Muscarinic Toxin 3

Useful because there has never been a good small-molecule alternative for M4, and not to be confused with MT7. Its muscarinic selectivity is real but modest, and the 2011 finding that it binds alpha-1A, alpha-1D and alpha-2A adrenoceptors in the same nanomolar range means an experiment using it without an adrenergic control is not finished.

MT3 has no human use, no human dose and no published human safety data; it exists as a research reagent. The clearest documented in vivo effect is behavioural: injected into the dorsal hippocampus of rats immediately after training, it produces retrograde amnesia for an inhibitory avoidance task, which is the basis of the argument that M4 receptors carry part of memory consolidation ^[3]. The practical hazard in the laboratory is interpretive rather than toxicological, because nanomolar binding at three adrenoceptor subtypes means any effect attributed to M4 blockade needs an adrenergic control run alongside it ^[4]. Systemic toxicity, immunogenicity and pharmacokinetics of the purified protein have not been reported, so nothing is known about exposure outside the tissue it is applied to.

MT3 was isolated from Dendroaspis angusticeps venom and sequenced by Jolkkonen and colleagues at Uppsala in 1994, who described it as the most selective m4 ligand then known. Potter's group in Miami reached the same toxin from the other direction and called it m4-toxin, which is why both names appear in the literature. Through the late 1990s it became the default tool for M4: it was used to map M4 density across the rat brain by autoradiography, and radiolabelled M1 and M4 toxins applied to hippocampus from Alzheimer patients showed a 50 percent reduction of M4 sites in the dentate gyrus. Its standing as a clean M4 probe was qualified in 2011 by a systematic screen that found nanomolar binding at three adrenoceptor subtypes.

MT3 is the reagent people reach for when they need to block M4, mostly for lack of a good small-molecule competitor. It is respected but no longer trusted blindly; careful groups now pair it with an adrenergic control. Outside muscarinic pharmacology and experimental myopia research it is essentially unheard of.