Acute and Chronic Ethanol Modulate Dopamine D2‐Subtype Receptor Responses in Ventral Tegmental Area GABA Neurons

The mesocorticolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAcc) is considered to be a key area in reward from natural behaviors such as feeding (Phillips et al., 2003), drinking (Agmo et al., 1995), and drug reward, including alcohol reward (Koob, 1996). Support for a role for this neural circuit in alcohol reward hinges on the evidence demonstrating that local injections of DA antagonists into the NAcc and/or afferent circuit systems prevent ethanol self-administration in rats (Hyytia and Koob, 1995; Pettit et al., 1984; Roberts et al., 1996; Vaccarino et al., 1985). Moreover, rats will self-administer ethanol directly into the VTA (Gatto et al., 1994), and an ethanol-induced increase of DA release in the NAcc, detected by microdialysis, has been reported extensively (Di Chiara and Imperato, 1988; Weiss et al., 1993; Wozniak et al., 1991; Yoshimoto et al., 1992). Acute ethanol increases the firing rate of midbrain DA neurons both in vivo and in vitro (Brodie et al., 1990a; Gessa et al., 1985), and chronic ethanol reduces both DA activity and release of DA in the NAcc during withdrawal (Diana et al., 1993). Although mesencephalic DA neurons in the midbrain are excited by ethanol, it has been suggested that their excitation may be attributed to disinhibition produced by a primary inhibitory effect on γ-aminobutyric acid (GABA)-containing neurons of the midbrain (Mereu and Gessa, 1985). Accordingly, we have demonstrated that acute ethanol reduces VTA GABA neuron firing rate (Gallegos et al., 1999) and cortico-tegmental excitatory synaptic responses (Stobbs et al., 2004) in vivo, with an IC50 of 1.0 g/kg (100 mg percentage blood alcohol level), a moderately intoxicating dose, at a substantial fraction of the EC50 for ethanol excitation of DA neurons in vitro (Brodie et al., 1999). Moreover, VTA GABA neurons become hyperexcitable during ethanol withdrawal and evince tolerance to ethanol inhibition of firing rate during chronic ethanol (Gallegos et al., 1999), suggesting that GABA neurons in the VTA constitute a critical substrate for the acute and chronic effects of ethanol in the mesocorticolimbic DA system (Diana et al., 2003). Theoretically, inhibition of VTA GABA neurons by acute ethanol would result in hyperexcitability of DA neurons and an increased amount of DA release, while chronic ethanol would result in hypoexcitability of DA neurons due to hyperexcitability of GABA neurons. Dopamine neurons in the midbrain are inhibited by DA via D2 autoreceptor activation (for review see Adell and Artigas, 2004). GABA neurons in the VTA are activated by DA (Lassen et al., 2007; Stobbs et al., 2004) via D2 receptors (Steffensen et al., 2008). As ethanol inhibits and DA enhances VTA GABA neuron activity, the aim of this study was to evaluate the role of DA in acute ethanol effects, as well as the effects of chronic ethanol consumption on the potential adaptation of VTA GABA neuron activity and D2 receptors. As others have shown that critical DA-related gene products in the VTA, such as tyrosine hydroxylase are up-regulated (Ortiz et al., 1995), and D2 receptors are down-regulated (Rommelspacher et al., 1992), during withdrawal from chronic ethanol, and we have shown that VTA GABA neurons become hyperexcitable with chronic ethanol (Gallegos et al., 1999), we hypothesized that D2 receptor expression in VTA GABA neurons would adapt in association with chronic ethanol consumption.