Figure 8
Upregulation of NMDAR phosphorylation in dorsal horn after activation of 5-HT 3 Rs and descending 5-HT-dependent spinal glial hyperactivity after inflammation. A. IL-1RI was colocalized with GluN1R in dorsal horn neurons. Enlarged regions (the three right panels) correspond to the rectangle area in the left panel. Scale bar=50 μm (the left panel) and 25 μm (the three right panels). B. p-GluN1R was increased in the dorsal horn at 2 h after i.t. SR57227 (10 pmol, i.t.; ***, p<0.001, n=3), which was significantly attenuated by pretreatment with CX3CR1 Ab (20 μg, i.t., n=3), IL18R Ab (20 μg, i.t., n=3) or IL-1ra (10 μg, i.t., n=3) (####, p<0.001 vs. saline + SR 57227; but *, p<0.05 or **, p<0.01, vs. saline + saline), respectively. C. CD11b and GFAP were increased in the dorsal horn after hindpaw injection of CFA in rats treated with control or Tph-2 shRNA in the RVM (*, <0.05 or **, p<0.01, vs. saline group), however, intra-RVM Tph-2 shRNA reduced CFA-produced increases of spinal CD11b and GFAP expression, in comparison with control Tph-2 shRNA (#, p<0.05) (n=3 for each group). Treatment with Tph-2 shRNA alone in the RVM didn’t affect basal expression of spinal CD11b and GFAP when compared to control shRNA. D. Proposed signal pathways involved in pain hypersensitivity after 5-HT3 receptor activation in the spinal cord. Some excitatory spinal neurons and primary afferent terminals expressing 5-HT3 receptors are activated by the 5-HT3R agonist. Fractalkine is released from the 5-HT3R-containing neurons or sensory afferents, and then acts on its receptor, CX3CR1 that is mainly expressed in microglia. Hyperactivity of microglia consequently evokes astrocytes through an IL18/IL18R signaling and induces astrocytic IL-1β release. The released IL-1β contributes to neuronal hyperexcitability and behavioral hypersensitivity via IL-1β receptors (IL-1RI) and enhancement of pGluN1R on spinal dorsal horn neurons.