Therapeutic potential of excitatory amino acid antagonists: channel blockers and 2,3-benzodiazepines

Budipine is a low affinity, N-methyl-D-aspartate receptor antagonist: patch clamp studies in cultured striatal, hippocampal, cortical and superior colliculus neurones

The NMDA receptor antagonistic effects of budipine were assessed using concentration- and patch-clamp techniques on cultured striatal, hippocampal, cortical and superior colliculus neurones. Inward current responses of striatal neurones to NMDA (200 microM) at -70 mV were antagonized by budipine in a concentration-dependent manner (50% inhibitory concentration (IC50) 59.4 +/- 10.7 microM, n = 17) with 24 times lower potency than memantine but similar potency to amantadine. In striatal neurones, budipine blocked outward currents at +70 mV with an IC50 of 827 microM, suggesting that the binding site is less deep in the channel (delta = 0.45) than for memantine. However, more detailed analysis of the fractional block by budipine 300 microM in hippocampal neurones gave a delta-value of 0.90, but revealed that 28% block is mediated at a voltage-independent site. This voltage-insensitive site was accessible in the absence of agonist. Budipine exhibited concentration-dependent open channel blocking kinetics (kappa(on) = 0.71 x 10(4) M(-1) s(-1)) whereas the fast offset rate was concentration-independent (kappa(off) = 0.63 s(-1)). Calculation of the ratio kappa(off)/kappa(on) revealed an apparent Kd value of 88.7 microM. Budipine, memantine and amantadine had similar effects against NMDA-induced currents in cultured hippocampal, cortical and superior colliculus neurones, although amantadine was somewhat more potent in cultured striatal neurones. The relevance of NMDA receptor antagonism to the anti-Parkinsonian effects of budipine remains to be established.

Adding ketamine in a multimodal patient-controlled epidural regimen reduces postoperative pain and analgesic consumption

We designed this double-blind study to evaluate the effect of adding small-dose ketamine in a multimodal regimen of postoperative patient-controlled epidural analgesia (PCEA). Ninety-one patients, ASA physical status I-III, undergoing major surgery, received a standardized general anesthesia and epidural catheterization in an appropriate intervertebral space after surgery. A PCEA device was programmed to deliver a regimen of morphine 0.02 mg/mL, bupivacaine 0.8 mg/mL, and epinephrine 4 microg/mL, with the addition of ketamine 0.4 mg/mL (ketamine, n = 45) or without (control, n = 46). The mean visual analog pain scale (VAS) scores during cough or movement for the first 3 days after surgery were higher in the control group than in the ketamine group (P < 0.05), whereas the mean VAS score at rest for the first 2 days were higher in the control group than in the ketamine group (P < 0.05). Furthermore, patients in the control group consumed more multimodal analgesics than patients in the ketamine group for the first 2 days (P < 0.05). The sedation scores and the incidence of side effects (pruritus, nausea, emesis, sleep deprivation, motor block, and respiration depression) were similar between the two groups. We conclude that adding ketamine 0.4 mg/mL in a multimodal PCEA regimen provides better postoperative pain relief and decreases consumption of analgesics.

IMPLICATIONS

Many studies have evaluated one or a combination of two analgesics for postoperative pain control, but few have examined a multimodal approach using three or four different epidural analgesics. This study demonstrates an additive analgesic effect when ketamine is added to a multimodal analgesic treatment.

7,8-Methylenedioxy-4H-2,3-benzodiazepin-4-ones as novel AMPA receptor antagonists

The synthesis and anticonvulsant activity of novel 7,8-methylenedioxy-4H-2,3-benzodiazepin-4-ones 3a-e, structurally-related to GYKI 52466 1, a well-known noncompetitive AMPA-receptor antagonist, are reported. The new compounds possess marked anticonvulsant properties and, in analogy to 1, antagonize seizures induced by AMPA. In addition, when compared to the model compound 1, compounds 3 show a longer-lasting anticonvulsant activity and a lower toxicity.

Prospects of glutamate antagonists in the therapy of Parkinson's disease

It has been suggested that the excitatory amino acid glutamate, acting as both a neurotoxin and a neurotransmitter, might play a central role in the pathophysiology of Parkinson's disease (PD). Intrinsic energetic defects of the neurons of the substantia nigra pars compacta, the brain area where the degenerative process of PD takes place, may render nigral neurons highly vulnerable to the effects of glutamate, which acts as a neurotoxin in the presence of impaired cellular energy metabolism. Degeneration of dopamine nigral neurons and striatal dopaminergic denervation cause a cascade of functional modifications in the activity of basal ganglia nuclei. Due to the close relationship that links dopaminergic and glutamatergic neurotransmission, glutamate is directly involved in the functional alterations of basal ganglia circuitry that lead to the development of parkinsonian motor symptoms. Drugs counteracting the effects of glutamate might therefore provide new protective and symptomatic strategies for therapy of PD.

Pharmacology of glutamate receptor antagonists in the kindling model of epilepsy

It is widely accepted that excitatory amino acid transmitters such as glutamate are involved in the initiation of seizures and their propagation. Most attention has been directed to synapses using NMDA receptors, but more recent evidence indicates potential roles for ionotropic non-NMDA (AMPA/kainate) and metabotropic glutamate receptors as well. Based on the role of glutamate in the development and expression of seizures, antagonism of glutamate receptors has long been thought to provide a rational strategy in the search for new, effective anticonvulsant drugs. Furthermore, because glutamate receptor antagonists, particularly those acting on NMDA receptors, protect effectively in the induction of kindling, it was suggested that they may have utility in epilepsy prophylaxis, for example, after head trauma. However, first clinical trials with competitive and uncompetitive NMDA receptor antagonists in patients with partial (focal) seizures, showed that these drugs lack convincing anticonvulsant activity but induce severe neurotoxic adverse effects in doses which were well tolerated in healthy volunteers. Interestingly, the only animal model which predicted the unfavorable clinical activity of competitive NMDA antagonists in patients with chronic epilepsy was the kindling model of temporal lobe epilepsy, indicating that this model should be used in the search for more effective and less toxic glutamate receptor antagonists. In this review, results from a large series of experiments on different categories of glutamate receptor antagonists in fully kindled rats are summarized and discussed. NMDA antagonists, irrespective whether they are competitive, high- or low-affinity uncompetitive, glycine site or polyamine site antagonists, do not counteract focal seizure activity and only weakly, if at all, attenuate propagation to secondarily generalized seizures in this model, indicating that once kindling is established, NMDA receptors are not critical for the expression of fully kindled seizures. In contrast, ionotropic non-NMDA receptor antagonists exert potent anticonvulsant effects on both initiation and propagation of kindled seizures. This effect can be markedly potentiated by combination with low doses of NMDA antagonists, suggesting that an optimal treatment of focal and secondarily generalized seizures may require combined use of both non-NMDA and NMDA antagonists. Given the promising results obtained with novel AMPA/kainate antagonists and glycine/NMDA partial agonists in the kindling model, the hope for soon having potentially useful glutamate antagonists for use in epileptic patients is increasing.

Antagonist properties of a phosphono isoxazole amino acid at glutamate R1-4 (R,S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid receptor subtypes

The activity of the (R, S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor antagonist, (R,S) -2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl] propionic acid (ATPO), at recombinant ionotropic glutamate receptors (GluRs) was evaluated using electrophysiological techniques. Responses at homo- or heterooligomeric AMPA-preferring GluRs expressed in human embryonic kidney (HEK) 293 cells (GluR1-flip) or Xenopus laevis oocytes (GluR1-4-flop or GluR1-flop + GluR2) were potently inhibited by ATPO with apparent dissociation constants (Kb values) ranging from 3.9 to 26 microM. A Schild analysis for kainate (KA)-activated GluR1 receptors showed ATPO to have a KB of 8.2 microM and a slope of unity, indicating competitive inhibition. The antagonism by ATPO at GluR1 was of similar magnitude at holding potentials between -100 mV and +20 mV. In contrast, ATPO (<300 microM), does not inhibit responses to kainate at homomeric GluR6 or heterooligomeric GluR6/KA2 expressed in HEK 293 cells but activated GluR5 and GluR5/KA2 expressed in X. laevis oocytes. ATPO produced <15% inhibition at the maximal concentration (300 microM) of current responses through NR1A + NR2B receptors expressed in X. laevis oocytes. Thus, ATPO shows a unique pharmacological profile, being an antagonist at GluR1-4 and a weak partial agonist at GluR5 and GluR5/KA2.

Sexually dimorphic MK801-induced c-fos in the rat hypothalamic paraventricular nucleus

MK801 induces Fos-like immunoreactivity (FLI) in the paraventricular nucleus (PVN) in a sex, age, hormone and dosage dependent manner. MK801 (1.0 mg/kg) elicited greater behavioural disruption, but less FLI in the PVN of cycling and lactating female rats, compared to like-treated males. Results from gonadectomized rats with or without acute steroid replacement resembled those seen in intact rats but sex differences were extinguished. At a lower dose (0.1 mg/kg), behavioural effects were diminished but females exhibited more behavioural disruption. At this dose however, more FLI was seen in the PVN of females compared to males, but only male-lactating female differences were significant. Taken together, the data suggest that the action of MK801 on the PVN is influenced by gonadal steroids.

Dextromethorphan and other N-methyl-D-aspartate receptor antagonists are teratogenic in the avian embryo model

N-Methyl-D-aspartate (NMDA) receptors are a calcium-conducting class of excitatory amino acid receptors that are involved in neuronal development and migration. Certain well known teratogens (e.g. homocysteine, ethanol, and chloroform) that induce congenital neural tube and neural crest defects also have the capacity to act as NMDA receptor antagonists. We hypothesized that teratogenicity was a general property of NMDA receptor antagonists, and that high affinity NMDA receptor antagonists would induce neural tube and neural crest defects. Chicken embryos were given 5, 50, or 500 nmol/d of selected NMDA receptor antagonists for 3 consecutive days during the process of neural tube closure, beginning 4 h after the beginning of incubation. Selected NMDA receptor antagonists represented three classes of antagonists: ion channel blockers, glycine site antagonists, and glutamate site agonists and antagonists. All classes of NMDA receptor antagonists induced embryonic death and congenital defects of the neural crest and neural tube; however, the channel blockers were the most potent teratogens. Dextromethorphan at 500 nmol/embryo/d killed more than half the embryos and induced congenital defects in about one-eighth of the survivors; dextromethorphan was also highly lethal at 50 nmol/embryo/d. Glutamate site NMDA receptor agonists (NMDA and homoquinolinic acid) displayed weak toxicity relative to their known NMDA receptor potency. Taken together, these data indicate that NMDA receptor antagonists, particularly channel blockers, are potent teratogens in the chicken embryo model. Because dextromethorphan is a widely used nonprescription antitussive, its strong teratogeneticity using this model is particularly noteworthy.

5-(N-oxyaza)-7-substituted-1,4-dihydroquinoxaline-2,3-diones: novel, systemically active and broad spectrum antagonists for NMDA/glycine, AMPA, and kainate receptors

A group of 5-aza-7-substituted-1,4-dihydroquinoxaline-2,3-diones (QXs) and the corresponding 5-(N-oxyaza)-7-substituted QXs were prepared and evaluated as antagonists of ionotropic glutamate receptors. The in vitro potency of these QXs was determined by inhibition of [3H]-5,7-dichlorokynurenic acid ([3H]DCKA) binding to N-methyl-D-aspartate (NMDA)/glycine receptors, [3H]-(S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) binding to AMPA receptors, and [3H]kainate ([3H]KA) binding to KA receptors in rat brain membranes. 5-(N-Oxyaza)-QXs 12a-e all have low micromolar or submicromolar potency for NMDA/glycine receptors and low micromolar potencies for AMPA and KA receptors. QXs 12a-e display 2-12-fold selectivity for NMDA/glycine receptors compared to AMPA receptors, and approximately 2-fold difference between AMPA and KA potency. In contrast to other QXs that either show high selectivity for NMDA (such as ACEA 1021) or AMPA (such as NBQX) receptors, these molecules are broad spectrum antagonists of ionotropic glutamate receptors. 7-Nitro-5-(N-oxyaza)-QX (12e) is the most potent inhibitor among 12a-e, having IC50 values of 0.69, 1.3, and 2.4 microM at NMDA, AMPA, and KA receptors, respectively. In functional assays on glutamate receptors expressed in oocytes by rat cerebral cortex poly(A+) RNA, 7-chloro-5-(N-oxyaza)-QX (12a) and 7-nitro-5-(N-oxyaza)-QX (12e) have Kb values of 0.63 and 0.31 microM for NMDA/glycine receptors, and are 6- and 4-fold selective for NMDA over AMPA receptors, respectively. 5-(N-Oxyaza)-7-substituted-QXs 12a-e all have surprisingly high in vivo potency as anticonvulsants in a mouse maximal electroshock-induced seizure (MES) model. 7-Chloro-5-(N-oxyaza)-QX (12a), 7-bromo-5-(N-oxyaza)-QX (12b), and 7-methyl-5-(N-oxyaza)-QX (12c) have ED50 values of 0.82, 0.87, and 0.97 mg/kg i.v., respectively. The high in vivo potency of QXs 12a-e is particularly surprising given their low log P values (approximately -2.7). Separate studies indicate that QXs 12a and 12e are also active in vivo as neuroprotectants and also have antinociceptive activity in animal pain models. In terms of in vivo activity, these 5-(N-oxyaza)-7-substituted-QXs are among the most potent broad spectrum ionotropic glutamate antagonists reported.

Synthesis and structure-activity relationships of 6,7-benzomorphan derivatives as antagonists of the NMDA receptor-channel complex

We have synthesized a series of stereoisomeric 6,7-benzomorphan derivatives with modified N-substituents and determined their ability to antagonize the N-methyl-D-aspartate (NMDA) receptor-channel complex in vitro and in vivo. The ability of the compounds to displace [3H]-MK-801 from the channel site of the NMDA receptor in rat brain synaptosomal membranes and to inhibit NMDA-induced lethality in mice was compared with their ability to bind to the mu opioid receptor. Examination of structure-activity relationships showed that the absolute stereochemistry is critically important for differentiating these two effects. (-)-1R,9 beta,2"S-enantiomers exhibited a higher affinity for the NMDA receptor-channel complex than for the mu opioid receptor. The aromatic hydroxy function was also found to influence the specificity of the compounds. Shift of the hydroxy group from the 2'-position to the 3'-position significantly increased the affinity for the NMDA receptor-channel complex and considerably reduced the affinity for the mu opioid receptor. From this series of 6,7-benzomorphan derivatives, the compound 15cr.HCl [(2R)-[2 alpha, 3(R*),6 alpha]-1,2,3,4,5,6-hexahydro-3-(2-methoxypropyl)-6,11,11-trimethyl -2,6-methano-3-benzazocin-9-ol hydrochloride] was chosen as the optimum candidate for further pharmacological investigations.

Non-NMDA receptor antagonist GYKI 52466 suppresses cortical afterdischarges in immature rats

GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzo-diaz epi ne), a non-competitive non-NMDA receptor antagonist, was tested against epileptic afterdischarges elicited by cortical stimulation in 12-, 18- and 25-day-old rats with implanted electrodes. Shortening of afterdischarges and a decrease in intensity of clonic movements accompanying both stimulation and afterdischarges were induced by the 20 mg/kg dose of GYKI 52466 in 18- and 25-day-old animals, whereas 12-day-old rat pups exhibited only shortening of electroencephalographic afterdischarges. The 10 mg/kg dose of GYKI 52466 did not significantly change afterdischarges in any age group. Motor skills were compromised after the 20 mg/kg dose of GYKI 52466. This effect was again more marked in 18- and 25-day-old animals than in the youngest group. In addition, anxiolytic-like action was observed in the jumping down test in 25-day-old rats. This effect was not influenced by a benzodiazepine antagonist flumazenil. On the contrary, the anticonvulsant action of GYKI 52466 was partly blocked by flumazenil, indicating thus multiple mechanisms of action of GYKI 52466.

Remacemide hydrochloride reduces cortical lesion volume following brain trauma in the rat

We evaluated the therapeutic effects of remacemide hydrochloride, an N-methyl-D-aspartate (NMDA) receptor-associated ionophore blocker with sodium channel blocking activity, on cortical lesion volume and memory dysfunction following parasagittal fluid-percussion brain injury in the anesthetized rat. We found that intravenous (i.v.) administration 15 min following injury of remacemide hydrochloride at both 25 and 10 mg/kg significantly reduced posttraumatic cortical lesion volume (P < 0.05), measured at 48 h postinjury using a tetrazolium salt tissue staining technique. However, neither of these doses nor the dosing regimen of 25 mg/kg i.v. 15 min postinjury plus a subcutaneous infusion over 24 h of 20 mg/kg remacemide hydrochloride improved posttraumatic memory function determined by a Morris water maze paradigm.

Regionally different effects of scopolamine on NMDA antagonist-induced heat shock protein HSP70

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.

Electrophysiological effects of the anticonvulsant remacemide hydrochloride and its metabolite ARL 12495AA on rat CA1 hippocampal neurons in vitro

The electrophysiological actions of the putative anticonvulsants remacemide hydrochloride and its des-glycine metabolite ARL 12495AA were examined using whole-cell recordings from CA1 hippocampal neurons in adult rat brain in vitro. Remacemide hydrochloride (4-400 microM) and ARL 12495AA (4-400 microM) limited sustained high frequency repetitive firing (SRF) induced by application of long duration depolarizing current pulses (20-400 pA, 500 msec). This SRF limitation was concentration-dependent, and equipotent IC50 values of 66 and 60 microM were calculated for remacemide hydrochloride and ARL 12495AA, respectively. Examination of the spike configuration revealed that, over the same concentration range, each compound caused a concentration-related reduction of: (a) the action potential amplitude; and (b) the rate-of-rise. Remacemide hydrochloride or ARL 12495AA increased spike duration and decreased or eliminated the spike after-hyperpolarization. Possible mechanisms for these electrophysiological actions including modulation of sodium and/or potassium channel activity are considered. It is suggested that such multiple mechanisms, including inhibition of SRF may be relevant to the anticonvulsant properties of remacemide hydrochloride and its metabolite, ARL 12495AA. The activity of both compounds as modulators of neuronal excitability indicates that metabolic conversion of remacemide hydrochloride to ARL 12495AA could enhance the therapeutic efficacy of the former.

Dextromethorphan ameliorates effects of neonatal hypoxia on brain morphology and seizure threshold in rats

Hypoxic injury to the brain is mediated in part by NMDA receptors. Therefore, NMDA receptor blockade with dextromethorphan (DM), a non-competitive channel blocker, was hypothesized to ameliorate injury even when given after the hypoxic insult. Rats were exposed to 8% oxygen for 3 h on postnatal day 7. Within 20 min of exposure, animals received 30 mg/kg i.p. DM or normal saline. Littermates maintained in room air for 3 h also received DM or saline. At 14 days of age, 7 days after exposure, cortical thickness and hippocampal area were measured. At 70-90 days of age, approximately two months after exposure, in a separate group of rats, seizure threshold using pentylenetetrazol (PTZ) and passive avoidance learning and retention were determined. There were no gross changes in cellular morphology and no evidence for cellular necrosis in any of the exposure groups. However, cortical thickness was decreased in animals exposed to hypoxia. DM administration prevented this decrease. Hippocampal area was unaffected. Seizure susceptibility in adulthood was increased in animals exposed to hypoxia in the neonatal period. DM prevented the decrease in seizure threshold. There was no difference in passive avoidance learning or retention as a function of neonatal exposure condition. Mild to moderate hypoxia, previously thought not to produce any histologic changes, causes significant short-term loss of cortical thickness and long-term decrease in seizure threshold. DM appears to ameliorate these effects even when given after the hypoxic insult. These results implicate the glutamate receptor system in the pathophysiology of hypoxia damage and suggest that treatment with a glutamate receptor blocker when neonatal asphyxia is suspected would help ameliorate the consequences of such an insult.

1-Aryl-3,5-dihydro-4H-2,3-benzodiazepin-4-ones: novel AMPA receptor antagonists

Our previous publication (Eur. J. Pharmacol. 1995, 294, 411-422) reported preliminary chemical and biological studies of some 2,3-benzodiazepines, analogues of 1-(4-aminophenyl)-4-methyl-7,8-(methylenedioxy)-5H-2,3-benzodiazepine (1, GYKI 52466), which have been shown to possess significant anticonvulsant activity. This paper describes the synthesis of new 1-aryl-3,5-dihydro-4H-2,3-benzodiazepin-4-ones and the evaluation of their anticonvulsant effects. The observed findings extend the structure-activity relationships previously suggested for this class of anticonvulsants. The seizures were evoked both by means of auditory stimulation in DBA/2 mice and by pentylenetetrazole or maximal electroshock in Swiss mice. 1-(4'-Aminophenyl)- (38) and 1-(3'-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4H-2,3-benzodiazepin- 4-one (39), the most active compounds of the series, proved to be more potent than 1 in all tests employed. In particular, the ED50 values against tonus evoked by auditory stimulation were 12.6 micromol/kg for derivative 38, 18.3 micromol/kg for 39, and 25.3 micromol/kg for 1. Higher doses were necessary to block tonic extension induced both by maximal electroshock and by pentylenetetrazole. In addition these compounds exhibited anticonvulsant properties that were longer lasting than those of compound 1 and were less toxic. The novel 2,3-benzodiazepines were also investigated for a possible correlation between their anticonvulsant activities against convulsions induced by 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) and their affinities for benzodiazepine receptors (BZR). The 2,3-benzodiazepines did not affect the binding of [3H]flumazenil to BZR, and conversely, their anticonvulsant effects were not reversed by flumazenil. On the other hand the 2,3-benzodiazepines antagonized seizures induced by AMPA and aniracetam in agreement with an involvement of the AMPA receptor. In addition, both the derivative 38 and the compound 1 markedly reduced the AMPA receptor-mediated membrane currents in guinea-pig olfactory cortical neurons in vitro in a noncompetitive manner. The derivatives 25 and 38-40 failed to displace specific ligands from N-methyl-D-aspartate (NMDA), AMPA/kainate, or metabotropic glutamate receptors.

Epidural morphine plus ketamine for upper abdominal surgery: improved analgesia from preincisional versus postincisional administration

Increased postoperative pain may be caused by central nervous system plasticity, which may be related to actions of N-methyl-D-aspartic acid (NMDA) receptors on neurons in the dorsal horn of the spinal cord. Opioids act mainly on presynaptic receptors and reduce neurotransmitter release, while ketamine antagonizes NMDA receptors and prevents wind-up and long-term potentiation. Thus, we postulated that central nervous system sensitization would be prevented more effectively by the preoperative use of these two drugs simultaneously, and the effect of preemptive analgesia would be demonstrated. Ketamine, 60 mg, and morphine, 2 mg, were injected epidurally through an indwelling catheter that was inserted at the T7-8 interspace in 60 ASA physical status class 1-2 patients. The drugs were injected before induction of anesthesia (Group 1; n = 30) or immediately after removal of a surgical specimen (Group 2; n = 30). An additional 2 mg of morphine was injected when the patients complained of resting pain. The analgesic effect was assessed by the time from first analgesic injection to second dose and the number of patients who needed supplemental injections. Complications were also noted. The duration of analgesia was longer (P < 0.01) in Group 1 (31.1 +/- 16.0 h) than in Group 2 (21.1 +/- 12.0 h), and the proportion of patients who needed supplemental injections was decreased (P < 0.05) in Group 1 (56.7%) compared with Group 2 (90.0%). The incidence of adverse effects was not different between the two groups. In conclusion, preoperative administration of morphine and ketamine is more effective in reducing postoperative pain than it is when given during the operation.

Mechanism of memantine block of NMDA-activated channels in rat retinal ganglion cells: uncompetitive antagonism

1. N-methyl-D-aspartic acid (NMDA)-activated currents were recorded from dissociated rat retinal ganglion cells using whole-cell recording. The NMDA open-channel blocking drug memantine was evaluated for non-competitive and/or uncompetitive components of antagonism. A rapid superfusion system was used to apply various drugs for kinetic analysis. 2. Dose-response data revealed that memantine blocked 200 microM NMDA-evoked responses with a 50% inhibition constant (IC50) of approximately 1 microM at -60 mV and an empirical Hill coefficient of approximately 1. The antagonism followed a bimolecular reaction process. This 1:1 stoichiometry is supported by the fact that the macroscopic blocking rate of memantine (kon) increased linearly with memantine concentration and the macroscopic unblocking rate (koff) was independent of it. The estimated pseudo-first order rate constant for macroscopic blockade was 4 x 10(5) M-1 S-1 and the rate constant for unblocking was 0.44 s-1. Both the blocking and unblocking actions of memantine were well fitted by a single exponential process. 3. The kon for 2 microM memantine decreased with decreasing concentrations of NMDA. By analysing kon behaviour, we estimate that memantine has minimal interaction with the closed-unliganded state of the channel. As channel open probability (Po) approached zero, a small residual action of memantine may be explained by the presence of endogenous glutamate and glycine. 4. Memantine could be trapped within the NMDA-gated channel if it was suddenly closed by fast washout of agonist. The measured gating process of channel activation and deactivation appeared at least 10-20-fold faster than the kinetics of memantine action. By combining the agonist and voltage dependence of antagonism, a trapping scheme was established for further kinetic analysis. 5. With low agonist concentrations, NMDA-gated channels recovered slowly from memantine blockade. By analysing the probability of a channel remaining blocked, we found that memantine binding appeared to stabilize the open conformation of the blocked channel and did not affect ligand affinity. Validity of the "trapping model' and stabilization of the open conformation were further suggested by agreement between the predicted dose-response curve for NMDA in the presence of 2 microM memantine and the empirically derived dose-response relationship. 6. Based on simple molecular schemes, the degree of blockade at various concentrations of agonist for "pure' non-competitive vs. uncompetitive inhibition was computer simulated. The measured degree of blockade by 6 microM memantine was close to ideal for pure uncompetitive antagonism. Taken together, we conclude that the predominant mechanism of open-channel blockade by memantine is uncompetitive. In general, the relative magnitude of the dissociation rate of an open-channel blocker from the open but blocked channel (the apparent off-rate) compared with the rate of leaving the closed and blocker-trapped state (the leak rate) will determine the contribution of uncompetitive vs. non-competitive actions, respectively. 7. Millimolar internal Cs+ competed with memantine for binding in the NMDA-gated channel, and reduced the association rate of memantine, but had no effect on the voltage dependence of the dissociation rate. After removal of Cs+, the calculated Ki for memantine remained voltage dependent. These observations would be difficult to reconcile with models in which memantine binds to a site outside the channel pore and instead strongly support the supposition that the blocking site for memantine is within the permeation pathway.

NMDA receptor-mediated increase in cyclic GMP in the rat cerebellum in vivo is blocked by alaproclate and GEA-857

The effects of alaproclate and GEA-857 (2-(4-chlorophenyl)-1,1-dimethylethyl 2-amino-3-methylbutanoate) on the production of cyclic GMP in the rat cerebellum in vivo induced by stimulation of N-methyl-D-aspartate (NMDA) receptors were studied. Alaproclate per se at a dose of 20 mg/kg subcutaneously, did not influence the basal cGMP level. The increase in cGMP induced by harmaline (20 mg/kg subcutaneously) was dose-dependently antagonized by alaproclate (5-40 mg/kg subcutaneously). S-(-)-Alaproclate was 2-5 times more potent than the R-(+)-enantiomer. GEA-857 which in contrast to alaproclate is a very weak 5-HT uptake inhibitor shared the ability of alaproclate to inhibit the effect of harmaline on cGMP accumulation with similar potency to S-(-)-alaproclate. Alaproclate at 15 mg/kg subcutaneously blocked the increase in cGMP in cerebellum caused by NMDA itself at 200 mg/kg subcutaneously. In contrast to alaproclate, the K+ channel antagonist, 4-aminopyridine, 5 mg/kg subcutaneously, produced per se an increase in cGMP levels in the rat cerebellum by 300% which was antagonized by the NMDA receptor antagonists, dizocilpine, phencyclidine and (+/-)-CCP, the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester and by alaproclate. Alaproclate. Alaproclate and GEA-857 antagonized seizures induced by NMDA, 200 mg/kg subcutaneously at doses similar to those antagonizing the harmaline- and NMDA-induced elevation of cerebellar cGMP. Neither alaproclate nor GEA-857 caused any behavioural effects typical for uncompetitive NMDA receptor antagonists except a slight increase in motor activity and sniffing. The effect of alaproclate on the NMDA receptor-mediated increase in cGMP in rat cerebellum in vivo might be due to blockade of the cation channel of the NMDA receptor complex previously observed in in vitro experiments and these compounds seems to belong to the group of low-affinity uncompetitive NMDA receptor antagonists that might have clinical interest.

Psychotogenicity and N-methyl-D-aspartate receptor antagonism: implications for neuroprotective pharmacotherapy

The development of neuroprotective agents for the prevention of neuronal loss in acute conditions such as stroke and epilepsy or chronic neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, Huntington's chorea, and motor neuron disease is currently focusing on drugs that inhibit excitatory amino acid neurotransmission or exhibit antioxidant properties. Unfortunately, potent antagonists at the N-methyl-D-aspartate (NMDA) type glutamate receptor, which is thought to mediate excitotoxic neuronal injury, e.g., MK-801 or phencyclidine (PCP), share a high probability of inducing psychotomimetic side effects. Further, these drugs have been associated with acute neurotoxicity in vitro and in vivo, precluding their clinical use. In contrast, low affinity NMDA receptor antagonists like amantadine and its dimethyl derivative, memantine, have been administered clinically for the management of Parkinson's disease, dementia, neuroleptic drug-induced side effects, and spasticity. These agents have only rarely induced significant psychotomimetic side effects. Recent pharmacologic advances have helped to elucidate how high drug affinity for the PCP binding site of the NMDA receptor may enhance psychotogenicity. Low affinity and associated fast voltage-dependent channel unblocking kinetics are likely to be responsible for the better tolerance of amantadine and memantine compared with MK-801 and PCP. Further factors apparently modulating psychotogenicity of glutamate receptor antagonists include differential actions on neuronal populations in various brain regions and interactions with neurotransmitter receptors other than the NMDA type glutamate receptor.

YM90K, a selective-amino-3-hydroxy5-methyl-4-isoxazole propionate (AMPA) receptor antagonist, prevents induction of heat shock protein HSP -70 and hsp -70 mRNA in rat retrosplenial cortex by phencyclidine

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist such as an abused drug phencyclidine (PCP) causes the induction of heat shock protein HSP-70, a sensitive marker of neuronal injury, in the retrosplenial cortex of rat brain. The present study was undertaken to examine the role of a -amino-3- hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor in the expression of heat shock protein HSP-70 and hsp-70 mRNA in the retrosplenial cortex by PCP. Administration of PCP (50 mg/kg, i.p.) caused the induction of heat shock protein HSP-70 in the retrosplenial cortex of rat brain, whereas no HSP-70 immunoreactivity was detected in the vehicle-treated group. Pretreatment with a potent and selective AMPA receptor antagonist YM90K (1, 3 or 10 mg/kg, i.p; 15 min) inhibited in a dose dependent manner, the induction of heat shock protein HSP-70 by PCP (50 mg/kg). Furthermore, administration of PCP (50 mg/kg, i.p) caused marked expression of hsp-70 mRNA in the retrosplenial cortex of rat brain, whereas the expression of hsp-70 mRNA was NOT found in the vehicle-treated group. Pretreatment with YM90K (1, 3 or 10 mg/kg, i p; 15 min) also inhibited the expression of hsp-70 mRNA by PCP (50 mg/kg), in a dose-dependent manner. These results suggest that AMPA receptor may play a role in the expression of heat shock protein HSP-70 and heat shock gene hsp-70 mRNA in the retrosplenial cortex of rat brain by non-competitive NMDA receptor antagonists such as PCP.

Trapping channel block of NMDA-activated responses by amantadine and memantine

We investigated the mechanisms by which the antiparkinsonian and neuroprotective agents amantadine and memantine inhibit responses to N-methyl-D-aspartic acid (NMDA). Whole cell recordings were performed using cultured rat cortical neurons or Chinese hamster ovary (CHO) cells expressing NMDA receptors. Both amantadine and memantine blocked NMDA-activated channels by binding to a site at which they could be trapped after channel closure and agonist unbinding. For neuronal receptors, the IC50s of amantadine and memantine at -67 mV were 39 and 1.4 microM, respectively. When memantine and agonists were washed off after steady-state block, one-sixth of the blocked channels released rather than trapped the blocker; memantine exhibited "partial trapping." Thus memantine appears to have a lesser tendency to be trapped than do phencyclidine or (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[1,d]cyclihepten-5,1 0-imine (MK-801). We next investigated mechanisms that might underlie partial trapping. Memantine blocked and could be trapped by recombinant NMDA receptors composed of NR1 and either NR2A or NR2B subunits. In these receptors, as in the native receptors, the drug was released from one-sixth of blocked channels rather than being trapped in all of them. The partial trapping we observed therefore was not due to variability in the action of memantine on a heterogeneous population of NMDA receptors in cultured cortical neurons. Amantadine and memantine each noncompetitively inhibited NMDA-activated responses by binding at a second site with roughly 100-fold lower affinity, but this form of inhibition had little effect on the extent to which memantine was trapped. A simple kinetic model of blocker action was used to demonstrate that partial trapping can result if the presence of memantine in the channel affects the gating transitions or agonist affinity of the NMDA receptor. Partial trapping guarantees that during synaptic communication in the presence of blocker, some channels will release the blocker between synaptic responses. The extent to which amantadine and memantine become trapped after channel block thus may influence their therapeutic effects and their modulation of NMDA-receptor-mediated excitatory postsynaptic potentials.

Study of potency, kinetics of block and toxicity of NMDA receptor antagonists using fura-2

NMDA receptor antagonists reduced NMDA-triggered increases in [Ca2+]i measured by fura-2 and showed qualitative differences in the potency and kinetics of block. High potency antagonists produced a slow block which allowed an initial increase in [Ca2+]i that was greater than the steady-state level, while compounds with moderate to low potency produced a rapid block that was at steady-state from the first measurement. The more potent antagonists showed the slowest unblocking rates. Using this simple method, novel NMDA antagonists can be screened to ascertain potency, kinetics of block and relative toxicity, prior to animal testing.

Memantine induces heat shock protein HSP70 in the posterior cingulate cortex, retrosplenial cortex and dentate gyrus of rat brain

High-affinity N-methyl-D-aspartate (NMDA) receptor antagonists like MK-801 are known to induce the heat shock protein, HSP70, in the posterior cingulate cortex and retrosplenial cortex of rat brain. Memantine, which is a low affinity uncompetitive NMDA receptor antagonist, has been used in the treatment of Parkinson's disease in Europe. The faster kinetics of memantine in blocking and unblocking the NMDA receptor-operated ion channel as opposed to high-affinity NMDA antagonists like MK-801 has been thought to account for the safety of memantine. The present study evaluated the neurotoxic potential of memantine and amantadine using the induction of HSP70 immunoreactivity in rat brain. Memantine (25, 50, 75 mg/kg) induced HSP70 in the posterior cingulate, retrosplenial cortex and dentate gyrus of rat brain. In contrast, amantadine (50, 100, 200 mg/kg) did not induce HSP70 in the rat brain. These results suggest that memantine has an antagonistic effect at NMDA receptor in vivo, and raises the possibility that high doses of memantine may cause neuronal damage similar to those observed with other high-affinity NMDA receptor antagonists.

Inhibition by memantine of the development of persistent oral dyskinesias induced by long-term haloperidol treatment of rats

1. Tardive dyskinesia (TD) is a serious side-effect of long-term treatment with neuroleptics. To investigate if neuroleptic-induced excessive stimulation of striatal glutamate receptors may underlie TD development, the effect of the NMDA antagonist, memantine (1-amino-3,5-dimethyladamantane), was studied in a rat model of TD. 2. In an acute experiment, six groups of rats were treated daily for 1 week with either vehicle or memantine 20 or 40 mg kg-1 day-1, and on the seventh day they received one injection of either haloperidol 1.0 mg kg-1 i.p. or saline i.p. In a subsequent long-term experiment lasting 20 weeks, the same treatment was continued, except that haloperidol was injected i.m. as decanoate (38 mg kg-1 every 4 weeks) and control rats received sesame oil. The behaviour was videotaped and scored at intervals during both experiments, and for 16 weeks after cessation of the long-term treatment. 3. In the acute experiment, haloperidol decreased motor activity and memantine increased moving and tended to attenuate the immobility induced by haloperidol. Memantine also enhanced the haloperidol-induced increase in the putative TD-analogue vacuous chewing movements (VCM). 4. In the long-term experiment, the most marked effect of haloperidol was a gradual increase in VCM and the increase persisted significantly for 12 weeks after cessation of treatment. Memantine dose-dependently increased VCM and moving during long-term treatment. However, only one week after stopping treatment, both these effects of memantine disappeared. In contrast to rats previously treated with haloperidol alone, rats co-treated with memantine (both doses) and haloperidol had VCM at the level of controls two weeks after stopping treatment. The blood levels of drugs were within the therapeutic range achieved in human subjects. 5. These results suggest that long-lasting changes induced by haloperidol are prevented by memantine, which supports the theory that excessive NMDA receptor stimulation may be a mechanism underlying the development of persistent VCM in rats and maybe also TD in human subjects.

Anticonvulsant activity of 5,7DCKA, NBQX, and felbamate against some chemoconvulsants in DBA/2 mice

The anticonvulsant effects of felbamate (10-300 mg/kg, intraperitoneally, IP), and those of two representative antagonists of the excitatory amino acid receptors, 5-7 dichlorokynurenic acid (5-7DCKA; 0.6-30 nmol/mouse, intracerebroventricularly, ICV), and 2, 3-dihydroxy-6 nitro-7-sulfamoylbenzo (F) quinoxoline (NBQX; 1.1-33.6 mg/kg, IP) were studied in the DBA/2 mice. All drugs protected the animals from sound-induced seizures. The drugs were also effective against seizures induced by stimulation of the excitatory amino acid receptor complex using the agonists N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5 methyl-4-isoxazolepropionic acid (AMPA). In separate studies, felbamate protected mice from seizures induced by ICV administration of the activator of dihydropyridine-sensitive calcium channels, methyl-1, 4-dihydro-2, 6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl) pyridine-5-carboxylate (Bay k 8644), with ED50 values of 26 and 46.9 mg/kg for tonus and clonus, respectively. Using Bay k 8644, NBQX (1-40 mg/kg IP) was uneffective, while 5,7DCKA (5-90 nmol/mouse, ICV) protected mice against tonus. Moreover, felbamate prevented seizures induced by blocking voltage-dependent K+ channels using alpha-dendrotoxin, with ED50 values of 22.6 mg/kg for tonus and of 34.8 mg/kg for clonus. Conversely, 5,7DCKA or NBQX did not significantly antagonize seizures induced by alpha-dendrotoxin. The present data indicate that felbamate is an effective anticonvulsant drug in DBA/2 mice with a broader anticonvulsant spectrum than 5,7DCKA and NBQX.

Effects of memantine on recombinant rat NMDA receptors expressed in HEK 293 cells

1. The actions of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, memantine (1-amino-3,5-dimethyladamantane) and (+)-MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate, dizocilpine), on recombinant NMDA receptors has been studied by use of the whole-cell patch clamp technique. 2. Human embryonic kidney (HEK) 293 cells were transiently transfected with different NMDA receptor subunit combinations (NR1a/NR2A, NR1a/NR2B and NR1a/NR2D). A mutant form of the green fluorescent protein (GFP) cotransfected with the NMDA receptor subunits to enable the visualization of transfected cells. 3. Memantine (0.3-30 microM) blocked L-glutamate (100 microM)-mediated currents in a concentration-dependent manner in NR1a/NR2A, NR1a/NR2B and NR1a/NR2D transfected cells with IC50 values (at -70 mV) of 0.93 +/- 0.15 microM, 0.82 +/- 0.12 microM and 0.47 +/- 0.06 microM (mean +/- s.c. mean), respectively. 4. The memantine-induced block was strongly voltage-dependent. Alteration of the holding potential from -70 mV to +60 mV resulted in an e-fold increase in the IC50 values per 30-33 mV change in membrane potential, for all 3 subunit combinations investigated. 5. The kinetics of the actions of memantine (30 microM) were investigated for the NR1a/2A combination, in 6 cells (13-15 determinations). At -70 mV, the block and recovery from block were both best described by two exponentials with time-constants of 201 +/- 23 ms (81 +/- 2%) and 3.9 +/- 0.6 s and 597 +/- 94 ms (18 +/- 1%) and 18.6 +/- 2.4 s, respectively. The predominant effect of depolarization was to increase the weight of the faster recovery time-constant. Kinetic analysis suggests that these results are consistent with previously proposed Markov models. 6. (+)-MK-801 was studied briefly for comparative purposes. (+)-MK-801 (200 nM) preferentially blocked NMDA receptor currents (at -70 mV) in NR1a/NR2A and NR1a/NR2B (82 +/- 10% and 93 +/- 2% depressions) compared to NR1a/NR2D (38 +/- 7%) transfected cells. (+)-MK-801 appeared to be less voltage-dependent than memantine on all three receptor combinations. 7. In conclusion, memantine was a voltage-dependent antagonist of recombinant rat NMDA receptors expressed in HEK 293 cells but showed little selectivity between the subunits investigated. Its actions on these recombinant receptor combinations are similar to its actions on native NMDA receptors.

NMDA antagonist blockade of AT8 tau immunoreactive changes in neuronal cultures

Antagonists at four distinct regulatory sites on the N-methyl-D-aspartate (NMDA) receptor were tested for their ability to attenuate NMDA-mediated chronic excitotoxicity and the consequences on AT8 tau immunoreactivity in neuronal cultures. Excitotoxicity was monitored in cultures by diacetate fluorescein staining. Immunoreactivity of tau phosphorylated at serine 202 was quantified by laser confocal microscopy. The NMDA-receptor antagonists MK801, AP7 and 7-chlorokynurenate significantly blocked NMDA-induced cell death and significantly reduced AT8 tau immunoreactivity. NMDA antagonism by the polyamine site antagonist, ifenprodil, did not completely reverse the increase in AT8 tau immunolabeling induced by NMDA and did not completely protect NMDA-sensitive neurons, suggesting an heterogeneity in the NMDA receptor population.

Felbamate protects CA1 neurons from apoptosis in a gerbil model of global ischemia

BACKGROUND AND PURPOSE

Felbamate, a novel anticonvulsant that binds to the glycine site of the N-methyl-D-aspartate receptor, has been shown to have neuroprotective properties in vitro and in vivo. In a rat pup model of hypoxia-ischemia, felbamate selectively reduced delayed death in hippocampal granule cells. The present study explores its neuroprotective potential in a gerbil model of global ischemia, in which good evidence exists that ischemia triggers apoptosis of CA1.

METHODS

Gerbils were subjected to bilateral carotid occlusion for 5 minutes and then treated with felbamate (100 or 200 mg/kg IV) or vehicle. They were killed 3 days later, and the numbers of live and dead neurons in the CA1 sector of the hippocampus were counted at sterotaxically defined levels.

RESULTS

Felbamate (200 mg/kg IV) administered after the release of carotid clamping did not change brain temperature but reduced neuronal death in CA1 from 332 +/- 60 cells per section of dorsal hippocampus in unmedicated gerbils to 62 +/- 12 cells in felbamate-treated animals (P<.001). A lower dose of felbamate (100 mg/kg post hoc) showed only a nonsignificant reduction of neuronal death. In the 200-mg/kg group, felbamate serum concentrations peaked at 162 microg/mL and were above 100 microg/mL for at least 3 hours, and brain levels reached 150 microg/mL at 1 hour. In the 100-mg/kg group, blood serum levels were well below 100 microg/mL.

CONCLUSIONS

These results suggest that felbamate given post hoc is remarkably effective in preventing delayed apoptosis secondary to global ischemia but that effective neuroprotection requires doses higher than those used for anticonvulsant treatment.

Voltage-dependent interaction of open-channel blocking molecules with gating of NMDA receptors in rat cortical neurons

1. The mechanisms by which four adamantane derivatives (IEM-1857, -1592, -1460 and -1754) block the open NMDA-activated channel were studied at membrane voltages (Vm) from -170 to +30 mV. The rate constants of channel block (k+) and of channel unblock (k-) were measured from the fully resolvable flicker of single-channel currents induced by each compound. 2. The k+ of each compound exhibited a similar exponential dependence on voltage over the Vm range studied. 3. The k- of IEM-1857 and IEM-1592 over the Vm range studied, and of IEM-1754 and IEM-1460 from -30 to -90 mV, exhibited similar exponential dependencies on voltage. However, the k- of IEM-1754 and IEM-1460 at Vm values more hyperpolarized than -90 mV were much more steeply voltage dependent, suggesting that at these Vm values the two drugs can occupy a deeper binding site. 4. Each of the drugs induced a concentration-dependent prolongation of the mean burst length at -90 mV, suggesting that while blocking they can interfere with channel closure. 5. The prolongation of mean burst length induced by the largest drug (IEM-1857) increased with hyperpolarization. The increase was consistent at each Vm with the predictions of the sequential scheme of block, suggesting that channel closure is prevented when IEM-1857 is bound. The prolongation of burst length induced by the smallest drug (IEM-1754) was less than predicted by the sequential scheme and the deviation increased with hyperpolarization. 6. The IEM-1857 concentration-dependence of number of blockages per unit open time had a slope equal to k+ at -150 mV. The IEM-1754 concentration-dependence of number of blockages per unit open time revealed a slope about two times less than k+ for this compound at -150 mV. 7. The mean patch current was not significantly altered by 3 microM IEM-1857 at Vm values from -90 to -150 mV, as expected of a drug that prevents channel closure when blocking. Mean patch current significantly decreased with hyperpolarization beyond -90 mV in the presence of 1 microM IEM-1754. 8. The data suggest that there are two blocking sites at different depths within the NMDA-activated channel. Channel closure is prevented when any of the IEM drugs occupy the shallow blocking site. Channel closure is permitted during occupation of a deeper blocking site that can be reached only by the smaller IEM drugs at hyperpolarized voltages.

Antiparkinsonian effect of flupirtine in monoamine-depleted rats

Excitatory amino acid receptor antagonists lead to marked suppression of parkinsonian-like symptoms in rodent and primate models of Parkinson's disease and are able to potentiate the ability of L-DOPA to reverse akinesia and ameliorate muscular rigidity displayed in these animal models. Flupirtine, which is clinically used as a non-opioid analgesic agent, has some N-methyl-D-aspartate (NMDA) antagonistic properties in several in vivo and in vitro experiments. We now report that in monoamine depleted rats (pretreated with reserpine, 5 mg/kg, and alpha-methyl-para-tyrosine, 250 mg/ kg i.p.) flupirtine dose-dependently (1-20 mg/kg i.p.) suppressed rigidity, measured as tonic EMG activity in the gastrocnemius muscle, but had no effect on akinesia, measured as locomotor activity. In addition, it potentiated the antiparkinsonian effect of L-DOPA on akinesia and rigidity in this rodent model of Parkinson's disease. These effects of flupirtine are of particular clinical relevance, since flupirtine is devoid of the typical side effects of NMDA-receptor antagonists.

EEG seizure activity and behavioral neurotoxicity produced by (+)-MK801, but not the glycine site antagonist L-687,414, in the rat

The objective of the present study was to compare the in vivo effects of the anticonvulsant/neuroprotective glycine-site partial agonists L-687,414 (3R-amino-1-hydroxy-4R-methylpyrrolidin-2-one) and (+)-HA966 (3-Amino-1-hydroxypyrrolidin-2-one) and the non-competitive N-methyl-D-aspartate (NMDA) antagonist (+)-MK801 on spontaneous cortical EEG activity and behavior in the unanesthetized rat. Comprehensive dose-response assessments demonstrated that acute i.v. injections of (+)-MK801 induced a behavioral neurotoxic syndrome comprised of head-weaving, ataxia, hyperlocomotion and myoclonic/clonic behaviors and associated with disruptions in normal EEG rhythms including paroxysmal EEG spike/wave complexes. Injections of (+)-HA966 produced behavioral sedation associated with high-amplitude, slow-wave synchronized EEG patterns; signs of ictal EEG activity were minimal (33% incidence) and only seen at the highest dose tested (100 mg/kg). Both (+)-MK801 and (+)-HA966 severely delayed the latency to slow-wave sleep (SWS). In contrast, the EEG dynamics and overt behavior associated with L687,414 were essentially indistinguishable from controls. There was no disruption in the latency to SWS and mild ataxia was evident only upon awakening. The calculated protective indices (EEG seizure ED50/anticonvulsant ED50) for (+)-MK801 and L-687,414 were 1.2 and > 4.5, respectively. The results of this study confirm that valuable pharmacological actions mediated via glycine site modulation of the NMDA receptor are possible without the clinical manifestation of unwanted neurotoxic side-effects.

Influence of aminophylline and strychnine on the protective activity of excitatory amino acid antagonists against maximal electroshock-induced convulsions in mice

Aminophylline reversed the protective action of both, D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid (D-CPP-ene-a competitive NMDA antagonist) and valproate (used as a conventional antiepileptic drug for comparative purposes) against maximal electroshock-induced seizures. The respective ED50 values of aminophylline were 55.7 and 98.4 mg/kg i.p. However, aminophylline (up to 100 mg/kg i.p.) did not influence the protective efficacy of 1-(4-aminophenyl)-4-methyl-7,8-methyl- enedioxy-5H-2,3-benzodiazepine (GYKI 52466-a non-NMDA antagonist). Strychnine affected the protection provided by D-CPP-ene, GYKI 52466, and valproate against maximal electroshock-the ED50 values of strychnine for the reversal of the anticonvulsive effects of D-CPP-ene, GYKI 52466 or valproate were 0.082, 0.35 and 0.28 mg/kg s.c., respectively. An involvement of strychnine sensitive glycinergic receptor-mediated events in the mechanism of the anticonvulsive activity of excitatory amino acid antagonists and valproate may be postulated. The ineffectiveness of aminophylline to reduce the anticonvulsive effects of GYKI 52466 may distinguish a new class of antiepileptic drugs offering an advantage over conventional antiepileptics in patients with epilepsy, requiring aminophylline for pulmonary reasons.

Interactions of 2,3-benzodiazepines and cyclothiazide at AMPA receptors: patch clamp recordings in cultured neurones and area CA1 in hippocampal slices

1. The 2,3-benzodiazepines GYKI 52466, GYKI 53405 and GYKI 53655 antagonized AMPA-induced currents in cultured superior colliculus neurones in a non use-dependent manner (steady state IC50s: GYKI 52466 9.8 +/- 0.6 microM; GYKI 53405 3.1 +/- 0.6 microM; GYKI 53655 0.8 +/- 0.1 microM). 2. Higher concentrations of all three antagonists slowed the onset kinetics and quickened the offset kinetics of AMPA-induced currents indicative of an allosteric interaction with the AMPA recognition site. 3. Cyclothiazide (3-300 microM) dramatically slowed desensitization of AMPA-induced currents and potentiated steady state currents (EC50 10.0 +/- 2.5 microM) to a much greater degree than peak currents. Both tau on and tau off were also increased by cyclothiazide in a concentration-dependent manner (EC50: tau on 42.1 +/- 4.5 microM; tau off 31.6 +/- 6.6 microM). 4. Cyclothiazide (10-100 microM) shifted the concentration-response curves of the 2,3-benzodiazepines to the right. For example, with 10 microM cyclothiazide the IC50s of GYKI 52466 and GYKI 53405 on steady-state AMPA-induced currents were 57.9 +/- 9.5 and 41.6 +/- 1.5 microM, respectively. 5. GYKI 53405 and GYKI 52466 concentration-dependently reversed the effects of cyclothiazide (100 microM) on offset kinetics (GYKI 53405 IC50 16.6 +/- 4.2 microM). However, the 2,3-benzodiazepines were unable to reintroduce desensitization in the presence of cyclothiazide and even concentration-dependently slowed the onset kinetics of AMPA responses further (GYKI 53405 EC50 8.0 +/- 2.8 microM). 6. GYKI 52466 decreased the peak amplitude of hippocampal area CA1 AMPA receptor-mediated excitatory postsynaptic currents (e.p.s.cs) (IC50 10.8 +/- 0.8 microM) with no apparent effect on response kinetics. Cyclothiazide prolonged the decay time constant of AMPA receptor-mediated e.p.s.cs (EC50 35.7 +/- 6.5 microM) with less pronounced effects in slowing e.p.s.c. onset kinetics and increasing e.p.s.c. amplitude. 7. Cyclothiazide (330 microM) shifted the concentration-response curve for the effects of GYKI 52466 on AMPA receptor-mediated e.p.s.c. peak amplitude to the right (GYKI 52466 IC50 26.9 +/- 9.4 microM). Likewise, GYKI 52466 (30-100 microM)) shifted the concentration-response curve for the effects of cyclothiazide on AMPA receptor-mediated e.p.s.c. decay time constants to the right. 8. In conclusion, cyclothiazide and the 2,3-benzodiazepines seem to bind to different sites on AMPA receptors but exert strong allosteric interactions with one another and with other domains such as the agonist recognition site. The interactions of GYKI 52466 and cyclothiazide on AMPA receptor-mediated e.p.s.cs in area CA1 of hippocampal slices provide evidence that the decay time constant of these synaptic events are not governed by desensitization.

Comparative patch-clamp studies with freshly dissociated rat hippocampal and striatal neurons on the NMDA receptor antagonistic effects of amantadine and memantine

Patch- and concentration-clamp techniques were used to compare the effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists (+)-MK-801 (dizocilpine, (+)-5-methyl-10, 11-dihydro-5H-dibenzocyclohepten-5, 10-imine maleate), ketamine, memantine (1-amino-3,5-dimethyladamantane) and amantadine (1-amino-adamantane) on agonist-induced inward currents in freshly dissociated rat hippocampal and striatal neurons. In hippocampal neurons, ketamine (5 microM), menantine (10 microM) and amantadine (100 microM) selectively antagonized inward current responses to NMDA (500 microM plus glycine 5 microM) in a voltage-dependent manner without affecting responses to (s)-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (100 microM) or gamma-aminobutyric acid (10 microM). The NMDA receptor antagonistic effect of all four agents was typical of open channel blockade. The kinetics of blockade/unblockade was inversely related to antagonist affinity. In hippocampal neurons amantadine was the least potent NMDA receptor antagonist (IC50 18.6 +/- 0.9 microM) and showed the fastest blocking kinetics, whereas (+)-MK-801 was the most potent (IC50 0.12 +/- 0.01 microM) and showed the slowest blocking kinetics. Memantine (IC50 1.04 +/- 0.26 microM) and ketamine (IC50 0.43 +/- 0.10 microM) were almost equipotent and had similar, intermediate blocking kinetics. In striatal neurons recorded under identical conditions (+)-MK-801, ketamine and memantine were 3- to 4-fold less potent whereas amantadine was somewhat more potent than on hippocampal neurons. This could offer an explanation for the better clinical profile of amantadine in Parkinson's disease, as therapeutically relevant concentrations of amantadine are likely to be more active in the striatum whereas memantine is likely to be more active in other structures.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Glutamate and Parkinson's disease

Altered glutamatergic neurotransmission and neuronal metabolic dysfunction appear to be central to the pathophysiology of Parkinson's disease (PD). The substantia nigra pars compacta--the area where the primary pathological lesion is located--is particularly exposed to oxidative stress and toxic and metabolic insults. A reduced capacity to cope with metabolic demands, possibly related to impaired mitochondrial function, may render nigral highly vulnerable to the effects of glutamate, which acts as a neurotoxin in the presence of impaired cellular energy metabolism. In this way, glutamate may participate in the pathogenesis of PD. Degeneration of dopamine nigral neurons is followed by striatal dopaminergic denervation, which causes a cascade of functional modifications in the activity of basal ganglia nuclei. As an excitatory neurotransmitter, glutamate plays a pivotal role in normal basal ganglia circuitry. With nigrostriatal dopaminergic depletion, the glutamatergic projections from subthalamic nucleus to the basal ganglia output nuclei become overactive and there are regulatory changes in glutamate receptors in these regions. There is also evidence of increased glutamatergic activity in the striatum. In animal models, blockade of glutamate receptors ameliorates the motor manifestations of PD. Therefore, it appears that abnormal patterns of glutamatergic neurotransmission are important in the symptoms of PD. The involvement of the glutamatergic system in the pathogenesis and symptomatology of PD provides potential new targets for therapeutic intervention in this neurodegenerative disorder.

Learning deficits induced by chronic intraventricular infusion of quinolinic acid--protection by MK-801 and memantine

The NMDA receptor agonist quinolinic acid (9 mM) was infused i.c.v. via ALZET osmotic minipumps for 2 weeks. This treatment produced a persistent, short-term memory deficit in the T-maze. Autoradiography revealed a decrease in the density of choline uptake sites in the hippocampus. Parallel s.c. infusion by another minipump of the uncompetitive NMDA receptor antagonist memantine (1-amino-3,5-dimethyladamantane, 20 mg/kg per day) or (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801, 0.31 mg/kg day) prevented the learning deterioration induced by quinolinic acid. The treatment with memantine resulted in steady-state serum levels of 1.2 mu M which, based on in vitro data, should assure inhibition of NMDA receptors and are similar to levels seen in the serum of demented patients treated with this agent. In naive animals this treatment had no effect on either learning or on ex vivo induction of long-term potentiation, indicating that under chronic conditions it is possible to obtain neuroprotective effects with NMDA receptor antagonists without negative effects on memory processes. This contrasts to some acute insults (e.g. ischaemia) where high doses of NMDA receptor antagonists that produce side effects are required.

Activity of 2,3-benzodiazepines at native rat and recombinant human glutamate receptors in vitro: stereospecificity and selectivity profiles

The activity and selectivity of the glutamate receptor antagonists belonging to the 2,3-benzodiazepine class of compounds have been examined at recombinant human non-NMDA glutamate receptors expressed in HEK293 cells and on native rat NMDA and non-NMDA receptors in vitro. The racemic 2,3-benzodiazepines GYKI52466, LY293606 (GYKI53405) and LY300168 (GYKI53655) inhibited AMPA (10 microM)-mediated responses in recombinant human GluR1 receptors expressed in HEK293 cells with approximate IC50 values of 18 microM, 24 microM and 6 microM, respectively and AMPA (10 microM) responses in recombinant human GluR4 expressing HEK293 cells with approximate IC50 values of 22 microM, 28 microM and 5 microM, respectively. GYKI 52466, LY293606 and LY300168 were non-competitive antagonists of AMPA receptor-mediated responses in acutely isolated rat cerebellar Purkinje neurons with approximate IC50 values of 10 microM, 8 microM and 1.5 microM, respectively. The activity of racemic compounds LY293606 and LY300168 was established to reside in the (-) isomer of each compound. At a concentration of 100 microM, GYKI52466, LY293606 and LY300168 produced < 30% inhibition of kainate-activated currents evoked in HEK293 cells expressing either human homomeric GluR5 or GluR6 receptors or heteromeric GluR6+KA2 kainate receptors. The activity of the 2,3-benzodiazepines at 100 microM was weak at kainate receptors, but was stereoselective. Similar levels of inhibition were observed for kainate-induced currents in dorsal root ganglion neurons. Intact tissue preparations were also used to examine the stereoselective actions of the 2,3-benzodiazepines. In the cortical wedge preparation, the active isomer of LY300168, LY303070, produced a non-competitive antagonism of AMPA-evoked depolarizations with smaller changes in depolarizations induced by kainate and no effect on NMDA-dependent depolarizations. LY303070 was also effective in preventing 30 microM AMPA-induced depolarizations in isolated spinal cord dorsal roots with an approximate IC50 value of 1 microM. Synaptic transmission in the hemisected spinal cord preparation was stereoselectively antagonized by the active isomers of LY300168 and LY293606. In summary, these results indicate that 2,3-benzodiazepines are potent, selective and stereospecific antagonists of the AMPA subtype of the non-NMDA glutamate receptor.