Parabrachial bombesin receptor subtype 3 neurons facilitate heat pain in persistent inflammation

The parabrachial nucleus (PBN) is a critical hub for pain processing that acts as a switchboard for nociceptive signals, relaying them to forebrain regions that integrate sensory signals with affective aspects of pain. Despite strong evidence for the PBN in pain modulation, the heterogeneity of parabrachial neurons poses a challenge in defining their specific contributions. Here, we identify bombesin receptor subtype 3 (Brs3)-expressing neurons as a distinct glutamatergic PBN subpopulation involved in inflammatory heat pain. Using Fos expression analysis and in vivo calcium imaging, we demonstrate that Brs3 neurons exhibit heightened activity in response to noxious and innocuous stimuli following an inflammatory insult. Chemogenetic activation of Brs3 neurons in uninjured mice induces pain-like behaviors, indicating their sufficiency in driving nociceptive responses. While inhibition of Brs3 neurons does not reverse inflammatory pain-induced mechanical allodynia, it effectively reduces heat hypersensitivity, suggesting a specific role in thermal pain processing. Brs3-expressing neurons encompass multiple previously identified pain-related PBN subpopulations, including those expressing the mu opioid receptor (Oprm1), tachykinin 1 receptor (Tacr1), and neuropeptide Y Y1 receptor (Npy1r), positioning Brs3 as a potential unifying marker of heat hypersensitivity circuits. These findings provide new insight into the organization of pain-processing networks in the PBN and highlight Brs3 neurons as a crucial population for inflammatory heat pain.