G protein-coupled receptors inhibit neurotransmitter release by modifying vesicle fusion properties

The process of neurotransmission is accomplished through the exocytosis of synaptic vesicles filled with neurotransmitter. The biophysics of this process is not completely understood. For example, do synaptic vesicles completely fuse with the plasma membrane, partially fuse, or both? What are the modulatory mechanisms that may alter the mode of vesicle fusion and what are the physiological consequences of this effect?; The presynaptic terminal contains an array of receptors that modify the properties of the terminal. G Protein-coupled receptors (GPCRs) are one type of receptor superfamilies that may modify presynaptic function via various second messenger systems. GPCRs that inhibit neurotransmitter release have been widely studied as modulators of synaptic transmission. These receptor-mediated effects that alter release are presumed to change the number of vesicles that fuse. Using the unique features present in the lamprey reticulospinal synapse, we have to set out to determine the mechanisms by which GPCRs modulate transmitter release at central synaptic terminals. Previous work on presynaptic 5-HT GPCRs has shown that activation of these receptors markedly inhibits synaptic transmission (Buchanan and Grillner, 1991) by an effect that is downstream of calcium entry into the presynaptic terminal (Takahashi et al., 2001). This 5-HT-mediated presynaptic inhibition utilizes the betagamma arm of the G-protein (Blackmer et al., 2001). We have now investigated the mechanism behind this inhibition and demonstrate a modification in vesicle fusion properties as a consequence GPCR activation. Using visualization of variants of styryl dyes in the intact spinal cord, we demonstrate that presynaptic GPCR receptor activation causes a switch between quantal neurotransmission and subquantal release. Our data suggest an incomplete fusion (also known as kiss-and-run) of synaptic vesicles with the presynaptic terminal plasma membrane, in which a fusion pore is formed with the presynaptic plasma membrane but with no subsequent collapse of the vesicle. The consequence of this incomplete fusion is a reduction in amount or rate of neurotransmitter release. These results suggest a novel mechanism of GPCR-mediated inhibition, whereby the quantity of neurotransmitter in the synaptic cleft is reduced by a direct action on the fusion event.