Leptinotarsin: A presynaptic neurotoxin that stimulates release of acetylcholine

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Abstract
Leptinotarsin, a toxin found in the hemolymph of the beetle Leptinotarsa haldemani can stimulate release of acetylcholine from synaptic termini. Leptinotarsin causes an increase in the frequency of miniature endplate potentials (mepps) of the rat phrenic nerve-diaphragm preparation. The increase in the frequency of mepps induced by leptinotarsin is biphasic: about 10% of the total mepps are released in an initial burst that lasts about 90 s, after which the remaining mepps are released over 10-20 min. Tetrodotoxin has no effect on the release induced by leptinotarsin, but low-Ca2+ conditions abolish the 1st phase. The 2 phases of release may represent 2 presynaptic pools of acetylcholine, both of which can be released in quantized form. In a 2nd study, rat brain synaptosomes were incubated with [3H]choline and were immobilized on Millipore filters. Leptinotarsin induced release of [3H]acetylcholine from this preparation, confirming the release seen by using neurophysiological methods. The ability of leptinotarsin to induce release from either intact nerve terminals or synaptosomes was abolished when the toxin was heated. The releasing activity of leptinotarsin from synaptosomes was also partially dependent on the presence of Ca2+ in the perfusing solution. Release from synaptosomes followed 1st-order kinetics, and was not inhibited by commercial antibodies to black widow spider antigens. Leptinotarsin acts as a presynaptic neurotoxin and may be of value as a mechanistic probe in understanding the storage and release of neurotransmitters.