Glutamic acid decarboxylase in different areas of the developing chick central nervous system.

The temporal course of the development of GAD activity in GABAergic neurons was studied in the chick retina, optic lobe and cerebellum. The developmental pattern of GAD activity was similar in the three areas studied, showing typical sigmoideal curves, which reached a maximal value at the 3rd post-hatching day. Kinetic studies during development revealed that Km remained unchanged while Vmax increased 3-fold in the retina (48.99 +/- 0.84 nmol/hr/mg protein), almost 4-fold in the optic lobe (162.77 +/- 4.32 nmol/hr/mg protein) and 3.5 fold in the cerebellum (69.30 +/- 1.26 nmol/hr/mg protein). The developmental pattern of GAD activity in homogenates of the three areas studied from dark-reared and light-reared chicks with respect to normal light-dark cycle animals showed no significant differences. These results indicate that the increase in GAD activity during development are not due to a change in the affinity for its substrate but rather to changes in the concentration of the enzyme. The developmental pattern of GAD activity in the chick visual system was not affected by environmental conditions suggesting that the developmental profile is light-independent.