GLUTAMATE RECEPTORS ON TYPE-I VESTIBULAR HAIR-CELLS OF GUINEA-PIG

Afferent nerve calyces which surround type I vestibular hair cells (VH CI) have recently been shown to contain synaptic-like vesicles and to be immunoreactive to glutamate antibodies. In order to understand the physiological significance of these observations, the presence of glut amate receptors on type I vestibular sensory cells has been investigat ed. The effect of excitatory amino acids applied by iontophoresis was examined by spectrofluorimetry using fura-2 sensitive dye. Glutamate a pplication caused a rapid and transient increase in intracellular calc ium concentration ([Ca2+]i), in a dose-dependent manner. The ionotropi c glutamate receptors agonists N-methyl-D-aspartic acid (NMDA), alpha- amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and quisqua lic acid (QA) induced an increase of [Ca2+]i. The NMDA receptor antago nist 2-amino-5-phosphonovaleric acid and the AMPA receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione partially blocked the glutamate resp onse, by 39 +/- 10 and 53 +/- 11% respectively. Metabotropic receptors were also revealed by the specific agonist trans-1-amino-cyclopentyl- 1,3-dicarboxylate. The presence of different glutamate receptors on th e VHCI membrane suggests two kinds of feedback. (i) At the base of the sensory cell, autoreceptors may locally control the synaptic transmis sion. (ii) At the apex, postsynaptic receptors may modulate sensory tr ansduction from glutamate release at the upper part of the afferent ne rve calyx. These feedbacks suggest presynaptic modulation of the vesti bular hair cell response which could affect its sensitivity.