Phosphate activated glutaminase is concentrated in mitochondria of sensoryhair cells in rat inner ear: a high resolution immunogold study

Glutamate has been implicated in signal transmission between sensory hair c ells and afferent fibers in the inner ear. However, the mechanisms responsi ble for glutamate replenishment in these cells are not known. Here we provi de evidence that phosphate activated glutaminase, which is thought to be th e predominant glutamate-synthesizing enzyme in the brain, is concentrated i n all types of hair cell in the organ of Corti and vestibular epithelium. B y use of two different antibodies (directed to the N and C terminus, respec tively) it was shown that glutaminase is largely restricted to mitochondria and that part of the enzyme pool is associated with the inner membrane of this organelle. Quantitative analysis of immunogold labelled Lowicryl secti ons revealed that the level of glutaminase immunoreactivity in mitochondria of supporting cells is less than 15% of that in hair cell mitochondria. Us ing triple labelling for glutaminase, glutamate, and glutamine, evidence wa s provided of a positive correlation between the glutamate/glutamine ratio and the level of glutaminase immunoreactivity, suggesting that the glutamin ase antibodies identify a functional enzyme pool. Our results strengthen th e idea that glutamate is a hair cell transmitter and indicate that the sens ory epithelia in the inner ear show a metabolic compartmentation analogous to that in the brain.