ReviewBarriers in the developing brain and Neurotoxicology
Introduction
In pregnancy and following birth, the developing brain is vulnerable to the effects of exposure to drugs and xenobiotics via the mother (Landrigan and Goldman, 2011). It is therefore essential to have available accurate information about both protective mechanisms and potential vulnerabilities in the developing brain. This will ensure that clinical advice to patients is based on reliable evidence and relevant agencies are able to formulate regulations that are in accord with the real hazards rather than assumed ones.
In the adult, the brain is protected from many drugs and xenobiotics by a series of mechanisms described by the term “blood–brain barrier”. In the embryo and fetus an important degree of protection is provided by the placenta, with potential additional protection provided by blood–brain barrier mechanisms in the developing brain. However, there is a widespread belief, particularly amongst neurotoxicologists, going back nearly 100 years that “the” blood–brain barrier is “immature” or even absent in the embryo, fetus and newborn. As it will be discussed below, this belief is promulgated in many reviews and textbooks (e.g. Costa et al., 2010, Ahmed et al., 2005) particularly in the journals Neurotoxicology (Table 1) and Environmental Health Perspectives (Table 2) as well as in official government sources such as the CDC in the US (ATSDR, 2011; see Table 3, Table 4). This belief is not supported by most evidence going back almost as far as when the original concept of the blood–brain barrier was suggested more than 100 years ago (Lewandowsky, 1900).
The permeability of the brain barrier interfaces during development has been inferred from measurements of the concentrations of marker compounds in the brain or CSF in relation to their concentrations in plasma (i.e. CSF/plasma and brain/plasma concentration ratios). However, these ratios only measure apparent permeability, not the actual permeability of the barrier interfaces. This is because the brain or CSF concentration of a compound is a function not only of its rate of influx, but also its rate of efflux and any developmental changes in the volume of the compartment that it is distributing in. Changes in any of these factors directly affect concentration ratios and thus the apparent permeability (Johansson et al., 2008). Unfortunately, apparent permeability changes have often been confused in the literature as only reflecting changes in influx and thus interpreted as changes in the maturity or tightness of the brain barrier interfaces. In addition, efforts to take into account the blood component present in brain tissue samples prior to analysis are often not employed.
In spite of the general advice to clinicians to avoid prescribing drugs to pregnant women and infants, the practice continues. In developed countries, a very high proportion of women are prescribed medications during their pregnancies; 93% in France (Lacroix et al., 2000), 85% in Germany (Egen-Lappe and Hasford, 2004), 69% in the Netherlands (Schirm et al., 2004) and 64% in the US (Andrade et al., 2004). In a survey by Crespin and colleagues (2011), it was found that more women were prescribed medications during pregnancy than before and some were prescribed potentially dangerous medications for the first time during their pregnancy. Lacroix and colleagues (2000) reported, that 79% of pregnant women in Southwest France were prescribed medications for which there is no information available from either animal or human studies about their safety in pregnancy. Over half the women (59%) received a prescription for drugs that are classed by the US Food and Drug Administration as category D (fetal risk, but benefits may be acceptable) and 1.6% received prescriptions for drugs classed as category X (fetal risk outweighs benefits). In the US, it has been estimated that between 30% and 35% of women have taken psychoactive drugs during their pregnancies (Goldberg, 1994). A World Health Organisation (WHO) sponsored investigation showed that women ingest an average of three prescription medications during pregnancy (range 1–15) (Collaborative Group on Drug Use in Pregnancy, 1992). In the US, Yaffe's index (Briggs et al., 2008) now lists more than 1200 drugs that have been given to pregnant women. This occurs in the light of summary information on untoward effects. It is perhaps surprising that there are relatively few drugs with “black box” warnings, but it is not clear whether, in our present state of knowledge, this is because the brain in the embryo or fetus is quite well protected or that we simply do not have the information to judge. Of course for some conditions such as epilepsy, severe hypertension and depression it may be clinically essential to advise a pregnant women to take an appropriate drug. However, it would be better for that to be done with the knowledge of whether there is in fact a risk to the baby and, if so, how frequent and of what potential severity.
In this review, we shall briefly outline the barrier interfaces and mechanisms in the adult and then summarise what is known in the developing brain, particularly where there is information available for the human brain. We shall also review why the papers frequently cited in the neurotoxicological literature provide inadequate evidence for support of the supposition that the blood–brain barrier in the fetus and newborn is absent or immature. We also try to explain how this inaccurate assumption has prevailed over the decades. Overall, it is now clear that many blood–brain barrier mechanisms are likely to be functionally effective in the embryo and fetus and that protection of the developing brain is also provided by similar mechanisms in the placenta. However, we shall also show that some mechanisms are unique to the developing brain, such as selective transport of plasma proteins across the choroid plexus. In addition, there are other mechanisms, such as transport of some amino acids, which are more active in the developing bran than in the adult. Because many xenobiotics and some drug may be transported into the brain and cerebrospinal fluid by these mechanisms in utero, they may render the developing brain more vulnerable than in the adult. Thus concern about possible involvement of the blood–brain barrier in the vulnerability of the brain to xenobiotics and drugs is justified. This selective vulnerability of the developing brain is due to essential normal physiological mechanisms and not because of an absence of barrier mechanisms. A consequence of this is that much more information is needed about whether or not individual drugs and xenobiotics do indeed enter the developing brain via the mother. This realisation contrasts with current assumption that there is an absence or immaturity of the blood–brain barrier; which, if correct, would mean that any drug or xenobiotic that crossed the placenta would have direct access to the vulnerable brain.
Section snippets
The blood–brain barrier in the adult
The term blood–brain barrier normally refers to an array of several barrier mechanisms that exist between the blood and the central nervous system (CNS) i.e. brain, cerebrospinal fluid (CSF) and spinal cord (Davson and Segal, 1996, Abbott et al., 2010). However, the term “barrier” is somewhat misleading, since as well as providing a set of morphological barrier mechanisms, it also encompasses a series of inward and outward transport mechanisms between blood and the CNS. To fully grasp the
Adult brain
The earliest experiments that led to the concept of a barrier between blood and the brain in the adult involved systemic injections of easily visualised compounds such as dyes, which coloured the whole body except for the brain and spinal cord (Ehrlich, 1885, Ehrlich, 1957, Goldmann, 1909). In contrast, when the dyes were injected directly into the brain all of the neural tissue became stained, but the rest of the body remained free of the dye (Goldmann, 1913). It was also found that some
Inward transport systems
This review will only provide a brief summary of inward transport systems into the CNS. These systems deal with molecules that are mainly water-soluble and the diffusion of which would be expected to be highly restricted by the brain barriers unless specifically transferred into the CNS.
The placental barrier and neurotoxicity
The placenta is not the focus of this review, however, it does need to be considered in order to understand overall neurotoxicity mechanisms in the fetus. It is possible that developmental changes in the placental barrier and brain barrier function influence each other and are linked in order to maintain protection of fetal brain throughout pregnancy. The placenta provides the fetus with oxygen and nutrients, but also protects it from harmful compounds such as xenobiotics in the mother's blood.
Cerebrospinal fluid secretion
One aspect of brain barrier function that might be considered immature is the secretion rate of CSF, although it is presumably appropriate for each stage of brain development. For technical reasons, it is difficult to measure CSF secretion in embryos and fetuses, but measurements have been made in sheep fetuses as early as E60 (Evans et al., 1974); gestation in sheep is 150 days. Estimates have also been obtained for neonatal rats (Bass and Lundborg, 1973). Both studies showed that the absolute
Susceptibility of the developing nervous system
As has been outlined above, there is a range of barrier mechanisms already functioning in the fetus and in the placenta. Anatomically restrictive junctions are present at all these sites and several detoxifying mechanisms are functioning. There are a few developmental aspects that most likely make the growing brain more susceptible to toxins than the adult. Firstly, developing neurons and glial cells undergo an intricate maturation processes that is still not fully understood. During this
Persistence of the belief that the blood–brain barrier in the embryo, fetus and newborn is absent, incomplete or immature
As outlined above, most of the original studies of the developing blood–brain barrier using dyes showed that this barrier was present as in the adult brain, although perhaps more fragile (Saunders, 1992). Evidence of functionally effective barrier mechanisms in the developing brain has been known for many years (Saunders, 1977, Saunders and Møllgård, 1981, Saunders and Møllgård, 1984, Møllgård and Saunders, 1986, Saunders, 1992, Saunders et al., 1999a, Saunders et al., 1999b, Saunders et al.,
The effect of medication on the developing child
One hopes that medication taken during pregnancy is taken for important clinical problems in the mother. However, as indicated in the introduction the proportion of pregnant women taking drugs is very high and the risks to the fetus, in particular the fetal brain, are generally unclear. Overall we know very little about how much of a drug reaches fetal tissues from the maternal circulation or how it can affect the normal development of the fetus. Drugs are categorised by pregnancy categories A,
Environmental toxins
There is increasing concern about the hazards of a wide range of compounds in the environment including heavy metals, pesticides, plastic products and various solvents. It is outside the scope of this review to give a detailed summary of potential or actual effects of neurotoxins on the developing brain. Thus, we shall concentrate mainly on the state of barrier functions in the fetus and newborn that may contribute to the amount of compounds entering the developing brain. There are two main
Metals
Some transition metals (such as manganese and iron) are used in our bodies for essential biological functions; however many metals found in our environment have no known essential biological function and are highly neurotoxic. The possible contributions of brain barrier mechanisms to neurotoxicology in the developing brain have been reviewed previously (Saunders et al., 2010).
Pesticides
Many pesticides, such as organochlorins, are lipid soluble and can therefore be expected to be able to pass through both the placenta and blood–brain barriers to reach neural tissues, resulting in adverse effects (Whyatt et al., 2006, Eskenazi et al., 2008). At least some pesticides seem to be P-gp substrates (Bain and LeBlanc, 1996, Schinkel et al., 1995) so both the placenta and blood–brain barrier may afford some protection to the fetus from these compounds even though they are lipid soluble.
Summary
The proportion of women taking medications during pregnancy is very high. The risks to the fetus, in particular the fetal brain, for most medications are generally unclear. However, a WHO report in 2001 did go somewhat towards acknowledging that there are indeed important functional barrier mechanisms present in the developing brain (WHO Environmental Health Criteria 223).
The evidence summarised in this review clearly indicates that the brain barrier mechanisms show significant functional
Conclusions
The widespread notion that “the” blood–brain barrier is absent or immature is long overdue to be laid to rest. However, the developing brain is undoubtedly vulnerable to drugs and toxins, thus caution in administering drugs to pregnant women remains important. The most vulnerable members of society, fetuses and neonates, are currently the least protected by the regulatory authorities. Most of the drugs used in pregnancy, neonates and children have never been fully evaluated for effectiveness or
Conflict of interest
The authors declare that there is no conflict of interest.
Acknowledgements
This work was supported by funding from the National Health and Medical Research Council (NHMRC, Australia) as part of an agreement with the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement No. HEALTH-F2-2009-241778.
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