Activation of the Intrinsic Pain Inhibitory Circuit from the Midcingulate Cg2 to Zona Incerta Alleviates Neuropathic Pain

Abstract

Neuropathic pain is one of the most common and notorious neurological diseases. The changes in cerebral structures after nerve injury and the corresponding contributions to neuropathic pain are not well understood. Here we found that the majority of glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2 ^Glu) were inhibited by painful stimulation in male mice. Optogenetic manipulation revealed that these neurons were tonically involved in the inhibitory modulation of multimodal nociception. We further identified the projections to GABAergic neurons in the zona incerta (ZI ^GABA) mediated the pain inhibitory role. However, MCC Cg2 ^Glu became hypoactive after nerve injury. Although a brief activation of the MCC Cg2 ^Glu to ZI ^GABA circuit was able to relieve the aversiveness associated with spontaneous ongoing pain, consecutive activation of the circuit was required to alleviate neuropathic allodynia. In contrast, glutamatergic neurons in the area 1 of MCC played opposite roles in pain modulation. They became hyperactive after nerve injury and only consecutive inhibition of their activity relieved allodynia. These results demonstrate that MCC Cg2 ^Glu constitute a component of intrinsic pain inhibitory circuitry and their hypoactivity underlies neuropathic pain. We propose that selective and persistent activation of the MCC Cg2 ^Glu to ZI ^GABA circuit may serve as a potential therapeutic strategy for this disease.

SIGNIFICANCE STATEMENT Glutamatergic neurons in the area 2 of midcingulate cortex (MCC Cg2 ^Glu) are tonically involved in the intrinsic pain inhibition via projecting to GABAergic neurons in the zona incerta. They are hypoactive after nerve injury. Selective activation of the circuit compensates the reduction of its analgesic strength and relieves neuropathic pain. Therefore, MCC Cg2 ^Glu and the related analgesic circuit may serve as therapeutic targets for neuropathic pain. In contrast, MCC Cg1 ^Glu have an opposite role in pain modulation and become hyperactive after nerve injury. The present study provides novel evidence for the concept that neuropathic pain is associated with the dysfunction of endogenous pain modulatory system and new perspective on the treatment of neuropathic pain.