KCl stimulation and ultrastructural responses of tannic acid-stained cholinergic synaptic terminals

The quantal-vesicular hypothesis equates miniature end-plate potentials (MEPPs) with fusions of synaptic vesicles. MEPP production thus predicts vesicle losses, increases in vesicle fusions and increases in terminal plasma membrane. MEPP production and these ultrastructural parameters have been evaluated in the cholinergic presynaptic terminals of skate electric organ following tannic acid saline incubation, known to promote capture and selective staining of dense-core granule fusions, and KCl stimulation, known to elevate MEPP production dramatically in these cholinergic terminals. After pretreatment in tannic acid-elasmobranch saline, KCl stimulation produced MEPPs at 40/s/microm(2)of terminal surface for several minutes with gradual reduction to spontaneous levels by 25-30 min. No loss of vesicles, no vesicle fusions, no expansions of plasma membrane and no tannic acid enhanced staining of vesicles or vacuoles accompanied the generation of 800 MEPPs/microm(3)of terminals having densities of 567 vesicles/microm(3). No ultrastructural footprints were found to support the notion that unnaturally high rates of vesicular exocytosis had occurred.