CELLULAR AND SYNAPTIC LOCALIZATION OF THE NEURONAL GLUTAMATE TRANSPORTERS EXCITATORY AMINO-ACID TRANSPORTER-3 AND TRANSPORTER-4

Glutamate transport is a primary mechanism for the synaptic inactivati on of glutamate. Excitatory amino acid transporter 4 (EAAT4) is a nove l glutamate transporter with properties of a ligand-gated chloride cha nnel that was recently cloned from human brain. The present study was an, investigation of the protein expression and cellular localization of EAAT4 in human and rat brain, and comparison with another neuronal glutamate transporter, EAAT3 (rabbit excitatory amino acid carrier 1; EAAC1). Regional immunoblot analysis of EAAT4, using a monospecific ol igopeptide (carboxy-terminal) affinity-purified polyclonal antibody, r evealed that the protein was restricted to the central nervous system. The EAAT4 protein was largely expressed in cerebellum, with a much lo wer expression in hippocampus, neocortex, striatum, brain stem and tha lamus. Immunohistochemical studies showed intense EAAT4 immunoreactivi ty in the human and rat cerebellar Purkinje cells with a somatodendrit ic localization. Other brain regions including neocortex, hippocampus, striatum showed faint neuropil staining of EAAT4. Immunogold localiza tion identified EAAT4 protein at plasma membranes of Purkinje cell den drites and spines. In the hippocampus and neocortex, EAAT4 immunoreact ivity was found mainly at small calibre dendrites. Rarely, EAAT4 immun oreactivity was found in astrocytic cell processes of forebrain. In th e cerebellum, EAAT4 localization partly overlapped with the neuronal l ocalization of EAAT3 (EAAC1). Immunoreactivity for EAAT3 was enriched in the somatodendritic compartment of the Purkinje cells like EAAT4, b ut EAAT3 was also found in Purkinje cell axons and in boutons in deep cerebellar nuclei, as well as in granular cells and stellate cells. Ou r results indicate that EAAT4 protein is largely localized to cerebell ar cortex and lower levels of EAAT4 protein are present in forebrain b y immunoblot and immunohistochemistry. Both neuronal glutamate transpo rter EAAT3 (EAAC1) and EAAT4 are located at somatodendritic compartmen t of Purkinje cells, and probably contribute to glutamate re-uptake me chanisms at Purkinje cell synapses. (C) 1997 IBRO. Published by Elsevi er Science Ltd.