Developmental changes of glutamate receptors in the rat cerebral cortex and hippocampus

The Chemokine CCL2 Promotes Excitatory Synaptic Transmission in Hippocampal Neurons via GluA1 Subunit Trafficking

The CC chemokine ligand 2 (CCL2, also known as MCP-1) and its cognate receptor CCR2 have well-characterized roles in chemotaxis. CCL2 has been previously shown to promote excitatory synaptic transmission and neuronal excitability. However, the detailed molecular mechanism underlying this process remains largely unclear. In cultured hippocampal neurons, CCL2 application rapidly upregulated surface expression of GluA1, in a CCR2-dependent manner, assayed using SEP-GluA1 live imaging, surface GluA1 antibody staining, and electrophysiology. Using pharmacology and reporter assays, we further showed that CCL2 upregulated surface GluA1 expression primarily via Gαq- and CaMKII-dependent signaling. Consistently, using i.p. injection of lipopolysaccharide to induce neuroinflammation, we found upregulated phosphorylation of S831 and S845 sites on AMPA receptor subunit GluA1 in the hippocampus, an effect blocked in Ccr2-/- mice. Together, these results provide a mechanism through which CCL2, and other secreted molecules that signal through G-protein coupled receptors, can directly regulate synaptic transmission.

GluA1-Shank3 interaction decreases in response to chronic neuronal depolarization

Interactions between AMPA receptors and synaptic scaffolding proteins are key regulators of synaptic receptor density and, thereby, synapse strength. Shank3 is one such scaffolding protein with high clinical relevance, as genetic variants and deletions of this protein have been linked to autism spectrum disorder. Shank3 acts as a master regulator of the postsynaptic density of glutamatergic synapses, interacting with ionotropic and metabotropic glutamate receptors and cytoskeletal elements to modulate synaptic structure. Notably, Shank3 has been shown to interact directly with the AMPAR subunit GluA1, and Shank3 knockout animals show deficits in AMPAR-mediated synaptic transmission. In this study, we sought to characterize the stability of GluA1-Shank3 interaction in response to chronic stimuli using a highly sensitive and specific proximity ligation assay. We found that GluA1-Shank3 interactions decrease in response to prolonged neuronal depolarization induced by elevated extracellular potassium, and that this reduced interaction is blocked by NMDA receptor antagonism. These results firmly establish the close interaction of GluA1 and Shank3 in cortical neurons in vitro, and that this select interaction is subject to modulation by depolarization.

Increased glutamatergic synaptic transmission during development in layer II/III mouse motor cortex pyramidal neurons

Postnatal maturation of the motor cortex is vital to developing a variety of functions, including the capacity for motor learning. The first postnatal weeks involve many neuronal and synaptic changes, which differ by region and layer, likely due to different functions and needs during development. Motor cortex layer II/III is critical to receiving and integrating inputs from somatosensory cortex and generating attentional signals that are important in motor learning and planning. Here, we examined the neuronal and synaptic changes occurring in layer II/III pyramidal neurons of the mouse motor cortex from the neonatal (postnatal day 10) to young adult (postnatal day 30) period, using a combination of electrophysiology and biochemical measures of glutamatergic receptor subunits. There are several changes between p10 and p30 in these neurons, including increased dendritic branching, neuronal excitability, glutamatergic synapse number and synaptic transmission. These changes are critical to ongoing plasticity and capacity for motor learning during development. Understanding these changes will help inform future studies examining the impact of early-life injury and experiences on motor learning and development capacity.

Investigation of GluA1 and GluA2 AMPA receptor subtype distribution in the hippocampus and anterior cingulate cortex of Long Evans rats during development

Preadolescent development is characterized by a reorganization of connectivity within and between brain regions that coincides with the emergence of complex behaviors. During the preadolescent period, the rodent hippocampus and regions of the frontal cortex are remodelled as the brain strengthens active connections and eliminates others. In the developing and mature brain, changes in the properties of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAr)-mediated synaptic responses contribute to experience-dependent changes in neural organization and function. AMPAr are made up of 4 subunits, of which GluA1 and GluA2 have been shown to play the most prominent role in functional plasticity. In this study, we sought to determine whether levels of these two subunits changed during the course of pre-adolescent development in the hippocampus and anterior cingulate cortex (ACC). To investigate the developmental changes in GluA1 and GluA2 AMPAr subunits, Western blotting and immunohistochemistry were performed on the ACC and hippocampus from P18 - P30 and compared to adult (P50) levels and distribution. Within the hippocampus, protein levels of GluA1 and GluA2 peaked around P26-30 whereby localized staining in the dentate gyrus reflected this pattern. GluA1 and GluA2 levels within the ACC showed little variation during this developmental period. These results indicate that changes in AMPAr subunits within the hippocampus coincide with developmental modifications that underlie the shift from juvenile- to adult-like capabilities. However, changes in AMPAr distribution in the ACC might not mediate changes that reflect preadolescent developmental shifts.

Do stereoisomers of homocysteic acid exhibit different convulsant action in immature rats?

Mechanism of ictogenesis of D- and L-stereroisomers of homocysteic acid was studied in 12-day-old rats by means of antagonists of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. There was no qualitative difference between the two stereoisomers in generation of emprosthotonic (flexion) as well as generalized tonic-clonic seizures. Moderate differences were observed in the first, nonconvulsive effects of the two isomers. As generation of the two types of seizures is concerned, NMDA and AMPA participate in generalized tonic-clonic seizures whereas NMDA receptors play a dominant role in generation of flexion seizures.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

The effect of AMPA receptor blockade on spatial information acquisition, consolidation and expression in juvenile rats

Improvement on spatial tasks in rats is observed during a late, postnatal developmental period (post-natal day (PND) 18 - PND 20). The developmental emergence of this spatial function occurs in conjunction with hippocampal connectivity changes and enhanced hippocampal-AMPA receptor-mediated synaptic responses. The current work investigated the effect of AMPAr blockade on the emergence and long-term storage of spatial information in juvenile rats and associated neural activity patterns in the dorsal hippocampus CA1 region. Male, Long Evans rats between the ages of PND 18 and PND 20 were systemically (i.p.) administered the AMPAr antagonist, NBQX, (0, 5 or 10mg/kg) every day prior to hidden platform water maze training (PND 18, 19 and 20), every day immediately post-training or immediately before the probe test (PND 41). NBQX administration prior to training prolonged latencies, pathlength and increased thigmotaxis during the acquisition phase. Administration of NBQX immediately posttraining had no effect on the day-to-day performance. When given a probe test 3weeks later, the saline group across all conditions spent more time in the target quadrant. Rats treated with pretraining 5mg NBQX dose showed a preference for the target quadrant while the posttraining and pretesting 5mg NBQX doses impaired the target quadrant preference. Groups injected with 10mg of NBQX pretraining, posttraining or pretesting did not show a preference for the target quadrant. c-Fos labeling in the CA1 reflected these differences in probe performance in that groups showing greater than chance dwell time in the target quadrant showed more c-Fos labeling in the CA1 region than groups that did not show a target quadrant preference. These findings provide support for the critical role of AMPA receptor-mediated function in the organization and long-term storage of spatial memories acquired during the juvenile period.

Seizure control by decanoic acid through direct AMPA receptor inhibition

See Rogawski (doi:10.1093/awv369) for a scientific commentary on this article. 

The medium chain triglyceride ketogenic diet is an established treatment for drug-resistant epilepsy that increases plasma levels of decanoic acid and ketones. Recently, decanoic acid has been shown to provide seizure control in vivo, yet its mechanism of action remains unclear. Here we show that decanoic acid, but not the ketones β-hydroxybutryate or acetone, shows antiseizure activity in two acute ex vivo rat hippocampal slice models of epileptiform activity. To search for a mechanism of decanoic acid, we show it has a strong inhibitory effect on excitatory, but not inhibitory, neurotransmission in hippocampal slices. Using heterologous expression of excitatory ionotropic glutamate receptor AMPA subunits in Xenopus oocytes, we show that this effect is through direct AMPA receptor inhibition, a target shared by a recently introduced epilepsy treatment perampanel. Decanoic acid acts as a non-competitive antagonist at therapeutically relevant concentrations, in a voltage- and subunit-dependent manner, and this is sufficient to explain its antiseizure effects. This inhibitory effect is likely to be caused by binding to sites on the M3 helix of the AMPA-GluA2 transmembrane domain; independent from the binding site of perampanel. Together our results indicate that the direct inhibition of excitatory neurotransmission by decanoic acid in the brain contributes to the anti-convulsant effect of the medium chain triglyceride ketogenic diet.

Selective regulation of GluA subunit synthesis and AMPA receptor-mediated synaptic function and plasticity by the translation repressor 4E-BP2 in hippocampal pyramidal cells

The eukaryotic initiation factor 4E-binding protein-2 (4E-BP2) is a repressor of cap-dependent mRNA translation and a major downstream effector of the mammalian target of rapamycin (mTOR) implicated in hippocampal long-term synaptic plasticity and memory. Yet, synaptic mechanisms regulated by 4E-BP2 translational repression remain unknown. Combining knock-out mice, whole-cell recordings, spine analysis, and translation profiling, we found that 4E-BP2 deletion selectively upregulated synthesis of glutamate receptor subunits GluA1 and GluA2, facilitating AMPA receptor (AMPAR)-mediated synaptic transmission and affecting translation-dependent chemically induced late long-term potentiation (cL-LTP). In 4E-BP2 knock-out (4E-BP2(-/-)) mice, evoked and miniature EPSCs were increased, an effect mimicked by short-hairpin RNA knockdown of 4E-BP2 in wild-type mice, indicating that 4E-BP2 level regulates basal transmission at mature hippocampal AMPAR-containing synapses. Remarkably, in 4E-BP2(-/-) mice, the AMPA to NMDA receptor (NMDAR) EPSC ratio was increased, without affecting NMDAR-mediated EPSCs. The enhanced AMPAR function concurred with increased spine density and decreased length resulting from greater proportion of regular spines and less filopodia in 4E-BP2(-/-) mice. Polysome profiling revealed that translation of GluA1 and GluA2 subunits, but not GluN1 or GluN2A/B, was selectively increased in 4E-BP2(-/-) hippocampi, consistent with unaltered I-V relation of EPSCs mediated by GluA1/GluA2 heteromers. Finally, translation-dependent cL-LTP of unitary EPSCs was also affected in 4E-BP2(-/-) mice, lowering induction threshold and removing mTOR signaling requirement while impairing induction by normal stimulation. Thus, translational control through 4E-BP2 represents a unique mechanism for selective regulation of AMPAR synthesis, synaptic function, and long-term plasticity, important for hippocampal-dependent memory processes.

Biophysical mechanisms regulating AMPA receptor accumulation at synapses

Controlling the number of AMPA receptors at synapses is fundamental for fast synaptic transmission as well as for long term adaptations in synaptic strength. In this review, we examine the biophysical mechanisms implicated in regulating AMPAR levels at the cell surface and at synapses. We first describe the structure and function of AMPARs, as well as their interactions with various proteins regulating their traffic and function. Second we review the vesicular trafficking mechanism involving exocytosis and endocytosis, by which AMPARs reach the cell surface and are internalized, respectively. Third, we examine the properties of lateral diffusion of AMPARs and their trapping at post-synaptic densities. Finally, we discuss how these two parallel mechanisms are integrated in time and space to control changes in synaptic AMPAR levels in response to plasticity protocols. This review highlights the important role of the extra-synaptic AMPAR pool, which makes an obligatory link between vesicular trafficking and trapping or release at synapses.

Sex and steroid hormones in early brain injury

Brain injury during development can have severe, long-term consequences. Using an array of animal models, we have an understanding of the etiology of perinatal brain injury. However, we have only recently begun to address the consequences of endogenous factors such as genetic sex and developmental steroid hormone milieu. Our limited understanding has sometimes led researchers to make over-generalizing and potentially dangerous statements regarding treatment for brain injury. Therefore this review acts as a cautionary tale, speaking to our need to understand the effects of sex and steroid hormone environment on the response to brain trauma in the neonate.

Expression of the AMPA Receptor Subunits GluR1 and GluR2 is Associated with Granule Cell Maturation in the Dentate Gyrus

The dentate gyrus produces new granule neurons throughout adulthood in mammals from rodents to humans. During granule cell maturation, defined markers are expressed in a highly regulated sequential process, which is necessary for directed neuronal differentiation. In the present study, we show that α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor subunits GluR1 and GluR2 are expressed in differentiated granule cells, but not in stem cells, in neonatal, and adult dentate gyrus. Using markers for neural progenitors, immature and mature granule cells, we found that GluR1 and GluR2 were expressed mainly in mature cells and in some immature cells. A time-course analysis of 5-bromo-2′-deoxyuridine staining revealed that granule cells express GluR1 around 3 weeks after being generated. In mice heterozygous for the alpha-isoform of calcium/calmodulin-dependent protein kinase II, a putative animal model of schizophrenia and bipolar disorder in which dentate gyrus granule cells fail to mature normally, GluR1 and GluR2 immunoreactivities were substantially downregulated in the dentate gyrus granule cells. In the granule cells of mutant mice, the expression of both presynaptic and postsynaptic markers was decreased, suggesting that GluR1 and GluR2 are also associated with synaptic maturation. Moreover, GluR1 and GluR2 were also expressed in mature granule cells of the neonatal dentate gyrus. Taken together, these findings indicate that GluR1 and GluR2 expression closely correlates with the neuronal maturation state, and that GluR1 and GluR2 are useful markers for mature granule cells in the dentate gyrus.

GluR2/3 label expression of the AMPA-type glutamate receptor in the hippocampal formation of the homing pigeon stabilizes just after birth

The compositions of the glutamate AMPA-type receptors influence the neural response and the subunits GluR2/3 has been referred to as essential for receptor trafficking and synapse consolidation. We investigate the GluR2/3 occurrence and expression in the hippocampal formation of newly born homing pigeons by a semi-quantitative approach, the Western-blotting technique and by immunohistochemistry. Immunoreactivity for GluR2/3 occurs before hatching has been evident in neuropil that was fully dispersed over the hippocampus proper (HP) and the area parahippocampalis (APH). Although many HP cells are NeuN-positives, a specific neuronal protein indicating that they are already differentiated as neurons while not one contains GluR2/3 at the hatching day (P0). Few neurons at the APH seem to express GluR2/3 at P0, but 3 days later (P3) the GluR2/3 labeling can be recognized in many HP neurons, showing a distribution pattern that resembles the adult, gradually increasing in intensity until P10. Also, the Western-blot shows an augment between P0 and P3, remaining stable after that. The enhancement of the neuronal label at P3 coincides with the retraction of the GluR2/3 label in neuropil, reducing their occurrence during the maturational period to become restricted to the dorsomedial portion as reported for adults. As the HP GluR2/3-containing cells are supposedly projecting neurons, taking together, the results signalize the relevance of the GluR2/3 in post-hatch formation of avian hippocampal circuitry in which the third day seems to be the critical period.

Chronic pretreatment with acetyl-L-carnitine and ±DL-α-lipoic acid protects against acute glutamate-induced neurotoxicity in rat brain by altering mitochondrial function

Cellular oxidative stress and energy failure were shown to be involved in Glutamate (L-Glu) neurotoxicity, whereas, acetyl-L-carnitine (ALCAR) and ±DL-α-lipoic acid (LA) are known to be key players in the mitochondrial energy production. To evaluate the effects of the above antioxidants, adult rats were pretreated with ALCAR (100 mg/kg i.p for 21 days) and both ALCAR and LA (100 mg/kg i.p + 50 mg/kg i.p for 21 days), before stereotactically administering L-Glu bolus (1 μmole/1 μl) in the cerebral cortex. Results showed that acute L-Glu increased ROS (P < 0.001), LPO (P < 0.001), Ca(2+) (P < 0.001), TNF-α (P < 0.001), IFN-γ (P < 0.001), NO (P < 0.001) levels and mRNA expression of Caspase-3, Casapase-9, iNOS, and nNOS genes with respect to saline-injected control group. Key antioxidant parameters such as SOD, CAT, GSH, GR along with mitochondrial transmembrane potential (Ψ∆m) were decreased (P < 0.05), while ALCAR pretreatment prevented these effects by significantly inhibiting ROS (P < 0.001), LPO (P < 0.001), Ca(2+) (P < 0.05), TNF-α (P < 0.05), IFN-γ (P < 0.001), NO (P < 0.01) levels and expression of the above genes. This chronic pretreatment of ALCAR also increased SOD, CAT, GSH, GR, and Ψ∆m (P < 0.0.01, P < 0.0.01, P < 0.05, P < 0.05, and P < 0.001, respectively) with respect to L: -Glu group. The addition of LA to ALCAR resulted in further increases in CAT (P < 0.05), GSH (P < 0.01), GR (P < 0.05), Ψ∆m (P < 0.05) and additional decreases in ROS (P < 0.001), LPO (P < 0.05), Ca(2+) (P < 0.05), TNF-α (P < 0.05) and mRNA expression of iNOS and nNOS genes with respect to ALCAR group. Hence, this "one-two punch" of ALCAR + LA may help in ameliorating the deleterious cellular events that occur after L-Glu.

Corticosteroids: way upstream

Studies into the mechanisms of corticosteroid action continue to be a rich bed of research, spanning the fields of neuroscience and endocrinology through to immunology and metabolism. However, the vast literature generated, in particular with respect to corticosteroid actions in the brain, tends to be contentious, with some aspects suffering from loose definitions, poorly-defined models, and appropriate dissection kits. Here, rather than presenting a comprehensive review of the subject, we aim to present a critique of key concepts that have emerged over the years so as to stimulate new thoughts in the field by identifying apparent shortcomings. This article will draw on experience and knowledge derived from studies of the neural actions of other steroid hormones, in particular estrogens, not only because there are many parallels but also because 'learning from differences' can be a fruitful approach. The core purpose of this review is to consider the mechanisms through which corticosteroids might act rapidly to alter neural signaling.

Ontogeny of subunits 2 and 3 of the AMPA-type glutamate receptors in Purkinje cells of the developing chick cerebellum

Several molecules, involved in cellular communication in the mature nervous system, appear to play important roles during neural development. These roles include neuronal growth, morphological changes of neurites, and neuronal survival. Such plasticity processes seem to be in part the result of activation of different receptor subtypes, which could cause Ca(2+) influx, a major candidate to be an outgrowth promoter. In this context, we performed immunohistochemical and in situ hybridization experiments to examine the following aspects of the development of chick cerebellum Purkinje cells: (i) expression of AMPA-type glutamate receptor GluR2/3 proteins; (ii) the levels of mRNAs coding for the GluR2 and GluR3 flip/flop isoforms; and (iii) expression of calbindin (CB) and parvalbumin (PV). Expression of GluR2/3 proteins, CB, PV, and the mRNAs coding for GluR2 and GluR3 splice variants all revealed a differential expression during development in chick Purkinje cells. GluR2/3 proteins and the GluR3 flop variant start to be expressed at E10, while the expression of CB, PV, the GluR3 flip isoform and the splice variants of GluR2 all started around E12-E14. All proteins showed an increasing expression from embryonic stages into the posthatching period. These results reveal a developmentally regulated expression of GluR2/3 proteins, including their splice variants, and of CB and PV in Purkinje cells. These findings may suggest a relationship between these proteins and specific cerebellar developmental processes.

Novel bioactive bromopyrrole alkaloids from the Mediterranean sponge Axinella verrucosa

The Mediterranean sponge Axinella verrucosa has been investigated for its alkaloid composition and has been found to produce a complex mixture of bromopyrrole alkaloids. Along with the previously isolated compounds 5-18, four novel alkaloids of this class, compounds 1-4, have been isolated, and their structures established through spectroscopic methods. Compounds 1-4 were found to display neuroprotective activity against the agonists serotonin and glutamate in vitro.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

In vivo effect of chronic hypoxia on the neurochemical profile of the developing rat hippocampus

The cognitive deficits observed in children with cyanotic congenital heart disease suggest involvement of the developing hippocampus. Chronic postnatal hypoxia present during infancy in these children may play a role in these impairments. To understand the biochemical mechanisms of hippocampal injury in chronic hypoxia, a neurochemical profile consisting of 15 metabolite concentrations and 2 metabolite ratios in the hippocampus was evaluated in a rat model of chronic postnatal hypoxia using in vivo 1H NMR spectroscopy at 9.4 T. Chronic hypoxia was induced by continuously exposing rats (n = 23) to 10% O2 from postnatal day (P) 3 to P28. Fifteen metabolites were quantified from a volume of 9-11 microl centered on the left hippocampus on P14, P21, and P28 and were compared with normoxic controls (n = 14). The developmental trajectory of neurochemicals in chronic hypoxia was similar to that seen in normoxia. However, chronic hypoxia had an effect on the concentrations of the following neurochemicals: aspartate, creatine, phosphocreatine, GABA, glutamate, glutamine, glutathione, myoinositol, N-acetylaspartate (NAA), phosphorylethanolamine, and phosphocreatine/creatine (PCr/Cr) and glutamate/glutamine (Glu/Gln) ratios (P < 0.001 each, except glutamate, P = 0.04). The increased PCr/Cr ratio is consistent with decreased brain energy consumption. Given the well-established link between excitatory neurotransmission and brain energy metabolism, we postulate that elevated glutamate, Glu/Gln ratio, and GABA indicate suppressed excitatory neurotransmission in an energy-limited environment. Decreased NAA and phosphorylethanolamine suggest reduced neuronal integrity and phospholipid metabolism. The altered hippocampal neurochemistry during its development may underlie some of the cognitive deficits present in human infants at risk of chronic hypoxia.

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.

Brain-derived neurotrophic factor signal enhances and maintains the expression of AMPA receptor-associated PDZ proteins in developing cortical neurons

Postsynaptic molecules with PDZ domains (PDZ proteins) interact with various glutamate receptors and regulate their subcellular trafficking and stability. In rat neocortical development, the protein expression of AMPA-type glutamate receptor GluR1 lagged behind its mRNA expression and rather paralleled an increase in PDZ protein levels. One of the neurotrophins, brain-derived neurotrophic factor (BDNF), appeared to contribute to this process, regulating the PDZ protein expression. In neocortical cultures, BDNF treatment upregulated SAP97, GRIP1, and Pick1 PDZ proteins. Conversely, BDNF gene targeting downregulated these same PDZ molecules. The BDNF-triggered increases in PDZ proteins resulted in the elevation of their total association with the AMPA receptors GluR1 and GluR2/3, which led to the increase in AMPA receptor proteins. When Sindbis viruses carrying GluR1 or GluR2 C-terminal decoys disrupted their interactions, GluR2 C-terminal decoys inhibited both BDNF-triggered GluR1 and GluR2/3 increases, whereas GluR1 C-terminal decoys blocked only the BDNF-triggered GluR1 increase. In agreement, coexpression of SAP97 and GluR1 in nonneuronal HEK293 cells increased both proteins compared with their single transfection, implying mutual stabilization. This work reveals a novel function of BDNF in postsynaptic development by regulating the PDZ protein expression.

Perinatal Iron Deficiency Alters the Neurochemical Profile of the Developing Rat Hippocampus

Cognitive deficits in human infants at risk for gestationally acquired perinatal iron deficiency suggest involvement of the developing hippocampus. To understand the plausible biological explanations for hippocampal injury in perinatal iron deficiency, a neurochemical profile of 16 metabolites in the iron-deficient rat hippocampus was evaluated longitudinally by 1H NMR spectroscopy at 9.4 T. Metabolites were quantified from an 11-24 microL volume centered in the hippocampus in 18 iron-deficient and 16 iron-sufficient rats on postnatal day (PD) 7, PD10, PD14, PD21 and PD28. Perinatal iron deficiency was induced by feeding the pregnant dam an iron-deficient diet from gestational d 3 to PD7. The brain iron concentration of the iron-deficient group was 60% lower on PD7 and 19% lower on PD28 (P < 0.001 each). The concentration of 12 of the 16 measured metabolites changed over time between PD7 and PD28 in both groups (P < 0.001 each). Compared with the iron-sufficient group, phosphocreatine, glutamate, N-acetylaspartate, aspartate, gamma-aminobutyric acid, phosphorylethanolamine and taurine concentrations, and the phosphocreatine/creatine ratio were elevated in the iron-deficient group (P < 0.02 each). These neurochemical alterations suggest persistent changes in resting energy status, neurotransmission and myelination in perinatal iron deficiency. An altered neurochemical profile of the developing hippocampus may underlie some of the cognitive deficits observed in human infants with perinatal iron deficiency.

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.

Differential expression of AMPA-type glutamate receptor subunits during development of the chick optic tectum

Glutamate receptors have been often associated with developmental processes. We used immunohistochemical techniques to evaluate the expression of the AMPA-type glutamate receptor (GluR) subunits in the chick optic tectum (TeO). Chick embryos from the 5th through the 20th embryonic day (E5-E20) and one-day-old (P1) chicks were used. The three types of immunoreactivity evaluated (GluR1, GluR2/3, and GluR4) had different temporal and spatial expression patterns in the several layers of the TeO. The GluR1 subunit first appeared as moderate staining on E7 and then increased on E9. The mature GluR1 pattern included intense staining only in layer 5 of the TeO. The GluR2/3 subunits presented low expression on E5, which became intense on E7. The staining for GluR2/3 changed to very intense on E14 in tectal layer 13. Staining of layer 13 neurons is the most prominent feature of GluR immunoreactivity in the adult TeO. The GluR4 subunit generally presented the lowest expression starting on E7, which was similar to the adult pattern. Some instances of transient expression of GluR subunits were observed in specific cell populations from E9 through E20. These results demonstrate a differential expression of the GluR subunits in the embryonic TeO, adding information about their possible functions in the developmental processes of the visual system.

Alpha7 nicotinic acetylcholine receptors occur at postsynaptic densities of AMPA receptor-positive and -negative excitatory synapses in rat sensory cortex

NMDA receptor (NMDAR) activation requires concurrent membrane depolarization, and glutamatergic synapses lacking AMPA receptors (AMPARs) are often considered "silent" in the absence of another source of membrane depolarization. During the second postnatal week, NMDA currents can be enhanced in rat auditory cortex through activation of the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). Electrophysiological results support a mainly presynaptic role for alpha7nAChR at these synapses. However, immunocytochemical evidence that alpha7nAChR is prevalent at postsynaptic sites of glutamatergic synapses in hippocampus and neocortex, along with emerging electrophysiological evidence for postsynaptic nicotinic currents in neocortex and hippocampus, has prompted speculation that alpha7nAChR allows for activation of NMDAR postsynaptically at synapses lacking AMPAR. Here we used dual immunolabeling and electron microscopy to examine the distribution of alpha7nAChR relative to AMPAR (GluR1, GluR2, and GluR3 subunits combined) at excitatory synapses in somatosensory cortex of adult and 1-week-old rats. alpha7nAChR occurred discretely over most of the thick postsynaptic densities in all cortical layers of both age groups. AMPAR immunoreactivity was also detectable at most synapses; its distribution was independent of that of alpha7nAChR. In both age groups, approximately one-quarter of asymmetrical synapses were alpha7nAChR positive and AMPAR negative. The variability of postsynaptic alpha7nAChR labeling density was greater at postnatal day (PD) 7 than in adulthood, and PD 7 neuropil contained a subset of small AMPA receptor-negative synapses with a high density of alpha7nAChR immunoreactivity. These observations support the idea that acetylcholine receptors can aid in activating glutamatergic synapses and work together with AMPA receptors to mediate postsynaptic excitation throughout life.

Ontogeny and localization of the neutral amino acid transporter ASCT1 in rat brain

ASCT1 is a protein that encodes System ASC, a sodium-dependent amino acid transport activity that transports primarily zwitterionic amino acids at physiological pH. ASCT1 has a 39-44% identity to the EAAT family of glutamate transporters. At extracellular pH values below 7.4, ASCT1 shifts substrate specificity to transport anionic amino acids. In this study we have examined the location of the ASCT1 transporter by immunohistochemistry in the developing rat brain. In addition, we have examined the cellular localization of ASCT1 in glial and neuronal cultures. The presence of ASCT1 immunoreactivity (ASCT1ir) in the developing brain was detectable as early as 14 days of gestation. At the cellular level, ASCT1ir was prominent in hippocampal pyramidal and dentate granule neurons. In the cerebellum, Purkinje cells and their dendrites were intensely labeled, whereas the granule and molecular layers were moderately labeled. In the cerebral cortex, neuronal cell bodies in all lamina and scattered astrocytes showed intense ASCT1ir. Double labeling experiments in vitro confirmed that ASCT1 was localized to both glia and neurons. These data illustrate that the rat ASCT1 transporter is expressed in the developing brain at levels equivalent to those observed in adult tissue. In addition, the expression and localization of ASCT1 are consistent with its possible role in pathophysiological processes that involve glutamate toxicity.

Transplanted neuroblasts differentiate appropriately into projection neurons with correct neurotransmitter and receptor phenotype in neocortex undergoing targeted projection neuron degeneration

Reconstruction of complex neocortical and other CNS circuitry may be possible via transplantation of appropriate neural precursors, guided by cellular and molecular controls. Although cellular repopulation and complex circuitry repair may make possible new avenues of treatment for degenerative, developmental, or acquired CNS diseases, functional integration may depend critically on specificity of neuronal synaptic integration and appropriate neurotransmitter/receptor phenotype. The current study investigated neurotransmitter and receptor phenotypes of newly incorporated neurons after transplantation in regions of targeted neuronal degeneration of cortical callosal projection neurons (CPNs). Donor neuroblasts were compared to the population of normal endogenous CPNs in their expression of appropriate neurotransmitters (glutamate, aspartate, and GABA) and receptors (kainate-R, AMPA-R, NMDA-R. and GABA-R), and the time course over which this phenotype developed after transplantation. Transplanted immature neuroblasts from embryonic day 17 (E17) primary somatosensory (S1) cortex migrated to cortical layers undergoing degeneration, differentiated to a mature CPN phenotype, and received synaptic input from other neurons. In addition, 23.1 +/- 13.6% of the donor-derived neurons extended appropriate long-distance callosal projections to the contralateral S1 cortex. The percentage of donor-derived neurons expressing appropriate neurotransmitters and receptors showed a steady increase with time, reaching numbers equivalent to adult endogenous CPNs by 4-16 weeks after transplantation. These results suggest that previously demonstrated changes in gene expression induced by synchronous apoptotic degeneration of adult CPNs create a cellular and molecular environment that is both permissive and instructive for the specific and appropriate maturation of transplanted neuroblasts. These experiments demonstrate, for the first time, that newly repopulating neurons can undergo directed differentiation with high fidelity of their neurotransmitter and receptor phenotype, toward reconstruction of complex CNS circuitry.

Functional ionotropic glutamate receptors emerge during terminal cell division and early neuronal differentiation of rat neuroepithelial cells

Ionotropic glutamate receptors mediate fast forms of excitatory synaptic transmission in mature neurons and may play critical roles in neuronal development. However, the developmental stage at which neuronal cells begin to express functional receptors and their roles in lineage progression remain unclear. In the present study, neural precursor cells were isolated from the cortical neuroepithelium of embryonic day 13 rats, and rapidly expanded in serum-free medium in response to basic fibroblast growth factor. RT-PCR revealed the presence of mRNAs encoding AMPA(A), AMPA(C), KA(1), KA(2), NMDA(1), and NMDA(2D) subunits after 3 days in culture. The functional expression of AMPA/kainate and NMDA receptors was investigated using Ca(2+) imaging and whole-cell patch-clamp recording techniques in cells pulse-labeled with bromodeoxyuridine (BrdU) for 1-4 hr. The recorded cells were then double-immunostained for BrdU incorporation and neuron-specific beta-tubulin (TuJ1). The results show that AMPA/kainate and NMDA induced increases in cytosolic Ca(2+) and inward currents only in differentiating neurons. In contrast, proliferating (BrdU(+)TuJ1(-)) cells failed to respond to any ionotropic glutamate receptor agonists. Interestingly, Ca(2+) imaging revealed that a subpopulation of BrdU(+)TuJ1(+) cells also responded to AMPA, indicating the emergence of functional ionotropic AMPA/kainate receptors during terminal cell division and the earliest commitment to neuronal cell lineage. These in vitro results were supported by flow cytometric sorting of AMPA-responsive cells pulse-labeled with BrdU for 1 hr in vivo, which revealed that functional AMPA receptors appear in BrdU(+)TuJ1(+) cells under physiological conditions and may play a role in terminal cell division.

Downregulation of glutamate receptor subunit 2(3) in subependymal giant-cell tumor

Immunohistochemical techniques were used to investigate the expression of glutamate receptor (GluR) subunits in samples of brain resected from children with and without tuberous sclerosis, using antibody to an epitope common to GluR subunits 2 and 3 [2(3)]. Our purpose was to characterize the phenotype of balloon cells in cortical tubers and tumor cells in subependymal giant-cell tumors. In cortical tubers, GluR 2(3) was expressed in the processes and cell bodies of balloon cells, demonstrating consistent immunoreactivity to vimentin. In subependymal giant-cell tumors, tumor cells also exhibited consistent immunoreactivity to vimentin but only faint immunoreactivity to GluR 2(3). The reason for the expression of subunit 2(3) in tubers but not in subependymal giant-cell tumors remains unknown. However, if one assumes that the presence of subunit 2 substantially reduces calcium conductance through alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid channel and maintains intracellular calcium homeostasis, one could speculate that downregulation of subunit 2(3) in tumor cells could result in increased calcium flux into these cells, causing tumorigenesis. Another explanation may be that receptor subunits cannot be produced sufficiently in tumor cells. Moreover, the pathogenetic pathways between balloon and giant-cells are distinctly different, despite the similarity in their phenotypical pathologic features.

Late embryonic expression of AMPA receptor function in the CA1 region of the intact hippocampus in vitro

Studies in slices suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic currents are not present in CA1 (Cornu ammonis) pyramidal neurons at birth (P0). We have re-examined this issue in the rat intact hippocampal formation (IHF) in vitro. Injections of biocytin or carbocyanine show that the temporo-ammonic, commissural and Schaffer collateral pathways are present at birth in the marginal zone of CA1. Electrical stimulation of these pathways evoked field excitatory postsynaptic potentials (fEPSPs) in the marginal zone of CA1 from embryonic day 19 (E19) to postnatal day 9 (P9). These fEPSPs are mediated by synaptic AMPA receptors as they are reduced or completely blocked by: (i) tetrodotoxin; (ii) high divalent cation concentrations; (iii) the adenosine A1 receptor agonist CPA; (iv) anoxic episodes; (v) the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI-53655) or the mixed AMPA-kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX). The amplitude of the fEPSPs is also reduced by D(-)-2-amino-5-phosphonopentanoic acid (D-APV) and its duration is increased by bicuculline suggesting the participation of N-methyl-D-aspartate (NMDA) and GABAA (gamma-aminobutyric acid) receptors. Finally, AMPA receptor-mediated fEPSPs are also recorded in P0 slices, but they are smaller and more labile than in the IHF. Our results suggest that in embryonic CA1 neurons, glutamate acting on AMPA receptors already provides a substantial part of the excitatory drive and may play an important role in the activity-dependent development of the hippocampus. Furthermore, the IHF may be a convenient preparation to investigate the properties of the developing hippocampus.

Glutamate receptor phenotypes in the auditory brainstem and mid-brain of the developing rat

Glutamate receptors mediate most excitatory synaptic transmission in the adult vertebrate brain, but their activation in developing neurons also influences developmental processes. However, little is known about the developmental regulation of the subunits composing these receptors. Here we have studied age-dependent changes in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits in the cochlear nucleus complex (CN), the superior olivary complex (SOC), the nuclei of the lateral lemniscus, and the inferior colliculus of the developing rat. In the lateral superior olive, the medial nucleus of the trapezoid body, and the ventral nucleus of the lateral lemniscus, the distribution of AMPA receptor subunits changed drastically with age. While GluR1 and GluR2 subunits were highly expressed in the first 2 postnatal weeks, GluR4 staining was detectable only thereafter. GluR1 and GluR2 immunoreactivities rapidly decreased during the third postnatal week, with the GluR1 subunits disappearing from most neurons. In contrast, the adult pattern of the distribution of AMPA receptor subunits emerged gradually in most of the other auditory nuclei. Thus, progressive as well as regressive events characterized AMPA receptor development in some nuclei, while a monotonically maturation was seen in other regions. In contrast, the staining patterns of NMDA receptor subunits remained stable or only decreased during the same period. Although our data are not consistent with a generalized pattern of AMPA receptor development, the abundance of GluR1 subunits is a distinctive feature of early AMPA receptors. As similar AMPA receptors are present during plasticity periods throughout the brain, neurons undergoing synaptic and structural remodelling might have a particular need for these receptors.

Changes in dendritic structure and function following hippocampal lesions: correlations with developmental events?

Recovery after nervous system lesions may lead to partial re-institution of developmental schemes and processes. Here we review several of these proposed schemes, with the conclusion that though some processes may involve re-expression of embryonic phenotypes, there are many processes invoked during recovery from lesions that do not mirror developmental phenomena. The inability to fully revert to embryonic schemes because of adult phenotype may partially account for the decreased recovery observed in adults compared to that noted after lesions during development.