The lethal expression of the GluR2flip/GluR4flip AMPA receptor in HEK293 cells

alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) -type glutamate receptors play a critical role in excitotoxicity associated with cerebral hypoxia, ischaemia and other acute brain insults. AMPA receptors are composed of GluR1-GluR4 subunits in homomeric and heteromeric assemblies, forming nonselective cation channels. In addition, each subunit has alternative splice variants, flip and flop forms. Heterologous expression studies showed that the AMPA receptor channels exhibit diverse properties depending on subunit/variant composition. For example, the absence of the GluR2 subunit makes AMPA receptor assemblies Ca2+-permeable. Excitotoxicity induced by activating AMPA receptor channels has been linked to excessive Ca2+ influx through the GluR2-lacking channels. Here we demonstrate that coexpression of the AMPA receptor GluR2flip and GluR4flip subunits exerts a lethal effect on HEK293 cells, whereas no lethal activity is observed in other homomeric or heteromeric combinations of AMPA receptor subunits. Patch clamp recordings and Ca2+ imaging analyses have revealed that this GluR2flip/GluR4flip receptor exhibits a low Ca2+ permeability. This subunit combination, however, showed prolonged Na+ influx following AMPA stimulation, even in the absence of cyclothiazide, which attenuates AMPA receptor desensitization. Furthermore, the GluR2flip/GluR4flip-mediated lethality was potentiated by the interruption of cellular Na+ extrusion mechanisms using ouabain or benzamil. These observations suggest that the GluR2flip/GluR4flip receptor-mediated excitotoxicity is attributed to Na+ overload, but not Ca2+ influx.

Characterization of the anticonvulsant and neuroprotectant BIIR 561 CL in vitro: effects on native and recombinant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors

BIIR 561 CL is a novel blocker of AMPA receptors and voltage-dependent sodium channels. In this study we further describe the effects of BIIR 561 CL on AMPA receptor-mediated membrane currents in rodent neurons, as well as in cells expressing recombinant human GluR1/2 receptors in more detail. BIIR 561 CL suppressed responses to kainate in neuronal cultures from rat cortex with an IC50 of 9.8 microM. Similar effects were observed using acutely dissociated neurons from the CA1 region of rat hippocampus (IC50 = 9.5 microM). Inhibition of kainate responses by BIIR 561 CL was prevented by preapplication of GYKI 53655, suggesting that both non-competitive inhibitors bind to a common site of the receptor. The effect of 10 microM BIIR 561 CL on kainate-induced currents was dependent on extracellular pH, with more pronounced block (84.1%) under acidic conditions (pHextern=6.4), compared to only 30.1% at a pHextern of 8.4. Thus, it can be hypothesized that BIIR 561 CL inhibits AMPA receptors in ischaemic brain regions more effectively than in healthy tissue. BIIR 561 CL inhibited responses to 1 mM glutamate in cells expressing recombinant human GluR1/2 receptors with similar potency, as compared to kainate responses in rat neurons (IC50=17.3 microM). The reference compound NBQX had an IC50 of 25.2 nM. None of the two compounds affected the glutamate-induced receptor desensitization at any tested concentration. The block by BIIR 561 CL was not use-dependent and had fast on- and off-kinetics (tauon=6.8 s; tauoff=1.3 s in hGluR1/2 receptors with 30 microM BIIR 561 CL). Thus, BIIR 561 CL can be anticipated to have a promising profile for the treatment of neurological disorders like brain ischaemia and head trauma.

Oral administration of hapten inhibits in vivo induction of specific cytotoxic CD8+ T cells mediating tissue inflammation: a role for regulatory CD4+ T cells

We investigated whether oral tolerance could block the development of an inflammatory response mediated by CD8+ T cells, using a mouse model of oral tolerance of contact sensitivity (CS) to the hapten 2, 4-dinitrofluorobenzene (DNFB). In this system, the skin inflammatory response is initiated by hapten-specific class I-restricted cytotoxic CD8+ T (CTL) cells, independently of CD4 help. Oral delivery of DNFB before skin sensitization blocked the CS response by impairing the development of DNFB-specific CD8+ effector T cells in secondary lymphoid organs. This was shown by complete inhibition of DNFB-specific CTL and proliferative responses of CD8+ T cells, lack of specific IFN-gamma-producing CD8+ T cells, and inability of CD8+ T cells to transfer CS in RAG20/0 mice. RT-PCR and immunohistochemical analysis confirmed that recruitment of CD8+ effectors of CS in the skin at the site of hapten challenge was impaired in orally tolerized mice. Sequential anti-CD4 Ab treatment showed that only depletion of CD4+ T cells during the afferent phase of CS abrogated oral tolerance induction by restoring high numbers of specific CD8+ effectors in lymphoid organs, whereas CD4 depletion during the efferent phase of CS did not affect oral tolerance. These data demonstrate that a single intragastric administration of hapten can block in vivo induction of DNFB-specific CD8+ CTL responsible for tissue inflammation and that a subset of regulatory CD4+ T cells mediate oral tolerance by inhibiting expansion of specific CD8+ effectors in lymph nodes.

Stable expression of human homomeric and heteromeric AMPA receptor subunits in HEK293 cells

Human homomeric and heteromeric alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors (GluRs) were stably expressed in HEK293 cells with cDNAs encoding the flip splice variant of GluR1, GluR2, GluR3, GluR4 subunit, and the GluR1/GluR2, GluR3/GluR2, and GluR4/GluR2 combination. The lethal combination of GluR2 and GluR4 subunits was found in high expression levels of both receptors. The AMPA-evoked current voltage relationships demonstrated the functional channel properties, such as a double rectification in GluR1, GluR3, and GluR4 receptors, and a linear relation in receptors assembled from GluR2 alone and coexpression of GluR2 with the other subunits. All the transfectants exhibited higher selectivity for AMPA than glutamate in dose-dependent current responses. [3H]AMPA binding revealed that the homomeric and heteromeric receptors displayed a single binding site in Scatchard analysis, with dissociation constant (Kd) values in the range of 14.5-49.3 nM. The Bmax values were in the range of 0.57-7.66 pmol/mg protein. The ligand displacement potency for [3H]AMPA binding was CNQX > glutamate > NS257 in all of the transfectants. These results suggest that stable transformants expressing human homomeric and heteromeric AMPA receptors will be useful tools to define selectivity and potential site of action for AMPA receptor modulators.

Localization of a G-protein-coupled inwardly rectifying K+ channel, CIR, in the rat brain

The cellular localization of a G-protein-coupled K+ channel, CIR, in the rat brain has been demonstrated using a CIR-specific antibody, in combination with in situ hybridization. The CIR protein and messenger RNA were found in the cerebellar cortex, hippocampal formation, olfactory system, cerebral cortex, basal ganglia, several nuclei of the lower brain stem and the choroid plexus. In contrast to the messenger RNA, which was concentrated in the cell soma, the CIR protein was found in a subset of nerve fibers and, in other cases, in axon terminals. In the cerebellar cortex and hippocampus, the CIR protein was concentrated in the axon terminals of basket cells which are known to be GABAergic interneurons. This discrepancy between the distribution of protein and messenger RNA was observed in the substantia nigra, the interpeduncular, trigeminal, hypoglossal, oculomotor and red nuclei of the lower brain stem, and the tufted and mitral cells of the olfactory bulb. These observations suggested the translocation of the CIR protein into the nerve fibers following synthesis in the cell soma. Furthermore, its specific neuronal localization, especially in GABAergic interneurons, suggested the importance of CIR in synaptic transmission in neuronal systems.

Selective effector coupling of muscarinic acetylcholine receptor subtypes

Attempts have been made by means of recombinant DNA technology to understand the molecular basis of the functional heterogeneity of the muscarinic acetylcholine receptor (mAChR). Molecularly defined mAChR subtypes have been produced from the cloned DNAs in Xenopus oocytes and NG108-15 neuroblastoma-glioma hybrid cells as transient and stable expression systems, respectively, and agonist-induced cellular responses have been examined. The results obtained provide evidence that mAChR subtypes are selectively coupled with different effector systems, albeit not exclusively.

Location of a region of the muscarinic acetylcholine receptor involved in selective effector coupling

Chimaeric muscarinic acetylcholine receptors (mAChR) in which corresponding portions of mAChR I and mAChR II are replaced with each other have been produced in Xenopus oocytes by expression of cDNA constructs encoding them. Functional analysis of the chimaeric mAChRs indicates that a region mostly comprising the putative cytoplasmic portion between the proposed transmembrane segments V and VI is involved in selective coupling of mAChR I and mAChR II with different effector systems. In contrast, the exchange of this region between mAChR I and mAChR II does not significantly affect the antagonist binding properties of the two mAChR subtypes.

Different sensitivities to agonist of muscarinic acetylcholine receptor subtypes

Muscarinic acetylcholine receptor (mAChR) III expressed in Xenopus oocytes, like mAChR I, mediates activation of a Ca2+-dependent Cl- current, whereas mAChR IV, like mAChR II, principally induces activation of Na+ and K+ currents in a Ca2+-independent manner. mAChR III has a sensitivity to agonist of about one order of magnitude higher than that of mAChR I in mediating the Ca2+-dependent current response in Xenopus oocytes and in stimulating phosphoinositide hydrolysis in NG108-15 neuroblastoma-glioma hybrid cells. The agonist-binding affinity of mAChR III is also about one order of magnitude higher than that of mAChR I.

Selective coupling with K+ currents of muscarinic acetylcholine receptor subtypes in NG108-15 cells

The primary structures of two muscarinic acetylcholine receptor (mAChR) species, designated as mAChR I and mAChR II, have been elucidated by cloning and sequence analysis of DNAs complementary to the porcine cerebral and cardiac messenger RNAs, respectively. mAChR I and mAChR II expressed in Xenopus oocytes differ from each other both in acetylcholine-induced response and in antagonist binding properties. These results, together with the differential tissue location of the two mAChR mRNAs, have indicated that pharmacologically distinguishable subtypes of the mAChR represent distinct gene products. The primary structures of two additional mammalian mAChR species, designated as mAChR III and mAChR IV, have subsequently been deduced from the nucleotide sequences of the cloned cDNAs or genomic DNAs. We report here that mAChR I and mAChR III expressed in NG108-15 neuroblastoma-glioma hybrid cells, but not mAChR II and mAChR IV, efficiently mediate phosphoinositide hydrolysis, activation of a Ca2+-dependent K+ current and inhibition of the M-current, a voltage-dependent K+ current sensitive to muscarinic agonists.

Primary structure of porcine muscarinic acetylcholine receptor III and antagonist binding studies

The complete amino acid sequence of porcine muscarinic acetylcholine receptor III has been deduced by cloning and sequencing the genomic DNA. The antagonist binding properties of muscarinic acetylcholine receptor III expressed from the cloned DNA in Xenopus oocytes correspond most closely to those of the pharmacologically defined M2 glandular (III) subtype.

Molecular distinction between muscarinic acetylcholine receptor subtypes

The muscarinic acetylcholine receptor (mAChR) mediates various cellular responses, including inhibition of adenylate cyclase, breakdown of phosphoinositides and modulation of potassium channels, through the action of guanine nucleotide-binding regulatory proteins. Pharmacologically distinguishable forms of the mAChR occur in different tissues and have provisionally been classified into M1 (I), M2 cardiac (II) and M2 glandular (III) subtypes on the basis of their difference in apparent affinity for antagonists. In an attempt to elucidate the molecular basis of the functional heterogeneity of the mAChR, we have cloned and sequenced DNAs complementary to porcine cerebral and cardiac messenger RNAs encoding mAChRs and have thereby deduced the primary structures of the receptor proteins. We report here that the messenger RNA generated by transcription of the cardiac complementary DNA directs the formation of a functional mAChR in Xenopus oocytes and that this mAChR differs from the mAChR formed by expression of the cerebral cDNA both in acetylcholine (ACh)-induced response and in antagonist binding properties. Our results provide evidence indicating that the mAChR encoded by the cerebral cDNA (designated as mAChR I) and the mAChR encoded by the cardiac cDNA (mAChR II) are of the M1 (I) and the M2 cardiac (II) subtype, respectively.

Primary structure of porcine cardiac muscarinic acetylcholine receptor deduced from the cDNA sequence

The complete amino acid sequence of the porcine cardiac muscarinic acetylcholine receptor has been deduced by cloning and sequencing the cDNA. The tissue location of the RNA hybridizing with the cDNA suggests that this muscarinic receptor species represents the M2 subtype.

Urinary sex-dependent low molecular weight proteins as a sign of narcotic dependence in male rats

The relationship between urinary excretion of sex-dependent low molecular weight proteins (LMWP) in male rats and narcotic dependence has been investigated in this study. Chronic administration of codeine (0.5 mg/g food) caused a significant decrease in urinary excretion of LMWP from the third day, without any change in urinary high molecular weight proteins. The decrease recovered to the control level after the withdrawal of codeine. Withdrawal symptoms including loss of body weight and diarrhea were observed following codeine withdrawal. In animals chronically treated with pethidine (1.0 mg/g food), however, neither changes in urinary LMWP excretion nor withdrawal symptoms were observed. These present findings suggest that the decrease in urinary excretion of sex-dependent LMWP is related to narcotic dependence in male rats, since we previously reported the decrease in urinary excreted LMWP in morphine-dependent rats.

A new approach for assessment of narcotic physical dependence using urinary sex-dependent low molecular weight proteins in male rats

Attempts have been made to examine the relationship between urinary excretion of sex-dependent low molecular weight proteins found only in male rats (LMWP) and morphine physical dependence. Chronic administration of morphine produced a dose-related decrease in urinary LMWP excretion, which was correlated to the intensity of withdrawal signs including body weight loss and abnormal behaviors recognized after naloxone challenge. Furthermore, a statistically high correlation was obtained between the decrease in urinary LMWP excretion and the loss of body weight precipitated by naloxone challenge. LMWP was identified immunologically in the livers, kidneys, and sera using an antibody against purified LMWP. The serum level of LMWP was increased rapidly following bilateral nephrectomy. After chronic treatment with morphine, the LMWP content in the livers, kidneys, and sera were decreased. These findings indicate that the decrease in urinary LMWP excretion induced by chronic administration of morphine can be a useful parameter to assess the development of physical dependence on narcotics on the peripheral level without requiring drug withdrawal and naloxone challenge. This decrease in urinary LMWP may be caused by the inhibition of LMWP synthesis in the liver.

[Relationship between the decrease in urinary sex-dependent low molecular weight proteins by morphine and hormonal parameters in male rats]

The relationship between the decrease in urinary sex-dependent low molecular weight proteins (LMWP), which exist only in the male rat, and the serum levels of some hormones were examined in this study. Castration of male rats reduced the urinary excretion of LMWP by about 50%. Replacement therapy with testosterone increased the urinary LMWP excretion. Adrenalectomy did not affect the urinary excretion of LMWP. In the adrenalectomized rat, however, corticosterone increased LMWP excretion. Therefore, it is considered that testosterone and corticosterone play a part in the urinary excretion of LMWP under physiological conditions and that the effect of testosterone is more specific than that of corticosterone. Serum concentration of testosterone and corticosterone tended to increase in comparison with the control on the 7th day after chronic treatment with morphine (0.5 mg/g food), when the urinary excretion of LMWP was significantly decreased. Furthermore, after rats were chronically administered morphine following castration or adrenalectomy, the urinary LMWP excretion was markedly decreased in the same way as found in intact animals. On the other hand, the serum thyroxine level of rats treated with morphine for 7 days was significantly lower than that of the control. Thyroxine increased dose-dependently the decreased urinary excretion of LMWP induced by morphine administration. These findings suggest that the decrease in urinary excretion of LMWP after chronic treatment with morphine may be caused by the change of serum thyroxine level via the action of morphine on the endocrine functions.

Narcotic physical dependence and urinary sex-dependent low molecular weight proteins in male rats

The relationship between urinary excretion of sex-dependent low molecular weight proteins (LMWP) in male rats and narcotic dependence is described in this study. Rats were intermittently infused with narcotics at one hour intervals through an implanted intravenous cannula. Development of physical dependence on morphine, pethidine, and pentazocine was detected by withdrawal signs including body weight loss and abnormal behaviors after naloxone challenge. In these animals, a significant decrease in urinary LMWP excretion was found following the second day of each drug treatment without significant changes in albumin excretion, and this decrease was observed continuously throughout the experiment. The markedly decreased level of LMWP recovered to the control level within 7 d after withdrawal of the drugs. These results suggest that the decrease in urinary excretion of sex-dependent LMWP in male rats is a phenomenon closely related to narcotic dependence.

[Plasma corticosterone concentration in morphine-dependent rats]

The effects of morphine on plasma corticosterone concentration in rats were studied using the drug-admixed food method. Morphine was mixed with the powder form of rat food in concentrations of 0.5 mg/g, 1 mg/g, and 2 mg/g of food. Plasma corticosterone concentration in rats treated with morphine-admixed food increased significantly, and the increment depended on the morphine concentration of drug-admixed food. The time course change of plasma corticosterone concentration in rats treated with morphine-admixed food (1 mg/g food) for 1 week was similar to that of non-treated rats without the concentration at 9:00. The plasma corticosterone concentration of morphine-treated rats at 9:00 was significantly increased in comparison with that of non-treated rats. Furthermore, the plasma corticosterone concentration after withdrawal in morphine-treated rats increased with time, and a significant increment in corticosterone was observed at 24 hr after the withdrawal. Naloxone, injected subcutaneously into morphine-dependent rats, significantly increased plasma corticosterone concentration. Increment of plasma corticosterone concentration in rats treated with morphine-admixed food for 3 or 4 weeks was significantly different from the non-treated group. The increment of plasma corticosterone concentration after the withdrawal was in a morphine treatment period-related manner. By these results, we suggest that tolerance to the increment action in plasma corticosterone concentration of morphine does not develop for at least 4 weeks using the drug-admixed food method, and the drug-admixed food method could induce morphine dependence without disturbance of the circadian rhythm in plasma corticosterone.