Establishment of CHO cell lines expressing four N-methyl-D-aspartate receptor subtypes and characterization of a novel antagonist PPDC

Mechanical stress activates NMDA receptors in the absence of agonists

While studying the physiological response of primary rat astrocytes to fluid shear stress in a model of traumatic brain injury (TBI), we found that shear stress induced Ca2+ entry. The influx was inhibited by MK-801, a specific pore blocker of N-Methyl-D-aspartic acid receptor (NMDAR) channels, and this occurred in the absence of agonists. Other NMDA open channel blockers ketamine and memantine showed a similar effect. The competitive glutamate antagonists AP5 and GluN2B-selective inhibitor ifenprodil reduced NMDA-activated currents, but had no effect on the mechanically induced Ca2+ influx. Extracellular Mg2+ at 2 mM did not significantly affect the shear induced Ca2+ influx, but at 10 mM it produced significant inhibition. Patch clamp experiments showed mechanical activation of NMDAR and inhibition by MK-801. The mechanical sensitivity of NMDARs may play a role in the normal physiology of fluid flow in the glymphatic system and it has obvious relevance to TBI.

Comparative pharmacology of the human NMDA-receptor subtypes R1-2A, R1-2B, R1-2C and R1-2D using an inducible expression system

Pharmacological characterization of N-methyl-D-aspartate (NMDA) receptors has been hampered by the difficulty to outwit cytotoxicity after functional expression in recombinant systems. In this study a muristerone-inducible expression system for the NNMDA-R1 subunit was used. This was combined with constitutive expression of NMDA-R2A, 2B, 2C and 2D in different cell clones. After establishment of the cell lines, quantitative RT-PCR demonstrated the inducibility of the NNMDA-R1 subunit, and verified the expression of the NMDA-R2 subunits in the different cell clones. Functional responses were characterized using calcium influx through the ion channel as a robust assay system. Stimulation of the NMDA-receptor subtypes in the different cell lines led to calcium transients which were rising gradually, peaked after 30-160 s and declined thereafter very slowly. The expression of the four different NMDA-receptor subtypes in the same cellular background allowed a direct pharmacological comparison of the different receptors. Glutamate showed the highest potency at the NMDA-R1-2D. NMDA displayed at all subtypes a lower potency compared to glutamate and was a partial agonist except at the NMDA-R1-2D. 20 antagonists were tested in this study and the pharmacological characterization of the inhibition of glutamate-evoked elevation of intracellular free Ca(2+) revealed a distinct rank order of antagonist potency for each receptor subtype. These data illustrate that assessment of calcium transients upon receptor stimulation in the same cellular background is a powerful tool to compare the functional effects of compounds acting at the different NMDA-R2 receptors.

Modulation of agmatine on calcium signal in morphine-dependent CHO cells by activation of IRAS, a candidate for imidazoline I1 receptor

The present study investigated the effects of agmatine action on imidazoline I1 receptor antisera-selected protein (IRAS), a candidate for imidazoline I1 receptor, on prolonged morphine-induced adaptations of calcium signal and long-lasting alterations in gene expression to further elucidate the role of IRAS in opioid dependence. Two cell lines, Chinese hamster ovary cells expressing mu opioid receptor alone (CHO-mu) and expressing mu opioid receptor and IRAS together (CHO-mu/IRAS), were used. After chronic treatment with morphine for 48 h, naloxone induced a significant elevation of intracellular calcium concentration ([Ca2+]i) in CHO-mu and CHO-mu/IRAS cells. Agmatine (0.01-3 microM) concentration-dependently inhibited the naloxone-precipitated [Ca2+]i elevation when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Efaroxan, an imidazoline I1 receptor-preferential antagonist, completely reversed the effect of agmatine in CHO-mu/IRAS. Agmatine (1-10 microM) administration after chronic morphine exposure for 48 h partially decreased the [Ca2+]i elevation in CHO-mu/IRAS which was entirely antagonized by efaroxan, but not in CHO-mu. In addition, agmatine (1 microM) co-pretreated with morphine attenuated the naloxone-precipitated increases of cAMP-responsive element binding protein and extracellular signal-regulated kinase 1/2 phosphorylations and c-Fos expression in CHO-mu/IRAS. These effects were blocked by efaroxan as well. Taken together, these results indicate that the agmatine-IRAS action system attenuates the up-regulations of Ca2+ signal and its downstream gene expression in morphine-dependent model in vitro, providing additional evidence to support the contribution of IRAS to opioid dependence.

Fyn Is Required for Haloperidol-induced Catalepsy in Mice

Fyn-mediated tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits has been implicated in various brain functions, including ethanol tolerance, learning, and seizure susceptibility. In this study, we explored the role of Fyn in haloperidol-induced catalepsy, an animal model of the extrapyramidal side effects of antipsychotics. Haloperidol induced catalepsy and muscle rigidity in the control mice, but these responses were significantly reduced in Fyn-deficient mice. Expression of the striatal dopamine D(2) receptor, the main site of haloperidol action, did not differ between the two genotypes. Fyn activation and enhanced tyrosine phosphorylation of the NMDA receptor NR2B subunit, as measured by Western blotting, were induced after haloperidol injection of the control mice, but both responses were significantly reduced in Fyn-deficient mice. Dopamine D(2) receptor blockade was shown to increase both NR2B phosphorylation and the NMDA-induced calcium responses in control cultured striatal neurons but not in Fyn-deficient neurons. Based on these findings, we proposed a new molecular mechanism underlying haloperidol-induced catalepsy, in which the dopamine D(2) receptor antagonist induces striatal Fyn activation and the subsequent tyrosine phosphorylation of NR2B alters striatal neuronal activity, thereby inducing the behavioral changes that are manifested as a cataleptic response.

IRAS, a candidate for I1-imidazoline receptor, mediates inhibitory effect of agmatine on cellular morphine dependence

Agmatine, an endogenous ligand for the I1-imidazoline receptor, has previously been shown to prevent morphine dependence in rats and mice. To investigate the role of imidazoline receptor antisera-selected protein (IRAS), a strong candidate for I1R, in morphine dependence, two CHO cell lines were created, in which mu opioid receptor (MOR) was stably expressed alone (CHO-mu) or MOR and IRAS were stably co-expressed (CHO-mu/IRAS). After 48 h administration of morphine (10 microM), naloxone induced a cAMP overshoot in both cell lines, suggesting cellular morphine dependence had been produced. Agmatine (0.1-2.5 microM) concentration-dependently inhibited the naloxone-precipitated cAMP overshoot when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Agmatine at 5-100 microM also inhibited the cAMP overshoot in CHO/mu and CHO-mu/IRAS. Efaroxan, an I1R-preferential antagonist, completely blocked the effect of agmatine on the cAMP overshoot at 0.1-2.5 microM in CHO-mu/IRAS, while partially reversing the effects of agmatine at 5-100 microM. L-type calcium channel blocker nifedipine entirely mimicked the effects of agmatine at high concentrations on forskolin-stimulated cAMP formation in CHO-mu and naloxone-precipitated cAMP overshoot in morphine-pretreated CHO-mu. Therefore, IRAS, in the co-transfected CHO-mu/IRAS cell line, appears necessary for low concentrations of agmatine to cause attenuation of cellular morphine dependence. An additional effect of agmatine at higher concentrations seems to relate to both transfected IRAS and some naive elements in CHO cells, and L-type voltage-gated calcium channels are not ruled out. This study suggests that IRAS mediates agmatine's high affinity effects on cellular morphine dependence and may play a role in opioid dependence.

A glycine receptor antagonist, strychnine, blocked NMDA receptor activation in the neonatal mouse neocortex

The NMDA receptor (NMDAR) is a Ca (2+)-permeable cation channel that plays a critical role in neural network formation during brain development. Since it is blocked in a voltage-dependent manner by extracellular Mg(2+), in order for the NMDA to be activated, the membrane must be strongly depolarized. Immature neurons in the developing neocortex can be depolarized by ligand-gated Cl(-) channels, such as the glycine receptor (GlyR) or GABA(A) receptor (GABA(A) R). We here assess the contribution of GlyRs to Ca(2+) influx via NMDARs in neonatal mouse cortical neurons. The GlyR antagonist, strychnine, was more effective in suppressing postsynaptic Ca(2+) influx than the GABA(A) R antagonist, picrotoxin, suggesting greater potentiation of NMDARs by GlyRs than by GABA(A) Rs. The GlyR, known to be endogenously activated at this stage, may play a critical role in neocortical development.