Tissue plasminogen activator contributes to morphine tolerance and induces mechanical allodynia via astrocytic IL-1β and ERK signaling in the spinal cord of mice

Abstract

Accumulating evidence indicates that activation of spinal cord astrocytes contributes importantly to nerve injury and inflammation-induced persistent pain and chronic opioid-induced antinociceptive tolerance. Phosphorylation of extracellular signal-regulated kinase (pERK) and induction of interleukin-1 beta (IL-1β) in spinal astrocytes have been implicated in astrocytes-mediated pain. Tissue plasminogen activator (tPA) is a serine protease that has been extensively used to treat stroke. We examined the potential involvement of tPA in chronic opioid-induced antinociceptive tolerance and activation of spinal astrocytes using tPA knockout (tPA(-/-)) mice and astrocyte cultures. tPA(-/-) mice exhibited unaltered nociceptive pain and morphine-induced acute analgesia. However, the antinociceptive tolerance, induced by chronic morphine (10mg/kg/day, s.c.), is abrogated in tPA(-/-) mice. Chronic morphine induces tPA expression in glial fibrillary acidic protein (GFAP)-expressing spinal cord astrocytes. Chronic morphine also increases IL-1β expression in GFAP-expressing astrocytes, which is abolished in tPA-deficient mice. In cultured astrocytes, morphine treatment increases tPA, IL-1β, and pERK expression, and the increased IL-1β and pERK expression is abolished in tPA-deficient astrocytes. tPA is also sufficient to induce IL-1β and pERK expression in astrocyte cultures. Intrathecal injection of tPA results in up-regulation of GFAP and pERK in spinal astrocytes but not up-regulation of ionized calcium binding adapter molecule 1 in spinal microglia. Finally, intrathecal tPA elicits persistent mechanical allodynia, which is inhibited by the astroglial toxin alpha-amino adipate and the MEK (ERK kinase) inhibitor U0126. Collectively, these data suggest an important role of tPA in regulating astrocytic signaling, pain hypersensitivity, and morphine tolerance.

Keywords: DRGs; ERK; ERK kinase; FBS; GAPDH; GFAP; IBA-1; IL-1β; L-2-AA; LTP; MEK; MMP-2; PWL; WT; acute opioid analgesia; chronic morphine exposure; dorsal root ganglia; extracellular signal-regulated kinase; extracellular signal-regulated kinase (ERK); fetal bovine serum; glial fibrillary acidic protein; glyceraldehyde3-phosphate dehydrogenase; interleukin-1 beta; interleukin-1 beta (IL-1β); ionized calcium binding adapter molecule 1; l-2-Aminoadipic acid; long-term potentiation; matrix metalloprotease-2; pERK; paw withdrawal latency; phosphoERK; protease; tPA; tPA knockout mice; tissue plasminogen activator; wild-type.