Relationship of the wasting syndrome to lethality in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Young adult male Sprague-Dawley rats treated with a LD95 dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibited a progressive reduction in feed intake and body weight until death occurred 15 to 32 days post-treatment. The time course and magnitude of weight loss and lethality of pair-fed control rats were essentially identical to that of TCDD-treated rats with each pair-fed control animal dying within 3 days of its TCDD-treated partner. Body composition analysis of the dead animals revealed that the total amounts of protein, fat, water, and ash in the carcasses of TCDD-treated and pair-fed control rats were each reduced to a similar extent. The temporal pattern of daily feed intake in TCDD-treated and pair-fed control rats (3 meals/day) or (1 meal/day) did not influence the results. Studies conducted at LD25-62 doses of TCDD in male Sprague-Dawley rats of different ages--weanling (90 g), young adult (275 g), and mature (450 g)--showed that the severity of the wasting syndrome in all age groups was greatest for animals that died. Also, young adult rats treated with a LD25 dose of TCDD that died displayed the same degree of hypophagia and weight loss prior to death as rats administered a LD95 dose. Histopathology of the liver and gastrointestinal tract was compared in TCDD-treated (LD95 dose) and pair-fed control rats killed 1 day before they otherwise would have died. Hepatocytes of TCDD-treated rats were enlarged relative to those of pair-fed control rats and contained nuclei that varied in size and number. Pair-fed control rats exhibited atrophy of the liver cords due to a decrease in the cytoplasmic volume of their hepatocytes. The stomach and small intestine of TCDD-treated rats were histologically similar to those of ad libitum-fed controls. In contrast, the glandular mucosa of the stomach of pair-fed control rats was ulcerated and the intestinal mucosa was atrophied. Stomach ulcers were the source of clotted blood found throughout the gastrointestinal tract of pair-fed control rats but not that of TCDD-treated animals. These findings demonstrate that hypophagia-induced weight loss is one of perhaps several responses that contribute to the death of TCDD-treated rats. That other responses are also involved is suggested by differences between pair-fed control and TCDD-treated rats in the weight and histopathology of certain organs. In addition, gastrointestinal blood loss contributes to the death of pair-fed control rats but not TCDD-treated animals.     

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.     

Androgenic deficiency in male rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the male reproductive system were investigated. Sexually mature (290 g) Sprague-Dawley rats were given single oral doses of TCDD sufficient to cause varying degrees of hypophagia and impaired body weight gain. The largest doses decreased plasma testosterone and dihydrotestosterone concentrations by 90 and 75%, respectively, from ad libitum-fed control values, while decreasing seminal vesicle and ventral prostate weights by 68 and 48%. On Day 7, the approximate ED50 for these responses was 15 micrograms TCDD/kg, a nonlethal dose. Reductions in caput epididymis and testis weights were also observed. The androgenic deficiency was seen as early as 2 days after dosing and persisted for at least 12 days. Based on data from pair-fed control rats, only about half the decreases in accessory sex organ weights and in plasma androgen concentrations could be accounted for by TCDD-induced hypophagia or body weight loss. These signs of androgenic deficiency were not the result of stress (based in part on plasma corticosterone assays), nor could they be accounted for by the known effects of TCDD on steroid metabolism. While the TCDD-induced depression in plasma testosterone concentrations appears to be the primary event observed, the mechanism by which testosterone concentrations were decreased remains unknown. The androgenic deficiency may account for the male reproductive pathology and dysfunction in animals treated with overtly toxic doses of TCDD.     

Body weight regulation in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Rats treated with a sublethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 15 μg/kg) exhibited reduced feed intake and loss of body weight for the first 3 weeks after treatment. During the next 10 weeks, TCDD-treated rats maintained their body weight at a lower nearly constant percentage of that of control rats fed ad libitum. At no time did rats treated with TCDD exhibit hyperphagia which would have returned their weight to a normal level. Control rats pair-fed to TCDD-treated rats for more than 7 weeks displayed compensatory hyperphagia when permitted to feed ad libitum and their weight recovered to a near-normal level. The lower level of body weight in TCDD-treated animals was apparently due to a reduction in the regulation level or “set-point” for body weight. The following findings in TCDD-treated and control rats fed ad libitum supported this idea. First, when the reduced weight of the TCDD group was challenged by changes in the caloric density or palatability of the diet, TCDD-treated rats exhibited adjustments in feed intake and body weight that were essentially identical to those of control rats. Second, when body weight was manipulated by feeding a high-calorie diet or by restricting feed intake, both TCDD-treated and control rats quickly returned to weight levels from which they had been displaced. Third, carcass analyses conducted 7 weeks after treatment revealed that TCDD-treated rats had lower absolute amounts of body fat, protein, and water. However, when these constituents were expressed as percentages of total body weight no remarkable differences from the control were observed. Fourth, when TCDD-treated rats were induced to overeat and restore body weight to the same levei as control rats fed ad libitum, TCDD-treated animals did not reassume a normal body composition but became obese. Obesity was also observed when control rats were induced to overeat. Thus, TCDD-treated rats regulate their body weight in the same fashion as control rats but at a weight regulation level or set-point that is markedly reduced.     

Thyroid status and thermogenesis in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Several key aspects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity resemble the effects of hypothyroidism, while in other ways the toxic responses are characteristics of hyperthyroidism. Whether thyroid dysfunction plays a role in TCDD toxicity remained unknown, however. We therefore determined the dose-related effects of TCDD treatment on plasma concentrations of L-thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3), and thyroid-stimulating hormone (TSH), and compared these changes with signs of TCDD toxicity. We also determined whether indices of functional thyroid status (and thermogenesis) were altered in response to TCDD treatment. Young adult male Sprague-Dawley rats were given single oral doses of TCDD (6.25-100 micrograms/kg) and evaluated 1 week later. Toxicity, measured by decreases in feed intake and body weight, ranged from minimal to severe. Plasma concentrations of T4 were greatly reduced at all doses tested, while T3 was increased in a dose-related fashion (up to 35%). TSH was elevated but was inversely proportional to dose. Thyroid histology was unremarkable, and TCCD treatment had little effect on the ability of rats to raise serum T4, T3, and TSH concentrations in response to acute cold stress. TCDD treatment caused a slight (8%) decrease in basal metabolic rate, yet comparable decreases were seen in pair-fed control animals. Thermogenesis, as measured by O2 consumption and colonic temperatures in rats exposed to various ambient temperatures, was only marginally affected. In summary, although thyroid hormone concentrations were markedly altered, rats given doses of TCDD sufficient to cause overt toxicity appeared to be essentially euthyroid. These results do not support proposals by other researchers that altered thyroid status is a major contributor to TCDD toxicity and/or a key response to TCDD exposure.     

Hepatic indices of thyroid status in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

The functional thyroid status of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is unknown. Therefore, activities of certain thyroid-responsive enzymes were examined in the livers of adult male Sprague-Dawley rats 1 week after treatment with TCDD (6.25, 25 or 100 micrograms/kg). Activity of the thyroid-responsive flavin L-glycerol-3-phosphate dehydrogenase (per mg mitochondrial protein) was decreased slightly in livers of TCDD-treated rats, while that of succinate dehydrogenase remained unchanged. In contrast, activities (per mg supernatant protein) of three thyroid-responsive NADP-dependent cytosolic enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were increased by TCDD treatment in a dose-dependent manner. Lactate dehydrogenase (activity per mg supernatant protein) was also augmented slightly 1 week after TCDD administration. Liver mass was increased by TCDD treatment in a dose-dependent manner, but DNA content per liver was similar at all doses examined. Total hepatic protein, expressed per liver or mg hepatic DNA, was increased in TCDD-treated rats when compared to their pair-fed counterparts. The decreased activity of the mitochondrial L-glycerol-3-phosphate dehydrogenase, in contrast to the increased activities of the supernatant enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, is not consistent with a shift in functional thyroid status following TCDD treatment.     

Inhibition of intestinal glucose absorption in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Male Sprague-Dawley rats were injected with either 125 micrograms 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg or vehicle (pair-fed and ad libitum-fed controls). Transfer of water, electrolytes and D-glucose as well as fats of a tracer dose of the non-metabolizable radioactive marker 3-O-methyl-D-[U-14C]glucose was studied in isolated perfused jejunal segments 1, 2, 7, and 21 d after treatment (TCDD-treated and pair-fed control rats) and after 26 d in ad libitum-fed controls. TCDD-treated rats demonstrated reduced feed consumption and loss of body weight. Active intestinal absorption of glucose was significantly inhibited 30 and 22% compared to pair-fed controls, respectively 2 and 7 d after TCDD treatment. After 21 d the inhibition (14%) was less significant. There were no differences in glucose transfer between severely starved pair-fed controls (body weights 370 +/- 26 g) and ad libitum-fed rats (body weights 512 +/- 15 g). Water absorption and transfer of sodium and calcium was not influenced by TCDD treatment. However, a significant increase of potassium transfer was observed in parallel with impaired glucose absorption. The uptake of 3-O-methylglucose into mucosal tissue was not impaired, whereas the transfer to the serosal side was significantly inhibited by 30-60% compared to pair-fed as well as ad libitum-fed animals from day 2 until the end of the experiment. These results suggest that TCDD is involved in an inhibition of glucose transport at the basolateral membrane.     

Digestible energy and efficiency of feed utilization in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Treatment of male rats (300 to 325 g) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 15 or 50 micrograms/kg) caused dose-dependent reductions in body weight, feed and water intakes, and fecal output. Urine output, however, was not altered by TCDD. Fecal energy loss, as a percentage of daily feed energy intake (kcal/day), was similar in control and TCDD-treated rats as was the percentage of feed energy absorbed by the gastrointestinal tract, i.e., digestible energy. These findings dispel the long-standing proposal that a gross malabsorption syndrome is responsible for weight loss in TCDD-treated rats and place greater emphasis on hypophagia as the reason for weight loss. In support of a central role for hypophagia, it was found that control rats pair-fed to rats treated with a sublethal dose of TCDD (15 micrograms/kg) lost almost the same amount of weight. However, from 15 to 50 days post-treatment, the pair-fed animals consistently maintained their weight at a 10- to 15-g higher level than age-matched TCDD-treated rats. To determine why this weight difference occurs, the efficiency of feed utilization from Day 30 to 45 post-treatment in ad libitum fed control and TCDD-treated rats (15 micrograms/kg) that were maintaining different levels of body weight was compared. First, daily feed intakes of TCDD-treated and control rats were determined from Day 25 to 30 post-treatment. Second, weights of both groups were lowered by reducing feed intake in two successive 5-day periods to 50 and 10% of the respective ad libitum level. Third, on Days 40 to 45, both groups were refed their prereduction level of intake but reduced in proportion to the intervening loss in metabolic tissue mass. At each level of feed energy reduction, weight losses observed in the TCDD-treated and control rats were equivalent. Furthermore, although given only prerestriction amounts of feed that were indexed to their reduced metabolic body size (body wt kg 0.75), both TCDD-treated and control rats gained weight rapidly and at similar rates during the refeeding period. Thus, rats treated with a sublethal dose of TCDD displayed normal efficiency of feed utilization but did so at a subnormal level of weight. That is, just like control rats, TCDD-treated rats increased their efficiency of feed utilization (weight gain/feed intake) but only when their body weight was caused to fall below the lower weight maintenance level determined by the TCDD dose administered.(ABSTRACT TRUNCATED AT 400 WORDS)     

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in cold-adapted rats

The toxicity of 60g/kg 2,3,7,8-tetrachlorodi-benzo-p-dioxin (TCDD) given IP in corn oil/5% acetone was examined in male Sprague-Dawley rats adapted to 25 °C or 4 °C ambient temperature. Cold exposure significantly reduced mean time to death and tended to increase mortality. Body weight at the time of death was reduced at both ambient temperatures to about the same extent. Thus, the rate of body weight loss was about twice as fast in nonsurvivors at 4°C than at 25 °C. There was a continuous decrease in feed intake of the non-survivors at 25 °C until death. However, no reduction in feed intake occurred in any of the rats at 4 °C ambient temperature. At 14 days after dosing all TCDD-dosed animals were hypothyroid in terms of T₄ but essentially euthyroid in terms of T₃. Oxygen consumption at 10 days after dosing was reduced to the same extent in all TCDD-dosed rats without regard to survival status. By day 20 after TCDD dosage, survivors increased their oxygen consumption at both ambient temperatures to nearly control levels whereas non-survivors were unable to do so. Body temperature of all animals remained within normal range except for the non-survivors, which showed reduced rectal temperature shortly before death. It is concluded (1) that cold adaptation aggravates the toxicity of TCDD, (2) that reduced feed intake alone cannot explain TCDD-induced wasting syndrome, (3) that reduced oxygen consumption in TCDD-treated rats may be due to reduced feed intake and/or altered thyroid hormone status, and (4) that TCDD is likely to activate metabolic pathways which represent a wasteful utilization of ingested and/or stored energy.     

Development of hepatocytes in the pancreas of hamsters treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Transdifferentiation is a process in which one differentiated cell type is converted to another. A unique example of transdifferentiation is the development of hepatocytes from pancreatic cells in adult hamsters and rats. In this communication we report the induction of pancreatic hepatocytes in hamsters that were given 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Two or 6 intraperitoneal (ip) injections of TCDD at a dose of 100 micrograms/kg body weight at 4-wk intervals induced pancreatic hepatocytes in 75% and 89% of the animals respectively. In animals given only two doses of TCDD each pancreas contained one to two hepatic foci, whereas when six injections were administered multiple hepatic foci were observed. By hematoxylin and eosin stain and by periodic acid Schiff stain, the pancreatic hepatocytes were morphologically identical to those in normal liver. Although the exact mechanism by which TCDD induces the transformation is not clear, it is conceivable that TCDD acting through receptor-mediated mechanisms is activating the repressed liver-specific genes in the pancreas.     

Characterization of the hyperlipidemia in guinea pigs induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Treatment of adult male guinea pigs with a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 6.2 nmol (2.0 μg)/kg body wt, induces a marked hyperlipidemia characterized by a 19-fold increase in very low density lipoproteins (VLDL) and a 4-fold increase in low-density lipoproteins (LDL) compared to pair-fed control animals. VLDL from TCDD-treated animals were similar in size and electrophoretic mobility to VLDL from pair-fed control animals, but they contained less cholesteryl ester and an altered pattern of C apoproteins on sodium dodecyl sulfate-polyacrylamide gels. LDL from TCDD-treated animals were larger than LDL from pair-fed controls and contained more phospholipid and less protein than LDL from pair-fed control animals. In addition, LDL from TCDD-treated animals contained increased amounts of apoprotein C as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. No change in the concentration or properties of serum high-density lipoproteins was observed. Serum free fatty acids, triglycerides, and cholesteryl esters from TCDD-treated animals were enriched in linoleic acid (18:2), a principal fatty acid of adipose tissue. This suggests that mobilization of adipose tissue fatty acids in TCDD-treated animals may lead to increased hepatic lipoprotein production. However, weight-paired control animals did not become hyperlipidemic. Thus, in addition to mobilizing adipose tissue fatty acids, TCDD may alter the relative rates of anabolic and/or catabolic processes controlling serum VLDL and LDL concentrations in the guinea pig.     

Metabolism and distribution of [14C]glucose in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Male Sprague-Dawley rats were given a single, usually lethal, dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 125 micrograms/kg ip in corn oil), or vehicle alone. Twenty-four hours after ip administration of TCDD the animals received an ip injection of 14C-labeled glucose, and the time course and amount of exhalation of 14CO2 were monitored for 8 h continuously and once daily for 20 min for the subsequent 5 d. TCDD treatment reduced the amount of 14CO2 exhaled within 8 h after the injection of [14C]glucose by 33%, as compared to pair-fed controls. Blood levels of radioactivity were affected by TCDD accordingly. No particular organ appeared to act as a sink for the radioactivity not exhaled during these 8 h by the treated animals. TCDD (125 micrograms/kg) induced significant changes in the disposition of radioactivity in heart and brown adipose tissue between 25 and 125 min after the iv injection of [14C]glucose. The areas under the curve of [14C]glucose-derived radioactivity were the same after either iv or ip injection in the blood of TCDD-treated rats, allowing a direct comparison of experiments with iv or ip injection of [14C]glucose. The half-lives of radioactivity in the exhaled air and in feces of treated animals were greatly elevated during the 5 d following administration of [14C]glucose. These results indicate that TCDD induces in rats, within 24 h after dosing, alterations in the metabolism of glucose that preceded changes in insulin homeostasis, because hypoglycemia and hypoinsulinemia in rats do not occur until about a week after TCDD treatment. Since overt signs of acute toxicity (reduced feed intake and body weight loss) are also not noticeable until several days after a lethal dose of TCDD, it is probable that this earlier disturbance of glucose metabolism is part of the biological changes that result in wasting away and eventually in death.     

Characterization of the hydrophobic properties of the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The hydrophobic character of the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been estimated by its tendency to adsorb to hydrophobically interacting matrices. The receptor is adsorbed to uncharged pentyl-Sepharose but not to butyl-Sepharose at 1 M NaCl. It is also adsorbed to phenyl-Sepharose or Cibacron blue-Sepharose at lower ionic strengths (0-0.15 M NaCl). Elution of adsorbed receptor could not be achieved under mild conditions (decreasing salt concentration, increasing glycerol concentration). A concentration of 0.2% (w/v) of the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) was required to desorb the receptor from pentyl-Sepharose with an approximate yield of 11-14% of the specific [3H]TCDD-binding activity. The CHAPS-treated receptor exhibited the same physicochemical characteristics as that in crude cytosol (4-5 S, Stokes radius approximately 60 A). Furthermore, the effects of detergents other than CHAPS on hydrodynamic parameters and on [3H]TCDD binding to the receptor were studied. In conclusion the TCDD receptor showed more pronounced hydrophobic properties than those reported for steroid hormone receptors.     

Hepatocarcinogenesis in female Sprague-Dawley rats following discontinuous treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

In this study, we investigated the time course of promotion of tumors and putatively preneoplastic altered hepatic foci in the livers of diethylnitrosamine (DEN)-initiated female Sprague-Dawley rats. These rats had been treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) under different dosing regimens, but we used the same administered biweekly dose of 1.75 microg/kg of body weight. Animals were treated continuously for up to 60 weeks, or continuously for 30 weeks, followed by cessation of treatment for up to 30 weeks. In addition, TCDD treatment in these groups was begun either 2 or 18 weeks after initiation with DEN. Liver tumors were only observed in animals after 60 weeks on the study and were increased by continuous TCDD treatment, relative to controls. The incidence of hepatocellular adenoma and carcinoma combined, in animals treated with TCDD for 30 weeks followed by no TCDD treatment for 30 weeks (17%), was lower than in animals receiving either TCDD (79%) or vehicle control (corn oil) alone (55%) for 60 weeks. The lower liver-tumor incidence after cessation of TCDD treatment paralleled time-dependent decreases in the volume fraction occupied by placental glutathione S-transferase-positive altered hepatic foci and the number of foci per unit volume, but not the mean focus volume that exhibited a time-dependent increase after cessation of TCDD treatment. Cessation of TCDD treatment led to reductions in liver TCDD levels, and these changes were reflected in a cessation of reduced body weight because of TCDD treatment. These data indicate that liver-tumor promotion by TCDD in female rats is dependent upon continuous exposure to TCDD, and that alterations in patterns of TCDD exposure can have significant effects on tumor incidence not reflected by standard measures of dioxin exposure.     

Enhanced liver DNA synthesis in partially hepatectomized rats pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on liver DNA synthesis was studied in rats after a $\text{1}\text{3}$ hepatectomy. The rats were maintained on a controlled feeding schedule and were treated with 5 μg/kg of TCDD or acetone/corn oil (control). Five days after treatment a $\text{1}\text{3}$ hepatectomy was performed and at designated times thereafter liver DNA synthesis was measured by [³H]thymidine incorporation into DNA. The main finding was that liver DNA synthesis was increased 8- to 10-fold by TCDD over that which was observed in control rats. This increase occurred after a latency period that was appropriate for the regenerative liver DNA synthesis response. Other experiments showed that increased incorporation of thymidine in TCDD-treated rats could be blocked by hydroxyurea, an inhibitor of semiconservative DNA synthesis, and that the increased incorporation was secondary to increased DNA synthesis and not increased thymidine kinase activity. Thus, hepatocytes in TCDD-treated rats respond in a quantitatively different manner than control rats to the same proliferative signal, $\text{1}\text{3}$ hepatectomy. Liver DNA synthesis in nonhepatectomized TCDD-treated rats tended to be greater than control, but the difference was not statistically significant.     

Reduced gluconeogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

The effect of a usually lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 g/kg) was studied on the conversion of¹⁴C-alanine into¹⁴C-glucose in male Sprague-Dawley rats by established procedures (determination of plasma alanine and blood glucose by enzymatic assays and isolation of¹⁴C-alanine and¹⁴C-glucose from whole blood by column chromatography). TCDD-treated rats converted significantly (p < 0.05) less¹⁴C-alanine into¹⁴C-glucose than did their pair-fed or ad libitum-fed counterparts, indicating reduced gluconeogenesis as a result of TCDD treatment. This finding suggests that reduced gluconeogenesis in TCDD-treated rats contributed to the progressively developing, severe hypoglycemia observed in these animals. Corticosterone, a key hormone in gluconeogenesis, provides partial protection from TCDD-induced toxicity in hypophysectomized rats. Therefore, the conversion of¹⁴C-alanine into¹⁴C-glucose was also determined in hypophysectomized rats dosed with TCDD (125 g/kg) and given corticosterone (25 g/ ml in drinking water). These rats also converted significantly (p <0.05) less¹⁴C-alanine into¹⁴C-glucose than did their pair-fed counterparts. However, in contrast to non-hypophysectomized TCDD-treated rats, these rats maintained marginal normoglycemia even at 64 days after dosing with TCDD, which suggests that the partial protective effect of corticosterone in hypophysectomized, TCDD-treated rats is unrelated to its effect on gluconeogenesis. The protection provided by corticosterone supplementation in TCDD toxicity is more likely due to reduced peripheral utilization of glucose enabling the animals to maintain marginal normoglycemia.Presented in part at the 27th Annual Meeting of the Society of Toxicology, Dallas, TX, 1988     

Characterization of the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-provoked strong and rapid aversion to unfamiliar foodstuffs in rats

A conspicuous but scantly studied feature of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity is avoidance of unfamiliar foodstuffs, which seems to be one of the very few exquisitely sensitive behavioural effects in adult laboratory animals. Here we characterized this peculiar response further after low doses of TCDD. The time-course of the novelty avoidance, the role of nutriment form and dependence of the aversion on the time lag between TCDD exposure and the presentation of a novel food item was determined using rats with different sensitivities to lethality of TCDD. Rats were offered chocolate, liquid nutriment or familiar feed with an unfamiliar texture and the consumptions were measured for varying periods. Aversion to a novel food item (chocolate) emerged within 5.5h after TCDD exposure. A lag of a week or more between TCDD exposure and the presentation of chocolate abolished the avoidance whereas simultaneous presentation of chocolate with TCDD treatment rendered the rats oblivious to the chocolate's presence for over 40 days. Rats avoided also liquid nutriments when these were coupled with TCDD administration but this faded much sooner than chocolate aversion. Even a change in feed texture at the exposure was able to elicit the response. However, habituation was found to interfere with the aversion. These findings indicate that temporal proximity to TCDD exposure is a requisite for the avoidance response which emerges rapidly and may linger on for extended periods, but is not strictly confined to any specific food type. The molecular mechanisms of this tantalizing behavioural alteration remain to be determined.     

Corticosterone decreases toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in hypophysectomized rats

Male Sprague-Dawley rats were hypophysectomized by an established surgical technique. Hypophysectomy aggravated the toxicity (mortality and mean time to death) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 micrograms/kg ip) when compared to sham-operated rats (100% mortality with 9 +/- 1 d mean time to death vs. 90% mortality with 32 +/- 6 d mean time to death, respectively). However, administration of corticosterone (25 micrograms/ml in drinking water) to hypophysectomized rats resulted in an attenuation of the toxicity (40-60% mortality with 40-90 d mean time to death) to a range of TCDD doses (125, 250, 500 micrograms/kg) much higher than the LD50 (about 60 micrograms/kg TCDD) in nonhypophysectomized rats (about 30 d mean time to death). Furthermore, thyroid hormone supplementation in hypophysectomized rats dosed with 125 micrograms/kg TCDD restored the toxicity of TCDD to approximately "normal." Based on these data it is concluded that one or more as yet unknown key factors that are important in the modulation of the toxicity of TCDD reside in the pituitary.