Presynaptic mechanisms of glycine-modulated synaptic transmission at central synapses

Glycine is a major inhibitory neurotransmitter in the central nervous system and plays a critical role in synaptic transmission and plasticity. Although its postsynaptic actions mediated by glycine receptors (GlyRs) are well characterized, the presynaptic mechanisms by which glycine regulates synaptic vesicle recycling, particularly endocytosis, remain poorly understood. In this study, we combined pharmacological manipulations with membrane capacitance measurements, calcium imaging, and pHluorin-based fluorescence assays to investigate this process. We found that 1 mM glycine inhibited both slow and rapid endocytosis at large calyx-type synapses and slowed vesicle retrieval at small, conventional hippocampal synapses. This inhibition was GlyR dependent and was associated with membrane depolarization and increased basal cytosolic Ca 2+ ; pharmacological experiments further implicated Ca 2+ -sensitive downstream signaling. Pharmacological manipulation of protein kinase C (PKC) and phospholipase D (PLD) signaling further suggests that PKC/PLD-related signaling contributes to glycine-induced inhibition of vesicle retrieval. Our findings support a previously unrecognized role for glycine in regulating vesicle recycling and provide a framework for examining how GlyR-dependent ionic changes interact with Ca 2+ -sensitive membrane-trafficking processes.