Localization of the glutamate-aspartate transporter, GLAST, in rat taste buds

A number of putative neurotransmitter substances have been found in vertebrate taste buds. Amongst these glutamate has been localized in fibres innervating the buds and uptake of glutamate has been shown to occur into receptor cells. It is therefore possible that, in common with other sensory systems, glutamate is a neurotransmitter in taste buds. In the inner ear and retina of mammals, the membranes of supporting cells have been shown to contain the glial glutamate transporter GLAST. In the brain, this protein is involved in glutamate re-uptake into glial cells where the glutamate is converted into glutamine for recycling into glutamatergic terminals. In this study, the presence of GLAST has been investigated in taste buds in the rat vallate papilla and its distribution compared with that of glutamine to determine whether there are cells in this system that play a glia-like role in glutamate handling. Immunofluorescent labelling showed that a subset of cells in the taste bud contains GLAST. Immunogold labelling indicated that it occurs in the plasma membranes of supporting cells, especially on the fine cytoplasmic processes of dark cells towards the basal region of the bud. A protein of molecular mass similar to that of cerebellar GLAST was detected in immunoblots of excised papillae. Double labelling and semiquantitative analysis of glutamine and GLAST immunoreactivity showed that the GLAST-positive cells have a higher level of cytoplasmic glutamine than the adjacent cells. It is proposed that these GLAST-positive cells play a glia-like role in the uptake of glutamate following its release at synapses within the taste bud although the precise location of the latter remains uncertain. The GLAST-positive cells may also be involved in its subsequent conversion to glutamine in a glutamate/glutamine cycle similar to that described in the brain.