The parabrachial to central amygdala circuit is a key mediator of injury-induced pain sensitization

Abstract

The spino-ponto-amygdaloid pathway is a major ascending circuit relaying nociceptive information from the spinal cord to the brain. Potentiation of excitatory synaptic transmission in the parabrachial nucleus (PbN) to central amygdala (CeA) pathway has been reported in rodent models of persistent pain. At the behavioral level, the PbN→CeA pathway has been proposed to serve as a general alarm system to potential threats that modulates pain-related escape behaviors, affective-motivational (but not somatosensory) responses to painful stimuli, aversion, and threat memory. Increased sensitivity to previously innocuous somatosensory stimulation is a hallmark of chronic pain. Whether the PbN→CeA circuit contributes to heightened sensitivity to somatosensory stimulation following an injury, however, remains unknown. Here, we demonstrate that activation of CeA-projecting PbN neurons contributes to injury-induced behavioral hypersensitivity but not baseline nociception in male and female mice. Using optogenetic assisted circuit mapping, we confirmed a functional excitatory projection from PbN→CeA that is independent of the genetic or firing identity of CeA cells. We then showed that peripheral noxious stimulation increases the expression of the neuronal activity marker c-Fos in CeA-projecting PbN neurons and chemogenetic inactivation of these cells reduces behavioral hypersensitivity in models of neuropathic and inflammatory pain without affecting baseline nociception. Lastly, we show that chemogenetic activation of CeA-projecting PbN neurons is sufficient to induce bilateral hypersensitivity without injury. Together, our results demonstrate that the PbN→CeA pathway is a key modulator pain-related behaviors that can amplify responses to somatosensory stimulation in pathological states without affecting nociception under normal physiological conditions.

Competing Interest Statement

The authors have declared no competing interest.