DISCRETE CELLULAR AND SUBCELLULAR-LOCALIZATION OF GLUTAMINE-SYNTHETASE AND THE GLUTAMATE TRANSPORTER GLAST IN THE RAT VESTIBULAR END-ORGAN

Glial cells play an important role in the removal and metabolism of sy naptically released glutamate in the central nervous system (CNS). It is not clear how glutamate is handled at peripheral glutamate synapses , which are not associated with glia. Glutamate is a likely transmitte r in the synapse between the hair cells and afferent dendrites of the vestibular end organ. Immunocytochemistry was performed to investigate the distribution at this site of the high affinity glutamate transpor ter GLAST and glutamate metabolizing enzyme glutamine synthetase. Conf ocal microscopy revealed that GLAST and glutamine synthetase were to-l ocalized in supporting cells apposed to the immunonegative hair cells. Postembedding immunoelectron microscopy revealed that GLAST was heter ogeneously distributed along the plasma membranes of the supporting ce lls, with higher concentrations basally (at the level of the afferent synapses) than apically. Both immunoreactivities were also present in non-neuronal cells in the vestibular ganglion. The present findings su ggest that glutamate released at the afferent synapse of vestibular ha ir cells may be taken up by adjacent supporting cells and converted in to glutamine. Thus, at this peripheral synapse, the supporting tells m ay carry out functions similar to those of glial cells in the CNS. (C) 1997 IBRO. Published by Elsevier Science Ltd.