Selective disruption of the mammalian secretory apparatus enhances or eliminates calcium current modulation in nerve endings

Modulation of secretion via G protein-coupled receptors (GPCRs) serves an important regulatory function in neuronal and nonneuronal secretory cells. Most secretory cells possess voltage-gated calcium channels, share homologues of the core complex of three proteins (the SNAREs) that constitute the secretory apparatus, and are modulated by GPCR activation. Activators of GPCRs generally inhibit the release of neurotransmitter substances to a maximum of only 50-60% of the control level, suggesting that complex protein-protein interactions may govern the efficacy of this form of modulation. In this article, molecular genetic approaches are used in combination with botulinum toxins (selective molecular scalpels that cleave the SNAREs at highly restricted loci) to address this issue. The results suggest that the cleavage of either of the plasma membrane SNAREs (syntaxin or SNAP-25) prevents modulation of calcium currents by A₁ adenosine receptors at mammalian motor nerve endings. In contrast, cleavage of the synaptic vesicle SNARE (synaptobrevin) in conjunction with deletion of the vesicledocking protein Rab3A greatly enhances the efficacy of calcium current modulation.