Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on stimulated DNA synthesis in the liver and kidney of the rat

No effect of targeted memory reactivation during sleep on retention of vocabulary in adolescents

Re-exposure of newly acquired vocabulary during sleep improves later memory recall in healthy adults. The success of targeted memory reactivation (TMR) during sleep presumably depends on the presence of slow oscillations (i.e., EEG activity at a frequency of about 0.75 Hz). As slow oscillating activity is at its maximum during adolescence, we hypothesized that TMR is even more beneficial at this developmental stage. In the present study, adolescents aged 11 to 13 learnt Dutch vocabulary in the evening and were tested on recall performance the next morning. Half of the words were presented via loudspeakers during post-learning periods of NREM (Non Rapid Eye Movement) sleep in order to stimulate memory reactivation. Unexpectedly, TMR during sleep did not improve memory on the behavioral level in adolescents. On the oscillatory level, successful reactivation during sleep resulted in the characteristic increase in theta power over frontal brain regions, as reported in adults. However, we observed no increase in spindle power during successful reactivation. Possible factors that may explain the lacking effect of TMR in adolescents in this study such as differences in learning abilities and pre-sleep performance levels are discussed.

Development of quantitative structure-activity relationship (QSAR) models for predicting risk of exposure from carcinogens in animals

Quantitative structure-activity relationship (QSAR) models capable of predicting acute toxicity and carcinogen potency of polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated hydrocarbons, and chlorinated insecticides have been formulated. Median lethal dose (LD50) for PCDD-exposed mice correlated negatively with polarity and positively with (H acceptor x 2 chi), whereas LD50 for PCDD-exposed guinea pigs correlated with (H acceptor x density). Both (H acceptor x 2 chi) and (H acceptor x density) exhibited parabolic relationship with log P (partition coefficient). Carcinogenic potency, determined from order of magnitude (OM) values, correlated negatively with log P and positively with (length x width). Thus, a hydrophobic mechanism plays a key role in the lethal effects of PCDD in mice, whereas both hydrophobic and electronic mechanisms are involved in the lethal effects of PCDD in guinea pigs. However, the molecule's lipophilicity, length, and width may play important roles in the carcinogenic effects of chlorinated compounds.

Inhibitory effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on rat hepatocyte proliferation induced by 2/3 partial hepatectomy

To better understand the mode of action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced alterations in hepatic cell proliferation and the potential link to tumour-promoting activity, we investigated the effects of TCDD on the expression of certain key genes involved in liver cell growth and the effects of TCDD on induced hepatocyte cell proliferation. Gene expression analysis was conducted, by Northern blot hybridization, using RNA isolated from female Sprague-Dawley rat livers collected at various times during a 14-day dosing period with TCDD known to produce alterations in cell proliferation. No major changes were observed in the expression of the transforming growth factors TGF-alpha and TGF-beta or in oncogenes ras, src and myc. However, the expression of the transcription factors C/EBP, HNF-1 alpha and HNF-4 decreased after 14 days of TCDD treatment. To investigate how TCDD affects hepatic growth, cell proliferation analysis was conducted in rats stimulated to undergo hepatocyte proliferation following either 2/3 partial hepatectomy or lead nitrate treatment. Cell proliferation was quantified by means of immunocytochemical detection of Proliferating Cell Nuclear Antigen (PCNA). Fourteen days of pretreatment with TCDD caused an overall inhibition of hepatocytes in the growth fraction (G1, S, G2 and M) from 61 +/- 3% in the control-partial hepatectomy group to 41 +/- 3% in the TCDD-partial hepatectomy group. A periportal pattern of cell proliferation was observed in the TCDD-partial hepatectomy group as compared to the panlobular pattern of cell proliferation in the control-partial hepatectomy group. TCDD pretreatment also produced an inhibition of cell proliferation induced by the liver mitogen lead nitrate. TCDD-induced inhibition of hepatocyte proliferation could play a role in TCDD tumour promotion and hepatocarcinogenesis through the creation of a environment whereby preneoplastic cells continue to expand while normal hepatocyte proliferation is inhibited.

Toxicity of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)

In summary, the toxicity of TCDD has been comprehensively examined in multiple acute, subchronic, and chronic studies. Acute toxicity studies have shown marked species differences, with up to a 10,000-fold difference between the single oral LD50 dose for the guinea pig and hamster. TCDD is capable of causing an acnegenic response in man and a similar skin response in certain animals. It is also a potent inducer of microsomal enzymes in some but not all species. A dose-related suppression of cell-mediated immunity has been observed at higher dose levels in laboratory animals but not in humans manifesting TCDD-induced acnegenic response. TCDD causes a dose-related teratogenic response in mice, with the no-adverse-effect level of 0.1 micrograms TCDD/kg/day. In rats, TCDD causes embryo- and fetotoxicity above the no-adverse-effect level of 0.03-0.125 micrograms/kg/day. Dose-related reproductive effects have also been noted in monkeys at doses that elicit maternal toxicity, and additional long-term studies are presently underway. A multigeneration reproduction study as well as a lifetime chronic toxicity study have been completed with TCDD in rats; in both studies, the no-adverse-effect level was found to be 0.001 microgram TCDD/kg/day. Numerous mutagenic studies have been performed using in vitro plant and microbial test systems as well as in vivo tests in mammals and man. A mutagenic response was noted in a few of the vitro test systems, but there are no definitive in vivo correlates of TCDD mutagenicity in higher mammals or man. TCDD has been studied for carcinogenic potential in rats and mice. There is good correlation of the results, with a carcinogenic response noted in both species only after long-term ingestion of higher dose levels that induce toxicity. No carcinogenic response occurred at continuous dose levels of 0.001-0.0014 micrograms/kg/day in rats and 0.001-0.03 micrograms/kg/day in mice. Data presently available are more supportive of a nongenetic (?promotor) rather than a genetic mechanism of carcinogenesis. The most recent research, some of which is still underway, indicates that the biologic uptake and toxicity of TCDD may be significantly decreased if the TCDD is adsorbed onto carbon or soil particles. This information is helpful in hazard assessment of exposure to TCDD.

A histochemical study of the liver lesion induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) in rats

A histochemical study of plasma-membrane associated enzymes in rat liver demonstrated a significant lesion 3 days after a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). The complete loss of canalicular ATPase reaction in the parenchymal cells of the centrilobular zone remained the prominent feature of the liver throughout the 6-wk period studied. Involvement of the periportal and midzonal regions occurred in moribund animals and improvement in the health of two surviving animals at 9 mth was associated with a normal distribution of ATPase in the liver. Qualitative changes in 5-nucleotidase and acid phosphatase were secondary to the parenchymal cell damage. This lesion supports the morphological evidence, reported previously, that the parenchymal cell plasma-membrane is a specific subcellular site of the toxic action of dioxin.