Light Adaptation of Retinal Rods, Adaptation Memory, and Afterimages
- PDF / 629,844 Bytes
- 7 Pages / 594 x 792 pts Page_size
- 114 Downloads / 210 Views
Light Adaptation of Retinal Rods, Adaptation Memory, and Afterimages A. Yu. Rotov, L. A. Astakhova, M. L. Firsov, and V. I. Govardovskii
Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 106, No. 4, pp. 462–473, April, 2020. Original article submitted February 1, 2020. Revised version received February 11, 2020. Accepted February 11, 2020. The GPCR signal cascade in retinal photoreceptor cells – rods and cones – supports powerful signal amplification and allows rods to respond reliably to single light quanta. Rods remain functional at light fluxes down to 105 quanta/sec, which is supported by the highly effective light adaptation system. Light adaptation is based on negative feedback circuits, mostly via changes in intracellular Ca2+ ion concentrations. Three calcium regulation loops have been reliably identified – acceleration of quenching of light-activated rhodopsin, acceleration of cGMP synthesis by guanylate cyclase, and increased affinity of cGMP-controlled ion cannels for nucleotide. However, one or two further highly effective mechanisms of adaptation are known to operate, one of which regulates the lifetime of activated phosphodiesterase; for others, the target of regulation is unknown. The mediators of this regulation are also unknown. Our studies of these mechanisms revealed a novel phenomenon, which has been missed in previous studies. We found that recovery of the dark current of rods after cessation of non-saturating adapting light can take 20–30 sec. Furthermore, after formal return of the membrane current to the dark level, cell sensitivity to test stimuli remains decreased for a further 1–2 min. We termed this phenomenon “adaptation memory.” Adaptation memory is reminiscent of the phenomenology of afterimages. The gradual return of the membrane current to the dark level may correspond to fading of the positive afterimage. Prolonged decreases in photoreceptor sensitivity to additional stimulation can create a negative afterimage. As far as we know, this is the first experimental physiological demonstration of the ability to generate afterimages at the level of single photoreceptors. Keywords: photoreceptor, rod, light adaptation, calcium feedback, adaptation memory, afterimages.
range is maintained by light adaptation mechanisms which regulate the sensitivity and speed of the cascade. Light adaptation provides negative feedback based on changes in the intracellular Ca2+ ion concentration on illumination (see Fig. 1). Three targets for calcium have been solidly established. Firstly, light-induced drops in [Ca2+] accelerate quenching of activated rhodopsin (R*), ultimately decreasing the catalytic activity of PDE. Secondly, the decrease in [Ca2+] activates guanylate cyclase, accelerating the restoration of cGMP to the dark level. Thirdly, at low [Ca2+], the affinity of cGMP-controlled channels for cGMP increases, such that they remain in the open state at lower messenger concentrations. All three mechanisms promote restoration of the “dark” state of the photoreceptor an
Data Loading...
