Postsynaptic response kinetics are controlled by a glutamate transporter at
cone photoreceptors
Lubor Gaal, Botond Roska, Serge A. Picaud, Samuel M. Wu, Robert Marc & Frank
S. Werblin.
Department of Molecular and Cell Biology, Division of Neurobiology,
University of California at Berkeley, Berkeley, CA 94720, Cullen Eye
Institute, Baylor College of Medicine, Houston, Texas 77030, Moran Eye Center,
University of Utah, Salt Lake City, Utah 84132.
APStracts 4:0252N, 1997.
ABSTRACT
We evaluated the role of the sodium/glutamate transporter at the synaptic
terminals of cone photoreceptors in controlling postsynaptic response
kinetics. The strategy was to measure the changes in horizontal cell response
rate induced by blocking transporter uptake in cones with dihydrokainate
(DHK). DHK was chosen as the uptake blocker because we show through
autoradiographic uptake measurements that DHK specifically blocked uptake in
cones without affecting uptake in Mueller cells. Horizontal cells depolarized
from about -70 mV to -20 mV as the exogenous glutamate concentration was
increased from about 1 µM to 40 µM, so horizontal cells can serve as
“glutamate electrodes” during the light response. DHK slowed the rate of
hyperpolarization of the horizontal cells in a dose-dependent way, but didn’t
affect the kinetics of the cone responses. At 300 µM DHK, the rate of the
horizontal cell hyperpolarization was slowed to only 17 ? 8.5 per cent of
control. Translating this to changes in glutamate concentration using the
slice dose response curve as calibration in Fig 2, DHK reduced the rate of
removal of glutamate from about 0.12 µM/sec to 0.031 µM/sec. The voltage
dependence of uptake rate in the transporter alone was capable of modulating
glutamate concentration: We blocked vesicular released glutamate with bathed
20 mM Mg++, then added 30 µM glutamate to the bath to re-establish a
physiological glutamate concentration level at the synapse and thereby
depolarize the horizontal cells. Under these conditions, a light flash
elicited a 17 mV hyperpolarization in the horizontal cells. When we
substituted kainate, which is not transported, for glutamate, horizontal cells
were depolarized, but light did not elicit any response, indicating that the
transporter alone was responsible for the removal of glutamate under these
conditions. This suggests that the transporter was both voltage-dependent and
robust enough to modulate glutamate concentration. The transporter must be at
least as effective as diffusion in removing glutamate from the synapse because
there is only a very small light response once the transporter is blocked. The
transporter, via its voltage dependence upon cone membrane potential, appears
to contribute significantly to the control of postsynaptic response kinetics.
Received 26 February 1997; accepted in final form 10 September 1997.
APS Manuscript Number J161-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997