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Characterization of glutamate receptor interacting protein‐immunopositive neurons in cerebellum and cerebral cortex of the albino rat

Glutamate receptor interacting protein (GRIP) binds to the C‐terminus of the glutamate receptor 2 (GluR2) subunit of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) receptors in vitro and may play an important role in the synaptic organization of these receptors. To determine the distribution of GRIP in vivo, GRIP was localized immunocytochemically in cerebellum and cerebral cortex of adult Sprague‐Dawley rats. In the cerebellar cortex, GRIP staining was prominent in perikarya and proximal dendrites of Purkinje cells, whereas Golgi cells were stained more weakly. Double labeling revealed that GRIP and GluR2 were colocalized in Purkinje cells but not in Golgi cells. In the cerebral cortex, GRIP‐stained dendrites and somata of nonpyramidal neurons were scattered throughout cortical layers, whereas pyramidal cells were only weakly immunopositive. GRIP was especially prominent in a subset of GluR2‐containing cells that also expressed a high level of GluR1. The large majority of strongly GRIP‐positive cells in neocortex were immunopositive for γ‐aminobutyric acid (GABA), including the overwhelming majority of calbindin‐positive cells in superficial cortical layers, most of the parvalbumin‐positive cells, and half of the calretinin‐positive interneurons. Staining in the neuropil became more punctate after antigen was unmasked with proteinase K. Electron microscopic localization in the cerebral cortex by postembedding immunogold showed that somatic GRIP was associated with rough endoplasmic reticulum and Golgi apparatus. GRIP was seen over the postsynaptic density of axospinous and axodendritic asymmetric synapses and at high levels in dendrites of GABA‐positive neurons. The present data support a role for GRIP in anchoring AMPA receptors and suggest that GRIP trafficking may be especially active in GABAergic neurons. J. Comp. Neurol. 411:601–612, 1999. © 1999 Wiley‐Liss, Inc.