POSTSYNAPTIC RESPONSES OF HORIZONTAL CELLS IN THE TIGER SALAMANDER RETINA ARE MEDIATED BY AMPA-PREFERRING RECEPTORS

The postsynaptic responses of sign-preserving second-order retinal neu rons (horizontal cells (HCs) and off-bipolar cells) are mediated by CN QX-sensitive AMPA/KA glutamate receptors. In this study we used recept or-specific allosteric regulators of desensitization and selected anta gonists to determine the glutamate receptor subtypes in tiger salamand er horizontal cells. Two approaches were employed in this study. The f irst was to measure postsynaptic currents induced by exogenously appli ed glutamate under voltage clamp conditions in living retinal slices; and the second was to record voltage responses controlled by endogenou s glutamate released from photoreceptors in whole retinas. Application of 100 mu M cyclothiazide (a specific AMPA receptor desensitization b locker) enhanced the glutamate-induced current by about 5 fold. In con trast, 300 mu gml(-1) ConA (a specific kainate receptor desensitizatio n blocker), had no effect. GYKI 52466 (a specific AMPA receptor antago nist) at 30 mu M almost completely suppressed the glutamate-induced in ward current in HCs. Cyclothiazide at 100 mu M depolarized the HC dark membrane potential by about 5 mV and reduced the amplitudes of the vo ltage responses to dim Lights, but enhanced the voltage responses to b right lights. Cyclothiazide had no effect on either the dark potential or the light responses of rods and cones. Con A at 300 mu gml(-1) had no effect on either the dark potential or the light responses of the HC. GYKI 52466 (30 mu M) hyperpolarized the HC dark membrane potential by about 55 mV and almost completely suppressed the light responses. We conclude from these results that the postsynaptic glutamate- and Li ght-induced responses in the tiger salamander retinal horizontal cells are mediated by AMPA-preferring, and not kainate-preferring glutamate receptors. The functional roles of AMPA receptors and their desensiti zation kinetics in visual information processing are discussed. (C) 19 98 Elsevier Science B.V. All rights reserved.