Effect Of Fasting On Estrogen Deficiency Induced Depression-like Behavior And Its Potential Mechanism

Depression is a common nervous system disease.The main clinical manifestation is persistent depression.Core symptoms also include mental retardation,decreased volitional activity,cognitive dysfunction,lack of sleep,lack of interest,and even suicide.The pathogenesis of depression is very complex,which is related to biological,psychological and social environmental factors.A large number of studies show that there are obvious gender differences in depression,and the incidence rate of female depression is about two times that of men.Estrogen may be the main reason for this gender difference.Studies have shown that sustained low levels of estrogen concentration is closely related to the occurrence of depression.Ovaries are the main source of estrogen in adult female animals.The removal of mouse ovaries can cause a drop in estrogen levels and trigger depression-like behavior.Studies have shown that fasting has estrogen-like effects.Further research shows that fasting can improve depression-like behavior induced by stress.Our previous studies showed that fasting significantly increased the expression of estrogen in mice.However,whether fasting can improve the depression-like behavior induced by estrogen deficiency and its specific mechanism is not clear,which needs to be further explored.It is worth noting that fasting can significantly increase the expression level of ghrelin in serum.In addition,ghrelin can improve stress-induced depression-like behavior.However,whether ghrelin can improve the depression-like behavior induced by estrogen deficiency has not been reported.In addition,the expression of ghrelin is regulated by estrogen.Whether ghrelin is involved in the effect of fasting on the improvement of estrogen deficiency induced depression-like behavior and whether this effect is closely related to estrogen remains to be further explored.The present study utilized a combination of approaches,including behavior pharmacology,Western blotting,Golgi staining,immunohistochemistry and electrophysiology,from the overall to cellular and molecular level,to examine the potential mechanisms underlying the antidepressant effect of fasting in animal model of the ovariectomized.This study will focus on fasting and ghrelin,which is closely related to fasting.First,we used behavioral pharmacology experiments to explore the effect of 9-hour fasting on estrogen deficiency induced depression-like behavior.In the open field test,the locomotor activity of ovariectomized mice was not affected by the 9-hour fasting.Forced swimming test indicated that 9-hour fasting significantly reduced the immobility time of ovariectomized mice,and this effect has the characteristics of rapid onset.The change of synaptic plasticity induced by the rapid activation of BDNFmTORC1 signaling pathway is considered to be an important mechanism of rapid antidepressant action.Therefore,we investigated the effect of 9-hour fasting on the levels of BDNF-mTORC1 signaling pathway by Western blotting.The western blotting indicated that fasting treatment reversed the reductions in BDNF,AKT,ERK,pERK,mTORC1,p70S6,4EBP1,PSD95 expression induced by ovariectomy.Rapamycin,an inhibitor of mTORC1,also antagonized the effect of fasting on the protein expression.Rapamycin significantly decreased the expression of mTORC1,PSD95 in hippocampus and frontal cortex.Then,we used Golgi staining experiment to explore the effect of 9-hour fasting on the density of dendritic spines in the hippocampus of ovariectomized mice.The results showed that 9-hour fasting significantly reversed the decrease of dendritic spine density in the CA1,CA3 and DG regions of hippocampus caused by ovariectomy.Furthermore,rapamycin reversed the effect of 9-hour fasting on dendritic spine density in area CA1,CA3 and DG of hippocampus.Rapamycin alone significantly increased the density of dendritic spines in the CA1 and CA3 regions of hippocampus compare to the ovariectomized group.Finally,we investigated the effect of 9-hour fasting on LTP in the CA1 region of hippocampus in ovariectomized mice.The result showed that 9-hour fasting significantly increased the LTP in the hippocampus CA1 region compared with the ovariectomized group.Next,we first used behavioral pharmacology experiments to investigate the effects of Ghrelin on estrogen-deficiency induced depression-like behavior and whether it is regulated by estrogen.In the open field test,ghrelin had no effect on locomotor activity in ovariectomized mice.Forced swimming test indicated that ghrelin significantly reduced the immobility time of ovariectomized mice.Furthermore,an estrogen antagonist tamoxifen reversed the effect of ghrelin on immobility time.Tamoxifen alone decreased immobility time compares to the ovariectomized group.The hippocampus and prefrontal cortex are brain regions where estrogen receptors are highly expressed.In order to further elucidate the effect of ghrelin on estrogen deficiency induced depression like behavior and its mechanism regulated by estrogen,brain sites related to the antidepressant effect of ghrelin were determined.We investigated the effect of ghrelin on c-fos protein expression in hippocampus and prefrontal cortex of ovariectomized mice by immunohistochemical experiments.Immunohistochemical study indicated that ovariectomy produced a significant reduction in c-fos expression in both hippocampus and prefrontal cortex.This effect was reversed by ghrelin in the CA1 and DG regions of hippocampus and then tamoxifen prevented the effect of ghrelin on c-fos expression in the DG region of hippocampus.Tamoxifen alone decreased c-fos expression in the CA1 and CA4 regions of hippocampus.The CREB-BDNF signaling pathway is an intersection of several antidepressants.To further explore the mechanism of ghrelin’s antidepressant effect.We used western blotting to detect the effect of ghrelin on the protein expression level of CREB-BDNF signaling pathway.The results showed that ghrelin normalized BDNF levels in the hippocampus.Finally,we used immunofluorescence staining to investigate the effect of ghrelin on neurogenesis.The results indicated that ghrelin treatment significantly increased BrdU expression.Ovariectomy reduced the expression of BrdU in DG regions of hippocampus.Tamoxifen also antagonized the effect of Ghrelin on the BrdU expression.Taken together,we conclude that fasting can improve estrogen-deficiency induced depression-like behavior.On the one hand,the 9-hour fasting could rapidly activate the BDNF-mTORC1 signaling pathway,promote the increase of dendritic spine density in the hippocampus and enhance the LTP in the hippocampal CA1 region.On the other hand,ghrelin can promote the activation of neurons in the hippocampus and prefrontal cortex of depressed mice,activate the CREB-BDNF signaling pathway,and promote neuronal regeneration.The completion of this study may help in the adjustment of the dietary structure of female patients with depression.