Role of intestinal permeability and inflammation in the biological and behavioral control of alcohol-dependent subjects

Abstract

Background and aims

Mood and cognition alterations play a role in the motivation for alcohol-drinking. Lipopolysaccharides are known to stimulate inflammation that was shown to induce mood and cognitive changes in rodents and humans. Enhanced intestinal permeability and elevated blood LPS characterize alcohol-dependent mice. However, no data have been published in non-cirrhotic humans. Our first goal was to test whether intestinal permeability, blood LPS and cytokines are increased in non-cirrhotic alcohol-dependent subjects before withdrawal and if they recover after withdrawal. Our second goal was to test correlations between these biochemical and the behavioral variables to explore the possibility of a role for a gut–brain interaction in the development of alcohol-dependence.

Methods

Forty alcohol-dependent-subjects hospitalized for a 3-week detoxification program were tested at onset (T1) and end (T2) of withdrawal and compared for biological and behavioral markers with 16 healthy subjects. Participants were assessed for gut permeability, systemic inflammation (LPS, TNFα, IL-6, IL-10, hsCRP) and for depression, anxiety, alcohol-craving and selective attention.

Results

Intestinal permeability and LPS were largely increased in alcohol-dependent subjects at T1 but recovered completely at T2. A low-grade inflammation was observed at T1 that partially decreased during withdrawal. At T1, pro-inflammatory cytokines were positively correlated with craving. At T2 however, the anti-inflammatory cytokine IL-10 was negatively correlated with depression, anxiety and craving.

Conclusion

Leaky gut and inflammation were observed in non-cirrhotic alcohol-dependent subjects and inflammation was correlated to depression and alcohol-craving. This suggests that the gut–brain axis may play a role in the pathogenesis of alcohol-dependence.

Introduction

Alcohol-dependence is a disorder that is present in 5–7% of the population of developed countries (Anderson and Baumberg, 2006, Grant et al., 2004). In addition to its familial, social and professional consequences, it is also a major risk factor for several pathologies including liver or pancreatic diseases, cardiac diseases, cancers and neurological or psychiatric disorders. This addiction is also the first cause of malnutrition in developed countries. Indeed, both human and animal studies have shown that heavy chronic alcohol consumption leads to mucosal damages that will interfere with the absorption of micro and macronutrients and with mucosal enzyme activities contributing to malnutrition (Bode and Bode, 2003). Alcohol-fed animals are characterized by an intestinal hyperpermeability and possibly a Gram-negative bacterial overgrowth in the upper small intestine that contribute to increased leakage of lipopolysaccharides (LPS) into the circulation (Adachi et al., 1995, Yan et al., 2010). LPS are potent pro-inflammatory agents that through a CD14-TLR4 receptor mechanism (Akira and Hemmi, 2003) activate the transcriptional factor NFκB to induce the production of inflammatory mediators such as pro-inflammatory cytokines, chemokines, nitric oxide and reactive oxygen species (Wheeler, 2003). This LPS activated inflammatory process has been thought to play an important role in the development of alcoholic liver disease (Wheeler, 2003). These mechanisms have been mainly studied in rodent models of alcohol-dependence (Adachi et al., 1995, Mathurin et al., 2000, Rao et al., 2004, Wheeler, 2003, Yan et al., 2010) but data on whether this also occurs in alcohol-dependent humans are scarce.

Among the human studies, increased circulating LPS levels have been reported in subjects that have already developed liver damage and cirrhosis (Fukui et al., 1991, Parlesak et al., 2000) where the increased LPS levels have been also be attributed to dysfunctional Kupffer cells with reduced ability to detoxify endotoxins (Rao et al., 2004). Therefore, the rise in plasma LPS and subsequent inflammation in these patients can also be the consequence of the liver disease. The analysis of the effects of alcohol consumption in non-cirrhotic alcoholics would allow us to test the hypothesis that changes in intestinal permeability, LPS and low-grade inflammation may not only be a consequence but also play a role in continuing alcohol-dependence.

The influence of circulating cytokines on the human brain functions and the development of depression is supported by several studies (Dantzer and Kelley, 2007, Dantzer et al., 2008, Maes, 1999). Furthermore, an infection (Dantzer et al., 2008, Konsman et al., 2002) or the injection of LPS (Reichenberg et al., 2001) will induce several symptoms (fever, fatigue, anorexia, sleep abnormalities, loss of interest in social activities), and cognitive disturbances (e.g. memory or attention dysfunctions) that are close to symptoms of depression. These symptoms are induced by the effect of inflammation on the brain through both humoral and autonomic pathways (Konsman et al., 2002). These observations support the idea that inflammation induces a Sickness Behavior, which may play a role in the pathophysiology of some psychiatric diseases. Consistent with this idea, depressed subjects have higher plasma pro-inflammatory cytokines concentrations than controls (Dowlati et al., 2010, Maes, 1999). On the other hand, the anti-inflammatory cytokine IL-10 seems to prevent LPS-mediated mood and cognition disturbances (Bluthe et al., 1999, Richwine et al., 2009, van den Boogaard et al., 2010). In addition, depression plays an important role in the behavioral mechanism of addiction (Schuckit, 1994) and has been shown to be strongly and consistently correlated to alcohol craving (Andersohn and Kiefer, 2004) which could be defined as the appetitive urge to drink alcohol (Anton, 1999). Also, recent psychological studies indicated that negative affects (depression, anxiety) and craving largely improve during the alcohol-withdrawal (Cordovil De Sousa Uva et al., 2010, De Timary et al., 2008). Selective attention also improved during the detoxification programme whereas other cognitive functions such as executive functions (inhibition, flexibility, decision making) remained unchanged (Cordovil De Sousa Uva et al., 2010).

Our first goal was to show that heavy chronic alcohol consumption induced an increase in intestinal permeability. Then, we tested whether this hyperpermeability was associated with a rise in plasma LPS levels (Cani et al., 2007, Cani et al., 2008) and systemic inflammatory response. Finally, we hypothesized that these biological changes would in turn influence the severity of depressive symptoms, the intensity of craving and consequently alcohol consumption. To test these hypotheses, alcohol-dependent subjects were carefully selected upon diagnosis as non-cirrhotic. They were assessed for intestinal permeability, plasma LPS levels, plasma pro- and anti-inflammatory cytokines levels, high-sensitivity C-reactive protein (hsCRP) levels and for behavioral measures both at the onset and end of alcohol-withdrawal. This alcohol-dependent group was compared to a control group consisting of individuals matched for age and body mass index (BMI) and who socially consume low amounts of alcohol.