Delayed teratogenic effect of methylmercury on hepatic cytochrome P-450-dependent monooxygenase systems of rats

Drosophila CYP6g1 and its human homolog CYP3A4 confer tolerance to methylmercury during development

Methylmercury (MeHg) is a persistent environmental toxicant that is commonly encountered through dietary fish and seafood. While the fetal nervous system is a well-known primary target for MeHg toxicity, the risks of MeHg exposures that are commonly experienced today through diet and environmental exposure remain uncertain. Despite knowledge of numerous cellular processes that are affected by MeHg, the mechanisms that ultimately influence tolerance or susceptibility to MeHg in the developing fetus are not well understood. Using transcriptomic analyses of developing brains of MeHg tolerant and susceptible strains of Drosophila, we previously identified members of the cytochrome p450 (CYP) family of monooxygenases/oxidoreductases as candidate MeHg tolerance genes. While CYP genes encode Phase I enzymes best known for xenobiotic metabolism in the liver, several classes of CYPs are required for synthesis or degradation of essential endobiotics, such as hormones and fatty acids, that are critical to normal development. We now demonstrate that variation in expression CYP genes can strongly influence MeHg tolerance in the developing fly. Importantly, modulating expression of a single CYP, CYP6g1, specifically in neurons or the fat body (liver equivalent) is sufficient to rescue development in the presence of MeHg. We also demonstrate a conserved function for CYP3A4, a human homolog of CYP6g1, in conferring MeHg tolerance to flies. Finally, we show that pharmacological induction of CYPs with caffeine parallels an increase in tolerance to MeHg in developing flies. These findings establish a previously unidentified role for CYPs in MeHg toxicity and point to a potentially conserved role of CYP genes to influence susceptibility to MeHg toxicity across species.

Prenatal exposure to methylmercury during late gestation affects cerebral opiatergic system in rat offspring

Pregnant female rats were orally administered a single dose (8 mg/kg) of methylmercury chloride (MMC) on Day 15 of gestation. The binding characteristics of opioid receptors were studied in the brain of developing rats at different stages of age. An increased density of opioid receptors was found in whole brain of MMC-exposed rats at 21 days (delta receptors) and 60 days (mu and delta receptors) of age, in comparison with matched controls. An enhanced response to morphine administration was detected in MMC-exposed rat offspring at Day 60 of postnatal life, which, however, was not apparently due to an impaired liver metabolization or renal excretion. Hence, it is reasonable to surmise a possible correlation between receptor up-regulation and increased response to pharmacological challenge. These data seem to indicate that neurochemical alterations produced in the rat developing organism by prenatal exposure to methylmercury involve the opiatergic system which undergoes a supersensitivity phenomenon. This effect, which is not detectable in the first postnatal period, shows a delayed onset, being detectable only at the adult stage. These findings seem to indicate that pre- and postnatal methylmercury exposure induces latent neurochemical and behavioral alterations which could last even after the clearance of the metal from the brain.

Evolution and current status of neurotoxicity risk assessment

The U.S. EPA neurotoxicity risk assessment guidelines define neurotoxicity as the capacity of chemical, biological, or physical agents to cause an adverse functional or structural change in the peripheral or central nervous system. Chemical-induced changes in the structure or persistent changes in behavior, neurochemistry, or neurophysiology of the nervous system are regarded as neurotoxic effects. Reversible effects occurring at doses that could endanger performance in the workplace, are associated with a known neurotoxicological mechanism of action, covary with a known neurotoxicological effect, or are latent effects uncovered by pharmacological or environmental challenge, could be considered to be neurotoxic effects. It should also be noted that even if a chemical-induced change in neurobiological function or structure does not meet the criteria established for a neurotoxic effect, the alteration could still be considered as adverse and used in a regulatory decision. Adverse effects detected by neurobiological procedures should not, however, necessarily lead to the conclusion that the chemical is a neurotoxicant. At the present time,the default RfD uncertainty factor approach is used for neurotoxicological data. A number of other safety factor approaches and quantitation dose-response models have been proposed for evaluating neurotoxicological data. However, a very limited number of comparisons have been made between the RfD approach and alternatives. In summary, neurotoxicity is one of several noncancer and points currently being used by regulatory agencies. The effective utilization of neurobiological measures to assess chemical-induced changes in the structure and function of the nervous system depends on properly designed studies and elimination of potential confounding variables that obscure interpretation of data from toxicological studies.

Long-term (imprinting) effects of transplacental treatment of mice with 3-methylcholanthrene or beta-naphthoflavone on hepatic metabolism of 3-methylcholanthrene

Foetal mice of genotype AhbAhd (responsive to induction of metabolism of polycyclic aromatic hydrocarbons [PAH]) or AhdAhd (non-responsive) were exposed transplacentally on gestation day 17 to a single dose of 3-methylcholanthrene (MC, 5-175 mg/kg) with or without prior treatment on day 15 with beta-naphthoflavone (beta NF, 150 mg/kg). The mothers were themselves either induction-responsive [(C57BL/6 x DBA/2)F1] or non-responsive (DBA/2). Metabolism of [14C]MC by homogenates of livers from the transplacentally-exposed offspring was quantified at 9 months of age (first experiment) or 13 months (second experiment) with or without prior inducing treatment with MC. The foetal exposure to MC had a permanent effect on MC metabolism by the adult hepatic homogenates in both experiments. In most instances the effect was positive in direction and small in magnitude (15-30%). It was dose-dependent with regard to transplacental MC, occurred in both induced (AhbAhd) and non-induced (AhdAhd) individuals, and was significant only when the mother and/or the foetus was inducible. beta NF itself did not have a positive imprinting effect. In some cases it either reduced or potentiated the long-term imprinting effect of MC, depending on the MC dose and the phenotype of the mother. These results confirm that transplacental exposure to a carcinogenic PAH may permanently alter metabolism of the chemical in later life, and indicate that this imprinting action is dependent on induced metabolism of the chemical in the mother and/or foetus.

Changes in central GABAergic function following acute and repeated stress

1. The function of gamma-aminobutyric acid (GABA)ergic systems in response to acute and repeated stressful manipulations was evaluated in both the corpus striatum and frontal cerebral cortex of the rat. 2. In the corpus striatum the activity of the synthetic enzyme for GABA (glutamic acid decarboxylase, GAD) and the levels of GABA were reduced by acute immobilization stress (1 h). GABA turnover was reduced only by acute cold stress (3 h, 4 degrees C). 3. In the frontal cerebral cortex no changes were observed after acute stressful manipulations, but repeated stress (0.5 h immobilization per day for 14 days) enhanced both GAD activity and GABA turnover, and reduced GABA levels. 4. In conclusion, it would appear that the GABAergic system in the corpus striatum of the rat is most sensitive to acute stress and that the system in the frontal cerebral cortex area is preferentially responsive to chronic stress. It is speculated that the cortical GABAergic system is responsible for adaptive responses to the adverse conditions prevailing during chronic stress.

Effects of methylmercury on hypnotic action of hexobarbital, liver hydroxylase and cytochrome P-450 in mice

The effects of methylmercury hydroxide (MeHgOH) on the hypnotic action of hexobarbital was investigated in adult 129/SvSl mice of both sexes. It was found that there was no sex difference in the response to MeHgOH treatment. The action of MeHgOH was intimately related to the interval between MeHgOH administration and the test of hexobarbital hypnosis. A biphasic effect was observed. An initial dose-dependent prolongation of hexobarbital hypnosis was observed in animals pretreated with MeHgOH 24 h earlier. If the interval was extended from 24 h to 1 week, a shortening of sleeping time was observed in MeHgOH treated animals. The animals recovered from the effects of MeHgOH in 3 weeks. The initial effect of MeHgOH was found closely related to the decrease in the rate of hexobarbital metabolism in the liver through lowering of cytochrome P-450 concentration. On the other hand, the delayed shortening of hexobarbital hypnosis was not related to the rate of hexobarbital metabolism. It is assumed that the delayed effect of MeHgOH on the hexobarbital hypnosis is due to MeHgOH acting on the central nervous system to decrease its sensitivity to hexobarbital via interaction with the barbiturate receptors on the GABA-chloride ionophore complex. In animals exposed in utero to MeHgOH, it was found that the duration of hexobarbital-induced sleeping time was significantly longer in the offsprings tested for hexobarbital hypnosis 3.5 months after birth following prenatal exposure to MeHgOH. Repeated administration of hexobarbital to adult offsprings prenatally exposed to MeHgOH and to control mice shortened hexobarbital sleeping time, however, the maximum shortening capacity was smaller in the treated group. It is concluded that the hypnotic action of hexobarbital in mice can be altered by MeHgOH exposure both prenatally and postnatally. The effects of prenatal exposure to MeHg were observed in adult offsprings, indicating that MeHg may have a functional teratogenic effect on barbiturate-induced hypnosis in the absence of gross anomalies.

Perinatal cannabinoid exposure: effects on hepatic cytochrome P-450 and plasma protein levels in male mice

Maternal exposure to the major psychoactive delta 9-tetrahydrocannabinol (THC), or to the nonpsychoactive cannabinol (CBN) or cannabidiol (CBD) on day 12 of gestation, or on day 1 postpartum, affected the concentrations of hepatic cytochromes P-450 in adult male offspring. Levels of P-450 were significantly increased in adult males prenatally exposed to cannabinoids, but were reduced after postnatal exposure. The response to exogenous testosterone was also differentially affected by perinatal cannabinoid exposure, with reduced plasma androgen in males prenatally exposed to THC, but increased levels of hormone in mice exposed postnatally to THC or CBN. There was a concomitant decrease in plasma albumin and increased gamma-globulin in adult males postnatally exposed to CBN. Beta-globulin levels were also significantly increased in adult males exposed to cannabichromene (CBC) on day 1 postpartum. Cannabinoid exposure during perinatal periods of development exert effects on hepatic function, plasma androgen levels, and on the immune system. These effects may reflect the ability of perinatal cannabinoid exposure to interfere with androgen-mediated processes of differentiation.

Acute and chronic effects of lithium chloride on GABA-ergic function in the rat corpus striatum and frontal cerebral cortex

The acute (1 h, i.p.) and chronic (14 days, p.o.) effects of LiCl treatment upon GABA-ergic neurons were studied in the rat corpus striatum and frontal cerebral cortex. One hour after a single injection of LiCl the activity of glutamic acid decarboxylase (GAD) was reduced by 29% in the striatum (2 meq/kg LiCl) and by 38% in the cerebral cortex (10 meq/kg LiCl). In contrast, striatal GAD was activated by 34% 1 h after the injection of 10 meq/kg of LiCl; this dose also reduced the endogenous striatal GABA level by 24%. After 14 days of oral LiCl administration (2 meq/kg/day): a) cortical GAD activity was enhanced by 50% and GABA concentration was decreased by 28%; b) no changes were observed in the striatum. These findings suggest that: LiCl administration stimulates GABA-ergic function in specific areas (depending on the dose and length of treatment) increasing both GAD activity and probably GABA release. This occurs in the striatum after acute treatment only with a high dose, and in the frontal cerebral cortex after chronic treatment with a low dose.

Review of the health effects of methylmercury

The study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies, including pre-and postnatal exposure, are also examined. The consumption of fish and fish produce represents the major source of methylmercury exposure in the general population. Reported mercury concentrations in fish throughout the world are examined, particularly in the Mediterranean Sea. Here there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families. The measurement of mercury in hair is now regarded as the most useful indicator of exposure but more experimental data are still required to increase the value of this index. The threshold levels of methylmercury in blood, hair and for dietary intake, as estimated by the World Health Organization, have been largely endorsed. However, new information from Japan and Canada suggests the existence of a latency period for some effects, so that the frequency or probability of their occurrence is inversely related to the duration of exposure. Incorporation of such findings would therefore lead to the designation of lower threshold values than are presently recognized. Pregnant women and the fetus have been identified as groups that are at special risk. The fetal blood mercury level is up to twice that of the mother and the sensitivity of both mother and fetus may be higher than in non-pregnant adults. This should be taken into account when assigning protective threshold concentrations.

Environmental influences on human foetal and placental xenobiotic metabolism

The human foetus is more capable of metabolizing xenobiotics than foetuses of common laboratory animal species. However, xenobiotic metabolism in animal foetuses is inducible by the exposure of the mother to various inducers during late pregnancy. Xenobiotic metabolism in neonates is more easily inducible than in foetal animals. With respect to the human foetus at mid-pregnancy, the hepatic enzyme systems do not seem to be readily inducible by exoaenous inducers, whereas the placental monooxygenase system is almost totaly dependent on maternal cigarette smoking. In the human newborn, indirect evidence points to the possibility of induction by potential inducers. The ontogenetic development of xenobiotic metabolism is probably regulated by endogenous hormones. It is possible that environmental factors may effect these normal regulatory and "imprinting" phenomena and thus lead to permanent disturbances in xenobiotic metabolism.