Optogenetic inhibition of ventrolateral orbitofrontal cortex astrocytes facilitates ventrolateral periaqueductal gray glutamatergic activity to reduce hypersensitivity in infraorbital nerve injury rat model.

Abstract

Background: Trigeminal neuropathic pain (TNP) is a chronic condition characterized by heightened nociceptive responses and neuroinflammatory changes. While astrocytes are recognized as critical players in pain modulation, their specific role in influencing descending trigeminal pain pathways via ventrolateral orbitofrontal cortex (vlOFC) activity modulation remains underexplored. Therefore, we investigated the impact of optogenetic modulation of astrocytes in the vlOFC on pain hypersensitivity in a rat model of chronic constriction injury of the infraorbital nerve (CCI-ION).

Method: Adult female Sprague Dawley rats underwent ION constriction to mimic TNP symptoms, with naive and sham animals serving as controls. AAV8-GFAP-hChR2-mCherry, AAV8-GFAP-eNpHR3.0-mCherry, or AAV8-GFAP-mCherry were delivered to the vlOFC for in vivo optogenetic manipulation. Pain behaviors were assessed using acetone, von Frey, and elevated plus maze tests, while electrophysiological recordings from the ventrolateral periaqueductal gray (vlPAG) and ventral posteromedial (VPM) thalamus were obtained.

Results: Orofacial hyperalgesia, reduced vlPAG activity, and thalamic hyperexcitability were associated with vlOFC astrocytic hyperactivity in the TNP animals. In contrast, optogenetic inhibition of vlOFC astrocytes restored vlOFC glutamatergic signaling, increased vlPAG glutamatergic neuronal activity, and reduced hyperactivity in the VPM thalamus. Behavioral assessments also revealed alleviation of hyperalgesia, allodynia, and anxiety-like behaviors during the stimulation-ON phase, alongside reduced neuroinflammatory markers, including P2 × 3 and Iba-1. However, astrocytic excitation and null virus controls did not alter TNP responses, underscoring the specificity of astrocytic inhibition.

Conclusion: These findings suggest that the astrocytic subpopulation in the vlOFC and its robust influence on vlPAG glutamatergic neurons play a crucial role in restoring descending pain processing pathways, potentially contributing to the development of novel therapeutic approaches for TNP management.