Effects of maternal chronic stress during pregnancy on learning and memory function and expression of hippocampal brain-derived neurotrophic factor in offspring rats

Background Chronic stress stimulation during pregnancy will have a long-term impact on the growth and development of offspring, and the most serious damage is the decline of learning and memory ability. Previous studies have found that maternal stress in early or late pregnancy has the most significant effect on the development of fetal brain, such as abnormal expressions of monoamine neurotransmitter and insulin-like growth factor Ⅱ in hippocampus, but there are few reports about the expression of brain derived neurotrophic factor (BDNF) and its relationship with learning and memory function.

Objective This experiment is designed to observe the changes of learning and memory function in offspring of pregnant rats with chronic stress, detect the expression of BDNF in hippocampus, and analyze the relationship between BDNF and learning and memory of offspring rats.

Methods Twenty female Wistar rats of SPF grade aged 80-90 days and not pregnant were randomly divided into a model group and a control group, with 10 rats in each group. Fifteen male Wistar rats of SPF grade aged 90-100 days were randomly divided into a model group with 10 rats and a control group with 5 rats. After one week of adaptive feeding, a stress model of female rats was established by chronic unpredictable mild stress (CUMS) for 21 consecutive days, while the female rats in the control group were fed in normal environment. On the 3rd day of the stress stimulation, the female and male rats in the model group were caged according to 1:1 sex ratio, and the control rats were caged according to 2:1. One day before the stress and 1, 7, and 14 days after the stress, blood samples were collected from the angular vein of the female rats, plasma cortisol content was measured by ¹²⁵I cortisol radioimmunoassay, and the concentration of corticosterone was calculated. If the concentration of corticosterone in the model group was higher than that in the control group, the model was considered successful. On postnatal day 21 (PND21), the offspring rats were weaned, and 16 pups (eight females and eight males) were randomly selected from different nests of the two groups, named correspondingly as model offspring group and control offspring group, and four pups were raised in normal environment in each cage after being grouped. On PND28, the body weight of the pups was recorded. On PND42, the body weight of the pups was recorded, the learning and memory ability of the pups was detected by Morris water maze (escape latency and the number of crossing platform) and Y maze (the training times for reaching learning criterion and correct response rate), the morphology of hippocampus was observed after HE staining using electron microscopy, and the mRNA and protein expressions of BDNF were measured by real-time fluorescence quantitative PCR and Western blotting respectively.

Results On the 7th day of stress, the plasma corticosterone level of the model female rat group (348.50±13.24) μg·L^-1 was higher than that of the control female rat group (224.00±39.84)μg·L^-1 and reached a peak (P < 0.05). On the 14th day of stress, the corticosterone level of the model female rat group (258.38±22.77)μg·L^-1 was still higher than that of the control female rat group (218.78±16.68) μg·L^-1, suggesting that the model group was stressed. The results of Morris water maze experiment showed that the escape latency of the model offspring group (32.00±3.61) s was longer than that of the control offspring group (20.01±4.12) s (F=7.30, P < 0.05); the number of crossing platform of the model offspring group (5.54±2.11) was less than that of the control offspring group (7.38±1.38) (t=2.26, P < 0.05). In the Y maze test, the training times in the model offspring group (31.32±17.12) were significantly higher than that in the control offspring group (19.38±11.22), while the correct response rate in the model offspring group (45%±15%) was lower than that in the control offspring group (89%±25%) (t=6.77, P < 0.05). The hippocampal structure of the model offspring group was not as clear as that of the control offspring group, the cell morphology was irregular, the cell body was reduced, the cell spacing was significantly increased, and the cell arrangement was loose. The mRNA relative expression level of BDNF in the model offspring group (0.61±0.15) was lower than that in the control offspring group (1.00±0.22) (t=3.79, P < 0.05), and the protein relative expression of BDNF in the model offspring group (0.15±0.10) was also lower than that in the control offspring group (0.50±0.02) (t=28.11, P < 0.05). The escape latency (r=-0.59, P < 0.05) and training times (r=-0.59, P < 0.05) were negatively correlated with the protein relative expression of BDNF, and the correct response rate (r=0.66, P < 0.05) was positively correlated with the protein relative expression of BDNF in the offspring rats.

Conclusion Chronic stress during pregnancy can decrease the learning and memory function and the relative expression of synaptic related protein BDNF in hippocampus of offspring rats, and the decline of learning and memory ability is related to the decline of BDNF protein relative expression.