Effects of GABA, THIP, and potassium on excitability of myelinated axons of isolated amphibian spinal roots.

The effects of direct applications of GABA (gamma-aminobutyric acid) and the GABAA agonist, THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) on the excitability of myelinated axons of individual dorsal and ventral spinal roots (lumbar VI and (or) VII) of the isolated bullfrog peripheral nerve are reported. Increases evoked by the GABA agonists (0.01-10 mM) in the amplitude of half-maximal A-fiber compound action potentials indicate the presence of depolarizing responses with apparently greater localization to the dorsal roots, and a sensitivity to GABA twofold greater than that for THIP. The changes evoked by GABA and THIP, as well as potassium have components that closely resemble those of sensory and motor fibers in the more distal, desheathed nerve bundle but are smaller and delayed, differences attributable to a closely attached root sheath that acts as a diffusion barrier. These results confirm the likely existence of GABAA receptors on both dorsal and ventral spinal roots.