Roles of mediodorsal thalamus in observational fear-related neural activity in mouse anterior cingulate cortex

Observational fear (OF) is the ability to vicariously experience and learn from another's fearful situation, enabling adaptive responses crucial for survival. It has been shown that the anterior cingulate cortex (ACC) and basolateral amygdala (BLA) are crucial for OF. A subset of neurons in the ACC is activated when observing aversive events in the demonstrator, which elicits OF. However, the neural circuit mechanisms underlying the expression of OF-related activity in the ACC remain unexplored. Previous studies have shown that the mediodorsal thalamus (MD) is crucial for OF, and MD neurons project to the ACC. Therefore, we hypothesize that the projection from MD to ACC may facilitate the OF-related activity in the ACC. By utilizing in vivo calcium imaging combined with the optogenetic terminal inhibition of MD-ACC pathway, we found that a subset of ACC neurons was activated when observing demonstrator's fearful situation in male mice. Furthermore, the optogenetic inhibition of the MD-ACC projection during the demonstrator's aversive moments significantly suppressed the OF-related activity in the ACC. Our data suggests that the MD-ACC projection plays a role in OF-related activity in ACC neurons.