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 μM) 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 μM) 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 μM) 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.