Journal of Signal Transduction -oPAKM sreounn ph e s i v o t h c a p e f r o o r n eb u um imm N 30 min CFA injection p-ERK p-p38 MAPK p-p38 MAPK control p-p38 MAPK 30 min p-ERK control p-ERK 7 h 7 h 24 h Time after CFA injection (a) (b) Figure 1: (a) Time courses of p-ERK1/2 and p-p38 MAPK in the RVM after CFA injection into the hindpaw. (b) Photomicrographs showing p-ERK1/2- and p-p38 MAPK-immunoreactive neurons in the RVM (bregma −11.00 mm) following hindpaw inﬂammation. Scale bar = 100 μm. hyperalgesia and tactile allodynia [18]. The activation of p38 MAPK is involved in tumor necrosis factor-α and interleukin-1 production [43]. Since these cytokines phos- phorylate NMDA receptor in the RVM neurons and cause allodynia [44], glial p38 MAPK may contribute to descending pain facilitation via cytokine production. In CA1 pyramidal 3 neurons, a small amount of Ca2+ ion inﬂux via NMDA receptor activates the Rap-p38 MAPK signaling pathway, which drives the removal of synaptic AMPA receptors [45]. The activation of neuronal p38 MAPK in the RVM may contribute to a decrease in RVM excitability via the removal of synaptic AMPA receptors. AMPA receptor sensitivity in the RVM was reduced at 3 hours after the hindpaw inﬂammation, [46, 47]. Phosphorylation of p38 MAPK has also been shown to activate the transcription of TPH [38]. The activation of p38 MAPK in RVM serotonergic neurons may also contribute to serotonin biosynthesis. Thus, it is possible that p38 MAPK activation induced by inﬂammation increases glial cytokines and neuronal TPH production, leading to the enhancement of hyperalgesia and allodynia. Chronic restraint stress (6 h daily for 3 weeks) induced thermal hyperalgesia and signiﬁcant increase in activation of ERK1/2 in the RVM [48]. This stress-induced ERK1/2 acti- vation in the RVM serotonergic neurons may also contribute to serotonin biosynthesis. The level of TPH in the RVM was signiﬁcantly increased in the rats with chronic restraint stress [48]. Meanwhile, many studies have reported that chronic stresses decrease growth-associated and cytoskeletal proteins and induce neuronal atrophy in the hippocampus [49–51]. Since the sustained activation of ERK1/2 has been shown to be involved in neuronal degeneration [52], the stress-induced activation of ERK1/2 may be associated with neuronal atrophy and dendritic reorganization in the RVM. 2.2. Locus Coeruleus (LC). Acute noxious stimulation induced by formalin injection into the hindpaw activated ERK1/2 in the LC for 1 hour after the injection. How- ever, CFA-evoked chronic inﬂammation did not induce a prolonged activation of ERK1/2 in the LC. After formalin injection, p-ERK1/2 was almost exclusively (more than 90%) located in the tyrosine hydroxylase- (TH-) positive neurons of the LC [17]. TH is the rate-limiting enzyme in biosynthesis. Short-term regulation of TH is accomplished by changes in the phosphorylation of this enzyme. ERK1/2 phosphorylates Ser31 in TH [53, 54]. The phosphorylation of Ser31 poten- tiates TH activity [55]. The activation of ERK1/2 in the LC after formalin injection might contribute to phosphorylation of TH and potentiation of TH activity. Furthermore, it has been shown that p-ERK1/2 activates c-fos, Fra-2, and CREB [56, 57]. The ﬁrst two and the last interact with the AP- 1 and CRE sites of the TH gene promoter, respectively [58]. Therefore, it has been speculated that activation of ERK1/2 in the LC increases TH gene transcription through the activation of several transcription factors [59, 60]. The activation of ERK1/2 in the LC might increase TH gene transcription and biosynthesis. Several studies have demonstrated that acute restraint stress increases p-ERK1/2 in the LC [17, 59–61]. On the other hand, chronic restraint stress (repeat 2–6 times) induced further marked activation of ERK1/2, JNK, and p38 MAPK in the LC [59]. Other study has reported that chronic restraint stress for 3 weeks decreases p-ERK1/2 in the LC [48]. The disparity among those experimental results may