The Regulatory Roles Of Lateral Septal Nucleus And Its Downstream Brain Area Lateral Hypothalamus In Innate Fear Behaviors In Mice

Objective: The innate fear response is an instinctive behavior of animals.When animals are threatened by the outside stimuli,they will exhibit fear responses,such as defensive behaviors,autonomic responses,hormonal change and so on.Olfactory and auditory cues can both induce innate fear behaviors.However,the neural mechanisms of these behaviors are largely unknown.The purpose of this study is to reveal the neural circuits that regulate auditory and olfactory cues induced innate fear behaviors,to clarify the regulatory roles of specific type of neurons in innate fear behaviors,to understand the neural mechanism of innate fear behavior,and to provide a theoretical basis for the study of mental disorders related to abnormal fear emotions.Methods: Auditory cues induced innate fear behavior paradigm was established to identify the acoustic stimuli that induced innate fear behaviors.The whole brain was screened with c-fos expression as a probe to find the key brain regions involved in the regulation of auditory cues induced innate fear behaviors.glutamatergic neurons or GABAergic neurons in LS were activated or inhibited by optogenetics.The changes of fear-related behaviors in the auditory experiments were observed.The anterograde tracer virus was injected into LS,and the output brain regions of LS were determined by sectioning after the virus expression,and its downstream the lateral hypothalamus(LH).The LS→LH excitatory projection pathway was activated or inhibited by optogenetics to observe the changes in the auditory and olfactory cues induced innate fear behaviors respectively.Results: When exposing to the auditory cues like cat sound,mice exhibited a series of fear responses including increased immobility and freeze time,decreased time to go to the center area,and increased rearing behavior.After observing the expression of c-fos protein in the whole brain,we found that the number of c-fos positive cells in LS was significantly higher than that in the control group.Optogenetic techniques were used to regulate the activities of glutamatergic neurons in LS.When glutamatergic neurons in LS were activated,the center time increased,the number of rearing behaviors decreased,and the degree of innate fear was diminished.When the glutamatergic neurons of LS were inhibited,the center time decreased,the number of rearing increased and the degree of innate fear was elevated.The anterograde tracing revealed that LH was the main downstream projection brain region of LS.Specific activation of glutamatergic projection from LS to LH resulted in increased center time,decreased rearing behaviors,and decreased fear to cat sound.When the glutamatergic projection from LS to LH was inhibited,the center time decreased,rearing behaviors increased,and the innate fear was strengthened.In the 2MT-mediated innate fear experiment,when the glutamatergic neurons of LH were activated,the average speed of mice increased,the investigation time to 2MT area increased,the open arm time in EPM increased,and the degree of fear to 2MT decreased.When inhibiting the glutamatergic projection from LS to LH,the investigation time to 2MT decreased,open arm time in EPM decreased,and the degree of fear to 2MT increased.Conclusions: LS is a key brain region involved in the regulation of auditory and olfactory cues mediated innate fear behaviors.Activation of glutamatergic neurons in LS inhibited auditory cues mediated innate fear behavior,while inhibition of glutamatergic neurons in LS enhanced auditory cues mediated innate fear behavior.The excitatory projection from LS to its downstream brain region LH is an important projection pathway in innate fear behavior.Activating the excitatory projection from LS to LH can inhibit the innate fear behavior,and inhibiting the excitatory projection from LS to LH can increase the innate fear behavior in mice.