Monocyte-Derived Macrophages Expressing Dopamine D2-Subtype Receptors Drive Alcohol Effects on Mesolimbic Neurons and Microglia

<b>Background/Objectives</b>: Microglia are the primary immune cells in the central nervous system (CNS) and are known as "resident" macrophages. The aim of this study was to determine the effect of acute ethanol (EtOH) on the microglia state and monocyte infiltration into the CNS, with particular attention to the role of peripheral and central dopamine (DA) D2 receptors (D2Rs) in mediating EtOH effects on peripheral and central substrates. We hypothesize that EtOH interacts with peripheral immune mediators via D2Rs including monocyte-derived macrophages (MDMs) to modulate midbrain neurons, DA transmission in the mesolimbic pathway from the ventral tegmental area (VTA) to nucleus accumbens (NAc), and the intoxicating effects of acute EtOH. <b>Methods</b>: Using the Macrophage FAS-Induced Apoptosis (MaFIA) mouse model (GFP+ on Csf1r promoter), we assessed the effects of three intraperitoneal (IP) doses of EtOH (1, 2, and 4 g/kg) at three time points (0.5, 1, and 2 h after injection) on D2R expression in blood leukocytes and microglia, as well as midbrain neuronal activity, DA release, and behavior. <b>Results</b>: Acute EtOH significantly enhanced lymphocyte and monocyte D2R expression at 1.0 g/kg by 2 h after injection in vivo but decreased D2R expression in vitro. Ethanol enhanced microglia D2R expression in the NAc, while not altering D2R expression in the VTA, but altered the microglia state in these areas, shifting them toward an inflammatory phenotype. Acute EtOH induced prolonged and progressive hypersensitivity of D2R activation of VTA GABA neurons. Intravenous injection of the macrophage depleter liposomal clodronate significantly reduced blood macrophages by 55.3% and blocked the typical inhibition of VTA GABA neurons by EtOH, as well as the enhancement of DA levels in the NAc, and the locomotor indices of intoxication produced by acute EtOH, but not choice place preference. <b>Conclusions</b>: These findings strongly suggest a neuroimmune peripheral connection for acute low-dose EtOH use and challenge the dogma that central actions of EtOH exclusively mediate its effect on DA neuronal activity and release.