| Following peripheral inflammation, NMDA receptor (NMDAR) activation in spinal cord dorsal horn (SCDH) neurons facilitates the generation of pain in response to low threshold inputs (allodynia) and activates downstream neuronal signaling cascades involved in injury-induced pain. Glia and proinflammatory cytokines are also implicated in the maintenance of inflammatory pain and the modulation of neuronal activity. However, the role of NMDARs and neuronal-glial-cytokine interactions that initiate and maintain inflammatory pain are not well defined. The goals of this thesis are (1) to identify the temporal contribution of NMDARs to the development and persistence of inflammatory pain, (2) to characterize the expression patterns of extracellular signal-regulated kinase (ERK), a critical pain signaling protein, and (3) to identify neuron-astrocyte interactions that underlie the persistence of inflammatory pain. To identify the contribution of NMDARs, we utilized a spatial knockout of NR1 (NR1 KO), the functionally essential subunit of the NMDAR, that is confined to SCDH neurons. In control mice, intraplantar complete Freund's adjuvant (CFA) induces inflammatory allodynia at 24 hours (hrs) after CFA and a concurrent increase in the phosphorylation of neuronal protein kinase C (pPKCgamma) and ERK2 (pERK2), but not glial pERK2 in SCDH. These effects are attenuated in the NR1 KO, suggesting that allodynia and signaling protein expression at 24 hrs after CFA is NMDAR-dependent. In contrast, at 96 hr after CFA, NR1 KO mice are no longer protected from allodynia and SCDH expression of pPKCgamma and pERK2 are increased in these mice. Therefore, at 96 hrs after CFA, allodynia and signaling protein expression become NMDAR-independent. At this time, the expression of the proinflammatory cytokine, IL-1beta, and pERK2 are increased in astrocytes in both control and NR1 KO mice at 96 hrs after CFA and allodynia is completely reversed by intrathecal IL-1 receptor (IL-1R) antagonist, acting on neuronal IL-1beta receptors. These results indicate that the onset of inflammatory pain depends on NMDAR activation while the maintenance depends on IL-1beta activation in astrocytes and subsequent IL-1R signaling in neurons. Therefore, the effective reduction of the initiation and maintenance of inflammatory pain requires targeting the neuron-astrocyte-cytokine interactions identified in this thesis. |
