Group-selective reagent modification of the sodium- and chloride-coupled glycine transporter under native and reconstituted conditions

Administration of thimerosal to infant rats increases overflow of glutamate and aspartate in the prefrontal cortex: protective role of dehydroepiandrosterone sulfate

Thimerosal, a mercury-containing vaccine preservative, is a suspected factor in the etiology of neurodevelopmental disorders. We previously showed that its administration to infant rats causes behavioral, neurochemical and neuropathological abnormalities similar to those present in autism. Here we examined, using microdialysis, the effect of thimerosal on extracellular levels of neuroactive amino acids in the rat prefrontal cortex (PFC). Thimerosal administration (4 injections, i.m., 240 μg Hg/kg on postnatal days 7, 9, 11, 15) induced lasting changes in amino acid overflow: an increase of glutamate and aspartate accompanied by a decrease of glycine and alanine; measured 10-14 weeks after the injections. Four injections of thimerosal at a dose of 12.5 μg Hg/kg did not alter glutamate and aspartate concentrations at microdialysis time (but based on thimerosal pharmacokinetics, could have been effective soon after its injection). Application of thimerosal to the PFC in perfusion fluid evoked a rapid increase of glutamate overflow. Coadministration of the neurosteroid, dehydroepiandrosterone sulfate (DHEAS; 80 mg/kg; i.p.) prevented the thimerosal effect on glutamate and aspartate; the steroid alone had no influence on these amino acids. Coapplication of DHEAS with thimerosal in perfusion fluid also blocked the acute action of thimerosal on glutamate. In contrast, DHEAS alone reduced overflow of glycine and alanine, somewhat potentiating the thimerosal effect on these amino acids. Since excessive accumulation of extracellular glutamate is linked with excitotoxicity, our data imply that neonatal exposure to thimerosal-containing vaccines might induce excitotoxic brain injuries, leading to neurodevelopmental disorders. DHEAS may partially protect against mercurials-induced neurotoxicity.

Structure, function and regulation of glycine neurotransporters

Glycine exerts multiple functions in the central nervous system, as an inhibitory neurotransmitter through activation of specific, Cl--permeable, ligand-gated ionotropic receptors and as an obligatory co-agonist with glutamate on the activation of N-methyl-D-aspartate (NMDA) receptors. In some areas of the central nervous system, glycine seems to be co-released with gamma-aminobutyric acid (GABA), the main inhibitory amino acid neurotransmitter. The synaptic action of glycine ends by active recapture through sodium- and chloride-coupled glycine transporters located in glial and neuronal plasma membranes, whose structure-function relationship is being studied. The trafficking and plasma membrane expressions of these proteins are controlled by regulatory mechanisms. Glycine transporter inhibitors may find application in the treatment of muscle tone defects, epilepsy, schizophrenia, pain and neurodegenerative disorders. This review deals on recent progress on localization, transport mechanisms, structure, regulation and pharmacology of the glycine transporters (GLYTs).

Phenylglyoxal suppresses cationic lysine/K+ symport under alkaline conditions in brush border membrane vesicles from larval Manduca sexta midgut

The arginine-specific reagent, phenylglyoxal, decreases the initial rate of lysine/K+ symport (cotransport) as well as maximum lysine accumulation at pH 9.2, by brush border membrane vesicles obtained from the larval midgut of the lepidopteran, Manduca sexta. The symport of a neutral amino acid, leucine, remained unaffected. Following exposure to phenylglyoxal, the apparent dissociation constant for lysine increased by a factor of 2.5 whereas the maximum uptake rate decreased by a factor of 0.4. More than one arginine residue appears to react with phenylglyoxal. Apparently phenylglyoxal reacts preferentially with arginine residues on a symporter that is specific for positively charged lysine. Phenylglyoxal shows promise as a specific covalent label for the identification of a cationic amino acid symporter.

Sulfhydryl groups essential for the volume-sensitive release of taurine from astrocytes

The volume-sensitive [3H]taurine efflux from cultured astrocytes was found to be strongly inhibited by the sulfhydryl group-modifying reagent N-ethylmaleimide (NEM). This maleimide inhibited taurine efflux evoked by 50% hyposmotic solutions with an IC50 of 54 microM. The inhibition by NEM followed pseudo-first order reaction kinetics. A double log plot of the pseudo-first order constant against NEM concentration gave a linear relationship with a slope of 1.2. The data are consistent with a simple bimolecular reaction mechanism in which one molecule of NEM reacts with one sulfhydryl group per transport unit. The membrane-impermeant maleimide derivative 5-eosin maleimide did not affect the volume-stimulated taurine efflux. The sulfhydryl-modifying mercurial reagents mersalyl and p-chloro mercuribenzenesulfonate (0.5-1 mM) increased [3H]taurine efflux under isosmotic conditions and concomitantly decreased the hyposmolarity-evoked efflux. The results demonstrate an essential requirement for sulfhydryl groups for the volume-sensitive taurine efflux.

The role of sulfhydryl groups and calcium in the mercuric chloride-induced inhibition of glutamate uptake in rat primary astrocyte cultures

Inhibition by mercuric chloride (MC) of the astrocytic uptake of the excitotoxic neurotransmitter L-glutamate (L-GLU) has been postulated to contribute to MC neurotoxicity. In the present study, we analyzed the ability of two sulfhydryl (SH)-protecting agents: a cell membrane non-penetrating compound-reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), to reverse the inhibitory action of MC on the initial rate of uptake of radiolabelled GLU (100 microM) in primary cultures of rat astrocytes. MC at 5 microM concentration reduced the uptake to 46% of control when present in the incubation medium during the 5 min of actual uptake, and to 27% of control when astrocytes were preincubated for 30 min in HEPES buffer containing MC prior to GLU uptake measurements. GLU uptake inhibition caused by 30 min preincubation with MC was partly relieved by the addition of 1 mM DTT during the actual 5 min uptake period. However, this inhibition could not be reversed by 1 mM GSH. Accordingly, it is postulated that the inhibitory effect exerted by MC on GLU uptake is associated with vulnerable SH groups located within, but not on the surface of the cell membrane. Neither 5 microM N-ethylmaleimide (NEM) nor 5 microM or 25 microM iodoacetate (IA) affected GLU uptake, indicating steric hindrance of the access of these two sulfhydryl reagents to the SH groups critical for the uptake.(ABSTRACT TRUNCATED AT 250 WORDS)