Alcoholism is a devastating disease that affects over 11 million individuals in the USA (Williams et al. 1987). Epidemiological studies have documented an important role of genetics in predisposition to alcoholism as evidenced by the higher concordance for alcoholism in identical twins than in fraternal twins and by the fourfold increased risk for alcoholism in children of alcoholics than in the general population (Cotton 1979; Goodwin 1979; Cloninger 1987; Schuckit and Gold 1988). The biological mechanisms underlying the predisposition to alcoholism are poorly understood. A target neurotransmitter for alcoholism is dopamine (DA) since it is believed to underlie the reinforcing effects of drugs of abuse including those of alcohol (Koob et al. 1987; Wise and Bozarth 1987; Di Chiara et al. 1992; Weiss 2000). It has been proposed that DA is one of the neurotransmitters that modulate the predisposition to alcohol abuse (Blum et al. 1996; George et al. 1995; Di Chiara et al. 1996; Repo et al. 1999; Li 2000). It is the projections of the DA cells to the nucleus accumbens (NAc) that have been implicated in the reinforcing effects of alcohol (Koob et al. 1987); where alcohol dose-dependently increases dopamine concentration (Le and Kiianmaa 1988; Weiss et al. 1993).
Chronic alcoholism has been shown to produce significant changes in dopamine D2 receptors (DRD2) concentrations (Tajuddin and Druse 1996). More importantly, of the dopamine receptor subtypes, the (DRD2) appear to be involved in transmitting the dopamine mediated reinforcing effects of alcohol (Stefanini et al. 1992; McBride et al. 1993b; Nowak et al. 2000). This was evidenced, by the reduced reinforcing effects of alcohol in DRD2 knockout mice (Phillips et al. 1998). Studies in human subjects also implicate DRD2 in alcoholism (Volkow et al. 1996a; Guardia et al. 2000). Alcoholics have reduced levels of DRD2 in brain (Volkow et al. 1996b) and epidemiological studies, though not always consistent (Cook et al. 1996), show a higher frequency of the DRD2 A1 Taq allele (Blum et al. 1990; Noble et al. 1991; Parsian et al. 1991; Persico et al. 1996; Eriksson et al. 2000), which is an allele associated with low DRD2 density (Noble et al. 1991). Because the higher frequency of the A1 Taq allele as well as the reduction in brain DRD2 has also been documented with other drugs of abuse, it has been hypothesized that low levels of DRD2 predispose subjects to use drugs or alcohol as a means to compensate for the decrease in the activation of the associated reward circuits (Phillips et al. 1998). Though this hypothesis could be perceived as being in apparent conflict with the results from animal studies showing that removal of DRD2 results in a decrease in alcohol consumption (Phillips et al. 1998; Myers and Robinson 1999), one can not assume that the behavioral effects of absence of DRD2 receptors can be extrapolated to the behavioral consequences of variability in DRD2 receptor levels. Nor is it possible to predict on the basis of the data from the DRD2 knockout mice, the dose effect relationship between the levels of DRD2 and the effects on the behavior.
The purpose of this study was to investigate if we could modulate alcohol intake by varying the levels of DRD2 in the NAc, which is the brain region associated with the reinforcing effects of drugs of abuse (Pontieri et al. 1996). Brain levels of DRD2 in rats were modified using a replication-deficient adenoviral vector containing the rat cDNA insert for DRD2 (AdCMV.DRD2), which was injected into the NAc. Gene transfer via adenoviral vector is an effective strategy that can be utilized to introduce particular genes into tissue and provides a high specificity targeting and delivery (Crystal 1992; Suhr and Gage 1993). The effectiveness of the present vector for intracerebral transfer of DRD2 as well as the expression of functional DRD2 effects has been previously well established (Ikari et al. 1995, 1999; Umegaki et al. 1997; Ingram et al. 1998; Ogawa et al. 2000). Specifically, injection of the DRD2 adenoviral vector in the rat brain has been shown to increase the expression of functional DRD2 and that in vitro autoradiography visualizes this overexpression. More recently, we demonstrated that positron emission tomography (PET) was able to image the overexpression of DRD2 induced by adenoviral-mediated gene transfer into the rat brain (Ogawa et al. 2000). In addition, this overexpression of DRD2 by adenoviral vector was not strain or species specific and could be visualized by both PET and autoradiography (Ogawa et al. 2000)., Ethanol intake was assessed using the popular sucrose-fading procedure (Samson 1986; Tolliver et al. 1988; Samson et al. 1989) and analyzed in terms of overall ethanol preference in a two-bottle choice paradigm. This technique has been widely used to train animals to drink ethanol and preference ratio is reflective of the CNS pharmacological effects of ethanol rather than attributed to taste as a factor (Samson et al. 1996).
Our working hypothesis was that DRD2 overexpression induced by vector administration would alter the reinforcing effects of alcohol and change ethanol intake.