Neural Circuit Mediates And Regulates Innate Fear Related Behaviors

The current global incidence of mental illness accounts for 13%of all diseases,higher than cardiovascular and cancer.Mental illness is defined by the National Institute of Mental Health as a disruption of the neural circuits.From the perspective of clinical diagnosis,mental illness is often considered to be"emotional disorder".Innate fear is one of the most important and indispensable basic emotional expression,which is well-conserved across species.Studies have shown that innate fear responses are abnormally expressed in some mental disorders such as autism and schizophrenia.Therefore,the accurate dissection of the neural circuitry mechanism of innate fear will not only help us to understand the mechanism of evolutionary conserved behavior across species,but also provides insights for the diagnose and treatment of mental illness.Selection of appropriate innate defensive responses?e.g.freezing and flight?to imminent predator-like threats is essential for the individual survival.It is well established that predators evoke innate fear responses through distinct neural circuits,which include the amygdala,periaqueductal grey?PAG?,and hypothalamus.Recent research has revealed that the superior colliculus?SC?,a retina-receipt structure,responds to looming stimuli in the upper visual field.and mediates the innate freezing,but not flight behavior evoked by looming,through a distinct SC-LP-Amygdala subcortical pathway.The ventral tegmental area?VTA?,a brain region implicated in motivation control,value evaluation,and salience detection,receives direct visual input from the SC.Previous studies in anesthetized rodents have demonstrated activation of this pathway following a flash of light,but its behavioral function,especially in term of innate fear,remains unknown.Accumulating evidence suggests the VTA is a crucial component in emotional state processing following an aversive stimulus.However,the role of VTA circuitry in mediating innate fear remains to be investigated.Here,using optogenetics,fiber photometry,viral genetic tracing,and multichannel recordings,we identified GABAergic VTA neurons activated by upper visual field looming stimulation,and demonstrated that inhibition of these neurons suppresses the defensive flight response to looming.These GABAergic VTA neurons received prominent glutamatergic inputs from intermediate/deep layers of the SC?IDSC?,but not from other sources of fast visual afferents,such as primary visual cortex?V1?and lateral geniculate nucleus?LGN?.Moreover,optogenetic activation and inhibition revealed lDSC-VTA pathway is necessary and sufficient to provoke innate-like defensive flight behavior.Viral tracing and in vivo and in vitro electrophysiological recordings showed that VTA^GABA+neurons receive direct excitatory inputs from the SC and that these afferents mediate the flight behavior.Further work showed that glutamatergic SC-VTA projections synapse onto VTA^GABA+neurons that project to the central nucleus of the amygdala?CeA?and that CeA is involved in mediating the flight behavior.Besides deciphering the SC-VTA-CeA circuit in mediating the innate flight behavior,which mostly discussed about the interaction of VTA DA+and GABA+neurons,the role of VTA glutamatergic?VGluT2?neurons were also studied in aversion and rewarding paradigm.Our in vivo fiber photometry data demonstrated that VTA^VGluT2+neuronal population response to aversive and rewarding conditioning are divergent,especially during retrieval of conditioning..So far,our research revealed a specific subcortical pathway mediating innate fear response and expaned the role of a classical midbrain dopaminergic nucleus-VTA to innate fear and aversion processing.As to the modulating of survival circuits,while expression of these responses is considered to be instinctive and unconditional,their magnitude may be affected by environmental and internal factors.The neural circuits underlying this modulation are still largely unknown.Based on the SC-VTA-CeA circuitry basis of innate defensive behavior,we found tthat repeated stress caused an anxiety-like state in mice and accelerated defensive responses to looming.Stress also induced c-fos activation in locus coeruleus?LC?tyrosine hydroxy-lase?TH?+neurons and modified adrenergic receptor expression in SC,suggesting a possible Th::LC-SC projection that may be involved in the accelerated defensive responses.Indeed,both anterograde and retrograde neural tracing confirmed the anatomical Th::LC-SC projection and that the SC-projecting TH+neurons in LC were activated by repeated stress.Optogenetic stimulation of either LC TH+neurons or the Th::LC-SC fibers also caused anxiety-like behav-iors and accelerated defensive responses to looming.Meanwhile,chemogenetic inhibition of LC TH+neurons and the infusion of an adrenergic receptor antagonist in SC abolished the enhanced looming defensive responses after repeated stress,confirming the necessity of this pathway.These findings suggest that the Th::LC-SC pathway plays a key role in the sophisticated adjustments of defensive behaviors induced by changes in physiological states.Upon imminent threating signals,well conserved neural circuit mediates innate fear responses,which is further subjected to sophisticated modulating to ensure appropriate response and response magnitude are executed to ensure individual survival.At the same time,at species level,the ability to detect conspecific's distress is crucial for animal survival.In rodent models,observational fear?OF?occurs when one animal perceives another's fear related negative emotions and emotions,which may model certain behaviors caused by witnessing traumatic experiences in humans.Anterior cingulate cortex?ACC?has been showed to play a crucial role in OF.However,cellular and neural circuit basis relating to ACC governing OF are poorly understood.Our data reveal that activating ACC PV interneurons caused decreased freezing time in OF without affecting freezing time in pavlovian fear conditioning.By using mono-trans-synaptic tracing,we found that ACC PV interneurons receive projections from anteromedial thalamic nucleus?AM?and mediodorsal thalamic nucleus?MD?,both reported to regulate OF learning.Abnormal function of ACC PV interneurons might contribute to the pathology of empathy-deficits related diseases,such as autism and schizophrenia.Taken together,we combine the cell-specific neural tracer,optogenetics,chemogenetics,fiber photometry and in vivo electrophysiological recording to dissect the neural circuits of innate fear response and its modulatory mechanism.A detailed mechanistic understanding of the neural basis of these circuits will provide new insights to the potential mechanisms of survival across species,as well as the maladaptive behavior in fear-and anxiety-related mental disorders.