Intracellular Ca2+ and N-methyl-D-aspartate-stimulated neuritogenesis in rat cerebellar granule cell cultures

Cognitive Deficits after Cerebral Ischemia and Underlying Dysfunctional Plasticity: Potential Targets for Recovery of Cognition

Cerebral ischemia affects millions of people worldwide and survivors suffer from long-term functional and cognitive deficits. While stroke and cardiac arrest are typically considered when discussing ischemic brain injuries, there is much evidence that smaller ischemic insults underlie neurodegenerative diseases, including Alzheimer's disease. The "regenerative" capacity of the brain relies on several aspects of plasticity that are crucial for normal functioning; less affected brain areas may take over function previously performed by irreversibly damaged tissue. To harness the endogenous plasticity mechanisms of the brain to provide recovery of cognitive function, we must first understand how these mechanisms are altered after damage, such as cerebral ischemia. In this review, we discuss the long-term cognitive changes that result after cerebral ischemia and how ischemia alters several plasticity processes. We conclude with a discussion of how current and prospective therapies may restore brain plasticity and allow for recovery of cognitive function, which may be applicable to several disorders that have a disruption of cognitive processing, including traumatic brain injury and Alzheimer's disease.

Characterization of dendritic morphology and neurotransmitter phenotype of thoracic descending propriospinal neurons after complete spinal cord transection and GDNF treatment

After spinal cord injury (SCI), poor regeneration of damaged axons of the central nervous system (CNS) causes limited functional recovery. This limited spontaneous functional recovery has been attributed, to a large extent, to the plasticity of propriospinal neurons, especially the descending propriospinal neurons (dPSNs). Compared with the supraspinal counterparts, dPSNs have displayed significantly greater regenerative capacity, which can be further enhanced by glial cell line-derived neurotrophic factor (GDNF). In the present study, we applied a G-mutated rabies virus (G-Rabies) co-expressing green fluorescence protein (GFP) to reveal Golgi-like dendritic morphology of dPSNs. We also investigated the neurotransmitters expressed by dPSNs after labeling with a retrograde tracer Fluoro-Gold (FG). dPSNs were examined in animals with sham injuries or complete spinal transections with or without GDNF treatment. Bilateral injections of G-Rabies and FG were made into the 2nd lumbar (L2) spinal cord at 3 days prior to a spinal cord transection performed at the 11th thoracic level (T11). The lesion gap was filled with Gelfoam containing either saline or GDNF in the injury groups. Four days post-injury, the rats were sacrificed for analysis. For those animals receiving G-rabies injection, the GFP signal in the T7-9 spinal cord was visualized via 2-photon microscopy. Dendritic morphology from stack images was traced and analyzed using a Neurolucida software. We found that dPSNs in sham injured animals had a predominantly dorsal-ventral distribution of dendrites. Transection injury resulted in alterations in the dendritic distribution with dorsal-ventral retraction and lateral-medial extension. Treatment with GDNF significantly increased the terminal dendritic length of dPSNs. The density of spine-like structures was increased after injury, and treatment with GDNF enhanced this effect. For the group receiving FG injections, immunohistochemistry for glutamate, choline acetyltransferase (ChAT), glycine, and GABA was performed in the T7-9 spinal cord. We show that the majority of FG retrogradely-labeled dPSNs were located in the Rexed Lamina VII. Over 90% of FG-labeled neurons were glutamatergic, with the other three neurotransmitters contributing less than 10% of the total. To our knowledge this is the first report describing the morphologic characteristics of dPSNs and their neurotransmitter expressions, as well as the dendritic response of dPSNs after transection injury and GDNF treatment.

Hypobaric hypoxia induces dendritic plasticity in cortical and hippocampal pyramidal neurons in rat brain

Hypobaric hypoxia (HH), a predisposing environmental condition at high altitude (HA) encountered by many mountaineers jeopardizes their normal physiology like motor coordination and cognitive functions. Our previous studies revealed that the HH induces oxidative stress and neurodegeneration, which is associated with spatial memory impairment in rats. However, the dendritic changes after exposure to different duration of HH remain largely unknown. The aim of the present study was to investigate the duration-dependent dendritic changes in CA1, CA3 and entorhinal cortex (EC) of hippocampus and layer II of prefrontal cortex (PFC) with spatial memory functions in rats on exposure to different duration of HH. The rats were exposed to simulated HA of 6100 m for 3, 7, 14 and 21 days and the spatial reference memory was investigated using Morris water maze (MWM) and the morphological alteration of CA1, CA3, EC and layer II of PFC were investigated. There was a significant decrease in dendritic arborization and spine number along with increased number of damaged neurons, after 3, 7 and 14 days of HH but after 21 days of HH exposure the structural recovery was noted in all the regions. There was impairment of spatial memory after 3 and 7 days of exposure, but slight improvement of spatial memory was noted after 14 and 21 days of exposure. Our studies suggested that HH induces dendritic plasticity of PFC and hippocampal pyramidal neurons of rat brain, which might be associated with improvement of spatial memory function after 21 days of HH exposure.

Effects of disrupting calcium homeostasis on neuronal maturation: early inhibition and later recovery

It has become increasingly clear that agents that disrupt calcium homeostasis may also be toxic to developing neurons. Using isolated primary neurons, we sought to understand the neurotoxicity of agents such as MK801 (which blocks ligand-gated calcium entry), BAPTA (which chelates intracellular calcium), and thapsigargin (TG; which inhibits the endoplasmic reticulum Ca(2+)-ATPase pump). Thus, E18 rat cortical neurons were grown for 1 day in vitro (DIV) and then exposed to vehicle (0.1% DMSO), MK801 (0.01-20 microM), BAPTA (0.1-20 microM), or TG (0.001-1 microM) for 24 h. We found that all three agents could profoundly influence early neuronal maturation (growth cone expansion, neurite length, neurite complexity), with the order of potency being MK801 < BAPTA < TG. We next asked if cultures exposed to these agents were able to re-establish their developmental program once the agent was removed. When we examined network maturity at 4 and 7 DIV, the order of recovery was MK801 > BAPTA > TG. Thus, mechanistically distinct ways of disrupting calcium homeostasis differentially influenced both short-term and long-term neuronal maturation. These observations suggest that agents that act by altering intracellular calcium and are used in obstetrics or neonatology may be quite harmful to the still-developing human brain.

NMDA receptor NR2B subunit over-expression increases cerebellar granule cell migratory activity

Glutamate acting on NMDA receptors (NMDARs) is known to influence cerebellar granule cell migration. Subunit composition of NMDARs in granule cells changes characteristically during development: NR2B subunit containing receptors are abundant during migration towards the internal granule cell layer but are gradually replaced by NR2A and/or NR2C subunits once the final position is reached. Cerebellar granule cell migration was investigated using mutant mouse lines either with a deletion of the NR2C gene (NR2C(-/-) mice) or expressing NR2B instead of the NR2C subunit (NR2C-2B mice). BrdU-labeling revealed that over-expression of NR2B increased granule cell translocation in vivo, while the lack of NR2C subunit did not have any detectable effects on cell migration. Cellular composition of wild-type and mutant dissociated cerebellar granule cell cultures isolated from 10-day-old cerebella were similar, but NR2C-2B cultures had elevated level of NR2B subunits and intracellular Ca2+ imaging revealed higher sensitivity towards the addition of NR2B-selective antagonist in vitro. Time-lapse videomicroscopic observations revealed that average migratory velocity and the proportion of translocating cell bodies were significantly higher in NR2C-2B than in wild-type cultures. Our results provide evidence that NR2B-containing NMDARs can have specialized roles during granule cell migration and can increase migratory speed.

Dendritic plasticity of CA1 pyramidal neurons after transient global ischemia

Dendrites and spines undergo dynamic changes in physiological conditions, such as learning and memory, and in pathological conditions, such as Alzheimer's disease and epilepsy. Long-term dendritic plasticity has also been reported after ischemia/hypoxia, which might be compensatory effects of surviving neurons for the functional recovery after the insults. However, the dendritic changes shortly after ischemia, which might be associated with the pathogenesis of ischemic cell death, remain largely unknown. To reveal the morphological changes of ischemia-vulnerable neurons after ischemia, the present study investigated the alteration of dendritic arborization of CA1 pyramidal neurons in rats after transient cerebral ischemia using intracellular staining technique in vivo. The general appearance of dendritic arborization of CA1 neurons within 48 h after ischemia was similar to that of control neurons. However, a dramatic increase of dendritic disorientation was observed after ischemia with many basal dendrites coursed into the territory of apical dendrites and apical dendrites branched into the region of basal dendrites. In addition, a significant increase of apical dendritic length was found 24 h after ischemia. The increase of dendritic length after ischemia was mainly due to the dendritic sprouting rather than the extension of individual dendrites, which mainly occurred in the middle segment of the apical dendrites. These results reveal a plasticity change in dendritic arborization of CA1 neurons shortly after cerebral ischemia.

Neonatal toluene exposure alters glutamate-induced calcium signaling in developing cerebellar granule neurons

Glutamatergic neurotransmission is critical both for neurogenesis and mature functioning of the central nervous system (CNS), and is thought to be one target for toluene-induced damage. It has been reported that toluene antagonizes the function of N-methyl-D-aspartate (NMDA) receptor. In this study, we examined the effect that neonatal toluene exposure has on NMDA receptor in cerebellar granule neurons. Sprague-Dawley rats were treated with 0, 200, 500, and 1000 mg/kg of toluene by intraperitoneal injection from postnatal day (PN) 4 to 7. After culture under toluene-free condition, Ca2+ signaling in response to glutamate and NMDA was measured using fura-2 Ca2+ imaging for up to 14 days. Toluene exposure dose-dependently reduced glutamate/glycine and NMDA/glycine induced Ca2+ signaling in culture cerebellar granule neurons at DIV5 (day in vitro), and the effects were gradually recovered. The effects of toluene exposure on NMDA-stimulated Ca2+ signals in response to NMDA receptor inhibitors were also compared. The results indicated that neonatal toluene exposure can induce long-term but reversible changes in NMDA-induced Ca2+ signaling pathway. Such changes could be involved in the impairment of CNS function and development observed in fetal solvent syndrome.

Neonatal Toluene Exposure Alters Agonist and Antagonist Sensitivity and NR2B Subunit Expression of NMDA Receptors in Cultured Cerebellar Granule Neurons

Toluene has been reported to antagonize the function of N-methyl-D-aspartate (NMDA) receptors. In this study, the effects of neonatal toluene exposure on NMDA receptors in primarily cultured cerebellar granule neurons were examined. Sprague-Dawley rats were treated with toluene (0, 200, 500, and 1000 mg/kg, i.p.) from postnatal day (PN) 4 to PN 7. Under toluene-free conditions, Ca2+ signals of cultured neurons in response to glutamate and NMDA were measured for up to 14 days. The expression of NMDA receptor subunits (NR1, NR2A, and NR2B) at 5-14 days in vitro (DIV) were also determined. Neonatal toluene exposure dose-dependently reduced intracellular Ca2+ signals in response to glutamate/glycine and NMDA/glycine in cultured cerebellar granule neurons, and these effects were gradually decreased with time. Such toluene exposure did not influence the inhibition of Mg2+ or MK801 on NMDA-evoked responses, but it decreased the potency of ifenprodil (an NR2B preferring antagonist). The protein levels of NMDA receptor subunit NR2B were consistently reduced by toluene exposure at 5 DIV, but not at 14 DIV. These results demonstrate that neonatal toluene exposure induces long-term but reversible changes in the function and composition of NMDA receptors. Such changes during developmental stages may contribute to the cerebellar dysfunction observed in fetal solvent syndrome.

Secretoneurin promotes pertussis toxin-sensitive neurite outgrowth in cerebellar granule cells

The neuropeptide secretoneurin (SN) is an endoproteolytic product of the chromogranin secretogranin II. We investigated the effects of SN on the differentiation of immature cerebellar granule cells derived from the external granular layer (EGL). Secretoneurin caused concentration-dependent increases in neurite outgrowth, reflecting differentiation. The maximum effect was reached at a concentration of 100 nm SN. Secretoneurin immunoneutralization using specific antiserum significantly decreased neurite outgrowth; however, neurite morphology was altered. An affinity chromatography-purified antibody significantly inhibited the outgrowth response to SN (p < 0.001) without altering the morphology. Binding studies suggest the existence of specific G-protein-coupled receptors on the surface of monocytes that recognize SN. Assuming that SN promotes neurite outgrowth in EGL cells by acting through a similar G-protein-coupled mechanism, we treated SN-stimulated EGL cultures with pertussis toxin. Exposure to pertussis toxin (0.1 micro g/mL) showed a significant inhibition of the SN-induced outgrowth. To establish a second messenger pathway we used the protein kinase C inhibitor staurosporine. We found that EGL cell viability was not enhanced following chronic SN treatment for 24 h. These data indicate that SN is a novel trophic substance that can affect cerebellar maturation, primarily by accelerating granule cell differentiation through a signalling mechanism that is coupled to pertussis toxin-sensitive G-proteins.

Relationship Between Intracellular Free Calcium Concentration and NMDA-induced Cerebellar Granule Cell Survival In Vitro

The survival of cerebellar granule cells in culture is stimulated by activation of the N-methyl-d-aspartate (NMDA) class of glutamate receptors. Activation of these receptors at the key period for cell survival in vitro (3 days; 3DIV) resulted in a sustained elevation of intracellular free calcium concentration [Ca2+]i over the same concentration range of NMDA that led to granule cell survival. Agents that release Ca2+ from intracellular stores led to only small, transient elevations of [Ca2+]i and were unable to stimulate granule cell survival. Addition of the Ca2+ ionophore ionomycin to granule cell cultures at 3DIV resulted in increased granule cell number at 7DIV. The ability of ionomycin to stimulate granule cell survival was related to the [Ca2+]i elicited, indicating that a rise in [Ca2+]i is sufficient to activate the processes leading to granule cell survival and that the extent of the elevation in [Ca2+]i is crucially important in determining granule cell fate.

Early activation of Ca(2+)-permeable AMPA receptors reduces neurite outgrowth in embryonic chick retinal neurons

Calcium entry through Ca(2+)-permeable AMPA/kainate receptors may activate signaling cascades controlling neuronal development. Using the fluorescent Ca(2+)-indicator Calcium Green 1-AM we showed that the application of kainate or AMPA produced an increase of intracellular [Ca(2+)] in embryonic chick retina from day 6 (E6) onwards. This Ca(2+) increase is due to entry through AMPA-preferring receptors, because it was blocked by the AMPA receptor antagonist GYKI 52466 but not by the N-methyl-D-aspartic acid (NMDA) receptor antagonist AP5, the voltage-gated Ca(2+) channel blockers diltiazem or nifedipine, or by the substitution of Na+ for choline in the extracellular solution to prevent the depolarizing action of kainate and AMPA. In dissociated E8 retinal cultures, application of glutamate, kainate, or AMPA reduced the number of neurites arising from these cells. The effect of kainate was prevented by the AMPA/kainate receptor antagonist CNQX and by GYKI 52466 but not by AP5, indicating that the reduction in neurite outgrowth resulted from the activation of AMPA receptors. Blocking Ca(2+) influx through L-type voltage-gated Ca(2+) channels with diltiazem and nifedipine prevented the effect of 10-100 microM kainate but not that of 500 microM kainate. In addition, joro spider toxin-3, a blocker of Ca(2+)-conducting AMPA receptors, prevented the effect of all doses of kainate. Neither GABA, which is depolarizing at this age in the retina, nor the activation of metabotropic glutamate receptors with tACPD mimicked the effects of AMPA receptor activation. Calcium entry via AMPA receptor channels themselves may therefore be important in the regulation of neurite outgrowth in developing chick retinal cells.

Vasopressin-induced neurotrophism in cultured neurons of the cerebral cortex: dependency on calcium signaling and protein kinase C activity

Neuronal process outgrowth has been postulated to be one of the fundamental steps involved in neuronal development. To test whether vasopressin can influence neuronal development by acting on the outgrowth of neuronal processes, we determined the neurotrophic action of the memory-enhancing peptide, vasopressin, in neurons derived from the cerebral cortex, a site of integrative cognitive function and long-term memory. Exposure to V(1) receptor agonist significantly increased multiple features of nerve cell morphology, including neurite length, number of branches, branch length, number of branch bifurcation points and number of microspikes. The dose-response profile of V(1) receptor agonist-induced neurotrophism exhibited a biphasic function, with lower concentrations inducing a significant increase while higher concentrations generally induced no significant effect. The neurotrophic effect of V(1) receptor activation did not require growth factors present in serum. Analysis of the regional selectivity of the vasopressin-induced neurotrophic effect revealed significant V(1) receptor agonist-induced neurotrophism in occipital and parietal neurons, whereas frontal and temporal neurons were unresponsive. Results of experiments to determine the mechanism of vasopressin-induced neurotrophism demonstrated that vasopressin-induced neurotrophism is dependent on V(1)a receptor activation, requires L-type calcium channel activation and activation of both pathways of the phosphatidylinositol signaling cascade, inositol trisphosphate and protein kinase C. These studies are the first to describe a functional cellular response for vasopressin in the cerebral cortex. The findings are discussed with respect to their implications for understanding the role of vasopressin-induced neurotrophism, the associated signaling pathways required for this response, and the ability of vasopressin to enhance memory function.

L-type Ca(2+) channel blocker inhibits mossy fiber sprouting and cognitive deficits following pilocarpine seizures in immature mice

Behavioral and cognitive deficits are one of the most frequent sequelae of childhood epilepsy. Accumulating evidence indicates that epilepsy induces aberrant development of the mossy fibers in the hippocampus, the region that is commonly accepted to play a key role in learning and memory. We have therefore proposed that such abnormal maturation of the central nervous system may cause the adverse prognoses following epilepsy. Based on this hypothesis, using primary cultures of the dentate granule cells, we showed that the L-type Ca(2+) channel blocker nicardipine was neuroprotective against excessive mossy fiber synaptogenesis induced by prolonged depolarization that was assumed to mimic epileptiform conditions. Therefore, we evaluated the in vivo effect of nicardipine on aversive sequelae following epileptiform seizures. We found aberrant sprouting of the mossy fibers and poor performance of spatial and contextual tasks in the mice that had received treatment with pilocarpine at their early postnatal age. Repetitive administration of nicardipine prevented the mossy fiber sprouting and ameliorated the cognitive deterioration, although it did not show anticonvulsant actions against pilocarpine seizures. In the present study, we proposed two in vitro and in vivo models for evaluating epilepsy sequelae and noticed that L-type Ca(2+) channel blocker nicardipine was effective in both models. L-type Ca(2+) channel blocker may be a good candidate for a preventive for childhood epilepsy sequelae. Likewise, these useful systems will disclose additional candidates in future.

Rat motor neuron plasticity induced by dorsal rhizotomy

The control of peripheral structural plasticity of motor neurons by primary sensory neurons was studied in rat extensor digitorum longus (EDL) muscle. Polyinnervation of muscle fibers, sprouting and the motor neuron peripheral field size following L4 dorsal root cutting were evaluated using three different approaches: intracellular recording of end plate potentials, histochemical demonstration of sprouting and polyinnervation and in vivo recording of nerve-evoked twitch. Nodal sprouting was found in rhizotomized rats but not in controls and consistently muscle polyinnervation appeared. The muscle portion innervated by L3 ventral root was relatively reduced and that innervated by L5 was relatively enlarged: a trend to caudal shift of muscle innervation arose in rhizotomized rats. A control of motor neuron plasticity by primary sensory neurons is suggested.

BAPTA-AM blocks both voltage-gated and Ca2+-activated K+ currents in cultured bovine chromaffin cells

The effects of the membrane permeant Ca2+ chelator BAPTA-AM on voltage-gated Na+, Ca2+, K+ (I(Na), I(Ca) I(K), respectively) and Ca2+-activated K+ (I(KCa)) currents in cultured bovine chromaffin cells were investigated using the whole-cell patch-clamp technique. Superfusion with BAPTA-AM (50 microM) induced a rapid (< 60 s) and reversible block of both I(KCa) and I(K) (approximately 50%), without affecting either I(Ca) or I(Na). Preincubation with BAPTA-AM (50 microM, 30 min) or cell loading with the nonpermeable active form of BAPTA (10 mM in the pipette solution) permanently blocked I(KCa). BAPTA-AM superfusion (50 microM) also blocked I(K) (approximately 53%) after BAPTA-loading or BAPTA-AM preincubation. In conclusion, we show a fast and reversible block of I(KCa) and I(K) by BAPTA-AM, acting directly on K+ channels before it operates as a Ca2+ chelator, in cultured bovine chromaffin cells.

Redistribution of NMDA receptors in the cochlear nucleus following cochleotomy

The major input to neurons of the cochlear nucleus comes from the glutamatergic cells of the spiral ganglion. We have studied the effect of unilateral destruction of the inner ear, including the spiral ganglion, with two antibodies against different types of NMDA receptor subunits, NMDAR1 and NMDAR2A/B, in the cochlear nucleus of the rat. Following cochleotomy, a dramatic redistribution of the receptor subunits was observed from a mostly perikaryal to a predominantly dendritic localization. Moreover, distinct changes in the composition of NMDA receptor complexes occurred. These effects were interpreted as compensatory responses to the massive loss of presynaptic release of the transmitter glutamate.

Neurotoxicology and amino acid intake during development: the case of threonine

The development of the central nervous system is highly dependent on an adequate supply of nutrients. In particular, protein and amino acid availability is of major concern during gestation and in early postnatal life. Numerous data have been published on some amino acids directly involved in brain functions as neurotransmitters or indirectly as precursors of neurotransmitters, but scant information is available on the possible consequences of hyperthreoninemia, a phenomenon repeatedly noted in clinical reports. The results of neurochemical and behavioral studies in the developing rat suggest that despite numerous possible effects of threonine on brain constituents, moderate hyperthreoninemia does not impair markedly the development of the central nervous system.

Characterization of calcium accumulation in the brain of rats administered orally calcium: the significance of energy-dependent mechanism

The characterization of calcium accumulation in the brain of rats administered orally calcium chloride solution was investigated. Rats received a single oral administration of calcium (15-50 mg/100 g body weight), and they were sacrificed by bleeding between 15 and 120 min after the administration. The administration of calcium (50 mg/100 g) produced a significant increase in serum calcium concentration and a corresponding elevation of brain calcium content, indicating that the transport of calcium into the brain is associated with the elevation of serum calcium levels. The increase in brain calcium content by calcium administration was not appreciably altered by the pretreatment with Ca2+ channel blockers (verapamil or diltiazem with the doses of 1.5 and 3.0 mg/100 g). In thyroparathyroidectomized rats, the administration of calcium (50 mg/100 g) caused a significant increase in brain calcium content, indicating that calcium-regulating hormones do not participate in the brain calcium transport. Now, brain calcium content was clearly elevated by fasting (overnight), although serum calcium level was not significantly altered. Calcium administration to fasted rats induced a further elevation of brain calcium content as compared with that of control (fasted) rats. The fasting-induced increase in brain calcium content was appreciably restored by refeeding. This restoration was also seen by the oral administration of glucose (0.4 g/100 g) to fasted rats. The present study demonstrates that serum calcium is transported to brain, and that the increased brain calcium is released promptly. The release of calcium from brain may be involved in energy metabolism, and this release may be weakened by the reduction of glucose supply into brain. The finding suggests a physiological significance of energy-dependent mechanism in the regulation of brain calcium.

Nitric oxide-mediated death of cultured neonatal retinal ganglion cells: neuroprotective properties of glutamate and chondroitin sulfate proteoglycan

The release of nitric oxide and stimulation of glutamate receptors by excitatory amino acids has been linked to neuronal degeneration and toxicity. In the rat retina approximately 60% of retinal ganglion cells (RGCs) die during the first postnatal week. In this study we examined the effects of nitric oxide synthase blockers and glutamate on the survival of neonatal RGCs in vitro over a 16 h assay period. Less than 10% of P1 RGCs survived in serum free defined media alone (control), however survival was increased, in a dose-dependent manner, when L-glutamate (10 microM-10 mM) was added to the media; a maximum of 70% of RGCs could be maintained with the addition of 5 mM glutamate. This effect was blocked by the NMDA and non-NMDA receptor blockers APV and DNQX and was age dependent; the survival of RGCs from P5 but not P7 rats was enhanced by the addition of glutamate even in high calcium concentrations (10 mM). When the nitric oxide synthase blockers L-NAME (5 mM) or haemoglobin (25 microM) were added to the culture media, up to 61% of P1 RGCs survived. The addition of the 480 kDa chondroitin sulfate proteoglycan (SCCP) previously shown to enhance RGC survival in vivo and in vitro, potentiated the action of glutamate and L-NAME and increased RGC survival to over 90% with almost all RGCs expressing a profusion of processes. These results suggest that the release of nitric oxide and glutamate by cells within the retina may contribute to the regulation of RGC numbers in vivo during development.

Treatment with NMDA receptor antagonists does not affect developmental changes in NMDA receptor properties in vivo

Effects of acute and long-term treatment of neonatal rats with N-methyl-D-aspartate (NMDA) receptor antagonists on changes in NMDA receptor properties were examined. Animals received either on postnatal day 6 a single dose of the antagonists MK-801 (1 mg/kg), or D-CPPene (2 mg/kg) or during the period from postnatal day 5 to 14, two daily injections of MK-801 (0.25 mg/kg) or D-CPPene (0.75 mg/kg). Control littermates received saline injections. In both cases animals were sacrificed one day after the last injection. NMDA receptor properties were examined in membrane preparations derived from the cerebral cortex by studying the modulation of [3H]MK-801 binding by glutamate, Mg2+ and D-CPPene. The density of agonist- and antagonist-binding sites in the CA1 region of the hippocampus were determined by autoradiography, using [3H]CGP39653 or [3H]glutamate as ligands. A significant developmental increase in NMDA receptor binding sites was detected both in preparations of cerebral cortical membranes and in the CA1 area of the hippocampus. In addition, the Mg2+ sensitivity of [3H]MK-801 binding was significantly higher in membrane preparations from the cerebral cortex of postnatal day 15 compared to postnatal day 7 animals. Neither the single nor the subchronic treatment with NMDA receptor antagonists exerted a significant influence on the density of antagonist binding sites or on the modulation of [3H]MK-801 binding by glutamate, Mg2+ or D-CPPene. We conclude therefore that neonatal treatment with NMDA receptor antagonists in vivo does not involve significant alterations in the properties and the densities of NMDA receptors in the brain regions studies, i.e., during the period when expression of these receptors is subject to pronounced developmental regulation.

Effect of threonine on the behavioural development of the rat

Rats received different levels of threonine (Thr), one, 1.7 and four times the normal dietary intake, from conception to adulthood. The mothers were fed the experimental diets before and during pregnancy. Their offspring received a daily oral load of Thr or placebo until weaning. Thereafter, the juveniles were fed the same diet as their mothers. Morphologic development, ingestive behaviour, homing, and locomotion were observed before weaning. Exploration and spontaneous alternation were studied thereafter. Animals exposed during gestation to 1.7 times the normal Thr intake consumed more food during the test of independent ingestion. Grooming showed inconsistent variations between days 12 and 29 in pups fed 1.7 times the normal Thr intake. Rats performed equally well on the other behavioural tasks independently of the dietary treatment. We conclude that Thr intake as much as four times higher than the levels found in normal diets does not impair the behavioural ontogenesis of the rat.

Hippocalcin in rat retina. Comparison with calbindin-D28k, calretinin and neurocalcin

The post-natal developmental expression in rat retina of four calcium-binding proteins belonging to the calmodulin-troponin-C family was investigated by immunohistochemistry using anti-calbindin-D28k, anti-calretinin, anti-hippocalcin and anti-neurocalcin polyclonal antibodies on paraffin sections from Wistar rat retinae aged from post-natal days 1 (P1), 5 (P5), 10 (P10), 20 (P20) to adulthood (8 weeks). Immunoblot using anti-hippocalcin and homogenates proteins from retina, cerebellar cortex, hippocampus and cerebellum was also performed. Hippocalcin immunoreactivity in adult rat retina was demonstrated by both immunohistochemistry and Western blot. During post-natal development, calbindin-D28k, calretinin and neurocalcin immunoreactivity were detected at P1 in ganglion cells, whereas hippocalcin immunoreactivity was seen later at P5 in this cell layer. In the amacrine cell layer, neurocalcin immunoreactivity was detected at P5 and hippocalcin at P10. Calbindin-D28k was labelling the immature horizontal cell, calretinin was detected in nearly all ganglion cells and in some amacrine cells since P1. These three calcium-binding proteins do not seem to play a role in synaptogenesis which takes place later. We confirmed that calbindin-D28k appeared to be a good marker for horizontal cells. The presence of hippocalcin, a myristoylated calcium-binding protein belonging to the recovering subfamily and previously localized in few brain areas has been detected for the first time in retina.

Low extracellular magnesium unmasks N-methyl-D-aspartate-mediated graft-host connections in rat neocortex slice preparation

The main purpose of this study was to investigate the role of N-methyl-D-aspartate receptors in host-graft synaptic transmission in the neocortex. The effects of low extracellular magnesium, the glutamate agonist N-methyl-D-aspartate and N-methyl-D-aspartate antagonists on the synaptic activation of connections between embryonic neocortical graft tissue and the surrounding host tissue were studied in 17 perfused slices of rat neocortex. In standard artificial cerebrospinal fluid, stimulation of the host white matter evoked field potentials in four of 17 grafts. However, in Mg(2+)-free medium, the same stimulation evoked field potentials in an additional six grafts, with significant increases in the mean duration of the evoked responses in the 10 responsive grafts. In five of these slices stimulation of the graft also evoked field potentials in the host tissue, suggesting reciprocal interaction between graft and host. Simultaneous extracellular recordings from graft and host tissues in Mg(2+)-free medium showed that spontaneous epileptiform discharges developed in the graft and host tissue synchronously. In Mg(2+)-free medium, application of N-methyl-D-aspartate induced a shift of the baseline with superimposed epileptiform discharges in both graft and host. Application of the non-competitive N-methyl-D-aspartate antagonist ketamine and the competitive antagonist D,L-2-amino-5-phosphonovaleric acid attenuated or reversibly blocked both the spontaneous epileptiform discharges and the evoked field potentials. Our data provides evidence that N-methyl-D-aspartate receptors are present at synapses created between fetal graft and host neocortex, and that the N-methyl-D-aspartate-activated receptor-channel complex plays an active role in mediating excitatory synaptic transmission in host-graft circuitry.

Mechanosensitivity of NMDA receptors in cultured mouse central neurons

Changes in osmotic and hydrostatic pressure were found to modulate NMDA responses of cultured embryonic mouse neurons recorded in various patch-clamp configurations. In nucleated patches, NMDA currents were potentiated by reductions in external osmolarity and were reduced in hyper-osmotic solutions. These changes, which were greater for low concentrations of NMDA, were not observed for responses to kainate, glycine, or GABA. They could be mimicked by directly changing the pipette pressure in nucleated, outside-out, inside-out, and cell-attached patches. Osmosensitivity of NMDA responses was also observed in the whole-cell mode, but only after prolonged dialysis. Mechanosensitivity of NMDA receptors could have an important role in neuronal regions experiencing changes in membrane tension, such as spines or growth cones.

Developmental alterations in NMDA receptor-mediated [Ca2+]i elevation in substantia gelatinosa neurons of neonatal rat spinal cord

Using spinal cord slices prepared from neonatal rats, the intracellular free Ca2+ concentration ([Ca2+]i) in neurons located in the dorsal horn substantia gelatinosa (SG) was measured with microscopic fluorometry by loading fura 2-AM into neurons. Developmental alterations in the elevation of [Ca2+]i elicited by the glutamate analogs, NMDA and AMPA, were investigated from postnatal day (PNDs) 1 to 17. During the 1st week of postnatal life, when neuronal maturation of the SG is known to take place, the NMDA response remained large or even slightly increased. It subsequently showed a gradual decline. This pattern of postnatal changes is consistent with previously reported autoradiographic studies on NMDA-binding sites. The affinity of receptors for NMDA was found to decrease constantly during the period examined. The AMPA response and resting [Ca2+]i showed no significant developmental changes. Neonatal treatment with capsaicin, which has been shown to degenerate fine primary afferent fibers terminating in the SG, delayed the developmental decline in the NMDA-induced [Ca2+]i response. It is suggested that the number and the molecular properties of NMDA receptors expressed in the SG change during early postnatal neuronal maturation. The temporal coincidence between postnatal alteration in NMDA-induced [Ca2+]i elevation and neuronal maturation of the SG may indicate that intracellular Ca2+ regulated by NMDA receptor activation is related to postnatal neuronal maturation. Activation of fine primary afferent fibers may contribute to the observed developmental alterations in the NMDA response of SG neurons.

Protein kinase C activation attenuates N-methyl-D-aspartate-induced increases in intracellular calcium in cerebellar granule cells

Activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor increases levels of intracellular calcium and can lead to stimulation of protein kinase C activity. Several reports have demonstrated that stimulation of protein kinase C can, in turn, increase electrophysiological responses to NMDA in certain cells or in oocytes expressing certain NMDA receptor subunits. In the present study, the effects of protein kinase C activation on NMDA receptor-mediated increases in intracellular Ca2+ level were investigated in primary cultures of rat cerebellar granule cells using fura-2 fluorescence spectroscopy. Pretreatment of the cells with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA), but not the inactive analogue 4 alpha-phorbol 12-myristate 13-acetate, inhibited NMDA-induced increases in intracellular Ca2+ levels. Coincubation of cells with PMA and the kinase inhibitor staurosporine or calphostin C blocked the PMA effect. The potency of NMDA was reduced twofold, and the potency of the NMDA receptor co-agonist, glycine, to enhance the response to NMDA was decreased fourfold by pretreatment of cells with PMA. The effect on glycine was mimicked by pretreatment with okadaic acid, a protein phosphatase inhibitor. PMA treatment did not significantly alter Mg2+ inhibition of the NMDA response but decreased the potency of the competitive antagonist CGS-19755. These data suggest that, in cerebellar granule cells, the function of the NMDA receptor may be subject to feed-back inhibition by protein kinase C stimulation. Under physiological conditions, this inhibition may result from a decreased effectiveness of the endogenous co-agonists, glutamate and glycine.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate and kainate differently affect neuronal cytoarchitecture of rat cerebellar granule cells

Rat cerebellar granule cells cultured in media containing 12 mM KCl showed short life-span, did not branch, and died after 10 days in vitro. The cell exposure to N-methyl-D-aspartate (NMDA) or to kainate promoted both neuron survival and branching, reproducing the viability and the neurite extension routinely observed in cultures maintained in media containing 25 mM KCl. Exposure of neurons to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) resulted in an increased survival not associated with neuritic arborization. These results suggest that the glutamate ionotropic receptor subtypes differently contribute in elaborating neuronal morphogenesis.

Neurotrophic effects of NMDA receptor activation on developing cerebellar granule cells

Glutamate acting on N-methyl-D-aspartate (NMDA) receptors controls a variety of aspects of neuronal plasticity in the adult and developing brain. This review summarizes its effects on developing cerebellar granule cells. The glutamatergic mossy fibre input to cerebellar granule cells exerts a neurotrophic effect on these cells during development. The investigation of potential neurotrophic agents can be carried out using enriched granule cell cultures. Considerable evidence now indicates that glutamate acting on N-methyl-D-aspartate receptors is an important neurotrophic factor that regulates granule cell development. In culture, neurite growth, differentiation and cell survival are all stimulated by N-methyl-D-aspartate receptor activation. The intracellular pathways involved following Ca2+ entry through the N-methyl-D-aspartate receptor channel are beginning to be elucidated. The cerebellar granule cell culture system may provide an ideal model to investigate the molecular mechanisms involved in long term N-methyl-D-aspartate receptor-mediated changes in neuronal function.

Phosphoproteins of cultured cerebellar granule cells and response to the differentiation-promoting stimuli NMDA, high K+ and ionomycin

In order to investigate signalling pathways involved in the control of granule cell differentiation, survival and other functions by depolarization or activation of NMDA receptors we have characterized protein phosphorylation in cerebellar granule cells. Cultures of cerebellar granule cells were incubated with 32P orthophosphate and then challenged with NMDA, K+ or the Ca2+ ionophore ionomycin, agents which raise [Ca2+]i and stimulate differentiation and survival. Upon separation of labelled phosphoproteins by two-dimensional gel electrophoresis three differences were found in response to all of these agents. These were an increase in acidity of two phosphoproteins of 87 and 48 kDa (p87 and p48) and increased 32P-incorporation into a phosphoprotein of 120 kDa (p120). Treatment with PMA which stimulates neurite outgrowth but not survival affected p87 (increased its acidity) but not p48. The acidic shift of p87, therefore, is not sufficient to stimulate granule cell survival. The identification of p87 as the actin-binding MARCKS protein and the demonstration of its presence in neurites and growth cones of granule cells suggests that it may be involved in NMDA-stimulated neurite outgrowth. The phosphoproteins p120 and p48 may potentially be involved in events linking the rise in [Ca2+]i to increased granule cell survival or other aspects of granule cell differentiation.

Modulation of neuronal migration by NMDA receptors

The N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor is essential for neuronal differentiation and establishment or elimination of synapses in a developing brain. The activity of the NMDA receptor has now been shown to also regulate the migration of granule cells in slice preparations of the developing mouse cerebellum. First, blockade of NMDA receptors by specific antagonists resulted in the curtailment of cell migration. Second, enhancement of NMDA receptor activity by the removal of magnesium or by the application of glycine increased the rate of cell movement. Third, increase of endogenous extracellular glutamate by inhibition of its uptake accelerated the rate of cell migration. These results suggest that NMDA receptors may play an early role in the regulation of calcium-dependent cell migration before neurons reach their targets and form synaptic contacts.

Metabotropic glutamate receptors in cultured cerebellar granule cells: developmental profile

Excitatory amino acid (EAA)-induced polyphosphoinositide (PPI) hydrolysis was studied during the development in culture of cerebellar granule cells. The developmental pattern was similar using metabotropic glutamate (Glu) receptor (mGluR) agonists, including L-Glu, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid: The stimulation of [3H]inositol monophosphate ([3H]-InsP) formation was low at 2 days in vitro (DIV), but the response increased steeply, reaching a peak at 4 DIV, followed by a progressive decline. In contrast, carbamylcholine-induced PPI hydrolysis exhibited a plateau after a pronounced increase during the first week in vitro. At 6 DIV, but not at 4 DIV, when the activity peaked, PPI hydrolysis elicited by Glu was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, indicating that in cultured granule cells, NMDA receptors contribute to [3H]-InsP formation and that this component of the response develops relatively late. Accordingly, NMDA-induced [3H]-InsP formation, estimated under Mg(2+)-free conditions, increased markedly from very low values at 2 DIV to a plateau at 8-10 DIV. The developmental pattern of EAA-induced PPI hydrolysis was paralleled by changes in the level of an mRNA for a specific mGluR subtype (mGluR1 mRNA). RNA blot analysis performed with the pmGR1 cDNA probe revealed that the hybridization signal in RNA extracts from cultures at 1 DIV was very weak, but mGluR mRNA levels increased dramatically between 1 and 3 DIV, followed by a progressive decrease, so that by 15 DIV the mRNA levels were only approximately 10% of the values at 3 DIV.(ABSTRACT TRUNCATED AT 250 WORDS)

A narrow window of intracellular calcium concentration is optimal for neurite outgrowth in rat sensory neurones

We have examined neurite outgrowth in rat sensory neurones when cytosolic free calcium concentration ([Ca2+]i) was varied in the range 0-60 nM. Neurite outgrowth was maximal at 35 nM [Ca2+]i and was reduced at higher and lower values of [Ca2+]i. These results provide direct evidence for Mattson and Kater's suggestion of an optimal calcium range for growth cone function.