Excitatory amino acid response in isolated spiral ganglion cells of guinea pig cochlea

The physiological and pharmacological properties of excitatory amino acid (EAA)-induced responses were investigated in acutely isolated spiral ganglion cells of guinea pig by a conventional patch-clamp technique combined with a rapid drug application (Y-tube) method. L-glutamate (Glu) and its agonists, quisqualate (QA) and kainate (KA), induced inward currents in a concentration-dependent manner at a holding potential (V(H)) of -70 mV. The values of half-maximal concentration (EC₅₀) were 4.0 x 10^-4 M for Glu, 2.3 x 10^-5 M for QA, and 1.4 x 10^-4 for KA. The Hill coefficients were 0.96, 1.00, and 1.56 for Glu, QA, and KA, respectively. However, one of Glu agonists, N-methyl-D-aspartate (NMDA), and another excitatory amino acid, L-aspartate (Asp), did not induce any responses even in Mg²⁺-free external solution containing 10^-6 M glycine (Gly). The current-voltage (I-V) relationships for the Glu-, QA-, and KA-induced responses were linear, and these reversal potentials were near 5 mV. Kynurenic acid (Kyn), 6,7-dichloro-3-hydroxy-2-quinoxalinecarboxylic acid (diCl-HQC), and quinoxalinediones such as 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) suppressed the Glu-, QA-, and KA-induced currents in a concentration-dependent manner. The inhibitory potency was in the order of DNQX = CNQX > diCl-HQC > Kyn. CNQX antagonized the Glu-, QA-, and KA-induced currents without affecting the maximum responses showing no voltage-dependency, indicating the competitive inhibition. The selective NMDA antagonists such as 2-amino-5-phosphonovalerate (APV) and 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphate (CPP) did not block the Glu-, QA-, and KA-induced responses. Effects of EAAs on the cochlear ganglion cells freshly isolated from the chicken were also investigated. All the Glu, QA, KA, NMDA, and Asp could evoke inward currents in Mg²⁺-free standard solution at a V(H) of -70 mV. The Glu and NMDA responses were facilitated by adding Gly and were suppressed by adding either external 1 mM Mg²⁺ or 3 x 10^-5 M APV. Our results suggested that the subtype of Glu receptor in spiral ganglion cells of the guinea pig is non-NMDA type (KA- and QA-sensitive), whereas that in cochlear ganglion cells of the chicken is mixed type (KA-, QA-, and NMDA sensitive).