Comparisons of increased levels of CCL2 in post-mortem human alcoholic brain and mouse brain following chronic ethanol treatment. Shown is data from different studies within our laboratory illustrating increased levels of the chemokine MCP-1 (CCL2) in humans and mice following ethanol exposure. (A) MCP-1 protein levels from human hippocampal homogenate measured using ELISA. Increased MCP-1 levels were also found in ventral tegmental area, substantia nigra, and amygdala (see He and Crews, 2008). (B) Levels of MCP-1 in mouse brain increased following chronic ethanol treatment. Mice (C57Bl/6) treated with 10 daily doses of ethanol (5.0 g/kg. i.g.) and brain MCP-1 levels were determined 24 h after the last ethanol administration (see Qin et al., 2008 for details). These studies indicate that ethanol upregulates the innate immune chemokine MCP-1 in post-mortem human alcoholic and mouse brain samples, which is consistent with ethanol activation of innate immune genes.

Ventral hippocampus shows greater activation (pMAPK + IR) and neurotoxicity (Silver stain) following binge ethanol treatment. Coronal sections of rats exposed to a 4-day binge ethanol model are shown [approximately −5.80 mm from bregma, adapted from Crews et. al. (2006a; The position of the coronal histological sections are depicted in the sagittal diagram in the upper left corner. The histological coronal sections show both dorsal (upper) and ventral (lower) dentate gyrus of hippocampus)]. Left: Mitogen-activated protein kinase (MAPK) is a family of kinases activated by phosphorylation. Phosphorylated MAPK (pMAPK) provides an index of kinase activation. Note the lower-ventral dentate gyrus, hippocampus, and entorhinal cortex (indicated by black arrows) contains more pMAPK + IR than the upper-dorsal sections consistent with greater activation of ventral hippocampus. Right: Silver stain identifies dying neurons (see Crews et. al., 2006a). Note that the ventral hippocampus contains many more silver stained neurons (black arrowheads) compared to the ventral hippocampus. Boxes on the right show higher magnification of silver stained dorsal and ventral hippocampus. These findings are consistent with ethanol causing greater emotional ventral hippocampus activation (pMAPK + IR) and cell death (silver stain).

Nuclear factor kappa-light-chain-enhancer of activated B cells transcription increases the expression of chemokines, cytokines, oxidases, and proteases. The transcription factor NF-κB is involved in the induction of innate immune genes (Ghosh and Hayden, 2008). Stimuli such as stress, drugs of abuse, peptides, chemokines, cytokines, reactive oxygen species (ROS), ultraviolet irradiation, bacteria, viruses, trauma, and other factors all increase NF-κB-DNA binding and transcription. Reactive oxygen species resulting from oxidases such as NADPH-oxidase or ethanol metabolism by CYP2E1 increase NF-κB transcription of NOX2^phox, a key NOX catalytic subunit (Cao et al., 2005) that produces ROS (Qin et al., 2008). Loops of activation also occur through induction of genes that stimulate further NF-κB activation leading to autocrine and paracrine amplification and persistent signals. Cytokines and chemokines, such as TNFα, IL1β, IL6, and MCP-1 as well as their receptors (TNFR in figure), are also induced resulting in amplification loops. Toll-like receptors are increased by ethanol (Dolganiuc et al., 2006; Alfonso-Loeches et al., 2010) as are other damage-associated molecular pattern receptors and there agonists resulting in the formation of positive activation loops (Garg et al., 2010). Toll-like receptors and HMGB1 interact to create another activation-amplification loop. Persistent and repeated activation occurs through positive cycles of activation. These loops spread innate immune signaling across the brain causing altered neurocircuitry and neurobiology. Figure abbreviations: CYP2E1, cytochrome P450 2E1; ECM, extracellular matrix; EtOH, ethanol; gp91, NADPH-oxidase flavocytochrome b components; HMGB1, high-mobility group box 1; IL-1β, interleukin-1 beta; IL1, interleukin-1; LPS, lipopolysaccharide; MMP, matrix metalloproteinase; MCP-1, monocyte chemoattractant protein-1; NOX, nicotinamide adenine dinucleotide phosphate (NADPH) oxidases; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; TACE, TNFα converting enzyme; TLR, toll-like receptor; TNFα, tissue necrosis factor-alpha; tPA, tissue plasminogen activator.

Chronic ethanol self-administration induces depression-like behavior and inhibits hippocampal neurogenesis. C57BL/6J mice self-administered either ethanol (10% v/v) or water for 28 days. (A) Abstinence-induced increase in immobility (seconds) on the forced swim test provides an index of depression-like behavior. Abstinence from chronic ethanol consumption resulted in increased negative affect. (B) Ethanol self-administration decreased PCNA, a marker of cell proliferation, in the neurogenic region of the hippocampal dentate gyrus. (C) Ethanol self-administration decreased doublecortin expression, a marker of neurogenesis, in the dentate gyrus. Reduced progenitor cell proliferation and neurogenesis is associated with increased depression-like behavior. Furthermore, these studies are consistent with the research suggesting that decreased hippocampal neurogenesis is linked to depression. Finally, desipramine treatment, an anti-depressant, reversed both the reduced hippocampal neurogenesis and the depression-like behavior in abstinent mice (see Stevenson et al., 2009).

The neurobiology of addiction. (A) Flow chart distinguishing the frontal–cortical and limbic changes associated with drug addiction. Both stress and drug abuse activate innate immune gene expression, which increases limbic activation and disrupts frontal–cortical function. (B) A simplified schematic of the frontal–cortical and limbic circuitry that contributes to addictive behavior. Depicted is a rat brain with internal structures highlighted and accompanying projections (as indicated by black arrows). The frontal–cortical areas include the medial prefrontal, anterior cingulate, and orbitofrontal cortices, and are involved in attention, goal setting, planning, and impulse control (Schoenbaum et al., 2006; Schoenbaum and Shaham, 2008). The limbic circuitry, comprising the nucleus accumbens (NAcc), amygdala (AMG), hippocampus (HPC), and ventral tegmental area (VTA), is involved in emotion, learning, and memory. Acute drug abuse activates frontal–cortical attention mechanisms, prompting limbic learning. Similarly, innate immune gene induction in the brain leads to PFC hyperexcitability (Zou and Crews 2005; Crews et al., 2006a) that inactivates frontal–cortical regulation of limbic structures (Gruber et al., 2010). Innate immune gene induction in limbic regions increases negative affect and depression-like behaviors prompting further drug abuse and self-medication. The harmful consequences of prolonged alcohol, opiate and stimulant drug dependence result in diminished activation of frontal–cortical circuits leading to a loss of attention and poor decision combined with increasing urgency and negative affect motivating persistent drug taking behaviors. Decreased ventral hippocampal activation likely contributes to frontal hyperexcitability and loss of cognitive flexibility (Gruber et al., 2010). Thus, an inactivated PFC, loss of behavioral flexibility, and increasing limbic negative emotion characterizes the drug-addicted brain.