Psychopharmacology

, Volume 195, Issue 3, pp 397–406

Acamprosate attenuates cocaine- and cue-induced reinstatement of cocaine-seeking behavior in rats

Authors

  • M. Scott Bowers
    • Ernest Gallo Clinic and Research Center, Department of NeurologyUniversity of California at San Francisco
  • Billy T. Chen
    • Ernest Gallo Clinic and Research Center, Department of NeurologyUniversity of California at San Francisco
  • Jonathan K. Chou
    • Ernest Gallo Clinic and Research Center, Department of NeurologyUniversity of California at San Francisco
  • Megan P. H. Osborne
    • Center for Drug and Alcohol Programs, Department of Psychiatry and Behavioral SciencesMedical University of South Carolina
  • Justin T. Gass
    • Center for Drug and Alcohol Programs, Department of Psychiatry and Behavioral SciencesMedical University of South Carolina
  • Ronald E. See
    • Center for Drug and Alcohol Programs, Department of Psychiatry and Behavioral SciencesMedical University of South Carolina
    • Department of NeurosciencesMedical University of South Carolina
  • Antonello Bonci
    • Ernest Gallo Clinic and Research Center, Department of NeurologyUniversity of California at San Francisco
    • Wheeler Center for the Neurobiology of AddictionUniversity of California at San Francisco
  • Patricia H. Janak
    • Ernest Gallo Clinic and Research Center, Department of NeurologyUniversity of California at San Francisco
    • Wheeler Center for the Neurobiology of AddictionUniversity of California at San Francisco
    • Center for Drug and Alcohol Programs, Department of Psychiatry and Behavioral SciencesMedical University of South Carolina
Original Investigation

DOI: 10.1007/s00213-007-0904-y

Cite this article as:
Bowers, M.S., Chen, B.T., Chou, J.K. et al. Psychopharmacology (2007) 195: 397. doi:10.1007/s00213-007-0904-y

Abstract

Rationale

Acamprosate (calcium acetylhomotaurinate) is a glutamatergic neuromodulator used for the treatment of alcoholism, but its potential efficacy in the treatment of psychostimulant addiction has not been explored.

Objectives

The purpose of this study was to assess the effects of acamprosate on cocaine-stimulated locomotor activity, cocaine self-administration, and cue- and cocaine-induced reinstatement of cocaine-seeking behavior.

Materials and methods

All experiments utilized once-daily treatment for 5 consecutive days. First, the effects of saline or acamprosate (100, 300, or 500 mg/kg intraperitoneally) on body weight were examined. On the last day of treatment, locomotor activity was assessed before and after drug treatment, after which all animals received an acute challenge of cocaine (10 mg/kg). Next, a separate group of rats were trained to intravenously (IV) self-administer cocaine (0.6 mg/kg per infusion), subjected to extinction procedures, and then tested for effects of acamprosate on cue- or cocaine-induced reinstatement. A third group of rats was trained to self-administer cocaine as described above and were treated with saline or acamprosate before daily IV self-administration sessions.

Results

Repeated administration of 500 mg/kg acamprosate but not lower doses produced reductions in both body weight and spontaneous locomotor activity, and thus this dose was not tested further. Acamprosate at 300 mg/kg but not 100 mg/kg attenuated both cocaine- and cue-induced reinstatement without altering baseline patterns of cocaine self-administration or cocaine-stimulated hyperlocomotion.

Conclusions

Acamprosate attenuates both drug- and cue-induced reinstatement of cocaine-seeking behavior, suggesting that this compound may serve as a potential treatment for preventing relapse in cocaine-addicted humans.

Keywords

CocaineSelf-administrationReinstatementRelapseCuesDrug primingAcamprosateGlutamateLocomotor activityBody weight

Introduction

Acamprosate prolongs abstinence and reduces subjective measures of alcohol craving in alcoholic patients (see Heilig and Egli 2006; Rosenthal 2006; Soyka and Roesner 2006 for recent reviews). However, the potential efficacy of acamprosate in reducing the propensity to relapse to the use of abused drugs other than alcohol has received little attention. We recently demonstrated that acamprosate prevents both the development and cocaine-induced reinstatement of a cocaine-conditioned place preference in mice (Mcgeehan and Olive 2003, 2006), suggesting that acamprosate may modify the rewarding and relapse-inducing effects of acute cocaine exposure.

Preclinical data indicate that blockade of either N-methyl-d-aspartate (NMDA) ionotropic or type 5 metabotropic (mGlu5) glutamate receptors or normalization of extracellular glutamate levels in regions such as the nucleus accumbens can attenuate reinstatement of cocaine-seeking behavior (Backstrom and Hyytia 2006, 2007; Baker et al. 2003; Lee et al. 2005; Maldonado et al. 2007; but see Famous et al. 2007; See et al. 2001). While the precise neuropharmacological mechanisms of action for acamprosate are not fully understood (De Witte et al. 2005), electrophysiological studies have shown that acamprosate modulates the function of NMDA receptors (Berton et al. 1998; Madamba et al. 1996; Popp and Lovinger 2000; Rammes et al. 2001; Zeise et al. 1990, 1993) and may also antagonize the mGlu5 receptor (De Witte et al. 2005; Harris et al. 2002, 2003). Additional studies utilizing in vivo microdialysis have shown that acamprosate dampens extracellular levels of glutamate when the organism is in a hyperglutamatergic state (Dahchour and De Witte 2000, 2003; Dahchour et al. 1998; Spanagel et al. 2005).

Given that acamprosate appears to exert some of its actions via modulating glutamate receptor function or extracellular levels of glutamate, the purpose of the present study was to examine the effects of acamprosate on cocaine-induced locomotion, cocaine self-administration, as well as cocaine- and cue-induced reinstatement of cocaine-seeking behavior.

Materials and methods

Subjects

All experimental procedures conformed to the 1996 National Institutes of Health Guide for the Care and Use of Laboratory Animals, the 2003 Guide for the Care and Use of Mammals in Neuroscience and Behavioral Research, and with the approval of an Institutional Animal Care and Use Committee. Male Sprague–Dawley rats (300–325 g, Charles River Laboratories, Wilmington, MA) were individually housed upon arrival and were weighed and handled daily for 1 week to assess health and allow acclimation to handling procedures. Animals were maintained on a 12-h light–dark cycle (lights on at 07:00 hours) with ad libitum access to food unless otherwise stated. All experimentation was conducted during the light phase of the light–dark cycle.

Drugs

Cocaine hydrochloride was obtained from either Sigma-Aldrich (St. Louis, MO) or the National Institute on Drug Abuse Drug Supply Program (Research Triangle Park, NC). Acamprosate was obtained from EstechPharma (Seoul, South Korea). Ketamine and xylazine were obtained from the University of California at San Francisco or Medical University of South Carolina hospital pharmacy. Pentobarbital sodium and chloral hydrate were obtained from Sigma-Aldrich. Methohexital was obtained from King Pharmaceuticals (Bristol, TN). Cocaine and acamprosate were dissolved in sterile saline (0.9% w/v sodium chloride, Hospira, Lake Forest, IL) before intravenous (i.v.) or intraperitoneal (i.p.) administration. All i.p. administrations were given in a volume of 1 ml/kg. Repeated dosing of acamprosate was utilized because it has been reported that plasma levels of acamprosate do not stabilize until at least 5 days of treatment (Durbin et al. 1996; Wilde and Wagstaff 1997).

Monitoring of locomotor activity and body weight

After acclimation to handling procedures, animals were randomized into one of four groups and received daily i.p. injections of saline or acamprosate (100, 300, or 500 mg/kg i.p.) for 4 days. Body weights were measured daily approximately 1 h before drug or saline administration. On the fifth day of treatment, animals were weighed and placed into 40 × 40-cm open-field activity-monitoring chambers equipped with photobeams (Med Associates, St. Albans, VT) and interfaced to a personal computer. Horizontal locomotor activity was monitored at 100-ms resolution for 60 min (habituation period) before administration of saline or acamprosate (as above). After treatment, animals were placed back in the activity monitoring chambers for 20 min. Next, all animals received an acute injection of cocaine (10 mg/kg i.p.), and locomotor activity was monitored for an additional 120 min.

Surgery and catheter maintenance

Before the initiation of self-administration training, rats were anesthetized with ketamine (66 mg/kg i.p.), xylazine (1.3 mg/kg i.p.), and Equithesin (0.5 ml/kg, consisting of 9.72 mg/ml pentobarbital sodium, 42.5 mg/ml chloral hydrate, and 21.3 mg/ml magnesium sulfate heptahydrate dissolved in a 44% v/v propylene glycol and 10% v/v ethanol solution, Sigma-Aldrich), and silastic catheters were inserted unilaterally 2.7–3.0 cm into the jugular vein. Catheters were sutured to the underlying muscle tissue with 4–0 Ethicon polyglactin absorbable sutures (Johnson and Johnson, Somerville, NJ) through Dacron impregnated mesh (PlasticsOne, Roanoke, VA). The other end of the catheter was externalized through a 3-mm dermal biopsy hole made between the scapulae. Catheters consisted of a stainless steel cannula (PlasticsOne) attached to silastic tubing (Fisher Scientific, Santa Clara, CA) and Dacron impregnated mesh via dental acrylic (Lang Dental, Wheeling, IL). Dorsal external incisions were closed with 4–0 Ethicon nonabsorbable silk sutures (Johnson and Johnson) and ventral external incisions closed with wound clips. Catheters were flushed daily with cefazolin (0.2 ml of 0.1 g/ml, Sandoz, Holzkirchen, Germany) in heparinized saline (0.2 ml of 100 IU, Elkins-Sinn, Cherry Hill, NJ) to help maintain catheter patency and to protect against infection. Seven days after surgery, rats were placed on food restriction for 2 days and then maintained at 90% normal body weight. This regimen aids in acquisition and maintenance of operant responding while allowing weight gain. Catheter patency was verified by infusion of methohexital sodium (to effect up to 5 mg/kg i.v., King Pharmaceuticals) after self-administration procedures.

Self-administration training

Animals were placed in standard operant self-administration chambers (Coulborn Instruments, Allentown, PA for basal self-administration studies; Med Associates for reinstatement studies) equipped with two stimulus lights above two retractable levers. Operant self-administration chambers were housed individually in sound-attenuating cubicles that also contained a house light and exhaust fan to mask external odors and noise. Animals were randomly assigned an active lever that, upon depression, resulted in reinforcer delivery. Assignment of the active lever remained constant throughout training and testing for any given animal. Depression of the inactive lever resulted in no programmed consequences. Active lever pressing was acquired during overnight, 15-h sessions on a modified fixed ratio-1 (FR1) schedule of reinforcement where each lever press resulted in delivery of a single 45-mg food pellet (Testdiet, Richmond, IN), which was paired with illumination of a stimulus light located above the active lever and activation of a tone for 5 s. Each reinforcer delivery was followed by a 20-s timeout period where lever presses were recorded but did not result in additional reinforcer delivery. The following day, sessions were reduced to 2 h, and the reinforcer was changed to cocaine (0.6 mg/kg per infusion, 2-s infusion time). Training continued with ad libitum home cage access to food and water until active lever pressing stabilized to less than 15% variation over 3–4 days.

Reinstatement testing

After reaching the above criterion for stability of responding, daily extinction trials began, whereby presses on the active lever no longer produced any programmed consequences. After reaching extinction criteria (<15% of the average number of active lever presses made during the last 2 days of cocaine self-administration, minimum 14 days), animals were subdivided into three groups balanced for self-administration patterns, each receiving daily injections of saline or acamprosate (100 or 300 mg/kg i.p.) in the home cage for 5 days 20 min before additional 2-h extinction sessions. On the fifth day of treatment, at 20 min after saline or acamprosate administration, half of the animals were given a single priming injection of cocaine (10 mg/kg i.p.) and placed immediately into the operant self-administration chamber for assessment of cocaine-seeking behavior (i.e., active lever presses) for 2 h. The 10-mg/kg priming dose of cocaine was chosen based on previous reports that lower or higher priming doses of cocaine result in lower magnitudes of reinstatement (c.f. Cornish and Kalivas 2000; see also Schmidt et al. 2005 for review). During the cocaine-induced reinstatement session, presses on either lever were recorded but had no programmed consequences. The remaining animals was placed into the operant self-administration chamber for cue-induced reinstatement of cocaine-seeking behavior for 2 h. For cue-induced reinstatement, lever presses on the lever that previously delivered cocaine resulted in illumination of the stimulus light and activation of the auditory tone for 5 s, but no cocaine was delivered. After each reinstatement test session, animals underwent daily 2-h extinction sessions until pretreatment patterns of extinction responding were re-established, and then 5-day dosing procedures were reinitiated. This sequence was performed three times so that all animals received saline and both doses of acamprosate (100 and 300 mg/kg i.p.) in a randomized, counterbalanced within-subjects dose–response design. This “crossover” design allowed quantification of reinstatement magnitude during each test session by examining the behavior of saline-treated animals that were tested in parallel with acamprosate-treated animals.

Self-administration testing

To assess the effects of acamprosate on baseline cocaine self-administration, a separate group of animals was trained as described above. Once stable baseline responding for cocaine was established, animals were administered saline or acamprosate (100 or 300 mg/kg i.p.) once daily in a randomized between-subjects design 20 min before five subsequent daily 2-h self-administration sessions.

Statistical analysis

Data were analyzed using SigmaStat 3.0 software (Systat Software, San Jose, CA). Pretreatment body weight values were averaged across the last 2 days before the initiation of drug treatment. Locomotor activity (in 10-min time bins) and body weight were analyzed by a mixed two-way analysis of variance (ANOVA, treatment day × treatment group). Locomotor activity (total distance traveled) was analyzed by one-way ANOVA. Cocaine intake (mg/kg) was averaged across the last 2 days of baseline self-administration for each group of animals and was ana-lyzed by one-way ANOVA. Effects of acamprosate on cocaine self-administration were analyzed using a two-way repeated-measures ANOVA with treatment day × lever or number of lever presses or reinforcers delivered as factors within each treatment group (saline, acamprosate 100 mg/kg or acamprosate 300 mg/kg). Effects of acamprosate on cocaine- or cue-induced reinstatement were analyzed by a two-way repeated-measures ANOVA (experimental phase [self-administration, extinction, or treatment before reinstatement] × lever) for each reinstatement modality (cue or cocaine). To test for possible order effects on reinstatement, a separate three-way ANOVA was conducted, with reinstatement test × lever × acamprosate dose as factors. All ANOVAs were followed by Holm–Sidak post-hoc multiple comparison procedures. For self-administration and extinction phases, data values were calculated as the average of active or inactive lever presses during the last 2 days of active cocaine self-administration or extinction-training procedures, respectively. All data are stated or depicted as mean ± SEM.

Results

Effects of acamprosate on locomotor activity and body weight

A total of 32 rats (n = 8 per group) were tested for effects of saline or acamprosate (100, 300, or 500 mg/kg i.p.) on body weight and spontaneous and cocaine-stimulated horizontal locomotor activity. These data are presented in Fig. 1. One animal treated with 500 mg/kg acamprosate died after the second day of treatment, and the corresponding data were excluded from analysis. For the remaining animals, there was an overall significant effect of treatment group on body weight (F[3, 137] = 4.46, p < 0.05) and a treatment group × treatment day interaction (F[15, 137] = 3.10, p < 0.001) but no effect of treatment day. Post-hoc analysis revealed that animals treated with the 500-mg/kg dose of acamprosate exhibited reduced body weight on days 3, 4, and 5 of treatment as compared with saline-treated animals (Fig. 1a). Treatment with the lower doses of acamprosate (100 or 300 mg/kg) had no effect on body weight (p > 0.05).
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Fig. 1

Effects of acamprosate on a body weight and b, c spontaneous and cocaine-stimulated locomotor activity. a Pretreatment (PreTx) body weight values were calculated as an average across the last 2 days before the commencement of repeated treatment with saline i.p. (open circles, n = 8) or acamprosate 100 mg/kg i.p. (filled circles, n = 8), 300 mg/kg i.p. (filled squares, n = 8), or 500 mg/kg i.p. (filled triangles, n = 7). Asterisk, p < 0.05 vs saline-treated animals on the same treatment day. b Locomotor activity testing (plotted in 10-min time bins) on day 5 of saline or acamprosate treatment. Saline (Sal) or acamprosate (Acamp) was administered at the first arrow according to the respective treatment group. All rats subsequently received cocaine (10 mg/kg i.p.) at the second arrow. Number sign, p < 0.05 for animals treated with acamprosate 300 mg/kg vs saline at same time point. Asterisk, p < 0.05 for animals treated with acamprosate 500 mg/kg vs saline at same time point. Plus sign, p < 0.05 for all groups vs time point immediately before cocaine. c Analysis of total distance traveled during the 60-min habituation period, 20-min postacamprosate period, or 120-min postcocaine period. Asterisk, p < 0.05 vs saline-treated animals. All data are presented as mean ± SEM

The effects of acamprosate on spontaneous and cocaine-stimulated locomotor activity are shown in Fig. 1b,c. Analysis of locomotor activity in 10-min time bins (Fig. 1b) revealed a significant effect of treatment group (F[3, 513] = 3.151, p < 0.05), time (F[19, 513] = 40.89, p < 0.001), and a treatment group × time interaction (F[57, 513] = 1.37, p < 0.05). Animals that had been pretreated with 100 mg/kg acamprosate for 4 days showed normal patterns of habituation to the novel locomotor activity testing apparatus during the 60-min habituation period as compared with saline-pretreated animals. Animals pretreated with the 300-mg/kg dose of acamprosate showed reduced locomotor activity at one time point (40 min) during the habituation period as compared with saline-pretreated animals. Animals pretreated with the 500-mg/kg dose of acamprosate showed reduced locomotor activity at two time points (30 and 40 min) as compared with saline-pretreated animals. An analysis of the overall total distance traveled during the 60-min habituation period revealed a significant effect of treatment group (F[3, 27] = 4.77, p < 0.01), with animals pretreated with 500 mg/kg acamprosate showing reduced overall locomotor activity (Fig. 1c).

After administration of saline or acamprosate (according to their respective treatment groups), animals pretreated with 100 mg/kg showed no differences in locomotor activity during the 20 min after drug treatment as compared to saline-pretreated animals. Rats treated with 300 mg/kg acamprosate showed reduced locomotor activity during the first 10 min after acamprosate treatment relative to saline-treated animals, and rats treated with 500 mg/kg acamprosate showed reduced locomotor activity during the 20-min period after acamprosate treatment relative to saline-treated animals. An analysis of the overall total distance traveled during the 20-min postacamprosate period revealed a significant effect of treatment group (F[3, 27] = 7.41, p < 0.001), with animals treated with 300 or 500 mg/kg acamprosate showing reduced overall horizontal locomotor activity (Fig. 1c).

All animals demonstrated an increase in locomotor activity during the first 50 min after acute injection of cocaine (10 mg/kg) as compared to their respective levels immediately before cocaine administration. Animals that had been pretreated with 100 mg/kg acamprosate exhibited an enhanced locomotor stimulant effect of cocaine during the first 10 min after cocaine administration as compared with saline-treated animals. Animals treated with the 500-mg/kg dose of acamprosate exhibited attenuated locomotor activity during the 60–80-min time points after cocaine administration. However, an analysis of the overall total distance traveled during the 120-min postcocaine period revealed no significant group differences (Fig. 1c).

Because we observed adverse effects of the high (500 mg/kg) dose of acamprosate (i.e., reductions in body weight and spontaneous locomotor activity as well as one case of mortality) and because previous reports have shown that doses of 400 and 450 mg/kg of acamprosate inhibit general behavioral output and reduce body weight in other rat strains and experimental settings (Czachowski et al. 2001; Escher and Mittleman 2006; Gewiss et al. 1991; Heyser et al. 1998; Hölter et al. 1997; Le Magnen et al. 1987), we elected not to test any doses of acamprosate higher than 300 mg/kg on cocaine self-administration and reinstatement of cocaine-seeking behavior.

Effects of acamprosate on cocaine and cue-induced reinstatement of cocaine-seeking behavior

A total of 24 animals were implanted with indwelling jugular vein catheters for reinstatement experiments. Of these, one animal died because of cocaine overdose during self-administration training, three animals were removed from the study because of failure to obtain stable patterns of cocaine self-administration, and two animals were removed because of catheter patency issues. Data from animals that underwent cocaine- or cue-induced reinstatement procedures (n = 9 per group) are shown in Fig. 2. Cocaine intake was similar between both groups during the last 2 days of the maintenance phase of self-administration (18.0 ± 1.2 mg/kg for the cocaine-induced reinstatement group, 20.0 ± 1.6 mg/kg for the cue-induced reinstatement group). A significant interaction was found between experimental phase (self-administration, extinction, or treatment before reinstatement) and lever for the cocaine-induced reinstatement group (F[4, 28] = 4.44, p < 0.01) and for the cue-induced reinstatement group (F[4, 28] = 7.02, p < 0.001). Post-hoc multiple comparisons revealed that in both groups, extinction procedures yielded significant reductions in presses on the active (i.e., previously cocaine-paired) lever but not the inactive lever vs the respective values observed during the maintenance phase of self-administration (p < 0.05). During cocaine- and cue-induced reinstatement testing, a significant increase in active but not inactive lever responding was observed in animals pretreated with saline or the low (100 mg/kg) dose of acamprosate (p < 0.05 vs extinction for both groups). However, in animals pretreated with the high (300 mg/kg) dose of acamprosate, a significant increase in active lever presses was not observed in either group (vs extinction for both groups). Inactive lever presses were not altered by acamprosate treatment in either the cocaine- or cue-induced reinstatement group. No ordering effects were observed across reinstatement tests for the cocaine-induced reinstatement group or the cue-induced reinstatement group. Treatment with saline or acamprosate during the 4 days before reinstatement testing in either group did not result in changes in the number of active or inactive lever presses on any of the treatment days (data not shown).
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Fig. 2

Acamprosate dose-dependently attenuated a cocaine- or b cue-induced reinstatement of cocaine-seeking behavior (n = 9 per group). Baseline self-administration (SA) represents the number of active or inactive lever presses during the last 2 days of self-administration before extinction. Extinction (Ext) represents the number of active or inactive lever presses during the last 2 days of extinction training. All data are presented as mean ± SEM. Number sign, p < 0.05 vs active lever presses during SA. Asterisk, p < 0.05 vs active lever presses during Ext

Effects of acamprosate on cocaine self-administration

A total of 38 animals were implanted with indwelling jugular vein catheters for examination of the effects of acamprosate on cocaine self-administration. Of these animals, five were removed from the study because of failure to obtain stable patterns cocaine self-administration, and three were removed because of catheter patency issues. Cocaine intake was similar across all groups on the last 2 days before initiation of saline or acamprosate treatment (12.2 ± 1.8 mg/kg for the saline group; 14.3 ± 1.9 mg/kg for the acamprosate 100 mg/kg group; 14.4 ± 1.7 mg/kg for the acamprosate 300 mg/kg group). As seen in Fig. 3, there was no effect of saline or the 100- or 300-mg/kg doses of acamprosate (n = 10 per treatment group) on active cocaine self-administration on any of the treatment days after administration. Post-hoc comparisons revealed a slight trend toward an effect of increased cocaine self-administration in animals treated with 100 mg/kg acamprosate on day 3 of administration; however, this effect failed to meet statistical significance (p = 0.08). A significant effect of lever was observed within each treatment group (F[2, 90] = 31.81, p < 0.001 for animals treated with saline, F[2, 90] = 25.92, p < 0.001 for animals treated with acamprosate 100 mg/kg, and F[2, 90] = 25.71, p < 0.001 for animals treated with acamprosate 300 mg/kg), reflecting the greater responding on the cocaine (active) lever as compared to the inactive lever.
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Fig. 3

Acamprosate had no effect on active cocaine self-administration. After establishment of stable baseline responding, rats were administered saline i.p. (a, n = 10), acamprosate 100 mg/kg i.p. (b, n = 10), or acamprosate 300 mg/kg i.p. (c, n = 10) 20 min before 2-h cocaine self-administration sessions once daily for 5 consecutive days. Active lever presses = black bars, number of cocaine infusions = hatched bars, inactive lever presses = white bars. p > 0.05 for all variables compared to pretreatment (PreTx) values. All data are presented as mean ± SEM

Discussion

Our data reveal that acamprosate attenuates cocaine- and cue-induced reinstatement of cocaine-seeking behavior. These observations are consistent with our previous report that acamprosate also attenuates cocaine-primed reinstatement of a cocaine-conditioned place preference in mice (Mcgeehan and Olive 2006). Taken together, these data suggest that acamprosate may provide some benefit in preventing relapse in human cocaine addicts. The ability of acamprosate to attenuate reinstatement of cocaine-seeking behavior may be relatively specific to cocaine, as other investigators have demonstrated that acamprosate does not alter heroin or stress-induced reinstatement of heroin-seeking behavior (Spanagel et al. 1998) and does not alter sucrose reinforcement (Czachowski et al. 2001).

Repeated dosing of acamprosate was utilized because it has been previously reported that at least 5 days of treatment with drug is necessary to obtain stable plasma levels (Durbin et al. 1996; Wilde and Wagstaff 1997), which may be attributable to poor bioavailability. We observed that only the 300-mg/kg dose of acamprosate was effective in attenuating cocaine- and cue-induced reinstatement of cocaine-seeking behavior. While statistical analyses revealed that active lever presses in animals treated with this dose were not significantly different from values after extinction training, a nonsignificant trend toward an increase in active lever presses during reinstatement was evident in rats treated with this dose (p = 0.12 for both groups). However, doses of acamprosate higher than 300 mg/kg were not tested in reinstatement experiments because we observed that repeated administration of a 500-mg/kg dose produced adverse side effects including reduced body weight and hypolocomotion. In addition, doses of acamprosate of 400–450 mg/kg have been previously reported to produce reductions in general fluid consumption and body weight (Czachowski et al. 2001; Escher and Mittleman 2006; Gewiss et al. 1991; Heyser et al. 1998; Hölter et al. 1997; Le Magnen et al. 1987).

During open-field locomotor experiments, we noted that distance traveled was reduced by 300 mg/kg acamprosate, although no overall effects of pretreatment with this dose were observed during the preceding 60-min habituation period. Nonetheless, several lines of evidence suggest that this reduction in motor activity following acute re-exposure to 300 mg/kg acamprosate does not underlie the attenuation of cocaine- and/or cue-induced reinstatement of cocaine-seeking behavior. First, acamprosate treatment at any dose tested did not reduce locomotor activity after an acute challenge with cocaine (10 mg/kg i.p.), which was the same dose utilized for cocaine-primed reinstatement. Second, no effects of acamprosate were observed on inactive lever presses during either reinstatement test. Moreover, no effects of acamprosate were observed on either active or inactive lever pressing or during ongoing cocaine self-administration. Finally, we have previously demonstrated, in mice, that 300 mg/kg i.p. acamprosate is devoid of aversive effects that might cause malaise or nonspecific effects on behavior (Mcgeehan and Olive 2003).

The precise mechanism by which acamprosate attenuated cocaine- and cue-induced reinstatement of cocaine-seeking behavior remains unknown, primarily because of the complex pharmacological profile of acamprosate. For example, acamprosate interferes with polyamine regulation of NMDA receptor function in a bimodal fashion (Berton et al. 1998; Madamba et al. 1996; Popp and Lovinger 2000; Rammes et al. 2001; Zeise et al. 1993; Zeise et al. 1990) that is dependent upon resting NMDA receptor activity (De Witte et al. 2005; Popp and Lovinger 2000). It is interesting to note that the NMDA receptor antagonist dizocilpine (MK-801) alters active cocaine self-administration and reinstatement induced by cues but not cocaine-primed reinstatement of drug seeking (Backstrom and Hyytia 2006; Hyytia et al. 1999; Lee et al. 2005; Park et al. 2002). Taken together, it is unlikely that the attenuation of both cocaine- and cue-induced reinstatement without altering active cocaine self-administration, as reported here, is simply a result of NMDA receptor antagonism.

Recent studies have demonstrated that acamprosate inhibits the neurotoxic effects of (+-)-1-amino-1,3-cyclopentane-trans-dicarboxylic acid, a group I and II mGluR agonist, in a manner similar to the mGluR5 antagonist SIB-1893 (Harris et al. 2002, 2003). These data suggest that acamprosate can antagonize metabotropic glutamate receptors as well as ionotropic glutamate receptors. Indeed, the mGlu5 antagonist 2-methyl-6-(phenylethynyl)-pyridine attenuates both cocaine- and cue-induced reinstatement of cocaine-seeking behavior (Backstrom and Hyytia 2006; Iso et al. 2006; Lee et al. 2005). However, mGlu5 antagonists also attenuate baseline cocaine self-administration patterns (Chiamulera et al. 2001; Kenny et al. 2003, 2005; Tessari et al. 2004), an effect that was not observed with acamprosate in the present study. Thus, it is uncertain whether acamprosate attenuates cocaine- and cue-induced reinstatement by acting at mGlu5 receptors.

Nonetheless, the importance of glutamatergic signaling in the ventral forebrain during cocaine-induced reinstatement of drug-seeking behavior has been elegantly studied. Basal extracellular glutamate levels are reduced during long-term withdrawal from repeated cocaine exposure, but cocaine-stimulated glutamate release is significantly elevated above that of acutely treated subjects in the nucleus accumbens (NAc) core (Bell et al. 2000; Hotsenpiller et al. 2001; Kalivas et al. 2003; Keys et al. 1998). This dichotomy in ventral forebrain glutamate transmission appears to be a critical mediator of relapse to cocaine seeking, as restoration of basal extracellular glutamate levels prevents cocaine-primed reinstatement of cocaine seeking by inhibiting additional cocaine-induced glutamate release (Baker et al. 2003; Moran et al. 2005). Moreover, stimulation of NMDA or AMPA glutamate receptors in the NAc facilitates reinstatement of cocaine-seeking (Cornish et al. 1999) while local blockade of these receptors reduces cocaine-induced reinstatement of cocaine-seeking behavior (Cornish et al. 1999; Cornish and Kalivas 2000; McFarland et al. 2003; Park et al. 2002).

Intriguingly, acamprosate has been shown to normalize elevated extracellular levels of glutamate in the NAc during ethanol withdrawal (Dahchour and De Witte 2000, 2003; Dahchour et al. 1998). However, as mentioned above, withdrawal from cocaine is associated with reduced, rather than elevated, extracellular levels of glutamate in the NAc (Baker et al. 2003; Bell et al. 2000; Hotsenpiller et al. 2001; Kalivas et al. 2003; Moran et al. 2005). Thus, additional microdialysis studies are needed to determine whether acamprosate can “normalize” extracellular levels of glutamate in the NAc when basal levels of this neurotransmitter are suppressed (i.e., during cocaine withdrawal), rather than elevated (i.e., during alcohol withdrawal).

We also observed that acamprosate elicited no effect on responding for cocaine in actively self-administering animals. This lack of effect was unanticipated because we have previously observed that acamprosate (at similar doses) attenuates the development of a conditioned place preference to cocaine in mice (Mcgeehan and Olive 2003). This apparent discrepancy underscores important differences in the effects of certain therapeutic agents on the conditioned rewarding effects of drugs of abuse, as measured in a place preference paradigm, as compared to consummatory or reinforcing aspects that are measured in an operant self-administration paradigm (Bardo and Bevins 2000; Calcagnetti et al. 1995; Tzschentke 1998). Accordingly, similar differential effects of acamprosate on relapse-like behavior, as compared to baseline drug self-administration, have also been reported in the alcohol literature (Heyser et al. 1998). It is possible, however, that acamprosate might alter active cocaine self-administration under other unit doses of reinforcement, higher response requirements, or in a progressive ratio paradigm. Along these lines, acamprosate has previously been demonstrated to reduce alcohol self-administration under higher response requirements as opposed to lower (i.e., FR1) response requirements (Heyser et al. 1998; Spanagel and Hölter 2000). Thus, additional studies are needed to determine if acamprosate reduces cocaine self-administration under conditions of higher response requirements.

With regards to the ability of acamprosate to attenuate ongoing cocaine self-administration, it is interesting to note that a Phase II pilot clinical trial examining the efficacy of acamprosate in reducing cocaine craving, withdrawal symptoms, and daily cocaine use was recently initiated (www.clinicaltrials.gov identifier: NCT00385268). The results of this clinical study will have important implications in determining the predictive validity of current preclinical animal models (i.e., IV self-administration and reinstatement procedures) for predicting the efficacy of potential therapeutic compounds for the treatment of cocaine addiction.

In summary, we have shown in a rat model of relapse that acamprosate attenuated both cue- and drug-induced reinstatement of cocaine-seeking behavior at doses that do not affect ongoing cocaine self-administration or cocaine-stimulated locomotion. Because of a broad mechanism of action, further investigation with acamprosate may uncover multiple receptor systems that contribute to relapse. Regardless of its many potential neurochemical targets, acamprosate lacks abuse potential and presents a mild side effect profile in humans (Grant and Woolverton 1989; Le Magnen et al. 1987; Mcgeehan and Olive 2003; Naassila et al. 1998; Nalpas et al. 1990; Schneider et al. 1998; Spanagel et al. 1996). While further preclinical studies are needed to determine if acamprosate attenuates cocaine relapse induced by other stimuli such as stress, our data suggest that acamprosate may be a novel pharmacological therapeutic particularly useful in reducing the incidence of relapse in cocaine addiction.

Acknowledgments

This work was supported by the Neurobiology of Addiction Research Center (DA015369) and funds provided by the State of California for medical research on alcohol and substance abuse through the University of California at San Francisco. The authors wish to thank Shannon Ghee and Brian Wheeler for technical assistance.

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© Springer-Verlag 2007