GABAA/benzodiazepine receptor-like immunoreactivity in rat and monkey cerebellum

The cellular and subcellular localization of GABAA/benzodiazepine receptor-like immunoreactivity in the rat and monkey cerebellum has been studied with a monoclonal antibody (E9) directed against the alpha-subunit of purified GABAA/benzodiazepine receptors. At both the light and electron microscopic level E9 immunoreactivity is located in all 3 layers of the cerebellar cortex and within the deep cerebellar nuclei. The reaction product accumulates within the cytoplasm of neurons and their dendrites but axons are not immunoreactive. Glial cells in the white matter and the cortical layers are also unlabeled, although in some instances Bergmann glia do contain reaction product. The overall distribution and cellular and subcellular localization of E9 immunoreactivity is identical for both monkey and rat cerebellum. On the basis of cell size, morphology, and location it is evident that E9 immunoreactivity occurs in examples of all 5 neuronal types in the cerebellar cortex: Purkinje cells, Golgi type II cells, granule cells, and stellate and basket cells. However, the distribution of the reaction product within the cells is more selective. For example, electron microscopy demonstrates that axonal processes and terminals are not E9 immunoreactive with the single exception of the mossy fiber terminals in the granular layer. Also, examples of unlabeled axon terminals resembling those derived from Golgi type II cells, basket cells, and stellate cells form synapses with immunoreactive dendrites and cell bodies in the cortical layers. Finally, in the deep cerebellar nuclei unreactive axon terminals make symmetric synapses with immunostained neurons and dendrites. These results show that E9 monoclonal antibodies label neurons and portions of their processes which are postsynaptic in GABA-mediated inhibitory circuits, and demonstrates that this antiserum can be used as a morphological marker for cells which make GABAA/benzodiazepine receptors.