Parabrachial CGRP neurons modulates conditioned active defensive behavior under a naturalistic threat

Recent studies suggest that calcitonin gene-related peptide (CGRP) neurons in the parabrachial nucleus (PBN) represent aversive information and signal a general alarm to the forebrain. If CGRP neurons serve as a true general alarm, activation of CGRP neurons can trigger either freezing or fleeing defensive behavior, depending on the circumstances. However, the majority of previous findings have reported that CGRP neurons modulate only freezing behavior. Thus, the present study examined the role of CGRP neurons in active defensive behavior, using a predator-like robot programmed to chase mice in fear conditioning. Our electrophysiological results showed that CGRP neurons encoded the intensity of various unconditioned stimuli (US) through different firing durations and amplitudes. Optogenetic and behavioral results revealed that activation of CGRP neurons in the presence of the chasing robot intensified fear memory and significantly elevated conditioned fleeing behavior during recall of an aversive memory. Animals with inactivated CGRP neurons exhibited significantly low levels of fleeing behavior even when the robot was set to be more threatening during conditioning. Our findings expand the known role of CGRP neurons in the PBN as a crucial part of the brains alarm system, showing they can regulate not only passive but also active defensive behaviors.