AMPA and NMDA receptors expressed by differentiating Xenopus spinal neurons

Novel Ca2+-dependent mechanisms regulate spontaneous release at excitatory synapses onto CA1 pyramidal cells

Although long thought to simply be a source of synaptic noise, spontaneous, action potential-independent release of neurotransmitter from presynaptic terminals has multiple roles in synaptic function. We explored whether and to what extent the two predominantly proposed mechanisms for explaining spontaneous release, stochastic activation of voltage-gated Ca²⁺ channels (VGCCs) or activation of Ca²⁺-sensing receptors (CaSRs) by extracellular Ca²⁺, played a role in the sensitivity of spontaneous release to the level of extracellular Ca²⁺ concentration at excitatory synapses at CA1 pyramidal cells of the adult male mouse hippocampus. Blocking VGCCs with Cd²⁺ had no effect on spontaneous release, ruling out stochastic activation of VGCCs. Although divalent cation agonists of CaSRs, Co²⁺ and Mg²⁺, dramatically enhanced miniature excitatory postsynaptic current (mEPSC) frequency, potent positive and negative allosteric modulators of CaSRs had no effect. Moreover, immunoblot analysis of hippocampal lysates failed to detect CaSR expression, ruling out the CaSR. Instead, the increase in mEPSC frequency induced by Co²⁺ and Mg²⁺ was mimicked by lowering postsynaptic Ca²⁺ levels with BAPTA. Together, our results suggest that a reduction in intracellular Ca²⁺ may trigger a homeostatic-like compensatory response that upregulates spontaneous transmission at excitatory synapses onto CA1 pyramidal cells in the adult hippocampus. NEW & NOTEWORTHY We show that the predominant theories for explaining the regulation of spontaneous, action potential-independent synaptic release do not explain the sensitivity of this type of synaptic transmission to external Ca²⁺ concentration at excitatory synapses onto hippocampal CA1 pyramidal cells. In addition, our data indicate that intracellular Ca²⁺ levels in CA1 pyramidal cells regulate spontaneous release, suggesting that excitatory synapses onto CA1 pyramidal cells may express a novel, rapid form of homeostatic plasticity.

Timing of developmental sequences in different brain structures: physiological and pathological implications

The developing brain is not a small adult brain. Voltage- and transmitter-gated currents, like network-driven patterns, follow a developmental sequence. Studies initially performed in cortical structures and subsequently in subcortical structures have unravelled a developmental sequence of events in which intrinsic voltage-gated calcium currents are followed by nonsynaptic calcium plateaux and synapse-driven giant depolarising potentials, orchestrated by depolarizing actions of GABA and long-lasting NMDA receptor-mediated currents. The function of these early patterns is to enable heterogeneous neurons to fire and wire together rather than to code specific modalities. However, at some stage, behaviourally relevant activities must replace these immature patterns, implying the presence of programmed stop signals. Here, we show that the developing striatum follows a developmental sequence in which immature patterns are silenced precisely when the pup starts locomotion. This is mediated by a loss of the long-lasting NMDA-NR2C/D receptor-mediated current and the expression of a voltage-gated K(+) current. At the same time, the descending inputs to the spinal cord become fully functional, accompanying a GABA/glycine polarity shift and ending the expression of developmental patterns. Therefore, although the timetable of development differs in different brain structures, the g sequence is quite similar, relying first on nonsynaptic events and then on synaptic oscillations that entrain large neuronal populations. In keeping with the 'neuroarcheology' theory, genetic mutations or environmental insults that perturb these developmental sequences constitute early signatures of developmental disorders. Birth dating developmental disorders thus provides important indicators of the event that triggers the pathological cascade leading ultimately to disease.

GABAergic control of neurite outgrowth and remodeling during development and adult neurogenesis: general rules and differences in diverse systems

During development, Gamma-aminobutyric acidergic (GABAergic) neurons mature at early stages, long before excitatory neurons. Conversely, GABA reuptake transporters become operative later than glutamate transporters. GABA is therefore not removed efficiently from the extracellular domain and it can exert significant paracrine effects. Hence, GABA-mediated activity is a prominent source of overall neural activity in developing CNS networks, while neurons extend dendrites and axons, and establish synaptic connections. One of the unique features of GABAergic functional plasticity is that in early development, activation of GABA_A receptors results in depolarizing (mainly excitatory) responses and Ca²⁺ influx. Although there is strong evidence from several areas of the CNS that GABA plays a significant role in neurite growth not only during development but also during adult neurogenesis, surprisingly little effort has been made into putting all these observations into a common framework in an attempt to understand the general rules that regulate these basic and evolutionary well-conserved processes. In this review, we discuss the current knowledge in this important field. In order to decipher common, universal features and highlight differences between systems throughout development, we compare findings about dendritic proliferation and remodeling in different areas of the nervous system and species, and we also review recent evidence for a role in axonal elongation. In addition to early developmental aspects, we also consider the GABAergic role in dendritic growth during adult neurogenesis, extending our discussion to the roles played by GABA during dendritic proliferation in early developing networks versus adult, well established networks.

Role of P2Y1 receptor in astroglia-to-neuron signaling at dorsal spinal cord

Several studies have shown that astrocytes release neurotransmitters into the extracellular space that may then activate receptors on nearby neurons. In the present study, the actions of adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS)-activated astrocyte conditioned medium (ADPbetaS-ACM) on cultured dorsal spinal cord neurons were evaluated by using confocal laser scanning microscopy and whole-cell patch-clamp recording. ADPbetaS caused astrocytic glutamate efflux (43 microM), which in turn induced inward currents in dorsal horn neurons with short time in culture. The inward currents were abolished by 2-amino-5-phosphonlanoicacid (AP-5; NMDAR antagonist) plus 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; non-NMDAR antagonist) but were unaffected by MRS2179 (selective P2Y(1) receptor antagonist). Furthermore, N6-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179) was used to block glutamate release from astrocytes. As a result, ADPbetaS-ACM-induced inward currents in neurons were significantly blocked. On the other hand, both NMDAR and non-NMDAR were involved in ADPbetaS-ACM (concentration was diluted to one-tenth)-evoked small [Ca(2+)](i) transients in neurons. Under this condition, the values of glutamate concentrations in the medium are close to values for extracellular glutamate concentrations under physiological conditions. For this reason, it is possible that astrocyte-derived glutamate is important for distant neuron under physiological conditions at dorsal spinal cord. These observations indicate that astrocytic P2Y(1) receptor activation triggered glutamate efflux, which acts on distant neurons to elevate calcium levels or acts on nearby neurons to evoke inward current. Finally, our results support the conclusion that the astrocytic P2Y(1) receptor plays an important role in bidirectional communication between astrocytes and neurons.

From Guidance Signals to Movement: Signaling Molecules Governing Growth Cone Turning

Directed growth cone movements in response to external guidance signals are required for the establishment of functional neuronal connections during development, adult nerve regeneration, and adult neurogenesis. Growth cone intrinsic properties permit different growth cone responses (e.g., attraction or repulsion) to a guidance signal, and alterations to these intrinsic properties often result in opposite growth cone responses. This article reviews the current knowledge of growth cone signaling, emphasizing the dependency of Ca2+ signaling on membrane potential shifts, and cyclic nucleotide and phosphoinositide signaling pathways during growth cone turning in response to guidance signals. We also discuss how asymmetrical growth cone signaling is achieved for the fine-tuned growth cone movement.

Developmental characteristics of AMPA receptors in chick lumbar motoneurons

Ca2+ fluxes through ionotropic glutamate receptors regulate a variety of developmental processes, including neurite outgrowth and naturally occurring cell death. In the CNS, NMDA receptors were originally thought to be the sole source of Ca2+ influx through glutamate receptors; however, AMPA receptors also allow a significant influx of Ca2+ ions. The Ca2+ permeability of AMPA receptors is regulated by the insertion of one or more edited GluR2 subunits. In this study, we tested the possibility that changes in GluR2 expression regulate the Ca2+ permeability of AMPA receptors during a critical period of neuronal development in chick lumbar motoneurons. GluR2 expression is absent between embryonic day (E) 5 and E7, but increases significantly by E8 in the chick ventral spinal cord. Increased GluR2 protein expression is correlated with parallel changes in GluR2 mRNA in the motoneuron pool. Electrophysiological recordings of kainate-evoked currents indicate a significant reduction in the Ca2(+)-permeability of AMPA receptors between E6 and E11. Kainate-evoked currents were sensitive to the AMPA receptor blocker GYKI 52466. Application of AMPA or kainate generates a significant increase in the intracellular Ca2+ concentration in E6 spinal motoneurons, but generates a small response in older neurons. Changes in the Ca(2+)-permeability of AMPA receptors are not mediated by age-dependent changes in the editing pattern of GluR2 subunits. These findings raise the possibility that Ca2+ influx through Ca(2+)-permeable AMPA receptors plays an important role during early embryonic development in chick spinal motoneurons.

Modulation of interleukin-6 and matrix metalloproteinase 2 expression in human fibroblast-like synoviocytes by functional ionotropic glutamate receptors

OBJECTIVE

Patients with rheumatoid arthritis (RA) have increased concentrations of the amino acid glutamate in synovial fluid. This study was undertaken to determine whether glutamate receptors are expressed in the synovial joint, and to determine whether activation of glutamate receptors on human synoviocytes contributes to RA disease pathology.

METHODS

Glutamate receptor expression was examined in tissue samples from rat knee joints and in human fibroblast-like synoviocytes (FLS). FLS from 5 RA patients and 1 normal control were used to determine whether a range of glutamate receptor antagonists influenced expression of the proinflammatory cytokine interleukin-6 (IL-6), enzymes involved in matrix degradation and cytokine processing (matrix metalloproteinase 2 [MMP-2] and MMP-9), and the inhibitors of these enzymes (tissue inhibitor of metalloproteinases 1 [TIMP-1] and TIMP-2). IL-6 concentrations were determined by enzyme-linked immunosorbent assay, MMP activity was measured by gelatin zymography, and TIMP activity was determined by reverse zymography. Fluorescence imaging of intracellular calcium concentrations in live RA FLS stimulated with specific antagonists was used to reveal functional activation of glutamate receptors that modulated IL-6 or MMP-2.

RESULTS

Ionotropic and metabotropic glutamate receptor subunit mRNA were expressed in the patella, fat pad, and meniscus of the rat knee and in human articular cartilage. Inhibition of N-methyl-D-aspartate (NMDA) receptors in RA FLS increased proMMP-2 release, whereas non-NMDA ionotropic glutamate receptor antagonists reduced IL-6 production by these cells. Stimulation with glutamate, NMDA, or kainate (KA) increased intracellular calcium concentrations in RA FLS, demonstrating functional activation of specific ionotropic glutamate receptors.

CONCLUSION

Our findings indicate that activation of NMDA and KA glutamate receptors on human synoviocytes may contribute to joint destruction by increasing IL-6 expression.

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.

Functional expression of metabotropic GABAB receptors in primary cultures of astrocytes from rat cerebral cortex

GABA(B) receptor subunits are widely expressed on neurons throughout the central nervous system (CNS), at both pre- and postsynaptic sites, where they mediate the late and slow component of the inhibitory response to the major inhibitory neurotransmitter GABA. Recently, GABA(B) receptors have been reported to be expressed in astrocytes and microglia in the rat CNS by immunocytochemistry. However, there are few reports available for the functional characterization of GABA(B) receptors on astrocytes. In the present study, we therefore investigated the functional expression and characteristics of GABA(B) receptors in primary cultures of astrocytes from rat cerebral cortex. In the presence of 10 microM GTP, forskolin concentration-dependently increased adenylylcyclase (AC) activity in membranes prepared from rat astrocytes. The selective GABA(B) agonist (R)-baclofen concentration-dependently reduced forskolin-stimulated AC activity in the presence of 10 microM GTP. This effect was reversed by the selective GABA(B) antagonists, CGP-55845 and CGP-54626, and was completely abolished by treatment of astrocytic membranes with pertussis toxin. In addition, RT-PCR, Western blotting, and immunocytochemistry clearly showed that metabotropic GABA(B) receptor isoforms (GABA(B)R1 and GABA(B)R2) are expressed in rat cerebrocortical astrocytes. Taken collectively, these results demonstrate that functionally active metabotropic GABA(B) receptors are expressed in rat cerebrocortical astrocytes.

The appearance and maturation of excitatory and inhibitory neurotransmitter sensitivity during retinal regeneration of the adult newt

Using living slice preparations from newt retinas at different stages of regeneration, we examined the time course of appearance and maturation of neurotransmitter-induced currents with whole-cell patch-clamp methods. Neurons from which currents were recorded were identified by Lucifer Yellow fills. All progenitor cells examined at the regenerating retinas did not express any voltage-gated Na+ currents and responsiveness to excitatory amino acid analogues (AMPA and NMDA) and inhibitory amino acids (GABA and glycine). Voltage-gated Na+ currents were first detected in premature ganglion cells with round cell body located at the most proximal level of the 'intermediate-II' regenerating retina. AMPA- GABA- and glycine-induced currents were simultaneously observed in many premature ganglion cells expressing Na+ channels, but not all, suggesting that the onset of the Na+ channels is slightly earlier than that of excitatory and inhibitory amino acid receptors in regeneration. NMDA-evoked currents were first observed in the 'intermediate-III' regenerating retina just before the synaptogenesis. Pharmacological properties and reversal potential values of the excitatory and inhibitory amino acid responses did not change substantially between regenerating ganglion cells and mature ganglion cells, while rectification properties of current-voltage relations for AMPA and NMDA responses were somewhat different between them.

Glutamate antagonists limit tumor growth

The management of malignancies in humans constitutes a major challenge for contemporary medicine. Despite progress in chemotherapy, bone marrow transplantation, surgical measures, and radiation technologies, and in immunological and immunomodulatory approaches, humans continue to succumb to cancer due to tumor recurrence and metastatic disease. The excitatory neurotransmitter glutamate, which regulates proliferation and migration of neuronal progenitors and immature neurons during the development of the mammalian nervous system, is present in peripheral cancers. Since both neuronal progenitors and tumor cells possess propensity to proliferate and to migrate, and since glutamate and glutamate receptors are known to modify these phenomena in the nervous system, we proceeded to investigate the possible influence of glutamate antagonists on the proliferation and migration of tumor cells. We found and recently reported that glutamate N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) antagonists inhibit the proliferation of human colon adenocarcinoma, astrocytoma, breast and lung carcinoma, and neuroblastoma cells in vitro. The antiproliferative effect of glutamate antagonists is Ca(2+)-dependent and results from decreased cell division and increased cell death. Glutamate antagonists produce morphological alterations in tumor cells, which consist of reduced membrane ruffling and pseudopodial protrusions, and decrease their motility and invasive growth. Furthermore, glutamate antagonists enhance in vitro cytostatic and cytotoxic effects of common chemotherapeutic agents used in cancer therapy. These findings demonstrate the anticancer potential of glutamate antagonists and suggest that they may be used as an adjunctive measure in the treatment of cancer.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Glutamate-mediated responses in developing retinal ganglion cells

Glutamate has been suggested to regulate the development of retinal ganglion cells, but little is known about the functional properties of glutamate receptors during ontogeny of these neurons. Using whole-cell and outside-out patch-clamp recordings, we have characterized the pharmacological, rectification, and kinetic properties of ionotropic glutamate receptors in ganglion cells isolated from fetal and postnatal cat retinas. The fetal cells were studied at embryonic day 33 (E33) to E38, before significant outgrowth of their dendritic processes and prior to the formation of synaptic contacts in the inner plexiform layer. In several respects, the functional properties of early fetal ganglion cells were found to be remarkably similar to those of postnatal cells. In both age groups, glutamate and AMPA produced fast desensitizing currents, kainate yielded large steady-state currents, while applications of NMDA resulted in multiple channel openings. The shapes and amplitudes of these glutamate-gated currents were also similar and the current-voltage relations were nearly linear, with reversal potentials near 0 mV. Moreover, the dose-response curves (to kainate) were virtually identical in the fetal and postnatal neurons. The proportion of neurons responsive to NMDA and non-NMDA agonists was nearly the same in both age groups. This early functional expression of glutamate receptors cannot be involved in the transmission of electrical information in the developing retina because at this stage few ganglion cells are capable of generating action potentials (Skaliora et al., 1993). It is suggested that the early activation of NMDA and non-NMDA receptors in fetal ganglion cells may regulate the outgrowth and stabilization of dendritic processes in these neurons. Our data also revealed some differences in the responses of fetal and postnatal cells to glutamate and its agonists. Thus, the unitary NMDA conductance was found to decrease with age, while the rate of glutamate receptor desensitization increased with age. Also, while virtually all postnatal cells responded to glutamate, the proportion of fetal cells that manifested glutamate-mediated responses was lower. These maturational changes presumably allow retinal ganglion cells to integrate synaptic inputs for the transmission of electrical signals to the visual centers of the brain.

Glutamate antagonists limit tumor growth

Neuronal progenitors and tumor cells possess propensity to proliferate and to migrate. Glutamate regulates proliferation and migration of neurons during development, but it is not known whether it influences proliferation and migration of tumor cells. We demonstrate that glutamate antagonists inhibit proliferation of human tumor cells. Colon adenocarcinoma, astrocytoma, and breast and lung carcinoma cells were most sensitive to the antiproliferative effect of the N-methyl-d-aspartate antagonist dizocilpine, whereas breast and lung carcinoma, colon adenocarcinoma, and neuroblastoma cells responded most favorably to the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate antagonist GYKI52466. The antiproliferative effect of glutamate antagonists was Ca(2+) dependent and resulted from decreased cell division and increased cell death. Morphological alterations induced by glutamate antagonists in tumor cells consisted of reduced membrane ruffling and pseudopodial protrusions. Furthermore, glutamate antagonists decreased motility and invasive growth of tumor cells. These findings suggest anticancer potential of glutamate antagonists.

Long-term memory formation in the chick requires mobilization of ryanodine-sensitive intracellular calcium stores

Training chicks (Gallus domesticus) on a one-trial passive avoidance task results in transient and time-dependent enhanced increases in N-methyl-d-aspartate- or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-stimulated intracellular calcium concentration in synaptoneurosomes isolated from a specific forebrain region, the intermediate medial hyperstriatum ventrale. This increase could result from either calcium entry from the extracellular medium or from mobilization of intracellular calcium stores. We have therefore examined the effects of dantrolene, an inhibitor of calcium release from the intracellular ryanodine-sensitive store, on these processes. Dantrolene, 50 nmol per hemisphere injected intracerebrally 30 min pre- or 30 min posttraining, blocked longer term memory for the passive avoidance task, whereas memory for the task was unaffected when dantrolene was injected at earlier or later times. Preincubation of synaptoneurosomes, isolated from the intermediate hyperstriatum ventrale 10 min after training, with 100 nM dantrolene abolished the enhanced training-induced increase in intracellular calcium concentration elicited by 0.5 mM N-methyl-d-aspartate. By contrast, the training-induced enhancement of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-stimulated increase in intracellular calcium concentration in synaptoneurosomes prepared 6 h posttraining was unaffected by preincubation with dantrolene, which was not amnestic at this time. Calcium release from ryanodine-sensitive intracellular stores may thus be a necessary stage in the early phase of the molecular cascade leading to the synaptic modulation required for long-term memory storage.

Perinatal development of lumbar motoneurons and their inputs in the rat

The rat is quite immature at birth and a rapid maturation of motor behavior takes place during the first 2 postnatal weeks. Lumbar motoneurons undergo a rapid development during this period. The last week before birth represents the initial stages of motoneuron differentiation, including regulation of the number of cells and the arrival of segmental and first supraspinal afferents. At birth, motoneurons are electrically coupled and receive both appropriate and inappropriate connections from the periphery; the control from supraspinal structures is weak and exerted mainly through polysynaptic connections. During the 1st postnatal week, inappropriate sensori-motor contacts and electrical coupling disappear, the supraspinal control increases gradually and myelin formation is responsible for an increased conduction velocity in both descending and motor axons. Both N-methyl-D-aspartate (NMDA) and non-NMDA receptors are transiently overexpressed in the neonatal spinal cord. The contribution of non-NMDA receptors to excitatory amino acid transmission increases with age. Activation of gamma-aminobutyric acid(A) and glycine receptors leads to membrane depolarization in embryonic motoneurons but to hyperpolarization in older motoneurons. The firing properties of motoneurons change with development: they are capable of more repetitive firing at the end of the 1st postnatal week than before birth. However, maturation does not proceed simultaneously in the motor pools innervating antagonistic muscles; for instance, the development of repetitive firing of ankle extensor motoneurons lags behind that of flexor motoneurons. The spontaneous embryonic and neonatal network-driven activity, detected at the levels of motoneurons and primary afferent terminals, may play a role in neuronal maturation and in the formation and refinement of sensorimotor connections.

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.

Involvement of peroxynitrite and hydroxyradical generated from nitric oxide in hypoxia/reoxygenation injury in rat cerebrocortical slices

The changes in nitric oxide (NO) formation during hypoxia and reoxygenation were measured in slices of rat cerebral cortex, and the possible involvement of NO and its decomposition products, including peroxynitrite and hydroxyradical, in the hypoxia/reoxygenation injury was subsequently investigated. NO formation estimated from cGMP accumulation in the extracellular fluids was enhanced during hypoxia and to a lesser extent in the reoxygenation period. The mRNA for inducible NO synthase (NOS) was detected 3-5 h after reoxygenation, although neuronal NOS mRNA decreased after reoxygenation. Several NOS inhibitors such as N(G)-monomethyl-L-arginine and N(G)-nitro-L-arginine blocked not only the NO formation but also the hypoxia/reoxygenation injury as determined by lactate dehydrogenase (LDH) leakage. The hypoxia/reoxygenation injury was prevented by peroxynitrite scavengers including deferoxamine and uric acid, or several hydroxyradical scavengers such as dimethylthiourea, 2-mercaptopropionylglycine and D(-) mannitol. In addition, the hypoxia/reoxygenation injury was attenuated by poly(ADP-ribose)synthetase inhibitors such as banzamide, 3-aminobenzamide and 1,5-isoquinolinediol. On the other hand, both N-morpholinosidnonimine, a peroxynitrite generator, and hydroxyradical-liberating solution containing FeCl(3)-ADP and dihydroxyfumarate caused a marked LDH leakage in normoxic slices. These findings suggest that the enhanced formation of NO causes hypoxia/reoxygenation injury after degradation to peroxynitrite and hydroxyradical and the resultant activation of poly(ADP-ribose)synthetase.

Ca(2+)-permeable AMPA receptors and spontaneous presynaptic transmitter release at developing excitatory spinal synapses

At many mature vertebrate glutamatergic synapses, excitatory transmission strength and plasticity are regulated by AMPA and NMDA receptor (AMPA-R and NMDA-R) activation and by patterns of presynaptic transmitter release. Both receptors potentially direct neuronal differentiation by mediating postsynaptic Ca(2+) influx during early development. However, the development of synaptic receptor expression and colocalization has been examined developmentally in only a few systems, and changes in release properties at neuronal synapses have not been characterized extensively. We recorded miniature EPSCs (mEPSCs) from spinal interneurons in Xenopus embryos and larvae. In mature 5-8 d larvae, approximately 70% of mEPSCs in Mg(2+)-free saline are composed of both a fast AMPA-R-mediated component and a slower NMDA-R-mediated decay, indicating receptor colocalization at most synapses. By contrast, in 39-40 hr embryos approximately 65% of mEPSCs are exclusively fast, suggesting that these synapses initially express predominantly AMPA-R. In a physiological Mg(2+) concentration (1 mM), mEPSCs throughout development are mainly AMPA-R-mediated at negative potentials. Embryonic synaptic AMPA-R are highly Ca(2+)-permeable, mEPSC amplitude is over twofold larger than at mature synapses, and mEPSCs frequently occur in bursts consistent with asynchronous multiquantal release. AMPA-R function in this motor pathway thus appears to be independent of previous NMDA-R activation, unlike other regions of the developing nervous system, ensuring a greater reliability for embryonic excitatory transmission. Early spontaneous excitatory activity is specialized to promote AMPA-R-mediated synaptic Ca(2+) influx, which likely has significant roles in neuronal development.

Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Subpopulations of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors that are either permeable or impermeable to Ca2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca2+ -permeable AMPA receptors provide a Ca2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca2+-permeable AMPA receptors was assayed using the kainate-induced cobalt loading technique first developed by Pruss et al. [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for gamma-aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca2+-binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate-induced cobalt uptake is selectively mediated by Ca2+-permeable AMPA receptors, and that a majority of GABA and NK1 receptor-expressing neurons express Ca2+-permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor-expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino-thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca2+-permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca2+-permeable AMPA receptors.

Development of rhythmic pattern generators

In contrast to the wealth of knowledge about the organizational rules of adult central pattern generators, far less is known about how these networks are assembled during development. The basic architecture for adult central pattern generators appears early in development but different generators may follow completely different developmental pathways to reach maturity. Recent evidence suggests that neuromodulatory inputs, in addition to their short-term adaptive control of central pattern generator activity, play a crucial role in both the final developmental tuning and the long-term maintenance of adult network function.

Properties of ectopic neurons induced by Xenopus neurogenin1 misexpression

We have examined cells cultured from ectoderm-misexpressing Neurogenin1 (Ngn1) to describe better the extent to which this gene can control aspects of neuronal phenotype including motility, morphology, excitability, and synaptic properties. Like primary spinal neurons which normally express Ngn1, cells in Ngn1-misexpressing cultures exhibit a motility-correlated behavior called circus movements prior to neuritogenesis. Misexpression of NeuroD also causes circus movements and later neuronal differentiation. GSK3beta, which inhibits NeuroD function in vivo, blocks both Ngn1-induced and NeuroD-induced neuronal differentiation, while Notch signaling inhibits only Ngn1-induced neuronal differentiation, confirming that NeuroD is downstream of Ngn1 and insensitive to Notch inhibition. While interfering with NeuroD function in ventral ectoderm inhibits both circus movements and neuronal differentiation, such inhibition in the neural plate inhibits only neuronal differentiation, suggesting that additional factors regulate circus movements in the neural ectoderm. Ngn1-misexpressing cells extend N-tubulin-positive neurites and exhibit tetrodotoxin-sensitive action potentials. Unlike the majority of cultured spinal neurons, however, Ngn1-misexpressing cells do not respond to glutamate and do not form functional synapses with myocytes, suggesting that these cells are either like Rohon-Beard sensory neurons or are not fully differentiated.