The Oxytocin System Regulates Neural Mechanisms By Which Long-term Social Defeat Stress Affects Behavior

Severe environmental pressure or social stress may interfere with human mental function,and lead to the development of mental disorders and mental illness.According to statistics,there are about 450 million people around the world suffering from mental or behavioral disorders in different degrees.However,women suffer from the most common diseases(such as anxiety and depression)almost twice as much as men.Therefore,studying how stress experience are transformed into behavioral responses and gender differences will not only help us understand the pathogenesis of mental illness,but also provide an important theoretical basis for the prevention and treatment of mental illness.Some behavioral symptoms of mood disorders in humans can be simulated in non-human animals.Chronic social defeat stress(CSDS)is one of the models that can elicit long-lasting stress behavioral responses in many species.Previous studies have shown that repeated exposure to social conflict can lead to strong behavioral responses,such as reduced social interaction,social avoidance,and low levels of aggression.Research on responses to CSDS in females is challenged,because the females of laboratory rodent species have the low level of aggression.Mandarin vole(Microtus mandarinus)is a kind of monogamous rodent with the high level of sociality.Both males and females display high levels of territorial protection and aggressive behaviors.Therefore,we can explore the influence of CSDS exposure on behavioral responses and sex differences by using mandarin voles.Oxytocin(OT),a stress buffering hormone,may be involved in the regulation of the influence of CSDS on emotional and social behaviors.However,whether CSDS can produce different effects on two sexes,whether OT is involved in different effects of CSDS on different sexes,what function OT play in alteration of behaviors during process of CSDS and what is downstream circuits mechanism underlying effects of CSDS on behaviors remain unclear.Following experiments were conducted to answer these questions.OT and arginine vasopressin(AVP)have similar structures,and can interact with each other's receptors and play roles.At the same time,the effect of OT and AVP may play different roles in different sexes,resulting in different behavioral outcomes.On this basis,the second part of the study focuses on the influence of CSDS on emotional and social behaviors,OT and AVP systems,and sex differences.The results showed that exposure of CSDS led to anxiety-like behaviors and social avoidance in both male and female mandarin voles.At the same time,only females showed more sensitive behavioral responses to CSDS,that is,they showed depressive-like behaviors and decreased locomotion.We found that OT/c-Fos and AVP/c-Fos co-expressing neurons were significantly increased in the CSDS group of males compared to the control group exposed to single social defeat stress by double-labeled OT-and AVP-positive neurons in the paraventricular nucleus(PVN,the main source of OT and AVP)with c-Fos(a marker of neuronal activation).On the contrary,CSDS decreased the activity level of OT neurons in females but had no effect on the activity level of AVP neurons.The nucleus accumbens(NAc)is a key brain region of the regulation of the reward,motivation and social behavior,and is closely related to the occurrence of psychiatric diseases.Therefore,we analyzed the protein and mRNA expression levels of OTR,AVP and V1aR in the NAc and PVN of mandarin voles.We found that CSDS significantly reduced the protein and mRNA levels of OTR in NAc of male and female mandarin voles,and down-regulated the levels of OTR in the PVN of males.In further studies using female mandarin voles,we found that CSDS resulted in a downregulation of the relative density of OT-positive neural projections in the shell of nucleus accumbens(NAcs).Microinjection of OT agonists in NAcs by pharmacological means reversed anxiety-like and depression-like behaviors induced by CSDS.We infer that OT neural microcircuits of PVN-NAcs may play an important role in regulating the influence of CSDS on emotional and social behaviors.Both exogenous and endogenous OT has been shown to have anti-stress and anti-anxiety effects.However,most studies have focused on the regulation of OT in post-stress.It is unclear whether the OT system will change during the CSDS process.Whether the intervention of the exogenous drug in the process of CSDS will have a protective effect on OT system needs further exploration.In addition,the regulation of OT neural circuits of PVN-NAcs during and after CSDS on resulting behaviors is still unknown.In the third part of this study,we first investigated the changes in activation levels of OT neurons in the PVN,bed nucleus of stria terminalis(BNST)and supraoptic nucleus(SON)during CSDS.The results showed that short-term social defeat stress could increase the activation of OT neurons in the PVN and BNST in both males and females.Medium-to-long term social defeat stress could lead to downregulation of OT neuron activities in the PVN of females.But for males,the sustained activation of OT neuron activities in the PVN was observed after exposed to medium-to-long term social defeat.At the same time,medium-to-long term social defeat stress has no significant effect on the OT neuron activation level of the BNST.In addition,the change of OT neuron activity during the CSDS process was not obvious in the SON.Then,by chronic exogenous administration of OT during CSDS,we found that OT can prevent anxiety-like behaviors and social avoidance caused by CSDS,as well as depression-like behavior,decreased locomotion,and low levels of OTR protein in NAc induced by CSDS in females.Then,we used chemogenetics and optogenetics techniques to explore the neural circuit mechanisms underlying alteration of emotional and social behavior in mandarin voles during and after CSDS.The results showed that the activation of PVNNAcs OT neuron projections during or after CSDS significantly reversed the anxiety-like behavior and social avoidance caused by CSDS.Our results suggest that PVN-NAcs neuron projections play a key role in the regulation of CSDS-induced behavioral abnormalities.OT and dopamine(DA)are two important neurochemicals in the brain regulating emotion and reward.OT is involved in the regulation of DA function,which is helpful to social reward,social cognition and emotion-related behaviors.Previous studies have shown that decreased DA transmission in the medial prefrontal cortex(mPFC)is associated with the pathophysiology of depression.However,interaction between OT and DA in regulating stress responses needs further investigation.In the fourth part of this study,the influence of exogenous administration of OT during CSDS on the DA system in the mPFC was investigated.Our results showed that CSDS induced a decrease in DA 1 receptor levels in the prelimbic cortex(PrL)and the infralimbic cortex(IL)in both male and female voles.CSDS also induced the decrease of the protein level of tyrosine hydroxylase in the PrL,which is a rate-limiting enzyme in DA synthesis,and these changes could be prevented by repeated administration with OT during the CSDS process.In addition,using multi-channel optical fiber recording system and the DA probe,we found that the DA release levels in the PrL in mandarin voles exposed to CSDS was significantly reduced upon social investigation.Meanwhile,using the combination of optogenetics and DA probe,we found that activation of PVN-The ventral tegmental area(VTA)OT pathway after exposure to CSDS significantly increased the DA release level in the PrL.These results provide direct evidence for the interaction between the OT and DA in the PrL on the regulation of effects of CSDS on behaviors.To sum up,combined with the behavioral analysis,pharmacology,neural pathway manipulation techniques,molecular biology,multi-channel fiber photometry and DA probe method,this study analyzed the neurobiological mechanism and neural pathway underlying the interaction between the OT and other system involved in regulating the influence of CSDS on emotional and social behavior.We demonstrated that(1)the behavioral abnormalities induced by CSDS and sex differences may be caused by the differences between OT and AVP systems involved in the regulation of CSDS;(2)there were significant sex differences in the changes of OT neuron activation in PVN during CSDS;(3)exogenous administration of OT during CSDS can protect OT and DA systems,and then prevent the alterations in behaviors caused by CSDS;(4)the PVN-NAcs OT neuron projections are involved in regulating the influence of CSDS on emotional and social behaviors;(5)the activation of PVN-VTA neuron projection can directly increase the release of DA in the PrL.In conclusion,the present study reveals the neurobiological mechanism of OT and AVP/DA systems underlying the regulation of effects of CSDS on emotional and social behaviors,as well as neural circuits mechanism underlying changes in emotional and social behaviors during or after CSDS.These findings will provide an important theoretical basis for the diagnosis,relief and treatment of stress-induced mental diseases.