gamma-Acetylenic GABA produces axon-sparing neurodegeneration after focal injection into the rat hippocampus.

In exploring the recently discovered phenomenon of indirect excitotoxicity, we noted that intrahippocampal injections of the nonspecific aminotransferase inhibitor gamma-acetylenic GABA (GAG; 60-240 nmol) caused excitotoxic lesions in rats. When assessed 3 days following the injection, GAG was shown to be approximately equally toxic to CA3/hilar neurons and CA1 pyramids, while CA2 neurons and granule cells were clearly less vulnerable. Choline acetyltransferase activity, a marker of extrinsic afferents, remained unchanged in the GAG-lesioned hippocampus, indicating the axon-sparing nature of the insult. In contrast, a lesion caused by 240 nmol of GAG resulted in a significant reduction in 3H-MK-801 binding, which was used as a marker for NMDA receptor-bearing hippocampal neurons. GAG-induced lesions were blocked by the NMDA receptor antagonists MK-801 and AP7 but were not influenced by the nature of the anesthetic used during surgery. Iontophoretic application of GAG did not excite CA1/CA3 cells in the rat hippocampus. In vitro, GAG proved to be a relatively potent inhibitor (IC50: 43 microM) of kynurenine aminotransferase, the biosynthetic enzyme of the endogenous neuroprotectant kynurenic acid. GAG also inhibited the neosynthesis of kynurenic acid in hippocampal slices (IC50: 790 microM). Thus, GAG shares several characteristics of the recently described indirect excitotoxin aminooxyacetic acid (AOAA; Exp. Neurol. 113: 378, 1991). GAG and AOAA appear to belong to a new family of excitotoxic agents which produce lesions indirectly by metabolic derangement and/or inhibition of kynurenate production.