Astrocyte PERK and IRE1 Signaling Contributes to Morphine Tolerance and Hyperalgesia through Upregulation of Lipocalin-2 and NLRP3 Inflammasome in the Rodent Spinal Cord

Background Endoplasmic reticulum (ER) stress plays a crucial role in the pathogenesis of neuroinflammation and chronic pain. Herein, we hypothesized that PRKR-like endoplasmic reticulum kinase (PERK) and Inositol-requiring enzyme type 1 (IRE1) regulate Lipocalin-2 (LCN2) and Nod-like receptor family pyrin domain containing 3 (NLRP3) expression in astrocytes, thereby contributing to morphine tolerance and hyperalgesia. Methods The study was performed in Sprague-Dawley rats and C57/Bl6 mice of both sexes. The expression of LCN2 and NLRP3 were assessed by Western blotting. The tail-flick, von Frey, and Hargreaves tests were used to evaluate nociceptive behaviors. Chromatin immunoprecipitation (ChIP) was conducted to analyze the binding of activating transcription factor 4 (ATF4) to the promoters of LCN2 and TXNIP. Whole-cell patch-clamp recordings were used to evaluate neuronal excitability. Results Pharmacological inhibition of the PERK and IRE1 attenuated the development of morphine tolerance and hyperalgesia in male [tail latency on day 7, 8.0 ± 1.13 s in the morphine + GSK2656157 (10 μg) group vs. 5.8 ± 0.65 s in the morphine group, P = 0.04, n = 6 rats per group] and female [tail latency on day 7, 6.0 ± 0.84 s in the morphine + GSK2656157 (10 μg) group vs. 3.1 ± 1.09 s in the morphine group, P = 0.0005, n = 6 rats per group] rats. Activation of PERK and IRE1 upregulated expression of LCN2 and NLRP3 in vivo and in vitro. ChIP analysis showed that ATF4 directly bound to the promoters of the LCN2 and TXNIP. LCN2 induced neuronal hyperexcitability in the spinal cord and dorsal root ganglia via melanocortin 4 receptor (MC4R). Conclusions Astrocyte ER stress sensors PERK and IRE1 facilitated morphine tolerance and hyperalgesia through upregulation of LCN2 and NLRP3 in the spinal cord.