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Two temporal phases of light adaptation in retinal rods.

Calvert PD, Govardovskii VI, Arshavsky VY, Makino CL - J. Gen. Physiol. (2002)

Bottom Line: 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 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.Other possible mechanisms are discussed.

View Article: PubMed Central - PubMed

Affiliation: Department of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA. pdcalvert@meei.harvard.edu

ABSTRACT
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.

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Mapping the onset of light adaptation with bright steps of light. (A) Accelerated recovery from saturation after longer light exposure. Each trace shows the average of two responses of rod 2 to a step of 802 photons μm−2 s−1. Step durations are given in the top right corner of each panel. The vertical gray lines emphasize the moment at which the light was extinguished for each trace. (B) Time course of the reduction in saturation time. The time required for 10% photocurrent recovery after each light pulse was extinguished is plotted as a function of step duration. The line is a single exponential function with τ = 17 s. (C) Time course of the slow phase of adaptation. The relative sensitivity was calculated from the difference in saturation time (ΔT) between the 80-s step and that at the indicated step duration and the τC from flash responses according to Eq. 3. For steps of <10-s duration, the relative sensitivities were divided by f0(step duration) (see , Eq. A5), where τE was taken as τC = 3.5 s measured for this cell to account for fractional, pre–steady-state PDE activity, and fitted with a single exponential with τ = 12 s. The results were expressed as desensitization relative to the amplitude of the exponential at time zero.
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fig4: Mapping the onset of light adaptation with bright steps of light. (A) Accelerated recovery from saturation after longer light exposure. Each trace shows the average of two responses of rod 2 to a step of 802 photons μm−2 s−1. Step durations are given in the top right corner of each panel. The vertical gray lines emphasize the moment at which the light was extinguished for each trace. (B) Time course of the reduction in saturation time. The time required for 10% photocurrent recovery after each light pulse was extinguished is plotted as a function of step duration. The line is a single exponential function with τ = 17 s. (C) Time course of the slow phase of adaptation. The relative sensitivity was calculated from the difference in saturation time (ΔT) between the 80-s step and that at the indicated step duration and the τC from flash responses according to Eq. 3. For steps of <10-s duration, the relative sensitivities were divided by f0(step duration) (see , Eq. A5), where τE was taken as τC = 3.5 s measured for this cell to account for fractional, pre–steady-state PDE activity, and fitted with a single exponential with τ = 12 s. The results were expressed as desensitization relative to the amplitude of the exponential at time zero.

Mentions: (column 2) q1/2, the half-saturating flash strength, calculated according to r/rmax = 1 − exp(−kq), where k = ln(2)/q1/2 and q is the flash strength. (column 3) ti, integration time integral of the dim flash response divided by response amplitude. (columns 4 and 5) τfast and τslow′, the time constants of the fast and slow phases of light adaptation derived from the 0.98 rmax criterion response as explained in Fig. 1 C legend. (column 6) τslow, time constant for the onset of the slow phase of light adaptation during continuous light determined from the recovery of saturated steps as described in Fig. 4 C legend. (columns 7–9) τc, the time constant of the rate-limiting step in the shutoff of phototransduction, estimated from the slope of the relation between saturation time and natural logarithm of either the flash strength or the step intensity. (columns 10–12) Magnitudes of desensitization that operated with fast and slow kinetics and total desensitization during continuous light determined as described in Fig. 1 C legend. (column 13) ΔT, difference in saturation time between the responses to short, 2.5-s steps and long, 80-s (rods 1–6) or 60-s (rods 7–10) steps taken from the brightest intensities applied to both step durations. (column 14) The magnitude of the desensitization that occurred between short and long steps calculated using Eq. 3, ΔT from Col. 13 and τc from Col. 7, except for rods 7–9 where the average of the flash and 2.5-s step τc values from Cols. 7 and 8 were used and for rod 10 where the 2.5-s step τc was used. The magnitudes were corrected for the fractional activation of PDE during the 2.5-s step as explained in the . (columns 15–18) Kinetic parameters of the exchanger current found from a fit of Eq. 4 to responses to saturating steps >25 s in duration.


Two temporal phases of light adaptation in retinal rods.

Calvert PD, Govardovskii VI, Arshavsky VY, Makino CL - J. Gen. Physiol. (2002)

Mapping the onset of light adaptation with bright steps of light. (A) Accelerated recovery from saturation after longer light exposure. Each trace shows the average of two responses of rod 2 to a step of 802 photons μm−2 s−1. Step durations are given in the top right corner of each panel. The vertical gray lines emphasize the moment at which the light was extinguished for each trace. (B) Time course of the reduction in saturation time. The time required for 10% photocurrent recovery after each light pulse was extinguished is plotted as a function of step duration. The line is a single exponential function with τ = 17 s. (C) Time course of the slow phase of adaptation. The relative sensitivity was calculated from the difference in saturation time (ΔT) between the 80-s step and that at the indicated step duration and the τC from flash responses according to Eq. 3. For steps of <10-s duration, the relative sensitivities were divided by f0(step duration) (see , Eq. A5), where τE was taken as τC = 3.5 s measured for this cell to account for fractional, pre–steady-state PDE activity, and fitted with a single exponential with τ = 12 s. The results were expressed as desensitization relative to the amplitude of the exponential at time zero.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2233805&req=5

fig4: Mapping the onset of light adaptation with bright steps of light. (A) Accelerated recovery from saturation after longer light exposure. Each trace shows the average of two responses of rod 2 to a step of 802 photons μm−2 s−1. Step durations are given in the top right corner of each panel. The vertical gray lines emphasize the moment at which the light was extinguished for each trace. (B) Time course of the reduction in saturation time. The time required for 10% photocurrent recovery after each light pulse was extinguished is plotted as a function of step duration. The line is a single exponential function with τ = 17 s. (C) Time course of the slow phase of adaptation. The relative sensitivity was calculated from the difference in saturation time (ΔT) between the 80-s step and that at the indicated step duration and the τC from flash responses according to Eq. 3. For steps of <10-s duration, the relative sensitivities were divided by f0(step duration) (see , Eq. A5), where τE was taken as τC = 3.5 s measured for this cell to account for fractional, pre–steady-state PDE activity, and fitted with a single exponential with τ = 12 s. The results were expressed as desensitization relative to the amplitude of the exponential at time zero.
Mentions: (column 2) q1/2, the half-saturating flash strength, calculated according to r/rmax = 1 − exp(−kq), where k = ln(2)/q1/2 and q is the flash strength. (column 3) ti, integration time integral of the dim flash response divided by response amplitude. (columns 4 and 5) τfast and τslow′, the time constants of the fast and slow phases of light adaptation derived from the 0.98 rmax criterion response as explained in Fig. 1 C legend. (column 6) τslow, time constant for the onset of the slow phase of light adaptation during continuous light determined from the recovery of saturated steps as described in Fig. 4 C legend. (columns 7–9) τc, the time constant of the rate-limiting step in the shutoff of phototransduction, estimated from the slope of the relation between saturation time and natural logarithm of either the flash strength or the step intensity. (columns 10–12) Magnitudes of desensitization that operated with fast and slow kinetics and total desensitization during continuous light determined as described in Fig. 1 C legend. (column 13) ΔT, difference in saturation time between the responses to short, 2.5-s steps and long, 80-s (rods 1–6) or 60-s (rods 7–10) steps taken from the brightest intensities applied to both step durations. (column 14) The magnitude of the desensitization that occurred between short and long steps calculated using Eq. 3, ΔT from Col. 13 and τc from Col. 7, except for rods 7–9 where the average of the flash and 2.5-s step τc values from Cols. 7 and 8 were used and for rod 10 where the 2.5-s step τc was used. The magnitudes were corrected for the fractional activation of PDE during the 2.5-s step as explained in the . (columns 15–18) Kinetic parameters of the exchanger current found from a fit of Eq. 4 to responses to saturating steps >25 s in duration.

Bottom Line: 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 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.Other possible mechanisms are discussed.

View Article: PubMed Central - PubMed

Affiliation: Department of Ophthalmology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA. pdcalvert@meei.harvard.edu

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus