Primary afferent depolarization. Distribution of the gamma-aminobutyric acid system in frog spinal cord

In the frog spinal cord primary afferent depolarization (PAD) constitutes a powerful inhibitory control mechanism. It has been suggested that gamma-aminobutyric acid (GABA) is the transmitter substance involved in the genesis of PAD. In these studies we show that maximal glutamic acid decarboxylase activity is localized roughly 400-600 micrometers from the dorsal surface, and that correlates well with the intraspinal distribution of field potentials associated with PAD. Measurements of GABA in serial spinal cord sections cut in a dorsal--ventral direction shows that high levels of GABA are seen at 400--600 micrometers, with a peak at 800 micrometers from the dorsal surface. Stimulation at frequencies shown to produce PAD augments the release of endogenous GABA from a superfused frog hemicord preparation.