Cell physiology of the pineal body

The results from recent experiments on the cellular physiology of the trout pineal photoreceptors are briefly reviewed. The arguments are mainly concerned with pineal phototransduction. These studies have stimulated further research on melatonin, a molecule produced in pineal as well as in retinal photoreceptors. A discussion follows on our actual research object, that is a study of the influences of endogenous melatonin upon retinal receptor cells activities

Retinal photoreceptors of Syrian hamsters undergo oxidative stress during streptozotocin-induced diabetes.

AIMS/HYPOTHESIS: The aim of this study was to verify whether retinal photoreceptors, like other tissues, are subject to oxidative stress during diabetes. METHODS: Oxidative stress was monitored by the oxidation of preloaded dehydrorhodamine 123 into fluorescent rhodamine 123, during a period of intense illumination of isolated rod retinal receptor cells. These were obtained from 22 Syrian hamsters injected with streptozotocin (50 mg/kg body weight., intraperitoneal route) 90 days before the study began. Eleven hamsters were treated daily with melatonin (0.4 mg/kg body wt., per os), an antioxidant synthesized within photoreceptors. Isolated photoreceptors were bathed on the stage of a Leitz Orthoplan microscope, where the fluorescent lamp also served as the light stimulus (485 nm). Fluorescence irradiance was measured by photometry and stored in a personal computer for further analysis. RESULTS: The light-induced oxidant production greatly decreased and was also delayed in the streptozotocin-injected hamsters compared with the control hamsters matched for age. Similar effects were obtained in control photoreceptors after 40 min incubation with 2-2'-azobis (2-amidinopropane) dihydrochloride, a potent lipoperoxidation inducer. The effect of melatonin was to partially restore the light-induced fluorescence response. CONCLUSION/INTERPRETATION: The depression of the light-induced oxidative response in diabetic photoreceptors could be ascribed to a hyperglycaemia-induced background of oxidative stress whereby the light-oxidizable substrate is actually lowered. Melatonin induces a larger fluorescence response during illumination, probably as a consequence of its antioxidant effect during diabetes, which would provide more oxidizable lipids

EFFECTS OF MELATONIN ON LIPID PEROXIDATION INDUCED BY OXYGEN RADICALS

We here report the activity of the neurohormone melatonin (MLT) as a scavenger of free radicals in two different experimental models: (a) linoleic acid peroxidation initiated by different free radical-generating systems and (b) a multilamellar vesicle system composed of dilinoleoylphosphatidylcholine. In system (a) linoleic acid peroxidation, induced by either the water-soluble initiator 2,2'-azobis (2-amidinopropane) dihydrochloride (ABAP) or Fe2+-EDTA addition to 2.6 mM linoleic acid dispersed in SDS-phosphate buffer, was evaluated as the formation of conjugated dienes, measured spectrophotometrically at 236 nm. MLT did not reduce the rate of peroxidation induced by ABAP, but did reduce, in a concentration-dependent fashion, the rate of the reaction activated by Fe2+-EDTA. In system (b) multilamellar vesicles were used as the substrate for lipid peroxidation, initiated by Fe2+-EDTA and determined by means of malonaldehyde (MDA) and 4-hydroxyalkenal (4-HDA) content. MLT was found to be slightly more effective in system (b) than in the dispersed linoleic acid system (see a). These results show that MLT inhibits lipid damage induced by oxygen free radicals. However, MLT is only about one one-hundredth as effective an antioxidant as vitamin E in the micelles system