Low SJ, Roland CL
Review of NMDA antagonist-induced neurotoxicity and implications for clinical development.
Int J Clin Pharmacol Ther. 2004 Jan;42(1):1-14.
NMDA receptor antagonists have been investigated for many years as therapeutic agents for the treatment of neurological disorders such as stroke, epilepsy, pain and Parkinson's disease. It has been discovered, however, that many of these compounds cause adverse behavioral (psychotomimetic) effects and can produce neurotoxicity characterized by neuronal vacuolization, induction of heat-shock protein, neuronal/axonal degeneration and regional brain cell death in several animal species. It is unknown whether NMDA antagonists induce neurotoxicity in humans. The mechanism of NMDA antagonist-induced neurotoxicity is not completely known, but some evidence suggests disinhibition of GABAergic inputs to the affected neurons. Several classes of compounds have been shown to prevent NMDA antagonist-induced neurotoxicity. The extent of neurotoxicity produced by NMDA antagonists is affected by many factors, including type of antagonist, dose, length of exposure, age, sex and species. While there are no published regulatory guidelines regarding how NMDA antagonist compounds should be evaluated, sponsors and investigators of these compounds should make every effort to assess the potential for neurotoxicity. NMDA receptor antagonists, as well as other CNS-active compounds need to be analyzed for neurotoxicity through careful experimental design, adequate tissue sampling and through the use of a sensitive method of detection. [Abstract]
Farber NB, Kim SH, Dikranian K, Jiang XP, Heinkel C.
mechanisms and circuitry underlying NMDA antagonist neurotoxicity.
"NMDA glutamate receptor antagonists are
used in clinical anesthesia, and are being developed as therapeutic agents for
preventing neurodegeneration in stroke, epilepsy, and brain trauma. However, the
ability of these agents to produce neurotoxicity in adult rats and psychosis in
adult humans compromises their clinical usefulness. In addition, an NMDA receptor
hypofunction (NRHypo) state might play a role in neurodegenerative and psychotic
disorders, like Alzheimer's disease and schizophrenia. Thus, understanding the
mechanism underlying NRHypo-induced neurotoxicity and psychosis could have significant
clinically relevant benefits. NRHypo neurotoxicity can be prevented by several
classes of agents (e.g. antimuscarinics, non-NMDA glutamate antagonists, and alpha(2)
adrenergic agonists) suggesting that the mechanism of neurotoxicity is complex.
In the present study a series of experiments was undertaken to more definitively
define the receptors and complex neural circuitry underlying NRHypo neurotoxicity.
Injection of either the muscarinic antagonist scopolamine or the non-NMDA antagonist
NBQX directly into the cortex prevented NRHypo neurotoxicity. Clonidine, an alpha(2)
adrenergic agonist, protected against the neurotoxicity when injected into the
basal forebrain. The combined injection of muscarinic and non-NMDA Glu agonists
reproduced the neurotoxic reaction. Based on these and other results, we conclude
that the mechanism is indirect, and involves a complex network disturbance, whereby
blockade of NMDA receptors on inhibitory neurons in multiple subcortical brain
regions, disinhibits glutamatergic and cholinergic projections to the cerebral
cortex. Simultaneous excitotoxic stimulation of muscarinic (m(3)) and glutamate
(AMPA/kainate) receptors on cerebrocortical neurons appears to be the proximal
mechanism by which the neurotoxic and psychotomimetic effects of NRHypo are mediated."
Li, Qiang, Clark, Suzanne, Lewis, Darrell V., Wilson,
NMDA Receptor Antagonists Disinhibit Rat Posterior Cingulate
and Retrosplenial Cortices: A Potential Mechanism of Neurotoxicity
Neurosci. 2002 22: 3070-3080
"NMDA receptor antagonists produce region-specific
neurodegeneration by an undetermined mechanism, but one proposed mechanism involves
disinhibition. In certain areas of the brain, NMDA receptors mediate excitatory
drive onto inhibitory interneurons. Thus, NMDA receptor/channel antagonists may
reduce inhibition (i.e., produce "disinhibition"). If a sufficient level
of disinhibition is produced, enhanced vulnerability to excitotoxicity may result.
Furthermore, if there are region-specific differences in NMDA antagonist-induced
disinhibition, this could underlie region-specific NMDA antagonist-induced neurotoxicity.
In the present study, we tested this hypothesis by exposing rat brain slices to
the NMDA receptor antagonist dizocilpine maleate (MK-801) and measuring MK-801-induced
disinhibition in areas of higher and lower vulnerability to neurodegeneration
[posterior cingulate/retrosplenial cortices (PCC/RSC) and parietal cortex, respectively].
Using whole-cell patch-clamp techniques, bicuculline-sensitive GABA(A) receptor-mediated
IPSCs were measured in biocytin-labeled pyramidal neurons in the PCC/RSC and parietal
cortex. In the PCC/RSC, bath-applied MK-801 (10-40 microm) produced disinhibition,
shown as a concentration-dependent decrease in spontaneous IPSC frequency and
amplitude; MK-801 (40 microm) also reduced evoked IPSC amplitudes. In parietal
cortex, MK-801 produced significantly less disinhibition. To determine whether
disinhibition is caused by presynaptic or postsynaptic mechanisms, we tested the
effects of MK-801 (40 microm) against miniature IPSC (mIPSC) frequency and amplitude
in tetrodotoxin (TTX; 0.5 microm)-treated slices and found that MK-801 did not
alter mIPSC frequency or amplitude. Taken together, these results suggest that
NMDA receptors regulate activity of inhibitory interneurons and, consequently,
GABA release in certain cortical areas. This region-specific reduction in inhibitory
input to pyramidal cells could underlie the region-specific neurotoxicity of NMDA
A. Haberny , Merle G. Paule , Andrew C. Scallet , Frank D. Sistare , David S.
Lester , Joseph P. Hanig , and William Slikker, Jr.
the N-Methyl-D-Aspartate (NMDA) Receptor System and Susceptibility to Neurotoxicity
Toxicol. Sci. 68: 9-17. 2002.
"The NMDA receptor
has been widely investigated in recent years as a target for the pharmacological
management of seizures, pain and a variety of neurological disorders. Its role
in normal central nervous system (CNS) activity and development, as well as in
the development of CNS abnormalities and neurodegeneration has also been of interest.
The NMDA receptor is one of three pharmacologically distinct subtypes of ionotropic
receptor channels that are sensitive to the endogenous excitatory amino acid,
L-glutamate. The ontogeny of the NMDA receptor, a multiple tetrameric and heteromeric
channel complex with at least six known subunits, is controlled by three gene
families and varies in developmental profile with species and regional brain area.
NMDA receptors play a role in excitatory synaptic transmission, in the activity-dependent
synaptic plasticity underlying learning and memory, and in pre- and postnatal
CNS development, including brain cell differentiation, axonal growth and degeneration
of unused neurons. The results of recent studies suggest that sustained alteration
of NMDA receptor activation during critical periods of development may have deleterious
effects on normal CNS development and function. Neonatal rats administered the
NMDA receptor antagonists 2-amino-5-phosphonovalerate (AP5) and MK-801 during
the first two weeks of life develop abnormal axonal arborization in the retinal
connections to the superior colliculus, interfering with normal visual responses.
Results from monkey studies suggest that chronic developmental exposure to high
doses of a NMDA antagonist, remacemide, has pronounced and long-lasting effects
on learning. Recent findings indicate that if NMDA receptors are blocked during
a specific period in neonatal life (first two weeks postnatally in the rat), massive
apoptotic neurodegeneration results, due not to excitotoxic overstimulation of
neurons but to deprivation of stimulation. These observations require further
laboratory evidence and support in order to establish their relevance to drug-induced
human neurodevelopmental concerns. It is necessary to investigate the relevance
of these findings in other animal species in addition to the rat, most notably,
nonhuman primates, where neuronal cytoarchitecture and development are significantly
different than the rodent but more like the human." [Abstract]
W, Hetman M, Nikolaev E, Quack G, Danysz W, Kaczmarek L.
evaluation of long-term neurotoxic effects of NMDA receptor antagonists.
Res. 2000 Apr;1(4):299-310.
"High doses of NMDA antagonists e.g. (+)MK-801
evoke neurodegeneration in retrosplenial cortex in rodents. To assess functional
consequences of such treatment, three paradigms of two-way active avoidance learning
(with visual or auditory conditioned stimuli) and additionally a spatial learning
paradigm - radial maze - were used. Female rats were treated i.p. with 5 mg/kg
of (+)MK-801. Recumbence, severe hypothermia and loss of body weight were observed
for 3-7 days. Despite that, there were no statistically significant differences
in performance of avoidance reaction between saline and (+)MK-801 treated animals
trained 10-40 days after the drug administration. However, in the radial maze
test (+)MK-801 impaired reference (but not working) memory in the experiment that
started 8 days after the treatment. Similar effect was observed on reversal learning.
The clinically used NMDA receptor antagonist memantine at the doses of 20 and
40 mg/kg had also no such long term negative effect on working memory during training
(even positive effect was seen at 20 mg/kg) but at 40 mg/kg impaired learning
on the first day of reversal. This indicates that (+)MK-801 neurotoxicity in the
retrosplenial cortex is connected with subtle alterations in the learning performance
that may be seen in some tests only. Moreover, memantine doses greatly exceeding
therapeutically relevant range produce minimal functional alteration. An additional
experiment revealed that the same dose of memantine results in two fold higher
serum levels of the antagonist in female than male rats. Hence, considering that
profiling studies are done in male rats, a safety factor of over 16 fold can be
calculated for memantine." [Abstract]
NB, Jiang X, Dikranian K, Nemmers B.
Muscimol prevents NMDA antagonist
neurotoxicity by activating GABAA receptors in several brain regions.
Res. 2003 Dec 12; 993(1-2): 90-100.
"N-Methyl-D-aspartate (NMDA) glutamate
receptor antagonists are being developed as therapeutic agents for several clinical
conditions. However, the ability of these agents to produce neurotoxicity and
psychosis can compromise their clinical usefulness. In addition, an NMDA receptor
hypofunction (NRHypo) state may play a role in neurodegenerative and psychotic
disorders. A better understanding of the mechanism underlying these adverse effects
should allow for the safer use of these agents and might clarify mechanisms underlying
certain clinical disorders. NRHypo neurotoxicity is mediated by a complex disinhibition
mechanism in which NMDA antagonists abolish GABAergic inhibition, resulting in
the simultaneous excessive release of acetylcholine and glutamate onto the vulnerable
retrosplenial cortex (RSC) neurons. Systemically administered GABAergic agents
are potent protectors against NRHypo neurotoxicity. To determine where in brain
GABAergic agents could be acting to protect against NRHypo neurotoxicity, we injected
the GABAergic agonist, muscimol, into different brain regions of rats treated
systemically with a neurotoxic dose of the potent NMDA antagonist, MK-801. We
report that muscimol injections into the anterior thalamus or diagonal band of
Broca provide substantial protection, suggesting that disinhibition of neurons
in these regions underlies NRHypo neurotoxicity. Muscimol injections into the
RSC also provide substantial protection possibly by directly inhibiting the vulnerable
RSC neuron. Injections of muscimol into other areas known to project to the RSC
(ventral orbital cortex, anterior cingulate cortex and subiculum) provide only
minimal protection. We conclude that GABAergic agents prevent NRHypo neurotoxicity
mainly by activating GABA receptors in the anterior thalamus, diagonal band of
Broca and RSC." [Abstract]
V, Wozniak DF, Powell S, Olney JW.
Propofol and sodium thiopental
protect against MK-801-induced neuronal necrosis in the posterior cingulate/retrosplenial
Brain Res. 2001 Sep 21;913(2):185-9.
(NMDA) antagonists act by an anti-excitotoxic action to provide neuroprotection
against acute brain injury, but these agents can also cause toxic effects. In
low doses they induce reversible neuronal injury, but in higher doses they cause
irreversible degeneration of cerebrocortical neurons. GABAmimetic drugs protect
against the reversible neurotoxic changes in rat brain. Here we show that two
GABAmimetic anesthetic agents--propofol and sodium thiopental--protect against
the irreversible neurodegenerative reaction induced by the powerful NMDA antagonist,
GG, Guerrero JM, Reiter RJ, Poeggeler BH, Bitzer-Quintero OK, Feria-Velasco A.
Arch Med Res. 1999 Mar-Apr;30(2):125-7.
The noncompetitive NMDA antagonists phencyclidine (PCP) and dizocilpine (MK-801)
have been considered for use as neuroprotective therapeutic agents, although both
produce injury in neurons of cingulate and retrosplenial cortices in rodents.
The low-affinity, noncompetitive NMDA antagonist dextrorphan has been considered
for use as a neuroprotective therapeutic drug. The aim of the present work was
to evaluate the neurotoxicity of dextrorphan. METHODS: Sprague-Dawley male rats
were used and injected with either saline or dextrorphan (30 mg/kg i.p.). The
animals were sacrificed 30 min later, and the brain was examined for histopathological
changes. RESULTS: After systemic administration of the drug, hyperchromatic and
shrunken nuclei with chromatin condensation and disruption were observed. Also,
granular and vacuolated cytoplasm was apparent in pyramidal neurons in the retrosplenial
(posterior cingulate) cortex. Status spongiosus (spongy degeneration) of the neuropil
was also detected. CONCLUSIONS: Morphological changes are similar to those described
previously, which are induced by high-affinity, noncompetitive NMDA antagonists,
such as MK-801." [Abstract]
P, Flood K, Lehmann A, Berge OG.
MK-801 neurotoxicity in the guinea
pig cerebral cortex: susceptibility and regional differences compared with the
J Neurosci Res. 1997 Aug 1;49(3):364-71.
(NMDA) receptor antagonists induce transient vacuole formation in neurons of the
retrosplenial cortex and, after higher doses, necrosis in the same region. To
our knowledge, all studies demonstrating these effects have been carried out in
rats or mice. The present study investigated whether vacuolization occurs in the
guinea pig, rats being used as controls. Female Dunkin-Hartley guinea pigs (age
15-18 weeks) were given a single subcutaneous injection of saline or the non-competitive
NMDA antagonist dizocilpine maleate [(+)-MK-801; 1, 4, or 12 mg/kg]. Female Sprague-Dawley
rats (age 16 weeks) received saline or MK-801 (1 mg/kg). Whatever the dose of
MK-801, guinea pigs showed only occasional vacuolated neurons in the retrosplenial
cortex. However, affected neurons (mainly large pyramidal cells of layer V) were
found in the frontoparietal neocortex. The reaction was limited after 1 mg/kg,
and seemed to reach a maximum at 4 mg/kg. Rats injected with 1 mg/kg MK-801 showed
an intense vacuole reaction in neurons from layers III-IV of the retrosplenial
cortex, but no affected neurons were noted in neocortical areas. We conclude that
there are significant species differences in susceptibility to, and location of,
vacuolization induced by NMDA receptor antagonists." [Abstract]
Effect of Age and Sex on N-Methyl-D-Aspartate Antagonist-Induced
Neuronal Necrosis in Rats
Stroke 27: 743-746
AND PURPOSE: Although N-methyl-D-aspartate (NMDA) antagonism may be a useful therapeutic
approach in stroke treatment, it has been found that these pharmacological agents
cause neuronal necrosis in restricted cortical regions of the rodent brain. METHODS:
To test the hypothesis that age and sex influence NMDA antagonist-induced neuronal
necrosis, male and female rats were studied at 2 months (young), 12 months (middle-aged),
and 24 months (old) of age. A dose of 5 mg/kg MK-801 was administered, followed
by quantitation of neuronal necrosis at nine coronal levels in the cingulate and
retrosplenial cortex at 1 week of survival. RESULTS: Mortality was dependent on
age but not sex and was higher in the old rats (P<.01). The number of necrotic
neurons per hemisphere was greater in female than in male rats at all ages (P<.0001).
Female rats also showed increasing neuronal necrosis with age (P<.05). CONCLUSIONS:
The results indicate a major sex difference in neuronal cytotoxicity caused by
NMDA antagonists and a minor increase in susceptibility with increasing age in
females. The findings may be relevant to development of drugs with NMDA antagonist
properties for use in human stroke." [Full
Farber NB, Wozniak DF, Price MT, Labruyere
J, Huss J, St Peter H, Olney JW.
Age-specific neurotoxicity in the
rat associated with NMDA receptor blockade: potential relevance to schizophrenia?
Psychiatry. 1995 Dec 15;38(12):788-96.
"Agents that block the N-methyl-D-aspartate
(NMDA) subtype of glutamate receptor induce a schizophrenialike psychosis in adult
humans and injure or kill neurons in several corticolimbic regions of the adult
rat brain. Susceptibility to the psychotomimetic effects of the NMDA antagonist,
ketamine is minimal or absent in children and becomes maximal in early adulthood.
We examined the sensitivity of rats at various ages to the neurotoxic effects
of the powerful NMDA antagonist, MK-801. Vulnerability was found to be age dependent,
having onset at approximately puberty (45 days of age) and becoming maximal in
early adulthood. This age-dependency profile (onset of susceptibility in late
adolescence) in the rat is similar to that for ketamine-induced psychosis or schizophrenia
in humans. These findings suggest that NMDA receptor hypofunction, the mechanism
underlying the neurotoxic and psychotomimetic actions of NMDA antagonists, may
also play a role in schizophrenia." [Abstract]
Noguchi KK, Nemmers B, Farber NB
Age has a similar influence on the susceptibility to NMDA antagonist-induced neurodegeneration in most brain regions.
Brain Res Dev Brain Res. 2005 Aug 8;158(1-2):82-91.
NMDA antagonists are of potential therapeutic benefit for several conditions. However, their ability to produce neurotoxicity and psychosis has hampered their clinical use. A better understanding of these side effects and the mechanism underlying them could result in their safer use and in improving our understanding of psychotic illnesses. By disinhibiting certain multisynaptic circuits, moderate doses of NMDA antagonists produce reversible neurotoxicity in the retrosplenial cortex in rats older than 1 month. Higher doses of these same agents result in the death of neurons in the retrosplenial cortex and several other brain regions. It is unknown whether susceptibility to this irreversible neurodegeneration has a similar age dependency profile. We, therefore, examined the sensitivity of rats of various ages (PND20-60) to the irreversible neurodegenerative effect of the selective NMDA antagonist, MK-801. Quantification of the severity of neurodegeneration with stereology revealed that the retrosplenial cortex, induseum griseum, and dentate gyrus had decreasing amounts of damage with decreasing age and onset of sensitivity around PND30. The piriform cortex also displayed a decreased amount of degeneration in younger age groups. However, a low level of degeneration continued to occur in the posterior piriform cortex in the PND20-25 animals. The stage of degeneration appeared to be more advanced, suggesting that these neurons were dying by a different mechanism. We conclude that for most neuronal populations, susceptibility to the irreversible and reversible neurodegenerative effects of NMDA antagonists has a similar age dependency profile, consistent with the proposal that the same disinhibitory mechanism underlies both neurotoxicities. [Abstract]
Assessing structural changes in the brain to evaluate neurotoxicological
effects of NMDA receptor antagonists.
"Like all pharmacologic agents known, N-methyl-D-aspartate
(NMDA) antagonist compounds have side effects. It is expected that neuroactive
molecules have effects, including side effects, in the central nervous system
(CNS). With NMDA antagonists in rodents, these side effects are remarkably focal
in the cingulate and retrosplenial cortex. The salient features of NMDA antagonist
neurotoxicity which should be underscored are hypermetabolism, lactate accumulation,
neuronal vacuolization in aldehyde fixed material, and neuronal death in older
rodents. The scope of this phenomenon must urgently be determined in non-rodent
species, specifically primates. This is important from both a regulatory and neurotherapeutic
point of view, since effective molecules having potential in human disease states
may also have NMDA antagonist properties." [Abstract]
Bueno A, De Olmos S, Heimer L, De Olmos J.
MK-801-induced neuronal degeneration in Wistar rat brain detected by the Amino-Cupric-Silver
Exp Toxicol Pathol. 2003 Mar; 54(4): 319-34.
neurotoxic effect following a single intraperitoneal injection of MK-801 (10 mg/kg)
in adult female Wistar rats at different survival times was studied with the 1994
version of de Olmos' Amino-Cupric-Silver (A-Cu-Ag) technique for detection of
neural degeneration. In addition to the well documented somatodendritic degeneration
observable in cortical olfactory structures, dentate gyrus, retrosplenial and
sensory cortices, we detected this type of neuronal degeneration also in the main
olfactory bulb, motor and anterior cingulate cortices, thalamus and cerebellum.
Terminal degeneration, not reported by previous authors, was detected in cortical
olfactory structures, hippocampal formation, sensory, infralimbic, prelimbic,
agranular insular, ectorhinal, perirhinal and lateral orbital cortices. These
results demonstrate that the A-Cu-Ag procedure is more efficient than other silver
methods for detecting the degeneration induced by MK-801. In fact, the use of
the A-Cu-Ag method has made it possible to infer the connectional relations between
the damaged cell bodies and corresponding terminal degeneration. Our results also
indicate that the A-Cu-Ag technique may be a suitable method for the staining
of neurons undergoing apoptotic-like degeneration. The probable degenerative mechanism
of MK-801 in the main olfactory system is discussed." [Abstract]
Tomitaka M, Tomitaka S, Rajdev S, Sharp FR.
prevents PCP- and MK801-induced HSP70 expression in injured limbic cortical neurons
Biol Psychiatry. 2000 May 1;47(9):836-41.
N-Methyl-D-aspartate (NMDA) receptor antagonists, including phencyclidine (PCP)
and dizocilpine (MK801), cause schizophrenialike psychosis in humans, and produce
vacuolated neurons in the cingulate and retrosplenial cortices of the rat brain.
Since psychotically depressed patients and schizophrenic depressed patients may
require treatment with selective serotonin reuptake inhibitors (SSRIs), it is
of interest to examine the relationship between SSRIs and NMDA antagonist neurotoxicity.
METHODS: The neurotoxicity of PCP and MK801 was assessed using heat shock protein
(HSP70) immunocytochemistry and HSP70 Western blots because HSP70 is expressed
in the injured, vacuolated neurons. Female rats were given fluoxetine (0, 5, 10,
and 20 mg/kg IP) followed 1 hour later by MK801 (1 mg/kg IP) or PCP (50 mg/kg
IP). RESULTS: Pretreatment with fluoxetine (20 mg/kg IP) 1 hour before MK801 prevented
the induction of HSP70 by MK801 in the cingulate and retrosplenial cortices. Pretreatment
with fluoxetine (10 or 20 mg/kg IP) 1 hour before PCP also prevented the HSP70
induction by PCP. CONCLUSIONS: Fluoxetine prevents the neurotoxicity of NMDA receptor
antagonists in rat brain." [Abstract]
P, Tortella FC, Bowery NG.
An autoradiographic study of dextromethorphan
high-affinity binding sites in rat brain: sodium-dependency and colocalization
Br J Pharmacol. 1997 Apr;120(7):1255-62.
The distribution and some pharmacological properties of centrally located dextromethorphan
high-affinity binding sites were investigated by in vitro autoradiography. 2.
Sodium chloride (50 mM) induced a 7 to 12 fold increase in dextromethorphan binding
to rat brain in all areas tested. The effect of sodium was concentration-dependent
with a higher dose (120 mM) exerting a smaller effect on binding. 3. [3H]-dextromethorphan
binding in the presence of sodium was inhibited in the presence of the anticonvulsant
phenytoin at a concentration of 100 microM, while the sigma ligand (+)-3-(-3-hydroxyphenyl)-N-(1-propyl)pipendine
((+)-PPP) had no effect on the binding, suggesting an interaction with the DM2
site. 4. The distribution of the sodium-dependent binding identified in this study
correlated significantly with the distribution of the selective 5-HT uptake inhibitor
[3H]-paroxetine, and paroxetine and dextromethorphan mutually displaced their
binding at concentrations in the low nanomolar range. 5. These data show that
dextromethorphan and paroxetine share a sodium-dependent high affinity binding
site in rat brain, and suggest that dextromethorphan might interact, in the presence
of sodium, with the 5-HT uptake mechanism in rat brain." [Abstract]
MG, Fuller RW.
Dextromethorphan antagonizes the acute depletion of
brain serotonin by p-chloroamphetamine and H75/12 in rats.
Res. 1992 Oct 30;594(2):323-6.
"A role for calcium in p-chloroamphetamine-induced
neurotoxicity has been inferred previously from protective effects of dextromethorphan.
We found that dextromethorphan reduces rat brain concentrations of 5-hydroxyindoleacetic
acid and blocks the acute, non-neurotoxic depletion of brain serotonin by p-chloroamphetamine
and by H75/12. Inhibition of the membrane transporter on brain serotonin neurons
by dextromethorphan in vivo might explain its protective effect against p-chloroamphetamine
GZ, Musacchio JM.
Computer-assisted modeling of multiple dextromethorphan
and sigma binding sites in guinea pig brain.
Eur J Pharmacol.
1991 Apr 25;206(4):261-9.
"Computer-assisted, simultaneous analysis of
self- and cross-displacement experiments demonstrated the existence of several
binding sites in guinea pig brain for dextromethorphan, (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine
((+)-3-PPP), and 1,3-di-o-tolyl guanidine (DTG). Dextromethorphan binds with high
affinity to two sites (R1 Kd 50-83 and R2 Kd 8-19 nM) and with low affinity to
two additional sites (R3 and R4). (+)-3-PPP binds to one high-affinity (R1 Kd
24-36 nM), to one intermediate-affinity (R3 Kd 210-320 nM), and to two (R2 and
R4) low-affinity sites. DTG binds with almost identical high affinity to two different
sites (R1 Kd 22-24 and R3 Kd 13-16 nM). These results confirm that dextromethorphan,
(+)-3-PPP, and DTG bind to the common DM1/sigma 1 site (R1). The binding of DTG
to two different sites with identical affinities precludes the use of this compound
as a specific marker for sigma receptors. Besides, haloperidol displaces labeled
ligands from both high-affinity DTG sites (R1 and R3) with high affinity. Thus,
haloperidol sensitivity should not be used as the single criterion to identify
a putative receptor. The resolution of these novel sites also may provide new
insights into the multiple effects of antipsychotic drugs. In addition, this investigation
has important implications regarding the methods that must be applied to characterize
multiple binding sites and their relations with putative receptors." [Abstract]
P, Leonardon J, Besnard F, Graham D, Depoortere H, Scatton B.
properties of eliprodil and other NMDA receptor antagonists at rat NR1A/NR2A and
NR1A/NR2B receptors expressed in Xenopus oocytes.
Lett. 1997 Feb 21;223(2):133-6.
"We have studied the effects of a variety
of N-methyl-D-aspartate (NMDA) antagonists acting at different sites of the NMDA
receptor complex on NMDA-induced currents in Xenopus oocytes expressing heteromeric
NR1A/NR2 and NR1A/NR2B receptors. The polyamine site antagonists eliprodil (IC50
= 3.0 microM) and ifenprodil (IC50 = 0.27 microM) antagonized NMDA responses at
NR1A/NR2B receptors but not at NR1A/NR2A receptors (IC50 > 100 microM). The
channel blockers dizocilpine, memantine and phencyclidine (PCP) were equally potent
antagonists at both receptor subtypes whereas dextromethorphan was four times
more potent at NR1A/NR2A receptors." [Abstract]
Jevtovic-Todorovic V, Wozniak DF, Powell S, Nardi
A, Olney JW.
Clonidine potentiates the neuropathic pain-relieving
action of MK-801 while preventing its neurotoxic and hyperactivity side effects.
Res. 1998 Jan 19;781(1-2):202-11.
"Antagonists of NMDA glutamate receptors
have been shown to alleviate neuropathic pain in rats and humans. However, NMDA
antagonists can cause significant side effects ranging from behavioral disturbances
to injury of neurons in the posterior cingulate/retrosplenial (PC/RS) cortex.
We have found that alpha-2 adrenergic agonists prevent the PC/RS neurotoxic side
effects of NMDA antagonists. In the present study of adult female rats subjected
to sciatic nerve ligation (Bennett neuropathic pain model) and tested for paw
withdrawal latency (PWL) following a thermal stimulus, we evaluated the ability
of the NMDA antagonist, MK-801, to alleviate neuropathic pain either by itself
or when administered together with the alpha-2 adrenergic agonist, clonidine.
We found that MK-801, at a dose (0.05 mg/kg s.c.) that is known to cause mild
hyperactivity but is subthreshold for producing PC/RS neurotoxic changes, relieved
the neuropathic pain state associated with sciatic nerve ligation. However, the
relief at this dose was very transient, and no neuropathic pain-relieving effect
was observed at a lower dose (0. 025 mg/kg s.c.) of MK-801. Clonidine, at a dose
(0.05 mg/kg s.c.) that prevents the cerebrocortical neurotoxic effects of MK-801,
decreased sensitivity to the thermal stimulus equally under all conditions (ligated,
sham ligated, unoperated), but did not specifically relieve neuropathic pain in
the ligated limb. Combining this dose of clonidine with an ineffective dose (0.025
mg/kg s.c.) of MK-801 provided specific, complete and long lasting (up to 4 h)
relief from neuropathic pain. Rats receiving this drug combination did not display
hyperactivity or any other behavioral disturbance typically associated with MK-801
treatment, nor show neurotoxic changes in cerebrocortical neurons. In separate
experiments on normal unoperated rats, we found that clonidine (0.05 mg/kg s.c.)
counteracted the hyperactivity induced by MK-801 (0.05 mg/kg s.c.) and returned
activity levels to a normal range. These findings signify that clonidine, which
does not specifically relieve neuropathic pain, can potentiate the neuropathic
pain-relieving action of MK-801, while also protecting against neurotoxicity and
hyperactivity side effects of MK-801. The potentiation is of a sufficient magnitude
that it permits cutting the MK-801 dose requirement in half, thereby achieving
prolonged neuropathic pain relief while doubling the margin of safety against
any type of side effect that might be mediated by blockade of NMDA receptors."
NB, Foster J, Duhan NL, Olney JW.
alpha 2 adrenergic agonists prevent
Neuropsychopharmacology. 1995 Jul;12(4):347-9.
of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor are of considerable
interest for various neurotherapeutic purposes, including preventing neuronal
degeneration in stroke and CNS trauma, suppressing neuropathic pain and preventing
the development of tolerance to opiate analgesics. Unfortunately, NMDA antagonists
can cause potentially serious side effects, including acute neurodegenerative
changes in corticolimbic regions of the adult rat brain and psychotic reactions
in adult humans. We have been investigating the mechanisms underlying the neuropathological
changes in rat brain and exploring methods of suppressing or preventing such changes.
Here we report that alpha 2 adrenergic agonists can prevent NMDA antagonist neurotoxicity.
Therefore, administering alpha 2 adrenergic agonists together with NMDA antagonists
may be a valuable strategy for preventing adverse side effects of NMDA antagonists
and making these agents safer for various neurotherapeutic purposes." [Abstract]
NB, Foster J, Duhan NL, Olney JW.
Olanzapine and fluperlapine mimic
clozapine in preventing MK-801 neurotoxicity.
Res. 1996 Jul;21(1):33-7.
"Antagonists of the N-methyl-D-Aspartate (NMDA)
subtype of glutamate receptor (e.g., phencyclidine, ketamine, MK-801) cause a
schizophrenia-like psychosis in humans and neurotoxicity in the adult rat brain.
We report here that clozapine and structurally related agents (olanzapine, fluperlapine,
loxapine, amoxapine) can prevent NMDA antagonist neurotoxicity in the rat with
a rank order corresponding to their ability to mimic the antipsychotic properties
of clozapine." [Abstract]
R, Nickolenko J, Chang J, Sagar SM, Sharp FR.
ketamine- and phencyclidine-induced HSP70 protein expression but not microglial
Exp Neurol. 1996 Feb;137(2):234-41.
N-methyl-D-aspartate (NMDA) receptor antagonists, inclu ding ketamine and phencyclidine
(PCP), produce abnormal intracellular vacuoles in posterior cingulate and retrosplenial
cortical neurons in the rat. Ketamine also induces 70-kDa heat shock protein (HSP70)
expression in pyramidal neurons in the posterior cingulate and retrosplenial cortex
and, as shown by this study, activates microglia in the retrosplenial cortex of
the rat. Whereas HSP70 protein expression was induced with ketamine doses of 40
mg/kg (ip) and higher, doses of 80 mg/kg and higher were required to activate
microglia. HSP70-positive neurons were observed in 30- to 90-day-old rats but
not in younger, 10- to 20- day old animals following ketamine (80 mg/kg, ip).
Pretreatment with the antipsychotic drug haloperidol at doses of 1.0 mg/kg and
above abolished all HSP70 immunostaining produced by ketamine (80 mg/kg). However,
a single dose of haloperidol (5 mg/kg, im) did not decrease the number of microglia
activated in retrosplenial cortex by ketamine (80-140 mg/kg). Similarly, PCP (10
and 50 mg/kg, ip)-induced microglial activation in the posterior cingulate and
retrosplenial cortex of adult rats was not blocked by haloperidol (10 mg/kg, im,
1 h prior to PCP). These results suggest that ketamine and PCP injure neurons
in the posterior cingulate and retrosplenial cortex of adults rats. Though haloperidol
may afford some protection against this injury since it inhibits induction of
HSP70 expression, the failure to prevent microglial activation suggests that single
doses of haloperidol do not completely protect neurons from NMDA antagonist toxicity."
Farber NB, Nemmers B, Noguchi KK
Acute D(2)/D(3) Dopaminergic Agonism but Chronic D(2)/D(3) Antagonism Prevents NMDA Antagonist Neurotoxicity.
Biol Psychiatry. 2006 Apr 7;
BACKGROUND: Antagonists of the N-methyl-D-aspartate (NMDA) glutamate receptor, most likely by producing disinhibtion in complex circuits, acutely produce psychosis and cognitive disturbances in humans, and neurotoxicity in rodents. Studies examining NMDA Receptor Hypofunction (NRHypo) neurotoxicity in animals, therefore, may provide insights into the pathophysiology of psychotic disorders. Dopaminergic D(2) and/or D(3) agents can modify psychosis over days to weeks, suggesting involvement of these transmitter system(s). METHODS: We studied the ability of D(2)/D(3) agonists and antagonists to modify NRHypo neurotoxicity both after a one-time acute exposure and after chronic daily exposure. RESULTS: Here we report that D(2)/D(3) dopamine agonists, probably via D(3) receptors, prevent NRHypo neurotoxicity when given acutely. The protective effect with D(2)/D(3) agonists is not seen after chronic daily dosing. In contrast, the antipsychotic haloperidol does not affect NRHypo neurotoxicity when given acutely at D(2)/D(3) doses. However, after chronic daily dosing of 1, 3, or 5 weeks, haloperidol does prevent NRHypo neurotoxicity with longer durations producing greater protection. CONCLUSIONS: Understanding the changes that occur in the NRHypo circuit after chronic exposure to dopaminergic agents could provide important clues into the pathophysiology of psychotic disorders. [Abstract]
JW, Farber NB.
Efficacy of clozapine compared with other antipsychotics
in preventing NMDA-antagonist neurotoxicity.
J Clin Psychiatry.
1994 Sep;55 Suppl B:43-6.
"The focus of this article will be on toxic
symptoms associated with blockade of the N-methyl-D-aspartate (NMDA) subtype of
the glutamate receptor. We have been studying two parallel phenomena: NMDA-antagonist
neurotoxicity (NAN) in rats and NMDA-antagonist psychotogenicity (NAP) in humans.
These phenomena have a common denominator--NMDA receptor hypofunction, which is
putatively a mechanism operative in schizophrenia. We have found that the NAN
reaction in rats can be prevented by specific drugs that prevent NAP in humans
and by certain antipsychotic agents, including clozapine, that ameliorate symptoms
in schizophrenia. By studying mechanisms by which clozapine prevents the NAN reaction
in rats, we hope to gain insight into mechanisms by which clozapine or other atypical
antipsychotics ameliorate symptoms in schizophrenia." [Abstract]
Farber NB, Heinkel C, Dribben WH, Nemmers B, Jiang X
In the adult CNS, ethanol prevents rather than produces NMDA antagonist-induced neurotoxicity.
Brain Res. 2004 Nov 26;1028(1):66-74.
Single doses of an NMDA antagonist cause an adult or a prepubertal form of neurodegeneration, depending on the age of the animal. Single doses of ethanol (EtOH) by blocking NMDA receptors produce apoptotic neurodegeneration in young animals. This capability could account, in part, for the ability of EtOH to produce the fetal alcohol syndrome. We investigated whether EtOH could produce NMDA antagonist-induced neurotoxicity (NAN), a different neurotoxicity that is seen only in adult animals. In spite of producing blood EtOH levels (30 to 600 mg/dl) known to block NMDA receptors, EtOH was unable to produce neurotoxicity in the adult central nervous system (CNS). Moreover, EtOH in a dose-dependent fashion (ED(50)=138 mg/dl) prevented the selective and powerful NMDA antagonist, MK-801, from producing NAN in adult animals, suggesting that activity at another site might be negating the neurotoxic effect of EtOH's inherent NMDA antagonistic activity. Because GABA(A) agonism and non-NMDA glutamate antagonism, properties which EtOH possesses, can prevent NAN, we proceeded to study whether GABA(A) antagonists (or agents capable of reversing EtOH's GABAergic effects) and non-NMDA agonists could reverse EtOH's protective effect. Bicuculline, Ro15-4513, finasteride, kainic acid or AMPA, alone or in combination, did not significantly reverse EtOH's protective effect. Given that EtOH has effects on a wide range of ion channels and receptors, determining the precise mechanism of EtOH's protective effect will take additional effort. The inability of EtOH to acutely produce NAN in the adult CNS indicates that, in contrast to fetuses, brief exposure of the adult CNS to EtOH is non-toxic for neurons. [Abstract]
NB, Jiang XP, Heinkel C, Nemmers B.
Antiepileptic drugs and agents
that inhibit voltage-gated sodium channels prevent NMDA antagonist neurotoxicity.
"N-methyl-D-aspartate (NMDA) glutamate receptor
antagonists are used in clinical anesthesia and are being developed as therapeutic
agents for preventing neurodegeneration in stroke, epilepsy, and brain trauma.
However, the ability of these agents to produce neurotoxicity in adult rats and
psychosis in adult humans compromises their clinical usefulness. In addition,
an NMDA receptor hypofunction (NRHypo) state might play a role in neurodegenerative
and psychotic disorders, like Alzheimer's disease, bipolar disorder and schizophrenia.
Thus, developing pharmacological means of preventing these NRHypo-induced effects
could have significant clinically relevant benefits. NRHypo neurotoxicity appears
to be mediated by a complex disinhibition mechanism that results in the excessive
stimulation of certain vulnerable neurons. Here we report our findings that five
agents (phenytoin, carbamazepine, valproic acid, lamotrigine, and riluzole), thought
to possess anticonvulsant activity because they inhibit voltage-gated sodium channels,
prevent NRHypo neurotoxicity. The ability of tetrodotoxin, a highly selective
inhibitor of voltage-gated sodium channels, to prevent the same neurotoxicity
suggests that inhibition of this ion channel is the likely mechanism of action
of these five agents. We also found that three other anticonvulsants (felbamate,
gabapentin and ethosuximide), whose mechanism is less clear, also prevent NRHypo
neurotoxicity, suggesting that inhibition of voltage-gated sodium channels is
not the only mechanism via which anticonvulsants can act to prevent NRHypo neurotoxicity.
Several of these agents have been found to be of clinical use in bipolar disorder.
It would be of interest to determine whether these agents might have therapeutic
benefits for conditions in which a NRHypo state may exist." [Abstract]
SI, Hashimoto K, Narita N, Minabe Y, Tamura A.
effects of scopolamine on NMDA antagonist-induced heat shock protein HSP70.
Res. 1997 Jul 25;763(2):255-8.
"Using immunohistochemical technique, we
investigated the regionally different roles of muscarinic receptors in the induction
of HSP-70 by NMDA receptor antagonists. The administration of memantine and phencyclidine
induced HSP-70 in the retrosplenial cortex of rat brain. Pretreatment with the
muscarinic receptor antagonist scopolamine (0.1-1 mg/kg) blocked induction of
HSP-70 in layer III of the retrosplenial cortex. However, induction of HSP-70
in layer V was augmented by scopolamine. These results suggest a regional difference
in the mechanism of neurotoxicity induced by NMDA receptor antagonists."
V, Kirby CO, Olney JW.
Isoflurane and propofol block neurotoxicity
caused by MK-801 in the rat posterior cingulate/retrosplenial cortex.
Cereb Blood Flow Metab. 1997 Feb;17(2):168-74.
"In acute brain injury
syndromes, the potent N-methyl-D-aspartate (NMDA) antagonist, MK-801, can prevent
neuronal degeneration, and the general anesthetics, isoflurane and propofol, may
also provide neuroprotective benefits. An obstacle to the use of NMDA antagonists
for neuroprotective purposes is that they can cause a neurotoxic vacuole reaction
in cerebrocortical neurons. This study demonstrates the ability of isoflurane
and propofol to prevent the neurotoxic vacuole reaction induced by MK-801. Low
sedative doses of inhaled isoflurane (1%) or intravenous (i.v.) propofol (7.5
mg/kg/h) were as effective as higher general anesthetic doses. Thus, in the clinical
management of acute brain injury conditions such as stroke and brain trauma, administration
of one of these anesthetic agents together with an NMDA antagonist may be an excellent
formula for obtaining optimal neuroprotection while eliminating serious side effects."
M, Fukamauchi F, Olney JW.
Halothane prevents MK-801 neurotoxicity
in the rat cingulate cortex.
Neurosci Lett. 1995 Jun 23;193(1):1-4.
administration of the N-methyl-D-aspartic acid (NMDA) antagonist, MK-801, to adult
rats causes a toxic vacuole reaction in neurons of the posterior cingulate cortex
which is readily detected in histological sections 4 h following MK-801 administration.
Certain drugs that facilitate neurotransmission at gamma-aminobutyric acidA (GABAA)
receptors block this neurotoxic action of MK-801. The anesthetic actions of halothane
(fluothane) are thought to be due, at least in part, to an interaction with GABAA
receptors. In the present study, we investigated the effect of halothane on MK-801
neurotoxicity. When halothane was administered for either 1 or 2 h, then terminated
immediately prior to MK-801 treatment, the vacuole reaction detected 4 h later
was almost as severe as in controls not exposed to halothane. Administration of
halothane for 1 h after MK-801 injection postponed but did not prevent a relatively
full vacuole reaction. However, when rats were kept under halothane anesthesia
continuously throughout the 4 h period following MK-801 administration, the vacuole
reaction was completely prevented. We postulate that halothane blocks MK-801 neurotoxicity
by a facilitative action at GABAA receptors. Because halothane's duration of action
is fleeting compared to the very long duration of action of MK-801, the efficacy
of halothane in blocking MK-801 neurotoxicity varies in direct proportion to the
length of time following MK-801 treatment that the rat brain is exposed to halothane."
NB, Hanslick J, Kirby C, McWilliams L, Olney JW.
that activate 5HT2A receptors prevent NMDA antagonist neurotoxicity.
1998 Jan;18(1):57-62. [Abstract]