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
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)
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: [email protected]
p1 Current Address: College of Physicians and Surgeons, Columbia University, New York, New York 10026.