Distribution of the glutamate/aspartate transporter GLAST in relation to the afferent synapses of outer hair cells in the guinea pig cochlea.

The glutamate/aspartate transporter GLAST is known to occur in the plasma membrane of supporting cells and their glialike processes around the synaptic region of inner hair cells of the mammalian cochlea. Its function there is presumably to take up glutamate following the release of this putative amino acid neurotransmitter from the inner hair cells. In this study, we have investigated whether GLAST is also associated with the outer hair cells using postembedding immunogold labeling. This is interesting because it is uncertain whether the outer hair cells have a functional synapse at which glutamate may be released. However, earlier ultrastructural studies of the afferent synapses in outer hair cells in several mammalian species have shown features normally associated with synaptic activity. These observations are confirmed and extended here in guinea pig where these afferent synapses have presynaptic bodies, putative synaptic vesicles, and coated pits associated with them. Immunoreactivity for GLAST was found along the plasma membranes of Deiters' cells, especially around the synaptic region of the hair cell, on processes wrapped around approaching nerve fibers. Semiquantitative analysis of the distribution of immunogold labeling of Deiters' cells confirmed that it was densest in the region adjacent to the synapses. There was also more labeling in apical than in basal regions of the cochlea in three of the four animals examined, suggesting an association with the number of afferent synapses, which are more numerous in apical regions. Interestingly, labeling also occurred in other regions of the cell membrane away from the afferent terminals. This suggests that glutamate uptake is also required away from the immediate vicinity of synapses, perhaps as a consequence of glutamate dispersal resulting from the mechanical displacement of the cochlear partition during stimulation. Nonetheless, the particular association of GLAST with the synaptic region of the outer hair cell implies that the latter have active afferent synapses at which glutamate is released.