Handbook of Photosensory Receptors (BRIGGS:PHOTORECEPTORS O-BK) || A Novel Light Sensing Pathway in the Eye: Conserved Features of Inner Retinal Photoreception in Rodents, Man and Teleost Fish
✍ Scribed by Briggs, Winslow R.; Spudich, John L.
- Book ID
- 102682114
- Publisher
- Wiley-VCH Verlag GmbH & Co. KGaA
- Year
- 2005
- Tongue
- German
- Weight
- 700 KB
- Edition
- 1
- Category
- Article
- ISBN
- 3527310193
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✦ Synopsis
Until recently the light sensing capabilities of the eye have been considered well understood. Rods and cones of the outer retina were thought to be the only photoreceptors of the eye, with the neurones of the inner retina providing the initial stage of visual processing. However, studies on mice lacking rod and cone photoreceptors have shown that these animals can adjust their circadian clocks, suppress pineal melatonin, modify locomotor activity, and regulate pupil size in response to environmental brightness. At least two of these responses, circadian regulation and pupillary constriction, are regulated by a previously uncharacterized opsin/vitamin A-based photopigment with a wavelength of maximum sensitivity λ max near 480 nm (OP 480 ). The use of calcium imaging in wholemount retinal preparations has demonstrated the existence of a heterogeneous network of directly light-sensitive neurones within the ganglion cell layer of the retina. At least some of these light sensitive neurones express melanopsin. The ablation of this gene in mice with no rods or cones abolishes all known responses to light. Thus rods, cones and melanopsin neurones fully account for all photoreception within the eye, and melanopsin remains the primary candidate for OP 480 . Studies in humans have identified an opsin/vitamin A based photopigment with a λ max ∼480 nm, and hence a probable homolog of OP 480 . This photopigment regulates both circadian responses to light and appears to act through a local retinal mechanism to drive diurnal changes in the primary visual pathway.
Whilst considerable attention has been paid to the inner retinal photoreceptors of mammals, little attention has been paid to the physiological function of these neurones in fish. Recent electrophysiological evidence suggests that one function of this inner retinal photoreceptor is to modulate the activity of retinal horizontal cells in response to environmental irradiance. The action spectrum for this depolarizing response fits a single opsin photopigment with a λ max of 477 nm. Both VA-opsin and melanopsin appear to be expressed in these intrinsically photosensitive horizontal cells, but it remains unclear which of these opsins form the photopigment.