Many patients with traumatic
spinal cord injury (SCI) report
pain that persists indefinitely and is resistant to available therapeutic approaches. We recently showed that microglia become activated after experimental SCI and dynamically maintain hyperresponsiveness of spinal cord nociceptive neurons and
pain-related behaviors. Mechanisms of signaling between microglia and neurons that help to maintain abnormal
pain processing are unknown. In this study, adult male Sprague Dawley rats underwent T9
spinal cord contusion injury. Four weeks after injury when lumbar dorsal horn multireceptive neurons became hyperresponsive and when behavioral nociceptive thresholds to mechanical and thermal stimuli were decreased, we tested the hypothesis that
prostaglandin E2 (
PGE2) contributes to signaling between microglia and neurons. Immunohistochemical data showed specific localization of phosphorylated
extracellular signal-regulated kinase 1/2 (pERK1/2), an upstream regulator of
PGE2 release, to microglial cells and a neuronal localization of the
PGE2 receptor E-
prostanoid 2 (EP2).
Enzyme immunoassay analysis showed that
PGE2 release was dependent on microglial activation and ERK1/2 phosphorylation. Pharmacological antagonism of
PGE2 release was achieved with the
mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor
PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-
benzopyran-4-one] and the microglial inhibitor
minocycline.
Cyclooxygenase-2 expression in microglia was similarly reduced by MEK1/2 inhibition.
PD98059 and EP2 receptor blockade with
AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) resulted in a decrease in hyperresponsiveness of dorsal horn neurons and partial restoration of behavioral nociceptive thresholds. Selective targeting of dorsal horn microglia with the Mac-1-SAP
immunotoxin, a chemical conjugate of mouse
monoclonal antibody to CD11b and the
ribosome-inactivating protein saporin, resulted in reduced microglia staining, reduction in
PGE2 levels, and reversed
pain-related behaviors [corrected]. On the basis of these observations, we propose a PGE2-dependent, ERK1/2-regulated microglia-neuron signaling pathway that mediates the microglial component of
pain maintenance after injury to the spinal cord.