Glucocorticoid is a major regulator of

Glucocorticoid is a major regulator of body's response to stress (Armario, 2006). Repeated injections of the glucocorticoid corticosterone (CORT) in mice have been shown to produce an animal model of depression (Zhao et al., 2008). Further study of this phenomenon suggested that prolonged CORT exposure significantly increases the phosphorylated form of Cx43, which modulates gap junction channel gating (Lampe et al., 2000, Moreno and Lau, 2007, Quesseveur et al., 2015). In addition, the glucocorticoid receptor participates in the regulation of connexin gene expression (Kojima et al., 1995). Furthermore, dexamethasone inhibits gap junction intercellular communication (GJIC) through glucocorticoid receptor in neural progenitor cell (Samarasinghe et al., 2011). Collectively, these studies indicate potential roles for CORT in regulating the function of gap junctions. Therefore, one of the main objectives of this study was to test the effects of CORT on GJIC in primary astrocytes from the rat prefrontal cortex and hippocampus, the bupropion hydrochloride sale areas most related with depression (Jaworska et al., 2016, Koenigs and Grafman, 2009). Ginsenoside Rg1 (Rg1), an extract from Panax ginseng C. A. Mey, exhibits antidepressant-like effects in a stress-induced depression model (Jiang et al., 2012, Liu et al., 2016), although the mechanism underlying this action is not clear. Recent studies indicate that the effects of antidepressants are related to the regulation of Cx43 expression (Fatemi et al., 2008, Morioka et al., 2014, Sun et al., 2012). For example, fluoxetine reverses the reduction of Cx43 in the prefrontal cortex of rats exposed to chronic unpredictable stress (Fatemi et al., 2008). However, it is not known whether gap junctions are involved in the antidepressant-like effects of Rg1. The second main objective of this study was to clarify this point by investigating the effects of Rg1 on GJIC and Cx43 in primary rat astrocytes exposed to CORT.
Materials and methods
Results
Discussion Mounting evidence suggests that gap junctions may be a new therapeutic target in major depressive disorder (Sarrouilhe and Dejean, 2015). However, there is currently no model of gap junctional integrity for screening antidepressant compounds. Our results showed that CORT (50μM) reduced GJIC and Cx43 expression but increased Cx43 phosphorylation at S368 in rat primary astrocytes. These data suggest that CORT inhibits the function of gap junctions and may be useful in establishing a gap junction damage model for testing potential antidepressant drugs. Rg1 was further investigated as a potential drug for this therapeutic target. Our results reveal that Rg1 reversed the effects of CORT on GJIC and Cx43 expression and phosphorylation, suggesting that Rg1 provides protection against CORT-induced inhibition of gap junctions. In our model system, we took advantage of the fact that gap junctions allow molecules less than 1kDa in size to pass through these channels. LY can pass through gap junctions but not through cell membranes; therefore, the transference of LY reflects the function of gap junctions (Laird, 2006, Zvalova et al., 2004). Our results show that CORT reduces the area of LY transference, indicating the inhibition of GJIC. This is consistent with a previous study that showed reduction of GJIC by dexamethasone (Samarasinghe et al., 2011). As inhibition of GJIC is related to deficiencies in Cx43, a major component of gap junctional channels, we examined the expression of Cx43 after CORT treatment. Our results show that total Cx43 expression is reduced by CORT treatment, suggesting that CORT-induced inhibition of gap junctions may result from the downregulation of Cx43 expression. This finding is supported by a previous study that showed that the glucocorticoid receptor participates in the regulation of Cx43 mRNA (Kojima et al., 1995). However, phosphorylation of Cx43 triggers gap junction disassembly, internalization and degradation (Laird, 2005), and we found that CORT increased the levels of phosphorylated Cx43 in our astrocyte cultures. Therefore, the observed reduction in Cx43 could also be the result of upregulated Cx43 phosphorylation and subsequent gap junction remodeling. Thus, the specific mechanisms by which CORT regulates the expression of Cx43 needs further investigation.