Stress reinstates cocaine-seeking behavior after prolonged extinction and a drug-free period

Corticotropin-releasing factor receptor type I mediates stress-induced relapse to opiate dependence in rats

The possible effect of different corticotropin-releasing factor receptor (CRFR) antagonists (alpha-helical CRF, CP-154,526 and AS-30) on the maintenance and reactivation of morphine-conditioned place preference (CPP) induced by morphine or footshock stress, respectively, were investigated in rats. The results show that morphine-induced maintenance of CPP was not affected by pretreatment with any CRFR antagonists. However, morphine-induced the reactivation of CPP was significantly attenuated by pre-administration of 10 microg alpha-helical CRF (i.c.v.). The maintenance of morphine CPP could be induced by repeated footshock and this effect was significantly attenuated by pretreatment of 10 microg alpha-helical CRF (i.c.v.) and 10 mg CP-154,526 (i.p.). Furthermore, following a 28-day extinction of morphine CPP, a single footshock could again elicit the reactivation of place preference that was blocked by pretreatment with 10 microg alpha-helical CRF (i.c.v.) and 1 or 10 mg CP-154,526 (i.p.). The present study demonstrates that CRFR type 1, but not CRFR type 2, mediates the stress-induced maintenance and reactivation of morphine CPP. These findings suggest that CRFR type 1 antagonists might be of some value in the treatment and prevention of stress-induced relapse to drug dependence long after detoxification.

Stress-induced relapse to heroin and cocaine seeking in rats: a review

Studies in humans suggest that exposure to stress increases the probability of relapse to drug use, but until recently there has been no animal model to study the mechanisms that mediate this effect. We have developed a reinstatement procedure that allows us to study the effect of stress on relapse to drug seeking in rats. Using this procedure, we have shown that exposure to intermittent footshock stress reliably reinstates heroin and cocaine seeking after prolonged drug-free periods. In the present paper, we summarize results from several studies on stress-induced reinstatement of heroin and cocaine seeking in rats. We first assess the degree to which the phenomenon of stress-induced relapse generalizes to other stressors, to behaviors controlled by other drugs of abuse, and to behaviors controlled by non-drug reinforcers. We then review evidence from studies concerned with the neurotransmitters, the brain sites, and the neural systems involved in stress-induced reinstatement of drug seeking. Finally, we consider the mechanisms that might underlie stress-induced relapse to drug seeking and the possible implications of the findings for the treatment of relapse to drug use in humans.

Impact of self-administered cocaine and cocaine cues on extracellular dopamine in mesolimbic and sensorimotor striatum in rhesus monkeys

Studies were conducted to determine the impact of self-administered cocaine on extracellular striatal dopamine in four rhesus monkeys. The extent to which external cue conditioning contributed to the effects of cocaine and whether there is activation of striatal dopaminergic neurotransmission during drug-seeking behavior was also examined. Microdialysis measurements were made at 2 min intervals in sensorimotor (dorsolateral) and mesolimbic (central and ventromedial) striatum. A fixed-ratio schedule of reinforcement was used, with cocaine availability signaled by a visual cue. Studies examined the effects of cocaine or cocaine cues against a drug-free baseline. Large (fivefold to eightfold) increases in extracellular dopamine after a self-administered infusion of 0.5 mg/kg cocaine were quite rapid and matched the time course of reported subjective effects in human laboratory studies. To determine if conditioning to external cues contributed to the cocaine-induced increases, saline was substituted for cocaine in the infusion, leaving all other visual and auditory stimuli unchanged. No increase in extracellular dopamine in either sensorimotor or mesolimbic striatal subdivisions was observed. Extracellular dopamine during extended periods of drug-seeking behavior triggered by a visual cue was determined in both central and ventromedial striatum. This procedure also did not result in any measurable changes in extracellular dopamine. These studies demonstrate rapid and pronounced pharmacological actions of self-administered cocaine. No apparent conditioned component of those actions was associated with external environmental cues, suggesting that cues that trigger drug-seeking behavior in nonhuman primates do not cause conditioned increases in mesolimbic striatal dopamine.

Involvement of the medial septum in stress-induced relapse to heroin seeking in rats

Intermittent footshock stress has been shown to reinstate extinguished drug-taking behaviour in rats, but the brain areas involved in this effect are to a large degree unknown. Here we studied the role of the septum in stress-induced reinstatement of heroin seeking. Rats were trained to self-administer heroin for 9-10 days (three 3-h sessions per day, 0.1 mg/kg per infusion). Following training, extinction sessions were given for 8-13 days by substituting saline for heroin, and then tests for reinstatement of heroin seeking were carried out. Reversible inactivation of the medial septum with tetrodotoxin (TTX; 1-5 ng, infused 25-40 min before the test sessions) reliably reinstated heroin seeking, mimicking the effect of 15 min of intermittent footshock. This effect of TTX was not observed after infusions made 1.5 mm dorsally into the lateral septum. In other experiments, it was found that infusions of a low, subthreshold dose of TTX (0.5 ng) into the medial septum, when combined with 2 min of footshock that in itself was ineffective, reinstated heroin seeking. Furthermore, electrical stimulation (400 microA pulses, 100 micros duration, 100 Hz frequency) of the medial septum during exposure to 10 min of intermittent footshock attenuated footshock-induced reinstatement of heroin seeking. These data suggest a role for the medial septum in stress-induced relapse to drug seeking. The septum is thought to be involved in neuronal processes underlying behavioural inhibition, thus we speculate that stressors provoke relapse by interfering with these processes.

Stress, corticotropin-releasing factor, and drug addiction

The neuropeptide corticotropin-releasing factor (CRE) and related neuropeptides not only mediate hormonal responses to stressors but also have a neurotropic role in the central nervous system to mediate behavioral responses to stressors. CRF antagonists effectively block CRF responses and have effects opposite those of CRF in many stress-related situations. Recent advances suggest that in addition to CRF itself there is another CRF-related neuropeptide, urocortin, that may be involved in stress-related responses, particularly those involving appetite. At least two CRF receptors have been discovered to date, CRF-1 and CRF-2. CRF may be involved in various aspects of the addiction cycle associated with drugs of abuse. CRF appears to be activated during stress-induced reinstatement of drug taking as well as acute withdrawal from all major drugs of abuse. CRF is hypothesized to be part of an allostatic change leading to vulnerability to relapse during prolonged abstinence from drugs of abuse.

Effects of N-methyl-D-aspartate receptor antagonists on reinstatement of cocaine-seeking behavior by priming injections of cocaine or exposures to cocaine-associated cues in rats

The reinstatement of extinguished cocaine self-administration behavior was studied in rats pretreated with N-methyl-D-aspartate receptor antagonists. Rats were trained to self-administer intravenous cocaine (0.32 mg/kg/infusion) during five consecutive daily sessions that were followed by five consecutive daily extinction sessions, during which cocaine was unavailable and cocaine-associated cues (sound and light) were absent. Neither the competitive N-methyl-D-aspartate receptor antagonist D-CPPene (0.3-3 mg/kg) nor the low-affinity N-methyl-D-aspartate receptor channel blocker memantine (1-10 mg/kg) reinstated extinguished responding. Priming injections of intravenous cocaine (Experiment 1), and exposures to cocaine-associated stimuli (buzzer and light; Experiment 2) engendered responding on the reinforced lever in excess of that on the non-reinforced lever. In Experiment 1, administration of D-CPPene or memantine prior to the priming injection of cocaine eliminated the difference between reinforced-lever and non-reinforced-lever response rates. For both D-CPPene and memantine, however, this effect was largely due to increased responding upon the non-reinforced lever rather than to decreased reinforced-lever responding. In Experiment 2, D-CPPene, but not memantine, abolished in a dose-dependent manner the selective increase in reinforced-lever over non-reinforced-lever responding that was induced by exposures to cocaine-related stimuli. This effect of D-CPPene was not due to increased non-reinforced-lever responding. These data help define the boundaries within which N-methyl-D-aspartate receptor antagonists can prevent reinstatement of cocaine-seeking behavior (e.g. type of antagonist used and reinstatement procedure).

Clonidine blocks stress-induced reinstatement of heroin seeking in rats: an effect independent of locus coeruleus noradrenergic neurons

Using a reinstatement procedure, it has been shown that intermittent footshock stress reliably reinstates extinguished drug-taking behaviour in rats. Here we studied the role of noradrenaline (NE), one of the main brain neurotransmitters involved in responses to stress, in reinstatement of heroin seeking. We first determined the effect of clonidine, an alpha-2 adrenergic receptor agonist that decreases NE cell firing and release, on stress-induced reinstatement of heroin seeking. Rats were trained to self-administer heroin (0.1 mg/kg per infusion, IV, three 3-h sessions per day) for 9-10 days. Extinction sessions were given for up to 11 days during which saline was substituted for the drug. Tests for reinstatement were then conducted after exposure to intermittent footshock (5, 15 and 30 min, 0.5 mA). During testing, clonidine was injected systemically (10-40 microgram/kg, i.p.) or directly into the lateral or fourth ventricles (1-3 microram). Clonidine (1-2 microgram per site) or its charged analogue, 2-[2, 6-diethylphenylamino]-2-imidazole (ST-91, 0.5-1 microgram per site), was also injected bilaterally into the locus coeruleus (LC), the main noradrenergic cell group in the brain. Clonidine blocked stress-induced reinstatement of drug seeking when injected systemically or into the cerebral ventricles. In contrast, neither clonidine nor ST-91 consistently altered stress-induced reinstatement when injected into the locus coeruleus. We therefore studied the effect of lesions of the lateral tegmental NE neurons on stress-induced reinstatement. 6-Hydroxydopamine (6-OHDA) lesions performed after training for heroin self-administration had no effect on extinction of heroin-taking behaviour, but significantly attenuated reinstatement induced by intermittent footshock. These data suggest that: (i) clonidine prevents stress-induced relapse to heroin seeking by its action on neurons other than those of the locus coeruleus; and (ii) activation of the lateral tegmental NE neurons contributes to stress-induced reinstatement of heroin seeking.

Anesthesia is a risk factor for drug and alcohol craving and relapse in ex-abusers

Relapse to cocaine, heroin, and alcohol is a common occurrence in ex-abusers of these substances. Although there are many potential causes for relapse, we postulate that one cause in some people may be re-exposure in an anesthesia setting to a drug similar to the formerly abused drug. We hypothesize, for example, that opioids given during and after surgery may reinstate craving for, and initiate subsequent seeking of, heroin in former abusers. There are a substantial number of animal studies and some human studies documenting a reinstatement phenomenon in which an experimenter-administered psychoactive drug can precipitate drug-seeking behavior in 'abstinent' animals and humans. There is concern amongst health professionals and patients alike on this issue, and we discuss possible avenues of research, both preclinical and clinical, to explore the validity of our hypothesis.

A role for corticotropin releasing factor and urocortin in behavioral responses to stressors

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides have an important role in the central nervous system to mediate behavioral responses to stressors. CRF receptor antagonists are very effective in reversing stress-induced suppression and activation in behavior. An additional CRF-like neuropeptide, urocortin, has been identified in the brain and has a high affinity for the CRF-2 receptor in addition to the CRF-1 receptor. Urocortin has many of the effects of CRF but also is significantly more potent than CRF in decreasing feeding in both meal-deprived and free-feeding rats. In mouse genetic models, mice over-expressing CRF show anxiogenic-like responses compared to wild-type mice, and mice lacking the CRF-1 receptor showed an anxiolytic-like behavioral profile compared to wild-type mice. Results to date have led to the hypothesis that CRF-1 receptors may mediate CRF-like neuropeptide effects on behavioral responses to stressors, but CRF-2 receptors may mediate the suppression of feeding produced by CRF-like neuropeptides. Brain sites for the behavioral effects of CRF include the locus coeruleus (LC), paraventricular nucleus (PVN) of the hypothalamus, the bed nucleus of the stria terminalis (BNST), and the central nucleus of the amygdala. CRF may also be activated during acute withdrawal from all major drugs of abuse, and recent data suggest that CRF may contribute to the dependence and vulnerability to relapse associated with chronic administration of drugs of abuse. These data suggest that CRF systems in the brain have a unique role in mediating behavioral responses to diverse stressors. These systems may be particularly important in situations were an organism must mobilize not only the pituitary adrenal system, but also the central nervous system in response to environmental challenge. Clearly, dysfunction in such a fundamental brain-activating system may be the key to a variety of pathophysiological conditions involving abnormal responses to stressors such as anxiety disorders, affective disorders, and anorexia nervosa.

Cocaine potentiates defensive behaviors related to fear and anxiety

Cocaine use has been associated with a number of psychiatric disturbances, and an emerging literature attests to its ability to enhance anxiety-like behaviors in animal models. Ethoexperimental analyses of defensive behaviors, and tests designed specifically to provide individual measures of these behaviors, have been shown to respond very selectively and appropriately to anxiolytic and panicogenic or panicolytic drugs, suggesting that these tests, and this approach, might provide a more detailed and comprehensive description of the emotionality effects of cocaine than is currently available. In a Mouse Defense Test Battery (MDTB) using mouse subjects and an anesthetized rat as the threat stimulus, cocaine consistently enhanced flight and escape, with effects seen at 10-30 mg/kg (i.p.) dose levels. The effect was so potent that a lack of cocaine effect on other behaviors may have been due to response competition, or to early distancing of cocaine-dosed subjects from the threat stimulus. In a Rat Runway Test (RRT) similar to the MDTB but with rat subjects, 4 mg/kg cocaine, i.v. produced an explosive, but well directed, flight response. Flight was still elevated, although of lesser magnitude than originally, 30 min. after the i.v. cocaine, and defensive threat/attack to the oncoming threat stimulus were also reliably increased. Cocaine enhancement of defense was also seen in tests of sniffing "stereotypy" in rats. Sniffing after 30 mg/kg cocaine, i.p. was found to be appropriately oriented toward the direction of incoming air flow, suggesting that it may be part of a defensive risk assessment pattern. In undosed rats, risk assessment is suppressed by the presence of high-magnitude threat stimuli such as a cat, and the same, durable, phenomenon was obtained after 30 mg/kg (i.p.) cocaine. Toy cat exposure initially suppressed sniffing in cocaine-dosed rats, but this suppression was removed and sniffing increased, over repeated dose/toy cat exposures. Crouching in the same animals over these testing regimes supported a "sniffing-suppression" interpretation of these changes and also provided data suggesting that cocaine may enhance crouching. These data, indicating that cocaine enhances a number of defensive behaviors--some more strikingly than others--have implications for the involvement of cocaine in defense-linked psychopathologies; and for the involvement of defense in both conditioning and "sensitization" phenomena associated with cocaine. These effects raise the issue of the relationship between the defense-enhancing and the reinforcing consequences of cocaine.

Encounters with aggressive conspecifics enhance the locomotor-activating effects of cocaine in the rat

Evidence suggests that stress enhances the behavioural actions of cocaine in the rat. Paradoxically, however, encounters with aggressive conspecifics lead to a pattern of cocaine self-administration indicative of a reduced functional impact of the drug. Hence, we examined the effects of aggressive encounters on another behavioural measure-locomotor activity. Encounters between Lister Hooded rats and rats of the aggressive Tryon Maze Dull strain significantly enhanced the locomotor-activating effects of cocaine (20 mg/kg) in the Lister Hooded rats. The results suggest that the discrepant findings derived from self-administration studies are a property of the paradigm rather than a property of the stressor.

Pharmacological and environmental determinants of relapse to cocaine-seeking behavior

Animal models have been developed that simulate relevant features of relapse to cocaine-seeking behavior in humans. These models have provided valuable information about pharmacological and environmental factors that precipitate reinstatement of extinguished cocaine-seeking in rats and monkeys, as well as new insights about potential pharmacotherapies for relapse prevention. Reinstatement of cocaine-seeking behavior in animals can be induced by cocaine priming or by cocaine-paired environmental stimuli: however, maximum reinstatement of drug-seeking appears to be induced when cocaine priming and cocaine-paired stimuli are combined. Drugs that share cocaine's indirect dopamine agonist properties or that act as direct agonists at D2-like dopamine receptors also induce reinstatement of cocaine-seeking behavior, whereas with some exceptions (e.g., caffeine, morphine) drugs from other pharmacological classes do not. D1-like receptor agonists block rather than mimic the priming effects of cocaine, suggesting different roles for D1- and D2-like receptor mechanisms in cocaine relapse. Although considerable overlap exists, drugs that exhibit cocaine-like discriminative stimulus and/ or reinforcing effects in other situations do not invariably induce cocaine-like reinstatement of drug-seeking and vice versa, implying that these effects are not simply different behavioral expressions of a unitary neurobiological process. Finally, recent findings with D1-like receptor agonists, partial agonists, and antagonists suggest that some of these drugs may be viable candidates for development as antirelapse pharmacotherapies.

A role for nucleus accumbens glutamate transmission in the relapse to cocaine-seeking behavior

This study investigated the effect of ionotropic glutamate receptor agonist or antagonist administration into the nucleus accumbens on the maintenance of cocaine self-administration and the reinstatement of cocaine-seeking behavior. The stimulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid or N-methyl-D-aspartate glutamate receptors in the nucleus accumbens with either alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid or 1-aminocyclobutane-cis-1,3-dicarboxylic acid, respectively, decreased the number of cocaine-reinforced responses, suggesting an enhancement in the rewarding properties of cocaine. In contrast, blockade of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors with N-methyl-D-aspartate, or N-methyl-D-aspartate receptors with dizocilpine maleate or 2-amino-5-phosphonovaleric acid had no selective effect on the maintenance of cocaine self-administration. Following one week of extinction from the reinforcing cue of the drug-paired lever, both alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid and 1-aminocyclobutane-cis-1,3-dicarboxylic acid treatment in the nucleus accumbens reinstated cocaine-seeking behavior. However, alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid treatment increased responding only on the drug-paired lever, while 1-aminocyclobutane-cis-1,3-dicarboxylic acid increased responding on both the drug-paired and non-drug-paired levers. These results suggest that stimulation of glutamate receptors in the nucleus accumbens augments the reinforcing effect of cocaine, yet glutamate transmission is not required to maintain cocaine self-administration. In addition, increased glutamate transmission in the nucleus accumbens may be involved in facilitating the relapse to cocaine-seeking behavior.

Ketoconazole does not block cocaine discrimination or the cocaine-induced reinstatement of cocaine-seeking behavior

Ketoconazole is an FDA-approved antifungal agent that also blocks the synthesis of adrenocorticosteroids and functions as a glucocorticoid receptor antagonist. It has been previously demonstrated that this drug blocks the stress-induced reinstatement of cocaine-seeking behavior and reduces low-dose cocaine self-administration in rats. In the present experiments, the effects of ketoconazole on the cocaine-induced reinstatement of extinguished cocaine-seeking behavior and on cocaine discrimination were investigated in male Wistar rats. In rats trained to self-administer cocaine (0.5 mg/kg/infusion) by pressing a lever under a fixed-ratio 4 schedule of reinforcement, cocaine (5-20 mg/kg, IP) dose dependently reinstated cocaine-seeking behavior following at least 10 days of extinction, during which responding on the cocaine lever resulted in no programmed consequences. Ketoconazole (50 mg/kg, IP) failed to block cocaine-induced reinstatement despite blocking cocaine-induced increases in plasma corticosterone. Ketoconazole (25 or 50 mg/kg) also failed to block cocaine discrimination in rats trained to discriminate 10 mg/kg cocaine from saline. In these rats, generalization to the training dose of cocaine was observed in the absence of increases in plasma corticosterone. The results of these experiments indicate that corticosterone may mediate the effects of stressors on cocaine-seeking behavior but not the direct effects of cocaine itself.

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

We showed previously that brief footshock stress and priming injections of heroin reinstate heroin-seeking after prolonged drug-free periods. Here, we examined whether the adrenal hormone, corticosterone, and brain corticotropin-releasing factor (CRF) were involved in such reinstatement. We tested the effects of adrenalectomy, chronic exposure to the corticosterone synthesis inhibitor metyrapone (100 mg/kg, s.c., twice daily), acute exposure to metyrapone, acute intracerebroventricular injections of CRF (0.3 and 1.0 microgram), and intracerebroventricular injections of the CRF antagonist alpha-helical CRF (3 and 10 micrograms). Rats were trained to self-administer heroin (100 micrograms/kg/infusion, i.v.) for 12-14 d. Extinction sessions were given for 4-8 d (saline substituted for heroin). Tests for reinstatement were given after priming injections of saline and of heroin (0.25 mg/kg, s.c.), and after intermittent footshock (15 or 30 min, 0.5 mA). Adrenalectomy (performed after training) did not affect reinstatement by heroin but appeared to potentiate the reinstatement by footshock. Chronic exposure to metyrapone (from the beginning of extinction) or an acute injection of metyrapone (3 hr before testing) did not alter the reinstatement of heroin-seeking induced by footshock or heroin. Acute exposure to metyrapone alone potently reinstated heroin-seeking. In addition, acute exposure to CRF reinstated heroin-seeking, and the CRF antagonist alpha-helical CRF attenuated stress-induced relapse. The effect of the CRF antagonist on reinstatement by heroin was less consistent. These results suggest that CRF, a major brain peptide involved in stress, contributes to relapse to heroin-seeking induced by stressors.

Drug-induced reinstatement of heroin- and cocaine-seeking behaviour following long-term extinction is associated with expression of behavioural sensitization

The present study was designed to evaluate the relationship between reinstatement of drug-seeking behaviour following long-term extinction of intravenous (i.v.) drug self-administration (an animal model for craving) and long-term behavioural sensitization. Rats were allowed to self-administer heroin (50 microg/kg per inj., 14 daily sessions), cocaine (500 microg/kg per inj., 10 daily sessions) or saline. Following a 3-week extinction period, reinstatement tests were performed to evaluate priming effects of amphetamine, cocaine and heroin on nonreinforced drug-seeking behaviour. In addition, the occurrence of long-term behavioural sensitization in rats with a history of heroin or cocaine self-administration was determined. Heroin-seeking behaviour was reinstated by heroin (0.25 mg/kg), amphetamine (1.0 mg/kg) and cocaine (10 mg/kg). In addition, animals with a history of heroin self-administration displayed locomotor sensitization to both heroin and amphetamine. Cocaine-seeking behaviour was reinstated by cocaine and amphetamine, but not by heroin. Interestingly, locomotor sensitization to amphetamine, but not heroin, was observed in animals with a history of cocaine self-administration. In other words, the induction of drug-seeking behaviour following a prolonged drug-free period was found to be associated with the expression of long-term behavioural sensitization. These data provide experimental evidence for a role of behavioural sensitization in the incentive motivation underlying drug-seeking behaviour. If drug hyperresponsiveness would indeed be a crucial factor in drug-induced craving in human addicts, pharmacological readjustment of the neuroadaptations underlying drug sensitization may prevent relapse to drug use long after detoxification.

Neural substrates of drug craving and relapse in drug addiction

Drug addiction is characterized by motivational disturbances such as compulsive drug taking and episodes of intense drug craving. Recent advances using animal models of relapse have shown that drug-seeking behaviour can be triggered by drug-associated cues, by stress and by 'priming' injections of the drugs themselves, events also known to trigger drug craving in human drug addicts. Current evidence suggests that these stimuli all induce relapse, at least in part, by their common ability to activate the mesolimbic dopamine system. Drug-associated cues and stress can activate this system via neural circuits from the prefrontal cortex and amygdala and through activation of the hypothalamic-pituitary-adrenal axis. Our studies suggest that dopamine triggers relapse to drug-seeking behaviour by stimulating D2-dopamine receptors which inhibit the cyclic AMP second messenger pathway in the neurones of the nucleus accumbens. In contrast, compounds which activate D1 receptors prevent relapse to drug-seeking behaviour, possibly through satiation of reward pathways. Chronic neuroadaptations in dopamine receptor signalling pathways in the nucleus accumbens caused by repeated drug use are hypothesized to produce tolerance to the rewarding effects of D1-receptor stimulation, leading to increased drug intake during drug self-administration. Conversely, these same neuroadaptations are hypothesized to enhance drug craving by potentiating D2 receptor-mediated signals during abstinence. These findings identify D1 and D2-dopamine receptor mechanisms as potential targets for developing anticraving compounds to treat drug addiction.

The role of corticotropin-releasing factor and corticosterone in stress- and cocaine-induced relapse to cocaine seeking in rats

We have shown previously that footshock stress and priming injections of cocaine reinstate cocaine seeking in rats after prolonged drug-free periods (Erb et al., 1996). Here we examined the role of brain corticotropin-releasing factor (CRF) and the adrenal hormone corticosterone in stress- and cocaine-induced reinstatement of cocaine seeking in rats. The ability of footshock stress and priming injections of cocaine to induce relapse to cocaine seeking was studied after intracerebroventricular infusions of the CRF receptor antagonist D-Phe CRF12-41, after adrenalectomy, and after adrenalectomy with corticosterone replacement. Rats were allowed to self-administer cocaine (1.0 mg/kg/infusion, i.v) for 3 hr daily for 10-14 d and were then placed on an extinction schedule during which saline was substituted for cocaine. Tests for reinstatement were given after intermittent footshock (10 min; 0.5 mA) and after priming injections of saline and cocaine (20 mg/kg, i.p.). Footshock reinstated cocaine seeking in both intact animals and animals with corticosterone replacement but not in adrenalectomized animals. The CRF receptor antagonist D-Phe CRF12-41 blocked footshock-induced reinstatement at all doses tested in both intact animals and animals with corticosterone replacement. Reinstatement by priming injections of cocaine was only minimally attenuated by adrenalectomy and by pretreatment with D-Phe CRF12-41. These data suggest that brain CRF plays a critical role in stress-induced, but only a modulatory role in cocaine-induced, reinstatement of cocaine seeking. Furthermore, the data show that although reinstatement of cocaine seeking by footshock stress requires minimal, basal, levels of corticosterone, stress-induced increases in corticosterone do not play a role in this effect.

Relapse to drug-seeking: neural and molecular mechanisms

A central determinant of addictive disorders in people is increased risk of relapse to drug use even after prolonged periods of abstinence. Recent advances in animal models of relapse indicate that drug-seeking behavior can be triggered by priming injections of the drugs themselves, by drug-associated environmental stimuli, and by footshock stress. The neural mechanisms underlying this relapse can be viewed in general terms as drug-like or proponent processes. Considerable evidence points to the mesolimbic dopamine system, and more specifically to activation of D2-like dopamine receptors in the nucleus accumbens, as a crucial neural substrate utilized by various stimuli that induce relapse. Drug-associated stimuli and stress may activate this system via neural circuits from the prefrontal cortex and amygdala as well as via the hypothalamo-pituitary-adrenal axis. There is also evidence for dopamine-independent mechanisms in relapse as well. A major effort of current research is to identify the long-lasting neuroadaptations within these various brain regions that contribute to relapse in addicted people. One potential neuroadaptation is up-regulation of the cAMP pathway in the nucleus accumbens, which occurs after chronic drug exposure, and represents a drug-opposite or opponent process. Modulation of this system has been related directly to relapse to drug-seeking behavior. Given the long-lasting nature of increased risk of relapse, it is likely that the relevant neuroadaptations are mediated via drug-induced changes in gene expression. A detailed understanding of the neural and molecular basis of relapse will facilitate efforts to develop truly effective treatments and preventive measures.

The role of stress in drug self-administration

Environmental experiences have an important effect on the sensitivity of an individual to drugs of abuse. Studies of drug self-administration in laboratory animals have shown that both physical and psychological stressors facilitate the acquisition of drug self-administration, probably by increasing the reinforcing efficacy of drugs of abuse. Stressors also facilitate the reinstatement of drug taking even after prolonged periods of withdrawal. The adrenal hormones, glucocorticoids, which increase the sensitivity of mesencephalic dopaminergic neurones to drugs, seem to be one of the biological substrates of the effects of stress on the propensity to develop drug intake. In this review, Pier Vincenzo Piazza and Michel Le Moal discuss theories of drug abuse, the influence of different stressful experiences on drug self-administration and their possible mechanisms of action.