General Anesthesia-activated Neurons in the Central Amygdala Mediate Antinociception: Distinct Roles in Acute versus Chronic Phases of Nerve Injury.

BACKGROUND: General anesthesia, such as isoflurane, induces analgesia (loss of pain) and loss of consciousness through mechanisms that are not fully understood. A distinct population of γ-aminobutyric acid-mediated neurons has been recently identified in the central amygdala (CeA) that can be activated by general anesthesia (CeA GA ) and exert antinociceptive functions. In this study, the authors aimed to explore the underlying cellular mechanisms of CeA GA neurons across different phases of nerve injury-induced nociceptive sensitization in mice. METHODS: This study used 107 mice, including 57 males and 50 females. The authors induced c-fos activation in the mice brains using 1.2% isoflurane and validated Fos expression via RNAscope (Advanced Cell Diagnostics, USA) in situ hybridization. Unlike previous studies using the capturing activated neuronal ensembles method, CeA GA neurons (tdTomato + ) were labeled using the Fos-Targeted Recombination in Active Populations (TRAP2) method. The authors then performed ex vivo electrophysiologic recordings to assess the properties of both Fos-positive/CeA GA neurons and Fos-negative CeA neurons. Using chemogenetic strategy to selectively activate the CeA GA neurons, the authors investigated pain-like behaviors and associated comorbidities in mice after spared nerve injury (SNI). RESULTS: Isoflurane induced robust Fos expression in CeA γ-aminobutyric acid-mediated neurons. Electrophysiologic recordings in brain slices revealed that compared to Fos-negative CeA neurons, CeA GA neurons had higher excitability and exhibited distinct patterns of action potentials. Chemogenetic activation of Fos-TRAPed CeA GA neurons increased nociceptive thresholds in naive mice and in mice 2 weeks after SNI, but demonstrated modest antinociception 8 weeks after SNI. Finally, Fos-negative CeA neurons, but not CeA GA neurons, exhibited increased excitability in the chronic phase of SNI, which was correlated with a downregulation of K + -Cl - cotransporter-2 (KCC2) in the CeA (sham vs . SNI 8 weeks). CONCLUSIONS: These results validate the antinociceptive power of CeA GA neurons using a different approach. Additionally, the authors highlight distinct roles of CeA GA neurons in governing physiologic pain, acute pain, and the transition to chronic pain through KCC2 dysregulation.