Expression of pERK in spinal microglia and neurons and its relation to thermal hyperalgesia in a rat model of CFA-induced arthritis

Abstract

Intraarticular injection of complete Freund’s adjuvant (CFA) into a single joint can induce osteoarthritis in rats. It has been reported that phosphorylation of ERK in spinal neurons in rats with CFA-induced osteoarthritis (OA) occurs after passive movement of the inflamed joint prior to perfusion (Cruz et al., 2005; Seino et al., 2006). Expression of the phospho-ERK (p-ERK) can also occur in activated spinal microglia that has recently been suggested to play an extensive role in the central mechanisms of nociception in a model of neuropathic pain. We here report on the expression of p-ERK in neurons and activated microglia in the spinal cords of rats in a model of OA of the ankle joint. Rats received single injections of 10 μl of CFA into one ankle and vehicle in the other ankle, 7 or 14 days prior to sacrifice, and the expression of p-ERK, NK1, and two different markers of microglia, Iba1 and OX42, was studied in the spinal cord using multiple immunofluorescence and confocal microscopy. In some rats, the ankles were immersed in water at 40°C (normally innocuous) for 20 min before perfusion. Compared to naive rats, the expression of Iba1 and OX42 was increased on both sides of the spinal cord, but was stronger in the dorsal horn ipsilateral to the inflamed ankle. While Iba1 and OX42 colocalized in most microglia, OX42-positive/Iba1-negative cells were observed in lamina I of the dorsal horn at 1 week after CFA injection. Expression of p-ERK was confined to microglia in the spinal cord of rats with OA alone. However, in rats with OA and subsequent thermal stimulation of the inflamed limb, p-ERK was also expressed in neurons in the superficial dorsal horn, including NK1-positive cells known to give rise to ascending projections in the pain pathway. These results suggest that the increased expression of p-ERK in spinal neurons is primarily related to thermal hyperalgesia, and that activation of spinal microglia, especially the OX42-positive cells in lamina I, may play a role in the central mechanisms of inflammatory pain in arthritis.