Immunoelectron microscopic localization of the M6a antigen in rat brain

The monoclonal antibody M6-7, which recognizes both native and denatured im munopurified M6a antigen, was used in the present immunocytochemical study to localize its corresponding antigen in young rat brain. Strong labelling was observed in the cerebellar molecular layer, which corresponds to heavil y stained axon terminals originating from granule cells. The immunodeposit, as observed by electron microscopy, is present only on the cytoplasmic sid e of the presynaptic membrane and on the membrane of synaptic vesicles. In contrast, the Purkinje cells and their processes are unstained. Stained syn apses are also found, although less frequently, in several other cerebral a reas. The pattern of staining at these synapses is similar to that observed in the cerebellar molecular layer. It is hypothesized, on the basis of its restricted distribution in certain neuronal endings and its high homology with myelin proteolipids, that the M6a antigen revealed by the M6-7 antibod y is probably involved in a specific biological function in these structure s.