Ibotenic acid

Ibotenic acid or (S)-2-amino-2-(3-hydroxyisoxazol-5-yl)acetic acid, also referred to as ibotenate, is a naturally occurring alpha-amino acid found in certain Amanita mushrooms, that primarily acts as a potent glutamate receptor agonist that precipitates neurological effects and is used experimentally as a brain-lesioning agent in mice and rats.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Ibotenic acid is a conformationally-restricted analogue of glutamate that acts as a non-selective glutamate receptor agonist, strongly activating NMDA, group I and II metabotropic glutamate receptors, and weakly activating AMPA and kainate receptors. It is a prodrug of muscimol, broken down by the liver into this more stable compound, which acts as a potent GABA_A and GABA_A-ρ receptor agonist. Although its psychoactive effects are not well understood, some researchers speculate that ibotenic acid itself may have stimulant properties. Ibotenic acid is biosynthesized from glutamic acid by hydroxylation catalyzed by an Fe(II)/2-oxoglutarate-dependent oxygenase, with subsequent conversion steps carried out by enzymes encoded within a linked biosynthetic gene cluster.

Ibotenic acid is commonly used in research to create site-specific hippocampal brain lesions in rats, allowing for task relearning due to its interaction with glutamate receptors, and is favored over other agents for its selectivity and long-term stability in saline solution.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> It induces excitotoxicity in mice and rats by overactivating NMDA and metabotropic glutamate receptors, leading to calcium overload and oxidative damage.

In contrast, it targets glutamate-gated chloride channels in invertebrates, causing increased chloride permeability without affecting their excitatory glutamate receptors.<ref name=":3" /><ref name=":4" />

Pharmacology

File:Amanita muscaria 3 vliegenzwammen op rij.jpg

Amanita muscaria, which contains ibotenic acid

Ibotenic acid acts as a potent agonist of the NMDA and group I (mGluR1 and mGluR5) and II (mGluR2 and mGluR3) metabotropic glutamate receptors.<ref name="LiljeforsKrogsgaard-Larsen2002">Template:Cite book</ref><ref name="Wantanabe1999">Template:Cite book</ref> It is inactive at group III mGluRs.<ref name="pmid14985045">Template:Cite journal</ref> Ibotenic acid also acts as a weak agonist of the AMPA and kainate receptors.<ref name="LiljeforsKrogsgaard-Larsen2002" /><ref name="Wantanabe1999" /> In addition, due to in vivo decarboxylation into muscimol, it acts indirectly as a potent GABA_A and GABA_A-ρ receptor agonist.<ref name="Wantanabe1999" /> Unlike muscimol—the principal psychoactive constituent of Amanita muscaria that is understood to cause sedation and delirium—ibotenic acid's psychoactive effects are not known independent of its serving as a prodrug to muscimol, although some researchers have speculated that it would act as a stimulant.<ref>Chilton 1975; Theobald et al. 1968</ref><ref>Chilton 1975; Ott 1976a</ref>

Biological properties

Mechanism of action

Ibotenic acid is an agonist of glutamate receptors, specifically at both the N-methyl-D-aspartate, or NMDA, and trans-ACPD receptor sites in multiple systems in the central nervous system. Ibotenic neurotoxicity can be enhanced by glycine and blocked by dizocilpine. Dizocilpine acts as an uncompetitive antagonist at NMDA receptors.<ref name="link.springer.com">Template:Cite journal</ref>

Ibotenic acid toxicity comes from activation of the NMDA receptors. NMDA receptors are related to synaptic plasticity and work with metabotropic glutamate receptors to establish long term potentiation or LTP. The process of long term potentiation is believed to be related to the acquisition of information. The NMDA receptor functions properly by allowing Ca²⁺ ions to pass through after activation at the receptor site.

File:Activated NMDAR.svg

Activated NMDAR

The binding of ibotenic acid allows excess Ca²⁺ into the system which results in neuronal cell death. Ca²⁺ also activates CaM-KII or Ca²⁺/Calmodulin Kinase which phosphorylates multiple enzymes. The activated enzymes then begin producing reactive oxygen species which damages surrounding tissue. The excess Ca²⁺ results in the enhancement of the mitochondrial electron transport system which will further increase the number of reactive oxygen species.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Biological effects

Ibotenic acid typically affects both NMDA and APCD or metabolotropic quisqualate receptor sites in the central nervous system.<ref name="link.springer.com"/> Due to their targeting of these systems the symptoms associated with ibotenic acid poisoning are often related to perception and control.

At least some ingested ibotenic acid is likely decarboxylated into muscimol so some of the effects of ingesting ibotenic acid are similar to muscimol's effects.<ref name="Michelot">Template:Cite journal</ref> Symptoms associated with ibotenic acid are usually onset within 30–60 minutes and include a range of nervous system effects. The most common symptoms include nausea, vomiting, and drowsiness. However, after the first hour symptoms begin to include confusion, euphoria, visual and auditory distortions, sensations of floating, and retrograde amnesia.<ref name=":0" />

Symptoms are slightly different for children, typically beginning after 30–180 minutes. Dominant symptoms in children include ataxia, obtundation, and lethargy. Seizures are occasionally reported, however, more commonly with children.<ref name=":0">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

In contrast, ibotenic acid has a completely different action in invertebrates. Instead of an excitatory effect, it increases the permeability of invertebrate skeletal muscle and nerve cell membranes to chloride ions but shows no affinity for invertebrate glutamate excitatory receptors. This effect was first observed in locust muscle fibers,<ref name=":3">Template:Cite journal</ref><ref name=":4">Template:Cite journal</ref><ref>Template:Cite journal</ref> leading to the discovery of a new ion channel, the glutamate-gated chloride channel (GluCl), which was later cloned from the soil nematode C. elegans.<ref>Template:Cite journal</ref>Similar effects have been observed in other invertebrate excitable cells, including Drosophila melanogaster neurons and crayfish muscle.<ref name=":1">Template:Cite journal</ref><ref name=":2">Template:Cite journal</ref>

Since GluCl does not exist in vertebrates, it has become a valuable target for anti-parasitic drugs such as Avermectin and Ivermectin.<ref name=":1" /><ref name=":2" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Treatment

Treatment of ibotenic acid toxicity centres around supportive care and treatment of symptoms; no antidote is available. Gastric decontamination with activated charcoal or gastric lavage can be of benefit if the patient presents early. The psychotropic effects and hallucinations ibotenic acid and its metabolite muscimol produce are best managed in a quiet environment with minimal stimulation. Benzodiazepines can be of benefit in agitated or panicked patients; they can also be used to control seizures if they occur. Benzodiazepines as a GABA-A PAM interacts with Muscimol as a GABA-A agonist and may cause a significantly increased risk of depressant effects. Airway management may be required if sedation is profound. Symptoms usually resolve within a few hours of ingestion but can last for days following significant exposures.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Monitoring for the presence of brain lesions may be required following a large or repeated exposure. Other measures may be required if the patient has been exposed to a mushroom such as Amanita muscaria as other active compounds may be present.<ref> Template:Cite journal </ref>

Use in research

Ibotenic acid used for the lesioning of rat's brains is kept frozen in a phosphate-buffered Saline Solution at a pH of 7.4, and can be kept for up to a year with no loss in toxicity. Injection of .05-.1 microliters of Ibotenic acid into the hippocampus at a rate of .1 microliter/min resulted in semi-selective lesioning. Hippocampal lesioning led to a considerable loss of cells in pyramidal cells (CA1-CA3) as well as granule cells in the dentate gyrus. Ibotenic acid lesioning also causes some damage to axons along the perforant pathway.

Typically, when lesioning is done with other chemicals the subject cannot relearn a task. However, due to Ibotenic acid's reactivity with glutamate receptors such as the NMDA receptor, Ibotenic acid lesioning does allow the subject to relearn tasks. Ibotenic acid lesioning is thus preferred in studies where re-learning a task after lesioning is essential. Compared to other lesioning agents, Ibotenic acid is one of the most site-specific; however, less-damaging alternatives are presently sought.<ref>Template:Cite journal</ref>

Biosynthesis

Ibotenic acid's biosynthetic genes are organized in a physically linked biosynthetic gene cluster. The biosynthetic pathway is initiated by hydroxylation of glutamic acid by a dedicated Fe(II)/2-oxoglutarate-dependent oxygenase. The reaction yields threo-3-hydroxyglutamic acid, which is converted into ibotenic acid, likely by enzymes encoded in the biosynthetic gene cluster.<ref>Template:Cite journal</ref>

See also

• Conotoxin
• Grayanotoxin
• Quisqualic acid
• Tetrodotoxin