Cholesterol depletion attenuates tonic release but increases the ambient level of glutamate in rat brain synaptosomes

The low level of ambient glutamate is important for the brain's spontaneous activity and proper synaptic transmission. Cholesterol deficiency has been implicated in the pathogenesis of several neurodegenerative disorders. It was examined whether membrane cholesterol modulated the extracellular glutamate level in nerve terminals and the processes responsible for its maintenance. The ambient l-[14C]glutamate level, being an equilibrium between Na+-dependent uptake and tonic release, was increased from 0.193 ± 0.013 nmol/mg protein to 0.282 ± 0.013 (extracellular endogenous glutamate—from 6.9 ± 2.0 to 16.6 ± 2.0, respectively) in rat brain synaptosomes treated with a cholesterol acceptor methyl-β-cyclodextrin (MβCD). This alteration was not due to the change in the activity of glutamine synthetase that was shown with the specific blocker l-methionine sulfoximine. In the presence of dl-threo-β-benzyloxyaspartate, which significantly reduced the contribution of glutamate transporters, net tonic release of l-[14C]glutamate was decreased by 38% and release in low-Na+ medium was attenuated by 41% after cholesterol extraction. Also, cholesterol-deficient synaptosomes showed a reduced content of cytosolic l-[14C]glutamate and a lower initial velocity of l-[14C]glutamate uptake. We suggested that cholesterol deficiency altered the intra-to-extracellular glutamate ratio by the reduction of the cytosolic level of the neurotransmitter and the augmentation of the ambient glutamate level, thereby favoring a decrease in tonic glutamate release. Thus, increased extracellular glutamate in cholesterol-deficient nerve terminals was not a result of the changes in tonic release and/or glutamine synthetase activity, but was set by lack function of glutamate transporters.