High-frequency stimuli preferentially release large dense-core vesicles located in the proximity of nonspecialized zones of the presynaptic membrane in sympathetic ganglia.

We characterized the effect of a brief high-frequency stimulus on the number, distribution, and optical density of large dense-core vesicles (LDCVs) in the nerve terminals of the rat superior cervical ganglia. From 4.21+/-0.37 LDCVs/bouton detected in control nerve terminals, a stimulus of 40 Hz for 1 min released 41% of LDCVs, decreasing their number to 2.48+/-0.14 LDCVs/bouton (p=0.0009). In control ganglia, most dense vesicles were located close to the plasma membrane (at <or=100 nm); in contrast, in stimulated ganglia they were broadly distributed with respect to the active zone. The mean distance of LDCVs to membrane and active zones was 95+/-8 nm and 473+/-15 nm, respectively. The analysis of the core density showed that both groups had a similar asymmetric distribution with the same average. Stimulation preferentially released those vesicles located <or=100 nm from the plasma membrane that had no apparent relationship with the active zone. After the stimulus, the average distances of LDCVs to the plasma membrane and active zone did not change, suggesting that the stimulus also caused the relocation of inner LDCVs. Interestingly, optical density analysis showed that the released vesicles had low range densities, and suggested that LDCVs release their entire content. We conclude that LDCV exocytosis mainly involves those vesicles located <or=100 nm from the plasma membrane and occurs in regions of synaptic boutons presumed to be nonspecialized. These results agree with the characteristics of the classical model that proposes full content release.