Abstract
The mammalian eye contains two systems for light perception: an image detecting system constituted primarily of the classical photoreceptors, rods and cones, and a non-image forming system (NIF) constituted of a small group of intrinsically photosensitive retinal ganglion cells driven by melanopsin (mRGCs). The mRGCs receive input from the outer retina and NIF mediates light entrainment of circadian rhythms, masking behavior, light induced inhibition of nocturnal melatonin secretion, pupillary reflex (PLR), and affect the sleep/wake cycle. This review focuses on the mammalian NIF and its anatomy in the eye as well as its neuronal projection to the brain. This pathway is known as the retinohypothalamic tract (RHT). The development and functions of the NIF as well as the knowledge gained from studying gene modified mice is highlighted. Furthermore, the similarities of the NIF between sighted (nocturnal and diurnal rodent species, monkeys, humans) and naturally blind mammals (blind mole rats Spalax ehrenbergi and the Iberian mole, Talpa occidentalis) are discussed in relation to a changing world where increasing exposure to artificial light at night (ALAN) is becoming a challenge for humans and animals in the modern society.
Highlights
The daily shift between day and night due to the rotation of the earth toward the sun defines the astronomical day of 24 h, which has shaped almost all life forms on the planet
Light is perceived through the retina and used for image formation primarily based on the classical photoreceptors, rods and cones, and a non-image forming (NIF) system using the photoreceptor, melanopsin (Do and Yau, 2010)
The different subtypes of melanopsin-expressing RGCs (mRGCs) are based on the pattern of dendritic aberration in the inner layers of the inner plexiform layer (IPL) and inner nuclear layer (INL), which form two distinct networks of dendritic projections: one inner stratifying layer located in the sublamina I of the IPL and one outer stratifying layer located in sublaminar V in the IPL close to the INL (Figure 3; Schmidt et al, 2011a; Reifler et al, 2014; Hannibal et al, 2017)
Summary
Neurology of Circadian Photoreception: The Retinohypothalamic Tract (RHT) in Sighted and Naturally Blind Mammals. The mRGCs receive input from the outer retina and NIF mediates light entrainment of circadian rhythms, masking behavior, light induced inhibition of nocturnal melatonin secretion, pupillary reflex (PLR), and affect the sleep/wake cycle. This review focuses on the mammalian NIF and its anatomy in the eye as well as its neuronal projection to the brain. This pathway is known as the retinohypothalamic tract (RHT). The similarities of the NIF between sighted (nocturnal and diurnal rodent species, monkeys, humans) and naturally blind mammals (blind mole rats Spalax ehrenbergi and the Iberian mole, Talpa occidentalis) are discussed in relation to a changing world where increasing exposure to artificial light at night (ALAN) is becoming a challenge for humans and animals in the modern society
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