Effects of spinal cord stimulation on the flexor reflex and involvement of supraspinal mechanisms: an experimental study in mononeuropathic rats

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✓ The physiological mechanisms responsible for pain relief caused by spinal cord stimulation (SCS) are essentially unknown and recent experimental data are sparse. In the present study the authors explored the possible involvement of supraspinal mechanisms in the effects of SCS applied in rats with experimental mononeuropathy produced by sciatic nerve ligation according to the method of Bennett and Xie or that of Seltzer, et al. Confirming results of a previous study undertaken by the authors, the thresholds of the early component of the flexor reflex (latency 8–12 msec), which is mediated by A fibers, were significantly lower in the nerve-ligated than in the intact leg. In halothane-anesthetized animals the spinal cord was exposed and SCS was applied with parameters similar to those used in clinical SCS. Ten minutes of SCS produced a significant elevation of the lowered threshold of the early flexor component only in the nerve-ligated leg, and this augmentatory effect of SCS persisted for 30 to 40 minutes after cessation of the stimulation. The threshold elevation amounted to between 50% and 80% of the prestimulatory value and it was related to the intensity of SCS. The threshold of the late, C-fiber—mediated component of the flexor reflex was not influenced in either of the legs. After transection of the spinal cord at the T-6 level, there was a moderate threshold increase in both the early and late components in both legs, but the threshold of the early component in the nerve-ligated leg remained lower. Spinal cord stimulation produced an almost identical threshold increase in the early component in the nerve-ligated leg with the same time course as before the transection. There was no effect on the late component of the reflex in either leg. The results indicate that this effect of SCS in mononeuropathic rats does not necessarily involve supraspinal mechanisms; instead SCS is operative at a spinal, segmental level. In view of the similarities between the effects of therapeutic SCS on cutaneous hypersensibility in patients with peripheral neuropathic pain and the effects demonstrated in neuropathic rats, the clinical pain relief achieved with SCS may be produced, at least partially, by intraspinal mechanisms.

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Address reprint requests to: Björn A. Meyerson, M.D., Department of Neurosurgery, Karolinska Hospital, S-17176 Stockholm, Sweden.

© AANS, except where prohibited by US copyright law.

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Figures

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    Graphs displaying the effects of spinal cord stimulation (SCS) on the thresholds of the early component of the flexor reflex recorded in nerve-ligated and the intact legs of rats before (A) and after (B) spinal cord transection at T-6. The SCS was applied for 10 minutes with different intensities expressed in percentages of that producing a motor response. The graphs illustrate the mean of threshold values obtained from 15 rats with experimental mononeuropathy, which was produced according to the method of Bennett and Xie3 or that of Seltzer, et al.26 There were no significant differences in the results obtained from rats treated according to these two methods, and the same applies to the results presented in the subsequent figures.

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    Graph depicting the duration of the post—spinal cord stimulation (SCS) augmentatory effect on the thresholds of the early flexor reflex component. The SCS was applied for 10 minutes with different intensities. Spinal cord transection did not influence the duration of the post-SCS effect.

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    Bar graph showing increases in thresholds of the early flexor reflex component produced by spinal cord stimulation (SCS) of different intensities. The thresholds were assessed 5 minutes after the cessation of a 10-minutes application of SCS; increases are presented as percentages of the pre-SCS values. Transection of the spinal cord did not influence the augmentatory effect of SCS on the thresholds. (The moderate differences between values before and after transection are not statistically significant.)

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