Modulation of Neurotransmission by GPCRs Is Dependent upon the Microarchitecture of the Primed Vesicle Complex

Gi/o-protein-coupled receptors (GPCRs) ubiquitously inhibit neurotransmission, principally via Gβγ, which acts via a number of possible effectors. GPCR effector specificity has traditionally been attributed to Gα, based on Gα's preferential effector targeting in vitro compared with Gβγ's promiscuous targeting of various effectors. In synapses, however, Gβγ clearly targets unique effectors in a receptor-dependent way to modulate synaptic transmission. It remains unknown whether Gβγ specificity in vivo is due to specific Gβγ isoform-receptor associations or to spatial separation of distinct Gβγ pathways through macromolecular interactions. We thus sought to determine how Gβγ signaling pathways within axons remain distinct from one another. In rat hippocampal CA1 axons, GABAB receptors (GABABRs) inhibit presynaptic Ca2+ entry, and we have now demonstrated that 5-HT1B receptors (5-HT1BRs) liberate Gβγ to interact with SNARE complex C terminals with no effect on Ca2+ entry. Both GABABRs and 5-HT1BRs inhibit Ca2+-evoked neurotransmitter release, but 5-HT1BRs have no effect on Sr2+-evoked release. Sr2+, unlike Ca2+, does not cause synaptotagmin to compete with Gβγ binding to SNARE complexes. 5-HT1BRs also fail to inhibit release following cleavage of the C terminus of the SNARE complex protein SNAP-25 with botulinum A toxin. Thus, GABABRs and 5-HT1BRs both localize to presynaptic terminals, but target distinct effectors. We demonstrate that disruption of SNARE complexes and vesicle priming with botulinum C toxin eliminates this selectivity, allowing 5-HT1BR inhibition of Ca2+ entry. We conclude that receptor-effector specificity requires a microarchitecture provided by the SNARE complex during vesicle priming.