Dysfunction of the fusion of pre-synaptic plasma membranes and synaptic vesicles caused by oxidative stress, and its prevention by vitamin E.

To define whether hyperoxia induces the dysfunction of membrane fusion between synaptic vesicles with pre-synaptic plasma membranes in the nerve terminals, and whether vitamin E prevents this abnormal event, we investigated the influence of hyperoxia on the fusion ability of isolated synaptic vesicles and the inside-out type pre-synaptic plasma membrane vesicles from rat brain using the fluorescence tracing method. The membrane fusion ability of both membranes from rats subjected to hyperoxia was markedly decreased compared with the membranes from a normal rat. Rats subjected to hyperoxia in the form of oxidative stress showed significant increases in the levels of thiobarbituric acid reactive substances (TBARS), conjugated dienes, and protein carbonyl moieties in both synaptic vesicles and pre-synaptic plasma membranes. When rats were supplemented with vitamin E, these abnormalities were inhibited even when rats were subjected to hyperoxia.

Influence of oxidative stress on fusion of pre-synaptic plasma membranes of the rat brain with phosphatidyl choline liposomes, and protective effect of vitamin E.

Influence of oxidative stress on fusion of pre-synaptic plasma membranes with phosphatidylcholine (PC) liposomes as a model of synaptic vesicle was investigated. The inhibitory effect of vitamin E on the decline in the fusion caused by oxidative stress was also assessed. Rats subjected to hyperoxia as oxidative stress showed significant increases in the levels of lipid hydroperoxides and protein carbonyl moieties in pre-synaptic plasma membranes in the brain. The zeta potential of pre-synaptic membrane surface was decreased markedly. When synaptosomes were incubated with PC liposomes labeled by either rhodamine B or calcein as a fluorescence probe, or 12-doxyl stearic acid as an ESR spin trapping agent, translocation of each probe into oxidatively damaged pre-synaptic membranes was decreased significantly. Fatty acid composition analysis in pre-synaptic membranes obtained from normal rats revealed a marked increase in linoleic acid and a moderate decrease in docosahexaenoic content after the incubation with liposomes. However, rats subjected to hyperoxia did not show marked changes in these fatty acid contents in their pre-synaptic membranes after the incubation. Such changes caused by hyperoxia were inhibited by vitamin E treatment of rats. These results suggest that oxidative damage of pre-synaptic membranes caused by oxidative stress lowers the lipid-mixing for the membrane fusion. The results of this study imply that vitamin E prevents the deficit in neurotransmission at nerve terminals due to the decline in fusion between pre-synaptic membrane and synaptic vesicles caused by oxidative membrane damage.