A radioautographic study of [ 3H] l-glutamate and [ 3H] l-glutamine uptake in the guinea-pig cochlea

Abstract

Since glutamate has been recently proposed as a possible transmitter of the sensory hair cells in the cochlea, a radioautographic study was performed to look for the in vitro uptake of [ 3H] l-glutamate and [ 3H] l-glutamine. Several experimental conditions were applied. The control experimental procedures consisted in an incubation with one of the labelled tracers (10 min), followed by a post-incubation (3 × 10 min) without tracer. In these experiments, either with [ 3H] l-glutamate or [ 3H] l-glutamine, the following structures were labelled: inner hair cells, glial cells of the osseous spiral lamina and areas of the inner spiral and tunnel spiral bundles. When these experiments were carried out in absence of Na +, these labellings were strongly decreased. When the incubation was not followed by a post-incubation, the results differed depending on the tracer: with [ 3H] l-glutamate, the glial cells and the areas of inner spiral and tunnel spiral bundles were labelled, whereas with [ 3H] l-glutamine, mainly the inner hair cells were labelled. An addition of l-methionine- dl-sulfoximine, a glutamine synthetase inhibitor, into the incubation and post-incubation media, produced a decrease of the labelling of the inner hair cells and of the glial cells. An addition of unlabelled glutamine to the post-incubation media decreased the inner hair cell labelling, while a similar addition of unlabelled glutamate did not. In either case, neither the outer hair cells, the second type of sensory cells, nor the spiral ganglion neurons were labelled. These results suggest that in the cochlea, glutamate and glutamine have their metabolisms linked together, as in some parts of the central nervous system. Correlated to biochemical and electro physiological data these results support the hypothesis that glutamate could be the neurotransmitter of the inner hair cells.