On the other hand, Moller and Villiger stated that although spinal p38MAPK activation following the production of proinflammatory cytokines (TNF-α and IL-1β) increased and caused the induction of rheumatoid arthritis, hyperalgesia reduced due to the higher opioid receptors expression in chronic phase of arthritis (Möller & Villiger, 2006). Here, TNF is linked to rheumatoid arthritis.