Alpha-2 adrenergic receptor agonists block stress-induced reinstatement of cocaine seeking

Access to a running wheel decreases cocaine-primed and cue-induced reinstatement in male and female rats

BACKGROUND

Relapse to drug use after a period of abstinence is a persistent problem in the treatment of cocaine dependence. Physical activity decreases cocaine self-administration in laboratory animals and is associated with a positive prognosis in human substance-abusing populations. The purpose of this study was to examine the effects of long-term access to a running wheel on drug-primed and cue-induced reinstatement of cocaine-seeking behavior in male and female rats. methods: Long-Evans rats were obtained at weaning and assigned to sedentary (no wheel) and exercising (access to wheel) groups for the duration of the study. After 6 weeks, rats were implanted with intravenous catheters and trained to self-administer cocaine for 14 days. After training, saline was substituted for cocaine and responding was allowed to extinguish, after which cocaine-primed reinstatement was examined in both groups. Following this test, cocaine self-administration was re-established in both groups for a 5-day period. Next, a second period of abstinence occurred in which both cocaine and the cocaine-associated cues were withheld. After 5 days of abstinence, cue-induced reinstatement was examined in both groups.

RESULTS

Sedentary and exercising rats exhibited similar levels of cocaine self-administration, but exercising rats responded less than sedentary rats during extinction. In tests of cocaine-primed and cue-induced reinstatement, exercising rats responded less than sedentary rats, and this effect was apparent in both males and females.

CONCLUSIONS

These data indicate that long-term access to a running wheel decreases drug-primed and cue-induced reinstatement, and that physical activity may be effective at preventing relapse in substance-abusing populations.

Pharmacotherapeutics directed at deficiencies associated with cocaine dependence: focus on dopamine, norepinephrine and glutamate

Much effort has been devoted to research focused on pharmacotherapies for cocaine dependence yet there are no FDA-approved medications for this brain disease. Preclinical models have been essential to defining the central and peripheral effects produced by cocaine. Recent evidence suggests that cocaine exerts its reinforcing effects by acting on multiple neurotransmitter systems within mesocorticolimibic circuitry. Imaging studies in cocaine-dependent individuals have identified deficiencies in dopaminergic signaling primarily localized to corticolimbic areas. In addition to dysregulated striatal dopamine, norepinephrine and glutamate are also altered in cocaine dependence. In this review, we present these brain abnormalities as therapeutic targets for the treatment of cocaine dependence. We then survey promising medications that exert their therapeutic effects by presumably ameliorating these brain deficiencies. Correcting neurochemical deficits in cocaine-dependent individuals improves memory and impulse control, and reduces drug craving that may decrease cocaine use. We hypothesize that using medications aimed at reversing known neurochemical imbalances is likely to be more productive than current approaches. This view is also consistent with treatment paradigms used in neuropsychiatry and general medicine.

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.

Exercise as a potential treatment for drug abuse: evidence from preclinical studies

Epidemiological studies reveal that individuals who engage in regular aerobic exercise are less likely to use and abuse illicit drugs. Until recently, very few studies had examined the causal influences that mediate this relationship, and it was not clear whether exercise was effective at reducing substance use and abuse. In the past few years, several preclinical studies have revealed that exercise reduces drug self-administration in laboratory animals. These studies have revealed that exercise produces protective effects in procedures designed to model different transitional phases that occur during the development of, and recover from, a substance use disorder (e.g., acquisition, maintenance, escalation, and relapse/reinstatement of drug use). Moreover, recent studies have revealed several behavioral and neurobiological consequences of exercise that may be responsible for its protective effects in these assays. Collectively, these studies have provided convincing evidence to support the development of exercise-based interventions to reduce compulsive patterns of drug intake in clinical and at-risk populations.

In vivo voltammetry monitoring of electrically evoked extracellular norepinephrine in subregions of the bed nucleus of the stria terminalis

Norepinephrine (NE) is an easily oxidized neurotransmitter that is found throughout the brain. Considerable evidence suggests that it plays an important role in neurocircuitry related to fear and anxiety responses. In certain subregions of the bed nucleus of the stria terminalis (BNST), NE is found in large amounts. In this work we probed differences in electrically evoked release of NE and its regulation by the norepinephrine transporter (NET) and the α(2)-adrenergic autoreceptor (α(2)-AR) in two regions of the BNST of anesthetized rats. NE was monitored in the dorsomedial BNST (dmBNST) and ventral BNST (vBNST) by fast-scan cyclic voltammetry at carbon fiber microelectrodes. Pharmacological agents were introduced either by systemic application (intraperitoneal injection) or by local application (iontophoresis). The iontophoresis barrels were attached to a carbon fiber microelectrode to allow simultaneous detection of evoked NE release and quantitation of iontophoretic delivery. Desipramine (DMI), an inhibitor of NET, increased evoked release and slowed clearance of released NE in both regions independent of the mode of delivery. However, the effects of DMI were more robust in the vBNST than in the dmBNST. Similarly, the α(2)-AR autoreceptor inhibitor idazoxan (IDA) enhanced NE release in both regions but to a greater extent in the vBNST by both modes of delivery. Since both local application by iontophoresis and systemic application of IDA had similar effects on NE release, our results indicate that terminal autoreceptors play a predominant role in the inhibition of subsequent release.

Clonidine blocks stress-induced craving in cocaine users

RATIONALE

Reactivity to stressors and environmental cues, a putative cause of relapse in addiction, may be a useful target for relapse-prevention medication. In rodents, alpha-2 adrenergic agonists such as clonidine block stress-induced reinstatement of drug seeking, but not drug cue-induced reinstatement.

OBJECTIVE

The objective of this study is to test the effect of clonidine on stress- and cue-induced craving in human cocaine users.

METHODS

Healthy, non-treatment-seeking cocaine users (n = 59) were randomly assigned to three groups receiving clonidine 0, 0.1, or 0.2 mg orally under double-blind conditions. In a single test session, each participant received clonidine or placebo followed 3 h later by exposure to two pairs of standardized auditory-imagery scripts (neutral/stress and neutral/drug). Subjective measures of craving were collected.

RESULTS

Subjective responsivity ("crave cocaine" Visual Analog Scale) to stress scripts was significantly attenuated in the 0.1- and 0.2-mg clonidine groups; for drug-cue scripts, this attenuation occurred only in the 0.2-mg group. Other subjective measures of craving showed similar patterns of effects but Dose × Script interactions were not significant.

CONCLUSIONS

Clonidine was effective in reducing stress-induced (and, at a higher dose, cue-induced) craving in a pattern consistent with preclinical findings, although this was significant on only one of several measures. Our results, though modest and preliminary, converge with other evidence to suggest that alpha-2 adrenergic agonists may help prevent relapse in drug abusers experiencing stress or situations that remind them of drug use.

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.

Effect of prazosin and guanfacine on stress-induced reinstatement of alcohol and food seeking in rats

RATIONALE AND OBJECTIVES

Relapse to alcohol use during abstinence or maladaptive eating habits during dieting is often provoked by stress. The anxiogenic drug yohimbine, which causes stress-like responses in humans and non-humans, reliably reinstates alcohol and food seeking in a rat relapse model. Yohimibine is a prototypical alpha-2 adrenoceptor antagonist, but results from studies on noradrenaline's role in yohimbine-induced reinstatement of drug and food seeking are inconclusive. Here, we further addressed this issue by studying the effect of the alpha-1 adrenoceptor antagonist prazosin and the alpha-2 adrenoceptor agonist guanfacine on yohimbine-induced reinstatement.

METHODS

In exp. 1, we trained rats to self-administer alcohol (12% w/v, 1 h/day), and after extinction of alcohol-reinforced lever pressing, we tested prazosin's (0.5, 1.0, and 2.0 mg/kg, i.p.) or guanfacine's (0.125, 0.25, and 0.5 mg/kg, i.p.) effect on yohimbine (1.25 mg/kg, i.p.)-induced reinstatement; we also examined prazosin's effect on intermittent-footshock-stress-induced reinstatement. In exp. 2, we trained food-restricted rats to self-administer 45 mg food pellets and first examined prazosin's or guanfacine's effects on food-reinforced responding, and then, after extinction of lever presses, on yohimbine-induced reinstatement.

RESULTS

Prazosin (0.5-2.0 mg/kg) blocked yohimbine-induced reinstatement of food and alcohol seeking, as well as footshock-induced reinstatement of alcohol seeking. Guanfacine attenuated yohimbine-induced reinstatement of alcohol seeking at the highest dose (0.5 mg/kg), but its effect on yohimbine-induced reinstatement of food seeking was not significant. Neither prazosin nor guanfacine affected high-rate food-reinforced responding.

CONCLUSIONS

Results demonstrate an important role of postsynaptic alpha-1 adrenoceptors in stress-induced reinstatement of alcohol and food seeking.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Cannabinoid receptor involvement in stress-induced cocaine reinstatement: potential interaction with noradrenergic pathways

This study examined the role of endocannabinoid signaling in stress-induced reinstatement of cocaine seeking and explored the interaction between noradrenergic and endocannabinergic systems in the process. A well-validated preclinical model for human relapse, the rodent conditioned place preference assay, was used. Cocaine-induced place preference was established in C57BL/6 mice using injections of 15 mg/kg cocaine. Following extinction of preference for the cocaine-paired environment, reinstatement of place preference was determined following 6 min of swim stress or cocaine injection (15 mg/kg, i.p.). The role of endocannabinoid signaling was studied using the cannabinoid antagonist AM-251 (3 mg/kg, i.p.). Another cohort of mice was tested for reinstatement following administration of the cannabinoid agonist CP 55,940 (10, 20, or 40 μg/kg, i.p.). The alpha-2 adrenergic antagonist BRL-44408 (5 mg/kg, i.p.) with or without CP 55,940 (20 μg/kg) was administered to a third group of mice. We found that: (1) AM-251 blocked forced swim-induced, but not cocaine-induced, reinstatement of cocaine-seeking behavior; (2) the cannabinoid agonist CP 55,940 did not reinstate cocaine-seeking behavior when administered alone but did synergize with a non-reinstating dose of the alpha-2 adrenergic antagonist BRL-44408 to cause reinstatement. These results are consistent with the hypothesis that stress exposure triggers the endogenous activation of CB1 receptors and that activation of the endocannabinoid system is required for the stress-induced relapse of the mice to cocaine seeking. Further, the data suggest that the endocannabinoid system interacts with noradrenergic mechanisms to influence stress-induced reinstatement of cocaine-seeking behavior.

Relevance of both type-1 and type-2 corticotropin releasing factor receptors in stress-induced relapse to cocaine seeking behaviour

The essential role of corticotropin releasing factor (CRF) and its type-1 receptor (CRF1) in stress-induced relapse to drug seeking has been demonstrated. The bed nucleus of the stria terminalis is the major anatomical substrate for this CRF/CRF1 receptor action. More recently, the role of type-2 CRF (CRF2) receptors in stress-induced relapse to cocaine seeking has also has been documented. The ventral tegmental area is the anatomical substrate for this CRF/CRF2 receptor action. The new information involving CRF2 receptors in stress-induced relapse to cocaine seeking has generated a need for a reappraisal of the existing anatomical and pharmacological evidence that have been used to support the critical role of CRF1 receptors. The role of CRF2 receptors in stress-induced relapse to drug seeking also opens the question of the putative role of the other peptides of the CRH family (urocotin-1, urocortin-2 and urocortin-3) that have high affinity for CRF2 receptors. In this commentary, the available evidence supporting the role of both CRF1 and CRF2 receptors in stress-induced relapse to drug seeking is reviewed.

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.

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

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

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.

Effects of the serotonin transporter (5-HTTLPR) and α2A-adrenoceptor (C-1291G) genotypes on substance use in children and adolescents: a longitudinal study

RATIONALE AND OBJECTIVE

Twin studies suggest that substance use initiation in children and adolescents is determined primarily by environmental influences, whereas the establishment of use patterns is strongly controlled by genetic factors. The present study analysed the effects of the serotonin transporter promoter polymorphism [5-HT transporter gene-linked polymorphic region (5-HTTLPR)] and the α(2A)-adrenoceptor C-1291G genotype (ADRA2A C-1291G) as well as their interaction effects on alcohol, tobacco and drug use from preadolescence to the late adolescence.

METHODS

Initial sample of 9-year-old children of Estonian Children Personality Behaviour and Health Study (n = 583) was recalled at ages 15 and 18. Participants reported in all waves how frequently they smoked and used alcohol and illicit drugs.

RESULTS

5-HTTLPR had age-dependent effects on alcohol, tobacco and drug use: substance use did not differ by genotype at age 9, but at age 15, the participants with the short (s)/s genotype had higher tobacco use, and at age 18, they were more active alcohol, drug and tobacco users. Effects of ADRA2A C-1291G on drug use were dependent on gender, age and 5-HTTLPR. Males (age 18) with ADRA2A CG genotype, when compared to other participants, tended to have higher drug use especially when they had s/s genotype of 5-HTTLPR.

CONCLUSIONS

Our results reveal that expression of genetic vulnerability for substance use in children and adolescents may depend on age, gender, interaction of genes, and type of substance.

Stimulation of α2-adrenergic receptors in the central nucleus of the amygdala attenuates stress-induced reinstatement of nicotine seeking in rats

Tobacco addiction is a chronic disorder that is characterized by craving for tobacco products, withdrawal upon smoking cessation, and relapse after periods of abstinence. Previous studies demonstrated that systemic administration of α2-adrenergic receptor agonists attenuates stress-induced reinstatement of drug seeking in rats. The aim of the present experiments was to investigate the role of noradrenergic transmission in the central nucleus of amygdala (CeA) in stress-induced reinstatement of nicotine seeking. Rats self-administered nicotine for 14-16 days and then nicotine seeking was extinguished by substituting saline for nicotine. The effect of the intra-CeA infusion of the α2-adrenergic receptor agonists clonidine and dexmedetomidine, the nonselective β1/β2-adrenergic receptor antagonist propranolol, and the α1-adrenergic receptor antagonist prazosin on stress-induced reinstatement of nicotine seeking was investigated. In all the experiments, exposure to footshocks reinstated extinguished nicotine seeking. The administration of clonidine or dexmedetomidine into the CeA attenuated stress-induced reinstatement of nicotine seeking. The administration of propranolol or prazosin into the CeA did not affect stress-induced reinstatement of nicotine seeking. Furthermore, intra-CeA administration of clonidine or dexmedetomidine did not affect operant responding for food pellets. This suggests that the effects of clonidine and dexmedetomidine on stress-induced reinstatement of nicotine seeking were not mediated by motor impairments or sedation. Taken together, these findings indicate that stimulation of α2-adrenergic receptors, but not blockade of α1 or β-adrenergic receptors, in the CeA attenuates stress-induced reinstatement of nicotine seeking. These findings suggest that α2-adrenergic receptor agonists may at least partly attenuate stress-induced reinstatement of nicotine seeking by stimulating α2-adrenergic receptors in the CeA.

A procedure for studying the footshock-induced reinstatement of cocaine seeking in laboratory rats

The most insidious aspect of drug addiction is the high propensity for relapse. Animal models of relapse, known as reinstatement procedures, have been used extensively to study the neurobiology and phenomenology of relapse to drug use. Although procedural variations have emerged over the past several decades, the most conventional reinstatement procedures are based on the drug self-administration (SA) model. In this model, an animal is trained to perform an operant response to obtain drug. Subsequently, the behavior is extinguished by withholding response-contingent reinforcement. Reinstatement of drug seeking is then triggered by a discrete event, such as an injection of the training drug, re-exposure to drug-associated cues, or exposure to a stressor. Reinstatement procedures were originally developed to study the ability of acute non-contingent exposure to the training drug to reinstate drug seeking in rats and monkeys. Reinstatement procedures have since been modified to study the role of environmental stimuli, including drug-associated cues and exposure to various forms of stress, in relapse to drug seeking. Over the past 15 years, a major focus of the reinstatement literature has been on the role of stress in drug relapse. One of the most commonly used forms of stress for studying this relationship is acute exposures to mild, intermittent, electric footshocks. The ability of footshock stress to induce reinstatement of drug seeking was originally demonstrated by Shaham and colleagues (1995) in rats with a history of intravenous heroin SA(5). Subsequently, the effect was generalized to rats with histories of intravenous cocaine, methamphetamine, and nicotine SA, as well as oral ethanol SA. Although footshock-induced reinstatement of drug seeking can be achieved reliably and robustly, it is an effect that tends to be sensitive to certain parametrical variables. These include the arrangement of extinction and reinstatement test sessions, the intensity and duration of footshock stress, and the presence of drug-associated cues during extinction and testing for reinstatement. Here we present a protocol for footshock-induced reinstatement of cocaine seeking that we have used with consistent success to study the relationship between stress and cocaine seeking.

Hippocampus, amygdala, and stress: interacting systems that affect susceptibility to addiction

Stress is one of the major factors in drug abuse, particularly in relapse and drug-seeking behavior. However, the mechanisms underlying the interactions between stress and drug abuse are unclear. For many years, studies have focused on the role of the dopaminergic reward system in drug abuse. Our results, for example, show that increased dopaminergic activity is induced by drug sensitization and different stressors via potentiation of the ventral subiculum-nucleus accumbens (NAc) pathway. Although the role of the norepinephrine (NE) system in stress is well known, its involvement in drug abuse has received less attention. This review explores the different mechanisms by which stressors can modulate the ventral subiculum-accumbens pathway, and how these modulations can induce alterations in the behavioral response to drug administration. In particular, we will focus on two main afferents to the NAc, the basolateral amygdala and the ventral subiculum of the hippocampus, and their interactions with the locus coeruleus-norepinephrine system.

Disulfiram attenuates drug-primed reinstatement of cocaine seeking via inhibition of dopamine β-hydroxylase

The antialcoholism medication disulfiram (Antabuse) inhibits aldehyde dehydrogenase (ALDH), which results in the accumulation of acetaldehyde upon ethanol ingestion and produces the aversive 'Antabuse reaction' that deters alcohol consumption. Disulfiram has also been shown to deter cocaine use, even in the absence of an interaction with alcohol, indicating the existence of an ALDH-independent therapeutic mechanism. We hypothesized that disulfiram's inhibition of dopamine β-hydroxylase (DBH), the catecholamine biosynthetic enzyme that converts dopamine (DA) to norepinephrine (NE) in noradrenergic neurons, underlies the drug's ability to treat cocaine dependence. We tested the effects of disulfiram on cocaine and food self-administration behavior and drug-primed reinstatement of cocaine seeking in rats. We then compared the effects of disulfiram with those of the selective DBH inhibitor, nepicastat. Disulfiram, at a dose (100 mg/kg, i.p.) that reduced brain NE by ∼40%, did not alter the response for food or cocaine on a fixed ratio 1 schedule, whereas it completely blocked cocaine-primed (10 mg/kg, i.p.) reinstatement of drug seeking following extinction. A lower dose of disulfiram (10 mg/kg) that did not reduce NE had no effect on cocaine-primed reinstatement. Nepicastat recapitulated the behavioral effects of disulfiram (100 mg/kg) at a dose (50 mg/kg, i.p.) that produced a similar reduction in brain NE. Food-primed reinstatement of food seeking was not impaired by DBH inhibition. Our results suggest that disulfiram's efficacy in the treatment of cocaine addiction is associated with the inhibition of DBH and interference with the ability of environmental stimuli to trigger relapse.

Involvement of noradrenergic neurotransmission in the stress- but not cocaine-induced reinstatement of extinguished cocaine-induced conditioned place preference in mice: role for β-2 adrenergic receptors

The responsiveness of central noradrenergic systems to stressors and cocaine poses norepinephrine as a potential common mechanism through which drug re-exposure and stressful stimuli promote relapse. This study investigated the role of noradrenergic systems in the reinstatement of extinguished cocaine-induced conditioned place preference by cocaine and stress in male C57BL/6 mice. Cocaine- (15 mg/kg, i.p.) induced conditioned place preference was extinguished by repeated exposure to the apparatus in the absence of drug and reestablished by a cocaine challenge (15 mg/kg), exposure to a stressor (6-min forced swim (FS); 20-25°C water), or administration of the α-2 adrenergic receptor (AR) antagonists yohimbine (2 mg/kg, i.p.) or BRL44408 (5, 10 mg/kg, i.p.). To investigate the role of ARs, mice were administered the nonselective β-AR antagonist, propranolol (5, 10 mg/kg, i.p.), the α-1 AR antagonist, prazosin (1, 2 mg/kg, i.p.), or the α-2 AR agonist, clonidine (0.03, 0.3 mg/kg, i.p.) before reinstatement testing. Clonidine, prazosin, and propranolol failed to block cocaine-induced reinstatement. The low (0.03 mg/kg) but not high (0.3 mg/kg) clonidine dose fully blocked FS-induced reinstatement but not reinstatement by yohimbine. Propranolol, but not prazosin, blocked reinstatement by both yohimbine and FS, suggesting the involvement of β-ARs. The β-2 AR antagonist ICI-118551 (1 mg/kg, i.p.), but not the β-1 AR antagonist betaxolol (10 mg/kg, i.p.), also blocked FS-induced reinstatement. These findings suggest that stress-induced reinstatement requires noradrenergic signaling through β-2 ARs and that cocaine-induced reinstatement does not require AR activation, even though stimulation of central noradrenergic neurotransmission is sufficient to reinstate.

Evaluation of the relationship between anxiety during withdrawal and stress-induced reinstatement of cocaine seeking

The initial termination of cocaine consumption in human addicts is associated with heightened anxiety states and low levels of craving. Craving, however, tends to increase progressively over time, remains high for extended periods of time, and can be exacerbated by stressors, leading to relapse. Laboratory rats, likewise, exhibit heightened states of anxiety after withdrawal from drug, and follow a time course of cocaine seeking that parallels the time course of craving reported in humans. In addition, laboratory rats show heightened susceptibility to relapse when exposed to stressors after extended periods of withdrawal, and exhibit persistent and heightened expressions of stress-induced anxiety. The general objective of this paper is to consider the relationship between anxiety states after withdrawal from cocaine and stress-induced reinstatement of cocaine seeking in laboratory rats, and to identify the neural substrates involved. The focus of the review is on studies addressing the roles of corticotropin-releasing factor (CRF) and noradrenaline pathways of the extended amygdala circuitry, and their direct or indirect interactions with the mesocorticolimbic dopamine system, in anxiety after withdrawal from cocaine and stress-induced reinstatement of cocaine seeking. Furthermore, the effects of time after withdrawal from cocaine and amount of cocaine exposure during self-administration on the activity of CRF, noradrenaline, and dopamine pathways of the extended amygdala and mesocorticolimbic systems will be considered. The review will highlight how changing levels of activity within these systems may serve to alter the nature of the relationship between anxiety and stress-induced reinstatement of cocaine seeking at different times after withdrawal from cocaine.

Individual differences in prefrontal cortex function and the transition from drug use to drug dependence

Several neuropsychological hypotheses have been formulated to explain the transition to addiction, including hedonic allostasis, incentive salience, and the development of habits. A key feature of addiction that remains to be explored is the important individual variability observed in the propensity to self-administer drugs, the sensitivity to drug-associated cues, the severity of the withdrawal state, and the ability to quit. In this review, we suggest that the concept of self-regulation, combined with the concept of modularity of cognitive function, may aid in the understanding of the neural basis of individual differences in the vulnerability to drugs and the transition to addiction. The thesis of this review is that drug addiction involves a failure of the different subcomponents of the executive systems controlling key cognitive modules that process reward, pain, stress, emotion, habits, and decision-making. A subhypothesis is that the different patterns of drug addiction and individual differences in the transition to addiction may emerge from differential vulnerability in one or more of the subcomponents.

Repeated agmatine treatment attenuates nicotine sensitization in mice: modulation by alpha2-adrenoceptors

Agmatine [2-(4-aminobutyl)guanidine] is an endogenous amine proposed as a neurotransmitter/neuromodulator that binds to multiple target receptors in brain. Besides, many central and peripheral functions, agmatine have been implicated in the process of drug addiction. The purpose of the present study was to examine the effects of centrally injected agmatine on nicotine induced locomotor sensitization in Swiss male mice. Our data shows that repeated injections of nicotine (0.4 mg/kg, sc, twice daily for 7 days) gradually increased locomotion during 7 days development period or after 3 days (nicotine) withdrawal phase challenged with nicotine (0.4 mg/kg, sc) on day 11. Mice were pretreated with agmatine (40-80 microg, icv) or agents known to increase endogenous brain agmatine levels [e.g. an agmatine biosynthetic precursor, L-arginine (80 microg, icv), ornithine decarboxylase inhibitor, difluoromethyl-ornithine (50 microg, icv), diamine oxidase inhibitor, aminoguanidine (25 microg, icv) and agmatinase inhibitor, arcaine (50 microg, icv)] 30 min before daily first nicotine injection or during nicotine withdrawal phase. All these treatments attenuated the development as well as incubation of locomotor sensitization to nicotine. Coadministration of agmatine (20 microg, icv) and alpha(2)-adrenoreceptors agonist, clonidine (0.1 microg, icv) evoked synergistic inhibition of nicotine sensitization. Conversely, prior administration of alpha(2)-adrenoceptor antagonist, yohimbine (5mg/kg, ip) or idazoxan (0.4 mg/kg, ip) reversed the inhibitory effect of agmatine on nicotine sensitization. There was no significant difference in activity between mice injected with any of these agents/saline alone and saline/saline groups. These data indicate that agmatine attenuates nicotine induced locomotor sensitization via a mechanism which may involve alpha(2)-adrenergic receptors. Thus, agmatine might have therapeutic implications in the treatment of nicotine addiction and deserve further investigations.

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

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

Prefrontal cortical regulation of drug seeking in animal models of drug relapse

Prefrontal cortical dysfunction is thought to underlie maladaptive behaviors displayed by chronic drug users, most notably the high propensity for relapse that severely impedes successful treatment of drug addiction. In animal models of drug relapse, exposure to drug-associated stimuli, small amounts of drug, and acute stressors powerfully reinstate drug seeking by critically engaging the prefrontal cortex, with the anterior cingulate, prelimbic, infralimbic, and orbitofrontal subregions making distinct contributions to drug seeking. Hence, from an addiction treatment perspective, it is necessary to fully explicate the involvement of the prefrontal cortex in drug relapse.

Cognitive enhancement as a pharmacotherapy target for stimulant addiction

BACKGROUND

No medications have been proven to be effective for cocaine and methamphetamine addiction. Attenuation of drug reward has been the main strategy for medications development, but this approach has not led to effective treatments. Thus, there is a need to identify novel treatment targets in addition to the brain reward system.

AIM

To propose a novel treatment strategy for stimulant addiction that will focus on medications enhancing cognitive function and attenuating drug reward.

METHODS

Pre-clinical and clinical literature on potential use of cognitive enhancers for stimulant addiction pharmacotherapy was reviewed.

RESULTS AND CONCLUSIONS

Cocaine and methamphetamine users show significant cognitive impairments, especially in attention, working memory and response inhibition functions. The cognitive impairments seem to be predictive of poor treatment retention and outcome. Medications targeting acetylcholine and norepinephrine are particularly well suited for enhancing cognitive function in stimulant users. Many cholinergic and noradrenergic medications are on the market and have a good safety profile and low abuse potential. These include galantamine, donepezil and rivastigmine (cholinesterase inhibitors), varenicline (partial nicotine agonist), guanfacine (alpha(2)-adrenergic agonist) and atomoxetine (norepinephrine transporter inhibitor). Future clinical studies designed optimally to measure cognitive function as well as drug use behavior would be needed to test the efficacy of these cognitive enhancers for stimulant addiction.

HPA axis response to psychological stress and treatment retention in residential substance abuse treatment: a prospective study

INTRODUCTION

Substance abuse treatment programs are often characterized by high rates of premature treatment dropout, which increases the likelihood of relapse to drug use. Negative reinforcement models of addiction emphasize an individual's inability to tolerate stress as a key factor for understanding poor substance use treatment outcomes, and evidence indicates that dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis contributes to an individual's inability to respond adaptively to stress. The aim of the current study was to examine whether HPA axis response to stress is predictive of treatment retention among a sample of drug users in residential substance abuse treatment.

METHOD

Prospective study assessing treatment retention among 102 individuals enrolled in residential substance abuse treatment. Participants completed two computerized stress tasks, and HPA axis response to stress was measured via salivary cortisol at five time points from baseline (pre-stress) to 30 min post-stress exposure.

RESULTS

The main outcome measures were treatment dropout (categorical) and total number of days in treatment (continuous). A significantly higher salivary cortisol response to stress was observed in treatment dropouts compared to treatment completers. Further, Cox proportional hazards survival analyses indicated that a higher peak cortisol response to stress was associated with a shorter number of days to treatment dropout.

CONCLUSIONS

Results indicate that a higher salivary cortisol level in response to stress is associated with an inability to remain in substance abuse treatment. These findings are the first to document a biological marker of stress as a predictor of substance abuse treatment dropout, and support the development and implementation of treatments targeting this vulnerability.

Modulation of glutamatergic synaptic transmission in the bed nucleus of the stria terminalis

Glutamate, catecholamine and neuropeptide signaling within the bed nucleus of the stria terminalis (BNST) have all been identified as key participants in anxiety-like behaviors and behaviors related to withdrawal from exposure to substances of abuse. The BNST is thought to serve as a key relay between limbic cognitive centers and reward, stress and anxiety nuclei. Human studies and animal models have demonstrated that stressors and drugs of abuse can result in long term behavioral modifications that can culminate in psychological diseases such as addiction and post-traumatic stress disorder. The ability of catecholamines and neuropeptides to influence synaptic glutamatergic transmission (stemming from cognitive centers) within the BNST may have profound consequences over these behaviors. In this review we highlight studies examining synaptic plasticity and modulation of excitatory transmission within the BNST, emphasizing how such modulation may result in alterations in anxiety and reward related behavior.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

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

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

Behavioral sensitization to cocaine: cooperation between glucocorticoids and epinephrine

RATIONALE

Stressful life experiences facilitate responsiveness to psychostimulant drugs. While there is ample evidence that adrenal glucocorticoids mediate these effects of stress, the role of the sympatho-adrenal system in the effects of psychostimulants is poorly understood.

OBJECTIVES

The present study investigated the role of the two adrenal stress hormones, corticosterone and epinephrine, in sensitization to the locomotor stimulant effects of cocaine.

MATERIALS AND METHODS

The DBA/2 mouse strain was used, as behavioral sensitization in this strain critically depends on adrenal hormones. Animals were subjected to adrenalectomy ("ADX", surgical removal of the adrenals) or SHAM surgery, and ADX mice were given replacement of epinephrine (5 x 10(-3) mg/kg subcutaneously (s.c.) just prior to each drug administration), corticosterone (20%, s.c., pellet), or both. Mice were subjected to a cocaine sensitization regimen (15.0 mg/kg cocaine on nine consecutive days followed by a 7.5 mg/kg cocaine challenge after a 5-day withdrawal).

RESULTS

In agreement with our previous observations, ADX prevented initiation and expression of cocaine-induced locomotor sensitization. Whereas neither corticosterone nor epinephrine alone were sufficient to reverse the ADX effect, both hormones were necessary to fully restore initiation and retention of sensitization to levels observed in SHAM animals.

CONCLUSIONS

The present findings indicate that corticosterone and epinephrine cooperate to facilitate behavioral responsiveness to cocaine. These data emphasize that in addition to the hypothalamic-pituitary-adrenal axis, the sympathetic nervous system plays a critical role in psychostimulant sensitivity.

The role of noradrenaline and 5-hydroxytryptamine in yohimbine-induced increases in alcohol-seeking in rats

RATIONALE AND OBJECTIVES

We previously showed that systemic administration of the prototypical alpha-2 noradrenaline (NA) receptor antagonist yohimbine increases alcohol self-administration and reinstatement. Yohimbine also acts as an agonist of 5-hydroxytryptamine (5-HT) 5-HT1A receptors, which have been shown to be involved in alcohol seeking. Here, we determined the contributions of the alpha-2 and 5-HT1A properties of yohimbine to its effects on alcohol seeking.

METHODS

The effects of lesions of the dorsal or ventral NA bundles with 6-OHDA on yohimbine-induced alcohol self-administration were first determined in male Wistar rats trained to self-administer alcohol (12% w/v, 0.19 ml per alcohol delivery), and then on reinstatement induced by yohimbine after extinction of the operant response. It was then determined whether the selective alpha-2 antagonist RS-79948 (0.1, 0.2, 0.4 mg/kg) would mimic the effects of yohimbine on self-administration and reinstatement. The effects of the alpha-2 receptor agonist clonidine, or the 5-HT1A antagonist WAY 100,635 were then determined on yohimbine-induced self-administration and reinstatement.

RESULTS

Lesions of the NA systems did not affect yohimbine-induced alcohol self-administration or reinstatement, and RS-79948 did not mimic the effects of yohimbine. Clonidine did not significantly affect increased alcohol self-administration induced by yohimbine, but did attenuate its effects on reinstatement. Blockade of 5-HT1A receptors reduced both yohimbine-induced self-administration and reinstatement.

CONCLUSIONS

These results suggest that alpha-2 antagonist properties of yohimbine may play a role in the reinstatement of alcohol-seeking, but not self-administration. On the other hand, yohimbine's actions on 5-HT1A receptors contribute to its effects on both alcohol self-administration and reinstatement.

Systemic propranolol acts centrally to reduce conditioned fear in rats without impairing extinction

BACKGROUND

Previous work has implicated noradrenergic beta-receptors in the consolidation and reconsolidation of conditioned fear. Less is known, however, about their role in fear expression and extinction. The beta-receptor blocker propranolol has been used clinically to reduce anxiety. With an auditory fear conditioning task in rats, we assessed the effects of systemic propranolol on the expression and extinction of two measures of conditioned fear: freezing and suppression of bar-pressing.

METHODS

One day after receiving auditory fear conditioning, rats were injected with saline, propranolol, or peripheral beta-receptor blocker sotalol (both 10 mg/kg, IP). Twenty minutes after injection, rats were given either 6 or 12 extinction trials and were tested for extinction retention the following day. The effect of propranolol on the firing rate of neurons in prelimbic (PL) prefrontal cortex was also assessed.

RESULTS

Propranolol reduced freezing by more than 50%, an effect that was evident from the first extinction trial. Suppression was also significantly reduced. Despite this, propranolol had no effect on the acquisition or retention of extinction. Unlike propranolol, sotalol did not affect fear expression, although both drugs significantly reduced heart rate. This suggests that propranolol acts centrally to reduce fear. Consistent with this, propranolol reduced the firing rate of PL neurons.

CONCLUSION

Propranolol reduced the expression of conditioned fear, without interfering with extinction learning. Reduced fear with intact extinction suggests a possible use for propranolol in reducing anxiety during extinction-based exposure therapies, without interfering with long-term clinical response.

Electrocardiographic effects of lofexidine and methadone coadministration: secondary findings from a safety study

STUDY OBJECTIVE

To determine the electrocardiographic effects of coadministration of lofexidine and methadone.

DESIGN

Prospective, double-blind study.

SETTING

Outpatient drug treatment research clinic.

PARTICIPANTS

Fourteen adults (mean +/- SD age 34.9 +/- 5.3 yrs) with physical dependence on opioids.

INTERVENTION

Participants were stabilized on methadone maintenance therapy, reaching a target dose of 80 mg/day. After 3 weeks of methadone stabilization, participants received lofexidine 0.4 mg or placebo once/day, each for 1 week, administered at the same time as methadone. From weeks 3-8, all subjects received lofexidine, with the dose escalated each week in 0.2-mg increments so that by week 8, participants were receiving lofexidine 1.6 mg/day. Electrocardiograms (ECGs) were obtained at baseline (before methadone), after stabilization with methadone, and after lofexidine coadministration during peak plasma lofexidine levels.

MEASUREMENTS AND MAIN RESULTS

Prespecified outcome measures of mean and maximal changes in heart rate, and PR, QRS, and QTc intervals were obtained after stabilization with methadone and after lofexidine 0.4 mg coadministration. Repeated-measures regression showed no significant changes in heart rate or PR, QRS, or QTc interval after methadone stabilization, but a significant decrease in heart rate (mean +/- SD -8.0 +/- 7.3 beats/min, p=0.0006) after starting lofexidine. When data were analyzed by using maximal ECG response, again, no significant changes were noted during methadone induction compared with baseline, but significant changes did occur in all four ECG parameters when lofexidine was coadministered: decreased heart rate (mean +/- SD -9.6 +/- 5.8 beats/min, p<0.0001) and increased PR interval (+11.1 +/- 19.8 msec, p=0.026), QRS interval (+3.7 +/- 4.3 msec, p=0.002), and QTc interval (+21.9 +/- 40.8 msec, p=0.018). In three female participants, the change in QTc interval from baseline was clinically significant (> 40 msec).

CONCLUSION

Our preliminary data suggest that coadministration of lofexidine and methadone induces QTc interval prolongation. This drug combination should be prescribed cautiously, with ECG monitoring. Furthermore, because the participants with the largest changes in QTc interval in our study were female, women may be at highest risk.

Norepinephrine and stimulant addiction

No pharmacotherapies are approved for stimulant use disorders, which are an important public health problem. Stimulants increase synaptic levels of the monoamines dopamine (DA), serotonin and norepinephrine (NE). Stimulant reward is attributable mostly to increased DA in the reward circuitry, although DA stimulation alone cannot explain the rewarding effects of stimulants. The noradrenergic system, which uses NE as the main chemical messenger, serves multiple brain functions including arousal, attention, mood, learning, memory and stress response. In pre-clinical models of addiction, NE is critically involved in mediating stimulant effects including sensitization, drug discrimination and reinstatement of drug seeking. In clinical studies, adrenergic blockers have shown promise as treatments for cocaine abuse and dependence, especially in patients experiencing severe withdrawal symptoms. Disulfiram, which blocks NE synthesis, increased the number of cocaine-negative urines in five randomized clinical trials. Lofexidine, an alpha(2)-adrenergic agonist, reduces the craving induced by stress and drug cues in drug users. In addition, the NE transporter (NET) inhibitor atomoxetine attenuates some of d-amphetamine's subjective and physiological effects in humans. These findings warrant further studies evaluating noradrenergic medications as treatments for stimulant addiction.

Interaction between noradrenaline and corticotrophin-releasing factor in the reinstatement of cocaine seeking in the rat

RATIONALE

Corticotropin-releasing factor (CRF) and noradrenaline (NA) have been shown in independent studies to mediate stress-induced reinstatement of drug seeking. To date, however, a functional interaction between the systems in reinstatement has not been demonstrated.

OBJECTIVES

The objectives of this study were to determine whether CRF and NA systems can interact to influence reinstatement responding and, if so, in what direction the interaction occurs.

MATERIALS AND METHODS

Rats were trained to self-administer cocaine (0.23 mg/kg per infusion) for 8-10 days. Subsequently, responding for drug was extinguished, and tests for reinstatement were conducted following: (1) pretreatment with the CRF receptor antagonist, D: -Phe CRF(12-41) [1 microg, intracerebroventricular (i.c.v.)], prior to i.c.v. injections of NA (10 microg; Experiment 1); (2) pretreatment with the alpha(2) adrenoceptor agonist, clonidine (40 microg/kg, i.p.), prior to i.c.v. injections of CRF (0.5 microg; Experiment 2); (3) pretreatment with D: -Phe (1, 5 microg, i.c.v.), prior to systemic injections of the alpha(2) adrenoceptor antagonist, yohimbine (1.25 mg/kg; Experiment 3A); or (4) pretreatment with clonidine (40 microg/kg, i.p.) prior to systemic injections of yohimbine (0.625 mg/kg, 1.25 mg/kg; Experiment 3B).

RESULTS

NA reliably induced reinstatement, an effect that was blocked by pretreatment with D: -Phe. In contrast, CRF-induced reinstatement was not attenuated by pretreatment with clonidine. Pretreatment with neither D: -Phe nor clonidine was effective in blocking yohimbine-induced reinstatement.

CONCLUSION

Together, the present findings suggest a functional interaction between NA and CRF systems in mediating stress-induced reinstatement of cocaine seeking, whereby activation of CRF receptors occurs subsequent to, and downstream of, the sites of action of NA.

Accumbal noradrenaline that contributes to the alpha-adrenoceptor-mediated release of dopamine from reserpine-sensitive storage vesicles in the nucleus accumbens is derived from alpha-methyl-para-tyrosine-sensitive pools

Alpha-adrenoceptors in the nucleus accumbens are known to inhibit accumbal dopamine release from reserpine-sensitive pools. The aim of this study was to test our previously reported hypothesis that accumbal noradrenaline that controls the dopamine release from these storage vesicles is derived from alpha-methyl-para-tyrosine-sensitive pools. The sensitivity of accumbal alpha-adrenoceptors to noradrenergic agents depends on the amount of noradrenaline that is available in the synapse. In case the synaptic noradrenaline levels decrease, the conformation of alpha-adrenoceptors changes into a state that makes these receptors more sensitive to its agonists. The effects of alpha-methyl-para-tyrosine, respectively reserpine, on the alpha-adrenoceptor-agonist-induced changes of accumbal dopamine release were investigated. Alpha-methyl-para-tyrosine, but not reserpine, made accumbal postsynaptic alpha-adrenoceptors more sensitive to phenylephrine. These results indicate that noradrenaline that inhibits the release of dopamine from reserpine-sensitive storage vesicles, via stimulation of accumbal postsynaptic alpha-adrenoceptors, is derived from alpha-methyl-para-tyrosine-sensitive pools. The clinical impact of these data is discussed.

Review. Psychological and neural mechanisms of relapse

Relapse, the resumption of drug taking after periods of abstinence, remains the major problem for the treatment of addiction. Even when drugs are unavailable for long periods or when users are successful in curbing their drug use for extended periods, individuals remain vulnerable to events that precipitate relapse. Behavioural studies in humans and laboratory animals show that drug-related stimuli, drugs themselves and stressors are powerful events for the precipitation of relapse. Molecular, neurochemical and anatomical studies have identified lasting neural changes that arise from mere exposure to drugs and other enduring changes that arise from learning about the relationship between drug-related stimuli and drug effects. Chronic drug exposure increases sensitivity of some systems of the brain to the effects of drugs and stressful events. These changes, combined with those underlying conditioning and learning, perpetuate vulnerability to drug-related stimuli. Circuits of the brain involved are those of the mesocorticolimbic dopaminergic system and its glutamatergic connections, and the corticotropin-releasing factor and noradrenergic systems of the limbic brain. This paper reviews advances in our understanding of how these systems mediate the effects of events that precipitate relapse and of how lasting changes in these systems can perpetuate vulnerability to relapse.

The comparative distributions of the monoamine transporters in the rodent, monkey, and human amygdala

The monoamines in the amygdala modulate multiple aspects of emotional processing in the mammalian brain, and organic or pharmacological dysregulation of these systems can result in affective pathologies. Knowledge of the normal distribution of these neurotransmitters, therefore, is central to our understanding of both the normal processes regulated by the amygdala and the pathological conditions associated with monoaminergic dysregulation. The monoaminergic transporters have proven to be accurate and reliable markers of the distributions of their substrates. The purpose of this review was twofold: First, to briefly recount the functional relevance of dopamine, serotonin, and norepinephrine transmission in the amygdala, and second, to describe and compare the distributions of the monoamine transporters in the rodent, monkey, and human brain. The transporters were found to be heterogeneously distributed in the amygdala. The dopamine transporter (DAT) is consistently found to be extremely sparsely distributed, however the various accounts of its subregional topography are inconsistent, making any cross-species comparisons difficult. The serotonin transporter (SERT) had the greatest overall degree of labeling of the three markers, and was characterized by substantial inter-species variability in its relative distribution. The norepinephrine transporter (NET) was shown to possess an intermediate level of labeling, and like the SERT, its distribution is not consistent across the three species. The results of these comparisons indicate that caution should be exercised when using animal models to investigate the complex processes modulated by the monoamines in the amygdala, as their relative contributions to these functions may differ across species.

Stress-induced reinstatement of cocaine seeking is mediated by the kappa opioid system

INTRODUCTION

Prior activation of the kappa opioid system by repeated stress or agonist administration has been previously shown to potentiate the rewarding properties of subsequently administered cocaine. In the present study, intermittent and uncontrollable footshock, a single session of forced swim, or acute administration of the kappa agonist U50,488 (5 mg/kg) were found to reinstate place preference in mice previously conditioned with cocaine (15 mg/kg) and subsequently extinguished by repeated training sessions without drug.

RESULTS AND DISCUSSION

Stress-induced reinstatement did not occur for mice pretreated with the kappa opioid receptor antagonist norbinaltorphimine (10 mg/kg) and did not occur in mice lacking either kappa opioid receptors (KOR -/-) or prodynorphin (Dyn -/-). In contrast, the initial cocaine conditioning and extinction rates were not significantly affected by disruption of the kappa opioid system. Cocaine-injection also reinstated conditioned place preference in extinguished mice; however, cocaine-primed reinstatement was not blocked by kappa opioid system disruption.

CONCLUSION

The results suggest that stress-induced drug craving in mice may require activation of the dynorphin/kappa opioid system.

Alpha1-adrenergic receptor-induced heterosynaptic long-term depression in the bed nucleus of the stria terminalis is disrupted in mouse models of affective disorders

The glutamatergic synapse in specific brain regions has been shown to be a site for convergence of stress and addictive substances. The bed nucleus of the stria terminalis (BNST), a nucleus that relays between higher order processing centers and classical reward and stress pathways, receives dense noradrenergic inputs that are known to influence behavioral paradigms of both anxiety and stress-induced relapse to drug seeking. Alpha(1)-adrenergic receptors (alpha(1)-ARs) within this region have been implicated in modulation of the HPA axis and anxiety responses. We found that application of an alpha(1)-AR agonist produced a long-term depression (LTD) of excitatory transmission in an acute mouse BNST slice preparation. This effect was mimicked by a 20 min, but not a 10 min, application of 100 microM norepinephrine (NE) in a prazosin-sensitive manner. This alpha(1)-AR LTD was independent of N-methyl-D-aspartate receptor (NMDAR) function unlike previously described alpha(1)-AR LTD in the hippocampus and visual cortex; however, it was dependent on the activation of L-type voltage gated calcium channels (VGCCs). In addition, alpha(1)-AR LTD was induced independently of the activation of mGluR5 which can also induce LTD in this region. Furthermore, alpha(1)-AR LTD was intact in mice receiving an intraperitoneal injection of cocaine but was disrupted in alpha(2a)-AR and NE transporter (NET) knockout (KO) mice. Thus a loss of this plasticity at glutamatergic synapses in BNST could contribute to affective behavioral phenotypes of these mice.

Noradrenergic transmission in the extended amygdala: role in increased drug-seeking and relapse during protracted drug abstinence

Studies reviewed here implicate the extended amygdala in the negative affective states and increased drug-seeking that occur during protracted abstinence from chronic drug exposure. Norepinephrine (NE) and corticotropin-releasing factor (CRF) signaling in the extended amygdala, including the bed nucleus of the stria terminalis, shell of the nucleus accumbens, and central nucleus of the amygdala, are generally involved in behavioral responses to environmental and internal stressors. Hyperactivity of stress response systems during addiction drives many negative components of drug abstinence. In particular, NE signaling from the nucleus tractus solitarius (NTS) to the extended amygdala, along with increased CRF transmission within the extended amygdala, are critical for the aversiveness of acute opiate withdrawal as well as stress-induced relapse of drug-seeking for opiates, cocaine, ethanol, and nicotine. NE and CRF transmission in the extended amygdala are also implicated in the increased anxiety that occurs during prolonged abstinence from chronic opiates, cocaine, ethanol, and cannabinoids. Many of these stress-associated behaviors are reversed by NE or CRF antagonists given systemically or locally within the extended amygdala. Finally, increased Fos activation in the extended amygdala and NTS is associated with the enhanced preference for drugs and decreased preference for natural rewards observed during protracted abstinence from opiates and cocaine, indicating that these areas are involved in the altered reward processing associated with addiction. Together, these findings suggest that involvement of the extended amygdala and its noradrenergic afferents in anxiety, stress-induced relapse, and altered reward processing reflects a common function for these circuits in stress modulation of drug-seeking.

A role for brain stress systems in addiction

Drug addiction is a chronically relapsing disorder characterized by compulsion to seek and take drugs and has been linked to dysregulation of brain regions that mediate reward and stress. Activation of brain stress systems is hypothesized to be key to the negative emotional state produced by dependence that drives drug seeking through negative reinforcement mechanisms. This review explores the role of brain stress systems (corticotropin-releasing factor, norepinephrine, orexin [hypocretin], vasopressin, dynorphin) and brain antistress systems (neuropeptide Y, nociceptin [orphanin FQ]) in drug dependence, with emphasis on the neuropharmacological function of extrahypothalamic systems in the extended amygdala. The brain stress and antistress systems may play a key role in the transition to and maintenance of drug dependence once initiated. Understanding the role of brain stress and antistress systems in addiction provides novel targets for treatment and prevention of addiction and insights into the organization and function of basic brain emotional circuitry.

Norepinephrine signaling through beta-adrenergic receptors is critical for expression of cocaine-induced anxiety

BACKGROUND

Cocaine is a widely abused psychostimulant that has both rewarding and aversive properties. While the mechanisms underlying cocaine's rewarding effects have been studied extensively, less attention has been paid to the unpleasant behavioral states induced by cocaine, such as anxiety.

METHODS

In this study, we evaluated the performance of dopamine beta-hydroxylase knockout (Dbh -/-) mice, which lack norepinephrine (NE), in the elevated plus maze (EPM) to examine the contribution of noradrenergic signaling to cocaine-induced anxiety.

RESULTS

We found that cocaine dose-dependently increased anxiety-like behavior in control (Dbh +/-) mice, as measured by a decrease in open arm exploration. The Dbh -/- mice had normal baseline performance in the EPM but were completely resistant to the anxiogenic effects of cocaine. Cocaine-induced anxiety was also attenuated in Dbh +/- mice following administration of disulfiram, a dopamine beta-hydroxylase (DBH) inhibitor. In experiments using specific adrenergic antagonists, we found that pretreatment with the beta-adrenergic receptor antagonist propranolol blocked cocaine-induced anxiety-like behavior in Dbh +/- and wild-type C57BL6/J mice, while the alpha(1) antagonist prazosin and the alpha(2) antagonist yohimbine had no effect.

CONCLUSIONS

These results indicate that noradrenergic signaling via beta-adrenergic receptors is required for cocaine-induced anxiety in mice.

Involvement of basolateral amygdala alpha2-adrenoceptors in modulating consolidation of inhibitory avoidance memory

These experiments investigated the role of the alpha(2)-adrenoceptors of the basolateral nucleus of the amygdala (BLA) in modulating the retention of inhibitory avoidance (IA). In Experiment 1, male Sprague Dawley rats implanted with bilateral cannulae in the BLA received microinfusions of a selective alpha(2)-adrenoceptor antagonist idazoxan 20 min either before or immediately after training. Retention was tested 48 h later. Idazoxan induced a dose-dependent enhancement of retention performance and was more effective when administered post-training. In Experiment 2, animals received pre- or post-training intra-BLA infusions of a selective alpha(2)-adrenoceptor agonist UK 14,304. The agonist induced a dose-dependent impairment of retention performance and, as with the antagonist treatments, post-training infusions were more effective. These results provide additional evidence that consolidation of inhibitory avoidance memory depends critically on prolonged activation of the noradrenergic system in the BLA and indicate that this modulatory influence is mediated, in part, by pre-synaptic alpha(2)-adrenoceptors.

Making a bad thing worse: adverse effects of stress on drug addiction

Sustained exposure to various psychological stressors can exacerbate neuropsychiatric disorders, including drug addiction. Addiction is a chronic brain disease in which individuals cannot control their need for drugs, despite negative health and social consequences. The brains of addicted individuals are altered and respond very differently to stress than those of individuals who are not addicted. In this Review, we highlight some of the common effects of stress and drugs of abuse throughout the addiction cycle. We also discuss both animal and human studies that suggest treating the stress-related aspects of drug addiction is likely to be an important contributing factor to a long-lasting recovery from this disorder.