Neuroendocrine Mechanism Of Comforting Behavior In Brown Voles

Empathy refers to sharing or relating to the affective states of another.As a highly preserved prosocial behavior,empathy ubiquitously exists both in lower and higher animals.In humans,empathy is essential for the production of altruistic behaviors(e.g.,prosocial and helping behavior),and impaired empathy may be associated with a few of antisocial behaviors such as apathy,assault,homicide and attempted homicide.On the other hand,deficits in empathy are present in many psychiatric diseases,including but not limited to autism,schizophrenia and psychopathy.Therefore,the study of neural mechanisms of empathy and its affecting factors have great means not only to scientific and clinical interests but also to sociological significance.According to the characteristics of complexity,empathy could be divided three levels from simple to complex,i.e.,the primary level of "emotional contagion",the more complex level of "consolation",and the most elaborate level of "perspective taking and targeted helping".Consolation behavior has been defined as an increase in affiliative contact in response to and directed toward a distressed individual by an uninvolved bystander,which produces a calming effect.As a more complex form of empathy,it has long been assumed to be restricted to species possessing more complex cognitive functions such as humans,apes,elephants,dolphins,canids and corvids.However,recent studies indicated that some social rodents might be capable of displaying consolation behaviors such as monogamous prairie vole(Microtus ochrogaster).Efforts have been made to examine the underlying neural mechanisms of empathy-like behavior.Although the neural basis of some lower-level empathy(emotion contagion)had been well studied,the occurrence of some higher-level empathy(consolation,perspective taking,etc.),influence factors,potential sex differences and the underlying neuroendocrine mechanisms remain largely unknown.The mandarin vole(Microtus mandarinus)is a high social monogamous rodent that is widely distributed across China.In this study,we determine to systematically examine consolation behaviors and the underlying neural mechanisms by using this animal model.In section one of this thesis,we first examined whether consolation behaviors could be elicited in mandarin voles by exposure to a social-defeated partner.We then investigated consolation-related brain regions by examining c-fos expression profiles(a cellular marker of neural activity)in some brain structures along with the behavior changes.Our results showed that after 15-min separation,the naive subjects greatly increased grooming(allogrooming)toward a socially defeated partner but not toward a partner who just underwent only separation.Follow up analysis indicated that the behavior changes in the subject had empathy-based characters,which including emotional contagion,state matching,familiarity bias,and self-other differentiation.Accompanied with these behavioral changes in the subjects,c-Fos expression was elevated in many of the brain regions relevant for emotional processing,including but not limited to the ACC,basal/basolateral and central nucleus of the amygdale,paraventricular nucleus(PVN),bed nucleus of the stria terminalis,medial preoptic area,and the lateral habenular nucleus.In particular,by using immunofluorescence double-labeling technique,we found the GAD67/c-Fos and oxytocin(OT)/c-Fos colocalization rates were elevated in the ACC and PVN,indicating selective activation of GABA and OT neurons in these regions.In the following pharmacological studies.we found that this partner-directed grooming was blocked by pretreatment with an OTR antagonist or a GABA-A receptor antagonist in the ACC.Overall,the above results suggested that consolation behavior could be elicited by a defeated partner in mandarin voles;many brain structures were activated when consolation happens;and the OT-GABA pathway within the ACC may be necessary for the expression of this consolation behavior.It is well known that impaired empathy is frequently observed in some depressed patients.But currently,the neural mechanisms underlying the impaired empathy,especially impaired-consolation,remains largely unknown.Chronic stress(physical or psychological)is a major precipitating factor for depression.Therefore,in section two,we compared the effect of chronic physical stress(PS,social defeat)and psychological stress(ES,vicariously experienced the defeat of their partners)on consolation behavior in mandarin voles.Owing to the important roles played by OT,vasopressin(AVP),dopamine(DA),and serotonin(5-HT)systems in empathy-like behaviors,we studied the expression profiles of OTRs,vasopressin V1a receptors(V1aRs),dopamine D1 and D2 receptors(D1R and D2R),and serotonin receptors(5-HT1ARs)within the ACC under chronic ES and PS exposure respectively.We further investigated which receptors are involved in the impaired consolation by using pharmacological methodology.We found that chronic PS but not ES exposure reduced allogrooming toward their distressed partners.In addition,PS-exposed voles also show a range of depression-like symptoms such as weight fluctuation,anxiety-and despair-like behaviors and elevated plasma corticosterone levels.Along with these behavior changes,the expressions of OTRs,D2Rs and 5-HT1ARs within the ACC were significantly down-regulated.Finally,we found that the reduced allogrooming and some anxiety-like syndromes in PS-voles could be alleviated by pretreatment with an OTR,D2R or 5-HT1AR agonist within the ACC,whereas injections of corresponding receptor antagonists to the control voles decreased the consolation behavior and increased some anxiety-like behaviors.Together,the above results indicate that the decreased OTR,D2R and 5-HT1AR activities in the ACC may be involved in the behavioral changes in the PS-voles.Serotonin systems within the ACC play important roles in emotion,motivation and cognition.It is well known that 5-HT neurons are mainly originated in dorsal raphe nucleus(DR).But currently,there are conflicting reulsts regarding the function role played by 5-HT in some empathy-like behaviors.Furthermore,the neural circuits underlying some empathy-like behaviors,especially consolation behavior,remain largely unknown.Therefore in the third section,we explore the function role of DR?ACC serotonergic circuit in the consolation behavior of mandarin voles by using optogenetic technology.Our results showed that although optogenetic activation of DR?ACC serotonergic circuit had no significant effect on allogrooming behavior towards a distressed partner,optogenetic inhibition of this circuit significantly reduced both the frequency and duration of allogrooming in mandarin voles.In addition,optogenetic inhibition decreased the subjects' sociability in the three-chamber test.In the following experiment,by using fiber photometry along with 5-HT fluorescent sensors,we found that 5-HT releases were largely elevated in the ACC when allogrooming behavior happens.These results provide empirical evidence that 5-HT release within the ACC play essential role in the consolation-like behaviors in mandarin voles.Altogether,in section one of this thesis,we found that consolation behavior could be elicited by a defeated partner in mandarin voles and that the OT-GABA pathway within the ACC may be necessary for the expression of this consolation behavior.In section two,we found that chronic physical stress exposure impaired consolation and induced depression-like symptoms in mandarin voles and OTRs,5-HT1ARs,and D2Rs within the ACC may play important roles in these processes.In section three,we found that DR?AC serotonergic circuits has an important role in consolation.It should be noted that although our studies are conducted both in males and females,sexually dimorphic effects were barely found in all the studies.This may provide additional evidence supporting the notion that comparable outcomes may allow males and females to similarly adapt to environmental challenges,particularly in species that adapt monogamous life strategies and require cooperative breeding.Our results are meaningful as we provide new understanding on empathy-like behaviors and also provide a basis for further examination of the impaired empathy which is frequently observed in many mood-related illnesses,such as depression,autism and schizophrenia.