Questioning photostasis

Abstract

Photostasis is a phenomenon where the photoreceptor outer segment (OS) length and its rhodopsin content vary depending on environmental lighting. When light is reduced for extended periods, it is argued that OS lengthen and its rhodopsin concentration rises to increase photon capture in darker environment. Increases in OS length may occur because the retinal pigment epithelium (RPE) cells reduce OS consumption in prolonged darkness. But sample sizes in assessing changes in OS length have been small, and results highly varied with no statistical analysis ever offered. Further, animals used were often albinos, which have abnormal RPE cells. Here we keep pigmented and albino mice for 21 days in darkness and compare OS length with those in a normal 12:12 light/dark environment. We measured approximately 1300 OS but found no statistically significant difference in their lengths between light and dark groups in either pigmentation phenotype, although there was a small trend in the data favoring OS extension in the dark. Given that earlier studies were undertaken on limited samples with no statistical analysis, our data pose serious questions for the notion of mammalian photostasis in terms of significant OS plasticity.

Adaptations in rod outer segment disc membranes in response to environmental lighting conditions

“…Recent studies on both pigmented and albino mice raise questions about whether or not photostasis or adaptations to long-term changes in environmental lighting occur in photoreceptor cells [33-35]. The current study utilizing AFM to directly visualize the packing arrangement of rhodopsin in ROS disc membranes revealed that structural adaptations occur in ROS discs, which can affect scotopic visual function and are consistent with the concept of photostasis.…”

“…7 and Table 1). Moreover, the size of the ROS as whole does not appear to contribute to improved visual function since the ROS length does not change after housing mice in constant darkness, especially in pigmented animals [33, 34]. Even though differences were observed in the sizes of nanodomains, because the most abundant size of nanodomains was the same in all lighting scenarios (Fig.…”

“…The concept of photostasis has been recently questioned based on the absence of significant changes observed in the length of the ROS in mice housed in different lighting regimes when improved statistics were utilized [33]. Furthermore, changes in ROS length and rhodopsin expression levels is minimal in pigmented mice compared to albino animals [34, 35], which raises the question of whether or not photostasis is a general adaptive mechanism in vertebrate rod photoreceptor cells and whether or not long-term changes occur in rod photoreceptor cells in response to changes in the lighting environment.…”

Light-Induced Thickening of Photoreceptor Outer Segment Layer Detected by Ultra-High Resolution OCT Imaging

“…In fact, we observed a slight (∼2.5 μm) elongation of “outer-retina” length for the eyes after 8 days in dark, which is consistent with photostasis mechanisms reported by others for rodent eyes. 15 – 17 …”

Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer

“…To estimate the degree of scatter from the TL in summer and winter eyes, whole-mounted TL preparations were placed flat on a microscope slide and a micromanipulator used to hold the probe vertically less than 1 mm above the specimen. The specimen was dimly illuminated with 1.76 Â 10 23 W m 22 at 510 nm which is similar to the max sensitivity of rhodopsin. Increased relative scatter should result in more light being detected in winter compared with that in summer in the sensing element of the probe.…”