Differential effects of metal ligands on synaptic membrane glutamate binding and uptake systems

Ammonia-induced alterations in glutamate and muscimol binding to cerebellar synaptic membranes

Binding of glutamate and muscimol (an agonist for GABAA receptors) to their respective receptors has been studied in the cerebellum of normal and hyperammonemic rats. There was a decrease in both high- and low-affinity binding of glutamate in the cerebellum during hyperammonemia. Kinetic studies revealed that the decrease is due to a reduction in the number of binding sites, but not due to changes in the binding affinities. Further studies also revealed that the decrease was only in the N-methyl-D-aspartate (NMDA)-specific binding sites without any alterations in the binding to non-NMDA sites represented by kianic acid (KA)- and quisqualic acid (QQ)-sensitive receptor sites. These effects were also mimicked when the membrane preparations from the cerebellum of normal animals were incubated with ammonium acetate. Enhancement of muscimol binding was observed in animals injected with ammonium acetate. It is concluded that hyperammonemic states, even in the presence of a functional liver, are capable of altering amino acid neurotransmission and this might play an important role in cerebral dysfunction under these conditions.

Magnetic resonance imaging in multiple sclerosis: decreased signal in thalamus and putamen

High-field strength (1.5 Tesla) magnetic resonance imaging in 15 patients with multiple and extensive white-matter lesions and clinically definite multiple sclerosis delineated a previously undescribed finding of abnormally decreased signal intensity on T2-weighted images in the thalamus and putamen. The decreased signal intensity (preferential decreased T2 relaxation time) is most likely to be related to abnormally increased iron accumulation causing local magnetic field heterogeneities.

Presence of proteins and glutamate as major constituents of the venom of the spider Araneus gemma

The venom from the spider Araneus gemma contains an inhibitor of physiologic glutamate receptors and of glutamate binding sites in brain synaptic membranes. In the present study the chemical composition of the venom was examined in order to determine the presence of constituents that may have physiologically important actions on the prey. The milked venom contains high concentrations of protein (approximately equal to 200 micrograms/microliter) and of glutamate (130-425 mM) and very low concentrations of epinephrine, epinine, dopamine and 3,4-dihydroxyphenylacetic acid. There is also marked heterogeneity in the venom peptides detected by SDS gel electrophoresis. The presence of free glutamate may be very important for the actions of the glutamate receptor inhibitor in the venom.

Iron concentration reduced in ventral pallidum, globus pallidus, and substantia nigra by GABA-transaminase inhibitor, gamma-vinyl GABA

Recent histochemical studies indicate that there is considerable overlap of brain areas accumulating iron in oligodendrocytes with those in which GABA neurons terminate. The ventral pallidum, globus pallidus, substantia nigra and cerebellar nuclei are iron-rich areas, receive GABA-containing efferents, and have high concentrations of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). The present study examines the effect of disruption of the metabolism of GABA on the accumulation of iron in GABAergic projection sites. Gamma-vinyl GABA, an enzyme activated inhibitor of GABA-transaminase, was injected unilaterally into the globus pallidus and adjacent striatum or into the substantia nigra of the rat brain. Additional animals received unilateral injections of saline into the same areas or an electrocoagulation lesion of the globus pallidus and surrounding striatum. Two days after injection or lesion all animals were perfused and 40 micron sections of the brain were processed with the Perls' + diaminobenzidine (DAB) histochemical method for iron. The intensity of iron stain was measured with densitometry. Gamma-vinyl GABA injection into the striatum/pallidum resulted in a significant reduction in iron concentration in the ipsilateral ventral pallidum, globus pallidus and substantia nigra. Gamma-vinyl GABA injected into the substantia nigra reduced iron in the injection site. This study provides evidence that the presence of iron in the brain is related to the utilization of GABA.

The regional distribution and cellular localization of iron in the rat brain

The regional distribution and cellular localization of iron throughout the rat brain was determined with iron histochemistry. Densitometry was used to measure the intensity of stain of 51 iron-concentrating sites. Among the areas of highest iron content are the circumventricular organs, islands of Calleja, globus pallidus, ventral pallidum, substantia nigra pars reticulata, interpeduncular nucleus, dentate nucleus, and interpositus nucleus. Iron occurs most commonly in oligodendrocytes and in the fibrous network of the neuropil, but is also found in the interstitial spaces of circumventricular organs and in the tanycytes of the organum vasculosum of the lamina terminalis, median eminence, and walls of the third ventricle. In diverse areas throughout the brain--among them, the islands of Calleja, dentate gyrus of the hippocampal formation, lateral septal nucleus, and central amygdala--iron is found in association with the perikarya and neuronal processes of nerve cells. The overlapping distribution patterns of iron and gamma-aminobutyric acid, enkephalin, and luteinizing hormone-releasing hormone suggest that the distribution of iron is related to its association with the metabolism of one or more neurotransmitters or neuroactive compounds.