The effects of in vivo inactivation of GABA-transaminase and glutamic acid decarboxylase on levels of GABA in the rat retina

Gabaculine and γ-vinyl GABA (GVG) are specific enzyme-activated irreversible inhibitors of GABA-transaminase (GABA-T). γ-Acetylenic GABA (GAG) irreversibly inhibits both GABA-T and glutamic acid decarboxylase (GAD). Subcutaneous injection of any of those compounds rapidly elevated levels of GABA in the retinae of rats. After injection of 10 mg/kg gabaculine, levels of retinal GABA climbed 5-fold in 4 h, and peaked 16 h after injection at levels approximately 7 times those from water-injected control rats. They remained significantly elevated compared to control levels for at least 6 days after injection. The postgabaculine increase in levels of retinal GABA was linear with time between 0.5 and 4 h after injection. In contrast, retinal GABA levels peaked at less than 3 times control levels within 8 h of injection of 50 mg/kg GAG and returned to baseline levels within 4 days. GAG, upon coadministration with gabaculine, significantly attenuated the postgabaculine rise in levels of GABA in retinae. Neither the rate of rise, nor the maximum levels, of retinal GABA was so great after injection of GAG plus gabaculine, compared to those after injection of gabaculine alone. The degree to which postgabaculine GABA accumulation was inhibited in the retina by 50 mg/kg GAG closely corresponded with the extent to which that dose of GAG inactivated retinal GAD activity. The results of this study extend previous reports from this laboratory that systematically administered gabaculine, GVG and GAG all inactivate target enzymes more potently in retina than in other brain regions⁷. The observation that retinal GABA accumulates linearly with time shortly after subcutaneous injection of gabaculine provides a simple approach to the study of regulation of retinal GABA synthesis in vivo. The correspondence between the inactivation of retinal GAD activity and the attenuation of postgabaculine GABA accumulation in the retina by GAG argues that GABA synthesis in rat retina occurs primarily via the decarboxylation of l-glutamate catalysed by GAD, and not via other metabolic pathways.