Proceedings of The Physiological Society

University of York (2002) J Physiol 539P, S025


Diffuse organization of central sensitization of withdrawal reflexes in the decerebrated, spinalized rabbit

John Harris and Rob W. Clarke

Division of Animal Physiology, University of Nottingham, School of Biosciences, Sutton Bonington Campus, Loughborough LE12 5RD, UK

In decerebrated rabbits, central sensitization of withdrawal reflexes shows a marked degree of somatotopic specificity, with reflexes to ankle (tibialis anterior, TA) and knee (semitendinosus, ST) flexors enhanced only after noxious stimulation of the plantar surface of the ipsilateral hindpaw, and responses in the ankle extensor medial gastrocnemius (MG) potentiated only after stimulation at the heel. Noxious stimuli applied at other sites produced inhibition of reflexes or no effect (Harris et al. 2001). We have now investigated the extent to which this pattern of inhibition and excitation is dependent on descending pathways, by performing a similar study in decerebrated, spinalized rabbits.

Experiments were performed in 22 rabbits decerebrated and spinalized at L1 under nitrous oxide/halothane (2-3 %) anaesthesia, in accordance with UK legislation. Reflexes were evoked in the left ST and TA muscles by electrical stimulation of the plantar skin near the two central toes, while responses were elicited in the MG muscle by electrical stimulation of the skin just distal to the heel. Reflexes were recorded as compound EMG signals and integrated. Up to eight conditioning stimuli, separated by intervals of at least 62 min, were applied to each rabbit depending upon conditions. The stimulus used was 100 µl 20 % mustard oil in paraffin oil, and care was taken to keep this stimulus away from the stimulating electrodes. Three stimulation sites were on the plantar surface of the ipsilateral foot (toe tips, mid-point of sole and heel); one on the dorsal surface at the ankle; and one on the contralateral heel. The other locations stimulated were the MG muscles of both hindlimbs and the ipsilateral ankle joint. Friedman's ANOVA on ranks was used to determine if a mustard oil stimulus had altered a reflex response significantly. Experiments were terminated by I.V. injection of saturated KCl solution.

Application of mustard oil to any of the sites on the ipsilateral hindlimb augmented the toes-TA and toes-ST reflexes (P < 0.03) for periods of ≥ 15 min, while the only significant effects on the heel-MG reflex were inhibition from the toe tips (P < 0.02) and facilitation from the heel (P < 0.02). Stimulation of contralateral sites had no significant effects on any reflex (P > 0.2).

Comparison of the present data with those obtained in non-spinalized animals shows that the flexor reflexes can be sensitized from a wider range of sites when descending pathways are interrupted, and that inhibition of any reflex is much less likely to be seen. These findings suggest that the spatial organization of central sensitization of withdrawal reflexes is largely imposed through the action of descending systems.

This work was supported by BBSRC.

Where applicable, experiments conform with Society ethical requirements