| Objective: The incidence of depression in high-stress populations such as clinical patients and nurses is significantly higher compared with general population,which deserves more attention.Studies have found that individuals with high resilience to stress are less likely to suffer from depression under stress.Thus trying to improve the resilience to stress and further reveal potential cerebral mechanism might help to prevent the occurrence of depression.Natural rewards such as food,water,sex and social interactions can bring pleasure and reduce the negative effects of stress.However,the specific role of reward during the process of stress and the impact of stress resilience under different stress levels remains unclear.Plenty of researches have explored the cerebral mechanism of stress and reward separately,but the cerebral mechanism behind the effect of reward on stress resilience is seldom.The ventral tegmental area(VTA)and the dorsal raphe nucleus(DRN)are key brain regions of reward network and are closely related to stress,but their roles in the effect of reward on stress resilience remain unclear.Studies have reported that the endogenous cannabinoid system and downstream metabolic glutamate receptor 5(mGluR5)are closely related to stress and reward,which might be the potential mechanism behind the effect of reward on stress resilience.Therefore,the current study,used social defeat stress to establish an animal depression model,applied different intensity of stress and reward during the modeling process,and observed the effect of reward on stress resilience under different intensity of stress through behavioral testing.Furthermore,this study also explored potential cerebral mechanism of the effect of reward on stress resilience,by focusing on reward and stress related endogenous cannabis system and downstream metabolic glutamate receptor 5(mGluR5)in VTA and DRN,which might help to reveal the cerebral mechanism behind the effect of reward on stress resilience,further give theoretical suggestions to build reward interventions for high stress populations such as nurses and patients.Methods:1.Social defeat stress model was developed in 21 days,during which,the C57 mice were placed and attacked in cages of aggressive CD-1 mice for 10 minutes everyday,after that they were ocated in the other side of the transparent partition,where they could smell and see the CD-1 without physical contact.Different stress intensity were reflected in escaping space,that is,small stress meant large cage,while large stress meant small cage.During the stress,24 hours accompany of female mice was given as a reward,after 21 days,behavioral tests including social interaction,sucrose preference and forced swimming were conducted.2.Protein expression of CB1 and mGluR5 in DRN and VTA was detected by western blot after chronic stress and reward;while expression of CB1 and mGluR5 at mRNA level was detected by qPCR;and the level of AEA,2-AG in DRN and VTA was determined by liquid chromatography coupled to tandem mass spectrometry(LC-MS/MS).3.During everyday social defeat stress,CB1 agonists or inhibitors was injected into mice,to observe the effect of medicine on the depression-like behavior of mice.Results:1.The large stress led to more depression-like behavior compare with the small stress.With the evidence that the social interaction index in the social interaction zone of large stress group was significantly lower than that of the small stress group(p = 0.039)and the control group(p < 0.001),while the social interaction index in the corner area was significantly higher than that of the small stress group(p = 0.019)and the control group(p = 0.003);in forced swimming test,the immobility time was longer in large stress group compare with the small stress group(p = 0.033)and the control group(p = 0.017).However there was no significant difference in the latency of immobility and sucrose preference between three groups.2.Under the large stress,the reward changed the depression-like behavior of the mice.The social interaction index in the social interaction zone of no reward group was significantly lower than that of the reward group(p = 0.006)and the control group(P<0.001),while the social interaction index in the corner area of no reward group was significantly higher than that of the reward group(p = 0.024)and the control group(p = 0.021).The immobility time of no reward group was also significantly higher than that of the reward group(p < 0.001)and the control group(p = 0.006),while no significant difference in the latency to immobility and the sucrose preference between three groups.With the small stress,the reward only changed the social avoidance behavior,and the social interaction index in the social interaction area of the reward group(p = 0.040)and the control group(p = 0.020)were significantly higher than that of the non-reward group,while the social interaction index in the corner zone and the immobility time of forced swimming and sucrose preference were not significantly differed between three groups.3.The differences on the social interaction index in the social interaction zone(t = 2.360,p = 0.036),social interaction index in the corner zone(t = 2.111,p = 0.048)and immobility time of forced swimming(t = 3.376,p = 0.003),between the reward group and the non-reward group,were significantly bigger under the large stress than that of small stress.That is,under the large stress,the reward has a greater effect on the depression-like behavior than that of small stress,which is more effective in improving the social interaction behavior of the mice and reducing the social avoidance behavior and despair behavior.4.The effect of reward on the mRNA expression of CB1 and mGluR5 in the VTA and DRN regions was detected by qPCR.Which found that in VTA,the expression level of CB1 in the non-reward group was significantly lower than that of the reward group(p = 0.021)and the control group(p < 0.001),the expression of mGluR5 was significantly lower in the no-reward group than that of the reward group(p = 0.028)and the control group(p < 0.001).Similarly,in the DRN,the CB1 expression level of the no-reward group was also significantly lower than that of the reward group(p = 0.043)and the control group(p < 0.001).Consistently the expression level of mGluR5 was also significantly lower in the no-reward group than that of the reward group(p < 0.001)and the control group(p < 0.001).5.The effect of reward on the protein expression of CB1 and mGluR5 in VTA and DRN regions was examined by Western Blot.Which found that in VTA,the expression level of mGluR5 in the reward group was significantly higher than that of the no-reward group(p = 0.010)and the control group(p = 0.030).In DRN,in contrast,the expression level of mGluR5 in the no-reward group was significantly lower than that of the reward group(p = 0.042)and the control group(p = 0.006).The expression of CB1 in VTA and DRN was not significantly differed between three groups.6.The effect of reward on the expression of AEA and 2-AG in VTA and DRN regions was examined by liquid chromatography coupled to tandem mass spectrometry(LC-MS/MS).Which found that in VTA,the expression level of 2-AG in the reward group was significantly higher than that of the no-reward group(p = 0.002)and the control group(p = 0.005).Similarly,in DRN,the expression of 2-AG was significantly higher in the reward group than that of the no-reward group(p = 0.014)and the control group(p = 0.043).The expression of AEA in VTA was not significantly differed between three groups,while in DRN,the expression of AEA in the reward group(p = 0.006)and no-reward group(p = 0.034)was significantly lower than that of control group.And the expression of AEA(t = 8.299,p < 0.001)and 2-AG(t = 2.254,p = 0.032)in VTA was significantly higher than that of DRN.7.CB1 agonist(URB-597)and CB1 inhibitor(AM251)were injected intraperitoneal during daily social deficit stress.The behavioral test found that compared with the CB1 agonist group(p = 0.004)and control group(p = 0.015),the CB1 inhibition group showed significant decrease in the social interaction index in the social interaction zone,they showed significant increase in the social interaction index in the corner area compared with the CB1 agonist group(p = 0.010)and control group(p = 0.003).The sucrose preference of CB1 inhibition group was also significantly lower than that of CB1 agonist group(p = 0.012)and the control group(p = 0.005).However,group difference on the immobility time and immobility latency of forced swimming was not significant.FollowingConclusionDifferent stress intensities can lead to different degrees of depression-like behavior,during which,reward can enhance stress resilience and reduces depression-like behavior,and the effect is larger under large stress.Reward changes the expression level of endocannabinoid system and the mGluR5 in VTA and DRN regions,and further reduces the damage caused by stress.Therefore,rewards can be an effective way to improve stress resilience and avoid depression for high-stress people(such as nurses and patients).However,the intervention programmer should be designed based on the stress characteristics of different populations.The endogenous cannabis system and mGluR5 might be potential biomarker to reflect the effects of intervention. |
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