Measuring pain-like behaviours in the monosodium iodoacetate (MIA) model of joint injury in the rat: primary hyperalgesia, referred allodynia or spontaneous measures?

Denise Richardson, Nick Andrews, Ian Machin, Janet Nicholson. Pfizer, Pain Research Unit, Sandwich, Kent, CT13 9NJ, United Kingdom.

Intraarticular injection of the glycolysis inhibitor monosodium iodoacetate (MIA) into the knee of the rat causes damage to the joint which reflects histopathology observed in the joints of osteoarthritis (OA) patients. Further to this, rats injected with MIA display pain-like behaviours, similar to those reported by patients, such as mechanical allodynia and weight bearing changes (Tanimoto et al., 2010; Pitcher et al., 2010).

In the studies described we aim to extend this characterisation to include investigation into the effect of MIA (1 or 2 mg) on evoked pain responses, and sensitivity to oxycodone, at the primary site of injury, the knee joint, as well as measuring referred allodynia in the hindpaw. These studies also aim to determine whether altered behaviour is observed in the absence of stimulation in these rats using a spontaneous burrowing behaviour (Deacon, 2006).

Male Sprague-Dawley rats (CRUK, 250-300 g at time of injection) were habituated and a baseline measure of mechanical hypersensitivity in both knee joints was taken using a digital Randall-Selitto device (WPI, UK) applied to the lateral aspect of the knee, whilst rats were gently restrained. Gradually increasing pressure was applied to the joint until a move to withdraw the limb was observed (compression threshold). Rats were then briefly anaesthetised, knees shaved and cleaned before receiving an intraarticular injection of either 1 mg (n = 8) or 2 mg (n = 16) MIA into the left knee joint. A time course of the development of mechanical hypersensitivity was carried out up to day 56 post-injection. In the same batch of rats mechanical allodynia scores were also obtained using the up-down method (Chaplan et al., 1994) to calculate 50% paw withdrawal threshold (PWT g). All pharmacology studies were carried out in the 2 mg MIA group, with rats randomised based on baseline scores and received either vehicle (saline, 1 ml/kg, S.C.) or oxycodone (0.3 mg/kg, S.C.), and tested 30 mins post-dose at either the knee or hindpaw. To obtain measures of burrowing activity rats were habituated and exposed to plastic tubes containing 2500g of gravel over 2 days. Then baseline burrowing during 30 min was established. Rats were then allowed 30 min to burrow freely and the gravel remaining in the tube was weighed.

Rats demonstrated mechanical allodynia and a decrease in compression threshold in the injured limb. Both the allodynia present in the hindpaw and the hypersensitivity of the knee were reversed by the opioid oxycodone (A & B, ANOVA, P < 0.05), at 35 (allodynia) or 42 days (hypersensitivity) post-injection, time points associated with chronic changes to the knee joint. In addition rats treated with 2 mg MIA showed a significant decrease in the amount of gravel burrowed (C, ANOVA, P < 0.05).

In summary, robust pain-like behaviour has been confirmed in this batch of MIA-treated rats, in response to stimulation at either the knee joint or hindpaw. This effect was reversed by a clinically used analgesic, demonstrating the potential of the mechanical hypersensitivity end-point to be used alongside more traditional measures. The reduction in burrowing observed in rats treated with 2 mg MIA suggests that this end point may also represent an additional measure to assess ongoing pain in this model of joint injury.

Chaplan et al., 1994, J. Neurosci. Methods 53: 55-63; Deacon, 2006, Nature Protocols. (1): 118-121;
Pitcher at al., 2010, Proceedings of the British Pharmacological Society at http://www.pA2online.org/abstracts/Vol7Issue4abst121P.pdf; Tanimoto-Mori et al., 2010, IASP Abstract PM192.