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W. C. Gordon; Amphibian Cone Photoreceptors With Paraboloids Utilize Glycogen During Hibernation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3076.
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© ARVO (1962-2015); The Authors (2016-present)
Endoplasmic reticulum is a Ca++ intracellular storage compartment involved with mitochondria in Ca++ regulation. However, disturbance of Ca++ homeostasis leads to ER dysfunction; hypoxia, hypoglycemia, and injuries initiate ER dysfunction and stress, leading to growth arrest and DNA damage-inducible genes. In photoreceptors of hibernating animals, constructive processes are halted and destructive mechanisms slowly alter cells, including dissociation of synaptic ribbons into cytoplasm, dramatic organizational changes in ER, and loss of outer segments. In this study we examined photoreceptors of frogs that had just emerged from hibernation to determine if rod and cone responses to conditions of hibernation were different.
Newly emerged adult frogs (Rana pipiens) and 1-month-emerged frogs were obtained from J.M. Hazen Frog Company, Alberg VT, and kept under 14L:10D conditions until sacrifice. Retinas were prepared for EM and photoreceptors examined.
Inner segment ER of rods and cones was examined. No significant differences were found between rods and cones of newly emerged frogs and frogs obtained 1 month later, with the exception of the 502 nm accessory cones. These cells showed packed granular glycogen-filled paraboloids in 1-month-emerged frogs, but all accessory cones in newly emerged frogs were devoid of these deposits. Large nucleus-sized arrays, up to 5 um across, of empty ER were found near inner segment distal ends at the usual paraboloid site. These arrays consisted of ER sheets arranged in spirals and concentric rings, with peripheral ribosomes. Many sheets were interconnected.
Glycogen is found throughout inner segments in amphibian rods and within cone paraboloids. Reptile and bird accessory cone paraboloids are composed of dense tubular ER arrays packed with glycogen, and liver glycogen is located within a loose ER array. This suggests that these large ER arrays in frog accessory cones are nascent paraboloids, depleated of glycogen. Moreover, glycogen loss during hibernation implies an active process to maintain these 502 nm cones. A huge glycogen store only within one cell type suggests some unique function in that cell, and because it takes 1 week to fully recover visual function after hibernation, maintenance of these 502 photopic green-sensitive receptors at a level above that of other photoreceptors may impart selective advantages to these animals.
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