Glutamate binding in the rat cerebral cortex during ontogeny

Delayed effects of neonatal immune activation on brain neurochemistry and hypothalamic-pituitary-adrenal axis functioning

During the postnatal period, the brain is highly sensitive to stress and inflammation, which are hazardous to normal growth and development. There is increasing evidence that inflammatory processes in the early postnatal period increase the risk of psychopathologies and cognitive impairment later in life. On the other hand, there are few studies on the ability of infectious agents to cause long-term neuroinflammation, leading to changes in the hypothalamic-pituitary-adrenal axis functioning and an imbalance in the neurotransmitter system. In this review, we examine short- and long-term effects of neonatal-induced inflammation in rodents on glutamatergic, GABAergic and monoaminergic systems and on hypothalamic-pituitary-adrenal axis activity.

Taurine Increases Zinc Preconditioning-Induced Prevention of Nitrosative Stress, Metabolic Alterations, and Motor Deficits in Young Rats following Intrauterine Ischemia

Oxygen deprivation in newborns leads to hypoxic-ischemic encephalopathy, whose hallmarks are oxidative/nitrosative stress, energetic metabolism alterations, nutrient deficiency, and motor behavior disability. Zinc and taurine are known to protect against hypoxic-ischemic brain damage in adults and neonates. However, the combined effect of prophylactic zinc administration and therapeutic taurine treatment on intrauterine ischemia- (IUI-) induced cerebral damage remains unknown. The present work evaluated this issue in male pups subjected to transient IUI (10 min) at E17 and whose mothers received zinc from E1 to E16 and taurine from E17 to postnatal day 15 (PND15) via drinking water. We assessed motor alterations, nitrosative stress, lipid peroxidation, and the antioxidant system comprised of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Enzymes of neuronal energetic pathways, such as aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH), were also evaluated. The hierarchization score of the protective effect of pharmacological strategies (HSPEPS) was used to select the most effective treatment. Compared with the IUI group, zinc, alone or combined with taurine, improved motor behavior and reduced nitrosative stress by increasing SOD, CAT, and GPx activities and decreasing the GSSG/GSH ratio in the cerebral cortex and hippocampus. Taurine alone increased the AST/ALT, LDH/ALT, and AST/LDH ratios in the cerebral cortex, showing improvement of the neural bioenergetics system. This result suggests that taurine improves pyruvate, lactate, and glutamate metabolism, thus decreasing IUI-caused cerebral damage and relieving motor behavior impairment. Our results showed that taurine alone or in combination with zinc provides neuroprotection in the IUI rat model.

Brain development in rodents and humans: Identifying benchmarks of maturation and vulnerability to injury across species

Hypoxic-ischemic and traumatic brain injuries are leading causes of long-term mortality and disability in infants and children. Although several preclinical models using rodents of different ages have been developed, species differences in the timing of key brain maturation events can render comparisons of vulnerability and regenerative capacities difficult to interpret. Traditional models of developmental brain injury have utilized rodents at postnatal day 7-10 as being roughly equivalent to a term human infant, based historically on the measurement of post-mortem brain weights during the 1970s. Here we will examine fundamental brain development processes that occur in both rodents and humans, to delineate a comparable time course of postnatal brain development across species. We consider the timing of neurogenesis, synaptogenesis, gliogenesis, oligodendrocyte maturation and age-dependent behaviors that coincide with developmentally regulated molecular and biochemical changes. In general, while the time scale is considerably different, the sequence of key events in brain maturation is largely consistent between humans and rodents. Further, there are distinct parallels in regional vulnerability as well as functional consequences in response to brain injuries. With a focus on developmental hypoxic-ischemic encephalopathy and traumatic brain injury, this review offers guidelines for researchers when considering the most appropriate rodent age for the developmental stage or process of interest to approximate human brain development.

Cytoskeleton as a potential target in the neuropathology of maple syrup urine disease: insight from animal studies

In this short review we provide evidence that the branched-chain keto acids accumulating in the neurometabolic disorder maple syrup urine disease disturb rat cerebral cytoskeleton in a developmentally regulated manner. Alterations of protein phosphorylation leading to brain cytoskeletal misregulation and neural cell death caused by these metabolites are associated with energy deprivation, oxidative stress and excitotoxicity that may ultimately disrupt normal cell function and viability.

alpha-Ketoisocaproic acid regulates phosphorylation of intermediate filaments in postnatal rat cortical slices through ionotropic glutamatergic receptors

In this study we investigated the effects of alpha-ketoisocaproic acid (KIC), the main keto acid accumulating in the inherited neurometabolic disorder maple syrup urine disease (MSUD), on the in vitro incorporation of 32P into intermediate filament (IF) proteins from cerebral cortex of rats during development. KIC decreased the in vitro incorporation of 32P into the IF proteins studied up to day 12, had no effect on day 15, and increased this phosphorylation in cortical slices of 17- and 21-day-old rats. A similar effect on IF phosphorylation was achieved along development by incubating cortical slices with glutamate. Furthermore, the altered phosphorylation caused by the presence of KIC in the incubation medium was mediated by the ionotropic receptors NMDA, AMPA and kainate up to day 12 and by NMDA and AMPA in tissue slices from 17- and 21-day-old rats. The results suggest that alterations of IF phosphorylation may be associated with the neuropathology of MSUD.

The effect of ammonia and pH on brain γ-glutamyl transpeptidase in young rats

Acute hyperammonemia, induced by two consecutive injections of ammonium acetate (550 and 450 mg per kg b.wt.), decreased the activity of gamma-glutamyl transpeptidase (GGT) in most brain regions of 18- and 30-day-old rats. This decrease in the brain GGT activity was more pronounced in younger than in older rats. After the addition of NH4Cl to the incubation medium, the inhibitory action of NH4+ on this enzyme activity was also demonstrated in crude synaptosomal membranes at pH 7.4, but in a range of NH4+ concentrations many-times higher than those found in the plasma or brains of young hyperammonemic rats. Because similar concentrations of NH4+ stimulated the activity of the purified enzyme from rat kidney (mainly at pH 9.0), the inhibition of GGT activity in the young rat brain is probably mediated indirectly and not by a direct interaction of ammonia with the enzyme molecules.

Specific [3H]glutamate binding in the cerebral cortex and hippocampus of rats during development: effect of homocysteine-induced seizures

Specific [3H]glutamate binding to synaptic membranes from the cerebral cortex and hippocampus of 7-, 12- and 18-day-old rats was examined, both in control animals and during seizures induced by homocysteine. In the cerebral cortex a transient peak of glutamate binding was observed in 7-day-old group, whereas in the hippocampus it occurred in 12-day-old animals. Total specific [3H]glutamate binding was not influenced by preceding seizure activity in either of the age groups and both the studied regions. NMDA- and QA-sensitive glutamate bindings represent the highest portion of the total binding. Moreover, NMDA-sensitive binding in the cerebral cortex of 7-day-old rats is significantly higher as compared to the two more mature groups. The proportion of individual receptor subtypes on total binding in each age group was not influenced by preceding seizure activity. However, NMDA-sensitive binding in the hippocampus of 12-day-old rats, sacrificed during homocysteine-induced seizures, was significantly increased as compared to corresponding controls. In contrast to the effect of NMDA, AMPA, kainate and quisqualate which displaced to a different extent [3H]glutamate binding, homocysteine had no effect when added to membrane preparations. Similarly, [3H]CPP and [3H]AMPA bindings were not affected in the presence of homocysteine. It thus seems unlikely that homocysteine is an effective agonist for conventional ionotropic glutamate receptors. Its potential activity at some of the modulatory sites at the NMDA receptor channel complex or at metabotropic receptors has to be clarified in further experiments.

Postnatal development of muscarinic receptor-stimulated phosphoinositide metabolism in mouse cerebral cortex: sensitivity to ethanol

An enhanced coupling of cholinergic muscarinic receptors to phosphoinositide metabolism had been previously observed in brain from immature rat. This study reports that the postnatal development of muscarinic receptor-stimulated phosphoinositide metabolism is also enhanced in cerebral cortex slices from immature Swiss-Webster and Balb-c mice, as compared to adults. Response to the agonist carbachol was lower on postnatal day 3, peaked between days 5 and 12 and then declined to adult levels. Density of muscarinic binding sites, measured with the M1 ligand [3H]telenzepine on postnatal day 7, was, on the other hand, only half of the adult value. Phosphoinositide hydrolysis stimulated by glutamate decreased with age, while that elicited by norepinephrine increased. These results are also similar to those previously reported in the rat. Ethanol has been found to inhibit muscarinic receptor-stimulated phosphoinositide metabolism in rat brain in an age-dependent manner. This was confirmed in mouse brain, where ethanol inhibited this response in cerebral cortex of immature but not adult animals. These results indicate that the enhanced muscarinic receptor-stimulated phosphoinositide metabolism, which coincides with the brain growth spurt, is similar in rats and mice. Mice may be a useful species in which to genetically manipulate muscarinic receptors to gain a better understanding of their potential role in brain development.

Developmental changes in the sensitivity of the neonatal rat brain to hypoxic/ischemic injury

Developmental changes in the response of the neonatal rat brain to hypoxic/ischemic injury were examined. Hypoxic/ischemic injury was produced by unilateral carotid ligation followed by exposure to hypoxia in 1- (D1), 3- (D3), 5- (D5) and 7-day-old (D7) rats. Injury was produced in most D7 animals exposed to > or = 120 min of 7.6 or 8% oxygen after carotid ligation. The extent of neuronal injury was variable, ranging from focal neuronal death to massive infarction. In D5 and D3 animals, there was a progressive decline in the extent of neuronal injury in response to hypoxia/ischemia. In the younger animals, bilateral injury was occasionally seen. Sham-operated animals exposed to hypoxia alone had numbers of karyorrhectic neurons similar to normal control animals in all age groups. The underlying developmental changes which account for these differences are not yet known but are likely to be multiple.

Regulation of D-aspartate release by glutamate and GABA receptors in cerebral cortical slices from developing and ageing mice

The basal release of D-[3H]aspartate, an unmetabolized analogue of glutamate, from cerebral cortical slices remained at the same level from three-day-old to 24-month-old mice, but the response to K+ stimulation (50 mM) was smaller in young than in adult or aged mice. Kainate, N-methyl-D-aspartate and quisqualate (0.1 mM) stimulated the basal release of D-aspartate in the cerebral cortex of seven-day-old mice, the effects of kainate and N-methyl-D-aspartate being reduced by their antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and dizocilpine maleate, respectively, indicating that in the immature cerebral cortex the kainate and N-methyl-D-aspartate types of the glutamate receptor are involved in the basal release. The K(+)-stimulated release was not affected by glutamate agonists in developing mice, though they markedly attenuated the evoked release in adults. The inhibitory amino acids GABA, taurine and glycine depressed the K(+)-stimulated release only in the adult cerebral cortex. The action of GABA was abolished by bicuculline, demonstrating the involvement of presynaptic GABAA receptors. The glycine effect was strychnine-insensitive, characteristic of the glycine modulatory site in the N-methyl-D-aspartate receptor. This kind of regulation by both kainate and N-methyl-D-aspartate receptors could be of physiological significance, particularly in the immature cerebral cortex.

Effect of repeated hyperammonemia on Na(+)-dependent binding of glutamate in rat cortical and hippocampal synaptic membranes

Na(+)-dependent binding of L-glutamate in cortical and hippocampal synaptic membranes from hyperammonemic rats was compared to corresponding data in the controls. In hippocampal membranes, repeated hyperammonemia resulted in a 13% and 18% decrease in binding in 20-day-old and 50-day-old rats, respectively. The decrease was statistically significant (P < 0.05) in the older animals and Scatchard analysis revealed a 19% reduction in the number of binding sites without any changes in the affinity. Within the hippocampal formation, the binding in the dentate gyrus was the most sensitive to hyperammonemia where a 21% decrease was found (P < 0.01), whilst the decline of binding in CA1 and CA3 areas of the hippocampus proper was not significant. The results support the idea that excessive accumulation of extracellular glutamate during hyperammonemia is a consequence not only of its increased release, but also of the blocking of Na(+)-dependent binding of glutamate to specific uptake sites.

Development of dopamine and N-methyl-D-aspartate systems in rat brain: the effect of prenatal phencyclidine exposure

Phencyclidine (PCP) inhibits the uptake of the neurotransmitter dopamine (DA), and blocks N-methyl-D-aspartate (NMDA) receptor-regulated ion channels. PCP also binds to sigma receptors in vivo and in vitro in rat brain. Prolonged exposure to PCP in adults has been observed to reduce the number of PCP binding sites in brain. We designed these experiments to evaluate whether prolonged prenatal exposure to PCP produces alterations in the development of DA and NMDA systems in brain. To do so, we characterized the normal course of development of basal and stimulated DA release in striatal slices, the ontogeny of striatal DA concentrations, and the development of NMDA receptor channels and associated glutamate binding sites in frontal cortex. We compared these developmental profiles to those in rats exposed to prenatal PCP, in an attempt to characterize the effect of prenatal PCP exposure on the pattern of brain development. Pregnant CD rats were injected s.c. with either 0, 10 or 20 mg/kg PCP daily on gestational days 8 through 20. On postnatal days (PND) 8, 21, 45, or 100, rats were sacrificed and brain tissues isolated for in vitro assessment. In vitro [3H]DA release from striatal slices evoked by either 40 microM glutamate or 15 mM K+ increased over 250% from PND 8 to PND 45, and glutamate-stimulated release was still significantly below adult levels at PND 45. In contrast, D-methamphetamine (D-METH)-evoked [3H]DA release, frontal cortical glutamate binding sites and NMDA channels developed early, reaching adult levels on or before PND 21.(ABSTRACT TRUNCATED AT 250 WORDS)

Phencyclidine-binding sites in mouse cerebral cortex during development and ageing: effects of inhibitory amino acids

The binding of N-[1-(2-thienyl)cyclohexyl]-[3H]piperidine ([3H]TCP) to the phencyclidine-binding sites in the N-methyl-D-aspartate (NMDA) receptor complex-associated ion channel was characterized in cerebral cortical membranes from 3-day-old to 24-month-old mice. The binding was saturable, exhibiting only one binding component during the whole life-span studied. The maximal binding capacity Bmax, calculated per protein content, decreased during postnatal development until 3 months of age, remaining thereafter constant in ageing mice, thus indicating the greatest availability of phencyclidine-binding sites in the immature cerebral cortex. The binding constant KD increased during the first postnatal week, remained thereafter unchanged and increased again during the second year of life, indicating a decreased affinity of the receptor sites for the ligand. The general properties of the binding; potentiation by glutamate and NMDA, as well as by glycine in a strychnine-insensitive manner, prevailed during development and ageing, certain of these effects being however less pronounced in the immature brain. Taurine and beta-alanine stimulated TCP binding, acting probably at the glycine modulatory site. The actions of these inhibitory amino acids were weak and inconsistent when compared to that of glycine. Since NMDA receptors have been suggested to be involved in neuronal plasticity and learning and memory processes, these modifications in the properties of cortical phencyclidine-binding sites might be of importance in the regulation of excitatory amino acid functions during development and ageing.

Changes in synaptosomal glutamate release during postnatal development in the rat hippocampus and cortex

The effectiveness of K+ depolarisation in inducing the release of [3H]L-glutamate from preloaded hippocampal and cortical synaptosomes was examined in rats aged from postnatal day 4 (PND 4) to adult. In the lower age groups studied (PND 4-PND 15), the response to depolarisation was always smaller than that seen in the adult. From PND 15, the sensitivity of the release process increased steadily to a maximum level in the adult. The relatively small amounts of glutamate released in response to K(+)-depolarisation in the younger age groups may be a factor which contributes to the relative insensitivity of neonatal brain to ischaemic damage. Discrete variations in the sensitivity to K+ depolarisation observed in animals aged from PND 4 to PND 15 may be involved in plastic changes in neural activity which are known to occur during this important development period.

Developmental changes in the effects of drugs acting at NMDA or non-NMDA receptors on synaptic transmission in the chick cochlear nucleus (nuc. magnocellularis)

The developmental pharmacology of excitatory amino acid (EAA) receptors in the chick cochlear nucleus (nucleus magnocellularis, NM) was studied by means of bath application of drugs and recording of synaptically-evoked field potentials in brain slices taken from chicks aged embryonic day (E) 14 through hatching (E21). The abilities of various EAA agonists (N-methyl-D-aspartate [NMDA], kainic acid, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) to suppress postsynaptic responses by depolarization block and of EAA antagonists ((3-[RS]-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CCP], dizocilpine [MK-801], 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3 dione [NBQX], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6,7-dinitroquinoxaline-2,3-dione [DNQX]) to suppress these responses directly were assessed quantitatively. The results support the existence of NMDA receptors in NM and suggest that the ability of these receptors to influence synaptically-evoked responses declines dramatically during the last week of embryonic life. The results similarly suggest that the non-NMDA receptors in NM undergo changes in density and/or function during a period of development when the cochlear nucleus is undergoing a variety of morphological and functional transformations.

Developmental changes in the activity of membrane-bound gamma-glutamyl transpeptidase and in the sialylation of synaptosomal membranes from the chick embryonic brain

gamma-Glutamyl transpeptidase (GGT) is a membrane-bound sialoglycoprotein. The developmental changes in GGT activity and in sialic acid content were determined in a crude synaptosomal membrane fraction from the cerebral hemispheres of the chick embryo between days 11 and 19 of incubation. The GGT activity increased almost eightfold during the examined developmental period, while sialic acid content rose significantly only between days 11 and 15. Cortical administered on day 13 significantly increased GGT activity. On the other hand, the content of membrane bound sialic acid was not substantially affected. The value of the GGT apparent Michaelis constant (Kmapp) for gamma-glutamyl-p-nitroanilide in the presence of 20 mmol.l-1 glycylglycine was 1.5 mmol.l-1 and cortisol did not influence it. However, Vmax was increased by this hormone. The affinity of GGT to concanavalin A (ConA) did not change during development. Neither the administration of cortisol nor neuroaminidase treatment had any effect on the interaction of GGT with ConA. Desialylation of crude synaptosomal fraction did not change GGT activity. The results presented here suggest no developmental nor functional relationship between the activity of GGT and the level of sialylation in synaptosomal membranes from the cerebral hemispheres of the chick embryo.

Ontogeny of glycine-enhanced [3H]MK-801 binding to N-methyl-D-aspartate receptor-coupled ion channels

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is thought to play a critical role in neuronal development, differentiation and plasticity. A number of studies have shown an enhanced sensitivity to NMDA receptor ligands in neonatal animals. This study examined the ontogenetic changes in the glycinergic modulation of NMDA-coupled cation channels in the developing central nervous system of rat pups. The nonequilibrium binding of the specific channel ligand [3H]MK-801 was used as a measure of NMDA channel access. Glycine (10(-5) M) enhancement of [3H]MK-801 binding at 2 h in forebrain membranes from adult rats was significantly greater than that observed in tissues from 8- to 28-day-old rat pups. This difference was due to changes in the efficacy, but not potency of glycine. The observed ontogenetic changes in the efficacy of glycine-enhanced [3H]MK-801 binding were attributable to developmental changes in receptor site density, as determined by equilibrium [3H]MK-801 saturation isotherms. Kinetic studies revealed that glycine increased the association rate constants of [3H]MK-801 in 8-day and adult membranes by a similar magnitude (0.111 +/- 0.021 vs 0.094 +/- 0.009 nM-1 h-1, respectively). Similarly, the fractional amount of [3H]MK-801 bound (i.e., amount bound at time t normalized to amount bound at equilibrium) in the presence of glycine was relatively constant throughout neonatal development. These findings suggest that the allosteric modulation of the NMDA ionophore by glycine is similar in postnatal and adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Three types of hyperthermic seizures in rats

Three types of rat hyperthermic seizures were observed. The first comprised generalized clonic convulsions preceded by intermittent myoclonus. Ictal EEG showed diffuse intermittent spikes and sequential rapid spike-wave bursts (type 1 seizures). In the other types of seizures, the paroxysmal discharges originated in the occipital region without (type 2a seizures) or with (type 2b seizures) secondary generalization. Type 2a seizures involved no convulsive movement whereas type 2b seizures involved clonic convulsions. The threshold temperatures for type 1, 2a and 2b seizures were 44.1 +/- 0.60, 40.9 +/- 1.47 and 42.1 +/- 0.75 degrees C, respectively. Of these seizures, the type 2 seizures (2a and 2b) did not severely affect the general condition of rats and thus may be an appropriate model for the investigation of febrile seizures.

Developmental expression, compartmentalization, and possible role in excitotoxicity of a putative NMDA receptor protein in cultured hippocampal neurons

The mechanisms regulating the expression and localization of excitatory amino acid (EAA) neurotransmitter receptors in neurons of the developing mammalian brain, and roles for these receptors in the plasticity and degeneration of neural circuits are not well understood. We previously isolated and characterized a 71 kDa glutamate binding protein (GBP) from rat brain, and have recently obtained evidence that this GBP is a component of a functional N-methyl-D-aspartate (NMDA) receptor-ion channel complex. We have now used antibodies to this putative NMDA receptor protein to examine its expression and localization, and consequences of its activation in cultured embryonic (18 day) rat hippocampal neurons. Immunocytochemistry and Western blots using monoclonal antibodies to the GBP demonstrated an increase in GBP-positive neurons and their staining intensity with time in culture. GBP was localized to the somata and dendrites of pyramidal-like neurons and was sparse or absent in the axons. The expression and compartmentalization of GBP occurred in isolated neurons indicating that direct cell interactions were not required for these processes. Cell surface staining for GBP occurred in patches on the soma and dendrites. The developmental expression of GBP immunoreactivity closely paralleled the expression of sensitivity to NMDA neurotoxicity. There was a direct relationship between GBP immunoreactivity and neuronal vulnerability to glutamate-induced degeneration; vulnerable neurons stained heavily whereas resistant neurons showed either low levels of staining or no staining. Finally, a GBP antiserum greatly reduced NMDA neurotoxicity (but not kainate neurotoxicity). Taken together, these findings demonstrate the expression of presumptive NMDA receptors within a subpopulation of embryonic hippocampal neurons, and their segregation to the soma and dentrites of pyramidal neurons. This spatial distribution of glutamate receptors among and within neurons is likely to play important roles in regulating the structure of neural circuitry during development, and may also be an important determinant of selective neuronal vulnerability in pathological conditions.

Regional development of carbachol-, glutamate-, norepinephrine-, and serotonin-stimulated phosphoinositide metabolism in rat brain

Phosphoinositide metabolism stimulated by activation of cholinergic muscarinic, glutamatergic, alpha-adrenergic and serotoninergic receptors was measured in brain regions of the developing rats. Accumulation of [3H]inositol phosphates ([3H]InsPs) in [3H]inositol-prelabeled slices from cerebral cortex, hippocampus, brainstem and cerebellum was measured as an index of phosphoinositide metabolism. Large age-, neurotransmitter receptor-, and brain region-dependent differences were found. Carbachol-stimulated [3H]InsPs accumulation peaked on postnatal day 7 in cerebral cortex and hippocampus while in cerebellum and brainstem the effect of muscarinic stimulation was maximal at birth and then declined to adulthood. The effect of glutamate also showed a peak on day 7 in hippocampus and brainstem and a developmentally related decrease in cerebral cortex. In the cerebellum, on the other hand, the response to glutamate remained sustained through adulthood. Stimulation of phosphoinositide metabolism by norepinephrine increased with age in hippocampus and cerebral cortex, but decreased in the cerebellum, while the effect of serotonin did not change significantly with age except in cerebellum. These changes in receptor-stimulated phosphoinositide metabolism do not parallel, for the most part, the ontogeny of receptor recognition sites. Activation of the phosphoinositide metabolism pathway leads to an increase in intracellular calcium levels and to stimulation of protein kinase C, which are believed to play significant roles in cellular proliferation and differentiation. Thus, the differential ability of neurotransmitters to stimulate phosphoinositide hydrolysis might play a role in the development of brain regions.

Parallel development of excitotoxic vulnerability to N-methyl-D-aspartate and kainate in dispersed cultures of the rat cerebral cortex

The development of excitotoxic cell death caused by L-glutamate, N-methyl-D-aspartate, quinolinate and kainate was examined in dispersed primary cultures of the rat cerebral cortex. Cell death was evaluated by phase-contrast microscopy and quantified by the measurement of lactic dehydrogenase activity in the culture medium. Cells obtained from embryonic cerebral cortex on days 16-18 of pregnancy, and maintained in a serum-supplemented medium, started to respond to glutamate N-methyl-D-aspartate quinolinate and kainate by cell death on day 7 in vitro. The sensitivity to the neurotoxins increased rapidly, and in a similar fashion, during the second week and remained unchanged up to day 21. Our findings indicate that, unlike the cerebral cortex in situ, the sensitivity of cultured cortical cells to the cytotoxicity mediated by N-methyl-D-aspartate and kainate receptors develops in a nearly parallel fashion.

Changes in messenger RNAs coding for neurotransmitter receptors and voltage-operated channels in the developing rat cerebral cortex

The ontogenetic development of poly(A)+ mRNAs coding for receptors to several neurotransmitters (kainate, glutamate, acetylcholine, and serotonin) and voltage-operated channels (sodium and calcium) was studied by isolating total poly(A)+ mRNA from the brains of rats at various developmental stages and injecting it into Xenopus oocytes. The oocytes translated the foreign mRNA and incorporated functional receptor/ion channel complexes into the cell membrane. Thus, recording of induced membrane currents in voltage-clamped oocytes gave a measure of the relative amounts of the different messengers. Responses induced by kainate, glutamate, acetylcholine, and serotonin all increased with age and reached a maximum in oocytes injected with mRNA from adult cortex. Messenger RNAs for the earliest ages examined, Embryonic Days 15 and 18, expressed little or no response to kainate, glutamate, or acetylcholine, while 50-70% of the adult response was reached by Postnatal Day 10. In contrast, the serotonin-induced response was already comparatively large (16% of the adult level) in oocytes injected with mRNA from Embryonic Day 15 brain and increased postnatally to adult levels. The expression of voltage-dependent sodium and calcium channels was small in oocytes injected with mRNA from embryonic animals and increased postnatally to reach a maximum in oocytes injected with mRNA from adult animals.

Postnatal development of the excitatory amino acid system in visual cortex of the rat. Changes in ligand binding to NMDA, quisqualate and kainate receptors

The postnatal development of the ligand binding to N-methyl-D-aspartate (NMDA), quisqualate and kainate receptor sites was examined in whole homogenates of the visual cortex of rats, aged 2-360 days. As selective ligands, [3H]CPP (3-(2-carboxypyperazine-4-yl)-propyl-1-phosphonic acid, [3H]AMPA (RS-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid) and [3H]KA (kainic acid) were used, respectively. The binding of CPP was low in newborns, rapidly increased from the second postnatal week, reached its maximum between weeks 2 and 3, then slowly declined up to the age of 1 year. In contrast, the binding of AMPA and kainate was high perinatally, increased rapidly up to day 6 after birth to reach an early maximum value, then gradually decreased to adult values which were attained at an age of 3-4 weeks. These age-related changes were derived from alterations in the density of binding sites, which, in the case of AMPA, was accompanied by an increase in binding affinity. The results, compared with the developmental time-course of excitatory synapses, indicate that, in the immature cerebral cortex, NMDA receptors may be primarily involved in synaptic transmission, whereas quisqualate and kainate receptors may play some other (e.g. trophic) roles.

Physiological and pathophysiological roles of excitatory amino acids during central nervous system development

Recent studies suggest that excitatory amino acids (EAAs) have a wide variety of physiological and pathophysiological roles during central nervous system (CNS) development. In addition to participating in neuronal signal transduction, EAAs also exert trophic influences affecting neuronal survival, growth and differentiation during restricted developmental periods. EAAs also participate in the development and maintenance of neuronal circuitry and regulate several forms of activity-dependent synaptic plasticity such as LTP and segregation of converging retinal inputs to tectum and visual cortex. Pre- and post-synaptic markers of EAA pathways in brain undergo marked ontogenic changes. These markers are commonly overexpressed during development; periods of overproduction often coincide with times when synaptic plasticity is great and when appropriate neuronal connections are consolidated. The electrophysiological and biochemical properties of EAA receptors also undergo marked ontogenic changes. In addition to these physiological roles of EAAs, overactivation of EAA receptors may initiate a cascade of cellular events which produce neuronal injury and death. There is a unique developmental profile of susceptibility of the brain to excitotoxic injury mediated by activation of each of the EAA receptor subtypes. Overactivation of EAA receptors is implicated in the pathophysiology of brain injury in several clinical disorders to which the developing brain is susceptible, including hypoxia-ischemia, epilepsy, physical trauma and some rare genetic abnormalities of amino acid metabolism. Potential therapeutic approaches may be rationally devised based on recent information about the developmental regulation of EAA receptors and their involvement in the pathogenesis of these disorders.

Excitatory amino acid receptors on sustained retinal ganglion cells in the kitten during the critical period of development

Effects of iontophoretically applied excitatory amino acid analogues, kainate, quisqualate and N-methyl-D-aspartate (NMDA) and their receptor antagonists on the sustained class of retinal ganglion cells were studied in the optically intact eye of pentobarbitone-anaesthetized kittens (7-9 weeks of age). These results were compared with the effects obtained in adult cats. All 3 excitatory amino acid agonists had excitatory actions on the majority of On- and Off-sustained ganglion cells in the kitten but at higher current levels than those required for adult cells, suggesting all 3 types of receptors of weaker sensitivity are present on the kitten cells. Whilst the relative potency of kainate, quisqualate and NMDA was 15:3:1 in the adult cells, it was 5:2:1 in the kitten cells. As for other neurones in the CNS, an increase in the potency of kainate receptors and a decrease in that of NMDA receptors appear, therefore, to characterize the postnatal development of the excitatory amino acid receptors on the retinal ganglion cells. In accordance with the agonist results, a broadband receptor antagonist, kynurenate, powerfully antagonised responses of kitten cells as well as those of adult cells. The pure NMDA receptor antagonist, 3((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), however, only suppressed spontaneous firing of kitten cells. Furthermore, in kitten cells, the visually-driven firing was depressed while the level of firing was raised by these excitatory amino acid analogous, and a long period of inhibition of firing followed the agonist-induced excitation.(ABSTRACT TRUNCATED AT 250 WORDS)

The ontogeny of excitatory amino acid receptors in rat forebrain--I. N-methyl-D-aspartate and quisqualate receptors

The ontogeny of radioligand binding to N-methyl-D-aspartate and quisqualate receptors in rat forebrain was studied quantitatively using in vitro receptor autoradiography. Specific binding to both receptors could be detected by postnatal day 1 in hippocampus and striatum. The adult pattern of binding to N-methyl-D-aspartate receptors emerged by postnatal day 14 with high densities of binding in CA1 (stratum oriens and stratum radiatum), dentate gyrus (molecular layer) and striatum (caudate-putamen). Binding to the outer laminae of frontal cortex was as much as 45% above adult levels during development. Binding of [3H]amino-3-hydroxy-5-methylisoxazole-4-propionic acid to quisqualate receptors showed a similar overshoot during development, but also manifested a unique distribution with CA3 and medial aspects of the amygdala exhibiting transient, intense labeling. Homogenate binding studies with [3H]amino-3-hydroxy-5-methylisoxazole-4-propionic acid demonstrated a 73% increase in quisqualate receptors in whole brain at postnatal day 21 compared with adult levels. The selectivity of excitatory amino acid binding to the quisqualate site in development was similar to the selectivity in adult brain. These data taken with other recent reports, suggest that quisqualate receptors may have a role in development distinct from their function in the adult brain.

Actions of excitatory amino acids on somatostatin release from cortical neurons in primary cultures

L-Glutamate, N-methyl-D-aspartic acid (NMDA), quisqualate, and kainate were found to increase endogenous somatostatin release from primary cultures of rat cortical neurons in a dose-dependent manner. The rank order of potency calculated from the dose-response curves was quisqualate greater than glutamate = NMDA greater than kainate, with EC50 values of 0.4, 20, and 40 microM, respectively. Alanine, glutamine, and glycine did not modify the release of somatostatin. The stimulation of somatostatin release elicited by L-glutamate was Ca2+ dependent, was decreased by Mg2+, and was blocked by DL-amino-5-phosphonovaleric acid (APV) and thienylphencyclidine (TCP), two specific antagonists of NMDA receptors. The NMDA stimulatory effect was strongly inhibited by APV in a competitive manner (IC50 = 50 microM) and by TCP in a noncompetitive manner (IC50 = 90 nM). The release of somatostatin induced by the excitatory amino acid agonists was not blocked by tetrodotoxin (1 microM), a result suggesting that tetrodotoxin-sensitive, sodium-dependent action potentials are not involved in the effect. Somatostatin release in response to NMDA was potentiated by glycine, but the inhibitory strychnine-sensitive glycine receptor did not appear to be involved. Our data suggest that glutamate exerts its stimulatory action on somatostatin release essentially through an NMDA receptor subtype.

Transient increase in ligand binding to quisqualate and kainate sites in cerebral cortex of immature rats

The postnatal changes in the specific binding of [3H]kainate and [3H]AMPA (RS-alpha-amino-3-hydroxy-5-methyl-4-isoxasolopropionic acid, an agonist of quisqualate receptors) were studied in cerebral cortex of rats, aged 2-360 days. The binding of the two ligands was assayed in whole-tissue homogenates. Similar developmental time courses were found for kainate and AMPA binding, characterized by high perinatal levels, a further increase during the first few days after birth, an early maximum value around the age of one week, and a gradual decrease to adult values which were attained at an age of 3-4 weeks. As revealed by Scatchard analysis, the transient elevation of ligand binding was derived from an increased density of binding sites, which, in the case of AMPA, was accompanied also by an increase in binding affinity. The results indicate that, in the immature cerebral cortex, kainate and quisqualate receptors may play a role other than in synaptic transmission.

Divergent ontogeny of sigma and phencyclidine binding sites in the rat brain

The postnatal developmental patterns of sigma (sigma) and phencyclidine (PCP) binding sites were compared in the rat brain. The results show diametrically different ontogenic patterns for the sites. While both the affinity and the density of sigma sites remain constant throughout the developmental period tested (postnatal day 1 to 1 year), the density of PCP binding sites increases from the time of birth, reaching the adult level by postnatal day 14. The differences in developmental patterns provide evidence for distinctive properties of cerebral sigma and PCP binding sites.

Ontogeny of glutamate accumulating activity in rat brain synaptic vesicles

The relationship between the ontogeny of the vesicular glutamate uptake system and synaptogenesis in rats was investigated. For this purpose we have developed a simplified procedure for the preparation of crude synaptic vesicles which are sufficiently pure to demonstrate a highly ATP-dependent glutamate uptake. ATP-dependent glutamate uptake into synaptic vesicles was found to increase dramatically starting on postnatal day 10 and reaching a maximum on day 30 (76 +/- 40 and 657 +/- 40 pmol/mg protein/10 min, respectively), correlating well with the active period of synaptogenesis. Stimulation of uptake by chloride also developed in parallel with the vesicular glutamate uptake. In contrast, combined non-ATP-dependent uptake and non-specific binding remained constant (21 +/- 6 pmol/mg protein/10 min). This development of vesicular glutamate uptake during the period of synaptogenesis supports the notion that synaptic vesicles play an important role in glutamate synaptic transmission.

Maturational changes in retinal excitatory amino acid receptors

The appearance, kinetics and pharmacological properties of receptors for n-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate (KA), L-glutamate (Glu) and L-aspartate (Asp) was investigated using 3H-ligand binding during the development of chick embryo retina. Receptors for AMPA are maximally concentrated at embryonic day 7 (ED 7) and decline 50% in subsequent days; L-Glu receptors are low until ED 11, and the same is true for Asp and NMDA receptors which increase at ED 14 and 18 respectively. All receptors studied underwent an increase in pharmacological specificity, whereas only AMPA-receptors showed an important change in affinity during ontogeny. Results demonstrate that receptors for excitatory amino acids in the retina suffer maturational changes and suggest that while NMDA and aspartate could interact with the same receptor, AMPA and glutamate seem to bind to different sites.

Development of hypothalamic obesity in growing rats

Administration of monosodium glutamate to neonate rats causes hypothalamic lesions in the region of the nucleus arcuatus and the eminentia mediana, followed by massive accumulation of triglycerides, diminished secretion of growth hormone, reduced body length and organ weights and diminished number of adipocytes (hypoplastic-hypertrophic obesity). Locomotor activity of obese animals is reduced by about 50%. Food intake is increased by about 10% during growth and development of obesity but decreased beneath the level of that in control animals in the stationary phase of obesity. Hyperinsulinemia coupled with insulin resistance develops in the stationary phase of obesity, i.e. when adipocyte diameter has reached approximately 100 microns. The effects of reduced secretion of growth hormone are considered to be a main factor of fat accumulation in this type of obesity.

Identification and characterization of an N-methyl-D-aspartate-specific L-[3H]glutamate recognition site in synaptic plasma membranes

Conditions have been developed for an L-[3H]glutamate binding assay in which 85-95% of the specific binding is to a site that corresponds to the N-methyl-D-aspartate subclass of acidic amino acid receptors. Incubation of synaptic plasma membranes with L-[3H]glutamate in 50 mM Tris/acetate, pH 7.4, for 2-20 min at 2 degrees C results in binding with pharmacological characteristics of the electrophysiologically defined N-methyl-D-aspartate receptor. The fraction of glutamate binding to this subclass of receptors, relative to the total, decreases with both increased time and temperature. This binding is reversible, is concentrated in the synaptic plasma membrane fraction, has a pH optimum of 7.0-7.4, and is linear with respect to tissue protein concentration. The binding is unaffected by 1 mM concentrations of the anions sulfate, chloride, bromide, thiocyanate, phosphate, acetate, nitrate, or carbonate and the monovalent cations potassium or ammonium. However sodium and the divalent cations copper, cobalt, zinc, cadmium, and manganese decrease binding to this N-methyl-D-aspartate site.

Cerebral GABA-ergic and glutamatergic function in hepatic encephalopathy

Measurement of amino acids in brain tissue obtained at autopsy from cirrhotic patients dying in hepatic coma revealed a threefold increase in glutamine and a concomitant decrease in brain glutamate. The GABA levels were found to be unaltered. Studies using an animal model of portal-systemic encephalopathy gave similar results. Glutamic acid decarboxylase (GAD) activities were within normal limits, both in the brains of cirrhotic patients and portocaval-shunted rats. A previous study reported normal [3H]GABA binding to synaptic membrane preparations from cerebral cortex in these animals. Taken together, these findings suggest that cerebral GABA function is not impaired in hepatic encephalopathy associated with chronic liver disease and portal-systemic shunting. On the other hand, there is evidence to suggest that the releasable pool of glutamate may be depleted in brain in hepatic encephalopathy. Data consistent with this hypothesis include: Reduction in the evoked release of endogenous glutamate by superfusion of hippocampal slices with pathophysiological levels of ammonia; ammonia-induced reduction of glutamatergic neurotransmission; and an increase in the number of [3H]glutamate binding sites in synaptic membrane preparations from hyperammonemia rats and from rats with portocaval shunts. Such neurochemical changes may be of pathophysiological significance in hepatic encephalopathy.

Maternal ethanol consumption: binding of L-glutamate to synaptic membranes from whole brain, cortices, and cerebella of offspring

We examined the influence of chronic maternal ethanol consumption on the Na+- and Ca2+-independent binding of L-glutamate to synaptic plasma membranes from whole brain as well as from cortices and cerebella of developing offspring. The maximum specific binding (Bmax) of L-glutamate to the Na+- and Ca2+-independent binding sites in synaptic plasma membranes of brain peaked at 17 days of age in the offspring of both control and ethanol-fed rats, although at that age there were significantly fewer binding sites in the brains of the offspring of ethanol-fed rats. The regional localization of this deficit is not now known. However, it appears that one major glutamatergic region (the cortex) does not reflect the transient deficiency of L-glutamate sites in brain. In fact, the concentration of L-glutamate binding sites in cortical synaptic plasma membranes was significantly increased in the 20-day-old offspring of ethanol-fed rats. In contrast to the cortex, binding to cerebellar synaptic plasma membranes was comparable in 20-day-old offspring of control and ethanol-fed rats. Despite transient alterations in the concentrations of L-glutamate binding sites in brain and synaptic plasma membranes, the affinity of the sites for L-glutamate (Kd) was consistently normal in the 14- to 26-day-old offspring of ethanol-fed rats.

The binding properties and regional ontogeny for [3H]glutamic acid Na+-independent and [3H]kainic acid binding sites in chick brain

The binding kinetics, pharmacological properties and regional ontogeny of L-[3H]glutamic acid Na+-independent and [3H]kainic acid binding sites were studied in preparations of chick brain. One binding component was found for L-[3H]glutamic acid with a Kd value of 176 x 10(9) M. For [3H]kainic acid two binding components were found in the hemispheres, optic lobes and brain stem, one with high affinity and a Kd value of 12.5 x 10(9) M and one with low affinity and a Kd value of 260 x 10(9) M. In cerebellum only one binding site was detected for [3H]kainic acid with a Kd value of 144 x 10(9) M. The ontogeny of L-[3H]glutamic acid and [3H]kainic acid binding sites was studied using membrane preparations (48,000 g pellet) of hemispheres, optic lobes, brain stem and cerebellum. Binding of L-[3H]glutamic acid was already significant in all brain regions by embryonic day 11 but major increases in total receptor number per brain region or per mg of protein were apparent by embryonic day 19 and especially after hatching. Cerebral hemispheres, optic lobes and brain stem showed few [3H]kainic acid binding sites by day 13 in ovo. An increase follows which, in hemispheres and optic lobes, continues at the same rate during the first two weeks after hatching. In cerebellum, by contrast, the kainic acid binding site is almost undetectable until embryonic day 15. The appearance of these binding sites in cerebellum takes place during the restricted period between days 15 in ovo and 5 post-hatching. This pattern of development of [3H]kainic acid binding sites almost parallels the developmental patterns of the molecular layer of chick cerebellum and it is consistent with the results of our autoradiographic study showing that the great majority of kainic acid binding sites are localized in the molecular layer.

Effects of short-term and prolonged aerogenic hypoxia on gamma-glutamyl transpeptidase activity in the brain, liver, and biological fluids of young rats

Posthypoxic fluctuations in the levels of two excitatory amino acids, glutamate and aspartate, may be related to changes in mechanism(s) which are responsible for their reuptake. As gamma-glutamyl transpeptidase (GGT) plays a role in mediating the uptake of glutamate and aspartate into various compartments of the brain, we studied changes in the activity of this enzyme in main regions of the brain in young and adult rats. We found a posthypoxic increase in bound GGT activity in some brain regions of 18-day-old animals after acute exposure, but no changes were observed after prolonged altitude hypoxia, with the exception of a decrease in cortical GGT activity. In contrast, acute hypoxia decreased GGT activity in the cortical capillaries to 59%, but prolonged hypoxic exposure was ineffective. However, the activity of soluble GGT in the cerebrospinal fluid of both groups of rats was several-times elevated in comparison with controls. At the same time, bound GGT activity was increased in the liver after acute or prolonged altitude hypoxia. The soluble GGT activity in plasma was only increased after prolonged exposure. Ninety days after prolonged hypoxic exposure the bound GGT activity was reduced in all brain regions to about 60-70% of controls (significantly higher in females than in males) as long-term developmental sequel from early postnatal hypoxia.

A comparison of the postnatal development of post-activation potentiation in the neocortex and dentate gyrus of the rat

The postnatal development of short-term potentiation (STP) and long-term potentiation (LTP) was examined in the neocortex and dentate gyrus of rats aged 7 days to adult. STP and LTP of the transcallosal response in the neocortex could not be demonstrated until the third postnatal week. STP and LTP of the perforant path-dentate response could not be demonstrated until the second postnatal week. In both cases, STP appeared several days before LTP. Structural and neurochemical correlates of STP/LTP development, and their implications for possible STP/LTP mechanisms, are discussed.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

Kainate and glutamate neurotoxicity in dependence on the postnatal development with special reference to hippocampal neurons

The particular vulnerability of hippocampal CA3/CA4 neurons to intracerebroventricularly administered kainate did not occur in rats until postnatal day 16. Glutamate applied in the same way failed to show comparable neurodegenerative signs, even when administered in large doses to adult animals. Alternatively, both kainate and glutamate exerted similar behavioral effects from postnatal day 15 onwards. No correlation appears between kainate/glutamate-induced behavioral changes and their neurodegenerative potency.

Alanine aminotransferase in the rat nervous system during the postnatal development referring to the glutamate transmitter metabolism

Alanine aminotransferase has been studied in various nervous tissues during the postnatal development of the rat. At birth the enzyme activity was low and showed similar levels in all tissues studied. In the hippocampal formation and in the cerebellum which are supposed to be endowed with glutamatergic structures, the enzyme activity increased significantly during the postnatal development. These results contrast markedly with dorsal root ganglia and superior cervical ganglia, in which glutamatergic transmission processes are obviously absent. In these peripheral ganglia the time course of the enzyme activity persisted on a very low level after birth. The participation of alanine aminotransferase in forming of transmitter glutamate is discussed.

Binding sites for L-glutamate in the central nervous system of the rat

The regional distribution of 2-amino-4-phosphonobutyrate (APB)/chloride-insensitive L-[3H]glutamate binding sites in the rat central nervous system was compared with that of APB/chloride-sensitive and with sodium-dependent binding sites. The distribution of APB-sensitive and APB-insensitive sites was not correlated, but the latter was identical to that of the sodium-dependent sites. The pharmacological specificity of the APB-insensitive sites was not consistent with that of an N-methylaspartate-preferring receptor, and was also different from the specificity determined for the sodium-dependent sites. The APB-insensitive sites appear to be unrelated to any other previously described excitatory amino acid binding site.

Purification, biochemical characterization, binding activity, and selectivity of a glutamate binding protein from bovine brain

A glutamate binding protein was purified from bovine brain to apparent homogeneity. The procedure used for the purification of this protein involved extraction of a crude synaptic membrane fraction with Na-cholate, followed by solubilization of the binding protein from the membranes by Triton X-100, and, finally, affinity batch separation of the protein on L-glutamate-loaded glass fiber. The molecular characteristics of the purified protein were similar to those previously described for the glutamate binding protein from rat brain synaptic membranes and included the following: small Mr (14,000), acidic (pI = 4.7) protein with a single NH2-terminal amino acid (tyrosine), and significant absorption at wave-lengths greater than 300 nm. Complete amino acid analysis of the protein was not achieved, either because of destruction of some amino acids or of incomplete hydrolysis of the protein. The protein bound L-glutamate with high affinity (KD = 0.87 microM), exhibited one class of L-glutamate binding sites, and bound glutamate with a stoichiometry of 0.7 mol ligand/mol protein. The displacement of protein-bound L-glutamic acid by other neuroactive amino acids had characteristics similar to those observed for the displacement of L-glutamate from rat brain synaptic membrane or purified protein binding sites. Finally, the metal ligand formers KCN and NaN3 inhibited the activity of this protein just as they have been shown to do in rat brain synaptic membranes or the purified protein.

Postnatal changes in the activity of glutamate dehydrogenase and aspartate aminotransferase in the rat nervous system with special reference to the glutamate transmitter metabolism

The activities of aspartate aminotransferase (AAT) and glutamate dehydrogenase (GIDH), the major glutamate metabolizing enzymes, were studied in hippocampal formation, cerebellar cortex, dorsal root ganglia, superior cervical ganglia and liver as a function of postnatal development. At birth, in all these nervous tissues the enzyme activities were quite low and showed similar levels (AAT 7-15 U/g wet weight; 0.18-0.23 U/mg protein; GIDH 3.4-13 U/g wet weight; 0.07-0.18 U/mg protein). Based on protein, AAT activity increased during the postnatal period studied 5.8 and 3.8 times in the hippocampal formation and cerebellar cortex, respectively, while the respective GIDH rise was 5.2 and 2.3 times. During postnatal maturation, enzyme activities in dorsal root ganglia showed only minor changes. In superior cervical ganglia, AAT and GIDH were remarkably constant. In liver the enzyme activities changed during postnatal development, but the activity curve profile was quite distinct from those obtained for brain regions. The steep rise of AAT and GIDH activities in brain regions is discussed as being a consequence of the maturation of preferably glutamatergic structures. Glutamatergic transmission processes obviously do not take place in superior cervical ganglia and dorsal root ganglia, and certainly not in liver. The present results suggest a quantitatively significant participation of glutamate transmitter metabolism in proportion to the whole glutamate metabolism of the CNS.