Synaptic integration of subquantal neurotransmission by co-localized G protein coupled receptors in presynaptic terminals

Abstract

AbstractIn presynaptic terminals, membrane-delimited Gi/o-mediated presynaptic inhibition is ubiquitous and acts through Gβγ to inhibit Ca2+ entry, or directly at SNARE complexes to inhibit Ca2+-dependent synaptotagmin-SNARE complex interactions. At CA1-subicular presynaptic terminals 5-HT1B and GABAB receptors colocalize. GABAB receptors inhibit Ca2+ entry, whereas 5-HT1B receptors target SNARE complexes. We demonstrate in male and female rats that GABAB receptors receptors alter Pr, whereas 5-HT1B receptors reduce evoked cleft glutamate concentrations allowing differential inhibition of AMPA and NMDA receptor 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 Pr changes, consistent with GABAB receptor effects, were straightforwardly represented by changes in numbers of synapses. The effects of 5-HT1B receptor-mediated inhibition are well-fit by simulated modulation of the release rate of glutamate into the cleft. Colocalization of different actions of GPCRs provide synaptic integration within presynaptic terminals. Train-dependent presynaptic Ca2+ accumulation forces frequency-dependent recovery of neurotransmission during 5-HT1B receptor activation. This is consistent with competition between Ca2+-synaptotagmin and Gβγ at SNARE complexes. Thus, stimulus trains in 5-HT1B receptor agonist unveil dynamic synaptic modulation and a sophisticated hippocampal output filter that itself is modulated by colocalized GABAB receptors which alter presynaptic Ca2+. In combination these pathways allow complex presynaptic integration.Significance StatementTwo G protein coupled receptors colocalize at presynaptic sites, to mediate presynaptic modulation by Gβγ, but one – a GABAB receptor inhibits Ca2+ entry while another – a 5-HT1B receptor competes with Ca2+-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. GABAB receptors alter Pr leaving synaptic properties unchanged, while 5-HT1B receptors fundamentally change properties of synaptic transmission, modifying AMPA receptor but sparing NMDA receptor responses. Co-activation of these receptors allows synaptic integration because of convergence of GABAB receptor alteration on Ca2+ and the effect of this altered Ca2+ signal on 5-HT1B receptor signaling. This presynaptic convergence provides a novel form of synaptic integration.