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alpha-conotoxin mii is a cone-snail peptide toxin and the foundational research probe for alpha6-containing nicotinic receptors; native mii blocks both alpha3beta2 and alpha6* subtypes, but engineered analogs (mii[h9a;l15a], mii[s4a,e11a,l15a]) reach ~590-1000-fold alpha6 selectivity at low-nanomolar potency; a lab tool, not a drug, but the one that unlocked alpha6* dopamine-terminal pharmacology.
- the defining research probe for alpha6-containing nicotinic receptors; radiolabeled MII is the standard a6* imaging ligand
- engineered analogs (H9A;L15A and S4A,E11A,L15A) achieve low-nanomolar, ~590-1000-fold alpha6-selective block
- the tool that proved alpha6 (not alpha3) partners beta2 on dopamine neurons
- underpins the entire rationale for alpha6beta2*-targeted smoking-cessation and Parkinson's drug discovery
- clean pharmacology: selective analogs spare alpha4beta2, alpha7 and muscle receptors
Mechanism
alpha-conotoxin mii is a 16-residue disulfide-bridged from the venom of the cone snail conus magus, and it is the single most important pharmacological tool for the alpha6 receptor field; not a therapeutic, but the reagent that made the target visible. it works as a competitive at nicotinic receptors, and its historical importance is that it was the first able to grip the closely related alpha3- and alpha6-containing subtypes with high affinity. the native toxin, however, does not cleanly separate them; it potently blocks both alpha3beta2 and alpha6beta2* receptors, which for a while muddied interpretation of 'mii-sensitive' release. the field's breakthrough came from structure-function engineering of the peptide. according to pubmed, mcintosh et al scanned mii with alanine substitutions and produced mii[h9a;l15a], which is 590-fold more selective for the alpha6/alpha3beta2 receptor than for alpha3beta2, functionally blocks the rat alpha6/alpha3beta2beta3 receptor with an ic50 around 2.4 nm, and shows brain binding (ki ~3.3 nm) that tracks its functional potency, with little or no activity at alpha2beta2, alpha3beta4, alpha4beta2 or alpha7 (doi 10.1124/mol.65.4.944). azam et al then pushed selectivity further with mii[s4a,e11a,l15a], which blocks alpha6/alpha3beta2beta3 at an ic50 near 1.2 nm and is roughly a thousand-fold weaker at alpha3beta2 and non-alpha6 subtypes; they even mapped the specific alpha6 residues (-152, aspartate-184, threonine-195) that confer the selectivity (doi 10.1074/jbc.m710288200). armed with these tools, the field established that alpha6* receptors are concentrated presynaptically on dopamine terminals and gate a large, reward-relevant fraction of nicotine-evoked dopamine release, and knockout genetics showed [125i]mii binding vanishes in alpha6-null brain, proving alpha6 (not alpha3) is the beta2 partner on dopamine neurons (champtiaux et al 2002, doi 10.1523/jneurosci.22-04-01208.2002). the radiolabeled toxin remains the standard way to image and quantify alpha6* receptors, and the whole rationale for alpha6beta2*-selective smoking-cessation and parkinson's agents rests on the receptor pharmacology this peptide family exposed. the honest framing: mii and its analogs are research reagents; peptide conotoxins don't make easy drugs (poor brain penetration, proteolysis), but as probes and design templates they are indispensable to alpha6 neuroscience.
receptor fingerprint
Alpha3beta2 (a3b2)competitive antagonist (native MII)
Alpha6beta2* (a6/a3b2b3, a6b2b3)competitive antagonist
Alpha6/alpha3beta2beta3 (analog MII[H9A;L15A])antagonist (a6-selective analog)
Alpha6/alpha3beta2beta3 (analog MII[S4A,E11A,L15A])antagonist (highly a6-selective analog)
Alpha4beta2 / alpha7 / muscle nAChRslittle or no activity (analogs)
Safetyrisks and cautions, not medical advice
Alpha-Conotoxin MII is a peptide isolated from the cone snail Conus magus and is used as a research ligand for alpha6/alpha3-beta2 nicotinic acetylcholine receptors; it has no human clinical safety profile because it exists only as a laboratory probe, not a drug. As a selective nAChR antagonist it interrupts cholinergic neurotransmission, and cone-snail venom peptides are potent neuroactive agents, so systemic exposure would carry theoretical neuromuscular and autonomic risk, yet no documented human toxicity data exist. Its disulfide-rich peptide structure means it is not orally active and would be broken down in the gut, limiting incidental exposure. It should be handled as a research reagent rather than anything consumable. The candid summary is that its risk profile in humans is uncharacterized, with no trials or labels to draw on.
Subjective profileweighing the evidence above
A superb research tool and not a drug in any sense; it exists to make the alpha6 nicotinic receptor visible in the lab, and it is not even orally active. Worth reading about if you follow smoking-cessation or Parkinson's pharmacology, and pointless to buy.
Resources
This entry is here for reference.
Research
- 2002first citedDistribution and pharmacology of alpha6-containing nicotinic acetylcholine receptors analyzed w…
- 2012most recentA novel α-conotoxin MII-sensitive nicotinic acetylcholine receptor modulates [(3) H]-GABA relea…
- 1.Analogs of alpha-conotoxin MII are selective for alpha6-containing nicotinic acetylcholine receptors
- 2.Amino acid residues that confer high selectivity of the alpha6 nicotinic acetylcholine receptor subunit to alpha-conotoxin MII[S4A,E11A,L15A].
- 3.Distribution and pharmacology of alpha6-containing nicotinic acetylcholine receptors analyzed with mutant mice
- 4.Striatal alpha6* nicotinic acetylcholine receptors: potential targets for Parkinson's disease therapy
- 5.A novel α-conotoxin MII-sensitive nicotinic acetylcholine receptor modulates [(3) H]-GABA release in the superficial layers of the mouse superior colliculus.
5 listed here; entry last updated August 2026
Reviews
My notesprivate to this device
FAQ
what is alpha-conotoxin mii?
it's a 16-amino-acid peptide toxin from a cone snail (conus magus), and it's the single most important research tool for studying alpha6-containing nicotinic receptors. it's not a drug you take; it's a probe used in labs to block specific nicotinic receptor subtypes so scientists can figure out which receptors do what. radiolabeled versions ([125i]alpha-conotoxin mii) are the standard way to visualize and count alpha6* receptors in brain tissue.
does it actually block alpha6 receptors specifically?
the original toxin is a bit of a blunt instrument here; native mii potently blocks both alpha3beta2 and alpha6-containing receptors and doesn't cleanly separate the two closely related subunits. the real breakthrough was engineering analogs. according to pubmed, mcintosh et al made mii[h9a;l15a], which is 590-fold more selective for alpha6/alpha3beta2 over alpha3beta2 and blocks the alpha6 receptor with an ic50 around 2.4 nm (doi 10.1124/mol.65.4.944); its brain binding (ki ~3.3 nm) matched the functional data beautifully.
how did we get an even more selective version?
by continuing the structure-function work. azam et al designed mii[s4a,e11a,l15a], which blocks the alpha6/alpha3beta2beta3 receptor with an ic50 around 1.2 nm while being roughly a thousand-fold weaker at the closely related alpha3beta2 and at other subtypes (doi 10.1074/jbc.m710288200). they even mapped the exact alpha6 residues (glu-152, asp-184, thr-195) that confer the selectivity, which is a nice piece of receptor cartography.
why does a snail toxin matter for human brains?
because it solved a problem nothing else could. the alpha3 and alpha6 nicotinic subunits are so similar that, before these toxins, nobody could tell their receptors apart pharmacologically. mii and its analogs finally gave researchers a clean handle on alpha6*, which turned out to be the receptor concentrated on dopamine terminals and central to nicotine reward and parkinson's biology. essentially every 'alpha6* does x' finding in the last two decades relied on this toxin family.
what did it reveal about dopamine and nicotine?
a lot. because a large fraction of nicotine-evoked striatal dopamine release is blocked by alpha-conotoxin mii, it showed that alpha6* receptors gate the reward-relevant slice of that dopamine signal. it also confirmed, via knockout mice where [125i]mii binding vanishes, that alpha6 (not alpha3) is the beta2 partner on dopamine neurons (champtiaux et al 2002, doi 10.1523/jneurosci.22-04-01208.2002). that's the empirical backbone of the 'target alpha6b2* for smoking cessation' idea.
could it ever be a drug itself?
directly, probably not; peptide conotoxins are hard to dose (they don't cross the blood-brain barrier well and get degraded), so mii's value is as a research probe and a template. but the selectivity lessons it taught are feeding real drug discovery; the whole rationale for alpha6-selective smoking-cessation and parkinson's agents rests on the receptor pharmacology mii made visible. think of it as the key that unlocked the target, not the therapy.
is it related to the other alpha6 conotoxins?
yes; it's the founding member of a small family of alpha6-relevant cone-snail peptides. alpha-conotoxin pia (from conus purpurascens) was the first to truly discriminate alpha6 from alpha3, and txib (from conus textile) is a later, highly alpha6b2*-selective antagonist. mii and its engineered analogs, though, remain the most widely used, and the radiolabeled form is the standard alpha6* imaging ligand.
so what's the honest bottom line?
alpha-conotoxin mii is a research reagent, not a supplement or medicine, and it's one of the most valuable tools in nicotinic-receptor neuroscience. its engineered analogs (h9a;l15a and s4a,e11a,l15a) are genuinely alpha6-selective at low-nanomolar potency, and they underpin most of what we know about alpha6* receptors in dopamine, reward and parkinson's disease. it belongs on this list as the defining alpha6 probe, with the clear caveat that it's a lab tool.