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

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. Papaindissociated retinal cells from adult zebrafish were exposed to GABAergic ligands while transmembrane potentials were monitored with a fluorescent voltage-sensitive dye (oxonol, DiBaC 4 (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.

Immunocytochemical evidence for SNARE protein‐dependent transmitter release from guinea pig horizontal cells

“…, 1985; Ayoub & Lam, 1987; O’Malley & Masland, 1989; Connaughton et al. , 2008; Nelson et al. , 2008).…”

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

“…The termination of the synaptic action of GABA is achieved by its uptake into presynaptic neuronal terminals and into the surrounding glial cell processes. In addition to neurons such as amacrine and interplexiform cells (Moran et al, 1986; Pow et al, 1996) and, in the fish retina, horizontal and bipolar cells (Nelson et al, 2008), Müller cells and, at least under pathological conditions, astrocytes and microglia take up GABA (Sarthy, 1982; Osborne et al, 1995). It was suggested that in the retinas of most lower vertebrates and birds, GABA removal is almost exclusively mediated by neurons, whereas in the mammalian retina, neurons, and Müller cells remove extracellular GABA (Yazulla, 1986; but, see below).…”

Bipolar cells in the zebrafish retina

“…Though GABA hyperpolarized most bipolar cells, a minority of isolated cells (∼5%) were depolarized by GABA (Nelson et al, 2008). GABA-elicited depolarizations were only observed in OFF-bipolar cells (unpublished observations).…”