Research on the H-reflex of Rabbits with Experimental Spinal Cord Injury; Effects of a GABAB Agonist and a GABAB Receptor Agonist Positive Modulator on the H-reflex
The H-reflex is sometimes used for evaluation of spasticity following spinal cord injury (SCI). Since it is difficult to perform any human experimental research related to changes in the H-reflex after the administration of new medicines or after invasive operations, I created an experimental SCI model using rabbits, and compared the waveforms of the H-reflex before and after the creation of SCI. The effects of intrathecal infusion of muscle relaxants on the H-reflex were also investigated. At first, appearance of the H-reflex before and 1 week after creating SCI was observed, and the amplitude ratio of the maximum H-reflex and the maximum M-wave (H/M ratio) was calculated. Then, the effects of baclofen (GABAB receptor agonist) and CGP7930 (GABAB receptor agonist positive modulator) on the H/M ratio of the SCI rabbits were investigated. The H-reflex was evoked by smaller amplitude stimulation after the creation of SCI than before. The H/M ratio after the creation of SCI was significantly higher than before the creation. The mean H/M ratio after infusion of baclofen (10 nmol) was significantly lower than before infusion. Single infusion of CGP7930 had no effect on the H/M ratio. The mean H/M ratio after combined infusion of baclofen (10 nmol) and CGP7930 (1 nmol) was significantly lower than before infusion, and the reduction was more significant than that after single infusion of baclofen. It was ascertained that the increase in the H/M ratio after the creation of SCI is caused by interception of central descending pathways projecting to Ia inhibitory interneurons and that baclofen activated the presynaptic GABAB receptors on Ia afferent terminals. If single infusion of baclofen is not effective in any patients, additional infusion of CGP7930 may be used for the management of severe spasticity in the future.