Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina

Vis Neurosci. Mar-Apr 2008;25(2):155-65. doi: 10.1017/S0952523808080310.

Abstract

GABA-mediated interactions between horizontal cells (HCs) and bipolar cells (BCs) transform signals within the image-processing circuitry of distal retina. To further understand this process, we have studied the GABA-driven membrane responses from isolated retinal neurons. Papain-dissociated retinal cells from adult zebrafish were exposed to GABAergic ligands while transmembrane potentials were monitored with a fluorescent voltage-sensitive dye (oxonol, DiBaC4(5)). In HCs hyperpolarizing, ionotropic GABA responses were almost never seen, nor were responses to baclofen or glycine. A GABA-induced depolarization followed by after hyperpolarization (dep/AHP) occurred in 38% of HCs. The median fluorescence increase (dep component) was 0.17 log units, about 22 mV. HC dep/AHP was not blocked by bicuculline or picrotoxin. Muscimol rarely evoked dep/AHP responses. In BCs picrotoxin sensitive, hyperpolarizing, ionotropic GABA and muscimol responses occurred in most cells. A picrotoxin insensitive dep/AHP response was seen in about 5% of BCs. The median fluorescence increase (dep component) was 0.18 log units, about 23 mV. Some BCs expressed both muscimol-induced hyperpolarizations and GABA-induced dep/AHP responses. For all cells, the pooled Hill fit to median dep amplitudes, in response to treatments with a GABA concentration series, gave an apparent k of 0.61 muM and an n of 1.1. The dep/AHP responses of all cells required both extracellular Na+ and Cl(-), as dep/AHP was blocked reversibly by Li+ substituted for Na+ and irreversibly by isethionate substituted for Cl(-). All cells with dep/AHP responses in zebrafish have the membrane physiology of neurons expressing GABA transporters. These cells likely accumulate GABA, a characteristic of GABAergic neurons. We suggest Na+ drives GABA into these cells, depolarizing the plasma membrane and triggering Na+, K+-dependent ATPase. The ATPase activity generates AHP. In addition to a GABA clearance function, these large-amplitude transporter responses may provide an outer plexiform layer GABA sensor mechanism.

Publication types

  • Research Support, N.I.H., Intramural

MeSH terms

  • Animals
  • Dose-Response Relationship, Drug
  • Electrophysiology
  • GABA Agonists / pharmacology
  • GABA Antagonists / pharmacology
  • GABA Plasma Membrane Transport Proteins / physiology*
  • Glutamic Acid / pharmacology
  • In Vitro Techniques
  • Membrane Potentials
  • Muscimol / pharmacology
  • Retinal Bipolar Cells / cytology
  • Retinal Bipolar Cells / drug effects*
  • Retinal Bipolar Cells / physiology*
  • Retinal Horizontal Cells / cytology
  • Retinal Horizontal Cells / drug effects*
  • Retinal Horizontal Cells / physiology*
  • Sodium Chloride / pharmacology
  • Zebrafish / physiology*
  • gamma-Aminobutyric Acid / administration & dosage
  • gamma-Aminobutyric Acid / pharmacology*

Substances

  • GABA Agonists
  • GABA Antagonists
  • GABA Plasma Membrane Transport Proteins
  • Muscimol
  • Glutamic Acid
  • Sodium Chloride
  • gamma-Aminobutyric Acid