(Investigative Ophthalmology and Visual Science. 2002;43:446-451.)
© 2002
by The Association for Research in Vision and Ophthalmology, Inc.
New Insight into the Functional Role of Acetylcholine in Developing Embryonic Rat Retinal Neurons
Hiroki Yasuyoshi1,
Satoshi Kashii1,
Masashi Kikuchi1,
Shen Zhang1,
Yoshihito Honda1 and
Akinori Akaike2
1 From the Departments of Ophthalmology and Visual Sciences, Graduate School of Medicine and
2 Pharmacology, Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto, Japan.
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Abstract
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PURPOSE. To examine the effects of acetylcholine (ACh) on glutamate-induced
neurotoxicity in embryonic rat retinal neurons.
METHODS. Primary cultures were obtained from rat retinas at embryonic days
17 to 19. Cultured cells were exposed to glutamate for 10 minutes,
followed by incubation in glutamate-free medium for 1 hour. Drugs were
added to the incubation medium for 1 to 24 hours until immediately
before glutamate exposure and were removed from culture medium
during glutamate exposure and the postincubation period. The neurotoxic
effects on retinal cultures were quantitatively assessed by the trypan
blue exclusion method.
RESULTS. Cell viability was markedly reduced by 10-minute exposure to 500 µM
glutamate followed by a 1-hour incubation in glutamate-free medium.
Incubating the cultures with 1 µM ACh for 12 hours before glutamate
exposure reduced glutamate neurotoxicity. A similar effect was induced
by application of carbachol (1 µM). The protective effect of ACh
against glutamate neurotoxicity was inhibited by a nicotinic
acetylcholine receptor (nAChR) antagonist, mecamylamine (0.5 µM),
whereas a muscarinic acetylcholine receptor (mAChR) antagonist,
atropine (0.5 µM) did not affect ACh-induced protection. In addition,
a similar protection was induced by application of nicotine (1 µM),
but not by muscarine (1 µM). Pretreatment with nicotine induced a
protective effect in a time-dependent manner, ranging from 1 to 12
hours. Pretreatment with nicotine at concentrations ranging from 0.001
to 1 µM induced dose-dependent protection against glutamate
neurotoxicity. Furthermore, the protective action of nicotine was
inhibited by simultaneous application of dopamine D1 receptor
antagonist, SCH23390 (1 µM), with nicotine, whereas a dopamine D2
receptor antagonist, domperidone (1 µM), did not affect
nicotine-induced protection.
CONCLUSIONS. These results suggest that pretreatment of cultured rat retinal neurons
with ACh or the nAChR agonists, nicotine and carbachol, has a
protective action against glutamate neurotoxicity through nAChRs and
that the dopamine release induced by nicotinic stimulation subsequently
protects the retinal neurons by way of dopamine D1
receptors.
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Introduction
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Cells are closely connected with surrounding cells
including neurons. Neurons respond to neurotransmitters released by
adjacent neurons and take part in the formation of neuronal networks,
whereas neurons maintain homeostasis and survive within the networks
among surrounding neurons.
In our previous study, we showed the involvement of glutamate in
ischemia-reperfusion-induced retinal injury in vivo.1
Glutamate is an excitatory neurotransmitter in the
retina,2
3
4
yet it has a toxic action5
6
7
on
postsynaptic neurons by stimulating its receptors when it is present in
excess under pathologic conditions such as retinal
ischemia.8
9
10
The
N-methyl-D-aspartate (NMDA) receptor,
a subtype of glutamate receptors, plays a predominant role in the
delayed retinal neuronal death induced by
glutamate.7
11
12
13
14
15
16
Furthermore, we have demonstrated that
dopamine, one of the chemical neuromodulators in the retina, has a
protective action on cultured embryonic rat retinal neurons against
NMDA receptormediated glutamate neurotoxicity through dopamine D1
receptors.15
Based on these findings, we suggest that neurotransmitters such as
glutamate and dopamine may contain some signals affecting neuronal
cells survival and death in addition to their signal informations of
the neuronal network. Acetylcholine (ACh), which is released from
cholinergic amacrine cells, is one of the major endogenous
neurotransmitters in the retina. ACh receptors (AChRs) are subdivided
into two main types, nicotinic AChRs (nAChRs) and muscarinic AChRs
(mAChRs), and both nAChRs and mAChRs are prevalently distributed in the
ganglion cell layer (GCL) and the inner nuclear layer (INL) of the rat
retina.17
18
Therefore, to elucidate whether or not ACh is
involved in neuronal cells survival and death in the retina, we
examined the effect of ACh on glutamate-induced neurotoxicity mediated
through NMDA receptors in cultured rat retinal neurons.
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Materials and Methods
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Cell Culture
Primary cultures were obtained from Wistar rat retinas
(embryonic days 1719). The procedures have been described
previously.15
16
19
20
21
In brief, retinal tissues were
mechanically dissociated and single-cell suspensions were plated on
plastic coverslips (1.0 x 106 cells/mL).
Ten coverslips were placed in a 60-mm dish (Falcon Labware, Oxnard,
CA). Approximately 15 to 20 dishes were obtained and used in a single
experiment. Retinal cultures were incubated with Eagles minimal
essential medium (Eagles salts; Nissui, Tokyo, Japan) containing 2 mM
glutamine, 11 mM glucose (total), 24 mM sodium bicarbonate, and 10 mM
HEPES with 10% heat-inactivated fetal calf serum added during the
first week and supplemented with 10% horse serum for the remaining 9
to 10 days. Ten micromolar cytosine arabinoside (ara-C) was added to
the culture on the sixth day to eliminate proliferating cells. We used
only those cultures maintained for 9 to 10 days in vitro and used only
isolated cells in this study. Clusters of cells were excluded from the
results, because cells located in the clusters could not be used for
histologic experiments.15
A previous immunocytochemical
study revealed that these isolated cells mainly consist of amacrine
cells.15
All animals were treated in accordance with the
ARVO Statement for the Use of Animals in Ophthalmic and Vision
Research.
Drug Application
In a previous study using cultured rat retinal neurons, we
demonstrated that cell viability was markedly reduced by exposure to
glutamate (0.5 and 1 mM) for 10 minutes followed by postincubation in
glutamate-free medium for more than 1 hour,7
15
16
and we
showed that there was no significant difference between the reduction
in cell viability during 1- and 24-hour incubations.7
Therefore, in this study, cultures were exposed to drugs as follows:
Glutamate neurotoxicity was assessed by 10-minute exposure to 500 µM
glutamate, followed by a 1-hour incubation in glutamate-free medium.
Effects of drugs were assessed by pretreatment with drugs before
glutamate exposure and by simultaneous application of the drugs with
glutamate. To assess the effects of pretreatment with the drugs, the
drugs were added to the incubation medium for 1 to 24 hours until
immediately before glutamate exposure and removed from the culture
medium during glutamate exposure and the postincubation period. To
investigate the effects of simultaneous drug application, drugs were
added to the incubation medium during glutamate exposure and removed
from culture medium during the postincubation period.
The following drugs were used: monosodium L-glutamate
(Nakalai Tesque, Kyoto, Japan), acetylcholine chloride (Research
Biochemicals, Natick, MA), carbachol (Research Biochemicals),
(-)-nicotine (Sigma, St. Louis, MO), muscarine (Research
Biochemicals), MK-801 (Research Biochemicals), mecamylamine
hydrochloride (Sigma), atropine sulfate monohydrate (Wako, Osaka,
Japan), SCH23390 hydrochloride (Research Biochemicals), and domperidone
(Research Biochemicals).
Measurement of Neurotoxicity
The neurotoxic effects of glutamate and the protective effects
of drugs on the retinal cultures were quantitatively assessed by the
trypan blue exclusion method, as described
previously.7
15
16
19
20
21
22
At each session of the
experiment, we randomly chose five coverslips from different dishes,
which constituted the number of samples (n = 5) for
measurement of neurotoxicity. All experiments were performed in
Eagles solution at 37°C. After the completion of drug treatment,
cell cultures were stained with 1.5% trypan blue solution at room
temperature for 10 minutes and then fixed with isotonic formalin (pH
7.0, 24°C). The fixed cultures were rinsed with physiological
saline and examined under Hoffman modulation microscopy at x400
(Hoffman Modulation Optics, Greenvale, NY). More than 200 cells on each
of five coverslips were randomly counted to determine the viability of
the cell culture. The cell counts were made by a blind observer.
Viability of culture was calculated as the percentage of the ratio of
the number of unstained cells (viable cells) to the total number of
cells counted (viable cells plus nonviable cells). In each experiment,
five coverslips were used to obtain mean values ± SEM of cell
viability. The significance of data were determined by the Dunnett
two-tailed test.
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Results
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Effects of ACh on Glutamate-Induced Neurotoxicity
Figure 1
demonstrates an example of the effect of ACh on glutamate-induced
neurotoxicity. Most cells in nontreated culture (control) were not
stained by trypan blue (Fig. 1A)
, which is normally excluded by living
cells. However, numerous cells were stained by trypan blue, and cell
viability was markedly reduced by 10-minute exposure to 500 µM
glutamate followed by 1-hour incubation in glutamate-free medium (Fig. 1B) . Incubating the culture with 1 µM ACh for 12 hours before
glutamate exposure reduced the number of cells stained by trypan blue,
and cell death was markedly reduced (Fig. 1C)
. Furthermore, as shown in
the Methods section, cholinergic antagonists and ACh (1 µM)
were added to the incubation medium for 12 hours until immediately
before glutamate exposure and removed from culture medium during
glutamate exposure, followed by a 1-hour incubation. Mecamylamine (0.5
µM), an nAChR antagonist, increased the number of stained cells, and
cell viability was markedly reduced (Fig. 1D)
, whereas atropine (0.5
µM), an mAChR antagonist, did not affect the number of stained cells,
and cell death was reduced (Fig. 1E)
.

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Figure 1. Photomicrographs showing the effect of ACh on glutamate-induced
neurotoxicity in cultured rat retinal neurons. All cultures were
photographed after trypan blue staining followed by formalin fixation,
by using modulation microscopy. Cells stained with trypan blue dye were
regarded as nonviable. (A) Nontreated cells (control). Cells
showed almost no stain. (B) Cells treated with glutamate
(500 µM) for 10 minutes, followed by a 1-hour incubation with
glutamate-free medium. Marked cell death occurred.
(CE) Cells pretreated with drugs for 12 hours
until exposure to glutamate, then treated with glutamate (500 µM) for
10 minutes, followed by a 1-hour incubation with glutamate-free medium.
(C) Cells pretreated with ACh (1 µM). Cell death was
markedly reduced. (D) Cells pretreated with both ACh (1
µM) and mecamylamine (0.5 µM). The number of stained cells
increased, and cell viability was markedly reduced. (E)
Cells pretreated with both ACh (1 µM) and atropine (0.5 µM). The
number of stained cells was reduced, and cell viability was increased.
Bar, 50 µm.
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The quantitative assessment of the effect of ACh on glutamate-induced
neurotoxicity and the effects of cholinergic antagonists on ACh-induced
action against glutamate neurotoxicity is shown in Figure 2
. ACh has a protective effect on neurotoxicity induced by glutamate. A
similar effect was induced by application of carbachol (1 µM).
Carbachol is similar to ACh but is not decomposed by
acetylcholinesterase. Mecamylamine (0.5 µM), an nAChR antagonist,
reversed the protective effects of ACh against glutamate neurotoxicity;
by contrast, atropine (0.5 µM), an mAChR antagonist, did not affect
ACh-induced protection. Cell viability was not affected by a 12-hour
exposure of the cells to cholinergic antagonists used in this study.

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Figure 2. The quantitative assessment of the effect of ACh and carbachol on
glutamate-induced neurotoxicity and effects of cholinergic antagonists
on ACh-induced protection against glutamate neurotoxicity. Incubating
the cultures with 1 µM ACh for 12 hours before glutamate exposure
reduced glutamate neurotoxicity. A similar effect was induced by
application of carbachol (1 µM). Simultaneous application of the
nicotinic receptor antagonist, 0.5 µM mecamylamine, with ACh for 12
hours before glutamate exposure reversed the protective effects of ACh
against glutamate neurotoxicity, whereas the muscarinic receptor
antagonist, 0.5 µM atropine, did not affect the ACh-induced
protection (**P < 0.01, compared with the
glutamate-only group). Error bars in this and the subsequent figure
represent the SEM (n = 5). CAR, carbachol; MEC,
mecamylamine; AT, atropine.
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Figure 3
shows the effects of the simultaneous application of drugs with
glutamate for 10 minutes on glutamate-induced neurotoxicity. The
neurotoxic effect of glutamate was greatly reduced by the use of a
selective NMDA channel blocker, MK-801. By contrast, ACh did not
inhibit glutamate neurotoxicity.

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Figure 3. Effects of simultaneous application of ACh with glutamate on
glutamate-induced neurotoxicity. Simultaneous application of a
selective NMDA channel blocker, MK-801 (1 µM), with glutamate
markedly reduced the neurotoxic effect of glutamate. By contrast,
simultaneous application of ACh (1 µM) with glutamate did not inhibit
glutamate-induced neurotoxicity (**P < 0.01,
compared with the glutamate-only group).
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Effects of Cholinergic Agonists against Glutamate Neurotoxicity
The following study was undertaken, using nicotine as a selective
nAChR agonist and muscarine as a selective mAChR agonist, to elucidate
whether indeed the protective effects of ACh against glutamate
neurotoxicity are mediated by nAChRs in cultured rat retinal neurons.
Figure 4
summarizes the effects of the cholinergic agonists, nicotine or
muscarine, on glutamate-induced neurotoxicity. Incubating the cultures
with 1 µM nicotine for 12 hours before glutamate exposure reduced
glutamate-induced neurotoxicity, but 1 µM muscarine did not inhibit
glutamate neurotoxicity. Cell viability was not affected by a 12-hour
exposure of the cells to nicotine or muscarine.

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Figure 4. The effects of the cholinergic agonists, nicotine and
muscarine, on glutamate-induced neurotoxicity. Incubating the
cultures with 1 µM nicotine for 12 hours before glutamate exposure
reduced glutamate-induced neurotoxicity. However, 1 µM muscarine did
not inhibit glutamate neurotoxicity. Cell viability was not affected by
a 12-hour exposure of the cells to nicotine or muscarine
(**P < 0.01, compared with the glutamate-only
group).
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Figure 5A
demonstrates the time dependence of the protective effects of nicotine
against glutamate neurotoxicity in cultured retinal neurons. Cell
viability was not affected by a 24-hour exposure of the cells to
nicotine (1 µM) alone. Incubating the cultures with 1 µM nicotine
for 1 to 12 hours before glutamate exposure reduced the
glutamate-induced neurotoxicity. Pretreatment with nicotine induced a
protective effect in a time-dependent manner ranging from 1 to 12
hours. A significant difference was noted between cell viability of
cultures pretreated with nicotine for more than 2 hours and that of
glutamate-treated cultures. Maximal protection was observed in the
culture pretreated with nicotine for 12 hours before glutamate
exposure.

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Figure 5. (A) Time dependence of the protective effect of nicotine, an
nAChR agonist, against glutamate neurotoxicity. Incubating the cultures
with 1 µM nicotine for 1 to 12 hours before glutamate exposure
reduced the glutamate-induced neurotoxicity in a time-dependent manner.
The cultures pretreated with nicotine (1 µM) for more than 2 hours
significantly inhibited the cell death induced by glutamate (500 µM).
Maximal protection was observed in the culture pretreated with nicotine
for 12 hours before glutamate exposure (**P < 0.01,
compared with the glutamate-only group). (B) The
doseresponse relationship of the protective action of nicotine
against glutamate neurotoxicity. Pretreatment with nicotine at
concentrations ranging from 0.001 to 1 µM for 12 hours before
glutamate exposure demonstrated dose-dependent protection against
glutamate-induced neurotoxicity. Maximal protection was observed in the
culture pretreated with nicotine at concentrations of 0.1 and 1 µM
(*P < 0.05, **P < 0.01, compared with
the glutamate-only group).
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Figure 5B
summarizes the doseresponse relationship of the protective
effects of nicotine against glutamate-induced neurotoxicity. The cells
were treated with various concentrations of nicotine for 12 hours
before glutamate exposure. Pretreatment with nicotine at concentrations
ranging from 0.001 to 1 µM induced dose-dependent protection against
glutamate neurotoxicity. A significant difference was noted between
cell viability of cultures pretreated with nicotine at concentrations
ranging from 0.01 to 1 µM for 12 hours and that of glutamate-treated
cultures. Maximal protection was observed in culture pretreated with
nicotine at concentrations of 0.1 and 1 µM.
Effects of Dopamine Receptor Antagonists on nAChR-Mediated
Protection against Glutamate Neurotoxicity
Dopamine is known to be released by nicotinic stimulation from
dopaminergic amacrine cells in the retina.23
Therefore, we
examined the interaction between nicotine and dopamine.
Figure 6
summarizes the effects of the dopamine receptor antagonists, SCH23390
and domperidone, on nicotine-induced protection against glutamate
neurotoxicity. As shown in the Methods section, dopamine receptor
antagonists (1 µM) and nicotine (1 µM) were added to the incubation
medium for 12 hours until immediately before glutamate exposure and
removed from the culture medium during glutamate exposure, followed by
1-hour incubation. SCH23390, a dopamine D1 receptor antagonist,
reversed the protective effects of nicotine against glutamate
neurotoxicity, whereas domperidone, a dopamine D2 receptor antagonist,
did not affect the nicotine-induced protection. Exposure of the cells
to 1 µM dopamine receptor antagonists for 12 hours did not affect the
cell viability of the cultures.

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Figure 6. The effects of dopamine receptor antagonists, SCH23390 and domperidone,
on nicotine-induced protection against glutamate neurotoxicity.
Dopamine receptor antagonists (1 µM) and nicotine (1 µM) were added
to the incubation medium for 12 hours until immediately before
glutamate exposure and were removed from the culture medium during
glutamate exposure followed by 1-hour incubation. SCH23390, a dopamine
D1 receptor antagonist, reversed the protective effects of nicotine
against glutamate neurotoxicity, whereas domperidone, a dopamine D2
receptor antagonist, did not affect the nicotine-induced protection.
Exposing the cells to 1 µM dopamine receptor antagonists for 12 hours
did not affect cell viability (**P < 0.01,
compared with the glutamate-only group). SCH, SCH23390; DOM,
domperidone.
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Discussion
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In the present study, we demonstrate that the neurotoxic effect of
glutamate was greatly reduced by pretreatment of 1 µM ACh for 12
hours in cultured rat retinal neurons. ACh is one of the endogenous
excitatory neurotransmitters in the retina. AChRs are subdivided into
two main types, nAChRs and mAChRs. Furthermore, to investigate whether
the ACh-induced protection against glutamate neurotoxicity depends on
the specific receptor-mediated effects, the effects of selective
cholinergic antagonists and agonists were examined. The results show
that the protective effects of ACh on glutamate-induced neurotoxicity
was mediated by nAChRs.
The neurotoxic effect of glutamate was not reduced by the simultaneous
application of ACh with glutamate. A long-term exposure is necessary
for ACh to inhibit glutamate neurotoxicity. Therefore, it is not likely
that the effect of ACh is caused by directly blocking NMDA receptors in
the cell. Thus, it is tempting to speculate that some other mechanism
must have been involved in this nAChR-mediated protection against
glutamate neurotoxicity.
Dopamine is known to be released by nicotinic stimulation from
dopaminergic amacrine cells in the retina23
and substantia
nigraderived dopaminergic nerve terminals in the
striatum.24
Furthermore, we have demonstrated that
dopamine has a protective action on cultured rat retinal neurons
against NMDA receptormediated glutamate neurotoxicity through
dopamine D1 receptors.15
To investigate whether the
nAChR-mediated neuroprotection against glutamate neurotoxicity depends
on dopamine, the effects of dopamine receptor antagonists were
examined. The results suggest that nAChR stimulation induces a release
of dopamine and subsequently protects the retinal neurons against
glutamate neurotoxicity through dopamine D1 receptors. However, nAChR
stimulation may not only facilitate dopamine release but may also
upregulate the expression of dopamine D1 receptors. Further studies are
needed to elucidate the possibility that nAChR stimulation is related
to upregulation of dopamine D1 receptors.
Activation of nAChRs may interact with NMDA receptors or downregulate
the expression of NMDA receptors. As a result, calcium influx through
the NMDA receptors may be decreased through a direct or indirect
pathway. In previous reports, this possibility was suggested. Aizenman
et al.25
reported that certain nicotinic agonists can
interact with the NMDA receptor and block its function. Wong and
Gallagher26
reported that the application of nicotinic
agonists to rat dorsolateral septal neurons demonstrate a direct
membrane hyperpolarization mediated by an increase in potassium
conductance. In our cultured retinal neurons, however, main protective
action of ACh appears to be exerted through D1 receptors, because its
protective action was inhibited by simultaneous application of D1
receptor antagonist. Therefore, it is suggested that these
possibilities play little role in the protective action of ACh against
glutamate neurotoxicity in the retina.
In this study, ACh protected retinal neurons against glutamate
neurotoxicity by stimulating the nAChRs, and dopamine was involved in
the nAChR-mediated protection. Glutamate, dopamine, and ACh are closely
interrelated in the retina. Close linkage and interactions between ACh
and dopamine in retinal glutamate-induced neurotoxicity mediated
through NMDA receptors suggests that certain neurotransmitters and
neuromodulators play crucial roles in neuronal cell viability in
addition to their primary role as a neuronal signal transmitter. During
development, neurons that constitute a neuronal network are kept alive,
but those that do not participate in the network are eliminated.
Recently, Kaczmarek et al.27
demonstrated that the NMDA
receptor plays an important role in neuronal development, plasticity,
and cell death in the central nervous system. Receptor studies in the
developing retina have demonstrated that nAChRs28
and
dopamine D1 receptors29
appear at embryonic days 13 to 15
in the rat retina. Glutamate, ACh, and dopamine, therefore, may play
important roles in the selection of which neurons are to live or die
and the formation of neuronal networks in the developing embryonic
retina. It is tempting to speculate that they deliver not only neuronal
signals but signals concerning survival or death. However, careful
consideration is required to generalize the results from cultured
embryonic rat retinal neurons, because glutamate neurotoxicity does not
seem to occur in embryonic rat retina.30
Further studies
are necessary to elucidate roles of the endogenous neurotransmitters in
neuronal death and survival in the developing retina.
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Footnotes
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Submitted for publication February 12, 2001; revised August 22, 2001;
accepted September 5, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Satoshi Kashii, Department of Ophthalmology and
Visual Sciences, Graduate School of Medicine, Kyoto University, Kyoto
606-8507, Japan; skashii{at}kuhp.kyoto-u.ac.jp
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