Dense-cored vesicles, smooth endoplasmic reticulum, and mitochondria are closely associated with non-specialized parts of plasma membrane of nerve terminals: Implications for exocytosis and calcium buffering by intraterminalorganelles

To determine whether there are anatomical correlates for intraterminal Ca2 stores to regulate exocytosis of dense-cored vesicles (DCVs) and whether t hese stores can modulate exocytosis of synaptic vesicles, we studied the sp atial distributions of DCVs, smooth endoplasmic reticulum (SER), and mitoch ondria in 19 serially reconstructed nerve terminals in bullfrog sympathetic ganglia. On average, each bouton had three active zones, 214 DCVs, 26 SER fragments (SERFs), and eight mitochondria. DCVs, SERFs and mitochondria wer e located, on average, 690, 624, and 526 nm, respectively, away from active zones. Virtually no DCVs were within "docking'' (i.e., less than or equal to 50 nm) distances of the active zones. Thus, it is unlikely that DCV exoc ytosis occurs at active zones via mechanisms similar to those for exocytosi s of synaptic vesicles. Because there were virtually no SERFs or mitochondr ia within 50 nm of any active zone, Ca2+ modulation by these organelles is unlikely to affect ACh release evoked by a single action potential. In cont rast, 30% of DCVs and 40% of SERFs were located within 50 nm of the nonspec ialized regions of the plasma membrane. Because each bouton had at least on e SERF within 50 nm of the plasma membrane and most of these SERFs had DCVs , but not mitochondria, near them, it is possible for Ca2+ release from the SER to provide the Ca2+ necessary for DCV exocytosis. The fact that 60% of the mitochondria had some part within 50 nm of the plasma membrane means t hat it is possible for mitochondrial Ca2+ buffering to affect DCV exocytosi s. J. Comp. Neurol. 403:378-390, 1999. (C) 1999 Wiley-Liss, Inc.