ZFIN ID: ZDB-PUB-181207-9
Localizing proton-mediated inhibitory feedback at the retinal horizontal cell-cone synapse with genetically-encoded pH probes
Beckwith-Cohen, B., Holzhausen, L.C., Wang, T.M., Rajappa, R., Kramer, R.H.
Date: 2018
Source: The Journal of neuroscience : the official journal of the Society for Neuroscience   39(4): 651-662 (Journal)
Registered Authors: Holzhausen, Lars, Kramer, Richard H.
Keywords: none
MeSH Terms:
  • Animals
  • Calcium Channels, L-Type/genetics
  • Calcium Channels, L-Type/physiology
  • Connexins/metabolism
  • Feedback, Physiological/physiology*
  • Female
  • Glutamates/physiology
  • Hydrogen-Ion Concentration
  • Male
  • Protons
  • Receptors, AMPA/metabolism
  • Retinal Cone Photoreceptor Cells/physiology*
  • Retinal Cone Photoreceptor Cells/ultrastructure
  • Retinal Horizontal Cells/physiology*
  • Retinal Horizontal Cells/ultrastructure
  • Synapses/physiology*
  • Synapses/ultrastructure
  • Synaptic Transmission/physiology
  • Vesicle-Associated Membrane Protein 2/genetics
  • Vesicle-Associated Membrane Protein 2/physiology
  • Zebrafish
PubMed: 30504272 Full text @ J. Neurosci.
ABSTRACT
Lateral inhibition in the vertebrate retina depends on a negative feedback synapse between horizontal cells (HCs) and rod and cone photoreceptors. A change in pH is thought to be the signal for negative feedback, but its spatial profile in the synaptic cleft is unknown. Here we use 3 different membrane proteins, each fused to the same genetically-encoded pH-sensitive GFP (pHluorin), to probe synaptic pH in retina from transgenic zebrafish (Danio rerio) of either sex. We used the cone transducin promoter to express SynaptopHluorin (pHluorin on VAMP2) or CalipHluorin (pHluorin on an L-type Ca2+ channel) and the HC-specific connexin-55.5 promoter to express AMPApHluorin (pHluorin on an AMPA receptor). Stimulus light led to increased fluorescence of all three probes, consistent with alkalinization of the synaptic cleft. The receptive field size, sensitivity to surround illumination, and response to activation of an alien receptor expressed exclusively in HCs, are consistent with lateral inhibition as the trigger for alkalinization. However, SynaptopHluorin and AMPApHluorin, which are displaced farther from cone synaptic ribbons than CalipHlourin, reported a smaller pH change. Hence, unlike feed-forward glutamatergic transmission, which spills over to allow cross-talk between terminals in the cone network, the pH change underlying HC feedback is compartmentalized to individual synaptic invaginations within a cone terminal, consistent with private line communication.SIGNIFICANCE STATEMENTLateral inhibition (LI) is a fundamental feature of information processing in sensory systems, enhancing contrast sensitivity and enabling edge discrimination. Horizontal cells (HCs) are the first cellular substrate of LI in the vertebrate retina, but the synaptic mechanisms underlying LI are not completely understood, despite decades of study. This paper makes a significant contribution to our understanding of LI, by showing that each HC-cone synapse is a "private-line" that operates independently from other HC-cone connections. Using transgenic zebrafish expressing pHluorin, a pH-sensitive GFP variant spliced onto 3 different protein platforms expressed either in cones or HCs we show that the feed-back pH signal is constrained to individual cone terminals, and more stringently, to individual synaptic contact sites within each terminal.
ADDITIONAL INFORMATION