Larkspur itself, Consolida ajacis, contains over thirty diterpenoid Aconitine-like alkaloids such as
MethylLycAconitine and Lycoctonine.
MethylLycAconitine (aka MLA) is based on Lycoctonine but with an additional benzoic acid moiety and a 3-methyl-2,5-dioxo-pyrrolidine group (
succinimide) attached to that. It is a highly poisonous diterpenoid (or more strictly, a nor-diterpenoid, since it has one less carbon atom than the required 20 for a diterpenoid) alkaloid which is found in many species of Larkspur and is the principle toxin therein. It has insecticidal properties. Lycoctonine, upon which it is based, is however far less poisonous by between 10 to 100-fold. MLA blocks neuromuscular transmission in skeletal muscle but not smooth muscle which is typical of acetylcholine antagonists at nicotinic (but not muscarinic) sites. It is highly poisonous to both humans and livestock causing complete paralysis and respiratory arrest.
Neostigmine with Atropine was found to be an effective antidote for sheep whereas for calves
Physostigmine reversed the effects. It has been used as a treatment for neurological disorders. The lethal dose (LD50) for man has not been determined, but it is likely to be less than 0.5mg/kg body weight, thus ruling out most if not all applications as a therapeutic pharmaceutical.
It is interesting to note that several anti-convulsant pharmaceuticals are based upon
Succinimide, such as
Phensuximide, which also has the phenyl group attached (but in a different place to that in MLA.
Larkspur also contains