Limitations of cyclosporin A inhibition of the permeability transition in CNS mitochondria

Activation of the mitochondrial permeability transition may contribute to e xcitotoxic neuronal death (Ankarcrona et al., 1996; Dubinsky and Levi, 1998 ). However, cyclosporin A (CsA), a potent inhibitor of the permeability tra nsition in liver mitochondria, only protects against neuronal injury by lim ited doses of glutamate and selected ischemic paradigms. The lack of consis tent CsA inhibition of the mitochondrial permeability transition was analyz ed with the use of isolated brain mitochondria. Changes in the permeability of the inner mitochondrial membrane were evaluated by monitoring mitochond rial membrane potential (Delta psi), using the distribution of tetraphenylp hosphonium, and by monitoring mitochondrial swelling, using light absorbanc e measurements. Metabolic impairments, large Ca2+ loads, omission of extern al Mg2+, or low doses of palmitic acid or the protonophore FCCP exacerbated Ca2+-induced sustained depolarizations and swelling and eliminated CsA inh ibition. BSA restored CsA inhibition in mitochondria challenged with 50 muM Ca2+, but not with 100 muM Ca2+. CsA failed to prevent Ca2+-induced depola rization or to repolarize mitochondria when mitochondria were depolarized e xcessively. Similarly, CsA failed to prevent mitochondrial swelling or PEG- induced shrinkage after swelling when the Ca2+ challenge produced a strong, sustained depolarization. Thus in brain mitochondria CsA may be effective only as an inhibitor of the permeability transition and the Ca2+-activated low permeability state under conditions of partial depolarization. In contr ast, ADP plus oligomycin inhibited both permeabilities under all of the con ditions that were tested. In situ, the neuroprotective action of CsA may be limited to glutamate challenges sufficiently toxic to induce the permeabil ity transition but not so severe that mitochondrial depolarization exceeds threshold.