The light chain of tetanus toxin inhibits calcium-dependent vasopressin release from permeabilized nerve endings

The effects of tetanus toxin and its light and heavy chain subunits
on vasopressin release were investigated in digitonin-permeabilized
neurosecretory nerve terminals isolated from the neural lobe of the
rat pituitary gland. Exocytosis was induced by challenging the
permeabilized nerve endings with micromolar calcium concentrations.
Tetanus toxin inhibited vasopressin release only in the presence of
the reducing agent dithiothreitol. This effect was irreversible.
The purified light chain of tetanus toxin strongly inhibited
exocytosis in a dose-dependent manner with half-maximal effect at
c. 10 nM. The action of the light chain was observed after only 2.5
min of preincubation. Separated heavy chain subunit had no effect
on hormone secretion. Inhibition of vasopressin release could be
prevented by preincubating the light chain of tetanus toxin with an
immune serum against tetanus toxin. The data clearly demonstrate
that in mammalian neurosecretory nerve endings tetanus toxin acts
at a step downstream from the activation by Ca2+ of the exocytotic
machinery and that the functional domain of this toxin is confined
to its light chain.