Of known neurotransmitters, glutamate is the most likely to be released from chick cochlear hair cells

1. Experiments were performed to identify the excitatory amino acid (EAA) released from cochlear hair cells isolated from chick. An isolated hair cell was transported and closely apposed to a cultured granule cell or a Purkinje cell, and current responses were studied in these cell pairs when the apposed hair cell was depolarized. 2. The apposed hair cell was voltage clamped at -65 mV by a nystatin perforated-patch recording technique and the membrane potential was step changed to -10 or 0 mV to induce a release of neurotransmitter. In the granule cell voltage clamped at +55 mV, the hair cell membrane depolarization induced outward currents through the activation of the N-methyl-D-aspartate subtype of glutamate receptors. In the Purkinje cell voltage clamped at -60 mV, however, the hair cell depolarization could not induce any current responses. 3. Concentration-response relations for three endogenous EAAs (L-glutamate, L-aspartate, and L-homocysteate) were studied in the granule cell and in the Purkinje cell. The granule cell was most sensitive to glutamate, whereas these three EAAs did not show great sensitivity differences in the Purkinje cell and the half-maximum current was obtained at EAA concentrations of 17-60 mu M. Concentrations of glutamate, aspartate, and homocysteate inducing the half-maximum current in the granule cell were 1.1, 11, and 10 mu M, respectively. 4. The depolarization of the hair cell induced outward current (70 +/- 33 pA, mean +/- SD, n = 8) in the apposed granule cell voltage clamped at +55 mV. This current was equivalent to the current size induced by 0.33 mu M glutamate, 5 mu M aspartate, or 2 mu M homocysteate. However, the critical concentration (0.33 mu M) of glutamate failed to induce any current in the Purkinje cell voltage clamped at -60 mV. But the critical concentrations of the other two amino acids did evoke inward currents in the Purkinje cell. 5. These observations suggest that, of known neurotransmitter candidates, glutamate is the most likely to be released by cochlear hair cells in the chick. 6. Purkinje cells had an EAA-induced current that was not suppressed by 2-amino-5-phosphonovalerate (200 mu M) or by 6-cyano-7-nitroquinoxaline-2.3-dione (20 mu M). The current was dominant at low concentrations of EAAs (similar to 1.0 mu M). This EAA-induced current was Na+ dependent and was likely carried through the high-affinity EAA transporter.