Objective  To investigate the behavioral changes and monoamine neurotransmitter levels in a rat model of chronic unpredictable mild stress (CUMS)-induced depression and explore the potential effects of Vitamin B₁₂ (VitB₁₂) on CUMS model rats and the underlying mechanisms.

Methods A total of 72 Sprague-Dawley (SD) rats were randomly assigned to 3 groups, a control group, a CUMS group (subjected to three weeks of CUMS), and a CUMS + VitB₁₂ group (CUMS rats receiving microinjections of VitB₁₂ in the neck). The body mass of the rats was measured, and behavioral assessments were conducted using the sucrose preference test, open field test, and forced swimming test. High-performance liquid chromatography (HPLC) was used to analyze the levels of monoamine neurotransmitters, including 5-hydroxytryptamine (5-HT), norepinephrine (NE), and dopamine (DA), in each group of rats. Hematoxylin-eosin (HE) staining was performed to observe pathological changes in hippocampal neurons. Western blot was performed to detect the expression of signal pathway-related proteins, including brain-derived neurotrophic factor (BDNF), tropomyosin receptor kinase B (TrkB), and cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) in the hippocampal tissue.

Results  Starting from week 5, rats in the CUMS group exhibited lower average body mass compared to the control group, while the CUMS + VitB₁₂ group showed a significant increase in body mass compared to the CUMS group (P < 0.05). At weeks 3 and 6, sucrose preference of rats in the CUMS group was significantly lower than that in the control group (P < 0.001). At week 3, sucrose consumption in the CUMS + VitB₁₂ group was significantly higher than that in the CUMS group (P < 0.01), with a more pronounced increase observed in week 6 (P < 0.001). Starting from week 4, the CUMS group showed reduced scores in grid crossing, grooming, and rearing activities in the open field test compared to the control group, indicating reduced locomotor activity and exploratory behavior (P < 0.001). The CUMS + VitB₁₂ group showed improved behavioral performance compared to the CUMS group (P < 0.01, P < 0.001). In the forced swimming test at weeks 3 and 6, the immobility time of rats in the CUMS group was significantly longer than that in the control group (P < 0.01). At week 6, the immobility time of rats in the CUMS + VitB₁₂ group was significantly shorter compared to that of the CUMS group (P < 0.01). HPLC results showed that the levels of 5-HT, NE, and DA in the cerebral cortex of rats in the CUMS group were significantly lower than those in the control group (P < 0.01, P < 0.001), while these neurotransmitter levels were significantly higher in the CUMS + VitB₁₂ group compared to those in the CUMS group (P < 0.05, P < 0.01). HE staining results showed that the number of hippocampal cells in the CUMS group was significantly reduced, with shrunken nuclei, while the CUMS + VitB₁₂ group showed an increased number of neurons with intact morphology compared to the CUMS group (P < 0.05). Western blot analysis showed that the expression levels of BDNF, TrkB, and CREB proteins in the hippocampus were significantly lower in rats in the CUMS group than those in the control group (P < 0.05), while the expression levels of BDNF, TrkB, and phosphorylated CREB (p-CREB) were significantly higher in the CUMS + VitB₁₂ group compared to the CUMS group (P < 0.05).

Conclusion In CUMS rats, the levels of monoamine neurotransmitters (5-HT, NE, and DA) in the cerebral cortex of the brain are decreased, accompanied by a decrease in neuronal cells, which results in anxiety- and depression-like behaviors. VitB₁₂ can upregulate the levels of these neurotransmitters, ameliorate the cytopathological conditions, and regulate the BDNF/TrkB/p-CREB signaling pathway, thereby alleviating depressive symptoms.