Cannabinoid CB2 Receptor Gene and Environmental Interaction in the Development of Psychiatric Disorders

Hiroki Ishiguro, Yasue Horiuch, Koichi Tabata, Qing-Rong Liu, Tadao Arinami and Emmanuel S. Onaivi

Molecules, 2018, 23, 1836;

doi : 10.3390/molecules23081836

Abstract :

CB2 cannabinoid receptor (CB2R) gene is associated with depression. We investigated the gene-environment interaction between CB2R function and diverse stressors. First, anxiety-like behavior during chronic-mild-stress (CMS) was evaluated in C57BL/6JJmsSlc mice following treatment with CB2R agonist JWH015 or inverse-agonist AM630. Second, locomotor activity and anxiety-like behavior were measured following exposure to an immune poly I:C stressor. Gene expressions of HPA axis related molecules, Fkbp5, Nr3c1 and Crf and pro-inflammatory cytokine Il-1b, as well as Bdnf as a key neurotrophin that supports neuron health, function, and synaptic plasticity, were determined in hippocampus of Cnr2 knockout mice, as indicators of stressful environment. CMS-induced anxiety-like behavior was enhanced by AM630 and reduced by JWH015 and fluvoxamine. Poly I:C reduced locomotor activity and increased anxiety-like behavior, and these effects were pronounced in the heterozygote than in the wild type mice. Fkbp5 and Nr3c1 expression were lower in the Cnr2 heterozygotes than in the wild type mice with Poly I:C treatment. These findings indicate that interaction between CB2R gene and stressors increases the risk of depression-like behaviors that may be linked with neuro-immune crosstalk. Further studies in human subjects are necessary to determine the role of CB2R and environmental interaction in the development of depression.

Keywords : endocannabinoid; cannabinoid CB2 receptor; Cnr2; depression; anxiety; chronic mild stress; neuro-immune signaling; HPA axis

1. Introduction
While most studies focus on the CB1 receptor (CB1R), recognition of the importance of understanding the roles of the cannabinoid CB2 receptor (CB2R) in the central nervous system in neuropsychiatric disorders has been increasing [1]. Reduced function of the CB2R is associated with several psychiatric disorders including schizophrenia, major depression, and substance abuse [2–4], but these genetic findings need to be validated by behavioral experiments with animal models of psychiatric disorders. The polymorphisms in the CB2R gene (CNR2) are frequent, and are commonly observed in human population [2] associated with psychiatric disorders [5]. In addition, our studies on the association of human CNR2 gene polymorphisms suggested that these polymorphisms are a common factor in some psychiatric disorders. We have previously demonstrated that Cnr2 gene expression was downregulated in the brains of the mice subjected to chronic-mild-stress (CMS) and also in mice that developed alcohol preference [3].

The endocannabinoid system appears to contribute to stress response, including activation of the hypothalamic-pituitary-adrenal (HPA) axis and increases in anxiety-related behavior during stress [6]. As stress responses involves the HPA axis and affect the onset and maintenance of stress-related disorders including depression [7], genes that play a role in the HPA-axis regulation may likely contribute to stress-related disorders. Within the canonical HPA axis, we examined the FKBP5, CRF and NR3C1 in this study. FKBP5, a co-chaperone of hsp90, is responsible for regulation of GR sensitivity to cortisol [8,9]. FKBP5 has been reported to be one of the biomarkers for stress-response and for effects of antidepressant action in patients with depression [8,10–12]. The NR3C1, encoding GR, is known as a major component of HPA axis against stress, and have been reported to be one of stress biomarkers with responses to psychosocial stress following the Trier Social Stress Test (TSST) [13]. The polymorphisms, methylation and expression of the NR3C1 gene have been indicated to be associated with depression and especially related to stress [14–19]. The CRF gene, that codes corticotropin-releasing factor, contributes to stress-related depressiveness [20]. We hypothesized that if the endocannabinoid system is involved in HPA axis, then the expression of Fkbp5, Crf and Nr3c1 genes may be associated with changes in CB2R function in mice. Neuro-immune crosstalk mediated by stressors is an important target of study in psychiatric disorders. Previous studies have indicated changes in expression of cytokines in psychiatric disorders [21–29]. Differences in proinflammatory cytokines produced by monocytes between patients with major depressive disorder and healthy controls [30]. Exposure to chronic restraint stress exacerbated allodynia and depressive-like behavior along with increase in Interleukin 1 (IL-1) gene expression [31]. GR antagonist reduced depression-like behaviors induced by Il-1b administration in rodent model [32]. Brain-derived neurotrophic factor (BDNF) gene expression in blood was found to be negatively associated with depression score (HAMD) [33], and Bdnf/trkB signaling was reduced in the hippocampus of genetic model of vulnerability to stress-induced depression [34]. We therefore examined Il1b and Bdnf gene expressions that have been implicated as biomarkers for depression. The endocannabinoid system is associated with stress-induced psychiatric disorders. Chronic-mild-stress (CMS) has been developed on the basis of stress-diathesis hypothesis of depression [35]. The viral mimic polyinosinic-polycytidilic acid, Poly I:C is a synthetic double-stranded RNA which activates the Toll-like receptor 3 (TLR3) pathway, and which is used in animal model of depression [36]. Therefore, the effects of CMS and Poly I:C treatments were analyzed in this study to simulate stressors such as emotional and immune stressors. The aim of this study was to determine the role of CB2Rs and environmental factors on behavioral characteristics of mouse models of depression using two stressors, and to evaluate neuro-inflammatory and HPA axis biomarkers in the stress models. This study is a first report of our research focusing on the gene-environment interaction between CB2R and stressors in the mature mouse.

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molecules-23-01836