Epigenetic basis of the dark side of alcohol addiction

Abstract

Alcoholism is a complex brain disease characterized by three distinct stages of the addiction cycle that manifest as neuroadaptive changes in the brain. One such stage of the addiction cycle is alcohol withdrawal and the negative affective states that promote drinking and maintain addiction. Repeated alcohol use, genetic predisposition to alcoholism and anxiety, and alcohol exposure during crucial developmental periods all contribute to the development of alcohol-induced withdrawal and negative affective symptoms. Epigenetic modifications within the amygdala have provided a molecular basis of these negative affective symptoms, also known as the dark side of addiction. Here, we propose that allostatic change within the epigenome in the amygdala is a prime mechanism of the biological basis of negative affective states resulting from, and contributing to, alcoholism. Acute alcohol exposure produces an anxiolytic response which is associated with the opening of chromatin due to increased histone acetylation, increased CREB binding protein (CBP) levels, and histone deacetylase (HDAC) inhibition. After chronic ethanol exposure, these changes return to baseline along with anxiety-like behaviors. However, during withdrawal, histone acetylation decreases due to increased HDAC activity and decreased CBP levels in the amygdala circuitry leading to the development of anxiety-like behaviors. Additionally, innately higher expression of the HDAC2 isoform leads to a deficit in global and gene-specific histone acetylation in the amygdala that is associated with a decrease in the expression of several synaptic plasticity-associated genes and maintaining heightened anxiety-like behavior and excessive alcohol intake. Adolescent alcohol exposure also leads to higher expression of HDAC2 and a deficit in histone acetylation leading to decreased expression of synaptic plasticity-associated genes and high anxiety and drinking behavior in adulthood. All these studies indicate that the epigenome can undergo allostatic reprogramming in the amygdaloid circuitry during various stages of alcohol exposure. Furthermore, opening the chromatin by inhibiting HDACs using pharmacological or genetic manipulations can lead to the attenuation of anxiety as well as alcohol intake. Chromatin remodeling provides a clear biological basis for the negative affective states seen during alcohol addiction and presents opportunities for novel drug development and treatment options. This article is part of the Special Issue entitled "Alcoholism".

Keywords: Addiction; Alcoholism; Anxiety; Dark side; Epigenetics; Gene expression.

Figure 1. The epigenetic allostasis model of the dark side of alcohol addiction

The epigenome is dynamically altered by acute alcohol, chronic alcohol, and alcohol withdrawal in the amygdala and is crucial for the progression from casual use to addiction. Acute alcohol exposure opens the chromatin by inhibiting histone deacetylases (HDACs), thereby increasing histone acetylation around crucial synaptic plasticity-related genes such as brain-derived neurotrohic factor (Bdnf), activity regulated cytoskeleton-associated protein (Arc), and neuropeptide Y (Npy) in the amygdala, leading to anxiolysis. However, these biological parameters normalize (=) with continued alcohol exposure. During ethanol withdrawal, HDAC activity is increased leading to condensed chromatin structure and decreased expression of these genes as well as dendritic spine density. As the amygdala controls negative affective states, amygdalar chromatin conformation is critical for the development of anxiety seen in ethanol-withdrawn animals. Additionally, the condensed chromatin state is associated with greater ethanol preference and self-administration, ostensibly to relieve the negative affective states experienced. This switch from consuming alcohol for its pro-social, anxiolytic effects to drinking as a means to relieve negative affective states is critical to the addiction cycle and is reflected in the underlying epigenome of the amygdala ( Krishnan et al., 2014; Pandey et al., 2008a, b; Sakharkar et al., 2012; You et al., 2014).

Figure 2. Innately condensed chromatin leads negative affective states and alcoholism

Animals bred for alcohol preference, such as alcohol-preferring (P) rats, display increased histone deacetylase (HDAC) activity and HDAC2 levels in the amygdala compared to non-preferring (NP) rats, leading to decreased histone acetylation around synaptic plasticity-related genes such as brain-derived neurotrophic factor (Bdnf) and activity regulated cytoskeleton-associated protein (Arc). The resultant decrease in BDNF and Arc expression contributes to decreased amygdaloid synaptic plasticity, increased anxiety-like behaviors, and increased alcohol preference. However, HDAC inhibition by trichostatin A (TSA) or specific inhibition of HDAC2 via small interfering RNA (siRNA) normalizes the histone acetylation deficits in the amgydala of P rats and also attenuates anxiety-like and alcohol drinking behaviors (Moonat et al., 2013; Sakharkar et al., 2014b).

Figure 3. Alcohol exposure during development leads to persistent changes in epigenetic mechanisms in amygdala and produces long-lasting anxiety and excessive alcohol intake in adulthood

Exposure to adolescent intermittent ethanol (AIE) during this critical developmental period causes increased HDAC2 in the amygdala leading to decreased global and Bdnf- and Arc-specific histone acetylation. Decreased BDNF and Arc expression in the adult amygdala of AIE animals is accompanied by increased alcohol preference and anxiety-like behaviors. Aside from histone acetylation, lysine demethylase 1 (LSD1) and particularly the neuron-specific splice variant Lsd1+8a, are decreased in the AIE adult amygdala. This decrease is accompanied by a resultant increase in histone H3K9 dimethylation (H3K9me2) both globally and at the Bdnf exon IV promoter, possibly explaining the decreased BDNF expression seen in these rats. Thus, increased HDAC2 and decreased LSD1 associated with AIE-induced chromatin remodeling may provide novel targets for intervention in alcohol addiction and comorbid anxiety (Kyzar et al., 2016a, b; Pandey et al., 2015; Sakharkar et al., 2014a).