A Single, Moderate Ethanol Exposure Alters Extracellular Dopamine Levels and Dopamine D2 Receptor Function in the Nucleus Accumbens of Wistar Rats

The mesolimbic dopamine (DA) circuit is an important substrate mediating appetitive or reward-related activities, including consumption of ethanol (EtOH) and other drugs of abuse (Di Chiara et al., 2004; Gonzales et al., 2004; Koob et al., 1998; Spanagel and Weiss, 1999). EtOH’s interactions with the mesolimbic DA system have been well investigated in order to elucidate the neurochemical correlates of alcohol abuse disorders. Research investigating the projections from DA cell bodies in the ventral tegmental area to terminals in the nucleus accumbens (NAc) has generated substantial support that this mesolimbic DA system mediates at least some of the adaptive effects of EtOH, as well as other drugs of abuse, that may underlie addictive properties of these drugs (see Pierce and Kumaresan, 2006 for review). Activation of the mesolimbic DA system by EtOH is evidenced by significant increases in extracellular DA levels in the NAc following operant oral self-administration of EtOH using in vivo microdialysis (Gonzales and Weiss, 1998; Melendez et al., 2002; Weiss et al., 1993, 1996). Similar increases in extracellular DA have been reported following local (Wozniak et al., 1991; Yoshimoto et al., 1992a) and systemic (Imperato and DiChiara, 1986; Smith and Weiss, 1999; Yoshimoto et al., 1992a) exposure to EtOH. Neuroadaptive effects of EtOH on the mesolimbic DA system have also been reported by a number of investigators. Smith and Weiss (1999), for example, utilized no-net-flux microdialysis (Justice, 1993; Parsons and Justice, 1992) and found alterations in DA levels in the NAc core subregion following repeated (5) 1.0 g/ kg intraperitoneal (i.p.) EtOH pretreatments. They observed that outbred Wistar and selectively bred alcohol-preferring (P) rats exhibited similar increases in basal DA levels following EtOH exposure. Moreover, these results were observed at least 15 hours after the last injection, indicating that the effects of EtOH pretreatment persisted after EtOH cleared the system (Smith and Weiss, 1999). Analyses of extraction fractions in these animals indicated that the observed neuroadaptions did not result from alterations in NAc DA clearance (Smith and Weiss, 1999). Many previous reports support the hypothesis that the observed elevations in NAc DA levels following EtOH exposure are not mediated by changes in clearance (e.g., Budygin et al., 2005; Engleman et al., 2003; Jones et al., 2006; Ramachandra et al., 2007). Contradictory results, however, have been reported in High Alcohol Drinking replicate line-1 (HAD-1) rats (Carroll et al., 2006), as well as in P rats, following periadolescent EtOH exposure (Sahr et al., 2004). Several methods have been employed to evaluate the manner in which EtOH experience alters DA neurotransmission in the NAc. One approach has been to investigate potential modifications in DA D2 autoreceptor regulation, following EtOH exposure. PET scan research has indicated that human alcoholics exhibit a diminished number of DA D2 receptors, but not transporters, in the striatum compared with nonalcoholics (Volkow et al., 1996). Further, this population of receptors has been found to be significantly greater in nonalcoholic family members of alcoholics, compared with those family negative for alcoholism, suggesting that the receptors may present a protective mechanism against the condition (Volkow et al., 2006). Prolonged EtOH drinking alters DA D2 receptor functioning in the NAc of P rats under both 1-hr limited (Engleman et al., 2003) and continuous (Thielen et al., 2004) EtOH access paradigms, as well as following EtOH deprivation (Thielen et al., 2004). Both of these studies reported a diminished response to DA D2-like receptor antagonist, (−)sulpiride (SUL), following EtOH experience. In addition, Levy and colleagues (1991) reported that intra-NAc SUL injections led to increases in EtOH intake in P rats. These studies are suggestive of a reciprocal relationship between EtOH drinking and DA D2 receptor function. Further, the studies by Engleman and colleagues (2003) and Thielen and colleagues (2004) indicate that chronic and/ or repeated EtOH exposure may alter the mesolimbic DA system via functional changes to DA D2 receptors in the NAc, and likely through associated changes in DA regulatory mechanisms. The previous studies examined changes in neurobiological measures after repeated or chronic exposure to EtOH. However, few studies have assessed the effects of brief EtOH administration on subsequent D2 autoreceptor function. Research in this area would be informative in characterizing the onset of vulnerability to alcoholism, particularly following the first experience with ethanol. The current study evaluated the effects of brief EtOH exposure on DA neurotransmission in the NAc. Experiment 1 was undertaken to determine whether systemic EtOH pretreatment would alter DA D2 receptor function, in a dose- and experience-dependent manner. Previous evidence (Engleman et al., 2003; Thielen et al., 2004) indicated that EtOH pretreatment attenuated SUL-induced increases in extracellular DA in the NAc. Further, the current study tested the hypothesis that repeated systemic EtOH exposures (similar to those employed by Smith and Weiss, 1999) would have a greater effect to diminish DA D2 receptor response to SUL than would a single exposure. Experiment 2 was performed to quantify the basal extracellular concentration and extraction fraction of NAc DA, following a single 1.0 g/ kg EtOH treatment. Previous reports by Smith and Weiss (1999) indicated repeated i.p. exposure to 1.0 g/ kg EtOH would alter DA neurotransmission, however, it was unclear whether a single EtOH exposure would be sufficient to produce similar results. This experiment tested the hypothesis that a single, moderate exposure to EtOH would alter DA neurotransmission, but not clearance, in the NAc.