The behavioral paradigm utilizing a low response requirement for intake assessment along with single extinction sessions to assess reinforcer seeking, and the separation of appetitive and consummatory responding within each session, made it possible to determine that each drug treatment had differential effects on the regulation of seeking and drinking behaviors. It is also important to note that we previously have demonstrated that the seeking response (extinction session lever-press behavior) in this model is stable over repeated measurements (Samson et al. 2001
) and reflects a prior association with the post-ingestive, pharmacological effects of ethanol (Samson et al. 2004
). Therefore, with the inclusion of the sucrose-reinforced control groups, the experiments allowed for detailed comparison between the treatment effects, the identification of the specific behaviors “targeted” by each drug treatment, and the selectivity of treatment effects for ethanol. Overall, naltrexone appeared to be the better pharmacotherapy, and most of the beneficial or “treatment-like” effects of acamprosate were limited to a dose that also caused weight loss and general health issues. This is consistent with most comparison assessments of both drugs in animal and human studies (Morley et al. 2006
; Richardson et al. 2008
; Stromberg et al. 2001
) and a follow-up of the COMBINE project (Combining Medications and Behavioral Interventions) that found that at 1 year post-treatment, naltrexone but not acamprosate was “associated with sustained efficacy beyond discontinuation” of the drugs (Donovan et al. 2008
To briefly summarize the overall findings, with regard to drinking behavior, both acamprosate and naltrexone had somewhat similar effects in both nondependent and dependent rats in that neither drug was selective for ethanol over sucrose intake. However, in nondependent animals, naltrexone was more efficacious at more doses than acamprosate, and acamprosate’s effects primarily were limited to a dose that also had adverse effects on body weight gain. In dependent animals, naltrexone was again more efficacious since acamprosate did not attenuate either ethanol or sucrose intake at the moderate dose tested. Both pharmacotherapies showed more selectivity when examining a specific reinforcer-seeking response. In nondependent rats, both acamprosate and naltrexone had response-attenuating effects in the ethanol groups, but not the sucrose groups. In fact, acamprosate increased sucrose seeking at the low dose, but unfortunately, the decreases in ethanol seeking were again only noted at the high dose. Of more concern in terms of treatment efficacy, in dependent animals, is that acamprosate had selective effects limited to a decrease in sucrose seeking with no effect on ethanol seeking. Naltrexone, on the other hand, selectively decreased ethanol seeking in nondependent rats with no effects on sucrose seeking and decreased reinforcer seeking in general in ethanol-dependent animals. There was no indication of the development of either tolerance or sensitivity to treatment effects over the 9 or 14-day dosing regimens and no motor impairment as indicated by the lack of effects on latency to respond. The present findings are limited to male Long Evans rats, and other rat strains and females should be examined to assess possible strain or sex-specific treatment effects.
Daily administration of naltrexone dose-dependently decreased ethanol consumption in nondependent rats, while the same treatment decreased sucrose intake, but to a slightly lesser degree at all doses and with a less pronounced dose/response pattern. Naltrexone doses in a similar range (0.15–0.45 mg/kg) previously have been shown to non-selectively decrease “free” home cage intake of sweetened ethanol and the sweetened control solution, with modest selectivity for ethanol responding at the lower dose (0.15 mg/kg) when tested in an operant paradigm in “binge-drinking” rats with baseline intake of approximately 0.8–1.0 g/kg (Ji et al. 2008
). While treatment selectivity was not assessed, a 1.0-mg/kg dose of naltrexone decreased cue-induced reinstatement responding for ethanol when tested in rats after a single and multiple ethanol withdrawals after vapor exposure (Ciccocioppo et al. 2003
). In the present experiments, when tested 3 days after
naltrexone/reinforcer sessions, subjects in the ethanol-reinforced group showed a decreased tendency to seek ethanol, while sucrose-reinforced subjects showed no such effect of the naltrexone/sucrose exposures (). Since subjects had no naltrexone on board at the time of these extinction tests, the decreases in ethanol seeking indicate a decrease in incentive motivation that likely resulted from the reinforcer devaluation caused by the repeated naltrexone/ethanol pairings during the reinforced sessions. It is unlikely that naltrexone produced a general malaise that interfered with subsequent reinforcer seeking, since no decrement in responding was observed in the sucrose-reinforced subjects. Moreover, the use of the sucrose control groups showed that while naltrexone’s effect on the consumption of a reinforcing substance was not specific to ethanol, the decrease in ethanol seeking did not generalize to a highly palatable calorie source, suggesting that the seeking attenuation in nondependent animals may be specific to the psychoactive reinforcing properties of ethanol and possibly related to the phenomenon of craving. This finding is in agreement with the clinical observation that naltrexone seems to be most efficacious in patients that continue to drink some ethanol (Killeen et al. 2004
), since some ethanol must be consumed in order to experience that it is less reinforcing with naltrexone on board and then for this experience to subsequently affect ethanol-motivated behavior (Heinälä et al. 2001
). In rats, the intake suppression produced by naltrexone over extended drug treatment and following termination of treatment was best accounted for by a learned association of the decreased reinforcing properties of consumed ethanol (Stromberg et al. 1998
). The present findings in nondependent rats suggest that naltrexone treatment may be effective for decreasing the motivation to seek ethanol in “binge-drinking” individuals without disrupting motivational processes in general. Moreover, the effects on continued ethanol seeking did not require naltrexone on board, indicating that this reinforcer blocking type of mechanism is perhaps long lasting and again consistent with the follow-up study of patients in the COMBINE Study (Donovan et al. 2008
In rats, naltrexone has been shown to suppress ethanol intake and ethanol-induced increases in dopamine in the nucleus accumbens during ethanol self-administration (Gonzales and Weiss 1998
). Recent evidence extends this finding to show that naltrexone also blocks ethanol-induced increases in tyrosine hydroxylase mRNA levels in the ventral tegmental area (Lee et al. 2005
), leading to the conclusion that naltrexone affects the reinforcing properties of ethanol by interfering with ethanol-induced stimulation of the mesolimbic dopaminergic pathway. In humans, naltrexone has been shown to decrease some of the positive reinforcing properties of ethanol on the ascending limb of the blood alcohol curve, such as simulation, vigor, “high”, and positive mood (Ray and Hutchison 2007
). Acamprosate has also been shown to affect the dopaminergic system in rats, producing increases in dopamine transporter density and decreases in dopamine D2-like receptor density in the nucleus accumbens, but tolerance develops quickly (within 3 days) to this effect (Cowen et al. 2005
). In addition, the combination of naltrexone plus acamprosate treatment showed no evidence of either additive or synergistic effects on ethanol-reinforced responding, suggesting that the two drugs are not working by a common mechanism (Stromberg et al. 2001
Our previous study using a similar operant model, nondependent rats, and 2 days of acamprosate treatment at the same doses used in this study (Czachowski et al. 2001b
) found that all doses decreased ethanol intake relative to saline after 2 days, again supporting the notion of “reinforcer blocking” or that ethanol and acamprosate needed to be experienced together to be effective. In that study, there were no effects on sucrose seeking or intake and no effects of acamprosate on any measure of ethanol seeking at doses that did not cause weight-related side effects. Overall, the short-term study suggested selective effects of acamprosate on ethanol intake, while the present study shows nonselective effects with extended treatment. Also, the low doses of acamprosate were effective at attenuating ethanol drinking early in treatment (first 9 days), but when examined over the whole 14-day treatment in this study, effectiveness was limited to the high dose that also had adverse side effects. We also observed these gastrointestinal problems/weight loss in our previous study after a single injection of 400 mg/kg, and Bowers et al. (2007)
reported a similar dose-dependent weight loss over 100- to 500-mg/kg doses, with the high dose reaching significant loss by day 3 and causing one fatality when administered only once daily, with deficits in locomotor activity at 300 and 500 mg/kg.
One rationale for assessing acamprosate in ethanol-dependent subjects was based on findings showing that dependent (LeMagnen et al. 1987
) or high-drinking (Boismare et al. 1984
) subjects showed greater responsiveness to acamprosate than non-selected nondependent animals (Heyser et al. 1998
). Indeed, there was one indication that acamprosate specifically affected ethanol seeking in dependent animals in the present study which was the increase in latency to first response, which was not seen in the sucrose-reinforced subjects and therefore not simple motor impairment. Total responding, however, was affected only in the sucrose-reinforced subjects, suggesting that even at moderate doses, acamprosate may have unwanted side effects. One hypothesis as to the process by which acamprosate decreases ethanol intake is that it inhibits the negative reinforcing properties of ethanol (Littleton 1995
; Spanagel and Zieglgansberger 1997
); however, the present paradigm tested animals after a withdrawal recovery period, so they were not exposed to the negative reinforcing (i.e., withdrawal-attenuating) properties of ethanol.
In ethanol-dependent rats, a moderate dose of naltrexone decreased ethanol consumption and decreased sucrose intake to a lesser but significant degree. The sustained “devaluation” of ethanol that resulted from multiple naltrexone/ethanol pairings and was evidenced by an attenuation of ethanol seeking in the nondependent rats, however, did not occur following dependence induction (). When tested with naltrexone on board (not previously tested in the nondependent rats), both ethanol and sucrose seeking were significantly decreased. When tested after experience with naltrexone/reinforcer pairings but with no naltrexone on board (similar to the nondependent animals), only sucrose seeking remained attenuated, while ethanol seeking had returned to baseline levels. It should be noted that the effectiveness of naltrexone in nondependent versus dependent animals may reflect differences in the number of naltrexone/ethanol pairings in each experiment which were greater in the nondependent animals (14) versus the dependent animals (4). Alternatively, naltrexone’s effectiveness in the nondependent versus dependent animals may be a result of differences in opiate system function resulting from the repeated dependence/withdrawal cycles. More specifically, recent findings from Walker and Koob (2008)
also show that naltrexone produced greater attenuation of ethanol-reinforced responding in nondependent rats as compared to dependent rats and that dysregulation of the κ-opioid system versus the μ-opioid system is primarily involved in dependence-induced ethanol drinking.
Interestingly, the ethanol dependence induction used in the present experiments did not result in an increase in “binge-like” ethanol drinking in the operant chamber (sucrose drinking was also unaffected). It should be noted that animals were tested after a week-long recovery period, so they were not in withdrawal at the time of the operant sessions. Ciccocioppo et al. (2003)
similarly observed that repeated ethanol vapor exposure did not augment ethanol cue-induced reinstatement responding and also attributed this to the absence of exposure to ethanol self-administration during the alleviation of withdrawal. Studies that report an increase in intake have used animals tested 6–12 h into withdrawal and are thus assessing the tendency to self-administer ethanol to avoid or decrease withdrawal symptoms (Roberts et al. 1996
; Walker and Koob 2007
). From a clinical perspective, those approaches would be examining patients with experience of drinking for negative reinforcement, that is, to alleviate ethanol withdrawal, while the present approach more closely models individuals with prolonged abstinence preceding relapse. In terms of “success” of the two pharmacotherapies, it should be noted that treatment-seeking individuals are not modeled by either the binge-drinking or ethanol-dependent animals assessed in the present experiments. Therefore, lack of specificity of the treatments for ethanol reinforcement may be of less concern when choosing a drug treatment. That is, motivated, treatment-seeking individuals may still greatly benefit from a pharmacotherapy that decreases the reinforcing properties of ethanol in order to overcome craving and, in conjunction with other forms of therapy, may be the best approach for controlling the excessive use of alcohol.