Abnormal Expression of MicroRNAs Induced by Chronic Unpredictable Mild Stress in Rat Hippocampal Tissues

Previous research has demonstrated that the behaviors observed in chronic unpredictable mild stressed (CUMS) rats are similar to the symptoms of depressed patients and that the abnormal expression of cerebral microRNAs is associated with depressive disorder. However, little is known regarding the expression profile of microRNAs induced by CUMS. In this study, we aimed to examine the hippocampal microRNA expression profile in CUMS rats. Forty adolescent male Sprague-Dawley rats were randomly divided into normal and model groups. The rats in the model group were stimulated daily with randomly applied mild stressors from among 14 different mild stressors. The stressors were changed every day and were applied for 35 consecutive days. On the 28th and 35th days after treatment, the weights, physical condition, sucrose preference, and open-field test scores of the rats of the two groups were evaluated. Successful induction of CUMS was considered if the differences of the above metrics between the two groups were statistically significant on the 28th and 35th days after treatment. Cerebral sucrose metabolism images of rats were obtained by 18F-FDG PET/CT. The rats were euthanized under anesthesia, and hippocampal tissues were collected for hematoxylin-eosin (HE) staining. In addition, the samples were used for microRNA array chip and qRT-PCR analysis. The target genes of different microRNAs were predicted using bioinformatic analysis, and the functions and signal pathways of these target genes were investigated by GO and KEGG analyses. Sixteen rats exhibited successful induction of CUMS. Cerebral ¹⁸F-FDG PET/CT imaging showed that the glucose metabolism rate of CUMS rats were significantly lower than normal rats in the central nucleus of the inferior colliculus (CIC, p = 0.022), the retrosplenial agranular area (RSA, p = 0.002), the second sensory cortex (S2, p = 0.028), the first auditory cortex (Au1, p = 0.012), the primary somatosensory cortex, barrel field (SIBF, p = 0.001), and the ventral posteromedial nucleus (VPM) of the right thalamus (p = 0.048). HE staining showed that hippocampal pyramidal cells CUMS rats were thinner, disordered, and exhibited irregular shapes, with many pyknotic cells. The microarray chip and qRT-PCR analysis revealed that five microRNAs were significantly up-regulated [miR-382-3p (p = 0.026), miR-183-5p (p = 0.018), miR-3573-5p (p = 0.042), miR-202-3p (p = 0.016), miR-493-3p (p = 0.009)], and only miR-370-3p was significantly down-regulated (p = 0.036). miRNA target gene prediction and functional annotation analysis showed significant enrichment in several GO terms and pathways associated with depression. Our findings provide supportive evidence for the abnormal expression of multiple CUMS-induced hippocampal microRNAs in rats as well as the involvement of these microRNAs in depressive disorder.