Antiepileptic drugs and the developing brain

Epilepsy is the most common neurological disorder in young humans. Antiepileptic drugs (AEDs) which are used to treat seizures in infants, children and pregnant women can cause cognitive impairment, microcephaly and birth defects. Ion channels, neurotransmitters and second messenger systems constitute molecular targets of AEDs. The same targets regulate brain processes essential both for propagation of seizures and for learning, memory and emotional behavior. Thus, AEDs can influence brain function and brain development in undesired ways. Here we review mechanisms of action of AEDs, examine clinical evidence for their adverse effects in the developing human brain, and present studies on cognitive and behavioral effects in animal models. Furthermore, we discuss mechanisms responsible for adverse effects of AEDs in the developing mammalian brain, including interference with cell proliferation and migration, axonal arborization, synaptogenesis, synaptic plasticity and physiological apoptotic cell death.

Glutamate antagonists limit tumor growth

Neuronal progenitors and tumor cells possess propensity to proliferate and to migrate. Glutamate regulates proliferation and migration of neurons during development, but it is not known whether it influences proliferation and migration of tumor cells. We demonstrate that glutamate antagonists inhibit proliferation of human tumor cells. Colon adenocarcinoma, astrocytoma, and breast and lung carcinoma cells were most sensitive to the antiproliferative effect of the N-methyl-d-aspartate antagonist dizocilpine, whereas breast and lung carcinoma, colon adenocarcinoma, and neuroblastoma cells responded most favorably to the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate antagonist GYKI52466. The antiproliferative effect of glutamate antagonists was Ca(2+) dependent and resulted from decreased cell division and increased cell death. Morphological alterations induced by glutamate antagonists in tumor cells consisted of reduced membrane ruffling and pseudopodial protrusions. Furthermore, glutamate antagonists decreased motility and invasive growth of tumor cells. These findings suggest anticancer potential of glutamate antagonists.

Neuronal death enhanced by N-methyl-D-aspartate antagonists

Glutamate promotes neuronal survival during brain development and destroys neurons after injuries in the mature brain. Glutamate antagonists are in human clinical trials aiming to demonstrate limitation of neuronal injury after head trauma, which consists of both rapid and slowly progressing neurodegeneration. Furthermore, glutamate antagonists are considered for neuroprotection in chronic neurodegenerative disorders with slowly progressing cell death only. Therefore, humans suffering from Huntington's disease, characterized by slowly progressing neurodegeneration of the basal ganglia, are subjected to trials with glutamate antagonists. Here we demonstrate that progressive neurodegeneration in the basal ganglia induced by the mitochondrial toxin 3-nitropropionate or in the hippocampus by traumatic brain injury is enhanced by N-methyl-d-aspartate antagonists but ameliorated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate antagonists. These observations reveal that N-methyl-d-aspartate antagonists may increase neurodestruction in mature brain undergoing slowly progressing neurodegeneration, whereas blockade of the action of glutamate at alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors may be neuroprotective.

Autoimmune encephalomyelitis ameliorated by AMPA antagonists

Multiple sclerosis is an immune-mediated disorder of the central nervous system leading to progressive decline of motor and sensory functions and permanent disability. The therapy of multiple sclerosis is only partially effective, despite anti-inflammatory, immunosuppresive and immunomodulatory measures. White matter inflammation and loss of myelin, the pathological hallmarks of multiple sclerosis, are thought to determine disease severity. Experimental autoimmune encephalomyelitis reproduces the features of multiple sclerosis in rodents and in nonhuman primates. The dominant early clinical symptom of acute autoimmune encephalomyelitis is progressive ascending muscle weakness. However, demyelination may not be profound and its extent may not correlate with severity of neurological decline, indicating that targets unrelated to myelin or oligodendrocytes may contribute to the pathogenesis of acute autoimmune encephalomyelitis. Here we report that within the spinal cord in the course of autoimmune encephalomyelitis not only myelin but also neurons are subject to lymphocyte attack and may degenerate. Blockade of glutamate AMPA receptors ameliorated the neurological sequelae of autoimmune encephalomyelitis, indicating the potential for AMPA antagonists in the therapy of multiple sclerosis.

N-Methyl-D-aspartate antagonists and apoptotic cell death triggered by head trauma in developing rat brain

Morbidity and mortality from head trauma is highest among children. No animal model mimicking traumatic brain injury in children has yet been established, and the mechanisms of neuronal degeneration after traumatic injury to the developing brain are not understood. In infant rats subjected to percussion head trauma, two types of brain damage could be characterized. The first type or primary damage evolved within 4 hr and occurred by an excitotoxic mechanism. The second type or secondary damage evolved within 6-24 hr and occurred by an apoptotic mechanism. Primary damage remained localized to the parietal cortex at the site of impact. Secondary damage affected distant sites such as the cingulate/retrosplenial cortex, subiculum, frontal cortex, thalamus and striatum. Secondary apoptotic damage was more severe than primary excitotoxic damage. Morphometric analysis demonstrated that the N-methyl-D-aspartate receptor antagonists 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonate and dizocilpine protected against primary excitotoxic damage but increased severity of secondary apoptotic damage. 2-Sulfo-alpha-phenyl-N-tert-butyl-nitrone, a free radical scavenger, did not affect primary excitotoxic damage but mitigated apoptotic damage. These observations demonstrate that apoptosis and not excitotoxicity determine neuropathologic outcome after traumatic injury to the developing brain. Whereas free radical scavengers may prove useful in therapy of head trauma in children, N-methyl-D-aspartate antagonists should be avoided because of their propensity to increase severity of apoptotic damage.

Blockade of NMDA receptors and apoptotic neurodegeneration in the developing brain

Programmed cell death (apoptosis) occurs during normal development of the central nervous system. However, the mechanisms that determine which neurons will succumb to apoptosis are poorly understood. Blockade of N-methyl-D-aspartate (NMDA) glutamate receptors for only a few hours during late fetal or early neonatal life triggered widespread apoptotic neurodegeneration in the developing rat brain, suggesting that the excitatory neurotransmitter glutamate, acting at NMDA receptors, controls neuronal survival. These findings may have relevance to human neurodevelopmental disorders involving prenatal (drug-abusing mothers) or postnatal (pediatric anesthesia) exposure to drugs that block NMDA receptors.

ZK200775: a phosphonate quinoxalinedione AMPA antagonist for neuroprotection in stroke and trauma

Stroke and head trauma are worldwide public health problems and leading causes of death and disability in humans, yet, no adequate neuroprotective treatment is available for therapy. Glutamate antagonists are considered major drug candidates for neuroprotection in stroke and trauma. However, N-methyl-D-aspartate antagonists failed clinical trials because of unacceptable side effects and short therapeutic time window. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonists derived from the quinoxalinedione scaffold cannot be used in humans because of their insolubility and resulting renal toxicity. Therefore, achieving water solubility of quinoxalinediones without loss of selectivity and potency profiles becomes a major challenge for medicinal chemistry. One of the major tenets in the chemistry of glutamate antagonists is that the incorporation of phosphonate into the glutamate framework results in preferential N-methyl-D-aspartate antagonism. Therefore, synthesis of phosphonate derivatives of quinoxalinediones was not pursued because of a predicted loss of their selectivity toward AMPA. Here, we report that introduction of a methylphosphonate group into the quinoxalinedione skeleton leaves potency as AMPA antagonists and selectivity for the AMPA receptor unchanged and dramatically improves solubility. One such novel phosphonate quinoxalinedione derivative and competitive AMPA antagonist ZK200775 exhibited a surprisingly long therapeutic time window of >4 h after permanent occlusion of the middle cerebral artery in rats and was devoid of renal toxicity. Furthermore, delayed treatment with ZK200775 commencing 2 h after onset of reperfusion in transient middle cerebral artery occlusion resulted in a dramatic reduction of the infarct size. ZK200775 alleviated also both cortical and hippocampal damage induced by head trauma in the rat. These observations suggest that phosphonate quinoxalinedione-based AMPA antagonists may offer new prospects for treatment of stroke and trauma in humans.

Alprazolam dependence prevented by substituting with the beta-carboline abecarnil

Abrupt termination of the treatment of humans with benzodiazepines (BDZs) leads to a rapid onset of discontinuation syndrome characterized by anxiety, muscle spasms, and occasionally convulsions. For this reason, it is recommended in clinical practice to reduce the dose of the BDZs gradually at the end of treatment. Nevertheless, many clinicians report signs of dependence even during gradual reduction of doses (tapering) of the BDZs in a large proportion of patients. Thus, there is considerable interest in discovering means of weaning patients away from BDZs without the risk of discontinuation syndrome. In the present study, mice withdrawn from chronic treatment with alprazolam showed anxiety, muscle rigidity, and seizures between days 1 and 28 after termination of the treatment. Replacement of alprazolam with the beta-carboline abecarnil for 7 days prevented the occurrence of the signs of dependence. In contrast, substitution of the beta-carboline antagonist ethyl-5-isopropoxy-4-methyl-beta-carboline-3-carboxylate (ZK93426) for alprazolam worsened the discontinuation syndrome. Replacement therapy with abecarnil after long-term treatment with the BDZs offers a novel method for rapid tapering.

Prevention of trauma-induced neurodegeneration in infant and adult rat brain: glutamate antagonists

The mechanisms of neuronal degeneration following traumatic head injury are not well understood and no adequate treatment is currently available for the prevention of traumatic brain damage in humans. Seven day old rat pups were subjected to mechanical percussion of the head. Cortical damage in infant rats was reduced by pre-treatment with the N-methyl-D-aspartate (NMDA) antagonists dizocilpine (MK-801) or 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-I-phosphonate (CPP). The AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo (f) quinoxaline (NBQX) did not significantly suppress cortical damage in infant rats. In adult rats, traumatic head injury leads to primary (at impact-cortex) and secondary (distant-hippocampus) damage to the brain. Morphometric analysis demonstrated that both cortical and hippocampal damage was mitigated by pre-treatment with either the NMDA antagonist CPP or the non-NMDA antagonist NBQX. Neither treatment prevented primary damage in the cortex when therapy was started after trauma. Delayed treatment of rats with NBQX, but not with CPP, beginning between 1 and 7 h after trauma prevented the hippocampal damage. No protection was seen when therapy with NBQX was started 10 h after trauma. These data indicate that NMDA antagonists may possess better neuroprotective properties against excitotoxic processes triggered by traumatic brain injury in young individuals whereas AMPA antagonists may be more beneficial in adults.

Traumatic brain damage prevented by the non-N-methyl-D-aspartate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f] quinoxaline

The mechanisms of neuronal degeneration following traumatic head injury are not well understood and no adequate treatment is currently available for the prevention of traumatic brain damage in humans. Traumatic head injury leads to primary (at impact) and secondary (distant) damage to the brain. Mechanical percussion of the rat cortex mimics primary damage seen after traumatic head injury in humans; no animal model mimicking the secondary damage following traumatic head injury has yet been established. Rats subjected to percussion trauma of the cortex showed primary damage in the cortex and secondary damage in the hippocampus. Morphometric analysis demonstrated that both cortical and hippocampal damage was mitigated by pretreatment with either the N-methyl-D-aspartate (NMDA) antagonist 3-((+/-)- 2-carboxypiperazin-4-yl)-propyl-1-phosphonate (CPP) or the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX). Neither treatment prevented primary damage in the cortex when therapy was started after trauma. Surprisingly, delayed treatment of rats with NBQX, but not with CPP, beginning between 1 and 7 hr after trauma prevented hippocampal damage. No protection was seen when therapy with NBQX was started 10 hr after trauma. These data indicate that both NMDA- and non-NMDA-dependent mechanisms contribute to the development of primary damage in the cortex, whereas non-NMDA mechanisms are involved in the evolution of secondary damage in the hippocampus in rats subjected to traumatic head injury. The wide therapeutic time-window documented for NBQX suggests that antagonism at non-NMDA receptors may offer a novel therapeutic approach for preventing deterioration of the brain after head injury.

Effects of the antiparkinsonian drug budipine on central neurotransmitter systems

Budipine is a novel antiparkinsonian drug which is particularly beneficial in the treatment of parkinsonian tremor. The mechanism of action of budipine is not fully understood. To study whether budipine has dopaminergic activity in vivo, we used the 6-hydroxydopamine rotational model of Parkinson's disease. Budipine (0.78-12.5 mg/kg i.p.) did not induce ipsilateral or contralateral rotations, suggesting that it does not possess direct or indirect dopaminergic activity. This conclusion is further supported by the observation that budipine (10 mg/kg) i.v. did not facilitate striatal dopamine release measured in vivo by brain microdialysis. To investigatate possible antimuscarinic and N-methyl-D-aspartic acid (NMDA) antagonistic properties of budipine, we compared budipine with the antimuscarinic antiparkinsonian drug biperiden and the NMDA receptor antagonist 3-[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid (CPP). In receptor-binding assays, budipine inhibited thienylcyclohexylpiperidyl-3,4-[3H](n) ([I3H]TCP) (2.5 nM)-binding with an IC50 of 36 microM and [3H]3-quinuclidinol benzilate-binding with an IC50 of 1.1 microM. The respective values for biperiden were 170 and 0.053 microM. In line with these findings, budipine and CPP increased the threshold for NMDA-induced seizures in mice with an ED50 of 10.2 and 4.4 mg/kg, respectively, whereas biperiden was not effective. In 6-hydroxydopamine-lesioned rats, budipine (3.13-12.5 mg/kg) and CPP (0.1-0.39 mg/kg) increased the number of contralateral rotations induced by apomorphine, whereas biperiden was not effective. The present data suggest that budipine acts by blocking muscarinic and NMDA transmission while facilitation of dopaminergic transmission does not appear to contribute to its in vivo action. In comparison to biperiden, which has also antimuscarinic and NMDA receptor antagonistic properties, the anti-NMDA action of budipine is more prominent.

Effects of 7-nitroindazole, NG-nitro-L-arginine, and D-CPPene on harmaline-induced postural tremor, N-methyl-D-aspartate-induced seizures, and lisuride-induced rotations in rats with nigral 6-hydroxydopamine lesions

The present behavioral study was undertaken to investigate whether neuronal nitric oxide (NO) synthase mediates the abnormal consequences of increased NMDA receptor-mediated synaptic transmission in models of postural tremor, Parkinson's disease and epilepsy. We used 7-nitroindazole, a selective inhibitor of neuronal NO synthase, and NG-nitro-L-arginine (L-NAME), an unspecific NO synthase inhibitor, and compared their action with that of the competitive NMDA receptor antagonist 3-[(R)-2-carboxypiperazin-4-yl]-prop-2-enyl-1-phosphonic acid (D-CPPene). In both mice and rats, 7-nitroindazole, L-NAME and D-CPPene dose dependently reversed the harmaline-induced increase of cerebellar cyclic guanosine-5'-monophosphate (cGMP) levels. For subsequent behavioral experiments we used doses of 7-nitroindazole, L-NAME and D-CPPene which were equipotent in preventing harmaline-induced cGMP increase. Harmaline-induced tremor in mice and rats was suppressed by D-CPPene, but not by 7-nitroindazole or by L-NAME. This effect of D-CPPene was not due to unspecific suppression of motor activity, since D-CPPene did not affect locomotor activity at doses which reduced tremor. D-CPPene, but not 7-nitroindazole and L-NAME potentiated the antiparkinsonian action of the dopamine agonist lisuride in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra. D-CPPene antagonized seizures induced by intracerebroventricular injection of NMDA in mice. In contrast, 7-nitroindazole and L-NAME had only a tendency to prevent seizures and to delay the latency to onset of seizures. We conclude from these results that neuronal NO synthase does not serve as a major mediator of increased NMDA receptor-mediated synaptic transmission in animal models of Parkinson's disease, postural tremor and epilepsy. The novel observation that D-CPPene suppresses harmaline-induced tremor leads us to suggest that NMDA receptor antagonists should be considered as novel therapeutics for postural tremor.

Glutathione depletion potentiates MPTP and MPP+ toxicity in nigral dopaminergic neurones

Glutathione levels are decreased in the substantia nigra of patients with Parkinson's disease. We studied whether glutathione depletion contributes to dopaminergic cell death using a specific inhibitor of glutathione biosynthesis, L-buthionine sulfoximine (BSO). We found no significant reduction of tyrosine hydroxylase-positive cells in the substantia nigra pars compacta (SNpc) when BSO was administered systemically to preweanling mice or locally to the SNpc of adult rats. However, the combination of BSO with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in preweanling mice and the combination of nigral injections of BSO with intrastriatal injections of MPP+ (1-methyl-4-phenylpyridinium), the active metabolite of MPTP in adult rats, potentiated the toxic effects of MPTP and MPP+ on nigral neurones. Our data show that glutathione depletion can result in cell death if the nigrostriatal system is metabolically compromised.

Neurodegenerative disorders: clues from glutamate and energy metabolism

It is well established that glutamate receptors play a major role in mediating acute ischemic neuronal degeneration in the CNS. Cerebral ischemia and head or spinal cord trauma are associated with excessive release and extracellular accumulation of glutamate, which leads to persistent activation of glutamate receptors and acute neurotoxic degeneration of the hyperstimulated neuron. It has been more difficult to link neuronal degeneration that occurs in chronic neurodegenerative disorders to an excitotoxic mechanism. However, accumulating evidence suggests that impairment of intracellular energy metabolism associated with hyperactivation of glutamate receptors may be a common mechanism contributing to neuronal death in such disorders. It is proposed that impaired energy metabolism results in deterioration of membrane function and loss of the voltage-dependent Mg2+ block of N-methyl-D-aspartate receptors, which allows persistent activation of these receptors by glutamate, even if concentrations of glutamate at the receptor are within the normal physiological range. Studies in rodents using mitochondrial respiratory chain toxins, such as aminooxyacetic acid, 1-methyl-4-phenylpyridinium ion, malonic acid, and 3-nitropropionic acid, suggest that these agents do induce CNS degeneration by a process involving an excitotoxic mechanism. Striatal and nigral degeneration induced by mitochondrial toxins in rodents resembles neuropathology seen in humans suffering from Huntington's or Parkinson's disease and can be attenuated by glutamate receptor antagonists and agents that improve energy metabolism. Such experimental observations suggest that disturbed energy metabolism and glutamate may be involved in neuronal death leading to abiotrophic/neurodegenerative disorders in humans. If so, glutamate antagonists or agents that improve energy metabolism may slow the degenerative process and offer a therapeutic approach for temporarily retarding the progression of these disabling disorders.

Excitotoxicity and neurodegenerative diseases

Glutamate is an excitatory neurotransmitter in the mammalian central nervous system and a neurotoxin (excitotoxin) that has the potential to destroy neurones by activation of ionotropic receptors. In contrast to the well documented role of glutamate in the pathogenesis of neuronal degeneration resulting from hypoxia/ischaemia, hypoglycaemia, status epilepticus and trauma, it has been difficult to establish a link between the excitotoxicity and neuronal death that occur in chronic neurodegenerative disorders. Impairment of energy metabolism has been shown to increase neuronal vulnerability to glutamate. The cause of this phenomenon lies in the attenuation of the Mg2+ blockade of the N-methyl-D-aspartate receptors that leads to persistent activation of these receptors by physiologic extracellular glutamate concentrations. The concept of increased neuronal vulnerability to excitotoxic injury establishes a link between slow neuronal degeneration and excitotoxicity and suggests that glutamate antagonists may prove beneficial in the treatment of chronic neurodegenerative diseases that have been resistant to therapy.

Spontaneous recurrent seizures in rats: amino acid and monoamine determination in the hippocampus

Rats subjected to structural brain damage induced by sustained convulsions triggered by systemic administration of pilocarpine (PILO) are a useful model for investigation of the mechanisms essential for seizure generation and spread in rodents. After PILO administration, three distinct phases are observed: (a) an acute period of 1-2 days' duration corresponding to a pattern of repetitive limbic seizures and status epilepticus; (b) a seizure-free (silent) period characterized by a progressive return to normal EEG and behavior of 4-44 days' duration; and (c) a period of spontaneous recurrent seizures (SRS) starting 5-45 days after PILO administration and lasting throughout the animal's life. PILO (320-350 mg/kg intraperitoneally, i.p.) was administered to rats, and the content of hippocampal monoamines and amino acids was measured in the acute, silent, and SRS periods by liquid chromatography. Norepinephrine (NE) level was decreased during all periods whereas dopamine (DA) content was increased. Serotonin (5-hydroxytryptamine, 5-HT) was increased only in the acute period. Utilization rate measurement of monoamines showed increased NE consumption and decreased DA consumption during all phases. 5-HT utilization rate was increased only in the acute period. Amino acid content showed a decrease in aspartate (ASP) and glutamate (GLU) concentrations associated with increased gamma-aminobutyric acid (GABA) level during the acute period. The silent phase was characterized by a decrease in glycine (GLY) and GABA levels and an increase in GLU concentration. The SRS period showed an increase in all amino acid concentrations. These findings show important neurochemical changes in the course of establishment of an epileptic focus after brain damage induced by status epilepticus triggered by pilocarpine.

Towards an understanding of the role of glutamate in neurodegenerative disorders: energy metabolism and neuropathology

It is thought that impairment of energy metabolism that results in deterioration of membrane function, leading to loss of the Mg2+ block on NMDA receptors, and allowing persistent activation of these receptors by glutamate, might be a cause of neuronal death in neurodegenerative disorders. Studies in rodents using mitochondrial respiratory chain toxins, such as aminooxyacetic acid, 1-methyl-4-phenylpyridinium, malonic acid and 3-nitropropionic acid, suggest that such processes may indeed be involved in neurotoxicity. Striatal and nigral degeneration induced by mitochondrial toxins in rodents resembles the neuropathology seen in humans suffering from Huntington's or Parkinson's disease, and can be prevented either by decortication or by NMDA receptor antagonists. Such experimental observations suggest that glutamate may be involved in neuronal death leading to neurodegenerative disorders in humans. If so, glutamate antagonists may offer a therapeutic approach for retarding the progression of these disabling disorders.

Effect of the beta-carboline abecarnil on spinal reflexes in mice and on muscle tone in genetically spastic rats: a comparison with diazepam

Abecarnil is a beta-carboline agonist at benzodiazepine receptors with potent anxiolytic activity but no muscle relaxant side effects in rodents. Clinical experience suggests that changes in the muscle tone induced by benzodiazepines are related to their effects on spinal reflexes. The authors therefore analyzed the effect of treatment with abecarnil on spinal monosynaptic (Hoffmann reflexes) and polysynaptic (flexor) reflexes in mice and the influence of abecarnil on muscle tone in genetically spastic rats. The i.v. administration of abecarnil in mice (dose range, 0.02-1 mg/kg) depressed flexor reflexes in a dose-dependent manner; Hoffmann reflexes remained unchanged. Administration of diazepam i.v. (0.01-1 mg/kg) also reduced flexor reflexes and had little or no effect on Hoffmann reflexes. In genetically spastic rats, i.v. administration of abecarnil (10-30 mg/kg) decreased the muscle tone in a dose- and time-dependent manner. A similar muscle relaxant effect was observed in such rats after i.v. administration of diazepam (0.1-0.8 mg/kg). By contrast, i.p. administration of abecarnil in mice did not influence spinal reflexes up to the dose of 1 mg/kg and, in genetically spastic rats, did not affect muscle tone up to the dose of 100 mg/kg. Administration of diazepam i.p. (1 mg/kg) depressed flexor reflexes in mice and over the range 0.2 to 5 mg/kg produced a dose- and time-dependent decrease of muscle tone in genetically spastic rats. The muscle relaxant effect of i.p. diazepam could be antagonized by i.p. administered abecarnil. These studies thus demonstrate that i.v. but not i.p. administration of abecarnil may result in muscle relaxant action in mice and in genetically spastic rats.

Kindling to the benzodiazepine receptor inverse agonist, FG 7142: evidence for involvement of NMDA, but not non-NMDA, glutamatergic receptors

Repeated administration of the beta-carboline FG 7142 to mice leads to the development of kindled convulsions. In order to investigate a role for glutamatergic mechanisms in the processes underlying FG 7142 kindling, the N-methyl-D-aspartate (NMDA) antagonist, 2-amino-7-phosphono-heptanoic acid (AP7; 25 nmol), was administered intracerebroventricularly (i.c.v.) daily before administration of FG 7142 (40 mg/kg, i.p.). Under these conditions, kindling to FG 7142 did not occur. Administration of two antagonists at non-NMDA excitatory amino acid receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and gamma-D-glutamylaminomethylsulphonic acid (gamma-D-GAMS; both 25 nmol) did not prevent the development of seizures; these doses were, however, adequate and selective in protecting against seizures induced by respectively quisqualic and kainic acids given by i.c.v. The susceptibility of mice kindled with FG 7142 to seizures induced by NMDA, or kainate or quisqualate was similar in mice which had shown 5 kindled seizures to that seen in drug-naive mice; mice which had shown 10 kindled seizures showed a decreased sensitivity to NMDA-induced convulsions (ED50 was increased from 0.24 to 0.31 nmol). No changes were seen in the convulsant thresholds of either NMDA or non-NMDA agonists. These observations suggest that although NMDA receptors appear to be involved in the processes underlying FG 7142 kindling, such kindling is not necessarily associated with an increased sensitivity of glutamate receptors, and in animals which have convulsed, a decreased sensitivity to NMDA agonists occurs.

Toward an understanding of the role of glutamate in experimental parkinsonism: agonist-sensitive sites in the basal ganglia

Increased glutamatergic transmission in the basal ganglia is implicated in the pathophysiology of Parkinson's disease. However, the mechanisms by which activation of glutamate receptors produce parkinsonism are unknown. Therefore, we examined whether the glutamate agonists N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate produce parkinsonism in rats after microapplication into different subregions of the basal ganglia. Electromyographic activity was used as a measure of parkinsonian rigidity. We found that in the rostral striatum, excitation mediated by NMDA but not by non-NMDA receptors led to parkinsonism. In the substantia nigra pars reticulata, internal pallidal segment/entopeduncular nucleus, and subthalamic nucleus, activation of AMPA/kainate and metabotropic receptors but not of NMDA receptors led to parkinsonian rigidity. Rigidity occurred also in animals bearing ibotenate-induced lesions of the posterior part of the striatum and of the external pallidal segment, but not in animals with lesions of the anterior striatum, subthalamic nucleus, internal pallidal segment/entopeduncular nucleus, or substantia nigra pars reticulata. These observations suggest that the activation of glutamate receptor subtypes in the basal ganglia may be differentially involved in the expression of parkinsonian symptoms.

Diazepam dependence prevented by glutamate antagonists

Long-term treatment leads to tolerance to and dependence on benzodiazepines. Abrupt termination of benzodiazepine administration triggers the expression of signs of dependence. Mice withdrawn from chronic treatment with diazepam showed a time-related evolution of anxiety, muscle rigidity, and seizures between days 4 and 21 after treatment discontinuation. A period between withdrawal days 1 and 3 was symptom-free. Surprisingly, during this "silent phase" the susceptibility of mice to alpha-amino-3-hydroxy-5-tert-butyl-4-isoxazolepropionate (ATPA) and kainate seizures and the magnitude of monosynaptic reflexes mediated by non-N-methyl-D-aspartate (NMDA) mechanisms were enhanced. In apparent contrast, the "active phase", between withdrawal days 4 and 21, was characterized by increased susceptibility to NMDA seizures and enhanced magnitude of polysynaptic reflexes, which are NMDA dependent. Treatment of mice with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonists 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466) or 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline but not with the NMDA antagonist 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) during the silent phase prevented signs of dependence. In contrast, treatment with CPP but not with GYKI 52466 during the active phase prevented the symptoms. The development of tolerance to and dependence on diazepam was prevented by concurrent treatment of mice with CPP but was not prevented by GYKI 52466. These data indicate that NMDA-dependent mechanisms contribute to the development of tolerance to diazepam and to the expression of signs of dependence in mice after termination of long-term treatment with diazepam. Nevertheless, the non-NMDA-mediated silent phase is essential for triggering the symptoms. Therefore, AMPA antagonists may offer a therapeutic approach for preventing dependence on benzodiazepines that is an alternative to NMDA antagonism.

The antiparkinsonian agent budipine is an N-methyl-D-aspartate antagonist

Budipine (1-t-butyl-4,4-diphenylpiperidine) is a novel antiparkinsonian agent. Its clinical efficacy has been proven in double-blind placebo-controlled trials. The mechanism of action of budipine, however, is unknown. Budipine selectively increased the threshold of N-methyl-D-aspartate (NMDA)-induced seizures in mice. Similar to known specific NMDA antagonist, budipine depressed polysynaptic spinal reflexes in mice, but had no consistent effect on spinal monosynaptic reflexes. In receptor binding experiments, budipine displaced thienylcyclohexylpiperidyl-3,4-[3H]-(n) ([3H]-TCP) from its binding site with an IC50 of 36 microM suggesting that budopine acts as a non-competitive NMDA antagonist with moderate receptor affinity. It is concluded that the newly discovered NMDA antagonistic action of budipine is at least partly responsible for its antiparkinsonian activity. Our findings are additional evidence for the hypothesis that NMDA antagonists may be useful in the treatment of Parkinson's disease (PD).

Modulation of the seizure threshold for excitatory amino acids in mice by antiepileptic drugs and chemoconvulsants

A novel method for the assessment of the threshold for clonic seizures induced by excitatory amino acids based on continuous infusion of the glutamate agonists [alpha-amino-3-hydroxy-5-terbutyl-4-isoxazolepropionate (ATPA), kainate or N-methyl-D-aspartate (NMDA)] into the lateral brain ventricle of unrestrained mice is reported. Using this novel method of seizure threshold determination, it was found that systemically administered diphenylhydantoin and carbamazepine elevated the threshold for ATPA and had negligible effects on the threshold for kainate and NMDA. Phenobarbital and trimethadione elevated the threshold for all excitatory amino acids tested, whereas valproate elevated the threshold for ATPA and kainate seizures. Ethosuximide elevated the threshold for ATPA and kainate and decreased the threshold for NMDA seizures. The quisqualate antagonists [2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine] elevated the threshold for ATPA and less so for kainate seizures, whereas the NMDA antagonist 3-((+-)2-carboxypiperazine-4-yl)-propyl-1-phosphonate elevated the threshold for NMDA seizures. 1-(4-Aminophenyl)4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine in higher doses was also active against NMDA seizures, whereas 3-((+-)2-carboxypiperazine-4-yl)-propyl-1-phosphonate did so with kainate seizures. Among seven different convulsants, pentylenetetrazol, picrotoxin and the beta-carboline methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate lowered the threshold for seizures induced by excitatory amino acids. Pentylenetetrazol and picrotoxin did so with kainate seizures, whereas methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate lowered ATPA thresholds. Bicuculline, 3-mercaptopropionate, strychnine and pilocarpine were inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term treatment with abecarnil does not induce diazepam-like dependence in mice

Abecarnil (isopropyl-6-benzyloxy-4-methoxymethyl-beta-carboline-3-carboxylate) is a metabolically stable anxiolytic and anticonvulsant beta-carboline derivative with few sedative and muscle relaxant effects in rodents. Abecamil binds with high affinity to benzodiazepine receptors. Because long-term treatment with benzodiazepines leads to development of dependence, we evaluated in mice whether abecarnil also possesses a potential for producing dependence, using electroencephalographic and electromyographic monitoring, and behavioral assessment of anxiety to detect withdrawal responses after chronic treatment. Diazepam was used as a reference. Mice withdrawn from chronic treatment with diazepam (15 mg/kg/day for 12 days) showed a time-related evolution of anxiety, muscle rigidity and seizures between days 4 and 21 after discontinuation of the treatment. A period between withdrawal days 1 and 3 was symptom free. Mice withdrawn from chronic administration of abecarnil (6 mg/kg/day for 12 days) showed no anxiety and no changes in seizure susceptibility and muscle tone. The doses of diazepam and abecarnil used for chronic treatment were equivalent in terms of kinetics and binding to benzodiazepine receptors. These data indicate that long-term treatment with abecarnil does not induce benzodiazepine-like dependence in mice. Thus, it may be predicted that chronic treatment with abecarnil in humans may offer an important alternative to benzodiazepines in the treatment of anxiety.

Inhibition of cisplatin-induced emesis in ferrets by the non-NMDA receptor antagonists NBQX and CNQX

The excitatory amino acid (EAA) receptor antagonists, 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(f)quinoxaline (NBQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), which preferentially block non-N-methyl-D-aspartate (non-NMDA) subtypes of EAA receptors, effectively inhibit cisplatin-induced emesis in ferrets. A high dose of cisplatin (10 mg/kg i.v.) was used which induced emesis in all saline-treated control ferrets. At 10 mg/kg i.v., NBQX totally prevented cisplatin-induced emesis in 5 of 6 ferrets and CNQX totally prevented emesis in 3 of 5 ferrets. By comparison, each of the 5-HT3 inhibitors, zacopride and ondansetron, at 1.0 mg/kg i.v. (a dose considered in the high therapeutic range for controlling emesis by these compounds), totally prevented emesis in 2 of 5 ferrets. It is concluded that non-NMDA antagonists effectively inhibit cisplatin-induced emesis. They are potential antiemetic compounds, alone or in combination with 5-HT3 antagonists or other more conventional drugs of choice.

Relief of experimental spasticity and anxiolytic/anticonvulsant actions of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline

Spasticity is characterized by pathological overactivity in spinal stretch reflex circuits and may be associated with disturbances in excitatory amino acid-mediated transmission in the cord. A genetically determined syndrome of spasticity in the rat permits the quantitative evaluation of the antispastic effects of drugs by recording activity in the electromyogram (EMG) from a hind limb extensor muscle. In genetically spastic rats, systemic administration of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F) quinoxaline (NBQX), normalized pathologically increased EMG activity, whereas the AMPA agonist, alpha-amino-3-hydroxy-5-tertbutyl-4-isoxazolepropionate (ATPA), exacerbated the EMG measures of spasticity. The reflex mechanisms in the spinal cord can be studied in mice using EMG recordings from the tibial muscle (Hoffmann reflex) or from the plantar foot muscle (flexor reflex) after electrical stimulation of the tibial nerve. Systemic and i.t. administration of NBQX blocked Hoffmann reflexes in mice, leaving flexor reflexes unchanged. ATPA enhanced Hoffmann, and had no effect on flexor reflexes. The effects of NBQX on spinal reflexes were seen in doses which do not affect locomotor activity, but show anxiolytic and some antiepileptic activity in rodents. These data suggest that the design of novel muscle relaxant drugs acting at the AMPA subtype of glutamate receptors may be feasible.

Excitatory amino acid antagonists protect mice against MPP+ seizures

Administration of 1-methyl-4-phenyl-pyridinium ion (MPP+) into the lateral ventricle of mice induced clonic convulsions and lethality in a dose- and age-dependent manner. MPP+ failed to induce seizures in 4-day-old mice, and the convulsant response to MPP+ was enhanced in aged mice. The seizures triggered by MPP+ in adult mice were blocked by coadministration of L-glutamate antagonists active at kainate/AMPA receptors such as gamma-D-glutamylaminomethylsulphonate and 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo[f]quinoxaline. The N-methyl-D-aspartate (NMDA) antagonist 2-amino-7-phosphonoheptanoate, but not kynurenate, also protected mice against MPP+ convulsions. Similarly, the benzodiazepine midazolam and the adenosine A1 agonist 2-chloroadenosine, but not antiepileptic drugs such as phenobarbital, trimethadione, ethosuximide, or acetazolamide, showed a protective efficacy against seizures. Additionally, the excitatory amino acid antagonists as well as phenobarbital, midazolam and 2-chloroadenosine protected mice against MPP+ lethality. These data suggest that convulsant action of MPP+ and its lethality in rodents may be mediated by excitatory amino acids.

The AMPA receptor antagonist NBQX has antiparkinsonian effects in monoamine-depleted rats and MPTP-treated monkeys

Abnormally increased subthalamic nucleus output to the internal pallidal segment and the reticular part of the substantia nigra plays a critical pathophysiological role in the development of parkinsonism. Because synaptic transmission of subthalamic output is glutamatergic and mediated, in part, by the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptor, AMPA receptor antagonists may possess antiparkinsonian properties. We report that in monoamine-depleted rats, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX) (Novo-Nordisk, Copenhagen, Denmark)--a selective antagonist of the AMPA subtype of glutamate receptor--suppressed muscular rigidity but had no effect on akinesia. NBQX microinjected into the subthalamic nucleus, internal pallidal segment, and reticular part of the substantia nigra, but not into the laterodorsal neostriatum of the rats, stimulated locomotor activity and reduced muscular rigidity. In aged Rhesus monkeys with bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism, intramuscular NBQX produced clinically apparent improvement in akinesia, tremor, posture, and gross motor skills. NBQX also potentiated the antiparkinsonian effects of L-3,4-dihydroxyphenylalanine in both rats and monkeys. Blockade of excitatory synaptic transmission by AMPA receptor antagonists may provide a new therapeutic strategy for Parkinson's disease (PD).

Long-term effects of pilocarpine in rats: structural damage of the brain triggers kindling and spontaneous recurrent seizures

Structural damage of the human brain (perinatal damage, cerebral trauma, head injury, cerebrovascular and degenerative diseases, intracranial tumor, metabolic diseases, toxins, drug-induced seizures) may lead to chronic epilepsy in survivors. Epidemiologic analyses show that a considerable time-delay occurs between the exposure of the brain to injury and the appearance of seizures. Such seizures are usually partial or mixed, may develop at any age, and are difficult to treat. In rats subjected to structural damage of the brain induced by sustained convulsions triggered by systemic administration of the cholinergic agent pilocarpine, spontaneous seizures may develop after a mean latency of 14-15 days. The mean frequency of spontaneous recurrent convulsions remains constant for several months. Evolution of these convulsions proceeds through several electrographic and behavioral stages resembling kindling. Kindling may be otherwise induced in rodents by repeated systemic administration of convulsants or by repeated electrical stimulation of sensitive brain regions. These observations demonstrate that structural damage of the brain may lead to spontaneously recurrent convulsions (chronic epilepsy) in rats and that kindling may be involved in the evolution of such a condition. This finding suggests that kindling mechanisms underlie the development of epileptic foci from structural brain lesions. Such mechanisms may be involved in the etiology of some forms of epilepsy in humans.

Synergism of the AMPA-antagonist NBQX and the NMDA-antagonist CPP with L-dopa in models of Parkinson's disease

Degeneration of dopaminergic nigrostriatal neurons in Parkinson's disease results in an overactivity of excitatory glutamatergic projections from the subthalamic nucleus to the output nuclei of the basal ganglia resulting in rigidity and akinesia. In theory pharmacological blockade of these overactive systems should improve parkinsonian symptomatology. The selective AMPA-antagonist NBQX and the competitive NMDA-antagonist CPP are not effective in animal models of Parkinson's disease when given alone but ameliorate parkinsonian symptomatology and stimulate locomotor activity when co-administered with a threshold dose of L-Dopa. These synergistic effects are seen in the MPTP-treated (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) common marmoset and the rat with unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra. Therefore competitive NMDA and non-NMDA antagonists may offer a new therapeutic strategy for the treatment of Parkinson's disease.

Protection of substantia nigra from MPP+ neurotoxicity by N-methyl-D-aspartate antagonists

Intake of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) leads to symptoms of Parkinson's disease and produces degeneration of nigrostriatal dopaminergic neurons in humans, giving rise to the hypothesis that this disorder may be caused by endogenous or environmental toxins. Excitation mediated by dicarboxylic amino acids such as L-glutamate or L-aspartate, has been claimed to be involved in pathogenesis of neurodegenerative disorders. We therefore sought to determine whether antagonists active at the NMDA or quisqualate subtypes of L-glutamate receptors prevent toxicity of either MPP+ (1-methyl-4-phenyl-pyridinium ion, the active metabolite of MPTP) or the selective dopaminergic neurotoxin 6-OHDA in the rat substantia nigra pars compacta. We report here that certain selective NMDA antagonists (AP7, CPP, MK-801), but not the preferential quisqualate antagonists CNQX and NBQX, provided short-term (up to 24 h) protection against MPP+ toxicity when coadministered into the substantia nigra. Systemic administration of CPP or MK-801 also offered temporary protection for up to 4 h against MPP+ toxicity. Repeated systemic administration of either compound prolonged protection against MPP+ challenge. Repeated administration for at least 24 h also led to permanent protection, still evident 7 days after intranigral administration of MPP+.

Paradoxical anticonvulsant activity of the gamma-aminobutyrate antagonist bicuculline methiodide in the rat striatum

Bicuculline methiodide (BMI), a gamma-aminobutyrate (GABA) antagonist, is a powerful convulsant agent when injected into the cerebral ventricles, amygdala, hippocampus, thalamus, neocortex, and deep prepiriform cortex in rats. In contrast, bilateral microinjection of BMI into the rat striatum confers protection against seizures induced by the cholinergic agonist pilocarpine (380 mg/kg, i.p.), with an ED50 of 94 fmol (range 45-195 fmol). No topographical variation in the anticonvulsant action of BMI was detected throughout rostrocaudal and dorsoventral aspects of the striatum. The anticonvulsant action of BMI in the striatum was reversed by coadministration of the GABA agonist muscimol or by blocking GABA-mediated inhibition in either the substantia nigra pars reticulata or in the entopeduncular nucleus. The results show that blockade of GABA-mediated inhibition in the striatum has a powerful anticonvulsant effect in the pilocarpine model, suggesting that GABAergic transmission in the striatum modulates the seizure propagation in the forebrain.

NMDA antagonists potentiate antiparkinsonian actions of L-dopa in monoamine-depleted rats

Systemically administered N-methyl-D-aspartate (NMDA) antagonists, MK-801 ((+)5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate) and CPP (3-[(+-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate), potentiate the ability of L-dopa (L-3,4-dihydroxyphenylalanine) to reverse akinesia and to alleviate muscular rigidity in monoamine-depleted rats. On the basis of these findings, it is proposed that NMDA antagonists may be beneficial as adjunctive treatment in the therapy of Parkinson's disease. CPP locally injected into the subthalamic nucleus, entopeduncular nucleus--the rat homologue of the internal pallidal segment--or substantia nigra pars reticulata of monoamine-depleted rats stimulates locomotor activity and alleviates rigidity, whereas local microinjection of CPP into the neostriatum is ineffective. These results make it unlikely that the neostriatum is the site of the antiparkinsonian action of NMDA antagonists in monoamine-depleted rats, whereas the subthalamic nucleus, internal pallidal segment, and substantia nigra pars reticulata appear to be important for the effects of NMDA antagonists.

Substantia nigra: a site of action of muscle relaxant drugs

Sites of action of centrally active muscle relaxant drugs are not well defined. Clinical experience with such drugs suggests that the spinal cord may be one of the important regions from which pathologically increased muscle tone may be relieved. Supraspinal centers that may also be involved in the expression of muscle relaxant action have not yet been defined. We report here that microinjections of therapeutically relevant muscle relaxants into the midbrain tegmentum of genetically spastic rats decrease muscle tone. The substantia nigra is the region from which midazolam, baclofen, and tizanidine (drugs used clinically in the treatment of spasticity), or gamma-vinyl-GABA, (-)-2-amino-7-phosphonoheptanoate, and [D-pro2-D-phe7-D-trp9]-substance P (experimental drugs active in animal models of spasticity), reduce muscle tone in genetically spastic rats and Hoffmann reflexes in normal rats. The effects of muscle relaxant drugs are topographically restricted to the substantia nigra pars reticulata and are receptor specific. These observations disclose a previously unknown function of the substantia nigra in mediating muscle relaxation.

Substantia nigra regulates action of antiepileptic drugs

The cholinergic agonist pilocarpine triggers sustained limbic seizures in rodents. Pilocarpine seizures were blocked by systemic administration of benzodiazepines, barbiturates, valproate and trimethadione, while diphenylhydantoin did not affect, and ethosuximide increased the susceptibility of rats to such seizures. This pattern of action of antiepileptic drugs is characteristic for pilocarpine seizures and different from other rodent models of epilepsy. Although the anatomical substrates in the forebrain involved in the expression of anticonvulsant activity are unknown, the basal ganglia are believed to be essential for the motor expression of pilocarpine seizures. Bilateral microinjections into the substantia nigra, a major output station of the basal ganglia, of midazolam (ED50 38.5 nmol; range 29-52 nmol), phenobarbital (ED50 16 nmol; range 7-39 nmol) and trimethadione (ED50 30 nmol; range 16-56 nmol) protected rats against pilocarpine seizures (380 mg/kg i.p.) Diphenylhydantoin (up to 100 nmol) remained inactive, while ethosuximide (ED50 38 nmol; range 22-65.5 nmol) reduced the threshold for pilocarpine seizures, converting subconvulsant doses of pilocarpine (200 mg/kg i.p.) into convulsant ones. The profiles of action of antiepileptic drugs on pilocarpine seizures were similar following intranigral and systemic administration. These observations suggest that the substantia nigra may mediate some actions of antiepileptic drugs.

Differential effects of the excitatory amino acid antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 3-((+-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), on spinal reflex activity in mice

Intrathecal administration of the preferential quisqualate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in anesthetized mice depressed Hoffmann (H)-reflexes, while flexor reflexes remained unaffected. The depressant effect of CNQX on H-reflexes was dose-dependent (range 0.1-10 nmol). The intrathecal administration of the selective N-methyl-d-aspartate (NMDA) antagonist 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) reduced flexor reflexes (range 10-100 nmol) and had no effect on H-reflexes. These results suggest that H-reflexes in mice are mediated by spinal non-NMDA receptors, while flexor reflexes involve NMDA receptors.

[The N-methyl-D-aspartate receptor complex. Various sites of regulation and clinical consequences]

Amino acids such as L-glutamate und L-aspartate are major excitatory neurotransmitters in the mammalian central nervous system (CNS) and potential neurotoxins (excitotoxins), which can destroy central neurons by excessive activation of respective receptors. In the last three decades evidence has accumulated that excitatory amino acids (EAA) are involved in many neurological diseases and that pharmacological intervention offers prospects of novel and more effective therapies. Three different receptor types for EAA have been identified, each being named by the selective agonist to which it is preferentially sensitive, i.e. N-methyl-D-aspartate- (NMDA), kainate- and quisqualate-receptors. In this review interest is focused primarily on the NMDA-receptor, whose structure has been subject of numerous electrophysiological and biochemical studies. Today, it is well established that the NMDA-receptor-ionophore complex has an agonist binding site for glutamate, NMDA and related EAAs which is coupled with an ion channel permeable to Na+, K+, Cl- and Ca2+. Four other binding sites for glycine, phencyclidine, Mg2+ and Zn2+ have been identified which can differentially modulate the function of the NMDA receptor. An additional polyamine binding site has recently been reported. Numerous studies on experimental animals demonstrate that modulators of NMDA-mediated neurotransmission may have antiepileptic, anxiolytic, muscle-relaxant and memory-enhancing effects. Particular interest has gained the possible neuroprotective efficacy of NMDA-receptor antagonists in neurological diseases such as hypoxia/ischemia, hypoglycemia, epilepsy and chronic neurodegenerative disorders (Huntington's, Alzheimer's and Parkinson's disease, amyotrophic lateral sclerosis, and AIDS encephalopathy).(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant action of the beta-carboline abecarnil: studies in rodents and baboon, Papio papio

Abecarnil (ZK 112119; isopropyl-6-benzyloxy-4-methoxymethyl-beta-carboxylate) is a metabolically stable beta-carboline derivative with potent anxiolytic and few sedative and ataxic effects in rodents. The anticonvulsant and muscle relaxant actions of abecarnil have been evaluated in mice, rats, gerbils and baboons. Abecarnil raised the threshold for tonic electroconvulsions in mice after corneal but not after auricular application, had no effect on maximal electroshock-induced tonic convulsions triggered by either method, protected mice against the tonic hindlimb extension in PTZ-, picrotoxin- and 3-mercaptopropionate-induced seizures and blocked clonus after PTZ, DMCM (methyl-4-ethyl-6,7-dimethoxy-9H-pyrido-(3,4-b)-indol-3-carboxylate) and 3-mercaptopropionate. Abecarnil had no effect on convulsions induced by bicuculline and strychnine. Furthermore, abecarnil blocked kindled seizures after chronic administration of PTZ and FG 7142 (beta-carboline-3-carboxylic acid methylamide) and protected mice and rats against limbic convulsions induced by pilocarpine. Severity and afterdischarge duration of amygdala-kindled seizures were reduced in rats treated with abecarnil. Abecarnil also antagonized selectively convulsions induced by i.c.v. administration of kainate, but not those triggered by N-methyl-D-aspartate or quisqualate. In genetic models of reflex epilepsy, abecarnil was effective against sound-induced convulsions in DBA/2 mice, against air blast-induced generalized seizures in gerbils and against myoclonus in baboons Papio papio. The anticonvulsant effect of abecornil in a PTZ seizure model in mice was potentiated by ethosuximide, whereas no significant potentiation was found with diazepam, clonazepam, diphenylhydantoin, carbamazepine and phenobarbital. Electromyographic monitoring in a etorphine model of muscle rigidity in rats showed no or little muscle relaxant effect of abecarnil.(ABSTRACT TRUNCATED AT 250 WORDS)

Abecarnil, a metabolically stable, anxioselective beta-carboline acting at benzodiazepine receptors

Abecarnil (isopropyl 6-benzyloxy-4-methoxymethyl-beta-carboline-3-carboxylate) is a novel ligand for central benzodiazepine (BZ) receptors, possessing anxiolytic and anticonvulsant properties, but with considerably reduced muscle relaxant effects in comparison to diazepam (DZP). In vitro, abecarnil inhibited the binding of the BZ [3H]lormetazepam to rat cerebral cortex membranes with an IC50 value of 0.82 nM in comparison to 56 nM for DZP. The ability of abecarnil to displace [3H]lormetazepam was enhanced 1.24-fold in the presence of 30 microM gamma-aminobutyric acid; the corresponding value for DZP was 2.8-fold. DZP and abecarnil were equally effective in enhancing the binding of t-[35S]butylbicyclophosphorothionate to rat cortical membranes. In vivo, abecarnil exhibited a 3- to 6-fold higher affinity to forebrain BZ receptors than DZP. Abecarnil was from 2 to 10 times more potent than DZP in most rodent tests of anxiolytic activity, and in reducing locomotor activity in mice and rats thoroughly habituated to the test chamber. However, in rats newly exposed to a novel cage, abecarnil was less potent than DZP in reducing locomotor activity. In tests of motor coordination, abecarnil, in contrast to DZP, showed no or only weak activity, and in potentiating the effects of ethanol and hexobarbital on motor performance abecarnil was 4 to 25 times less potent than DZP. Abecarnil antagonized the effects of BZs in the chimney and loss of righting reflex tests, but not in the rotarod test.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of excitatory neurotransmission in the globus pallidus induces rigidity and akinesia in the rat: implications for excitatory neurotransmission in pathogenesis of Parkinson's diseases

Bilateral microinjections of the selective N-methyl-D-aspartate (NMDA) antagonist, (-)-2-amino-7-phosphonoheptanoate (AP7), 0.02-0.5 nmol, into the globus pallidus and ventral-posterior portions of the caudate-putamen result in an increase in the muscle tone (rigidity) and catalepsy (akinesia) in rats. NMDA blocked the actions of AP7 on motility in sensitive regions of the globus pallidus and caudate-putamen. Topographical differences in the action of AP7 in the striatum were detected in the dorsal-ventral and rostral-caudal direction. Microinjections of AP7 into the nucleus accumbens induced neither an increase in the muscle tone nor catalepsy in rats, while ventral regions of the caudate-putamen were sensitive to both actions of AP7. Microinjections of AP7 into the dorsal caudate-putamen resulted in a moderate or no increase in the muscle tone. AP7 failed to induce catalepsy from dorsal regions of the caudate-putamen. These data identify the globus pallidus and a defined subregion of the caudate-putamen as crucial sites where excitatory neurotransmission acts to regulate the final set-point of the respective output neurons providing modulation of the passage of motor information through the basal ganglia.

The entopeduncular nucleus regulates muscle tone in genetically spastic rats: role of substance P and gamma-aminobutyric acid

Microinjections of the substance P (SP) antagonist (D-pro2,D-phe7,D-trp9)-SP, or the gamma-aminobutyric acid (GABA) agonist, muscimol, into the entopeduncular nucleus reduced muscle tone in genetically spastic rats in a dose- and time-dependent manner. Similar injections into the ventral thalamus, zona incerta or amygdala had no effect on muscle tone. The muscle relaxant effect of (D-pro2,D-phe7,D-trp9)-SP injected into the entopeduncular nucleus was blocked by co-injections of SP, and that of muscimol by the GABAA antagonist, bicuculline methiodide. These results suggest that SP- and GABA-dependent mechanisms in the entopeduncular nucleus mediate regulation of the muscle tone.