Whole-cell currents in cultured hippocampal neurons were recorded to investigate the effects of SO-3, a new O-superfamily conopeptide derived from Conus striatus, on voltage-sensitive channels.SO-3 had no effect on voltage-sensitive sodium currents, delayed rectifier potassium currents, and transient outward potassium currents.Similar to the selective N-type calcium channel blocker ω-conotoxin MVIIA (MVIIA), SO-3 could concentration-dependently inhibit the high voltage-activated (HVA) calcium currents (ICa).MVIIA(3 μM), 10 μM nimodipine, and 0.5 μM ω-agatoxin IVA (Aga) could selectively block the N-, L-, and P/Q-type ICa, which contributed ∼32, ∼38, and ∼21% of the HVA currents in hippocampal neurons, respectively. About 31% of the total HVA currents were inhibited by 3 μM SO-3. SO-3 (3 μM) and 3 μM MVIIA inhibited the overlapping components of HVA currents, whereas no overlapping component was inhibited by 3 μM SO-3 and 10 μM nimodipine, or by 3 μM SO-3 and 0.5 μM Aga. Also, 3 μM SO-3 had no effect on R-type currents.SO-3 had less inhibitory effects on non-N-type HVA currents than MVIIA at higher concentrations (30 and 100 μM).The inhibitory effects of SO-3 and MVIIA on HVA currents were almost fully reversible. However, the recovery from block by MVIIA was more rapid than recovery from block by SO-3.It is concluded that SO-3 is a new ω-conotoxin selectively targeting N-type voltage-sensitive calcium channels. Considering the significance of N-type calcium channels for pain transduction, SO-3 may have therapeutic potential as a novel analgesic agent.British Journal of Pharmacology (2005) 145, 728–739. doi:10.1038/sj.bjp.0706223; published online 9 May 2005 [ABSTRACT FROM AUTHOR]