Changes in primary afferent depolarization after administration of γ-acetylenic γ-aminobutyric acid (GAG), a γ-aminobutyric acid (GABA) transaminase inhibitor

γ-Acetylenic γ-aminobutyric acid (GAG), an irreversible inhibitor of GABA transaminase, increased the concentration of GABA in feline spinal cords to 239% of the control value by 225 min after its injection. After administration of GAG to spinally transected cats, the height of the segmentally evoked dorsal root potential (DRP), which is generated at one point via a GABA synapse, was increased to more than twice the control value although the area increased only slightly. However, GAG had no effect on the segmental DRP in the decerebrate cat. In contrast, the DRP evoked in decerebrate cats by electrical stimulation of the brain stem, which is probably mediated by GABA, was decreased by administration of GAG. These effects of GAG were accompanied by the development of spontaneous primary afferent depolarizations which resembled spontaneous DRPs in both spinal and decerebrate cats. The temporal and size correlation between spontaneous DRPs occurring in different spinal roots indicate they are generated by an interneuronal pathway that is released by the action of GAG. The action of GAG on the segmental DRP in the spinal but not decerebrate preparation is also most easily explained by GAG-induced effects on interneuronal pathways. These data suggest GABA transaminase inhibition does not affect the axoaxonic GABA synapse mediating the DRP.