Subcellular localization of a putative kainate receptor in Bergmann glial cells using a monoclonal antibody in the chick and fish cerebellar cortex.

A monoclonal antibody, IX-50, that was raised against a kainate binding protein (Mr = 49,000) from chicken cerebellum, was used in light and electron microscopic immunocytochemical studies to localize putative kainate receptors. Pre- and postembedding immunoperoxidase and immunogold methods were used in the cerebellar cortices of one to 26-day old chickens and adult rainbow trout. Immunoreactivity was detected only in association with Golgi epithelial/Bergmann glial cells. Intracellular immunoreactivity was present in the granular and agranular endoplasmic reticulum, Golgi apparatus and in lysosomes, representing the sites of synthesis, glycosylation and degradation of the protein. In the fish the granular endoplasmic reticulum was not immunoreactive. Extracellular immunoreactivity was associated with the plasma membrane. In the fish it was established that the epitope is on the outer surface of the membrane. The protein seems to be uniformly distributed along the membrane including the somata, the radial stem processes and the leafy lamellae surrounding Purkinje cell dendrites. Areas of the glial membrane in contact with other glial cells were also immunopositive. High-resolution light microscopy demonstrated all the Bergmann glial plasma membrane in the cortex, providing a "negative" image of Purkinje cell dendrites. It is apparent that Bergmann glial processes selectively outline the dendrites of the Purkinje cells by surrounding the parallel fibre terminal/Purkinje cell spine synaptic complexes. The parallel fiber terminals were highly immunoreactive for glutamate, as shown by an immunogold procedure. The association of Bergmann glial processes, carrying the Mr = 49,000 kainate binding protein, with the Purkinje cell dendrites and spine synapses could provide a basis for neuronal signalling to the Bergmann glia, possibly by glutamate.