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Possible role of excitatory amino acids in the convulsant action of catechol
European Journal of Pharmacology, 145 (1988) 209-212
209
Elsevier EJP 20045
Short communication
Possible role of excitatory amino acids in the convulsant action of catechol D a v i d G. D e w h u r s t Department of Biological Science, Sheffield City Polytechnic, Pond Street, Sheffield $1 l WB, U.K. Received 15 October 1987, accepted 3 November 1987
The effects of several excitatory amino acid receptor antagonists on sensory-evoked electromyographic activity induced by catechol have been studied in urethane-anaesthetised rats. 2-Amino-5-phosphono-valearic acid (1.2 /~mol/kg i.c.), cis-2,3-piperidine dicarboxylic acid (1.4 #mol/kg i.c.), y-D-glutamyl-glycine (2.0 /Lmol/kg i.c.), 2-amino-7-phosphono-heptanoic acid (230 /~mol/kg i.v.) and MK-801 (5 mg/kg i.p.) all significantly decreased the frequency of occurrence of those components of the sensory evoked EMG dependent on supraspinal structures, but were without effect on the spinal component. Catechol; Convulsions; Excitatory amino acid antagonists
1. Introduction
Catechol (1,2-dihydroxybenzene) is a potent convulsant agent with a central site of action. Administration of catechol to anaesthetised rats produce, with increasing dose, tremor, a stimulussensitive state during which brief myoclonic jerks can be evoked by a variety of sensory stimuli, and convulsions. The mechanism of action of catechol is as yet unknown, although it has been shown to increase the release of acetylcholine at the neuromuscular junction by a presynaptic action (Gallagher and Blaber, 1973), to selectively increase the K+-evoked release of tritiated D-aspartate from preloaded slices of rat thalamus and cortex (Minchin and Pearson, 1981), and endogenous aspartate from rat olfactory cortex slices (Collins and Dewhurst, 1986). Preliminary experiments have also indicated that excitatory amino acid antagonists significantly reduced catechol-induced, sensory-evoked convulsions (Dewhurst, 1986). Thus catechol convulsions may result from a facilitation of the release of neurotransmitters such as aspartate at other sites in the CNS.
This work investigates further the action of a number of excitatory amino acid antagonists against sensory-evoked convulsions induced by catechol in the anaesthetised rat.
2. Materials and methods
Female albino rats (Sheffield strain) in the weight range 190-210 g were used in all experiments. Animals were anaesthetised with urethane (1.2-1.4 g / k g i.p.) and convulsions were induced by continuous infusion of catechol (2.5 m g / k g per min) into a jugular vein. Convulsant activity was assessed by measuring the percentage probability of occurrence of each of the three components (M1, M2 and M3) of the sensory-evoked electromyogram (Angel and Lemon, 1973; Dewhurst, 1984). Evoked electromyographic (EMG) activity was recorded from Flexor carpii, by means of needle electrodes inserted into the muscle. Cutaneous sensory afferents were stimulated electrically, stimuli (0.02 ms duration, 0.17 Hz, 12-20 V) being
0014-2999/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
210
applied at the wrist, Recorded E M G responses were amplified, displayed on an oscilloscope (Nicolet 3091), and simultaneously stored on magnetic tape (Trio KX-440). Analysis was performed by measuring the number of times each of the three components occurred per 20 applied stimuli throughout the control period (usually 15 rain) and after administration of a test drug (for a further 40-50 min). The frequency of occurrence of M1, M2 and M3 was expressed as a % probability of occurrence and drug effects were then evaluated by plotting the % probability of occurrence of each component against time, for the post-drug period and subtracting from the area under this curve the area under the control curve, obtained by extrapolation of the mean % probability of occurrence during the control period (see fig. 1). Results are then shown as the % difference in area between control and test curves i.e. ((Test -Control)/Test) × 100 in arbitrary units. Thus test drugs which decrease the probability of occurrence of one of the components of the evoked E M G will show a negative result. All of the test drugs were dissolved in warmed 0.9% NaC1 and the pH adjusted to pH 6-7. 2Amino-5-phosphono-valearic acid (APV), cis-2,3piperidine dicarboxylic acid (PDA) and "/-Dglutamyl glycine (DGG) were slowly injected (in a volume of 0.02 ml over 1 min) intracysternally (i.c.) via the cysterna magna, and the syringe needle left in position. Since this method of administration is imprecise, the exact dose of the drug administered is unknown. Those doses quoted are the maximum the animal could have received
assuming all of the injected volume entered the CNS and none escaped. Control animals were treated identically, receiving 0.9% saline alone. To satisfy any doubts that the effects seen with APV, PDA and D G G might be due to the admnistration procedure, 2-amino-7-phosphono-heptanoic acid (APH), which is known to be active following systemic administration (Rostain et al., 1985), was given i.v. (femoral vein) and (+)-5-methyl-10,11dihdyro-5 H-dibenzo-(a,d)-cyclohepten-5,10 (MK801), an orally active excitatory amino acid antagonist (Kemp et al., 1986), was given i.p. Control animals were again treated identically but received vehicle alone.
3. Results The effects of the amino acid receptor antagonists (APH, APV, PDA, D G G and MK801) on the probability of occurrence of the M1, M2 and M3 components of the catechol-induced sensory evoked electromyogram are shown in table 1. All of the drugs tested significantly decreased the probability of occurrence of the M2 and M3 components but were without effect on M1. None of the drugs significantly affected the latency of each component. The time course of effects was similar for each of the drugs after i.c. administration, and is shown in fig. 1 for APV. The frequency of occurrence of M2 and M3 decreased within 2-4 rain of administration, and peak effects were observed between 20 and 25 min later. Partial re-
TABLE 1 The effects of excitatory amino acid receptor antagonists on the probability of occurrence of the M1, M2 and M3 components of the c a t e c h o l - i n d u c e d s e n s o r y - e v o k e d m u s c l e r e s p o n s e in a n a e s t h e t i s e d rats. R e s u l t s a r e e x p r e s s e d as t h e m e a n % d i f f e r e n c e _ + S.D. in a r e a b e t w e e n c o n t r o l a n d test c u r v e s for e a c h c o m p o n e n t o f t h e e v o k e d E M G . Drug
APV DGG PDA APH MK-801
Dose
M e a n % d i f f e r e n c e _ + S.D.
(/* m o l / k g i.c.)
M1
M2
1.2 2.0 1.4 230.0 (i.v.) 5 m g / k g (i.p.)
-
-53.7+17.4 - 34.2 _+ 19.3 - 66.0 _+ 10.0 - 29.6 _+ 10.6 - 52.1 _+ 18.5
a p < 0.001; b p < 0.01, S t u d e n t ' s p a i r e d t-test.
n M3 a
b a a a
-57.2_+30.7 59.2 _+ 18.5 - 65.0 ~ 11.2 - 22.1 ___13.6 - 63.3 ___11.4 --
b a a b a
4 5 6 5 6
211
covery was seen in all experiments, 40-50 rain after administration of the drug. After APH (i.v.), peak effects were observed 25-30 min after administration. In contrast, with Mk-801 (i.p.) the time course of effects was similar to that of APV administered intracysternally.
4. Discussion
i
,
t
i
i
,
,
20ms
t
M1
T
,2
lOmin
APV
Fig. 1. The effects of APV (1.2 /~mol/kg i.c.) on the sensoryevoked electromyogram induced by systemic infusion of catechol (2.5 m g / k g per min). The records on the left show the evoked EMGs recorded from Flexor carpii, to electrical stimulation of cutaneous afferents at the wrist: (a) before catechol, (b) 10-15 min after catechol, (c) 10-15 rain after APV, (d) 35-40 min after APV. Each record is of 20 superimposed responses. Note the significant decrease in the frequency of occurrence of M2 and M3 in (c) and partial recovery in (d). Note also the absence of any E M G activity before infusion of catechol in (a). The time course of these effects is shown in histogram form below. The results are of a single representative experiment and iullustrate how the results in table 1 were calculated. Similar results were obtained in all other experiments with APV.
The catechol-induced, stimulus-sensitive state, during which brief myoclonic convulsions can be evoked by sensory stimulation, has been shown to be a sensitive model for studying the convulsant action of catechol. The sensory-evoked E M G typically consists of three temporally distinct components, each representing the motor expression of the activation of a different reflex pathway: M1 is the result of a polysynaptic, propriospinal reflex; M2, a long-loop reflex dependent on the integrity of the sensorimotor cortex; and M3 is probably a cerebellar reflex (Angel and Lemon, 1973; Dewhurst, 1984). All of the excitatory amino acid antagonists tested significantly decreased the probability of occurrence of the longer latency components (M2 and M3) of this response but had no effect on M1 (table 1). APH, APV and MK-801 have been shown to antagonise excitation mediated by dicarboxylic amino acids with a preferential action at N-methyl-D-aspartate (NMDA) receptors while PDA and P G G are less selective, with actions at N M D A , kainate and quisqualate receptors (Watkins and Evans, 1981; Kemp et al., 1986). These drugs, particularly those with selective actions at N M D A receptors, have been shown to be anticonvulsant against a number of other experimental models of seizures (Jones et al., 1982; Croucher et al., 1982). The lack of effect on M1, particularly of those drugs given either systemically or parenterally, suggests that the site of action of these agents is supraspinal. A comparison of the time course of action of MK-801 and APH, suggests a rapid penetration of MK-801 into the brain, even after systemic administration, and is consistent with other findings (Kemp et al., 1986). The time course of effects of APH is also consistent with the
212
reported rate of penetration of APH into brain after systemic administration (Rostain et al., 1985). In these experiments, since drugs were administered as single doses and by different routes, it is impossible to make direct comparisons between the different drugs. However all of the excitatory amino acid antagonists significantly decreased the probability of occurrence of M2 and M3, whatever their route of administration, suggesting that synaptic transmission at sites involving aspartate or glutamate is important in the pathways leading to the motor expression of these components. Equally, since amino acids play an important excitatory role in the CNS, it is possible that the convulsive response is decreased as a consequence of the generalised CNS depression, which would result from antagonism of excitatory amino acid transmission. There is evidence of catechol causing a selective increase in the release of aspartate from cortex and olfactory cortex slices of the rat (Minchin and Pearson, 1981; Collins and Dewhurst, 1986), possibly by a presynaptic action. A similar action at other sites in the CNS could explain the convulsant action of catechol and the protective action of the excitatory amino acid antagonists. In conclusion, excitatory amino acid antagonists significantly reduce the probability of occurrence of those components of the catechol-induced EMG dependent on supraspinal structures, indicating that excitatory amino acids may be involved as neurotransmitters in the pathways subserving these responses.
References Angel, A. and R.N. Lemon, 1973, An analysis of the myoclonic jerks produced by 1,2 dihydroxybenzene in the rat, Electroenceph. Clin. Neurophysiol. 35, 589. Collins, G.G.S. and D.G. Dewhurst, 1986, Sites and mechanisms of action of catechol (1,2 dihydroxybenzene) in the rat olfactory cortex slice, Br. J. Pharmacol. 88, 433. Croucher, M.J., J.F. Collins and B.S. Meldrum, 1982, Anticonvulsant action of excitatory amino-acid antagonists, Science 216 (4584), 899. Dewhurst, D.G., 1984, Some characteristics of the long-latency component of the evoked muscle response induced by administration of catechol to the anaesthetised rat: a neurophysiological and neuropharmacological investigation, Br. J. Pharmacol. 83, 83. Dewhurst, D.G., 1986, Effects of excitatory amino acid antagonists on catechol-induced seizures, Br. J. Pharmacol. Proc. (Suppl.) 89, 640P. Gallagher, J.P. and L.C. Blaber, 1973, Catechol, a facilitatory drug that demonstrates only a prejunctional site of action, J. Pharmacol. Exp. Ther. 184, 129. Jones, A.W., M.J. Croucher, B.S. Meldrum and J.C. Watkins, 1984, Suppression of audiogenic seizures in DBA/2 mice by two new dipeptide N M D A receptor antagonists, Neurosci. Lett. 45, 157. Kemp, J.A., T. Priestley and G.N. Woodruff, 1986, MK-801, a novel, orally active anticonvulsant, is a potent, non-competitive N-methyl-D-aspartate-receptor antagonist, Br. J. Pharmacol. Proc. (Suppl.) 89, 535P. Minchin, M.C.W. and G. Pearson, 1981, The effect of the convulsant agent, catechol, on transmitter uptake and release in rat brain slices, Br. J. Pharmacol. 74, 715. Rostain, J.C., B. Wardley-Smith and B.S. Meldrum, 1985, Effects of 2-amino-7-phosphonoheptanoic acid on EEG of rats, Electroenceph. Clin. Neurophysiol. 60, 367. Watkins, J.C. and R.H. Evans, 1981, Excitatory amino acid transmitters, Ann. Rev. Pharmacol. Toxicol. 21, 165.
209
Elsevier EJP 20045
Short communication
Possible role of excitatory amino acids in the convulsant action of catechol D a v i d G. D e w h u r s t Department of Biological Science, Sheffield City Polytechnic, Pond Street, Sheffield $1 l WB, U.K. Received 15 October 1987, accepted 3 November 1987
The effects of several excitatory amino acid receptor antagonists on sensory-evoked electromyographic activity induced by catechol have been studied in urethane-anaesthetised rats. 2-Amino-5-phosphono-valearic acid (1.2 /~mol/kg i.c.), cis-2,3-piperidine dicarboxylic acid (1.4 #mol/kg i.c.), y-D-glutamyl-glycine (2.0 /Lmol/kg i.c.), 2-amino-7-phosphono-heptanoic acid (230 /~mol/kg i.v.) and MK-801 (5 mg/kg i.p.) all significantly decreased the frequency of occurrence of those components of the sensory evoked EMG dependent on supraspinal structures, but were without effect on the spinal component. Catechol; Convulsions; Excitatory amino acid antagonists
1. Introduction
Catechol (1,2-dihydroxybenzene) is a potent convulsant agent with a central site of action. Administration of catechol to anaesthetised rats produce, with increasing dose, tremor, a stimulussensitive state during which brief myoclonic jerks can be evoked by a variety of sensory stimuli, and convulsions. The mechanism of action of catechol is as yet unknown, although it has been shown to increase the release of acetylcholine at the neuromuscular junction by a presynaptic action (Gallagher and Blaber, 1973), to selectively increase the K+-evoked release of tritiated D-aspartate from preloaded slices of rat thalamus and cortex (Minchin and Pearson, 1981), and endogenous aspartate from rat olfactory cortex slices (Collins and Dewhurst, 1986). Preliminary experiments have also indicated that excitatory amino acid antagonists significantly reduced catechol-induced, sensory-evoked convulsions (Dewhurst, 1986). Thus catechol convulsions may result from a facilitation of the release of neurotransmitters such as aspartate at other sites in the CNS.
This work investigates further the action of a number of excitatory amino acid antagonists against sensory-evoked convulsions induced by catechol in the anaesthetised rat.
2. Materials and methods
Female albino rats (Sheffield strain) in the weight range 190-210 g were used in all experiments. Animals were anaesthetised with urethane (1.2-1.4 g / k g i.p.) and convulsions were induced by continuous infusion of catechol (2.5 m g / k g per min) into a jugular vein. Convulsant activity was assessed by measuring the percentage probability of occurrence of each of the three components (M1, M2 and M3) of the sensory-evoked electromyogram (Angel and Lemon, 1973; Dewhurst, 1984). Evoked electromyographic (EMG) activity was recorded from Flexor carpii, by means of needle electrodes inserted into the muscle. Cutaneous sensory afferents were stimulated electrically, stimuli (0.02 ms duration, 0.17 Hz, 12-20 V) being
0014-2999/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
210
applied at the wrist, Recorded E M G responses were amplified, displayed on an oscilloscope (Nicolet 3091), and simultaneously stored on magnetic tape (Trio KX-440). Analysis was performed by measuring the number of times each of the three components occurred per 20 applied stimuli throughout the control period (usually 15 rain) and after administration of a test drug (for a further 40-50 min). The frequency of occurrence of M1, M2 and M3 was expressed as a % probability of occurrence and drug effects were then evaluated by plotting the % probability of occurrence of each component against time, for the post-drug period and subtracting from the area under this curve the area under the control curve, obtained by extrapolation of the mean % probability of occurrence during the control period (see fig. 1). Results are then shown as the % difference in area between control and test curves i.e. ((Test -Control)/Test) × 100 in arbitrary units. Thus test drugs which decrease the probability of occurrence of one of the components of the evoked E M G will show a negative result. All of the test drugs were dissolved in warmed 0.9% NaC1 and the pH adjusted to pH 6-7. 2Amino-5-phosphono-valearic acid (APV), cis-2,3piperidine dicarboxylic acid (PDA) and "/-Dglutamyl glycine (DGG) were slowly injected (in a volume of 0.02 ml over 1 min) intracysternally (i.c.) via the cysterna magna, and the syringe needle left in position. Since this method of administration is imprecise, the exact dose of the drug administered is unknown. Those doses quoted are the maximum the animal could have received
assuming all of the injected volume entered the CNS and none escaped. Control animals were treated identically, receiving 0.9% saline alone. To satisfy any doubts that the effects seen with APV, PDA and D G G might be due to the admnistration procedure, 2-amino-7-phosphono-heptanoic acid (APH), which is known to be active following systemic administration (Rostain et al., 1985), was given i.v. (femoral vein) and (+)-5-methyl-10,11dihdyro-5 H-dibenzo-(a,d)-cyclohepten-5,10 (MK801), an orally active excitatory amino acid antagonist (Kemp et al., 1986), was given i.p. Control animals were again treated identically but received vehicle alone.
3. Results The effects of the amino acid receptor antagonists (APH, APV, PDA, D G G and MK801) on the probability of occurrence of the M1, M2 and M3 components of the catechol-induced sensory evoked electromyogram are shown in table 1. All of the drugs tested significantly decreased the probability of occurrence of the M2 and M3 components but were without effect on M1. None of the drugs significantly affected the latency of each component. The time course of effects was similar for each of the drugs after i.c. administration, and is shown in fig. 1 for APV. The frequency of occurrence of M2 and M3 decreased within 2-4 rain of administration, and peak effects were observed between 20 and 25 min later. Partial re-
TABLE 1 The effects of excitatory amino acid receptor antagonists on the probability of occurrence of the M1, M2 and M3 components of the c a t e c h o l - i n d u c e d s e n s o r y - e v o k e d m u s c l e r e s p o n s e in a n a e s t h e t i s e d rats. R e s u l t s a r e e x p r e s s e d as t h e m e a n % d i f f e r e n c e _ + S.D. in a r e a b e t w e e n c o n t r o l a n d test c u r v e s for e a c h c o m p o n e n t o f t h e e v o k e d E M G . Drug
APV DGG PDA APH MK-801
Dose
M e a n % d i f f e r e n c e _ + S.D.
(/* m o l / k g i.c.)
M1
M2
1.2 2.0 1.4 230.0 (i.v.) 5 m g / k g (i.p.)
-
-53.7+17.4 - 34.2 _+ 19.3 - 66.0 _+ 10.0 - 29.6 _+ 10.6 - 52.1 _+ 18.5
a p < 0.001; b p < 0.01, S t u d e n t ' s p a i r e d t-test.
n M3 a
b a a a
-57.2_+30.7 59.2 _+ 18.5 - 65.0 ~ 11.2 - 22.1 ___13.6 - 63.3 ___11.4 --
b a a b a
4 5 6 5 6
211
covery was seen in all experiments, 40-50 rain after administration of the drug. After APH (i.v.), peak effects were observed 25-30 min after administration. In contrast, with Mk-801 (i.p.) the time course of effects was similar to that of APV administered intracysternally.
4. Discussion
i
,
t
i
i
,
,
20ms
t
M1
T
,2
lOmin
APV
Fig. 1. The effects of APV (1.2 /~mol/kg i.c.) on the sensoryevoked electromyogram induced by systemic infusion of catechol (2.5 m g / k g per min). The records on the left show the evoked EMGs recorded from Flexor carpii, to electrical stimulation of cutaneous afferents at the wrist: (a) before catechol, (b) 10-15 min after catechol, (c) 10-15 rain after APV, (d) 35-40 min after APV. Each record is of 20 superimposed responses. Note the significant decrease in the frequency of occurrence of M2 and M3 in (c) and partial recovery in (d). Note also the absence of any E M G activity before infusion of catechol in (a). The time course of these effects is shown in histogram form below. The results are of a single representative experiment and iullustrate how the results in table 1 were calculated. Similar results were obtained in all other experiments with APV.
The catechol-induced, stimulus-sensitive state, during which brief myoclonic convulsions can be evoked by sensory stimulation, has been shown to be a sensitive model for studying the convulsant action of catechol. The sensory-evoked E M G typically consists of three temporally distinct components, each representing the motor expression of the activation of a different reflex pathway: M1 is the result of a polysynaptic, propriospinal reflex; M2, a long-loop reflex dependent on the integrity of the sensorimotor cortex; and M3 is probably a cerebellar reflex (Angel and Lemon, 1973; Dewhurst, 1984). All of the excitatory amino acid antagonists tested significantly decreased the probability of occurrence of the longer latency components (M2 and M3) of this response but had no effect on M1 (table 1). APH, APV and MK-801 have been shown to antagonise excitation mediated by dicarboxylic amino acids with a preferential action at N-methyl-D-aspartate (NMDA) receptors while PDA and P G G are less selective, with actions at N M D A , kainate and quisqualate receptors (Watkins and Evans, 1981; Kemp et al., 1986). These drugs, particularly those with selective actions at N M D A receptors, have been shown to be anticonvulsant against a number of other experimental models of seizures (Jones et al., 1982; Croucher et al., 1982). The lack of effect on M1, particularly of those drugs given either systemically or parenterally, suggests that the site of action of these agents is supraspinal. A comparison of the time course of action of MK-801 and APH, suggests a rapid penetration of MK-801 into the brain, even after systemic administration, and is consistent with other findings (Kemp et al., 1986). The time course of effects of APH is also consistent with the
212
reported rate of penetration of APH into brain after systemic administration (Rostain et al., 1985). In these experiments, since drugs were administered as single doses and by different routes, it is impossible to make direct comparisons between the different drugs. However all of the excitatory amino acid antagonists significantly decreased the probability of occurrence of M2 and M3, whatever their route of administration, suggesting that synaptic transmission at sites involving aspartate or glutamate is important in the pathways leading to the motor expression of these components. Equally, since amino acids play an important excitatory role in the CNS, it is possible that the convulsive response is decreased as a consequence of the generalised CNS depression, which would result from antagonism of excitatory amino acid transmission. There is evidence of catechol causing a selective increase in the release of aspartate from cortex and olfactory cortex slices of the rat (Minchin and Pearson, 1981; Collins and Dewhurst, 1986), possibly by a presynaptic action. A similar action at other sites in the CNS could explain the convulsant action of catechol and the protective action of the excitatory amino acid antagonists. In conclusion, excitatory amino acid antagonists significantly reduce the probability of occurrence of those components of the catechol-induced EMG dependent on supraspinal structures, indicating that excitatory amino acids may be involved as neurotransmitters in the pathways subserving these responses.
References Angel, A. and R.N. Lemon, 1973, An analysis of the myoclonic jerks produced by 1,2 dihydroxybenzene in the rat, Electroenceph. Clin. Neurophysiol. 35, 589. Collins, G.G.S. and D.G. Dewhurst, 1986, Sites and mechanisms of action of catechol (1,2 dihydroxybenzene) in the rat olfactory cortex slice, Br. J. Pharmacol. 88, 433. Croucher, M.J., J.F. Collins and B.S. Meldrum, 1982, Anticonvulsant action of excitatory amino-acid antagonists, Science 216 (4584), 899. Dewhurst, D.G., 1984, Some characteristics of the long-latency component of the evoked muscle response induced by administration of catechol to the anaesthetised rat: a neurophysiological and neuropharmacological investigation, Br. J. Pharmacol. 83, 83. Dewhurst, D.G., 1986, Effects of excitatory amino acid antagonists on catechol-induced seizures, Br. J. Pharmacol. Proc. (Suppl.) 89, 640P. Gallagher, J.P. and L.C. Blaber, 1973, Catechol, a facilitatory drug that demonstrates only a prejunctional site of action, J. Pharmacol. Exp. Ther. 184, 129. Jones, A.W., M.J. Croucher, B.S. Meldrum and J.C. Watkins, 1984, Suppression of audiogenic seizures in DBA/2 mice by two new dipeptide N M D A receptor antagonists, Neurosci. Lett. 45, 157. Kemp, J.A., T. Priestley and G.N. Woodruff, 1986, MK-801, a novel, orally active anticonvulsant, is a potent, non-competitive N-methyl-D-aspartate-receptor antagonist, Br. J. Pharmacol. Proc. (Suppl.) 89, 535P. Minchin, M.C.W. and G. Pearson, 1981, The effect of the convulsant agent, catechol, on transmitter uptake and release in rat brain slices, Br. J. Pharmacol. 74, 715. Rostain, J.C., B. Wardley-Smith and B.S. Meldrum, 1985, Effects of 2-amino-7-phosphonoheptanoic acid on EEG of rats, Electroenceph. Clin. Neurophysiol. 60, 367. Watkins, J.C. and R.H. Evans, 1981, Excitatory amino acid transmitters, Ann. Rev. Pharmacol. Toxicol. 21, 165.