreactivity was measured at one of three timepoints: pre-session, post-session or 3 hours post-session. LgA, but not ShA, rats showed increased total session and first-hour cocaine intake. CRF immunoreactivity increased within the dorsal raphe (DR) and basolateral, but not central, nucleus of the amygdala (BLA, CeA) of ShA rats from pre-session to 3 hours post-session. In LgA rats, CRF immunoreactivity increased from pre-session to 3 hours post-session within the CeA and DR but tended to decrease in the BLA. LgA rats showed higher CRF levels than ShA rats in the DR and, pre-session, in the BLA. Thus, voluntary cocaine intake engages stress-regulatory CRF systems of the DR and amygdala. Increased availability of cocaine promotes greater tissue CRF levels in these extrahypothalamic brain regions, changes associated here with a model of cocaine dependence.

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.

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.

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.

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.

Inverse agonism of cannabinoid CB1 receptors potentiates LiCl-induced nausea in the conditioned gaping model in rats

BACKGROUND AND PURPOSE

Cannabinoid CB(1) receptor antagonists/inverse agonists, potentiate toxin-induced nausea and vomiting in animal models. Here, we sought to determine if this potentiated nausea was mediated by inverse agonism or neutral antagonism of the CB(1) receptor, and if the potentiated nausea would be produced by intracerebroventricular (icv) administration of an inverse agonist.

EXPERIMENTAL APPROACH

The conditioned gaping model of nausea in rats was used to compare the CB(1) receptor antagonist/inverse agonist, AM251, and the CB(1) receptor neutral antagonists, AM6527 (centrally and peripherally active) and AM6545 (peripherally active), in potentiating conditioned gaping produced by lithium chloride (LiCl) solution. The effect of icv (lateral ventricle and 4th ventricle) administration of AM251 on LiCl-induced gaping in this model was also evaluated.

KEY RESULTS

At a dose that did not produce conditioned gaping on its own, systemically administered AM251 (1.25 mg.kg(-1)) potentiated LiCl-induced conditioned gaping and reduced sucrose palatability; however, even doses as high as 8 mg.kg(-1) of AM6545 and AM6527 neither potentiated LiCl-induced conditioned gaping nor reduced sucrose palatability. Infusions of AM251 into the lateral ventricles (1.25, 12.5 and 125 microg) or the 4th ventricle (2.5, 12.5 and 125 microg) did not potentiate LiCl-induced conditioned gaping reactions, but all doses attenuated saccharin palatability during the subsequent test.

CONCLUSIONS AND IMPLICATIONS

Inverse agonism, but not neutral antagonism, of CB(1) receptors potentiated toxin-induced nausea. This effect may be peripherally mediated or may be mediated centrally by action on CB(1) receptors, located distal to the cerebral ventricles.

Disrupting protein complexes using Tat-tagged peptide mimics

Protein-protein interaction is a widely existing phenomenon and is essential for almost all biological processes, extending from the formation of cellular macromolecular structures and enzymatic complexes to the regulation of signal transduction pathways. Proteins interact with each other through the dynamic associations between modular protein domains within different cellular compartments and with distinct temporal dynamics. Disrupting protein interactions has emerged as an effective way to specifically modulate certain signaling pathways. Tat-tagged peptide mimics are a recently developed experimental tool that is used to disrupt specific interactions between protein complexes. TAT, an 11-amino acid protein transduction domain from HIV Tat protein, is tagged to peptides that mimic the functional fragment of protein interaction domains, and facilitates the delivery of peptides into cells to disrupt the associated protein both competitively and selectively. Here we provide a technical description on the utilization of Tat-tagged peptide mimics as a tool to disrupt protein interaction in cultured neurons and in the rat brain.

Dissection of corticotropin-releasing factor system involvement in locomotor sensitivity to methamphetamine

Sensitivity to the euphoric and locomotor-activating effects of drugs of abuse may contribute to risk for excessive use and addiction. Repeated administration of psychostimulants such as methamphetamine (MA) can result in neuroadaptive consequences that manifest behaviorally as a progressive escalation of locomotor activation, termed psychomotor sensitization. The present studies addressed the involvement of specific components of the corticotropin-releasing factor (CRF) system in locomotor activation and psychomotor sensitization induced by MA (1, 2 mg/kg) by utilizing pharmacological approaches, as well as a series of genetic knockout (KO) mice, each deficient for a single component of the CRF system: CRF-R1, CRF-R2, CRF, or the CRF-related peptide Urocortin 1 (Ucn1). CRF-R1 KO mice did not differ from wild-type mice in sensitization to MA, and pharmacological blockade of CRF-R1 with CP-154,526 (15, 30 mg/kg) in DBA/2J mice did not selectively attenuate either the acquisition or expression of MA-induced sensitization. Deletion of either of the endogenous ligands of CRF-R1 (CRF, Ucn1) either enhanced or had no effect on MA-induced sensitization, providing further evidence against a role for CRF-R1 signaling. Interestingly, deletion of CRF-R2 attenuated MA-induced locomotor activation, elucidating a novel contribution of the CRF system to MA sensitivity, and suggesting the participation of the endogenous urocortin peptides Ucn2 and Ucn3. Immunohistochemistry for Fos was used to visualize neural activation underlying CRF-R2-dependent sensitivity to MA, identifying the basolateral and central nuclei of the amygdala as neural substrates involved in this response. Our results support further examination of CRF-R2 involvement in neural processes associated with MA addiction.

Role of innate and drug-induced dysregulation of brain stress and arousal systems in addiction: Focus on corticotropin-releasing factor, nociceptin/orphanin FQ, and orexin/hypocretin

Stress-like symptoms are an integral part of acute and protracted drug withdrawal, and several lines of evidence have shown that dysregulation of brain stress systems, including the extrahypothalamic corticotropin-releasing factor (CRF) system, following long-term drug use is of major importance in maintaining drug and alcohol addiction. Recently, two other neuropeptide systems have attracted interest, the nociceptin/orphanin FQ (N/OFQ) and orexin/hypocretin (Orx/Hcrt) systems. N/OFQ participates in a wide range of physiological responses, and the hypothalamic Orx/Hcrt system helps regulate several physiological processes, including feeding, energy metabolism, and arousal. Moreover, these two systems have been suggested to participate in psychiatric disorders, including anxiety and drug addiction. Dysregulation of these systems by chronic drug exposure has been hypothesized to play a role in the maintenance of addiction and dependence. Recent evidence demonstrated that interactions between CRF-N/OFQ and CRF-Orx/Hcrt systems may be functionally relevant for the control of stress-related addictive behavior. The present review discusses recent findings that support the hypotheses of the participation and dysregulation of these systems in drug addiction and evaluates the current understanding of interactions among these stress-regulatory peptides.

Corticotropin releasing factor and neuroplasticity in cocaine addiction

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

Chronic cocaine enhances corticotropin-releasing factor-dependent potentiation of excitatory transmission in ventral tegmental area dopamine neurons

Current concepts suggest that stress-induced release of neuromodulators such as corticotropin-releasing factor (CRF) can drive drug-dependent behaviors. Although previous drug exposure can enhance behavioral and neurochemical responses to stress, it is unclear how such drug exposure alters the CRF modulation of excitatory synapses onto ventral tegmental area (VTA) dopamine neurons, a key locus of drug- and stress-induced neuroadaptation. Here, we demonstrate that, after repeated cocaine exposure, the magnitude and duration of the CRF-induced potentiation of NMDA receptor (NMDAR)-mediated neurotransmission was significantly increased compared with naive and saline-treated mice. Furthermore, CRF enhanced AMPA receptor (AMPAR)-mediated transmission only in mice that were exposed to cocaine. Increased frequency of AMPAR-mediated spontaneous miniature EPSCs and the intracellular blockade of CRF potentiation of AMPAR-mediated transmission suggest both presynaptic and postsynaptic effects of CRF. Importantly, pharmacological experiments revealed that CRF receptor 1 and protein kinase A pathways were newly recruited after repeated cocaine for the enhancement of CRF-induced NMDAR potentiation and the appearance of AMPAR potentiation. Thus, enhanced CRF-induced potentiation of excitatory synaptic transmission onto VTA dopamine neurons after cocaine preexposure is likely to produce an abnormal increase in dopamine release during stressful events and could augment activation of addictive behaviors in response to stress.

Behavioral characteristics and neurobiological substrates shared by Pavlovian sign-tracking and drug abuse

Drug abuse researchers have noted striking similarities between behaviors elicited by Pavlovian sign-tracking procedures and prominent symptoms of drug abuse. In Pavlovian sign-tracking procedures, repeated paired presentations of a small object (conditioned stimulus, CS) with a reward (unconditioned stimulus, US) elicits a conditioned response (CR) that typically consists of approaching the CS, contacting the CS, and expressing consummatory responses at the CS. Sign-tracking CR performance is poorly controlled and exhibits spontaneous recovery and long-term retention, effects that resemble relapse. Sign-tracking resembles psychomotor activation, a syndrome of behavioral responses evoked by addictive drugs, and the effects of sign-tracking on corticosterone levels and activation of dopamine pathways resemble the neurobiological effects of abused drugs. Finally, the neurobiological profile of individuals susceptible to sign-tracking resembles the pathophysiological profile of vulnerability to drug abuse, and vulnerability to sign-tracking predicts vulnerability to impulsive responding and alcohol self-administration. Implications of sign-tracking for models of drug addiction are considered.

A neuropeptide-centric view of psychostimulant addiction

Drugs of abuse all share common properties classically observed in human beings and laboratory animals. They enhance neural firing and dopamine tone within the nucleus accumbens and produce progressively greater drug-induced motor responses defined as behavioural sensitization. They produce conditioned place preference, a behavioural model of incentive motivation, which highlights the role of environmental cues in drug addiction. They increase brain reward function as seen by a lowering of intracranial self-stimulation thresholds. And last but not least, they are self-administered, and sometimes even abused, and can trigger reinstatement of drug-seeking behaviour in animals extinguished from drug self-administration. It has long been considered that the reinforcing properties of virtually all drugs of abuse, more specifically psychostimulants, are primarily dependent on activation of the mesolimbic dopamine system. However, recent evidence raises the importance of dopamine-independent mechanisms in reward-related behaviours. The overwhelming body of evidence that indicates a critical role for the mesolimbic dopamine system in the reinforcing effect of psychostimulants should not mask the key contribution of other modulatory systems in the brain. This review summarizes the complex and subtle role of several neuropeptidergic systems in various aspects of addictive behaviours observed in laboratory animals exposed to psychostimulants. A special emphasis is given to the cannabinoid, opioid, nociceptin/orphanin FQ, corticotropin-releasing factor and hypocretin/orexin systems. The relevance of these systems viewed as potential therapeutic targets for drug addiction is discussed in the light of their narrow pharmacological profile and their effectiveness in preventing drug addiction at doses usually not accompanied by severe side effects.

Adrenocorticotropic hormone and cortisol plasma levels directly correlate with childhood neglect and depression measures in addicted patients

Hypothalamic-pituitary-adrenal (HPA) axis dysfunction has been reported to be involved in vulnerability to alcohol and drug dependence in humans, possibly underlying both addictive behaviour and depression susceptibility. The aim of the present study was to investigate the possible interactions between childhood adverse experiences, depressive symptoms and HPA axis function in addicted patients, in comparison with healthy control. Eighty-two abstinent heroin or cocaine dependent patients and 44 normal controls, matched for age and sex, completed the symptoms Check List-90 (SCL-90), measuring depressive symptoms, and the Childhood Experience of Care and Abuse Questionnaire. Blood samples were collected to determine adrenocorticotropic hormone (ACTH) and cortisol basal plasma levels at 8:00 and 8:30 a.m. Addicted individuals showed significantly higher neglect and depression scores and ACTH-cortisol plasma levels respect to control subjects. Depression scores at SCL-90 in addicted patients positively correlated with plasma ACTH and cortisol values. In turn, plasma ACTH levels were directly associated with childhood neglect measures, reaching statistical significance with 'mother-neglect' scores. Plasma cortisol levels were related to 'father antipathy' among cocaine addicts. These findings suggest the possibility that childhood experience of neglect and poor parent-child attachment may have a persistent effect on HPA axis function as an adult, partially contributing, together with genetic factors and other environmental conditions, to both depressive traits and substance abuse neurobiological vulnerability.

Enhanced sensitivity to stress and drug/alcohol craving in abstinent cocaine-dependent individuals compared to social drinkers

Chronic exposure to cocaine is associated with neuroadaptions in stress and reward circuits that may increase susceptibility to relapse. We examined whether there are alterations in stress response and craving in abstinent cocaine-dependent individuals compared with a demographically matched group of non-addicted socially drinking community controls. Forty treatment-engaged abstinent cocaine patients (17F/23M) and 40 controls (19F/21M) were exposed to a brief 5 min guided imagery of individually calibrated stressful situations, personal drug/alcohol-related situation and a neutral-relaxing situation, one imagery per session, presented in random order. Craving, anxiety, emotion rating scales, and physiological measures were assessed. Cocaine patients reported significantly higher and more persistent stress- and cue-induced drug/alcohol craving, negative emotions, and physiological responses compared with social drinkers. In cocaine patients, stress- and cue-induced drug craving was accompanied by increased anger, fear, sadness, heart rate, and SBP. Controls reported minimal stress-induced craving and only increases in anxiety and SBP during stress exposure. Cue-induced alcohol craving was accompanied only by an increase in relaxed state. Females reported increased stress-induced anxiety and sadness compared with males, while males were emotionally and physiologically more reactive in the cue condition. These findings are the first to document functional alterations in stress- and reward-related affect and physiology in recently abstinent cocaine patients that is marked by an enhanced sensitivity to stress- and drug-related cue exposure. These data suggest that recovery from chronic cocaine abuse could be hampered by a hyper-responsive stress- and drug-craving state that increases cocaine relapse susceptibility.

Stressor- and corticotropin releasing factor-induced reinstatement and active stress-related behavioral responses are augmented following long-access cocaine self-administration by rats

RATIONALE

Stressful events during periods of drug abstinence likely contribute to relapse in cocaine-dependent individuals. Excessive cocaine use may increase susceptibility to stressor-induced relapse through alterations in brain corticotropin-releasing factor (CRF) responsiveness.

OBJECTIVES

This study examined stressor- and CRF-induced cocaine seeking and other stress-related behaviors in rats with different histories of cocaine self-administration (SA).

MATERIALS AND METHODS

Rats self-administered cocaine under short-access (ShA; 2 h daily) or long-access (LgA; 6 h daily) conditions for 14 days or were provided access to saline and were tested for reinstatement by a stressor (electric footshock), cocaine or an icv injection of CRF and for behavioral responsiveness on the elevated plus maze, in a novel environment and in the light-dark box after a 14- to 17-day extinction/withdrawal period.

RESULTS

LgA rats showed escalating patterns of cocaine SA and were more susceptible to reinstatement by cocaine, EFS, or icv CRF than ShA rats. Overall, cocaine SA increased activity in the center field of a novel environment, on the open arms of the elevated plus maze, and in the light compartment of a light-dark box. In most cases, the effects of cocaine SA were dependent on the pattern/amount of cocaine intake with statistically significant differences from saline self-administering controls only observed in LgA rats.

CONCLUSIONS

When examined after several weeks of extinction/withdrawal, cocaine SA promotes a more active pattern of behavior during times of stress that is associated with a heightened susceptibility to stressor-induced cocaine-seeking behavior and may be the consequence of augmented CRF regulation of addiction-related neurocircuitry.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

A role for corticotropin-releasing factor in the long-term expression of behavioral sensitization to cocaine

Corticotropin-releasing factor (CRF) has been implicated in a number of the behavioral and biochemical effects of cocaine. We recently reported that central injections of CRF produce a potentiated locomotor response in animals that had been given repeated injections of cocaine up to 4 weeks earlier. We now report that with as few as 1 or 3 exposures to cocaine (total of 45 mg/kg, i.p., per day), and a drug-free period of 28 days, i.c.v. injections of CRF (0.5 microg) produce augmented locomotor responses, similar to those induced by cocaine (10 mg/kg, i.p.) itself. In addition, in animals pre-exposed to cocaine for 3 days, pre-treatment with the CRF receptor antagonist, D-Phe CRF(12-41) (1 microg, i.c.v.), blocks the expression of behavioral sensitization to a cocaine challenge after a 28-day drug-free period. These results demonstrate that short-term exposure to cocaine produces a form of long-term sensitization within systems upon which CRF acts and that activation of CRF receptors is importantly involved in the expression of behavioral sensitization to cocaine.

Central injections of CRF reinstate cocaine seeking in rats after postinjection delays of up to 3 h: an influence of time and environmental context

RATIONALE

In drug addicts, the induction of drug craving is not always associated with an immediate opportunity to take drugs again. It is, therefore, important to study how delays in opportunity for drug seeking affect the time-course of relapse. Intracerebroventricular (i.c.v.) injection of corticotropin-releasing factor (CRF) is a stressor that reinstates heroin and alcohol seeking in rats, when administered just before tests for reinstatement. It is not known whether CRF reinstates cocaine seeking; moreover, the effect of delaying testing for reinstatement, after CRF injection, has not been studied.

OBJECTIVES

To determine whether i.c.v. CRF induces reinstatement of cocaine seeking after postinjection delays of up to 3 h and to determine whether the context in which a delay is experienced influences the time-course of CRF-induced reinstatement.

METHODS

Rats self-administered cocaine (1.0 mg/kg per infusion) for 8-10 days. Subsequently, responding for drug was extinguished and testing for reinstatement by i.c.v. CRF (0.5 microg) was conducted. Animals were tested after postinjection delays of up to 3 h; the delays were experienced either in the self-administration (SA) chamber or home cage (HC).

RESULTS

When delays were spent in the SA chambers, CRF induced reinstatement in all delay groups. When delays were spent in the HC, CRF did not induce reinstatement after a 2-h delay.

CONCLUSIONS

We argue that the effects we observed are consistent with a contextual conditioning account of reinstatement, whereby CRF that was experienced in the context of the SA chamber served to elicit a conditioned excitatory response developed to that context during training.

Increased vulnerability to stress following opiate exposures: behavioral and autonomic correlates

The authors used rats to study the impact of a history of opiate exposures on behavioral and autonomic responses to restraint stress. Brief restraint (30 min) provoked tachycardia and a pressor response, anxiety (as indexed by social interaction), grooming, and reduced exploration. The pressor response was reduced at 1 day, but not 7 days, after last opiate exposure; tachycardia was unaffected (Experiment 1). Stress-induced anxiety was potentiated 1 and 7 days after last opiate exposure (Experiment 2), and this potentiation was a function of dose (Experiment 3) and duration (Experiment 4) of opiate exposure. The results show that a history of opiate exposures alters vulnerability to stress and has implications for understanding coping, anxiety, and emotionality in former opiate users.

Activation of central neurotensin receptors reinstates cocaine seeking in the rat: modulation by a D1/D5, but not D2/D3, receptor antagonist

RATIONALE

Neurotensin (NT) has been implicated in some of the behavioral effects of psychostimulants. Thus, there is reason to think that NT may play a role in the reinstatement of cocaine seeking, and that it may do so via an interaction with dopamine (DA).

OBJECTIVES

To assess (1) whether NT and an NT analog, D-TYR[11]NT, induce reinstatement of cocaine seeking; (2) whether the effects of NT receptor activation on reinstatement can be modulated by D1/D5 or D2/D3 antagonists; (3) the specificity of the effects of NT receptor activation on the reinstatement of cocaine seeking.

METHODS

In Experiment 1, rats were initially trained to self-administer cocaine. Following a subsequent period of extinction training, they were tested for the reinstatement of cocaine seeking by NT or D-TYR[11]NT (15, 30 microg i.c.v.). In Experiment 2, rats were pretreated with the D1/D5 antagonist, SCH 23390 (0.05, 0.10 mg/kg i.p.) or the D2/D3 antagonist, raclopride (0.25, 0.50 mg/kg i.p.), prior to testing for reinstatement by D-TYR[11]NT (15 microg i.c.v.). In Experiment 3, rats that had been trained to self-administer sucrose pellets were tested for the reinstatement of sucrose seeking by D-TYR[11]NT (15, 30 microg i.c.v.).

RESULTS

(1) Both NT and D-TYR[11]NT produced robust reinstatement of cocaine seeking; (2) the effect of the analog was attenuated by pretreatment with the D1/D5, but not D2/D3, receptor antagonist; (3) the analog did not induce the reinstatement of sucrose seeking.

CONCLUSIONS

The findings suggest that an interaction between NT and DA may contribute to the neurobiology of reinstatement in animals with a history of cocaine self-administration.

Cocaine pre-exposure enhances CRF-induced expression of c-fos mRNA in the central nucleus of the amygdala: an effect that parallels the effects of cocaine pre-exposure on CRF-induced locomotor activity

There is evidence that cocaine pre-exposure produces changes in the responsivity of central corticotropin-releasing factor (CRF) systems and that these systems mediate some of the drug-related behavioural effects of acute stressors. The present experiment was conducted to assess the effects of repeated cocaine exposure on CRF-induced neuronal activation within two regions of the extended amygdala, the central nucleus of the amygdala (CeA) and lateral bed nucleus of the stria terminalis (BNST). In addition, CRF-induced neuronal activation was compared with CRF-induced locomotor activity. Rats were injected for 7 days with cocaine (days 1 and 7 in test chambers; days 2-6 in homecages) or saline. After 10 drug-free days, locomotor responsiveness to intracerebroventricular (i.c.v.) injections of CRF and Vehicle was assessed over 2-h test periods. Twenty-four to 48 h following testing for locomotor activity, animals were injected with either CRF or Vehicle, 30 min before being sacrificed. Subsequently, the brains were processed by in situ hybridization for c-fos mRNA, a widely used marker of neuronal activation, in the CeA and BNST. In CeA, i.c.v. CRF enhanced the expression of c-fos mRNA in cocaine, but not saline, pre-exposed animals; in the same animals, i.c.v. CRF resulted in enhanced locomotor activity in cocaine, but not saline, pre-exposed animals. The results demonstrate that repeated exposure to cocaine changes the neuronal response to CRF in the CeA; furthermore, they suggest that these changes in the CeA could potentially be of functional significance in the effects of repeated cocaine exposure on CRF-induced locomotor activity.

Cocaine pre-exposure produces a sensitized and context-specific c-fos mRNA response to footshock stress in the central nucleus of the AMYGDALA

In recent years, there has been growing interest in the putative relationship between stress and vulnerability to relapse in former drug addicts. In animal studies aimed at exploring this relationship, it has been shown that brief exposure to intermittent footshock stress produces reliable reinstatement of drug seeking after prolonged drug-free periods. Whereas footshock reinstates drug seeking, it does not reinstate behaviors maintained by non-drug reinforcers, suggesting that prior drug experience may produce a form of sensitization within neuronal systems that mediate stress-induced reinstatement. The primary objective of the present experiments was to determine whether pre-exposure to cocaine produces a long-lasting, sensitized neuronal response to footshock stress within two brain regions known to mediate footshock-induced reinstatement; the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). In experiment 1, animals were injected for 7 days with cocaine (days 1 and 7 in test chambers; days 2-6 in homecages) or saline. After 21 drug-free days, they were exposed to footshock or no footshock. In experiment 2, rats were injected daily for 7 days with cocaine in one of two contexts and saline in the alternate context. After 21 drug-free days, they were given footshock either in the same context that they were given cocaine in or the alternate context. In CeA, footshock produced enhanced expression of c-fos mRNA in cocaine, but not saline, pre-exposed animals. Furthermore, this effect was gated by the environmental context in which cocaine was given; footshock only enhanced c-fos mRNA expression when it was given in a context that had previously been paired with cocaine. Although footshock induced c-fos mRNA expression in the BNST, its effects in this region were not dependent on drug history. The major findings are that a history of cocaine exposure produces sensitization to an acute stressor within CeA, and this effect is gated by environmental context.

Effects of chronic cocaine exposure on corticotropin-releasing hormone binding protein in the central nucleus of the amygdala and bed nucleus of the stria terminalis

The neuropeptide, corticotropin-releasing hormone (CRH), has been shown to play a role in behavioral and neurobiological effects of drugs of abuse. An important modulator of CRH, the CRH binding protein (CRH-BP), has not, on the other hand, been assessed for its role in drug-associated effects. The primary objective of the present experiment was to assess whether prior, chronic exposure to cocaine modulates expression of CRH-BP, and to compare expression of the BP with that of the peptide itself. We assessed CRH-BP and CRH mRNA expression in two brain regions where CRH is known to affect responses to drugs of abuse; namely, the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). Male Long-Evans rats were given 14 daily injections of cocaine (30 mg/kg, i.p.) or saline. One, 3, 10, 28, or 42 days post-treatment, animals were killed and adjacent brain sections through the CeA and BNST were processed for CRH-BP and CRH by in situ hybridization. In the CeA, cocaine pre-exposure increased both CRH and CRH-BP mRNA expression 1 day post-treatment. In the dorsal BNST, cocaine pre-exposure elevated levels of CRH-BP, but not CRH, mRNA 3 days post-treatment. Taken together, the results suggest that withdrawal-induced changes in the expression of the CRH-BP, and CRH itself, are relatively short-lived and that a dysregulation in basal expression of either gene is not likely responsible for long-lasting behavioral effects noted with cocaine and other drugs of abuse.