Glutamate is the main neurotransmitter at the synapses between sensory cells and primary afferents in the peripheral vestibular system. Evidence has recently been obtained demonstrating that the atypical amino acid d-serine is the main endogenous co-agonist of the N-methyl-d-aspartate receptors in the CNS. We studied the distribution of d-serine and its synthesizing and degrading enzymes, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelium using immunocytochemistry. d-Serine, serine racemase and d-amino acid oxidase were localized in the transitional cells, which are parasensory cells located between the sensory epithelium and the dark cells. The dark cells expressed only serine racemase. d-Serine was also detected in the supporting cells of the sensory epithelium. These cells, which are in close contact with glutamatergic synapses, express GLAST, a glial specific transporter for glutamate. They may have similar functions to glial cells in the CNS and thus expression of d-serine suggests a neuromodulator role for d-serine at the glutamatergic synapses in the peripheral vestibular system. Our data also indicate that the metabolism of d-serine is not restricted to glial cells suggesting that the amino acid may play an additional role in the peripheral nervous system.
