31. Therapeutic morphine prolongs neuropathic pain in rats: A role for TLR4 and inflammasome signaling in the lumbar spinal cord

Abstract

Microgliosis occurs after morphine and peripheral nerve injury alone, but the behavioral and molecular impact in tandem is unknown. We hypothesized that sciatic chronic constriction injury (CCI)-allodynia would be enhanced by subsequent repeated morphine in rats, involving TLR4, P2X7 receptor (P2X7R) and caspase-1, facilitating release of interleukin (IL)-1β. Beginning 10 days after CCI, morphine (5 mg/kg b.i.d.) or saline was administered for 5 days. Compared to vehicle, morphine significantly prolonged the duration of CCI-induced allodynia ( n = 6/group; p < 0.05). Morphine also significantly elevated TLR4 mRNA, P2X7R, NFkappaB, NLRP3 and caspase-1 protein levels ( p < 0.05) in the ipsilateral lumbar dorsal quadrant (iLDQ), 5 weeks after dosing conclusion. Supporting a causal role for NLRP3 inflammasome activation in morphine-prolonged CCI-allodynia, continuous intrathecal infusion of inhibitors of TLR4 ([+]-naloxone; 60 micrograms/h), P2X7R (Brilliant Blue G; 30 ng/h), or caspase-1 (ac-YVAD-cmk; 1 μg/h) prevented prolonged allodynia when administered concomitantly with morphine, and abolished established morphine-prolonged CCI-allodynia when administered 5 weeks after morphine dosing ( n = 6/group; p < 0.05). A single intrathecal IL-1 receptor antagonist dose (100 micrograms) also attenuated morphine-prolonged CCI-allodynia ( n = 6/group; p < 0.05). In keeping with known pro-nociceptive roles for IL-1β, phosphorylation of the NR1 NMDA subunit was elevated, while GRK2 levels and GLT-1 mRNA were decreased in iLDQ 5 weeks after dosing conclusion ( p < 0.05). These data suggest that morphine and the products of nerve injury interact, resulting in prolonged neuropathic pain via sustained inflammasome signaling.