Circadian organization of the rodent retina involves strongly coupled, layer‐specific oscillators

Abstract

Rhythmic physiology is central to retinal function and survival and adapts vision to daily light intensity changes. Mammalian retina rhythmically releases melatonin when cultured under constant conditions, and the occurrence of clock gene [e.g., Period (Per)] expression has been shown for most cellular layers. However, contribution of the distinct layers to genesis of circadian rhythms within the retina is still debated. To characterize their endogenous oscillatory capacity and their communication at the whole-tissue level, we used a vibratome-based method to isolate individual or paired retina cellular layers from the mPer2Luc mouse and Per1-luciferase (Per1-Luc) rat, and realtime recorded bioluminescence. We report that each layer of the mouse retina harbors a self-sustained oscillator whose period is significantly longer (∼26 hours) than in whole-retina explants (∼22.9 hours), indicating that the period is correlated with the degree of coupling. Accordingly, the maximal period (∼29 hours) is reached upon complete enzymatic dissociation of the retina. By using pharmacological approaches, we demonstrate that connection between retina oscillators involves gap junctions but only minor contribution from the main retina neurochemicals. Taken together with results from Per1-Luc rats, these data show that mammalian retina consists of a network of layer-specific oscillators whose period is determined by their connectivity.

Neuropsin (OPN5)-mediated photoentrainment of local circadian oscillators in mammalian retina and cornea

“…The luciferase luminescence from these retinas comes predominantly from the inner retina (28,29), although autonomous rhythms have also been found in the outer retina (28). Pairs of retinas from these mice were exposed ex vivo to 9-h/15-h light/dark cycles for 4 d, with the two retinas in each pair subjected to opposite-phase light/dark entrainment (7) (SI Materials and Methods).…”

“…5C). The simplest interpretation of these findings is that the majority of the rhythmic cells in the retina do not express OPN5, although their exact locations are still unclear (28,29). If OPN5 cells are indeed distinct from the rhythmic cells, the important question arises of how light signals from OPN5 are transmitted to these rhythmic cells, especially if the OPN5-generated signal is nonelectrical downstream.…”

“…2A), raising the possibility that although OPN3 is not necessary for local photoentrainment of most inner retinal cells, it nonetheless may have a role in influencing the intrinsic retinal rhythmic activity, perhaps by synchronizing the rhythmic cells with a mechanism that is still unknown. For example, we cannot rule out the possibility that OPN3 is required for the photoentrainment of rhythmic outer retinal neurons, which contribute a minority of the overall Per2::Luciferase signal in the retina (28,29).…”

The retinal clock in mammals: role in health and disease

“…This differential pattern of circadian rhythm alterations between the retina and the SCN was proposed to relate to the fact that cellular coupling among clock cells/layers is not uniform throughout the retina, 22 whereas oscillators are strongly coupled in the SCN. 23 …”

“…50 Moreover, the cellular diversity of mammalian retina renders difficult the analysis of the circadian clock at the whole-tissue level because differences in phasing and period between cells/layers may result in low-amplitude rhythmicity or masking of coherent clock gene expression. 22,38,[51][52][53][54][55] It is therefore essential to determine which cell types possess clock properties to understand rhythm generation in the retina. Based on the molecular and physiological data from amphibians and birds, the initial prevailing model for circadian organization in vertebrate retinas proposed that photoreceptors contain self-sustained circadian clocks.…”