Synaptic Integration of Subquantal Neurotransmission by Colocalized G Protein-Coupled Receptors in Presynaptic Terminals.

In presynaptic terminals, membrane-delimited G-mediated presynaptic inhibition is ubiquitous and acts via Gβγ to inhibit Ca entry, or directly at SNARE complexes to inhibit Ca-dependent synaptotagmin-SNARE complex interactions. At CA1-subicular presynaptic terminals, 5-HT and GABA receptors colocalize. GABA receptors inhibit Ca entry, whereas 5-HT receptors target SNARE complexes. We demonstrate in male and female rats that GABA receptors alter P, whereas 5-HT receptors reduce evoked cleft glutamate concentrations, allowing differential inhibition of AMPAR and NMDAR EPSCs. This reduction in cleft glutamate concentration was confirmed by imaging glutamate release using a genetic sensor (iGluSnFR). Simulations of glutamate release and postsynaptic glutamate receptor currents were made. We tested effects of changes in vesicle numbers undergoing fusion at single synapses, relative placement of fusing vesicles and postsynaptic receptors, and the rate of release of glutamate from a fusion pore. Experimental effects of P changes, consistent with GABA receptor effects, were straightforwardly represented by changes in numbers of synapses. The effects of 5-HT receptor-mediated inhibition are well fit by simulated modulation of the release rate of glutamate into the cleft. Colocalization of different actions of GPCRs provides synaptic integration within presynaptic terminals. Train-dependent presynaptic Ca accumulation forces frequency-dependent recovery of neurotransmission during 5-HT receptor activation. This is consistent with competition between Ca-synaptotagmin and Gβγ at SNARE complexes. Thus, stimulus trains in 5-HT receptor agonist unveil dynamic synaptic modulation and a sophisticated hippocampal output filter that itself is modulated by colocalized GABA receptors, which alter presynaptic Ca In combination, these pathways allow complex presynaptic integration. Two G protein-coupled receptors colocalize at presynaptic sites, to mediate presynaptic modulation by Gβγ, but one (a GABA receptor) inhibits Ca entry whereas another (a 5-HT receptor) competes with Ca-synaptotagmin binding to the synaptic vesicle machinery. We have investigated downstream effects of signaling and integrative properties of these receptors. Their effects are profoundly different. GABA receptors alter P leaving synaptic properties unchanged, whereas 5-HT receptors fundamentally change properties of synaptic transmission, modifying AMPAR but sparing NMDAR responses. Coactivation of these receptors allows synaptic integration because of convergence of GABA receptor alteration on Ca and the effect of this altered Ca signal on 5-HT receptor signaling. This presynaptic convergence provides a novel form of synaptic integration.