Grayanotoxins are a group of closely relatedneurotoxins named afterLeucothoe grayana, a plant native to Japan and named for 19th-century American botanistAsa Gray.[1] Grayanotoxin I (grayanotoxane-3,5,6,10,14,16-hexol 14-acetate) is also known asandromedotoxin,acetylandromedol,rhodotoxin andasebotoxin.[2] Grayanotoxins are produced byRhododendron species and other plants in the familyEricaceae.Honey made from thenectar and so containingpollen of these plants also contains grayanotoxins and is commonly referred to asmad honey.[3]
Consumption of the plant or any of its secondary products, including mad honey, can cause a rare poisonous reaction called grayanotoxin poisoning, mad honey disease, honey intoxication, or rhododendron poisoning.[3][4] It is most frequently produced and consumed in regions ofTurkey andNepal as arecreational drug andtraditional medicine.[5][6]
Grayanotoxins are produced by plants in the familyEricaceae, specifically members of thegeneraAgarista,Craibiodendron,Kalmia,Leucothoe,Lyonia,Pieris andRhododendron.[3][7] The genusRhododendron alone encompasses over 750 species that grow around the world in parts of Europe, North America, Japan, India, Bhutan, Nepal and Turkey. They can grow at a variety of altitudes, ranging from sea level to more than 3 kilometres (9,800 ft).[6] While many of these species contain grayanotoxins, only a few contain significant levels. Species with high concentrations of grayanotoxins, such asR. ponticum andR. luteum, are most commonly found in regions of Turkey bordering theBlack Sea, and in Nepal.[5]
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Nearly all parts of grayanotoxin-producing rhododendrons contain the molecule, including the stem, leaves, flower, pollen and nectar. Grayanotoxins can also be found in secondary plant products, such as honey,labrador tea, cigarettes, and herbal medicines.[3]
| Grayanotoxin | R1 | R2 | R3 |
| Grayanotoxin I | OH | CH3 | Ac |
| Grayanotoxin II | CH2 | H | |
| Grayanotoxin III | OH | CH3 | H |
| Grayanotoxin IV | CH2 | Ac | |
Grayanotoxins are lowmolecular weighthydrophobic compounds.[8] They are structurally characterized as polyhydroxylated cyclicditerpenes. The base structure is a 5/7/6/5 ring system that does not containnitrogen.[3] More than 25 grayanotoxin isoforms have been identified fromRhododendron species[6], but grayanotoxin I and III are thought to be the principal toxic isoforms. DifferentRhododendron species contain multiple different grayanotoxin isoforms, contributing to differences in plant toxicity.[3]
The toxicity of grayanotoxin is derived from its ability to interfere withvoltage-gated sodium channels located in thecell membrane ofneurons. The Nav1.x channels consist of four homologousdomains (I-IV), each containing six transmembranealpha-helical segments (S1-S6). Grayanotoxin has abinding affinity (IC50) of approximately 10 μM and binds the group II receptor site located on segment 6 of domains I and IV (IS6 and IVS6).[3] Other toxins that bind to this region include the alkaloidsveratridine,batrachotoxin andaconitine.[8]
Experiments using squid axonal membranes indicate that sodium channel binding likely occurs on the internal face of the neuron.[9] Additionally, grayanotoxin only binds to the activated conformation of sodium channels. Normally, voltage gated sodium channels are activated (opened) only when the cellmembrane potential reaches a specific threshold voltage. This activated conformation allows for an influx of sodium ions resulting in celldepolarization, followed by the firing of anaction potential. At the peak of the action potential, voltage-gated sodium channels are quickly inactivated and are only reset once the cell has repolarized toresting potential. When grayanotoxin is present, binding induces further conformational changes that prevent sodium channel inactivation and lead to a prolonged depolarization. Owing to its transient ability to activate channels and increase membrane permeability to sodium ions, grayanotoxin is classified as a reversible Nav1.xagonist.[8]
Although mad honey is used in traditional medicine in Turkey,[3][5] the majority of grayanotoxin poisoning cases occur in middle-aged males who use the honey for perceived sexual enhancement.[10]Slowing of heart rate andlowering of blood pressure are typical effects reported in one review of cases.[5]Dizziness,nausea, fainting, and weakness were reported as commonneurological outcomes.[3][5][11] Other early-onset symptoms may includedoubled and blurred vision,hypersalivation, perspiration, andparesthesia in the extremities and around the mouth. In higher doses, symptoms can include loss of coordination, severe and progressive muscular weakness,electrocardiographic changes ofbundle branch block orST-segment elevations as seen in ischemic myocardial threat, and nodal rhythm orWolff-Parkinson-White syndrome.[5][12]
The primary mediator of this grayanotoxinpathophysiology is the pairedvagus nerve (tenth cranial nerve).[3] The vagus nerve is a major component of theparasympathetic nervous system (a branch of theautonomic nervous system) and innervates various organs including the lungs, stomach, kidney andheart. Vagal stimulation of the heart is mediated byM2-subtypemuscarinic acetylcholine receptors (mAChR).[13] In severe cases of grayanotoxin poisoning,atropine – a non-specific "mAChR antagonist" orMuscarinic antagonist – can be used to treat bradycardia and other heart rhythm malfunctions.[11] In addition to correcting rhythm disorders, administration of fluids andvasopressors can also help treat hypotension and mitigate other symptoms.[11]
Patients exposed to low doses of grayanotoxin typically recover within a few hours. In more severe cases, symptoms may persist for 24 hours or longer and may require medical treatment (as described above). Despite the risk from cardiac problems, grayanotoxin poisoning is rarely fatal in humans.[11]
In contrast to humans, grayanotoxin poisoning can be lethal for other animals.[3] Nectar containing grayanotoxin can kill honeybees, though some seem to have resistance to it and can produce honey from the nectar (see below). According to a team of researchers from the UK and Ireland, workerbumblebees are not harmed and may be preferable as pollinators because they transfer more pollen. Consequently, it may be advantageous for plants to produce grayanotoxin to be pollinated by bumblebees.[14]
Bees that collect pollen and nectar from grayanotoxin-containing plants often producehoney that also contains grayanotoxins.[3][11] This so-called "mad honey" is the most common cause of grayanotoxin poisoning in humans. Small-scale producers of mad honey typically harvest honey from a small area or singlehive to produce a final product containing a significant concentration of grayanotoxin. In contrast, large-scale honey production often mixes honey gathered from different locations, diluting the concentration of any contaminated honey.[11]
Mad honey is produced in specific world regions, notably the Black Sea region of Turkey (91% of poisoning cases in one analysis) and Nepal (5%).[5] In Turkey, mad honey known asdeli bal is used as a recreational drug and traditional medicine. It is most commonly made from the nectar ofRhododendron luteum andRhododendron ponticum in theCaucasus region.[15] In Nepal, this type of honey is used by theGurung people for both its hallucinogenic properties and supposed medicinal benefits.[16]
In the 18th century, this honey was exported to Europe to add to alcoholic drinks to give them extra potency. In modern times, it is consumed locally and exported to North America, Europe and Asia.[11][17][18]
In addition to variousRhododendron species, mad honey can also be made from several other grayanotoxin-containing plants. Honey produced from the nectar ofAndromeda polifolia contains high enough levels of grayanotoxin to cause full bodyparalysis and potentially fatalbreathing difficulties due todiaphragm paralysis.[11][19] Honey obtained fromspoonwood and allied species such assheep-laurel can also cause illness.[11] The honey fromLestrimelitta limao also produces a similar paralyzing effect to that of the honey fromA. polifolia and is also toxic to humans.[20]
The intoxicating effects of mad honey have been suspected for centuries, including records fromXenophon,Aristotle,Strabo,Pliny the Elder[17][21] andColumella, all reporting illness from eating "maddening" honey believed to be from the pollen or nectar ofRhododendron luteum andRhododendron ponticum.[22] According toXenophon'sAnabasis, an invading Greek army was accidentally poisoned by harvesting and eating the local Asia Minor honey, but they all made a quick recovery without any fatalities.[23] Having heard of this incident, and realizing that foreign invaders would be ignorant of the dangers of the local honey,King Mithridates later used the honey as a deliberate poison whenPompey's army attacked the Heptakometes in Asia Minor in 69 BC.[24] The Roman soldiers became delirious and nauseated after being tricked into eating the toxic honey, at which point Mithridates' army attacked.[25][26][27]
