Responses mediated by excitatory and inhibitory amino acid receptors in solitary spiking cells from normal newt retina.

Whole-cell currents activated by L-glutamate, kainate (KA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABA), and glycine were recorded from spiking cells enzymatically dissociated from adult newt retina. Most spiking cells responded to both AMPA and KA. KA tended to be a more potent agonist for them than AMPA. The reversal potential of both AMPA and KA responses was about -7 mV; this value was similar to that (about -5 mV) of glutamate response. AMPA- and KA-induced currents may be carried by monovalent cations, such as Na+, because their reversal potentials were sensitive to external Na+. About half of the spiking cells examined responded to mixtures of NMDA and glycine. Extracellular Mg2+ blocked completely the response to NMDA plus glycine in spiking cells held at negative membrane potentials, but not at positive membrane potentials. Almost all spiking cells responded to the inhibitory amino acids GABA and glycine. Dose-dependent desensitization was observed in both GABA and glycine responses. Currents activated by GABA and glycine were carried by Cl-. Bicuculline and strychnine strongly suppressed the responses to GABA and glycine, respectively, suggesting the existence of GABAA receptors and conventional glycine receptors in the spiking cells.