The Release of Amino Acids From the Hemisected Spinal Cord During Stimulation

Isolated frog or toad hemicords were incubated for 40 min with either [ 14 C]glycine, [ 3 H]GABA, l ‐[ 14 C]glutamate. l ‐[ 14 C]aspartate, l ‐[ 14 C]serine, l [ 14 C]threonine or l ‐[ 3 H]leucine, and the release of these compounds from the cord was measured under resting conditions and during electrical stimulation. Stimulation of spinal roots produced no significant change in the efflux of any of the compounds tested. Direct stimulation of the rostral cord however, produced a large increase in the efflux of [ 14 C]glycine, [ 3 H]GABA, l ‐[ 14 C]glutamate and l ‐[ 14 C]aspartate. These increased effluxes were calcium dependent, the effects of stimulation being reduced in a calcium‐free, or magnesium‐supplemented (10 mM) medium. Stimulation failed to produce an increase in the efflux of l ‐[ 14 C]serine, l ‐[ 14 C]threonine, l ‐[ 14 H]leucine, [ 14 C]mannitol or [ 14 C]urea. These results are consistent with the suggestions that glycine, GABA, glutamate and aspartate may be synaptic transmitters in the spinal cord.