Effects of lead and hyperthermia on prenatal brain growth of guinea pigs

Arrhenius thermodynamics and birth defects: chemical teratogen synergy. Untested, testable, and projected relevance

This article addresses the issue of hyperthermia-induced birth defects with an accompanying additional teratogen, be it a chemical or a physical agent (i.e., a simultaneous "combinational" exposure to two teratogens, one of which is hyperthermia). Hyperthermia per se is a recognized human and animal teratogen. An excellent example of such combinational exposures is an epileptic woman who becomes pregnant while taking valproic acid (VPA) to control seizures. VPA is a recognized chemical teratogen, and fever (hyperthermia) is not an uncommon event during pregnancy. While VPA also may occasionally induce fever as a side effect, we are concerned here with fevers arising from other, unrelated causes. There is a small but internally consistent literature on these combinational-teratogen exposures involving hyperthermia plus a chemical teratogen; in each instance, the effect level has been observed to be synergistically elevated above levels induced by the separate teratogenic components. The data were empirical. The observed synergy is, however, consistent with Arrhenius thermodynamics, a well-known chemical rate equation. The need for information about combinational teratogen exposures is acute; fever is a common occurrence during pregnancy; and there are many instances whereby there is also the simultaneous presence of some other teratogen(s). Given that the rate of autism spectrum disorders in the United States was recently presented as 1 in 88 births, it seems reasonable to suspect that such combinational regimens are much more prevalent than previously thought. Our hypothesis is that synergistic birth defect levels from combinational regimens are consistent with Arrhenius thermodynamics.

Review: Hyperthermia and fever during pregnancy

An episode of hyperthermia is not uncommon during pregnancy. The consequences depend on the extent of temperature elevation, its duration, and the stage of development when it occurs. Mild exposures during the preimplantation period and more severe exposures during embryonic and fetal development often result in prenatal death and abortion. Hyperthermia also causes a wide range of structural and functional defects. The central nervous system (CNS) is most at risk probably because it cannot compensate for the loss of prospective neurons by additional divisions by the surviving neuroblasts and it remains at risk at stages throughout pre- and postnatal life. In experimental animals the most common defects are of the neural tube, microphthalmia, cataract, and micrencephaly, with associated functional and behavioral problems. Defects of craniofacial development including clefts, the axial and appendicular skeleton, the body wall, teeth, and heart are also commonly found. Nearly all these defects have been found in human epidemiological studies following maternal fever or hyperthermia during pregnancy. Suggested future human studies include problems of CNS function after exposure to influenza and fever, including mental retardation, schizophrenia, autism, and cerebral palsy.

Marshall J. Edwards: Discoverer of maternal hyperthermia as a human teratogen

In a series of animal studies performed over a career spanning 40 years at the University of Sydney, Professor Marshall J. Edwards investigated the hypothesis that maternal hyperthermia during gestation can be teratogenic to the developing fetus. He is one of few investigators to have discovered a known human teratogen primarily through animal studies. In 1970 he earned his Ph.D. from the University of Sydney, writing a doctoral thesis entitled "A Study of Some Factors Affecting Fertility of Animals with Particular Reference to the Effects of Hyperthermia on Gestation and Prenatal Development of the Guinea-Pig." He went on to prove that hyperthermia-induced malformations in animals involve many organs and structures, particularly the central nervous system. Other defects include craniofacial anomalies, heart defects and hypodactyly, cataracts and coloboma, kyphoscoliosis, renal anomalies, dental agenesis, and abdominal wall defects. In a series of carefully planned and executed experiments, he demonstrated that the type of defect is related to the timing of the hyperthermic insult, and analyzed the underlying mechanisms. Cell death, membrane disruption, vascular disruption, and placental infarction were all implicated in causing embryonic damage. This special article reviews the scientific discoveries and personal philosophy of Marshall J. Edwards, the discoverer of maternal hyperthermia as a human teratogen.

Teratogenicity of elevated egg incubation temperature and egg vitamin A status in Atlantic salmon, Salmo salar L

The present study was undertaken to investigate the possibility that high egg vitamin A (VA) status in combination with elevated egg incubation temperatures may cause deformities in Atlantic salmon, Salmo salar L. Egg batches selected for their total VA concentration were exposed to low (normal, 8 degrees C) or elevated (14 degrees C) egg incubation temperatures. Temperature was the main factor causing bone deformities such as warped gill opercula, fin and jaw deformities, but not for the development of spinal deformities where all groups displayed a 'baseline' occurrence of mild deformity (decreased vertebral size in the cephalic region) and no systematic variation in the occurrence of serious spinal deformities (fused vertebrae). A possible effect of egg incubation temperature fluctuation was found for the groups reared at low temperatures. An indication of a negative effect of elevated egg VA status for the development of organ deformities such as missing septum transversum and situs inversus was found in addition to temperature effects, however, no firm conclusions could be drawn from the present data. The phenotypes for temperature-induced deformities resembled the phenotype of VA-induced deformities, but no clear conclusions on the causality of the deformities found in the present study could be drawn. Egg incubation temperatures, both absolute temperature and temperature variations, should therefore be strictly controlled.

Folic acid supplementation of pregnant mice suppresses heat-induced neural tube defects in the offspring

Neural tube defects (NTD) are a group of malformations that result from the failure of the neural tube to close early in embryonic development and among the most common congenital malformations in humans. It has been reported that a substantial proportion of NTD in humans can be prevented by folic acid (FA) supplementation prior to conception and during the first months of pregnancy, and myo-inositol (MI) was shown to reduce the incidence of NTD in curly tail mice which are not prevented by FA. Brief maternal hyperthermia (HT) early in pregnancy has been implicated in NTD both in humans and laboratory animals, and anterior NTD including exencephaly and anencephaly are induced frequently when pregnant mice are exposed to HT. We examined the effect of FA or MI supplementation of pregnant mice on the occurrence of heat-induced NTD in the offspring. When pregnant mice were treated with FA (3 mg/kg) daily from gestational day (GD) 0.5 through GD 9.5 and heated at GD 8.5, the prevalence of NTD in the fetuses (26.6%) was significantly lower than the corresponding figure in the HT alone group (38.6%; P < 0.05). However we failed to detect the preventive effect of MI (500 mg/kg). The results of this study suggest that prenatal FA supplementation decreases HT-induced NTD in mice and sufficient FA intake during early pregnancy may be recommended to avoid the birth of malformed children.

Teratogen update: gestational effects of maternal hyperthermia due to febrile illnesses and resultant patterns of defects in humans

This review has covered the pertinent literature concerning the teratogenic effects of hyperthermia in man and experimental animals. This is the first teratogen that was initially discovered in animals and then subsequently found to be a cause for concern in humans when similar patterns of defects were observed. Hyperthermia is a physical agent with a dose-response curve for abortions and malformations, but these effects can be mitigated in some circumstances by the heat shock response (HSR). We have reviewed the known functions of HSR and provided some insight into why embryos have some protection following an initial dose of heat, if it is sufficient to initiate the response. Thus, by reviewing the effects of hyperthermia in experimental animals, as well as malformative and protective mechanisms of teratogenesis, we have attempted to understand the effects of human hyperthermia teratogenesis.

Growth and behavioral changes in mice prenatally exposed to methylmercury and heat

Postnatal and behavioral changes in mice exposed prenatally to methylmercury and heat were investigated. Pregnant ICR mice were immersed in water at 37 degrees C or 42 degrees C for 10 min once or twice daily from day 12 through day 15 of gestation. Two hours after the heat exposure on day 12 of age, mice were injected s.c. with 5 mg Hg/kg of methylmercury (MeHg, as chloride) or saline. Prenatal exposure to heat significantly induced inactivity in an open field test (OPF) in males and retarded walking ability in both males and females. Prenatal exposure to MeHg caused significant inactivity in the OPF in females. Although heat did not enhance the effect of MeHg on physical growth or the behavior of pups and vice versa, there were some interactions between the effects of these two agents. Thus, the difference in walking ability in both sexes caused by heat was more distinctive in the saline-treated groups than in the MeHg-treated groups; the difference in locomotion in OPF caused by MeHg in females was more distinctive in the normothermic group than among the hyperthermic groups. The mechanisms underlying these behavioral changes need to be further examined.

Involvement of polyamines in the contragestational effect of hyperthermia

Although hyperthermia produces teratogenic effects in a great variety of animal species, the molecular mechanisms by which hyperthermia exerts its action remain unknown. We have studied the implications of polyamines in contragestational hyperthermia in rats. Our results show that the contragestational action of hyperthermia when applied in consecutive periods during days 8, 9 and 10 of rat pregnancy could be completely prevented by the previous administration of polyamines (putrescine and spermidine, 0.6 mmoles/kg and 0.03 mmoles/kg respectively) in combination with the diamine oxidase inhibitor aminoguanidine or by this inhibitor alone (0.12 mmoles/kg). The administration of polyamines alone partially prevented the fetotoxic effect of hyperthermia but produced a marked mortality (50%) in the pregnant rats. These findings support a major and complex role of polyamines in the mechanisms of hyperthermia-mediated teratogenesis, and suggest that the oxidative catabolism of polyamines could be in part responsible of the deleterious effect produced by hyperthermia in rat pregnancy.

Interactions in developmental toxicology: a literature review and terminology proposal

Developmental toxicologists have investigated the interactive effects from concurrent exposures to a variety of chemical and physical agents, including therapeutic drugs, industrial agents, and some biological organisms or their toxins. Of approximately 160 reports of concurrent exposures reviewed in this paper, about one third report no interactive effects (including additive effects--usually referring to response--as opposed to dose-additivity); another one third report antagonistic effects, and the final third report potentiative or synergistic effects. The quality of the studies is highly variable. Frequently, only small numbers of animals were included, and very few dose levels were evaluated. Maternal toxicity was rarely discussed. Time-effect relationships were examined infrequently. In addition, these studies are also inconsistent in the use of terms to describe interactive effects, and more than 90% of the terms were not in harmony with currently accepted definitions in toxicology. Because interaction studies will continue to be important in the future, this paper proposes uniform usage of terms for additivity and interactions in developmental toxicology: additivity (the combined effect of two or more developmental toxicants approximates the sum of the effects of the agents administered separately); antagonism (the combined effect of two or more agents, one or more of which are present at doses that would be developmentally toxic if given individually, is significantly less than the sum of the effects of the agents administered separately); potentiation (the increased effect of a developmental toxicant by concurrent action of another agent at a dose that is not developmentally toxic); synergism (the combined effect of two or more developmental toxicants is significantly greater than the sum of the effects of each agent administered alone); and, interaction if more precise terminology does not apply.

Effects of different factors in lead- and cadmium-induced hypothermia in mice

The aim of this study was to investigate the relationship between the hypothermic effect induced by lead or cadmium chloride treatments and the cerebral metal levels attained. Mice were injected intraperitoneally with different doses of lead acetate or cadmium chloride at ambient temperatures of 22 degrees C and 35 degrees C, and rectal temperatures and brain metal levels were determined. At 22 degrees C, doses of lead acetate exceeding 25 mg/kg caused significant hypothermia and a rise of lead levels in the brain; this hypothermic effect was significantly inhibited at 35 degrees C. In mice repeatedly treated with 2 or 5 mg/kg lead acetate and exposed to 22 degrees C or 35 degrees C, no significant changes were observed in body temperature after the repeated exposure, although brain lead levels increased significantly. Cadmium chloride at 22 degrees C produced a significant fall in body temperature, and a rise in brain cadmium levels at doses of 2-4 mg/kg; the decrease in body temperature was partially inhibited at 35 degrees C. The results of the present study suggest that the hypothermic effect elicited by these heavy metals is related to both the level of metal absorbed by the brain and its rate of uptake.

Teratogenic effect of hyperthermia during early organogenesis period in mice

Pregnant Swiss albino mice were subjected to 41, 42, or 43 degrees C temperature for 10 minutes on day 6.5 of gestation. Another group of animals treated at 37 degrees C was used as control. All animals were killed on the 18th day of gestation and fetuses were examined for prenatal mortality, growth retardation, and microphthalmia incidence. Results indicated a dose dependent increase in the mortality rates with a 42% death in the 43 degrees C group. Treatment with the higher temperatures (42 and 43 degrees C) resulted in a significant increase in the number of growth retarded fetuses and in the incidence of microphthalmia. Reduction in head length and decrease in brain weight were observed in the group exposed to 43 degrees C, particularly in the growth retarded fetuses. However, the percent brain weight(g)-body weight(g) ratio did not show any significant difference from the control values.

The individual and combined effects of X-irradiation and hyperthermia on early somite mouse embryos in culture

The effects of 1) X-irradiation and 2) hyperthermia at a temperature of 43 degrees C individually and in combination have been investigated using cultured 8-day mouse embryos. B6C3F1 embryos were exposed to 0.3-2.0 Gy of X-rays, 5-20 min of heating, or 5 min of heating and irradiation at 0.3, 0.6, and 0.9 Gy. Irradiation alone at 0.3 Gy showed no apparent effect on embryonic development, but irradiation at 0.6-2.0 Gy caused a dose-dependent increase in malformed embryos. Heating alone for 5 min produced no malformed embryos, while heating for 10-20 min caused malformations as a function of heating time. Combined treatments produced higher frequencies (22.2-100%) of malformations than those of the sum of the separate treatments (0-41.7%). Malformations observed were primarily microphthalmia, microcephaly, and open neural tubes. The results indicate that in cultured mouse embryos irradiation combined with a "nonteratogenic dose" of hyperthermia directly exerts an additive effect on formation of the malformed embryos. In addition, a single occurrence of left-sided tail was produced by hyperthermia alone, while four occurrences were produced in combination with radiation.

Hyperthermia as a teratogen: a review of experimental studies and their clinical significance

Although hyperthermia is teratogenic in birds, all the common laboratory animals, farm animals, and primates and satisfies defined criteria as a teratogen, its study as a human teratogen has been neglected. Homeothermic animals, including humans, can experience body temperature elevations induced by febrile infections, heavy exercise and hot environments which exceed the thresholds (1.5-2.5 degrees C elevation) which are known to cause a syndrome of embryonic resorptions, abortions, and malformations in experimental animals. Hyperthermia is particularly damaging to the central nervous system, and if a threshold exposure occurs at the appropriate stages of embryonic development, exencephaly, anencephaly, encephalocoele, micrencephaly, microphthalmia, neurogenic talipes, and arthrogryposis can be produced in a high proportion of exposed embryos, the incidence and type of defect depending on the species and strain within species, the stage of development, and the severity of hyperthermic exposure. Other defects which can be induced experimentally include exomphalos, hypoplasia of toes and teeth, renal agenesis, vertebral anomalies, maxillary hypoplasia, facial clefting, cataract, coloboma, and heart and vascular defects. Proliferating cells are particularly sensitive to temperature elevations, resulting in arrest of mitotic activity and immediate death of cells in mitosis with threshold elevations (1.5-2.5 degrees C) and delayed death of cells probably in S phase with higher elevations (3.5 degrees C). In general, lower temperature elevations (2.5 degrees C) require longer durations of elevation to cause defects than a simple spike at a higher elevation (4.5 degrees C). The death of cells is largely confined to the brain and in the day 21 guinea pig embryo to the alar regions of the brain. Cell death probably accounts for most of the defects in the central nervous system, but microvascular disturbances leading to leakage, oedema and haemorrhage, placental necrosis, and infarction are other known effects of hyperthermia; and these are probably involved in the pathogenesis of many defects of the heart, limbs, kidneys, and body wall. Recent experiments have demonstrated protection of rat embryos in culture against a known teratogenic exposure by a brief nonteratogenic exposure given at least 15 min earlier. This protection is associated with the synthesis of heat-shock proteins, and temporary arrest of the cell proliferative cycle. Hyperthermia appears to be capable of causing congenital defects in all species and may act alone or synergistically with other agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Congenital lead encephalopathy in monkeys

Thirty-one squirrel monkey fetuses were exposed to lead acetate given to the mothers perorally during the last three-fourths or two-thirds of pregnancy. The mean maternal blood lead concentration of the group was 37 micrograms/100 ml and the individual means ranged from 22-82 micrograms/100 ml. Examination of the central nervous system was performed in 15 of the offspring (one abortion, eight stillborns, three neonatal deaths, two killed fetuses and one killed newborn). The mean cerebral weight was reduced for the fetal age (about 10%). Three cerebra were paradoxically overweight for the fetal age as well as for the body weight, probably due to edema. Neurohistology revealed large numbers of characteristic perivascular, petechial hemorrhages in the white matter in six of the fifteen cerebra. In two of these cases, such hemorrhages were also found in the white matter of the cerebellum, brain stem and spinal cord. Examination of four recovered placentas (two after delivery and two at hysterotomies) indicated a lead dose-dependent weight reduction and revealed various pathological lesions. The extensive brain hemorrhages, as well as varying degrees of edema, were seen in stillborns and neonates, while prenatally sacrificed fetuses showed few or no petechial hemorrhages and no signs of edema. It is suggested that lead is involved in the parenchymal growth retardation and the endothelial changes in the prenatal brain, as well as the placental damage, and that prematurity, birth-associated mechanical stress and asphyxia contribute to or precipitate vascular lesions, which may form the basis of acute, or later apparent, neurobehavioral disturbances.