Phenylarsine oxide inhibits alpha-latrotoxin-stimulated [3H]GABA release from rat brain synaptosomes

Phosphatidylinositol 4,5-biphosphate has been implicated in a variety of membrane-trafficking processes, including exocytosis of neurotransmitters. However, there are contradictory findings concerned ability of phenylarsine oxide (PAO), an inhibitor of phosphatidylinositol 4-kinase, to affect exocytotic release of different types of neurotransmitters. We bent our efforts to a detailed analysis of action of PAO on Ca(2+)-dependent and Ca(2+)-independent [3H]GABA release produced by exposure of rat brain synaptosomes to different concentrations of alpha-latrotoxin. We also compared PAO action on alpha-latrotoxin- and 4-aminopyridine (4-AP)-evoked [3H]GABA release. The experiments have shown that release of [3H]GABA evoked by the depolarization with 4-AP was decreased by 80% as a result of action of 3 microM PAO and the complete inhibition of release was observed with 10 microM PAO. When alpha-latrotoxin as a stimulant was applied, release of [3H]GABA was increased as toxin concentration used was elevated from 0.5 to 3.0 nM, however, concomitantly, the response of the toxin-induced [3H]GABA release to PAO became attenuated: 10 microM PAO led to almost complete inhibition of the effect of 0.5 nM alpha-latrotoxin and only partly decreased (by 40%) the response to 3.0 nM alpha-latrotoxin. To test whether the efficacy of PAO depended on the toxin-induced outflow of cytosolic [3H]GABA, synaptosomes with depleted cytosolic [3H]GABA pool were also exploited. Depletion was performed by means of heteroexchange of cytosolic [3H]GABA with nipecotic acid. The experiments have shown that treatment of loaded synaptosomes with nipecotic acid resulted in some increase of [3H]GABA release evoked by 0.5 nM alpha-latrotoxin, but in the two-fold decrease of the response to 3.0 nM alpha-latrotoxin. PAO essentially inhibited [3H]GABA release from depleted synaptosomes irrespective of alpha-latrotoxin concentration used. Therefore, the amount of [3H]GABA released from cytosolic pool determined, in considerable degree, the insensitivity of alpha-latrotoxin action to PAO. Thus, our data show that subnanomolar concentrations of alpha-latrotoxin may be used for stimulation of exocytotic release of [3H]GABA. Exposure of synaptosomes with nanomolar toxin concentrations leads not only to stimulation of exocytosis, but also to leakage of [3H]GABA from cytosolic pool. PAO potently inhibits exocytotic release of [3H]GABA and its inhibitory effectiveness is diminished as far as the outflow of [3H]GABA is elevated.