The fine structural localization of glutamate decarboxylase in developing axonal processes and presynaptic terminals...

The immunocytochemical localization of L-glutamate decarboxylase (GAD), the enzyme which which forms gamma-aminobutyric acid (GABA), has been studied in developing rodent cerebellum. During the first 3-4 postnatal days, GAD is distributed along non-terminal portions of axonal processes in close association with small vesicles. Some of the axonal processes emanate from profiles which resemble growth cone varicosities, and are presumed to be foliopodia which extend distally from axonal growth regions. At the end of the first postnatal week the GAD-containing axonal processes are seen to form protosynaptic contacts, and GAD is localized around synaptic vesicles and at presynaptic junctional membranes. During the second and third postnatal weeks GAD gradually becomes localized to mature synaptic terminals in association with synaptic vesicle, mitochondrial, and presynaptic junctional membranes. The results suggest that GAD is present in growing neurites in close association with small vesicles prior to the time the neurites make protosynaptic contacts, and that differentiation of these contacts results in a sequestering of GAD into synaptic terminals.