Prolongation of actions of Ca2+ early in phototransduction by 9-demethylretinal

During adaptation Ca2+ acts on a step early in phototransduction, which is normally available for only a brief period after excitation. To investigate the identity of this step, we studied the effect of the light-induced decline in intracellular Ca2+ concentration on the response to a bright flash in normal rods, and in rods bleached and regenerated with 11-cis 9-demethylretinal, which forms a photopigment with a prolonged photoactivated lifetime. Changes in cytoplasmic Ca2+ were opposed by rapid superfusion of the outer segment with a 0Na+/0Ca2+ solution designed to minimize Ca2+ fluxes across the surface membrane. After regeneration of a bleached rod with 9-demethlyretinal, the response in Ringer's to a 440-nm bright flash was prolonged in comparison with the unbleached control, and the response remained in saturation for 10-15s. If the dynamic fall in Ca2+i induced by the flash was delayed by stepping the outer segment to 0Na+/0Ca2+ solution just before the flash and returning it to Ringer's shortly before recovery, then the response saturation was prolonged further, increasing linearly by 0.41 +/- 0.01 of the time spent in this solution. In contrast, even long exposures to 0Na+/0Ca2+ solution of rods containing native photopigment evoked only a modest response prolongation on the return to Ringer's. Furthermore, if the rod was preexposed to steady subsaturating light, thereby reducing the cytoplasmic calcium concentration, then the prolongation of the bright flash response evoked by 0Na+/0Ca2+ solution was reduced in a graded manner with increasing background intensity. These results indicate that altering the chromophore of rhodopsin prolongs the time course of the Ca2+-dependent step early in the transduction cascade so that it dominates response recovery, and suggest that it is associated with photopigment quenching by phosphorylation.     

A novel Ca2+-feedback mechanism extends the operating range of mammalian rods to brighter light

Sensory cells adjust their sensitivity to incoming signals, such as odor or light, in response to changes in background stimulation, thereby extending the range over which they operate. For instance, rod photoreceptors are extremely sensitive in darkness, so that they are able to detect individual photons, but remain responsive to visual stimuli under conditions of bright ambient light, which would be expected to saturate their response given the high gain of the rod transduction cascade in darkness. These photoreceptors regulate their sensitivity to light rapidly and reversibly in response to changes in ambient illumination, thereby avoiding saturation. Calcium ions (Ca²⁺) play a major role in mediating the rapid, subsecond adaptation to light, and the Ca²⁺-binding proteins GCAP1 and GCAP2 (or guanylyl cyclase–activating proteins [GCAPs]) have been identified as important mediators of the photoreceptor response to changes in intracellular Ca²⁺. However, mouse rods lacking both GCAP1 and GCAP2 (GCAP^−/−) still show substantial light adaptation. Here, we determined the Ca²⁺ dependency of this residual light adaptation and, by combining pharmacological, genetic, and electrophysiological tools, showed that an unknown Ca²⁺-dependent mechanism contributes to light adaptation in GCAP^−/− mouse rods. We found that mimicking the light-induced decrease in intracellular [Ca²⁺] accelerated recovery of the response to visual stimuli and caused a fourfold decrease of sensitivity in GCAP^−/− rods. About half of this Ca²⁺-dependent regulation of sensitivity could be attributed to the recoverin-mediated pathway, whereas half of it was caused by the unknown mechanism. Furthermore, our data demonstrate that the feedback mechanisms regulating the sensitivity of mammalian rods on the second and subsecond time scales are all Ca²⁺ dependent and that, unlike salamander rods, Ca²⁺-independent background-induced acceleration of flash response kinetics is rather weak in mouse rods.     

Physiological and microfluorometric studies of reduction and clearance of retinal in bleached rod photoreceptors

The visual cycle comprises a sequence of reactions that regenerate the visual pigment in photoreceptors during dark adaptation, starting with the reduction of all-trans retinal to all-trans retinol and its clearance from photoreceptors. We have followed the reduction of retinal and clearance of retinol within bleached outer segments of red rods isolated from salamander retina by measuring its intrinsic fluorescence. Following exposure to a bright light (bleach), increasing fluorescence intensity was observed to propagate along the outer segments in a direction from the proximal region adjacent to the inner segment toward the distal tip. Peak retinol fluorescence was achieved after approximately 30 min, after which it declined very slowly. Clearance of retinol fluorescence is considerably accelerated by the presence of the exogenous lipophilic substances IRBP (interphotoreceptor retinoid binding protein) and serum albumin. We have used simultaneous fluorometric and electrophysiological measurements to compare the rate of reduction of all-trans retinal to all-trans retinol to the rate of recovery of flash response amplitude in these cells in the presence and absence of IRBP. We find that flash response recovery in rods is modestly accelerated in the presence of extracellular IRBP. These results suggest such substances may participate in the clearance of retinoids from rod photoreceptors, and that this clearance, at least in rods, may facilitate dark adaptation by accelerating the clearance of photoproducts of bleaching.     

Electrical and adaptive properties of rod photoreceptors in bufo marinus. I. Effects of altered extracellular Ca(2+) levels

The effects of altering extracellular Ca(2+) levels on the electrical and adaptive properties of toad rods have been examined. The retina was continually superfused in control (1.6 mM Ca(2+)) or test ringer’s solutions, and rod electrical activity was recorded intracellularly. Low-calcium ringer’s (10(-9)M Ca(2+)) superfused for up to 6 min caused a substantial depolarization of the resting membrane potential, an increase in light-evoked response amplitudes, and a change in the waveform of the light-evoked responses. High Ca(2+) ringer’s (3.2 mM) hyperpolarized the cell membrane and decreased response amplitudes. However, under conditions of either low or high Ca(2+) superfusion for up to 6 min, in both dark-adapted and partially light-adapted states, receptor sensitivity was virtually unaffected; i.e., the V-log I curve for the receptor potential was always located on the intensity scale at a position predicted by the prevailing light level, not by Ca(2+) concentration. Thus, we speculate that cytosol Ca(2+) concentration is capable of regulating membrane potential levels and light-evoked response amplitudes, but not the major component of rod sensitivity. Low Ca(2+) ringer’s also shortened the period of receptor response saturation after a bright but nonbleaching light flash, hence accelerating the onset of both membrane potential and sensitivity recovery during dark adaptation.

Exposure of the retina to low Ca(2+) (10(-9)M) ringer’s for long periods (7-15 min) caused dark-adapted rods to lose responsiveness. Response amplitudes gradually decreased, and the rods became desensitized. These severe conditions of low Ca(2+) caused changes in the dark-adapted rod that mimic those observed in rods during light adaptation. We suggest that loss of receptor sensitivity during prolonged exposure to low Ca(2+) ringer’s results from a decrease of intracellular (intradisk) stores of Ca(2+); i.e., less Ca(2+) is thereby released per quantum catch.

     

Regulation of Mammalian Cone Phototransduction by Recoverin and Rhodopsin Kinase

Cone photoreceptors function under daylight conditions and are essential for color perception and vision with high temporal and spatial resolution. A remarkable feature of cones is that, unlike rods, they remain responsive in bright light. In rods, light triggers a decline in intracellular calcium, which exerts a well studied negative feedback on phototransduction that includes calcium-dependent inhibition of rhodopsin kinase (GRK1) by recoverin. Rods and cones share the same isoforms of recoverin and GRK1, and photoactivation also triggers a calcium decline in cones. However, the molecular mechanisms by which calcium exerts negative feedback on cone phototransduction through recoverin and GRK1 are not well understood. Here, we examined this question using mice expressing various levels of GRK1 or lacking recoverin. We show that although GRK1 is required for the timely inactivation of mouse cone photoresponse, gradually increasing its expression progressively delays the cone response recovery. This surprising result is in contrast with the known effect of increasing GRK1 expression in rods. Notably, the kinetics of cone responses converge and become independent of GRK1 levels for flashes activating more than ∼1% of cone pigment. Thus, mouse cone response recovery in bright light is independent of pigment phosphorylation and likely reflects the spontaneous decay of photoactivated visual pigment. We also find that recoverin potentiates the sensitivity of cones in dim light conditions but does not contribute to their capacity to function in bright light.     

Intracellular recordings from gecko photoreceptors during light and dark adaptation

Intracellular recordings were obtained from rods in the Gekko gekko retina and the adaptation characteristics of their responses studied during light and dark adaptation. Steady background illumination induced graded and sustained hyperpolarizing potentials and compressed the incremental voltage range of the receptor. Steady backgrounds also shifted the receptor's voltage-intensity curve along the intensity axis, and bright backgrounds lowered the saturation potential of the receptor. Increment thresholds of single receptors followed Weber's law over a range of about 3.5 log units and then saturated. Most of the receptor sensitivity change in light derived from the shift of the voltage-intensity curve, only little from the voltage compression. Treatment of the eyecup with sodium aspartate at concentrations sufficient to eliminate the beta-wave of the electroretinogram (ERG) abolished initial transients in the receptor response, possibly indicating the removal of horizontal cell feedback. Aspartate treatment, however, did not significantly alter the adaptation characteristics of receptor responses, indicating that they derive from processes intrinsic to the receptors. Dark adaptation after a strongly adapting stimulus was similarly associated with temporary elevation of membrane potential, initial lowering of the saturation potential, and shift of the voltage-intensity curve. Under all conditions of adaptation studied, small amplitude responses were linear with light intensity. Further, there was no unique relation between sensitivity and membrane potential suggesting that receptor sensitivity is controlled at least in part by a step of visual transduction preceding the generation of membrane voltage change.     

New insights into the fatty acid-binding protein (FABP) family in the small intestine

Cyclic GMP is essential for the ability of rods and cones to respond to the light stimuli. Light triggers hydrolysis of cGMP and stops the influx of sodium and calcium through the cGMP-gated ion channels. The consequence of this event is 2-fold: first, the decrease in the inward sodium current plays the major role in an abrupt hyperpolarization of the cellular membrane; secondly, the decrease in the Ca²⁺ influx diminishes the free intracellular Ca²⁺ concentration. While the former constitutes the essence of the phototransduction pathway in rods and cones, the latter gives rise to a potent feedback mechanism that accelerates photoreceptor recovery and adaptation to background light. One of the most important events by which Ca²⁺ feedback controls recovery and light adaptation is synthesis of cGMP by guanylyl cyclase. Two isozymes of membrane photoreceptor guanylyl cyclase (retGC) have been identified in rods and cones that are regulated by Ca²⁺-binding proteins, GCAPs. At low intracellular concentrations of Ca²⁺ typical for light-adapted rods and cones GCAPs activate RetGC, but concentrations above 500 nM typical for dark-adapted photoreceptors turn them into inhibitors of retGC. A variety of mutations found in GCAP and retGC genes have been linked to several forms of human congenital retinal diseases, such as dominant cone degeneration, cone-rod dystrophy and Leber congenital amaurosis.     

Light adaptation in retinal rods of the rabbit and two other nonprimate mammals

The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.     

Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors

The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the light-induced activity of the phototransduction cascade. In vertebrate photoreceptor cells, this rate-limiting reaction is thought to be either shutoff of catalytic activity in the photopigment or shutoff of the pigment''s effector, the transducin-GTP–phosphodiesterase complex. In suction pipette recordings from isolated salamander L-cones, we found that preventing changes in internal [Ca²⁺] delayed the recovery of the light response and prolonged the dominant time constant for recovery. Evidence that the Ca²⁺-sensitive step involved the pigment itself was provided by the observation that removal of Cl⁻ from the pigment''s anion-binding site accelerated the dominant time constant for response recovery. Collectively, these observations indicate that in L-cones, unlike amphibian rods where the dominant time constant is insensitive to [Ca²⁺], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation.     

Metabolic constraints on the recovery of sensitivity after visual pigment bleaching in retinal rods

The shutoff of active intermediates in the phototransduction cascade and the reconstitution of the visual pigment play key roles in the recovery of sensitivity after the exposure to bright light in both rod and cone photoreceptors. Physiological evidence from bleached salamander rods suggests this recovery of sensitivity occurs faster at the outer segment base compared with the tip. Microfluorometric measurements of similarly bleached salamander rods demonstrate that the reduction of all-trans retinal to all-trans retinol also occurs more rapidly at the outer segment base than at the tip. The experiments reported here were designed to test the hypothesis that these two phenomena are linked, e.g., that slowed recovery of sensitivity at the tip of outer segments is rate limited by the reduction of all-trans retinal and results from a shortage of cytosolic nicotinamide adenine dinucleotide phosphate (NADPH), the reducing agent for all-trans retinal reduction. Extracellular measurements of membrane current and sensitivity were made from isolated salamander rods under dark-adapted and bleached conditions while intracellular NADPH concentration was varied by dialysis from a micropipette attached to the inner segment. Sensitivity at the base and tip of the outer segment was assessed before and after bleaching. After exposure to a light that photoactivates 50% of the visual pigment, rods were completely insensitive for nearly 10 minutes, after which the base recovered sensitivity and responsiveness with a time constant of ∼200 seconds, but tip sensitivity recovered more slowly with a time constant of ∼680 seconds. Dialysis of 5 mM NADPH into the rod promoted an earlier recovery and eliminated the previously observed tip/base difference. Dialysis of 1.66 mM NADPH failed to eliminate the tip/base recovery difference, suggesting the steady-state NADPH concentration in rods is ∼1 mM. These results indicate the inner segment is the primary source of reducing equivalents after pigment bleaching, with the reduction of all-trans retinal to all-trans retinol playing a key step in the recovery of sensitivity.     

Two temporal phases of light adaptation in retinal rods

Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady light. Light adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level. We found that adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1-2-s time constant for the onset of adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every light intensity tested. The slow phase was observed only at light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of adaptation of approximately 3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca(2+)-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow adaptation is unclear. We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow adaptation phase, this hypothesis was ruled out because the intensity of light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.     

Increased intracellular sodium mimics some but not all aspects of photoreceptor adaptation in the ventral eye of Limulus

The effects of the intracellular iontophoretic injection of Na+ ions have been quantitatively compared with adaptation in ventral photoreceptors of Limulus. We find that: (a) both light adaptation and sodium injection are associated with a decrease in the variability of the threshold response amplitued; (b) both light adaptation and sodium injection are associated with a decrease in the absolute value of the temporal dispersion of the threshold response time delay; (c) the same template curve adequately fits the intensity response relationships measured under light adaptation and Na+ injection; (d) both light adaptation and Na+ injection produce a fourfold decrease in response time delay for a desensitization of 3 log units; (e) the time coures of light adaptation and dark adaptation is significantly faster than the onset of and recovery from desensitization produced by Na+ injection; (f) unlike local illumination, Na+ injection does not produce localized desensitization of the photoreceptor. These findings suggest that a rise in intracellular Na+ concentration makes at most only a minor contribution (probably less than 5%) to the total adaptation of these receptors in the intensity range we have examined (up to 3 log units above absolute threshold). However, changes in intracellular Na+ concentration may contribute to certain components of light and dark adaptation in these receptors.     

Phosducin regulates the expression of transducin betagamma subunits in rod photoreceptors and does not contribute to phototransduction adaptation

For over a decade, phosducin's interaction with the βγ subunits of the G protein, transducin, has been thought to contribute to light adaptation by dynamically controlling the amount of transducin heterotrimer available for activation by photoexcited rhodopsin. In this study we directly tested this hypothesis by characterizing the dark- and light-adapted response properties of phosducin knockout (Pd−/−) rods. Pd−/− rods were notably less sensitive to light than wild-type (WT) rods. The gain of transduction, as measured by the amplification constant using the Lamb-Pugh model of activation, was 32% lower in Pd−/− rods than in WT rods. This reduced amplification correlated with a 36% reduction in the level of transducin βγ-subunit expression, and thus available heterotrimer in Pd−/− rods. However, commonly studied forms of light adaptation were normal in the absence of phosducin. Thus, phosducin does not appear to contribute to adaptation mechanisms of the outer segment by dynamically controlling heterotrimer availability, but rather is necessary for maintaining normal transducin expression and therefore normal flash sensitivity in rods.     

Modulation of Mouse Rod Photoreceptor Responses by Grb14 Protein

Previous experiments have indicated that growth factor receptor-bound protein 14 (Grb14) may modulate rod photoreceptor cGMP-gated channels by decreasing channel affinity for cGMP; however, the function of Grb14 in rod physiology is not known. In this study, we examined the role of Grb14 by recording electrical responses from rods in which the gene for the Grb14 protein had been deleted. Suction-electrode recordings from single mouse rods showed that responses of dark-adapted Grb14^−/− mice to brief flashes decayed more rapidly than strain-controlled wild type (WT) rods, with decreased values of both integration time and the exponential time course of decay (τ_REC). This result is consistent with an increase in channel affinity for cGMP produced by deletion of Grb14. However, Grb14^−/− mouse rods also showed little change in dark current and a large and significant decrease in the limiting time constant τ_D, which are not consistent with an effect on channel affinity but seem rather to indicate modulation of the rate of inactivation of cyclic nucleotide phosphodiesterase 6 (PDE6). Grb14 has been reported to translocate from the inner to the outer segment in bright light, but we saw effects on response time course even in dark-adapted rods, although the effects were somewhat greater after rods had been adapted by exposure to bleaching illumination. Our results indicate that the mechanism of Grb14 action may be more complex than previously realized.     

Effect of blocking the Na+/K+ ATPase on Ca2+ extrusion and light adaptation in mammalian retinal rods.

Membrane current and light response were recorded from rods of monkey and guinea pig by means of suction electrodes. The correlation between adaptation and the Na+/K+ pump was investigated by measuring light-dependent changes in sensitivity with and without inhibition of Na+/K+ ATPase by strophanthidin. Strophanthidin was found to reduce the dark current, to slow the time course of the photoresponse, and to increase light sensitivity. At concentrations between 20 and 500 nM, the pump inhibitor suppressed in a reversible way the current re-activation occurring during prolonged illumination and modified the light-dependent decrease in sensitivity, which in control conditions approximates to a Weber-Fechner function. The effects of the pump inhibitor on the adaptive properties of rods are associated with an increased time constant of the membrane current attributed to the operation of the Na+:Ca2+,K+ exchanger. The effects of rapid application of the pump inhibitor on the current re-activation are consistent with the idea that significant changes in the internal sodium occur in rods of mammals during background illumination and that they play an important role in the process of light adaptation.     

Photoreceptor adaptation in intrinsically photosensitive retinal ganglion cells

A rare type of mammalian retinal ganglion cell (RGC) expresses the photopigment melanopsin and is a photoreceptor. These intrinsically photosensitive RGCs (ipRGCs) drive circadian-clock resetting, pupillary constriction, and other non-image-forming photic responses. Both the light responses of ipRGCs and the behaviors they drive are remarkably sustained, raising the possibility that, unlike rods and cones, ipRGCs do not adjust their sensitivity according to lighting conditions ("adaptation"). We found, to the contrary, that ipRGC sensitivity is plastic, strongly influenced by lighting history. When exposed to a constant, bright background, the background-evoked response decayed, and responses to superimposed flashes grew in amplitude, indicating light adaptation. After extinction of a light-adapting background, sensitivity recovered progressively in darkness, indicating dark adaptation. Because these adjustments in sensitivity persisted when synapses were blocked, they constitute "photoreceptor adaptation" rather than "network adaptation." Implications for the mechanisms generating various non-image-forming visual responses are discussed.     

Responses of retinal rods of the frog to light

To study the effect of the intensity, duration, spectral composition, and diameter of the light spot on the amplitude and shape of the response of single rods of the frog retina, potentials were recorded intracellularly. The rods tested could be divided into two groups on the basis of their responses to light spots of different spectral composition: those with maxima of sensitivity at 507 ± 8 nm and 442 ± 8 nm. With an increase in the intensity of light the response amplitude rose gradually and the time for the response to rise to its maximum was shortened. A bright flash temporarily inhibited the sensitivity of the cell to subsequent test flashes. If light spots of larger diameter (1000–1500 µ) were presented a delayed depolarization wave, due to illumination of the distant surroundings of the receptor, was observed in the course of recovery of the photic response; this effect was maximal for stimulation with red light and it was evidently induced by horizontal cell activity. The possible functional role of the depolarizing effect of illumination of the distant surroundings of the receptor is discussed.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 7, No. 1, pp. 84–92, January–February, 1975.     

Adaptive potentiation in rod photoreceptors after light exposure

Photoreceptors adapt to changes in illumination by altering transduction kinetics and sensitivity, thereby extending their working range. We describe a previously unknown form of rod photoreceptor adaptation in wild-type (WT) mice that manifests as a potentiation of the light response after periods of conditioning light exposure. We characterize the stimulus conditions that evoke this graded hypersensitivity and examine the molecular mechanisms of adaptation underlying the phenomenon. After exposure to periods of saturating illumination, rods show a 10–35% increase in circulating dark current, an adaptive potentiation (AP) to light exposure. This potentiation grows as exposure to light is extended up to 3 min and decreases with longer exposures. Cells return to their initial dark-adapted sensitivity with a time constant of recovery of ∼7 s. Halving the extracellular Mg concentration prolongs the adaptation, increasing the time constant of recovery to 13.3 s, but does not affect the magnitude of potentiation. In rods lacking guanylate cyclase activating proteins 1 and 2 (GCAP^−/−), AP is more than doubled compared with WT rods, and halving the extracellular Mg concentration does not affect the recovery time constant. Rods from a mouse expressing cyclic nucleotide–gated channels incapable of binding calmodulin also showed a marked increase in the amplitude of AP. Application of an insulin-like growth factor-1 receptor (IGF-1R) kinase inhibitor (Tyrphostin AG1024) blocked AP, whereas application of an insulin receptor kinase inhibitor (HNMPA(AM)₃) failed to do so. A broad-acting tyrosine phosphatase inhibitor (orthovanadate) also blocked AP. Our findings identify a unique form of adaptation in photoreceptors, so that they show transient hypersensitivity to light, and are consistent with a model in which light history, acting via the IGF-1R, can increase the sensitivity of rod photoreceptors, whereas the photocurrent overshoot is regulated by Ca-calmodulin and Ca²⁺/Mg²⁺-sensitive GCAPs.     

Rhodopsin kinase and recoverin modulate phosphodiesterase during mouse photoreceptor light adaptation

Light stimulates rhodopsin in a retinal rod to activate the G protein transducin, which binds to phosphodiesterase (PDE), relieving PDE inhibition and decreasing guanosine 3′,5′-cyclic monophosphate (cGMP) concentration. The decrease in cGMP closes outer segment channels, producing the rod electrical response. Prolonged exposure to light decreases sensitivity and accelerates response kinetics in a process known as light adaptation, mediated at least in part by a decrease in outer segment Ca²⁺. Recent evidence indicates that one of the mechanisms of adaptation in mammalian rods is down-regulation of PDE. To investigate the effect of light and a possible role of rhodopsin kinase (G protein–coupled receptor kinase 1 [GRK1]) and the GRK1-regulating protein recoverin on PDE modulation, we used transgenic mice with decreased expression of GTPase-accelerating proteins (GAPs) and, consequently, a less rapid decay of the light response. This slowed decay made the effects of genetic manipulation of GRK1 and recoverin easier to observe and interpret. We monitored the decay of the light response and of light-activated PDE by measuring the exponential response decay time (τ_REC) and the limiting time constant (τ_D), the latter of which directly reflects light-activated PDE decay under the conditions of our experiments. We found that, in GAP-underexpressing rods, steady background light decreased both τ_REC and τ_D, and the decrease in τ_D was nearly linear with the decrease in amplitude of the outer segment current. Background light had little effect on τ_REC or τ_D if the gene for recoverin was deleted. Moreover, in GAP-underexpressing rods, increased GRK1 expression or deletion of recoverin produced large and highly significant accelerations of τ_REC and τ_D. The simplest explanation of our results is that Ca²⁺-dependent regulation of GRK1 by recoverin modulates the decay of light-activated PDE, and that this modulation is responsible for acceleration of response decay and the increase in temporal resolution of rods in background light.     

Interhemispheric EEG asymmetries during unilateral bright-light exposure and subsequent sleep in humans

We tested whether evening exposure to unilateral photic stimulation has repercussions on interhemispheric EEG asymmetries during wakefulness and later sleep. Because light exerts an alerting response in humans, which correlates with a decrease in waking EEG theta/alpha-activity and a reduction in sleep EEG delta activity, we hypothesized that EEG activity in these frequency bands show interhemispheric asymmetries after unilateral bright light (1,500 lux) exposure. A 2-h hemi-field light exposure acutely suppressed occipital EEG alpha activity in the ipsilateral hemisphere activated by light. Subjects felt more alert during bright light than dim light, an effect that was significantly more pronounced during activation of the right than the left visual cortex. During subsequent sleep, occipital EEG activity in the delta and theta range was significantly reduced after activation of the right visual cortex but not after stimulation of the left visual cortex. Furthermore, hemivisual field light exposure was able to shift the left predominance in occipital spindle EEG activity toward the stimulated hemisphere. Time course analysis revealed that this spindle shift remained significant during the first two sleep cycles. Our results reflect rather a hemispheric asymmetry in the alerting action of light than a use-dependent recovery function of sleep in response to the visual stimulation during prior waking. However, the observed shift in the spindle hemispheric dominance in the occipital cortex may still represent subtle local use-dependent recovery functions during sleep in a frequency range different from the delta range.