A neuroendocrine mechanism of co-morbidity of depression-like behavior and myocardial injury in rats

Abstract

Depression is generally a recurrent psychiatric disorder. Evidence shows that depression and cardiovascular diseases are common comorbid conditions, but the specific pathological mechanisms remain unclear. The purpose of this study is to determine the effects of depression induced by chronic unpredictable mild stress (CUMS) on myocardial injury and to further elucidate the biological mechanism of depression. Rats were used as a model. The CUMS procedure lasted for a total of 8 weeks. After 4 weeks of CUMS, treated rats exhibited a reduced sucrose preference and changes in scores on an open field test, body weight and content of 5-HT in the brain as compared with the values of these variables in controls. These changes indicated depression-like changes in CUMS rats and demonstrated the feasibility of the depression model. In addition, pathological changes in the myocardium and increased cardiomyocyte apoptosis demonstrated that myocardial injury had occurred after 6 weeks of CUMS and had increased significantly by the end of 8 weeks of CUMS. Plasma serotonin (5-HT), norepinephrine (NE) and epinephrine (E), all depression-related neuroendocrine factors, were measured by HPLC-ECD techniques, and the content of plasma corticosterone (GC) was evaluated by an I(125)-cortisol radioactivity immunoassay in control and CUMS rats. The results indicated that 5-HT had decreased, whereas NE, E and GC had increased in CUMS rats, and these factors might be associated with depression-induced myocardial injury. The effects of 5-HT, NE and GC on the survival rate of cultured cardiomyocytes were determined using an orthogonal design. The results showed that 5-HT was a more important factor affecting cell survival than GC or NE. The results suggested that normal blood levels of 5-HT had a cytoprotective effect. The neuroendocrine disorders characterized by decreased 5-HT combined with increased GC and NE mediated the occurrence of depression-induced myocardial injury.

Figure 1. Unpredictable chronic mild stress induces comorbidity of depression-like behavior and myocardial injury in rats.

The depression-like behavior and myocardial injury of CUMS rats were measured at 2, 4, 6 and 8 weeks. A. The rats were weighed weekly, and the weights are shown. B. Locomotor activity and exploratory behavior were evaluated as described in the Materials and Methods section. Values are the mean ± SD from 3 independent experiments, *P<0.05 compared with the control. C. A 2-bottle sucrose preference test was used to measure rats’ preferences for sweetened solution over water. Preference was calculated as the percentage of sucrose solution consumed compared to the total fluid intake (sucrose/(sucrose+water) x 100). D. Myocardial damage was observed by Nagar Olsen staining. The size and scale of the red-stained range represent the severity of the injury. There was no injury in control group rats (a) or in 2 and 4 week CUMS rats (b, c). Myocardial injury occurred after 6 weeks of CUMS (d) and increased after 8 weeks of CUMS (e). E. An electron microscope was used to detect pathological ultrastructural alteration. A myocardial ultrastructure showing normal cardiac nuclear, myofibril structure and mitochondria (×10000) in control group rats is shown (a). Abnormal ultrastructural changes such as myofibril breakage were observed after 8 weeks of CUMS (×10000) (b). Mitochondrial disarrangement, mitochondrial interrupted inner membrane and cristae and mitochondrial vacuoles (×10000) are shown (c, f). Nuclear membrane shrinkage and chromatin margination (×10000) were also seen in 8-week CUMS rats (d, e, f). F. Restraint stress increased cardiomyocyte apoptosis as measured by TUNEL. Values represent the group mean ± structural equation modeling (SEM) (n = 8 rats per group). *P<0.05 compared with control, repeated measures ANOVA followed by Tukey’s multiple comparison test.

Figure 2. Effect of unpredictable chronic mild stress on plasma neuroendocrine transmitter in rats.

The concentrations of serotonin, norepinephrine and epinephrine were measured by HPLC-ECD, and the concentration of glucocorticoid was evaluated by an I¹²⁵–cortisol radioactivity immunoassay in both control and CUMS rats. A. The glucocorticoid content in plasma increased significantly after 2 and 4 weeks of CUMS but returned to normal levels after 6 and 8 weeks of CUMS. B. The 5-HT content in plasma increased significantly after 6 and 8 weeks of CUMS. C. The norepinephrine content in plasma increased significantly after 6 and 8 weeks of CUMS. D. The epinephrine content in plasma increased significantly after 6 and 8 weeks of CUMS. Values represent the group mean ± structural equation modeling (SEM) (n = 8 rats per group). *P<0.05 compared with control, repeated measures ANOVA followed by Tukey’s multiple comparison test.

Figure 3. 5-HT-induced cardiomyocyte injury required the presence GC or NE in vitro.

An MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was utilized to detect cell viability. 5-HT (10⁻⁵, 10⁻⁶, 10⁻⁷ and 10⁻⁸ M) was used to treat cultured cardiomyocytes with 10⁻⁶ M GC or 10⁻⁶ M NE. A. The cell viability of cardiomyocytes treated with 10⁻⁸ M 5-HT and 10⁻⁶ M GC decreased significantly compared with the control. B. The cell viability of cardiomyocytes treated with 10⁻⁸ M 5-HT and 10⁻⁶ M NE decreased significantly compared with the control. C. The cell viability of cardiomyocytes treated with only 5-HT was not changed significantly compared with the control. D. The apoptosis rate of cardiomyocytes treated with 5-HT and 10⁻⁶ GC increased significantly compared with the control. E. The apoptosis rate of cardiomyocytes treated with 5-HT and 10⁻⁶ NE increased significantly compared with the control. Values represent the group mean ± structural equation modeling (SEM) (n = 8 rats per group). *P<0.05 compared with control, repeated measures ANOVA followed by Tukey’s multiple comparison test.

Figure 4. Sertraline inhibits depression-like behavior and myocardial injury in CUMS rats.

Body weights, open field test scores and sucrose preference were measured as described in Figure 1. A. Sertraline inhibited weight loss induced by CUMS. B. Sertraline inhibited the decline of the open field test score induced by CUMS. C. Sertraline inhibited the decline of sucrose preference induced by CUMS. D. Sertraline inhibited the myocardium injury induced by CUMS. E. Sertraline inhibited the apoptosis induced by CUMS. Values represent the group mean ± structural equation modeling (SEM) (n = 8 rats per group). *P<0.05 compared with control, ^# P<0.05 compared with the 8-week CUMS group, repeated measures ANOVA followed by Tukey’s multiple comparison test.