Amino acid levels in the guinea pig spinal gray matter after axotomy of primary sensory and descending tracts.

Abstract

This study attempts to determine if the axonal endings of dorsal root sensory fibers and of descending axons to the spinal gray matter in the guinea pig store glutamate and/or aspartate. Bilateral dorsal rhizotomy (spinal segments C5-T1) and partial cordotomy (segment C5, right side) were used to interrupt primary sensory and descending tracts, respectively. At 1 and 2 days after surgery, amino acid levels were determined in regions microdissected from areas of the gray matter of spinal segment C7 that receive heavy projections from the primary sensory and the descending tracts. These regions were identified by visualizing the degeneration of axons and their terminal fields in silver-impregnated light microscopic preparations of the spinal cord. After dorsal rhizotomy, the heaviest degeneration in the spinal gray appeared centrally in laminae II-IV and medially in laminae IV-VI. The levels of aspartate, glutamate, and gamma-aminobutyrate were reduced by 34, 21, and 26% in laminae II-IV and 28, 33, and 23% in medial laminae IV-VI. The levels of glycine, alanine, and threonine-serine-glutamine (unseparated) were increased. After partial cordotomy, the heaviest degeneration in the spinal gray appeared laterally in laminae IV-VI, dorsolaterally in lamina VII, and in lamina IX. The levels of aspartate and glutamate were reduced by 22 and 28% in lateral laminae IV-VI and by 26 and 28% in dorsolateral laminae VII and IX. Glycine levels were reduced by 9% in dorsolateral laminae VII and IX. The levels of gamma-aminobutyrate, alanine, and threonine-serine-glutamine were either unchanged or raised. These findings suggest that the axonal endings of the primary sensory and of one or more of the descending tracts probably contain relatively high levels of glutamate and aspartate, and that they may use these amino acids as transmitters. The partial deafferentation of spinal interneurons and the destruction of some propriospinal fibers probably caused the losses of gamma-aminobutyrate and glycine, and contributed modestly to those of aspartate.