CGP 37849 and CGP 39551: novel and potent competitive N-methyl-D-aspartate receptor antagonists with oral activity

1. The pharmacological properties of CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid; 4-methyl-APPA) and its carboxyethylester, CGP 39551, novel unsaturated analogues of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoate (AP5), were evaluated in rodent brain in vitro and in vivo. 2. Radioligand binding experiments demonstrated that CGP 37849 potently (Ki 220 nM) and competitively inhibited NMDA-sensitive L-[3H]-glutamate binding to postsynaptic density (PSD) fractions from rat brain. It inhibited the binding of the selective NMDA receptor antagonist, [3H]-((+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), with a Ki of 35 nM, and was 4, 5 and 7 fold more potent than the antagonists [+/-)-cis-4-phosphonomethylpiperidine-2-carboxylic acid) (CGS 19755), CPP and D-AP5, respectively. Inhibitory activity was associated exclusively with the trans configuration of the APPA molecule and with the D-stereoisomer. CGP 39551 showed weaker activity at NMDA receptor recognition sites and both compounds were weak or inactive at 18 other receptor binding sites. 3. CGP 37849 and CGP 39551 were inactive as inhibitors of L-[3H]-glutamate uptake into rat brain synaptosomes and had no effect on the release of endogenous glutamate from rat hippocampal slices evoked by electrical field stimulation. 4. In the hippocampal slice in vitro, CGP 37849 selectively and reversibly antagonized NMDA-evoked increases in CA1 pyramidal cell firing rate.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of CGP 37849 and CGP 39551, competitive NMDA receptor antagonists, in the forced swimming test.

The present study examined the effects of CGP 37849 and CGP 39551, competitive NMDA receptor antagonists, in the forced swimming test in rats and mice. Administered in a single dose or three times both examined compounds reduced the immobility time in rats. Active doses used in that test either did not change the locomotor activity or decreased it. A similar effect in both tests was shown by active (R)-enantiomers CGP 40116 and CGP 43487. Reduction of the immobility time induced by CGP 37849 and CGP 39551 in the forced swimming test in rats was antagonized by haloperidol and (+/-)-sulpiride, but not by SCH 23390 or prazosin. CGP 37849, but not CGP 39551, also reduced the immobility time in the forced swimming test in mice. The results obtained indicate that CGP 37849 and CGP 39551 induce an antidepressant-like effect in the forced swimming test, probably via an indirect dopamine activation resulting from blockade of NMDA receptors.     

The competitive NMDA receptor antagonists CGP 37849 and CGP 39551 are potent,orally-active anticonvulsants in rodents

Summary Anticonvulsant properties of CGP 37849 and CGP 39551, two novel phosphono-amino acids which are competitive NMDA receptor antagonists, were examined in rodents. At optimal pretreatment times CGP 37849 suppressed electroshock-induced seizures in mice and rats with ED₅₀ ^s ranging from 8 to 22 mg/kg after oral administration, and 0.4 to 2.4 mg/kg after i. v. and i. p. injection. Relative to CGP 37849, CGP 39551 was more potent after p. o. (ED₅₀ 3.7–8.1 mg/kg), and less potent after i.v. or i.p. treatment (ED₅₀ 2.7–8.7 mg/kg). Following oral treatment, the duration of action of CGP 37849 was about 8 h, while CGP 39551 still showed good activity after 24 h (ED₅₀ 8.7 mg/kg, mouse; 21 mg/kg, rat). Both compounds were anticonvulsant at doses below those at which overt behavioural side effects were apparent. CGP 39551 delayed the development of kindling in rats at doses of 10 mg/kg p. o. and above, and showed weak anticonvulsant activity against pentylenetetrazolevoked seizures. CGP 37849 and CGP 39551 are the first competitive NMDA antagonists to show oral anticonvulsant properties in a therapeutically-useful dose-range, and hence are interesting candidates for novel antiepileptic therapy in man. Send offprint requests to M. Schmutz at the above address     

Weak anticonvulsant activity of CGP 37849 and CGP 39551 against kindled seizures following systemic administration.

The anticonvulsant and behavioural actions of CGP 37849 and CGP 39551, two novel competitive NMDA receptor antagonists, were examined in fully amygdala kindled rats following systemic administration. Only weak anticonvulsant effects were observed following either i.p. or i.v. injection of the antagonists. Moreover, behavioural abnormalities (ataxia, hyperactivity, muscular hypotonia) were apparent at all anticonvulsant doses. These results suggest that CGP 37849 and CGP 39551 may be of limited therapeutic usefulness against complex partial seizures in man, the seizure type showing greatest refractoriness to presently available medication.     

The effects of cyclic dicarboxylic acids on spontaneous and amino acid-evoked activity of rat cortical neurones.

1 A series of cyclic dicarboxylic acids were applied by microiontophoresis to neurones in the cerebral cortex of rats anaesthetized with urethane. The object was to examine effects on spontaneous firing rates and any ability to antagonize responses to excitatory amino acids. 2 At relatively low ejecting currents (10-25 nA) cis-2,3-piperidine dicarboxylic acid (cis-2,3-PDA) had no effect on spontaneous firing but selectively antagonized the excitation evoked by n-methyl-D-aspartate (NMDA) without affecting responses to quisqualaife or kainate. At higher ejecting currents (60-100 nA) responses to all three agonists were reduced. 3 Other cis-piperidine dicarboxylic acids and piperazine-2,3-dicarboxylic acid had only weak and variable effects on cell firing and responses to NMDA, quisqualate, kainate, glutamate and aspartate. 4 2, 3-Pyridine dicarboxylic acid (quinolinic acid) produced excitation of all cortical neurones tested. 5 2-Amino-5-phosphono-valeric acid, an NMDA antagonist, reduced responses to quinolinate, implying that this compound can act at NMDA receptors. 6 It is suggested that quinolinic acid may be of physiological interest as a potential endogenous excitant in the nervous system and that cis-2,3-PDA and its N-methyl derivative may be of use in studies of receptor pharmacology and the identification of synaptic transmitters.     

Competitive and non-competitive NMDA receptor antagonists in spatial learning tasks.

The effects of the competitive N-methyl-D-aspartate (NMDA) receptor antagonists, CGP37849 (3 or 6 mg/kg i.p.) and its ethyl ester CGP39551 (5 or 15 mg/kg i.p.) and of the non-competitive NMDA receptor antagonist, dizocilpine (0.16 mg/kg; i.p.) on acquisition by rats of different spatial orientation tasks in an 8-arm radial maze were evaluated. Neither of the CGP compounds influenced locomotor activity during spontaneous alternation, only dizocilpine increased the number of arm entries (locomotion). Preferred angles between consecutive arm entries were changed by the high doses of CGP37849 (6 mg/kg), CGP39551 (15 mg/kg) and dizocilpine (0.16 mg/kg). The high doses of both CGP compounds as well as dizocilpine produced impairments in the acquisition of an egocentric orientation task and an allocentric reversal task indicated by an increased number of arm entries and re-entries. Such amnesic effects did not occur after administration of low doses of CGP37849 (3 mg/kg) and CGP39551 (5 mg/kg), doses which are sufficient to produce anticonvulsant and anticataleptic effects. In contrast, the non-competitive NMDA receptor antagonist, dizocilpine, produced amnesic effects over the entire behaviourally effective dose range.     

Anticonvulsant activity of competitive antagonists of NMDA receptor in genetically epilepsy-prone rats.

The potency of some competitive antagonists of N-methyl-D-aspartate (NMDA) to antagonize audiogenic seizures was evaluated in genetically epilepsy-prone rats following intraperitoneal or oral administration. The anticonvulsant effects were evaluated on seizures evoked by auditory stimulation (109 dB, 12-16 kHz) in animals placed singly under a perspex dome. Sixty and hundred-twenty minutes after intraperitoneal or 120 min after oral administration all compounds showed antiseizure activity with ED50 against clonus ranging from 11.6 to 384 mumol/kg after intraperitoneal and higher doses after oral administration. DL-(E)-2-Amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849) and its carboxyethylester (CGP 39551) and 3-((+/-)-2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid (CPPene), were the most potent compounds when administered intraperitoneally and orally. 3-((+/-)-2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid ((-)-CPP) and (+)-CPP showed minor potency as anticonvulsants and 2-amino-7-phosphonoheptanoic acid (2AP7) was the least potent. Following oral treatment, the duration of action of CPPene was about 8 h, while CGP 39551 still protected against audiogenic seizures after 16 h. All compounds were anticonvulsant at doses below those at which overt behavioural side-effects were apparent. CPPene and (+/-)-CPP showed the best therapeutic index among the NMDA receptor antagonists studied. Since some of these compounds showed anticonvulsant properties after oral administration, potential use of these or other selective NMDA antagonists for antiepileptic therapy in man is suggested.     

Sensitivity of hippocampal neurones to kainic acid, and antagonism by kynurenate.

1. The sensitivity to kainic acid of neurones in the CA1 and CA3 regions of rat hippocampal slices has been examined by microiontophoresis and by superfusion methods. 2. When the iontophoretic currents needed to produce comparable plateaux of firing were compared, neurones in the pyramidal cell layer of the CA3 region were approximately 5 times more sensitive than cells in the CA1 region. No difference was noted in sensitivity to N-methyl-D-aspartate (NMDA) or quisqualate. 3. When kainate was superfused at known concentrations, the threshold for eliciting excitation in CA1 was 2.1 microM. The threshold concentration in CA3 was 0.24 microM. 4. Two weeks after the stereotaxic intrahippocampal injection of colchicine, the granule cells of the dentate gyrus and thus the mossy fibre projections to CA3 were destroyed. In slices prepared from animals thus treated the threshold concentration of kainate for eliciting excitation had risen to 1.64 microM. 5. Kainate was less effective in promoting the development of epileptiform bursts of neuronal firing in colchicine-treated slices than in controls. 6. Kynurenic acid antagonized the excitation of CA1 neurones elicited by kainate, NMDA or quisqualate. In the CA3 region kynurenate antagonized selectively responses to microiontophoretic NMDA, with little effect on responses to kainate or quisqualate. 7. In slices taken from colchicine-treated rats kynurenate was able to block responses to kainate in the CA3 area in parallel with responses to NMDA. 8. Taken together the results suggest that the excitatory responses to kainate in the CA3 region may be partly due to a presynaptic action on mossy fibre terminals to release endogenous amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protection from kainic acid neuropathological syndrome by NMDA receptor antagonists: effect of MK-801 and CGP 39551 on neurotransmitter and glial markers.

Systemic administration of kainic acid results in the development of a characteristic convulsive syndrome, accompanied by neuropathological alterations and loss of transmitter markers in some forebrain regions. Since some of these effects appear to involve the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptors, the protection given by a non-competitive (MK-801) and a competitive (CGP 39551) NMDA receptor antagonist against the loss of glutamatergic and gamma-amino butyric acid (GABAergic) neurochemical markers was compared. Appropriate doses of both compounds (1 mg/kg MK-801 and 25 mg/kg CGP 39551) completely reversed the decrease of high affinity uptake of glutamate and activity of glutamate decarboxylase in the olfactory cortex, amygdala, hippocampus and lateral septum. In addition, they also essentially counteracted the increase of a glial marker, the enzyme glutamine synthetase, consequent to neuronal degeneration. The results confirmed that involvement of NMDA receptors is essential for the full expression of neuropathological effects of kainic acid. They also support the use of a competitive antagonist of the NMDA receptor, such as CGP 39551, to afford substantial protection against the excitotoxic damage, whilst giving fewer side effects and motor disturbances than MK-801.     

The influence of the benzodiazepine receptor antagonist flumazenil on the anxiolytic-like effects of CGP 37849 and ACPC in rats

In this paper we examined the effect of flumazenil (Ro 15-1788, 10 mg/kg), a benzodiazepine receptor antagonist, on the anticonflict activity of DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, and 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at glycine(B) receptors, in the Vogel conflict drinking test in rats. The effect of flumazenil on the anxiolytic-like (in the plus-maze test) and the anticonvulsant (in the maximal electroshock-induced seizures) activities of CGP 37849 in rats was also studied. Diazepam was used as a reference drug. CGP 37849 (2. 5-5 mg/kg), ACPC (50-200 mg/kg) and diazepam (2.5-5 mg/kg) significantly and dose-dependently increased the number of shocks accepted during experimental sessions in the conflict drinking test. Flumazenil partly but significantly reduced the anticonflict effect of CGP 37849, and it fully blocked the anticonflict effect of ACPC and diazepam. CGP 37849 (2.5-5 mg/kg) and diazepam (2.5-5 mg/kg) were also active in the plus-maze test, as they significantly increased the percentage of the time spent in and entries into the open arms of the plus-maze, both those effects having been antagonized by flumazenil. Flumazenil alone was inactive in both the conflict drinking and the plus-maze tests. In the maximal electroshock-induced seizures, both CGP 37849 (2.5-5 mg/kg) and diazepam (5-10 mg/kg) produced anticonvulsant effects, of which only that of diazepam was antagonized by flumazenil. The results of the present study showing antagonism of flumazenil towards the anxiolytic-like effects of CGP 37849 and ACPC suggest involvement of benzodiazepine receptors in such an activity of the NMDA and glycine(B) receptor ligands, respectively, which may be due to a possible interaction between NMDA and GABA/benzodiazepine systems. The lack of effect of the benzodiazepine antagonist on the anticonvulsant activity of CGP 37849 indicates that involvement of benzodiazepine receptors in the pharmacological action of the NMDA antagonist is not a general phenomenon.     

A quantitative study of the actions of excitatory amino acids and antagonists in rat hippocampal slices.

1. A quantitative pharmacological investigation of the actions of excitatory amino acids on hippocampal CA1 neurones has been made using a new slice preparation developed for grease gap recording; d.c. potential was measured across a grease barrier placed between alvear fibres and the bathing medium. 2. In Mg2+-free perfusate, N-methyl-D-aspartate (NMDA, 1-100 microM), quisqualate (1-500 microM), kainate (1-200 microM) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1-100 microM) caused dose-dependent depolarizations. 3. The dose-response relationships were fitted to logistic expressions. The maximum responses to AMPA, NMDA and kainate were similar; their respective EC50 values were 5, 13 and 23 microM. Quisqualate had a smaller maximum; its EC50 value was 10 microM. The slopes of the dose-response relationships were different for the 4 agonists; the order of steepness of the slopes was NMDA greater than AMPA greater than kainate greater than quisqualate. 4. Similar amino acid-induced depolarizations were observed in slices of just the CA1 region or in whole slices bathed in tetrodotoxin. Isolated alvear fibres, however, were insensitive to the excitatory amino acids. 5. D-2-Amino-5-phosphonovalerate (APV, 50 microM) selectively and reversibly antagonized responses induced by NMDA (apparent pA2 = 5.21). 6. Kynurenic acid (1 mM) reversibly depressed responses to the three agonists tested. The dose-ratios for antagonism of AMPA, kainate and quisqualate were 6.9, 5.6 and 4.6 respectively. 7. This preparation has a different sensitivity profile to agonists from those of previously reported preparations of spinal cord, neocortex and cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Competitive NMDA receptor antagonists and agonists: effects on spontaneous alternation in mice exposed to cerebral oligemia

The purpose of the present study was to investigate the effects of competitive NMDA receptor antagonists,D,L-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849) and its ethyl ester (CGP 39551), or agonist, N-methyl-D-aspartate (NMDA) on spontaneous alternation in mice exposed to cerebral oligemia. Alternation behavior was evaluated in an Y-maze. Transient cerebral oligemic hypoxia was induced by bilateral clamping of carotid arteries (BCCA) for 30 min under pentobarbital anesthesia. In BCCA mice, CGP 37849 (5 mg/kg, ip) impaired spontaneous alternation when given 48 h or 7 days after surgery. CGP 39551 (5 mg/kg, ip) had no effect.NMDA (50 mg/kg, sc) improved performance of the task in BCCA mice when tested 48 h after surgery. These results suggest that cerebral oligemic hypoxia induced by BCCA leads to functional disturbances in the central nervous system, such as spontaneous alternation impairment and increased susceptibility to NMDA receptor-related drugs. Adverse potential of cerebral oligemia may limit a therapeutic use of NMDA receptor antagonists.     

Ketamine acts as a non-competitive N-methyl-D-aspartate antagonist on frog spinal cord in vitro

The effects of the dissociative anaesthetic, ketamine, the divalent cation, magnesium and the organic antagonist, 2-amino-5-phosphonovalerate (APV), were examined on responses of motoneurones to excitatory amino acids in the hemisected spinal cord of the frog in vitro. The amino acids, N-methyl-D-aspartate (NMDA), kainate and quisqualate, produced dose-dependent depolarizations. Ketamine, magnesium and APV depressed responses to NMDA but had no effect on those to quisqualate and kainate. Magnesium and APV produced Schild plot slopes that were not different from unity, whereas ketamine had a slope significantly less than unity, indicative of a non-competitive action. Experiments in which combinations of drugs were tested indicated that these substances act by three distinct mechanisms to cause antagonism of the actions of NMDA.     

Excitatory amino acid-induced convulsions in neonatal rats mediated by distinct receptor subtypes

We found that N-methyl-D-aspartate (NMDA) was a very potent, systematically active convulsant in the rat in the early period of postnatal development (7-11 days of age). Other receptor subtype-selective excitatory amino acid agonists were then examined for their convulsant effects following i.p. administration to neonatal rats. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) was the most potent convulsant (ED50 0.6 mg/kg), followed by kainate (ED50 1.5 mg/kg), N-methyl-D-aspartate (NMDA) (ED50 3.1 mg/kg), then quisqualate (ED50 5.1 mg/kg). NMDA-induced convulsions were antagonized in a dose-related manner by prior administration of the NMDA antagonists cis-(+/- )-4-phosphonomethyl-2-piperidine carboxylic acid (CGS19755), cis-(+/- )-4-(2H-tetrazol-5-yl)methyl-piperidine-2-carboxylic acid (LY233053), (+/- )3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), D,L-2-amino-5-phosphonovalerate (D,L-AP5) and MK801. NMDA antagonists did not protect against AMPA- or kainate-induced convulsions. 6,7-Dinitroquinoxaline-2,3-dione (DNQX) selectively prevented the effect of AMPA at doses which had no effect on NMDA or kainate convulsions. Quisqualate-induced convulsions were antagonized by NMDA antagonists or DNQX. The greater sensitivity of neonatal rats to systemically administered excitatory amino acid agonists appears useful for evaluating the selectivity of antagonists acting at ionotropic excitatory amino acid receptors in the central nervous system. Using neonatal rats three pharmacologically distinct excitatory amino acid receptor effects were demonstrated following administration of NMDA, AMPA or kainate.     

A comparison between the in vivo and in vitro activity of five potent and competitive NMDA antagonists.

1. Phosphonate analogues of glutamate have been tested and compared as N-methyl-D-aspartate (NMDA) antagonists in electrophysiological and binding experiments. The compounds tested were three established NMDA antagonists: D-2-amino-5-phosphonopentanoate (D-AP5), DL-2-amino-7-phosphonoheptanoate (DL-AP7), 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), and two novel putative NMDA antagonists: 3-(2-carboxypiperidin-4-yl)propyl-1-phosphonate (CPPP) and 3-(2-carboxy-piperidin-4-yl)methyl-1-phosphonate (CPMP). 2. When administered electrophoretically to rat spinal neurones in vivo, these compounds were found to be selective NMDA antagonists with little effect on excitations evoked by quisqualate and kainate. CPMP and CPPP were approximately equipotent with CPP and about 5 times more potent than D-AP5. 3. Following systemic administration, 2-5 mg kg-1 i.v. of CPP, CPMP and CPPP reduced NMDA-evoked excitations by 70-100% whereas 50-100 mg kg-1 of D-AP5 and DL-AP7 produced a similar effect. The onset of the effects required 20-30 min and lasted more than six hours. 4. On bath application to cortical wedges, the IC50 values (microM) for antagonism of 40 microM NMDA were: CPP, 0.64 +/- 0.06 (mean +/- s.e.mean; n greater than 4); CPMP, 1.65 +/- 0.13; CPPP 0.89 +/- 0.09; D-AP5, 3.7 +/- 0.32; DL-AP7, 11.1 +/- 2.1; and DL-AP4 and DL-AP6 were inactive at 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antagonism of monosynaptic excitations in the mouse olfactory cortex slice by 6,7-dinitroquinoxaline-2,3-dione

The effects of 6,7-dinitroquinoxaline-2,3-dione (DNQX) have been tested in slices of olfactory cortex of the mouse against responses evoked by N-methyl-D-aspartate, kainate and quisqualate and on the surface field potentials evoked on electrical stimulation of the lateral olfactory tract. At a concentration of 5 microM, DNQX competitively antagonized responses evoked by kainate and quisqualate, with only a small reduction in the responses to N-methyl-D-aspartate. In contrast, DL-(+-)-2-amino-5-phosphonopentanoic acid (APP, 50 microM) selectively antagonized depolarizations to N-methyl-D-aspartate. The amplitude of the field potential known as the N-wave was reduced by DNQX in a concentration-dependent reversible manner (IC50 = 2.92 +/- 0.33 microM; mean +/- SE mean, n = 4). DL-(+-)-2-Amino-5-phosphonopentanoic acid (50 microM) did not significantly affect this action of DNQX. It is concluded that DNQX inhibits monosynaptic excitations in the olfactory cortex by selectively blocking kainate and/or quisqualate receptors, although it is unclear whether the receptors are located at pre- and/or postsynaptic sites.     

Selective potentiating effect of beta-p-chlorophenylglutamate on responses induced by certain sulphur-containing excitatory amino acids and quisqualate

In dorsal horn neurones of the cat spinal cord iontophoretically administered (+/-)-beta-p-chlorophenylglutamate (chlorpheg) markedly enhanced the excitatory responses induced by L-homocysteate, L-homocysteine sulphinate, S-sulpho L-cysteine, L-cysteate and quisqualate, while responses to NMDA, kainate, L-glutamate, L-aspartate and L-cysteine sulphinate were generally unaffected. Preliminary data obtained on frog spinal cord in vitro supports the possibility that such selective potentiation may be due to differential inhibition by chlorpheg of amino acid uptake. No potentiating effects of chlorpheg were observed on spinal synaptic excitation.     

Differential effects of CGP 37849 and MK-801, competitive and noncompetitive NMDA antagonists,with respect to the modulation of sensorimotor gating and dopamine outflow in the prefrontal cortex of rats

In the present study we compared effects of the competitive and non-competitive NMDA antagonists CGP 37849 and MK-801, respectively, on sensorimotor gating in rats, measured as prepulse-induced inhibition of the acoustic startle response, and the outflow of dopamine in the rat prefrontal cortex. CGP 37849 (10, 20 mg/kg), decreased the amplitude of the acoustic startle response, but was without effect on the prepulse-induced inhibition of the acoustic startle response. MK-801 (0.4 but not 0.2 mg/kg) enhanced the amplitude of the acoustic startle response and its doses of 0.2 and 0.4 mg/kg markedly attenuated the prepulse-induced inhibition of the acoustic startle response. The effects of MK-801 (0.4 mg/kg) on the prepulse-induced inhibition of the acoustic startle response were not antagonized by the selective antagonists of D-2 and D-1 dopaminergic receptors, S(–)sulpiride (25 mg/kg) and SCH 23390 (0.1 mg/kg), respectively. When given alone, S(–)sulpiride attenuated the amplitude of the acoustic startle response and failed to altered the prepulse-induced inhibition of the acoustic startle response. SCH 23390 (0.1 mg/kg) failed to alter the amplitude and prepulse-induced inhibition of the acoustic startle response. The effects of CGP 37849 and MK-801 also differed with respect to dopamine outflow. MK-801 (0.2 and 0.4 mg/kg) enhanced the outflow of dopamine in the rat prefronatl cortex, while CGP 37849 (10 and 20 mg/kg) was without any effect on the extracellular concentration of dopamine. Our data indicate that the blockade of phencyclidine binding sites, exerted by the noncompetitive antagonist MK-801, evoked effects qualitatively different from those induced — via blockade of the NMDA recognition — by the competitive NMDA receptor antagonist CGP 37849. It is postulated that — in contrast to the non-competitive antagonist of NMDA receptors — the competitive NMDA antagonist CGP 37849 is/ should be devoid of psychotomimetic and abusing properties. It is also evident that disruption of sensorimotor gating in rats induced by MK-801 does not involve any dopaminergic mechanisms, since it is not modulated by drugs blocking D-1 and D-2 dopamine receptors. Correspondence to: K. Wdzony at the above address     

6-Cyano-7-nitroquinoxaline-2,3-dione as an excitatory amino acid antagonist in area CA1 of rat hippocampus.

1. A quantitative pharmacological investigation of the excitatory amino acid antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) has been made in area CA1 of rat hippocampal slices bathed in 1 mM Mg2+ containing medium. 2. At a concentration of 10 microM, CNQX reversibly antagonized responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualate and kainate; it produced a parallel shift in their log dose-response curves. Responses to N-methyl-D-aspartate (NMDA) were not antagonized by 10 microM CNQX (dose-ratio: 1.04 +/- 0.06, n = 3). 3. Schild plots (constructed over the range 1-100 microM) yielded the following estimated pA2 values, AMPA 5.8, quisqualate 5.9, and kainate 5.9. NMDA was antagonized by 100 microM CNQX, giving an apparent log K of 4.44 +/- 0.06. 4. The slopes (+/- s.e. mean) of the Schild plots were for AMPA 0.84 +/- 0.06, quisqualate 0.79 +/- 0.04 and kainate 0.68 +/- 0.07. These were all significantly less than unity. 5. Synaptic responses elicited by low frequency activation of the Schaffer collateral-commissural pathway were blocked completely by CNQX (10 microM) providing that a low stimulus intensity was used. With high intensity stimulation a small component remained that was blocked by the selective NMDA antagonist D-2-amino-5-phosphonovalerate (APV). 6. These results suggest that CNQX does not differentially affect the responses of CA1 neurones to AMPA, quisqualate and kainate. It does, however, depress responses to these agonists to a greater degree than it does responses to NMDA and it is a highly effective synaptic antagonist.     

Effects of antagonists at the NMDA receptor complex in two models of anxiety

The effects of an antagonist at the strychnine insensitive glycine site (5,7-dichlorokynurenic acid, i.c.v.), and of noncompetitive (MK-801, i.p.) and competitive (CGP 37849, i.p.; CGP 39551, i.p.; AP-7, i.c.v.) NMDA antagonists were compared with diazepam (i.p.) in two animal models of anxiety (the open field exploratory behavior of non-habituated rats, and the Vogel conflict test). All drugs when applied in appropriate doses increased punished drinking in the Vogel test, without producing any significant changes in free drinking and the stimulus threshold at their lowest anticonflict doses. The effective doses were as follows: diazepam 1.5 and 2.5 mg/kg; MK-801 0.005 and 0.01 mg/kg; CGP 39551 5.0 and 20.0 mg/kg; CGP 37849 1.0 and 2.5 mg/kg; 5,7-dichlorokynurenic acid 5.0 μg (i.c.v); AP-7 0.5 μg (i.c.v.). In the open field diazepam (0.05 mg/kg), MK-801 (0.1 mg/kg), CGP 37849 (0.01, 0.1, 1.0 mg/kg), and AP-7 2.5 μg (i.c.v.) significantly increased exploratory activity in the central sectors of the open field (anti-neophobic reaction), without changing motor activity of the rat. MK-801 at the highest tested dose of 0.2 mg/kg significantly stimulated animal locomotor activity. CGP 37849 in the largest dose examined (10 mg/kg) significantly depressed the motor behavior of rats. Overall, it appeared that different NMDA antagonists showed an anxiolytic-like profile, similar to that of the benzodiazepine diazepam. Among different NMDA receptor complex antagonists studied, CGP 37849 was characterized by the largest distinction between the doses showing an anxiolytic-like action in the open field test, and changing rat motor behavior.