Effects Of Maternal Deprivation On The Locomotion, Learning And Memory In Rats And Its Neuro-biological Mechanisms

Background and objective: Studies indicate that Early Life Stress (ELS) is one of the important risk factors in many types of mental disorders including major depression, posttraumatic stress disorder (PTSD), children attention deficit and schizophrenia. Most studies about Early Life Stress focus on the neuroendocrine system, trying to explain its mechanisms. However, studies on human could not be carried out in such field, due to the constraint of ethics and limitation of methods. Therefore, more and more animal models are utilized to research deeply and completely. The most popular one is maternal deprivation (MD) model that is employed in rodents, since it accords with the feature of mother-infant separation in human. Meanwhile, most studies on the mechanisms of maternal deprivation model concentrate on the hyper-responsiveness of hypothalamic-pituitary-adrenal (HPA) axis and the lower function of 5-hydroxytryptamine (5-HT) system in hippocampus. Our previous studies also indicate that the number of positive cell of 5-HT1A receptor in CA1 region of hippocampus is significantly less after maternal deprivation. Considerable evidences indicate that mental disorders, especially depression, are not just functional diseases, but also accompanying with neuro-biological changes,such as the decrease of neurons in hippocampus and the disturbance of neuranagenesis in dentate gyrus, and structural changes, such as the decreased volume of hippocampus. On the basis of the findings above, we consider that early life stress plays a key role in the onset of many types of mental disorders, especially depression in human, one of the possible reasons of which is the neuro-biological and structural changes in hippocampus. Therefore, the present research employs maternal deprivation model to observe its effects on the locomotion, learning and memory in rats, and then explore the neuro-biological mechanisms by observing the damage of hippocampus and the disturbance of neuranagenesis in dentate gyrus. Thus, the present study can provide evidence about the role of early life stress on the generation and development of mental disorders in human, and conduct the early education and mental intervention on children.Materials and methods: A couple of healthy and clean male and female Sprague-Dawley rats, the weight of which is 200-250g, was reared in one cage. Their male offspring were assigned to two grougs randomly, normal control and maternal deprivation group. Rats in control group were reared normally, while rats in maternal deprivation group experienced two types of stresses in two stages: (1) maternal separation stage, the infants were separated from their mothers for 3 hours per day, from postnatal day 3 to 21. (2)social isolation stage, the newborn rats which experienced maternal separation were reared individually from postnatal day 22 to 60. The locomotion and Morris water maze test were performed on day 22 and 61 respectively, to detect the locomotion , ability of learning and memory of rats. And then rats in both groups were sacrificed, while their brains were made into paraffin section. The numbers of cells and the percentage of apoptotic neurons in hippocampus and dentate gyrus were observed with Nissl staining and TUNEL methods. Data were presented as mean±?standard deviation (x±S), and statistically analyzed by two independent samples t test with EXCEL 2003 and SPSS13.0, when normally distributed. Otherwise, two independent samples Nonparametric test was applied and variables were presented as median?(? interquartile range) [M(QR)]. A level of P<0.05 was accepted as statistically significant.Results: Rats in maternal deprivation group experienced maternal separation and social isolation from postnatal day 3 to 21 and from 22 to 60 respectively, while rats in control group were reared normally. The locomotion test and Morris Water Maze test were performed on postnatal day 22 and 61. And then rats were sacrificed, while their brainswere made into paraffin section. To observe the number of cell and the percentage of positive apoptosis nerons in hippocampus and dentate gyrus, the Nissl staining and TUNEL are performed.1.Effect of maternal deprivation on the locomotion in rats①on postnatal day 22: Results showed that rats in maternal deprivation group had more activities, especially in periphery of arena in locomotion test, comparing with rats in control group (P<0.05). It indicates that the locomotion of rats in maternal deprivation group increases, which suggests that maternal deprivation induces anxiety-like behavior in rats.②on postnatal day 61: The time of activities of rats in maternal deprivation group was significantly increased, compared with rats in control group (P<0.01), while there were no difference in distance of activity between two groups. It indicates that after experiencing maternal separation and social isolation stress, rats in maternal deprivation group appear anxiety-like and/or depression-like behavior at the same time. 2. Effect of maternal deprivation on the learning and memory in rats Morris Water Maze test had been performed for 5 days. The first 4 days were called place navigation, during which rats were trained twice a day to examine the learning ability by comparing their escape latency. The memory ability of rats were reflected in test on the fifth day, which was called spatial probe by comparing the following variables, the swimming time in the four quadrants, the percentage of swimming pathway in every area around the platform and the times of crossing the platform. Morris water maze test was performed on postnatal day 22 and 61.①on postnatal day 22: Our results showed that rats in maternal deprivation group needed more time to find the insidious platform in place navigation, especially on the third day(P<0.05). And the swimming time in the third quadrant of rats in maternal deprivation group was less than that of rats in control group in spatial probe (P<0.05). It indicates that rats in maternal deprivation group have worse ability of learning and memory, which suggests that the ability of learning and memory is damaged after maternal separation.②on postnatal day 61: Our results showed that rats in maternal deprivation group needed more time to find the insidious platform on all of the four days in place navigation (P<0.05 or P<0.01), and their swimming time in the fourth quadrant was more than that of rats in control group (P<0.05), while the times of crossing the platform was significant less (P<0.05).The results indicate that rats still have learning and memory deficits after experiencing maternal deprivation and social isolation stress.3. Effect of maternal deprivation on the number of neurons in hippocampus and dentate gyrus①on postnatal day 22: The results showed that maternal deprivation significantly decreased the numbers of neurons in CA2 and CA3 regions of hippocampus and in dentate gyrus compared with that in control group (P<0.01). It indicates that maternal deprivation can induce decreasing of cells in hippocampus and dentate gyrus, which suggests that hippocampus and dentate gyrus are damaged by maternal separation stress.②on postnatal day 61: The results showed cells decreasing in CA3 region of hippocampus of rats in maternal deprivation group (P<0.01). It suggests that the hippocampus is still damaged after experiencing maternal deprivation and social isolation stress. 4. Effect of maternal deprivation on the neuronal apoptosis in hippocampus and dentate gyrus①on postnatal day 22: The results showed that rats in maternal deprivation group had a higher rate of neuronal apoptosis in CA3 region of hippocampus (P<0.05) and in dentate gyrus (P<0.01), compared with that in control group. It indicates that maternal separation can increase the neuronal apoptosis in hippocampus and dentate gyrus.②On postnatal day 61: The results showed that rats in maternal deprivation group had a high rate of neuronal apoptosis in CA3 region of hippocampus (P<0.05), which indicates that rats in maternal deprivation group still have more apoptotic neurons in hippocampus after experiencing maternal deprivation and social isolation stress.Conclusions: Early Life Stress induce Anxiety-like behavior, and/or depression-like behavior and deficit of learning and memory in rats. One of the neuro-biological reasons of that may be the decreased numbers of neurons in hippocampus and dentate gyrus. Meanwhile, the increase of neuronal apoptosis may engage in the damage of hippocampus and dentate gyrus.