A subpopulation of projections from the parabrachial nucleus to the central amygdala mediates itch

The parabrachial nucleus (PBN) plays a crucial role in transmitting itch and affective pain signals to the brain regions such as the central amygdala (CeA). While CGRP⁺ PBN neurons have been implicated in itch processing, the specific projections involved remain unclear. This study aimed to determine the proportion of itch-responsive PBN-CeA projections that express CGRP and to assess their functional role in itch and anxiety behaviors in mice. Using the Targeted Recombination in Active Populations system, we labeled itch-responsive PBN neurons with serotonin. Retrograde tracing revealed that approximately half of serotonin-responsive PBN neurons projecting to the CeA are CGRP⁺. Optogenetic stimulation of these PBN-CeA neurons elicited scratching behavior but did not enhance pruritogen-induced scratching or affect anxiety-like behaviors. In a mouse model of chronic itch, the inhibition of PBN-CeA neurons significantly reduced spontaneous scratching without impacting anxiety-like behaviors. These findings suggest that serotonin-responsive PBN-CeA neurons include both CGRP⁺ and non-CGRP⁺ populations and selectively mediate itch signaling.