A subpopulation of outer hair cells possessing GABA receptors with tonotopic organization

The olivocochlear innervation has been postulated to regulate active mechanical processes in the mammalian cochlea. Histochemical studies led to the suggestion that a subpopulation of these efferent nerves, which predominantly terminate on outer hair cells (OHCs), are gamma-aminobutyric acid (GABA)-ergic. By means of two monoclonal antibodies, we were able to visualize GABAA-receptor immunoreactivity at the basal pole of isolated sensory cells. Both subunits of the GABAA receptor, the alpha- and beta-subunit, are known to form the transmembranous GABA/benzodiazepine-receptor complex and were present on OHCs. In addition, these inhibitory receptors were more numerous in the apical turns of the cochlea, indicating another criterion for distinguishing the apical from basal turns of the cochlea. These results support the concept that a subpopulation of axosomatic synapses at the basal pole of OHCs liberate the inhibitory neurotransmitter GABA into the synaptic cleft. Binding of the transmitter to these newly observed subsynaptic receptors is possibly followed by a change in OHC motility and a subsequent modulation of the movement of the basilar membrane.