Effects Of Repeated Exposure To Predator Odor During Adolescence On Social Behavior And Neuroendocrine Parameters In Brandt's Voles

Adolescence is a key period for the growth and development.During this period,the central nervous system develops and matures,so it is very sensitive to the changes of external environment which can cause the behavioral and neuroendocrine remodeling.During adolescence,individuals are establishing their independence which is thought to be facilitated by increasing novelty-seeking and peer-directed social behaviors.As an ecologically relevant stressor for rodents,predator odor can affect the behavior,brain and endocrine development of small mammals.However,most studies focus on the defense,foraging and reproduction of prey,researches on social behavior have been little reported.In particular,it is not known whether predator odor exposure during adolescent period has any effect on social behavior in adolescent adult Brandt's voles,and its neuroendocrinological mechanism.In present study,adolescent Brandt's voles,Lasiopodomys brandtii,were used to investigate the effects of repeated exposure to cat urine(predator odor),rabbit urine(non-predator odor)and distilled water for 21 days on social behavior,serum ACTH and CORT levels,hypothalamic CRH and hippocampal GR expression,and the synthesis and release of central OT and AVP via ELISA,Western blot and immunohistochemistry.Social behavior tests included social play,alloparental behavior,same-sex and different-sex social interaction,partner preference,parental care,triple recognition(including social recognition,object recognition and spatial recognition),and social novelty preference tests.The results were as follows:(1)In the social play test,we did not find any effect of cat odor(CO)exposure during adolescence on play behaviors such as pinning behavior and play-fighting behavior,but this CO exposure increased social contact to their siblings.In the alloparental behavior test,we also did not find any effect of this experience on the behavioral response to an unrelated pup.This phenomenon also occurred in the same-sex social interaction when the CO voles were exposed to a same-sex stranger.However,we found that the voles repeatedly exposed to rabbit odor(RO)showed more self-grooming behavior than the control and CO groups.In different-sex social interaction test,repeated RO and CO exposure during adolescence significantly inhibited the investigating behavior of Brandt's voles to the different-sex strangers.(2)In the partner preference test,we found that both male and female Brandt's voles repeatedly exposed to CO during adolescence displayed partner preference 48 hours after pairing,and continued to show this preference even 1 week later.However,male and female Brandt's voles exposed to distilled water(DW)showed no preference for a partner and a stranger,or males showed stranger preference only at 72 hours after pairing and females showed stranger preference at 48 hours after pairing.Interestingly,male Brandt's voles exposed to RO at adolescence also showed a partner preference at 72 hours,and females showed a partner preference at 24 hours and at 1 week after pairing.(3)In parental behavioral test,voles exposed to CO during adolescence showed increased licking and grooming behavior to their own pups,and males showed increased pup retrieval.These results suggest that predation risk during adolescence alters reproductive strategies in adult Brandt's voles.In order to cope with the predation risk,Brandt's voles tend to choose familiar partner and spent more time in parental care.Furthermore,the litter size of male CO voles did not change,but the female ratio in their offspring significantly increased.The weight gain of their offspring was inhibited,which showed an intergenerational transmission effect as the weight gain was slowed down when exposed to CO during adolescence.Females exposed to CO during adolescence produced more,but the sex ratio did not change,suggesting some adaptive strategies for coping with high predation risk.(4)In the triple recognition paradigm,repeated exposure to CO significantly inhibited the investigating behavior to the unrelated individuals in male Brandt's voles to,but not on females.The index of novel object recognition and spatial recognition showed that repeated exposure to CO during adolescence significantly inhibited the recognition to the novel object and location in adult Brandt's voles,suggesting an inhibition effect on the ability of object recognition and spatial recognition.In the social preference test,male Brandt's voles from the CO group showed reduced preference for social stimuli and social novelty stimuli,showing the social dysfunction and impaired social recognition.These results suggest that adolescent exposure to predation risk affects adult object and spatial recognition abilities,and affects sociality and social recognition abilities in a sex-specific manner at adult.(5)Repeated exposure to CO during adolescence had no effect on serum ACTH and CORT levels,and CRH expression in the hypothalamus in adult Brandt voles.However,this experience significantly increased GR protein expression in the hippocampus.These results suggest that the HPA axis tends to stabilize after long-term exposure to predation risk during adolescence,but in the negative feedback process,the recovery of CORT level may lead to the accumulation of GR in the hippocampus,and the change of hippocampal plasticity may lead to the impairment of cognitive ability.(6)Immunohistochemical results showed that OT expression in the paraventricular hypothalamic nucleus(PVN)and supraoptic nucleus(SON)were significantly increased in CO Brandt's voles,and OT in the anterior hypothalamic nucleus(AH)also was increased after adolescent CO exposure.However,OT in the basolateral amygdala(BLA),central amygdala nucleus(CEA)and medial amygdala(MEA)were significantly decreased.In addition,AVP expression in the PVN and SON were significantly decreased in the CO group,and AVP in the AH and lateral hypothalamic(LH)was also decreased,but AVP in the MeA and lateral septal nucleus(LS)were significantly increased.These results suggest that predation risk in adolescence affects OT and AVP expression and release in the brain-specific fashion,and these changes may be involved in the expression of social behavior.In conclusion,predation risk exposure during adolescence enhances adolescent social contact,adult partner preference and parental care,but inhibits the object recognition and spatial cognition,and disrupts the social recognition in a sex-specific manner.These changes may be related to the GR expression in the hippocampus,central OT and AVP expression,and the changes in their secretion levels in different brain regions.Under long-term predation risk,adolescent Brandt's voles may improve their individual and inclusive fitness by changing their behavior and neuroendocrine systems.