Gamma-aminobutyric Acid and Other Blocking Compounds in Crustacea. Iii. Their Relative Concentrations in Separated Motor and Inhibitory Axons.

IN THE TWO PRECEDING PAPERS it was reported that gamma-aminobutyric acid (GABA) was the most active of ten blocking substances extracted from the nervous systems of lobsters and crabs. The concentrations of GABA in several peripheral nerves were measured and found to be highest in a nerve that contained only one motor and one inhibitory axon. It was natural to wonder if GABA was specifically concentrated in the inhibitory axon. In the present study, therefore, we have isolated individual axons and found, within the limits of sensitivity of our enzymic assay, that motor fibers contained no GABA while inhibitory neurons contained surprisingly high concentrations. Other blocking compounds were also extracted from separated motor and inhibitory axons, but in contrast to GABA, these are present in both neuron types, as is the precursor of GABA, glutamic acid. The current studies strongly suggest that GABA has a specific physiological role confined to inhibitory neurons.