Release of vasopressin from isolated permeabilized neurosecretory nerve terminals is blocked by the light chain of botulinum A toxin

The intracellular action on exocytosis of botulinim A toxin and
constituent chains was studied using permeabilized isolated nerve
endings from the rat neural lobe. The release of the neuropeptide
vasopressin was measured by radioimmunoassay. In the presence of
the reducing agent dithiothreitol, the two-chain form of botulinum
A toxin inhibited vasopressin release induced by 10 μM free
calcium. Half maximal inhibition was obtained with 15 nM botulinum
A toxin. In the absence of the heavy chain the light chain of the
toxin strongly inhibited exocytosis with a half maximal effect of
2.5 nM. The inhibitory effects on secretion could be prevented by
incubating the light chain with an immune serum against botulinum A
toxin. The heavy chain of botulinum A toxin did not affect
vasopressin release. However, it prevented the inhibitory effects
of the light chain on stimulated exocytosis. It is concluded that
botulinum A toxin inhibits the calcium-dependent step leading to
exocytosis by interfering with a target present in the isolated and
permeabilized nerve terminals. The functional domain of this
neurotoxin, which is responsible for the inhibition of vasopressin
release, is present in its light chain.