Developmental expression of GABA and subunits of the GABAA receptor complex in an inhibitory synaptic circuit in the rat cerebellum.

The temporal relationship between the expression of a transmitter and its corresponding receptor may provide important insights into the development of synaptic circuits in the central nervous system. Here we examined the emergence of the inhibitory transmitter GABA, and subunits of the GABAA/benzodiazepine receptor complex in a well-characterized cerebellar circuit formed by granule cells and the synapses they make with Golgi II neurons in the cerebella of rats ranging in age from birth to 21 days. The presence of GABA was determined immunocytochemically. The presence of the GABAA receptor was demonstrated by localizing the alpha 1 subunit of the receptor using in situ hybridization and immunochemical localization of a 50 kDa benzodiazepine-binding subunit using monoclonal antibodies. Germinal cells of the external granular layer which give rise to granule cells did not express the GABAA receptor at any age. Similarly, receptor labeling could not be detected in granule cells during their postmitotic migratory period. In the internal granular layer, immature postmigratory granule cells are unlabeled. The expression of GABAA receptor subunits was first observed on the fifth postnatal day (P5) and then only in the more mature granule cells which have well elaborated dendrites in contact with presynaptic elements. The number of labeled neurons increased over the subsequent ages examined. Presynaptic elements in association with the dendrites of labeled granule cells had ultrastructural features characteristic of Golgi II cell axon terminals. These elements demonstrate GABA transmitter as early as P3, preceded by 2-3 days receptor labeling in the granular layer. Therefore, granule cells express GABAA receptor subunits only after they have completed migration and their dendrites have become involved in specific synaptic circuits, including innervation by GABAergic afferents.