Feedback from luminosity horizontal cells mediates depolarizing responses of chromaticity horizontal cells in the Xenopus retina

Paul Witkovsky, Robert Gabriel, David Krizaj, Abram Akopian

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It has been proposed that the depolarizing responses of chromaticity horizontal cells (C-HCs) to red light depend on a feedback signal from luminosity horizontal cells (L-HCs) to short-wavelength-sensitive cones in the retinas of lower vertebrates. In this regard we studied the C-HCs of the Xenopus retina. C-HCs and L-HCs were identified by physiological criteria and then injected with neurobiotin. The retina then was incubated with peanut agglutinin, which stains red-but not blue-sensitive cones. Electron microscopic examination revealed that L-HCs contact all cone classes, whereas C-HCs contact only blue-sensitive cones. Simultaneous recordings from C- HC/L-HC pairs established that when the L-HC was saturated by a steady bright red light, C-HCs alone responded to a superimposed blue stimulus. In response to red test flashes, the C-HC response was delayed by ≃30 msec with respect to the L-HC response. Isolated HCs of both subtypes were examined by whole- cell patch clamp. Both responded to kainate with sustained inward currents and to quisqualate or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) with desensitizing currents from a negative holding potential; i.e., both have AMPA-type glutamate receptors. γ-Aminobutyric acid or glycine opened a chloride channel in the L-HC, whereas the C-HC was unresponsive to either inhibitory amino acid. Since glycine has been shown to abolish selectively the depolarizing response of the C-HC, this finding and other pharmacological data strongly implicate the L-HC in the underlying circuit. Moreover, because the C-HC does not respond to γ-aminobutyric acid, the neurotransmitter of the L-HC, by elimination, a feedback synapse from L-HC to blue cone is the most plausible mechanism for the creation of depolarizing responses in C-HCs.

Original languageEnglish
Pages (from-to)3556-3560
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number8
Publication statusPublished - Apr 11 1995



  • color vision
  • glutamate
  • neurobiotin

ASJC Scopus subject areas

  • General

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