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The Journal of Neuroscience, December 15, 1998, 18(24):10594-10602
Neurotransmitter Coupling through Gap Junctions in the Retina
David I.
Vaney,
J. Charles
Nelson, and
David V.
Pow
Vision, Touch and Hearing Research Centre, Department of Physiology
and Pharmacology, The University of Queensland, Brisbane 4072, Australia
Although all bipolar cells in the retina probably use the
excitatory transmitter glutamate, approximately half of the cone bipolar cells also contain elevated levels of the inhibitory
transmitter glycine. Some types of cone bipolar cells make heterologous
gap junctions with rod amacrine cells, which contain elevated levels of
glycine, leading to the hypothesis that the bipolar cells obtain their
glycine from amacrine cells. Experimental support for this hypothesis
is now provided by three independent lines of evidence. First, the
glycine transporter GLYT1 is expressed by the glycine-containing amacrine cells but not by the glycine-containing bipolar cells, suggesting that only the amacrine cells are functionally glycinergic. Second, the gap-junction blocker carbenoxolone greatly reduces exogenous 3H-glycine accumulation into the bipolar cells
but not the amacrine cells. Moreover, when the endogenous glycine
stores in both cell classes are depleted by incubating the retina with
a glycine-uptake inhibitor, carbenoxolone blocks the subsequent glycine
replenishment of the bipolar cells but not the amacrine cells. Third,
intracellular injection of rod amacrine cells with the gap-junction
permeant tracer Neurobiotin secondarily labels a heterogenous
population of cone bipolar cells, all of which show glycine
immunoreactivity. Taken together, these findings indicate that the
elevated glycine in cone bipolar cells is not derived by high-affinity
uptake or de novo synthesis but is obtained by
neurotransmitter coupling through gap junctions with glycinergic
amacrine cells. Thus transmitter content may be an unreliable indicator
of transmitter function for neurons that make heterologous gap junctions.
Key words:
gap junction; neurotransmitter coupling; glycine; glycine
transporter; GLYT1; retina; amacrine cell; bipolar cell; tracer
coupling; Neurobiotin; carbenoxolone; sarcosine
Copyright © 1998 Society for Neuroscience 0270-6474/98/182410594-09$05.00/0
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