RESUMEN
OBJECTIVE Fear can be learned indi-rectly,but excessive transmission of fear is essential for the development of mental illness.Previous research has indicated that the anterior insular cortex(AIC)may play a crucial role in the process of fear transmission,and abnormal AIC activity is a possible mechanism under-lying various affective disorders.Inhibitory neurons are crucial for maintaining local microcircuit homeostasis.With the support of novel specific neuroregulatory tech-niques,it is now possible to monitor and regulate differ-ent types of neurons in real-time.Therefore,investigating distinct subtypes of inhibitory neurons in the AIC that are involved in fear contagion may provide valuable insights into potential mechanisms underlying mental disorders.METHODS We established a modified observational fear(OF)model.A demonstrator(DM)mouse was placed in an acrylic cup at the center of the apparatus,and two observer(OB)mice were allowed to explore the DM mouse simultaneously from separate areas on either side.During the OF training,electric foot shocks were administered to the DM mouse and freezing,the side and corner time,and social interaction behavior were scored.Next,we characterized the activity patterns of distinct neuronal subtypes in the AIC using GCaMP-based calcium recording.Finally,we employed a Cre-dependent optogenetic approach to selectively modulate excitatory or inhibitory neurons in the AIC,and investigat-ed empathic fear behavior across different Cre transgenic mouse lines(CK2-Cre,PV-Cre,SOM-Cre,VIP-Cre).RESULTS During the training phase,the OB mice exhib-ited significantly higher levels of fear compared to the control group(which did not observe a traumatic event),as evidenced by increased freezing time,decreased interaction time,and increased corner zone time.Calcium fiber recording results suggested that CK2 neurons are involved in risk prediction,while PV and VIP neurons exert inhibitory control on this behavior.Optogenetic silencing of CK2-positive neurons in the AIC through injection of AAV-DIO-NpHR-mCherry in mice demon-strated a significant reduction in empathic fear.Similarly,activation of PV or VIP inhibitory neurons expressing ChR2-eYFP also resulted in a similar effect.However,activation of SOM neurons led to a significant increase in empathic fear.CONCLUSION Our study demonstrated that VIP and PV neuron activity in the AIC attenuates empathetic fear,while SOM and CK2 neuron activity enhances fear expression.These findings shed light on the distinct contributions of various inhibitory interneu-rons in the AIC to fear contagion,indicating their mutual interaction for maintaining local microcircuit homeostasis that regulates empathetic fear behaviors.
RESUMEN
OBJECTIVE Human beings possess the ability to indirectly acquire the emotions of others.This also known as emotional contagion or empathy,enables us to rapidly perceive the emotions of others.However,an excessive empathy may result in heightened fear and sensitivity to pain.Therefore,the establishment of appropri-ate animal models for analyzing neural mechanisms underlying empathy would contribute to pharmacological research on pain sensitivity caused by psychological sus-ceptibility.METHODS We used the observed fear para-digm for assessing contagion of negative emotions in mice.To minimize the impact of emotional contagion dif-ferences caused by the subject change,we established a bilateral observation area and the two mice were trained to observe fear simultaneously.First,two observer(OB)mice were placed on either side of the observational area.Next,a demonstrator(DM)mouse was introduced into the cylindrical shock cage located at the center of the apparatus.The shock cage is made of transparent organic plastic with air holes and has provided ample space for free movement by the DM mouse.During the shock stage,DM mice were subjected to electric stimulation while the behaviors of OB mice on both sides was observed,including freezing,the side and corner time,social interaction behavior.Additionally,c-Fos staining was utilized to confirm distinct local brain activities.RESULTS In the habituation stage,OB mice on both sides showed more social preference for DM mouse,as evidenced by an increase in duration time in the designat-ed interaction zone.During the shock phase,OB mice observed the DM mouse receiving electric shocks and displayed significantly higher levels of fear contagion;however,their fear behavior was not entirely consistent.Some mice exhibited a significant increase in freezing time,while others demonstrated a significant increase in corner and side exploration time.We utilized Z-normal-ization to evaluate changes in emotionality across vari-ous behaviors and identified mice with distinct susceptibil-ities.Fos-positive neurons exhibited higher expression levels in susceptible OB mice,primarily concentrated within brain regions associated with the ascending path-ways of pain perception,such as thalamus,the anterior insular cortex,and anterior cingulate cortex.CONCLU-SION In this study,we have developed an innovative experimental facility that integrates various behavioral tests to evaluate empathic behavior in mice.Our findings highlight the robustness of emotionality measures obtained from individual mice by combining this experi-mental model with the Z-scoring method,facilitating screening for empathic fear or pain-susceptible mice and will helpful for pharmacological evaluation.