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Routes of administration | By mouth, IM |
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Formula | C16H15N |
Molar mass | 221.303 g·mol−1 |
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Melting point | 68.75 °C (155.75 °F) |
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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. [1] 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. [2] 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. [3] 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 [4] and has been found to impair the acquisition of difficult, but not easy, learning tasks in rats [5] [6] and primates. [7] 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.
Dizocilpine has also been found to act as a nicotinic acetylcholine receptor antagonist. [8] [9] [10] It has been shown to bind to and inhibit the serotonin and dopamine transporters as well. [11] [12]
Dizocilpine has a great deal of potential to be used in research in creating animal models of schizophrenia. Unlike dopaminergic agonists, which mimic only the positive symptoms of schizophrenia, a single injection of dizocilpine was successful in modelling both the positive and negative symptoms of schizophrenia. [13] Another study found that, although repeated low doses of dizocilpine were only successful in mimicking behavioral changes such as a slight hyperlocomotion and decreased prepulse inhibition, repeated administration of a higher dose mimicked both the above changes as well as the neurochemical alterations found in first-episode schizophrenic patients. [14] Not only has temporary use been shown to mimic psychosis but chronic administration in laboratory animals resulted in similar neuropathological changes as in schizophrenia. [15]
The effects of dizocilpine at NMDA receptors are clear and significant. NMDA receptors are key in the progression of excitotoxicity (a process in which an excessive amount of extracellular glutamate overexcites glutamate receptors and harms neurons). Thus NMDA receptor antagonists including dizocilpine have been extensively studied for use in treatment of diseases with excitotoxic components, such as stroke, traumatic brain injury, and neurodegenerative diseases such as Huntington's, Alzheimer's, and amyotrophic lateral sclerosis. Dizocilpine has shown effectiveness in protecting neurons in cell culture and animal models of excitotoxic neurodegeneration. [16] [17] [18] The administration of dizocilpine protected the hippocampus from ischemia-induced neurodegeneration in the gerbil. The ED50 (effective dose 50) for neuroprotection was 0.3 mg/kg and the majority of the animals were protected against the ischemia-induced damage at doses greater than or equal to 3 mg/kg, when dizocilpine was given one hour prior to the occlusion of the carotid arteries, although other studies have shown protection up to 24 hours post-insult. Excitatory amino acids, such as glutamate and aspartate, are released in toxic amounts when the brain is deprived of blood and oxygen and NMDA antagonists are thought to prevent the neurodegeneration through the inhibition of these receptors. [19] [20]
Behavioural studies have shown that NMDA receptors are involved in the development of psychological dependence caused by chronic administration of morphine. Dizocilpine suppressed the morphine-induced rewarding effect. It is suggested that stimulating NR2B subunits of the NMDA receptor and its associated kinases in the nucleus accumbens leads to the rewarding effect caused by morphine. Inhibition of this receptor and its kinases in the nucleus accumbens by co-treatment with NMDA antagonists prevents morphine-associated psychological dependence. [21] An earlier study has shown that the prevention of morphine-associated psychological dependence was not due to state-dependency effects induced by dizocilpine [22] but rather reflect the impairment of learning that is caused by NMDA antagonists. [23] This is consistent with studies showing that dizocilpine potentiates the addictive potential of morphine and other drugs (see below).
As an antidepressant, positive results were found in animal models of depression. [24] NMDA antagonists like dizocilpine have been shown in animal models to attenuate the hearing loss caused by aminoglycosides It is thought that aminoglycosides mimic endogenous polyamines at NMDA receptors and produce excitotoxic damage, leading to hair cell loss. Antagonizing NMDA receptors to reduce the excitotoxicity would prevent that hearing loss. [25] [26] Dizocilpine was found to block the development of kindled seizures, although it does not have any effect on completed kindled seizures. [27] Oddly, it was discovered to decrease rabies virus production and is believed to be the first neurotransmitter antagonist to present with antiviral activity. Rat cortical neuron cells were infected with the rabies virus and those incubated with dizocilpine had virus produced reduced about 1000-fold. It is not known how MK-801 has this effect; the rabies virus suspension, without cells, was inoculated with dizocilpine and the drug failed to produce a virucidal effect, indicated that the mechanism of action is something other than direct discontinuation of virus reproduction. It was also tested against herpes simplex, vesicular stomatitis, poliovirus type I, and HIV. It did not have activity against these other viruses, however. [28] Dizocilpine was also shown to potentiate the ability of levodopa to ameliorate akinesia and muscular rigidity in a rodent model of parkinsonism. [29] When dizocilpine was administered to rats 15 minutes after a spinal trauma, the long-term neurological recovery of the trauma was improved. [30] However, NMDA antagonists like dizocilpine have largely failed to show safety in clinical trials, possibly due to inhibition of NMDA receptor function that is necessary for normal neuronal function. Since dizocilpine is a particularly strong NMDA receptor antagonist, this drug is particularly likely to have psychotomimetic side effects (such as hallucinations) that result from NMDA receptor blockade. Dizocilpine had a promising future as a neuroprotective agent until neurotoxic-like effects, called Olney's Lesions, were seen in certain brain regions of lab rats. [31] [32] Merck, a drug company, promptly dropped development of dizocilpine.
Dizocilpine, along with other NMDA antagonists, induce the formation of brain lesions first discovered by John W. Olney in 1989. Dizocilpine leads to the development of neuronal vacuolization in the posterior cingulate/retrosplenial cortex. [31] Other neurons in the area expressed an abnormal amount of heat shock protein [33] as well as increased glucose metabolism [34] in response to NMDA antagonist exposure. Vacuoles began to form within 30 minutes of a subcutaneous dose of dizocilpine 1 mg/kg. [35] Neurons in this area necrotized and were accompanied by a glial response involving astrocytes and microglia. [36]
![]() | This section possibly contains original research .(February 2015) |
This section needs additional citations for verification .(February 2015) |
Dizocilpine may be effective as a recreational drug. Little is known in this context about its effects, dosage, and risks. The high potency of dizocilpine makes its dosage more difficult to accurately control when compared to other similar drugs. As a result, the chances of overdosing are high. Users tend to report that the experience is not as enjoyable as other dissociative drugs, and it is often accompanied by strong auditory hallucinations. Also, dizocilpine is much longer-lasting than similar dissociative drugs such as ketamine and phencyclidine (PCP), and causes far worse amnesia and residual deficits in thinking, which have hindered its acceptance as a recreational drug.[ citation needed ] Several animal studies have demonstrated the addictive potential of dizocilpine. Rats learned to lever-press in order to obtain injections of dizocilpine into the nucleus accumbens and frontal cortex, however, when given a dopamine antagonist at the same time, the lever-pressing was not altered, which shows that the rewarding effect of dizocilpine is not dependent on dopamine. [37] Intraperitoneal administration of dizocilpine also produced an enhancement in self-stimulation responding. [38] Rhesus monkeys were trained to self-administer cocaine or phencyclidine, then were offered dizocilpine instead. None of the four monkeys who were used to cocaine chose to self-administer dizocilpine but three out of the four monkeys who had been using phencyclidine self-administered dizocilpine, suggesting again that dizocilpine has potential as a recreational drug for those seeking a dissociative anaesthetic type of experience. [39] It was found that dizocilpine administration elicited conditioned place preference in animals, again demonstrating its reinforcing properties. [40] [41]
A multiple drug fatality involving dizocilpine, benzodiazepines, and alcohol has been reported. [42]
Dizocilpine has been sold online as a Designer drug. [43]
Phencyclidine or phenylcyclohexyl piperidine (PCP), also known in its use as a street drug as angel dust among other names, is a dissociative anesthetic mainly used recreationally for its significant mind-altering effects. PCP may cause hallucinations, distorted perceptions of sounds, and violent behavior. As a recreational drug, it is typically smoked, but may be taken by mouth, snorted, or injected. It may also be mixed with cannabis or tobacco.
N-methyl-D-aspartic acid or N-methyl-D-aspartate (NMDA) is an amino acid derivative that acts as a specific agonist at the NMDA receptor mimicking the action of glutamate, the neurotransmitter which normally acts at that receptor. Unlike glutamate, NMDA only binds to and regulates the NMDA receptor and has no effect on other glutamate receptors. NMDA receptors are particularly important when they become overactive during, for example, withdrawal from alcohol as this causes symptoms such as agitation and, sometimes, epileptiform seizures.
The N-methyl-D-aspartatereceptor (also known as the NMDA receptor or NMDAR), is a glutamate receptor and ion channel found in neurons. The NMDA receptor is one of three types of ionotropic glutamate receptors, the other two being AMPA and kainate receptors. Depending on its subunit composition, its ligands are glutamate and glycine (or D-serine). However, the binding of the ligands is typically not sufficient to open the channel as it may be blocked by Mg2+ ions which are only removed when the neuron is sufficiently depolarized. Thus, the channel acts as a “coincidence detector” and only once both of these conditions are met, the channel opens and it allows positively charged ions (cations) to flow through the cell membrane. The NMDA receptor is thought to be very important for controlling synaptic plasticity and mediating learning and memory functions.
In excitotoxicity, nerve cells suffer damage or death when the levels of otherwise necessary and safe neurotransmitters such as glutamate become pathologically high, resulting in excessive stimulation of receptors. For example, when glutamate receptors such as the NMDA receptor or AMPA receptor encounter excessive levels of the excitatory neurotransmitter, glutamate, significant neuronal damage might ensue. Excess glutamate allows high levels of calcium ions (Ca2+) to enter the cell. Ca2+ influx into cells activates a number of enzymes, including phospholipases, endonucleases, and proteases such as calpain. These enzymes go on to damage cell structures such as components of the cytoskeleton, membrane, and DNA. In evolved, complex adaptive systems such as biological life it must be understood that mechanisms are rarely, if ever, simplistically direct. For example, NMDA in subtoxic amounts induces neuronal survival of otherwise toxic levels of glutamate.
Olney's lesions, also known as NMDA receptor antagonist neurotoxicity (NAT), is a form of brain damage observed in rats and certain other model animals exposed to large quantities of psychoactive drugs that inhibit the normal operation of the neuronal NMDA receptor. Such lesions are common in anesthesia, as well as certain psychiatric treatments.
The metabotropic glutamate receptors, or mGluRs, are a type of glutamate receptor that are active through an indirect metabotropic process. They are members of the group C family of G-protein-coupled receptors, or GPCRs. Like all glutamate receptors, mGluRs bind with glutamate, an amino acid that functions as an excitatory neurotransmitter.
Glutamate receptors are synaptic and non synaptic receptors located primarily on the membranes of neuronal and glial cells. Glutamate is abundant in the human body, but particularly in the nervous system and especially prominent in the human brain where it is the body's most prominent neurotransmitter, the brain's main excitatory neurotransmitter, and also the precursor for GABA, the brain's main inhibitory neurotransmitter. Glutamate receptors are responsible for the glutamate-mediated postsynaptic excitation of neural cells, and are important for neural communication, memory formation, learning, and regulation.
NMDA receptor antagonists are a class of drugs that work to antagonize, or inhibit the action of, the N-Methyl-D-aspartate receptor (NMDAR). They are commonly used as anesthetics for human and non-human animals; the state of anesthesia they induce is referred to as dissociative anesthesia.
Quisqualic acid is an agonist of the AMPA, kainate, and group I metabotropic glutamate receptors. It is one of the most potent AMPA receptor agonists known. It causes excitotoxicity and is used in neuroscience to selectively destroy neurons in the brain or spinal cord. Quisqualic acid occurs naturally in the seeds of Quisqualis species.
Glutamate [NMDA] receptor subunit epsilon-2, also known as N-methyl D-aspartate receptor subtype 2B, is a protein that in humans is encoded by the GRIN2B gene.
Glutamate [NMDA] receptor subunit 3A is a protein that in humans is encoded by the GRIN3A gene.
Glutamate [NMDA] receptor subunit 3B is a protein that in humans is encoded by the GRIN3B gene.
Midafotel is a potent, competitive antagonist at the NMDA receptor. It was originally designed as a potential therapy for excitotoxicity, epilepsy or neuropathic pain. It looked very promising in in vitro trials proving to be a potent competitive antagonist at the NMDA without affecting other receptors. Research continued through to in vivo cat studies where it proved to limit damage after occluding the middle cerebral artery, leading to ischaemia. It also blocked photosensitive epilepsies in baboons.
Selfotel (CGS-19755) is a drug which acts as a competitive NMDA antagonist, directly competing with glutamate for binding to the receptor. Initial studies showed it to have anticonvulsant, anxiolytic, analgesic and neuroprotective effects, and it was originally researched for the treatment of stroke, but subsequent animal and human studies showed phencyclidine-like effects, as well as limited efficacy and evidence for possible neurotoxicity under some conditions, and so clinical development was ultimately discontinued.
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.
CGP-37849 is a competitive antagonist at the NMDA receptor. It is a potent, orally active anticonvulsant in animal models, and was researched for the treatment of epilepsy. It also has neuroprotective activity and shows antidepressant and anxiolytic effects.
2-Methyl-6-(phenylethynyl)pyridine (MPEP) is a research drug which was one of the first compounds found to act as a selective antagonist for the metabotropic glutamate receptor subtype mGluR5. After being originally patented as a liquid crystal for LCDs, it was developed by the pharmaceutical company Novartis in the late 1990s. It was found to produce neuroprotective effects following acute brain injury in animal studies, although it was unclear whether these results were purely from mGluR5 blockade as it also acts as a weak NMDA antagonist, and as a positive allosteric modulator of another subtype mGlu4, and there is also evidence for a functional interaction between mGluR5 and NMDA receptors in the same populations of neurons. It was also shown to produce antidepressant and anxiolytic effects in animals, and to reduce the effects of morphine withdrawal, most likely due to direct interaction between mGluR5 and the μ-opioid receptor.
Traxoprodil is a drug developed by Pfizer which acts as an NMDA antagonist, selective for the NR2B subunit. It has neuroprotective, analgesic, and anti-Parkinsonian effects in animal studies. Traxoprodil has been researched in humans as a potential treatment to lessen the damage to the brain after stroke, but results from clinical trials showed only modest benefit. The drug was found to cause EKG abnormalities and its clinical development was stopped. More recent animal studies have suggested traxoprodil may exhibit rapid-acting antidepressant effects similar to those of ketamine, although there is some evidence for similar psychoactive side effects and abuse potential at higher doses, which might limit clinical acceptance of traxoprodil for this application.
Conantokins are a small family of helical peptides that are derived from the venom of predatory marine snails of the genus Conus. Conantokins act as potent and specific antagonists of the N-methyl-D-aspartate receptor (NMDAR). They are the only naturally-derived peptides to do so. The subtypes of conantokins exhibit a surprising variability of selectivity across the NMDAR subunits, and are therefore uniquely useful in developing subunit-specific pharmacological probes.
PD-137889 (N-methylhexahydrofluorenamine) is a chemical compound that is active as an NMDA receptor antagonist in the central nervous system at roughly 30 times the potency of the "flagship" of its class, ketamine, and substitutes for phencyclidine in animal studies. Ki [3H]TCP binding = 27 nM versus ketamine's Ki = 860 nM.
original publications for MK-801: