A Study Of The Effects And Mechanisms Of Early Life Environment On Social Behaviors Of Adult Mice

Background: The network formed by neurons and their processes provides the structural basis for information transmission and information processing in the brain.Excessive stress or persistent stress in early life affects neuron development,the morphology of neuronal processes,and the excitation/inhibition balance of local microcircuits,and leads to abnormal neuron regulation,and increases the risk of abnormal social behaviors in adulthood.Oxytocin(OT)-oxytocin receptor(OTR)system is critical for social behaviors and neuron development.It is unclear how early life stress affects the OT-OTR signaling system and neuron development in different brain regions.And it is unclear whether environmental enrichment reverses the abnormal social behaviors and neuronal morphology changes caused by early-life stress.The study established an early life stress model via long-term maternal deprivation(MD)which interfered with the mother-infant relationship.After weaning,the mice were reared in an enriched environment(EE)for intervention.We aimed to explore the effects and mechanism of early life stress and postweaning EE on the social behaviors of adult mice by observing the social behaviors,the neuron morphology in social brain regions,and the OT-OTR system.Methods: BALB/c mice were used to establish a neonatal MD model(4 hours per day during PND3-21)and a post-weaning EE model(PND22-110).The social-related behaviors were evaluated using the behavioral tests including the new object recognition test,three-chamber social approach,social avoidance experiment,marbles burying test,dark avoidance test,etc.The changes in the basolateral amygdala(BLA),prelimbic cortex(PL),and hippocampus(HIP)were observed by DAPI staining for cell nucleus density,Nissl staining for Nissl bodies,Golgi-Cox staining for neuronal dendrites and spines,and immunofluorescence for synaptic structure labeled by synaptophysin(SYP)and postsynaptic compact-95(PSD95).The relationships of the OT-OTR system and social behaviors,as well as changes in the above-mentioned regions,were examined by immunohistochemistry combined with semi-quantitative technology as well as correlation analysis.The effects of the OT-OTR system and the corticotropin-releasing hormone(CRH)-CRH receptor 1(CRHR1)system on hippocampal neuron morphology were observed by immunohistochemistry combined with semi-quantitative technology of OTR,CRH,and CRHR1 in the subregions of the HIP.Results: 1.The results of male BALB/c mice showed that: 1)Neonatal MD caused social deficits,cognitive impairment,and repetitive stereotyped behaviors.Postweaning EE restored these defects except for the decreased pleasure and repetitive stereotyped behaviors caused by MD.2)Neonatal MD increased the branch number and dendritic spine density of projection neurons in the BLA and pyramidal neurons in the CA1 region of the HIP.EE increased the branch number and dendritic spine density of small pyramidal neurons in the PL and pyramidal neurons in the CA3 region of the HIP.EE increased the synaptic connections in the BLA,PL,and hippocampal CA3.3)Neonatal MD increased OTR and CRHR1 levels in the CA1 of HIP and reduced the OT neurons in the magnocellular hypothalamic paraventricular nucleus(m PVH).EE increased the OTR levels in the PL and CA3 region of the HIP and the OT neurons in the parvocellular hypothalamic paraventricular nucleus(v PVH).2.The results of female BALB/c mice showed that neonatal MD caused an impairment of object-object and object-female recognition,an increase of OTR levels and branches of projection neurons in the BLA,and a decrease of OT neurons in the m PVH.EE restored the social recognition impairment induced by neonatal MD.EE caused an increase of OT neurons in the v PVH and the synaptic connections in the BLA and layer II/III of PL.Different from the male,neonatal MD had no obvious effect on layer II/III of the PL in female BALB/c mice.Conclusions: Postweaning EE of male BALB/c mice can partially restore the abnormal behaviors caused by neonatal MD.Neonatal MD affects the neuron development and OT-OTR signaling system in the BLA,PL,and HIP.Postweaning EE increases OT neurons in the v PVH and the OTR levels in the PL and CA3 region of the HIP.OTR and CRHR1 have a synergistic effect on neuronal development.Neonatal MD has little effect on neuronal morphology of the PL in adult BALB/c female mice.