Ibotenic acid or (S)-2-amino-2-(3-hydroxyisoxazol-5-yl)acetic acid, also referred to as ibotenate, is a chemical compound and psychoactive drug which occurs naturally in Amanita muscaria and related species of mushrooms typically found in the temperate and boreal regions of the northern hemisphere. It is a prodrug of muscimol, broken down by the liver to that much more stable compound. It is a conformationally-restricted analogue of the neurotransmitter glutamate, and due to its structural similarity to this neurotransmitter, acts as a non-selective glutamate receptor agonist. Because of this, ibotenic acid can be a powerful neurotoxin in high doses, and is employed as a "brain-lesioning agent" through cranial injections in scientific research. The neurotoxic effects appear to be dose-related and risks are unclear through consumption of ibotenic-acid containing fungi, although thought to be negligible in small doses.
Dizocilpine (INN), also known as MK-801, is a pore blocker of the NMDA receptor, a glutamate receptor, discovered by a team at Merck in 1982. Glutamate is the brain's primary excitatory neurotransmitter. The channel is normally blocked with a magnesium ion and requires depolarization of the neuron to remove the magnesium and allow the glutamate to open the channel, causing an influx of calcium, which then leads to subsequent depolarization. Dizocilpine binds inside the ion channel of the receptor at several of PCP's binding sites thus preventing the flow of ions, including calcium (Ca2+), through the channel. Dizocilpine blocks NMDA receptors in a use- and voltage-dependent manner, since the channel must open for the drug to bind inside it. The drug acts as a potent anti-convulsant and probably has dissociative anesthetic properties, but it is not used clinically for this purpose because of the discovery of brain lesions, called Olney's lesions (see below), in laboratory rats. Dizocilpine is also associated with a number of negative side effects, including cognitive disruption and psychotic-spectrum reactions. It inhibits the induction of long term potentiation and has been found to impair the acquisition of difficult, but not easy, learning tasks in rats and primates. Because of these effects of dizocilpine, the NMDA receptor pore blocker ketamine is used instead as a dissociative anesthetic in human medical procedures. While ketamine may also trigger temporary psychosis in certain individuals, its short half-life and lower potency make it a much safer clinical option. However, dizocilpine is the most frequently used uncompetitive NMDA receptor antagonist in animal models to mimic psychosis for experimental purposes.
Picrotoxin, also known as cocculin, is a poisonous crystalline plant compound. It was first isolated by the French pharmacist and chemist Pierre François Guillaume Boullay (1777–1869) in 1812. The name "picrotoxin" is a combination of the Greek words "picros" (bitter) and "toxicon" (poison). A mixture of two different compounds, picrotoxin occurs naturally in the fruit of the Anamirta cocculus plant, although it can also be synthesized chemically.
The GABAA receptor (GABAAR) is an ionotropic receptor and ligand-gated ion channel. Its endogenous ligand is γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system. Accurate regulation of GABAergic transmission through appropriate developmental processes, specificity to neural cell types, and responsiveness to activity is crucial for the proper functioning of nearly all aspects of the central nervous system (CNS). Upon opening, the GABAA receptor on the postsynaptic cell is selectively permeable to chloride ions and, to a lesser extent, bicarbonate ions.
Molecular neuroscience is a branch of neuroscience that observes concepts in molecular biology applied to the nervous systems of animals. The scope of this subject covers topics such as molecular neuroanatomy, mechanisms of molecular signaling in the nervous system, the effects of genetics and epigenetics on neuronal development, and the molecular basis for neuroplasticity and neurodegenerative diseases. As with molecular biology, molecular neuroscience is a relatively new field that is considerably dynamic.
Kainate receptors, or kainic acid receptors (KARs), are ionotropic receptors that respond to the neurotransmitter glutamate. They were first identified as a distinct receptor type through their selective activation by the agonist kainate, a drug first isolated from the algae Digenea simplex. They have been traditionally classified as a non-NMDA-type receptor, along with the AMPA receptor. KARs are less understood than AMPA and NMDA receptors, the other ionotropic glutamate receptors. Postsynaptic kainate receptors are involved in excitatory neurotransmission. Presynaptic kainate receptors have been implicated in inhibitory neurotransmission by modulating release of the inhibitory neurotransmitter GABA through a presynaptic mechanism.
Cicutoxin is a naturally-occurring poisonous chemical compound produced by several plants from the family Apiaceae including water hemlock (Cicuta species) and water dropwort (Oenanthe crocata). The compound contains polyene, polyyne, and alcohol functional groups and is a structural isomer of oenanthotoxin, also found in water dropwort. Both of these belong to the C17-polyacetylenes chemical class.
Adinazolam is a tranquilizer of the triazolobenzodiazepine (TBZD) class, which are benzodiazepines (BZDs) fused with a triazole ring. It possesses anxiolytic, anticonvulsant, sedative, and antidepressant properties. Adinazolam was developed by Jackson B. Hester, who was seeking to enhance the antidepressant properties of alprazolam, which he also developed. Adinazolam was never FDA approved and never made available to the public market; however, it has been sold as a designer drug.
Oenanthotoxin is a toxin extracted from hemlock water-dropwort and other plants of the genus Oenanthe. It is a central nervous system poison, and acts as a noncompetitive antagonist of the neurotransmitter gamma-aminobutyric acid. A case has been made for the presence of this toxin in local Oenanthe species playing a causative role in euthanasia in ancient Sardinia. It was crystallized in 1949 by Clarke and co-workers. It is structurally closely related to the toxins cicutoxin and carotatoxin. Oenanthotoxin is a C17 polyacetylene isomer of cicutoxin.
Loreclezole is a sedative and an anticonvulsant which acts as a GABAA receptor positive allosteric modulator. The binding site of loreclezole has been shown experimentally to be shared by valerenic acid, an extract of the root of the valerian plant. Structurally, loreclezole is a triazole derivative. In animal seizure models, loreclezole is protective against pentylenetetrazol seizures but is less active in the maximal electroshock test. In addition, at low, nontoxic doses, the drug has anti-absence activity in a genetic model of generalized absence epilepsy. Consequently, loreclezole has a profile of activity similar to that of benzodiazepines. A potential benzodiazepine-like interaction with GABA receptors is suggested by the observation that the anticonvulsant effects of loreclezole can be reversed by benzodiazepine receptor inverse agonists. The benzodiazepine antagonist flumazenil, however, fails to alter the anticonvulsant activity of loreclezole, indicating that loreclezole is not a benzodiazepine receptor agonist. Using native rat and cloned human GABA-A receptors, loreclezole strongly potentiated GABA-activated chloride current. However, the activity of the drug did not require the presence of the γ-subunit and was not blocked by flumazenil, confirming that loreclezole does not interact with the benzodiazepine recognition site.
Remacemide is a drug which acts as a low-affinity NMDA antagonist with sodium channel blocking properties. It has been studied for the treatment of acute ischemic stroke, epilepsy, Huntington's disease, and Parkinson's disease.
A convulsant is a drug which induces convulsions and/or epileptic seizures, the opposite of an anticonvulsant. These drugs generally act as stimulants at low doses, but are not used for this purpose due to the risk of convulsions and consequent excitotoxicity. Most convulsants are antagonists at either the GABAA or glycine receptors, or ionotropic glutamate receptor agonists. Many other drugs may cause convulsions as a side effect at high doses but only drugs whose primary action is to cause convulsions are known as convulsants. Nerve agents such as sarin, which were developed as chemical weapons, produce convulsions as a major part of their toxidrome, but also produce a number of other effects in the body and are usually classified separately. Dieldrin which was developed as an insecticide blocks chloride influx into the neurons causing hyperexcitability of the CNS and convulsions. The Irwin observation test and other studies that record clinical signs are used to test the potential for a drug to induce convulsions. Camphor, and other terpenes given to children with colds can act as convulsants in children who have had febrile seizures.
Tutin is a poisonous plant derivative found in New Zealand tutu plants. It acts as a potent antagonist of the glycine receptor, and has powerful convulsant effects. It is used in scientific research into the glycine receptor. It is sometimes associated with outbreaks of toxic honey poisoning when bees feed on honeydew exudate from the sap-sucking passion vine hopper insect, when the vine hoppers have been feeding on the sap of tutu bushes. Toxic honey is a rare event and is more likely to occur when comb honey is eaten directly from a hive that has been harvesting honeydew from passionvine hoppers feeding on tutu plants.
Pentylenetetrazol (PTZ), also known as pentylenetetrazole, leptazol, metrazol, pentetrazol (INN), pentamethylenetetrazol, Corazol, Cardiazol, or Deumacard, is a drug formerly used as a circulatory and respiratory stimulant. High doses cause convulsions, as discovered by Hungarian-American neurologist and psychiatrist Ladislas J. Meduna in 1934. It has been used in convulsive therapy, and was found to be effective—primarily for depression—but side effects such as uncontrolled seizures were difficult to avoid. In 1939, pentylenetetrazol was replaced by electroconvulsive therapy, which is easier to administer, as the preferred method for inducing seizures in England's mental hospitals. In the US, its approval by the Food and Drug Administration was revoked in 1982. It is used in Italy as a cardio-respiratory stimulant in combination with codeine in a cough suppressant drug.
The rostral ventromedial medulla (RVM), or ventromedial nucleus of the spinal cord, is a group of neurons located close to the midline on the floor of the medulla oblongata. The rostral ventromedial medulla sends descending inhibitory and excitatory fibers to the dorsal horn spinal cord neurons. There are 3 categories of neurons in the RVM: on-cells, off-cells, and neutral cells. They are characterized by their response to nociceptive input. Off-cells show a transitory decrease in firing rate right before a nociceptive reflex, and are theorized to be inhibitory. Activation of off-cells, either by morphine or by any other means, results in antinociception. On-cells show a burst of activity immediately preceding nociceptive input, and are theorized to be contributing to the excitatory drive. Neutral cells show no response to nociceptive input.
Quisqualamine is the α-decarboxylated analogue of quisqualic acid, as well as a relative of the neurotransmitters glutamate and γ-aminobutyric acid (GABA). α-Decarboxylation of excitatory amino acids can produce derivatives with inhibitory effects. Indeed, unlike quisqualic acid, quisqualamine has central depressant and neuroprotective properties and appears to act predominantly as an agonist of the GABAA receptor and also to a lesser extent as an agonist of the glycine receptor, due to the facts that its actions are inhibited in vitro by GABAA antagonists like bicuculline and picrotoxin and by the glycine antagonist strychnine, respectively. Mg2+ and DL-AP5, NMDA receptor blockers, CNQX, an antagonist of both the AMPA and kainate receptors, and 2-hydroxysaclofen, a GABAB receptor antagonist, do not affect quisqualamine's actions in vitro, suggesting that it does not directly affect the ionotropic glutamate receptors or the GABAB receptor in any way. Whether it binds to and acts upon any of the metabotropic glutamate receptors like its analogue quisqualic acid however is unclear.
Dioscorine is an alkaloid toxin isolated from the tubers of tropical yam on several continents. It has been used as a monkey poison in some African countries, and as an arrow poison to aid in hunting in several parts of Asia. It was first isolated from Dioscorea hirsute by Boorsma in 1894 and obtained in a crystalline form by Schutte in 1897, and has since been found in other Dioscorea species. Dioscorine is a neurotoxin that acts by blocking the nicotinic acetylcholine receptor. Dioscorine is generally isolated in tandem with other alkaloids such as dioscin but is usually the most potent toxin in the mixture. It is a convulsant, producing symptoms similar to picrotoxin, with which it shares a similar mechanism of action.
IPTBO is a bicyclic phosphate convulsant. It is an extremely potent GABA receptor antagonist that can cause violent convulsions in mice.
A GABAA receptor negative allosteric modulator is a negative allosteric modulator (NAM), or inhibitor, of the GABAA receptor, a ligand-gated ion channel of the major inhibitory neurotransmitter γ-aminobutyric acid (GABA). They are closely related and similar to GABAA receptor antagonists. The effects of GABAA receptor NAMs are functionally the opposite of those of GABAA receptor positive allosteric modulators (PAMs) like the benzodiazepines, barbiturates, and ethanol (alcohol). Non-selective GABAA receptor NAMs can produce a variety of effects including convulsions, neurotoxicity, and anxiety, among others.
TBPO is an extremely toxic bicyclic phosphate convulsant and GABA receptor antagonist. It is the most toxic bicyclic phosphate known, with an LD50 of 36 μg/kg in mice.
