The differential effects of GABA-transaminase inactivation in the chick retina and brain.

The inactivation of gamma-aminobutyrate (GABA)-transaminase by the highly specific and potent neurotoxin gabaculine leads to different neurochemical consequences in the chick brain as opposed to the chick retina. In the brain, GABA levels continually climb, reaching approximately eightfold increases over control values after 24 h. The elevation in GABA levels leads to a time-dependent and coincident fall in glutamate decarboxylase and cysteine-sulfinate decarboxylase activities, to approximately 50% of control values. On the other hand, in the retina GABA levels only increase to a plateau level two- to threefold that of control after inactivation of GABA-transaminase. Furthermore, although the glutamate decarboxylase activity decreases to about 50% of control values, cysteinesulfinate decarboxylase activity is not affected. These studies show that the processing of GABA in the retina differs from that in the brain, and that cysteinesulfinate and glutamate decarboxylase activity probably reside in different enzyme molecules in the retina, although they may reside in the same enzyme in the brain.