Full length articleVoluntary wheel running protects against the increase in ethanol consumption induced by social stress in mice
Introduction
Social stress is deeply implicated in the neural and behavioral alterations that contribute to the development of mental health disturbances and drug addiction (Beutel et al., 2018). Stressful experiences modify the reward system and are involved in the transition from drug abuse to addiction, causing an increase of intake and drug-seeking behaviors (Koob and Schulkin, 2019; Miczek et al., 2008; Montagud-Romero et al., 2016, Montagud-Romero et al., 2018; Ruisoto and Contador, 2019). Social defeat (SD) is one of the most commonly used animal models to study the effects of stressful experiences. In this model, the experimental subject is repeatedly confronted with an aggressive opponent mouse (Miczek et al., 2004). SD induce a short-term increase in consumption of ethanol (EtOH) using oral self-administration (SA) with a higher motivation to get the drug (Van Erp and Miczek, 2001; Norman et al., 2015). In a previous study, we found that the effects of repeated social defeat (RSD) can be long-lasting, since mice exposed to RSD during adolescence showed higher ethanol consumption rates and a greater motivation to get the drug during adulthood (Rodríguez‐Arias et al., 2016). Even after 6 months since the last stress exposure, defeated animals showed an enhanced motivation for ethanol intake (Riga et al., 2014). In addition, many studies support that social stress is one of the most important factors that influence the increase and escalation in ethanol consumption. Using voluntary ethanol intake, in the two bottle choice (TBC) task, SD produces an escalation in the consumption of alcohol after 10 days since the last exposure of stress (Norman et al., 2015; Hwa et al., 2016; Karlsson et al., 2017; Newman et al., 2018), although this effect is not observed immediately after being exposed to stress (Lopez et al., 2016). This increase in ethanol consumption induced by social stress could be due to stress-induced neuroadaptations, which ultimately produce changes in the hypothalamic, extrahypothalamic and mesocorticolimbic circuits, which are related to stress and reward (Holly et al., 2016; Hwa et al., 2016; Laine et al., 2017; Newman et al., 2018).
Nowadays, physical activity has emerged as a modulator of higher mental functions. Voluntary wheel running (VWR) in rodents produces enhanced learning, neurogenesis, angiogenesis, increases in neurotrophic factors and changes in several signaling molecules, as well as a reduction in behaviors associated with stress (Salam et al., 2009; Mul, 2018). VWR exercise after SD reduced social avoidance and anhedonia in rodents (Mul et al., 2018; Watanasriyakul et al., 2018; Zhang et al., 2019). It is known that physical exercise regulates some components of the hypothalamic-pituitary-adrenal axis (HPA), generating an adaptive response to stress (Pietrelli et al., 2018). Moreover, rats exposed to long-term access to VWR showed alterations in gene transcription factors involved in reward and dopaminergic neurotransmission in the mesolimbic reward pathway, developing conditioned place preference (CPP) to the compartment associated with physical exercise (Greenwood et al., 2011). Therefore, mice consumed significantly less ethanol in the unlimited access TBC model when they had access to the wheel (Ehringer et al., 2009; Darlington et al., 2014, 2016).
A number of recent reports have studied the relationship between stress, addiction and the immune system. Both exposure to stress and ethanol consumption activate the immune system and induce neuroinflammation (Calcia et al., 2016; Finnell and Wood, 2016; Rodríguez‐Arias et al., 2017; Ferrer-Pérez et al., 2018; Montagud‐Romero et al., 2018). Moreover, deregulation in chemokine signaling and neuroinflammation have been proposed to contribute to cognitive dysfunction and mental illness (Keogh and Parker, 2011; Wohleb et al., 2013; Pascual et al., 2015). SD-induced neuroinflammation has been clearly demonstrated, characterized by an activation of microglia (Stankiewicz et al., 2015), an increase of pro-inflammatory cytokines (Wohleb et al., 2011, 2012, 2014; Ferrer-Pérez et al., 2018), or the cross of peripheral immune cells to the CNS due to higher blood-brain barrier (BBB) permeability (Rodríguez‐Arias et al., 2017).
There are no current studies evaluating the role of VWR in ameliorating the increase in EtOH consumption induced by SD. In mice and humans, several studies suggest that excessive or forced physical exercise produces brain injury and neuroinflammation (Svensson et al., 2016; Paolucci et al., 2018). However, physical exercise also upregulates tight-junction associated proteins of the BBB and protects the brain from injury, reducing the activation of microglia and cytokine levels in the hippocampus in mice (Park et al., 2016; Spielman et al., 2017) and humans (Paolucci et al., 2018). Therefore, it is necessary to evaluate if the neuroinflammatory process induced by RSD mediates the increase in EtOH consumption and if VWR could modify it. The aim of the present study was, firstly, to confirm that RSD induces a long-lasting increase in EtOH consumption using oral EtOH SA when experienced during adulthood; secondly, to evaluate if VWR could decrease these RSD effects on EtOH; and finally, to evaluate the neuroinflammatory response induced by RSD and EtOH, measuring the striatal levels of two chemokines fractalkine (CX3CL1) and SDF-1 (CXCL12). Chemokines are a family of small cytokines with chemo-attraction characteristics. Social stress is known to intervene in the signaling of chemokines on microglial morpho-functional activity (Wohleb et al., 2013; Sawicki et al., 2015; Milior et al., 2016) and, in addition, the striatal levels CX3CL1 increase after EtOH intake (Pascual et al., 2015).
Section snippets
Subjects
A total of 115 male OF1 mice (Charles River, France) were delivered to our laboratory at postnatal day (PND) 21 (4 animals were discarded during the training phase of SA). All mice (except those used as aggressive opponents) were housed in groups of five in plastic cages (25 × 25 × 14.5 cm). Mice used as aggressive opponents were individually housed in plastic cages (23 × 13.5 × 13 cm) for a month before the experiments to induce heightened aggression (Rodrı́guez-Arias et al., 1998) (n = 15
VWR did not affect behaviors during RSD
The ANOVA revealed a significant effect of the variable Day for Defensive/Submissive [F(1,18) = 50.932; p = 0.000], for Attack [F(1,18) = 16.357; p = 0.001], and Threat [F(1,18) = 5.872; p = 0.026] behaviors (Table 1). All mice showed an increase of the time spent in these behaviors in the last RSD compared to the first (p = 0.001, d = 2.244; p = 0.001, d = 1.290; and p = 0.026, d = 0.557 respectively).
RSD increase CX3CL1 and CXCL12 levels in the striatum
The ANOVA indicated that the exposure to RSD induced a significant increase in CX3CL1
Discussion
The present study confirmed that social stress experienced during adulthood increases consumption and motivation for ethanol and that VWR reverted this effect. In addition, we corroborated that RSD produces a neuroinflammatory response by increasing protein levels of the chemokines CX3CL1 and CXCL12. VWR was also able to revert the neuroinflammatory response caused by stress and exposure to ethanol.
In a previous study from our laboratory, we observed that mice subjected to social stress during
Conclusions
In conclusion, our results suggest that VWR is a beneficial environmental intervention that is capable of blocking the increased ethanol intake and the neuroinflammation induced by social stress. Our work highlights the complexity of the brain mechanisms involved in the inflammatory process in response to social stress. To sum up, VWR could be a promising preventive and therapeutic target to avoid and reduce the detrimental effects induced by social stress.
Data availability
The datasets generated for this study are available on request to the corresponding author.
Funding
This work was supported by the Ministerio de Economía, Industria y Competitividad, Gobierno de España (MINECO), Dirección General de Investigación, PSI2014-51847-R and PSI 2017-83023-R; Instituto de Salud Carlos III, Red de Trastornos Adictivos (RTA; RETICS RD16/0017/0007) and Unión Europea, Fondos FEDER “A way to build Europe.’’ Reguilón, MD received FPI grant from the Spanish Ministry of Science, Innovation and Universities (PRE2018-084159).
The authors have no possible conflict of interest in
Declaration of competing interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
CRediT authorship contribution statement
M.D. Reguilón: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing - original draft. C. Ferrer-Pérez: Formal analysis, Investigation, Software, Validation, Writing - original draft. R. Ballestín: Investigation, Methodology. J. Miñarro: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing. M. Rodríguez-Arias: Conceptualization, Funding acquisition, Methodology, Project administration, Resources,
Acknowledgments
We wish to thank Guillem Chuliá for his English language editing.
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