Reinstatement of cocaine seeking in 129X1/SvJ mice: effects of cocaine priming,cocaine cues and food deprivation

RATIONALE AND OBJECTIVES: The mechanisms underlying relapse to cocaine seeking induced by exposure to priming cocaine injections, cues associated with cocaine self-administration and environmental stressors have been studied in rats. Here we describe a reinstatement method for studying relapse to cocaine seeking in mice, a suitable species for studying the effect of genetic manipulations such as gene knockout or gene over-expression on compulsive drug use.METHODS: Male mice of the 129X1/SvJ strain were trained for 14-16 days to self-administer cocaine (0.75 mg/kg/infusion; 4 h/day; fixed-ratio-1 schedule of reinforcement; infusions were paired with a light-tone compound cue). Next, the lever-pressing behavior was extinguished by removing the cocaine syringes in the presence (Exps. 1 and 3) or absence (Exp. 2) of the cocaine cue. Subsequently, tests for reinstatement were conducted after exposure to priming injections of cocaine (0, 1.5, 3.0 and 6.0 mg/kg, IV; Exp. 1), response-contingent presentations of the cocaine-associated cue (Exp. 2), or food deprivation stress (1 and 22 h; Exp. 3).RESULTS: The effect of cocaine priming on reinstatement was modest and was only observed at the highest dose tested. On the other hand, reinstatement of cocaine seeking was observed following exposure to the cocaine-associated cue and food deprivation stress.CONCLUSIONS: The present data suggest that factors contributing to relapse to drugs can be studied in the reinstatement model using the common 129X1/SvJ mouse inbred strain.     

The role of corticotrophin-releasing factor in stress-induced relapse to alcohol-seeking behavior in rats

RATIONALE: Intermittent footshock stress reliably reinstates extinguished alcohol-taking behavior in drug-free rats, but the neurochemical events involved in this effect are not known. OBJECTIVE: We studied here whether two main modulators of stress responses, corticotropin-releasing factor (CRF) and corticosterone, are involved in reinstatement of alcohol seeking induced by the intermittent footshock stressor. METHDOS: Rats were given alcohol in a two-bottle choice procedure (water versus alcohol) for 30 days and were then trained for 60 min per day to press a lever for alcohol (12% w/v) for 24-30 days in operant conditioning chambers. After stable drug-taking behavior was obtained, lever pressing for alcohol was extinguished by terminating drug delivery for 5-8 days. Reinstatement of alcohol seeking was then determined after exposure to intermittent footshock (0.8 mA; 10 min) in different groups of rats that were pretreated with CRF receptor antagonists or underwent adrenalectomy (ADX) to remove endogenous corticosterone from the body. RESULTS: The CRF receptor antagonists, d-phe-CRF (0.3 or 1.0 microg; ICV) and CP-154,526 (15, 30 or 45 mg/kg; IP) attenuated footshock-induced reinstatement of alcohol seeking in a dose dependent manner. In contrast, the removal of circulating corticosterone by ADX had no effect on footshock stress-induced reinstatement of alcohol-taking behavior. In addition, the prevention of the footshock-induced rise in corticosterone while maintaining basal levels of the hormone by providing adrenalectomized rats with corticosterone pellets (50 mg/kg per day), had no effect on stress-induced reinstatement. CONCLUSIONS: These data suggest that CRF contributes to stress-induced relapse to alcohol seeking via its actions on extra-hypothalamic sites. The present data, and previous data with heroin- and cocaine-trained rats, point to a general role of CRF in relapse to drugs induced by stressors.     

Stress reinstates nicotine seeking but not sucrose solution seeking in rats

  Rationale: Intermittent footshock stress effectively reinstates extinguished heroin-, cocaine- and alcohol-taking behaviors, but not behaviors previously maintained by food reinforcers. Here we tested further the generality of the phenomenon of stress-induced reinstatement by determining the effect of footshock on reinstatement of operant responding previously maintained by nicotine or palatable sucrose solutions. Methods: Groups of rats were trained to self-administer either nicotine (0.03 mg/kg per infusion, 14 days) or sucrose (10 or 30% w/v, 14–20 days). After extinction of the nicotine- or the sucrose-reinforced behaviors for 5–15 days, the rats were exposed to intermittent footshock stress (5 and 15 min, 0.8 mA) during tests for reinstatement. Results: Footshock reliably reinstated nicotine seeking after extinction of the drug-reinforced behavior. In contrast, the same parameters of footshock stress did not consistently reinstate operant responding previously maintained by sucrose solutions. Conclusions: These and previous data suggest that stressors may be more effective stimuli for reinstatement of behaviors previously maintained by drug reinforcers as compared with non-drug reinforcers. Received: 15 October 1998 / Final version: 10 December 1998     

Reinstatement and spontaneous recovery of nicotine seeking in rats

 Reinstatement and spontaneous recovery of previously extinguished nicotine-taking behavior were examined in rats. Male subjects were trained to self-administer nicotine (30 μg/kg per infusion, IV; one 60-min session per day for 3 weeks). Extinction sessions were then given for 5–10 days during which saline was substituted for nicotine. Subsequently, in the first set of tests for nicotine seeking, the reinstatement of lever presses that previously delivered nicotine was examined after priming injections of saline and nicotine (75, 150 and 300 μg/kg, SC; and 30 and 60 μg/kg, IV). In the second set of tests for nicotine-seeking, rats were tested after an additional 21-day drug-free period during which they were not exposed to the self-administration chambers (a test for the spontaneous recovery of drug seeking), and after priming injections of nicotine (150 and 300 μg/kg, SC). Reinstatement of extinguished food-reinforced behavior after exposure to nicotine was also determined. Priming injections of nicotine reinstated nicotine seeking regardless of the route of administration. In addition, previously extinguished nicotine seeking recovered spontaneously after a 21-day period during which rats were not exposed to the drug-taking environment. Nicotine also reinstated extinguished food-reinforced behavior in rats with a history of nicotine self-administration, but not in drug-naive rats. The present results extend previous work with opioid and stimulant drugs on reinstatement of drug seeking by the self-administered drug. It also appears that, as with other positive reinforcers, the mere passage of time is a sufficient condition for the spontaneous recovery of extinguished nicotine seeking. Received: 15 November 1996 / Final version: 10 January 1997     

Effects of unconditioned and conditioned social defeat on alcohol self-administration and reinstatement of alcohol seeking in rats

Rationale and objectives We and others have shown that a stressor commonly used in laboratory studies, intermittent footshock, reinstates alcohol seeking in a rat relapse model. The effects of more ethologically relevant stressors on reinstatement have not been examined. Here, we characterized the effects of social defeat (a naturalistic stressor) or a cue associated with the defeat experience on reinstatement of alcohol seeking. We also examined the effect of unconditioned and conditioned social defeat on alcohol self-administration.Methods Rats were trained to self-administer alcohol (12% w/v, 1 h day⁻¹), and after stable responding, one group of animals received five exposures to social defeat paired with peppermint odor prior to daily self-administration sessions. After three more self-administration sessions, these rats were tested for the effects of the peppermint odor cue on self-administration. In another group of rats, the effects of three daily exposures to social defeat paired with peppermint odor on extinction of responding were examined. After further extinction sessions, the effect of the odor cue on reinstatement was tested in these animals. The acute effect of social defeat on reinstatement was examined in another group of animals.Results Acute exposure to social defeat decreased alcohol self-administration, reduced rates of responding during extinction, and did not reinstate alcohol seeking. Exposure to a discrete odor cue previously paired with social defeat decreased alcohol self-administration but induced modest reinstatement of alcohol seeking.Conclusions Results provide the first demonstration of reinstatement of alcohol seeking by a cue paired with social defeat and are also in agreement with previous findings on the suppressive effect of social defeat stress on alcohol self-administration.     

Role of alpha-2 adrenoceptors in stress-induced reinstatement of alcohol seeking and alcohol self-administration in rats

Rationale and objectives Alpha-2 adrenoceptors are known to be involved in stress-induced reinstatement of heroin and cocaine seeking in laboratory animals. Here, we studied the involvement of these receptors in stress-induced reinstatement of alcohol seeking by using an agonist (lofexidine) and an antagonist (yohimbine) of these receptors, which inhibit and activate, respectively, noradrenaline transmission. We also tested the effect of lofexidine and yohimbine on alcohol self-administration. Lofexidine is used clinically for treating opiate withdrawal symptoms and yohimbine induces stress-like responses in humans and non-humans.Methods Rats were trained to self-administer alcohol (12% w/v, 1 h/day) and after extinction of the alcohol-reinforced behavior, they were tested for the effect of lofexidine (0, 0.05 and 0.1 mg/kg, IP) on reinstatement of alcohol seeking induced by intermittent footshock stress (10 min, 0.8 mA) or for the effect of yohimbine (0, 1.25 and 2.5 mg/kg, IP) on reinstatement of alcohol seeking. Other rats were trained to self-administer alcohol, and after stable responding, the effects of lofexidine and yohimbine on alcohol self-administration were determined.Results Pretreatment with lofexidine (0.05 mg/kg and 0.1 mg/kg) attenuated stress-induced reinstatement of alcohol seeking and also decreased alcohol self-administration. In contrast, yohimbine pretreatment potently reinstated alcohol seeking after extinction and also induced a profound increase in alcohol self-administration.Conclusions Results indicate that activation of alpha-2 adrencoceptors is involved in both stress-induced reinstatement of alcohol seeking and alcohol self-administration. To the degree that the present results are relevant to human alcoholism, alpha-2 adrencoceptor agonists should be considered in the treatment of alcohol dependence.     

Stress and relapse to drug seeking in rats: studies on the generality of the effect

RATIONALE: Intermittent footshock reinstates drug-taking behavior in rats, but not behaviors previously maintained by food reinforcers. Here we tested further the generality of this phenomenon by determining whether restraint and food deprivation stressors would reinstate heroin seeking, whether the environment in which footshock is given modulates footshock-induced reinstatement, and whether footshock would reinstate operant responding previously maintained by brain stimulation reward (BSR). METHODS: Groups of rats were trained to self-administer for 10 days either heroin (0.05-0.1 mg/kg/infusion, IV, three 3-h sessions/day) or brain stimulation into the septal area (trains of monopolar cathodal pulses of 100 micros for 500 ms, one 60-min session/day). After extinction of the heroin-reinforced behavior (10-13 days), the rats were tested for reinstatement after exposure to food deprivation (1 and 21 h), restraint given outside the self-administration environment (5, 15 and 30 min), or intermittent footshock (0.8 mA, 15 min) given in the self-administration environment or in a novel (non-drug) environment. For BSR-trained rats, the effect of footshock on reinstatement after extinction (6-10 days) was compared with that induced by noncontingent brain stimulation (three or six discrete stimulations at the start of the test sessions). RESULTS: Food deprivation reinstated heroin seeking. Footshock reliably reinstated heroin seeking when given in the drug environment, but not when given in a non-drug environment. Similarly, restraint given outside the self-administration environment failed to reinstate heroin seeking. In addition, footshock was as effective as priming brain stimulation in reinstating operant responding previously maintained by BSR. CONCLUSIONS: The effect of footshock on reinstatement of heroin seeking generalizes to food deprivation, and appears to be dependent on the environment in which the stressor is given. The data with BSR indicate that the phenomenon of footshock-induced reinstatement is not selective for drug reinforcers.     

Cocaine-seeking produced by experimenter-administered drug injections: dose-effect relationships in rats

Rationale: Relapse to drug taking is a major obstacle to the effective treatment of cocaine abuse. Animal studies have determined that various drugs are able to reinstate extinguished drug-taking behavior. Objectives: This study was designed to determine whether there is specificity in the ability of drugs to lead to cocaine-seeking and to compare potency and efficacy of a variety of drug primes. Another objective was to compare the effect of drugs with a primary dopaminergic mechanism with those having a secondary effect on dopaminergic substrates. Methods: Following acquisition of cocaine self-administration, the ability of injections of cocaine (5.0–20.0 mg/kg), amphetamine (0.30–3.0 mg/kg), methylphenidate (2.0–20.0 mg/kg), nicotine (0.0375–0.60 mg/kg), caffeine (1.25–20.0 mg/kg), morphine (0.10–10.0 mg/kg) or ^Δ9THC (0.3–3.0 mg/kg) to reinstate extinguished drug taking was measured. Tests were conducted in a single day and were comprised of three phases. The first phase consisted of a 60-min period of cocaine self-administration. During phase 2, the cocaine solution was replaced with saline and responding was extinguished during the next 3-h period. During phase 3, in which saline again was the only solution available for self-administration, responding was monitored for 3–8 h following an injection of a drug prime. Results: Reinstatement was produced by experimenter-administered injections of cocaine, amphetamine, methylphenidate and caffeine but not nicotine, morphine or ^Δ9THC. The potency and efficacy of cocaine, methylphenidate and caffeine were comparable, whereas amphetamine was more potent and efficacious. Cocaine seeking occurred primarily during the first hour following the injection. Conclusions: These findings suggest that cocaine seeking is only produced following administration of specific drugs. It is suggested that effective drug primes are those that produce a discriminative stimulus that generalizes to the stimulus produced by the reinforcing effects of cocaine. Received: 13 February 1999 / Final version: 25 June 1999     

Effects of opioid and dopamine receptor antagonists on relapse induced by stress and re-exposure to heroin in rats

The effects of blockade of opioid and dopamine receptors on relapse to heroin-seeking induced by footshock stress and re-exposure to heroin were examined in a reinstatement procedure. Male rats were trained to self-administer heroin (100 µg/kg per infusion, IV; four 3-h sessions/day for 8–11 consecutive days). Extinction sessions were given for 5–7 days during which saline was substituted for heroin. In nine groups, the effects on relapse induced by footshock (10 min, 0.5 mA, 0.5 s on with a mean off period of 40 s), heroin priming (0.25 mg/kg), and saline priming were studied after pretreatment with either naltrexone (1 or 10 mg/kg, SC), the D₁-like receptor antagonist SCH 23390 (0.05 or 0.1 mg/kg, IP), the D₂-like receptor antagonist raclopride (0.25 or 0.5 mg/kg, IP), the mixed dopamine antagonist flupenthixol decanoate (3 or 6 mg/kg, IM), or IP injection of saline (control condition). Naltrexone, flupenthixol, raclopride, and the highest dose of SCH 23390 attenuated heroin-induced relapse: only the mixed DA receptor antagonist, flupenthixol, attenuated footshock-induced relapse. These results, and those from microdialysis showing that heroin elicits greater locomotor activity and DA release in the nucleus accumbens than footshock, suggest that the neurochemical events underlying stress- and heroin-induced relapse are not identical.     

Baclofen attenuates cue-induced reinstatement of alcohol-seeking behavior in Sardinian alcohol-preferring (sP) rats

The GABA_B receptor agonist, baclofen, suppressed alcohol deprivation effect (a proposed experimental model of alcohol relapse) in Sardinian alcohol-preferring rats. The present study was designed to extend the characterization of the “anti-relapse” properties of baclofen to the reinstatement of alcohol-seeking behavior (another proposed model of alcohol relapse). Rats of the sP line were first trained to lever press for alcohol under a fixed ratio 4 schedule of reinforcement. Subsequently, rats were exposed to two within-session 70-min extinction/reinstatement tests with saline or baclofen administered in a counterbalanced, within-subject design. After a 60-min extinction phase, an alcohol-associated stimulus complex was presented (reinstatement phase). Saline or baclofen (3 mg/kg) were administered via a permanent intraperitoneal catheter, 30 min before the reinstatement phase. During the reinstatement phase, baclofen administration: (a) reduced by approximately 60% responses on the previously active lever, (b) increased latency to the first response and (c) decreased the response rate. These results indicate that baclofen reduced cue-induced reinstatement of alcohol-seeking behavior in sP rats.     

Effects of opiate antagonist treatment on the alcohol deprivation effect in long-term ethanol-experienced rats

Rationale: Opiate antagonists are promising pharmacotherapeutic agents for the treatment of alcohol dependence, reducing craving and relapse rates in weaned alcoholics. However, preclinical findings indicate that they can also increase ethanol consumption and preference in animals with a strong liking for ethanol, depending on the dose and treatment regimen. Objective: The present study examined the effects of chronic, intermittent and acute opiate antagonist treatment on the alcohol deprivation effect (ADE) in long-term ethanol- experienced rats, which is an animal model of craving and relapse. Methods: Long-term ethanol-experienced rats were either implanted with mini-osmotic pumps delivering 0, 0.5 or 1 mg/kg per hour naloxone (chronic treatment) or received intermittent naltrexone injections (2×5 mg/kg per day SC). Effects of chronic and intermittent treatment on the ADE were studied in a four-bottle home cage drinking paradigm. In a second experiment, long-term ethanol-experienced rats trained in an operant ethanol self-administration paradigm received acute naltrexone treatment (0, 0.1, 1 or 10 mg/kg SC) before a 23-h session either during basal drinking or during the ADE. Results: Chronic naloxone treatment increased ethanol preference during the ADE. Intermittent naltrexone treatment at a dose comparable to the lower dose of chronic treatment moderately attenuated the ADE. Acute naltrexone treatment selectively reduced lever pressing for ethanol both during the ADE and during basal drinking only at the lowest dose, whereas higher doses also suppressed water intake. The ethanol-specific suppressant effect on the ADE was long lasting. Concerning basal drinking, however, naltrexone had a long lasting reductive effect only on lever pressing for water. Conclusions: A low dose of naltrexone and an intermittent treatment regimen seem to be necessary to maintain a specific reduction in ethanol intake in individuals with a high motivation to consume ethanol. These findings are consistent with the notion that, at low doses, opiate antagonists reduce the reward value of reinforcers. Received: 29 September 1998 / Final version: 15 March 1999     

Nicotine increases alcohol self-administration and reinstates alcohol seeking in rats

Rationale and objective Alcohol and tobacco are often co-abused in humans and previous studies found that nicotine increases alcohol consumption in rats. Here, we studied whether nicotine would reinstate alcohol-taking behavior in drug-free rats and whether this effect would be enhanced by prior exposure to nicotine during alcohol self-administration training. Methods Rats were trained to press a lever for alcohol (12% w/v, 1 h/day), and following stable alcohol intake groups of rats (n=11–12) were given daily vehicle or nicotine (0.2, 0.4 or 0.8 mg/kg, SC) injections just prior to the self-administration sessions for 10 days. Rats were then given 6 days of alcohol self-administration in the absence of nicotine and an additional 5–10 drug-free days during which lever presses were not reinforced (extinction). Subsequently, rats were tested for reinstatement of alcohol seeking following exposure to priming injections of vehicle or nicotine (0.4 mg/kg, SC). Results Nicotine increased alcohol self-administration in a dose- and time-dependent manner over the 10-day period. Nicotine also reinstated alcohol seeking after extinction of the alcohol-reinforced behavior, and this effect was strongly enhanced by prior nicotine exposure. Conclusions The present data extend previous studies on the effect of nicotine on alcohol self-administration, and further indicate that nicotine is an effective stimulus for reinstatement of alcohol seeking during drug-free periods.     

Reinstatement of drug-seeking behavior produced by heroin-predictive environmental stimuli

 The current study examined whether stimuli predictive of heroin availability were capable of inducing a relapse of drug-seeking behavior in an operant runway task. Olfactory stimuli (orange and almond food extract) served as discriminative cues about the availability (S+) or unavailability (S–) of heroin reinforcement (a single 0.1 mg/kg IV infusion) in the goal box of a straight arm runway. Following discrimination training, the running response was extinguished in the absence of the olfactory cues. On a single trial, the discriminative stimuli were then tested for their ability to reinstate running behavior prior to presentation of the heroin reinforcer. Subjects presented with the S+ on test day took significantly less time to traverse the alley than they did on the final day of extinction, while those subjects presented with the S– on test day continued to run slowly. These results demonstrate, in an animal model of drug self-administration, that environmental discriminative cues can produce a relapse in drug seeking behavior following a period of abstinence. The response-reinstating properties of the S+ odor were unaltered by pretreatment with any of three doses of haloperidol (0.0, 0.15 or 0.3 mg/kg IP), suggesting that the motivating properties of heroin-predictive stimuli or cues remain intact during dopamine receptor antagonism. Received: 22 October 1996 / Final version: 14 January 1997     

CP-154,526, a selective, non-peptide antagonist of the corticotropin-releasing factor1 receptor attenuates stress-induced relapse to drug seeking in cocaine- and heroin-trained rats

We have found that peptide antagonists of corticotropin-releasing factor (CRF) receptors attenuate reinstatement of heroin and cocaine seeking induced by footshock. Here we examined the effect of a non-peptide, selective CRF₁ receptor antagonist, CP-154,526, on reinstatement of heroin and cocaine seeking induced by footshock. Rats were trained to self-administer heroin or cocaine (0.1 and 1.0 mg/kg per infusion, IV, respectively) for 9–12 days. Extinction sessions were given for up to 14 days, during which saline was substituted for the drugs. Tests for reinstatement were then conducted after exposure to intermittent footshock (10 or 15 min, 0.5 mA). The footshock stressor reliably reinstated extinguished cocaine- and heroin-taking behavior. Pretreatment with CP-154,526 (15 and 30 mg/kg, SC) significantly attenuated the reinstatement effect of the stressor in both heroin- and cocaine-trained rats. CP-154,526, administered in the absence of the footshock stressor, did not affect extinguished drug seeking. In addition, in a separate experiment, CP-154,526 was shown not to alter high rates of lever pressing for a 10% sucrose solution, suggesting that the suppression of lever pressing in stress-induced reinstatement is not caused by a performance deficit. These results extend previous reports on the role of CRF in reinstatement of drug seeking induced by stressors. The present data also suggest that, to the extent that exposure to environmental stressors provoke relapse to drug use in humans, systemically effective CRF receptor antagonists may be of use in the treatment of relapse to drug use. Received: 12 July 1997/Final version: 20 October 1997     

Acamprosate suppresses the expression of morphine-induced sensitization in rats but does not affect heroin self-administration or relapse induced by heroin or stress

Acamprosate (calcium-acetyl homotaurinate) is a new compound used in the treatment of alcohol abuse. Because of the putative link between alcoholism and the endogenous opioid systems in both humans and laboratory animals, we tested in rats the effects of acamprosate on behavioral and neurochemical effects of opioid drugs related to their abuse potential. These included sensitization to the behavioral effects of morphine, morphine-induced dopamine (DA) release in the nucleus accumbens (NAS), intravenous (IV) heroin self-administration and relapse to heroin seeking in drug-free rats. In experiment 1, rats were injected daily with either morphine (10 mg/kg, SC) or saline for 14 days. Three days later in a test for the expression of sensitization, an injection of morphine (10 mg/kg) resulted in increased locomotor activity and enhanced DA release in the NAS in rats previously exposed to morphine. Acamprosate (two injections of 200 mg/kg; 12 h apart; IP) suppressed the expression of the sensitized responses, but did not alter the effects of morphine in drug-naive control rats. In experiment 2, it was found that acamprosate (two injections of 50–200 mg/ kg; IP) had no consistent effects on IV heroin self-administration (50–100 μg/kg per infusion) and, in experiment 3, that acamprosate (100–200 mg/ kg, IP) did not alter reinstatement of drug seeking induced by priming injections of heroin (0.25 mg/kg, SC) or a footshock stressor (15 min; 0.5 mA) after a 5- to 8-day period of extinction. Thus, although acamprosate attenuated the expression of sensitized locomotor activity and DA release in the NAS, it did not have any consistent effect on either the intake of heroin during the maintenance phase or the relapse to heroin seeking in a drug-free state. Thus, to the extent that the self-administration and the reinstatement procedures provide valid preclinical models for drug use and relapse in humans, our data suggest that acamprosate may not be effective in altering drug-taking behavior in heroin users. Received: 4 November 1997/Final version: 25 January 1998     

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

 We have shown previously, using an animal model of relapse, that acute exposure to intermittent footshock stress induces reinstatement of heroin-taking behavior in rats. Here we report that in rats trained to self-administer cocaine, exposure to acute intermittent footshock stress induces reinstatement of cocaine-taking behavior after prolonged extinction sessions and after a 4- to 6-week drug-free period; an effect comparable to that induced by a priming injection of cocaine. Animals were initially allowed to self-administer cocaine HCl (1.0 mg/kg per infusion, IV) during one 3-h session/day for 12 days. Subsequently, extinction conditions were introduced by substituting saline for cocaine so that lever-pressing resulted in IV infusions of saline rather than of drug. Extinction conditions were maintained until animals made 15 responses or less in the 3 h, after which animals were given saline infusions at the start of each daily session to establish baseline responding of ten responses or less. Subsequently, animals were tested for reinstatement of responding for saline infusions following a non-contingent injection of cocaine (2.0 mg/kg, IV) and exposure to intermittent footshock (10 min, 0.5 mA, 0.5 s on, mean off period of 40 sec). After an additional 4- to 6-week drug-free period, tests for reinstatement were repeated. Reinstatement of cocaine-taking behavior was observed in both sets of tests in response to footshock and cocaine. These results extend previous reports from this laboratory that footshock stress is an effective stimulus for reinstatement of drug-taking behavior in the rat. Received: 21 March 1996 / Final version: 13 July 1996     

Effects of baclofen on maintenance and reinstatement of intravenous cocaine self-administration in rats

  Rationale: Recent studies suggest that the GABA_B receptor agonist, baclofen, may be a useful pharmacotherapy for cocaine abuse. Objectives: To investigate further the effects of baclofen on maintenance and reinstatement of cocaine-reinforced behavior in rats. Methods: Two groups of rats were trained to self-administer IV cocaine (0.2 or 0.4 mg/kg per infusion) during daily 7-h sessions under a fixed-ratio 1 schedule. Rats were pretreated with baclofen (1.25, 2.5 or 5 mg/kg IP) or saline before the session for 5 consecutive days. An additional group of rats was trained to self-administer IV cocaine (0.4 mg/kg per infusion) during the first 2 h of daily 7-h sessions. Cocaine was replaced by saline for the remaining 5 h of the session. Once behavior had stabilized over the 7-h period, priming injections of saline (IV), cocaine (3.2 mg/kg IV) or baclofen (1.25 or 2.5 mg/kg IP) were administered prior to hour 4. Injections of baclofen (1.25 or 2.5 mg/kg IP) or saline were also given before priming injections of cocaine. Results: Pretreatment with the two higher doses of baclofen (2.5 and 5 mg/kg) decreased the number of cocaine infusions in both maintenance groups (0.2 and 0.4 mg/kg) over the 5-day treatment period. Baclofen had a greater suppressant effect on responding maintained by the lower dose of cocaine. Priming injections of baclofen (1.25 and 2.5 mg/kg) or saline did not reinstate responding. However, these same doses of baclofen dose-dependently reduced the reinstatement of responding produced by priming injections of cocaine. Conclusions: 1) The magnitude of the suppressant effects of baclofen on maintenance of cocaine self-administration depends upon the maintenance dose, 2) baclofen may be useful in preventing reinstatement of cocaine-seeking behavior, and 3) compared to maintenance, reinstatement of responding is more sensitive to the suppressant effects of baclofen. Received: 10 August 1998 / Final version: 31 October 1998     

The CRF<Subscript>1</Subscript> receptor antagonist antalarmin attenuates yohimbine-induced increases in operant alcohol self-administration and reinstatement of alcohol seeking in rats

Rationale and objectives Yohimbine is an alpha-2 adrenoreceptor antagonist that provokes stress- and anxiety-like responses in both humans and laboratory animals. In rats, yohimbine increases operant alcohol self-administration and reinstates alcohol seeking. In this study, we assess whether these effects of yohimbine are attenuated by systemic injections of the corticotrotropin-releasing factor 1 (CRF₁) receptor antagonist antalarmin. Materials and methods In Exp. 1, we trained rats to lever press for alcohol solutions (12% w/v, 1 h/day) over several weeks; during training, the response requirement was increased from a fixed-ratio-1 (FR-1) to a fixed-ratio-3 (FR-3) reinforcement schedule. We then tested the effect of antalarmin (10 or 20 mg/kg) on yohimbine (1.25 mg/kg)-induced increases in operant alcohol self-administration (FR-3 reinforcement schedule). Subsequently, we assessed the effect of antalarmin on yohimbine-induced increases in plasma corticosterone levels in the previously self-administering rats. In Exp. 2, we trained the rats to self-administer alcohol as in Exp. 1, and after extinction of the alcohol-reinforced lever responding over 13 days, we tested antalarmin’s effect on yohimbine-induced reinstatement of alcohol seeking. Results Yohimbine increased operant alcohol self-administration and reinstated alcohol seeking after extinction. These effects of yohimbine were attenuated by antalarmin. Antalarmin injections in the absence of yohimbine had no effect on either operant alcohol self-administration or extinction responding. Antalarmin had no effect on yohimbine-induced corticosterone release in alcohol-experienced rats. Conclusions These results suggest that extrahypothalamic CRF₁ receptors are involved in the effect of yohimbine on operant alcohol self-administration and on relapse to alcohol seeking and support the notion that CRF₁ receptor antagonists should be considered in alcohol addiction treatment.     

Ketoconazole blocks the stress-induced reinstatement of cocaine-seeking behavior in rats: relationship to the discriminative stimulus effects of cocaine

  Rationale: Ketoconazole (Keto) is an antifungal agent that also inhibits the synthesis of adrenocorticosteroids and has been reported to act as a glucocorticoid receptor antagonist. Objective: The present experiments investigated the effects of Keto on the stressor-induced reinstatement of extinguished cocaine-seeking behavior and on the generalization of a stressor-induced discriminative stimulus to cocaine in rats. Methods: In the first experiment, male Wistar rats were trained to self-administer cocaine (0.5 mg/kg per infusion, IV) under a fixed-ratio 4 schedule of reinforcement with a 90-s limited hold. Following ten consecutive extinction sessions, the effects of Keto (25 or 50 mg/kg, IP) or vehicle on the ability of EFS (electric footshock; 15 min) to reinstate extinguished cocaine-lever responding were investigated. In the second experiment, rats were trained to discriminate cocaine (10 mg/kg, IP) from saline using a two-lever, food-reinforced drug discrimination design. The effects of Keto (50 mg/kg, IP) or vehicle on the EFS-induced generalization to cocaine were determined. Results: EFS reinstated extinguished cocaine- but not food-reinforced responding. Keto (25 and 50 mg/kg, IP) blocked the EFS-induced reinstatement of cocaine-seeking behavior and significantly attenuated the plasma corticosterone response to EFS. These same doses of Keto failed to affect responding in rats trained to self-administer food pellets under an FR4 schedule of reinforcement. EFS also produced significant cocaine-appropriate responding in rats trained to discriminate the drug from saline. However, Keto (50 mg/kg) failed to block the EFS-induced generalization to cocaine. Conclusions: Overall, these data suggest that corticosterone contributes to the stressor-induced reinstatement of extinguished cocaine-seeking behavior. Received: 5 June 1998 / Final version: 7 October 1998     

Reinstatement of ethanol and sucrose seeking by the neurosteroid allopregnanolone in C57BL/6 mice

Rationale  Recent work in our laboratory documented that the “sipper” method of operant ethanol self-administration produced high ethanol intake and blood ethanol concentrations as well as the typical extinction “burst” in responding under nonreinforced conditions in male C57BL/6 mice. However, the neurochemical basis for reinstatement of responding following extinction has not been examined in mice with this model. Objectives  Based on findings that the GABAergic neurosteroid allopregnanolone (ALLO) significantly increased the consummatory phase of ethanol self-administration, the present study determined the effect of ALLO on the reinstatement of extinguished ethanol-seeking behavior and compared this effect to the reinstatement of responding for sucrose reward. Materials and methods  Separate groups of male C57BL/6 mice were trained to lever press for access to a 10% ethanol (10E) or a 5% sucrose (5S) solution. A single response requirement of 16 presses (RR16) on an active lever resulted in 30 min of continuous access to the 10E or 5S solution. After the animals responded on the RR16 schedule for 14 weeks, mice were exposed to 30 min extinction sessions where responding had no scheduled consequence. Once responding stabilized below the preextinction baseline, mice received an intraperitoneal injection of ALLO (0, 3.2, 5.6, 10, or 17 mg/kg) 15 min prior to the extinction session in a within-subjects design. Results  ALLO produced a dose-dependent increase in responding under nonreinforced conditions in both the 10E and 5S groups. Additional work documented the ability of a conditioned cue light or a compound cue (light+lever retraction) to reinstate nonreinforced responding on the previously active lever. Conclusions  These findings definitively show that conditioned cues and priming with ALLO are potent stimuli for reinstating both ethanol- and sucrose-seeking behavior in C57BL/6 mice.