The effects of various K+concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal45Ca2+influx by ω-conotoxin GVIA (ω-CgTx) and neomycin were examined. K+(15-75 mM) caused a concentration-dependent release of [3H]-norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of ω-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+and was reduced substantially at higher concentrations of K+.ω-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25mM K+), 22% (50mM K+), and 12% (75mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with ic50values of 210 μM (15mM K+), 150 μM (25mM K+), 450 μM (50mM K+), and 1500 μM (75mM K+). ω-CgTx (1μM) had little effect (10% or less inhibition) on hippocampal synaptosomal45Ca2+influx at any concentration of K+whereas 3 mM neomycin caused at least 75% inhibition of45Ca2+influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K+evoked release of [3H]norepinephrine. The comparative absence of (ω-CgTx-sensitive synaptosomal45Ca2+-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit (ω-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC. © 1993.
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