Effects Of Background Stress On Regional Homogeneity Of Spontaneous Neural Activities And Stress-induced Neural Activities

Psychological stress refers to the process in which the body produces a series of physiological,psychological and behavioral responses to cope with internal and external challenges and maintain the homeostasis of the body when individuals are confronted with social psychological challenges or threats.Although the functions of stress response systems can maintain stability to a certain extent,studies have found that the functions of these systems may also be affected by factors such as previous stress experiences,thus showing individual differences in stress regulation.Therefore,researchers put forward the concept of background stress,which refers to the frequent or/and long-term stressors experienced by individuals in the social life environment,to explore the impact of these accumulated stress experiences on the individual's stress response system and physical and mental health.A large number of studies have found that the increase of background stress is related to the onset of various stress-related physiological and psychological diseases,and the effect of excessive stress exposure on the function of stress endocrine system plays a crucial role in the development of these diseases.Based on this,a large number of studies focused on the influence of background stress on the function of the hypothalamic-pituitary-adrenal axis(HPA axis),and revealed the influence process from two perspectives of the basic state(activity)and the acute stress response(reactivity)of the HPA axis.In terms of HPA axis activity,high level of background stress is related to the enhancement of HPA axis activity.In terms of HPA axis reactivity,it was found that after experiencing more background stress,individuals had a lower cortisol level in the context of acute psychological stress.The brain is a core organ in stress processing.The brain defines stressors,which determine what is threatening to an individual.At the same time,the brain orchestrates physiological and psychological mechanisms to respond to stressors.In addition,the brain and the experience of stress have a wide range of two-way effects,with the brain changing both structurally and functionally as a result of the experience.A large number of studies have shown that high background stress leads to changes in the activity and reactivity of the HPA axis,and the activity and reactivity of the HPA axis is highly regulated by the brain regions related to the limbic system.However,it is still unknown whether the effect of background stress on the HPA axis can be reflected in the stress related neural activities.Based on this,this study attempts to explore the influence of background stress on stress related neural activities from two aspects: spontaneous neural activities in resting state and stress-induced neural activities.Study 1 used the resting-state fMRI to explores how the cumulative effects of background stress are reflected in the spontaneous neural activity of the brain.48 healthy college students were selected as participants.Their sum of the total of the impact rating of daily life events that occurred in a 24-hour period was quantified as an index of background stress.Meanwhile,the resting state fMRI method was used to collect the spontaneous neural activity of the brain in the resting state,and regional homogeneity(ReHo)was used as an indicator.The results showed that the higher the background stress was,the stronger the spontaneous neural activity was in the resting state of the right hippocampus(which extends to the thalamus and brainstem).The results suggest that individuals experiencing high background stress exhibit chronic activation of the limbic system.Study 2 used task-state fMRI to explore the influence of background stress on stress-induced neural activities.48 healthy college students were selected as participants.Their sum of the total of the impact rating of daily life events that occurred in a 24-hour period was quantified as an index of background stress.Paiticipants were also took part in the Montreal Imaging Stress Task(MIST)while undergoing functional magnetic resonance imaging(fMRI)scans,and their saliva samples were collected for assessment of levels of the stress hormone cortisol.Results revealed that the individual that had a higher level of background stress showed a lower salivary cortisol stress responses and lower activation of the left hippocampus in response to the MIST.Further,the individual that had a higher level of background stress showed a stronger functional connectivity between the left hippocampus and the ventral medial prefrontal cortex extended to subgenual anterior cingulate cortex.The results suggested that HPA axis acute stress response was down-regulated due to high background stress,and this process was accompanied by changes in hippocampal stress regulation function.In summary,two experiments were conducted in this research to find the effects of background stress on stress related neural activities from two aspects: spontaneous neural activities in resting state and stress-induced neural activities.It was found that higher level of background stress was associated with chronic activation in resting states of limbic system-related brain regions such as the hippocampus,thalamus and brainstem.In the context of acute psychological stress,higher level of background stress predicted a blunted cortisol response,accompanied by a decrease in hippocampal activity and an increase in functional connections between the hippocampus and the ventromedial prefrontal cortex.These results suggest that the effect of background stress on the HPA axis is partly mediated by the brain regions related to the limbic system,especially the hippocampus.These results provide evidence of neural activity in the brain to understand the changes in the function of the stress response system caused by background stress.