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D-aminaphosphonovalerate is 100-fold more powerful than D-alpha-aminoadipate in blocking N-methylaspartate neurotoxicity
Brain Research, 221 (1981) 207-210
207
Elsevier/North-Holland Biomedical Press
D-Aminaphosphonovalerate is 100-fold more powerful than D-alphaaminoadipate in blocking N-methylaspartate neurotoxicity
JOHN W. OLNEY, JOANN LABRUYERE, JAMES F. COLLINS and K E N N E T H CURRY
Department of Psychiatry, Washington University School of Medicine, 4940 Audubon ave., St. Louis, Mo 63110 (U.S.A.) ~ t (Jr. F. C. and K.C.) Department of Chemistry, Sir John Cass School of Science and Technology, City of London Polytechnic, 31 Jewry St. London EC3N 3EY ( U.K.) (Accepted May 14th, 1981)
Key words : a-aminoadipate - - aminophosphonovalerate - - N-methylaspartate - - excitotoxin antagonists - - glutamate neurotoxicity
Here we report that the D-isomers of 2-amino-5-phosphonovalerate (D-APV) and a-aminoadipate (D-aAA) effectively protect arcuate hypothalamic neurons from the potent excitotoxic activity of N-methylaspartate (NMA). Consistent with evidence that APV is much more powerful than aAA in antagonizing the neuroexcitatory activity of NMA, we found D-APV nearly 100 times more powerful than D-~AA in preventing NMA from destroying arcuate neurons.
Glutamate (Glu), aspartate (Asp) and certain structural analogs excite neurons when introduced into the central nervous system in tiny amounts by microelectrophoresis 10, or destroy central neurons when administered in larger amounts, either systemically8 or intracranially 5. We have proposed that an excitatory mechanism underlies the neurotoxic activity of these compounds based on evidence that they have parallel orders of potency for their excitatory and neurotoxic actions and that both actions appear to be exerted primarily against dendrosomal portions of the neuron where excitatory receptors are located. Recent findings suggest that there may be at least three subtypes of excitatory amino acid receptors1°; these have been designated respectively as N-methylaspartate (NMA), kainic acid (KA) and quisqualic acid (QA) receptors on the basis of differential responses elicited in electrophysiological experiments employing these potent agonists, either alone or together with certain antagonists. Alpha-aminoadipate (aAA) and 2-amino-5-phosphonovalerate (APV) are increasingly being recognized as useful tools for studying NMA receptors as these molecules, in appropriate dosage, selectively antagonize the excitatory activity of NMA while displaying little or no blocking action against KA, QA or non-amino acid excitants ~0. In early studies, when only the DL racemic form of aAA was available, it was shown to be effective, but only moderately potent, in antagonizing either the excitatoryl,10 or neurotoxic 7 actions of NMA; it was later clarified that only D-aAA is effective in antagonizing NMA excitotoxic activity and that L-aAA is actually an
208 excitotoxic agonist a,6,1°. We are aware of no published evidence concerning either the agonist or antagonist properties of the separate I)- and L-isomers of APV, although it has been speculated that D-APV may be an exceedingly potent NMA antagonist based on evidence that DL-APV is more powerful than D-aAA in antagonizing the excitatory action of N M A 1°. A recent study .~, soon to be published, corroborates this speculation. D-APV and D-aAA were recently synthesized (by J. F. C. and K. C'.~ m sufficient quantity to permit the present study aimed at evaluating their potencies m antagonizing the in vivo neurotoxic action of N M A against arcuate hypothalamic IAH) neurons. A total of 69 H A - I C R albino mice IA.R. Schmidt. Madison Wll of mixed sex was employed' the sexes were divided equally between control and experimental groups. We used 25-day-old mice. since we have found (J. W. Olney and J. Labruyere. unpublished) that neonatal mice are rapidly killed by doses of N M A required to produce a hypothalamic lesion, but after 20 d of age, mice readily tolerate a lesioning dose without accompanying mortality. In prior experiments aimed at examining the antagonist potential of DL-(zAA, we found that administration of the antagcmst 15 min prior to N M A provided a reliable approach for testing antagonist efficacy. We obtained N M A (N-methyl-DL-aspartic acid~ from Sigma Chemical Co., St. Louis. MO. Our supply of D-nAA was partially from a batch synthesized by J. F. (7.. as described previously 6, and partially from that recently made commercially available by Sigma Chemical Co. D-APV was obtained by first synthesizing Dt,-APV by the method of Chambers and lsbell 2. then resolving the racemate as described elsewhere ~ for resolving DL-(~AA. By circular dichroism it was established that ( - )APV has the D-configuration. Immediately prior to use. all compounds were dissolved in sterile distilled water and adjusted to neutral p H with N a O H . Animals were injected s.c. with D-aAA (250, 500 or 750 mg/kg) or D-APV ~2.5, 5. 7.5. 10. 25, 50 or 75 mg/kg) 15 rain prior to s.c. administration of a standard dose of N MA (50 mg/kg). Controls received the standard dose of N M A without pretreatment. All animals were sacrificed by paraformaldehyde/glutaraldehyde perfusion fixation 4 h following treatment and their brains processed for histopathological evaluation by methods described previously s. Quantification of N M A toxicity was achieved by serially sectioning the brains transversely through the A H and counting the number of necrotic neuronal profiles per section. The point of maximal damage to AH was thereby established for each animal and the number of necrotic neurons per section at this level of sectioning was used as an index of lesion severity for a given animal. The mean numbers of necrotic neurons per section 4- S.E.M. for treatment groups were compared and cvaluated statistically by Student's r-test. Our findings (Table 1 and Fig. l) indicate that the D-isomers of ~zAA and APV are both effective in blocking the neurotoxic activity of NMA. that the blocking acuon has linear dose response characteristics with 100 o/,i protection against the neurotoxic effect being demonstrable as the dose of antagonist is increased and that D-APV is nearly 100 times more powerful than D-~zAA in displaying this blocking action. It is noteworthy that, while many of the early toxicology studies pertaining to Glu and its low-potency analogs involved administration of such large doses that questions were
209
:mist potencies o f D-(tAA and D-APV gents
MA MA MA MA MA MA MA MA
I ~ F i " ~
D-aAA D-~AA D-(tAA D-APV D-APV D-APV D-APV
Dose (mg/kg)
Lesion severiO ,~
50 50 50 50 50 50 50 50
23.8 13.3 1.5 0.7 17.0 5.5 0.5 0.0
: t J I i ! -
250 500 750 2.5 5,0 7.5 7.5
_-- 1.2 ± 2.1 e i 0.7 o k 0.6 c " 3.0 b ± 2.1 e J 0.2 e ~- 0.0 e
teral nucleus. Counts given here are mean (~_ S.E.M.) number of necrotic AH neurons leus) per transverse section at point of maximal damage to AH. A typical lesion is deI ; t~p • 0.05 compared to N M A control; e p < 0.001 compared to N M A control.
l e r n o n s p e c i f i c m e c h a n i s m s s u c h as excessive s o l u t e l o a d , p H p e r t u r b a f o r t h m i g h t c o n t r i b u t e t o t h e o b s e r v e d effects, t h e d o s e s e m p l o y e d in t h e ,~riments a r e n o t h i g h e n o u g h to w a r r a n t s u c h o b j e c t i o n s . I n d e e d , t h e ) - A P V a t 2.5, 5 a n d
7.5 m g / k g
t o give 29, 77 a n d
980/~ p r o t e c t i o n ,
against the powerful neuron-necrotizing activity of NMA
a t 50 m g / k g
t D - A P V is a n e x c e e d i n g l y p o w e r f u l a n t a g o n i s t . : o m p l e t i n g t h i s w o r k we l e a r n e d o f a v e r y r e c e n t s t u d y in w h i c h P e r k i n s et - A P V s u b s t a n t i a l l y m o r e p o w e r f u l t h a n D L - A P V ( w h i c h is m o r e p o t e n t ~10) in a n t a g o n i z i n g
the excitatory activity of NMA
on rat cortical
us, D - A P V is m u c h m o r e p o w e r f u l t h a n D-c~AA in a n t a g o n i z i n g e i t h e r t h e
w
w
¸¸
~
•
~
v~
•
dcrographs of the arcuate nucleus of the hypothalamus of a control mouse (a) treated mg/kg) and an experimental (b) treated with N M A (50 mg/kg) plus D-APV (5 mg/kg). ly necrotic neurons (arrows) in a which are not present in b ( ~ 150).
210 excitatory or n e u r o t o x i c activities o f N M A . These s t r u c t u r e - a c t i v i t y correlations. t o g e t h e r with o t h e r relevant datal,a, 5-s.10. c o m p r i s e s t r o n g evidence that the neurotoxic a c t i o n o f at least one G l u / A s p a n a l o g --- N M A is m e d i a t e d by an excitatory m e c h a n i s m involving specific excitatory receptors on the d e n d r o s o m a l surfaces o f central neurons. While it c a n n o t be stated categorically that these receptors are synaptic receptors, it is n o t e w o r t h y t h a t b o t h D - a A A a n d DL-APV are highly effective at spinal cord synapses in b l o c k i n g excitations i n d u c e d either by a p p l i c a t i o n o f N M A o r by electrical s t i m u l a t i o n o f the afferent limb o f the synaptic p a t h w a y ~-~'' M u c h a t t e n t i o n has been focused recently on the heterocyclic G l u analog, kainic acid ( K A ) , which is a m o n g the most p o t e n t o f the excitatory a m i n o acid neurotoxins yet discovered. A m a j o r I m p e d i m e n t in clarifying the m e c h a n i s m o f action o f K A has been the lack o f specific a n t a g o n i s t s for evaluating the r e c e p t o r specificity o f its excitatory a n d toxic properties. Conversely, evidence suggesting t h a t D - ~ A A and DA P V do specifically a n t a g o n i z e b o t h the excitatory a n d n e u r o t o x i c actions o f N M A m a k e s b o t h N M A and its specific a n t a g o n i s t s potentially powerful tools that should substantially facilitate progress in clarifying the role o f excitatory a m i n o acids in central n e u r o t r a n s m i s s i o n a n d possibly also in n e u r o p s y c h i a t r i c disease processes 4,'5 If a role for e n d o g e n o u s excitotoxic a m i n o acids in neuropsychiatric disorders can be established, it m a y be possible to e m p l o y a n t a g o n i s t s o f these agents for p r o p h y l a c t i c or t h e r a p e u t i c purposes, p r o v i d e d a n t a g o n i s t molecules t h a t readily cross b l o o d brain barriers can be developed. I t s h o u l d be k e p t in mind, o f course, that an agent which crosses b l o o d b r a i n barriers a n d is as p o t e n t as D-APV in b l o c k i n g both N M A i n d u c e d a n d s y n a p t i c excitations, m a y also be a p o t e n t b e h a v i o r altering agent. S u p p o r t e d by U S P H S G r a n t s NS-09156. DA-00259 a n d R S A - M H - 3 8 8 9 4 ( J . W . O . ) ; K.C. is a n S R C - s u p p o r t e d student. 1 Biscoe, T. J., Evans, R. H., Francis, A. A., Martin, M. R., Watkins, J. C., Davies, .I. and Dray, A., D-a-Aminoadipate as a selective antagonist of amino acid-induced and synaptic excitation of mammalian spinal neurones, Nature (Lond.), 270 (1977) 743-745. 2 Chambers, J. R. and Isbell, A. S., Chemical synthesis of phosphonate analogs of carboxylic acids, J. org. Chem., 29 (1964) 832-836. 3 Hall, J. G., Hicks, T. P. and McLennan. H., Kainic acid and the glutamate receptor, Neurosci. Lett., 8 (1978) 171. 4 Kim, J. S., Kornhuber, H., Schmidt-Burgk, W. and Holzmuller, B., Low cerebrospinal fluid glutamate in schizophrenic patients and a new hypothesis on schizophrenia, Neurosci. Left.. 20 (1980) 379-382. 5 Olney, J. W. and de Gubareff, T., Glutamate neurotoxicity and Huntington's "chorea, Nature, 271 (1978) 557-559. 6 Olney, J. W., de Gubareff, T. and Collins. J. F., Stereospecificity of the gliotoxic and antineurotoxic actions of alpha-aminoadipate, Neurosci. Lett., 19 (1980) 277-282. 7 Olney, J. W., de Gubareff, T. and Labruyere, J., a-Aminoadipate blocks the neurotoxic action of N-methyl aspartate, Life Sci., 25 (t979) 537-540. 8 Olney, J. W., Ho, O. L. and Rhee, V. Cytotoxic effects of acidic and sulphur containing amino acids on the infant mouse central nervous system, Exp. Brain Res.. 14 (1971) 61-67. 9 Perkins, N. M., Stone, T. W., Collins, J. F. and Curry, K., Phosphonate analogs of carboxylic acids as amino acid antagonists on rat cortical neurons, Neurosci. Lett.. in press. 10 Watkins. J. C.. Davies J., Evans, R. H.. Francis. A. A. and Jones. A. W , Pharmacology of receptors for excitatory amino acids. In G. Di Chiara and G. L. Gessa (Eds.). Glutamate as a Neurotransmitter, Raven Press, New York, 1981, pp. 263-273.
207
Elsevier/North-Holland Biomedical Press
D-Aminaphosphonovalerate is 100-fold more powerful than D-alphaaminoadipate in blocking N-methylaspartate neurotoxicity
JOHN W. OLNEY, JOANN LABRUYERE, JAMES F. COLLINS and K E N N E T H CURRY
Department of Psychiatry, Washington University School of Medicine, 4940 Audubon ave., St. Louis, Mo 63110 (U.S.A.) ~ t (Jr. F. C. and K.C.) Department of Chemistry, Sir John Cass School of Science and Technology, City of London Polytechnic, 31 Jewry St. London EC3N 3EY ( U.K.) (Accepted May 14th, 1981)
Key words : a-aminoadipate - - aminophosphonovalerate - - N-methylaspartate - - excitotoxin antagonists - - glutamate neurotoxicity
Here we report that the D-isomers of 2-amino-5-phosphonovalerate (D-APV) and a-aminoadipate (D-aAA) effectively protect arcuate hypothalamic neurons from the potent excitotoxic activity of N-methylaspartate (NMA). Consistent with evidence that APV is much more powerful than aAA in antagonizing the neuroexcitatory activity of NMA, we found D-APV nearly 100 times more powerful than D-~AA in preventing NMA from destroying arcuate neurons.
Glutamate (Glu), aspartate (Asp) and certain structural analogs excite neurons when introduced into the central nervous system in tiny amounts by microelectrophoresis 10, or destroy central neurons when administered in larger amounts, either systemically8 or intracranially 5. We have proposed that an excitatory mechanism underlies the neurotoxic activity of these compounds based on evidence that they have parallel orders of potency for their excitatory and neurotoxic actions and that both actions appear to be exerted primarily against dendrosomal portions of the neuron where excitatory receptors are located. Recent findings suggest that there may be at least three subtypes of excitatory amino acid receptors1°; these have been designated respectively as N-methylaspartate (NMA), kainic acid (KA) and quisqualic acid (QA) receptors on the basis of differential responses elicited in electrophysiological experiments employing these potent agonists, either alone or together with certain antagonists. Alpha-aminoadipate (aAA) and 2-amino-5-phosphonovalerate (APV) are increasingly being recognized as useful tools for studying NMA receptors as these molecules, in appropriate dosage, selectively antagonize the excitatory activity of NMA while displaying little or no blocking action against KA, QA or non-amino acid excitants ~0. In early studies, when only the DL racemic form of aAA was available, it was shown to be effective, but only moderately potent, in antagonizing either the excitatoryl,10 or neurotoxic 7 actions of NMA; it was later clarified that only D-aAA is effective in antagonizing NMA excitotoxic activity and that L-aAA is actually an
208 excitotoxic agonist a,6,1°. We are aware of no published evidence concerning either the agonist or antagonist properties of the separate I)- and L-isomers of APV, although it has been speculated that D-APV may be an exceedingly potent NMA antagonist based on evidence that DL-APV is more powerful than D-aAA in antagonizing the excitatory action of N M A 1°. A recent study .~, soon to be published, corroborates this speculation. D-APV and D-aAA were recently synthesized (by J. F. C. and K. C'.~ m sufficient quantity to permit the present study aimed at evaluating their potencies m antagonizing the in vivo neurotoxic action of N M A against arcuate hypothalamic IAH) neurons. A total of 69 H A - I C R albino mice IA.R. Schmidt. Madison Wll of mixed sex was employed' the sexes were divided equally between control and experimental groups. We used 25-day-old mice. since we have found (J. W. Olney and J. Labruyere. unpublished) that neonatal mice are rapidly killed by doses of N M A required to produce a hypothalamic lesion, but after 20 d of age, mice readily tolerate a lesioning dose without accompanying mortality. In prior experiments aimed at examining the antagonist potential of DL-(zAA, we found that administration of the antagcmst 15 min prior to N M A provided a reliable approach for testing antagonist efficacy. We obtained N M A (N-methyl-DL-aspartic acid~ from Sigma Chemical Co., St. Louis. MO. Our supply of D-nAA was partially from a batch synthesized by J. F. (7.. as described previously 6, and partially from that recently made commercially available by Sigma Chemical Co. D-APV was obtained by first synthesizing Dt,-APV by the method of Chambers and lsbell 2. then resolving the racemate as described elsewhere ~ for resolving DL-(~AA. By circular dichroism it was established that ( - )APV has the D-configuration. Immediately prior to use. all compounds were dissolved in sterile distilled water and adjusted to neutral p H with N a O H . Animals were injected s.c. with D-aAA (250, 500 or 750 mg/kg) or D-APV ~2.5, 5. 7.5. 10. 25, 50 or 75 mg/kg) 15 rain prior to s.c. administration of a standard dose of N MA (50 mg/kg). Controls received the standard dose of N M A without pretreatment. All animals were sacrificed by paraformaldehyde/glutaraldehyde perfusion fixation 4 h following treatment and their brains processed for histopathological evaluation by methods described previously s. Quantification of N M A toxicity was achieved by serially sectioning the brains transversely through the A H and counting the number of necrotic neuronal profiles per section. The point of maximal damage to AH was thereby established for each animal and the number of necrotic neurons per section at this level of sectioning was used as an index of lesion severity for a given animal. The mean numbers of necrotic neurons per section 4- S.E.M. for treatment groups were compared and cvaluated statistically by Student's r-test. Our findings (Table 1 and Fig. l) indicate that the D-isomers of ~zAA and APV are both effective in blocking the neurotoxic activity of NMA. that the blocking acuon has linear dose response characteristics with 100 o/,i protection against the neurotoxic effect being demonstrable as the dose of antagonist is increased and that D-APV is nearly 100 times more powerful than D-~zAA in displaying this blocking action. It is noteworthy that, while many of the early toxicology studies pertaining to Glu and its low-potency analogs involved administration of such large doses that questions were
209
:mist potencies o f D-(tAA and D-APV gents
MA MA MA MA MA MA MA MA
I ~ F i " ~
D-aAA D-~AA D-(tAA D-APV D-APV D-APV D-APV
Dose (mg/kg)
Lesion severiO ,~
50 50 50 50 50 50 50 50
23.8 13.3 1.5 0.7 17.0 5.5 0.5 0.0
: t J I i ! -
250 500 750 2.5 5,0 7.5 7.5
_-- 1.2 ± 2.1 e i 0.7 o k 0.6 c " 3.0 b ± 2.1 e J 0.2 e ~- 0.0 e
teral nucleus. Counts given here are mean (~_ S.E.M.) number of necrotic AH neurons leus) per transverse section at point of maximal damage to AH. A typical lesion is deI ; t~p • 0.05 compared to N M A control; e p < 0.001 compared to N M A control.
l e r n o n s p e c i f i c m e c h a n i s m s s u c h as excessive s o l u t e l o a d , p H p e r t u r b a f o r t h m i g h t c o n t r i b u t e t o t h e o b s e r v e d effects, t h e d o s e s e m p l o y e d in t h e ,~riments a r e n o t h i g h e n o u g h to w a r r a n t s u c h o b j e c t i o n s . I n d e e d , t h e ) - A P V a t 2.5, 5 a n d
7.5 m g / k g
t o give 29, 77 a n d
980/~ p r o t e c t i o n ,
against the powerful neuron-necrotizing activity of NMA
a t 50 m g / k g
t D - A P V is a n e x c e e d i n g l y p o w e r f u l a n t a g o n i s t . : o m p l e t i n g t h i s w o r k we l e a r n e d o f a v e r y r e c e n t s t u d y in w h i c h P e r k i n s et - A P V s u b s t a n t i a l l y m o r e p o w e r f u l t h a n D L - A P V ( w h i c h is m o r e p o t e n t ~10) in a n t a g o n i z i n g
the excitatory activity of NMA
on rat cortical
us, D - A P V is m u c h m o r e p o w e r f u l t h a n D-c~AA in a n t a g o n i z i n g e i t h e r t h e
w
w
¸¸
~
•
~
v~
•
dcrographs of the arcuate nucleus of the hypothalamus of a control mouse (a) treated mg/kg) and an experimental (b) treated with N M A (50 mg/kg) plus D-APV (5 mg/kg). ly necrotic neurons (arrows) in a which are not present in b ( ~ 150).
210 excitatory or n e u r o t o x i c activities o f N M A . These s t r u c t u r e - a c t i v i t y correlations. t o g e t h e r with o t h e r relevant datal,a, 5-s.10. c o m p r i s e s t r o n g evidence that the neurotoxic a c t i o n o f at least one G l u / A s p a n a l o g --- N M A is m e d i a t e d by an excitatory m e c h a n i s m involving specific excitatory receptors on the d e n d r o s o m a l surfaces o f central neurons. While it c a n n o t be stated categorically that these receptors are synaptic receptors, it is n o t e w o r t h y t h a t b o t h D - a A A a n d DL-APV are highly effective at spinal cord synapses in b l o c k i n g excitations i n d u c e d either by a p p l i c a t i o n o f N M A o r by electrical s t i m u l a t i o n o f the afferent limb o f the synaptic p a t h w a y ~-~'' M u c h a t t e n t i o n has been focused recently on the heterocyclic G l u analog, kainic acid ( K A ) , which is a m o n g the most p o t e n t o f the excitatory a m i n o acid neurotoxins yet discovered. A m a j o r I m p e d i m e n t in clarifying the m e c h a n i s m o f action o f K A has been the lack o f specific a n t a g o n i s t s for evaluating the r e c e p t o r specificity o f its excitatory a n d toxic properties. Conversely, evidence suggesting t h a t D - ~ A A and DA P V do specifically a n t a g o n i z e b o t h the excitatory a n d n e u r o t o x i c actions o f N M A m a k e s b o t h N M A and its specific a n t a g o n i s t s potentially powerful tools that should substantially facilitate progress in clarifying the role o f excitatory a m i n o acids in central n e u r o t r a n s m i s s i o n a n d possibly also in n e u r o p s y c h i a t r i c disease processes 4,'5 If a role for e n d o g e n o u s excitotoxic a m i n o acids in neuropsychiatric disorders can be established, it m a y be possible to e m p l o y a n t a g o n i s t s o f these agents for p r o p h y l a c t i c or t h e r a p e u t i c purposes, p r o v i d e d a n t a g o n i s t molecules t h a t readily cross b l o o d brain barriers can be developed. I t s h o u l d be k e p t in mind, o f course, that an agent which crosses b l o o d b r a i n barriers a n d is as p o t e n t as D-APV in b l o c k i n g both N M A i n d u c e d a n d s y n a p t i c excitations, m a y also be a p o t e n t b e h a v i o r altering agent. S u p p o r t e d by U S P H S G r a n t s NS-09156. DA-00259 a n d R S A - M H - 3 8 8 9 4 ( J . W . O . ) ; K.C. is a n S R C - s u p p o r t e d student. 1 Biscoe, T. J., Evans, R. H., Francis, A. A., Martin, M. R., Watkins, J. C., Davies, .I. and Dray, A., D-a-Aminoadipate as a selective antagonist of amino acid-induced and synaptic excitation of mammalian spinal neurones, Nature (Lond.), 270 (1977) 743-745. 2 Chambers, J. R. and Isbell, A. S., Chemical synthesis of phosphonate analogs of carboxylic acids, J. org. Chem., 29 (1964) 832-836. 3 Hall, J. G., Hicks, T. P. and McLennan. H., Kainic acid and the glutamate receptor, Neurosci. Lett., 8 (1978) 171. 4 Kim, J. S., Kornhuber, H., Schmidt-Burgk, W. and Holzmuller, B., Low cerebrospinal fluid glutamate in schizophrenic patients and a new hypothesis on schizophrenia, Neurosci. Left.. 20 (1980) 379-382. 5 Olney, J. W. and de Gubareff, T., Glutamate neurotoxicity and Huntington's "chorea, Nature, 271 (1978) 557-559. 6 Olney, J. W., de Gubareff, T. and Collins. J. F., Stereospecificity of the gliotoxic and antineurotoxic actions of alpha-aminoadipate, Neurosci. Lett., 19 (1980) 277-282. 7 Olney, J. W., de Gubareff, T. and Labruyere, J., a-Aminoadipate blocks the neurotoxic action of N-methyl aspartate, Life Sci., 25 (t979) 537-540. 8 Olney, J. W., Ho, O. L. and Rhee, V. Cytotoxic effects of acidic and sulphur containing amino acids on the infant mouse central nervous system, Exp. Brain Res.. 14 (1971) 61-67. 9 Perkins, N. M., Stone, T. W., Collins, J. F. and Curry, K., Phosphonate analogs of carboxylic acids as amino acid antagonists on rat cortical neurons, Neurosci. Lett.. in press. 10 Watkins. J. C.. Davies J., Evans, R. H.. Francis. A. A. and Jones. A. W , Pharmacology of receptors for excitatory amino acids. In G. Di Chiara and G. L. Gessa (Eds.). Glutamate as a Neurotransmitter, Raven Press, New York, 1981, pp. 263-273.