| IntroductionVascular depression(VD) is a unique subtype of the late-life depression(LLD), usually company with cerebrovascular disease, which can promote the development and continuity of the depressive symptoms of VD. The mechanism of how cerebrovascular disease cause depression has been unclear, and the diagnostic criteria of vascular depression are also unclear. Alexopoulos et al analyzed a series of studies on the association between inflammation and depression, then found that excessive and sustained immune inflammatory response could lead to emotional and cognitive change related to depression. However, some of the antidepressant drugs may reduce expression of the inflammatory cytokines, and some anti-inflammatory drugs may have antidepressant effect. Based on these findings, Alexopoulos et al then put forward the “inflammatory hypothesis” to the pathogenesis of vascular depression, which considered that the inflammatory process related to age and cerebrovascular disease could promote and mediate the neuropathological and metabolic change which may cause the following depressive syndrome. With the development of imaging technology, the white matter hyperintensity(WMH) displayed in the T2 sequences of magnetic resonance imaging(MRI) and depressive executive dysfunction(DED) are considered to be important characteristics of vascular depression as a subtype of LLD. Generally, the WMH displayed in the t2-weighted images indicates white matter lesions(WMLs), whose locations are closely related to depression and cognitive impairment, and the emergence of executive dysfunction often indicates poor antidepressant treatment response.Traditional monoamine hypothesis theory thought that the occurrence of depressive symptoms was related to the absence and decrease of serotonin(5-HT) and/or norepinephrine(NE) in the synaptic cleft. It is also the theoretical basis on which antidepressant drugs play a role of antidepressant effect such as selective serotonin reuptake inhibitors(SSRI). However these antidepressant treatments have a poor effect in the therapy of vascular depression. Recent years, studies found that inflammation factors can reduce the neurotransmitter synthesis and promote its reuptake during the neural synaptic cleft through various pathways, which could fight against the antidepressant effect of SSRI.Thus, we speculate that the inflammation in white matter caused by cerebral ischemia can damage oligodendrocytes, then result in demyelination and myelinization disorder of white matter, which may affect the functions of neurons and its efficient delivery of information, meanwhile, the release of inflammatory factors could influence the neurotransmitter metabolism, which may be an important link in the pathogenesis of vascular depression. In this study, we proposed to establish a mice model of vascular depression by repeated bilateral common carotid artery occlusion(BCCAO), and use enzyme-linked immunosorbent assay, immunohistochemistry, immunofluorescence and high performance liquid chromatography(HPLC) to detect the expression of inflammatory factor, neurotransmitters, and oligodendrocytes etc. Meanwhile, the open-field test, tail suspension test and Morris water maze were used to test the depressive and cognitive behavior changes in mice. The mechanisms of how ischemic inflammation participate the pathogenesis of vascular depression through oligodendrocytes injury and the following disorder of repair and regeneration were discussed. MethodsPart I: The observational study of the characteristics of oligodendrocytes injury and endogenous regeneration in vascular depression mice.Adult BALB/c males mice were randomly divided into model group and sham group with 24 mice in each group. Mice in model group were anesthetized with 0.3% pentobarbital, then subjected to repeated bilateral carotid artery occlusion and reperfusion to establish the animal model of vascular depression. Mice in sham group were conducted the same surgery expect occluding carotid artery. All mice were administrated 5-bromine deoxidization uracil nucleoside(150 mg/kg, intraperitoneal injection, qd) from the post operation day(POD) 4 to 6. On POD 14 and 28, 12 mice from each group were deeply anesthetized and transcardially perfused with phosphate buffered saline(PBS) respectively. Brain frozen sections were prepared for immunohistochemical and immunofluorescence double staining to observe the new cell proliferation and neuronal regeneration in sub ventricular zone on POD 14 and POD 28, and the regeneration of oligodendrocytes and expression of newborn astrocytes in dentate gyrus on POD 28, and the inflammatory cytokine levels in serum and brain homogenate on POD 28.Part II: The impact of inflammatory inhibition on behavior and neurotransmitters in vascular depression mice.Male BALB/c mice were subject to repeated common carotid artery occlusion and reperfusion to establish the vascular depression model, then randomly divided into experimental group(n=10), control group(n=10) and sham group(n=10). Minocycline(30 mg/kg, i.p.) and same dose of saline were administrated immediately after the surgery and subsequently the consecutive 7 days in experimental group and control group respectively. Mice in sham group were conducted the same surgery expect occluding carotid artery, then administered the same dose of saline as the control group. After the administration, tail suspension test and open-field test were used to assess depression behaviors of mice on the post operation day(POD) 8 and 9 respectively, and Morris water maze was used to assess cognitive function on the POD 10 to 12. On POD 12, mice were deeply anesthetized and euthanized and transcardially perfused with phosphate buffered saline(PBS). Expressions of tumor necrosis factor α(TNF-α), interleukin-1β(IL-1β) and interleukin-6(IL-6) in hippocampus were measured by enzyme-linked immunosorbent assay(ELISA) kit. Contents of 5-hydroxytryptamine(5-HT), norepinephrine(NE) and dopamine(DA) were measured by high performance liquid chromatography(HPLC). Myelin basic protein(MBP) expression was measure by immunohistochemical staining. ResultsPart I: The observational study of the characteristics of oligodendrocytes injury and endogenous regeneration in vascular depression mice.The immobility time was significantly longer in mice from model group(196.43±9.94 s vs 170.44±6.33 s, t=7.64, P<0.001). On POD 14 and 28, there were both significant differences in the number of newborn cells in sub ventricular zone between two groups(161.08±17.58/mm2 vs 85.70±7.61/mm2, t=13.64, P < 0.001; 99.17±11.49/mm2 vs 29.08±5.92/mm2, t=18.79, P<0.001). Compared with sham group, on POD 28, the number of newborn neuron in sub ventricular zone of mice from model group increased significantly(66.25±4.63/mm2 vs 37.92±5.93/mm2, t=13.04, P<0.001). Compared with sham group, the number of oligodendrocytes progenitor cells in dentate gyrus was significantly increased in model group on POD 28(70.33±9.11/mm2, 30.42±9.98/mm2, t=10.24, P<0.001), with pre-oligodendrocytes reduced(66.83±11.36/mm2 vs 80.50±10.47/mm2, t=10.24, P=0.006) and mature oligodendrocytes decreased but with no statistically significant difference(22.21 ± 2.01/mm2 vs 23.38 ± 1.84/mm2, t=1.48, P=0.152). Compared with sham group, the number of newborn astrocyte in the dentate gyrus increased significantly in model group(8.01 ± 2.89/mm2 vs 5.25 ± 2.18/mm2, t=2.631, P=0.015). There were significant differences of the contents of TNF-α and IL-1β in serum between the two groups respectively(P<0.001), but no significant difference was observed in the content of IL-6(t = 0.93, P=0.362). There were significant differences of the contents of TNF-α, IL-1β and IL-6 in cerebral homogenate between the two groups(P<0.001).Part II: The impact of inflammatory inhibition on behavior and neurotransmitters in vascular depression mice.Among the three groups, the immobility time was significantly different([174.75±11.37] s vs [194.32±14.32] s vs [169.62±19.27] s, F=6.59, P=0.005), and the immobility time of experimental group and sham group was shorter than control group significantly. The times of exploring holes, prolonged time and distance of movement were significantly different(F=6.17, P=0.008; F=11.55, P < 0.001; F=13.47, P < 0.001), the times of exploring holes of experimental group and sham group was more than control group significantly, and so did both prolonged time and distance of movement of those two groups. But there was significant difference in escape latency on the 3rd day of MWM([51.6±7.55] s vs [55.80±6.25] s vs [25.40±3.98] s, F=72.81, P<0.001) among the three groups, the escape latency of experimental group and control group was significantly longer than sham group. The expression of TNF-α, IL-1β and IL-6 in hippocampus were down-regulated in experimental group and sham group compared with control group(TNF-α: [141.10±24.36] pg/100 mg vs [167.60±15.91] pg/100 mg vs [123.80±15.53] pg/100 mg, F=13.42, P<0.001; IL-6:[20.01±3.62] pg/mg vs [24.39±5.04] pg/mg vs [18.40±3.78] pg/mg, F=5.49, P=0.010; IL-1β:([5.32±1.89] pg/mg vs [10.31±2.83] pg/mg vs [4.50±2.07] pg/mg, F=18.69, P<0.001). There were no significant difference in the level of NE among the three groups([3.97±1.35] ng/ml vs [3.16±1.55] ng/ml vs [4.68±1.99] ng/ml, F=2.13, P=0.139), but there is a increased level of DA in experimental group and sham group compared with control group([10.72±2.65] ng/ml, [11.76±3.10] ng/ml vs [7.99±2.31] ng/ml). Immunohistochemical staining of MBP showed that the optical density(OD)ratio of experimental group and control group compared to sham group were 0.57±0.06 and 0.69±0.15 respectively, and the difference between the two groups was significant(t=2.72,P=0.013). ConclusionThere was endogenous neurogenesis in vascular depression mice established by repeated common carotid artery occlusion and reperfusion, but the regeneration of mature oligodendrocytes in dentate gyrus was dysfunction. Promoting the regeneration of the ischemic injury after oligodendrocytes matured may become the effective strategy of vascular depression treatment.Minocycline can restrain the expression of inflammatory cytokines in vascular depression mice, and inhibited inflammation may improve their depression behaviors and protect the myelin sheath, but no improvement found in the cognitive impairment. Among those relevant neurotransmitters, the content of DA changes most significantly. These results indicated that the abnormalities of DA metabolic regulation may be the important reason of cognitive impairment in vascular depression. |
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