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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.  Here, we used Designer Receptor Exclusively Activated by a Designer Drug (DREADD) system to demonstrate the cell type specific circuit mechanism of ACC in OF. Firstly, inhibitory hM4D (Gi) designer receptor together with clozapine N-oxide (CNO) injection was applied to inactivate ACC neurons in the observer mice. We found that, chemogenetic inhibition of ACC resulted in a decreased freezing response in the observer mice. Next, combining PV-ires-Cre mice and Cre-dependent DREADD system, we selectivel targeted the ACC parvalbumin (PV) interneurons with the excitatory hM3D (Gq) designer receptor. Activation of ACC PV interneurons following CNO injection reduced freezing response in the observer mice, while had no effect on freezing response in the demonstrator mice. Finally, monosynaptic rabies retrograde tracing revealed that ACC PV interneurons receive inputs from the mediodorsal thalamic nucleus (MD) and the ventromedial thalamic nucleus (VM), both known for their roles in OF. Taken together, these findings reveal that ACC activation is important for OF, during which PV interneurons in ACC play an important regulatory role. Abnormal function of ACC PV interneurons might contribute to the patholog of empathy-deficits related diseases, such as autism and schizophrenia.

Figure 1. Mapping pre-synaptic inputs of ACC PV interneurons.

Cellular and neural circuit basis relating to ACC governing OF are poorly understood. By using mono-trans-synaptic tracing, the authors found that ACC PV interneurons receive projections from nucleus anteromedial thalamic nucleu (AM) and mediodorsal thalamic nucleus (MD), both reported to regulate OF learning.
 
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