The γ-aminobutyric acid system in the developing chick embryo cerebellum

Abstract

The time-sequence of the development of the components of the γ-aminobutyric acid (GABA) system in the chick embryo cerebellum was correlated with development as observed at both light and electron microscopic levels. Study also was made of the subcellular distribution pattern of the GABA system with the age of the embryo. Characteristic synapse-like structures were observed in the chick cerebellum as early as 11 days of incubation, the degree of synaptogenesis increasing greatly thereafter. The enzymes of the GABA system, l-glutamic decar☐ylase (GAD) and GABA-transaminase (GABA-T), began to increase much later in development than did the weight and protein content of the cerebellum. All of the data are consistent with the interpretation that the development of the whole GABA system temporally is better correlated with the development and increase in recognizable synaptic structures than with the accretion of the total mass of the cerebellum. The fractionation data at all stages showed GAD to be more highly concentrated in presynaptic endings than elsewhere and the GABA-T to be particularly high in the free mitochondria, which probably come from postsynaptic neuronal sites and from glial and endothelial cells. The results fit the suggestion that in the chick cerebellum GABA largely is formed at presynaptic sites and metabolized at postsynaptic sites onto which it is liberated, but definitive proof of this idea will only come when it will be possible to visualize GAD, GABA-T, and GABA at an ultrastructural level in specific sites of sections of the cerebellum.