Cocaine reverses the naltrexone-induced reduction in operant ethanol self-administration: The effects on immediate-early gene expression in the rat prefrontal cortex

Abstract

Naltrexone is a clinically approved medication for alcoholism. We aimed to investigate the effectiveness of naltrexone co-administered with cocaine and the association of these substances with immediate-early gene expression in the rat prefrontal cortex. We used chronic operant ethanol self-administration and oral treatments prescribed for alcoholism and available in pharmacies to maximise the predictive validity in humans. We performed real-time PCR analysis to determine gene expression levels in the prefrontal cortex. Only the highest dose of naltrexone (1, 3, and 10 mg/kg, p.o.) reduced the response to ethanol. Cocaine increased ethanol self-administration in a dose-dependent manner (2.5, 10, 20 mg/kg, i.p.) and reversed the naltrexone-induced reduction. Naltrexone failed to prevent the cocaine-induced increase in locomotor activity observed in these animals. Chronic self-administration of ethanol reduced the expression of the C-fos gene 4- to 12-fold and increased expression of the COX-2 (up to 4-fold) and Homer1a genes in the rat prefrontal cortex. Chronic ethanol self-administration is prevented by naltrexone, but cocaine fully reverses this effect. This result suggests that cocaine may overcome naltrexone's effectiveness as a treatment for alcoholism. The ethanol-induced reduction in C-fos gene expression in the prefrontal cortex reveals an abnormal activity of these neurons, which may be relevant in the compulsive consumption of ethanol, the control of reward-related areas and the behavioural phenotype of ethanol addiction.

Introduction

Alcoholism is a disabling addiction disorder (WHO, GISAH, 2011). An effective treatment for alcoholism remains elusive despite the advances that have been made including the development of naltrexone (i.e., Antaxone^®) and acamprosate (i.e., Campral^®), two clinically approved medications to treat binge ethanol consumption, ethanol abuse and dependence and to prevent relapse (Fuller and Gordis, 2001; Johnson, 2010; Mason, 2003; O'Brien et al., 1996; Spanagel and Zieglgänsberger, 1997). Treatment with oral naltrexone has been associated with a significant reduction in alcohol-related healthcare costs (Kranzler et al., 2010). Naltrexone is a non-selective opioid antagonist theorised to reduce ethanol consumption by blocking central opioid receptors that subsequently directly or indirectly modulate the effects of ethanol (Hillemacher et al., 2011; Hubbell and Reid, 1990). The mechanism of action of acamprosate is less well understood. Acamprosate is hypothesised to reduce neuronal hyperexcitability through its putative agonist-like effects at GABA receptors and its antagonist effects at the glutamate N-methyl-d-aspartate (NMDA) receptor (Kiefer and Mann, 2010; Littleton, 1995; Stromberg et al., 2001). The effects of combining the two compounds on ethanol consumption have also been assessed. Stromberg et al. (2001) reported no evidence of an additive or synergistic effect resulting from such a combination nor was it more effective than naltrexone alone in reducing ethanol consumption by rats.

The co-abuse of ethanol and cocaine occurs with high frequency and persistence in human populations worldwide. For example, Miller et al. (1989) reported that 94% of the American patients diagnosed as cocaine-dependent were also diagnosed with an ethanol dependence. In another study, approximately 12 million members of the general population had used both ethanol and cocaine within the previous twelve months (Grant and Harford, 1990). In a more recent study in Europe, 64% of the cocaine powder users (excluding crack cocaine users) reported frequent ethanol consumption (Gossop et al., 2006), and the concomitant use of ethanol was evident by analyses of blood and urine samples in 76% of cocaine-related cases of sudden death (Lucena et al., 2010). Finally, heavy chronic alcohol use is linked to a three-fold increased risk of cocaine use (Kulaga et al., 2010). These data support the idea that cocaine use could increase the vulnerability to the development of ethanol dependence (Rubio et al., 2008) and vice versa. A careful examination of the clinical data indicates that the concurrent use of ethanol and cocaine is associated with increased mortality and morbidity resulting from cerebro- and cardiovascular complications (Cami et al., 1998; O'Connor et al., 2005; Randall, 1992; Vroegop et al., 2009) as well as hepatotoxicity and compromised mental status (Odeleye et al., 1993; Vanek et al., 1996). Taken together, these facts underscore the urgency and necessity to develop pharmacotherapeutic interventions for alcoholism and the comorbidity of alcoholism with cocaine use.

Currently, despite the increasing number of studies investigating the effects of naltrexone or acamprosate on ethanol/cocaine co-consumption (Hersh et al., 1998; Oslin et al., 1999; Pettinati et al., 2008a, 2008b; Sable et al., 2004; Schmitz et al., 2004, 2009; Stromberg et al., 2002; Suh et al., 2008), knowledge in this area remains incomplete. For example, a high dose of naltrexone modestly reduced heavy ethanol consumption in individuals dependent on both cocaine and ethanol (Schmitz et al., 2009). Therefore, the aim of this study was to gain deeper knowledge regarding the treatment of alcohol addiction with naltrexone and acamprosate when cocaine is co-administered. For this purpose, we used chronic operant ethanol self-administration in rats, an animal model with one of the highest levels of predictive validity in humans (Koob et al., 2003) for the development of pharmacological treatments for substance abuse disorders. Furthermore and important in this study, we investigated the expression of immediate-early genes as biomarkers of neural stimulation in the prefrontal cortex. We focused on the prefrontal cortex because of its contribution to addictive behaviour (Lüscher and Malenka, 2011), its involvement in compulsive ethanol drinking, its demonstrated sensitivity to naltrexone and acamprosate treatment (Burattini et al., 2008; Li et al., 2010; Yu et al., 2011), and its critical role in integrating and regulating cognitive behaviour in rodents and in humans (e.g., Abernathy et al., 2010; Dayas et al., 2007; Vengeliene et al., 2009).