Synaptic vesicle pool-specific modification of neurotransmitter release by intravesicular free radical generation

Synaptic transmission is mediated by the release of neurotransmitters from synaptic vesicles in response to stimulation or through the spontaneous fusion of a synaptic vesicle with the presynaptic plasma membrane. There is growing evidence that synaptic vesicles undergoing spontaneous fusion versus those fusing in response to stimuli are functionally distinct. In this study, we acutely probe the effects of intravesicular free radical generation on synaptic vesicles that fuse spontaneously or in response to stimuli. By targeting vesicles that preferentially release spontaneously, we can dissociate the effects of intravesicular free radical generation on spontaneous neurotransmission from evoked neurotransmission and vice versa. Taken together, these results further advance our knowledge of the synapse and the nature of the different synaptic vesicle pools mediating neurotransmission.

Earlier studies suggest that spontaneous and evoked neurotransmitter release processes are maintained by synaptic vesicles which are segregated into functionally distinct pools. However, direct interrogation of the link between this putative synaptic vesicle pool heterogeneity and neurotransmission has been difficult. To examine this link, we tagged vesicles with horseradish peroxidase (HRP) - a haem-containing plant enzyme - or antibodies against synaptotagmin-1 (syt1). Filling recycling vesicles in hippocampal neurons with HRP and subsequent treatment with hydrogen peroxide (H₂ O₂ ) modified the properties of neurotransmitter release depending on the route of HRP uptake. While strong depolarization-induced uptake of HRP suppressed evoked release and augmented spontaneous release, HRP uptake during mild activity selectively impaired evoked release, whereas HRP uptake at rest solely potentiated spontaneous release. Expression of a luminal HRP-tagged syt1 construct and subsequent H₂ O₂ application resulted in a similar increase in spontaneous release and suppression as well as desynchronization of evoked release, recapitulating the canonical syt1 loss-of-function phenotype. An antibody targeting the luminal domain of syt1, on the other hand, showed that augmentation of spontaneous release and suppression of evoked release phenotypes are dissociable depending on whether the antibody uptake occurred at rest or during depolarization. Taken together, these findings indicate that vesicles that maintain spontaneous and evoked neurotransmitter release preserve their identity during recycling and syt1 function in suppression of spontaneous neurotransmission can be acutely dissociated from syt1 function to synchronize synaptic vesicle exocytosis upon stimulation.