Glutamate transporters in the guinea-pig cochlea: partial mRNA sequences, cellular expression and functional implications.

In the cochlea, glutamate plays a major role in synaptic transmission between the inner hair cell and the primary auditory neurons. Extracellular glutamate concentration must be regulated to prevent excitotoxicity. This regulation is mediated by excitatory amino acid transporters, membrane proteins that remove glutamate from the synaptic cleft. In this study, we investigated the distribution and activity of three excitatory amino acid transporters subtypes in the guinea-pig cochlea: glutamate aspartate transporter, glutamate transporter and excitatory amino acid carrier. A partial messenger ribonucleic acid sequence was determined for each of these transporters, by polymerase chain reaction with degenerate primers, using guinea-pig brain complementary deoxyribonucleic acid as the template. Primers specific for each transporter were then designed and used to screen a dissected organ of Corti complementary deoxyribonucleic acid library. The cellular distribution of each transporter was examined by immunocytochemistry. We investigated the functional consequences of inhibiting glutamate uptake by recording cochlear potentials during intracochlear perfusion with either l-trans-pyrrolidine-2,4-dicarboxylic acid or dihydrokainate. At the end of the electrophysiological session, cochleas were processed for electron microscopy. Only the glutamate aspartate transporter messenger ribonucleic acid was detected in the organ of Corti. Consistently, glutamate aspartate transporter protein was detected in the inner hair cell-supporting cells and in the ganglion of Corti satellite cells. Glutamate transporter and excitatory amino acid carrier were found in the afferent auditory neurons. Only intracochlear perfusions with l-trans-pyrrolidine-2,4-dicarboxylic acid resulted in a dose-dependent decrease in the amplitude of the cochlear compound action potential, leaving cochlear microphonic potential unaffected. After l-trans-pyrrolidine-2,4-dicarboxylic acid perfusion, cochleas displayed a swelling of the afferent endings typical of excitotoxicity. [(-)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamyl-2,3-benzodiazepine], a selective alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist protects the cochlea against l-trans-pyrrolidine-2,4-dicarboxylic acid effect.