Two Temporal Phases of Light Adaptation in Retinal Rods

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 ∼3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca2+-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.

Phosphorylation of G Protein-coupled Receptor Kinase 1 (GRK1) Is Regulated by Light but Independent of Phototransduction in Rod Photoreceptors

“…Although phototransduction has been studied extensively, there are still unknown factors that influence the timing and amplification of signaling under conditions of sustained illumination (26,27). Previously, we showed that recombinant GRK1 can be phosphorylated in retina extracts by an endogenous kinase (12).…”

“…Several groups have attempted to characterize the various factors responsible for light adaptation and determined that different mechanisms might be functioning during distinct phases of adaptation. For example, Calvert et al (26) noted two phases of adaptation: the better characterized fast phase with a time constant of 1-2 s, and a slower phase observed only at high light intensities. Burns and co-workers also identified a novel mechanism of adaptation, known as adaptive acceleration, which accelerates recovery and persists for many seconds (27).…”

“…An antibody directed against GRK1 phosphorylated at Ser 21 (anti-pGRK1) was generated against a phosphopeptide corresponding to amino acids [16][17][18][19][20][21][22][23][24][25][26][27][28] Mice-All mice were in a C57BL/6 background, except for the Tr␣ Ϫ/Ϫ mice, which were in a 129SV background. Therefore, in experiments using the Tr␣ Ϫ/Ϫ mice, wild-type mice in the 129SV background were used as controls.…”

The Glutamic Acid-rich Protein-2 (GARP2) Is a High Affinity Rod Photoreceptor Phosphodiesterase (PDE6)-binding Protein That Modulates Its Catalytic Properties

“…Electrophysiological evidence supports the hypothesis that factors in addition to transducin deactivation are involved in regulating the lifetime of light-activated PDE6 during light adaptation of rod photoreceptors (5,6). Several potential feedback mechanisms for modulating activated PDE6 have been proposed (7-9) but have not been explored in sufficient detail to validate their relevance to the phototransduction pathway.…”

Activation of RGS9-1GTPase Acceleration by Its Membrane Anchor, R9AP

“…All of these domains have been found to participate in the regulation of GAP activity and substrate specificity (9 -11). The inhibitory PDE␥ subunit of the effector regulated by G␣ t , cGMP phosphodiesterase (PDE6), interacts with both G␣ t and the catalytic core of RGS9-1 and enhances RGS9-1 GAP activity (12)(13)(14) in vitro, but it is not clear how this enhancement is accomplished in a physiological setting in which tight PDE␥ binding to PDE6 catalytic subunits blocks GAP enhancement (15)(16)(17)(18).…”