Postnatal changes in N-methyl-D-aspartate receptor binding and stimulation by glutamate and glycine of [3H]-MK-801 binding in human temporal cortex

D-Serine in neurobiology: CNS neurotransmission and neuromodulation

Homochirality is fundamental for life. L-Amino acids are exclusively used as substrates for the polymerization and formation of peptides and proteins in living systems. However, D- amino acids were recently detected in various living organisms, including mammals. Of these D-amino acids, D-serine has been most extensively studied. D-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl- D-aspartate receptor (NMDAr). D-Serine binds with high affinity to a co-agonist site at the NMDAr and, along with glutamate, mediates several vital physiological and pathological processes, including NMDAr transmission, synaptic plasticity and neurotoxicity. Therefore, a key role for D-serine as a determinant of NMDAr mediated neurotransmission in mammalian CNS has been suggested. In this context, we review the known functions of D-serine in human physiology, such as CNS development, and pathology, such as neuro-psychiatric and neurodegenerative diseases related to NMDAr dysfunction.

Sudden infant death syndrome, sleep, and seizures

benign febrile seizures seen in 7% of infants before 6 months play a role in the terminal pathway in a subset of sudden infant death syndrome victims. Supporting evidence: (1) lack of 5-hydroxitryptamine, one consistent finding in sudden infant death syndrome that Kinney et al coined a developmental serotonopathy, is consistent with risk for seizures. (2) Non-rapid eye movement sleep increasing during the age of highest risk for sudden infant death syndrome facilitates some seizures (seizure gate). (3) Sudden unexpected death in epilepsy is associated with severe hypoxemia and hypercapnia during postictal generalized electroencephalographic (EEG) suppression. In toddlers, sudden unexplained deaths are associated with hippocampal abnormalities and some seizures. (4) The sudden nature of both deaths warrants an exploration of similarities in the terminal pathway. Moreover, sudden infant death syndrome, febrile seizures, sudden unexplained death in childhood, and sudden unexpected death in epilepsy share some of the following risk factors: prone sleeping, infections, hyperthermia, preterm birth, male gender, maternal smoking, and mutations in genes that regulate sodium channels. State-of-the-art molecular studies can be exploited to test this hypothesis.

Aldosterone Signals the Onset of Depressive Behaviour in a Female Rat Model of Depression along with SSRI Treatment Resistance

Depression is a serious condition that occurs more frequently in women and is often associated with treatment resistance. The main hypotheses of this study are that (a) aldosterone is an early marker of depression onset and (b) a tryptophan (TRP) depletion model of depression previously validated in male rats is treatment resistant in females. To explore possible underlying mechanisms, we have focused on factors shown to be altered in patients with treatment-resistant depression. Female Sprague-Dawley rats were treated with a control or low-TRP-containing diet for various time periods up to 21 days. The results show that aldosterone secretion increased after 4 days of TRP depletion and prior to corticosterone. Optimal effects of TRP depletion occurred at 14 days. In addition to neurochemical and behavioural changes observed previously in males, TRP depletion in females was associated with a significant decline in serum magnesium concentrations, increased serum interleukin-6, enhanced gene expression of orexin A in the frontal cortex and induced a rise in N-methyl-D-aspartate (NMDA) receptor Bmax in the amygdala. Depression-like behaviour, NMDA receptor upregulation, enhancement of the kynurenine-to-kynurenic acid ratio and magnesium were resistant to paroxetine treatment (10 mg/kg/day in drinking water for 14 days). In conclusion, aldosterone may represent an important early marker for the onset of depression-like behaviour. With respect to treatment resistance, the underlying mechanisms may involve pro-inflammatory cytokines, the kynurenine pathway, magnesium, glutamate neurotransmission and the orexin pathway. This model of treatment-resistant depression may be useful for the future development of new compounds with novel antidepressant properties.

D-serine: the right or wrong isoform?

Only recently, d-amino acids have been identified in mammals. Of these, d-serine has been most extensively studied. d-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl-d-aspartate receptor (NMDAr), similar to glycine. Therefore, d-serine may well play a role in all physiological and pathological processes in which NMDArs have been implied. In this review, we discuss the findings implying an important role for d-serine in human physiology (CNS development and memory and learning) and pathology (excitotoxicity, perinatal asphyxia, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, epilepsy, schizophrenia and bipolar disorder). We will debate on the relative contribution of d-serine versus glycine and conclude with clinical applications derived from these results and future directions to progress in this field. In general, adequate concentrations of d-serine are required for normal CNS development and function, while both decreased and increased concentrations can lead to CNS pathology. Therefore, d-serine appears to be the right isoform when present in the right concentrations.

Therapeutic effect of hypothermia and dizocilpine maleate on traumatic brain injury in neonatal rats

This study was undertaken to evaluate the therapeutic effect of hypothermia and dizocilpine maleate in traumatic brain injury (TBI) on newborn rats. After induction of TBI, physiologic and histopathological assessments were performed on both the control and therapeutic groups to evaluate the effects of both agents. Rats were assigned into four groups as follows: normothermic (n = 23), hypothermic (n = 18), normothermia plus dizocilpine maleate (n = 18) and hypothermia plus dizocilpine maleate (n = 18). All the rats were injured using a weight-drop head injury model, artificially ventilated with a 33% O(2) and 66% NO(2) mixture, and physiological parameters, intracranial pressure, and brain and rectal temperatures were recorded. Mortality, physiological, neurological parameters, and histopathological changes were assessed after 24 h. As a result, intracranial pressure, cerebral perfusion pressure, morbidity, weight loss, and microscopic changes were significantly worse in the normothermic group (p <0.05). There was no statistical difference between other groups (p > 0.05). Hypothermia and dizocilpine maleate displayed similar neuroprotective effects in TBI on newborn rats, but no additive effect was observed.

Persistent NKH with transient or absent symptoms and a homozygous GLDC mutation

Three of four nonketotic hyperglycinemia patients homozygous for a novel GLDC mutation (A802V) were treated by assisted respiration and/or sodium benzoate with or without ketamine and had transient neonatal or absent symptoms and normal developmental outcome, despite persisting biochemical evidence of nonketotic hyperglycinemia. This exceptional outcome may be related to the high residual activity of the mutant protein (32% of wild type) and therapeutic intervention during a critical period of heightened brain exposure and sensitivity to glycine.

Neurobiology of Rett syndrome

Girls with Rett syndrome display signs of neuronal dysfunction including mental retardation, seizures, stereotyped movements, and abnormal breathing and autonomic control. Decelerating head growth during infancy might reflect a disorder in production or pruning of neuronal synapses or both. Recent immunocytochemical studies in rodent brain investigating development of MeCP2, the transcription factor mutated in Rett syndrome, suggest that expression is delayed until the time of synapse formation. These findings are consistent with other evidence that Rett syndrome disrupts genetic programs that establish and refine synaptic connections.

Expression of NMDA receptor NR1, NR2A and NR2B subunit mRNAs during development of the human hippocampal formation

The N-methyl-d-aspartate receptor plays a critical role in the formation and maintenance of synapses during brain development. In the rodent, changes in subunit expression and assembly of the heteromeric receptor complex accompany these maturational processes. However, little is known about N-methyl-d-aspartate receptor subunit expression during human brain development. We used in situ hybridization to examine the distribution and relative abundance of NR1, NR2A and NR2B subunit messenger ribonucleic acids in the hippocampal formation and adjacent cortex of 34 human subjects at five stages of life (neonate, infant, adolescent, young adult and adult). At all ages, the three messenger ribonucleic acids were expressed in all subfields, predominantly by pyramidal neurons, granule cells and polymorphic hilar cells. However, their abundance varied across ontogeny. Levels of NR1 messenger ribonucleic acid in CA4, CA3 and CA2 subfields were significantly lower in the neonate than all other age groups. In the dentate gyrus, subiculum and parahippocampal gyrus, NR2B messenger ribonucleic acid levels were higher in the neonate than in older age groups. NR2A messenger ribonucleic acid levels remained constant, leading to an age-related increase in NR2A/2B transcript ratio. We conclude that N-methyl-d-aspartate receptor subunit messenger ribonucleic acids are differentially expressed during postnatal development of the human hippocampus, with a pattern similar but not identical to that seen in the rodent. Changes in subunit composition may thus contribute to maturational differences in human hippocampal N-methyl-d-aspartate receptor function, and to their role in the pathophysiology of schizophrenia and other neurodevelopmental disorders.

Neurobiology of Rett syndrome: a genetic disorder of synapse development

Rett syndrome is a developmental disorder that restricts brain growth beginning in the first year of life and evidence from neuropathology and neuroimaging indicates that axonodendritic connections are especially vulnerable. In a study of amino acid neurotransmitter receptors using receptor autoradiography in tissue slices of frontal cortex and the basal ganglia, we found a biphasic age-related pattern with relatively high receptor densities in young RS girls and lower densities at later time. Using microarray analysis of gene expression in frontal cortex, we found that some of the most prominent alterations occurred in gene products related to synapses, including the NMDA receptor NR1 subunit, the cytoskeletal protein MAP-2 and synaptic vesicle proteins. Using a new antibody that recognizes MeCP2, the transcription factor mutated in RS, we established that most neurons in the rodent brain express this transcription factor. We hypothesize that a major effect of mutations in the MeCP2 protein is to cause age-related disruption of synaptic proliferation and pruning in the first decade of life.

Differential effects of two NMDA receptor antagonists on cognitive--behavioral performance in young nonhuman primates II

The present experiment examined the effects of chronic exposure to remacemide (an NMDA antagonist that also blocks fast sodium channels) or MK-801 (which blocks NMDA receptors more selectively) on the acquisition of color and position discrimination and short-term memory behavior in juvenile rhesus monkeys. Throughout the 2-year dosing period, a conditioned position responding (CPR) task was used to assess color and position discrimination and a delayed matching-to-sample (DMTS) task was used to assess memory. Chronic exposure to high doses of either drug delayed the acquisition of accurate color and position discrimination without altering response rates. In the case of MK-801, these effects abated within 6 months of the start of treatment. In the case of remacemide, the effects persisted for 17 months of dosing. Neither compound significantly altered performance of the short-term memory task at any time point or at any dose tested. The fact that the effects of remacemide on behavioral performance were more persistent than those seen for MK-801 suggests that tolerance may develop to the behavioral effects of MK-801, which does not develop to the effects of remacemide. Alternatively, these results may suggest that the concurrent antagonism of NMDA receptors and fast sodium channels may have more profound consequences for behavior than does the antagonism of NMDA receptors alone.

Differential effects of two NMDA receptor antagonists on cognitive-behavioral development in nonhuman primates I

The present experiment examined effects of chronic exposure to remacemide (an N-methyl-D-aspartate [NMDA] antagonist which also blocks fast sodium channels) or MK-801 (which blocks NMDA receptors, exclusively) on learning and motivation in young rhesus monkeys. Remacemide (20 or 50 mg/kg/day) or MK-801 (0.1 or 1.0 mg/kg/day) was administered every day to separate groups of animals via orogastric gavage for up to 2 years. Immediately prior to dosing, 5 days per week (M--F), throughout the 2-year dosing period, an incremental repeated acquisition (IRA) task was used to assess learning and a progressive ratio (PR) task was used to assess motivation. The results indicate an effect of 50 mg/kg/day remacemide to impair learning (IRA) which persisted even after drug treatment was discontinued. MK-801 had no effect on learning but transiently increased motivation. Because the effects of remacemide occurred independently of changes in motivation or response rates, they are likely due to specific cognitive impairments and are not due to an inability of subjects to fulfill the motoric requirements of the task. The fact that MK-801 did not alter learning suggests that NMDA antagonism alone may be insufficient to produce learning deficits in young monkeys and that such deficits may rely on the ancillary blockade of fast sodium channels.

Ontogeny of ionotropic glutamate receptor expression in human fetal brain

Glutamate receptors have multiple roles in the central nervous system. Recent evidence suggests that the iontropic glutamate receptors are critical during brain development, particularly for corticogenesis, neuronal migration, and synaptogenesis. In this study, we examined subunit mRNA expression and binding sites of the NMDA, AMPA, and kainate receptors from gestational weeks 8-20 in human fetal brain. Expression of glutamate receptors was high during several periods in these brains. Different levels of expression of each NMDA, AMPA, and kainate receptor subunit transcripts were present during development, with a greater abundance of NR1, NR2B, NR2D, GluR7, and KA1 mRNA at most gestational ages. Binding sites for NMDA, AMPA, and kainate receptors were all detected, but each had a unique pattern of expression. These results demonstrate that glutamate receptors are expressed early in human brain development, and undergo complex changes over time consistent with their role in normal development.

Effect of dexamethasone treatment on maturational changes in the NMDA receptor in sheep brain

The objective of the present study was to examine the effect of antenatal or postnatal treatment with corticosteroids on the NMDA receptor, one of the mediators of both normal brain development and hypoxic-ischemic injury, by determining the characteristics of the receptor MK-801 binding site in untreated and corticosteroid-treated fetal and newborn lambs. (3)H-MK-801 binding was performed in cerebral cortical cell membranes from fetal sheep at 88, 120, and 136 d gestation (term = 150 d), and from 5-d-old lambs and adult ewes. Animals were randomized to receive dexamethasone [fetuses: 6 mg, i.m. every 12 hr for four doses to mother; lambs: 0.01 mg/kg (low dose) or 0.25 mg/kg (high dose) every 12 hr for four doses] or placebo. During development, B(max) (apparent number of receptors) increased, reaching a maximum in 5-d-old lambs (p < 0.05) and decreasing in the adult brain. K(d) (dissociation constant) did not change, suggesting that receptor affinity was not altered during maturation. Dexamethasone treatment had no effect on MK-801 binding in the fetus or adult, but in lambs was associated with a significant decrease in B(max) from 2.17 +/- 0.18 pmol/mg protein in placebo-treated animals to 1.65 +/- 0.8 and 1.62 +/- 0.07 pmol/mg protein in low-dose and high-dose animals, respectively. Affinity for (3)H-MK-801 decreased 20% after dexamethasone treatment in lambs only (p < 0.05). Thus, dexamethasone treatment appears to modify the NMDA receptor only during a specific period of brain development.

Effect of dexamethasone treatment on maturational changes in the NMDA receptor in sheep brain

The objective of the present study was to examine the effect of antenatal or postnatal treatment with corticosteroids on the NMDA receptor, one of the mediators of both normal brain development and hypoxic-ischemic injury, by determining the characteristics of the receptor MK-801 binding site in untreated and corticosteroid-treated fetal and newborn lambs. (3)H-MK-801 binding was performed in cerebral cortical cell membranes from fetal sheep at 88, 120, and 136 d gestation (term = 150 d), and from 5-d-old lambs and adult ewes. Animals were randomized to receive dexamethasone [fetuses: 6 mg, i.m. every 12 hr for four doses to mother; lambs: 0.01 mg/kg (low dose) or 0.25 mg/kg (high dose) every 12 hr for four doses] or placebo. During development, B(max) (apparent number of receptors) increased, reaching a maximum in 5-d-old lambs (p < 0.05) and decreasing in the adult brain. K(d) (dissociation constant) did not change, suggesting that receptor affinity was not altered during maturation. Dexamethasone treatment had no effect on MK-801 binding in the fetus or adult, but in lambs was associated with a significant decrease in B(max) from 2.17 +/- 0.18 pmol/mg protein in placebo-treated animals to 1.65 +/- 0.8 and 1.62 +/- 0.07 pmol/mg protein in low-dose and high-dose animals, respectively. Affinity for (3)H-MK-801 decreased 20% after dexamethasone treatment in lambs only (p < 0.05). Thus, dexamethasone treatment appears to modify the NMDA receptor only during a specific period of brain development.

Development of amino acid receptors in frontal cortex from girls with Rett syndrome

To determine whether a disorder of excitatory neurotransmission plays a role in the pathophysiology of Rett syndrome (RS), N-methyl-D-aspartate (NMDA), adenosine monophosphate acid (AMPA), kainate, and metabotropic types of glutamate receptors were labeled autoradiographically in the superior frontal gyrus (SFG) from 9 RS patients and 10 female controls. The results showed a trend for the densities of NMDA, AMPA, gamma-aminobutyric acid, and metabotropic glutamate receptors to be higher in younger patients than in controls and for densities in older patients to fall below those of controls. The age-related changes in SFG NMDA receptor density may be correlated with the shift from psychomotor regression and seizures in younger stage II/III RS girls to the less epileptic plateau stage in older girls.

Consequences of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor blockade during status epilepticus in the developing brain

To investigate if AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor activation contributes to acute manifestations and long term consequences of status epilepticus (SE), we administered the AMPA receptor antagonist NBQX to P35 rats undergoing kainic acid (KA)-induced SE. NBQX (30 mg/kg/dose) given intraperitoneally (i.p.) at 30, 60 and 90 min after i.p. KA injection (12 mg/kg) reduced severity of SE. When tested as adults, rats that had received KA and NBQX were similar to controls with no long term impairment in visuospatial memory (assessed by the water maze test), or histologic damage in the CA1 or CA3 hippocampal subfields. However, both P35 groups, those receiving KA alone and those receiving KA and NBQX, had similar rates of spontaneous recurrent seizures (SRS). In P15 rats, NBQX resulted in increased acute mortality from KA associated SE. These results indicate that the effects of NBQX on KA-induced SE are age dependent, and that non-NMDA receptor activation contributes to the acute manifestations and to the long term sequelae seen after KA-induced SE in the prepubescent rat brain.

The modulatory effect of spermine on the glutamate-NMDA receptor is regionally variable in normal human adult cerebral cortex

The MK-801, glutamate and polyamine binding sites on the N-methyl-D-aspartate class of glutamate receptors labelled with [3H]MK-801 were characterized in four cortical areas (sensorimotor, superior temporal, mid-frontal and occipital) from seven human adult control cases. Age, post-mortem delay, tissue storage time and sex had no significant effects on any of the parameters measured. Dissociation constants (K(D) values) for MK-801 showed similar mean values in the four cortical areas, whereas receptor densities (B(max) values) showed significant differences between sensorimotor or occipital and superior temporal or mid-frontal cortex. There were marked regional differences in the profiles of the spermine- and glutamate-incremented enhancement of specific [3H]MK-801 binding. The EC(50) for the glutamate enhancement was significantly higher in the occipital than in the mid-frontal and sensorimotor cortex, whereas maximal glutamate-enhanced binding values did not differ. The maximal enhancement of [3H]MK-801 binding by spermine and glutamate varied between the cases, ranging from zero to 40.4+/-9.3 fmol x mg protein(-1) for spermine, and from 85+/-5 to 111+/-10 fmol x mg protein(-1) for glutamate. Maximal spermine enhancement of [3H]MK-801 binding was significantly more variable in superior temporal or mid-frontal than in sensorimotor or occipital cortex. The results suggest that N-methyl-D-aspartate receptor sites, especially the polyamine site, are heterogeneous in human cerebral cortex, and show a high degree of regional and individual variability.

Ontogenetic aspects of traumatic brain edema--facts and suggestions

Diffuse brain swelling (DBS) after severe traumatic brain injury (TBI) occurs more commonly in children than adults. Most of the recent clinical studies suggest that young children are more negatively affected by DBS. Until now studies in young animals in which the pathophysiology of DBS was evaluated remained seldom. However, pathogenetic mechanisms of edema formation after TBI in the immature brain appeared to be different in comparison to adult brains. There are evidences that vasogenic as well as cytotoxic edema components may be responsible for the development of DBS. Besides mechanical disturbance, the blood-brain barrier seems to be strongly endangered by oxidative stress after TBI because regional antioxidative capacity is obviously diminished. In addition, cytotoxic components of DBS may be caused by at least two different mechanisms. First, it was shown that a sustained posttraumatic cerebral hypoperfusion occurs in the immature brain. Moreover, a transient increase of NMDA receptor expression at this period of life may be responsible for an increased threat of intracellular sodium ion accumulation in brain cells. Obviously, brain swelling can be detrimental because it can elevate intracranial pressure, impair CBF, and may represent ongoing secondary brain injury.

PGE2, via EP3 receptors, regulates brain nitric oxide synthase in the perinatal period

We tested the hypothesis that high prostaglandin levels during the perinatal period might regulate brain nitric oxide synthase (nNOS) expression. nNOS and cyclooxygenase (COX)-2 mRNAs were higher in brain cortex and the periventricular area of newborn rats and pigs compared with adult brain. Nitric oxide synthase activity was also 2. 5- to 4-fold higher in newborn than in adult brain. Administration of nonselective COX inhibitor ibuprofen or COX-2 inhibitor nimesulide every 8 h for 24 h to newborn rats and pigs reduced prostaglandin levels and caused comparable reductions in nNOS mRNA, protein, and activity to levels of adults; COX inhibitor-induced changes were prevented by cotreatment with PGE2 analog, 16, 16-dimethyl-PGE2, and agonist for the EP3 receptor of PGE2, sulprostone, but not by PGI2 analog carbaprostacyclin, PGD2, EP1 receptor agonist 17-phenyl trinor-PGE2, and EP2 agonist butaprost. Concordant observations were made in vitro and revealed that nNOS expression (detected by NADPH diaphorase reactivity) mostly present in neurons of the deeper cortical layers was reduced by COX inhibitor, and this effect was prevented by EP3 agonist. In conclusion, high levels of PGE2 in neonatal brain contribute to the increased expression of nNOS by acting on EP3 receptors; this positive interaction between PGE2 and nNOS might be required physiologically for normal brain development.

Comparison of glutamate and gamma-aminobutyric acid uptake binding sites in frontal and temporal lobes in schizophrenia

BACKGROUND

Theories of schizophrenia proposing deficiencies of amino acid [glutamate, gamma-aminobutyric acid (GABA)] neurons are in accord with the observed temporal lobe pathology of the disease rather than with the newer theory of glutamate hyperinnervation and hyperfunction in areas of prefrontal cortex. This study addresses the issue by measuring specific uptake sites as indices of glutamatergic and GABAergic neuron densities in frontal and temporal lobes.

METHODS

Frontal cortex (six areas) and temporal lobe (six areas of cortex, amygdala, and hippocampus) were dissected from 19 control autopsy brains and 12 brains from neuroleptic drug-treated schizophrenic patients. Groups had similar ages, postmortem intervals, and storage times. Membranes, prepared from tissue homogenates, were incubated with D-[3H]aspartate to measure neuronal and glial glutamate uptake site binding in 14 areas and with [3H]nipecotic acid to measure neuronal GABA uptake site binding in 11 areas.

RESULTS

Glutamate and GABA uptake sites were not reduced in prefrontal and temporal areas. Instead, we found small increases in glutamate uptake sites in prefrontal areas. Some tendency toward increased GABA uptake sites were not disease-related.

CONCLUSIONS

Our findings concur with other studies that propose locally overabundant glutamate systems in prefrontal cortex in schizophrenia. Losses of amino acid neurons do not accompany the temporal lobe pathology.

Long-term changes of ionotropic glutamate and GABA receptors after unilateral permanent focal cerebral ischemia in the mouse brain

Long-term hyperexcitability was found after unilateral, permanent middle cerebral artery occlusion in exofocal neocortical areas of the adult mouse [Mittmann et al. (1998) Neuroscience 85, 15-27]. The aim of the present study was to test the hypothesis in an identical paradigm of ischemia. whether alterations in the densities of both excitatory and inhibitory amino acid receptors may underlie these pathophysiological changes. Alterations in densities of [3H]dizocilpine, [3H]D,L-amino-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, [3H]kainate and [3H]muscimol binding sites were demonstrated with quantitative in vitro receptor autoradiography. All binding sites were severely reduced in the core of the ischemic lesion. A completely different reaction was found in the exofocal, histologically inconspicuous parts of the somatosensory cortex and the more remote neocortical areas of both hemispheres. The [3H]muscimol binding sites were significantly reduced four weeks after ischemia in the motor cortex, hindlimb representation area and exofocal parts of the primary and secondary somatosensory cortices of both hemispheres. The focus of the reduction in [3H]muscimol binding sites was found in lower layer V and upper layer VI. Contrastingly, the densities of [3H]dizocilpine binding sites were found to be increased in these areas, whereas those of [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [3H]kainate binding sites did not show significant changes. The [3H]dizocilpine binding site density increased predominantly in layers III and IV. All binding sites were also reduced in the retrogradely reacting, gliotic part of the ipsilateral ventroposterior thalamic nucleus, whereas the [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites were increased in the surround of the ipsilateral nucleus and no changes in binding sites were seen in the whole contralateral nucleus. We conclude that permanent local ischemia leads to a long-term and widespread impairment of the normal balance between binding sites of excitatory and inhibitory neurotransmitter receptors in neocortical areas far away from the focus of the post-ischemic tissue damage. The imbalance comprises an up-regulation of the [3H]dizocilpine binding sites in the ion channels of N-methyl-D-aspartate receptors and a down-regulation of [3H]muscimol binding sites of the GABA(A) receptors in the ipsi- and contralateral neocortex. These changes at the receptor level explain the previously observed hyperexcitability with the appearance of epileptiform field potentials and the long duration of excitatory postsynaptic potentials four weeks after ischemia.

Neurotransmission and the ontogeny of human brain

The early appearance of neurotransmitters in brain tissue refers to their regulative functions on the neuronal circuits. Many neurotransmitters have direct effects on neuronal outgrowth and differentiation during brain development, which precede their role in synaptic information coding. Both the neurotrophic and neurotoxic properties of excitatory amino acids (EAAs) have focused special interest on glutamatergic neurotransmission during brain development. Therefore, this work intends to review and discuss developmental alterations of the EAA neurotransmitter system in the human brain, their relation to human brain maturation and implications for pathological processes during early human brain development.

Modulation by magnesium of N-methyl-D-aspartate receptors in developing human brain

AIM

To investigate age related alterations in glutamate N-methyl-D-aspartate (NMDA) receptor binding produced by the modulatory compounds glutamate, glycine, and magnesium (Mg2+) sulphate.

METHODS

The effects produced by glutamate plus glycine, and Mg2+ on the binding of [3H]MK-801, a ligand for the N-methyl-D-aspartate ion channel phencyclidine site, were measured in membrane preparations made from prefrontal cortex from human neonate (n = 5), infant (n = 6), and adult (n = 6) necropsy brains.

RESULTS

Neonatal brains had the least [3H]MK-801 binding, suggesting either a low density of NMDA receptors or a more restricted access of [3H]MK-801 to cation channel sites. Infant brains had the most [3H]MK-801 binding which was stimulated to a greater extent by L-glutamate (100 microM) and glycine (10 microM) than in neonatal and adult brains. MG2+ invariably inhibited [3H]MK-801 binding. However, the Mg2+ IC50 value was higher in neonatal brain (3.6 mM) than infant (1.4 mM) and adult (0.87 mM) brains.

CONCLUSION

Infant brain may have excess NMDA receptors which are hyper responsive to glutamate and glycine. The lower potency of Mg2+ to inhibit [3H]MK-801 binding in neonatal cortex may be because newborn babies have NMDA receptors without the normal complement of Mg2+ sites. The findings suggest that therapeutic NMDA receptor block in neonates requires higher concentrations of magnesium sulphate in brain tissue.

Acute and long-lasting effects of neonatal hypoxia on (+)-3-[125I]MK-801 binding to NMDA brain receptors

The NMDA receptor subtype is the major excitatory mediator for glutamate neurotoxicity. To assess its participation in the noxious effects of postnatal hypoxia, we have characterized the binding of the ionophoric marker of NMDA receptor, dizocilpine (MK-801). Binding of (+)-3-[125I]MK-801 to NMDA brain receptors under nonequilibrium conditions was quantified by in vitro autoradiography in rats exposed to hypoxia induced by 93% N2/6.5% O2 exposure for 70 min on Postnatal Day 4. Acute and long-lasting effects were investigated at 4 h after injury and on Postnatal Day 40. At the acute stage, a transient decrease in binding was found in several specific brain areas, hypothalamus, amygdaloid nuclei, entorhinal cortex, perirhinal cortex, and hippocampus, and no differences were found in temporal cortex, thalamus, and geniculate nucleus, when compared to sham-treated animals. At this early age, there was no increase of binding when slices from both groups were incubated in the presence of glutamate and glycine (Glu/Gly), positive allosteric modulators of MK-801 binding. In the 40-day-old brains, the binding to the NMDA receptors of hypoxiatreated animals was not different with respect to controls in most of the areas studied, but the Glu/Gly stimulation of binding in hypoxic rats showed a reduced, or absent, response to the allosteric modulators. In contrast, control rats showed a remarkable increase of the specific binding induced by the presence of the modulators in the incubation buffer. Binding of (+)-3-[125I]MK-801 was also performed at a higher concentration to clarify whether the altered response to Glu/Gly may be due to differences in the number of channels; however, the density of NMDA receptors at this concentration was similar in both control and hypoxia-treated rats. We conclude that the effect of exposure of newborn rats to hypoxia can generate acute and long-lasting effects on the NMDA receptor. The deleterious action of this kind of noxa on the CNS could be exerted by interference with normal glutamatergic transmission and hence over normal growth and development.

Synchronized overproduction of AMPA, kainate, and NMDA glutamate receptors during human spinal cord development

Quantitative receptor autoradiography was used to map the distribution in the developing human spinal cord of the three types of ionotropic glutamate receptors. N-methyl-D-Aspartate (NMDA) receptors were labeled with [3H]glutamate, kainic acid (KA) receptors were labeled with [3H]KA, and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) receptors were labeled with [3H]AMPA. In the adult, labeling of all three receptor subtypes is largely restricted to the substantia gelatinosa (SG) in the dorsal horn, with very low level labeling elsewhere in the spinal gray matter. In marked distinction, in late fetal life, high level ligand binding is seen throughout the spinal gray matter. In early postnatal life, binding sites diminish in all regions, but least so in the SG, until the adult pattern emerges. Thus a coordinated transient high level of ionotropic glutamate receptor expression occurs within the developing spinal cord. Saturation analysis of ligand binding shows that the affinity of [3H]KA and [3H]AMPA binding is not developmentally regulated. In contrast, the affinity of [3H]glutamate binding to the NMDA receptor in the fetal ventral horn is three-fold greater than in the adult ventral horn. Thus, in addition to quantitative changes in glutamate receptor expression, qualitative changes occur in the expression of NMDA receptors during development. The distinct glutamate receptor phenotype of fetal and early postnatal spinal cord cells suggests that alterations in the excitable properties of these cells plays an important role in activity-dependent development and in susceptibility to excitotoxic injury.

Head injuries in children and adults

Motor vehicle-related accidents account for the largest number of head injuries in all ages. This article reviews types of injury, neurologic assessment, secondary injury, brain swelling, seizures, resuscitation, and intensive care.

A two-process theory of schizophrenia: evidence from studies in post-mortem brain

Glutamate and GABA are the principle neurotransmitters of the cerebral cortex and are known to modulate dopaminergic function. Evidence of structural abnormalities in the cortex raises the possibility that schizophrenia involves disturbances of cortical amino-acid neurotransmission. The psychotomimetic effects of phencyclidine, a glutamate antagonist, have been taken to suggest that schizophrenia involves reduced brain glutamate function. Direct evidence for diminished glutamate function in schizophrenia is lacking. However, in polar temporal cortex and hippocampus we reported evidence of an asymmetric loss of glutamate terminals, and of reduced GABA function, which may be secondary to the loss of glutamatergic input. Glutamate cell body markers are spared in temporal lobe; the neurones which degenerate may originate in frontal cortex. A number of studies have reported increases in markers of glutamatergic cell bodies and terminals in orbital frontal cortex in schizophrenia. These findings are consistent with the presence of an abnormally abundant glutamatergic innervation, which may be the result of an arrest in the normal process of cellular and synaptic elimination which occurs during development. There is evidence that frontal abnormalities in schizophrenia are genetically determined. We suggest that glutamatergic abnormalities in anterior temporal cortex in schizophrenia are the result of the degeneration of fronto-temporal projections. Orbital frontal projections to polar temporal cortex may be prone to degeneration because they arise from an unstable frontal cortical cytoarchitecture which has not completed the normal process of post-natal remodelling. The structural abnormality of the orbital frontal region may confer vulnerability to some intrinsic or extrinsic mechanism, which brings about a progressive degeneration of projections to polar temporal lobe.

Prognostic clues and outcome of early treatment of nonketotic hyperglycinemia

Nonketotic hyperglycinemia presenting in the neonatal period is considered a devastating neurological condition. Most patients die soon after birth, and those who survive manifest severe neurological deficits, psychomotor retardation, and convulsions that are difficult to control. These abnormalities probably result from overstimulation of the N-methyl-D-aspartate receptor in the brain. In the present study, we wished to delineate prognostic clues for patients with neonatal (classic) nonketotic hyperglycinemia and to evaluate the results of early treatment with glycine-receptor modulators. The charts of six children treated in our department since 1991 were reviewed for details that might contribute to the prediction of prognosis. Postnatal clinical presentation and initial cerebrospinal fluid and plasma glycine levels were not predictive of outcome, but early treatment with N-methyl-D-aspartate modulators appeared to reduce late neurological complications and probably is as important as treatment with sodium benzoate. Attainment of developmental milestones, however, varies from one patient to another, presumably because of a factor or factors still unknown.

Glutamate receptor binding in the human hippocampus and adjacent cortex during development and aging

Distinct patterns of age-related alterations in NMDA (MK801 binding) and non-NMDA, AMPA (CNQX), and kainate binding have been identified in human hippocampus and parahippocampal gyrus in normal individuals with no evidence of degenerative brain disease ranging in age from 24 gestational weeks to 94 years. Whereas MK801 binding did not alter substantially over this age range, CNQX binding rose from low levels in the fetus to maximum levels between neonate and middle age, and kainate binding declined extensively from the perinatal to adult stage. Following maturity, there were no significant changes in kainate binding, although MK801 binding increased in CA1 and CA3 and CNQX binding declined in several regions, particularly CA2 and subiculum. For each receptor binding the timing of these fluctuations ocurring during development and aging varied within different regions of the dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex examined. The transient peaks of receptor binding are likely to reflect processes of synaptogenesis and pruning and may provide clues regarding the role of the different glutamate receptor subtypes in various pathologies of the hippocampus and adjacent cortex associated with developmental disorders (of genetic origin or due to perinatal trauma or insult). The absence of substantial changes in any subtype examined from middle to old age suggests alterations in transmitter binding to these glutamate receptors are not involved in senescent neurodegeneration.

Will a critical level of hyperventilation-induced hypocapnia always induce an absence seizure?

We wished to determine if the degree of hypocapnia correlates with increased frequency of absence seizures and if there is a critical pCO2 at which absence seizures are reliably provoked. Twelve untreated children with newly diagnosed absence epilepsy were continuously monitored by EEG and end-expiratory CO2 recording during quiet respiration and hyperventilation (to absence seizure or exhaustion) while breathing four gas mixtures: (a) room air, (b) 100% O2, (c) 4% CO2 in room air, or (d) 4% CO2 + 96% O2). In quiet respiration, a reduction in number of spike and wave bursts and total seconds of spike and wave was noted in children breathing supplemental CO2 (gases c and d vs. gases a and b), p < 0.05. Supplemental O2 had no effect. Eight subjects had absence seizures elicited with each trial of hyperventilation. All subjects had their own critical pCO2, ranging from 19 to 28 mmHg. Three children had no seizures, two despite hypocapnia to pCO2 of 19 and 21 and 1 who achieved a pCO2 of only 25. In 1, absence seizures were provoked in only six of nine hyperventilation trials to pCO2 of 17-23. In 67% of subjects, absence seizures were reliably provoked by hypocapnia. Critical pCO2 varied among children with absence. Determination of whether variation in sensitivity to hypocapnia may be helpful in determining response to antiepileptic drugs (AEDs) or remission of seizures will require further study.

Effects of hypothermia on traumatic brain injury in immature rats

Hypothermia is beneficial in adult models of traumatic brain injury (TBI), but it has not been evaluated in an immature animal model. We hypothesized that brief hypothermia applied after TBI would reduce cerebral edema and lesion volume in immature rats. Male Wistar rats (3-4 weeks of age, 90-140 g) were anesthetized, intubated, mechanically ventilated, and subjected to TBI by weight drop onto the exposed right parietal cortex. Hypothermic rats were then cooled to a brain temperature of 32.0 +/- 0.5 degrees C for 4 h, and control rats were maintained at a brain temperature of 37.0 +/- 0.5 degrees C. Cerebral edema (wet - dry weight method) was assessed at 5 days. At 4 h, a reduction of percent brain water in the traumatized hemisphere was observed in hypothermic versus normothermic rats (81.75 +/- 0.60 vs. 82.53 +/- 0.67%; p<0.05), but by 24 h posttrauma, the groups were similar (p = 0.82). Total lesion volume (47.2 +/- 8.5 vs. 44.4 +/- 10.0 mm3; p = 0.51) and necrotic volume (20.2 +/- 6.3 vs. 20.0 +/- 7.9 mm3; p = 0.95) were similar in the hypothermic and normothermic groups. We conclude that in this model, a transient (4-h) application of moderate (32 degrees C) hypothermia reduces the cerebral edema characteristically seen in immature rats at 4 h, but this reduction is not sustained at 24 h. Attenuating or delaying the development of cerebral edema could have important therapeutic relevance after TBI. Transient hypothermia, however, did not reduce lesion volume at 5 days posttrauma.

Dizocilpine binding to cerebral cortical membranes from developing and ageing mice

The binding of [3H]dizocilpine (MK-801) to the N-methyl-D-aspartate (NMDA)-gated ion channel was characterized in cerebral cortical membranes during the major portion of the mouse life-span (from 7-day- to 22-month-olds). The binding was saturable, consisting of only one component at all ages studied. The maximal binding capacity Bmax was very substantial in 14-day-old mice when compared to adults (3-month-olds), decreasing thereafter during ageing. The binding constant KD remained unchanged during development and increased only slightly in aged mice. Glutamate and glycine potentiated dizocilpine binding concentration-dependently. Their efficacy varied markedly with age. Both glutamate and glycine had considerably less effect on the immature cerebral cortex and in the oldest group of mice (22-month-old) than in young adults. The marked increase in dizocilpine binding sites at the age of 2 weeks coincides with the previously reported transient increase in NMDA binding sites in the cerebral cortex. The weak potentiation of dizocilpine binding by glutamate and glycine in the immature brain could be a factor which protects neurons during this period from excitotoxicity and increased susceptibility to seizures induced by acidic amino acids. The decrease in the number of dizocilpine binding sites during ageing could result partly from the loss of cortical neurons.

Developmental rearrangements of cortical glutamate-NMDA receptor binding sites in late human gestation

NMDA-preferring glutamate receptor biding sites were characterized using the site-selective ligand [3H]MK801, in synaptic membranes prepared from cerebral cortex tissue obtained postmortem from human infants who had died with minimal neurological and neuropathological impairment between 22 and 42 weeks' gestation. It proved necessary to modify the assay protocol used with adult tissue before reliable data could be obtained. In the four cortical region studied (prefrontal, motor, occipital, temporal), [3H]MK801 bound to a single class of sites which showed significant variations in affinity only in motor cortex. The density of [3H]MK801 binding sites (calculated at constant affinity) showed marked increases in all cortical regions over this period. The extent to which glutamate could enhance [3H]MK801 binding became significantly lower in prefrontal and motor cortex as gestation progressed, so that at term, little activation was apparent. In occipital and temporal cortex, this parameter was low throughout late gestation. The evidence suggests that Glutamate-NMDA binding sites may undergo structural rearrangements which alter their ability to interact with ligands during the later stages of human gestation, and that such changes are regionally variable.

Basic mechanisms of generalized absence seizures

Generalized absence seizures are neurophysiologically, pharmacologically, and developmentally unique and comprise the primary seizure type in a number of different absence epilepsy syndromes. Over the last 10 years, the availability of a number of animal models of generalized absence seizures and of sophisticated in vitro electrophysiological techniques that allow investigation of cortical and thalamic networks has begun to shed light on the pathogenesis of this disorder. The basic underlying mechanism appears to involve thalamocortical circuitry and the generation of abnormal oscillatory rhythms from that particular neuronal network. Biochemical mechanisms operative within thalamocortical circuitry during this neuronal oscillation seem to entail phase-locked gamma-aminobutyric acid (GABA)B-mediated inhibition alternating with glutamate-mediated excitation. The basic cellular mechanism operative within this tension between excitation and inhibition appears to involve the T-type calcium current. Local circuitry within the thalamus may influence these oscillatory rhythms by GABAA-mediated inhibition. Pharmacological factors at play external to thalamocortical circuitry include cholinergic, dopaminergic, and noradrenergic mechanisms. Pathways that utilize these various neurotransmitters project onto the thalamus and/or cortex from sites distant to those structures and may modulate the process either up or down. Perturbation of one or more of these neuronal networks may lead to abnormal neuronal oscillatory rhythms within thalamocortical circuitry, with a resultant generation of bilaterally synchronous spike wave discharges that characterize generalized absence seizures. Our increasing understanding of the basic mechanisms that underlie generalized absence seizures promises to allow, for the first time, a rational design of drug treatment for a seizure disorder based on the pathogenesis of that disorder.