Synthesis and characterization of N,N-dichlorinated amino acids: taurine, homotaurine, GABA and L-leucine.

Epilepsy, trauma and other circumstances leading to hyperexcitable conditions in the CNS tend neurochemically to be associated with excessive stimulated release of glutamic acid and/or a failure of GABA modulated inhibition. Somewhat to a lesser extent, taurine and its homologue homotaurine, have also been shown to antagonize the excitatory actions of glutamic acid. Here we report the successful synthesis and isolation in pure form of N,N-dichlorinated GABA, taurine, homotaurine and leucine. These compounds are much more lipophilic than their parent compounds and may therefore more readily penetrate the blood-brain barrier systems into the neural tissue, where they can be easily dechlorinated. Very preliminary biological testing shows that this may indeed occur. The synthesis and purification methodology will likely also be applicable to a number of other amino acids as well as certain peptides or selected proteins.

Effects of antipsychotic drugs on dopamine and serotonin contents and metabolites, dopamine and serotonin transporters, and serotonin1A receptors.

The effects of neuroleptics have been attributed to dopamine (DA) receptor blockade; however, other neurotransmitters, in particular serotonin (5-HT), have also been implicated. In this study, we examined the effects of clozapine and haloperidol on the distribution of DA and 5-HT transporters, on endogenous DA, 5-HT and their major metabolites, and on 5-HT1A receptors. Adult male Sprague-Dawley rats were treated with either haloperidol (1 mg/kg/day, i.p.), clozapine (20 mg/kg/day, i.p.) or saline for 21 days, and following 3 days of withdrawal, the brains were removed. Tissue levels of DA and 5-HT and their metabolites were measured by high-performance liquid chromatography in 16 brain regions, while quantitative autoradiography with [125I]RTI-121, [3H]citalopram and [3H]8-OH-DPAT was employed to label DA transporters, 5-HT transporters and 5-HT1A receptors, respectively. After haloperidol, densities of 5-HT transporters were increased in the dorsal insular cortex and in the ventral half of caudal neostriatum, while 5-HT1A receptors augmented in cingulate cortex but decreased in the entorhinal area. After clozapine, [3H]citalopram labelling was increased in ventral hippocampus, ventral caudal neostriatum and nucleus raphe dorsalis, but decreased in medio-dorsal and latero-dorsal neostriatum as well as in substantia nigra. Binding of [3H]8-OH-DPAT following clozapine was decreased in frontal, parietal, temporal and entorhinal cortices but increased in the CA3 division of Ammon's horn. The changes in 5-HT transporters in nucleus raphe dorsalis and substantia nigra, as well as the 5-HT1A receptor down-regulations caused by clozapine but not by haloperidol, may explain effects obtained with clozapine and other atypical neuroleptics. There were no modifications in densities of DA transporters, nor of tissue DA levels, after the chronic neuroleptic treatments. The results are in line with previous suggestions that a certain degree of tolerance to neuroleptics develops, in spite of profound D1 and D2 receptor changes that persist during the entire chronic treatment with these psychotropic agents.

Imbalance of Individual Plasma Amino acids Relative to Valine and Taurine as Potential Markers of Childhood Malnutrition.

In the present study a typical plasma amino acid profile for a defined population of healthy Venezuelan children was established and, further, the possibility was examined that deviations from such normalized amino acid patterns can be of use to warn of an impending nutritional deficiency, caused, in part, by adverse socio-economic conditions. This study comprised 152 children of both sexes ranging in age from 1 to 6 years. Classification into different socio-economic strata, ranging from impoverished to privileged, was evaluated by Graffar's method, as previously adapted by Mendez Castellano for Venezuela. The results of clinical and anthropometric examinations were used to group these children into 5 classes of nutritional sufficiency, ranging from adequate nutrition to severe undemutrition. The present data indicate that deviations in the plasma amino acid concentration profile, standardized for a defined population, can be used in combination with clinical evaluations to determine the type as well as the severity of inadequate nutrition. Abnormal ratios of several individual amino acids relative to Val and Tau may serve as early signs of (impending) undemutrition or malnutrition in children; the amino acid changes are detectable even in groups of children without any clinical signs but where sociological circumstances suggest a possibility of inadequate nutrition. Other uses for such plasma amino acid profiles may be to distinguish whether the detected amino acid abnormalities are of dietary or genetic origin, provided that the selected groups or individuals studied derive from a population with more or less the same genetic homogeneity and similar dietary customs.

Neuroleptics and dopamine transporters.

The effects of neuroleptic treatments on dopamine transporters and on dopamine receptors was investigated in the forebrain of adult rats treated for 21 days with either haloperidol, clozapine or saline. The dopamine D1 receptors, labeled with [3H]SCH23390, increased in nucleus accumbens, latero-dorsal rostral neostriatum and substantia nigra, after clozapine but not haloperidol. The dopamine D2 receptors, studied with [3H]raclopride, increased in nucleus accumbens and in dorso-lateral, ventro-medial and dorso-medial quadrants of the rostral neostriatum after either haloperidol or clozapine treatments, and also in latero-ventral rostral neostriatum but only after haloperidol. Haloperidol also up-regulated D2 receptors in rostral and caudal neostriatum, but clozapine produced a more uneven increase, especially in caudal neostriatum. In contrast, the densities of dopamine uptake sites, or transporters, labeled with [125I]RTI-121, remained unchanged after both neuroleptic treatments. The observation that dopamine transporters are resistant to treatments that modify D1 and D2 receptors indicates that these uptake sites can probably be ruled out as the target of neuroleptic drugs, and that dopamine receptor up-regulations can indeed occur independently of the densities of nerve endings at the terminal fields of innervation.

Effects of chronic neuroleptic treatments on dopamine D1 and D2 receptors: homogenate binding and autoradiographic studies.

The antipsychotic effects of neuroleptics are believed to be mediated via dopamine D2 receptor blockade; however, the anatomical and pharmacological targets of these drugs remain somewhat controversial. The purpose of this study was to examine the effects of chronic clozapine (CLZ) and haloperidol (HAL) treatments on the densities of DA D1 and D2 receptors. Adult male Sprague-Dawley rats (300-350 g) were treated for 21 days with either HAL (1 mg/kg/day, i.p.), CLZ (20 mg/kg/day, i.p.) or saline. Three days after ending the treatments, the brains were removed and used for biochemical assays of tissue DA and metabolites as well as for receptor studies. DA D1 and D2 receptors were labelled with [3H]SCH23390 and [3H]raclopride, respectively, and measured in the neostriatum by binding studies, and in autoradiograms of forebrain sections by quantitative densitometry. The autoradiographic measurements revealed significant increases in the densities of D2 receptors in nucleus accumbens, in the medio-ventral, latero-dorsal and latero-ventral quadrants of the rostral neostriatum, in caudal neostriatum and in globus pallidus of both HAL-(28-44%) and CLZ-treated (15-85%) animals. The HAL-induced up-regulation of D2 receptors in rostral and caudal neostriatum was homogenous, but CLZ produced a more uneven increase, with the highest absolute densities measured in latero-dorsal neostriatum, as well as with changes in the medio-dorsal rostral neostriatum. For D1 receptors, only CLZ and not HAL, produced significant increases in five regions, namely nucleus accumbens (43%) latero-dorsal rostral neostriatum (16%), caudal neostriatum (30%), globus pallidus (67%) and substantia nigra (12%). The observation that CLZ, contrary to HAL, also has an effect on D1 receptor densities may explain the greater therapeutic and selective efficacy with fewer side-effects of this agent, in comparison to other neuroleptics.

Alkylation of [3H]8-OH-DPAT binding sites in rat cerebral cortex and hippocampus.

The binding of tritiated 8-hydroxy-2-(di-n-propyl-amino)tetralin, or [3H]8-OH-DPAT, to membranes from rat cerebral cortex and hippocampus could be inhibited by serotonin (5-HT) and buspirone, and by the 5-HT antagonists propranolol, NAN-190, pindolol, pindobind-5-HT(1A), WAY1OO135, spiperone and ritanserin. All competition curves, except for ritanserin, best fitted a two-site model. In vitro treatment of the membranes with N-ethylmaleimide (NEM), to alkylate sulfhydryl groups, caused dose-dependent decreases of binding; the inhibition curves were biphasic, and the effects irreversible. Reduction of disulfide bonds with L-dithiothreitol (L-DTT) also decreased binding, but in a monophasic way; these effects were fully reversible in cortex, but only partially reversible in hippocampus. In the latter region, but not in cerebral cortex, previous occupancy by [(3)H]8-OH-DPAT partially protected binding from the effects of both L-DTT and NEM, suggesting that the thiol groups in the receptor recognition site(s) of this brain region are readily accessible. The binding characteristics were examined with the aid of saturation curves, carried out with increasing concentrations, up to 140 nM, of [(3)H]8-OH-DPAT. The saturation data were suggestive of a two-site receptor model incorporating a high-affinity site (K(H) of 0.3-0.5 nM) corresponding to the 5-HT(1A) receptor, and a low-affinity site (KL of ca 25 nM). After in vivo alkylations, carried out by treating rats with N-ethoxycarbonyl-2-ethoxy-1,2-dihydro-quinoline (EEDQ), the saturation curves from both control and EEDQ-treated rats were again best fitted to a two-site model. For EEDQ-treated animals, a drastic decrease of 5-HT(1A) receptor binding activity was noted; this loss was greater in hippocampus than in cerebral cortex. Since the decrease in 5-HT(1A) receptors was not associated with changes in low-affinity binding, the results suggest independent regulations of the two [(3)H]8-OH-DPAT binding proteins. Altogether, the present data further supports the notion that [(3)H]8-OH-DPAT, besides labelling 5-HT(1A) receptors, also binds to other structures in rat cerebral cortex and hippocampus.

Dopamine receptor alterations correlate with increased GABA levels in adult rat neostriatum: effects of a neonatal 6-hydroxydopamine denervation.

The effects of neonatal intracerebroventricular 6-hydroxydopamine (6-OHDA) injection on the densities of dopamine (DA) receptors and GABA levels were determined in the rostral neostriatum of adult rats. Measurement of GABA turnover indicated that increased tissue GABA in the DA-lesioned neostriatum is a consequence of higher GABA synthesis rate (205%). Binding experiments with [3H]SCH23390 (D1 receptors) and [3H]raclopride (D2 receptors) point to a correlation between tissue GABA content and altered DA receptors. Three months after the lesion there was a 27% decrease in D1 receptors and a 22% increase in D2 receptors. In control neostriatum, GABA levels were inversely related to D2 receptors and this relationship was reversed after 6-OHDA treatment. In contrast, the positive correlation between GABA and D1 receptors remained unchanged after the lesion. Irreversible blockade of DA receptors by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) decreased both D1 and D2 sites (73-87%) in both control and lesioned neostriatum, but increased GABA levels by 25% only in animals which have received 6-OHDA just after birth. Following acute inhibition of DA synthesis or of DA catabolism, GABA levels remained unchanged. The present results indicated that DA depletion by itself is not the cause for the increase in GABA levels. The augmented GABAergic activity following neonatal 6-OHDA is seemingly influenced primarily by DA receptor status; presumably, changes in D2 receptor properties during maturation may be a principal cause for an increase in neostriatal GABA content.

Changes of amino acid and monoamine levels after neonatal 6-hydroxydopamine denervation in rat basal ganglia, substantia nigra, and Raphe nuclei.

The effects of a neonatal dopaminergic deafferentation with the neurotoxin 6-hydroxydopamine (6-OHDA) on endogenous tissue levels of catecholamines, indoleamines, and amino acids were investigated in discrete rat brain regions. After producing the lesion at postnatal day 3 by intraventricular injections of 6-OHDA, with a desipramine pretreatment to protect noradrenaline neurons, the animals were kept for 3 months. Their brains were dissected to obtain samples of neostriatum, Globus pallidus, Substantia nigra, and Raphe nuclei, which were then analyzed by high-performance liquid chromatography, coupled either to electrochemical detection for aromatic monoamines, or to post-column ninhydrin derivatization with spectrophotometry for amino acids. The neonatal 6-OHDA treatment depleted dopamine (DA) levels in neostriatum, Globus pallidus, and Substantia nigra, but in Raphe nuclei DA was increased. The main metabolites of DA were also decreased in neostriatum, Globus pallidus, and Substantia nigra but remained unchanged in Raphe nuclei. Serotonin (5-HT) and its metabolite 5-hydroxy-indole-3-acetic acid increased in neostriatum and Raphe nuclei; in Substantia nigra there was a slight increase in 5-HT only. The 6-OHDA lesion caused heterogeneous alterations in amino acid contents, which varied according to the region. In the neostriatum there were increases of gamma-aminobutyric acid (GABA), aspartic acid, and glycine. In the Globus pallidus taurine, GABA, glutamic acid, glutamine, aspartic acid, serine, and alanine were elevated. In the Substantia nigra only increases in taurine, GABA, glutamic acid, and glutamine could be documented. This study shows important changes in amino acid levels and in some of their ratios, occurring in different anatomical subdivisions of the basal ganglia and related brainstem nuclei following a neonatal treatment with 6-OHDA. The results thus demonstrate major biochemical modifications in amino acids in the aftermath of a DA denervation and/or a 5-HT hyperinnervation during an early developmental period.

Metabolic manipulation of neural tissue to counter the hypersynchronous excitation of migraine and epilepsy.

Very prominent in the large biochemical data bank on epilepsy, is the almost "universal" finding that a familial or environmental predisposition towards epilepsy, as well as the earliest signs preceding other forms of hypersynchronous excitation, coincide with an altered glutamate metabolism. Hence, it has become increasingly apparent that glutamate occupies a central position in the development of epilepsy or in the onset of a migraine incident. The importance of glutamate is explained by a variety of functions in the CNS: as a dominant constituent of many proteins, by its intermediary role in linking energy metabolism to that of many other amino acids, and as the virtually exclusive precursor of GABA. Moreover, glutamate serves as the primary substrate in ammonia detoxification and the product, glutamine, actively participates in CSF water homeostasis. Finally, by its direct electrophysiological and metabolic actions on neurons and glia, via at least four distinct types of receptor proteins, glutamate is implicated in a number of critical mechanisms of information. These include neuronal excitatory modulation, intracellular Ca2+ redistribution, and key metabolic (phosphorylation) mechanisms. The phenomena, when exaggerated due to excessive extracellular glutamate levels, may cause pathological effects such as hypersynchrony--epilepsy, Spreading Depression-migraine, high internal Ca(2+)--damage, impaired phosphorylation/dephosphorylation-necrosis, among others. Not surprising therefore that severe epilepsy may eventually cause CNS cytoarchitectural and metabolic damage, or conversely, that neural tissue trauma not infrequently gives rise to epilepsy many years later. Both conditions are associated with a persistent, excessive leakage or release of glutamate into the extracellular milieu. An electrophysiological and neurochemical commonality between migraine and epilepsy has also been noted.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino acid levels and gamma-aminobutyric acidA receptors in rat neostriatum, cortex, and thalamus after neonatal 6-hydroxydopamine lesion.

The amino acid gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in brain, and GABAergic neurons have been proposed to play a major role in basal ganglia physiology. In the neostriatum (caudate putamen), medium-sized aspiny interneurons, as well as neostriatal output neurons that project to several brain regions, use GABA as their neurotransmitter. Dopamine fibers arising from the substantia nigra represent a major input to the neostriatum where, besides their classic neurotransmitter role, they are seemingly involved in the regulation of amino acid neurotransmitter release. To further characterize the nature of some of the amino acid/dopamine interactions, selective dopaminergic deafferentations were produced in neonatal rats (3 days postnatal) by intraventricular administration of the neurotoxin 6-hydroxydopamine (6-OHDA); the noradrenergic neurons were protected by prior administration of desmethylimipramine. After a 3-month survival, levels of catecholamines, indoleamines, and amino acids were determined in cingulate cortex, thalamus, and neostriatum. In addition, GABAA receptors were measured in membrane preparations from these three regions, using the specific agonist [3H]muscimol. In the 6-hydroxydopamine-lesioned rats, levels of dopamine and its metabolites homovanillic acid, 3,4-dihydroxyphenylacetic acid, and 3-methoxytyramine were decreased, as expected, in cortex and neostriatum, but remained unmodified in thalamus. In all three regions, serotonin content was increased; its metabolite, 5-hydroxyindole-3-acetic acid, was also elevated, but only in cortex and neostriatum. The levels of GABA were increased in neostriatum and thalamus, but remained unmodified in cortex. Glycine was increased in all three regions examined. There were also increases of phosphatidylethanolamine and serine in thalamus, and of aspartic acid and alanine in neostriatum. The density of GABAA binding sites was increased in neostriatum, but remained unchanged in cortex and thalamus. The changes in amino acid levels and [3H]muscimol binding sites induced by a neonatal 6-hydroxydopamine treatment differ from those found after similar lesions in adult animals, possibly because of the plastic and synaptic rearrangements that can still occur during early postnatal development. The present results also demonstrate that adaptations occur in response to a dopaminergic deafferentation at an early age and that these exhibit a regional specificity.

The establishment of essential and metabolically derived amino acid profiles in developing chick embryo organs.

The accumulation of certain essential and metabolically derived amino acids in the free amino acid pools of three excitable tissues has been studied in the chick embryo. Valine together with leucine are at the onset present in the yolk at higher concentrations than any of the other essential amino acids. By 15 days all the amino acids studied have accumulated in the embryonic pools at a higher rate than valine, although certain amino acids, such as phenylalanine or methionine, always remain at lower relative concentrations than valine. This reflects their low supply in the yolk, rather than a more rapid rate of disappearance (utilization). During early embryogenesis (E2-E4), tissues preferentially concentrate glutamic acid, besides taurine and phosphoethanolamine (6). The next distinct stage of development (E4-E7) is marked in the brain by a gradual rise in glutamic acid, glutamine and aspartic acid; the same three amino acids do not demonstrate a further increase in the pool of the heart, while in the whole eye the amino acid profile begins to resemble the blood. Leucine in all three tissues declines rapidly, to reach isoleucine levels by day 7 of development; tyrosine increases slowly in apparent reciprocity to an equally gradual phenylalanine decrease. Into the second week of embryo growth (E7-E15), GABA appears in the mesencephalon (E7) and the eye (E9-E10). In the mesencephalon, the free amino acid pool composition exhibits a rather sudden increase of most metabolically derived amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Embryonic exposure to high taurine: a possible nutritional contribution to Friedreich's ataxia.

When fertilized eggs (50 g) are injected on day 1 with 100 mumol taurine (0.2 ml), 15-day-old embryos exhibit increased taurine content in heart and brain. Lethal toxicity was no higher than with equimolar injections of NaCl (50 mumol) or valine (100 mumol) of the same volume. That same dose of taurine injected either on embryonic day 1 or 7 produced in hatchlings a typical syndrome of ataxia, reduced muscle strength and motor incoordination. When injected on embryo day 15 (E15) most chicks appeared incapable of pecking out of the egg, and the resulting delay in hatching, if if did occur, precluded presuming that the poor condition of such chicks was exclusively due to the late taurine injections. In view of the tendency of Friedreich's ataxia patients to exhibit increased alimentary absorption of taurine, and to demonstrate an excessive accumulation of taurine in the cerebellum and heart tissue on autopsy, fetal exposure to high taurine levels or neonatal high taurine milk ingestion may, by analogy, contribute to the slowly progressing disease process.

Methods to study changing free amino acid pools during embryonic chick development.

During early embryogenesis of the chick, the yolk represents the only source of free amino acids. Leucine and glutamic acid, together with valine, predominate in this nonrenewable nutrient supply; taurine and phosphoethanolamine are present in the lowest concentration. The tyrosine level is already twice as high as phenylalanine. At embryo day 2 (E2), the not-yet fully functioning vitelline blood plexus demonstrates an amino acid profile that rather closely resembles that of the yolk, except for three amino acids. During the transfer process from yolk to plexus, a certain amount of glutamic acid appears to be metabolized to glutamine and aspartic acid. Taurine and phosphoethanolamine are highly concentrated in the circulation, with a ten-fold increase of taurine and a 50-fold rise in phosphoethanolamine. The tyrosine-to-phenylalanine ratio remains 2:1, but their levels relative to valine and other essential amino acids fall by roughly 50%. In the already contracting heart tube and the developing neural tissue on E2, taurine and phosphoethanolamine also accumulate preferentially; the levels in neural tissue are ten- and fivefold higher than in the circulation for the respective amino acids, whereas the heart has three time the plexus content of taurine and two times the amount of phosphoethanolamine. Tyrosine and phenylalanine, still in the same proportion, have risen to approximately twice the circulation values. The heart tube, unlike the primitive brain, concentrates leucine, whereas isoleucine accumulates in both organs. Injections of valine (50 mumol) and taurine (200 mumol) into the yolk on E1 demonstrate that a higher yolk content of an amino acid can result in a two- to threefold tissue increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of the amino acid pools in chick embryo brain, heart, and eye: taurine, valine, glutamine, and phosphoethanolamine.

The redistribution of valine, from the nonrenewable yolk supply into excitable tissues, was studied during the first 15 days of chick embryogenesis. Valine levels in the extraembryonic circulation (the vitelline plexus) peak between days 7-9 (E7-9) and then decline steeply. In their first phase of differentiation (E2-E4), all embryonic tissues contain more valine than the blood plexus. From E4 to E7, the heart and brain exhibit initially a rapid fall in valine, but from day 7 on the decrease becomes more gradual. The eye during the same period reaches an equilibrium with circulating valine; as these levels fall from E9 to E15, the eye retains the valine that accumulated. Against this pattern of change, characteristic for an essential amino acid during embryogenesis, glutamine levels are at any time from two- to threefold higher than valine in all tissues. In the circulation, this ratio remains constant throughout the 15 days of embryonic development. Eye glutamine, higher on day 4, by E7 has entered into an equilibrium with glutamine in the plexus. A steady but two times higher glutamine level is maintained in the heart, although during the later stages of development it gradually tends to approach the plexus content. In sharp contrast, starting on E7 and accelerating on E9, a large increase of glutamine relative to valine or other essential amino acids is seen in developing brain tissue. This appears typical for most metabolic amino acids, suggesting that by days 9-10 the essential amino acid supply in the brain is being exhausted.(ABSTRACT TRUNCATED AT 250 WORDS)

Contributions of basic neurochemistry towards a novel concept of epilepsy.

Epilepsy is an ancient disorder which treatment over the centuries has been guided by preconceptions regarding its origin. The major improvements in epilepsy management came following the discovery of the EEG and the development of seizure suppressing agents. These advances in diagnosis and anticonvulsant therapy have further ingrained the conviction that epilepsy is a disease of neurons. Evidence presented here is intended to support a different point of view which suggests that the metabolic modifications in epileptogenic tissue denote subtle alterations in the anatomical and biochemical relationship between neurons and their glial envelopes. As a result the extracellular environment of these cells contain higher than normal levels of glutamic acid. This creates an unnatural functional connectivity between neurons so that they establish abnormal synchronous activity between them and become hyperexcitable due to the depolarizing milieu. To compensate for these biochemical changes it is suggested that some thought might be given to epilepsy management by metabolic manipulation. The measures should be directed specifically towards improving the ability of glia to remove glutamic acid from the extracellular milieu. Two obvious possibilities are to enhance glial glutamine synthesis and to improve the interstitial "wash-out" of glutamic acid in epileptogenic epicenters. Such a therapy would anticipate to gradually diminish seizure incidence and susceptibility without, however, having a direct action on convulsive episodes per se. The approach must be considered an adjunct to current epilepsy treatment and not a substitute for the use of anticonvulsants.

Amino acid and catecholamine markers of metabolic abnormalities in human focal epilepsy.

Studies of various parameters of amino acid and catecholamine metabolism in human cerebral cortex have provided a number of biochemical markers that appear to delineate areas of focal epileptic activity. These observations have been consolidated further by investigations of a number of experimental models of epilepsy in animals. In appraising this data, it is important to take into consideration whether the tissue samples were obtained during an actual seizure state or in an interictal period. It is also important when possible to assess the extent of astrogliosis and neuronal loss. Sites of spontaneously active epileptic spiking in the cerebral neocortex have a somewhat different amino acid profile when compared to gray matter obtained from surrounding nonspiking gyri several centimeters away. There is an elevation in glycine content, a relative diminution in taurine, and a trend towards lowered glutamic acid levels. However, the concentrations of the eight amino acids measured appear in both the foci and surround to still be within the general range for normal tissue. Measurements of key enzymes involved in the synthesis and regulation of neurotransmitters provide a complementary method of evaluating functional changes in epileptic brain as they are generally less labile than their substrates. There is a moderate increase in the activity of glutamic acid dehydrogenase, an enzyme that plays an important role in the synthesis of glutamic acid from glucose. In some patients a decrease in glutamic acid decarboxylase has also been reported: this enzyme forms gamma-aminobutyric acid (GABA) from glutamic acid and is thus important for inhibition in the central nervous system. Moreover, there is a striking increase in the activity of tyrosine hydroxylase, the rate-limiting enzyme responsible for catecholamine synthesis. The possibility of a focal abnormality in catecholamine metabolism is reinforced by the simultaneous finding of a relative decrease in the number of alpha-1 postsynaptic receptor sites. An important marker of energy metabolism in neural tissue, Na+,K+-ATPase activity, has also been found to be decreased in actively spiking human cerebral cortex. Data from experimental animal foci produced by topical application of convulsant agents show a consistent drop in glutamic acid tissue content. This can be matched to an efflux of glutamic acid from the cortical surface, which in turn is proportional to the electrographic activity of the spike focus. In addition, there is often also a decrease in taurine and GABA in such foci, as well as an increase in the levels of a number of neutral amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)