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Once every twenty-four hours the planet Earth completes a single rotation along its axis, crucial for the survival of all life. The most essential feature of this rotation is the exposure to light during certain times, and darkness at others. Given the latitudinal position and time of year, the amount time spent in light or dark can shift drastically. Yet so immense is the impact that the evolution of thousands of species and their adaptation to certain behaviors is a result of this process. Light is responsible for vision, certain wavelengths for heating, and entrainment of the central biological clock. Animals need light to determine their feeding and reproductive cycles, through activation and stimulation of certain areas of the brain. In human beings, we use light to modify our natural environment to adapt to changing conditions. Light will affect cardiac rhythm, core body temperature, cognitive processing and alertness. Light changes sleep by promoting wake through the suppression of melatonin, a hormone produced by the pineal gland, or change mood states, such as in people suffering from Seasonal Affective Disorder (Vandewalle, Gais et al. 2007).
2.1.2 Light and the brain
Light enters the brain via the eye, which is composed of several key structures: the pupil, the iris, and the retina. The pupil controls the amount of the light able to pass into the eye. The iris, a circular structure surrounding the pupil will expand or contract depending on the requirements, meaning the further the pupil expands, the higher the quantity of light can enter, and inversely as it contracts, the lower. After light has entered the pupil it passes through the interior chamber of the eye to the retina, a thin membrane of cells on the outer wall, sensitive to changes in light (see Fig. 2.1). At this point, light, both visual and non-visual, activates specific subtypes of cells within the retina and transmits this information to various areas of the brain to be interpreted.
Figure 2.1: Model of the human eye
2.2 - The Retina and Visual Light
The existence of the retina and its structure has been known for many years and was clearly illustrated in 1900 by famed anatomist Santiago Ramon y Cajal. The mammalian retina is composed of distinct types of cells which serve a variety of functions, principally the interpretation of visual and non-visual light produced by the environment. Visual light, composed of wavelengths within the observable electromagnetic spectrum, activates specialized cells in the outer layer called rods and cones (Fig. 2.2).
Rods, named for their elongated cylindrical shape, are extremely light sensitive cells important for vision, also known as scotopic vision, in environments with extremely low luminosity. The cell itself is composed of an inner segment, consisting of the organelles and nucleus, an outer segment, which functions as the light absorption area, and a synaptic terminal to transmit information to bipolar neurons or horizontal cells.
Color vision is constructed based on the reactivity of cone cells sensitive to specific wavelengths, and functions most efficiently under high luminosity. Though the basic cellular structure is nearly identical to rods, their physical shape is drastically different due to the requirements of photosensitivity. Several subtypes of cones are present: (i) the short-wavelength (S-cone), which is sensitive to blue, (ii) a medium-wavelength (M-cone), sensitive to green, and (iii) a long-wavelength (L-cone), sensitive to red, which makes up the majority of light entering the eye.
Thus it is a combination of photosensitive information from these cells which produces the interpretation of color. However, the distribution and density of rods and cones can vary widely depending on the animal, from over one hundred million in human beings, to far less in species of birds and reptiles. Following synaptic transmission to the bipolar and amacrine cells, which help to organize information from rods and cones, visual light, activates the retinal ganglion cells, neurons containing photo-opsins, which in turn communicate to the brain via the retinohypothalamic tract.
Figure 2.2: Simplified schematic of the mammalian retina outlining location of cell types and interrelation ââ‚¬" Lok, 2011 (Lok 2011)
2.2.2 Visual light
Visual light is extremely important for the way organisms interact with their environment, and varies depending on the species. Light itself is only one small part of the overall electromagnetic spectrum, which includes radio and microwaves, as well as more exotic types of energy. Human beings have a relatively limited visual spectrum which they directly view (Fig. 2.3) ranging from 380nm (violet) to 740nm (red), and a frequency of 405 THz to 790 THz. Outside of this spectrum of light on either side are ultraviolet (10-~380nm) and infrared (750-1000nm). Certain species of insects and birds are able to see in the ultraviolet spectrum due to adaptations for discerning certain contrasting objects. Certain damages to cone structure in the retina in humans can cause color blindness which removes the ability to perceive certain colors within the visible spectrum, a disease with a prevalence as high as 5% in the general population.
Figure 2.3: Visible spectrum of light for the human eye surrounded by UV and IR bands.
2.3 - Non-visual Light and Melanopsin
2.3.1 Non-visual light
For many years, non-visual light was not considered substantial in its influence upon key regulatory systems in the brain, and the assumption was that previously known photoreceptors involved in vision were responsible for processes such as circadian regulation. Thus these subsets of intrinsically photosensitive retinal ganglion cells (ipRGCs), which are now known to specifically transmit this information, were largely ignored upon their initial discovery. Though first identified in 1923 by Clyde Keeler, it was not until the end of the 20th century that their significance was truly understood. The principle finding in 1991 by Foster et al., was that this highly specialized cell was directly responsible for sending information to a variety of brain areas responsible for different functions (Foster, Provencio et al. 1991). It was shown that non-visual light mediated the circadian cycle through activation of the SCN, and additionally, controlled the pupillary light reflex, a function of the olivary pretectum nucleus. Furthermore, the function of melatonin, a hormone secreted by the pineal gland and critical for inducing sleep, was altered depending on changes to environmental non-visual light. However, it was the discovery of melanopsin (Opn4), a new photopigment within these cells, in 2000 by Provencio and colleagues, which showed the most promise for scientific advancement (Provencio, Rodriguez et al. 2000).
Opn4, is a photopigment whose existence was initially suggested by the group of Foster, who in 1999 identified a continued response to light in mice which possessed neither rods nor cones (Lucas, Freedman et al. 1999). These mice when placed under different light and dark conditions were still able to entrain their circadian cycle despite a total lack of visual input. Following this research axis, Berson underlined that these photosensitive cells were completely separate and were able to depolarize in response to light even when synaptic transmissions from rods and cones were blocked (Berson 2007). Later this same team would identify the sensitivity of Opn4 by using various levels of light and darkness to describe its responsiveness. Critical to understanding the contribution of this non-visual melanopsin-mediated information was to identify the areas of the brain being targeted by projections from the retinohypothalamic tract (RHT). Groups lead by Saper and Hannibal (Gooley, Lu et al. 2001; Hannibal and Fahrenkrug 2004) demonstrated that these melanopsin ganglion cells projected directly via the RHT to the SCN and other areas of the brain, thus confirming the initial experiments by Foster. Several years later researchers identified the absorption peak of the photopigment as centered to the blue light (460-480nm), close to the visual spectrum of the S-cones, suggesting wavelength specific adaptation of the system (Fig. 2.4). Use of a rodless-coneless animal was extremely important in proving the existence of Opn4, however it was not until the use of a melanopsin knock-out mice that the contributions were specifically elucidated. In 2008 and 2009, concurrent research studies using this model from Tsai et al. and Altimus et al, and Lupi et al., identified a direct non-circadian influence of non-visual light on sleep and alertness, mediated by Opn4 (Altimus, Guler et al. 2008; Lupi, Oster et al. 2008; Tsai, Hannibal et al. 2009). These findings will be more rigorously discussed in a review presented in chapter 3.
Figure 2.4: Melanopsin photopigment absorption peak adapted from Hankins et al. (Hankins, Peirson et al. 2008)
These ipRGCs though extremely small in number, accounting for less than 3% of mammalian ganglion cells, have now been highly implicated in a variety of regulatory processes. As previously discussed, melanopsin is able to directly control circadian rhythmsÂ through modulation of photic phases (Panda, Sato et al. 2002; Ruby, Brennan et al. 2002).
2.3.5 Innervations from the retina to the brain
Signals coming from the retina are transduced down the RHT, specifically innervating several cerebral structures (summarized in Fig. 2.5). However not all areas are innervated evenly. The areas of highest innervation are the SCN in the hypothalamus, the intergeniculate leaflet, and the olivary pretectum, responsible for circadian synchronization, interpretation of photic information, and the pupillary reflex, respectively. To a lesser extent is the VLPO in the hypothalamus, an area with a high concentration of sleep-promoting (galanin-containing GABAergic) neurons. An additional area of high innervation is the superior colliculus, which has a variety of functions including control of eye movements, and sleep regulation. The weakest projections are seen to the dorsal lateral geniculate nucleus, which functions mainly as a visual relay, and the subparaventricular zone (SPVZ), involved in regulating body temperature and food-energy intake (Hattar, Liao et al. 2002; Hannibal and Fahrenkrug 2004).