|
|
||||||||
From the Department of Pathophysiology of Vision and Neuro-Ophthalmology, Division of Experimental Ophthalmology, University Eye Hospital, Röntgenweg, Tübingen, Germany.
| Abstract |
|---|
|
|
|---|
-amino-3-hydroxy-5-methyl-4-isoxasole-propionate (AMPA) receptors in
retinal development, the spatial distribution and temporal regulation
of all AMPA receptor subunit proteins was studied in rats. METHODS. Immunohistochemistry was performed on retinal sections between embryonic days (E)20 and E21 and the adult stage by using specific antibodies against AMPA subunits GluR1 to 4.
RESULTS. All AMPA subunits were expressed in the ganglion cell layer from E21 on. In the inner plexiform layer (IPL), discernible bands of labeling appeared at distinct retinal ages for the different subunits. GluR1 immunoreactivity (IR) was concentrated in two broad bands by postnatal day (P)3, whereas three bands were visible beginning on P9. Two bands were located in a region of the IPL where off-cells terminate, and one band was found in the innermost part of the IPL where on-cells terminate. In contrast, two bands of GluR2/3- and GluR4-IR in the IPL were only discernible beginning on P14 and seemed to be located between the bands of GluR1-IR. GluR2/3 and GluR4 were observed both in horizontal cells and in the outer plexiform layer from early developmental stages on. GluR1 was not found in the outer retina, indicating that horizontal and bipolar cell processes in the rat express AMPA receptors composed of subunits GluR2 to 4. Double-labeling experiments with cell-specific markers revealed the expression of subunits GluR1 to 4 in cholinergic and AII amacrine cells.
CONCLUSIONS. AMPA receptors are expressed before synapse formation, indicating a role not only in fast signal transmission but also in the establishment of inner retinal circuits. The differences in spatial and temporal subunit expression suggest that different retinal cell types selectively express distinct types of AMPA receptors during development of the rat retina.
| Introduction |
|---|
|
|
|---|
Three main subtypes of ionotropic glutamate receptor have been
characterized by pharmacologic studies and have been named according to
their selective agonists:
N-methyl-D-aspartate (NMDA), kainate,
and
-amino-3-hydroxy-5-methyl-4-isoxasole-propionate (AMPA; see
Hollmann and Heinemann4
for a review). The AMPA receptor,
which is involved in fast glutamatergic transmission in the adult
mammalian central nervous system (CNS) is made up of four
subunits5
termed GluR1, GluR2, GluR3, and GluR4, each of
which contains two major splice variants, flip and flop.4
Each subunit can form a functional homomeric receptor when expressed in
oocytes, although it is generally presumed that in vivo AMPA receptors
are heteromeric and are composed of at least two different subunits. It
is known that the distinct functional properties of AMPA receptors are
due to differences in subunit composition6
7
; for
instance, AMPA receptors containing the GluR2 subunit confer low
calcium permeability. Thus, changes in response to glutamate during
neuronal development can occur through changes in channel properties
due to alterations in subunit composition and/or the transient
appearance of specific glutamate receptor subtypes. Because synaptic
receptor heterogeneity is a key factor underlying different functional
properties in neurons, it is important to determine which receptor
subunits are expressed in a given neuronal type at different stages of
development.
The expression of AMPA receptor subunits in the mammalian retina has been studied mainly by in situ hybridization.8 9 10 11 Only a few immunohistochemical studies exist, and all were performed in the adult vertebrate retina.12 13 14 15 From these studies we know that the high transcript levels of AMPA receptor mRNAs observed in the adult vertebrate retina are indeed reflected by their high expression in protein. This underscores the important role of AMPA receptors in mediating light-induced synaptic transmission.16 17
However, not much is known about developmental aspects of AMPA receptor expression in the mammalian retina. The present study is the first to immunocytochemically analyze the temporal regulation and distribution of AMPA receptor subunits to elucidate when AMPA receptors are first expressed and whether there are distinct patterns of expression that may change during retinal development because of different functions AMPA receptors may have during the establishment of retinal circuitry.
| Methods |
|---|
|
|
|---|
Immunohistochemistry
The endogenous peroxidase was first blocked with 3%
H2O2 in 40% methanol, and
sections were incubated for 1 hour with 10% normal goat serum (NGS;
Sigma, Munich, Germany) and 0.3% Triton X-100 in phosphate-buffered
saline (PBST) to reduce background staining. The primary antibodies
were diluted in PBST containing 10% NGS and incubated for 3 hours at
room temperature or overnight at 4°C. After washing with PBS, the
samples were incubated for 1 hour with the biotin-conjugated secondary
antibody (dilution 1:200; Vectastain Elite Kit; Vector Laboratories,
Burlingame, CA) in PBST with 5% NGS. After rinsing in PBS, retinal
sections were processed with an avidin-biotin-peroxidase complex
(Vectastain Elite Kit, Vector), and staining was visualized with
diaminobenzidine reaction products. For double-labeling
immunofluorescence, the preblocking step and primary antibody
incubation were performed in 20% NGS plus 2% bovine serum albumin
(BSA; Sigma). The secondary antibodies were conjugated either to Cy3 or
fluorescein isothiocyanate (FITC; both from Sigma; dilutions were 1:100
for FITC and 1:1000 for Cy3).
Antibodies
Polyclonal antibodies against GluR1, GluR2/3, and GluR4
(PharMingen, San Diego, CA) were raised against synthetic peptides
corresponding to the C-terminal sequences in the intracellular domain
of the rat GluR clones and were affinity purified by using the
corresponding peptides.13
18
The antibody against subunit
GluR1 does not cross-react with other AMPA subunits. The GluR2/3
antibody does not cross-react with GluR1. It recognizes the GluR4c
splice variant but not GluR4. The antibody against subunit GluR4 does
not cross-react with GluR1, GluR2, or GluR3, or GluR4c. Each antibody
used recognizes both flip and flop slice variants. Primary antibodies
were used at the following concentrations: anti-GluR1 and anti-GluR4, 4
µg/ml, and anti-GluR2/3, 2 µg/ml. Cholinergic amacrine cells were
immunodetected using a specific monoclonal antibody against choline
acetyltransferase (clone 1E6; dilution 1:250; Chemicon), and AII
amacrine cells were visualized with a monoclonal antibody against
parvalbumin (clone PA-235 diluted 1:200; Sigma). Control retina
sections were processed as described earlier, except that the first
antisera were omitted, resulting in no staining.
| Results |
|---|
|
|
|---|
GluR1 Subunit Expression
GluR1 immunoreactivity was found in most cells of the GCL and in
cells in the inner part of the neuroblast layer (NBL) in early
developmental stages (Fig. 1 P3
, arrows). No staining was observed in the outer part of the NBL
where differentiating horizontal cells are localized. Later in
development, GluR1 immunoreactivity was never observed in horizontal
cells or the OPL. This is in line with what has been observed in the
adult rat retina12
but not in the adult cat retina where
GluR1 was expressed in cone bipolar cell dendrites in the
OPL.14
|
|
GluR2/3 Subunit Expression
The staining pattern of GluR2 was indistinguishable from that of
GluR3 with the antibody used in the present study. However, previous
immunohistochemical studies in the adult vertebrate
retina12
suggested that distribution patterns of GluR2 and
GluR3 are similar and that GluR2 rather than GluR3 is predominantly
expressed in the rat retina during development as well as in adulthood.
GluR2/3 expression was found in most cells of the GCL and in various subpopulations of amacrine cells in the inner part of the INL at all developmental stages (Fig. 2 , thick arrows). The double-labeling study demonstrated that, as for GluR1, AII amacrine cells (Fig. 4C , arrowhead) and cholinergic amacrine cells (Fig. 4D , arrowhead) express the AMPA subunits GluR2/3. In contrast to GluR1, staining for GluR2/3 was also observed in horizontal cells beginning on P3 (Fig. 2 , inset, arrowheads). Moreover, labeling in the OPL appeared around P9, thus indicating a synaptic localization of GluR2/3 at the beginning of the second postnatal week. These results are in agreement with previous in situ hybridizations8 9 10 and immunohistochemical studies,12 13 14 15 which also found GluR2/3 expression in the outer part of the INL and the OPL of adult mammalian retinas.
|
GluR4 Subunit Expression
GluR4 expression was found in most cells of the GCL and in
subpopulations of amacrine cells in the inner part of the INL at all
developmental stages (Fig. 3
, thick arrows and stars). Expression of GluR4 in AII amacrine cells
(Fig. 4E
, arrowhead) and cholinergic amacrine cells (Fig. 4F
, arrowhead) was
verified by double-labeling. Similarly to GluR2/3, a clear GluR4
staining was observed in horizontal cells (Fig. 3
, arrowheads) and in
the OPL beginning on P3. GluR4 expression in the outer part of the INL
and in the OPL of the adult retina has previously been demonstrated by
in situ hybridization8
9
10
and
immunohistochemistry,12
13
14
15
and our present results show
that this specific pattern of expression is already present at early
developmental stages.
|
| Discussion |
|---|
|
|
|---|
AMPA receptor subunits show distinct distribution patterns in both synaptic layers and among the various retinal cell types, indicating that different types of glutamate receptors exist at all levels of retinal information processing and that they may be differentially distributed in the on and off pathways. Moreover, the developmental alterations and differences in the distribution of the different subunits suggest that the functional properties of AMPA receptors in distinct cell types may change with retinal age. It has to be pointed out, however, that our immunocytochemical data can give no information on how the subunits are combined in functional receptors and that electrophysiological analysis as well as ultrastructural examinations will be necessary to clarify this point. Nonetheless, our finding of multiple subunits expressed in individual cells such as cholinergic amacrine cells supports the assumption made by other investigators that AMPA subunits in vivo are heteromeric.6 7 22
Glutamate Receptor Expression in the Outer Retina
Previous in situ hybridization studies performed in the adult rat
have demonstrated labeling for GluR2/3 and GluR4 mRNA in the outer part
of the INL.8
9
10
In the current study, we have shown that
GluR2/3 and GluR4 subunits are present in horizontal cells and that
they are expressed in early postnatal stages. Moreover, the staining
observed in the OPL indicates a synaptic localization of these subunits
and therefore suggests that input to second-order neurons in the rat
retina may be mediated by different types of AMPA receptors in addition
to NMDA receptors.23
In the middle part of the INL where bipolar cell bodies are located, only a few cells were occasionally immunoreactive for GluR1, whereas staining for GluR2/3 and GluR4 was never above background. This is comparable to a recent immunohistochemical study in cat in which cell bodies and primary dendrites of bipolar cells were only weakly labeled.15 Those investigators, however, reported strong staining of subunits GluR2/3 and/or GluR4 in invaginating and basal dendrites of bipolar cells in the OPL and concluded that all bipolar cell classes express at least one GluR subunit. This observation, together with the finding that AMPA receptormediated currents can be elicited in bipolar cells of adult rats,24 suggests that in rat, GluR2/3 and GluR4 subunits may be located not only on processes of horizontal cells but also on bipolar cell dendrites. However, ultrastructural examinations are needed to determine the exact localization of the different GluR subunits in the OPL.
Glutamate Receptor Expression in the Inner Retina
Up to 30 different types of amacrine cells exist in the mammalian
retina, and our data indicate that every ionotropic glutamate receptor
subunit expressed in the rat retina is also expressed by amacrine
cells. Because of considerable differences in the distribution of
glutamate receptor subunits, we suggest that a differential expression
of glutamate receptor subtypes is important for generating functional
heterogeneity among amacrine cells.
Our results indicate that AII amacrine cells express AMPA receptors, which is in agreement with previous electrophysiological findings indicating the presence of functional non-NMDA receptors on AII amacrine cells.19 Electrophysiological evidence for the expression of both NMDA and non-NMDA receptors in some cholinergic amacrine cells25 26 is also supported by our results, because we found expression of all AMPA subunits in cholinergic amacrine cells in the INL. Moreover, virtually all cells in the GCL expressed AMPA receptor subunits, and some displaced amacrine cells in the GCL are reportedly cholinergic.27
We found that the IPL is immunoreactive for all AMPA subunits but
with differences in temporal expression and localization within the
different sublayers. This suggests that there is a rather precise
pattern of stratification of the IPL with respect to the expression of
specific glutamate receptor subunits. GluR1 labeling had already
concentrated in two bands at P3, indicating that this subunit may play
an important role in establishing early synaptic connections in the
inner retina. In contrast, although a first expression of GluR2 to 4
subunits in the IPL was found around P9, distinct bands of labeling
only appeared at approximately P14 which is shortly before eye opening
(
P16). Thus, the main alterations in the expression of AMPA receptor
subunits GluR2 to 4 take place within the period in which bipolar cells
form synapses with postsynaptic dendrites.28
29
Because
all ionotropic glutamate receptor subunits studied so far have been
found exclusively in processes postsynaptic to bipolar cell ribbon
synapses,14
30
31
different types of glutamate receptors
may be involved in the establishment of distinct synaptic connections
and may specify the roles of the different types of bipolar cells in
the adult rat retina. This is supported by our findings that different
subunits seemed to colocalize in distinct bands within the IPL.
All AMPA receptor subunits were expressed in a large number of cells located in the GCL, a finding that is in line with previous in situ hybridization studies.8 9 10 Individual ganglion cells have been shown to coexpress multiple subunits of the AMPA receptor by in situ hybridization on serial semithin sections10 and single-cell RT-PCR.32 In addition, differences in the Ca2+ permeability of AMPA- or kainate-induced whole cell currents were found in rat retinal ganglion cells,33 giving rise to the possibility that AMPA receptor subunits coassemble in heteromeric receptors with distinct functional properties.
In conclusion, our data suggest that different types of AMPA receptors are expressed in different cell types, both in the outer and inner retina. Because alterations in the expression of AMPA subunits occur almost exclusively within the first two postnatal weeks, we propose that they are related to processes of synapse formation in the retina. At the time of eye opening the expression patterns are already very similar to those observed in adult retinas indicating that visual experience prompts no further developmental changes.
| Footnotes |
|---|
Submitted for publication March 10, 2000; revised May 31, 2000; accepted June 23, 2000.
Commercial relationships policy: N.
Corresponding author: Elke Guenther, Experimental Ophthalmology, University Eye Hospital, Röntgenweg 11, D-72076 Tübingen, Germany. elke.guenther{at}uni-tuebingen.de
| References |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
N. J. Sucher, K. Kohler, L. Tenneti, H.-k. Wong, T. Grunder, S. Fauser, T. Wheeler-Schilling, N. Nakanishi, S. A. Lipton, and E. Guenther N-Methyl-D-Aspartate Receptor Subunit NR3A in the Retina: Developmental Expression, Cellular Localization, and Functional Aspects Invest. Ophthalmol. Vis. Sci., October 1, 2003; 44(10): 4451 - 4456. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. S. Correia, C. B. Duarte, C. J. Faro, E. V. Pires, and A. L. Carvalho Protein Kinase Cgamma Associates Directly with the GluR4 alpha -Amino-3-hydroxy-5-methyl-4-isoxazole Propionate Receptor Subunit. EFFECT ON RECEPTOR PHOSPHORYLATION J. Biol. Chem., February 14, 2003; 278(8): 6307 - 6313. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. Haverkamp, U. Grunert, and H. Wassle The Synaptic Architecture of AMPA Receptors at the Cone Pedicle of the Primate Retina J. Neurosci., April 1, 2001; 21(7): 2488 - 2500. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |