Okadaic acid and cyclosporin A modulate [ 3H]GABA release from rat brain synaptosomes

Rat brain synaptosomes were used to investigate the effect of okadaic acid, an inhibitor of protein phosphatase 1 and 2A, and cyclosporin A, an inhibitor of protein phosphatase 2B (calcineurin), on [ 3H]GABA release. Release of [ 3H]GABA was evoked by 4-aminopyridine in the presence of calcium and by α-latrotoxin in the presence and absence of calcium. Pretreatment of synaptosomes with 1 μM okadaic acid reduced [ 3H]GABA release evoked by 4-aminopyridine by about 40%. The effect of α-latrotoxin on [ 3H]GABA release was stimulated by okadaic acid. This stimulation was equal in both media. The stimulating effect of 4-aminopyridine and α-latrotoxin on [ 3H]GABA release was activated when synaptosomes were pretreated with cyclosporin A. Activation of 4-aminopyridine-evoked [ 3H]GABA release was observed at 1 μM cyclosporin A, but the toxin effect was enhanced only when concentration of cyclosporin A was increased to 10 μM. The level of cyclosporin A activation depended on α-latrotoxin concentrations used — a higher stimulating effect of cyclosporin A was observed with lower toxin concentration. These results suggest that in calcium medium 4-aminopyridine- and α-latrotoxin-evoked [ 3H]GABA release was realized by different mechanisms.