A novel ω-conopeptide for the presynaptic localization of calcium channels at the mammalian neuromuscular junction

Voltage-sensitive Ca²⁺ channels are essential to transmitter release at the chemical synapse. To demonstrate the localization of voltage-sensitive Ca²⁺ channels in relation to the site of transmitter release, mouse neuromuscular junctions were double-labelled with α-bungarotoxin and a novel voltage-sensitive Ca²⁺ channel probe, SNX-260, a synthetic analog of ω-conopeptide MVIIC. Similar to ω-conopeptide MVIIC, biotinylated SNX-260 blocked nerve-stimulated transmitter release at the mouse neuromuscular junction. Fluorescently-tagged biotinylated SNX-260 labelled the nerve terminal which appeared thinner than and was outlined by acetylcholine receptor clusters as seen in en face view. This SNX-260 labelling was inhibited by preincubation with unconjugated SNX-260. Side-views of the neuromuscular junction indicated that the SNX-260 labelling was on the synaptic side facing the acetylcholine receptor rather than on the nonsynaptic side of the nerve terminal. This presynaptic binding was confirmed by the absence of SNX-260 labelling in denervated muscles following a nerve cut or disjunction after collagenase treatment. Confocal microscopy revealed spots of SNX-260 labelling that may correlate with active zones. The SNX-260 labelling pattern was not affected by preincubation with unconjugated SNX-111 (ω-conopeptide MVIIA), an N-type voltage-sensitive Ca²⁺ channel blocker. These findings suggest that SNX-260 is a novel probe for localizing non-N type voltage-sensitive Ca²⁺ channels and that these voltage-sensitive Ca²⁺ channels are localized near the transmitter release sites at the mammalian motor nerve terminal membrane. The results are consistent with the suggestion that non-N, probably P/Q type voltage-sensitive Ca²⁺ channels mediate evoked transmitter release at the mammalian neuromuscular junction.