Studies on the role of NGF in arthritis-induced pain transmission using gait and weight bearing as outcome measures

Abstract: Pain is one of the most common reasons for seeking healthcare, with approximately forty percent of those suffering from chronic pain having joint pain. Osteoarthritis is the most common cause of joint pain, but currently there are few treatments available. The search for new, effective pain treatment has been mostly unsuccessful, in spite of the discovery of mechanisms that are involved in the transmission of nociceptive signals from the periphery to the central nervous system where pain is experienced. This work focuses on the evaluation of rodent joint pain models, the behavioural manifestations of the injuries, and the possibility to detect treatment effects in these models. Three models have been evaluated in rats; intra-articular injection of carrageenan, Freund´s complete adjuvant (CFA), and monoiodoacetate (MIA) into one hind leg. In mice, two models have been evaluated; intra-articular injection of CFA, and the surgical model of anterior cruciate ligament transection (ACLT). Carrageenan injection resulted in an acute, robust inflammation, CFA injection caused a more long-lasting strong joint inflammation, and MIA injection resulted in an almost complete loss of joint cartilage after a few weeks. The model more resembling osteoarthritis was the surgical model, ACLT, which gave severe cartilage degeneration, osteophytes, and pathophysiological changes in synovia and ligaments. Gait and weight bearing during locomotion have been tested in all models. The degree of weight bearing reduction in the affected limb was largest in the CFA- and carrageenan-induced model, followed by the MIA model and least effect was seen in the ACLT surgical model. Thus the ACLT model was not possible to use for pharmacological evaluation of drugs, whereas carrageenan- and CFA-induced monoarthritis resulted in a big enough difference between animals with monoarthritis and those without, to test drugs commonly used for pain as well as those under investigation for effects on pain. Conventional pain relieving drugs such as non-stereoidal anti-inflammatory drugs (NSAIDs) and opioids were able to normalize effects on weight bearing caused by both the carrageenan- and the CFA-induced monoarthritis, as were treatments based on inhibiting the NGF-TrkA pathway; an anti-NGF antibody and two pan-Trk compounds. However, an antagonist of the TRPV1 receptor lacked effect. We also investigated mice with a mutation in the R100 NGFß gene (hR100E), in comparison with mice possessing a human wild-type NGF (hWT), similar but not exactly like the one found in a hereditary sensory and autonomic neuropathy type V (HSAN V) disorder. This disorder leads to insensitivity to deep pain in homozygous patients, with sensory and autonomic functions remaining almost normal. In mice with the hR100E mutation, we found similar behavioural outcome; normal peripheral sensory functions but less pain-like behaviour when assessing joint pain with gait and weight bearing. In summary, this work shows that in order to detect translatable effects on joint pain, models need to be robust enough, especially for pharmacological testing, but more important, the methods of testing need to be relevant for the study aim.