| Introduction and Objective: Previous studies about the impact of stress mainly focused on stress intensity and endurance. However, it was probably ignored individual active adaptive behavior in stress coping. Facing to stressor, individual can learn to flee from the dilemma via continual study, it is an active challenge and study process; while on the other hand, it is also probable to induce various stress-related diseases because of stress fault. Based on successfully establishment of Forced Swimming Test animal model, changes of active and passive behavior after FST were observed; to explore the impact of different behavior response on explore behavior, endocrine, hippocampal volume and metabolin, and to illuminate psychopathological implication of active and passive behaviors of stressed rats in Forced Swimming Test and neuronal mechanism.Materials and Methods: Twenty-four adult male Sprague-Dawley rats, which body weight ranged from 180g to 200g, were randomly allocated into 2 groups (n=6 for each group): control and experiment group. Experiment group consists of single stress rats (Acute stress group), rats stressed daily for 14 consecutive days (2-week stress group) and rats stressed daily for 28 consecutive days (4-week stress group). Forced Swimming Test was utilized, recording times and duration of swimming, climbing and immobility during stress, and rats were redivided into active behavior group and passive behavior group based on behaviors present in the last stress. Spontaneous activities were measured at the beginning and end of stress by the Open-Field Test. Bilateral hippocampal volume and NAA ratio were accomplished by MRI and ~1H-MRS respectively. Serum corticosterone concentration was measured by ELISA. Concentrations of NAA and Cr in hippocampus structure were measured by high performance liquid chromatography coupled with ultraviolet detector. All values in the text and figure were expressed as Mean±Standard Error(x±SE). The data were analyzed by analysis of variance and T test with SPSS 13.0 for Windows and Excel 2003. Factor analyis explore interaction and independent effects between stress intensity and behavior adaptation. Pearson relation analyzed the relationship between behavior in FST and OFT.Results: (1) Relationship between behaviors in OFT and FST: coefficient between swimming duration and number of crossing and decoration are 0.472 and 0.531, P＜0.05; number of rearing and vertical movement are highly correlated to swimming duration, coefficient were 0.625 and 0.637 respectively, P＜0.01; climbing duration is correlate to vertical movement, r=0.469, P＜0.05, whereas there is no correlation between climbing duration and crossing, decoration and rearing, coefficient was 0.194, 0.454 and 0.399 respectively; immobility duration is negativly related to crossing, rearing, decoration and vertical movement, with the coefficient of-0.477, -0.672, -0.654 and -0.792 respectively. Coefficient between central grille detention time and swimming duration, climbing time and immobility duration are -0.139, -0.158 and 0.154 respectively, no statistically significant was found.(2) Rats were redivided into active behavior group and passive behavior group according to the duration of active behavior and passive behavior in the last stress. Standard of active behavior group: active behavior duration≥30seconds or≥10times(n=8); rests were passive behavior group.(3) Behavior differences after stress: no difference found among central grille detention time of control (2.4±0.4), passive behavior group (4.9±2.0) and active behavior group (3.0±0.6), P=0.447; control group (47.5±12.1) shows more crossing number than passive behavior group (15.4±4.4), P=0.003, no difference found among active behavior group (27.3±4.2), passive behavior group and control; there is more rearing times in active behavior group (5.9±1.8) than passive behavior group (2.1±0.7) and control (3.2±0.3), while no statistic difference found; locomotion of control (50.7±12.3) is more than passive behavior group (17.5±4.9), P=0.004, no difference was found between active behavior group (36.8±5.8) and control; vertical movement of passive behavior group(5.6±0.9) is less than active group (12.3±3.4), P=0.007, compare with control group(10.2±0.8), no difference found; passive behavior group (3.5±0.6) present less decoration times than control (7.0±0.7), P is 0.01, no difference between active behavior group (6.4±1.9) and passive behavior and control; no statistically significant was found about defecation times among active behavior (1.5±0.6) group, passive behavior group (2.9±0.7) and control (1.7±0.6), P=0.260.(4) Serum corticosterone concentration change induced by stress: passive behavior group (846.393±58.094ng/ml) presents higher corticosterone concentration than control (191.599±47.526ng/ml) and active behavior group (508.843±12.010ng/ml), P were 0.007 and 0.000. Active behavior group is higher than control group, P=0.022.(5) Comparison of hippocampal volume among three groups after stress: no difference was found of left hippocampal volume among active behavior group (68.912±2.264), control group (69.552±1.705) and passive behavior group (64.241±2.247), P=0.186; fight hippocampal volume of passive behavior group (63.702±1.541) is smaller than control (71.905±1.541) and active behavior group (70.062±2.129), P were 0.006 and 0.017 respectively; ratio of hippocampal volume to total brain volume of active behavior group left side (0.075±0.006) is larger than that of passive behavior group (0.065±0.008), P is 0.006; at the right side, control (0.072±0.005) and active behavior group (0.077±0.007) are significant largger than passive behavior group (0.064±0.006), P is 0.024 and 0.000. No difference between control and active behavior group.(6) Comparison of hippocampal metabolin ratio among three groups: relative NAA ratio of left hippocampal in control (1.304±0.088) is significant higher than passive behavior group (1.107±0.103), P is 0.004; similar phenomenon can be found in the right side, control group (1.268±0.070) is higher than that of passive behavior group (1.119±0.133), P is 0.021; compare to control group, no difference was found between both sides of active behavior group (left: 1.200±0.132; right: 1.224±0.117).(7) Comparison of left hippocampal metabolin concentration among three groups: left hippocampal NAA concentration of control group (7.143±0.366) is higher than passive behavior group (5.874±0.451), P is 0.044, no difference between active behavior group (6.419±0.310) and both of them; according to ratio of NAA/Cr tested by HPLC, control (1.323±0.094) is much higher than passive behavior group (1.110±0.160), P is 0.015, no difference was found between control and that of active behavior group (1.299±0.065).(8) Result of interaction and independent effect of stress intensity and behavior response: there is no interaction between two factors of following index: center grille detention time, crossing, rearing, decoration, defecation, vertical movement, locomotionm, bilateral hippocampal volume, hippocampal metabolin level and serum corticosterone concertration.Conclusion: (1) Active behavior in Forced Swimming Test is positivly related to crossing, rearing and decoration behavior in OFT; while passive behavior is negativly related to these behaviors. (2) Behavior response is an ideal index for whether rats successfully adapted to surroundings in FST.(3) Compared to active behavior rats, passive behavior rats show decrease of auto exploring behaviors, elevation of main stress hormone, and reduction of hippocampus volume and metabolin.(4) Factor analysis shows no interaction between stress intensity and behavior response, indicating that the effect of behavior response is independent.
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