In this study we investigated the role of the activation of the extracellular signal-regulated kinases 1 and 2 (ERK) in chronic inflammatory articular nociception. Monoarthritis was induced in the left ankle of Wistar rats by injection of complete Freund's adjuvant (CFA). Movement of the inflamed joint increased ERK phosphorylation in neurones of the superficial and deep ipislateral dorsal horn laminae of L3-L5 spinal cord segments. Spinal immunoreactivity to phosphoERK was more intense in animals in which the inflammation lasted longer, 7 days or more, than in rats with less time of inflammation. PhosphoERK levels were transient, since 2 h after ankle stimulation spinal immunoreaction had almost disappeared. PhosphoERK immunoreactivity was not induced by movement of ankles from non-arthritic control animals, neither in monoarthritic rats in which the inflamed ankle was not stimulated. Intrathecal administration of PD 98059, an inhibitor of ERK phosphorylation, reduced nociceptive behaviour induced by the ankle bend test in monoarthritic rats. The anti-nociceptive effect of PD 98059 was more prominent and in animals with short lasting (4 days) than in animals with longer (14 days) monoarthritis. Taken together, these findings suggest that ERK phosphorylation in spinal cord neurones plays an important role in chronic inflammatory articular pain and that its inhibition may provide significant anti-nociception.
aInstitute of Histology and Embryology, Faculty of Medicine and IBMC, University of Porto, Alameda Hernâni Monteiro, 4200-319 Porto, Portugal
bDepartment of Urology, Faculty of Medicine, University of Porto, Alameda Hernâni Monteiro, 4200-319 Porto, Portugal
cNeurorestoration Group, Wolfson CARD, King's College, London, UK
*Corresponding author. Address: Institute of Histology and Embryology, Faculty of Medicine of Porto, Alameda Hernâni Monteiro, 4200-319 Porto, Portugal. Tel.: +351 22 5091468; fax: +351 22 5505728.
Received 1 February 2005; received in revised form 22 April 2005; accepted 27 May 2005.