Papers of the Week

Electroacupuncture (EA) is widely used in clinical practice to relieve migraine pain. 5-HT receptor (5-HTR) has been reported to play an excitatory role in neuronal systems and regulate hyperalgesic pain and neurogenic inflammation. 5-HTR could influence phosphorylation of protein kinase A (PKA)- or extracellular signal-regulated kinase (ERK)-mediated signaling pathways, which mediate sensitization of nociceptive neurons via interacting with cyclic adenosine monophosphate (cAMP). In this study, we evaluated the role of 5-HTR in the antihyperalgesic effects of EA and the underlying mechanism through regulation of PKA and ERK in trigeminal ganglion (TG) and trigeminal nucleus caudalis (TNC). Hyperalgesia was induced in rats with dural injection of inflammatory soup (IS) to cause meningeal neurogenic inflammatory pain. Electroacupuncture was applied for 15 min every other day before IS injection. Von Frey filaments, tail-flick, hot-plate, and cold-plated tests were used to evaluate the mechanical and thermal hyperalgesia. Neuronal hyperexcitability in TNC was studied by an electrophysiological technique. The 5-HTR antagonist (SB269970) or 5-HTR agonist (AS19) was administered intrathecally before each IS application at 2-day intervals during the 7-day injection protocol. The changes in 5-HTR and 5-HTR-associated signaling pathway were examined by real-time polymerase chain reaction (RT-PCR), Western blot, immunofluorescence, and enzyme-linked immunosorbent assay (ELISA) analyses. When compared with IS group, mechanical and thermal pain thresholds of the IS + EA group were significantly increased. Furthermore, EA prevented the enhancement of both spontaneous activity and evoked responses of second-order trigeminovascular neurons in TNC. Remarkable decreases in 5-HTR mRNA expression and protein levels were detected in the IS + EA group. More importantly, 5-HTR agonist AS19 impaired the antihyperalgesic effects of EA on p-PKA and p-ERK. Injecting 5-HTR antagonist SB-269970 into the intrathecal space of IS rats mimicked the effects of EA antihyperalgesia and inhibited p-PKA and p-ERK. Our findings indicate that 5-HTR mediates the antihyperalgesic effects of EA on IS-induced migraine pain by regulating PKA and ERK in TG and TNC.