Department of Anatomy and Cell Biology, Wayne State University School of Medicine, Detroit, Michigan 48201
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ABSTRACT |
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Pan, Zhuo-Hua and
Hui-Juan Hu.
Voltage-Dependent Na+ Currents in Mammalian
Retinal Cone Bipolar Cells.
J. Neurophysiol. 84: 2564-2571, 2000.
Voltage-dependent
Na+ channels are usually expressed in neurons
that use spikes as a means of signal coding. Retinal bipolar cells are
commonly thought to be nonspiking neurons, a category of neurons in the
CNS that uses graded potential for signal transmission. Here we report
for the first time voltage-dependent Na+ currents
in acutely isolated mammalian retinal bipolar cells with whole cell
patch-clamp recordings. Na+ currents were
observed in ~45% of recorded cone bipolar cells but not in rod
bipolar cells. Both ON and OFF cone bipolar
cells were found to express Na+ channels. The
Na+ currents were activated at membrane
potentials around 50 to
40 mV and reached their peak around
20 to
0 mV. The half-maximal activation and steady-state inactivation
potentials were
24.7 and
68.0 mV, respectively. The time course of
recovery from inactivation could be fitted by two time constants of 6.2 and 81 ms. The amplitude of the Na+ currents
ranged from a few to >300 pA with the current density in some cells
close or comparable to that of retinal third neurons. In current-clamp
recordings, Na+-dependent action potentials were
evoked in Na+-current-bearing bipolar cells by
current injections. These findings raise the possibility that
voltage-dependent Na+ currents may play a role in
bipolar cell function.
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INTRODUCTION |
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Retinal bipolar cells are
second-order neurons that relay the signal from photoreceptors to
amacrine and ganglion cells in the retina. It is commonly believed that
bipolar cells are nonspiking neurons and that signals propagate in a
passive manner in these cells. However, the recent findings of
spontaneous and light-evoked Ca2+-spikes in Mb1
retinal bipolar cells in the goldfish suggest that bipolar cells may be
more excitable than what has been previously thought (Burrone
and Lagnado 1997; Protti et al. 2000
;
Zenisek and Matthews 1998
). Nevertheless
voltage-dependent Na+ currents have not been
previously reported in retinal bipolar cells. However, an early study
found that a portion of mammalian bipolar cells displayed
immunoreactivity to Na+ channel
subunits
(Miguel-Hidalgo et al. 1994
).
Bipolar cells are classified into ON and OFF
types based on their response polarity to light (Kaneko
1970; Werblin and Dowling 1969
). Bipolar cells
are also divided into rod and cone bipolar cells based on their
synaptic inputs. In mammals, only a single type of rod bipolar cells,
ON type, has been reported (Boycott and Dowling
1969
; Boycott and Kolb 1973
; Dacheux and
Raviola 1986
; Greferath et al. 1990
). On the
other hand, multiple subtypes of cone bipolar cells have been reported
(Euler and Wässle 1995
; Famiglietti
1981
; Kolb et al. 1981
; Pourcho and
Goebel 1987
). Most of the previous studies of membrane currents
in bipolar cells were performed in lower vertebrates
(Connaughton and Maguire 1998
; Kaneko and
Tachibana 1985
; Lasater 1988
;
Maguire et al. 1989
; Tessier-Lavigne et al.
1988
). Studies of mammalian bipolar cells were mainly limited
to rod bipolar cells (Gillette and Dacheux 1995
;
Karschin and Wässle 1990
). The properties of
voltage-activated membrane currents in mammalian cone bipolar cells are
less clear.
We previously reported the capability of distinguishing mammalian
rod and cone bipolar cells after enzymatic dissociation (Pan
2000). We characterized and compared voltage-activated membrane channels between these two types of bipolar cells. Here we report that
a portion of cone bipolar cells in the rat retina display voltage-dependent Na+ currents. A brief report of
this work has been presented in abstract form (Pan
1999
).
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METHODS |
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Bipolar cells were isolated from 4-wk-old Long Evans rats by
dissociation methods previously described (Pan 2000
;
Pan and Lipton 1995
). In brief, animals were deeply
anesthetized with CO2 and killed by decapitation.
Retinas were removed and placed in a Hanks' solution (normal Hanks')
containing (in mM) 138 NaCl, 1 NaHCO3, 0.3 Na2HPO4, 5 KCl, 0.3 KH2PO4, 1.25 CaCl2, 0.5 MgSO4, 0.5 MgCl2, 5 HEPES, and 22.2 glucose, with phenol
red, 0.001% vol/vol; adjusted to pH 7.2 with 0.3 N NaOH. The retinas
were incubated for ~50 min at 34-37°C in an enzyme solution that
consisted of the Hanks' solution, supplemented with 0.2 mg/ml
DL-cysteine, 0.2 mg/ml bovine serum albumin, and 1.6 U/ml
papain, adjusted to pH 7.2 with 0.3 N NaOH. Following several rinses in
Hanks' solution, the retinas were mechanically dissociated by gently triturating with a glass pipette. The resulting cell suspension was
plated onto culture dishes. Cells were used for recordings within
5 h after dissociation.
Bipolar cells were identified based on their characteristic morphology
(Karschin and Wässle 1990; Pan and Lipton
1995
; Yeh et al. 1990
). Identification of rod
and cone bipolar cells has been previously described (Pan
2000
). In brief, rod bipolar cells had long and thick axons and
usually retained axon terminals with relative large synaptic boutons.
Their dendritic trees were thick and bush-like. Cone bipolar cells, on
the other hand, had sparser dendritic trees and thinner axons. Synaptic
boutons were small or absent.
Recordings with patch electrodes in the whole cell configuration were
made by standard procedures (Hamill et al. 1981) at room
temperature (20-25°C) with an EPC-9 amplifier and PULSE software (Heka Electronik, Lambrecht/Pfalz, Germany). Electrodes were coated with silicone elastomer (Sylgard, Dow Corning, Midland, MI) and fire-polished. The resistance of the electrode was 7-14 M
. Series resistance ranged from 12 to 40 M
and was not routinely compensated. Cell capacitance was canceled and recorded by PULSE software. In some
recordings, leak currents were subtracted with an on-line P/4 protocol
provided by PULSE software. Most recordings were made either in the
normal Hanks' solution described in the preceding text with or without
Co2+ (4 mM) or in a
high-Ca2+ (10 mM) solution.
Ca2+ was not added in normal Hanks' when
Co2+ was used. The high
Ca2+ solution contained (in mM) 95 NaCl, 5 KCl,
10 CsCl, 20 TEA-Cl, 1 MgCl2, 10 CaCl2, 5 HEPES, and 22.2 glucose, with phenol
red, 0.001% vol/vol; pH 7.2. The electrode solution contained (in mM) 40 CsCl, 80 Cs-acetate, 20 TEA-Cl, 1 MgCl2, 0.5 CaCl2, 5 EGTA, 10 HEPES, 0.5 Na-GTP, and 2 Na-ATP, pH adjusted with CsOH to 7.4. In some recordings, Cs-acetate
was replaced by CsCl. In the recordings of action potentials, the
electrode solution contained (in mM) 133 K-gluconate, 7 KCl, 1 MgCl2, 0.5 CaCl2, 5 EGTA,
10 HEPES, 0.5 Na-GTP, and 2 Na-ATP, pH adjusted with KOH to 7.4. Liquid junction potentials were measured according to the procedure described by Neher (1992)
and corrected. Chemical agents were
applied by local perfusion by gravity-driven superfusion pipettes
placed ~200-300 µm away from the cell being recorded. Tetrodotoxin
(TTX) was purchased from Research Biochemicals (Natick, MA). All other chemicals were purchased from Sigma (St. Louis, MO). Each result reported in this study was based on the observations obtained from at
least five cells unless otherwise indicated.
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RESULTS |
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Na+ currents in morphologically identified cone bipolar cells
When cone bipolar cells were depolarized to 50 mV or more
positively from the holding potential of
70 or
80 mV, fast
transient inward currents were frequently observed. An example of a
morphologically identified cone bipolar cell that displayed such
currents is shown in Fig. 1A.
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The fast transient currents were still present in the recording
solution containing 4 mM Co2+ and without added
Ca2+ (Fig. 1, B and C). The
currents were activated around 50 to
40 mV and reached their peak
around
20 to 0 mV from the holding potential of
80 mV (Fig.
1B). The rapid activation and inactivation of the currents
resemble the typical pattern of Na+ currents.
Furthermore the currents were reversibly blocked by TTX (0.5-1 µM)
as exemplified in Fig. 1C (n > 10). These
results indicate that the fast transient currents are
Na+ currents.
GABAC receptor-mediated currents in Na+-current-bearing bipolar cells
To further ensure that the recorded
Na+-current-bearing cells were bipolar cells, we
examined GABAC receptor-mediated currents in
these cells. This is because bipolar cells but not third-order neurons
in the rat retina were reported to express GABAC
receptors, which are insensitive to bicuculline, a
GABAA receptor antagonist (Euler and
Wässle 1998; Feigenspan et al. 1993
;
Pan 2000
). Over 80 Na+-current-bearing cone bipolar cells were
examined. All of them displayed GABA-evoked currents in the presence of
bicuculline. As a typical example shown in Fig.
2, the presence of
Na+ currents in this cone bipolar cell was
confirmed by depolarizing voltage pulses (Fig. 2A). In the
same cell, co-application of GABA (100 µM) and bicuculline (200 µM)
elicited a sustained inward current when the membrane potential was
held at
80 mV (Fig. 2B). No significant current was
observed when GABA was co-applied with bicuculline in retinal
third-order neurons (data not shown) (see Pan 2000
),
confirming that GABAA receptors were blocked
under our recording conditions. The third-order neurons were identified by their multipolar morphology and, usually, the presence of much larger voltage-activated Na+ currents.
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Ca2+ currents in Na+-current-bearing bipolar cells
Mammalian cone bipolar cells have been reported to express both
low-voltage-activated (LVA) Ca2+ currents and
L-type high-voltage-activated Ca2+ channels
(de la Villa et al. 1998; Hartveit 1999
;
Pan 2000
; Satoh et al. 1998
). We have
previously reported that the LVA Ca2+ currents of
rod and cone bipolar cells display distinct activation and inactivation
kinetics (Pan 2000
). When recordings were made in normal
Hanks', inward currents with slower activation and inactivation than
those of above-described Na+ currents were also
observed in cone bipolar cells. These were Ca2+
currents since the currents were blocked by Co2+
but not by TTX (Fig. 3A). In
many of these cells, however, Na+ currents were
significantly larger than Ca2+ currents when
cells were depolarized from
80 mV and recorded in the physiological
Ca2+ concentration. A typical example is shown in
Fig. 3B. The I-V relationships for the peak
Na+ (
) and Ca2+ (
)
currents are shown in Fig. 3C. The amplitude of the peak Ca2+ currents did not find to show too much
variation among cone bipolar cells with the average value of 17.4 ± 5.6 pA (mean ± SD; n = 9). When recordings
were made in high-Ca2+ (10 mM) solution, as
expected, Ca2+ currents become much larger (also
see Pan 2000
). For comparison, the currents shown in
Fig. 3D were recorded in high-Ca2+
from the same cell shown in Fig. 3B. For this cell, the
Na+ currents were masked by the
Ca2+ currents and were barely noticed. But, for
many other cells, the presence of Na+ currents
could still be observed in high-Ca2+ solution
(data not shown). Voltage-activated Ca2+ currents
in Na+-current-bearing bipolar cells appeared to
be activated at slightly more negative potentials (around
60 to
50
mV) than that of Na+ currents (around
50 to
40 mV as described in the preceding text). Furthermore the
inactivation kinetics of the LVA Ca2+ currents
resembles that of the LVA Ca2+ current of cone
bipolar cells previously described (Pan 2000
).
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Na+ currents in both ON and OFF cone bipolar cells
Since Na+ currents were observed only
in a portion of cone bipolar cells, we investigated whether
Na+ channels were specifically expressed only in
the ON or OFF type of bipolar cells. It has
been known that ON and OFF bipolar cells express different glutamate receptors: metabotropic glutamate receptors
in ON bipolar cells and ionotropic glutamate receptors in
OFF bipolar cells. Particularly activation of the
metabotropic glutamate receptors by
L-2-amino-4-phosphonobutyric acid
(L-AP4), a selective agonist of metabotropic
glutamate receptors, closes cGMP-gated channels (Nawy and Jahr
1990; Shiells and Falk 1990
; Slaughter
and Miller 1981
). Mammalian OFF cone bipolar cells
have been reported to express kainate subtype of glutamate receptors (DeVries and Schwartz 1999
). Thus we examined the
response of Na+-current-bearing cone bipolar
cells to kainate, L-AP4, or glutamate. In these recordings,
cGMP (1 mM) was added in the electrode solution. Cells were usually
held at
70 or
80 mV. Both types of responses were observed among
Na+-current-bearing cone bipolar cells. Figure
4A illustrates a typical example of Na+-current-bearing cone bipolar cells
responding to L-AP4 (2 µM) or glutamate (1 mM) with an
outward current or a decrease of the holding inward current
(n = 31). A typical example of
Na+-current-bearing cone bipolar cells responding
to kainate (300 µM) with an inward current is shown Fig.
4B (n = 19). These results indicate that
Na+ channels are expressed in both ON
and OFF cone bipolar cells.
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Na+-current distribution and amplitude
Na+ currents were observed in ~45% of recorded cone bipolar cells. For example, among 493 recorded cone bipolar cells, 223 of them displayed Na+ currents. In contrast, Na+ currents were never observed in rod bipolar cells (n > 700). Although it was not possible for us to tell whether a cone bipolar cell could have Na+ currents by its morphology, Na+ currents were more frequently observed in the cone bipolar cells with a smaller soma and longer axon. Consistent with this observation, the average whole cell capacitance of cone bipolar cells with Na+ currents was 2.46 ± 0.41 pF (mean ± SD; n = 121). In contrast, the average whole cell capacitance of cone bipolar cells without Na+ currents was 2.74 ± 0.44 pF (n = 109). These two values are significantly different (P < 0.001; by t-test).
Na+ currents were observed in cone bipolar cells that did not retain axon terminals. In a few cases, we were able to identify cone bipolar cells without axon and Na+ currents were also observed in some of these cone bipolar cells. Furthermore we did not notice there were any correlations between the amplitude of the Na+ current and the presence or absence of axon or axon terminals. Our results suggest that the Na+ channels are at least located in the soma.
When recordings were made in normal Hanks', the peak Na+ current of cone bipolar cells ranged from a few to >300 pA. Figure 5 shows the distribution of the peak Na+ currents and the current densities for 28 cone bipolar cells. The Na+ current density was obtained by dividing the peak current by the cell membrane capacitance. The average Na+ current is 93.1 ± 95.8 pA (n = 28). The average Na+ current density is 39.1 ± 44.6 pA/pF (n = 28). As shown in Fig. 5, in some cells, Na+ current densities are >100 pA/pF.
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Voltage dependence of activation and inactivation
Voltage dependence of activation was calculated from the
I-V relationships of the Na+ currents.
The averaged data () for five cells were shown in Fig.
6A (right). The
half-maximum activation potential and the slope factor were
24.7 and
5.9 mV, respectively. Steady-state inactivation was determined by
conditioning pulses ranging from
100 to
30 mV followed by a test
pulse at
10 mV. The averaged data (
) from six cells were also
shown in Fig. 6A (left). The half-maximum
inactivation potential and the slope factor were
68 and 11.8 mV,
respectively.
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The activation and inactivation kinetics of the Na+ currents were determined by measuring the time to peak and the decay time constant at different test potentials. The decay time constants were obtained by fitting the decay current with a single exponential. The average values of the time to peak (n = 8) and decay time constant (n = 8) were plotted versus the test potentials in Fig. 6, B and C.
Recovery from inactivation
The recovery of Na+ currents from
inactivation was determined by a series of paired pulses (Fig.
7A). In each paired-pulse, cells were first depolarized from the holding potential of 80 to
10
mV for 100 ms (condition pulse) to fully inactivate the Na+ currents. Then, after a varied time delay, a
second test pulse (20 ms at
10 mV) was applied. The time course of
recovery was constructed by plotting the ratios of the peak
Na+ currents evoked by the test pulse to that
evoked by the condition pulse versus time delays (n = 8; Fig. 7B). The time course of recovery could be fitted by
a sum of two exponential functions with time constants of 6.2 and 81 ms.
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Na+-dependent action potentials in cone bipolar cells
We further determined whether Na+-dependent
spike activities could be present in cone bipolar cells. The recordings
were made in normal Hanks'. K-gluconate was used in intracellular
solution without blocking K+ currents (see
METHODS for details). Under these recording conditions, Na+-dependent action potentials were observed. An
example of such recordings is shown in Fig.
8. The cone bipolar cell was first recorded in the voltage-clamp mode to confirm the presence of voltage-dependent Na+ currents in this cell (Fig.
8A). Then the recordings were switched to the current-clamp
mode resulted in the cell being clamped at 3.8 pA. Stepwise current
pulses were applied to depolarize the cell from the holding current of
3.8 pA to 5 and 10 pA for 400 ms. A single spike was observed at the
beginning of the depolarization evoked by current injections (Fig.
8B). After an application of 500 nM TTX, the same current
injections did not evoke any spike (Fig. 8C). Similar
results were observed in four other cone bipolar cells.
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DISCUSSION |
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Voltage-dependent Na+ currents in cone bipolar cells
In this study, we report voltage-dependent
Na+ currents in rat retinal bipolar cells with
patch-clamp recordings. Bipolar cells were identified by their
characteristic morphology. Two lines of evidence further ensure that
the Na+-current bearing cells recorded in this
study were bipolar cells. First, these cells showed
bicuculline-insensitive GABA, or GABAC receptor-mediated, responses. GABAC receptors
have been reported to be expressed in bipolar cells but not in
third-order neurons in the rat retina (Euler and Wässle
1998; Feigenspan et al. 1993
; Pan
2000
). Second, we also showed that a portion of these cells responded to L-AP4 or glutamate with a decrease in
conductance, a unique property of ON bipolar cells. Thus
the Na+ currents observed in this study could not
be recorded from retinal third-order neurons.
The bipolar cells displaying Na+ currents were
morphologically identified as cone bipolar cells, which have been
previously described in detail (Pan 2000). In addition,
the LVA Ca2+ currents in these
Na+-current bearing bipolar cells displayed the
characteristic properties of the LVA Ca2+
currents of cone bipolar cells (Pan 2000
), further
supporting that Na+ currents are expressed in
cone bipolar cells.
It should be mentioned that the present study was performed on acutely dissociated bipolar cells. Large Na+ currents were also observed shortly after the dissociation (<20 min). Moreover, Na+ currents were never observed in rod bipolar cells even though a large number of rod bipolar cells were recorded. Therefore it is very unlikely that the Na+ current could arise as some type of culture artifact.
Taken together, our results indicate that voltage-dependent Na+ currents are expressed in cone bipolar cells of the rat retina. Markedly, a significant portion (~45%) of cone bipolar cells was observed to show Na+ currents in this study. The magnitude of the Na+ currents among cone bipolar cells varied widely. The variation was not found to be correlated to the presence or absence of the axon and axon terminals. Furthermore the Na+ currents were found to be expressed in both ON and OFF of cone bipolar cells. However, it remains to be determined whether Na+ channels are expressed in specific subtypes of ON and OFF cone bipolar cells.
The finding of Na+ currents is consistent with an
early report of immunolocalization of Na+ channel
subunits in a portion of cat and monkey bipolar cells (Miguel-Hidalgo et al. 1994
). Surprisingly, however,
this is the first report of voltage-activated Na+
currents in retinal bipolar cells with electrophysiological recordings. Why have voltage-dependent Na+ currents of
bipolar cells not been observed in previous studies? There are several
possible reasons. First, because only a portion of cone bipolar cells
express Na+ currents, these bipolar cells might
have been missed in previous recordings. Second, bipolar cells
displaying Na+ currents recorded in previous
studies might have been considered to be third-order neurons. Finally,
voltage-dependent Na+ currents may only be
expressed in mammalian cone bipolar cells. In fact, most of the
previous studies of membrane currents of bipolar cells were carried out
in lower vertebrates (Connaughton and Maguire 1998
;
Kaneko and Tachibana 1985
; Lasater 1988
;
Maguire et al. 1989
; Tessier-Lavigne et al.
1988
). Studies of mammalian retinal bipolar cells were
previously mainly performed on rod bipolar cells (Gillette and
Dacheux 1995
; Karschin and Wässle 1990
)
which do not express Na+ currents as further
confirmed in this study.
Properties of Na+ currents
The biophysical properties of the Na+
currents in cone bipolar cells are largely similar to that of
Na+ currents reported in retinal third-order
neurons (Barnes and Werblin 1986; Hidaka and
Ishida 1998
; Kaneda and Kaneko 1991
; Lipton and Tauck 1987
). The potential range of the peak
current occurring from
20 to 0 mV is also similar to that of retinal third-order neurons (Barnes and Werblin 1986
;
Lipton and Tauck 1987
). Both the activation and
inactivation are rapid and voltage dependent. In addition, there is an
overlap in the activation and steady-state inactivation curves (from
40 to
30 mV), suggesting Na+ currents may be
constantly activated within this potential range. Recovery of
Na+ currents from inactivation is rapid. Thus a
brief hyperpolarization of membrane potential could partially remove
the inactivation of Na+ channels.
Under normal physiological conditions, the amplitude of
Na+ currents of cone bipolar cells is at least
more than one order of magnitude smaller than that of retinal ganglion
cells. However, since bipolar cells are much smaller than retinal
ganglion cells, the difference in the Na+ current
density between cone bipolar cells and retinal third-order cells, such
as ganglion cells, does not appear to be so large. Na+ current densities in retinal ganglion cells
were reported to be 100-300 pA/pF (Hidaka and Ishida
1998; Lipton and Tauck 1987
). In fact,
Na+ current densities observed in some cone
bipolar cells in this study are close to or comparable to these values
(see Fig. 5). Bipolar cells with such a large Na+
current density would be expected to generate Na+
spikes. Indeed, we showed in this study that
Na+-dependent action potentials could be evoked
by current injections.
However, under our vitro current-clamp recording conditions, only a
single spike could be observed. Interestingly, such property resembles
that of amacrine cells in vivo. As having been demonstrated previously
for retinal amacrine cells (Eliasof et al. 1987), the lack of the capability to fire a series of action potentials could be
due to the intrinsic properties of the channel itself and/or other
membrane currents, such as voltage-activated K+
and Ca2+-activated currents.
The dark membrane potentials of bipolar cells are believed to be around
40 to
45 mV with ON bipolar cells depolarizing and OFF bipolar cells hyperpolarizing in responding to light
stimulation. The steady-state inactivation curve of the
Na+ current suggests that most of the
Na+ channels in ON bipolar cells
would be in the inactivated state(s). On the other hand, the
light-evoked hyperpolarization may partially remove the inactivation of
the Na+ channels in OFF bipolar
cells. Therefore the voltage-activated Na+
channels would be more likely to be activated in OFF cone
bipolar cells in vivo.
Functional implications of expression Na+ channels in bipolar cells
What might be the possible role of expression Na+ channels in bipolar cells? First, as mentioned in the preceding text, because there is an overlap in the activation and steady-state inactivation curves, Na+ channels might be constantly activated around bipolar cell resting membrane potentials, which could support cells' electrical excitability. Second, activation of Na+ channels, or Na+ spikes, in bipolar cells may speed up the membrane depolarization and shape light-response waveform and, in turn, affect Ca2+ current activation and transmitter release at the axon terminals.
Furthermore Na+ channels might play a role in
bipolar cell signal propagation. It is commonly thought that the space
constant of bipolar cells is much longer than the axons of bipolar
cells and, thus the membrane potentials can propagate from dendrites to
axon terminals passively without significant loss. However, at least in
rats, both the dendrites and axons of most cone bipolar cells after
dissociation appeared to be markedly thinner than that of rod bipolar
cells (Pan 2000). In addition, membrane resistance of
bipolar cells in vivo has been reported to be lower than the value
obtained in vitro probably due to cell coupling and constant activation
of membrane ion channels (Tessier-Lavigne et al. 1988
). Together these factors could significantly reduce the space constant for certain groups of bipolar cells. Particularly, as mentioned in the
preceding text, voltage-dependent Na+ currents
were more frequently observed in cone bipolar cells with smaller soma
and longer axon. Therefore it is possible that the expression of
Na+ channels in bipolar cells may serve as a
mechanism for boosting or facilitating membrane potential propagation.
However, a previous study reported that co-application of TTX and
cadmium was not found to show effect on the multi-quantal release of
glutamate from mouse bipolar cells in light-adapted retinal slices
(Tian et al. 1998). This appears to suggest that retinal
bipolar cells don't generate spontaneous Na+ or
Ca2+ activities under the studied conditions. On
the other hand, a recent study reported that Mb1-type bipolar cells in
dark-adapted goldfish retinal slices were capable of generating
light-evoked Ca2+ spikes (Protti et al.
2000
). Further study will be needed to determine the functional
role of the expression of voltage-gated Na+
currents in bipolar cells. Particularly it would be interesting to
determine whether Na+ currents could shape the
light response or generate light-evoked Na+
spikes in mammalian cone bipolar cells in vivo. Furthermore, experiments with paired recordings of bipolar cells and third-order neurons in retinal slices may be able to determine whether
Na+ currents play a role in bipolar cell signal propagation.
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ACKNOWLEDGMENTS |
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This research was supported by National Eye Institute Grant EY-12180.
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FOOTNOTES |
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Address for reprint requests: Z.-H. Pan, Dept. of Anatomy and Cell Biology, Wayne State University School of Medicine, 540 E. Canfield Ave., Detroit, MI 48201 (E-mail: zhpan{at}med.wayne.edu).
Received 14 December 1999; accepted in final form 8 August 2000.
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REFERENCES |
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