Ontogenesis of GABA receptor sites in chick embryo cerebellum.

The time-course of the development of GABA receptor sites in chick embryo cerebellum was correlated with the appearance of synaptic junctions in the cerebellar cortex. At 13 days of incubation, the earliest stage examined, specific [3H]GABA binding was only 19% of that found in cerebella of adult chicks. Between 15 days of incubation and hatching, specific [3H]GABA binding increased 3-fold, already reaching at birth adult values. During this period the number of synaptic junctions also increased. Scatchard analysis of the binding data obtained at birth revealed two binding sites of KdS 40 and 174 nM and a maximal number of binding sites (n) of about 1.6 and 4.0 pmol/mg protein, respectively. The high-affinity binding site for [3H]GABA was inhibited by muscimol, GABA, imidazoleacetic acid and bicuculline (IC50: 0.007, 0.020, 0.1 and 10 microM, respectively). These values correspond to the potencies shown by those compounds in the binding to the synaptic GABA receptor. Treatment of the synaptic membranes with Triton X-100 enhanced [3H]GABA binding depending on the developmental stage studied, suggesting that GABA-modulin that inhibits the binding also appears during that period.