PubMed Health⌕ Search

Biomedical subjects

Lynn A Raymond

Publications and source records attributed to Lynn A Raymond.

11 recordsLinked to original sources
PubMed2004

Enhanced striatal NR2B-containing N-methyl-D-aspartate receptor-mediated synaptic currents in a mouse model of Huntington disease.

Huntington disease (HD) is an inherited neurodegenerative disease caused by expansion of a polyglutamine tract near the N terminus of the protein huntingtin, leading to dramatic loss of striatal medium-sized spiny GABAergic projection neurons (MSNs). Evidence suggests overactivation of N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs) contributes to selective degeneration of MSNs in HD. Striatal MSNs are enriched in NR2B, and whole cell current and excitotoxicity mediated predominantly by the NR2B subtype of NMDARs is increased with expression of mutant huntingtin in transfected cell lines and striatal MSNs from mice models. To test whether synaptic NMDAR current is altered by mutant huntingtin expression, we recorded striatal MSN excitatory postsynaptic currents (EPSCs) evoked by stimulation of cortical afferents in corticostriatal slices from YAC72 mice and their wild-type (WT) littermates at age 21-31 days. The ratio of NMDAR- to AMPAR-mediated EPSC amplitude was significantly increased in YAC72 compared to WT mice. Furthermore, using a paired-pulse stimulation protocol as a measure of presynaptic glutamate release probability, we found no significant differences between YAC72 and WT striatal MSN responses. These data suggest selective potentiation of postsynaptic NMDAR activity at corticostriatal synapses in YAC72 mice. Measurements of EPSC decay kinetics, as well as the effects of NR2B-subtype selective antagonists and glycine concentration on EPSC amplitude, are consistent with the majority of postsynaptic NMDARs being triheteromers of NR1/NR2A/NR2B in both WT and YAC72 mice. Together with previous results, our data suggest that enhanced activity of NR2B-containing NMDARs is one of the earliest changes leading to neuronal degeneration in HD.

Animals↗
PubMed2004

Competition between phasic and asynchronous release for recovered synaptic vesicles at developing hippocampal autaptic synapses.

Developing hippocampal neurons in microisland culture undergo rapid and extensive transmitter release-dependent depression of evoked (phasic) excitatory synaptic activity in response to 1 sec trains of 20 Hz stimulation. Although evoked phasic release was attenuated by repeated stimuli, asynchronous (miniature like) release continued at a high rate equivalent to approximately 2.8 readily releasable pools (RRPs) of quanta/sec. Asynchronous release reflected the recovery and immediate release of quanta because it was resistant to sucrose-induced depletion of the RRP. Asynchronous and phasic release appeared to compete for a common limited supply of release-ready quanta because agents that block asynchronous release, such as EGTA-AM, led to enhanced steady-state phasic release, whereas prolongation of the asynchronous release time course by LiCl delayed recovery of phasic release from depression. Modeling suggested that the resistance of asynchronous release to depression was associated with its ability to out-compete phasic release for recovered quanta attributable to its relatively low release rate (up to 0.04/msec per vesicle) stimulated by bulk intracellular Ca2+ concentration ([Ca2+]i) that could function over prolonged intervals between successive stimuli. Although phasic release was associated with a considerably higher peak rate of release (0.4/msec per vesicle), the [Ca2+]i microdomains that trigger it are brief (1 msec), and with asynchronous release present, relatively few quanta can accumulate within the RRP to be available for phasic release. We conclude that despite depression of phasic release during train stimulation, transmission can be maintained at a near-maximal rate by switching to an asynchronous mode that takes advantage of a bulk presynaptic [Ca2+]i.

Animals↗
PubMed2004

Potentiation of NMDA receptor-mediated excitotoxicity linked with intrinsic apoptotic pathway in YAC transgenic mouse model of Huntington's disease.

Evidence suggests N-methyl-D-aspartate receptor (NMDAR) activation is involved in the degeneration of striatal medium-sized spiny neurons (MSNs) in Huntington's disease (HD). We tested the hypothesis that enhanced NMDAR-mediated excitotoxicity is mediated by the mitochondrial-associated apoptotic pathway in cultured MSNs from YAC transgenic mice expressing full-length huntingtin (htt) with a polyglutamine (polyQ) expansion of 46 or 72 (YAC46 or YAC72). NMDAR-mediated Ca(2+) transients and mitochondrial membrane depolarization were significantly increased in YAC compared to wild-type mice MSNs. Inhibitors of the mitochondrial permeability transition (mPT), cyclosporin A and bongkrekic acid, and coenzyme Q10 (an anti-oxidant involved in bioenergetic metabolism) dramatically diminished NMDA-induced cell death and eliminated genotypic differences. In YAC46 MSNs, NMDA stimulated significantly higher activation of caspase-3 and caspase-9 but not caspase-8, and NMDA-induced caspase-3 and -9 activation was markedly attenuated by cyclosporin A. Agents that improve mitochondrial function or inhibit the permeability transition may eliminate increased caspase activation and cell death associated with enhanced NMDAR activity in HD.

Animals↗
PubMed2004

Site within N-Methyl-D-aspartate receptor pore modulates channel gating.

N-methyl-d-aspartate-type glutamate receptors (NMDARs) are ligand-gated ion channels activated by coagonists glutamate and glycine. NMDARs play a critical role in synaptic plasticity and excitotoxicity, largely because of their high calcium permeability and slow deactivation and desensitization kinetics. NR1 is an obligate subunit in all NMDAR complexes, where it combines with NR2A, 2B, 2C, and/or 2D. NR1 binds glycine, and residue Asn598 in the re-entrant membrane loop M2 largely determines NMDAR calcium permeability. In contrast, NR2 subunits bind glutamate and contain regions that regulate receptor desensitization and deactivation. Here, we report that mutations of NR1(Asn598) in combination with wild-type NR2A, expressed in human embryonic kidney 293 cells, exhibit altered glycine-independent desensitization. In the absence of extracellular calcium, substitution of Arg for Asn598 (NR1R) slowed desensitization by 2- to 3-fold compared with wild-type NR1/NR2A, and glutamate-evoked peak current EC50 and deactivation rate were also affected. Replacement of Asn by Gln (NR1Q) produced two distinct rates of calcium- and glycine-independent desensitization. Moreover, in the presence of extracellular calcium, the voltage-dependent pore block by calcium for the NR1Q mutant mimicked the effects of the positively charged Arg at this site in NR1R on slowing desensitization and deactivation. A kinetic model of the NMDA receptor-channel suggests that these results can be explained by altered gating and not ligand binding. Our data increase understanding of the role that amino acids within the NMDAR pore play in channel gating.

Asparagine↗
PubMed2003

Developmental decrease in NMDA receptor desensitization associated with shift to synapse and interaction with postsynaptic density-95.

NMDA receptors (NMDARs) play a crucial role in neuronal development, synaptic plasticity, and excitotoxicity; therefore, regulation of NMDAR function is important in both physiological and pathological conditions. Previous studies indicate that the NMDAR-mediated synaptic current decay rate increases during development because of a switch in receptor subunit composition, contributing to developmental changes in plasticity. To test whether NMDAR desensitization also changes during development, we recorded whole-cell NMDA-evoked currents in cultured rat hippocampal neurons. We found that glycine-independent desensitization of NMDARs decreases during development. This decrease was not dependent on a switch in subunit composition or differential receptor sensitivity to agonist-, Ca2+-, or Zn2+-induced increase in desensitization. Instead, several lines of evidence indicated that the developmental decrease in desensitization was tightly correlated with synaptic localization of the receptor, suggesting that association of NMDARs with proteins selectively expressed at synapses in mature neurons might account for developmental alterations in desensitization. Accordingly, we tested the role of interactions between PSD-95 (postsynaptic density-95) and NMDARs in regulating receptor desensitization. Overexpression of PSD-95 reduced NMDAR desensitization in immature neurons, whereas agents that interfere with synaptic targeting of PSD-95, or induce movement of NMDARs away from synapses and uncouple the receptor from PSD-95, increased NMDAR desensitization in mature neurons. We conclude that synaptic localization and association with PSD-95 increases stability of hippocampal neuronal NMDAR responses to sustained agonist exposure. Our results elucidate an additional mechanism for differentially regulating NMDAR function in neurons of different developmental stages or the response of subpopulations of NMDARs in a single neuron.

Animals↗
PubMed2003

Disruption of the endocytic protein HIP1 results in neurological deficits and decreased AMPA receptor trafficking.

Huntingtin interacting protein 1 (HIP1) is a recently identified component of clathrin-coated vesicles that plays a role in clathrin-mediated endocytosis. To explore the normal function of HIP1 in vivo, we created mice with targeted mutation in the HIP1 gene (HIP1(-/-)). HIP1(-/-) mice develop a neurological phenotype by 3 months of age manifest with a failure to thrive, tremor and a gait ataxia secondary to a rigid thoracolumbar kyphosis accompanied by decreased assembly of endocytic protein complexes on liposomal membranes. In primary hippocampal neurons, HIP1 colocalizes with GluR1-containing AMPA receptors and becomes concentrated in cell bodies following AMPA stimulation. Moreover, a profound dose-dependent defect in clathrin-mediated internalization of GluR1-containing AMPA receptors was observed in neurons from HIP1(-/-) mice. Together, these data provide strong evidence that HIP1 regulates AMPA receptor trafficking in the central nervous system through its function in clathrin-mediated endocytosis.

Animals↗
PubMed2003

Functional NMDA receptor subtype 2B is expressed in astrocytes after ischemia in vivo and anoxia in vitro.

NMDA-type glutamate receptors play a critical role in neuronal synaptogenesis, plasticity, and excitotoxic death. Recent studies indicate that functional NMDA receptors are also expressed in certain glial populations in the normal brain. Using immunohistochemical methods, we detected the presence of the NMDA receptor 2B (NR2B) subunit of the NMDA receptor in neurons but not astrocytes in the CA1 and subicular regions of the rat hippocampus. However, after ischemia-induced neuronal death in these regions, double immunohistochemical labeling revealed that NR2B subunits colocalized with the astrocyte marker glial fibrillary acid protein and with NR1 subunits that are required for functional NMDA receptors. NR2B expression was first observed 3 d after ischemia and reached a peak at 28 d. At 56 d, only a few NR2B-expressing astrocytes were still present. In vitro, when postnatal hippocampal cultures were subjected to 5 min of anoxia, it resulted in NR2B expression on astrocytes in the glial feed layer. Imaging of intracellular calcium with postanoxic cultures and astrocytes isolated acutely from the ischemic hippocampus revealed a rise in intracellular [Ca2+] after stimulation with the specific agonist NMDA. The response could be blocked reversibly with the competitive antagonist 2-amino-5-phosphonovalerate and attenuated by the NR2B-selective antagonist ifenprodil. Control astrocytes were not responsive to NMDA but responded to glutamate. An understanding of the role of astrocytes that express functional NMDA receptors in response to ischemia may guide development of novel stroke therapies.

2-Amino-5-phosphonovalerate↗
PubMed2003

Role of NR2B-type NMDA receptors in selective neurodegeneration in Huntington disease.

N-Methyl-D-aspartate receptor (NMDAR)-mediated excitotoxicity has been proposed to play a role in Huntington disease (HD), caused by expansion of a polyglutamine tract in the protein huntingtin. HD is characterized by selective neurodegeneration most severely affecting striatal medium-sized spiny projection neurons (MSNs), where expression of the NMDAR subunit NR2B is increased relative to other NR2 subunits. Here, we review our data that NR2B-type NMDAR currents are selectively potentiated by mutant huntingtin in transfected non-neuronal cells and acutely dissociated striatal MSNs from the YAC72 transgenic mouse model of HD. As well, we report increased striatal MSN NMDAR-mediated synaptic currents in corticostriatal slice recordings from YAC72 compared with wild-type mice. This effect was associated with a larger NMDAR- to AMPAR-mediated current ratio, suggesting specific potentiation of postsynaptic NMDARs. Enhanced NMDAR current likely involves increased surface receptor numbers or activity, since we observed no differences between genotypes in striatal NR2B expression. Potentiation of NR2B-containing NMDAR current in striatal MSNs expressing mutant huntingtin may help explain the exquisite vulnerability of these neurons to degeneration in HD.

Animals↗
PubMed2002

Increased sensitivity to N-methyl-D-aspartate receptor-mediated excitotoxicity in a mouse model of Huntington's disease.

Previous work suggests N-methyl-D-aspartate receptor (NMDAR) activation may be involved in degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we show that these neurons are more vulnerable to NMDAR-mediated death in a YAC transgenic FVB/N mouse model of HD expressing full-length mutant huntingtin, compared with wild-type FVB/N mice. Excitotoxic death of these neurons was increased after intrastriatal injection of quinolinate in vivo, and after NMDA but not AMPA exposure in culture. NMDA-induced cell death was abolished by an NR2B subtype-specific antagonist. In contrast, NMDAR-mediated death of cerebellar granule neurons was not enhanced, consistent with cell-type and NMDAR subtype specificity. Moreover, increased NMDA-evoked current amplitude and caspase-3 activity were observed in transgenic striatal neurons. Our data support a role for NR2B-subtype NMDAR activation as a trigger for selective neuronal degeneration in HD.

Animals↗
PubMed2002

Differential regulation of synaptic and extra-synaptic NMDA receptors.

A variety of processes limit NMDA (N-methyl-D-aspartate) receptor (NMDAR) activity in response to agonist exposure, including rundown--the decline of peak current with repeated, sustained agonist application. Here we report that calcium and tyrosine phosphorylation differentially regulate rundown of synaptic versus extrasynaptic NMDAR-mediated current in rat hippocampal pyramidal neurons.

Animals↗