Maternal immune activation is associated with a lower number of dopamine receptor 3-expressing granulocytes with no alterations in cocaine reward, resistance to extinction or cue-induced reinstatement
Introduction
Schizophrenia is a complex mental disorder that affects 23 million worldwide (World Health Organization, 2018). This condition is characterized by positive, negative, cognitive and mood symptoms affecting perception, language, thought and overt behaviour. Due to the high heterogeneity of this wide cluster of symptoms among the patients, the diagnosis is a challenge for the clinicians (Gaebel and Zielasek, 2015).
There is a high prevalence of substance use disorders (SUDs) among people with schizophrenia, which tend to worsen as the disease progresses. Although the drugs more frequently abused by this population are nicotine and caffeine, others such as alcohol, cannabis and cocaine are not uncommon. Comorbidity with cocaine use disorder has not been sufficiently studied as compared to other drugs. However, the prevalence of cocaine use disorder has been found to range between 15% and 50% in schizophrenia patients (Thoma and Daum, 2013).
There are just a few studies that have analysed cocaine self-administration in animal models of schizophrenia. Chambers and Self, using the neonatal ventral hippocampal lesion (NVHL) observed a more robust acquisition of cocaine self-administration, resistance to extinction and high rate of relapse as compared with sham-operated animals (Chambers and Self, 2002). In the same vein, NVHL rats exhibited an increase in cue-induced reinstatement compared to the control group (Karlsson et al., 2013). Maternal immune activation (MIA) models recapitulate several aspects of schizophrenia and are rooted in the fact that prenatal infections are a predisposition factor to develop the condition (Brown and Meyer, 2018). These MIA models offer an excellent means to examine the causal relationships that exist between schizophrenia and the increased prevalence of addictive disorders in these patients. With this in mind, in a previous work, we set out to test the idea that the rats that have undergone prenatal immune activation would show increased cocaine self-administration behaviour. Contrary to our expectations, we observed no increased cocaine self-administration in the offspring of rats exposed to LPS during pregnancy (Santos-Toscano et al., 2016). However, in this previous study, we did not test for other aspects cocaine-related behaviours that are relevant to addiction such as sensitivity to the rewarding actions of the drug in a dose-response setting, motivation to obtain the drug, resistance to extinction or vulnerability to cue-induced relapse. The main goal of the experiments here reported is to examine these behaviours.
Due to the strong relationship between the dopaminergic system and schizophrenia, certain components of this neurotransmission pathway have been postulated as biomarkers of the disorder. In this regard, positive symptoms correlate positively with the levels of dopamine receptor 2 (D2R) mRNA in lymphocytes (Liu et al., 2013). Also, the severity of symptoms correlates with DRD3 mRNA levels in lymphocytes (Kwak et al., 2001). The mRNA expression level of DRD3 in T lymphocytes was also significantly higher in patients with schizophrenia than in controls (Cui et al., 2015). In another study, DRD2 mRNA levels in lymphocytes were increased in schizophrenic patients, while DRD4 mRNA levels in CD4+ T cells were decreased (Boneberg et al., 2006).
There is growing evidence that supports the relationship between the endocannabinoid system and schizophrenia (Bossong et al., 2014; Fakhoury, 2017; Gupta et al., 2014; Ibarra-Lecue et al., 2018; Saito et al., 2013). Moreover, the activation of this system produces the release of circulating cytokines and different neurotransmitters, including dopamine (Foster et al., 2016; Kucerova et al., 2014). Therefore, the endocannabinoid system could also act as a possible biomarker for this disorder. As regards this, a decrease in the cannabinoid receptor 2 (CNR2) mRNA in the peripheral blood mononuclear cells (PBMC) has been found both in the first outbreaks and in chronic patients (Bioque et al., 2013; Ferretjans et al., 2014). In another study, patients with schizophrenia showed an increase in CNR1 expression in CD4+ T-cells, while the CNR1 and CNR2 expression in B-cells was decreased (Campos-carli et al., 2017). Regarding schizophrenic symptoms, the levels of CNR1 and CNR2 mRNA in PBMCs were significantly higher in schizophrenic patients compared to controls and these levels correlated positively with positive and negative schizophrenia symptoms and negatively with cognitive functioning (Chase et al., 2016).
In the light of these data, we decided to examine the relative levels of DRD3+ and CNR2+ immune cells in the MIA model as a proof of concept of their utility as potential biomarkers of schizophrenia.
Section snippets
Animals
The experiments described here were performed on the 8–10-week-old male offspring of Lewis rats obtained from Charles River (France). All animals were housed in transparent Plexiglas cages and kept in a temperature and humidity-controlled environment (21 ± 1 °C/50–60%), with artificial lighting (12 h/12 h light/dark cycle, lights on at 8 am), and ad libitum access to food (commercial diet for rodents A04: Panlab, Barcelona, Spain) and tap water. Rats were maintained and handled in accordance
The first LPS injection reduced body weight increment, but daily LPS administration had no overall effects on the body weight of the dams
The bodyweight increment on the day after the first injection was higher in Saline animals than in LPS animals (t12 = 2372 p < .05 η2 = 0.419, Welch's correction applied) (see Fig. 2-A). However, no significant differences were found among the treatments (F(1,13) = 0.584, p = .459) or due to the Treatment * Gestational Day interaction (F(1,13) = 1.331, p = .165) in the increment of body weight across gestation. However, the statistical analyses do show a significant effect for the Gestational
Discussion
In the experiments here reported we have employed a widely used MIA model, that recapitulates several aspects of the complex constellation of symptoms in schizophrenia, to test if the motivation for cocaine, the sensitivity to the rewarding actions of the drug and the propensity to relapse were potentiated. We also explored the potential of some immune populations to serve as diagnostic tools or biomarkers of the disease. While there were no alterations in cocaine-related behaviours, consistent
Conclusions
In a widely used MIA model of schizophrenia there is no evidence for increased cocaine self-administration, motivation for the drug, sensitivity to the dose, extinction or cue-induced reinstatement of drug-seeking behaviour, suggesting that the clinical relationship between increased drug use and schizophrenia does not originate as a result of the primary neural alterations associated to the disease. In considering the results obtained in our flow cytometry experiments, we suggest
Contributors
RS-T, MU and EB performed the experiments. RS-T, MU also performed the statistical analyses. RS-T wrote the first draft of the manuscript. RS-T, MU and AH-M wrote the final version of the manuscript. EA and AH-M designed the experiments, provided funding and supervised the final version of the manuscript. All authors have approved the final manuscript.
Funding
Funding provided by the Spanish Ministry of Economy and Competitiveness (PSI2016-80541-P to EA and AH-M); Spanish Ministry of Health, Social Services and Equality (Network of Addictive Disorders – Project n°: RTA-RD16/0017/0022 of the Institute of Health Carlos III to EA and Plan Nacional Sobre Drogas, Project n°: 2016I073 to EA and 2017I042 to AH-M); The General Directorate of Research of the Community of Madrid (Project n°: S-2011/BMD-2308; Program of I + D + I Activities CANNAB-CM to EA);
Declaration of competing interest
The authors do not have any conflicts of interest that may affect the work reported in this article.
Acknowledgements
We thank Rosa Ferrado, Luis Carrillo, Gonzalo Moreno and Alberto Marcos for their excellent technical assistance.
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Present address: School of Medicine. The University of Central Lancashire, Preston, UK.
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These authors contributed equally to this work.