NMDA receptor antagonists are a class of anesthetics that work to antagonize, or inhibit the action of, the N-methyl d-aspartate receptor (NMDAR). They are used as anesthesia for animals and, less commonly, for humans; the state of anesthesia they induce is referred to as dissociative anesthesia. However, there is evidence that NMDA receptor antagonists can cause a certain type of brain damage referred to as Olney's Lesions (in rodents).
Some NMDA receptor antagonists, such as ketamine and phencyclidine (PCP), are popular as recreational drugs for their hallucinogenic properties. When used recreationally, they are classified as dissociative drugs. Because some users use them for spiritual reasons, these recreational NMDA receptor antagonists are sometimes considered entheogens.
Uses and effects
NMDA receptor antagonists induce a state called dissociative anesthesia, which is marked by catalepsy, amnesia, and analgesia. Ketamine and other NMDA receptor antagonists are most frequently used in conjunction with diazepam as anesthesia in cosmetic or reconstructive plastic surgery and in the treatment of burn victims. Ketamine is a favored anesthetic for emergency patients with unknown medical history because it depresses breathing and circulation less than other anesthetics. The NMDA receptor antagonist dextromethorphan is one of the most commonly used cough suppressants in the world.
Depressed NMDA receptor function is associated with an array of negative symptoms. For example, NMDA receptor hypofunction that occurs as the brain ages may be partially responsible for memory deficits associated with aging. Schizophrenia may also have to do with inadequate NMDA receptor function (the "glutamate hypothesis" of schizophrenia). NMDA receptor antagonists can mimic these problems; they sometimes induce "psychotomimetic" side effects, symptoms resembling psychosis. Such side effects caused by NMDA receptor inhibitors include hallucinations, paranoid delusions, confusion, difficulty concentrating, agitation, alterations in mood, nightmares, catatonia, ataxia, anaesthesia, and learning and memory deficits.
Because of these psychotomimetic effects, NMDA receptor antagonists, especially phencyclidine, ketamine, and dextromethorphan, are used as recreational drugs. At subanesthetic doses, these drugs have mild stimulant effects, and at higher doses, begin inducing dissociation and hallucinations.
Most NMDA receptor antagonists are metabolized in the liver. Frequent administration of most NMDA receptor antagonists can lead to tolerance, whereby the liver will more quickly eliminate NMDA receptor antagonists from the bloodstream.
Exposure to NMDA receptor antagonists may cause a serious brain damage in the cingulate cortex and retrosplenial cortex regions of the brain. The experimental NMDA receptor antagonist MK-801 has been shown to cause neural vacuolization in test rodents that later develop into irreversible lesions called "Olney's Lesions." Many drugs have been found that lessen the risk of neurotoxicity from NMDA receptor antagonists. Centrally acting alpha 2 agonists such as clonidine and guanfacine are thought to most specifically target the etiology of NMDA neurotoxicity. Other drugs acting on various neurotransmitter systems known to inhibit NMDA antagonist neurotoxicity include: anticholinergics, diazepam, barbiturates, ethanol, 5-HT2A serotonin agonists, and muscimol.
Potential for treatment of excitotoxicity
Since NMDA receptors are one of the most harmful factors in excitotoxicity, antagonists of the receptors have held much promise for the treatment of conditions that involve excitotoxicity, including traumatic brain injury, stroke, and neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. However, because of the neurotoxicity caused by NMDA receptor antagonists, research has slowed and studies have started to find agents that prevent this neurotoxicity. Most clinical trials involving NMDA receptor antagonists have failed due to unwanted side effects of the drugs; since the receptors also play an important role in normal glutamatergic function, blocking them has harmful effects. This interference with normal function could be responsible for neuronal death that sometimes results from NMDA receptor antagonist use.
Mechanism of action
The NMDA receptor is an ionotropic receptor that allows for the transfer of electrical signals between neurons in the brain and in the spinal column. For electrical signals to pass, the NMDA receptor must be open. To remain open, an NMDA receptor must bind to glutamate and to glycine. An NMDA receptor that is bound to glycine and glutamate and has an open ion channel is called "activated."
The receptor can be deactivated by inhibitors that can cause the NMDAR to close by binding to allosteric sites. Chemicals that deactivate the NMDA receptor are called antagonists. NMDAR antagonists fall into four categories: Competitive antagonists, which bind to and block the binding site of the neurotransmitter glutamate; glycine antagonists, which bind to and block the glycine site; noncompetitive antagonists, which inhibit NMDARs by binding to allosteric sites; and uncompetitive antagonists, which block the ion channel by binding to a site within it.
Uncompetitive channel blockers include:
Noncompetitive antagonists include:
- Dizocilpine (MK-801) – an experimental drug.
- Aptiganel (Cerestat, CNS-1102) – binds the Mg2+ binding site within the channel of the NMDA receptor.
- Memantine (Axura®, Akatinol®, Namenda®, Ebixa®, 1-amino-3,5-dimethylada-mantane) – moderate affinity, voltage-dependent uncompetitive antagonist. Approved in the U.S. by the Food and Drug Administration for the treatment of Alzheimer's disease.
- Remacimide – principle metabolite is an uncompetitive antagonist with a low affinity for the binding site.
Glycine antagonists (drugs that act at the glycine binding site) include:
Competitive antagonists include:
- AP7 (2-amino-7-phosphonoheptanoic acid)
- APV (R-2-amino-5-phosphonopentanoate)
- CPPene (3-[(R)-2-carboxypiperazin-4-yl]-prop-2-enyl-1-phosphonic acid)
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