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Journal of Neuroscience, Vol 14, 2648-2658, Copyright © 1994 by Society for Neuroscience
GABA transporters and GABAC-like receptors on catfish cone- but not rod- driven horizontal cells
CJ Dong, SA Picaud and FS Werblin
Department of Molecular and Cell Biology, University of California at Berkeley 94720.
The effects of GABA and related agents were studied in solitary rod- and
cone-driven horizontal cells, acutely isolated from the catfish retina
using enzymatic and mechanical treatment. Both types of horizontal cells,
which normally receive glutamatergic input from photoreceptors, responded
to pressure ejection of the glutamate analog kainate (50 microM) with an
inward current of 300-800 pA when voltage- clamped at -70 mV using the
whole-cell patch-clamp technique. But pressure ejection of GABA (500
microM) elicited an inward current only in cone-driven horizontal cells.
This current, ranging between 100 and 400 pA, consisted of two components:
(1) a GABA receptor-gated chloride current that reversed near the chloride
equilibrium potential and was blocked by bath application of picrotoxin
(100-500 microM), and (2) a GABA transporter-mediated current that was
picrotoxin resistant but was blocked by NO-711 (1 microM) and
cis-4-hydroxynipecotic acid (250 microM), two potent GABA transporter
blockers. The GABA transporter current could also be eliminated when sodium
was replaced by either choline or lithium in the bathing medium. The
picrotoxin-sensitive receptor-gated current could not be elicited by the
GABAB receptor agonist baclofen, nor could it be blocked by the potent
GABAB receptor antagonist 2-hydroxysaclofen. The picrotoxin-sensitive
current could be divided into two components based on their sensitivity to
the specific GABAA receptor antagonist bicuculline methiodide. The
bicuculline- sensitive component was found only in some cells, whereas the
bicuculline-resistant, picrotoxin-sensitive component was found in all
cells tested. The bicuculline-resistant current was insensitive to
pentobarbital, an allosteric modulator of GABAA receptor. To confirm the
effectiveness of the specific batch of bicuculline methiodide and
pentobarbital, we tested both drugs in ganglion cells in the salamander
retinal slice preparation, where the GABA-elicited current is almost
exclusively mediated by GABAA receptors. Bicuculline methiodide almost
completely blocked, while pentobarbital significantly enhanced, the GABA
current recorded in ganglion cells. Thus, in catfish cone horizontal cells
the bicuculline-resistant GABA receptor current is most likely mediated by
the GABAC receptor based on the above pharmacological profile. The relative
effectiveness of GABA, muscimol, trans- and cis-4-aminocrotonic acid (TACA
and CACA) was determined at this GABAC receptor site after cells were
bathed in choline Ringer to eliminate the transporter current and in the
presence of 100 microM bicuculline methiodide to block GABAA receptor
current.(ABSTRACT TRUNCATED AT 400 WORDS)
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