The Journal of Neuroscience, August 6, 2003, 23(18):6965-6971
Previous Article | Next Article 
Prolonged Photoresponses and Defective Adaptation in Rods of G
5-/- Mice
Claudia M. Krispel,1,2
Ching-Kang Chen,3
Melvin I. Simon,4 and
Marie E. Burns1,2
1Center for Neuroscience and
2Department of Psychiatry, University of California,
Davis, Davis, California 95616, 3Department of
Ophthalmology, Eccles Institute of Human Genetics, University of Utah, Salt
Lake City, Utah 84112, and 4Division of Biology,
California Institute of Technology, Pasadena, California 91125
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Abstract
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Timely deactivation of G-protein signaling is essential for the proper
function of many cells, particularly neurons. Termination of the light
response of retinal rods requires GTP hydrolysis by the G-protein transducin,
which is catalyzed by a protein complex that includes regulator of G-protein
signaling RGS9-1 and the G-protein 
subunit G5-L. Disruption of
the G5 gene in mice (G5-/-) abolishes the expression of
G5-L in the retina and also greatly reduces the expression level of
RGS9-1. We examined transduction in dark- and light-adapted rods from
wild-type and G5-/- mice. Responses of G5-/-
rods were indistinguishable in all respects from those of RGS9-/-
rods. Loss of G5-L (and RGS9-1) had no effect on the activation of the
G-protein cascade, but profoundly slowed its deactivation and interfered with
the speeding of incremental dim flashes during light adaptation. Both
RGS9-/- and G5-/- responses were consistent with
another factor weakly regulating GTP hydrolysis by transducin in a manner
proportional to the inward current. Our results indicate that a complex
containing RGS9-1-G5-L is essential for normal G-protein deactivation
and rod function. In addition, our light adaptation studies support the notion
than an additional weak GTPase-accelerating factor in rods is regulated by
intracellular calcium and/or cGMP.
Key words: phototransduction; G-protein; transducin; adaptation; RGS; calcium; cGMP
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Introduction
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Regulator of G-protein signaling (RGS) proteins stimulate GTP hydrolysis by
G-protein 
subunits. RGS genes are expressed in unique patterns
throughout the brain (Gold et al.,
1997
), suggesting that their physiological functions are tissue
specific. Several members of the RGS family (RGS6, -7, -9, and -11) contain
G-protein 
-like (GGL) domains through which they strongly interact with
a neuronal G-protein subunit called G5
(Watson et al., 1996).
Despite the seemingly clear function of RGS proteins in vitro,
surprisingly little is known about how these proteins might regulate the time
course of G-protein signaling in neurons. An exceptional model system for
studying signal transduction in real time is the phototransduction cascade of
retinal rod photoreceptors, in which the inward current through cGMP-gated ion
channels in the plasma membrane can be used to measure the activation and
deactivation of the cascade (Pugh and
Lamb, 2000). Transduction begins with the absorption of a photon
by the G-protein-coupled receptor rhodopsin in the outer segment of the rod.
Photoexcited rhodopsin catalyzes the exchange of GDP for GTP on the
subunit of the G-protein transducin (T). Each activated T binds
to the subunit of cGMP phosphodiesterase (PDE), removing the
inhibition by PDE of the catalytic subunits of PDE
(Hurley and Stryer, 1982). As
long as GTP-T is bound to PDE, the catalytic subunits of PDE are
free to hydrolyze cGMP. Because cGMP is responsible for gating the rod
membrane cation channels, the decrease of cGMP concentration results in the
closure of some of the channels, leading to a decrease in inward current. The
reduction in inward current leads to a hyperpolarization of the cell membrane
that passively spreads to the synaptic terminal, where it causes a reduction
in neurotransmitter release.
Like all G-proteins, T requires GTP hydrolysis for deactivation.
Upon hydrolyzing GTP, T releases PDE, which then re-inhibits the
catalytic subunits of PDE. In photoreceptors, GTP hydrolysis by T is
speeded by RGS9-1 (He et al.,
1998), a short, membrane-associated, and photoreceptor-specific
isoform of the RGS9 gene. RGS9-1 stimulates GTP hydrolysis preferentially when
GTP-T is bound to PDE (Tsang
et al., 1998), because the T-PDE complex has a higher
affinity for RGS9 (Skiba et al.,
2000). RGS9-1, like other members of its subfamily (for review,
see Cowan et al., 2001),
contains a GGL domain between the N terminal and the RGS homology domain
(Snow et al., 1998;
Kovoor et al., 2000;
Lishko et al., 2002) that
associates with G5. The catalytic activity of RGS9-1 is greatly enhanced
by the association of G5-L (Makino
et al., 1999), the retina-specific long-splice variant of the
G5 gene, as well as a protein anchor that mediates its attachment to the
disc membrane (Lishko et al.,
2002; Hu et al.,
2003).
Previous work demonstrated that photoreceptors of mice lacking functional
RGS9 genes (RGS9-/-) show abnormally slowed recovery of their light
responses (Chen et al., 2000;
Lyubarsky et al., 2001) and
slowed rates of GTP hydrolysis by transducin
(Chen et al., 2000). In
addition, retinas of RGS9-/- mice do not express G5-L,
despite abundant G5-L mRNA. Similarly, disruption of the G5 gene
(G5-/-) results in very low (<5% normal) levels of RGS9-1,
and undetectable levels of other subfamily members RGS6, RGS7, or RGS11 in the
retina, despite the abundance of mRNA transcripts for these proteins
(Chen et al., 2003). Thus, the
G5-/- rods not only lack G5-L but also essentially lack
RGS9-1 and other RGS proteins that may be present in photoreceptors at low
levels.
The purpose of this study was to quantitatively assess transduction in the
G5-/- rods and to compare them with the known characteristics
of RGS9-/- responses.
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Materials and Methods
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Suction electrode recording. Mice were cared for and handled
following an approved protocol from the Animal Care and Use Committee of
University of California, Davis, and in compliance with National Institutes of
Health guidelines for the care and use of experimental animals. Mice were
housed in 12 hr cyclic light and were dark-adapted overnight before an
experiment. Under infrared light, animals were anesthetized and euthanized,
and the retinas removed and stored in L15 solution with 10 mM
glucose and 0.1 mg/ml bovine serum albumin on ice. Retinas were then chopped
in a chopping dish with a razor blade and placed in the recording chamber. The
recording chamber was perfused with a solution containing (in mM):
112.5 NaCl, 3.6 KCl, 2.4 MgCl2, 1.2 CaCl2, 10 HEPES, 0.2
EDTA, 20 sodium bicarbonate, 3 disodium succinate, and 10 glucose. The
solution was bubbled with 95% O2-5% CO2 oxygen and
warmed to 35-37°C, and the pH was adjusted to 7.4 with KOH. Small pieces
of retina were visualized with a sensitive CCD camera (Stanford Photonics,
East Palo Alto, CA) using infrared light. Individual mouse rods were drawn
into a glass electrode containing (in mM): 140 NaCl, 3.6 KCl, 2.4
MgCl2, 1.2 CaCl2, 3 HEPES, 0.2 EDTA, and 10 glucose. The
pH of this solution was also 7.4 at 37°C. The bath and suction electrodes
were connected to calomel half-cells by agar bridges, and the bath voltage was
maintained at ground potential by an active feedback circuit. The rod membrane
current was amplified (Axopatch 1B; Axon Instruments, Foster City, CA) and
filtered at 20 Hz with an eight pole Bessel filter. Data was digitized
continuously at 200 Hz using NiDAQ (National Instruments, Austin, TX) for
IgorPro (Wavemetrics, Lake Oswego, OR). Tissue in the chamber was presented
with 10 msec flashes of 500 nm light either in darkness or in the presence of
steady, 520 nm light. Light intensity was controlled by calibrated neutral
density filters, and at the end of each experiment, the lamp power was
measured at 500 and 520 nm light using a silicon photodiode detector (United
Detector Technology, Baltimore, MD). We could detect only minimal qualitative
morphological differences between wild-type and the G5-/-
rods, with the G5-/- rod outer segments seeming slightly
shorter and of slightly larger diameter. The resting dark currents of
wild-type and G5-/- rods were not significantly different
(Table 1).
The average response to a large number (>20) of flashes was considered
to be in the linear range if its mean amplitude was <25% of the maximal
response amplitude. These dim flash responses were used to estimate the form
of the single photon response using the variance-to-mean method, as previously
described (Mendez et al.,
2000). Briefly, the mean dim flash response was squared and scaled
until its rising phase coincided with the rising phase of the ensemble
time-dependent variance. Assuming that the predominant source of variance at
this early time in the response arises from Poisson fluctuations in the number
of photoisomerizations, the scaling factor that brings these two traces into
alignment is proportional to 1/n, where n is the mean number
of photoisomerizations per flash. The mean response was then divided by
n to yield the form of the single photon response. The small
underestimation introduced by the inclusion of cells whose dim flash responses
exceeded 15% of the maximal amplitude is within experimental error (SEM).
Integration time was used as a measure of the duration of the incremental
flash response and is defined as the time integral of the average linear
response divided by its peak amplitude
(Baylor and Hodgkin, 1973). The
time that a bright flash response remained in saturation was calculated as the
time interval between the midpoint of the flash and the time at which the
current recovered by 10%.
Adaptation experiments. Adaptation was assessed by comparing the
responses of wild-type and G5-/- rods with flashes in
darkness and in the presence of steady light. Cells were presented with test
flashes in darkness, then in the presence of a background light, and then
again in darkness. Cells were kept for analysis if the dark currents measured
before and after the steady light did not vary by >20%. We define dim
background light as those intensities that turned off between 10 and 49% of
the dark current in the steady state.
Time course of light-activated PDE activity. Time course of
light-activated PDE activity [P*(t)] was calculated
according to Pugh and Lamb,
(1993):
where r(t) is the time course of the response,
rmax is the maximal response amplitude to a bright flash,
and n is the cooperativity of the cGMP-gated channels. We used this
equation to calculate P*(t), assuming n = 3
(Haynes et al., 1986;
Zimmerman and Baylor, 1986).
The initial rate of change of the light-activated PDE activity
(dP*/dt) was determined by linear regression of the initial
rate of change of P*(t) as described in Tsang et al.
(1998). The corner frequency
of the low-pass filter (20 Hz) may have contributed to the saturation of the
rate of change of P*(t) in response to bright flashes
(>1000 photons/µm2) but did not interfere with the rising
phases of the responses over most of the experimental range of flash strengths
presented in Figure
2c.
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Results
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Loss of G5 slows recovery of dim-flash responses
To study the effects of inactivating the G5 gene on
phototransduction, we used suction electrodes to record the light responses of
rods from wild-type mice (G5+/+) and rods from G5
hemizygous (G5+/-) and homozygous (G5-/-)
knock-out mice. Representative families of responses from such rods across a
wide range of flash strengths are shown in
Figure 1. There were no
detectable differences in the amplitude or kinetics of wild-type and
G5+/- rods (Fig.
1, Table 1),
consistent with the normal level of expression of both RGS9-1 and G5-L
in these retinas (Chen et al.,
2003). However, the responses of G5-/- rods
showed a specific defect in the recovery phase of the flash response
(Fig. 1). Exponential functions
fit to the final falling phases of the average dim flash responses of
G5-/- rods had a time constant >10-fold longer than those
fitted to wild-type responses (Table
1). These values are very similar to those of dim flash responses
in RGS9-/- rods (Chen et al.,
2000) (see Fig. 3)
consistent with the complete functional loss of the RGS9-1-G5-L complex
in the G5-/- rods.
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Figure 1. Families of responses to increasing flash strengths from representative
wild-type, knock-out, and hemizygote mice. Responses have been normalized
(r/rmax) by the maximal response amplitudes, which were
the following (in pA): 16.3 (+/+), 16.1 (-/-), and 17.7 (+/-). Flash strengths
(in photons/µm2) ranged from 11 to 4546 (+/+), 11 to 8563 (+/-),
and 19 to 8277 (-/-).
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As observed previously in RGS9-/- responses, the defect in the
G5-/- responses was limited to the recovery phases of the
responses. There were no significant differences in the sensitivity of these
dark-adapted rods: the flash sensitivity [as measured in
pA/(photons/µm2)], single photon response amplitude, and the
flash strength needed to elicit a half-maximal response (in
photons/µm2) were also very similar in each group of mice
(Table 2). Furthermore, the
rising phases of the single photon response in all three lines of mice were
indistinguishable (Fig.
2a,b). This suggests that the loss of the
RGS9-1-G5-L protein complex in G5-/- rods does not
affect the activation or the amplification of the G-protein cascade. To
further test this idea, we calculated the rate of change of PDE activity from
the rising phases of the responses to a wide range of flash strengths using
the method developed by Pugh and Lamb
(1993) (see Materials and
Methods). The light-activated change in PDE activity was similar for rods from
each line of mouse (Fig.
2c), supporting the idea that there were no changes in
the activities or level of expression of any of the proteins involved in the
activation stages of the cascade.
Loss of G5 more strongly affects recovery of bright-flash
responses than dim-flash responses
In normal rods, increasing the strength of a saturating flash results in
responses that remain in saturation for longer times. Because the cGMP and
calcium levels both fall to a minimum during the time that the response is
saturated, a calcium- and cGMP-independent measurement of the rate limiting,
or dominant, time constant of saturating response recovery can be found by
plotting the time that responses remain in saturation as a function of the log
of the flash strength (Pepperberg et al.,
1992; Lyubarsky et al.,
1996). This time constant was 0.2 sec for the wild-type rods and 9
sec for the G5-/- rods
(Table 1). These results are
very similar to results of previous studies of the RGS9-/- mice
(Chen et al., 2000) and are
consistent with the RGS9-/- mice examined in this study for direct
comparison (Fig. 3). Thus,
whereas there was a 
10-fold difference in recovery kinetics of the dim
flash responses of G5-/- and wild-type rods, there was a
50-fold difference in the time constant of recovery of bright flash
responses.
The profound slowing of recovery in the G5-/- bright flash
responses was not irreversible. Instead, the response recovery appeared to
speed up as the current was restored (Fig.
3c). In contrast, saturating responses of wild-type mouse
rods recovered with an exponential time constant of 0.2 sec throughout the
entire falling phase of the response (data not shown)
(Chen et al., 2000), as
evidenced by the similarity between the dim flash and bright flash recovery
time constants (both 0.2 sec). The acceleration of recovery that we observed
in G5-/- rods also occurs in RGS9-/- responses
(Fig. 3c)
(Chen et al., 2000). This
suggests that none of the other RGS proteins that are lacking in the
G5-/- mice (RGS6, -7, and -11) contribute to the deactivation
of T in the absence of RGS9. In addition, the residual RGS9-1 (<5%)
found in G5-/- retinal homogenates
(Chen et al., 2003) does not
accelerate transducin GTP hydrolysis under our experimental conditions (see
Discussion).
Our previous modeling work suggested that the progressive acceleration of
recovery in the absence of RGS9-1 could result if the rate of T
deactivation was regulated by calcium or cGMP
(Chen et al., 2000) (see also
Discussion). We therefore investigated the effect of sustained reduction in
calcium and cGMP on the kinetics of the responses of G5-/-
rods, such as occurs during light adaptation.
Adaptation of wild-type and
G5-/- rods
Continuous illumination causes photoreceptors to adapt; that is, they
undergo a drastic decrease in sensitivity and a speeding of the incremental
response kinetics (Baylor and Hodgkin,
1973; Fain et al.,
1989). Both of these characteristics decrease the steady-state
response to continuous illumination. Because the recovery kinetics change with
the amount of inward current in the G5-/- rods (see above),
we chose to focus our studies to a narrow range of light intensities that
caused similar changes in the circulating currents in wild-type and
G5-/- rods. The average percentage of dark current turned off
by the background light was similar for both wild-type and knock-out mice.
Assuming that the knockout and wild-type rods have channels with similar
sensitivities for cGMP and have similar Na+-Ca2+,
K+ exchanger activities, both groups experienced on average the
same fall in intracellular messengers cGMP and Ca2+.
During dim continuous illumination that turned off approximately
one-quarter of the dark current (wild-type rods, 24 ± 6%, n =
7; G5-/- rods, 29 ± 5%, n = 7), the
incremental flash sensitivity of both wild-type and G5-/-
rods was reduced to similar extents (wild-type rods, mean ± SEM, 71
± 12%; n = 7; G5-/- rods, 56 ± 4;
n = 7). However, there was a striking difference in the effect of
light adaptation on the time course of the response. In the presence of dim
background lights, the integration times of the wild-type dim-flash response
(n = 7) shortened slightly, on average to 89 ± 9% of their
dark values (Fig. 4), with a
few of these cells having no detectable change in integration time
(B). In contrast, the integration times of all of the knock-out rods
(seven of seven cells) significantly increased in the presence of background
lights to an average of 167 ± 12% of their dark values (n = 7)
(Fig. 4). Similar results were
also observed with the RGS9-/- rods in the presence of background
lights (n = 2; data not shown). We interpret this to indicate that in
the absence of the RGS9-1-G5-L complex, the suppression of current by
background light further slows GTP hydrolysis in a manner that interferes with
the speeding up of the incremental response that usually occurs during
adaptation (see Discussion).
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Figure 4. Impaired adaptation of dim flash responses in G5-/- rods.
a, Average dim flash response of representative
G5+/+ (left) and G5-/- (right) rods before,
after, and during steady light exposure. The intensities of the background
light were 30 photons/µm2/sec (G5+/+) and 7.2
photons/µm2/sec (G5-/-), which turned off 10
and 16% of the original dark current, respectively. The G5+/+
integration times shortened slightly in the presence of background light,
whereas the G5-/- integration times slowed significantly in
the presence of background light. Dim flash responses were normalized
(r') by peak amplitude for comparison of response durations
(integration time) (see Materials and Methods). Dark currents (in pA) were
15.2 (G5+/+) and 16.0 (G5-/-). In this
example, flash strengths were 10 photons/µm2 in the presence and
absence of background light (G5+/+), and 10
photons/µm2 in darkness and 19 photons/µm2 in
background light (G5-/-). b, Integration times from
G5+/+ (left) and G5-/- (right) dim flash
responses before, during, and after exposure to steady light. The background
light intensities ranged from 30 to 1340 photons/µm2/sec for the
wild-type rods, turning off 10-49% of the original dark current. The
background intensities for the G5-/- rods ranged from 5.9 to
25 photons/µm-2/sec, turning off 16-40% of the original dark
current. c, Average fractional change in integration times during
light adaptation for the cells in b. The integration time in the dark
is the average of the integration times before and after background light.
Error bars represent SEM.
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In addition to accelerating the incremental dim flash response, the time
that a bright flash response remains in saturation is also normally shorter in
the presence of background light (Baylor
and Hodgkin, 1973; Fain et al.,
1989; Matthews,
1995) (Fig.
5a). This form of adaptation is also observed in mouse
rods; in our experiments, on average, the saturation time of a wild-type
response shortened to 68 ± 8% of the dark value in the presence of dim
background lights that turned off 24 ± 6% of the dark current
(n = 7) (Fig.
5c). A similar decrease of saturation times (to 63
± 6% of the dark value) was also seen for the knock-out rods in the
presence of background light that turned off comparable current levels (28
± 3%; n = 5). In addition, the presence of background light
had no effect on the dominant time constant of recovery from saturating
flashes in wild-type rods, nor in G5-/- rods (data not
shown), consistent with previous reports
(Lyubarsky et al., 1996;
Calvert et al., 2002). Thus,
unlike the dim incremental responses that show slowed deactivation during
steady light, bright flash responses adapt normally and show no change in the
dominant time constant of recovery, indicating that this adaptation mechanism
is unaffected by the loss of the RGS9-1-G5-L complex.
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Figure 5. Adaptation of bright-flash responses in G5-/- rods.
a, Saturating responses of representative G5+/+
(left) and G5-/- (right) rods before, after, and during
background light exposure. The intensities of the background lights (in
photons/µm2/sec) were 340 (G5+/+) and 17
(G5-/-), and the flash strengths (in darkness and in the
presence of the background light) were 2452 (G5+/+) and 788
(G5-/-) photons/µm2. The time that both
G5+/+ and G5-/- responses remained in
saturation shortened in the presence of background light. b, Time
spent in saturation (Tsat) for G5+/+
(left) and G5-/- (right) rods before, during, and after
exposure to background light. The background light intensities ranged from 30
to 1340 photons/µm2/sec for wild-type rods, turning off 10-49%
of the original dark current. The background intensities for
G5-/- rods ranged from 7.1 to 25
photons/µm2/sec, turning off 16-37% of the original dark
current. c, Average fractional change in time spent in saturation
during adaptation for the cells in b. The time in saturation in the
dark is the average of the saturation times before and after background light.
Error bars represent SEM.
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Discussion
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Our experiments support the notion that the RGS9-1-G5-L complex is
essential for normal, rapid recovery of rod photo-responses. Although Western
blots of retinal homogenates suggest that G5-/- rods express
5% normal levels of RGS9-1 (Chen et
al., 2003), the similarities between the responses of the
RGS9-/- rods and the G5-/- rods lead us to
conclude that there is no functional expression of RGS9-1 in the outer
segments of the G5-/- rods. Thus, just as G5-L is not
expressed in RGS9-/- rods, RGS9-1 is functionally absent without
G5-L. This supports the hypothesis that RGS9-1 and G5-L are
obligate binding partners (Makino et al.,
1999) whose activity and targeting are inextricably linked
(Chen et al., 2000;
Kovoor et al., 2000;
Witherow et al., 2000;
Chen et al., 2003).
The G5+/- rods yielded responses that were not
significantly different from those of wild-type rods, consistent with the
observation that retinas of G5+/- mice contain normal levels
of both G5-L and RGS9-1 protein (Chen
et al., 2003). It has been proposed that the abundance of RGS9-1
in cones may contribute to their faster recovery kinetics
(Cowan et al., 1998). This
attractive idea has still to be supported, and future experiments will test
this idea in rods by expressing intermediate and excess levels of
RGS9-1-G5-L and determining whether there are corresponding changes in
the recovery rates of the flash responses.
Bright flash responses of both G5-/- and
RGS9-/- rods initially recovered slowly, but the recovery
accelerated as the current returned. This was previously found to be
consistent with a simple model whereby the rate of transducin deactivation
varied linearly with the inward current and led to the hypothesis that
RGS9-/- rods contained an additional calcium- or cGMP-dependent
factor with weak GTPase-accelerating activity
(Chen et al., 2000). This same
model also fits well to response families of G5-/- rods (data
not shown), further supporting the notion that, in the absence of
RGS9-1-G5-L, another factor can speed GTP hydrolysis in the dark.
Because G5-/- retinas also do not express RGS6, -7, and -11
(Chen et al., 2003), these less
abundant retinal RGS proteins cannot be the factor.
Additional evidence for the calcium and/or cGMP dependence of this factor
is the slowing of the incremental response of G5-/- rods in
the presence of steady light. Normally, light adaptation speeds the kinetics
of incremental flash responses (Baylor and
Hodgkin, 1973; Fain et al.,
1989; Pugh et al.,
1999). The mechanisms responsible for the speeding of the adapted
dim incremental response are not entirely known
(Baylor and Hodgkin, 1973;
Fain et al., 1989;
Pugh et al., 1999), although
recent evidence suggests that the increased steady-state PDE activity in the
presence of steady light can account for a great deal of the kinetic changes
(Nikonov et al., 2000). Here,
we showed that the G5-/- responses do not speed up in the
presence of background light, indicating that the usual mechanisms—such
as the increased steady-state PDE activity—are swamped out by slowed
deactivation mechanisms. Somehow, continuous light that lowers calcium and
cGMP levels further slows deactivation (and thus GTP hydrolysis) in
RGS9-/- and G5-/- rods.
Saturating responses of wild-type rods shortened in the presence of
background light as expected. In amphibian rods, the shortening of the time in
saturation requires a fall in intracellular calcium at or near the time of the
flash (Matthews, 1997). The
fall in calcium that accompanies light adaptation exerts numerous effects on
the cascade, all of which might be expected to make the response come out of
saturation sooner. The shortening of the time in saturation has little
(Calvert et al., 2002) or no
effect (Lyubarsky and Pugh,
1996; Lyubarsky et al.,
1996) on the dominant time constant of recovery in amphibians.
Likewise, in our experiments on wild-type mouse rods, we also observed that
the dominant time constant was not significantly different between darkness
and in the presence of background lights. Saturating responses from
G5-/- rods also shorten in the presence of background lights,
suggesting that this aspect of adaptation is functioning normally in knock-out
rods.
The similarity of G5-/- and RGS9-/- responses
during adaptation further supports the idea that both RGS9-1 and G5-L
are necessary for proper function. Our experiments have also strengthened the
hypothesis that an additional weak GTPase-accelerating factor exists in rod
photoreceptors and operates in the dark when both calcium and cGMP levels are
high. Future experiments will further investigate the identity and biochemical
regulation of this putative factor.
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Footnotes
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Received Feb. 26, 2003;
revised Jun. 3, 2003;
accepted Jun. 5, 2003.
This work was supported by the National Institutes of Health Vision Core
Grant EY12576-01 to University of California, Davis; University of California,
Davis, School of Medicine MD/PhD program (C.M.K.); and individual National
Institutes of Health Grants EY14047-01 (M.E.B.) and AG12288 (M.I.S.). M.E.B.
is an Alfred P. Sloan research fellow.
Correspondence should be addressed to Dr. Marie E. Burns, Center for
Neuroscience, University of California, Davis, 1544 Newton Court, Davis, CA
95616. E-mail:
meburns{at}ucdavis.edu.
Copyright © 2003 Society for Neuroscience
0270-6474/03/236965-07$15.00/0
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Top
Abstract
Introduction
) as indicated. Dark currents were 17.1 pA (G
