GABA-mediated control of glutamate decarboxylase (GAD) in cell aggregate culture of chick embryo retina.

Glutamate decarboxylase (L-glutamate 1-carboxylase, (EC 4.1.1.15) activity increased 7-fold during the course of differentiation of retina cell aggregate cultures prepared from 8-day-old embryos. The addition of 5 mM GABA in the culture medium almost completely prevented the appearance of GAD activity normally observed during the differentiation of the aggregates. This effect was readily reversible after shifting the aggregates to a GABA-free medium. Differentiated cultures, characterized by high GAD specific activity were also sensitive to GABA treatment, which promoted 50% decrease in the enzyme activity after 7 h incubation of the aggregates in the presence of 0.01 mM GABA. Maximal inhibition was obtained at 0.1 mM GABA. However, GABA up to 10 mM concentration had no effect on GAD activity when added directly to homogenates of retinal tissue. The GABA-mediated inhibition of GAD was antagonized by picrotoxin. The ED50 for picrotoxin to revert GAD inhibition by 0.01 mM GABA was approximately 2 microM. The time course for the decay in GAD activity of cultures exposed to 5 mM GABA, revealed two first-order kinetic components. For an 8-day-old culture the first component had a T1/2 of approximately 60 min and the second decayed with a T1/2 of approximately 315 min. In 13-day-old cultures the first component of GAD decay was identical to the one observed in 8-day-old cultures. The second component, however, was insensitive to GABA inhibition.