KA1-like kainate receptor subunit immunoreactivity in neurons and glia using a novel anti-peptide antibody

Functional kainate receptors can be formed by various combinations of subunits with low (GluR5, GluR6 and GluR7) or high affinity (KA1 and KA2) for kainate. The precise contribution of each subunit to native receptors, as well as their distribution within the central nervous system (CNS) is still unclear. Here, we describe the presence of KA1-like immunoreactivity in both neurons and glial cells of the CNS, using a newly developed antiserum to a specific carboxy terminus epitope of the KA1 subunit. Intense immunoreactivity was observed in the CA3 area of the rat hippocampus. Electron microscopy revealed that immunostaining was present in dendritic structures postsynaptic to commissural-associational fibers, rather than in those contacted by mossy fiber terminals. We also observed immunostaining of CA1 pyramidal cell apical dendrites. In the cerebral cortex, KA1-like immunostaining was observed in many pyramidal neuron somata, mainly in layer V, and along their apical dendrites. A subset of gamma-amino-butyric acidic cells were also intensely stained. In the cerebellum, the antiserum selectively stained Purkinje cell somata and their dendrites as well as Bergmann glial processes. Other types of macroglia were also labeled by the KA1 antiserum. Thus, optic nerve oligodendrocytes both in vitro and in situ and cultured astrocytes were densely stained. Our results indicate that KA1-type subunits are more widely distributed throughout the CNS than previously thought. This newly developed antiserum may help to clarify the properties of kainate receptors containing KA1 or KA1-type subunits within the normal and pathological brain.