Peripheral inflammation induces tumor necrosis factor dependent AMPA receptor trafficking and Akt phosphorylation in spinal cord in addition to pain behavior

In the present study, intraplantar carrageenan induced increased mechanical allodynia, phosphorylation of PKB/Akt and GluR1 ser 845 (PKA site) as well as GluR1, but not GluR2 movement into neuronal membranes. This change in membrane GluR1/GluR2 ratio is indicative of Ca2+ permeable AMPA receptor insertion. Pain behavior was reduced and biochemical changes blocked by spinal pretreatment, but not post-treatment, with a tumor necrosis factor (TNF) antagonist, Etanercept (100 μg). Pain behavior was also reduced by spinal inhibition of phosphatidylinositol 3-kinase (PI-3K) (wortmannin; 1 and 5 μg) and LY294002; 50 and 100 μg) and Akt (Akt inhibitor IV; 3 μg). Phosphorylated Akt was found exclusively in neurons in grey matter and in oligodendrocytes in white matter. Interestingly, this increase was seen first in superficial dorsal horn and α-motor neurons (peak 45 min) and later (peak 2 h post-injection) in deep dorsal horn neurons. Akt and GluR1 phosphorylation, AMPA receptor trafficking and mechanical allodynia were all TNF dependent. Whether phosphorylation of Akt and of GluR1 are in series or in parallel or upstream of pain behavior remains to be determined. Certainly, TNF-mediated GluR1 trafficking appears to play a major role in inflammatory pain and TNF-mediated effects such as these could represent a path by which glia contribute to neuronal sensitization (spinal LTP) and pathological pain.