Long-term effects of alpha-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 alpha-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. alpha-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 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate was selected for further experiments. [3H]alpha-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 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-treated animals, the specific binding of [3H]alpha-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 alpha-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]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate binding but it failed to inhibit totally the astroglial reaction induced by alpha-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 alpha-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. alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-induced injury is accompanied by a slight reduction of alpha-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.     

Differential expression of AMPA receptor subunits in substance P receptor-containing neurons of the caudate-putamen of rats

Previous evidence has suggested that glutamate-driving neurotransmission and glutamate-excitotoxicity are modulated by substance P in the basal ganglia, but the assembly of glutamate receptors mediating this process remains to be delineated. By using a double immunofluorescence, cellular expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits (GluR1-4) in substance P receptor (SPR)-containing neurons was examined in the striatum of rats. It revealed that distribution of SPR-immunoreactive neurons completely overlapped with that of GluR1, 2, 3 or 4-immunoreactive neurons in the caudate-putamen. Neurons showing both SPR and AMPA receptor subunits (except of GluR3)-immunoreactivity were observed: all (100%) of SPR-positive neurons displayed GluR1-, GluR2- or GluR4-immunoreactivity, and the double-labeled neurons constituted about 33, 3 or 29% of total GluR-positive ones. In contrast, the neurons exhibiting both SPR- and GluR3-immunoreactivity were not detected, though numerous GluR3-positive neurons were still distributed in the caudate-putamen regions. Co-localization of SPR and distinct AMPA receptor subunits in the striatal neurons has provided a basis for functional modulation of neuronal APMA receptors by substance P in the caudate-putamen of rodents. Taken together with previous observations, this study has also suggested that, through interaction with AMPA receptors composed of subunits 1, 2 and 4, substance P or neurokinin peptides may play important roles in regulating neuronal properties and protecting neurons from excitotoxicity in the basal ganglia of mammals.     

Changes of alpha-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 alpha-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 24h 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.     

Quisqualate agonists occlude kainate-induced current in cultured striatal neurons.

We employed the whole cell patch-clamp technique to examine the ionic currents induced via activation of kainate/quisqualate receptors on striatal neurons in primary culture when N-methyl-D-aspartate receptors were blocked by selective antagonists. Bath perfusion of 10 microM-1 mM each of quisqualate, glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (a selective quisqualate agonist) or kainate, induced only a sustained current, but more rapid application by pressure ejection of each of the first three agonists (but not kainate) also activated a rapidly desensitizing current. The current induced by a near-saturating concentration of kainate (1 mM) was, on average, 16-fold larger than the maximum sustained current induced by quisqualate (10 microM), or 7.5-fold larger than that induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (100 microM) or glutamate (100 microM). When kainate (100 microM-10 mM) was co-applied with each of the agonists (1 microM-1 mM), the sustained current was not the algebraic sum of the currents activated by kainate or the other agonist alone; rather, the kainate-induced current was increasingly occluded by co-application with increasing concentrations of another agonist. The potency to occlude kainate-induced current had a rank order of quisqualate greater than alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate approximately glutamate; although at sufficiently high concentrations all three agonists could occlude the kainate-induced current completely. When kainate and quisqualate were co-applied during the continued presence of quisqualate, the onset of the kainate-induced sustained current was dramatically slowed. However, the steady-state occlusion by quisqualate could be abolished when the ratio kainate to quisqualate was raised to 100:1; therefore, the occlusion appears to involve a competition between kainate and quisqualate at some shared receptor binding sites which have a higher affinity for quisqualate than kainate.     

The mRNA expression of AMPA type glutamate receptors in the primary motor cortex of patients with amyotrophic lateral sclerosis: an in situ hybridization study

The pathogenetic mechanisms leading to progressive neurodegeneration in amyotrophic lateral sclerosis (ALS) have not been fully elucidated. One possible factor responsible for the selective motor neuron loss in the motor cortex, brain stem and spinal cord is glutamate-induced excitotoxicity particularly mediated via alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) type glutamate receptors. Data about the expression pattern of AMPA receptors in the primary motor cortex are lacking so far. The pharmacological and physiological properties of AMPA receptors are defined by the heteromeric composition of the four different receptor subunits. Different expression patterns of these subunits at motor neurons may provide a molecular basis for increased vulnerability to excitotoxic damage. Using in situ hybridization histochemistry we did not detect any significant differences in the distribution of AMPA receptor mRNA in the motor cortex of ALS patients compared to controls.     

GluR2-R4 AMPA subunit study in rat vestibular nuclei after unilateral labyrinthectomy: an in situ and immunohistochemical study

In the present investigation, we address the question of whether the expression of GluR2-R4 subunits mRNAs and GluR2 and GluR4 subunits protein of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptors are modulated in the vestibular nuclei following unilateral labyrinthectomy. Specific GluR2-R4 radioactive oligonucleotides were used to probe sections of rat vestibular nuclei according to in situ hybridization methods. The signal was detected by means of film or emulsion photography. GluR2 and GluR4 subunit expression were also measured in control and operated rats by use of specific monoclonal GluR2 and GluR4 antibodies. Animals were killed at different stages following the lesion: 1, 3 or 8 days for the in situ hybridization study and 4 and 8 days for the immunohistochemical study.In normal animals, several brainstem regions including the lateral, medial, superior and inferior vestibular nuclei expressed all the GluR2, GluR3 and GluR4 subunit mRNAs. Moreover, numerous vestibular nuclei neurons are endowed with AMPA receptors containing the GluR2 and the GluR4 subunits.In unilaterally labyrinthectomized rats, no asymmetry could be detected on autoradiographs between the two medial vestibular nuclei probed with the GluR2 and the GluR4 oligonucleotide probes regardless of the delay following the lesion. However, compared to control, a bilateral decrease (-22%) in GluR3 gene expression was observed in the medial vestibular nuclei 3 days after the lesion followed by a return to normal at day 8 post-lesion. No significant asymmetrical changes in the density of GluR2- and GluR4-immunopositive cells could be detected between the intact and deafferented sides in any part of the vestibular nuclear complex and at any times (day 4 or day 8) following the lesion.Our data show that the removal of glutamatergic vestibular input induced an absence of modulation of GluR2 and GluR4 gene and subunits expression. This demonstrates that GluR2 and GluR4 expression do not play a role in the recovery of the resting discharge of the deafferented medial vestibular nuclei neurons and consequently in the functional restoration of the static postural and oculomotor deficits. The functional role of the slight and bilateral GluR3 mRNA decrease in the vestibular nuclei remains to be elucidated.     

Cellular and subcellular distribution of the amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit GluR2 in the rat dorsal vagal complex

Amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) type glutamate receptors are ligand gated ion channels made up of various combinations of four subunits termed GluR1-4. The GluR2 subunit controls several key features of the receptor including calcium permeability and inward rectification. In the present study, we analysed by immunocytochemistry the cellular and subcellular distribution of the GluR2 subunit in neurons of the dorsal vagal complex of the rat. GluR2 immunoreactivity was found both in the neuropile and in neuronal cell bodies. Perikaryal staining was strong in the dorsal motor nucleus of the vagus nerve and moderate in the medial part of the nucleus tractus solitarii as well as in the area postrema. The lateral part of the nucleus tractus solitarii was almost devoid of immunoreactivity except for the interstitial subnucleus which was filled with numerous strongly immunoreactive perikarya and large cell processes. Ultrastructural examination was carried out in the interstitial subnucleus. Peroxidase staining indicative of GluR2 immunoreactivity was observed in neuronal cell bodies and dendrites. No labeled axon terminal or glial cell body was found. Additional experiments performed using pre-embedding immunogold showed that most of the labeling in immunoreactive dendrites was intracytoplasmic.These results indicate that GluR2 immunoreactivity is differentially distributed among neurons in the dorsal vagal complex, thereby suggesting differences in the functional properties of AMPA receptors between neuronal populations. These results also suggest that AMPA receptors, at least those containing the GluR2 subunit, have no major role as presynaptic receptors within this region. Finally, they indicate the existence of large intracellular pools of GluR2 subunits within dendrites of immunoreactive neurons.     

Differential expression of AMPA receptor subunits in dopamine neurons of the rat brain: a double immunocytochemical study

We have examined the distribution of dopamine neurons expressing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits (glutamate receptors 1, 2/3 and 4) in the A8-A15 regions of the rat brain using double immunofluorescence. The distribution of glutamate receptor 1- or 2/3-like immunoreactive neurons completely overlapped that of tyrosine hydroxylase-like immunoreactive neurons in dopamine cell groups in the retrorubral field (A8), the substantia nigra (A9), the ventral tegmental area and the nucleus raphe linealis (A10), and the rostral hypothalamic periventricular nucleus (A14, A15). In the caudal hypothalamic periventricular nucleus (A11), arcuate nucleus (A12) and zona incerta (A13), the distribution was partially overlapping. Neurons double-labeled for tyrosine hydroxylase and glutamate receptor 1 or 2/3 immunoreactivities were, however, exclusively found in certain dopamine cell regions: in areas A14-A15, 85-88% of tyrosine hydroxylase-containing neurons expressed glutamate receptor 1 and 22-25% expressed glutamate receptor 2/3, while in areas A8-A10, 20-43% expressed glutamate receptor 1 and 63-84% expressed glutamate receptor 2/3. In contrast, the double-labeled neurons were hardly detected in the A11-A13 regions. No tyrosine hydroxylase-positive neurons displayed glutamate receptor 4 immunoreactivity, though a partially overlapping distribution of tyrosine hydroxylase- and glutamate receptor 4-immunopositive neurons was also seen in regions A8-10, A11 and A13.The present study has demonstrated the morphological evidence for direct modulation of dopamine neurons via AMPA receptors in rat mesencephalon and hypothalamus. This distribution may provide the basis for a selective dopamine neuron loss in neurodegenerative disorders, such as Parkinson's disease.     

Lack of protection from ischemic injury of monoamine oxidase B-deficient mice following middle cerebral artery occlusion

Recently, it has been shown that a single leucine-to-tyrosine mutation in the agonist binding domains of the homomerically expressed alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors GluR3 and GluR1 is sufficient to completely block receptor desensitization. In the present study we tested heteromeric subunit combinations of AMPA receptors to demonstrate that the block of desensitization afforded by this mutation is dominant in heteromeric subunit complexes containing the leucine-to-tyrosine mutation in at least one of the subunits. In addition, by comparing mutated, desensitization-deficient forms of unedited GluR1 and GluR1 edited at the Q/R-site of the ion pore we demonstrate that the desensitization properties of AMPA receptors are not linked to the editing state of the ion pore domain and thus are independent of the permeability properties of the ion channel.     

Decreased dendrite growth from cultured mouse cortical neurons surviving excitotoxic activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptors

During cerebral ischemia, massive glutamate release leads to cell death through ionotropic glutamate receptor activation. An early consequence of this excitotoxicity is dendrite injury, which can precede cell death. We therefore tested whether cells that survived an excitotoxic insult triggered by overactivation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate (KA) subtype of ionotropic glutamate receptors displayed altered dendrite growth. We demonstrate that 24 h exposure of cultured cortical neurons to AMPA or KA dramatically reduced dendrite growth from surviving neurons. AMPA or KA exposure decreased primary dendrite number and length, and also reduced dendritic branching. The AMPA/KA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the deleterious effect of AMPA and KA on dendrite growth. These results suggest that AMPA/KA receptor overactivation may contribute to dendritic injury from neurons that survive an ischemic insult.     

Polymorphisms in the GluR2 gene are not associated with amyotrophic lateral sclerosis

Excitotoxicity is thought to play a pathogenic role in amyotrophic lateral sclerosis (ALS). Excitotoxic motor neuron death is mediated through the Ca²⁺-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type of glutamate receptors and Ca²⁺ permeability is determined by the GluR2 subunit. We investigated whether polymorphisms or mutations in the GluR2 gene (GRIA2) predispose patients to ALS. Upon sequencing 24 patients and 24 controls no nonsynonymous coding variants were observed but 24 polymorphisms were identified, 9 of which were novel. In a screening set of 310 Belgian ALS cases and 794 healthy controls and a replication set of 3157 cases and 5397 controls from 6 additional populations no association with susceptibility, age at onset, or disease duration was observed. We conclude that polymorphisms in the GluR2 gene (GRIA2) are not a major contributory factor in the pathogenesis of ALS.     

Phosphorylation of Ca2+/calmodulin-dependent protein kinase type ii and the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (ampa) receptor in response to a threonine-devoid diet

The anterior piriform cortex (APC) functions as a chemosensor for indispensable amino acid deficiency and responds to this deficiency with increased activity, as indicated by observations including averaged evoked-potentials and c-fos expression in the APC. Little is known of the intracellular signaling mechanisms that mediate this deficiency-related increase in neuronal excitability, but previous studies have shown effects on intracellular Ca2+ in deficient APC slices in vitro. In the present study we hypothesized that indispensable amino acid deficiency increases intraneuronal Ca2+, resulting in autophosphorylation of calcium/calmodulin-dependent protein kinase type II (CaMKII) in vivo. Results demonstrated that phosphorylation levels of CaMKII (pCaMKII) in APC neurons increased at 20 and 40 min after a single meal of threonine-devoid diet. Phosphorylation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunit (GluR1) at the serine 831 (S831) site was modestly increased in the APC in response to a threonine-devoid meal. The GluR1 subunit also showed increased phosphorylation at the 845 (S845) site, suggesting additional signaling mechanisms. Although phosphorylation of CaMKII was sustained, phosphorylation of the GluR1 subunit returned to control levels by 40 min. These effects of amino acid deficiency did not occur throughout the brain as neither CaMKII nor GluR1 showed increased phosphorylation in the neocortex. These findings support the notion that calcium and glutamate signaling in the APC, but not throughout the brain, are triggered during early responses to amino acid deficiency. They also suggest that longer-term changes in APC neurons in response to such a deficiency may be mediated at least in part by CaMKII.     

Calcium-permeable AMPA receptors promote misfolding of mutant SOD1 protein and development of amyotrophic lateral sclerosis in a transgenic mouse model

Mutant Cu/Zn-superoxide dismutase (SOD1) protein aggregation has been suggested as responsible for amyotrophic lateral sclerosis (ALS), although the operative mediating factors are as yet unestablished. To evaluate the contribution of motoneuronal Ca2+-permeable (GluR2 subunit-lacking) alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors to SOD1-related motoneuronal death, we generated chat-GluR2 transgenic mice with significantly reduced Ca2+-permeability of these receptors in spinal motoneurons. Crossbreeding of the hSOD1G93A transgenic mouse model of ALS with chat-GluR2 mice led to marked delay of disease onset (19.5%), mortality (14.3%) and the pathological hallmarks such as release of cytochrome c from mitochondria, induction of cox2 and astrogliosis. Subcellular fractionation analysis revealed that unusual SOD1 species first accumulated in two fractions dense with neurofilaments/glial fibrillary acidic protein/nuclei and mitochondria long time before disease onset, and then concentrated into the former fraction by disease onset. All these processes for unusual SOD1 accumulation were considerably delayed by GluR2 overexpression. Ca2+-influx through atypical motoneuronal AMPA receptors thus promotes a misfolding of mutant SOD1 protein and eventual death of these neurons.     

N-methyl-D-aspartate receptor-dependent long-term potentiation in CA1 region affects synaptic expression of glutamate receptor subunits and associated proteins in the whole hippocampus

Long term potentiation in hippocampus, evoked by high-frequency stimulation, is mediated by two major glutamate receptor subtypes, alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate receptors and N-methyl-D-aspartate receptors. Receptor subunit composition and its interaction with cytoplasmic proteins constitute different pathways regulating synaptic plasticity. Here, we provide further evidence that N-methyl-D-aspartate receptor-mediated long term potentiation evoked at hippocampal CA1 region of rats induced by high-frequency stimulation of the Schaffer collateral-commissural pathway in vivo is not dependent on N-methyl-D-aspartate receptor subunit NR2B. Applying semi-quantitative immunoblotting, we found that in the whole tetanized hippocampus, synaptic expression of the N-methyl-D-aspartate and alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate receptor subunits (NR1, NR2A, glutamate receptor 1) and their associated partners, e.g. synaptic associated protein 97, postsynaptic density protein 95, alpha subunit of Ca2+/calmodulin-dependent protein kinase II, neuronal nitricoxide synthase, increased 180 min post-high-frequency stimulation. Moreover, phosphorylation of Ca2+/calmodulin-dependent protein kinase II at thr286 and glutamate receptor 1 at ser831 was increased 30 min post-high-frequency stimulation and blocked by N-methyl-D-aspartate receptor antagonists (AP-5 and MK-801). In sham group and controls, these changes were not observed. The expression of several other synaptic proteins (NR2B, glutamate receptors 2/3, N-ethylmaleimide sensitive factor) was not affected by long term potentiation induction. In hippocampal homogenates, the level of these proteins remained unchanged. These data indicate that N-methyl-D-aspartate receptor-dependent long term potentiation in CA1 region in vivo mainly affects the synaptic expression of glutamate receptor subunits and associated proteins in the whole hippocampus. The alteration of molecular aspects can play a role in regulating the long-lasting synaptic modification in hippocampal long term potentiation in vivo.     

Long-term changes in hippocampal n-methyl-D-aspartate receptor subunits following unilateral vestibular damage in rat

Previous studies have indicated that damage to the peripheral vestibular system results in dysfunction of hippocampal place cells and an impairment of spatial learning and memory. The aim of this study was to determine whether lesions of one vestibular labyrinth (unilateral vestibular deafferentation, UVD) result in changes in the expression of the NR1 and NR2A subunits of the N-methyl-D-aspartate (NMDA) receptor, and the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor, in subregions of the rat hippocampus (CA1, CA2/3 and the dentate gyrus) at 10 h or 2 weeks following UVD. Compared with sham surgery controls and anaesthetic controls, the expression of the NR1 subunit was significantly reduced in the ipsilateral CA2/3 region at 2 weeks post-UVD. The expression of the NR2A subunit was also significantly reduced in the ipsilateral CA2/3 and, to a smaller extent, in the contralateral CA2/3 region, at 2 weeks post-UVD. The only other change in NR2A expression was an increase in the ipsilateral CA1 at 10 h post-UVD. No other changes in NR1, NR2A or GluR2 expression were observed in any hippocampal subregion, at any time point, or in cortical tissue at any time point. These results suggest that UVD may result in long-term changes in NMDA receptor subunit expression in the rat hippocampus.     

Proteins interactions implicated in AMPA receptor trafficking: a clear destination and an improving route map

The mechanisms that regulate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), synthesis, transport, targeting and surface expression are of fundamental importance to understand the molecular basis of fast excitatory neurotransmission and synaptic plasticity in the mammalian CNS. An area of intense current interest is how AMPARs are directed to the correct locations in the neuron as and when required. This is a multi-layered problem, which involves complex spatio-temporal coordination of multiple protein interactions. Considerable progress has been achieved in identifying a number of proteins that bind directly to AMPAR subunits and the functional consequences of blocking some of these interactions have been determined. This review highlights recent developments in the field.     

Deficits in the expression of the NR2B subunit in the hippocampus of aged Fisher 344 rats

The NMDA receptor (NMDAR) has been implicated in the induction of LTP at hippocampal synapses, and has been proposed to play a significant role in the involvement of the hippocampus with learning and memory. Aged rats are known to have deficits in LTP, learning and memory. We tested the hypothesis that aged rats might have deficits in expression of NMDAR subunits. Aged rats have significantly lower levels of NR2B mRNA and protein compared to young animals. This complements a recent report which showed improved learning and memory in mice which overexpress NR2B. No changes were seen in either the mRNA or the protein levels of the NMDAR subunit NR2A, nor in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate receptor (AMPAR) subunit GluR2. Our data support the hypothesis that age related alterations in the expression of the NMDAR NR2B subunit might underlie deficits in LTP and learning and memory in aged animals.