May 2006
Volume 47, Issue 13
ARVO Annual Meeting Abstract  |   May 2006
Transporter–Like GABA Excitation of Horizontal and Bipolar Cells in Zebrafish Distal Retina
Author Affiliations & Notes
  • R.F. Nelson
    Neural Circuits Unit, NIH/NINDS, Rockville, MD
  • A.M. Bender
    Neural Circuits Unit, NIH/NINDS, Rockville, MD
    College of Physicians and Surgeons, Columbia University, New York, NY
  • V.P. Connaughton
    Neural Circuits Unit, NIH/NINDS, Rockville, MD
    Department of Biology, American University, Washington, DC
  • Footnotes
    Commercial Relationships  R.F. Nelson, None; A.M. Bender, None; V.P. Connaughton, None.
  • Footnotes
    Support  None
Investigative Ophthalmology & Visual Science May 2006, Vol.47, 390. doi:
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      R.F. Nelson, A.M. Bender, V.P. Connaughton; Transporter–Like GABA Excitation of Horizontal and Bipolar Cells in Zebrafish Distal Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):390.

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      © ARVO (1962-2015); The Authors (2016-present)

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Purpose: : To identify GABA signaling pathways for horizontal and bipolar cells in the outer plexiform layer of the zebrafish retina.

Methods: : Retinas removed from adult zebrafish eyes were papain–dissociated and plated on poly–D–lysine–coated plastic culture dishes. Dissociated horizontal cells (HCs) and bipolar cells (BCs) were identified morphologically (Connaughton & Dowling, 1998). Changes in membrane potential induced by GABA, selective GABAergic ligands, or treatments affecting GABA responses were obtained by photometry using the fluorescent voltage–sensitive dye oxonol (DiBaC4(5), 80 nM; Nelson et al, 2003). Only the dendro–somatic GABA responses of axotomized BCs were considered. HC recordings were mainly of the HB type (Connaughton et al, 2004).

Results: : Most BCs (194/289, 67%) hyperpolarized in response to GABA (1–20 µM, but HCs almost never did (2/73, 3%). A subpopulation of HCs (27/73, 37%) either depolarized and/or responded with after hyperpolarization (dep/AHP) in response to GABA, as did a minor group of BCs (13/289; 4%). In GABA depolarized BCs or HCs the mean fluorescence increase was .24 log units (N=27), about 30 mV (Nelson et al, 1999). Muscimol (10 µM), an iontropic GABA agonist, readily hyperpolarized BCs (56/91, 62%), but was ineffective at evoking dep/AHP in either HCs (3/44, 7%) or BCs (2/91, 2%). GABA–induced hyperpolarizations in BCs were blocked or partially blocked by picrotoxin (24/24, 100%); however, the dep/AHP GABA response, wherever seen in dissociated retinal neurons, was not (0/16, 0%). Dep/AHP responses required both extracellular Na+ and Cl. They were reversibly blocked by Li+ substitution for Na+ (15/16; 94%). In some cases (8/16, 50%) a hyperpolarizing GABA response was unmasked by this substitution. Na isethionate substituted for NaCl resulted in irreversible loss of GABA dep/AHP (13/14, 93%). Among 14 dep/AHP responses studied with GABA concentration series, the median concentration for half saturation of response was 0.7 µM.

Conclusions: : Ionotropic GABA receptors are expressed on most zebrafish BCs, but not HCs. Many HCs and rarely, BCs, express an Na+ –dependent, Cl –dependent, picrotoxin–insensitive, and muscimol–resistant membrane transporter for GABA. A depolarized Na+ gradient drives GABA into these cells, depolarizing the plasma membrane and triggering Na+, K+–dependent ATPase generating AHP. In effect many HCs and a few BCs in zebrafish exhibit a GABA–selective, Na+–dependent excitation within the outer plexiform layer.

Keywords: horizontal cells • neurotransmitters/neurotransmitter systems • bipolar cells 

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