CGS 19755 is a potent and competitive antagonist at NMDA-type receptors

The N-Methyl-D-aspartate (NMDA)-type receptor blocking properties of CGS 19755, a novel, rigid analog of 2-amino-5-phosphonopentanoate, were demonstrated in vitro by the ability of the compound to block NMDA-evoked [3H]acetylcholine release (pA2 = 5.93). CGS 19755 (0.045 and 0.224 mmol/kg i.p.) was shown to be active in vivo as well by its ability to block harmaline-induced increases in cerebellar cGMP. Finally, CGS 19755 blocked sound-induced seizures in DBA/2 mice completely at doses of 1.0 nmol i.c.v. or 0.1 mmol/kg i.p. Taken together, these data indicate that CGS 19755 is a potent and competitive NMDA antagonist in vitro which is also active in vivo.

Decrease in excitatory transmission within the lateral habenula and the mediodorsal thalamus protects against limbic seizures in rats

We have used limbic convulsions induced by systemic pilocarpine in rats combined with focal intracerebral injections concurrently to study the initiation and spread of seizure activity. Protection against pilocarpine-seizure development by antagonism of excitatory or facilitation of inhibitory neurotransmission at focal sites establishes the anatomical circuits involved in the propagation of seizures. The excitatory amino acid antagonist 2-amino-7-phosphonoheptanoate (APH, selective for the NMDA preferring glutamate receptor subtype) is potently anticonvulsant after bilateral focal injections into the habenula or mediodorsal thalamus. The dose of APH required to give sustained protection against pilocarpine-induced convulsions is 10 pmol for lateral habenula and 50 pmol for mediodorsal thalamus. The GABA agonist muscimol produces a similar sustained protection following focal injections (100 pmol/side) into either the lateral habenula or the mediodorsal thalamus. An overall decrease in the efferent neurotransmission of these two brain regions results in a strong anticonvulsant effect indicating their importance in modulating limbic seizure activity.

Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway

The afferent pathway involved in initiation of audiogenic seizures in the genetically epilepsy-prone rat was investigated by bilateral microinfusion of the excitant amino acid antagonist 2-amino-7-phosphonoheptanoate into the major brain stem and subcortical nuclei of the auditory system. This antagonist has been shown to possess anticonvulsant properties in other seizure models, and an excitant amino acid has been implicated as a putative neurotransmitter in several of these nuclei. Seizure severity was significantly reduced following infusion of this agent into the cochlear nucleus, superior olivary complex, inferior colliculus, and medial geniculate body. Many of these animals exhibited a complete blockade of seizures. The smallest effective dose in the cochlear nucleus and the medial geniculate body was 5 nmol per side. The smallest effective dose in the olive was 1 nmol, and in the inferior colliculus 0.1 nmol per side was protective. The onset of anticonvulsant effectiveness was earliest in the inferior colliculus. These findings showed that the inferior colliculus was the most sensitive auditory center to the anticonvulsant action of 2-amino-7-phosphonoheptanoate and that imbalance between inhibitory and excitatory transmission within this brain structure may be crucial in the initiation of audiogenic seizures in the genetically epilepsy-prone rat.

Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. II. Efferent seizure propagating pathway

Previous studies indicate that the inferior colliculus is the brain stem auditory nucleus most sensitive to the chemical blockade of audiogenic seizures in the genetically epilepsy-prone rat. Other auditory structures do not appear to be as important. This study attempted to define the efferent pathways involved in propagation of the seizure from the colliculus to the spinal cord where the motor components of the convulsion are generated. This study also determined whether certain nuclei which have been implicated in the propagation of seizures in other epilepsy models are involved in audiogenic seizures. The excitant amino acid antagonist, 2-amino-7-phosphonoheptanoate, was infused bilaterally into several of those sites. The drug was effective in significantly reducing seizure severity with infusion of 5 nmol bilaterally into the midbrain and the pontine reticular formation or the substantia nigra. However, similar drug doses were not effective when infused into the entopeduncular nucleus even though prominent behavioral effects were observed with this infusion. Infusion of 2-amino-7-phosphonoheptanoate into the prepiriform cortex resulted in a small but significant reduction in seizure severity. These results suggest that inhibition of excitatory transmission within the substantia nigra and the reticular formation effectively blocks the output pathway for the audiogenic seizures, whereas the role of the prepiriform cortex in this process is relatively minor.

Long-term development of selective neuronal loss and the mechanism of protection by 2-amino-7-phosphonoheptanoate in a rat model of incomplete forebrain ischaemia

Excitatory neurotransmission at the N-methyl-D-aspartate (NMDA) receptor is selectively blocked by 2-amino-7-phosphonoheptanoate acid (2-APH). Acute focal microinjection of 2-APH into the rat hippocampus partially protects against cytopathology developing in selectively vulnerable neurons after 30 min of ischaemia and 2 h of reperfusion. We show that this protective action of 2-APH does not involve alterations in local cerebral blood flow (CBF). Intermediate cytopathology and long-term neuronal survival has been assessed in rats receiving focal injections of (+/-) 2-APH, 20 micrograms in 1 microliter, into one dorsal hippocampus prior to and 4 and 10 h after a 10-min period of forebrain ischaemia. Cytopathology assessed 4 or 24 h after ischaemia shows no difference between the buffer and 2-APH-injected hippocampi. Assessment after 7 days survival shows a significant protection against neuronal loss in the CA1 zone of the 2-APH-injected hippocampus compared with the contralateral, buffer-injected hippocampus. Systemic injection of D(-)2-APH (675 mg/kg i.v. at 0 h, 4 h, and 10 h) affords significant protection to CA1 hippocampal neurones (as assessed after 7 days). These results suggest that maintained blockade of neurotransmission at the NMDA receptor in the postischaemic period can protect against delayed cell loss. The mechanism may be through antagonism of the excitotoxic action of an endogenous neurotransmitter acting in the postischaemic period.

Anticonvulsant effects of some calcium entry blockers in DBA/2 mice

1. The behavioural and anticonvulsant effects of several drugs acting by various mechanisms on calcium-channels or affecting intracellular Ca2+ concentrations were studied after both systemic and intracerebroventricular administration in DBA/2 mice, a strain genetically susceptible to sound-induced seizures. 2. The anticonvulsant effects were evaluated on seizures evoked by means of auditory stimulation (109 dB) in animals placed singly under a perspex dome. 3. Flunarizine and dihydropyridine derivatives, belonging to class I of calcium entry blockers, administered intraperitoneally, were the most potent compounds. 4. Diltiazem, a benzothiazepine derivative belonging to class III, and HA 1004, a calcium antagonist, acting by inhibiting Ca2+ mobilization from intracellular stores, injected intraperitoneally, were 3-7.6 fold and 5.8-10.7 fold less potent than flunarizine respectively. 5. Verapamil and methoxyverapamil, two phenylalkylamine derivatives, given intraperitoneally, were completely ineffective in preventing sound-induced seizures in DBA/2 mice. In addition, high doses of verapamil and its methoxyderivative occasionally produced spontaneous tonic-clonic seizures. 6. After intracerebroventricular administration of the hydrosoluble calcium entry blockers, belonging to different classes, the anticonvulsant effects were similar to those observed after systemic administration. 7. The systemic administration of Bay K 8644, a dihydropyridine analogue, having the ability to stimulate calcium entry into cells produced a dose-dependent increase in clonic and tonic convulsions and other neurological side effects. 8. The present results strongly support the idea that some Ca2+ antagonists may be useful in human epilepsy.

The involvement of excitatory amino acid receptors within the prepiriform cortex in pilocarpine-induced limbic seizures in rats

The prepiriform cortex (PPCx) shows high sensitivity to the epileptogenic action of chemo-convulsants and to the protective action of the NMDA receptor antagonist, 2-amino-7-phosphono-heptanoate (APH) against pilocarpine-induced (motor) limbic seizures in rats. In this study the interaction between agonists acting selectively on the three main excitatory amino acid receptor subtypes in the PPCx and the muscarinic agonist, pilocarpine, within the PPCx have been investigated. Kainate (KA) or quisqualate (QUIS) injected focally into the PPCx (100 pmoles or 5 nmoles per side respectively) induced motor limbic seizures when administered after a subconvulsant dose of pilocarpine (250 mg/kg, i.p.). KA, 100 pmoles injected into the same site in olfactory-bulboectomized rats (bulbectomy results in protection against pilocarpine-induced seizures) also facilitated seizures. However, activation of the NMDA receptor in the PPCx by focal injection of NMDA (250 fmoles-10 nmoles) failed to produce seizures after a subconvulsant dose of pilocarpine. Moreover NMDA in the same range of doses injected into the PPCx protected rats against the seizures induced by a fully convulsant dose of pilocarpine.

An adenosine analogue, 2-chloroadenosine, protects against long term development of ischaemic cell loss in the rat hippocampus

Delayed ischaemic damage was assessed by light microscopy following 10 min of incomplete forebrain ischaemia and 7 days of reperfusion. Iterative focal injections of 2-chloroadenosine, a stable analogue of adenosine, protect against selective hippocampal CA1 loss (P less than 0.01), when given either immediately before ischaemia and after 4 and 10 h of reperfusion or after 1 min, 4 h and 10 h of reperfusion. Delayed focal injection of 2-chloroadenosine after 10 and 24 h of reperfusion fails to protect against CA1 cell loss. The mechanism of cerebroprotection may involve attenuation of excitatory neurotransmission.

Kynurenate inhibition of cell excitation decreases stroke size and deficits

Pharmacological inhibition of excitatory neurotransmission attenuates cell death in models of global ischemia/reperfusion and hypoglycemia. The current investigations extend these observations to a model of focal ischemia. Kynurenic acid, a broad-spectrum antagonist at excitatory amino acid receptors, was used as treatment (300 mg/kg; 3 doses at 4-hour intervals) before and after focal cerebral ischemia in rats (n = 54). Preischemia but not 1 hour postischemia treatment with kynurenate attenuated infarction size (p less than 0.001) and improved neurological outcome (p less than 0.001) studied at 24 hours after injury. These data support the role of excitatory neurotransmission in acute neuronal injury and support pharmacological inhibition of cell excitation as a potential therapy for stroke.

Bidirectional effects of beta-carbolines in reflex epilepsy

Derivatives of ethyl-beta-carboline-3-carboxylate, ZK 91296, ZK 93423 and ZK 95962 have potent anticonvulsant activity against sound-induced seizures in audiogenic DBA/2 mice and against photically-induced seizures in the baboon, Papio papio. The convulsant beta-carbolines, DMCM and beta-CCM, have proconvulsant and convulsant activity in the same animal models. DMCM and beta-CCM are similar in potency as convulsants in DBA/2 mice (ED50 value for DMCM: 1.3 mg/kg; ED50 value for beta-CCM; 0.8 mg/kg), but differ with respect to their profiles for protection by anticonvulsant drugs. The anticonvulsant potencies of diazepam and clobazam are similar against both types of beta-carboline-induced seizures, whereas quazepam protects better against beta-CCM seizures (4 fold elevation in ED50 value at 1 mg/kg quazepam IP) than against DMCM seizures (1.7 fold elevation in ED50 value), supporting a preferential action of beta-CCM on BZ1 receptors. Valproate (400 mg/kg) and gamma-vinyl-GABA (1.5 g/kg) protect better against beta-CCM seizures (9.5 and 5.9 fold elevations in ED50 values respectively) than against DMCM seizures (1.8 and 2.7 fold elevations in ED50 values respectively). The excitatory amino acid antagonist, 2-amino-7-phosphonoheptanoic acid, has significant anticonvulsant activity against DMCM seizures. The elevated regional GABA levels in brains of DBA/2 mice observed during beta-CCM seizures are eliminated by the pretreatment with Ro 15-1788, which also blocks the seizure activity.

GABA uptake inhibitors. Synthesis and effects on audiogenic seizures of ester prodrugs of nipecotic acid, guvacine and cis-4-hydroxynipecotic acid

The pivaloyloxymethyl esters 4 and 5 of the amino acid GABA uptake inhibitors guvacine and nipecotic acid, respectively, were synthesized as potential prodrugs. The half-lives of 4 and 5 for conversion into the parent amino acids were determined under approximate physiological conditions in the presence or absence of human serum. Under the former conditions the half-lives for 4 and 5 were 6.3 hr and 0.8 hr, and, in the absence of serum, 15.5 hr and 1.2 hr, respectively. The compounds 4 and 5 were administered intracerebroventricularly (i.c.v.) or intraperitoneally (i.p.) to DBA/2 mice and their effects on audiogenic seizures determined. In agreement with earlier findings for 5, all phases of the seizure response of the animals were suppressed by compound 4 at doses above 2mmol/kg i.p. At anticonvulsant doses of compound 4, as well as of 5, side effects such as sedation and impairments of motor activities were observed. The ethyl and pivaloyloxymethyl esters 9 and 11 of cis-4-acetoxynipecotic acid, designed as 'double' ester prodrugs of the GABA uptake inhibitor cis-4-OH-nipecotic acid, were synthesized and shown to have very weak anticonvulsant effects. Compounds 9 and 11 did, however, show a broad spectrum of cholinergic side effects. These apparent interactions of 9 and 11 with muscarinic cholinergic receptors have been explained on the basis of the similarity of the structures of 9 and 11 to that of the muscarinic agonist 1-methyl-4-acetoxypiperidine. Furthermore, the structural similarity of 9 and the muscarinic agonist nipecotic acid ethyl ester may, to some extent, underlie the cholinergic profile of 9.

Anticonvulsant action and biochemical effects in DBA/2 mice of CPP (3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate), a novel N-methyl-D-aspartate antagonist

CPP has a potent anticonvulsant effect against sound-induced seizures in audiogenic DBA/2 mice. Pretreatment with CPP (0.01-10 nmol i.c.v., 45 min) protects against successive phases of sound-induced seizures in a dose-dependent fashion (ED50, tonic phase, 0.023 nmol; clonic phase, 0.039 nmol; wild running, 0.17 nmol). Systemic administration of CPP (0.001-0.1 mmol/kg i.p., 45 min) produces a similar protection (ED50, tonic phase, 0.0012 mmol/kg; clonic phase, 0.0026 mmol/kg; wild running, 0.021 mmol/kg). Following the administration of a fully anticonvulsant dose of CPP (0.1 mmol/kg i.p., 45 min) to adult DBA/2 mice regional brain glucose (cerebellum and striatum) levels are elevated and lactate (striatum and hippocampus) levels decrease. The CPP-induced changes in alanine, serine and glycine paralleled those of lactate. Aspartate levels are significantly decreased by CPP in the striatum (-21%) and the hippocampus (-23%).

Redistribution of transmitter amino acids in rat hippocampus and cerebellum during seizures induced by L-allylglycine and bicuculline: an immunocytochemical study with antisera against conjugated GABA, glutamate and aspartate

The effects of the convulsants L-allylglycine and bicuculline on the distribution of gamma-amino-butyric acid (GABA), glutamate and aspartate in rat brains were assessed immunocytochemically, using antisera raised against glutaraldehyde-protein conjugates of the respective amino acids. In accord with previous biochemical studies of GABA content, L-allylglycine treatment was followed by a decreased immunoreactivity for GABA in the hippocampus and cerebellum, whereas treatment with bicuculline led to an increased immunoreactivity in the hippocampus, but not in the cerebellum. Different cells and zones were affected differentially. With both convulsants the hippocampus showed the most pronounced changes in the neuropil of the pyramidal and granular cell layers. L-Allylglycine treatment led to a substantial decrease in the concentration of detectable GABA-immunoreactive bouton-like dots in the stratum oriens, radiatum and lacunosum-moleculare and in the deep hilar region, but did not produce statistically significant changes in this parameter in the outer and intermediate zones of the dentate molecular layer. In the cerebellum, the decrease in GABA immunoreactivity after L-allylglycine treatment was less in the basket cell terminals than in other GABA-containing elements. Neither convulsant altered the average staining intensity for aspartate or glutamate in the two regions studied, but L-allylglycine reduced the level of aspartate-like immunoreactivity in hippocampal hilar cells. All the changes described were evident after 20 min of seizure activity and were qualitatively similar after 60 min of seizure (animals paralysed and ventilated). Our results indicate that L-allylglycine or bicuculline given intravenously exerts specific effects on cerebral amino acid metabolism. The nature and magnitude of these effects show inter-regional variations and also differ among cellular compartments within each region. Amino acid immunocytochemistry may prove to be a valuable tool for the investigation of metabolic changes associated with epileptic seizures and should be particularly useful in regions showing heterogeneous changes that would tend to cancel each other in biochemical analyses.

2-Amino-7-phosphonoheptanoic acid inhibits insulin-induced convulsions and striatal aspartate accumulation in rats with frontal cortical ablation

Pretreatment of rats with the excitatory amino acid antagonist 2-amino-7-phosphonoheptanoic acid (2-APH; 0.5 mmol/kg, i.p.) protected against insulin-induced clonic seizures. Complete protection was observed in 38% of the rats and partial protection in an additional 50%. Lesioning of the corticostriatal pathway by frontal cortical ablation caused decreases in the striatal levels of aspartate (-28%) and glutamate (-18%), an increase in striatal glutamine level (45%), and decreased high-affinity uptake of D-[3H]aspartate (-27%) in the lesioned dorsal neostriatum. Insulin-induced hypoglycemia caused a predicted sharp increase in aspartate level (165%) and decreased glutamate (-20%) and glutamine (-38%) levels in the intact striatum. Pretreatment of rats with 2-APH significantly reversed the insulin-induced changes in striatal aspartate, glutamate, and glutamine levels, especially in the intact hemisphere. In normoglycemic control rats, the "metabolic," i.e., concentration in the lesioned hemisphere, aspartate pool constituted 72% and the "synaptic," i.e., the concentration difference between the intact and lesioned hemispheres, 28% of the total striatal aspartate pool. 2-APH had no effect on the level of "metabolic" aspartate in the striata of normoglycemic rats but caused an almost complete suppression of "synaptic" aspartate. Following insulin-induced hypoglycemia, the "metabolic" aspartate pool doubled, whereas the "synaptic" aspartate pool increased 3.5-fold in the absence of 2-APH. The insulin-induced rise in "synaptic" aspartate level was almost completely blocked by 2-APH (a 5% rise instead of a 3.5-fold rise).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of two novel dipeptide antagonists of excitatory amino acid neurotransmission on the high pressure neurological syndrome in the rat

Intracerebroventricular injection in the rat of beta-D-aspartyl aminomethylphosphonate (Asp-Amp) 1 mumol, or Y-D-glutamylaminomethylsulphonate (GAMS) 1 mumol, increases the onset pressure for the initial tremor phase of the high pressure neurological syndrome (HPNS) by 50%. Asp-Amp also significantly increases the onset pressures for myoclonus and for tonic-clonic seizures. GAMS did not significantly change the onset pressures for myoclonus or tonic clonic seizures, but it caused the appearance of brief clonic seizures prior to the onset of the HPNS.

Evidence that antagonism at non-NMDA receptors results in anticonvulsant action

The effect of gamma-D-glutamylaminomethylsulphonate (gamma-D-GAMS) and 1-(p-bromobenzoyl)-piperazine-2,3-dicarboxylate (pBB-PzDA) on convulsions elicited by intracerebroventricular application of kainate (KA) and N-methyl-D-aspartate (NMDA) was studied in mice. gamma-D-GAMS, 0.0025-1.0 mumol, and pBB-PzDA, 0.001-0.2 mumol, were preferentially active against myoclonic seizures induced by kainate, but had also pronounced anticonvulsant action against NMDA. Although pBB-PzDA was a more potent anticonvulsant relative to gamma-D-GAMS, gamma-D-GAMS displayed higher kainate-selectivity. gamma-D-GAMS, 0.025 and 0.5 mumol, and pBB-PzDA, 0.1 mumol, blocked myoclonic seizures induced by kainate in the presence of 2-amino-7-phosphonoheptanoate, a selective antagonist at the NMDA receptor, with potency comparable to that for antagonism of seizures produced by kainate alone. These results indicate that antagonism at kainate receptors may contribute to anticonvulsant drug action.

Paradoxical anticonvulsant activity of the excitatory amino acid N-methyl-D-aspartate in the rat caudate-putamen

We used limbic seizures induced in rats by systemic injection of the cholinergic agonist pilocarpine (380 mg/kg; i.p.) to study the neuronal pathways within the basal ganglia that modulate seizure threshold. N-Methyl-D-aspartate (N-Me-D-Asp) is an excitatory amino acid derivative that is a powerful convulsant agent when injected into the cerebral cortex, amygdala, or hippocampus in rats. Bilateral microinjections of N-Me-D-Asp into the caudate-putamen, however, protected against limbic seizures induced by pilocarpine (injected systemically), with an ED50 of 0.7 nmol (range 0.5-1.0 nmol). Lesioning the caudate-putamen (by bilateral microinjection of the excitotoxin ibotenate) converted subconvulsant doses of pilocarpine into convulsant ones. The anticonvulsant action of N-Me-D-Asp in the caudate-putamen was reversed by blocking gamma-aminobutyrate-mediated inhibition in the substantia nigra pars reticulata or in the entopeduncular nucleus. The results suggest that the caudate-putamen and its gamma-aminobutyrate-dependent efferent pathways modulate the threshold for seizures in the limbic forebrain.

Neurotransmission in the pedunculopontine nucleus and pilocarpine-induced motor limbic seizures in rats

Systemic injection of the cholinergic agonist, pilocarpine (380 mg/kg, i.p.) initiates a sequence of events leading to motor limbic seizure activity. Focal injection of the excitatory amino acid antagonist, 2-amino-7-phosphonoheptanoic acid (50 pmol-1 nmol) into the pedunculopontine nucleus (PPN), prior to pilocarpine injection, results in a powerful anticonvulsant action. The GABA agonist, muscimol (25-50 pmol) also afforded protection against pilocarpine-evoked convulsions when injected focally into the PPN. The results suggest that an overall inhibition of PPN output neurons is required for anticonvulsant action.

Unusual interactions of excitatory amino acid receptor agonists: alpha- and beta-kainate antagonize motor responses to N-methyl-D-aspartate in rodents

The alpha- and beta-stereoisomers of kainate correspond sterically to the L- and D-isomers of glutamate. Alpha-Kainate is a potent excitant at a specific membrane receptor site (kainate receptor). Beta-Kainate has been proposed as a functional N-methyl-D-aspartate antagonist in vivo. Because of the structural similarities between the alpha- and beta-stereoisomers of kainate we have investigated the interactions of both compounds with N-methyl-D-aspartate-mediated excitation in two well established animal models for assessing the action of excitatory amino acids and their antagonists in vivo: determination of CD50 (convulsant dose) for myoclonic seizures in mice and electromyographic measurement of muscle tone in genetically spastic rats. We find that alpha-kainate and beta-kainate produce myoclonic seizures in mice when given intracerebroventricularly and increase the muscle tone in genetically spastic rats when given intrathecally. Alpha-Kainate is about 5000 times more potent than beta-kainate as a convulsant and about 1000 times more active than beta-kainate in increasing the muscle tone. The excitatory actions of alpha-kainate and of beta-kainate are blocked by gamma-D-glutamylaminomethylsulphonate, a preferential kainate/quisqualate antagonist, but not by (+/-)-2-amino-7-phosphonoheptanoate, a specific N-methyl-D-aspartate antagonist. Surprisingly, alpha-kainate and beta-kainate antagonize the myoclonic seizures and the increase in muscle tone produced by N-methyl-D-aspartate, and potentiate both the anticonvulsant and myorelaxant actions of (+/-)2-amino-7-phosphonoheptanoate. Quisqualate induces myoclonic seizures in mice after intracerebroventricular application and increases muscle tone in genetically spastic rats following intrathecal injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant action of a kainate antagonist gamma-D-glutamyl aminomethylsulphonic acid injected focally into the substantia nigra and entopeduncular nucleus

Focal injection of gamma-D-glutamylaminomethylsulphonic acid (GAMS) into basal ganglia areas, the substantia nigra and entopeduncular nucleus, protects against electroshock (50 mA, 60 Hz, 0.2 s) and pilocarpine (380 mg/kg i.p.) induced seizures in rats. GAMS infusion into substantia nigra pars reticulata significantly protects against generalised (5, 10 nmol) and limbic (10, 50 nmol) seizures. The highest dose of GAMS results in similar suppression of seizure activity following injection into substantia nigra pars compacta against generalised (10 nmol) and limbic (50 nmol) seizures. The anticonvulsant action of GAMS is less marked after infusion into the entopeduncular nucleus. High doses of GAMS are required to provide protection against electroshock test (10 nmol) and limbic (40 nmol) seizures. Protection against limbic and electroshock seizures is more pronounced after infusion of GAMS into substantia nigra than into the entopeduncular nucleus. The substantia nigra pars reticulata is particularly sensitive to the anticonvulsant action of GAMS suggesting that antagonism at the kainate receptor is functionally important.

Excitatory amino acid and calcium content of subarachnoid spinal fluid during hypoglycemia in the rat

The concentrations of amino acids and calcium were measured in the subarachnoid cerebrospinal fluid (CSF) of rats prior to, and following, profound hypoglycemia. Calcium concentrations fell and amino acid changes were non-specific following hypoglycemia. In particular, elevations in excitatory amino acids did not occur, even though typical neuropathologic changes were found. These data do not support a role for excitatory amino acids or calcium as a 'CSF-borne toxin' in hypoglycemic brain injury. Moreover, the histopathological patterns of cell loss in the dentate gyrus often could not be reconciled with the concept of a CSF-borne toxin.

Olfactory bulbectomy protects against pilocarpine-induced motor limbic seizures in rats

Systemic injection of pilocarpine (380 mg/kg) results in the development of motor limbic seizures in rats. Olfactory bulbectomy performed 7 days prior to pilocarpine administration has a strong protective effect against the seizures. Neuropathological changes which normally occur in this seizure model do not develop in the brains of bulbectomized animals.

Excitotoxic mechanisms in hypoglycaemic hippocampal injury

Light and electron microscopy were used to study the effect of hypoglycaemia on selectively vulnerable neurons of rat hippocampus with and without pharmacologic blockade of the N-methyl-D-aspartate (NMDA)-preferring receptor with 2-amino-7-phosphonoheptanoic acid (AP-7). In control hypoglycaemic hippocampi, dark cell change occurs predominantly in dentate granule cells. The topography and ultrastructural appearance of these changes is distinct from that produced by ischaemia or status epilepticus. In hypoglycaemia, mitochondrial calcium accumulation characteristic of ischaemia or status epilepticus is not seen. NMDA receptor blockade markedly attenuates the hypoglycaemic cell injury. Similar attenuation of ischaemic and epileptic brain damage by NMDA receptor blockade suggest that excessive neuronal excitation is a common mechanism of injury in each of the three conditions.