Retinas

The retina comprises two principal components, the non-neural retinal pigment epithelium and the neural retina. The retinal pigment epithelium is an essential component of the visual system both structurally and functionally. It is important for the turnover and phagocytosis of photoreceptor outer segments, the metabolism of retinoids, the exchange of nutrients between the photoreceptors, and the choroidal blood vessels and the maintenance of an efficient outer blood-retinal barrier. 

It is now widely appreciated that polyunsaturated fatty acids (PUFAs) are highly susceptible to oxidative damage. Indeed, the process of lipid peroxidation was broadly defined as the oxidative deterioration of polyunsaturated lipids by Tappel (1979). The presence of a double 

The longitudinal effects of experimental vitamin E deficiency on visual function in the rat have been studied by Goss-Sampson et al. (1992). After 12 months of deficiency, visual function as assessed by electroretinography was absent or grossly abnormal. This was associated with 

Patients with abetalipoproteinaemia, a rare inborn disorder of lipoprotein metabolism, are totally deficient in vitamin E fiom birth and, if untreated, invariably develop a characteristic pigmentary retinopathy similar to that seen in retinitis pigmentosa and peroxisomal disorders. The same retinopathy has been observed in other patients with severe and chronic vitamin E deficiency. A essive vitamin E replacement therapy in all these patients has been shown either to prevent, to halt the progression of, or in some cases, to improve the characteristic visual abnormalities (Muller and Lloyd, 1982). 

In the retina, Glu functions as an inhibitory transmitter in certain photoreceptor-bipolar cell synapses. When light hyperpolarizes the photoreceptor, the release of Glu in the photoreceptor-bipolar cell synapse diminishes. This results in inhibition of off-center bipolar cells, as expected from diminished release of an excitatory transmitter, whereas on-center bipolar cells are excited (Copenhagen, 1991). The Glu excitation of off-center bipolar cells is due to activation of ionotropic Glu receptors, whereas the Glu-induced inhibition of on-center bipolar cells is dependent on metabotropic (mGluR6) receptors (Nakajima et al., 1993 Euler et al., 1996 Sasaki and Kaneko, 1996 Brandstatter et al., 1997 Vardi and Morigiwa, 1997 DeVries and Schwartz, 1999 Morigiwa and Vardi, 1999). 

The presence of Glu has also been examined in the cerebral cortex of humans (Aas et al., 1993). Slices of macroscopically normal parietal or temporal neocortex were obtained 

Aas J-E, Laake JH, Brodal P, Ottersen OP (1992) Immunocytochemical evidence for in vitro release of glutamate and GABA from separate nerve terminal populations in the rat pontine nuclei. Exp Brain Res S9 540-548. 

Aas J-E, Berg-Johnsen J, Hegstad E, Laake JH, Langmoen lA, Ottersen OP (1993) Redistribution of glutamate and glutamine in slices of human neocortex exposed to combined hypoxia and glucose deprivation in vitro. J Cereb Blood Flow Metab 7 503-515. 

Afsharpour S (1985) Topographical projections of the cerebral cortex to the subthalamic nucleus. J Comp Neurol 236 14-28. 

The high level of polyunsaturated fatty acids of the rod outer segment membranes probably insures a fluid membrane at all times, but makes the membranes particularly vulnerable to oxidative processes. However, bovine rod outer segments possess unusually high concentrations of a-tocopherol (Dilley and McConnell 1970, Farnswoth and Dratz 1976). In addition, there is a high level of superoxide dismutase (Hall and Hall 1975). 

Electron spin resonance spin trapping analysis of the signals obtained from the perfused rabbit retina showed that hydroxyl radical adducts were generated during the ischaemic period with no further increase at reperfusion (Muller et al. 1997). Electron spin resonance detection of the ascorbyl free radical/dimethyl sulfoxide complex in the retina confirmed that an oxidative stress occurred under ischaemia and was not increased under reperfusion. 

The flux of hydroxyl radicals, as measured directly by conversion of salicylate to 2,3- and 2,5-dihydroxybenzoic add (formula [177]), was significantly lower in gallium-desferrioxamine-treated retinas of cat eyes subjected to 90 min retinal ischaemia followed by 5 min of reperfusion (Banin et al. 2000). Gallium-desferrioxamine caused a significant reduction, by 2.56-fold, in lipid peroxidation, as reflected by levels of malondialdehyde. Acorbic acid, a natural antioxidant present in the retina, was severely depleted in untreated eyes. In contrast, in gallium-desferrioxamine-treated eyes, levels were 10 times higher than the control. 

In the turtle, Pseudemys scripta elegans, two morphologically different types of amacrine cells were nNOS/glycine-immunoreactive and three types are nNOS/v-aminobutyric acid-immuno-reactive (Haverkamp et al. 2000). 

Nitric oxide ( NO) stabilises whereas nitroso-nium (NO ) enhances filopodial outgrowth by rat retinal ganglion cells in vitro (Cheung et al. 2000). 

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The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

Melatonin. Melatonin (A/-acetyl-5-metlioxytryptaniine) [73-31-4] C 2H gN2O2(250) is secreted from the pineal gland and retina during dark periods of the vertebrate circadian rhythm (65). Melatonin regulates biological rhythms and neuroendocrine function and is formed from serotonin (5-HT). 

Melatonin produces its effects via the GPCR, ML-1. A second lower affinity form, ML-2, has been described on the basis of binding data. Activation of melatonin receptors can inhibit DA release in the retina. 

George Wald medicine, physiology research on chemical processes that allow pigments in eye retina to convert light into vision... 

The distribution of rods and cones is shown in Figure 3b centered about the fovea, the area of the retina that has the highest concentration of cones with essentially no rods and also has the best resolving capabiUty, with a resolution about one minute of arc. The fovea is nominally taken as a 5° zone, with its central 1° zone designated the foveola. There are about 40 R and 20 G cones for each B cone in the eye as a whole, whereas in the fovea there are almost no B cones. A result of this is that color perception depends on the angle of the cone of light received by the eye. The extremely complex chemistry involved in the stimulation of opsin molecules, such as the rhodopsin of the rods, and the neural connections in the retinal pathway are well covered in Reference 21. 

Threshold contrast is illustrated in Fig. 10-2. / is the intensity of light received by the eye from the object, and / + A/ represents the intensity coming from the surroundings. The threshold contrast can be as low as 0.018-0.03 and the object can still be perceptible. Other factors, such as the physical size of the visual image on the retina of the eye and the brain s response to the color of the object, influence the perception of contrast. 

Vision is vital for human activities, and eyes are very sensitive to a number of toxic insults induced by chemical compounds. The most serious outcome is permanent eye damage which may be so severe as to cause loss of vision. The eye consists of the cornea and conjunctiva, the choroid, the iris, and the ciliary body. It also contains the retina, which is of neural origin, and the optic nerve. The retina contains photoreceptors, a highly specific light-sensitive type of neural tissue. The eye also contains the lens and a small cerebrospinal fluid system, the aqueous humor system, that is important for the maintenance of the steady state of hydration of the lens and thus the transparency of the eye. 

Methanol intoxication can cause blindness due to damage to ganglion cells in the retina. The blindness results from the accumulation of formaldehyde and formic acid, which are metabolites of methanol. Chemical compounds can also damage the visual cortex, for example, visual damage was observed among the victims of organic mercury intoxication in Japan (the fishermen of Minamata Bay). ... 

When the retina becomes detached from the back of the eye, as in the case of nearsightedness or injury sustained in car accidents, it can be weighted down by injecting liquid fluorocarbon After the retina is reattached b> laser surgery or by... 

The retinol that is delivered to the retinas of the eyes in this manner is accumulated by rod and cone cells. In the rods (which are the better characterized of the two cell types), retinol is oxidized by a specific retinol dehydrogenase to become 2iW-trans retinal and then converted to 11-eis retinal by reti-... 

The complex thioamide lolrestat (8) is an inhibitor of aldose reductase. This enzyme catalyzes the reduction of glucose to sorbitol. The enzyme is not very active, but in diabetic individuals where blood glucose levels can. spike to quite high levels in tissues where insulin is not required for glucose uptake (nerve, kidney, retina and lens) sorbitol is formed by the action of aldose reductase and contributes to diabetic complications very prominent among which are eye problems (diabetic retinopathy). Tolrestat is intended for oral administration to prevent this. One of its syntheses proceeds by conversion of 6-methoxy-5-(trifluoroniethyl)naphthalene-l-carboxyl-ic acid (6) to its acid chloride followed by carboxamide formation (7) with methyl N-methyl sarcosinate. Reaction of amide 7 with phosphorous pentasulfide produces the methyl ester thioamide which, on treatment with KOH, hydrolyzes to tolrestat (8) 2[. 

On the basis of this work and other research, in 1849 Helmholtz was appointed professor of physiology at the University of Koonigsberg. There he devoted himself to the physiology of the eye, first explaining the mechanism of lens accommodation. In 1851 Ins invention of the ophthalmoscope, still the basic instrument used by eye doctors to peer at the retina of the eye, immediately made Helmholtz famous. In 1852 he also became the first experimentalist to measure the speed of nerve impulses in the human body. 

Photoreceptors in the eyes convert radiation in the visible band into neural signals that reach the brain. Photoreceptors are located throughout the retina, a sensoiy membrane that covers the entire back of the... 

The photoreceptron consists of three basic parts (1) the retina, which is made... 

There are two main types of light-sensitive receptor cells in the retina of the human eye, rod cells and cone cells. The 3 million or so rod cells are... 

Sulfur hexafluoride is a gas used as a long-term tamponade (plug) for a retinal hole to repair detached retinas in the eye. If 2.50 g of this compound is introduced into an evacuated 500.0-mL container at 83°C, what pressure in atmospheres is developed ... 

A disease process characterized by deterioration of the macula of the retina that results in a loss of shaip central vision. AMD is the leading cause of central vision loss in the developed countries today for those over the age of fifty years. 

AQP4 is the predominant water channel in the central nervous system (CNS), where it is involved in maintaining brain water balance and neural signal transduction. It is mainly expressed in astroglial cells, which support the neurons. Outside the CNS, AQP4 has been found in the basolateral membrane of renal principal cells as well as in various glandular epithelia, airways, skeletal muscle, stomach, retina and ear. 

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