Biochemical changes, early brain growth suppression and impaired detour learning in nicotine-treated chicks

Vascular and morphogenetic abnormalities associated with exposure of cigarette smoke condensate during chicken and murine embryogenesis

OBJECTIVE

Embryonic movements (EM) and angiogenesis pathways are evolutionarily conserved mechanisms which are essential for proper embryonic development. Deviations in these processes by exposure to cigarette smoke condensate (CSC) may cause vascular and morphogenetic disorders.

METHODS

Using chicken and mouse embryos, we have demonstrated the in vivo effects of CSC on EM, vascular development, and organogenesis.

RESULTS

Examination of the CSC exposed chicken embryos revealed a significant reduction in EM, stunted growth, deviated pattern of blood vessels, hemorrhages, and localized necrosis. Likewise, mouse embryos that were exposed to CSC at E8.5 and E9.5 died between E11.5 and E12.5, respectively. These mouse embryos showed defects in morphogenesis and remodeling of the embryonic vasculature, while littermate controls showed normal development.

CONCLUSIONS

Cigarette smoking during pregnancy is fatal for growing embryos. CSC may induce the remodeling of embryonic vasculature, leading to various pathologies.

Anti-angiogenic and teratological activities associated with exposure to total particulate matter from commercial cigarettes

Angiogenesis and the embryonic movement (EM) pathway are evolutionarily conserved mechanisms, which are essential for embryonic development. Deviation in these processes from exposure to cigarette total particulate matter (TPM) may produce vascular, morphogenetic, and teratological disorders. The anti-angiogenic and teratogenic potential of TPM from commercially available cigarettes was studied. In vitro effects of TPM on angiogenesis were determined with different assays utilizing human umbilical vein endothelial cells (HUVEC). A chicken embryo model was used to demonstrate the in vivo effects of TPM on EM, vascular development, and organogenesis. The current study provides evidence that cigarette TPM plays an impeding role in endothelial cell proliferation, migration, tube formation, and sprouting, which are crucial factors in angiogenesis. Video recordings and kinematic analyses of the TPM exposed chicken embryos revealed a striking decrease in EM. Likewise, exposure of TPM to embryos resulted in ocular, mandibular, and abdominal hemorrhaging. Several teratologies including ectopia cordis, as well as bi-trunked and mammoth headed embryos were frequent findings among TPM treated embryos. These results are strongly reminiscent of morphogenetic and teratogenic deformities in TPM exposed embryos. This shows that cigarette smoking during pregnancy can be fatal to growing embryos. In addition, TPM may produce defective morphogenesis, leading to various pathologies.

Growth retardation of fetal rats exposed to nicotine in utero: possible involvement of CYP1A1, CYP2E1, and P-glycoprotein

To elucidate the possible metabolic mechanism of intrauterine growth retardation induced by nicotine, this study determines the effects of prenatal nicotine exposure on fetal development and cytochrome P4501A1 (CYP1A1), CYP2E1, and P-glycoprotein (Pgp) expression in maternal liver and placenta. Pregnant rats were given 1.0 mg/kg nicotine subcutaneously twice a day from gestational day (GD) 8 to GD 15, 18, or 21. In nicotine-treated groups, fetal developmental parameters including body weight were significantly lower. The activities of CYP1A1 and CYP2E1 in maternal liver microsomes in nicotine-treated groups increased significantly with progressing gestation when compared with the corresponding control, but returned to the level similar to the control in late pregnancy. Nicotine-treated groups induced pathological changes and increased malondialdehyde (MDA) content in the placenta when compared with the control. The gene expressions of CYP1A1 and CYP2E1 in the placenta increased significantly in nicotine-treated groups on GD 15 and GD 18, but returned to the level similar to the corresponding control on GD 21. In nicotine group, there was a decrease of mdr1a expression on GD 15, GD 18, and GD 21, with the most significant decrease on GD 15. In contrast, no significant difference was found in mdr1b mRNA expression between the nicotine-treated animals and the corresponding control. In comparison with the corresponding control, the placental Pgp protein significantly decreased on GD 15 and GD 18. Our results showed that prenatal nicotine exposure resulted in inhibition of fetal growth significantly. The induction of CYP2E1 and CYP1A1 gene expression by nicotine in the maternal liver and placenta may be involved with the observed increase in oxidative stress and lipid peroxidation. The inhibition of the placental Pgp expression by nicotine may also contribute to an increased susceptibility of the fetus to environmental toxins.

Diminished embryonic movements of developing embryo by direct exposure of sidestream whole smoke solutions

Embryonic movements (EM) are considered to be the first sign of life and cigarette smoking during pregnancy has been linked to affect EM. Exposure to sidestream smoke, produced from the emissions of a smoldering cigarette, may result in poor pregnancy outcome and increased risk of serious perinatal morbidity and mortality. In this study, the chicken embryo bioassay was used to systematically assess the effects of short-term exposure to sidestream whole smoke solutions (SSWSS) on EM, recorded in real time by a video camera for 60 min and each EM was counted for every 3-min interval. Application of different types of SSWSS to the embryos caused significant changes in all types of EM from 15 to 18 min of recording time. Extensive reduction (P<0.001) and some time complete stoppage of swing-like movements and whole-body movements were observed in almost all treated embryos. Our data clearly link between exposure of SSWSS and substantial decrease in EM. It is unclear whether nicotine and/or other ingredients present in sidestream smoke are responsible for these alterations in EM. This article provides an outline of the relevance of SSWSS on EM for evolutionary developmental biology and this assay can be used to investigate the complex mixtures with regard to their effects on EM.

Toxicological effects of mainstream whole smoke solutions on embryonic movements of the developing embryo

Cigarette smoking is unrivaled among developmental toxicants in terms of total adverse impact on the human population. Maternal tobacco use during pregnancy adversely affects prenatal and postnatal growth and increases the risk of behavioral and developmental defects in children and adolescents. In the current study, the effects of different preparations of nicotine and mainstream whole smoke solutions (MSWSS) on embryonic movements during neonatal development were examined in vivo, using the chicken embryo model, recorded in real-time by a video camera. It was observed that low doses of nicotine induced hyperactivity and higher doses induced hypoactivity. Accordingly, a significant (p < 0.01) decrease in movements was observed by application of 10 microg of nicotine and different preparations of MSWSS. A dose-dependent decrease in embryonic movements was observed, which did not recover by the end of experiment. It was concluded that nicotine could alter embryonic movements, which are important during embryogenesis for differentiation and maturation of the body systems.

Cholinergic systems in brain development and disruption by neurotoxicants: nicotine, environmental tobacco smoke, organophosphates

Acetylcholine and other neurotransmitters play unique trophic roles in brain development. Accordingly, drugs and environmental toxicants that promote or interfere with neurotransmitter function evoke neurodevelopmental abnormalities by disrupting the timing or intensity of neurotrophic actions. The current review discusses three exposure scenarios involving acetylcholine systems: nicotine from maternal smoking during pregnancy, exposure to environmental tobacco smoke (ETS), and exposure to the organophosphate insecticide, chlorpyrifos (CPF). All three have long-term, adverse effects on specific processes involved in brain cell replication and differentiation, synaptic development and function, and ultimately behavioral performance. Many of these effects can be traced to the sequence of cellular events surrounding the trophic role of acetylcholine acting on its specific cellular receptors and associated signaling cascades. However, for chlorpyrifos, additional noncholinergic mechanisms appear to be critical in establishing the period of developmental vulnerability, the sites and type of neural damage, and the eventual outcome. New findings indicate that developmental neurotoxicity extends to late phases of brain maturation including adolescence. Novel in vitro and in vivo exposure models are being developed to uncover heretofore unsuspected mechanisms and targets for developmental neurotoxicants.

Exposure to low concentrations of nicotine during cranial nerve development inhibits apoptosis and causes cellular hypertrophy in the ventral oculomotor nuclei of the chick embryo

Maternal cigarette use during pregnancy is associated with increased incidence of neural impairments in offspring, but nicotine's unique contribution to any neuropathology remains unclear, and nicotine's neurodevelopmental effects assessed in animal models vary with concentration. During ontogenesis, the chick oculomotor complex (OMN) is regulated by central nervous system (CNS) afferent-derived and target-derived trophic factors, allowing assessment of nicotine's potential interference in receptor-mediated CNS trophic phenomena, unconfounded by myriad other compounds in cigarette smoke. In the current study, 100 ng nicotine applied daily in ovo to yolk during embryonic days (E) 1-7 mimicked maternal plasma nicotine concentrations during fetal cranial nerve development. Nicotine-treated embryos exhibited a 15% decrease in whole body weight and 7% decrease in brain weight at E16. However, at E16, nicotine-treated embryos had 37% and 15% increases in the combined ventromedial+lateral (v) OMN motoneuron density and soma area, respectively, effects not observed in the optic tectum, in which nicotine cholinergic receptor expression is delayed until E8-12. Incorporation of tritiated thymidine into whole brain DNA demonstrated that the nicotine treatment did not cause increased rates of whole brain mitosis, suggesting that the dosage regimen did not elicit a cytotoxic, wound-healing, response of differentiating cells. As determined by DNA fragment-labeling assay during the normal period of cell death, vOMN apoptosis occurs maximally on E11 during a normal period of declining cell density, and a dose-response study demonstrated 78% E11 vOMN apoptotic suppression at approximately 0.30 microM cumulative yolk nicotine with an inhibition threshold between 0.10 and 0.20 microM. These results suggest that plasma nicotine concentrations resulting from tobacco use or nicotine replacement therapy (NRT) are sufficient to inhibit motoneuron apoptosis and enhance neuronal growth.

Conditional expression in corticothalamic efferents reveals a developmental role for nicotinic acetylcholine receptors in modulation of passive avoidance behavior

Prenatal nicotine exposure has been linked to attention deficit hyperactivity disorder and cognitive impairment, but the sites of action for these effects of nicotine are still under investigation. High-affinity nicotinic acetylcholine receptors (nAChRs) contain the beta2 subunit and modulate passive avoidance (PA) learning in mice. Using an inducible, tetracycline-regulated transgenic system, we generated lines of mice with expression of high-affinity nicotinic receptors restored in specific neuronal populations. One line of mice shows functional beta2 subunit-containing nAChRs localized exclusively in corticothalamic efferents. Functional, presynaptic nAChRs are present in the thalamus of these mice as detected by nicotine-elicited rubidium efflux assays from synaptosomes. Knock-out mice lacking high-affinity nAChRs show elevated baseline PA learning, whereas normal baseline PA behavior is restored in mice with corticothalamic expression of these nAChRs. In contrast, nicotine can enhance PA learning in adult wild-type animals but not in corticothalamic-expressing transgenic mice. When these transgenic mice are treated with doxycycline in adulthood to switch off nAChR expression, baseline PA is maintained even after transgene expression is abolished. These data suggest that high-affinity nAChRs expressed on corticothalamic neurons during development are critical for baseline PA performance and provide a potential neuroanatomical substrate for changes induced by prenatal nicotine exposure leading to long-term behavioral and cognitive deficits.

Nicotine and alpha-bungarotoxin modify the dendritic growth of cholinoceptive neurons in the developing chick tectum

Nicotinic acetylcholine receptors have been suggested to participate in morphogenetic processes during development of the nervous system. In this study, nicotine applied both in ovo and in vitro produced a reduction of the neuritic length of cholinoceptive neurons of the developing chick tectum, whereas alpha-bungarotoxin produced the opposite effect. Taken together with previous data, our results are indicative of a role of the alpha-bungarotoxin-sensitive nicotinic receptors in neural development.

Environmental factors associated with a spectrum of neurodevelopmental deficits

A number of environmental agents have been shown to demonstrate neurotoxic effects either in human or laboratory animal studies. Critical windows of vulnerability to the effects of these agents occur both pre- and postnatally. The nervous system is relatively unique in that different parts are responsible for different functional domains, and these develop at different times (e.g., motor control, sensory, intelligence and attention). In addition, the many cell types in the brain have different windows of vulnerability with varying sensitivities to environmental agents. This review focuses on two environmental agents, lead and methylmercury, to illustrate the neurobehavioral and cognitive effects that can result from early life exposures. Special attention is paid to distinguishing between the effects detected following episodes of poisoning and those detected following lower dose exposures. Perinatal and childhood exposure to high doses of lead results in encephalopathy and convulsions. Lower-dose lead exposures have been associated with impairment in intellectual function and attention. At high levels of prenatal exposure, methylmercury produces mental retardation, cerebral palsy and visual and auditory deficits in children of exposed mothers. At lower levels of methylmercury exposure, the effects in children have been more subtle. Other environmental neurotoxicants that have been shown to produce developmental neurotoxicity include polychlorinated biphenyls (PCBs), dioxins, pesticides, ionizing radiation, environmental tobacco smoke, and maternal use of alcohol, tobacco, marijuana and cocaine. Exposure to environmental agents with neurotoxic effects can result in a spectrum of adverse outcomes from severe mental retardation and disability to more subtle changes in function depending on the timing and dose of the chemical agent.

Neurotransmitter regulation of neural development: acetylcholine and nicotinic receptors

Several neurotransmitter systems have been related to developmental processes during the past decade. In this review, we discuss the evidence that the nicotinic acetylcholine receptors could have an additional function during development that may be unrelated to their role in cholinergic neurotransmission in the vertebrate brain. Both temporal expression data and in vitro and in vivo studies with nicotinic agonists and antagonists have provided direct support for a role of nicotinic receptors in neural developmental processes such as neurite outgrowth and differentiation. A similar picture has emerged for other neurotransmitter and receptor systems as well, which generates a new view of neural processes during both development and mature life.

Ethanol- and nicotine-induced membrane changes in embryonic and neonatal chick brains

In order to study the effects of EtOH and/or nicotine on brain membrane fatty acid composition, various concentrations of EtOH and/or nicotine were injected into the air sac of chicken eggs at 0 days of incubation. Controls were injected with saline. Experimental groups were injected with either 200 micromol EtOH/kg egg, 100 micromol nicotine/kg egg, 200 micromol nicotine/kg egg, 200 micromol EtOH/kg and 100 micromol nicotine/kg egg, or 200 micromol EtOH/kg and 200 micromol nicotine/kg egg. In all experimental groups, EtOH- and nicotine-induced decreases in brain long-chain polyunsaturated membrane fatty acids were observed in stage 44 embryos, stage 45 embryos, and neonatal chicks. These EtOH- and nicotine-induced decreases in brain membrane polyunsaturated fatty acids correlated with elevated levels of brain lipid hydroperoxides and reduced brain acetylcholinesterase (AChE; EC. 3.1.1.7) activities.

Perinatal exposure to environmental tobacco smoke induces adenylyl cyclase and alters receptor-mediated cell signaling in brain and heart of neonatal rats

Perinatal exposure to environmental tobacco smoke (ETS) has adverse effects on neurobehavioral development. In the current study, rats were exposed to ETS during gestation, during the early neonatal period, or both. Brains and hearts were examined for alterations in adenylyl cyclase (AC) activity and for changes in beta-adrenergic and m2-muscarinic cholinergic receptors and their linkage to AC. ETS exposure elicited induction of total AC activity as monitored with the direct enzymatic stimulant, forskolin. In the brain, the specific coupling of beta-adrenergic receptors to AC was inhibited in the ETS groups, despite a normal complement of beta-receptor binding sites. In the heart, ETS evoked a decrease in m2-receptor expression. In both tissues, the effects of postnatal ETS, mimicking passive smoking, were equivalent to (AC) or greater than (m2-receptors) those seen with prenatal ETS mimicking active smoking; the effects of combined prenatal and postnatal exposure were equivalent to those seen with postnatal exposure alone. These data indicate that ETS exposure evokes changes in cell signaling that recapitulate those caused by developmental nicotine treatment. Since alterations in AC signaling are known to affect cardiorespiratory function, the present results provide a mechanistic link reinforcing the participation of ETS exposure, including postnatal ETS, in disturbances culminating in events like Sudden Infant Death Syndrome.

Establishment of a chick embryo shell-less culture system and its use to observe change in behavior caused by nicotine and substances from cigarette smoke

Early detection and removal of harmful factors are essential to the proper physical and psychological development of the fetus, presumably showing the effects during the prenatal period and after birth. As one procedure to aid in understanding such factors, we have established a shell-less culture system for video monitoring to observe change in behavior of 7-day-old chick embryos. Nicotine and aqueous cigarette smoke extract (ACSE) were selected for the present experiments, and the results showed a complete stoppage of swing-like movements by administrations of 10 microg nicotine and 1xACSE, possibly displaying paralytic symptoms. Quantitative analysis of nicotine in 1xACSE indicated that more than 10 microg of nicotine were contained in 100 microl of the extract. The present system, although in initial stage of development, may be a useful preliminary screening procedure for perhaps supervision and warning about the environment surrounding pregnant women.

Perinatal exposure to environmental tobacco smoke alters cell signaling in a primate model: autonomic receptors and the control of adenylyl cyclase activity in heart and lung

Perinatal exposure to environmental tobacco smoke (ETS) is known to have adverse effects on respiratory function in conjunction with changes in autonomic responses. In the current study, Rhesus monkeys were exposed to ETS during late gestation and in the early neonatal period. Hearts and lungs were examined for changes in beta-adrenergic and m2-muscarinic cholinergic receptors, and for alterations in adenylyl cyclase activity. Whereas there were no changes in the heart, there was robust induction of adenylyl cyclase in the lung; previous work with prenatal nicotine exposure in rodent models has shown that adenylyl cyclase induction is associated with a shift towards predominance of cholinergic over adrenergic responses. These data indicate that perinatal ETS exposure evokes changes in cells signaling that they are selective for the lung and that may ultimately reflect adverse effects at the level of physiological function.

Changes in brain glucose levels and glucose transporter protein isoforms in alcohol- or nicotine-treated chick embryos

Suppression of fetal brain growth during pregnancy as the result of maternal smoking or alcohol consumption leads to significant problems for the offspring as well as for the society who must care for these individuals. Chronic maternal intake of cigarette smoke is frequently observed in humans and studies using animal models suggest that in utero nicotine exposure is an important component of the growth suppression that results. Similarly, maternal consumption of alcohol (ethanol) has a profound, negative effect on fetal growth. The developing fetal central nervous system (CNS) is sensitive to the growth inhibitory effect of nicotine or alcohol and morphological as well as functional CNS deficits may result from fetal exposure. Using an embryonic chick model which minimizes drug-induced changes in maternal nutrition and behavior, the studies presented here indicate that nicotine or alcohol exposure during early embryonic development inhibits brain growth to a degree comparable to that seen in the rest of the organism, i.e., there was no 'brain sparing' in this model. Glucose content per milligram tissue was markedly decreased in brains of the nicotine-treated embryos but was not significantly different in the alcohol-exposed embryos. Western blots of fetal brain glucose transporter protein isoforms showed no change in the Glut 3 transporter content in the growth suppressed brains compared to vehicle-treated brains. The Glut 1 55 kilodalton (kd) isoform protein content was significantly decreased in the nicotine-treated brains but unchanged in the ethanol-treated brains, while the reverse was true for the Glut 1 45 kd isoform. Thus, the changes in the 55 kd isoform protein content were correlated with tissue glucose levels in the ethanol- and nicotine-treated embryos.