Diverse types of ganglion cell photoreceptors in the mammalian retina

Abstract

Photoreceptors carry out the first step in vision by capturing light and transducing it into electrical signals. Rod and cone photoreceptors efficiently translate photon capture into electrical signals by light activation of opsin-type photopigments. Until recently, the central dogma was that, for mammals, all phototransduction occurred in rods and cones. However, the recent discovery of a novel photoreceptor type in the inner retina has fundamentally challenged this view. These retinal ganglion cells are intrinsically photosensitive and mediate a broad range of physiological responses such as photoentrainment of the circadian clock, light regulation of sleep, pupillary light reflex, and light suppression of melatonin secretion. Intrinsically photosensitive retinal ganglion cells express melanopsin, a novel opsin-based signaling mechanism reminiscent of that found in invertebrate rhabdomeric photoreceptors. Melanopsin-expressing retinal ganglion cells convey environmental irradiance information directly to brain centers such as the hypothalamus, preoptic nucleus, and lateral geniculate nucleus. Initial studies suggested that these melanopsin-expressing photoreceptors were an anatomically and functionally homogeneous population. However, over the past decade or so, it has become apparent that these photoreceptors are distinguishable as individual subtypes on the basis of their morphology, molecular markers, functional properties, and efferent projections. These results have provided a novel classification scheme with five melanopsin photoreceptor subtypes in the mammalian retina, each presumably with differential input and output properties. In this review, we summarize the evidence for the structural and functional diversity of melanopsin photoreceptor subtypes and current controversies in the field.

Amphioxus photoreceptors - insights into the evolution of vertebrate opsins, vision and circadian rhythmicity

“…Vertebrate ipRGCs do not have elaborate membrane protrusions (Sand et al, 2012) of the kind found in rhabdomeric photoreceptors, leading to less efficient photon capture. In agreement with this finding, ipRGCs are less sensitive than most invertebrate visual photoreceptors (Do and Yau, 2015).…”

Adaptation to steady light by intrinsically photosensitive retinal ganglion cells

“…mammal | melanopsin | phototransduction | gain control I t has become clear over the past decade that the mammalian retina contains another photoreceptor class besides rods and cones, consisting of a subpopulation of retinal ganglion cells (RGCs) that are intrinsically photosensitive (ipRGCs) by virtue of their expression of the visual pigment, melanopsin (1)(2)(3)(4)(5)(6)(7). In mouse, these cells have been distinguished into subtypes: M1 through M5 (4)(5)(6)(7).…”

“…In mouse, these cells have been distinguished into subtypes: M1 through M5 (4)(5)(6)(7). The first three can be visualized immunocytochemically with antibodies against melanopsin (with the M1 cells showing a higher melanopsin level than the other two) and have distinct morphological and physiological properties (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). M4 and M5 cells cannot be labeled by conventional immunocytochemistry, presumably because they have too little melanopsin, and are generally visualized more indirectly (12) [most recently, however, M4 cells have been revealed by immunofluorescence after signal amplification (15)].…”

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“…Their classifications, morphological, and physiological characteristics have been reviewed from different point of views [15][16][17][18][19]. Not all ipRGCs are in one retinal layer [16,20,21].…”

“…A distinction has been made on whether they were placed in the ganglion cell layer (known as orthotopic or normal placed) or in the inner nuclear layer (known as displaced). Moreover, five subtypes (M1-M5) of ipRGCs have been identified each with different soma size and dendrites stratifications [16,17,19]. Every subtypes projects to specific retino-recipient brain structures or central targets, e.g., SCN, olivary pretectal nucleus (OPN), and dorsal lateral geniculate nucleus (dLGN).…”