Recovery of amino acid neurotransmitters from the spinal cord during posterior epidural stimulation: a preliminary study.

Continuous posterior epidural spinal cord stimulation (SCS) has been an effective method for treating spasticity. The mechanisms of SCS include activation of inhibitory segmental neuronal systems and suprasegmental structures that produce inhibitory descending control. The neurochemical correlates of the mechanism of action have not been clearly defined. Microdialysis of the spinal cord extracellular space in an in vivo preparation during continuous epidural SCS was performed. The recovery of amino acid neurotransmitters, aspartate, glutamate, gamma-aminobutyric acid (GABA), glycine, and taurine from stimulated animals was compared to non-stimulated animals. Evoked potentials from the cortex and spinal cord of the stimulated animals were recorded to insure that there had been adequate epidural stimulation and normal segmental cord function. A significant increase in the concentration of glycine was seen after 90 minutes of continuous stimulation. The levels of the other amino acids were not significantly elevated. These results suggest that amelioration of spasticity with epidural SCS may involve enhanced glycine release, the major inhibitory neurotransmitter of the spinal cord.