Glutamate modulation of GABA transport in retinal horizontal cells of the skate

Abstract

Transport of the amino acid GABA into neurons and glia plays a key role in regulating the effects of GABA in the vertebrate retina. We have examined the modulation of GABA‐elicited transport currents of retinal horizontal cells by glutamate, the likely neurotransmitter of vertebrate photoreceptors. Enzymatically isolated external horizontal cells of skate were examined using whole‐cell voltage‐clamp techniques. GABA (1 mm) elicited an inward current that was completely suppressed by the GABA transport inhibitors tiagabine (10 μm) and SKF89976‐A (100 μm), but was unaffected by 100 μm picrotoxin. Prior application of 100 μm glutamate significantly reduced the GABA‐elicited current. Glutamate depressed the GABA dose‐response curve without shifting the curve laterally or altering the voltage dependence of the current. The ionotropic glutamate receptor agonists kainate and AMPA also reduced the GABA‐elicited current, and the effects of glutamate and kainate were abolished by the ionotropic glutamate receptor antagonist 6‐cyano‐7‐nitroquinoxaline. NMDA neither elicited a current nor modified the GABA‐induced current, and metabotropic glutamate analogues were also without effect. Inhibition of the GABA‐elicited current by glutamate and kainate was reduced when extracellular calcium was removed and when recording pipettes contained high concentrations of the calcium chelator BAPTA. Caffeine (5 mm) and thapsigargin (2 nm), agents known to alter intracellular calcium levels, also reduced the GABA‐elicited current, but increases in calcium induced by depolarization alone did not. Our data suggest that glutamate regulates GABA transport in retinal horizontal cells through a calcium‐dependent process, and imply a close physical relationship between calcium‐permeable glutamate receptors and GABA transporters in these cells.

Pharmacological characterization, localization, and regulation of ionotropic glutamate receptors in skate horizontal cells

“…Previous studies have shown that skate horizontal cells respond to glutamate in a steep concentration-dependent manner (Kreitzer et al, 2003). The concentration dependence of such glutamateinduced currents was fit to a Hill equation of the form:…”

“…The dissociation protocol used in this study was identical to that reported in Kreitzer et al (2003). Isolated retinae were bubbled for 45 min in a skate-modified L-15 media at pH 7.6 containing 2 mg/ml papain (Calbiochem-Novabiochem, San Diego, CA) and 1 mg/ml l-cysteine.…”

Early development of GABAergic cells of the retina in sharks: An immunohistochemical study with GABA and GAD antibodies

“…It is thought that GABA plays a neurotransmitter role in interneurons involved in lateral pathways (amacrine cells, horizontal cells and interplexiform cells; for a review see Kalloniatis and Tomisich, 1999), although recently it has been proposed that in horizontal cells GABA also acts as a neuromodulator (Kreitzer et al, 2003(Kreitzer et al, , 2007. GABA and its synthesizing enzyme glutamic acid decarboxylase (GAD) have been localized in amacrine cells in all vertebrates, but their presence in horizontal, interplexiform and ganglion cells is dependent on the species, the GAD isoform or the developmental stage.…”

NMDA-Receptor-Initiated Modulatory Effect on GABA Transporter Current: Experiment and Modelling

“…Ca 2+ -influx via AMPA receptor downregulates the GABA uptake of the transporter [12]. In addition to AMPA receptors, it was recently found that functional NMDA receptor is expressed in carp H1 type horizontal cells [3].…”