The role of glycine in spinal shock.
Suppression of increased muscle tone by epidural spinal cord stimulation, an invasive method for treating spasticity, increases segmental concentrations of inhibitory amino acid neurotransmitters, particularly glycine. The role of glycine in spasticity and spinal shock was explored further in rabbits with ischemic spinal cord injuries that produced spastic paraparesis or flaccid paraplegia. H-reflexes were monitored following posterior tibial nerve stimulation and plantar surface recording. Spasticity was quantified by using H/M ratios. Spastic animals were intrathecally infused with 100 mmol/l solutions of glycine and related compounds. Glycine agonists suppressed tone whereas glycine antagonists increased tone. In addition, microdialysis sampling from the cord was done in injured, non-infused animals and aspartate, GABA, glutamate, glycine and taurine were measured. Flaccid animals had glycine levels two-three times higher than spastic or control animals. High concentrations of glycine within spinal cord segments is associated with spinal shock. Glycine and related compounds may be useful as treatment for excessive tone.