The phosphatidylinositol 4-kinase inhibitor phenylarsine oxide blocks evoked neurotransmitter release by reducing calcium entry through N-type calcium channels

1 The effects of the phosphatidylinositol 4-kinase inhibitor, phenylarsine oxide (PAO), on acetylcholine (ACh) release and on prejunctional Ca2+ currents were studied at the frog neuromuscular junction using electrophysiological recording techniques. 2 Application of PAO (30 mu M) increased both spontaneous ACh release reflected as miniature endplate potential (mepp) frequencies and evoked ACh release reflected as end-plate potential (epp) amplitudes with a similar time course. 3 Following the initial increase in epp amplitudes produced by PAO, epps slowly declined and were eventually abolished after approximately 20 min. However, mepp frequencies remained elevated over this time period. 4 PAO (30 mu M) also inhibited the perineural voltage change associated with Ca2+ currents through N-type Ca2+ channels (prejunctional Ca2+ currents) at motor nerve endings. Addition of British anti-lewisite (BAL, 1 mM), an inactivator of PAO, partially reversed both the inhibition of epps and the inhibition of the prejunctional Ca2+ current. 5 The effects of PAO on N-type Ca2+ channels were investigated more directly using the whole cell patch clamp technique on acutely dissociated sympathetic neurons. Application of PAO (30-40 mu M) to these neurons decreased the voltage-activated calcium currents through N-type Ca2+ channels, an effect that was partially reversible by BAL. 6 In combination, these results suggest that inhibition of neurotransmitter release by PAO occurs as a consequence of the inhibition of Ca2+ entry via N-type calcium channels. The relationship between the effects of PAO on N-type Ca2+ channels in motor nerve endings and in neuronal soma is discussed.