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.