Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina.
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.
Nelson
Ralph
R
Basic Neurosciences Program, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Rockville, Maryland 20892-9406, USA. nelsonr@ninds.nih.gov
Bender
Anna M
AM
Connaughton
Victoria P
VP
D000818:Animals; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins / Q000502:physiology*; D018698:Glutamic Acid / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009118:Muscimol / Q000494:pharmacology; D051245:Retinal Bipolar Cells / Q000166:cytology / Q000187:drug effects* / Q000502:physiology*; D051248:Retinal Horizontal Cells / Q000166:cytology / Q000187:drug effects* / Q000502:physiology*; D012965:Sodium Chloride / Q000494:pharmacology; D015027:Zebrafish / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000008:administration & dosage / Q000494:pharmacology*
D018755:GABA Agonists; D018756:GABA Antagonists; D050485:GABA Plasma Membrane Transport Proteins; D009118:Muscimol; D018698:Glutamic Acid; D012965:Sodium Chloride; D005680:gamma-Aminobutyric Acid