Effects of Chronic Alcohol and Repeated Deprivations on Dopamine D1 and D2 Receptor Levels in the Extended Amygdala of Inbred Alcohol-Preferring Rats

The dense dopaminergic (DA) projection from the ventral tegmental area (VTA) to the nucleus accumbens (Acb) is a key substrate mediating the natural rewarding properties of sex, food, and novelty (Hernandez and Hoebel, 1988; Pfaus et al., 1995). A consequence of obtaining these natural rewards is increased extracellular levels of DA in the Acb (Bozarth, 1991; Wise and Rompre, 1989). Similarly, administration of drugs of abuse (Di Chiara and Imperato, 1988; Imperato and Di Chiara, 1986) and ethanol (Melendez et al., 2002; Thielen et al., 2004) increase extracellular levels of DA in the Acb. While it is clear that the DA projection from the VTA to the Acb plays a key role in the rewarding effects of drugs of abuse, including ethanol, it is also apparent that multiple brain structures and neurotransmitter systems modulate this DA pathway (c.f., McBride, 2002; McBride and Li, 1998). In particular, the mesocorticolimbic circuit originates from the VTA with projections to the Acb, olfactory tubercle, frontal cortex, and in particular the amygdala complex (Asan, 1998). These DA innervations from the VTA are most intensely distributed to the central nucleus of amygdala (CeA), the bed nucleus of the stria terminalis (BNST), and Acb, collectively called the “extended amygdala” (EA) as has been suggested previously (Killcross et al., 1997; Palacios et al., 1990). The EA has extensive reciprocal connections with the VTA (Kalivas, 1993; Oades and Halliday, 1987) and is connected to the raphe nuclei, locus coeruleus, hippocampus, and ventral pallidum as well as other limbic structures implicated in reward (Azmitia and Segal, 1978; Fallon and Moore, 1978; Holstege et al., 1985; Krettek and Price, 1974; Nauta et al., 1978). Therefore, the EA and its interconnections with the mesocorticolimbic system form a complex reward circuit critical in mediating the rewarding properties of drugs of abuse, including ethanol, and the development of addiction and alcoholism (Koob and Le Moal, 2001; Koob et al., 1998a, 1998b). It has been shown that innate differences in the mesocorticolimbic DA system are associated with high ethanol consumption (McBride and Li, 1998; Murphy et al., 2002). In particular, alcohol-preferring (P) rats have lower levels of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the Acb and anterior striatum, compared with their non–alcohol-preferring (NP) counterparts (Murphy et al., 1982, 1987); our laboratory (Zhou et al., 1995) has reported a decreased number of DA neuronal projections from the VTA to the Acb in P rats, relative to NP rats. Moreover, operant oral self-administration of ethanol was found to increase DA overflow to a greater extent in the Acb of P rats than in unselected Wistar rats (Weiss et al., 1993), suggesting that the VTA DA system of the P line of rat may be more sensitive to the reinforcing actions of ethanol. Further support for this difference comes from the finding that P rats self-administer ethanol at lower concentrations directly into the VTA than Wistar rats (Rodd et al., 2004). Additionally, a role for DA activity in the Acb in mediating ethanol self-administration has been indicated by studies demonstrating changes in alcohol consumption following alterations in mesolimbic DA neurotransmission (Nowak et al., 2000; Rassnick et al., 1993; Samson et al., 1993). In vitro and in vivo electrophysiological and microdialysis findings indicate that ethanol can activate VTA neurons (Brodie and Appel, 1998; Brodie et al., 1999). Systemic administration of ethanol increases activity of VTA neurons (for a review, see Gessa et al., 1985) and extracellular levels of DA within the Acb and CeA. Moreover, oral ethanol self-administration and anticipation of ethanol have both been reported to increase extracellular levels of DA in the Acb as well (Weiss et al., 1996; c.f., McBride and Li, 1998). Studies have also shown that chronic alcohol altered DA release in central reward circuitry (Nishiguchi et al., 2002; Murphy et al., 1983; Syvalahti et al., 1988; Rothblat et al., 2001). Most of these studies were carried out with relatively short periods of drinking and addressed direct responses of DA neuro-transmission due to alcohol exposure. Overall, these results suggest that oral alcohol self-administration activates DA inputs to major components of the EA, and anticipation of ethanol alone can have a similar effect on these DA projections (McBride and Li, 1998). To better understand the effects of various patterns of chronic ethanol consumption on DA receptor levels, we investigated the effects of chronic alcohol including continuous access (C-Alc) and a repeated deprivation (RDAlc) protocol on dopamine D1 and D2 receptor binding site densities in brain structures associated with the EA. To date, there have been no published studies on changes in D1 and D2 receptor levels in the EA of iP, or P, rats using our extended 14-week chronic and repeated deprivation drinking protocols. The RD-Alc protocol was used to mimic the episodic drinking of alcohol abusers, where periods of high intake are interspersed with periods of abstinence, whether forced or voluntary (Burish et al., 1981; Hilbrom, 1990; McMillen, 1997).