Two distinct vesicle pools for depolarization-induced exocytosis in somata of dorsal root ganglion neurons

Ca ²⁺-regulated exocytosis is essential for neurotransmitter and hormone release. As well as this type of exocytosis, the somata of dorsal root ganglion (DRG) neurons also show Ca ²⁺-independent but voltage-dependent exocytosis. It is unclear whether these two types of exocytosis use the same or different vesicle pools in DRG neurons. Here, we found that they were separable in response to the same stimulation in low external Ca ²⁺ solution. Depletion of the Ca ²⁺-dependent vesicle pool did not affect the Ca ²⁺-independent but voltage-dependent exocytosis. These results show that DRG neurons exhibit two distinct types of exocytosis that use different vesicle pools. Abstract The somata of dorsal root ganglion (DRG) neurons release neurotransmitters and neuropeptides. In addition to the conventional Ca ²⁺-dependent secretion (CDS), Ca ²⁺-independent but voltage-dependent secretion (CIVDS) also occurs in the somata of DRG neurons. Electrical stimulation induces both CDS and CIVDS, which differ in size and are coupled with different types of endocytosis contributed by CIVDS and CDS, respectively. However, it is unclear whether they use a common vesicle pool, so we investigated the relationship between the vesicle pools of CDS and CIVDS. Membrane capacitance recording and photolysis of a caged-Ca ²⁺ compound showed that, in low external Ca ²⁺ solutions, the depolarization-induced exocytosis contained two (fast and slow) phases, which were contributed by CIVDS and CDS, respectively. Depletion of the CDS readily releasable pool using photolysis did not affect the CIVDS. When the CIVDS and CDS vesicle pools were depleted by electrical stimulation, the pools had different sizes. Their kinetics of exocytosis-coupled endocytosis were also different. Thus, CIVDS and CDS used different vesicle pools in DRG neurons.