Aim of investigation
In order to model chronic pain states in humans, several models with long-lasting inflammatory or neurogenic pain in the rat have been developed. Injection of lambda-carrageenan into an ankle joint is commonly used as an inflammatory pain model of arthritis. It results in limping and a reluctance to place weight on the injected limb, which is usually measured by manual scoring of the guarding behaviour [2]. More objective ways of assessing this behaviour, i.e. by measuring the paw-elevation time on a rotating cylinder [6] or the weight-bearing of each hind limb [5] have been described. The aim of the present study was to investigate whether a new computer assisted method of analysing locomotor behaviour (named "CatWalk"), previously used to investigate neurological outcome after spinal cord injury, could provide more detailed quantitative information.

Material and methods
A saline solution of 300 µg lambda-carrageenan (Cg) was injected under anaesthesia into the left tibio-tarsal joint in male Wistar rats. Control animals were injected with saline. Cg produces a localised inflammation with peak guarding behaviour 4-6 hours after injection. Before and up to one week after injection of Cg, the rats were made to traverse a corridor over a sheet of glass in which light shone via one of the long edges. Light rays are completely reflected internally except when a paw is placed on the glass surface, resulting in a sharp image of areas of contact at a given moment [1]. We used an automated version [4] of the original method [3], and paw prints were videotaped from underneath for parameters related to paw usage to be analysed (Figure 1).

Results and conclusion
Rats injected with carrageenan spend significantly longer time to cross the corridor at peak effect, compared to control animals. They perform fewer steps by the injected paw, but compensate by increasing the number of steps and decreasing the stride length of the front paws. The foot print (both length and width) of the injected paw is significantly decreased, while the opposite is true for the contralateral hind paw. In addition, pressure exerted by the injected paw, measured as intensity of the paw image during walking, is lower than for the contralateral side, and also lower than hind paw pressure of control rats. In conclusion, this computer-assisted method of assessing inflammatory pain provides objective, detailed quantitative information on the changes in locomotor behaviour caused by the experimental arthritis.

Figure 1. A rat walking, from right to left, through the CatWalk corridor. Top panel: Snapshot of the rat in the corridor where false colours represent the different intensities of the signal (red=low, blue=high). Middle panel: Track of footprints from one passage (right paws in green, left paws in red; front paws light, hind paws dark colour). Bottom panel: The time of floor-contact by the different paws. Blue dotted lines indicate seconds, and the white dotted line the snapshot in the top panel

References

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