Association between the levels of biogenic amines and superoxide anion production in brain regions of rats after subchronic exposure to TCDD

Abstract

The effects of TCDD on the distribution of biogenic amines and production of superoxide anion (SA) in different brain regions of rats have been studied after subchronic exposure. Groups of females Sprague–Dawley rats were administered daily dose of 46 ng TCDD/(kg day) (treated groups), or the vehicle used to dissolve TCDD (control group), for 90 days. The rats were sacrificed at the end of the exposure period and their brains were dissected into different regions including, hippocampus (H), cerebral cortex (Cc), cerebellum (C), and brain stem (Bs). The levels of different biogenic amines and some of their metabolites, including, norepinephrine (NE), dopamine (DA), 3,4-dihydroxy phenyl acetic acid (DOPAC), 4-hydroxy-3-methoxy-phenyl acetic acid (HVA), 5-hydroxy tryptamine (5-HT), and 5-hydroxy indole 3-acetic acid (5-HIAA), were determined in those brain regions, using a high performance liquid chromatography (HPLC) system with an electrochemical detector. SA production was also determined in those regions, using the cytochrome c reduction method. Results of analyses indicate significant increases in the levels of DA, NE and DOPAC in H, NE and HVA in Cc, NE and DA in Bs and NE in C. SA production was significantly increased in H and Cc, but not in Bs or C. The results also indicated strong correlations between DA and DOPAC, and SA and NE in all of the brain regions, and also between SA and 5-HT/HIAA in H and Cc. These results may indicate the contribution of biogenic amines, especially NE and 5-HT/HIAA to SA overproduction in some brain regions and may also indicate the potential of long term neurotoxic effects of those biogenic amines, in response to subchronic exposure to TCDD.

Introduction

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant that produces a wide range of acute and long term toxic, and biochemical effects in experimental animals (Alsharif et al., 1999, DeVito and Birnbaum, 1994, Devito et al., 1997, Hunter et al., 1999, Kimbrough, 1974, Kociba et al., 1976, Kociba et al., 1978, Pohjanvirta and Tuomisto, 1994, Poland and Knutson, 1982, Safe, 1990). One of the most prominent acute effects in rats, is hypohagia or feed intake reduction that can lead to the death of animals (Rozamn et al., 1991, Stahl et al., 1991, Tuomisto et al., 1990, Unkila et al., 1993, Unkila et al., 1995, Unkila et al., 1999). In an effort to understand the mechanism of hypophagia, studies were focused on determining the effects of acute doses of TCDD on the levels of biogenic amines in the brains of the animals (Rozamn et al., 1991, Tuomisto et al., 1990, Unkila et al., 1993). Biogenic amines are a class of low-molecular weight transmitters that includes the catecholamines, such as dopamine (DA), norepinephrine (NE) and epinephrine (E), and the serotonin or 5-hydroxytryptamine (5-HT), which are synthesized in brain. DA is synthesized from tyrosine, and about half of the DA formed in the cytoplasm is actively transported into DA beta hydroxylase-containing storage vesicles, where it is converted to NE. The remainder is deaminated to 3,4-dihydroxyphenylacetic acid (DOPAC), and subsequently o-methylated to 4-hydroxy-3-methoxy-phenylacetic acid (HVA) by monoamine oxidase (MAO) and catechol o-methyl transferase (COMT), respectively. 5-HT is synthesized from tryptophan, and could be degraded by MAO, giving rise to 5-hydroxyindole-3 acetic acid (5-HIAA). Rozamn et al. (1991) have observed progressive, and time-dependent increases in tryptophan levels in plasma and brain of TCDD-treated rats, which were paralleled by increases in brain 5-HT and 5-HIAA, and have suggested a serotonergic mechanism for TCDD-induced hypophagia. However, studies by Tuomisto et al. (1990) have demonstrated minor changes in brain neurotransmitter systems of rats, in response to lethal doses of TCDD, and suggested that the observed changes are not likely the key mediators of TCDD-induced hypophagia. Similarly, Unkila et al. (1993) argued against a crucial role for biogenic amines as mediators of TCDD-induced hypophagia, although they found that TCDD increased tryptophan level in some brain regions of rats. Also, Stahl et al. (1991) have shown that depletion of brain serotonin does not alter TCDD-induced hypophagia in rats.

Studies on the tissue distribution of TCDD in mice have found significant and dose-related recovery of the compound in different tissues, including the brain (Diliberto et al., 1995). Long term exposure of rodents to TCDD was found to result in dose-dependent increases in the production of reactive oxygen species (ROS), lipid peroxidation (LP) and DNA damage in brain tissues (Hassoun et al., 1998, Hassoun et al., 2000, Hassoun et al., 2002). Studies on the long term TCDD effects on various brain regions of rats have demonstrated significant increases in different biomarkers of oxidative stress, associated with suppression of different antioxidant enzyme activities in the cerebral cortex (Cc) and hippocampus (H), but not in the cerebellum (C) or brain stem (Bs) (Hassoun et al., 2003).

Oxidative deamination of biogenic amines by the mitochondrial and cytosolic MAO is found to be associated with production of large concentrations of ROS (Agostinelli et al., 2004, Cadenas and Davies, 2000, Siraki and O’Brien, 2002, Toninello et al., 2004). Catecholamines were also found to cause a reduction in intracellular glutathione level and accumulation of ROS in oligodendrocytes, in culture (Korchid et al., 2002). Incubation of some DA precursors and related catechols with DNA, proteins and lipids was shown to result in oxidative damage to these molecules, the majority of which appeared to be mediated by ROS, such as superoxide anion (SA) and hydroxyl radicals, in addition to semiquinone radicals and quinones (Pattison et al., 2002).

This study was undertaken to investigate the long term effect of TCDD exposure on the levels of some biogenic amines and their metabolites in different brain regions of rats, and also the possible association between those changes and TCDD-induced production of ROS, in select brain regions of those rats.