Interleukin 33-mediated inhibition of A-type K(+) channels induces sensory neuronal hyperexcitability and nociceptive behaviors in mice

Background: Interleukin-33 (IL-33) has been implicated in nociceptive pain behaviors. However, the underlying molecular and cellular mechanisms remain unclear. Methods: Using electrophysiological recording, immunoblot analysis, immunofluorescence labeling, reverse transcription-PCR, siRNA-mediated knockdown approach and behavior tests, we determined the role of IL-33 in regulating sensory neuronal excitability and pain sensitivity mediated by A-type K(+) channels. Results: IL-33 decreased A-type transient outward K(+) currents (I(A)) in small-sized DRG neurons in a concentration-dependent manner, whereas the delayed rectifier currents (I(DR)) remained unaffected. This IL-33-induced I(A) decrease was dependent on suppression of the tumorigenicity 2 (ST2) receptor and was associated with a hyperpolarizing shift in the steady-state inactivation. Antagonism of Syk abrogated the IL-33-induced I(A) response, while inhibition of JAK2 and PKA elicited no such effect. Exposure of DRG cells to IL-33 increased the activity of Akt, but surprisingly, neither Akt nor PI3K influenced the IL-33-induced I(A) response. IL-33 increased the level of phosphorylated p38 mitogen-activated protein kinase (MAPK). Chemical inhibition of p38 and genetic siRNA knockdown of p38 beta (p38β), but not p38α, abrogated the I(A) response induced by IL-33. Moreover, IL-33 increased neuronal excitability of DRG neurons and facilitated peripheral pain sensitivity in mice; both of these effects were occluded by I(A) blockade. Conclusions: Our present study reveals a novel mechanism by which IL-33/ST2 suppresses I(A) via a Syk-dependent p38β signaling pathway. This mechanism thereby increases DRG neuronal excitability and pain sensitivity in mice. Targeting IL-33/ST2-mediated p38β signaling may represent a therapeutic approach to ameliorate pain behaviors.