Depression is a common psychiatric disorder characterized by low mood with complex pathophysiological mechanisms and poor effect of pharmacological treatment.The animals were placed in greater sensory, physical and/or social stimuli than those of the standard feeding environment, so that they can obtain positive plasticity and adaptability.Environmental enrichment(EE) is a common intervention to improve brain function in laboratory.A large number of studies have shown that EE had significant ameliorative effects on various animal models of depression, but the mechanisms have not been yet fully understood with outcome heterogeneity in ethology.There was no universally accepted and unified paradigm and standard for EE due to its multi-dimensionality and complexity.Therefore, it is necessary to improve the structural components and implementation steps of EE by integrating the existing data.Combined with recent studies on animal models of depression, this paper reviewed the anti-depression mechanism of EE from promoting hippocampal neurogenesis, reducing neuroinflammation, regulating neuroendocrine and affecting epigenetic modifications, in order to provide new ideas for mechanisms research and treatment of depression.As the rise of precision medicine and individualized medicine brings human growing interest in exploring the sources and mechanisms of inter-individual differences and intra-group effects of depression, it will be a challenge to translate EE to the human society in a rational way.

Objective:To explore the effects of intrahippocampal injection of ferroptosis inducer Erastin on depressive- and anxiety-like behavior and the expression of ferroptosis-related proteins in rats.Methods:Forty 6-week-old healthy male Sprague-Dawley rats were randomly divided into five groups ( n=8/group): Control group, Erastin low-dose(200 ng/μL) group, Erastin medium-dose(400 ng/μL) group, Erastin high-dose group(600 ng/μL) and lipopolysaccharide (LPS, 10 μg/L) group.After the intrahippocampal injection of Erastin(2.5 μL per side), body weight, and behavioral tests, including sucrose preference test (SPT), forced swimming test (FST), open field test (OFT), and elevated plus maze (EPM), were performed to evaluate depressive- and anxiety-like phenotypes from the fourth day after injection.The levels of ferroptosis-related proteins and mRNA, including glutathione peroxidase 4 (GPX4), cyclo-oxygenase 2 (COX2), ferritin heavy polypeptide 1 (FTH1), long-chain fatty acyl-CoA synthetase 4 (ACSL4), solute carrier family 7 member 11 (SLC7A11) were measured using real-time quantitative PCR and Western blot analysis.SPSS 22.0 software was used for statistical analysis.One-Way ANOVA was used for multi-group comparison, and LSD was used for further pound-wise comparison. Results:(1)Body weight and behavioral tests: there were no statistically significant differences in baseline body weight and behavioral tests in these groups ( F=0.02-1.15, all P>0.05). After intrahippocampal injection, compared with the control group, medium-dose Erastin induced depression-like behaviors in rats more significantly, as indicated by reduced bodyweight ((245.20±5.24)g, (267.45±13.16)), sucrose preference in SPT ((32.14±8.51)%, (68.17±13.67)%), central time in OFT ((6.01±2.57)s, (16.49±7.21)s), percentage of time in open arm in EPM ((5.00±3.83)%, (19.63±5.91)%) and increased immobility time in FST ((37.00±7.58)s, (12.50±5.51)s) and percentage of time in closed arm in EPM ((89.43±4.77)%, (59.96±9.91)%), and there were statistically significant differences in these groups (all P<0.05). (2)The expression of ferroptosis-related indicators: after intrahippocampal injection, the expression of mRNA ( F=2.23, 8.37, 2.91, 7.60, 3.16, all P<0.05) and protein ( F=3.31, 40.13, 8.52, 3.70, 70.79, all P<0.05) of FTH1, GPX4, SLC7A11, COX2 and ACSL4 in hippocampus were statistically significant differences in the 5 groups.The mRNA and protein levels of FTH1, GPX4 and SLC7A11 in Erastin medium-dose group were lower than those in the control group (all P<0.05), while the mRNA and protein levels of COX2 and ACSL4 were higher than those in the control group (all P<0.05). Conclusion:Intrahippocampal microinjection of Erastin(400 ng/μL) can induce ferroptosis in hippocampus of rats and can also induce depressive-like behaviors in rats.

As a common psychiatric disorder, the etiology and pathogenesis of depression are complex and not yet fully elucidated.The diagnosis of depression mainly depends on the patients’ medical history, clinical symptoms and related examinations.Identification of biomarkers will provide important clues for the specific diagnosis and targeted treatment of depression.In addition to the widely recognized neurotransmitter dysregulation, hypothalamus-pituitary-adrenal axis hyperactivity, neuroplasticity, and neuro-inflammation theory, oxidative stress is also involved in the pathogenesis of depression in multiple ways.Many studies showed that the heat shock protein 70(HSP70)levels will increase in early stage to cope with the stress in patients with depression.However, lower HSP70 levels are often correlated with more severe depressive symptoms.HSP70 may be involved in depression through multiple pathways of oxidative stress, glucocorticoid receptors, neuroinflammation and neuroplasticity.Furthermore, increasing HSP70 expression results in significant improvement in depression-like behavior in animals.Thus, HSP70 possesses potential value as an early warning marker for depression as well as a therapeutic target.