Immunocytochemical localization of the glutamate transporter GLT-1 in goldfish (Carassius ouratus) retina

Glutamate is the major excitatory neurotransmitter in the retina of vertebr ates. Electrophysiological experiments in goldfish and salamander have show n that neuronal glutamate transporters play an important role in the cleara nce of glutamate From cone synaptic clefts. In this study, the localization of the glutamate transporter GLT-1 has been investigated immunocytochemica lly at the light and electron microscopical levels in the goldfish retina u sing a GLT-1-specific antibody. GLT immunoreactivity (IR) was observed at t he light microscopical level in Muller cells, bipolar cells, the outer plex iform layer (OPL), and the inner plexiform layer (IPL). At the electron mic roscopical level, membrane-bound and cytoplasmic GLT-1R in the OPL was loca ted in finger-like protrusions of the cone terminal located near the invagi nating postsynaptic processes of bipolar and horizontal cells. GLT-1R was n ot observed in the vicinity of synaptic ribbons. This location of GLT-1 all ows modulation of the glutamate concentration in the synaptic cleft, thereb y shaping the dynamics of synaptic transmission between cones and second-or der neurons. In the inner TPL, GLT-1R was observed in the cytoplasm and was membrane bound in mixed rod/cone bipolar cell terminals and cone bipolar c ell terminals. The membrane-bound GLT-1 was generally observed at some dist ance from the synaptic ribbon. The morphology of the bipolar cell terminal together with the localization of GLT-1 suggests that at least these glutam ate transporters are not primarily involved in rapid uptake of glutamate re lease by the bipolar cells. The GLT-1R in the cytoplasm of Muller cells was located throughout the entire goldfish retina from the outer limiting memb rane to the inner limiting membrane. The location of GLT-1 in Muller cells is consistent with the role of Muller cells in converting glutamate to glut amine. (C) 2000 Wiley-Liss, Inc.