**2.1 Ethanol consumption in MOP-R knockout mice**

206 Neuroscience – Dealing with Frontiers

Naltrexone has been shown to effectively reduce ethanol consumption and seeking in a large number of studies (Ciccocioppo et al., 2002a; Critcher et al., 1983; Franck et al., 1998; Gardell et al., 1996; Le et al., 1999; Nielsen et al., 2008; Simms et al., 2008; Stromberg et al., 1998a; Walker & Koob, 2008). Furthermore, intramuscular injections of naltrexone reduce intravenous (i.v.) or oral self-administration of ethanol in rhesus monkeys (Altshuler et al., 1980; Williams et al., 2001). Naltrexone reduces ethanol consumption in both low ethanol consuming rats (Stromberg et al., 1998a) and high ethanol consuming rats (Nielsen et al., 2008; Simms et al., 2008). However, the effects of naltrexone on ethanol consumption are non-selective as fat, sucrose and water intake are also reduced (Corwin & Wojnicki, 2009; Nielsen et al., 2008; Rao et al., 2008; Simms et al., 2008; Wong et al., 2009). Also, naltrexone reduces alcohol- and cueinduced reinstatement, but not foot-shock stress-induced reinstatement of ethanol-seeking in

A number of clinical studies have reported that naltrexone effectively reduces relapse in a subset of alcohol-dependent humans (Anton et al., 1999; Anton et al., 2006; O'Malley et al., 1992; Volpicelli et al., 1992) and is more effective at reducing heavy drinking (Pettinati et al., 2006). Alcohol-dependent patients that have a polymorphism in the OPRM1 gene encoding the mu opioid peptide receptor (MOP-R) with a mutation at A118G (Asn40Asp) have greater euphoric responses to alcohol, increased pain thresholds, greater susceptibility to AUDs and greater responses treatment with naltrexone (Anton et al., 2008; Bart et al., 2005; Oroszi et al., 2009; Oslin et al., 2003). More recently, the use of naltrexone in an extended– release intra-muscular (i.m.) depot formulation (Vivitrol) is more effective in patients who are able to abstain from drinking prior to treatment (O'Malley et al., 2007; Pettinati et al., 2009). Taken together, treatment with naltrexone appears to have the most consistent effects

**1.4 Pharmacological activity of naltrexone in reducing ethanol-mediated behaviors**  Naltrexone has the highest affinity for the MOP-R, moderate activity at the delta (DOP-R) and kappa (KOP-R) opioid peptide receptors, but without activity at nociceptin (NOP-R) and the sigma (SIG-R) receptors (Ananthan et al., 1999; Goldstein & Naidu, 1989; Takemori & Portoghese, 1984). There is a large body of evidence that suggests that the MOP-R plays a significant role in ethanol-mediated reward behavior (Becker et al., 2002; Ciccocioppo et al., 2002a; Gardell et al., 1996; Hall et al., 2001; Le et al., 1999; Reid & Hunter, 1984; Roberts et al., 2000) (Table 1). Ethanol has been reported to stimulate the activity of the endogenous opioids which target the MOP-R leading to increased basal dopamine release in the mesolimbic pathway Herz, 1997). Naltrexone's mechanism of action has been proposed to result from inhibition of ethanol-induced activity of endogenous opioid peptides and dopamine release *in vivo* (Benjamin et al., 1993; Gonzales & Weiss, 1998; Zalewska-Kaszubska et al., 2006; Zalewska-Kaszubska et al., 2008). In comparison, the roles of the other opioid subtypes on ethanol-mediated behaviors, such as the DOP-R (Krishnan-Sarin et al., 1995a; Roberts et al., 2001; Stromberg et al., 1998a) and KOP-R (Kovacs et al., 2005;

rodents (Ciccocioppo et al., 2002a; Le et al., 1999; Liu & Weiss, 2002).

on drinking outcomes in subjects with A118G mutation.

Lindholm et al., 2001; Mitchell et al., 2005) are less well-defined.

**1.2 Preclinical studies with naltrexone** 

**1.3 Clinical studies with naltrexone** 

Mice with a genetic deletion of the MOP-R have reduced levels of ethanol intake and seeking in comparison to wild-type mice with a mixed C57/129sv background (Becker et al., 2002; Hall et al., 2001; Roberts et al., 2001; Roberts et al., 2000) (Table 1). Although one study using purebred C57BL/6 mice failed to show any difference in ethanol intake (van Rijn & Whistler, 2009), the differences in these studies may reflect the varying degrees of ethanol intake in mice with different genetic backgrounds (Yoneyama et al., 2008) and also the models of ethanol consumption employed.
