Anticonvulsant properties of phencyclidine-like drugs in mice

In Silico Analysis, Anticonvulsant Activity, and Toxicity Evaluation of Schisandrin B in Zebrafish Larvae and Mice

The aim of this study is to evaluate the anticonvulsant potential of schisandrin B, a main ingredient of Schisandra chinensis extracts. Schisandrin B showed anticonvulsant activity in the zebrafish larva pentylenetetrazole acute seizure assay but did not alter seizure thresholds in the intravenous pentylenetetrazole test in mice. Schisandrin B crosses the blood-brain barrier, which we confirmed in our in silico and in vivo analyses; however, the low level of its unbound fraction in the mouse brain tissue may explain the observed lack of anticonvulsant activity. Molecular docking revealed that the anticonvulsant activity of the compound in larval zebrafish might have been due to its binding to a benzodiazepine site within the GABAA receptor and/or the inhibition of the glutamate NMDA receptor. Although schisandrin B showed a beneficial anticonvulsant effect, toxicological studies revealed that it caused serious developmental impairment in zebrafish larvae, underscoring its teratogenic properties. Further detailed studies are needed to precisely identify the properties, pharmacological effects, and safety of schisandrin B.

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

KETAMINE REDUCES LIDOCAINE-INDUCED SEIZURES IN MICE

Systemic toxic reactions to local anesthetics are brought about by absolute overdosage, and, most commonly, inadvertent intravascular injections. The anti-convulsant action of ketamine has been studied. However, the effect of ketamine on lidocaine-induced convulsions has not been reported. This study investigated the effect of ketamine on lidocaine-induced seizures in mice. Mice (32-41 g) were divided into 2 groups, 15 in each group, and were pretreated with intraperitoneal normal saline solution or intraperitenoeal (ip) ketamine before lidocaine. Group 1 (N = 15) received 75 mg kg ip lidocaine; Group 2 (N = 15) received 20 mg kg ketamin ip; 5 min later 75 mg kg lidokaine ip were applied. Clinical features, incidences, latencies, durations, and mortality rate of convulsions were recorded. After 75 mg kg lidocaine injection, ataxia, loss of righting reflex, and generalized tonic-clonic convulsions were seen within 2-5 min in Group 1. Generalized tonic-clonic convulsions were seen in 8 mice and deep sedation was seen in 7 mice in Group 2 (p < .05). Generalized status epilepticus occurred in one mouse in both groups. Three mice from Group l and one mouse from Group 2 died during convulsions. There were no differences between the two groups with regard to the onset and duration of seizures (p > .05). It was concluded that ketamine significantly prevented lidocaine-induced generalized tonic-clonic seizures; on the other hand, the lethality of lidocaine was least reduced by ketamine.

Molecular evidence for the involvement of NR2B subunit containing N-methyl-D-aspartate receptors in the development of morphine-induced place preference

The present study was designed to clarify the role of the NR1, NR2A and NR2B subunits of N-methyl-D-aspartate receptors in the development of morphine-induced place preference using specific antibodies to N-methyl-D-aspartate receptor subunits in the mouse. An i.c.v. injection with specific antibodies against the carboxyl-terminal region of either NR1, NR2A or NR2B subunits caused the attenuation of seizures induced by i.v. infusion of N-methyl-D-aspartate in the mouse. Using western blotting, NR1, NR2A and NR2B subunits were found to be highly expressed in the cerebral cortex and hippocampus of the mouse brain, which are key areas in producing seizures regulated by N-methyl-D-aspartate receptors. These findings indicate that all three subunits of the N-methyl-D-aspartate receptor in these areas are likely to be critical for the seizure induced by i.v. infusion of N-methyl-D-aspartate. Furthermore, present data provide evidence that these antibodies when given into the brain specifically act on the target sites, resulting in the blockade of the respective N-methyl-D-aspartate receptor subunit. Under these conditions, i.c.v. treatment with the antibody against NR2B subunits abolished the morphine-induced place preference, whereas antibodies against NR1 and NR2A subunits did not affect the rewarding effect of morphine. Furthermore, the NR2B subunit protein was specifically up-regulated in the limbic forebrain of morphine-conditioned mice, whereas the protein levels of either NR1 or NR2A subunits were not altered.From these results, we suggest that the stimulation of the carboxyl-terminal region of NR2B subunits in the limbic forebrain may contribute to the development of morphine-induced rewarding effect.

Rigid phencyclidine analogues. Binding to the phencyclidine and sigma 1 receptors

Three phencyclidine (PCP) analogues possessing a highly rigid carbocyclic structure and an attached piperidine ring which is free to rotate were synthesized. Each analogue has a specific fixed orientation of the ammonium center of the piperidinium ring to the centrum of the phenyl ring. The binding affinities of the rigid analogues 1-piperidino-7,8-benzobicyclo[4.2.0]octene (14), 1-piperidinobenzobicyclo[2.2.1]heptene (16), and 1-piperidinobenzobicyclo[2.2.2]octene (13) for the PCP receptor ([3H]TCP) and th-receptor (NANM) were determined. The three analogues show low to no affinity for the PCP receptor but good affinity for the th-receptor and can be considered th-receptor selective ligands with PCP/th ratios of 13, 293, and 368, respectively. The binding affinities for the th-receptor are rationalized in terms of a model for the th-pharmacophore.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. IV. Effects of CGP 39551

We determined anticonvulsant effects of CGP 39551 [(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid 1-ethylester] against pentylenetetrazol-induced seizures in developing, 7-90 day old, rats. The rats received CGP 39551 in doses of 10, 20 or 40 mg/kg IP 30 min prior to the pentylenetetrazol administration (100 mg/kg s.c.). In addition, the 20 mg/kg dose of CGP 39551 was injected 120 min prior to pentylenetetrazol. In adult rats, all doses of CGP 39551 blocked generalized tonic-clonic pentylenetetrazol-induced seizures. In younger rats, higher doses of CGP 39551 and/or a longer delay between the CGP 39551 pretreatment and pentylenetetrazol administration was necessary for similar anticonvulsant effects against tonic-clonic seizures. In contrast, there was no effect of CGP on pentylenetetrazol-induced clonic seizures. The results indicate that CGP 39551 has anticonvulsant features similar to other competitive NMDA receptor antagonists. High doses of CGP 39551 and long pretreatment latency which are necessary in young rats for anticonvulsant effects may reflect the overexpression of NMDA transmission during the second and third postnatal week of the rat. Alternatively in adult rats, we can speculate an anticonvulsant role of a CGP 39551 metabolite or maturation of brain uptake mechanism for CGP 39551.

Suppression of cortical epileptic afterdischarges by ketamine is not stable during ontogenesis in rats

Anticonvulsant action of ketamine, a noncompetitive NMDA antagonist, was studied in three groups of immature rats (12-, 18-, and 25-day-old) using cortically elicited epileptic afterdischarges (ADs) as a model. Rats with implanted electrodes were used, so that EEG and motor phenomena could be recorded. Stimulation (bipolar pulses of 1-ms duration and 8-Hz frequency) lasting 15 s was repeated four times with intervals of 10 min. Ketamine was administered IP 5 min after the first AD in doses of 5, 10, 20, or 40 mg/kg. Control groups did not receive any drug. Ketamine shortened ADs and suppressed motor correlates of stimulation as well as of ADs in a dose-dependent manner in 12- and 25-day-old rats. No significant changes were observed in 18-day-old animals, demonstrating thus a rather complicated development of anticonvulsant action of ketamine. Not only NMDA antagonism, but also other possible effects of ketamine must be taken into account.

Anticonflict effect of MK-801 in rats: time course and chronic treatment studies

The present study examined the time course and chronic treatment effects of the noncompetitive N-methyl-D aspartate (NMDA) antagonist, MK-801 (dizocilpine), on conflict behavior in the conditioned suppression of drinking (CSD) paradigm, a repeated-measures conflict task. In daily 10-min sessions, water-restricted rats drank from a tube that was occasionally electrified (0.25- or 0.5-mA shocks signaled by a tone). Trained subjects (4 weeks of CSD testing) exhibited stable baselines for both punished responding and unpunished responding. In the first experiment, the effects of MK-801 administered IP were determined in female and male rats following a range of pretreatment intervals (i.e., 0.5-48 h). In female rats, 0.2 mg/kg MK-801 exerted an anticonflict effect at pretreatment intervals of 10-36 h, but not before 10 h or after 36 h. In male rats, qualitatively similar results were obtained; MK-801 (0.4 mg/kg) exerted anticonflict effects following pretreatment intervals of 6-14 h, but not before 6 or after 14 h. In the second experiment, chronic treatment of female rats with 0.04, 0.01, or 0.2 mg/kg MK-801 resulted in a dose-dependent anticonflict effect in CSD paradigm, which remained stable over the course of 5 weeks of chronic treatment. Punished responding returned to pretreatment levels within 2-3 days after discontinuation of chronic treatment with MK-801. These data suggest that MK-801 exerts a delayed anticonflict effect in both female and male rats with a qualitatively similar pattern, and that there is no tolerance to the anticonflict effect of MK-801 with chronic treatment.

Tolerance to and dependence on MK-801 (dizocilpine) in rats

Rats were trained to respond under a fixed-ratio 30 schedule for food presentation during four daily 0.5-h sessions occurring every 6 h. After stable baseline response was established, osmotic minipumps were implanted that infused vehicle or (+)-5 methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine hydrogen maleate (dizocilpine; MK-801), SC. Behavioral sessions continued to be conducted daily. After 10 days the infusion pumps were removed. Vehicle and 0.10 mg/kg per day MK-801 did not affect behavior during infusions or after cessation of dosing. Dosing with 0.32 and 0.56 mg/kg per day initially suppressed responding, but tolerance developed to these effects. After the infusions were stopped, a dose-dependent disruption of operant behavior occurred. Response rates for the 0.32 and 0.56 mg/kg per day infusion groups were suppressed to 41 and 27% of preinfusion control response rates, respectively, the day after dosing stopped; however, no physical signs of abstinence were observed. Response rates recovered toward control over the next 2-4 days. In a separate experiment, the suppression of response produced by abstinence from 0.32 mg/kg per day of MK-801 (SC) for 10.5 days was reversed by readministration of MK-801 (IP). These results demonstrate that MK-801 produces dependence, as evidenced by the emergence of a behavioral abstinence syndrome after cessation of dosing.

Chronic neonatal phencyclidine treatment produces age-related changes in pentylenetetrazol-induced seizures

Although excitatory amino acids are known to play a critical role in the plasticity of developing brain, the behavioral effects of blocking the N-methyl-D-aspartate (NMDA) receptor-gated ion channel during development are not clear. Here we report the effects of chronic postnatal administration of 1-phenylcyclohexylpiperidine (phencyclidine or PCP), a NMDA channel blocker, on seizure susceptibility. To study the short-term effects of chronic PCP administration on pentylenetetrazol (PTZ)-induced seizures, rats were treated with PCP (5 mg/kg, i.p.) for 11 days from postnatal days 5-15, 24-34 or 44-54 and tested in the PTZ-induced seizure paradigm on postnatal days 21, 40 and 60, respectively. Administration of PCP in 5-15-day-old rats resulted in increased seizure susceptibility at day 21, while administration of PCP in postweanling rats (days 24-34) markedly attenuated their susceptibility to seizures at day 40. PCP injection had little effect on the seizure susceptibility of older rats. To study the long-term effects of postnatal PCP treatment, rats were injected with PCP (5 mg/kg from postnatal day 5-15, i.p.) and were tested for PTZ-induced seizures on postnatal days 40 and 60; each rat was tested only once. When tested for PTZ-induced seizure on day 40, PCP-treated rats did not differ from saline-treated controls. When tested on day 60, PCP-treated rats had a lower incidence of seizures and in the rats that did have seizures their latencies were significantly prolonged compared to controls. Together, our data suggest that chronic PCP administration alters PTZ-induced seizure susceptibility in an age-dependent manner and chronic PCP administration in postnatal rats produces long-term changes that persist into adulthood.

The glycine/NMDA receptor antagonist, R-(+)-HA-966, blocks activation of the mesolimbic dopaminergic system induced by phencyclidine and dizocilpine (MK-801) in rodents

1. The effects of the glycine/N-methyl-D-aspartate (NMDA) receptor antagonist, R-(+)-HA-966 on the neurochemical and behavioural responses to phencyclidine (PCP) and dizocilpine (MK-801) have been determined in rodents. 2. In rats, pretreatment with PCP (5 and 10 mg kg-1) or MK-801 (0.25 and 0.5 mg kg-1) dose-dependently stimulated dopamine turnover in nucleus accumbens, amygdala and medial prefrontal cortex, but had no effect in striatum. In contrast, pretreatment with (+)-HA-966 (10 and 30 mg kg-1) did not affect dopamine turnover in any brain region investigated. 3. Pretreatment with (+)-HA-966 (10 and 30 mg kg-1) significantly antagonized the stimulation of dopamine turnover induced by both PCP (10 mg kg-1) and MK-801 (0.5 mg kg-1) in rat nucleus accumbens, amygdala and medial prefrontal cortex. 4. Intracerebral dialysis studies in conscious rats demonstrated that systemic injection of PCP (10 mg kg-1) markedly stimulated dopamine release from the nucleus accumbens, an effect that was abolished by pretreatment with (+)-HA-966 (30 mg kg-1). 5. Pretreatment with PCP (3-30 mg kg-1) or MK-801 (0.1-1.6 mg kg-1) significantly increased locomotor activity in mice. In contrast, subcutaneous injection of (+)-HA-966 (10-100 mg kg-1) failed to stimulate activity. 6. Pretreatment with (+)-HA-966 (10 and 30 mg kg-1) dose-dependently antagonized both PCP (10 mg kg-1) and MK-801 (0.4 mg kg-1) induced hyperactivity in mice. 7. Blockade of PCP-induced hyperactivity by (+)-HA-966 is unlikely to be explained by the induction or potentiation of sedation/ataxia since PCP-induced rotarod deficits were not significantly different in mice pretreated with (+)-HA-966 (30 mg kg-1) or saline.8. The results demonstrate that (+ )-HA-966 antagonizes both the neurochemical and behavioural effects of PCP and MK-801, possibly through interactions at the glycine/NMDA receptor.

Metaphit-induced audiogenic seizures in mice: I. Pharmacologic characterization

Metaphit [an analogue of phencyclidine (PCP) with an acylating isothiocyanate group] induced audiogenic clonic to clonic-tonic seizures in mice exposed to audio stimulation 24 h after metaphit administration. The incidence of seizures was reduced by treatment 30 min before audio stimulation with specific PCP-like compounds [5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK-801), and PCP itself], competitive N-methyl-D-aspartate antagonists 2-amino-5-phosphonopentanoic acid (AP-5 and NPC-12626), antiepileptic drugs [phenobarbital (PB), phenytoin (PHT)], and gamma-aminobutyric acid (GABA) agonists (muscimol and diazepam). In contrast, when given in conjunction with metaphit, most of these drugs were ineffective in protecting animals from audiogenic seizures 24 h later. Only compounds with long half-lives (t1/2) such as MK-801, PB, and PHT had a protective effect. High-performance liquid chromatography (HPLC) determination of [3H]MK-801 showed its long-term presence in the brain after intraperitoneal (i.p.) administration of [3H]MK-801. Audiogenic seizures observed 24 h after metaphit administration were potentiated by administration of the GABA antagonist picrotoxin 15 min before audio stimulation, and picrotoxin-induced spontaneous seizures were enhanced by pretreatment (24 h earlier) with a dose of metaphit that in itself did not produce spontaneous seizures at the time of the picrotoxin test. Similar observations were made with N-methyl D-aspartic acid (NMDA) instead of picrotoxin. Thus, an interplay exists between excitatory glutaminergic and inhibitory GABAergic circuitries in the metaphit seizure model.

Metaphit-induced audiogenic seizure and its inhibition by MK-801: electroencephalographic and behavioral characterization

Adult male Wistar rats were subjected to intense sound stimulation from an electric bell (100 dB and 12 KHz for 60 s) after a single intraperitoneal (i.p. 50 mg/kg) injection of metaphit [1-(1-/3 isothiocyanatophenyl-cyclohexyl) piperidine]. EEG recordings demonstrated appearance of paroxysmal activity and spike-wave complexes from cortical electrodes, with frequency and amplitude increasing with time. Metaphit-induced audiogenic seizures in the rats were tested 24 h after metaphit administration. The seizures consisted of wild running followed by clonic and tonic convulsions, and the seizure pattern could be elicited at hourly intervals for the next 24 h in all tested animals. Forty-eight hours after metaphit administration, susceptibility to sound stimulation began to decrease gradually. The first component of seizure response to disappear was tonic extension, followed by disappearance of clonic convulsion; the last component to disappear was running behavior. Each behavioral seizure response had a characteristic EEG correlate. After approximately 50 h, no animal responded to sound stimulation. The noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptors, MK-801 [5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5,10-imine maleate] was evaluated as an anticonvulsant against metaphit-induced audiogenic seizures in two experiments. In the first experiment, MK-801 was administered in a single dose of 0.5 mg/kg i.p. 23.5 h after metaphit injection and 30 min before sound stimulation, which completely blocked both the EEG and the behavioral response to sound stimulation for 37 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant properties of non-competitive antagonists of the N-methyl-D-aspartate receptor in genetically epilepsy-prone rats: comparison with CPPene

Some non-competitive antagonists of N-methyl-D-aspartate (NMDA) were evaluated for potency to antagonize audiogenic seizures in genetically epilepsy-prone rats, following intraperitoneal administration. Phencyclidine (PCP), dizocilpine (MK-801), ketamine, ifenprodil and dextromethorphan, displayed anticonvulsant activity at doses similar to those which impaired performance in the rotarod equilibrium procedure. The noncompetitive NMDA receptor antagonists, at doses which slightly overlapped with the doses required for a full anticonvulsant protection against audiogenic seizures in genetically epilepsy-prone rats, induced profound untoward behavioural effects. This behavioural syndrome was characterized by marked ataxia, hyperactivity, stereotypes and wet dog shakes. In contrast, the effective anticonvulsant dose of 3-(2-carboxypiperazin-4-yl)propenyl-1-phosphonic acid (CPPene) was less than that required to impair rotarod performance and did not produce the PCP-like syndrome. A potential use in antiepileptic therapy, of CPPene or other new selective NMDA antagonists, with fewer neurotoxic effects but not for non-competitive antagonists such as MK-801, is suggested.

Inhibitory influence of morphinans on ictal and interictal EEG changes induced by cortical application of penicillin in rabbits: a comparative study with NMDA antagonists and pentobarbitone

The effects of dextrorphan (DX) and dextromethorphan (DM) were tested using the electroencephalogram (EEG) and behavioral effects induced by topical cortical application of penicillin in rabbits. For comparison, the influence of the NMDA antagonists, dizocilpine (MK 801) and 3-((+-(-)2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP), and of pentobarbitone was investigated. Intracortical injection of 500 IU of penicillin produced an EEG spiking followed by a repeated generalization of the electrical and behavioral symptoms. Within a few minutes, DX (5-15 mg/kg, IV) or pentobarbitone (5-10 mg/kg, IV) reduced dose dependently and significantly (p less than 0.01) the interictal and ictal EEG and behavioral effects elicited by cortical injection of 500 IU of penicillin. Higher doses of pentobarbitone (20 mg/kg, IV) but not of DX (20 mg/kg, IV) completely blocked the ictal behavioral and EEG effects elicited by cortical injection of 500 IU of penicillin. Within a few minutes, MK 801 (0.1-0.2 mg/kg, IV) or CPP (10-20 mg/kg, IV) reduced dose dependently and significantly (p less than 0.01) the ictal EEG and behavioral effects elicited by cortical injection of 500 IU of penicillin, while they did not affect the penicillin-induced interictal EEG changes. Higher doses of MK 801 (0.3 mg/kg, IV) completely blocked the ictal behavioral and EEG effects elicited by cortical injection of 500 IU of penicillin. Within a few minutes, DM (10-20 mg/kg, IV) blocked the behavioral effects, but failed to affect either the interictal or the ictal EEG effects induced by cortical injection of 500 IU of penicillin.(ABSTRACT TRUNCATED AT 250 WORDS)

Extract of kava (Piper methysticum) and its methysticin constituents protect brain tissue against ischemic damage in rodents

The purpose of the present study was to test whether kava extract and its constituents kawain, dihydrokawain, methysticin, dihydromethysticin and yangonin provide protection against ischemic brain damage. To this end, we used a model of focal cerebral ischemia in mice and rats. Ischemia was induced by microbipolar coagulation of the left middle cerebral artery (MCA). To quantify the size of the lesion in mice, the area of the infarct on the brain surface was assessed planimetrically 48 h after MCA occlusion by transcardial perfusion of carbon black. In the rat model infarct volume was determined 48 h after MCA occlusion by planimetric analysis and subsequent integration of the infarct areas on serial coronal slices. Compounds were administered i.p., except the kava extract, which was administered orally. The effects of the kava extract and its constituents were compared with those produced by the typical anticonvulsant, memantine. The kava extract, methysticin and dihydromethysticin produced effects similar to those of the reference substance memantine. The kava extract (150 mg/kg, 1 h before ischemia) diminished the infarct area (P less than 0.05) in mouse brains and the infarct volume (P less than 0.05) in rat brains. Methysticin, dihydromethysticin (both 10 and 30 mg/kg, 15 min before ischemia) and memantine (20 mg/kg, 30 min before ischemia) significantly reduced the infarct area in mouse brains. All other compounds failed to produce a beneficial effect on the infarct area in mouse brains. In conclusion, the kava extract exhibited neuroprotective activity, which was probably mediated by its constituents methysticin and dihydromethysticin.

Electroencephalographic characteristics of audiogenic seizures induced in metaphit-treated small rodents

Adult male mice, rats, and guinea pigs were subjected to intense sound stimulation of an electric bell (100 dB, 12 kHz for 60 s) after a single intraperitoneal (i.p.) injection of metaphit (1-(1-(3 isothiocyanatophenyl)-cyclohexyl)piperidine) (50 mg/kg). When the animals were tested 24 h after administration of metaphit, audiogenic seizures were observed. None of the control saline-injected animals had convulsions. EEG recordings demonstrated the appearance of paroxysmal activity and spike-wave complexes in the trace from cortical and hippocampal electrodes, with frequency and amplitude increasing with time. Behaviorally, myoclonic jerks of facial muscles, ears, and neck appeared, but no correlation was noted between EEG and the motor phenomena. Auditory stimulation was necessary to elicit the full-blown sequence of seizure responses consisting of wild running followed by clonic and then tonic extension. At the time of seizures, repetitive high-amplitude spikes and waves appeared in the EEG, followed by profound EEG and behavioral depression. None of the animals died during or immediately after seizures. The seizure response to sound stimulation of mice, rats, and guinea pigs was phenomenologically similar, with minor differences in quantitative pattern of convulsive components, which suggests that all three animal species share the common property of extreme susceptibility to audiogenic stimulation caused by metaphit administration.

Behavioural and biochemical evidence of the interaction of the putative antipsychotic agent, BMY 14802 with the 5-HT1A receptor

The behavioural and biochemical profile of the sigma ligand and putative antipsychotic agent, BMY 14802 (alpha-(4-fluorophenyl)-4-(5-fluoro-2- pyrimidinyl)-1-piperazine butanol) has been determined in the mouse and rat. In mice, pretreatment with BMY 14802 attenuated both amphetamine-induced hyperactivity and conditioned avoidance responding, consistent with its previously reported antipsychotic potential. In common with 5-HT1A receptor agonists or partial agonists, BMY 14802 induced (a) a dose-dependent hypothermia in mice; (b) aspects of the 5-HT behavioural syndrome in rats, (c) antagonised mescaline-induced head twitches in mice and (d) generalised to the 8-hydroxy-2-(di-n-propylamino)tetralin discriminative stimulus over the dose range of 3-15 mg/kg. BMY 14802 had appreciable affinity for the 5-HT1A receptor (pIC50 = 6.7 compared to 7.3 for sigma binding) and antagonised forskolin-stimulated adenylate cyclase activity with a pEC50 of 6.2, consistent with an agonist action at this receptor. The results support the involvement of 5-HT1A receptors, but not the sigma binding site, in the behavioural profile of BMY 14802.

Inhibitory effects of NMDA receptor antagonists on hypoxia-induced seizures in dietary Mg(2+)-deficient mice

The effects of the non-competitive N-methyl-D-aspartate (NMDA) antagonists, (+)-MK-801 hydrogen maleate (MK-801) and phencyclidine hydrochloride (PCP), were tested against hypoxia-induced tonic convulsions and hippocampal epileptiform discharges. Systemic administration of MK-801 and PCP suppressed the hypoxia-induced tonic convulsions in dietary Mg(2+)-deficient mice in a dose-dependent manner. The ED50 values (and their 95% confidence limits) of MK-801 and PCP were 0.19 (0.14-0.26) and 1.14 (0.32-4.11) mumol/kg, respectively. These values were lower than those of the conventional anticonvulsants tested. Induction of epileptiform discharges following hypoxia was also prevented by the presence of MK-801 and PCP at concentrations of 5-30 microM. The hypoxia-induced epileptiform discharges, however, were reduced, but not blocked completely, by the application of MK-801 and PCP. These results strongly suggest that activation of NMDA receptors in the hippocampus plays a pivotal role in the induction of hypoxia-induced tonic convulsions in dietary Mg(2+)-deficient mice.

NMDA antagonists: lack of antipunishment effect in squirrel monkeys

Effects of the noncompetitive N-methyl-D-aspartate (NMDA) antagonist phencyclidine (PCP) and competitive antagonists 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 12626) were studied in 6 squirrel monkeys trained under a multiple schedule of unpunished and punished lever pressing. PCP (0.03-0.3 mg/kg, IM) failed to produce increases in punished responding, even at doses that produced extreme response-rate decreases in nonpunishment components. Similarly, CPP (1-17 mg/kg) and NPC 12626 (3-30 mg/kg) did not produce increases in punished responding at any dose tested. Repeated administration of NPC 12626 (17 mg/kg) for 4 consecutive days did not result in increased rates of punished responding. The benzodiazepine anxiolytic midazolam (0.3 mg/kg) and, to a lesser extent, the barbiturate pentobarbital (5.6 mg/kg), produced increases in punished responding in the same subjects at doses that did not markedly affect unpunished responding. Coadministration of PCP (0.03 mg/kg) with doses of midazolam ranging from 0.03-3 mg/kg did not produce changes in the midazolam dose-response curve for either unpunished or punished responding. These results fail to support findings in rats that NMDA antagonists produce antipunishment effects similar to those of benzodiazepine anxiolytics.

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. II. The effects of MK-801

MK-801 is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist with anticonvulsant and neuroprotective properties. The action of MK-801 (0.05-10 mg/kg IP) was assessed against pentylenetetrazol-induced seizures (PTZ; 100 mg/kg SC; 30 min after MK-801) in rats aged 7, 12, 18, 25, and 90 days (N = 263). We observed pronounced ataxia and hypermobility after MK-801 pretreatment during the whole ontogenesis, and the animals exhibited head-weaving and body-rolls. After the combination of MK-801 and PTZ "wet dog shakes" were detected in 18-, 25-, and 90-day-old rats (never seen in controls receiving PTZ only). MK-801 only insignificantly modified the latencies of minimal (clonic) seizures in 18-day-old and older rats where this seizure type is regularly elicited. In 12-day-old rats an increased incidence of minimal seizures was detected. MK-801 nearly completely blocked or strongly delayed major (generalized tonic-clonic) seizures and attenuated the seizure severity during ontogenesis in a dose-dependent manner. Present results suggest the important role of NMDA receptors in the genesis of generalized tonic-clonic seizures whilst the role of NMDA receptors in minimal seizures appears to be negligible during the whole ontogenetic development.

Ketamine suppresses both bicuculline- and picrotoxin-induced generalized tonic-clonic seizures during ontogenesis

An anticonvulsant action of ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist (5-40 mg/kg IP), on the bicuculline-induced (3-8 mg/kg IP) or picrotoxin-induced seizures (3-6 mg/kg IP) was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused moderate ataxia which was more pronounced in younger animals. In combination with both aforementioned convulsants, ketamine exerted anticonvulsant effects against generalized tonic-clonic seizures in all developmental stages studied. This effect was more pronounced in bicuculline-treated animals. Moreover, ketamine also suppressed the lethality induced by both drugs during all the development. On the contrary, the action of ketamine on minimal (clonic) seizures was moderate or absent. Our results suggest an important role of ketamine-affected transmission in the generation of the generalized tonic-clonic seizure pattern; moreover, an action of high doses of ketamine on GABA-A receptors might be present.

Specificity of phencyclidine-like drugs and benzomorphan opiates for two high affinity phencyclidine binding sites in guinea pig brain

Recently, the presence of two high affinity binding sites for phencyclidine were described in guinea pig brain, with one site coupled to the glutamate excitatory amino acid receptor, specifically activated by N-methyl-D-aspartate (NMDA) (site 1) and the other site associated with the dopamine (DA) reuptake carrier (site 2). Phencyclidine and its analogs, as well as the benzomorphan opiates, are known to interact with binding sites for phencyclidine. In this study, the equilibrium dissociation constants (Kd) of these compounds for the two binding sites for phencyclidine were determined. Phencyclidine and 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP), an analog of PCP, were essentially non-selective between the two sites and also were the two drugs of the group observed to have the highest affinity for site 2. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5,10-imine [(+)MK801] was the most selective agent for site 1, while none of the drugs tested showed selectivity for site 2. In humans, phencyclidine produces psychotomimetic effects, while (+)MK801 has been reported to produce minimal, if any, psychotomimetic effects, at doses sufficient to reduce seizures. These clinical observations, in conjunction with the present biochemical binding data, suggest that (+)MK801 may serve as a "marker" for site 1 and that the psychotomimetic effects of phencyclidine might be mediated by site 2.

The effect of the NMDA receptor antagonist, MK-801, on the course and outcome of kindling

A rapid kindling procedure was used to distinguish between the anticonvulsant activity of drugs and their ability to retard the kindling process. MK-801 is a specific ligand at the phencyclidine (PCP) recognition site, and acts as a noncompetitive antagonist of NMDA-type glutamate/aspartate receptors. Intraperitoneal injections of MK-801 (0.5-4.0 mg/kg IP) significantly reduced the cumulated effect of 12 2-hr kindling stimulations, as determined from behavioral measures of seizure activity in immediately ensuing 24-hr drug-free kindling sessions; however, the corresponding electrographic effects did not reach significance. MK-801 also showed significant anticonvulsant activity when injected in fully kindled rats. Higher doses tested were accompanied by locomotor and postural effects. The anticonvulsant benzodiazepine, clonazepam, formulated with a proprietary diluent (as Rivotril, Roche), injected in anticonvulsant doses during the first 12 kindling sessions (0.64 mg/kg IP, repeated after 9 hr) did not significantly affect the course of subsequent sessions of drug-free kindling. Systemic injections of kynurenic acid (300-600 mg/kg IP 4 hours), a nonspecific antagonist of glutamate receptors in vitro, were without significant anticonvulsant or antikindling activity. Activity of NMDA-sensitive glutamate/aspartate receptors associated with the PCP recognition site may induce lasting facilitation of neural transmission; this facilitation may be responsible for the remote propagation and progressive enhancement of seizure activity kindled in the amygdala. The facilitatory process appears to be antagonised by MK-801.

Modulation of seizure susceptibility in the mouse by the strychnine-insensitive glycine recognition site of the NMDA receptor/ion channel complex

1. In order to determine whether the strychnine-insensitive glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex is fully activated in vivo, the ability of the selective glycine receptor agonist, D-serine, to modulate seizure susceptibility in the mouse has been examined. 2. D-Serine (10-200 micrograms per mouse, i.c.v.) dose-dependently increased the potency of NMDLA in inducing seizures in Swiss Webster mice by approximately 3 fold. L-Serine was without significant effect. 3. The potency of pentylenetetrazol in inducing seizures was also enhanced by D-, but not L-serine, although the magnitude of the shift (1.6 fold) was considerably less than for NMDLA. 4. Similar doses of D-serine were also able to block the anticonvulsant effect of the non-selective glycine receptor antagonist, kynurenic acid, against seizures induced by NMDLA, but were without effect on the anticonvulsant effect of the competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). 5. D-Serine completely antagonized the protective effect of the selective glycine receptor antagonist, 7-chlorokynurenic acid, against sound-induced seizures in DBA/2 mice, but was less effective in this model against the less selective antagonist, kynurenic acid. 6 The results indicate that in vivo, NMDA receptors are not maximally potentiated by endogenous glycine and suggest an important involvement of the glycine modulatory site on the NMDA receptor/ion channel complex in the pathophysiology of epilepsy.

Differential effects of antiepileptic drugs and beta-carbolines on seizures induced by excitatory amino acids

Agonists acting at subtypes of glutamate receptors, N-methyl-D-aspartate, kainate and quisqualate, induce convulsions in rodents. Clonic seizures induced in mice by intracerebral administration of N-methyl-D-aspartate, kainate or quisqualate were used to study the anti- and proconvulsant potential of antiepileptic drugs and beta-carbolines. Systemic administration showed that the benzodiazepines clonazepam and midazolam blocked convulsions induced by kainate and had no effect on seizures triggered by N-methyl-D-aspartate and quisqualate. In contrast, diazepam blocked convulsions induced by either excitatory amino acid, as did valproate. The benzodiazepine receptor agonist beta-carboline ZK 93423 blocked convulsions induced by kainate but had no effect on seizures induced by N-methyl-D-aspartate or quisqualate. The antagonist beta-carboline ZK 93426 did not affect convulsions induced by excitatory amino acids, while the inverse agonists FG 7142 and ethyl-beta-carboline-3-carboxylate increased the sensitivity of mice to kainate. Phenobarbital and 2-chloroadenosine protected mice against seizures induced by quisqualate and kainate, while baclofen was active against convulsions produced by kainate. MK-801 selectively blocked convulsions induced by N-methyl-D-aspartate, and enhanced the susceptibility of mice to seizures triggered by kainate and quisqualate. Ethosuximide increased the susceptibility of mice to N-methyl-D-aspartate and had little or no effect on other types of seizures. Diphenylhydantoin enhanced the convulsant potential of quisqualate. Trimethadione and carbamazepine did not affect convulsions induced by N-methyl-D-aspartate, kainate or quisqualate. Intracerebral administration of midazolam protected mice against seizures induced by kainate. Ethosuximide increased the susceptibility of mice to N-methyl-D-aspartate, while diphenylhydantoin to quisqualate convulsions.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acid antagonists and pentylenetetrazol-induced seizures during ontogenesis. I. The effects of 2-amino-7-phosphonoheptanoate

The anticonvulsant action of 2-amino-7-phosphonoheptanoate (AP7) was assessed during ontogenesis of the rat. Animals of five age groups (7, 12, 18, 25, and 90 days) were pretreated with AP7 i.p. in the doses from 15 to 60 mg/kg 30 min prior to pentamethylenetetrazol (PTZ; metrazol; 100 mg/kg s.c.). The incidence and latency of minimal seizures (pure clonic without the loss of righting ability) and of generalized tonic-clonic seizures (major) were evaluated and compared with the control groups. Minimal metrazol seizures were not regularly observed in controls between ages 7 and 12 days. An increased incidence was noticed in AP7-treated groups. In animals of 18 days of age and older the AP7-pretreatment did not influence incidence of minimal seizures; the latencies were significantly lengthened only in 18-day-old animals. Major seizures were significantly suppressed with the highest dose of AP7 (60 mg/kg) in all groups except 7-day-old rats. In 90-day-old rats all doses of AP7 were effective in the suppression of major seizures. The latencies of major seizures were increased in 7 and 18 days old rats. It appears that the blockade of NMDA receptor substantially influences the major seizures induced by PTZ, whereas minimal (clonic) seizures are affected weakly. This suggests an important role of NMDA receptor-mediated transmission in the genesis of generalized tonic-clonic seizure pattern.

Role of N-methyl-D-aspartate (NMDA) receptors in experimental catalepsy in rats

N-methyl-D-aspartic acid (NMDA; 40 mg/kg, i.p.) did not elicit catalepsy, but it potentiated the cataleptic effect of haloperidol and GABAB receptor agonist, baclofen. MK-801 (0.2 mg/kg, i.p.), NMDA-receptor antagonist, reversed haloperidol- but not baclofen-induced catalepsy. MK-801 also potentiated the anticataleptic effect of scopolamine and bromocriptine against haloperidol-induced catalepsy. Dihydropyridine (DHP) calcium-channel antagonists such as nimodipine and nitrendipine (10 mg/kg, i.p.), reversed the anticataleptic effect of MK-801, and potentiated the cataleptic effect of haloperidol, as well as baclofen. These observations indicate the involvement of NMDA receptors in catalepsy, and suggest a potential clinical implication of NMDA-receptor antagonists in Parkinson's disease.

Anticonvulsant actions of phencyclidine receptor ligands: correlation with N-methylaspartate antagonism in vivo

1. Drugs with phencyclidine (PCP)-like activity in behavioural discrimination and [3H]PCP binding studies share anticonvulsant properties. 2. We have compared the rank order potency of a series of PCP-like compounds as N-methylaspartate (NMA) antagonists, determined from previously published studies from our laboratory, with their rank order anticonvulsant potencies as determined by two independent research groups in three different in vivo models of experimentally-induced epilepsy. 3. Rank order potency for NMA antagonism correlated well with rank order anticonvulsant potency. Furthermore, the systemic doses required for an effective blockade of NMA-evoked excitations were, in most cases, similar to those which produced anticonvulsant activity. 4. The results suggest that functional NMA antagonism may underlie the shared anticonvulsant properties of structurally dissimilar compounds with PCP-like activity.

A comparison of the anticonvulsant effects of competitive and non-competitive antagonists of the N-methyl-D-aspartate receptor

The anticonvulsant activity of two competitive antagonists of the N-methyl-D-aspartate (NMDA) receptor, 2-amino-7-phosphonoheptanoic acid (APH) and 3-[2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP), and two non-competitive NMDA antagonists, phencyclidine (PCP) and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), were compared in 4 models of induced seizures in mice. All 4 drugs protected against tonic extensor seizures induced by pentylenetetrazol (PTZ), by submaximal (15 mA) electroconvulsive shock (ECS) and by maximal (50 mA) ECS. Similar orders of potency (i.e., MK-801 greater than PCP greater than or equal to CPP greater than APH) were seen in each of the 3 seizure models. All 4 drugs failed to block clonic seizures induced by picrotoxin in the dose ranges that protected from tonic seizures. These data are consistent with other data demonstrating that competitive and non-competitive NMDA antagonists have similar pharmacologic effects. These results also support the suggestion that the anticonvulsant effects of competitive and non-competitive NMDA antagonists are mediated by the NMDA receptor-ionophore complex.

Non-competitive N-methyl-D-aspartate antagonists protect against sound-induced seizures in DBA/2 mice

Non-competitive antagonists of the N-methyl-D-aspartate (NMDA) receptor have been evaluated as anticonvulsants against sound-induced seizures in DBA/2 mice. The ED50 values for protection against sound-induced clonic seizures 15 min following the intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) administration are: MK-801, ED50 = 0.5 nmol (i.c.v.); 0.14 mumol/kg (i.p.); phencyclidine, ED50 = 14 nmol (i.c.v.); 1.9 mumol/kg (i.p.); dextrorphan, ED50 = 35 nmol (i.c.v.); 18.5 mumol/kg (i.p.); tiletamine, ED50 = 40 nmol (i.c.v.); 5.6 mumol/kg (i.p.); SKF-10047, ED50 = 50 nmol (i.c.v.); 23.5 mumol/kg (i.p.); dextromethorphan, ED50 = 70 nmol (i.c.v.); 28.0 mumol/kg (i.p.); ketamine, ED50 = 110 nmol (i.c.v.); 15.5 mumol/kg (i.p.). The anticonvulsant effects of ketamine and tiletamine are of short duration (10-30 min), whereas the anticonvulsant effects of MK-801 and dextromethorphan last for 45 min or longer. The effects of phencyclidine, SKF-10047 and dextrorphan are of intermediate duration. Mild to moderate behavioural excitation is associated with the anticonvulsant activity of all the non-competitive NMDA antagonists. For MK-801, phencyclidine, dextrorphan, SKF-10047 and ketamine there is a close correlation between their relative anticonvulsant potencies and their potencies for displacing [3H]MK-801. The anticonvulsant effect is likely to be primarily mediated via NMDA antagonism at the PCP/MK-801 site.

Efflux of rubidium in rat cortical synaptosomes is blocked by sigma and dextromethorphan binding site ligands

Large concentrations of potassium were used to stimulate the release of rubidium-86 from preloaded cortical synaptosomes, so that the pharmacological sensitivity of this efflux could be examined. Potassium channel blockers, 4-aminopyridine and tetraethylammonium, inhibited the evoked release of rubidium. Sigma ligands, e.g. pentazocine, cyclazocine, rimcazole, 1,3-di(2-tolyl)guanidine (DTG) and haloperidol, as well as the antitussives, carbetapentane, caramiphen and dextromethorphan, significantly reduced potassium-stimulated efflux of rubidium. By contrast, 3-hydroxyphenyl-propylpiperidine (3-PPP), 5-methyl-10,11-dihydro-5H-dibenzo(a,d)-cyclohepten-5,10-imine (MK-801), phencyclidine (PCP), ketamine and D-2-amino-5-phosphonovalerate (D-AP5) were all inactive. This suggests that inhibition of potassium-stimulated efflux of rubidium is correlated with activity at the sigma and/or dextromethorphan binding sites rather than at the N-methyl-D-aspartate (NMDA)/PCP receptor-channel complex.

Metaphit, an isothiocyanate analog of PCP, induces audiogenic seizures in mice

Metaphit induces audiogenic seizures in mice. The most severe clonic/tonic seizures occur 18-24 h after metaphit administration. After 48 h the incidence of the seizure episodes begin to diminish. These audiogenic seizures can be prevented by the administration of either PCP or MK-801 24 h after metaphit and 30 min prior to audio stimulation. These seizures may be due to a modulation of the PCP recognition site by metaphit which results in an enhanced probability that the NMDA/PCP ion channels are open.

Anaesthetic properties of phencyclidine (PCP) and analogues may be related to their interaction with Na+ channels

Among other properties, phencyclidine (PCP) and analogues display anaesthetic and anticonvulsant properties. Interaction of PCP and some analogues with the voltage-sensitive Na+ channels have been investigated and compared with their interaction with the PCP receptor. PCP and TCP inhibit apparently in a competitive manner the veratridine stimulated 22Na+ synaptosomal uptake with Ki values of 8.6 and 12.7 microM, respectively, close to those obtained in the inhibition of [3H]BTX-B binding (IC50 = 4.1 and 3.8 microM, respectively). The specific [3H]TCP binding to synaptosomes in ionic near physiological conditions is inhibited by PCP and TCP with IC50 values of 1.25 and 0.29 microM, respectively. Other PCP derivatives (GK3 and GK4) and PCP-like drugs (ketamine and MK801) inhibit 22Na+ uptake in an order of potency (GK3 greater than GK4 greater than PCP greater than TCP greater than MK801 greater than ketamine) which is different from that obtained in the inhibition of [3H]TCP binding (MK801 greater than TCP greater than PCP greater than ketamine greater than GK4 greater than GK3). Ketamine inhibits the veratridine-stimulated Na+ uptake at a concentration where its anesthetic effect occurs. It was concluded that the interaction of these drugs with the Na+ channel may reflect their anaesthetic properties while the interaction with the PCP receptors may be mainly related to their anticonvulsant and ataxic properties.

Selective depression of N-methyl-D-aspartate-mediated responses by dextrorphan in the hippocampal slice in rat

The effects of dextrorphan (DX) and dextromethorphan (DM) on responses to excitatory amino acids in the CA1 region of the hippocampus of the rat were studied using extracellular and intracellular recording in in vitro slices of brain. Dextrorphan selectively and non-competitively blocked depolarizations evoked by focally-applied N-methyl-D,L-aspartate (NMA), recorded by both extracellular and intracellular techniques. Quisqualate (QUIS) responses and evoked field potentials were not affected by DX. Epileptiform activity elicited in Mg2+-free solution was suppressed by DX. Dextrorphan had no effect on resting membrane potential or input resistance. The antagonism of NMA by DX was dose-dependent with an EC50 of 0.65 microM; DM was also effective but considerably less potent. In the paradigm used in the present study, DX did not produce the clear use-dependent block observed in the presence of MK-801. These data suggest that DX, the metabolite of the widely used antitussive DM, is a potent NMDA antagonist with a potential role as an anticonvulsant and neuroprotective agent.

Anticonvulsant activity of MK-801 and nimodipine alone and in combination against pentylenetetrazole and strychnine

The effects of the N-methyl-D-aspartate receptor antagonist MK-801 and the dihydropyridine calcium channel antagonist nimodipine were assessed for their anticonvulsant activity alone and in combination against clonic convulsions to pentylenetetrazole (PTZ) and strychnine (STR) in mice. Nimodipine (2-20 mg/kg) and MK-801 (0.1 and 0.5 mg/kg) did not affect the number of mice displaying clonic convulsions to PTZ. However, nimodipine in a dose-dependent manner increased (100%) the latency to clonic convulsions and lethality (mortality from tonic extension convulsions and respiratory failure) following PTZ. In contrast, MK-801 did not increase the latency to PTZ convulsions, but prevented the lethal effects of PTZ. When combined, MK-801 and nimodipine produced a significant reduction in the number of animals (40-60%) displaying PTZ convulsions and a greater increase in the latency to PTZ convulsions than did nimodipine alone. In contrast, MK-801 decreased the onset time, and increased the severity of STR convulsions. A combination of MK-801 and nimodipine which afforded significant protection against PTZ convulsions did not affect STR convulsions. These findings suggest that MK-801 and nimodipine, while possessing significant anticonvulsant activity on their own, produce a potent anticonvulsant synergism against PTZ but not STR.

mRNA from NCB-20 cells encodes the N-methyl-D-aspartate/phencyclidine receptor: a Xenopus oocyte expression study

The mouse neuroblastoma--Chinese hamster brain hybrid cell line NCB-20 is the only clonal cell line in which binding studies indicate the presence of phencyclidine (PCP) receptors. We report here that Xenopus oocytes injected with NCB-20 cell poly(A)+ RNA express N-methyl-D-aspartate (NMDA)-activated channels and that these channels include the PCP receptor site. In injected oocytes, NMDA application evoked a partially desensitizing inward current that was potentiated by glycine, blocked by the competitive antagonist D-2-amino-5-phosphonovaleric acid, blocked by Mg2+ and by Zn2+, and blocked in a use-dependent manner by the PCP receptor ligands PCP and MK-801. There was little or no response to kainate or quisqualate (agonists of the other excitatory amino acid receptors), to gamma-aminobutyric acid (an inhibitory transmitter), or to glycine (an inhibitory transmitter as well as an allosteric potentiator of NMDA channels). Thus, NMDA/PCP receptors expressed from NCB-20 cell mRNA exhibit properties similar to those of the neuronal receptors. The absence of expression of other excitatory amino acid receptors in this system makes it particularly useful for study of NMDA-evoked responses without interference from responses mediated by other receptors. Moreover, NCB-20 mRNA may be an appropriate starting material for cloning the cDNA(s) encoding the NMDA/PCP-receptor complex.

Effects of ketamine on metrazol-induced seizures during ontogenesis in rats

An anticonvulsant action of ketamine, a noncompetitive NMDA receptor antagonist (1-40 mg/kg IP), on the metrazol-induced seizures was assessed in male Wistar rats aged 7, 12, 18, 25 and 90 days. Ketamine alone caused ataxia even in the lowest dose used. As concerens its interaction with metrazol it exerted a clearcut anticonvulsant effect against generalized tonic-clonic seizures at all developmental stages. On the contrary, the effects on clonic (i.e., minimal) seizures were only moderate or absent. Higher efficacy of ketamine was observed in young animals. Our results suggest a role of excitatory amino acids in the generation of generalized tonic-clonic metrazol seizures, but their share on the induction of clonic (minimal) seizures seems to be very small.

Phencyclidine (PCP) reduces the intensity of caffeine-induced convulsions in rats

The effects of phencyclidine (PCP) on the threshold and intensity of caffeine-induced convulsions in rats were examined. There was a dose-dependent effect of PCP on convulsion intensity with significant reduction in intensity at 4.0 and 8.0 mg/kg PCP. At 16.0 mg/kg PCP, convulsant intensity was reduced in 50% of subjects but potentiated to the point of death in the remaining 50%. PCP had no significant effect on threshold for caffeine-induced convulsions. These results suggest that PCP antagonizes caffeine-induced convulsions and further suggests that the mechanisms involved in onset of caffeine-induced convulsions and the decrease of convulsion intensity are pharmacologically dissociable.

Drug discrimination based on the competitive N-methyl-D-aspartate antagonist, NPC 12626

The discriminative stimulus effects of the N-methyl-D-aspartate (NMDA) antagonists 3-([+/-]-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and phencyclidine were assessed in a drug discrimination based on the competitive NMDA antagonist 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 12626). Adult male Sprague-Dawley rats were trained to discriminate NPC 12626 from saline using a standard two-lever fixed ratio 32 schedule of food reinforcement. NPC 12626 dose-dependently substituted for the training dose (20 mg/kg IP) with an ED50 of 9.5 mg/kg. The competitive NMDA antagonist CPP completely substituted for NPC 12626 (ED50 = 1.4 mg/kg IP). The non-competitive NMDA antagonist phencyclidine, as well as pentobarbital and NMDA, failed to substitute completely for NPC 12626, even at doses of these drugs that reduced response rates. These data indicate that the discriminative stimulus properties of NPC 12626 are selective and shared by CPP but not by phencyclidine, pentobarbital or NMDA. The emerging evidence for differences in the discriminative stimulus effects of competitive NMDA antagonists and phencyclidine suggests that competitive antagonists such as NPC 12626 and CPP may not have phencyclidine-like abuse liability.