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