Nicotinic receptor ligands reduce ethanol intake by high alcohol–drinking HAD-2 rats

Abstract

Neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in the reinforcing effects of many drugs of abuse, including ethanol. The present study examined the efficacy of cytisine, a nAChR partial agonist, and lobeline, a putative nAChR antagonist, on the maintenance of ethanol drinking by HAD-2 rats. Adult male HAD-2 rats were given access to ethanol (15 and 30%, with ad libitum access to water and food) 22 h/day for 12 weeks, beginning at 60 days of age, after which cytisine (0.0, 0.5, and 1.5 mg/kg) was tested for 3 consecutive days. The rats were given an 18-day washout period and were then tested with lobeline (0.0, 1.0, and 5.0 mg/kg) for 3 consecutive days. Ethanol intake was measured at 1, 4, and 22 h postinjection. Rats were injected intraperitoneally just before lights out (1200 h). There was a significant main effect of cytisine treatment on the second test day, with the 1.5 mg/kg dose significantly reducing ethanol intake at the 1- and 4-h time-points, relative to saline, and the 0.5 mg/kg dose inducing a significant reduction at the 4-h time-point. Conversely, lobeline treatment resulted in significant main effects of treatment for all three time-points within each test day, with the 5.0 mg/kg dose significantly reducing ethanol intake, relative to saline, at each time-point within each test day. These findings provide further evidence that activity at the nAChR influences ethanol intake and is a promising target for pharmacotherapy development for the treatment of alcohol dependence and relapse.

Introduction

Alcohol abuse and dependence remain public health problems, with the worldwide prevalence of alcohol use disorders being 1.7% and the prevalence in the United States being 8.5% with an annual cost of 185 billion dollars (National Institute on Alcohol Abuse and Alcoholism, 2007). Whereas previous estimates of the ratio of men to women having an alcohol use disorder has varied between 1:2 and 1:3 (Brienza and Stein, 2002), more recent data suggest that the “gender gap” has been narrowing in younger and older populations (Brienza and Stein, 2002, Nelson et al., 1998, Wilsnack et al., 1991). The deleterious effects of alcohol abuse on societal health are staggering (cf, Brienza and Stein, 2002, Room et al., 2005). For example, the mortality of women with substance-associated diseases is four times that of breast cancer alone (Blumenthal, 1996, Blumenthal, 1997), and a causal relationship has been shown between alcohol use and at least 50 different medical conditions (Rehm et al., 2003).

The central cholinergic system has been implicated in the development of alcohol and/or drug abuse (cf, Larsson and Engel, 2004, Narahashi et al., 2001, Ribeiro-Carvalho et al., 2008, Rahman et al., 2008, Soderpalm et al., 2000). For example, there is ample evidence that both ethanol and nicotine increase extracellular dopamine (DA) in the nucleus accumbens (Acb; Tizabi et al., 2007) and particular subunits for the nicotinic acetylcholine receptor (nAChR: e.g., alpha6 and beta4) are implicated in ethanol's stimulating effects (Kamens and Phillips, 2008). One possible mechanism for this effect is through activity at nAChRs in the ventral tegmental area (VTA, Blomqvist et al., 1997), particularly those expressing alpha4 and beta2 subunits (Olausson et al., 2007). Additionally, there is some evidence for a preferential role of anterior Acb nAChRs mediating ethanol's effects on Acb-DA (Ericson et al., 2008). Moreover, nicotine pretreatment alters ethanol-induced DA release in the Acb shell (Lopez-Moreno et al., 2008). Genetically, the alpha3, alpha5, alpha7, and beta4 subunits of the nAChR have been implicated in a predisposition for ethanol stimulation and/or adolescent use of ethanol and tobacco (e.g., Greenbaum et al., 2006, Kamens et al., 2008, Rigbi et al., 2008, Schlaepfer et al., 2008). Pharmacologically, ligands for nAChRs with the alpha3, alpha6, beta2, or beta3 subunits alter ethanol self-administration and ethanol-induced increases in extracellular DA in the Acb (Kuzmin et al., 2008, Lof et al., 2007).

Animal models have been successfully used in developing treatments for a number of medical and psychiatric disorders (e.g., Griffin, 2002, McKinney, 2001). The selectively bred high alcohol-drinking (HAD-2) line of rat meets most of the proposed criteria (Cicero, 1979, Lester and Freed, 1973, McBride and Li, 1998) for a valid animal model of alcoholism (Bell et al., 2005, McBride and Li, 1998, Murphy et al., 2002). These animals will (1) as peri-adolescents (Bell et al., 2004) or adults (Bell et al., 2008), freely self-administer approximately 8 g/kg/day, or greater amounts, of ethanol under home-cage free-choice access conditions, when multiple concentrations of ethanol are made available; (2) achieve blood alcohol concentrations approximating 70 mg% (∼2.3 g/kg/1 h) after short deprivation periods and 150 mg% (∼6 g/kg/2 h) after longer deprivation periods under home-cage, free-choice self-administration conditions (Bell et al., 2008, Rodd et al., 2008, respectively); (c) self-administer ethanol under operant conditions (Files et al., 1998, Oster et al., 2006, Samson et al., 1998); and (d) display an alcohol deprivation effect (a model of relapse-like drinking [Bell et al., 2005, Rodd et al., 2004, Sinclair and Senter, 1967, Sinclair et al., 1973]) after repeated extended deprivation cycles under home cage (Rodd et al., 2008, Rodd-Henricks et al., 2000) and operant self-administration conditions (Oster et al., 2006). Thus, the HAD-2 rat line, as an animal model of alcoholism, provides an opportunity to assess the efficacy of different pharmacological agents in reducing excessive ethanol consumption.

In the present study, we examined the effects of two nAChR ligands, cytisine and lobeline, on the maintenance of home-cage, free-choice ethanol self-administration by HAD-2 rats. Cytisine is a plant alkaloid with a relatively rigid conformation (Mihalak et al., 2006). In binding assays, cytisine is found to be selective for the alpha4beta2 nAChR subunit combination, compared with other important nAChR subtypes such as the alpha3beta4 and alpha7 (Parker et al., 1998; Stauderman et al., 1998; Xiao et al., 2004). Furthermore, cytisine shows greater potency at alpha4beta2 nAChRs compared with many other subunit combinations in functional assays (Chavez-Noriega et al., 2000, Slater et al., 2003). Cytisine is a high-efficacy agonist at alpha7 nAChRs and at various beta4-containing nAChRs, such as alpha3beta4, and functions as a low-efficacy partial agonist at alpha4beta2 and other beta2-containing nAChRs (Carbonnelle et al., 2003, Papke and Porter-Papke, 2002; Stauderman et al., 1998). Recently, cytisine has shown potential as a smoking cessation treatment (Tutka and Zatonski, 2006) and a derivative of cytisine has been found to inhibit ethanol intake and ethanol-seeking behavior (Coe et al., 2005, Steensland et al., 2007). However, the behavioral effects of cytisine on ethanol consumption in an animal model of alcoholism have not been investigated thus far.

Lobeline is a naturally occurring alkaloid obtained from the Asian plant, Lobelia inflata. It is generally considered to be an agonist at nAChRs present in the central nervous system (see Dwoskin and Crooks, 2002, for review). Like cytisine, it shows promise as a treatment for smoking cessation (Nunn-Thompson and Simon, 1989). However, lobeline has high affinity for nAChRs and acts as a competitive and a nonselective antagonist at alpha4beta2 and alpha3beta2 nAChRs (Dwoskin and Crooks, 2002, Teng et al., 1997). Furthermore, lobeline was found to inhibit the function of dopamine transporter and vesicular monoamine transporter (VMAT) (Miller et al., 2004, Teng et al., 1997, Wilhelm et al., 2004). Lobeline inhibits the effect of psychostimulants in behavioral and neurochemical assays, such that lobeline reduces amphetamine-induced endogenous DA release from rat striatal slices (Miller et al., 2001). Lobeline was also found to inhibit nicotine-evoked [3H] DA overflow from rat striatal slices (Miller et al., 2000). Furthermore, lobeline has been shown to attenuate methamphetamine self-administration (Harrod et al., 2001). It is believed that the efficacy of lobeline to inhibit psychostimulant-induced effects is likely mediated by its activity at nAChRs and/or its ability to alter presynaptic DA storage and release (Miller et al., 2007). In addition to these effects, lobeline was found to function as a mu opioid receptor antagonist; however, it has less affinity than classical opioid receptor antagonist (Miller et al., 2007). Recently, lobeline has been shown to have some effect on alcohol preference in mice (Farook et al., 2009). Although the existing literature indicates that lobeline diminishes the behavioral and neurochemical effects of psychostimulants, as with cytisine, the effects of lobeline on ethanol consumption in an animal model of alcoholism have not been investigated thus far.