Thomas K. C. Leung

The effects of cadmium, manganese and aluminium on sodium-potassium-activated and magnesium-activated adenosine triphosphatase activity and choline uptake in rat brain synaptosomes

The effects of Cd2+, Mn2+ and Al3+ on rat brain synaptosomal sodium-potassium-activated and magnesium-activated adenosine triphosphatase (Na-K-ATPase and Mg-ATPase) activity and choline uptake were studied. All three types of metal ions inhibited Na-K-ATPase activity more markedly than Mg-ATPase activity. The rank order of inhibition of Na-K-ATPase was: Cd2+ (ic50 = 5.4 μM) > Mn2+ (ic50 = 955 μm) > Al3+ (ic50 = 8.3 mM). The rank order of inhibition of Mg- was:Cd2+ (ic50 = 316 μM > Mn2+ (ic50 = 5.5 mM > Al3+ (ic50 = 21.9 mM). Al3+ was most potent in inhibiting synaptosomal choline uptake (ic50 = 24μM in the absence of Ca2+ and 123 μ.M in the presence of 1 mM Ca2+). Cd2+ (ic50 = 363 μM) was a more effective inhibitor of choline uptake than Mn2+(ic50 = 1.2−1.5 mM) . The presence of 1 mM Ca2+ did not alter choline uptake, nor did it antagonize the inhibitory actions of the three metals. Our observations that Cd2+ and Al3+ inhibited synaptosomal choline uptake, but did not show parallel inhibitory effects on Na-K-ATPase activity directly contradicts the ionic gradient hypothesis. These results are also discussed in relation to the in vivo neurotoxicity of cadmium, manganese and aluminium.

Brain Regional Distribution of Glutamic Acid Decarboxylase, Choline Acetyltransferase, and Acetylcholinesterase in the Rat: Effects of Chronic Manganese Chloride Administration after Two Years

Abstract: Rats were treated chronically with manganese chloride from conception onward for a period of over 2 years in order to study the effects of manganese and aging on the activities of glutamic acid decarboxylase (GAD), choline acetyltransferase (ChAT), and acetylcholinesterase (AChE) in hypothalamus, cerebellum, pons and medulla, striatum, midbrain, and cerebral cortex (which included the hippocampus). Manganese‐treated 2‐month‐old and 24‐ to 28‐month‐old rats and age‐matched controls were studied. In control rats during aging the activities of GAD decreased in hypothalamus (19%), pons and medulla (28%), and midbrain (22%) whereas the activities of AChE decreased in all regions (20–48%), particularly in the striatum (44–48%). Changes in ChAT activities in aging were observed only in one region—a decrease (23%) in the striatum. Life‐long treatment with manganese appeared to abolish partially the decreases in aging in AChE activities in hypothalamus, cerebellum and striatum, and striatal ChAT activity. Manganese treatment also seemed to abolish the age‐related decreases in GAD activities, since GAD activities in various brain regions of manganese‐treated senescent rats were not significantly different from those of control young rats. These results are discussed in relation to other metabolic changes associated with aging and manganese toxicity.

The monoamine oxidase inhibitors clorgyline and l-deprenyl also affect the uptake of dopamine, noradrenaline and serotonin by rat brain synaptosomal preparations

Abstract Clorgyline and l -deprenyl are, respectively, specific type A and type B monoamine oxidase (MAO) inhibitors. We investigated the effects of these two drugs as differential inhibitors of synaptosomal amine uptake and determined how far these effects might be predicted from their properties as specific MAO-A and MAO-B inhibitors. The rank order of inhibition of uptake by clorgyline was found to be: serotonin ( ic 50 = 10 μM) dopamine ( ic 50 = 56 μM) noradrenaline ( ic 50 = 66 μM). The rank order of inhibition of uptake by l -deprenyl was: noradrenaline ( ic 50 = 26 μM) serotonin ( ic 50 = 460 μM) dopamine ( ic 50 = 530 μM). The observation that clorgyline is a more specific inhibitor of the uptake of serotonin (a type A MAO substrate) is consistent with its activity as a type A MAO inhibitor. Paradoxically, L-deprenyl, though a type B MAO inhibitor, is fairly effective at inhibiting the uptake of noradrenaline (a type A MAO substrate).

Localization of a human heat-shock HSP 70 gene sequence to chromosome 6 and detection of two other loci by somatic-cell hybrid and restriction fragment length polymorphism analysis

SummaryThe human 70 kdalton heat-shock protein (HSP 70) is a member of a multigene family which is expressed in response to various physiological stresses including elevated temperatures. Using a cloned genomic HSP 70 DNA sequence we demonstrate by somatic cell hybrid and restriction fragment length polymorphism (RFLP) analyses that there are a minimum of three distinct HSP 70 loci in the human genome, one of which is located on chromosome 6.

The regional distribution of monoamine oxidase activities towards different substrates: effects in rat brain of chronic administration of manganese chloride and of ageing.

Abstract: The effects of chronic manganese chloride administration (1 mg MnCl2 4H2O/ml of drinking water) and ageing on the regional distribution of monoamine oxidase (MAO, EC 1.4.3.4) were studied in 2‐month‐ and 24–28‐month‐old rats. In both the control and Mn‐treated rats, the serotonin oxidation (type A) rates decreased in hypothalamus, pons and medulla, striatum, midbrain and cerebral cortex, but not in cerebellum, in ageing. On the other hand the benzylamine oxidation (type B) rates in hypothalamus, striatum and cerebral cortex increased in ageing. In all regions except the cerebellum, there was a uniform decrease in the A/B ratio. This decrease was verified by differential inhibition studies using clorgyline and l‐deprenyl, specific type A and type B inhibitors respectively. The dopamine‐oxidising rates decreased in all regions, except the cerebral cortex and the cerebellum, in ageing control rats. This age‐related decrease was not seen in the striatum and midbrain of manganese‐treated rats. In these rats the other effect was an age‐related increase in the rate of oxidation of all the amines in the cerebellum, not observed in control rats. These selective effects of manganese are only seen when comparing age‐related changes in both groups of animals, since comparison of manganese‐treated rats with age‐matched controls showed a significant difference only in the rate of serotonin oxidation in the cerebellum of 2‐month‐old rats. The relationship of these observations to the effects of ageing and manganese encephalopathy on specific amine systems is discussed.

Neurochemical changes in rats chronically treated with a high concentration of manganese chloride

Several neurochemical parameters were studied in brain regions of rats chronically treated with a high concentration of manganese chloride (20 mg MnCl2.4H2O per ml. of drinking water) throughout development until adulthood. Large increases in Mn accumulation were found in all brain regions (hypothalamus, +530%; striatum, +479%; other regions, +152 to +250%) of Mn-treated adult rats. In these animals, Ca levels were decreased (−20 to −46%) in cerebellum, hypothalamus, and cerebral cortex but were increased (+186%) in midbrain. Mg levels were decreased (−12 to −32%) in pons and medulla, midbrain, and cerebellum. Fe levels were increased (+95%) in striatum but were decreased (−28%) in cerebral cortex. Cu levels were increased (+43 to +100%) in pons and medulla and striatum but Zn levels were decreased (−30%) in pons and medulla. Na levels were increased (+22%) in striatum but those of K and Cl remained unchanged. Type A monoamine oxidase activities were decreased (−13 to −16%) in midbrain, striatum, and cerebral cortex, but type B monoamine oxidase activities decreased (−13%) only in hypothalamus. Acetylcholinesterase activities were increased (+20 to +22%) in striatum and cerebellum. The results are consistent with out hypothesis that chronic manganese encephalopathy not only affects brain metabolism of Mn but also that of other metals.

Chronic Manganese Treatment of Rats Alters Synaptosomal Uptake of Dopamine and the Behavioural Response to Amphetamine Administration

Abstract: Chronic manganese treatment from conception onward resulted in increased striatal synaptosomal uptake of dopamine, but not of a variety of other neurotransmitters/precursors in 80‐day‐old rats. The open‐field behaviour of these manganese‐treated 80‐day‐old rats was no different from that of untreated age‐matched rats. However, amphetamine administration (1 mg/kg body weight) increased activity to a significantly lower extent in manganese‐treated rats. These observations indicate that chronic manganese treatment results in marked alterations of activities associated with the dopaminergic system.

Differential Effects of Metal Ions on Type A and Type B Monoamine Oxidase Activities in Rat Brain and Liver Mitochondria

To investigate the hypothesis that neurotoxic metals can exert their toxicity through the direct inhibition of monoamine oxidases (MAOs), the effects of several neurotoxic metal ions on type A (MAO-A) and type B (MAO-B) monoamine oxidase activities in rat forebrain nonsynaptic mitochondria and rat liver mitochondria were studied. At pathophysiological levels (10–100 μM), Cu2+ and Cd2+ are good inhibitors of brain mitochondrial MAO-A and, to a lesser extent, liver mitochondrial MAO-A. The inhibition of MAO-B activities in brain and liver mitochondria by Cu2+ and Cd2+ is only detected at the higher end of the concentration range (i.e., 50–100 μM). At the pathophysiological level of 0.5 mM, Al3+ only inhibits brain mitochondrial MAO-A but at the higher level of 2.5 mM, it inhibits both forms of MAO in brain as well as liver mitochondria. Even at toxic levels (e.g., 5 mM), neither Mn2+ nor Li+ inhibits the activities of MAO-A and MAO-B in brain and liver mitochondria. Our results are consistent with the hypothesis that some neurotoxic metals can exert their toxicity through the direct inhibition of the isoforms of MAO. Our data also suggest that the selective inhibition of brain MAO-A by Cu2+ and Cd2+ may assume pathophysiological importance in the neurotoxicity of copper and cadmium.

The Effects of Chronic Manganese Chloride Treatment Expressed as Age-Dependent, Transient Changes in Rat Brain Synaptosomal Uptake of Amines

Abstract: As a result of chronic manganese treatment of rats from conception onwards, a decrease was observed in the uptake of dopamine, but not of noradrenaline or serotonin, by synaptosomes isolated from hypothalamus, striatum, and midbrain and in choline uptake by hypothalamic synaptosomes obtained from 70–90‐day‐old animals. In 100–120‐day‐old manganese‐treated rats the only difference observed was increased choline uptake by striatal synaptosomes. All comparisons were with age‐matched controls. These results, which are consistent with views of a dopaminergic and cholinergic involvement in manganese encephalopathy, point out that changes in these systems are observable only at specific times during manganese intoxication.

The effects of chronic manganese feeding on the activity of monoamine oxidase in various organs of the developing rat.

1. Chronic manganese feeding of rat with doses as high as 10 mg/ml in drinking water had no effect on body weight increases during postnatal development. The organ weight increases in brain, liver, heart and kidney also remained unaffected, but spleen weight was consistently lower than in the age-matched controls after Mn-feeding, being more marked at the higher doses. An increase in the Mn concentration to 20 mg/ml led to drastic body weight losses not unlike that seen in malnutrition. 2. There were differential developmental changes in monoamine oxidase (MAO) with respect to tissue type and substrate used. Manganese feeding did not affect the developmental patterns of MAO in brain, heart and kidney. However, hepatic MAO activities towards 5-HT and BzNH2 were found to increase after 10--15 days of postnatal life. 3. In contrast, the activity in the spleen towards 5-HT was lower in the high Mn-treated group in the first few days post-partum.