Effects of stimulus intensity on the inhibition by ω-conotoxin GVIA and neomycin of K +-evoked [ 3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

Abstract

The effects of various K +concentrations on the inhibition of [ 3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca 2+ 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 ic 50 values 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 synaptosomal 45Ca 2+ influx at any concentration of K + whereas 3 mM neomycin caused at least 75% inhibition of 45Ca 2+ 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 synaptosomal 45Ca 2+-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.