Maternal separation increases alcohol-drinking behaviour and reduces endocannabinoid levels in the mouse striatum and prefrontal cortex
Introduction
Early postnatal life is a period of high vulnerability in which prolonged exposure stressors may lead to long-lasting deleterious effects on brain neurodevelopment and function. Therefore, preclinical (Kaufman et al., 2000, Sarro et al., 2014) and clinical studies (Lupien et al., 2009, Teicher and Samson, 2013) suggest that early life events exert a sustained influence over neural systems mediating stress reactivity. Maternal separation is a validated rodent model of early life stress frequently used to replicate early adversities, entailing the early separation of pups from dams for long periods after birth (Tractenberg et al., 2016). Previous studies have shown that maternal separation affects the formation of neuronal networks and exerts long-lasting effects on neural function (Nishi et al., 2014). Moreover, maternal separation leads to high levels of anxiety-like behaviour and high stress hormone responsiveness, depression-like behaviour assessed as anhedonia, despair behaviour and a decrease in behavioural responses to novelty (George et al., 2010, Gracia-Rubio et al., 2016a, Lukkes et al., 2017, Matthews and Robbins, 2003, Rüedi-Bettschen et al., 2004). In addition, maternal separation decreases neurogenesis (Lajud et al., 2012), reduces 5-HT1A receptor levels (Bravo et al., 2014, Gracia-Rubio et al., 2016b), and increases pro-inflammatory cytokines levels in serum (Réus et al., 2015) associated with increased neuroinflammatory responses (Gracia-Rubio et al., 2016a). Behavior and molecular alterations induced by maternal separation appear during adolescence and persist until adulthood (Gracia-Rubio et al., 2016a).
In this sense, adolescence is a critical period for brain development and maturation and is a sensitive period to develop psychiatric illnesses, including anxiety, mood and substance abuse disorders (Paus et al., 2008). The earlier drug use is initiated, the more likely it is for addiction to progress (Degenhardt et al., 2008) and at the same time, risk taking and novelty seeking are hallmarks of typical adolescent behaviour (Wolf et al., 2013). Alcohol is the most commonly abused drug during adolescence and alcohol intoxication was reported to induce brain damage (Pascual et al., 2014). Therefore, alcohol intake during adolescence is considered one of the main risk factors contributing to the development of neuropsychiatric disorders later in life (Skogen et al., 2014), including alcohol use disorder (Kyzar et al., 2016). Additionally, the individual risk of developing alcohol use disorder is affected by early life stress (Sinha, 2008), suggesting that the dysregulation of the brain reward function induced by early life adverse experiences may be related to the development of drug addiction (Cheetham et al., 2010), presumably through the modulation of the dopaminergic and endocannabinoid (EC) systems (Parsons and Hurd, 2015), which are involved in reward, mood and stress processing. Accordingly, previous reports showed that maternally separated animals present a lower density of dopamine transporter sites in the striatum, greatly reduced D2 (Gracia-Rubio et al., 2016b) and D3 dopamine receptor binding (Brake et al., 2004). Moreover, regarding the EC system, a decreased CB1 receptor expression was described in the hippocampus and prefrontal cortex (PFC) due to maternal separation, while in the striatum, an increase was reported (López-Gallardo et al., 2012, Romano-López et al., 2012), which may contribute to a proclivity to alcohol ingestion (Parsons and Hurd, 2015). Several studies have concluded that animals exposed to maternal separation exhibit high patterns of alcohol consumption (García-Gutiérrez et al., 2016, Gondré-Lewis et al., 2016, Roman and Nylander, 2005) during adulthood. Nevertheless, only a few studies have evaluated the effects of maternal separation on alcohol consumption during adolescence (García-Gutiérrez et al., 2016), and previous literature has not investigated the effects of maternal separation during alcohol abstinence and relapse.
It is, therefore, of interest to understand the underlying neurobiological mechanisms by which early life stress may contribute to the vulnerability to develop alcohol use disorders during adolescence, specially the contribution of the EC and the monoaminergic systems as related key targets in the abuse liability of alcohol. In this context, the aim of the present study was first to examine whether maternal separation may increase the propensity to alcohol consumption, and second to determine if it modifies the EC and the monoaminergic systems in the PFC and striatum in adolescent male C57BL/6 mice. For this purpose, to verify the effects of maternal separation on despair-like and social behaviour, the tail suspension test (TST) and the three-chamber social test were assessed respectively. Moreover, to evaluate the effects of maternal separation on alcohol rewarding properties, voluntary alcohol intake, binge drinking and relapse following abstinence were analysed. Finally, monoaminergic precursors and metabolites, such as tyrosine (Tyr), tryptophan (Trp) and 5-hydroxyindoleacetic acid (5-HIAA), and EC compounds were assessed in the striatum and PFC of maternally separated mice.
Section snippets
Animals
C57BL/6 mice male and female adult mice purchased from Charles River (France) were used as breeders. On arrival, breeding pairs were formed and housed in plastic boxes. After 2 weeks, the male was removed, and pregnant females were housed individually. A final group of 150 offspring male mice were used to conduct the different experiments. Offspring female mice were dismissed for the experiment and were used for other experiments. Animals were housed in an acclimatized room (temperature 21 ±
Effects of maternal separation on despair-like behaviour and social behaviour: TST and 3-chamber social test
MSEW mice exposed to the TST exhibited a significant increase in despair-like behaviour. There was a significant increase of immobility time in MSEW mice compared with the SN group [t(28) = −3.565; p = 0.001] (Figure 2A). In the 3-chamber social test, MSEW group showed a decrease in the percentage of novel interaction time [t(18) = 2.771; p < 0.05], without differences in the percentage of sociability time [t(18) = −1.714; n.s.]. No bias or initial preference for one of the two compartments was
Discussion
The present study demonstrates that adolescent mice exposed to MSEW showed behavioural alterations, such as increased despair-like behaviour, deficits in social behaviour and decreased EC levels in the striatum and PFC. Such alterations were also associated with a higher vulnerability to develop alcohol drinking behaviour due to a decreased sensitivity to alcohol-induced rewarding effects. Moreover, MSEW adolescent mice showed a facilitation of alcohol drinking following a period of abstinence
Acknowledgements
Authors thank to Gerald Fanon for the linguistic revision of the manuscript.
Role of the funding source
This study was supported by MINECO (SAF2016-75966-R, FEDER), Plan Nacional sobre Drogas (2014/020), ISCIII and FEDER (CPII16/00027), UE Medbioinformatic project (grant number 634143) and grants from DIUE, Generalitat de Catalunya (2014SGR 680). RL-A’s position was funded by an institutional program of the University of Barcelona in collaboration with the “Obra Social de la Fundació Bancària La Caixa”. None of these funding sources played any further role in the study’s design, in the
References (77)
Neuroactive steroids and stress axis regulation: pregnancy and beyond
J. Steroid Biochem. Mol. Biol.
(2016)- et al.
Differential role of anandamide and 2-arachidonoylglycerol in memory and anxiety-like responses
Biol. Psychiatry
(2011) - et al.
The role of affective dysregulation in drug addiction
Clin. Psychol. Rev.
(2010) - et al.
Stereological analyses of reward system nuclei in maternally deprived/separated alcohol drinking rats
J. Chem. Neuroanat.
(2016) - et al.
Maternal separation induces neuroinflammation and long-lasting emotional alterations in mice
Prog. Neuropsychopharmacol. Biol. Psychiatry
(2016) - et al.
Maternal separation modifies behavioural and neuroendocrine responses to stress in CCR7 deficient mice
Behav. Brain Res.
(2014) - et al.
Long-term effects of neonatal stress on adult conditioned place preference (CPP) and hippocampal neurogenesis
Behav. Brain Res.
(2012) - et al.
Effects of early adverse experiences on brain structure and function: clinical implications
Biol. Psychiatry
(2000) - et al.
Systemic ethanol administration elevates deoxycorticosterone levels and chronic ethanol exposure attenuates this response
Brain Res.
(2005) - et al.
Periodic maternal separation decreases hippocampal neurogenesis without affecting basal corticosterone during the stress hyporesponsive period, but alters HPA axis and coping behavior in adulthood
Psychoneuroendocrinology
(2012)