Toxicity and evidence for metabolic alterations in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated guinea pigs fed by total parenteral nutrition

Craniofacial form is altered by chronic adult exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in Han/Wistar and Long-Evans rats with different aryl hydrocarbon receptor (AhR) structures

Mammalian bone has shown a variety of responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in experimental and wildlife studies. Although many responses have been well characterized in the postcranial skeleton, dioxin-induced effects on the cranium are largely unknown. In this study, we investigated the effects of chronic adult exposure to TCDD on cranial size and shape in dioxin-resistant Han/Wistar (H/W) and dioxin-sensitive Long–Evans (L–E) rat strains. Three-dimensional landmark configurations for the face, vault, and base of the cranium were recorded and analyzed using geometric morphometrics (GM) and dose–response modeling. The strongest effects were shown by L–E and H/W rats with daily exposures of 100 and 1000 ng TCDD/kg bw/day, respectively, resulting in significant reductions in centroid size (CS) in all three cranial modules for both strains except for the vault in H/W rats. Consistent with previous evidence of intraspecific variation in TCDD resistance, the benchmark doses (CEDs) for cranial size reduction in L–E rats were roughly 10-fold lower than those for H/W rats. For both strains, the face showed the greatest size reduction from the highest doses of TCDD (i.e., 3.6 and 6.3% decreases in H/W and L–E rats, respectively), most likely related to dose-dependent reductions in limb bone size and body weight gain. However, intrinsic morphological differences between strains were also observed: although the control groups of H/W and L–E rats had vaults and bases of comparable size, the face was 6.4% larger in L–E rats. Thus, although H/W rats possess an altered aryl hydrocarbon receptor (AhR) that appears to mediate and provides some resistance to TCDD exposure, their smaller reductions in facial size may also relate to strain-specific patterns of cranial development and growth. Future research will be aimed at understanding how ontogenetic factors may modulate toxic effects of prenatal and lactational exposure on the mammalian skeleton.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

TCDD-induced anorexia and wasting syndrome in rats: effects of diet-induced obesity and nutrition

Interactions of diet and diet-induced obesity, and the characteristic wasting syndrome caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in TCDD-resistant Han/Wistar and TCDD-sensitive Long-Evans rats. The rats were made obese by feeding them either a high-energy diet (consisting of chocolate, cheese, and chow) or force feeding. TCDD reduced body weight in a parallel manner in lean and obese rats. The high-energy diet diminished the body weight loss and increased the survival time in L-E rats after a lethal dose of TCDD, while energy supplement with high-fat/low-protein food had an opposite effect. In conclusion, diet-induced obesity and TCDD had additive effects on body weight. Dietary manipulations were able to modify the weight loss and survival time after TCDD. Fat seems to have a negative impact, while carbohydrate or protein may have a positive impact in this respect. The results are in agreement with a view that TCDD-exposed rats have a negative fat balance favoring fat loss.

2,3,7,8-Tetrachlorodibenzo-p-dioxin increases reliance on fats as a fuel source independently of diet: evidence that diminished carbohydrate supply contributes to dioxin lethality

The environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes a wasting syndrome marked by hypophagia, loss of body fat, changes in intermediary metabolism and death. Use of conventional laboratory animals has not resolved whether or not TCDD affects intermediary metabolism independently of hypophagia. We used the chick embryo, which does not require an exogenous food supply for energy, to answer this question. Our results show that TCDD treatment increases dependence on fats as a fuel source independently of changes in food intake and therefore can affect intermediary metabolism independently of hypophagia. Results of experiments using aminocarnitine to inhibit fatty acid oxidation suggest that TCDD treatment impairs carbohydrate production rather than its utilization and that the former effect contributes to TCDD lethality.

Comparative effects of pair-feeding and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on various biochemical parameters in female rats

Hypophagia is a common characteristic of the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and may be responsible for some of the toxic manifestations. Pair-feeding has been used in control animals to compensate for the hypophagia, but relatively few studies have assessed biochemical changes associated with pair-feeding versus weight loss induced by TCDD. Rats were treated with TCDD and killed 7 days post-treatment while pair-fed animals received an amount of diet equivalent to TCDD-treated partner animals. Ad libitum-fed rats were also used. No correlations were seen in altered calcium and iron homeostasis between pair-feeding and TCDD administration relative to ad libitum-fed animals. Pair-feeding resulted in greater alterations than TCDD administration in the subcellular distribution of iron in mitochondria, microsomes and cytosol. Pair-feeding also resulted in greater accumulation of calcium in mitochondria and microsomes in pair-fed as compared to TCDD-treated animals. Greater lipid peroxidation was observed in whole liver and nuclei of rats receiving TCDD relative to pair-fed animals. A significantly greater incidence of DNA single strand breaks occurred in hepatic nuclei of TCDD-treated animals as compared to pair-fed and ad libitum-fed animals. Significantly greater inhibition of hepatic glutathione peroxidase activity and thymic involution were observed in TCDD treated animals as compared to the pair-fed group. Although some similarities existed between TCDD-treated animals and pair-fed rats, the overall biochemical changes which were observed following TCDD administration cannot be attributed to weight loss associated with hypophagia.

Mode of metabolism is altered in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

Male Sprague-Dawley rats were fed either a high-fat (HF) or a high-carbohydrate (HC) diet and subsequently injected with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (125 micrograms/kg) or vehicle (pair-fed controls). In all TCDD-treated animals, a reduction in caloric intake was evident as early as 1 day after dosage. Respiratory quotients (RQ) were determined at 5-day intervals. Their pattern for the HC-fed but not for the HF-fed TCDD-treated rats was different from that of the corresponding pair-fed controls. After an initial parallel decrease the RQ values remained low for TCDD-treated rats whereas they increased again for pair-fed controls. Serum total thyroxine (T4) was significantly lower in TCDD-treated animals and this reduction was not influenced by the composition of the diet. Serum triiodothyronine (T3) was neither altered by diet nor by TCDD. Thymic atrophy was as severe in pair-fed as in TCDD-treated rats fed the HC diet but not in rats fed the HF diet. Our results suggest that TCDD-treated rats are in a different mode of metabolism from pair-fed rats and that this difference is related to gluconeogenesis.

Modulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in F344 rats by di(2-ethylhexyl)phthalate

The effects of cotreatment with a hyperlipidemic chemical, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and a hypolipidemic agent, di(2-ethylhexyl)-phthalate (DEHP), on lipid metabolism and toxicologic responses were studied in F344 rats. Treatment with TCDD alone (160 micrograms/kg) caused an increase in serum triglycerides and cholesterol while treatment with DEHP alone (2 g/kg/day) caused a decrease in triglycerides and cholesterol versus untreated controls. When administered before or after TCDD, DEHP caused a decrease in TCDD-induced hyperlipidemia. This change was attributed to enhanced hepatic peroxisomal beta-oxidation and decreased hepatic lipid synthesis resulting from treatment with DEHP. TCDD treatment produced a fatty liver, as determined by gravimetric analysis of extracted lipid and microscopic examination of liver sections which revealed extensive cytoplasmic vacuolization that stained positive with Oil Red 0, but did not induce peroxisomal beta-oxidation. Thus, an increase in hepatic or serum lipid levels is not sufficient for induction of peroxisome proliferation. Neither TCDD nor DEHP treatment affected mitochondrial beta-oxidation. Pretreatment of rats with DEHP, followed by daily exposure to this hypolipidemic agent after treatment with TCDD, had a partial protective effect against TCDD-induced fatty liver, body weight loss and mortality. Microscopic examination of liver sections confirmed the suppression of TCDD-induced fatty liver by pretreatment with DEHP. When DEHP treatment was initiated after the TCDD dose, there was less protection against the above parameters of TCDD toxicity. This study demonstrates that TCDD-induced fatty liver, hyperlipidemia and mortality can be antagonized by treatment with a hypolipidemic agent such as DEHP.

Composition of diet modifies toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in cold-adapted rats

The effect of a high carbohydrate, high fat or high protein diet was studied on the acute toxicity of TCDD (125 micrograms/kg) in cold-adapted (4 +/- 1 degrees C) rats. Within 10 days after dosing, TCDD-treated rats fed a high carbohydrate or a high protein diet reduced their caloric intake by 25% whereas those fed a high fat diet consumed only 15% fewer kcal/MBS (metabolic body size). TCDD-treated rats fed a high protein diet lost body weight at the same rate as their pair-fed controls, whereas body weight loss in high fat-fed rats was significantly higher than in their pair-fed controls. In contrast, TCDD-treated rats fed a high carbohydrate diet effectively maintained their body weight in the 4 days immediately after TCDD dosage, whereas their pair-fed controls lost weight. Mortality in TCDD-treated animals was 100% irrespective of the diet; all pair-fed control rats (except one fed a high protein diet) were terminated on days corresponding to the spontaneous death of their TCDD-treated pairs. Mean time to 50% mortality and mean time to death were significantly longer in TCDD-treated rats fed a high carbohydrate diet in comparison with the other two TCDD-treated groups (p less than 0.05), although caloric intake was comparable. Serum triiodothyronine (T3) was reduced in TCDD-treated animals fed a high fat or a high carbohydrate diet but not in those fed a high protein diet; serum thyroxine (T4) was reduced in all the treated groups, irrespective of diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle glucocorticoid receptor and glutamine synthetase activity in the wasting syndrome in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

This study demonstrated specific changes in rat skeletal muscles after a single oral dose (100 micrograms/kg) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The development of the wasting syndrome was characterized by marked body weight loss, as well as atrophy of plantaris and gastrocnemius muscles. Fourteen days after administration of TCDD, gastrocnemius muscle cytosolic glucocorticoid receptor binding, measured at a single saturating concentration of [3H]triamcinolone acetonide, was significantly diminished, while plantaris muscle glutamine synthetase activity was strikingly elevated, indicating that specific biochemical alterations occur in skeletal muscle in the wasting syndrome.