Regulatory Roles Of Lateral Septum Glutamatergic Neurons In Predator Odor Mediated Innate Fear In Mice

Objective: Innate fear refers to the fear emotion that animals do not need to learn from their acquired experiences when they face certain dangerous information.With the fear emotion,animals will produce a series of behavioral responses.For example,rodents react by defensive behaviors like fleeing,hiding and stalling,autonomic and hormonal responses when they detect the predator odor.Until now,less is known about the neural mechanism of predator odor-evoked innate fear behaviors.This study is aiming to reveal the key brain regions regulating innate fear behavior mediated by the predator odor,clarify the regulatory roles of different types of neurons in innate fear,understand the neural mechanism of innate fear behavior,and provide theoretical basis for studying mental disorders relating to abnormity in fear.Methods: Innate fear behaviors were induced by 2-methyl-2-thiazoline(2MT)in mice.The expression pattern of early immediate gene c-fos was used as a probe for whole-brain screening to determine the key brain regions involved in the regulation of innate fear behaviors.Using brain stereotaxic injection technique,the recombinant adeno-associated virus carrying light-activated protein Chr2 was injected into the lateral septum nucleus(LS)in Vglut2-cre and Vgat-cre transgenic mice.Optical fibers were implanted into LS after virus injection.2-3weeks later,the glutamatergic neurons or GABAergic neurons in LS were activated by blue light stimulation.Innate fear behaviors of mice were detected.Using chemogenetic technique,after stable expression of Hm3 dq or Hm4 di gene in glutamatergic neurons or GABAergic neurons in LS,CNO was intraperitoneally injected to change the activity of glutamatergic or GABAergic neurons in LS,and the change of innate fear behaviors in mice was observed.Results: when exposed to 2MT odor stimulus,mice exhibited innate fear responses.Compared with control odor,the exploration time of 2MT significantly reduced,the mean speed decreased,the freezing and immobile time increased,the plasma cortisol level increased,and the anxiety behaviors were observed.Through detecting the expression of c-fos protein in the whole brain,we found that after exposing to 2MT,the positive signals of c-fos in LS were significantly elevated than that in the control group.After activating glutamatergic neurons in LS by optogenetic and chemogenetic techniques,the exploration time of 2MT odor increased,the mean speed increased,the freezing and immobile time decreased,and the anxiety level decreased.When the glutamatergic neurons in LS were inhibited,the mice spent less time exploring the area of 2MT,the freezing score decreased,and the anxiety level increased.After activating LS GABAergic neurons by CNO,no significant changes in2MT-mediated innate fear behaviors were observed.After inhibiting LS GABAergic neurons by CNO,no significant changes in mean speed and freezing behavior were observed.However,2MT avoidance time and open arms investigation time increased.Conclusion: LS is a key brain region involved in the regulation of predator odor-mediated innate fear behavior.The activation of glutamatergic neurons in LS inhibits the fear behavior induced by 2MT,while the decreased activity of glutamate neurons enhances the fear behavior induced by 2MT.