A correlation between changes in gamma-aminobutyric acid metabolism and seizures induced by antivitamin B6.
The effects of DL-penicillamine (DL-PeA), hydrazine and toxopyrimidine (TXP, 2-methyl-6-amino-5-hydroxymethylpyrimidine) on gamma-aminobutyric acid (GABA) metabolism in mouse brain were studied. All these compounds inhibited the activity of glutamate decarboxylase [EC 220.127.116.11] (GAD) and slightly inhibited that of 4-aminobutyrate: 2-oxoglutarate aminotransferase [EC 18.104.22.168] (GABA-T). In contrast, very different effects were observed on GABA levels; hydrazine caused a marked increase, DL-PeA had no effect, and TXP caused a slight decrease in the content of the amino acid. These results could be described by an equation which related the excitable state to changes in the flux of the GABA bypass. Since the values obtained from the equation clearly reflect the seizure activity, it is suggested that the decreased GABA flux might be a cause of convulsions induced by these drugs. (+info)
Mechanisms involved in the metabotropic glutamate receptor-enhancement of NMDA-mediated motoneurone responses in frog spinal cord.
1. The metabotropic glutamate receptor (mGluR) agonist trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) (10-100 microM) depolarized isolated frog spinal cord motoneurones, a process sensitive to kynurenate (1.0 mM) and tetrodotoxin (TTX) (0.783 microM). 2. In the presence of NMDA open channel blockers [Mg2+; (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK801); 3,5-dimethyl-1-adamantanamine hydrochloride (memantine)] and TTX, trans-ACPD significantly potentiated NMDA-induced motoneurone depolarizations, but not alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA)- or kainate-induced depolarizations. 3. NMDA potentiation was blocked by (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) (240 microM), but not by alpha-methyl-(2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (MCCG) (290 microM) or by alpha-methyl-(S)-2-amino-4-phosphonobutyrate (L-MAP4) (250 microM), and was mimicked by 3,5-dihydroxyphenylglycine (DHPG) (30 microM), but not by L(+)-2-amino-4-phosphonobutyrate (L-AP4) (100 microM). Therefore, trans-ACPD's facilitatory effects appear to involve group I mGluRs. 4. Potentiation was prevented by the G-protein decoupling agent pertussis toxin (3-6 ng ml(-1), 36 h preincubation). The protein kinase C inhibitors staurosporine (2.0 microM) and N-(2-aminoethyl)-5-isoquinolinesulphonamide HCI (H9) (77 microM) did not significantly reduce enhanced NMDA responses. Protein kinase C activation with phorbol-12-myristate 13-acetate (5.0 microM) had no effect. 5. Intracellular Ca2+ depletion with thapsigargin (0.1 microM) (which inhibits Ca2+/ATPase), 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetracetic acid acetyl methyl ester (BAPTA-AM) (50 microM) (which buffers elevations of [Ca2+]i), and bathing spinal cords in nominally Ca2+-free medium all reduced trans-ACPD's effects. 6. The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W7) (100 microM) and chlorpromazine (100 microM) diminished the potentiation. 7. In summary, group I mGluRs selectively facilitate NMDA-depolarization of frog motoneurones via a G-protein, a rise in [Ca2+]i from the presumed generation of phosphoinositides, binding of Ca2+ to calmodulin, and lessening of the Mg2+-produced channel block of the NMDA receptor. (+info)
An inhibitor of exported Mycobacterium tuberculosis glutamine synthetase selectively blocks the growth of pathogenic mycobacteria in axenic culture and in human monocytes: extracellular proteins as potential novel drug targets.
Mycobacterium tuberculosis and other pathogenic mycobacteria export abundant quantities of proteins into their extracellular milieu when growing either axenically or within phagosomes of host cells. One major extracellular protein, the enzyme glutamine synthetase, is of particular interest because of its link to pathogenicity. Pathogenic mycobacteria, but not nonpathogenic mycobacteria, export large amounts of this protein. Interestingly, export of the enzyme is associated with the presence of a poly-L-glutamate/glutamine structure in the mycobacterial cell wall. In this study, we investigated the influence of glutamine synthetase inhibitors on the growth of pathogenic and nonpathogenic mycobacteria and on the poly-L-glutamate/glutamine cell wall structure. The inhibitor L-methionine-S-sulfoximine rapidly inactivated purified M. tuberculosis glutamine synthetase, which was 100-fold more sensitive to this inhibitor than a representative mammalian glutamine synthetase. Added to cultures of pathogenic mycobacteria, L-methionine- S-sulfoximine rapidly inhibited extracellular glutamine synthetase in a concentration-dependent manner but had only a minimal effect on cellular glutamine synthetase, a finding consistent with failure of the drug to cross the mycobacterial cell wall. Remarkably, the inhibitor selectively blocked the growth of pathogenic mycobacteria, all of which release glutamine synthetase extracellularly, but had no effect on nonpathogenic mycobacteria or nonmycobacterial microorganisms, none of which release glutamine synthetase extracellularly. The inhibitor was also bacteriostatic for M. tuberculosis in human mononuclear phagocytes (THP-1 cells), the pathogen's primary host cells. Paralleling and perhaps underlying its bacteriostatic effect, the inhibitor markedly reduced the amount of poly-L-glutamate/glutamine cell wall structure in M. tuberculosis. Although it is possible that glutamine synthetase inhibitors interact with additional extracellular proteins or structures, our findings support the concept that extracellular proteins of M. tuberculosis and other pathogenic mycobacteria are worthy targets for new antibiotics. Such proteins constitute readily accessible targets of these relatively impermeable organisms, which are rapidly developing resistance to conventional antibiotics. (+info)
Retinal ganglion cell response properties in the transcorneal electrically evoked response of the visual system.
To identify the retinal origin of a cortical evoked potential elicited by transcorneal electrical stimulation of the visual system (EER), the response properties of retinal ganglion cells (RGCs) of cats to transcorneal electrical stimuli were studied. The discharge latency of RGCs to transcorneal stimulation had two peaks with a high temporal resolution. The latency of early components of the EER is associated with the discharge latency of RGCs. Some RGCs showed prominent oscillatory discharges after transcorneal stimulation. Discharges of ON-bipolar cells responding to transcorneal stimulation were significantly inhibited by intravitreal injection of DL-2-amino-4-phosphonobutyrate (APB), which blocks the ON-pathway. These findings indicate that the EER has far-field potentials that might relate to oscillatory discharges of RGCs, and that ON bipolar cells and their related synaptic sites are involved in transcorneal electrical stimuli. The far-field potentials of the EER may have clinical applications, similar to those of somatosensoric evoked potentials and auditory brain stem potentials. (+info)
Analysis of pharmacologically isolated components of the ERG.
An harmonic analysis was applied to the electroretinogram (ERG) measured in intact cat eyes in control conditions and after pharmacological isolation of the components attributed to photoreceptors (PIII) and bipolar neurons (PII). The frequency response curves obtained in various conditions showed that the bandwidth of the PII component extends over a range of stimulus frequencies higher than the bandwidth of PIII. The enhancement of the PII response to stimuli of high temporal frequency suggests the presence of a frequency dependent gain control located either pre- and/or post-synaptically in the transmission line between the phototransductive cascade and bipolar neurons. A possible role of these processes is to enhance relevant visual information whilst selectively attenuating low frequency signals originating in the transductive cascade. (+info)
NPY inhibits glutamatergic excitation in the epileptic human dentate gyrus.
Neuropeptide Y (NPY) has been shown to depress hyperexcitable activity that has been acutely induced in the normal rat brain. To test the hypothesis that NPY can also reduce excitability in the chronically epileptic human brain, we recorded intracellularly from dentate granule cells in hippocampal slices from patients with hippocampal seizure onset. NPY had a potent and long-lasting inhibitory action on perforant path-evoked excitatory responses. In comparison, the group 3 metabotropic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid (L-AP4) evoked a mild and transient decrease. NPY-containing axons were found throughout the hippocampus, and in many epileptic patients were reorganized, particularly in the dentate molecular layer. NPY may therefore play a beneficial role in reducing granule cell excitability in chronically epileptic human tissue, and subsequently limit seizure severity. (+info)
A novel domain of the inhibitory glycine receptor determining antagonist efficacies: further evidence for partial agonism resulting from self-inhibition.
Different amino side chains in the N-terminal extracellular region of the inhibitory glycine receptor (GlyR) have been shown to be crucial for ligand recognition. Here we describe a novel domain of the GlyRalpha1 subunit that constitutes an important determinant of antagonist activity. The antagonists strychnine, nipecotic acid, and isobutyric acid displayed reduced potencies at recombinant GlyRs formed from alpha1 subunits, in which lysine 104, phenylalanine 108, or threonine 112 were replaced by alanine. Agonist affinities, in contrast, were slightly increased at these mutant receptors. Taurine and beta-aminoisobutyric acid, which are partial agonists at the wild-type GlyR, behaved as full agonists at the mutant GlyRs and failed to inhibit glycine-induced currents. This is consistent with apolar residues at positions 104, 108, and 112 of the alpha1 subunit reducing the antagonistic, but not the agonistic, binding of beta-amino acids. Our data support a model in which the partial agonism of beta-amino acids results from their self-inhibitory activity. (+info)
Role of Ngamma-acetyldiaminobutyrate as an enzyme stabilizer and an intermediate in the biosynthesis of hydroxyectoine.
Strain CHR63 is a salt-sensitive mutant of the moderately halophilic wild-type strain Halomonas elongata DSM 3043 that is affected in the ectoine synthase gene (ectC). This strain accumulates large amounts of Ngamma-acetyldiaminobutyrate (NADA), the precursor of ectoine (D. Canovas, C. Vargas, F. Iglesias-Guerra, L. N. Csonka, D. Rhodes, A. Ventosa, and J. J. Nieto, J. Biol. Chem. 272:25794-25801, 1997). Hydroxyectoine, ectoine, and glucosylglycerate were also identified by nuclear magnetic resonance (NMR) as cytoplasmic organic solutes in this mutant. Accumulation of NADA, hydroxyectoine, and ectoine was osmoregulated, whereas the levels of glucosylglycerate decreased at higher salinities. The effect of the growth stage on the accumulation of solutes was also investigated. NADA was purified from strain CHR63 and was shown to protect the thermolabile enzyme rabbit muscle lactate dehydrogenase against thermal inactivation. The stabilizing effect of NADA was greater than the stabilizing effect of ectoine or potassium diaminobutyrate. A (1)H NMR analysis of the solutes accumulated by the wild-type strain and mutants CHR62 (ectA::Tn1732) and CHR63 (ectC::Tn1732) indicated that H. elongata can synthesize hydroxyectoine by two different pathways-directly from ectoine or via an alternative pathway that converts NADA into hydroxyectoine without the involvement of ectoine. (+info)