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

A number of putative neurotransmitter substances have been found in vertebr ate taste buds. Amongst these glutamate has been localized in fibres innerv ating the buds and uptake of glutamate has been shown to occur into recepto r cells. It is therefore possible that, in common with other sensory system s, glutamate is a neurotransmitter in taste buds. In the inner ear and reti na of mammals, the membranes of supporting cells have been shown to contain the glial glutamate transporter GLAST. In the brain, this protein is invol ved in glutamate re-uptake into glial cells where the glutamate is converte d into glutamine for recycling into glutamatergic terminals. In this study, the presence of GLAST has been investigated in taste buds in the rat valla te papilla and its distribution compared with that of glutamine to determin e whether there are cells in this system that play a glia-like role in glut amate handling. immunofluorescent labelling showed that a subset of cells i n the taste bud contains GLAST. Immunogold labelling indicated that it occu rs in the plasma membranes of supporting cells, especially on the fine cyto plasmic processes of dark cells towards the basal region of the bud. A prot ein of molecular mass similar to that of cerebellar GLAST was detected in i mmunoblots of excised papillae. Double labelling and semiquantitative analy sis 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 th e uptake of glutamate following its release at synapses within the taste bu d although the precise location of the latter remains uncertain. The GLAST- positive cells may also be involved in its subsequent conversion to glutami ne in a glutamate/glutamine cycle similar to that described in the brain.