Keith M. Erikson

Iron Deficiency Decreases dopamine D1 and D2 receptors in rat brain

Iron deficiency (ID) in early life is known to alter neurological development and functioning, but data regarding specific effects on dopamine biology are lacking. The objective of this study was to determine the extent of functional alterations in dopamine receptors in two dopaminergic tracts in young, growing, iron-deficient rats. Forty male and 40 female weanling Sprague-Dawley rats were fed either an iron-deficient (ID) diet or control (CN) diet for 6 weeks. ID decreased densities of D(1) and D(2) receptors in the caudate-putamen and decreased D(2) receptor densities in the nucleus accumbens. There were no apparent effects of ID on the affinities for the ligands in either receptor in several brain regions. In situ hybridization studies for both dopamine receptors revealed no significant effect of ID on mRNA expression for either receptor. Iron-deficient rats had a significantly higher ED(50) for raclopride-induced hypolocomotion in male and female rats compared to control rats of each sex. The loss of iron in the striatum due to dietary ID was significantly correlated with the decrease in D(2) receptor density; however, this relationship was not apparent in other brain regions. These experiments thus demonstrate abnormal dopamine receptor density and functioning in several brain regions that are related to brain regional iron loss. Importantly, the impact of ID on dopamine was more pronounced in males than females, demonstrating sex-related different sensitivities to nutrient deprivation.

Manganese and its Role in Parkinson's disease: from Transport to Neuropathology.

The purpose of this review is to highlight recent advances in the neuropathology associated with Mn exposures. We commence with a discussion on occupational manganism and clinical aspects of the disorder. This is followed by novel considerations on Mn transport (see also chapter by Yokel, this volume), advancing new hypotheses on the involvement of several transporters in Mn entry into the brain. This is followed by a brief description of the effects of Mn on neurotransmitter systems that are putative modulators of dopamine (DA) biology (the primary target of Mn neurotoxicity), as well as its effects on mitochondrial dysfunction and disruption of cellular energy metabolism. Next, we discuss inflammatory activation of glia in neuronal injury and how disruption of synaptic transmission and glial-neuronal communication may serve as underlying mechanisms of Mn-induced neurodegeneration commensurate with the cross-talk between glia and neurons. We conclude with a discussion on therapeutic aspects of Mn exposure. Emphasis is directed at treatment modalities and the utility of chelators in attenuating the neurodegenerative sequelae of exposure to Mn. For additional reading on several topics inherent to this review as well as others, the reader may wish to consult Aschner and Dorman (Toxicological Review 25:147–154, 2007) and Bowman et al. (Metals and neurodegeneration, 2009).

Manganese neurotoxicity and glutamate-GABA interaction.

Brain extracellular concentrations of amino acids (e.g. aspartate, glutamate, taurine) and divalent metals (e.g. zinc, copper, manganese) are primarily regulated by astrocytes. Adequate glutamate homeostasis is essential for the normal functioning of the central nervous system (CNS). Glutamate is of central importance for nitrogen metabolism and, along with aspartate, is the primary mediator of the excitatory pathways in the brain. Similarly, the maintenance of proper manganese levels is important for normal brain functioning. Several in vivo and in vitro studies have linked increased manganese concentrations with alterations in the content and metabolism of neurotransmitters, namely dopamine, gamma-aminobutyric acid, and glutamate. It has been reported by our laboratory and others, that cultured rat primary astrocytes exposed to manganese displayed decreased glutamate uptake, thereby increasing the excitotoxic potential of glutamate. Furthermore, decreased uptake of glutamate has been associated with decreased gene expression of glutamate:aspartate transporter (GLAST) in manganese-exposed astroctyes. Additional studies have suggested that attenuation of astrocytic glutamate uptake by manganese may be a consequence of reactive oxygen species (ROS) generation. Collectively, these data suggest that excitotoxicity may occur due to manganese-induced altered glutamate metabolism, representing a proximate mechanism for manganese-induced neurotoxicity.

Manganese accumulates in iron-deficient rat brain regions in a heterogeneous fashion and is associated with neurochemical alterations

Previous studies have shown that iron deficiency (ID) increases brain manganese (Mn), but specific regional changes have not been addressed. Weanling rats were fed one of three semipurified diets: control (CN), iron deficient (ID), or iron deficient/manganese fortified (IDMn+). Seven brain regions were analyzed for Mn concentration and amino acid (glutamate, glutamine, taurine, γ-aminobutyric acid) concentrations. Both ID and IDMn+ diets caused significant (p<0.05) increases in Mn concentration across brain regions compared to CN. The hippocampus was the only brain region in which the IDMn+ group accumulated significantly more Mn than both the CN and ID groups. ID significantly decreased GABA concentration in hippocampus, caudate putamen, and globus pallidus compared to CN rats. Taurine was significantly increased in the substantia nigra of the IDMn+ group compared to both ID and CN. ID also altered glutamate and glutamine concentrations in cortex, caudate putamen, and thalamus compared to CN. In the substantia nigra, Mn concentration positively correlated with increased taurine concentration, whereas in caudate putamen, Mn concentration negatively correlated with decreased GABA. These data show that ID is a significant risk factor for central nervous system Mn accumulation and that some of the neurochemical alterations associated with ID are specifically attributable to Mn accumulation.

Interactions between excessive manganese exposures and dietary iron-deficiency in neurodegeneration

For nearly a century, manganese has been recognized as an essential nutrient for proper bone formation, lipid, amino acid and carbohydrate metabolism. While manganese deficiency is characterized by symptoms ranging from stunted growth and poor bone remodeling to ataxia, it is manganese toxicity that is far more devastating from a public health standpoint. Most cases of manganese toxicity are the result of occupational exposure to high levels of the metal, and are characterized by specific neurological symptoms referred to as manganism. While manganism shares many common features with Parkinsons disease, there are distinct differences between the two disorders suggesting that manganism might indirectly affect nigrostriatal dopaminergic function. Recent studies from our laboratory show that dietary iron deficiency is a risk factor for brain manganese accumulation and that the striatum is particularly vulnerable. This review briefly discusses manganese from nutritional and toxicological aspects.

The effects of manganese on glutamate, dopamine and γ-aminobutyric acid regulation

Exposure to high levels of manganese (Mn) results in a neurological disorder, termed manganism, which shares a similar phenotype to Parkinsons disease due to the involvement of the basal ganglia circuitry in both. The initial symptoms of manganism are likely due to the involvement of the globus pallidus, a region rich in γ-aminobutyric acid (GABA) projections, while those of Parkinsons disease are related to the degeneration of the substantia nigra, a dopaminergic nucleus. Additionally, it is known that glutamate regulation is affected by increases in brain Mn levels. As Mn predominantly accumulates in the basal ganglia, it potentially could affect the regulation and interactions of all three neurotransmitters. This review will focus on the circuitry of these neurotransmitters within the basal ganglia and address potential sites for, as well as the temporal relationship, between Mn exposure and changes in the levels of these neurotransmitters. While most research has focused on perturbations in the dopaminergic system, there is evidence to support that early consequences of manganism also include disturbances in GABA regulation as well as glutamatergic-related excitotoxicity. Finally, we suggest that current research focus on the interdependence of these basal ganglial neurochemicals, with a greater emphasis on the GABAergic and glutamatergic systems.

Manganese causes differential regulation of glutamate transporter (GLAST), taurine transporter, and metallothionein in cultured rat astrocytes

Neurotoxicity due to excessive brain manganese (Mn) can occur due to environmental (air pollution, soil, water) and/ or metabolic aberrations (decreased biliary excretion). Manganese is associated with oxidative stress, as well as alterations in neurotransmitter metabolism with concurrent neurobehavioral deficits. Based on the few existing studies that have examined brain regional [Mn], it is likely that in pathological conditions it can reach 100-500 microM. Amino acid (e.g. aspartate, glutamate, taurine), as well as divalent metal (e.g. zinc, manganese) concentrations are regulated by astrocytes in the brain. Recently, it has been reported that cultured rat primary astrocytes exposed to Mn displayed decreased glutamate uptake, thereby, increasing the excitotoxic potential of glutamate. Since the neurotoxic mechanism(s) Mn employs in terms of glutamate metabolism is unknown, a primary goal of this study was to link altered glutamate uptake in Mn exposed astrocytes to alterations in glutamate transporter message. Further, we wanted to examine the gene expression of metallothionein (MT) and taurine transporter (tau-T) as markers of Mn exposure. Glutamate uptake was decreased by nearly 40% in accordance with a 48% decrease in glutamate/aspartate transporter (GLAST) mRNA. Taurine uptake was unaffected by Mn exposure even though tau-T mRNA increased by 123%. MT mRNA decreased in these Mn exposed astrocytes possibly due to altered metal metabolism, although this was not examined. These data show that glutamate and taurine transport in Mn exposed astrocytes are temporally different.

Neurobehavioral analysis of developmental iron deficiency in rats

Iron deficiency (ID) in early life alters the course of behavioral and cognitive development in humans, causing decreased physical activity and responsiveness to the environment. The effects of ID on behavior are similar in rats and hypothesized to be related to ID-related impairments in central dopamine pathways. The objective of this study was to examine the association between brain iron measures of dopamine function, and behavioral measures of activity and reactivity. Male and female weanling rats were fed either an iron deficient diet or control diet for 6 weeks. The iron deficient rats showed significantly decreased activity and increased anxiety-like behaviors. Iron deficient rats also showed significant decrements in brain iron content in the corpus striatum, prefrontal cortex, and midbrain and decreases in dopamine receptors and the transporter in the same areas. Multiple regression analysis showed ventral midbrain iron concentration and dopamine D(1) receptor density to be highly associated with exploration and repeated movements, respectively. In addition, the results showed anxiety-like behaviors to be related to prefrontal cortex dopamine transporter and dopamine D(1) receptor densities. We conclude from these analyses that iron concentration in dopamine containing regions and densities of dopamine receptors and the transporter, are significant predictors of measures of activity and reactivity. These observations also strengthen the argument that the Fe-dopamine link is fundamental to understanding biobehavioral difficulties seen in children with ID anemia.

Ferroportin is a manganese-responsive protein that decreases manganese cytotoxicity and accumulation.

J. Neurochem. (2010) 112, 1190–1198.

Functional coding variation in recombinant inbred mouse lines reveals multiple serotonin transporter-associated phenotypes

The human serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT, SLC6A4) figures prominently in the etiology and treatment of many prevalent neurobehavioral disorders including anxiety, alcoholism, depression, autism, and obsessive-compulsive disorder (OCD). Here, we use naturally occurring polymorphisms in recombinant inbred (RI) lines to identify multiple phenotypes associated with altered SERT function. The widely used mouse strain C57BL/6J, harbors a SERT haplotype defined by 2 nonsynonymous coding variants [Gly-39 and Lys-152 (GK)]. At these positions, many other mouse lines, including DBA/2J, encode, respectively, Glu-39 and Arg-152 (ER haplotype), amino acids found also in hSERT. Ex vivo synaptosomal 5-HT transport studies revealed reduced uptake associated with the GK variant, a finding confirmed by in vitro heterologous expression studies. Experimental and in silico approaches using RI lines (C57BL/6J × DBA/2J = BXD) identify multiple anatomical, biochemical, and behavioral phenotypes specifically impacted by GK/ER variation. Among our findings are several traits associated with alcohol consumption and multiple traits associated with dopamine signaling. Further bioinformatic analysis of BXD phenotypes, combined with biochemical evaluation of SERT knockout mice, nominates SERT-dependent 5-HT signaling as a major determinant of midbrain iron homeostasis that, in turn, dictates iron-regulated DA phenotypes. Our studies provide an example of the power of coordinated in vitro, in vivo, and in silico approaches using mouse RI lines to elucidate and quantify the system-level impact of gene variation.