Effects of enriched environment on behavior and synaptic transmission in hippocampal pyramidal neurons of depressive-like mice
Objective To investigate the effects of enriched environment on the behavior of depres-sive-like mice induced by chronic stress and the synaptic transmission function of hippocampal pyramidal neurons.Methods Thirty-six 7-week-old SPF grade male C57BL/6J mice were divided into control group,model group and enriched environment group according to the random zone method,with 12 mice in each group.The mice in model group and enriched environment group were subjected to chronic unpredictable mild stress to establish depression models.During the modeling period,the mice in enriched environment group lived in the enriched environment for 6 hours per day,and lived in the routine environment for the rest time just like the mice in control group and model group.The depressive behaviors of mice were detected by sucrose preference test(SPT),forced swimming test(FST)and tail suspension test(TST).The motor func-tion and anxious behavior of mice were detected by open field test(OFT)and the cognitive function was de-tected by Morris water maze test.In vitro electrophysiological techniques were used to record field excitatory post-synaptic potentials(fEPSP)in the hippocampus,and whole-cell recording was used to record spontane-ous excitatory post-synaptic currents(sEPSC)and miniature excitatory post-synaptic currents(mEPSC)in hippocampal pyramidal neurons.Statistical analysis was conducted by SPSS 23.0 software.Multiple group comparisons of data were conducted by one-way ANOVA and repeated measures ANOVA.Results(1)Be-havioral results showed statistically significant differences among the three groups in terms of total activity time in the central area of OFT,sugar water preference rate of SPT and immobility time of TST and FST(F=17.12,26.07,41.13,60.18,all P＜0.01).The activity time in the central area of OFT and sugar water pref-erence rate of model group were lower than those in the control group(both P＜0.01),and the immobility time of TST and FST were higher than those in the control group(both P＜0.01).The activity time in the central area of OFT((56.56±3.47)s)and sugar water preference rate((71.22±8.37)％)of enriched environment group were higher than those in the model group((52.56±3.47)s,(59.53±8.72)％)(all P＜0.05),and the immobility time of TST((94.19±10.77)s)and FST((76.98±12.10)s)were lower than those in the model group((104.58±8.24)s,(111.41±9.56)s)(all P＜0.05).(2)The results of spatial learning memory showed that there was a significant difference of group and time interaction effects for laten-cy to find the platform(F=12.02,P＜0.01),and the latency to find the platform in model group was higher than that in control group and enriched environment group(both P＜0.01).The results of spatial exploration showed that there were statistically significant differences in the target quadrant residence time and the times crossing the platform among the three groups(F=19.67,20.27,both P＜0.01).The both indexes in the mod-el group were lower than those in control group and enriched environment group(all P＜0.01).(3)The re-sults of electrophysiological experiment showed that the interaction effect of fEPSP between groups and stimu-lations intensity among the three groups was significant(F=1.86,P＜0.01).At the stimulation level of 150 mA and 200 mA,the fEPSP of the model group was lower than those of control group and enriched envi-ronment group(all P＜0.05).(4)The results of the long-term potentiation(LTP)showed a statistically sig-nificant difference among the three groups(F=15.32,P＜0.01).The LTP of the model group mice((137.42±4.48)％)was lower than those of the control group((156.62±4.79)％)and the enriched en-vironment group((147.86±9.14)％)(both P＜0.05).(5)The whole-cell recordings results showed that there was a statistically significant difference in the frequencies of sEPSC and mEPSC among the three groups(F=15.30,13.72,both P＜0.01).The sEPSC and mEPSC frequencies of the model group mice((0.60±0.31)Hz,(0.63±0.26)Hz)were lower than those of the control group((1.27±0.46)Hz,(1.02±0.28)Hz)and the enriched environment group((0.96±0.45)Hz,(0.83±0.22)Hz)(all P＜0.05).Conclusion Environmental enrichment can alleviate the depressive behaviors induced by chronic stress and enhance cognitive performance,hipp-ocampal pyramidal neuron synaptic transmission and synaptic plasticity in depressive-like mice.
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