Actions of aminoacids on the isolated hemisected spinal cord of the toad.

The actions of a series of amino-acids related to gamma-aminobutyric acid and glutamic acid have been determined upon the isolated and sagittally hemisected spinal cord of the toad, Bufo marinus. Slow and fast components of the ventral root reflex responses following dorsal root stimulation were depressed by gamma-aminobutyric acid and by a series of neutral amino-acids having a similar structure. The relative depressant potencies of the members of this series were determined by comparison of the concentrations of each required to cause the same reduction in the electrically integrated slow components of these reflex responses. Glutamic acid and closely related substances facilitated reflex responses in low concentrations and depressed these responses in high concentrations. The actions of these substances resulted in the depolarization of motoneurones which was recorded as a negative potential in the ventral root. The relative potencies of the substances were estimated from the concentrations of each required to produce negative potentials of the same magnitude.Several amino-acids not previously tested proved to have remarkably strong actions on this preparation. 3-Aminopropanesulphonic acid was the most potent depressant tested; homocysteic acid and N-methylaspartic acid were the most powerful excitatory substances. The D forms of optically active depressants or excitants were always stronger than the corresponding L forms where both enantiomorphs were available. Points of similarity and dissimilarity between these results and those of related investigations are discussed.