Modulation of NMDA receptors

Therapeutic potential of progesterone in spinal cord injury-induced neuropathic pain: At the crossroads between neuroinflammation and N-methyl-D-aspartate receptor

In recent decades, an area of active research has supported the notion that progesterone promotes a wide range of remarkable protective actions in experimental models of nervous system trauma or disease, and has also provided a strong basis for considering this steroid as a promising molecule for modulating the complex maladaptive changes that lead to neuropathic pain, especially after spinal cord injury. In this review, we intend to give the readers a brief appraisal of the main mechanisms underlying the increased excitability of the spinal circuit in the pain pathway after trauma, with particular emphasis on those mediated by the activation of resident glial cells, the subsequent release of proinflammatory cytokines and their impact on N-methyl-D-aspartate receptor function. We then summarize the available preclinical data pointing to progesterone as a valuable repurposing molecule for blocking critical cellular and molecular events that occur in the dorsal horn of the injured spinal cord and are related to the development of chronic pain. Since the treatment and management of neuropathic pain after spinal injury remains challenging, the potential therapeutic value of progesterone opens new traslational perspectives to prevent central pain.

Commemorating John F. MacDonald and the Art of Being a Mentor

John F. MacDonald was a close friend and mentor whose life was ended far too soon on April 22, 2014. To those who knew him, John was an endearing blend of fiery Scotsman, compassionate socialist, dedicated family man, and tireless investigator. Those close to him valued his loyalty and friendship, relished his biting wit, and puzzled at his self-deprecating manner. His career spanned a remarkable period of discovery from the early identification of excitatory amino acid, to the molecular cloning and characterization of glutamate receptors and the elucidation of mechanisms responsible for regulating their function. A true pioneer in each of these areas, John's research has had a lasting impact on our understanding of excitatory synaptic transmission and its plasticity. Our intent in commemorating John's work is to focus on some notable discoveries that highlight the impact and innovative aspects of John's work. In doing so, we also wish to highlight just how greatly our understanding of the glutamate transmitter systems has advanced since the late 1970s, when John first launched his independent neuroscience career.

Casein kinase II inhibition reverses pain hypersensitivity and potentiated spinal N-methyl-D-aspartate receptor activity caused by calcineurin inhibitor

Clinically used calcineurin inhibitors, including tacrolimus (FK506) and cyclosporine A, can induce calcineurin inhibitor-induced pain syndrome (CIPS), which is characterized as severe pain and pain hypersensitivity. Increased synaptic N-methyl-D-aspartate receptor (NMDAR) activity in the spinal dorsal horn plays a critical role in the development of CIPS. Casein kinase II (CK2), a serine/threonine protein kinase, can regulate synaptic NMDAR activity in the brain. In this study, we determined whether spinal CK2 is involved in increased NMDAR activity and pain hypersensitivity caused by systemic administration of FK506 in rats. FK506 treatment caused a large increase in the amplitude of NMDAR-mediated excitatory postsynaptic currents (EPSCs) evoked by primary afferent stimulation and in the frequency of miniature EPSCs of spinal dorsal horn neurons. CK2 inhibition with either 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) or 4,5,6,7-tetrabromobenzotriazole (TBB) completely normalized the amplitude of evoked NMDAR-EPSCs of dorsal horn neurons in FK506-treated rats. In addition, DRB or TBB significantly attenuated the amplitude of NMDAR currents elicited by puff application of N-methyl-D-aspartate to dorsal horn neurons in FK506-treated rats. Furthermore, treatment with DRB or TBB significantly reduced the frequency of miniature EPSCs of spinal dorsal horn neurons increased by FK506 treatment. In addition, intrathecal injection of DRB or TBB dose-dependently reversed tactile allodynia and mechanical hyperalgesia in FK506-treated rats. Collectively, our findings indicate that CK2 inhibition abrogates pain hypersensitivity and increased pre- and postsynaptic NMDAR activity in the spinal cord caused by calcineurin inhibitors. CK2 inhibitors may represent a new therapeutic option for the treatment of CIPS.

Reinforcement learning with modulated spike timing dependent synaptic plasticity

Spike timing-dependent synaptic plasticity (STDP) has emerged as the preferred framework linking patterns of pre- and postsynaptic activity to changes in synaptic strength. Although synaptic plasticity is widely believed to be a major component of learning, it is unclear how STDP itself could serve as a mechanism for general purpose learning. On the other hand, algorithms for reinforcement learning work on a wide variety of problems, but lack an experimentally established neural implementation. Here, we combine these paradigms in a novel model in which a modified version of STDP achieves reinforcement learning. We build this model in stages, identifying a minimal set of conditions needed to make it work. Using a performance-modulated modification of STDP in a two-layer feedforward network, we can train output neurons to generate arbitrarily selected spike trains or population responses. Furthermore, a given network can learn distinct responses to several different input patterns. We also describe in detail how this model might be implemented biologically. Thus our model offers a novel and biologically plausible implementation of reinforcement learning that is capable of training a neural population to produce a very wide range of possible mappings between synaptic input and spiking output.

The mGluR5 antagonist MPEP selectively inhibits the onset and maintenance of ethanol self-administration in C57BL/6J mice

RATIONALE

Many of the biochemical, physiological, and behavioral effects of ethanol are known to be mediated by ionotropic glutamate receptors. Emerging evidence implicates metabotropic glutamate receptors (mGluRs) in the biobehavioral effects of ethanol and other drugs of abuse, but there is little information regarding the role of mGluRs in the reinforcing effects of ethanol.

MATERIALS AND METHODS

Male C57BL/6J mice were trained to lever-press on a concurrent fixed ratio 1 schedule of ethanol (10% v/v) vs water reinforcement during 16-h sessions. Effects of mGluR1, mGluR2/3, and mGluR5 antagonists were then tested on parameters of ethanol self-administration behavior.

RESULTS

The mGluR5 antagonist MPEP (1-10 mg/kg, i.p.) dose-dependently reduced ethanol-reinforced responding but had no effect on concurrent water-reinforced responding. Analysis of the temporal pattern of responding showed that MPEP reduced ethanol-reinforced responding during peak periods of behavior occurring during the early hours of the dark cycle. Further analysis showed that MPEP reduced the number of ethanol response bouts and bout-response rate. MPEP also produced a 13-fold delay in ethanol response onset (i.e., latency to the first response) with no corresponding effect on water response latency or locomotor activity. The mGluR1 antagonist CPCCOEt (1-10 mg/kg, i.p.) or the mGluR2/3 antagonist LY 341495 (1-30 mg/kg, i.p.) failed to alter ethanol- or water-reinforced responding.

CONCLUSIONS

These data indicate that mGlu5 receptors selectively regulate the onset and maintenance of ethanol self-administration in a manner that is consistent with reduction in ethanol's reinforcement function.

Synaptic targeting of N-methyl-D-aspartate receptor splice variants is regulated differentially by receptor activity

The formation of postsynaptic clusters of various ligand-gated ion channels is regulated by receptor activity. Here we describe the developmental- and activity-dependent modification of N-methyl-D-aspartate (NMDA) receptor clustering in spinal cord neurons in vitro detected by immunofluorescence analysis using subunit and splice variant specific antibodies. NMDA receptors form synaptic and extrasynaptic clusters with sequential changes in subunit composition during in vitro development. During the first week of in vitro culture, a NR1 splice variant containing the C2-carboxy terminus and lacking the N1-cassette and the NR2B subunit are the prevailing components of receptor clusters at synaptic and extrasynaptic sites. After 3 weeks in culture (days in vitro [DIV] 22), the numbers of postsynaptic receptor clusters with N1-containing NR1 splice variants and NR2A subunits are upregulated. At DIV22, C2-specific clusters are abundant and are predominantly localized at postsynaptic sites, whereas the total number of C2'-clusters in dendrites is much lower and these clusters are localized mostly extrasynaptically. However, upon chronic inhibition of NMDA receptor activity in DIV8 and DIV22 cultures with MK801, the number of postsynaptic NR1-C2' subunit clusters is strongly upregulated. In contrast, numbers of NR1-C2 clusters are only modestly increased in DIV8 and not changed in DIV22 cultures upon MK801 treatment, suggesting a specific role of NR1 carboxy-terminal sequences in the activity-dependent synaptic targeting of NMDA receptor clusters of spinal cord neurons.

Protein phosphorylation networks in motor neuron death

The disorder amyotrophic lateral sclerosis (ALS) is characterized by the death of specific groups of neurons, especially motor neurons, which innervate skeletal muscle, and neurons connecting the cerebral cortex with motor neurons, such as corticospinal tract neurons. There have been numerous attempts to elucidate why there is selective involvement of motor neurons in ALS. Recent observations have demonstrated altered activities and protein levels of diverse kinases in the brain and spinal cord of transgenic mice that overexpress a mutant superoxide dismutase (mSOD) gene that is found in patients with the familial form of ALS, as well as in patients who have died with ALS. These results suggest that the alteration of protein phosphorylation may be involved in the pathogenesis of ALS. The changes in protein kinase and phosphatase expression and activity can affect the activation of important neuronal neurotransmitter receptors such as NMDA receptors or other signaling proteins and can trigger, or modify, the process producing neuronal loss in ALS. These various kinases, phosphatases and signaling proteins are involved in many signaling pathways; however, they have close interactions with each other. Therefore, an understanding of the role of protein kinases and protein phosphatases and the molecular organization of protein phosphorylation networks are useful to determine the mechanisms of selective motor neuron death.

cGMP-induced presynaptic depression and postsynaptic facilitation at glutamatergic synapses in visual cortex

The mechanisms by which the intracellular messenger cGMP can modulate synaptic efficacy remain poorly understood. Here we report that cGMP, acting through cGMP-dependent protein kinase (PKG), has multiple rapid and reversible effects on synaptic transmission in slices and cultures of rodent visual cortex. Extracellular application of the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and the PKG specific activator beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate sp-isomer (Sp-8-Br-PET-cGMPS) reduced stimulus-evoked EPSPs in slices. In cortical cultures, both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. In both slices and cultures, intracellular perfusion of the postsynaptic neurons with a pseudosubstrate inhibitory peptide specific for PKG had no effect on the reduction in EPSPs and EPSCs, indicating that the inhibition occurred at presynaptic sites. Whole-cell calcium currents in cultured cortical neurons were also reduced by both analogs, which may account for the effect on synaptic release. To determine whether cGMP was also acting at postsynaptic sites, we applied exogenous kainate/AMPA and NMDA to the recorded cells directly. cGMP and its analogs showed little effect on the postsynaptic kainate/AMPA responses but produced a dramatic enhancement of NMDA responses. cGMP-induced NMDA potentiation was prevented by the specific PKG inhibitory peptide infused into the postsynaptic cell. In summary, cGMP, acting through PKG, had depressive presynaptic and facilitatory postsynaptic actions at excitatory synapses in the visual cortex. We suggest that these opposing actions may be useful for altering the balance of synaptic inputs to cortical neurons in ways that enhance signals important for synaptic facilitation and neuronal plasticity.

Alteration in the voltage dependence of NMDA receptor channels in rat dorsal horn neurones following peripheral inflammation

1. It has been proposed that the activation of NMDA receptors and upregulation of protein kinase C (PKC) underlie the exaggerated and persistent pain experienced in the inflammatory state. However, there is no direct evidence to show that inflammation alters the function of NMDA receptors. 2. We examined the voltage-dependent properties of NMDA receptor channels in rat dorsal horn neurones that receive sensory inputs from an inflamed hindpaw. 3. Peripheral inflammation was induced by injections of complete Freund's adjuvant (CFA). Membrane currents were measured using the perforated patch-clamp technique. 4. After CFA treatment, the current-voltage relationship of NMDA receptor channels was shifted in the hyperpolarized direction. This resulted in enhanced NMDA responses at negative potentials. 5. The change was mediated by PKC because the voltage shift was blocked by the selective PKC inhibitors chelerythrine and bisindolylmaleimide I. 6. Furthermore, the Mg(2+) blockade of NMDA receptors was reduced. This reduction could account for the shift in the voltage dependence of NMDA receptor channels. 7. These results indicate that NMDA receptor channel characteristics in the dorsal horn are altered by inflammation, and that the changes observed could contribute to the hyperalgesia and allodynia associated with tissue injury.

Alternating dominance of NMDA and AMPA for learning and recall: a computer model

Physiological studies reveal a dichotomy in biological Hebbian learning: NMDA receptors are utilized for induction of long term potentiation (LTP) whereas AMPA is used for LTP expression. We propose that this dichotomy would have functional value: preventing previously stored memories from interfering with the storage of new memories. A previous hypothesis reduces this interference by temporarily reducing associative weights during learning. Complementary to this model, we propose a dual transmission algorithm in which one set of synaptic weights are used primarily for learning and another primarily for recall. This algorithm shows good performance in a simple neural network model. Biologically, the model could be mediated by a cholinergic switch from dominance of learning-insensitive NMDA receptors to dominance of learning-modifiable AMPA receptors.

Modulation of NMDA-mediated excitotoxicity by protein kinase C

Excessive activation of N-methyl-D-aspartate (NMDA) receptors leads to cell death in human embryonic kidney-293 (HEK) cells which have been transfected with recombinant NMDA receptors. To evaluate the role of protein kinase C (PKC) activation in NMDA-mediated toxicity, we have analyzed the survival of transfected HEK cells using trypan blue exclusion. We found that NMDA-mediated death of HEK cells transfected with NR1/NR2A subunits was increased by exposure to phorbol esters and reduced by inhibitors of PKC activation, or PKC down-regulation. The region of NR2A that provides the PKC-induced enhancement of cell death was localized to a discrete segment of the C-terminus. Use of isoform-specific PKC inhibitors showed that Ca(2+)- and lipid-dependent PKC isoforms (cPKCs), specifically PKCbeta1, was responsible for the increase in cell death when phorbol esters were applied prior to NMDA in these cells. PKC activity measured by an in vitro kinase assay was also increased in NR1A/NR2A-transfected HEK cells following NMDA stimulation. These results suggest that PKC acts on the C-terminus of NR2A to accentuate cell death in NR1/NR2A-transfected cells and demonstrate that this effect is mediated by cPKC isoforms. These data indicate that elevation of cellular PKC activity can increase neurotoxicity mediated by NMDA receptor activation.

An NMDA receptor ER retention signal regulated by phosphorylation and alternative splicing

Formation of mature excitatory synapses requires the assembly and delivery of NMDA receptors to the neuronal plasma membrane. A key step in the trafficking of NMDA receptors to synapses is the exit of newly assembled receptors from the endoplasmic reticulum (ER). Here we report the identification of an RXR-type ER retention/retrieval motif in the C-terminal tail of the NMDA receptor subunit NR1 that regulates receptor surface expression in heterologous cells and in neurons. In addition, we show that PKC phosphorylation and an alternatively spliced consensus type I PDZ-binding domain suppress ER retention. These results demonstrate a novel quality control function for alternatively spliced C-terminal domains of NR1 and implicate both phosphorylation and potential PDZ-mediated interactions in the trafficking of NMDA receptors through early stages of the secretory pathway.

Protein targeting and calcium signaling microdomains in neuronal cells

Over the last several years, a number of optical imaging, physiological, and molecular studies have clarified the mechanisms underlying differential calcium signaling in the postsynaptic neuron. These studies have revealed the existence of membrane-associated calcium microdomains, which are often specifically coupled to distinct protein signaling pathways. In this review, we discuss how these signaling microdomains are organized and regulated, emphasizing the structural and molecular features of synaptic protein complexes containing the metabotropic and N-methyl-D-aspartate (NMDA) glutamate receptors and the L-type voltage-dependent calcium channels (VDCCs). We conclude with a discussion of how these different signaling complexes may interact with one another, relationships which may be important in orchestrating the complex calcium signaling underlying developmental and activity-dependent changes in synaptic function.