Pharmacological characterization of voltage-clamped catfish rod horizontal cell responses to kainic acid

Excitatory amino acid-induced currents were examined in voltage-clamped rod horizontal cells dissociated from the catfish retina. The cells responded to glutamate (GLU) and the GLU analogues kainate (KA), quisqualate (QA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), while N-methyl-D-aspartate (NMDA) produced inconsistent responses. Of the effective agonists, only KA produced large, concentration-dependent current responses. While QA, AMPA, GLU, and NMDA were poor agonists, these compounds were able to block rod horizontal cell responses to KA. The rank order potency for this inhibition was: QA greater than AMPA greater than or equal to L-GLU much greater than D-GLU = NMDA. Several excitatory amino acid receptor antagonists were also able to inhibit rod horizontal cell responses to KA. The rank order potency for the inhibition by the compounds tested was: kynurenate greater than cis-piperidine-dicarboxylic acid much greater than D,L-alpha-amino-adipate. Comparison of the potency of several ligands to inhibit rod and cone horizontal cell responses to KA suggested similarities in the KA binding sites of both cell types.