Teratogenic effects of alcohol: current status of animal research and in vitro models

The effects of ethanol on CNS development in the chick embryo

Human and animal studies show that the central nervous system (CNS) is particularly vulnerable to developmental exposure to alcohol across all stages of development. New critical periods of ethanol sensitivity continue to be defined. The aim of this study was to further examine the stage-specific effects of ethanol on CNS development using a relatively simple programme of neuronal migration and differentiation, the chick embryo spinal cord, and treating at the immediate post-neurulation stage. Embryos (HH-stage 10-12) were explanted into shell-less culture and treated with ethanol (20 microl/40%) or saline (20 microl). At 6,12, 24 and 48 h post-treatment specimens were processed for resin histology. In addition, levels of cell death were analysed using Lysotracker Red, neural crest cell migration patterns were examined using HNK-1 staining and effects on DNA synthesis were evaluated on autoradiographs prepared 1h after exposure to 3H-TdR. This treatment protocol produced significant growth retardation in ethanol specimens examined at 48 h post-treatment. This effect was shown to involve increased levels of cell death, perturbation of DNA synthesis and an abnormal translocation and subsequent loss of cells into the neural tube lumen. No gross malformations were observed. We conclude that these results further highlight the stage-specific effects of ethanol on neurodevelopment.

Binge alcohol exposure in the second trimester attenuates fetal cerebral blood flow response to hypoxia

Alcohol is detrimental to the developing brain and remains the leading cause of mental retardation in developed countries. The mechanism of alcohol brain damage remains elusive. Studies of neurological problems in adults have focused on alcohol's cerebrovascular effects, because alcoholism is a major risk factor for stroke and cerebrovascular injuries. However, few studies have examined similar cerebrovascular effects of fetal alcohol exposure. We examined the effect of chronic binge alcohol exposure during the second trimester on fetal cerebrovascular and metabolic responses to hypoxia in near-term sheep and tested the hypothesis that fetal alcohol exposure would attenuate cerebrovascular dilation to hypoxia. Pregnant ewes were infused with alcohol (1.5 g/kg) or saline intravenously at 60–90 days of gestation (full term = 150 days). At 125 days of gestation, we measured fetal cerebral blood flow (CBF) and oxygen metabolism at baseline and during hypoxia. Maternal blood alcohol averaged 214 ± 5.9 mg/dl immediately after the 1.5-h infusion, with similar values throughout the month of infusion. Hypoxia resulted in a robust increase in CBF in saline-infused fetuses. However, the CBF response to hypoxia in fetuses chronically exposed to alcohol was significantly attenuated. Cerebral oxygen delivery decreased in both groups of fetuses during hypoxia but to a greater degree in the alcohol-exposed fetuses. Prenatal alcohol exposure during the second trimester attenuates cerebrovascular responses to hypoxia in the third trimester. Altered cerebrovascular reactivity might be one mechanism for alcohol-related brain damage and might set the stage for further brain injury if a hypoxic insult occurs.

The importance of alcohol misuse, malnutrition and genetic susceptibility on brain growth and plasticity

The "dyad: alcoholic mother and foetus" is a very complex entity in which several elements such as genes, metabolism, diet, drugs and social habits play a role at different stages in the development of the fetal brain damage. The literature on the effects of alcohol consumption on the developing brain is extensive but very few evidences have been reported regarding the combined neurotoxic effects of poor nutrition and alcohol consumption. The consequences of ethanol intake alone or combined with poor maternal nutrition appear to be severe and life-long. Alcohol exerts its neurotoxic effects on the developing brain directly by acting on fetal brain tissues, and indirectly either by interfering with placental physiology or by impairing the mother's physiology. Alcohol misuse in pregnancy is also frequently associated with other conditions that can potentially increase the brain damage such as poor nutrition and smoking. This article reviews the effects of poor nutrition and alcohol misuse during pregnancy on the development of the fetal brain and discusses the cumulative effects of these two environmental factors and their interaction with maternal and fetal genetic make-ups.

Prenatal alcohol exposure affects galactosyltransferase activity and glycoconjugates in the Golgi apparatus of fetal rat hepatocytes

Prenatal exposure to alcohol affects the morphological, structural, and functional features of the Golgi apparatus (GA), thus altering the glycosylation process in fetal hepatocytes. To elucidate the cellular mechanisms underlying these alterations, we have studied the effect of alcohol exposure in utero on the activity of liver galactosyltransferase, an enzyme involved in the glycosylation process, and on the hepatic glycoprotein sugar composition. For this, livers from 21-day-old fetuses obtained from control and ethanol-fed rats were used. Galactosyltransferase (GT) activity was determined in isolated GA cis and trans fractions. Colloidal gold-labeled lectin cytochemistry was used to analyze sugar residues in hepatocytes at the subcellular level. Finally, the integrity of the GA after alcohol treatment was assessed by electron microscopy and by evaluating the distribution of the Golgi beta-COP, a protein involved in vesicular trafficking. Prenatal alcohol exposure induces a significant increase in both liver weight and total protein content in the trans Golgi. Moreover, this treatment decreases the activity of galactosyltransferase, increases alpha-L-Fuc residues, and reduces the number of alpha-Man, GlcNAc(beta1,4,GlcNAc)1,2, GalNAc alpha1,3GalNAc, alpha-GalNAc, and a-Gal residues. Alcohol exposure also causes the Golgi cisternae to disappear in about 30% of the hepatocytes, and reduces 75% the number of anti-Golgi beta-COP protein binding sites. Our results suggest that the decrease in galactosyltransferase activity, the alterations in the oligosaccharide chain composition, and the reduction in the amount of Golgi beta-COP protein could be involved in the alterations in the glycosylation process, as well as in the accumulation of hepatic proteins observed after prenatal alcohol exposure. These alterations could contribute, therefore, to the alcohol-induced injury in the developing liver.

Physiological factors predisposing to neurotoxicity

Many factors determine individual susceptibility to toxic agents in addition to their primary interaction with the target site. Absorption, delivery to target tissues, bio-activation, bio-inactivation, elimination, and adaptive or protective responses all play important parts in determining the overall response of the individual. In addition changes in the physiological significance of the function which is disrupted may be crucially important. Pulmonary absorption can be limited by ventilation or perfusion, both of which increase with work rate. Tissue uptake can be limited by local blood flow, which is strongly influenced by local functional activity. In areas with a blood-tissue barrier, such as brain and testis, tissue uptake can be strongly influenced by developmental state, protein binding or vascular damage. Metabolic transformation can show marked inter-individual variations at both hepatic and extra-hepatic sites, due to genetic or nutritional influences. The capacity for adaptation to toxicological insult can also vary markedly, depending on functional reserve capacity as well as on inherent plasticity. Examples used to illustrate these factors include: the influence of motor activity on the toxicity of carbon monoxide; of noise on the ototoxicity of aminoglycoside antibiotics; of brain activity on the neurotoxicity of dinitrobenzene; of acid-base balance on the toxicity of nicotine; and of developmental stage on the neurotoxicity of haloperidol. In addition disease states can influence sensitivity. Thus anaemia sensitises to manganese; calcium deficiency to lead; nerve trauma to hexane; and Wilson's disease to copper overload.