An acute noxious stimulus (20 % mustard oil) generates adaptive changes in withdrawal reflexes to individual muscles according to the site at which is applied (Harris and Clarke, 2003). In this study we have investigated whether inflammation causes similar patterns of changes in reflexes as those obtained with mustard oil.

Sixteen rabbits were immunised against ovalbumin using the 2-injection, 4-week regime described by Cooke (1988). At the end of the immunisation period, the animals were terminally anaesthetised with sodium pentobarbitone (average dose 42 mg kg^-1, followed by an infusion of 19 mg kg^-1 h^-1). Reflexes were evoked in the medial gastrocnemius (MG) muscle by electrical stimulation of the skin at the heel, and in S.E.M. tendinosus (ST) and tibialis anterior (TA) muscles by electrical stimulation of the skin at the base of the toes. Reflexes were averaged and integrated by computer. After a control period of 1 h, 5 mg ovalbumin in 100 µl of 0.9 % saline was injected subcutaneously either at the heel or at the base of the toes. Injection sites were always distal to the position of the nearby stimulating electrodes.

Six hours after injection of ovalbumin at the toes (n = 8), the circumference of the foot at the injection point increased from a mean of 21 ± 0.6 mm (S.E.M.) to 23 ± 0.5 mm. Over the same time, the toes-TA and toes-ST reflexes increased to medians of 163 % (inter-quartile range, IQR 119-305 %) and 206 % (IQR 179-288) of pre-injection values respectively and the heel-MG reflex decreased to 33 % (IQR 19-63 %) of controls. When ovalbumin was injected at the heel (n = 8), the circumference of the foot at the point of injection increased from 10.0 ± 0.3 to 10.7 ± 0.4 mm over 6 h. The heel-MG and toes-ST reflexes increased to medians of 167 % (IQR 128-209 %) and 250 % (IQR 189-432 %) of pre-injection values respectively and the toes-TA reflex was 104 % (IQR 72-128 %) of controls. With the exception of the TA response to inflammation at the heel, all changes in reflexes were statistically significant (Friedman’s ANOVA, P < 0.03). Alterations in reflexes were not readily apparent until 1 h after the induction of inflammation and persisted thence until the end of the recording period.

The acute inflammation generated by injection of antigen in pre-immunized rabbits (Cooke, 1988) represents a much more persistent noxious stimulus than that provided by mustard oil. Nonetheless, the pattern of changes in reflexes obtained with antigen-induced inflammation was exactly the same as that obtained with mustard oil, albeit expressed over a longer timecourse.

This work was supported by BBSRC