Disposition of ethanol and activity of hepatic and placental alcohol dehydrogenase and aldehyde dehydrogenases in the third-trimester pregnant guinea pig for single and short-term oral ethanol administration

Prenatal Alcohol Exposure as a Case of Involuntary Early Onset of Alcohol Use: Consequences and Proposed Mechanisms From Animal Studies

Prenatal alcohol exposure has been found to be an important factor determining later consumption of this drug. In humans, despite the considerable diversity of variables that might influence alcohol consumption, longitudinal studies show that maternal alcohol intake during gestation is one of the best predictors of later alcohol use from adolescence to young adulthood. Experimental studies with animals also provide abundant evidence of the effects of prenatal alcohol exposure on later alcohol intake. In addition to increased consumption, other effects include enhanced palatability and attractiveness of alcohol flavor as well as sensitization to its sensory and reinforcing effects. Most of these outcomes have been obtained after exposing rats to binge-like administrations of moderate alcohol doses during the last gestational period when the fetus is already capable of detecting flavors in the amniotic fluid and learning associations with aversive or appetitive consequences. On this basis, it has been proposed that one of the mechanisms underlying the increased acceptance of alcohol after its prenatal exposure is the acquisition (by the fetus) of appetitive learning via an association between the sensory properties of alcohol and its reinforcing pharmacological effects. It also appears that this prenatal appetitive learning is mediated by the activation of the opioid system, with fetal brain acetaldehyde playing an important role, possibly as the main chemical responsible for its activation. Here, we review and analyze together the results of all animal studies testing these hypotheses through experimental manipulation of the behavioral and neurochemical elements of the assumed prenatal association. Understanding the mechanisms by which prenatal alcohol exposure favors the early initiation of alcohol consumption, along with its role in the causal pathway to alcohol disorders, may allow us to find strategies to mitigate the behavioral effects of this early experience with the drug. We propose that prenatal alcohol exposure is regarded as a case of involuntary early onset of alcohol use when designing prevention policies. This is particularly important, given the notion that the sooner alcohol intake begins, the greater the possibility of a continued history of alcohol consumption that may lead to the development of alcohol use disorders.

Does moderate drinking harm the fetal brain? Insights from animal models

Although public health campaigns advise pregnant women to abstain from ethanol, drinking during pregnancy is pervasive. Here, we highlight recent studies that have clearly demonstrated long-lasting neurobehavioral deficits in the offspring of laboratory animals exposed to moderate levels of ethanol during development. Alterations in learning, memory, motor coordination, social behavior, and stress responses were identified in these animals. Increased vulnerability to substance abuse was also demonstrated. These behavioral alterations have been associated with impairments in neurotransmitter systems, neuromodulators, and/or synaptic plasticity in several brain regions. With this review we hope to contribute to a better appreciation of the potential effects of developmental exposure to moderate ethanol levels, leading to better interventions aimed at relieving fetal alcohol spectrum disorders.

Chronic prenatal ethanol exposure and increased concentration of fatty acid ethyl esters in meconium of term fetal Guinea pig

In humans, the occurrence of prenatal exposure to ethanol is difficult to validate objectively. Increased concentration of fatty acid ethyl esters (FAEE) in the meconium of the newborn may be a biomarker of prenatal ethanol exposure. The validity of this proposed biomarker was tested in pregnant guinea pigs that received chronic oral administration of 4 g ethanol/kg maternal body weight/day (n=8), isocaloric-sucrose/pair-feeding (n=8) or water (n=2) throughout gestation. At gestational day 65 (term, gestational day 66 to 69), each dam and her offspring were euthanized, and meconium was collected from the term fetal large intestine. Eight individual FAEE (lauric, myristic, palmitic, palmitoleic, stearic, oleic, linolenic and arachidonic AEE) were measured by gas chromatography--flame ionization detection and confirmed by gas chromatography--mass spectrometry. The chronic maternal ethanol regimen decreased fetal body weight and brain weight. There was virtually no measurable FAEE in the meconium for the water group (n=3 fetuses). For meconium of the ethanol offspring (n=25 fetuses) compared with the sucrose offspring (n=23 fetuses), the total FAEE concentration was 8-fold higher; and lauric, palmitic, stearic and oleic AEE concentrations were at least 5-fold higher for the ethanol group. The data indicate that fetal meconium FAEE constitute a biomarker of prenatal ethanol exposure for a maternal ethanol regimen that restricts fetal development, with an inverse relationship between meconium total FAEE concentration and both body weight and brain weight.

Chronic maternal methanol inhalation in nonhuman primates (Macaca fascicularis): exposure and toxicokinetics prior to and during pregnancy

Toxicokinetic studies were conducted following daily inhalation exposure to methanol vapor prior to and throughout pregnancy in adult female Macaca fascicularis monkeys. They were part of a larger study to investigate the effects of chronic methanol exposure on maternal reproductive performance and early offspring effects. In a two-cohort study design, 48 females (24/cohort) were assigned to parallel exposure groups at 0 (control), 200, 600, or 1800 ppm methanol vapor for approximately 2.5 h/day, 7 days/week throughout breeding and pregnancy. Blood methanol at 30 min postexposure was monitored biweekly. The time course for the clearance of blood MeOH concentrations following exposure was characterized on four occasions: twice during the prebreeding period and during mid- and late pregnancy. Average blood methanol concentrations at 30 min postexposure were 5, 11, and 35 microg/ml across all four toxicokinetic studies in the 200, 600 and 1800 ppm groups, respectively. Blood concentrations in the 200 ppm group were barely above basal (preexposure) blood methanol concentrations or those observed in the control group (approximately 3 microg/ml). Nonlinear elimination kinetics were observed in most of the 1800 ppm group females. There was a decrease in elimination half-life (7-20%) and an increase in clearance (30%) after 3-months of daily MeOH exposure compared to the initial exposure. There were no statistically significant changes in the first-order blood methanol half-life or clearance during pregnancy, but the mean distribution volume per kilogram body weight decreased by 22% and 17% in the 600 and 1800 ppm groups. Plasma formate levels did not differ between the methanol and control exposure groups. Plasma formate and serum folate concentrations increased slightly over the course of this study in both the exposed and control groups but these increases were not related to methanol exposure.

Effects of chronic prenatal ethanol exposure on cGMP content and glutamate release in the hippocampus of the neonatal guinea pig

The glutamate-N-methyl-D-aspartate (NMDA) receptor-nitric oxide synthase (NOS)-cGMP signal transduction system plays key neurotrophic and intercellular communication roles in the hippocampus. In the guinea pig, chronic prenatal ethanol exposure (CPEE), via maternal ethanol administration, suppresses the hippocampal glutamate-NMDA receptor-NOS pathway in the near-term fetus and decreases stimulated glutamate release in the hippocampus of young postnatal offspring, with no effect on NMDA receptor number or NOS activity. At present, the effect of CPEE on cGMP, a key second messenger of the glutamate signal transduction system, in the hippocampus is not known. The objective of this study was to test the hypothesis that CPEE suppresses the hippocampal glutamate signal transduction system in the neonatal guinea pig at the levels of cGMP content and glutamate release. Timed pregnant guinea pigs received chronic oral administration of 4 g ethanol/kg maternal body weight/day, isocaloric-sucrose/pair-feeding, or water treatment throughout gestation. CPEE decreased brain and hippocampal weights at postnatal day (PD) 1 and PD 5 (P<.05). CPEE did not affect basal, NMDA (1, 10, or 100 microM)-stimulated, or K(+) (15 or 30 mM)-stimulated cGMP content in transverse hippocampal slices at PD 1 or 5. At 60 mM K(+), however, CPEE decreased stimulated hippocampal cGMP content at PD 1 (P<.05) and increased stimulated cGMP content at PD 5 (P<.05). In transverse hippocampal slices, CPEE did not affect basal or K(+) (40 or 45 mM)-stimulated glutamate release at PD 1 or 5, or NMDA (50 microM)-stimulated glutamate release at PD 1, but did decrease NMDA (50 microM)-stimulated glutamate release at PD 5 (P<.05). The data demonstrate that the effects of CPEE on stimulated cGMP content and glutamate release in the hippocampus of the neonatal guinea pig are stimulating agent- and age-dependent.

Ethanol neurobehavioral teratogenesis and the role of the hippocampal glutamate-N-methyl-D-aspartate receptor-nitric oxide synthase system

The purpose of this review is to evaluate a proposed mechanism for ethanol neurobehavioral teratogenesis in the hippocampus, involving suppression of the glutamate-N-methyl-D-aspartate (NMDA) receptor-nitric oxide synthase (NOS) system. It is postulated that suppression of this signal transduction system in the fetus by chronic maternal consumption of ethanol plays a key role in hippocampal dysmorphology and dysfunction in postnatal life. This mechanism is evaluated critically based on the current literature and our research findings. In view of the apparent time course for loss of CA1 pyramidal cells in the hippocampus produced by chronic prenatal ethanol exposure that manifests in early postnatal life, it is proposed that therapeutic intervention, which targets the glutamate-NMDA receptor-NOS system, may prevent or lessen the magnitude of postnatal hippocampal dysfunction.

Effects of ethanol on calcium homeostasis in the nervous system: implications for astrocytes

Ethanol is a major health concern, with neurotoxicity occurring after both in utero exposure and adult alcohol abuse. Despite a large amount of research, the mechanism(s) underlying the neurotoxicity of ethanol remain unknown. One of the cellular aspects that has been investigated in relationship to the neuroteratogenicity and neurotoxicity of ethanol is the maintenance of calcium homeostasis. Studies in neuronal cells and other cells have shown that ethanol can alter intracellular calcium levels and affect voltage and receptor-operated calcium channels, as well as G protein-mediated calcium responses. Despite increasing evidence of the important roles of glial cells in the nervous systems, few studies exist on the potential effects of ethanol on calcium homeostasis in these cells. This brief review discusses a number of reported effects of alcohol on calcium responses that may be relevant to astrocytes' functions.

Placental transfer of selected substances of abuse

The placenta was classically considered a barrier, but in 1957 this notion was shattered. This organ is only a selective filter and metabolic site. In this review, the placental transfer of the top 17 substances of abuse are analyzed. In the National Library of Medicine only 41 papers that documented placental transfer of the major substances of abuse could be located, and these data excluded approximately one-sixth of the most commonly abused substances. Nonetheless, it was possible to evaluate placental transfer of all substances of abuse based on their physical chemical properties. It is with despair that it must be reported that virtually all substances of abuse freely cross the placenta, exposing the embryo/fetus to whatever substances the mother may be using.

Ontogeny of the activity of alcohol dehydrogenase and aldehyde dehydrogenases in the liver and placenta of the guinea pig

The objectives of this study were to elucidate the ontogeny of the activity of alcohol dehydrogenase (ADH), low Km aldehyde dehydrogenase (ALDH) and high Km ALDH in the liver and placenta of the guinea pig, and to determine the relationship between the relative activity of each enzyme in the guinea pig maternal-placental-fetal unit and the disposition of ethanol and its proximate metabolite, acetaldehyde. The enzyme activities were determined in maternal liver, fetal liver, and placenta of the guinea pig at 34, 50, 60 and 65 days of gestation (term, about 66 days), in the liver of the 2-day-old neonate, and in adult liver. There was low ADH activity in fetal liver and placenta throughout gestation and in neonatal liver. The fetal liver low Km ALDH activity increased progressively and, at 60 days of gestation, was similar to adult liver activity, as was also the case for neonatal liver enzyme activity. Placental low Km ALDH activity was less than adult liver activity throughout gestation. Fetal hepatic high Km ALDH activity increased during gestation, but was less than adult liver activity, as was also the case for neonatal liver enzyme activity. Placental high Km ALDH activity was low throughout gestation. For oral administration of 0.5 g ethanol/kg maternal body weight to pregnant guinea pigs at mid-gestation (34 days), the maternal blood and fetal body ethanol concentration-time curves were similar. Acetaldehyde was measurable in maternal blood and fetal body at similar concentrations, which were 100- to 1000-fold less than the respective ethanol concentrations. The major difference in the disposition of ethanol and acetaldehyde at near-term pregnancy, compared with mid-gestation, was the lack of measurable acetaldehyde in fetal blood. These results indicate that the guinea pig fetus throughout gestation has virtually no capacity to oxidize ethanol, and its duration of exposure to ethanol is regulated by maternal hepatic ADH-catalyzed biotransformation of ethanol. The fetus, however, appears to have increasing low Km ALDH-dependent capacity to oxidize ethanol-derived acetaldehyde during development, and would appear to be increasingly protected from exposure to acetaldehyde as gestation progresses.

Disposition of acute, multiple-dose ethanol in the near-term pregnant ewe

The disposition of ethanol and its proximate metabolite, acetaldehyde, was determined in seven conscious instrumented pregnant ewes (127 to 132 days of gestation; term, 147 days) for intravenous infusion of four dosages of 0.5 gm ethanol/kg maternal body weight, administered over 5 hours to the mother. The maternal and fetal blood had ethanol concentrations that were maximal at 5 hours and were virtually identical during the 24-hour study. There was delayed transfer of ethanol into the amniotic and allantoic fluids during the dosing period, followed by higher ethanol concentrations in these fluids during the elimination phase compared with fetal blood. The ethanol elimination rate was similar for the four biologic fluids. Acetaldehyde concentrations in the four fluids were a thousandfold less than the respective ethanol concentrations. The maternal blood acetaldehyde concentration was greater than that in fetal blood. The data indicate that for a binge-type drinking episode during near-term pregnancy, there is unimpeded bidirectional placental transfer of ethanol between the mother and the fetus; the amniotic fluid surrounding the fetus is a reservoir for ethanol in utero; elimination of ethanol from the maternal-fetal unit is regulated by maternal hepatic biotransformation of ethanol; and there is appreciable acetaldehyde-oxidizing capacity in the maternal liver and at extrahepatic sites.