The emerging literature implicates a role for glia/
cytokines in persistent
pain. However, the mechanisms by which these non-neural elements contribute to CNS activity-dependent plasticity and
pain are unclear. Using a trigeminal model of inflammatory
hyperalgesia, here we provide evidence that demonstrates a mechanism by which glia interact with neurons, leading to activity-dependent plasticity and
hyperalgesia. In response to masseter
inflammation, there was an upregulation of glial fibrillary acidic
proteins (GFAPs), a marker of astroglia, and
interleukin-1beta (IL-1beta), a prototype proinflammatory
cytokine, in the region of the trigeminal nucleus specifically related to the processing of deep orofacial input. The activated astroglia exhibited
hypertrophy and an increased level of
connexin 43, an astroglial
gap junction protein. The upregulated IL-1beta was selectively localized to astrocytes but not to microglia and neurons.
Local anesthesia of the masseter nerve prevented the increase in GFAP and IL-1beta after
inflammation, and
substance P, a prototype
neurotransmitter of primary afferents, induced similar increases in GFAP and IL-1beta, which was blocked by a
nitric oxide synthase inhibitor
N(G)-nitro-L-arginine methyl ester. Injection of
IL-1 receptor antagonist and
fluorocitrate, a glial inhibitor, attenuated
hyperalgesia and
NMDA receptor phosphorylation after
inflammation. In vitro application of IL-1beta induced NR1 phosphorylation, which was blocked by an
IL-1 receptor antagonist, a PKC inhibitor (
chelerythrine), an
IP3 receptor inhibitor (2-aminoethoxydiphenylborate), and inhibitors of
phospholipase C [1-[6-((17b-3-methoxyestra-1,3,5(10)-
trien-17-yl)amino)hexyl]-1H-
pyrrole-2,5-dione] and
phospholipase A2 (arachidonyltrifluoromethyl
ketone). These findings provide evidence of astroglial activation by tissue injury, concomitant IL-1beta induction, and the coupling of
NMDA receptor phosphorylation through
IL-1 receptor signaling.