Visual Neuroscience

Research Article

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

RALPH NELSONa1 c1, ANNA M. BENDERa1 p1 and VICTORIA P. CONNAUGHTONa2

a1 Basic Neurosciences Program, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Rockville, Maryland

a2 Department of Biology, American University, Washington, DC

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 μM 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.

(Received November 29 2007)

(Accepted February 01 2008)

Correspondence:

c1 Address correspondence and reprint requests to: Ralph F. Nelson, Basic Neurosciences Program National Institute of Neurological Disorders and Stroke, National Institutes of Health, 5625 Fisher's Lane, Room TS-09, Rockville, MD 20892–9406. E-mail: nelsonr@ninds.nih.gov

p1 Current Address: College of Physicians and Surgeons, Columbia University, New York, New York 10026.