Changes In Glutamate Receptors Research Articles

Hippocampal synaptic plasticity and glutamate receptor regulation: influences of diabetes mellitus

Diabetes mellitus is an endocrine disorder of carbohydrate metabolism resulting primarily from inadequate insulin release (Type 1 insulin-dependent diabetes mellitus) or insulin insensitivity coupled with inadequate compensatory insulin release (Type 2 non-insulin-dependent diabetes mellitus). Previous studies involving behavioural and electrophysiological analysis indicate that diabetes mellitus induces cognitive impairment and defects of long-term potentiation in the hippocampus. Considered to be an important mechanism of learning and memory in mammals, long-term potentiation is known to require regulation of the glutamate receptor properties. According to many studies, defects of long-term potentiation in the hippocampus of diabetic animals are due to abnormal glutamate receptors. We review here the changes in glutamate receptors that may account for modifications of long-term potentiation in various models of diabetes mellitus. As glutamate receptors are also involved in the appearance of neurodegenerative states, we discuss the possibility that deficits in long-term potentiation during chronic diabetes might arise from dysfunction of the N-methyl- d-aspartate (NMDA) subtype of glutamate receptors in early stages of the disease. This review addresses the possible role of hyperglycaemia and insulin in regulating these receptors.

Neuregulin1 downregulates postsynaptic GABAa receptors at the hippocampal inhibitory synapse

The growth factor neuregulin 1 (NRG1) has been proposed to contribute to the formation and maturation of neuromuscular and interneuronal synapses by upregulating the expression of specific neurotransmitter receptor subunits. In the present report, we show that, in the hippocampus, NRG1 is expressed in a pattern suggesting that it regulates synapse development in the CA1 region. However, in contrast to what has been shown in other synapses, NRG1 reduces the expression of gamma-aminobutyric acid (GABA)A receptors alpha subunits in hippocampal slices, and the mean amplitude of GABAergic miniature inhibitory postsynaptic currents (IPSCs) in hippocampal CA1 pyramidal neurons, without affecting IPSC kinetics or frequency. These effects of NRG1 occur without concomitant changes in glutamate receptors and other synaptic proteins. We propose that the role of NRG1 in the formation and maturation in the hippocampal inhibitory synapse is downregulation, rather than upregulation, of receptor subunit expression. These results suggest that NRG1 may contribute to the reduction in GABAergic synaptic activity in hippocampal CA1 pyramidal neurons that normally occurs during early postnatal development, and that alterations in NRG1 signaling in the hippocampus may contribute to schizophrenia and epilepsy.

Glutamate and Schizophrenia

Schizophrenia is a complex and chronic disease that affects multiple aspects of cognition and behavior, including attention, perception, thought processes, emotion and volition. However, the pathophysiology of schizophrenia has not yet been fully clarified. For more than a decade, the dopamine hypothesis has been the most influential hypothesis in schizophrenia research. It postulates that schizophrenia is a manifestation of a hyperdopaminergic state in some regions of the brain. However, several studies have reported results that are inconsistent with the dopamine theory of schizophrenia and it is now believed that other neurotransmitter systems are probably involved. Interest in the role of excitatory amino acids in schizophrenia is rapidly increasing. Excitatory amino acids and their receptors are thought to play a role in the pathology of schizophrenia, based on several lines of evidence, such as the psychomimetic effects of glutamate receptor antagonists in non-schizophrenic subjects and patients with schizophrenia, postmortem studies of reporting changes in glutamate receptors in schizophrenia, and studies that have treated schizophrenic patients with glutamate receptor agonists. This review will focus on clinical evidence contributing to the “glutamate hypothesis of schizophrenia”. Keywords: glutamate, schizophrenia, cognition, hyperdopaminergic, excitatory amino acids, glutamate receptor antagonists, glutamate hypothesis

Ischemia induces short- and long-term remodeling of synaptic activity in the hippocampus.

One of the most vulnerable areas to ischemia or hypoglycemia is CA1 hippocampal region due to pyramidal neurons death. Glutamate receptors are involved together with protein-kinase C and nitric oxide synthase. Long-term potentiation (LTP) is generated in anoxic or hypoglycemic conditions via activation of NMDA while inhibition of these receptors attenuates this response. Protein-kinase C and nitric oxide synthase are involved in anoxic LTP mechanism. Postischemic neurons are hyperexcitable in CA3 area while CA1 pyramidal neurons degenerate and disappear. Changes of glutamate receptors triggered by ischemia and hypoglycemia are discussed in this review.

Glutamate stimulates neurotrophin expression in cultured Müller cells

The uptake of excess extracellular glutamate and the secretion of neurotrophins by glial cells have been suggested to protect CNS neurons from glutamate-induced toxicity. In the retina, perturbation of glutamate transport and decreased retrograde transport of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) may contribute to ganglion cell death in experimental glaucoma. Although many studies show a clear relationship between glutamate and neurotrophic factors, such relationship has not been thoroughly investigated in the retinal environment. In the following study, we determined the effects of glutamate on early passaged rat Müller cells, specifically their expression of neurotrophic factors including BDNF, nerve growth factor (NGF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), and glial-cell line derived neurotrophic factor (GDNF); and of glutamate receptors and transporters using immunoblots or enzyme-linked immunosorbent assays. Binding of BDNF to its cognate receptor TrkB was also determined using co-immunoprecipitation studies. Cultured Müller cells grown in the presence of glutamate were also assayed for survival using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS). Our study showed that while glutamate treatment did not promote cell death, it upregulated secretion of BDNF, NGF, NT-3, NT-4, and GDNF by Müller cells. While solitary bands at ∼13–14 kDa were observed for NGF, NT-3, and NT-4; two BDNF-reactive bands were observed in immunoblots: a faster migrating band at the reported size of the BDNF monomer (∼13 kDa); and a more intense band at ∼36 kDa. GDNF-reactive bands were observed at ∼22, ∼28, and ∼55 kDa. Glutamate also induced significant changes in glutamate receptor and transporter proteins, as well maintained the association of BDNF to TrkB in Müller cells. The decreased N-methyl- d-aspartate receptor (NMDAR) levels and sustained activation of TrkB by BDNF could serve as protective mechanisms for Müller cell survival. Moreover, the increased secretion of neurotrophic factors and upregulation of l-glutamate/ l-aspartate transporter (GLAST) expression in Müller cells may protect retinal neurons from glutamate toxicity.

Postsynaptic Depolarization Scales Quantal Amplitude in Cortical Pyramidal Neurons

Pyramidal neurons scale the strength of all of their excitatory synapses up or down in response to long-term changes in activity, and in the direction needed to stabilize firing rates. This form of homeostatic plasticity is likely to play an important role in stabilizing firing rates during learning and developmental plasticity, but the signals that translate a change in activity into global changes in synaptic strength are poorly understood. Some but not all of the effects of long-lasting changes in activity on synaptic strengths can be accounted for by activity-dependent release of the neurotrophin brain-derived neurotrophic factor (BDNF). Other candidate activity signals include changes in glutamate receptor (GluR) activation, changes in firing rate, or changes in the average level of postsynaptic depolarization. Here we combined elevated KCl (3-12 mm) with ionotropic receptor blockade to dissociate postsynaptic depolarization from receptor activation. Chronic (48 hr) depolarization, ranging between -62 and -36 mV, parametrically reduced the quantal amplitude of excitatory synapses in a BDNF-independent manner. This effect of depolarization did not depend on AMPA, NMDA, or GABA(A) receptor signaling, action-potential generation, or metabotropic GluR activation. Together with previous work, these data suggest that there are two independent signals that regulate activity-dependent synaptic scaling in pyramidal neurons: low levels of BDNF cause excitatory synapses to scale up in strength, whereas depolarization causes excitatory synapses to scale down in strength.

Neuroprotective properties of 17beta-estradiol, progesterone, and raloxifene in MPTP C57Bl/6 mice.

Previous work from our laboratory showed prevention of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) induced dopamine depletion in striatum of C57Bl/6 mice by 17beta-estradiol, progesterone, and raloxifene, whereas 17alpha-estradiol had no effect. The present study investigated the mechanism by which these compounds exert their neuroprotective activity. The hormonal effect on the dopamine transporter (DAT) was examined to probe the integrity of dopamine neurons and glutamate receptors in order to find a possible excitotoxic mechanism. Drugs were injected daily for 5 days before MPTP (four injections, 15 mg/kg ip at 2-h intervals) and drug treatment continued for 5 more days. MPTP induced a decrease of striatal DAT-specific binding (50% of control) and DAT mRNA in the substantia nigra (20% of control), suggesting that loss of neuronal nerve terminals was more extensive than cell bodies. This MPTP-induced decrease of striatal [(125)I]RTI-121 specific binding was prevented by 17beta-estradiol (2 microg/day), progesterone (2 microg/day), or raloxifene (5 mg/kg/day) but not by 17alpha-estradiol (2 microg/day) or raloxifene (1 mg/kg/day). No treatment completely reversed the decreased levels of DAT mRNA in the substantia nigra. Striatal [(125)I]RTI-121 specific binding was positively correlated with dopamine concentrations in intact, saline, or hormone-treated MPTP mice. Striatal NMDA-sensitive [(3)H]glutamate or [(3)H]AMPA specific binding remained unchanged in intact, saline, or hormone-treated MPTP mice, suggesting the unlikely implication of changes of glutamate receptors in an excitotoxic mechanism. These results show a stereospecific neuroprotection by 17beta-estradiol of MPTP neurotoxicity, which is also observed with progesterone or raloxifene treatment. The present paradigm modeled early DA nerve cell damage and was responsive to hormones.

Glutamic acid decarboxylase and glutamate receptor changes during tolerance and dependence to benzodiazepines.

Protracted administration of diazepam elicits tolerance, whereas discontinuation of treatment results in signs of dependence. Tolerance to the anticonvulsant action of diazepam is present in an early phase (6, 24, and 36 h) but disappears in a late phase (72-96 h) of withdrawal. In contrast, signs of dependence such as decrease in open-arm entries on an elevated plus-maze and increased susceptibility to pentylenetetrazol-induced seizures were apparent 96 h (but not 12, 24, or 48 h) after diazepam withdrawal. During the first 72 h of withdrawal, tolerance is associated with changes in the expression of GABA(A) (gamma-aminobutyric acid type A) receptor subunits (decrease in gamma(2) and alpha(1); increase in alpha(5)) and with an increase of mRNA expression of the most abundant form of glutamic acid decarboxylase (GAD), GAD(67). In contrast, dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor GluR1 subunit mRNA and cognate protein, which are normal during the early phase of diazepam withdrawal, increase by approximately 30% in cortex and hippocampus in association with the appearance of signs of dependence 96 h after diazepam withdrawal. Immunohistochemical studies of GluR1 subunit expression with gold-immunolabeling technique reveal that the increase of GluR1 subunit protein is localized to layer V pyramidal neurons and their apical dendrites in the cortex, and to pyramidal neurons and in their dendritic fields in hippocampus. The results suggest an involvement of GABA-mediated processes in the development and maintenance of tolerance to diazepam, whereas excitatory amino acid-related processes (presumably via AMPA receptors) may be involved in the expression of signs of dependence after withdrawal.

Changes of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors in layer v of epileptogenic, chronically isolated rat neocortex

In vivo chronic partial isolation of neocortical islands results in epileptogenesis that involves pyramidal neurons of layer V. To test whether an alteration in glutamate receptors might contribute to the epileptiform activity, we analysed the time-course of light microscopic changes in expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors using subunit-specific antibodies. The isolation caused a rapid down-regulation of immunoreactivity for GluR1 and GluR2/3 subunits in deep layer V pyramidal neurons within the neocortical island which was evident 24 h post-lesion, and within three days was reduced to about 40–60% of the control level. Many pyramidal cells in deep layer V completely lacked GluR2. Between one and four weeks of survival, down-regulation of GluR2/3 and GluR2 involved the majority of pyramidal layer V neurons, except for cells in the upper part of layer V, and those within narrow areas of all sub-laminae of layer V (“micro-islands”). Initial down-regulation was also observed one to three days post-lesion for subunits 1 and 2 of the N-methyl- d-aspartate receptor, but in contrast to GluR2/3 immunoreactivity, NMDAR2A/B immunoreactivity was enhanced three weeks post-lesion. The present data provide evidence for plastic changes in glutamate receptors in neurons of partially isolated neocortical island. A sub-population of layer V neurons remains relatively unaffected, and would presumably be capable of generating fast glutamatergic synaptic potentials necessary for the development of synchronous epileptiform activity.

Neuroplasticity in specific limbic system circuits may mediate specific kindling induced changes in animal affect—implications for understanding anxiety associated with epilepsy

In two complementary experiments, we studied the effects of low frequency stimulation (LFS) of the amygdala on behavioral effects of kindling in rats and cats. These studies tested the hypothesis that kindling induced long term potentiation (KLTP) in amygdala circuits underlies interictal behavioral change. Since LFS can depotentiate LTP, it was predicted that LFS should both depotentiate KLTP and reverse behavioral effects of kindling. In cats, the effects of LFS on KLTP of amygdala efferents was studied, and related to behavioral effects. Partial ventral hippocampal kindling in cats and right amygdala kindling in rodents lastingly increased defensive response to rats in cats, and anxiety-like behavior (ALB) in the elevated plus-maze in rats. In addition, partial kindling reduced predatory attack behavior in cats independent of its effects on defensive response. Partial kindling also induced KLTP of amygdala efferent transmission to ventromedial hypothalamus (VMH) and periaqueductal gray (PAG) in left and right hemispheres. Depotentiation of amygdala efferent KLTP by bilateral amygdala LFS selectively reduced KLTP in right amygdala efferents. At the same time, defensive behavior, but not attack behavior, was returned to levels seen prior to partial kindling. Defensiveness returned to post kindling levels between 44 and 76 days after LFS. At the same time, LTP was restored in the right Amygdalo-PAG pathway only. These findings suggest that lasting change in affect produced by kindling depends on LTP of right amygdala efferent transmission to PAG, replicating studies of the effects of FG-7142 on brain and behavior in the cat. The findings suggest further that the spectrum of behavioral changes produced by partial kindling are dependent on changes in a variety of neural circuits, and that amygdala efferent transmission changes are responsible for changes in defensive behavior, but not attack behavior. Effects of LFS were not due to damage, as thresholds to evoke amygdala efferent response were unchanged. Other data suggest KLTP and depotentiation in right Amygdalo-PAG may reflect changes in glutamate receptor density/synapse number. Kindling effects on rat ALB persisted for at least 1 month. Bilateral but not unilateral amygdala LFS reversed the effects of kindling on risk assessment in the plus maze for at least 3 weeks. Bilateral LFS also reversed the effects of kindling on open arm exploration, but effects were shorter lived, appearing at 1 day but not 3 weeks after kindling and LFS. These findings are consistent with other studies which suggest that amygdala neuroplasticity in separable amygdala circuits mediates lasting changes in open arm avoidance and risk assessment. Taken together, the findings of both studies support the hypothesis that a form of LTP of specific amygdala circuits underlies lasting changes in affect produced by limbic kindling. Clinical implications of these findings are discussed.

Unilateral destruction of the medial agranular cortical projection zone in the dorsocentral striatum produces severe neglect in rats

Physiological studies have indicated that neglect and recovery from neglect induced by unilateral lesions of the medial agranular cortex (AGm) in rats is correlated with reductions of metabolic activity and plastic changes in glutamate receptors in the dorsolateral quadrant of the striatum. The dorsolateral quadrant of the striatum encompasses the dorsocentral striatum (DCS), the projection zone of AGm efferents. Although these changes in function in the dorsolateral striatum are correlated with neglect and behavioral recovery, direct evidence for the role of this region is lacking. In the present study, we examined the behavioral effects of unilateral destruction of the DCS on neglect, as assessed by orientation to visual, auditory, and tactile stimulation. Subjects received a unilateral N-methyl-d-aspartate lesion of the DCS, a control lesion in a laterally adjacent site in the striatum, or an injection of vehicle into the DCS. The subjects were tested for neglect three times per week for 3 weeks by rating the magnitude of orientation to visual, tactile, and auditory stimulation. The results indicate that the DCS group demonstrated significantly more severe neglect than did the lesion control group or the vehicle controls. The results suggest that the DCS is part of a system for appropriate orientation to sensory stimuli in rats and that the DCS may play a pivotal role in behavioral recovery from neglect produced by AGm lesions.

Long-term effects of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate and 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione in the rat basal ganglia: calcification, changes in glutamate receptors and glial reactions

Previous data from our laboratory indicate that 25 mM ibotenic acid induces intracellular calcifications in the rat basal forebrain. Because of the lack of specificity of ibotenic acid for a glutamate receptor subtype, a dose–response study with α-amino-3-hydroxy-5-methyl-4-isoxazole propionate was undertaken and calcified areas (identified with Alizarin Red staining) as well as astro- and microglial reactions (by autoradiography with [ 3H]lazabemide and [ 3H]Ro 5-4864) were quantified at one month post-lesion. α-Amino-3-hydroxy-5-methyl-4-isoxazole propionate administered into the globus pallidus induced, in a dose-dependent manner, the formation of calcium deposits and the activation of both glial cells, the microglial reaction being particularly robust. From this study, a dose of 5.4 mM α-amino-3-hydroxy-5-methyl-4-isoxazole propionate was selected for further experiments. [ 3H]α-amino-3-hydroxy-5-methyl-4-isoxazole propionate, [ 3H]dizocilpine maleate and [ 3H]PN 200-110 binding in vitro were performed to assess autoradiographically whether the tissue damage was associated with changes in glutamate receptors and calcium channel binding sites. In the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate-treated animals, the specific binding of [ 3H]α-amino-3-hydroxy-5-methyl-4-isoxazole propionate was significantly reduced by 28% in the lesioned ventral pallidum, whereas it was unchanged in the globus pallidus and substantia innominata. In these three nuclei, calcifications developed and an increase in both glial markers was measured. In contrast, the binding of [ 3H]PN 200-110 and [ 3H]dizocilpine maleate were unaffected. Co-injection of 15 mM 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione, a selective α-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptor antagonist, prevented the formation of calcium concretions, the microglial reaction and the decrease in [ 3H]α-amino-3-hydroxy-5-methyl-4-isoxazole propionate binding but it failed to inhibit totally the astroglial reaction induced by α-amino-3-hydroxy-5-methyl-4-isoxazole propionate. This may suggest that the microglial reaction and calcification take place through different mechanisms from the astrogliosis associated with the neuronal loss. In conclusion, acute administration of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate in the rat globus pallidus elicits a dose-dependent calcification process associated with a chronic reaction of astrocytes and microglia. α-amino-3-hydroxy-5-methyl-4-isoxazole propionate-induced injury is accompanied by a slight reduction of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors in the ventral pallidum, whereas the binding of N-methyl- d-aspartate and L-type calcium channels receptors remains unchanged in any lesioned nucleus.

Roles of neuroactive amino acids in ammonia neurotoxicity.

Many neurologic disorders are related to congenital or acquired hyperammonemia (HA). Advanced symptoms of HA range from seizures in acute stages to stupor and coma in more chronic conditions, manifesting variable imbalance between the inhibitory and excitatory neurotransmission. Evidence obtained with the use of experimental HA models suggests that acute neurotoxic effects of ammonia are mediated by overactivation of ionotropic glutamate (GLU) receptors, mainly the N-methyl-D-aspartate (NMDA) receptors, and to a lesser degree the KA/AMPA receptors. NMDA receptor-mediated neurotoxicity may be potentiated by impaired control of their function by metabotropic GLU receptors, which are inactivated by ammonia. Prolonged overactivation of the NMDA receptors upon extended ammonia exposure causes their downregulation. The GLU receptor changes may be related to their excessive exposure to extrasynaptic GLU. Ammonia promotes GLU accumulation in the extrasynaptic space by enhancing its release from neurons, and/or by decreasing its reuptake to the nerve endings and astrocytes, where the effect results from inactivation (downregulation) of the astrocytic glutamate transporter GLT1. Excitotoxic effects of ammonia are augmented by increased synthesis of nitric oxide (NO), which is associated with NMDA receptor activation and/or increased synaptic transport of arginine (ARG). A shift toward neural inhibition is promoted by positive modulation of the gamma-aminobutyric acid (GABA)ergic tone resulting from excessive accumulation in the brain of endogenous central benzodiazepine receptor agonists, and from upregulation of astrocytic peripheral benzodiazepine receptors leading to elevated levels of prognenelone-derived neurosteroids, which positively modulate the GABA(A) receptor complex. Inhibitory neurotransmission may also be favored by enhanced release from astrocytes of an inhibitory amino acid, taurine.

Age-related functional changes of the glutamate receptor channels in rat Meynert neurones.

1. The developmental changes of glutamate receptors (GluRs) in acutely dissociated rat Meynert neurones were investigated using the conventional whole cell and nystatin perforated patch recording modes under voltage-clamp conditions. 2. The neurones became less responsive to N-methyl-D-aspartic acid (NMDA) with age, most dramatically between 1 day and 2 weeks, while the responses to kainic acid (KA) and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) gradually increased. The metabotropic GluR response appeared a few days after birth, but thereafter no further change was observed. 3. The decrease in the NMDA response during postnatal development was due to an abrupt reduction in the number of receptors without affecting the affinity, voltage-dependent Mg2+ blockade or high Ca2+ permeability (PCa/PCs approximately 7.0). 4. PCa/PCs in the presence of KA decreased from 2.8 in the 1-day-old (1D) rat neurones to 1.1 and 0.44 in the 2-week-old (2W) and 6-month-old (6M) rat neurones, respectively. The concentration-response relationship for KA shifted to the left with age. The KA response was not affected by NS-102, a KA-selective antagonist, thus indicating that the increased affinity of the receptor for the ligand resulted from the change in the AMPA receptor channel subunits. 5. The AMPA response in the presence of 10(-4) M cyclothiazide showed a change in the inward rectifying current-voltage relationship with age. The KA response was strongly cross-desensitized by the addition of AMPA and was also blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), whereas a rapid desensitization of the AMPA response was removed in a concentration-dependent manner by cyclothiazide. These results indicate that the non-NMDA receptor channels are assembled from the subunits of the AMPA receptor family without the GluR-2 subunit, thus resulting in a high Ca2+ permeability. 6. The L-glutamate (Glu)-induced responses were more sensitive to DL-2-amino-5-phosphonopentanoic acid (APV) in the 1D rat neurones than in the adult rat neurones. 7. Both NMDA and KA raised the intracellular Ca2+ concentration ([Ca2+]i) in all neurones of 1D, 2W and 6M rats, though the charybdotoxin-sensitive Ca(2+)-activated K+ current (IK(Ca)) did not appear in the 1D rat neurones. An age-related prolongation of both IK(Ca) decay and [Ca2+]i clearance was also seen after the removal of KA. 8. It was thus concluded that the age-related changes of ionotropic receptors appear to play a key role in the activities of immature and mature rat Meynert cholinergic neurones. The KA-induced IK(Ca), which developed with ageing, may thus function as one of the negative feedback systems, and thereby prevent excess cell excitation and neural damage, especially in adult rats.

Alteration in NMDA receptor subunit mRNA expression in vulnerable and resistant regions of in vitro ischemic rat hippocampal slices

Brain insults, including cerebral ischemia, can alter glutamate receptor subunit expression in vulnerable neurons. Understanding these post-ischemic changes in glutamate receptors could enhance our ability to identify specific, novel neuroprotective compounds. Reverse transcription-polymerase chain reaction (RT-PCR) amplification was used to quantify the altered expression of the N-methyl-d-aspartate (NMDA) NR2A, NR2B and NR2C subunits relative to one another in rat hippocampal slices in resistant and vulnerable regions following in vitro oxygen-glucose deprivation. Ninety minutes after re-oxygenation and return to 10 mM glucose, there was a significant increase in the expression of NR2C relative to NR2B and NR2A in the slice as a whole, as well as in the selectively vulnerable CA1 region and the resistant CA3 and dentate gyrus regions.

Involvement of hippocampal AMPA glutamate receptor changes and the cAMP/protein kinase A/CREB-P signalling pathway in memory consolidation of an avoidance task in rats.

Training in step-down inhibitory avoidance (0.3-mA footshock) is followed by biochemical changes in rat hippocampus that strongly suggest an involvement of quantitative changes in glutamate AMPA receptors, followed by changes in the dopamine D1 receptor/cAMP/ protein kinase A (PKA)/CREB-P signalling pathway in memory consolidation. AMPA binding to its receptor and levels of the AMPA receptor-specific subunit GluR1 increase in the hippocampus within the first 3 h after training (20-70%). Binding of the specific D1 receptor ligand, SCH23390, and cAMP levels increase within 3 or 6 h after training (30-100%). PKA activity and CREB-P levels show two peaks: a 35-40% increase 0 h after training, and a second increase 3-6 h later (35-60%). The results correlate with pharmacological findings showing an early post-training involvement of AMPA receptors, and a late involvement of the D1/cAMP/PKA/CREB-P pathway in memory consolidation of this task.

Changes in glutamate receptors, c- fos mRNA expression and activator protein-1 (AP-1) DNA binding activity in the brain of phenobarbital-dependent and -withdrawn rats

We studied changes in glutamate receptors, expression of immediate early genes, and AP-1 DNA binding activity in the brains of phenobarbital (PB)-dependent and -withdrawn rats to investigate the possible involvement of activation of glutamate receptors in PB withdrawal syndrome. PB-dependent rats were prepared by feeding drug-admixed food for 5 weeks. Autoradiographic analysis showed that binding of [ 3H](+)-5-methyl-10,11-dihydro-5 H-dibenzo[ a, d]cyclohepten-5,10-imine (MK-801), an antagonist of N-methyl- d-aspartic acid (NMDA) receptors, increased significantly in the cerebral cortices of PB-dependent and 24-h-withdrawn rats. However, [ 3H]MK-801 binding in the hippocampus and [ 3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and [ 3H]kainic acid binding in the hippocampus and cerebral cortex were essentially unchanged in both groups. PB withdrawal seizures were followed by increased expression of c- fos mRNA in the hippocampus and cerebral cortex and of c- jun mRNA in the cerebral cortex. The induction of c- fos and c- jun mRNA was suppressed by administration of MK-801. Furthermore, PB withdrawal enhanced AP-1 DNA binding activity in the brain. The present findings suggest functional enhancement of glutamatergic neurotransmission during the development of PB withdrawal syndrome.

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

We studied the immunohistochemical localization of the glutamate receptors (GluR-1, -2, and -3,) in the developing rat cerebral cortex and hippocampus using antibodies to GluR1 and to an epitope common to GluR2 and GluR3 (GluR2/3) subunits. In the cerebral cortex, GluR1 immunoreactivity appeared in the neurons from postnatal day (PND) 0, increased with maturation, was highest at PND 10, decreased until PND 30, and thereafter remained at the same level as on PND 0. GluR2/3 immunoreactivity appeared earlier in scattered neurons on embryonal day (ED) 18, increased with maturation and reached a peak between PND 10 and PND 15, after which the immunoreactivity gradually decreased and reached a plateau at PND 30. For both GluR1 and GluR2/3, some of the pyramidal neurons showed intense staining. In the pyramidal layers of the hippocampus, GluR1 and GluR2/3 immunoreactivity was found in all the pyramidal neurons of the CA1-4 area from ED 20. In the dentate gyrus of the hippocampus, GluR1 and GluR2/3 immunoreactivity was found in the neurons of the granule cells after PND 0. Immunoreactivity in the neurons of the subiculum was found after PND 5 and that of the polymorphic cell layers was found after PND 15-20. Our results indicate that the development of glutamate receptor subunits in the rat cerebral cortex and hippocampus is expressed in different spatial patterns and distinct temporal patterns throughout development and is scheduled during the early postnatal period, when synaptic plasticity or synaptic connection occurs in these regions.

Glutamate and Parkinson's disease.

Altered glutamatergic neurotransmission and neuronal metabolic dysfunction appear to be central to the pathophysiology of Parkinson's disease (PD). The substantia nigra pars compacta--the area where the primary pathological lesion is located--is particularly exposed to oxidative stress and toxic and metabolic insults. A reduced capacity to cope with metabolic demands, possibly related to impaired mitochondrial function, may render nigral highly vulnerable to the effects of glutamate, which acts as a neurotoxin in the presence of impaired cellular energy metabolism. In this way, glutamate may participate in the pathogenesis of PD. Degeneration of dopamine nigral neurons is followed by striatal dopaminergic denervation, which causes a cascade of functional modifications in the activity of basal ganglia nuclei. As an excitatory neurotransmitter, glutamate plays a pivotal role in normal basal ganglia circuitry. With nigrostriatal dopaminergic depletion, the glutamatergic projections from subthalamic nucleus to the basal ganglia output nuclei become overactive and there are regulatory changes in glutamate receptors in these regions. There is also evidence of increased glutamatergic activity in the striatum. In animal models, blockade of glutamate receptors ameliorates the motor manifestations of PD. Therefore, it appears that abnormal patterns of glutamatergic neurotransmission are important in the symptoms of PD. The involvement of the glutamatergic system in the pathogenesis and symptomatology of PD provides potential new targets for therapeutic intervention in this neurodegenerative disorder.

Glial cells in the mouse hippocampus express AMPA receptors with an intermediate Ca2+ permeability.

Recently, we could demonstrate the 'complex' glial cells in mouse hippocampal slices express glutamate receptor changes of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate subtypes. In the present study, we further characterized this glial receptor. Since voltage-clamp control is imperfect and diffusion barriers hinders the quantitative analysis of the receptor currents in situ, the patch-clamp technique was applied to glial cells acutely isolated from the mouse hippocampal CA1 stratum radiatum subregion. A concentration-clamp technique was used which enabled very fast exchange of the extracellular solutions. Thus, it was possible to characterize the transient receptor currents with high time resolution. Application of L-glutamate, AMPA and L-homocysteate induced rapidly activating and fast desensitizing receptor currents in the suspended glial cells. In contrast, kainate induced non-desensitizing currents. The corresponding dose-response curve revealed a half-maximum of current activation at 350 microM. The current/voltage relationship of the kainate-evoked response was linear with a reversal potential at approximately 9 mV. Analysis of the reversal potential in solutions containing high concentrations of CaCl2 confirmed earlier in situ data by demonstrating significant Ca2+ permeability of the glial glutamate receptor channels in the hippocampus. The kainate-induced receptor currents were markedly increased by cyclothiazide, a substance which selectively potentiates glutamate receptors of the AMPA subtype. We conclude that glial cells of the juvenile hippocampus mainly express heteromeric high-affinity AMPA receptors. Most probably, the receptor channels are assembled from the low Ca(2+)-permeable glutamate receptor-2 subunit together with Ca(2+)-permeable AMPA-preferring subunits.