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

Developmental regulation and lateralization of N-methyl-d-aspartate (NMDA) receptors in the rat hippocampus

N-methyl-d-aspartate receptors (NMDARs) are expressed abundantly in the brain and play a crucial role in the regulation of central nervous system (CNS) development, learning, and memory. During early neuronal development, NMDARs modulate neurogenesis, neuronal differentiation and migration, and synaptogenesis. The present study aimed to examine the developmental expression of NMDARs subunits, NR1 and NR2B, in the developing hippocampus of neonatal rats during the first two postnatal weeks. Fifty-four male offspring were randomly divided into three age groups, postnatal days (P) 0, 7, and 14. Real-time-PCR, western blotting, and immunohistochemistry (IHC) analyses were employed to examine and compare the hippocampal expression of the NMDA receptor subunits. The highest mRNA expression of NR1 and NR2B subunits was observed at P7, regardless of its laterality. The mRNA expression of both subunits in the right hippocampus was significantly higher than that of the left one at P0 and P7. Similarly, the highest protein level expression of NR1 and NR2B subunits was also observed at P7 in both sides hippocampi. Although the protein expression of NR1 was significantly higher on the right side in all studied days, the NR2B was significantly higher in the right hippocampus only at P7. The analysis of optical density (OD) has shown a marked increase in the distribution pattern of the NR1 and NR2B subunits at P7 in all hippocampal subregions. In conclusion, there is a marked right-left asymmetry in the expression of NR1 and NR2B subunits in the developing rat hippocampus, which might be considered as a probable mechanism for the lateral differences in the structure and function of the hippocampus in rats.

Postnatal alterations in induction threshold and expression magnitude of long-term potentiation and long-term depression at hippocampal synapses

Activity-dependent synaptic plasticity refines neural networks during development and subserves information processing in adulthood. Previous research has revealed postnatal alterations in synaptic plasticity at nearly all forebrain synapses, suggesting different forms of synaptic plasticity may contribute to network development and information processing. To assess possible relationships between modifications in synaptic plasticity and maturation of cognitive ability, we examined excitatory synaptic function in area CA1 of the mouse hippocampus ∼3 weeks of age, when hippocampal-dependent learning and memory abilities first emerge. Long-term potentiation (LTP) and depression (LTD) of synaptic efficacy were observed in slices from juvenile animals younger than 3 weeks of age. Both pre- and postsynaptic mechanisms supported LTP and LTD in juveniles. After the third postnatal week, the magnitude of LTP was reduced and the threshold for postsynaptic induction was reduced, but the threshold for presynaptic induction was increased. The reduced threshold for postsynaptic LTP appeared to be due, partly, to an increase in baseline excitatory synaptic strength, which likely permitted greater postsynaptic depolarization during induction. Low frequency stimulation did not induce LTD at this more mature stage, but it blocked subsequent induction of LTP, suggesting metaplastic differences across age groups. Late postnatal modifications in activity-dependent synaptic plasticity might reflect attenuation of mechanisms more closely tied to network formation (presynaptic potentiation and pre- and postsynaptic depression) and unmasking of mechanisms underlying information processing and storage (associative postsynaptic potentiation), which likely impact the integrative capacity of the network and regulate the emergence of adult-like cognitive abilities.

Developmental alteration of endocannabinoid retrograde signaling in the hippocampus

Endocannabinoids are lipid derivatives that mediate paracrine and juxtacrine signaling between cells. In the hippocampal CA1 region, a retrograde endocannabinoid signal suppresses GABA release by acting on presynaptic cannabinoid receptor-1 (CB1) and can be functionally manifested as depolarization-induced suppression of inhibition (DSI). In the present study, whole cell patch-clamp recordings in hippocampal slices were made to examine DSI in rats from P7-P21. Robust DSI develops in rat hippocampus at postnatal ages greater than two weeks, but only modest DSI is observed in P7-9 rat. DSI in neonatal rats can be enhanced by activation of group I metabotropic glutamate receptors (mGluRs) or muscarinic acetylcholine receptors in those neonatal rats. The DSI is also enhanced by sustained low-frequency (1 Hz) stimulation (5 min). This stimulus-enhanced DSI was prevented in the presence of 6-methyl-2-(phenylethynyl)-pyridine (10 microM), a group I mGluR antagonist. WIN55212-2, a synthetic CB1 agonist, produced a similar level of inhibition of GABAergic synaptic transmission at different postnatal time points. Therefore postsynaptic mechanisms appear to be mainly responsible for developmental changes in DSI, although presynaptic mechanisms cannot be ruled out entirely. We have also obtained evidence that tonic endocannabinoid release suppresses GABAergic transmission in the mature but not the neonatal hippocampus. The differential DSI magnitude at different stages of maturation could alter synaptic plasticity and learning and memory during hippocampal development.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

NMDA-induced stimulation of glycolysis in developing hippocampal cell cultures

Developmental changes in energy metabolism of primary hippocampal cell cultures from newborn rats were investigated during the first 3 weeks. These changes were measured by intensity of and number of cells exhibiting NAD(P)H fluorescence in response to NMDA-induced activation of neuronal activity. We observed gradual changes of stimulation-evoked NAD(P)H signaling over the first 3 weeks, such that at day 7 and 16, this stimulation is minimal, while at 5 and 12 days, it is maximal. These results describe a biphasic pattern that was similar to earlier findings from experiments investigating developmental changes in population spike amplitudes or glutamate release in young rats. Inhibition of mitochondrial respiration by KCN revealed that the NMDA-evoked stimulation of energy metabolism is mainly due to increased glycolytic activity.

Late postnatal maturation of excitatory synaptic transmission permits adult-like expression of hippocampal-dependent behaviors

Sensorimotor systems in altricial animals mature incrementally during early postnatal development, with complex cognitive abilities developing late. Of prominence are cognitive processes that depend on an intact hippocampus, such as contextual-configural learning, allocentric and idiocentric navigation, and certain forms of trace conditioning. The mechanisms that regulate the delayed maturation of the hippocampus are not well understood. However, there is support for the idea that these behaviors come "on line" with the final maturation of excitatory synaptic transmission. First, by providing a timeline for the first behavioral expression of various forms of learning and memory, this study illustrates the late maturation of hippocampal-dependent cognitive abilities. Then, functional development of the hippocampus is reviewed to establish the temporal relationship between maturation of excitatory synaptic transmission and the behavioral evidence of adult-like hippocampal processing. These data suggest that, in rats, mechanisms necessary for the expression of adult-like synaptic plasticity become available at around 2 postnatal weeks of age. However, presynaptic plasticity mechanisms, likely necessary for refinement of the hippocampal network, predominate and impede information processing until the third postnatal week.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Protective cap over CA1 synapses: extrasynaptic glutamate does not reach the postsynaptic density

Numerous data indicate that nonsynaptic release of glutamate occurs both in normal and pathophysiological conditions. When reaching receptors in the postsynaptic density (PSD), glutamate (Glu) could affect the synaptic transmission. We have tested this possibility in the hippocampal CA1 synapses of rats, either by applying exogenous Glu to the CA1 neurons or by disruption of Glu transporter activity. L-Glu (400 microM) was directly applied to the hippocampal slices acutely isolated from the rats. It produced a strong inhibition of both ortho- and antidromically elicited action potentials fired by CA1 neurons while the excitatory postsynaptic current (EPSC) measured in these neurons remained totally unaffected. The optical isomer D-Glu which is not transported by the systems of Glu uptake inhibited not only orthodromic and antidromic spikes, but also EPSC. Non-specific glutamate transporter inhibitor DL-threo-beta-hydroxyaspartic acid (THA, 400 microM) mimicked the effects of exogenous Glu and produced strong inhibition of both orthodromic and antidromic spikes, without any influence on the amplitude of EPSCs. Dihydrokainate (DHK, 300 microM), selective inhibitor of GLT-1 subtype of glutamate transporter, exerted a significant inhibitory action on the orthodromically evoked spikes and also on the EPSC. Our results indicate that extrasynaptic and PSD membranes of CA1 neurons form separate compartments differing in the mechanisms and efficiency of external Glu processing: the protection of PSD markedly prevails.

Maturation of astrocyte morphology and the establishment of astrocyte domains during postnatal hippocampal development

Mature protoplasmic astrocytes exhibit an extremely dense ramification of fine processes, yielding a 'spongiform' morphology. This complex morphology enables protoplasmic astrocytes to maintain intimate relationships with many elements of the brain parenchyma, most notably synapses. Recently, it has been demonstrated that astrocytes establish individual cellular-level domains within the neuropil, with limited overlap occurring between the extents of neighboring astrocytes. The highly ramified nature of protoplasmic astrocytes is closely associated with their ability to create such domains. This study was an attempt to characterize the development of spongiform processes and the establishment of astrocyte domains. A combination of immunolabeling for the astrocyte-specific markers glial fibrillary acidic protein and S100beta with intracellular dye labeling in fixed tissue slices allowed for the identification of immature astrocytes and the elucidation of their complete, well-preserved morphologies. We find that during the first two postnatal weeks astrocytes extend stringy, filopodial processes. Fine, spongiform processes appear during the third week. Protoplasmic astrocytes are quite heterogeneous in morphology at 1-week postnatum, but there is a remarkable consistency in morphology by 2 weeks of age. Finally, protoplasmic astrocytes initially extend long, overlapping processes during the first two postnatal weeks. The subsequent elaboration of spongiform processes results in the development of boundaries between neighboring astrocyte domains. Stray processes that encroach on neighboring domains are eventually pruned by 1 month of age. These observations suggest that domain formation is largely the consequence of competition between astrocyte processes, similar to the well-studied competitive interactions between certain neuronal dendritic fields.

Expression of Glutamate Transporters in the Medial and Lateral Vestibular Nuclei during Rat Postnatal Development

The postnatal developmental expression and the distribution of the glutamate transporters (GLAST, GLT-1 and EAAC1) were analyzed in rat vestibular nuclei (VN), at birth and during the following 4 weeks. Analyses were performed using reverse transcriptase-polymerase chain reaction and immunoblotting of GLAST, GLT-1 and EAAC1 mRNA and protein during the postnatal development of the VN neurons and their afferent connections. We also studied the distribution of each glutamate transporter in the medial and lateral VN by use of immunocytochemistry and confocal microscopy. GLAST, GLT-1 and EAAC1 mRNA and protein were present in the VN at each developmental stage. GLAST was highly expressed mainly in glia from birth to the adult stage, its distribution pattern was heterogeneous depending on the region of the medial and lateral VN. GLT-1 expression increased dramatically during the second and third postnatal weeks. At least during the first postnatal week, GLT-1 was expressed in the soma of neurons. EAAC1 was detected in neurons and decreased from the third week. These temporal and regional patterns of GLAST, GLT-1 and EAAC1 suggest that they play different roles in the maturation of glutamatergic synaptic transmission in the medial and lateral VN during postnatal development.

Ontogeny of ionotropic glutamate receptor subunit expression in the rat hippocampus

The ionotropic glutamate receptors play key roles in multiple developmental mechanisms, including regulation of neuronal migration and differentiation, and synaptic organization. In this study, we investigated the developmental expression of these glutamate receptors in the postnatal rat hippocampus. We examined the transcripts encoding the subunits composing the N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and kainate (KA) subtypes of glutamate receptors by in situ hybridization at multiple time points from postnatal day (PND) 1 to PND 35. In the case of the AMPA receptor, gluR1 expression did not change over this time period, while gluR2, gluR3, and gluR4 did. These three subunits each underwent a transient period of increased expression at either PND 7 or PND 18. All five of the kainate receptor subunits changed during this time, all starting at relatively high levels of expression that declined by PND 35. Similar to most of the AMPA subunits, all of the kainate subunits had transient periods of significantly increased expression. The NMDA receptors all changed during over time as well, and each had a period of increased expression. The periods of transiently increased expression of all of these subunits coincide with known periods of plasticity and other critical times in development. These results suggest the different glutamate receptor subtypes may be critical at specific times during postnatal brain development.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Ontogeny of synaptic transmission in the rat hippocampus

Electrophysiological characteristics of the hippocampal slices of juvenile (14-27 days) or young (36-40 days) Wistar rats have been compared. In the juvenile rats measurements were taken daily, from postnatal day (PN) 14 to PN27. Input-output curves were used to quantify the ontogeny of excitatory processes. The dynamic of population spike (PS) maturation was not even during the investigated postnatal period. After day 19 transient decrease of PS amplitude was observed until day 22. There were also some differences between the shape of input-output curves from the slices of rats of different ages. In general, PS was saturated at lower intensities in younger animals. The slices from 19-day-old rats did not display saturated input-output curve with 2-20 V stimuli intensities. But input-output curves on PN20-22 were rather similar to that obtained before PN19. The periods of gradual increase and subsequent decrease of PS amplitudes during early ontogeny correlate with the appearance of certain forms of behaviour. This fact suggests that hippocampal PS amplitude depression may be relevant functionally.

Extrasynaptic glutamate spillover in the hippocampus: evidence and implications

In the mammalian brain most excitatory transmission is mediated by glutamate binding to AMPA and NMDA receptors. These receptors have markedly different biophysical properties, and at synapses in the CAI region of the hippocampus they play complementary roles in long-term potentiation (LTP): while postsynaptic NMDA receptor activation is necessary for the induction of this form of plasticity, AMPA receptors play a larger role in its expression. Recent studies in hippocampal slices have revealed a further striking difference in the behaviour of the two receptor types: NMDA receptors consistently sense a larger number of quanta of glutamate released from presynaptic terminals than do AMPA receptors. Two alternative explanations for this are either that AMPA receptors are functionally silent at a proportion of synapses (although they can be uncovered by LTP), or that glutamate can spill over from neighbouring synapses and selectively activate NMDA (but not AMPA) receptors. Both of these competing hypotheses have extensive implications for the mechanisms of expression of LTP. Extrasynaptic glutamate diffusion appears to depend critically on the recording temperature, but if excitatory synapses are sufficiently close for cross-talk to occur under physiological conditions, it could have profound implications for the specificity of synaptic communication in the brain.

Differential developmental expression of the two rat brain glutamate transporter proteins GLAST and GLT

The extracellular concentration of the excitatory neurotransmitter glutamate is kept low by the action of glutamate transporters in the plasma membranes of both neurons and glial cells. These transporters may play important roles, not only in the adult brain, but also in the developing brain, as glutamate is thought to modulate the formation and elimination of synapses as well as neuronal migration, proliferation and apoptosis. Here we demonstrate the developmental changes in the expression of two glutamate transporters, GLAST and GLT, by quantitative immunoblotting and by light and electron microscopic immunocytochemistry. At birth, GLT is not detectable, but GLAST is present at significant concentrations both in the forebrain and in the cerebellum. GLT is first detected in the forebrain and cerebellum in the second and third week, respectively. Both transporters reach adult levels by postnatal week 5. The development of the total glutamate uptake activity in the forebrain, as determined by solubilization and reconstitution of the transporters in liposomes, parallels that of GLT, in agreement with the observation that GLT is the predominant transporter in the adult brain. The regional distributions of both GLAST and GLT in the tissue are similar in young and adult rats. Only GLAST is detectable in the external germinal layer of the cerebellar cortex. Electron microscopical investigation demonstrated GLAST and GLT exclusively in glial cells in young as well as in adult animals.

Topography of hypoxic injury proved by argyrophilia in postnatal rat brain

The argyrophil III method, a new esterification-silver staining approach, was used to elucidate regional differences in the susceptibility of developing brain to hypoxic-ischemic (H-I) injury. We created a unilateral common carotid artery-ligation model with hypoxia (8% oxygen) in postnatal day (P) 7, P14 and P21 rats. The argyrophil (i.e., deteriorated) neurons were apparent in the ipsilateral hippocampus, cortex, and striatum in each age group. Argyrophil neurons exhibited some morphological signs of the "early phase" of injury preceding the loss of structure and/or cell death in the "late phase," as indicated by hematoxylin-eosin (H-E) staining. The argyrophil neurons were apparent as early as 12 hours after the insult, whereas the histological changes revealed by H-E staining were subtle. The early phase and late phase histological changes had a stereotyped pattern of appearance in all ages studied. However, the duration of H-I situation required to produce argyrophil cells differed according to age. The most resistive age was P14 (P14 > P7 > P21) in this observation. Therefore, argyrophil III staining is feasible for H-I brain damage model in neonates. The results suggest that both the early phase and the late phase pathological processes after H-I injury have a characteristic topographical vulnerability that does not change during development but have a differing susceptibility according to age.

Emergence of activity-dependent, bidirectional control of microtubule-associated protein MAP2 phosphorylation during postnatal development

Pronounced changes in neuronal morphology occur as synapses mature; however, little is known about how synaptic transmission regulates the developing neuronal cytoskeleton. The postsynaptic, microtubule-associated protein MAP2 is a target of multiple, calcium-dependent signaling pathways activated by synaptic transmission. Here we demonstrate that MAP2 phosphorylation is differentially regulated across development. In 32P-labeled hippocampal slices prepared from adult rats, depolarization stimulated a bidirectional change in the phosphorylation of immunoprecipitated MAP2. A transient increase was mediated by metabotropic glutamate receptors (mGluRs) and stimulation of mitogen-activated protein kinases (MAPKs), Ca2+/calmodulin-dependent protein kinases (CaMKs), and protein kinase C (PKC). This increase was followed by a persistent dephosphorylation mediated by NMDA receptors and activation of protein phosphatase 2B (PP2B or calcineurin). In contrast, depolarization of neonatal hippocampal slices stimulated exclusively a net increase in MAP2 phosphorylation, which was attenuated by inhibitors of MAPKs, but not CaMKs or PKC. Furthermore, although incubation in NMDA induced a time-dependent decrease in MAP2 phosphorylation in both adults and neonates, this effect was both less robust and less sensitive to calcineurin inhibitors in neonates than in adults. These data indicate that the mechanisms coupling glutamate release to MAP2 dephosphorylation are relatively lacking in the neonatal hippocampus. Highly phosphorylated MAP2 is impaired in its ability to stabilize microtubules and actin filament bundles in vitro. The neonatal propensity toward glutamate-stimulated MAP2 phosphorylation may serve to reduce cytoskeletal stability and permit dendritic arborization early in postnatal development. In mature neurons, the bidirectional control of MAP2 phosphorylation may participate in activity-dependent synaptic remodeling.

The characteristics of arginine transport by rat cerebellar and cortical synaptosomes

The uptake of L-[3H]arginine into synaptosomes prepared from rat cerebellum and cortex occurred by a high-affinity carrier-mediated process. The uptake of arginine appeared to be potentiated by removal of extracellular Na+, inhibited by high levels of extracellular K+, but not by depolarization with veratridine or 4-amino pyridine. The effect of Na+ removal or K+ elevation did not seem to be due to changes in intracellular Ca2+ or pH. In both brain regions, uptake was significantly inhibited by L-arginine, L-lysine, L-ornithine, and L-homoarginine, but not by D-arginine nor L-citrulline. Uptake was also inhibited by NG-monomethyl-L-arginine acetate, but not by NG-nitro-L-arginine methyl ester nor NG-nitro-L-arginine except in the cortex at a concentration of 1 mM. The results indicate that the carrier system operating in synaptosomes showed many of the characteristics of the ubiquitous y+ system seen in many other tissues, although its apparent sensitivity to variations in extracellular Na+ was unusual.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

The uptake of gamma-aminobutyric acid and glutamate by synaptosomes from the visual cortex of albino and pigmented rabbits

The synaptosomal uptake of glutamate and gamma aminobutyric acid [GABA] in the visual cortex of albino and pigmented rabbits was compared. GABA uptake was similar in both pigmented and albino rabbits, but glutamate uptake was greater in the pigmented rabbit. The kinetics of glutamate uptake in albino and pigmented rabbits suggested that the number of functioning glutamate synapses may be lower in the albino. The significance of this with respect to the differences in visual processing in the two types of rabbit is discussed.

Excitatory amino acid-, except 1S,3R-ACPD, induced transient high stimulation of phosphoinositide metabolism during hippocampal neuron development

Rat hippocampal neurons in culture extended their neurites until day 5 in vitro (DIV). Then, the mean neuritic length slightly decreased. Excitatory amino acid (EAA)-elicited inositol phosphate (IP) formation increased from 0.5 to 2 DIV, reached a plateau between 2 and 4-5 DIV, and then gradually decreased until 10 DIV. This decrease was likely not due to neuronal death. This developmental pattern was observed for N-methyl-D-aspartate, kainate, glutamate, ibotenate and quisqualate (QA). Interestingly, the 1S,3R-aminocyclopentane dicarboxylate (1S,3R-ACPD) response slightly increased during neuronal culture development. At 3 DIV, the ionotropic antagonists 6,7-dinitro-quinoxalin-2,3-dion and D-2-amino-5-phosphonopentanoate efficiently blocked N-methyl-D-aspartate and kainate-elicited IP formation, and partially inhibited glutamate and ibotenate responses. QA and 1S,3R-ACPD responses were not affected, suggesting a metabotropic action for these two compounds. Furthermore, QA and 1S,3R-ACPD potencies significantly increased between 3 and 10 DIV. The transient high activity periods induced by EAA, except for 1S,3R-ACPD, are not observed for norepinephrine, carbachol and potassium chloride responses. Taken together, these data suggest that: (i) QA and 1S,3R-ACPD can act on two different glutamate metabotropic receptors subtypes during development; and (ii) the EAA-induced transient peaks of IP stimulation, which are specific with respect to other neuroactive substances profiles, could be involved in the development of hippocampal neurons. Indeed, these transient high activities take place when the neuritic length regularly increases in vitro.

Postnatal development of the calcium-dependency of glutamate release from rat cortical synaptosomes: comparison with 5-hydroxytryptamine release

The ontogeny of the Ca(2+)-dependency of the depolarisation-induced release of preloaded [3H]glutamate and [3H]5-hydroxytryptamine [5-HT] from rat cortical synaptosomes was examined. 5-HT release was found to be exclusively Ca(2+)-dependent at all ages studied. In contrast, glutamate release only showed a significant Ca(2+)-dependent component from postnatal day 10 [PND 10] onwards. This correlated with the ontogeny of the glutamate accumulating activity of synaptic vesicles, a finding consistent with vesicles being the site of Ca(2+)-dependent release. The effectiveness of K(+)-depolarisation in inducing the Ca(2+)-dependent release of both transmitters increased during the early neonatal period, reaching near adult levels at PND20 for 5-HT and PND30 for glutamate.

Developmental change from inhibition to facilitation in the presynaptic control of glutamate exocytosis by metabotropic glutamate receptors

We have addressed the role of presynaptic metabotropic glutamate receptors in the control of glutamate release from cerebrocortical nerve terminals. The metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid enhances the release evoked by a submaximal depolarization in the presence of low concentrations of arachidonic acid and in a staurosporine-sensitive manner. In contrast, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid and L(+)-2-amino-4-phosphonobutyrate inhibit the release evoked by a maximal depolarization, in the absence of arachidonic acid and by a staurosporine-insensitive mechanism. Interestingly, the effects of the metabotropic glutamate receptors that inhibit glutamate release are only observed in the nerve terminals from young rats (one to three weeks), while the facilitatory effects are better seen in latter developmental stages (three to four weeks) and adult (two to three months) rats, coinciding with the development of the maximal capacity of glutamate uptake. These results indicate the existence of important developmental changes in the presynaptic control of glutamate release.

On the role of nitric oxide as a cellular messenger in brain

The characteristics of the high-affinity uptake of [3H]-L-arginine into cerebellar and cortical synaptosomes were investigated. Uptake into cerebellar synaptosomes was often greater than seen in cortical synaptosomes under similar experimental conditions, and this was reflected by a higher Vmax in synaptosomes from this brain region. Uptake into synaptosomes prepared from both brain regions was markedly enhanced by removing extracellular Na+, adn inhibited by high concentrations of extracellular K+. Depolarisation with 4-aminopyridine or veratridine has no effect on uptake. Uptake was also unaffected by hyperpolarisation. The profile of inhibition of arginine uptake by related amino acids was similar to that seen for the y+ carrier, but the other characteristic alluded to above suggest that the carrier is distinct from the classical y+ system. The possible relationship between the carrier and the metabolism of arginine through the nitric oxide [NO] pathway, and the role of NO in the central nervous system is discussed.