Background:The major source of NADPH is considered to be the pentose phosphate pathway. Results: In isolated mouse rod photoreceptors, blocking metabolite entry into mitochondria substantially reduces NADPH generation. Mitochondrial metabolic substrates support NADPH generation. Conclusion: Mitochondria-linked pathways contribute substantially to NADPH generation in mouse rod photoreceptors. Significance: A wide range of photoreceptor cell functions depend on adequate NADPH supply.

G-protein receptor kinase and arrestin 1 are required for inactivation of photoactivated vertebrate rhodopsin. Frederiksen et al. show that they additionally regulate the subsequent decay of inactive rhodopsin into opsin and all-trans retinal and therefore dark adaptation.

High-dose remifentanil decreases rCBF and mean CBFv without impairing cerebrovascular carbon dioxide reactivity. This, together with its known short duration of action, makes remifentanil a useful agent in the intensive care unit when sedation that can be titrated rapidly is required.

SUMMARY The bis-retinoid N-retinylidene-N-retinylethanolamine (A2E) is one of the major components of lipofuscin, a fluorescent material that accumulates with age in the lysosomes of the retinal pigment epithelium (RPE) of the human eye. Lipofuscin, as well as A2E, exhibit a range of cytotoxic properties, which are thought to contribute to the pathogenesis of degenerative diseases of the retina such as Age-related Macular Degeneration. Consistent with such a pathogenic role, high levels of lipofuscin fluorescence are found in the central area of the human RPE, and decline toward the periphery. Recent reports have however suggested a surprising incongruence between the distributions of lipofuscin and A2E in the human RPE, with A2E levels being lowest in the central area and increasing toward the periphery. To appraise such a possibility, we have quantified the levels of A2E in the central and peripheral RPE areas of 10 eyes from 6 human donors (ages 75–91 years) with HPLC and UV/VIS spectroscopy. The levels of A2E in the central area were on average 3–6 times lower than in peripheral areas of the same eye. Furthermore, continuous accumulation of selected ions (CASI) imaging mass spectrometry showed the presence of A2E in the central RPE, and at lower intensities than in the periphery. We have therefore corroborated that in human RPE the levels of A2E are lower in the central area compared to the periphery. We conclude that the levels of A2E cannot by themselves provide an explanation for the higher lipofuscin fluorescence found in the central area of the human RPE.

Interphotoreceptor retinoid-binding protein (IRBP) is a specialized lipophilic carrier that binds the all- and 11- isomers of retinal and retinol, and this facilitates their transport between photoreceptors and cells in the retina. One of these retinoids, all-retinal, is released in the rod outer segment by photoactivated rhodopsin after light excitation. Following its release, all-retinal is reduced by the retinol dehydrogenase RDH8 to all-retinol in an NADPH-dependent reaction. However, all-retinal can also react with outer segment components, sometimes forming lipofuscin precursors, which after conversion to lipofuscin accumulate in the lysosomes of the retinal pigment epithelium and display cytotoxic effects. Here, we have imaged the fluorescence of all-retinol, all-retinal, and lipofuscin precursors in real time in single isolated mouse rod photoreceptors. We found that IRBP removes all-retinol from individual rod photoreceptors in a concentration-dependent manner. The rate constant for retinol removal increased linearly with IRBP concentration with a slope of 0.012 min μm IRBP also removed all-retinal, but with much less efficacy, indicating that the reduction of retinal to retinol promotes faster clearance of the photoisomerized rhodopsin chromophore. The presence of physiological IRBP concentrations in the extracellular medium resulted in lower levels of all-retinal and retinol in rod outer segments following light exposure. It also prevented light-induced lipofuscin precursor formation, but it did not remove precursors that were already present. These findings reveal an important and previously unappreciated role of IRBP in protecting the photoreceptor cells against the cytotoxic effects of accumulated all-retinal.