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Nature 501, 93–96. http://dx.doi.org/10.1016/j.cub.2014.09.018 Vision: Melanopsin as a Novel elegant approach that combines the use of a genetically modified mouse, Irradiance Detector at the Heart where the spectral sensitivity of cones has been long-wavelength shifted, of Vision together with metameric silent substitution to probe the impact of selectively activating or not activating A recent study defines a novel role of melanopsin-expressing ipRGCs, showing melanopsin during the presentation that these inner retinal photoreceptors function as retinal irradiance detectors of photopic visual stimuli. Allen et al. and provide a local measure of luminance to regulate functional adaptation in convincingly show reversible changes the mammalian retina. in the photopic flash electroretinogram (ERG) between ‘daylight’ and ‘mel-low’ Mark W. Hankins* and Steven Hughes pathway and explore the mechanisms lighting conditions — lighting of luminance-dependent adaptation in conditions that activate both classes The photopigment melanopsin (Opn4) the retina, a feature that is fundamental of cones equally but differ significantly has come a long way since the end of to visual function. For many years it in their activation of melanopsin (while the last century. What began as a quest was naturally assumed that all light largely saturating rod responses). to identify the circadian photoreceptor detection in the retina was driven by Under daylight conditions cone ERG critically led to the discovery of a new rod and cone photoreceptors, so that responses are reduced at high light class of inner retinal photoreceptor the mechanisms that regulate both intensities, but this adaptive response comprising a population of retinal photoreceptor and retinal network is lacking under mel-low conditions ganglion cells that are intrinsically adaptation were assumed to be driven where activation of melanopsin photosensitive (ipRGCs) [1–3]. These by these same cells. The emergence is selectively reduced. Critically, ipRGCs express Opn4 [4], a blue light of inner-retinal photoreceptors simultaneous recording in the dorsal sensitive opsin protein capable of essentially overthrew this dogma lateral geniculate nucleus (dLGN) rendering cells intrinsically light and raised the possibility that some revealed changes in feature selectivity responsive [5]. In the decade that of these systems are driven by of visual circuits in both temporal and followed their discovery, we have learnt melanopsin-expressing ipRGCs. The spatial dimensions depending on levels a lot about melanopsin cells and how first piece of evidence that this might of melanopsin activation. A substantial they provide photic input to the be the case came from a study of fraction of units preferred finer spatial suprachiasmatic nucleus (SCN) and human vision, where it was first patterns in the daylight condition, while other retino-recipient areas demanding shown that a diurnal rhythm in the the population of direction-sensitive of a robust and highly reliable measure human cone electroretinogram (ERG) units became tuned to faster motion. of irradiance. It has been widely was regulated by a photoreceptor with By studying the responses to simple assumed that such an irradiance signal a melanopsin-like spectral sensitivity movies they conclude that the dLGN is required by the SCN, principally [6]. Melanopsin was later shown to contained a richer encoding of natural because rod and cone photoreceptors be critical in the diurnal and circadian scenes when melanopsin was show profound levels of adaption regulation of the mouse photopic activated. to background light levels and are ERG [7]. What are the implications of these themselves an unreliable reporter of In their latest work, reported in this phenomena to vision? It has become overall environmental light levels. issue of Current Biology, Allen et al. clear that visual coding is a highly Following this analogy it becomes [8] present new data on the role of dynamic process and is continuously interesting to revisit the classical visual melanopsin in vision, employing an adapting to the current viewing context Current Biology Vol 24 No 21 R1056 Figure 1. Interactions between melanopsin ipRGCs and central visual pathways. (A) Cross-section image showing the location and anatomy of cone photoreceptors (green) and different ipRGC subtypes (red) of the mouse retina. M1 ipRGCs express high levels of melanopsin and have dendrites (white arrows) located in the OFF layer of the IPL. M2 ipRGCs express lower levels of melanopsin and their dendrites stratify the ON layers of the IPL (dotted white arrows). Displaced M1 ipRGCs are located within the inner nuclear layer, and in this case can be seen to extend processes towards the outer retina (asterisk) — a feature only rarely observed for ipRGCs. (B,C) Collectively, M1–M5 ipRGCs innervate a range of non-image forming areas of the brain including the SCN (almost exclusively M1 ipRGCs [2]), and also innervate a number of visual centres including the dLGN (predominately M4 ipRGCs [13]). ipRGCs labelled using a highly characterised anti-melanopsin antibody (UF006); cones labelled using anti-b-gal antibodies following transgenic insertion of a LacZ reporter cassette within the SWS1 locus (unpublished data). DAPI nuclear counter stain shown in blue. Retino-recipient areas of M1–M5 type ipRGCs identified using an Opn4. Cre-based transgenic reporter line [12,20]. ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer; ON, ON layer of the inner plexiform layer; OFF, OFF layer of the inner plexiform layer; 3V, third ventricle; SCN, suprachiasmatic nucleus; dLGN, dorsal lateral geniculate nucleus; IGL, intergeniculate leaflet; vLGN, ventral lateral geniculate nucleus. (Images courtesy of Steven Hughes.) [9,10]. It appears that in addition to visual areas of the brain, including recently, studies have shown that a classical photoreceptor adaptation, the dLGN and superior colliculus small number of M1 ipRGCs (w7%) neural circuitry (network) adaptation in [11–14] (Figure 1), both of which have recurrent axon collaterals that both the retina and brain are critical to represent primary relay centres for terminate in the inner plexiform layer maximizing information coding from image-forming vision. It remains to [19], and on rare occasions ipRGC the visual world. Much of these be resolved if these innervations are projections can also be observed adaptations involve referencing the providing additional parallel signals for extending deeper into the retina photoreceptoral (rod/cone) signal to retinal luminance. It may be that the towards the outer plexiform layer the local luminance. However, in order primary influence is at the retinal level (Figure 1). At present the function to create efficient representations of and that the additional projections are of these retrograde connections is dynamic natural scenes it is often there to safeguard against the problem unknown, but it is clear that they necessary to adjust localised retinal known as ‘coding catastrophe’ [15], represent a potential mechanism for contrast circuits according to levels of so that downstream processes can transmitting irradiance information to luminance present across the visual be calibrated against levels of retinal the outer retina. field. At present, the mechanisms by adaptation in order to prevent It is now becoming increasingly clear which this is achieved are not well misinterpretation of the visual scene. that the function of melanopsin is not defined.