Calcium-permeable AMPA/kainate receptors mediate toxicity and . activates both AMPA and kainate receptors (24), in a dose-dependent manner (EC50 = The α-aminohydroxymethylisoxazolepropionic acid receptor is an ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system (CNS). It has been traditionally classified as a non-NMDA-type receptor, along with .. The starting signal for AMPAR endocytosis is an NMDAR-dependent calcium. Both a general antagonist of AMPA/kainate receptors (CNQX) and a that the toxicity of these mutants is calcium-dependent and provide direct.
Calcium Dependent. AMPA/Kainate Toxicity Is
However hypofunction of NMDA receptors due to glutathione deficiency or other causes may be involved in impairment of synaptic plasticity  and could have other negative repercussions. The main problem with the utilization of NMDA receptor antagonists for neuroprotection is that the physiological actions of the NMDA receptor are essential for normal neuronal function.
To be clinically accepted, successful clinical application of NMDA antagonists would need to block excessive activation without interfering with normal functions. In the early s, NMDA receptors were shown to be involved in several central synaptic pathways. These findings led to vigorous campaign in the pharmaceutical industry. A fortuitous finding was made in when a woman was taking amantadine as flu medicine and experienced remarkable remission of her Parkinson's symptoms.
This finding, reported by Scawab et al. The purpose was to develop a hypoglycemic drug, but it showed no such efficacy. It was not until that a possible therapeutic importance of memantine for treating neurodegenerative disorders was discovered. From memantine has been recognized to be an uncompetitive antagonist of the NMDA receptor. The NMDA receptor is a glutamate and ion channel protein receptor that is activated when glycine and glutamate bind to it.
NR1 has eight different subunits generated by alternative splicing from a single gene. Six separate genes encode for NR2 and NR3. The NR2B subunit has been involved in modulating activity such as learning, memory, processing and feeding behaviors, as well as being implicated in number of human derangements.
It is claimed that three binding sites within the receptor, A on the NR2B subunit and A and N on the NR1 subunit, are important for binding of memantine and related compounds as seen in figure 2. A related gene family of NR3 A and B subunits have an inhibitory effect on receptor activity. Multiple receptor isoforms with distinct brain distributions and functional properties arise by selective splicing of the NR1 transcripts and differential expression of the NR2 subunits.
Each receptor subunit has modular design and each structural module also represents a functional unit:. The glycine-binding modules of the NR1 and NR3 subunits and the glutamate-binding module of the NR2A subunit have been expressed as soluble proteins, and their three-dimensional structure has been solved at atomic resolution by x-ray crystallography.
This has revealed a common fold with amino acid-binding bacterial proteins and with the glutamate-binding module of AMPA-receptors and kainate-receptors. Excitotoxicity is implied to be involved in some neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and Huntington's disease. This can possibly be achieved by uncompetitive antagonists, blocking the receptors ion channel when excessively open. Uncompetitive NMDA receptor antagonists, or channel blockers, enter the channel of the NMDA receptor after it has been activated and thereby block the flow of ions.
The off-rate of an antagonist from the receptors channel is an important factor as too slow off-rate can interfere with normal function of the receptor and too fast off-rate may give ineffective blockade of an excessively open receptor. Memantine is an example of an uncompetitive channel blocker of the NMDA receptor, with a relatively rapid off-rate and low affinity.
At physiological pH its amine group is positively charged and its block of the channel is voltage-dependent. During normal receptor activity the channels only stay open for several milliseconds and under those circumstances memantine is unable to bind within the channels and therefore doesn't interfere with normal synaptic activity. Strong evidence shows that the genes encoding the NR2 subunits in vertebrates have undergone at least two rounds of gene duplication.
More importantly, each NR2 subunit has a different intracellular C-terminal domain that can interact with different sets of signalling molecules. One particular subunit, NR2B, is mainly present in immature neurons and in extrasynaptic locations, and contains the binding-site for the selective inhibitor ifenprodil. The detailed time course of this switch in the human cerebellum has been estimated using expression microarray and RNA seq and is shown in the figure on the right.
NMDA receptors have been implicated by a number of studies to be strongly involved with excitotoxicity. There is ample evidence to support the dual nature of NMDA receptors based on location, and the hypothesis explaining the two differing mechanisms is known as the "localization hypothesis".
In order to support the localization hypothesis, it would be necessary to show differing cellular signaling pathways are activated by NMDA receptors based on its location within the cell membrane. These types of experiments have shown that different pathways are being activated or regulated depending on the location of the signal origin. This information is then transported to the nucleus.
This allows the transcription factors in the nucleus to respond differently based in the phosphorylation state of Jacob. NMDA receptors are also associated with synaptic plasticity. In addition, both synaptic and extrasynaptic are involved in expressing a full LTD. Another factor that seems to affect NMDAR induced toxicity is the observed variation in subunit makeup. Although both subunits are found in synaptic and extrasynaptic NMDARs there is some evidence to suggest that the GluN2B subunit occurs more frequently in extrasynaptic receptors.
This observation could help explain the dualistic role that NMDA receptors play in excitotoxicity. Despite the compelling evidence and the relative simplicity of these two theories working in tandem, there is still disagreement about the significance of these claims.
Some problems in proving these theories arise with the difficulty of using pharmacological means to determine the subtypes of specific NMDARs.
Excitotoxicity has been thought to play a role in the degenerative properties of neurodegenerative conditions since the late s. Most notably excitotoxic events involving NMDA receptors have been linked to Alzheimer's disease and Huntington's disease as well as with other medical conditions such as strokes and epilepsy.
These side effects are, in part, observed because the NMDA receptors do not just signal for cell death but also play an important role in its vitality. Activation of NMDA receptors requires binding of glutamate or aspartate aspartate does not stimulate the receptors as strongly. D -Serine has also been found to co-agonize the NMDA receptor with even greater potency than glycine. Recently, it has been shown that D -serine can be released both by neurons and astrocytes to regulate NMDA receptors.
This property is fundamental to the role of the NMDA receptor in memory and learning , and it has been suggested that this channel is a biochemical substrate of Hebbian learning , where it can act as a coincidence detector for membrane depolarization and synaptic transmission. Positive allosteric modulators include:. An example of memantine derivative is neramexane which was discovered by studying number of aminoalkyl cyclohexanes , with memantine as the template, as NMDA receptor antagonists.
Neramexane, which can be seen in figure 6, binds to the same site as memantine within the NMDA receptor associated channel and with comparable affinity.
It does also show very similar bioavailability and blocking kinetics in vivo as memantine. Neramexane went to clinical trials for four indications, including Alzheimer's disease.
Other weak partial agonists of the glycine site of the NMDA receptor such as rapastinel GLYX and apimostinel NRX are now viewed for the development of new drugs with antidepressant and analgesic effects without obvious psychotomimetic activities.
Antagonists of the NMDA receptor are used as anesthetics for animals and sometimes humans, and are often used as recreational drugs due to their hallucinogenic properties, in addition to their unique effects at elevated dosages such as dissociation. When certain NMDA receptor antagonists are given to rodents in large doses, they can cause a form of brain damage called Olney's lesions. NMDA receptor antagonists that have been shown to induce Olney's lesions include ketamine , phencyclidine , and dextrorphan a metabolite of dextromethorphan , as well as some NMDA receptor antagonists used only in research environments.
So far, the published research on Olney's lesions is inconclusive in its occurrence upon human or monkey brain tissues with respect to an increase in the presence of NMDA receptor antagonists.
Some common agents in which weak NMDA receptor antagonism is a secondary or additional action include:. The NMDA receptor is regulated via nitrosylation and aminoadamantane can be used as a target-directed shuttle to bring nitrogen oxide NO close to the site within the NMDA receptor where it can nitrosylate and regulate the ion channel conductivity. Unlike many other NO donors, alkyl nitrates do not have potential NO associated neurotoxic effects.
Alkyl nitrates donate NO in the form of a nitro group as seen in figure 7, -NO 2 -, which is a safe donor that avoids neurotoxicity. The nitro group must be targeted to the NMDA receptor, otherwise other effects of NO such as dilatation of blood vessels and consequent hypotension could result. Provisional studies in animal models show that nitromemantines are more effective than memantine as neuroprotectants, both in vitro and in vivo.
Memantine and newer derivatives could become very important weapons in the fight against neuronal damage. Negative allosteric modulators include:. The NMDA receptor is modulated by a number of endogenous and exogenous compounds: The main problem with the development of NMDA antagonist for neuroprotection is that the physiological NMDA receptor activity is essential for normal neuronal function. Complete blocking of all NMDA receptor activity therefore results in adverse side effects such as hallucination , agitation and anesthesia.
To be clinically accepted the NMDA receptor antagonist must block excessive activation without blocking the normal function. Competitive NMDA receptor antagonists, which were developed first, are not a good option because they compete and bind to the same site NR2 subunit on the receptor as the agonist, glutamate, and therefore block normal function also.
These antagonists can be displaced from the receptor by high concentration of glutamate which can exist under excitotoxic circumstances. Uncompetitive block refers to a type of block that increased concentration of glutamate cannot overcome and is dependent upon prior activation of the receptor by the agonist, i. This was confirmed by inclusion of a cannabinoid receptor antagonist, SRA Fig.
Neither THC or cannabidiol neuroprotection was affected by cannabinoid receptor antagonist. Effect of THC, cannabidiol, and cannabinoid receptor antagonist on glutamate induced neurotoxicity.
See Materials and Methods for experimental details. Studies have suggested that ROS damage may be involved in glutamate neurotoxicity 5 , 6. To investigate whether cannabinoids could protect neurons against glutamate by reacting with ROS, the antioxidant properties of cannabidiol and other cannabinoids were assessed. Cyclic voltametry, a procedure that measures the ability of a compound to accept or donate electrons under a variable voltage potential, was used to measure the oxidation potentials of several natural and synthetic cannabinoids.
Anandamide arachidonyl-ethanolamide , which is not a cannabinoid in structure but is an endogenous ligand for the cannabinoid receptor, did not undergo oxidation in this assay Fig. Three other cannabinoids, cannabinol, nabilone, and levanantrodol, also were tested, and they, too, exhibited oxidation profiles similar to cannabidiol and THC data not shown.
A A comparison of the oxidation potentials of cannabinoids and the antioxidant BHT. Anandamide, a cannabinoid receptor ligand with a noncannabinoid structure, was used as a nonresponsive control. Experiments were repeated three times with essentially the same results. B Effect of cannabidiol and THC on dihydrorhodamine oxidation. Cannabinoids were compared with BHT for their ability to prevent tert -butyl hydroperoxide-induced oxidation of dihydrorhodamine.
This experiment was repeated four times with essentially the same results. The ability of cannabinoids to be oxidized readily suggests that they may possess antioxidant properties comparable to BHT. These properties were examined further in a Fenton reaction iron-catalyzed ROS generation. Tert -butyl hydroperoxide was used to generate ROS and oxidize dihydrorhodamine into the fluorescent compound rhodamine. To confirm that cannabinoids act as antioxidants in the intact cell, neurons were incubated with tert -butyl hydroperoxide and varying concentrations of cannabidiol Fig.
The oxidant was chosen for its solubility in both aqueous and organic solvents, thereby facilitating oxidation in both cytosolic and membrane cell compartments. As observed in studies with glutamate, cannabidiol protected neurons against ROS toxicity in a concentration-related manner.
A The effect of cannabidiol on oxidative toxicity in neuronal cultures. Tert -butyl hydroperoxide-induced toxicity was examined in the presence or absence of cannabidiol. B Comparison of antioxidants and cannabidiol for their ability to prevent glutamate toxicity in neurons. All drugs were present throughout the glutamate exposure period. Each experiment represents the mean of four replicates repeated on three occasions. Significant differences between cannabidiol and other antioxidants are indicated with an asterisk.
The nonpsychoactive marijuana constituent cannabidiol was found to prevent both glutamate neurotoxicity and ROS-induced cell death. The psychoactive principle of Cannabis , THC, also blocked glutamate neurotoxicity with a similar potency to cannabidiol. In both cases, neuroprotection was unaffected by cannabinoid receptor antagonist. This suggests that cannabinoids may have potentially useful therapeutic effects that are independent of psychoactivity-inducing cannabinoid receptors 12 and so are not necessarily accompanied by psychotropic side effects.
Cannabidiol blocked glutamate toxicity in cortical neurons with equal potency regardless of whether the insult was mediated by NMDAr, AMPA receptors, or kainate receptors. This suggests that either cannabinoids antagonize all three glutamate receptors with the same affinity, or, more likely, their site of action is downstream of initial receptor activation events. Neurotoxic concentrations of glutamate induce massive calcium influx through NMDAr 4 that ultimately kills the cell.
However, the mixture of calcium channel inhibitors and NMDAr antagonist did not eliminate completely glutamate toxicity or reduce cell death as efficiently as EDTA. Cannabidiol and THC were found to be comparable with BHT antioxidant in both their ability to prevent dihydrorhodamine oxidation Fenton reaction and their cyclic voltametric profiles.
Synthetic cannabinoids such as HU, nabilone, and levanantradol also exhibited similar profiles. Anandamide, which is a natural cannabinoid receptor ligand but is not structurally related to cannabinoids, did not give an antioxidant-like profile by cyclic voltametry, which indicates that cannabinoids can act as reducing agents in a chemical system.
To confirm that cannabinoids also can function as antioxidants in living cells, a lipid hydroperoxide was used to generate ROS toxicity in neuronal cultures. As observed in the Fenton reaction system, cannabidiol attenuated this ROS-induced neurotoxicity. These observations indicate that many cannabinoids exert a considerable protective antioxidant effect in neuronal cultures.
The similarity of the voltamagrams observed with cannabidiol, HU, and several other cannabinoids also suggests that the reported antioxidant effect of HU is not a feature unique to this atypical cannabinoid, as previously implied; e.
The potency of cannabidiol as an antioxidant was examined by comparing it on an equimolar basis with other commonly used antioxidants. As in the Fenton reaction system, cannabidiol protected neurons with comparable efficacy to the potent antioxidant BHT. The similar antioxidant abilities of cannabidiol and BHT in this chemical system and their comparable protection in neuronal cultures implies that cannabidiol neuroprotection is caused by an antioxidant effect.
The antioxidative properties of cannabinoids suggest a therapeutic use as neuroprotective agents, and the particular properties of cannabidiol make it a good candidate for such development. Although cannabidiol was similar in neuroprotective capacity to BHT, cannabidiol has no known tumor-promoting effects [unlike BHT 25 , 26 ].
The lack of psychoactivity associated with cannabidiol allows it to be administered in higher doses than would be possible with psychotropic cannabinoids such as THC. Furthermore, the ability of cannabidiol to protect against neuronal injury without inhibiting NMDAr may reduce the occurrence of toxicity or side effects associated with NMDAr antagonists In vivo studies to examine the efficacy of cannabidiol as a treatment for experimentally induced ischemic stroke are currently in progress.
Cannabidiol was more protective against glutamate neurotoxicity than either ascorbate or alpha-tocopherol, indicating it to be a potent antioxidant. The snippet could not be located in the article text. This may be because the snippet appears in a figure legend, contains special characters or spans different sections of the article. Accepted Apr This article has been cited by other articles in PMC. Abstract The neuroprotective actions of cannabidiol and other cannabinoids were examined in rat cortical neuron cultures exposed to toxic levels of the excitatory neurotransmitter glutamate.
Open in a separate window. Abrahamov A, Mechoulam R. Ann NY Acad Sci. J Pharmacol Exp Ther. Amin N, Pearce B. Hack N, Balazs R.
excitotoxicity, depending on the receptor type that is activated. 1To whom . alone. Toxicity of AMPA applied together with CTZ and of kainate was abolished by CNQX. .. activation (e.g., voltage-gated calcium channels or the sodium- calcium. calcium, and astrocyte toxicity occurred only after 24 hr expo- sure to high ( alone was not toxic, was concentration-dependent for each of NMDA toxicity, they do have functional non-NMDA iono- tropic receptors. induced toxicity has been shown to be calcium dependent; this study demonstrates that .. AMPA/Kainate Toxicity Is Calcium Dependent. Increased calcium.