Melanopsin activates divergent phototransduction pathways in intrinsically photosensitive retinal ganglion cell subtypes

Two-photon activation, deactivation, and coherent control of melanopsin in live cells

Intrinsically photosensitive retinal ganglion cells are photoreceptors discovered in the last 20 years. These cells project to the suprachiasmatic nucleus of the brain to drive circadian rhythms, regulated by ambient light levels. The photopigment responsible for photoactivation in these cells, melanopsin, has been shown to exhibit many unique activation features among opsins. Notably, the photopigment can exist in three states dependent on the intensity and spectrum of ambient light, which affects its function. Despite increasing knowledge about these cells and melanopsin, tools that can manipulate their three states, and do so with single-cell precision, are limited. This reduces the extent to which circuit-level phenomena, and studying the implications of melanopsin tri-stability in living systems, can be pursued. In this report, we evoke and modulate calcium transients in live cells and intrinsically photosensitive retinal ganglion cells from isolated retinal tissues following two-photon excitation using near-infrared light pulses. We demonstrate that two-photon activation of melanopsin can successfully stimulate melanopsin-expressing cells with high spatio-temporal precision. Moreover, we demonstrate that the functional tri-stability of the photopigment can be interrogated by multiphoton excitation using spectral-temporal modulation of a broadband, ultrafast laser source.

Diurnal biological effects of correlated colour temperature and its exposure timing on alertness, cognition, and mood in an enclosed environment

Artificial lighting, which profits from the non-visual effects of light, is a potentially promising solution to support residents' psychophysiological health and performance at specific times of the day in enclosed environments. However, few studies have investigated the non-visual effects of daytime correlated colour temperature (CCT) and its exposure timing on human alertness, cognition, and mood. However, the neural mechanisms underlying these effects are largely unknown. The current study evaluated the effects of daytime CCT and its exposure timing on markers of subjective experience, cognitive performance, and cerebral activity in a simulated enclosed environment. Forty-two participants participated a single-blind laboratory study with a 4 within (CCT: 4000 K vs. 6500 K vs. 8500 K vs. 12,000 K) × 2 between (exposure timing: morning vs. afternoon) mixed design. The results showed time of the day dependent benefits of the daytime CCT on subjective experience, vigilant attention, response inhibition, working memory, emotional perception, and risk decisions. The results of the electroencephalogram (EEG) revealed that lower-frequency EEG bands, including theta, alpha, and alpha-theta, were quite sensitive to daytime CCT intervention, which provides a valuable reference for trying to establish the underlying mechanisms that support the performance-enhancement effects of exposure to CCT in the daytime. However, the results revealed no consistent intervention pattern across these measurements. Therefore, future studies should consider personalised optimisation of daytime CCT for different cognitive demands.

Flp-recombinase mouse line for genetic manipulation of ipRGCs

Light has myriad impacts on behavior, health, and physiology. These signals originate in the retina and are relayed to the brain by more than 40 types of retinal ganglion cells (RGCs). Despite a growing appreciation for the diversity of RGCs, how these diverse channels of light information are ultimately integrated by the ~50 retinorecipient brain targets to drive these light-evoked effects is a major open question. This gap in understanding primarily stems from a lack of genetic tools that specifically label, manipulate, or ablate specific RGC types. Here, we report the generation and characterization of a new mouse line (Opn4FlpO), in which FlpO is expressed from the Opn4 locus, to manipulate the melanopsin-expressing, intrinsically photosensitive retinal ganglion cells. We find that the Opn4FlpO line, when crossed to multiple reporters, drives expression that is confined to ipRGCs and primarily labels the M1-M3 subtypes. Labeled cells in this mouse line show the expected intrinsic, melanopsin-based light response and morphological features consistent with the M1-M3 subtypes. In alignment with the morphological and physiological findings, we see strong innervation of non-image forming brain targets by ipRGC axons, and weaker innervation of image forming targets in Opn4FlpO mice labeled using AAV-based and FlpO-reporter lines. Consistent with the FlpO insertion disrupting the endogenous Opn4 transcript, we find that Opn4FlpO/FlpO mice show deficits in the pupillary light reflex, demonstrating their utility for behavioral research in future experiments. Overall, the Opn4FlpO mouse line drives Flp-recombinase expression that is confined to ipRGCs and most effectively drives recombination in M1-M3 ipRGCs. This mouse line will be of broad use to those interested in manipulating ipRGCs through a Flp-based recombinase for intersectional studies or in combination with other, non-Opn4 Cre driver lines.

Genetic tuning of intrinsically photosensitive retinal ganglion cell subtype identity to drive visual behavior

The melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) comprise a subset of the ∼40 retinal ganglion cell types in the mouse retina and drive a diverse array of light-evoked behaviors from circadian photoentrainment to pupil constriction to contrast sensitivity for visual perception. Central to the ability of ipRGCs to control this diverse array of behaviors is the distinct complement of morphophysiological features and gene expression patterns found in the M1-M6 ipRGC subtypes. However, the genetic regulatory programs that give rise to subtypes of ipRGCs are unknown. Here, we identify the transcription factor Brn3b (Pou4f2) as a key genetic regulator that shapes the unique functions of ipRGC subtypes and their diverse downstream visual behaviors.

Coexistence within one cell of microvillous and ciliary phototransductions across M1- through M6-IpRGCs

Intrinsically photosensitive retinal ganglion cells (ipRGCs) serve as primary photoceptors by expressing the photopigment, melanopsin, and also as retinal relay neurons for rod and cone signals en route to the brain, in both cases for the purpose of non-image vision as well as aspects of image vision. So far, six subtypes of ipRGCs (M1 through M6) have been characterized. Regarding their phototransduction mechanisms, we have previously found that, unconventionally, rhabdomeric (microvillous) and ciliary signaling motifs co-exist within a given M1-, M2-, and M4-ipRGC, with the first mechanism involving PLCβ4 and TRPC6,7 channels and the second involving cAMP and HCN channels. We have now examined M3-, M5-, and M6-cells and found that each cell likewise uses both signaling pathways for phototransduction, despite differences in the percentage representation by each pathway in a given ipRGC subtype for bright-flash responses (and saturated except for M6-cells). Generally, M3- and M5-cells show responses quite similar in kinetics to M2-responses, and M6-cell responses resemble broadly those of M1-cells although much lower in absolute sensitivity and amplitude. Therefore, similar to rod and cone subtypes in image vision, ipRGC subtypes possess the same phototransduction mechanism(s) even though they do not show microvilli or cilia morphologically.