Book - Brain and behavioural development 4

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Dickerson JWT. and McGurk H. Brain And Behavioural Development. (1982) Blackie & Son Ltd., Glasgow.

Brain and Behavioural Development - 1982: 1 Neural Development | 2 Comparative Neural | 3 Malnutrition | 4 Hormones and Growth Factors | 5 Cortical Activity | 6 Functional Asymmetry | 7 Plasticity | 8 Sex Differences

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Chapter Four - Effects of Hormonal and Other Factors on Growth and Development

Brian L. G. Morgan


Introduction

Various hormones, especially thyroid hormone and growth hormone, are important in the control of normal growth (Zamenhof et al , 1971; Winick, 1976). Optimum growth can occur only when such hormones are at their optimum level. There is some evidence to show that malnutrition impairs brain growth and development via this hormonal system (Monckeberg et al , 1963; Muzzo et al , 1973).

Some infants suffer from inborn errors of metabolism. Such defects involve the enzymes responsible for metabolism of circulating substrates and also the mechanisms for transportation of nutrients into the brain (Pardridge, 1977). If the inability to metabolize a key circulating nutrient lowers its level in the plasma beyond the point when it can satisfy the brain’s needs, or elevates the level to the point of impairing the brain’s uptake of other key nutrients, then the brain’s nutrition will be compromised and continuing growth and development will be impaired. The functional consequences of these inborn errors of metabolism can be extremely debilitating. Inborn errors of metabolism have been discovered concerning phenylalanine (Foiling, 1934), tryptophan (Hsia, 1972), tyrosine (Gentz et al, 1965), valine, isoleucine and leucine (Wiltse and Menkes, 1972), lysine (Fellows and Carson, 1974), proline (Selkoe, 1969), hydroxyproline (Efron et al, 1965), glycine (Hsia et al , 1971), histidine (Wiltse and Menkes, 1972), homocysteine (Gerritsen et al, 1962), cystathionine (Gaitonde, 1970), methionine (Guroff, 1977), arginine (Allan et al, 1958), various carbohydrates (Austin, 1972; Kalckar et al, 1965), and nucleic acid precursors (Nyhan, 1973).


Much recent evidence has shown that the maternal diet during pregnancy and lactation is critical to normal brain growth and development of the offspring. As shown in an earlier chapter, protein energy malnutrition at this time has devastating effects. However toxic substances such as alcohol (Warren, 1977) and marijuana (Fried, 1976) reaching the foetus via the maternal circulation can also be extremely hazardous to normal brain growth.

This chapter will briefly review the literature in these areas of study.

Thyroid hormones

Neonatal hypothyroidism—animal studies

Animals made hypothyroid in the neonatal period show slow movement, delayed development, low body temperature, thickened skin, abnormal hair and subnormal intelligence (Simpson, 1908,1913,1924a, b).

By 15 days of age a significant reduction in brain growth becomes obvious in hypothyroid rat pups. By contrast, body weights do not become significantly lower until 24 days of age (Eayrs and Taylor, 1951). The reduced brain growth is accompanied by reductions in DNA, RNA and protein in both cerebrum and cerebellum (Faryna et al ., 1972; Gourdon et al ., 1973; Geel and Tamiras, 1967; Balazs et al, 1968). By 21 days postnatally, when normal hyperplasia in the brain ceases, the reduction in cellularity is considerable. However, hypothyroidism, as well as reducing the rate of cell division in brain, extends the time period during which cell division continues to 35 days. Ultimate cell numbers achieved may be even greater than normal (Winick, 1976). However the reductions in protein and RNA are permanent (Gourdon et'cd., 1973).

The neurones in the sensory cortex tend to be smaller and more closely packed than normal (Eayrs and Taylor, 1951). The basal dendritic processes tend to be shorter and show less branching in hypothyroid animals (Horn, 1955). A decrease in the vasculature of the cerebral cortex has also been reported (Eayrs, 1954). However if neonatal ablation of the thyroid gland is followed by treatment with thyroid hormone from day 24 to 60 after birth these organizational changes in the cerebral cortex can be reversed (Horn, 1955).

In the cerebellum there is a delayed migration of the external granular layer, and there is an increase in granule cells over and above normal by day 30. By contrast there is a reduced number of basket cells and an impaired development of astroglial processes. The Purkinje cells show a decreased synaptic content and retarded arborizations. There is also a retarded synaptogenesis in the molecular layer, and the overall pattern of growth in the cerebellum is thus impaired. However, if thyroid hormone is administered to hypothyroid rat pups before the end of the second week of life, many of these changes may be overcome. It is worthy of note that the timetable of cerebellar development may be accelerated in mice and rats by administration of thyroid hormone to normal newborn rats (Legrand et al , 1961; Legrand, 1965; Nicholson and Altman, 1972a, b,c; Hajos et al , 1973; Pesetsky, 1973).

Other histological changes in the central nervous system resulting from hypothyroidism are a decrease in the Nissl granules of the cells in the cerebral cortex, cerebellum, medulla and ventral horn of the spinal cord. The spinal cord also shows other characteristic changes. It is lighter and has fewer fibre tracts than normal (Barrnett, 1948).

The effects of hypothyroidism on myelination and on the lipid composition of the brain exceed those induced by malnutrition and are more reversible, i.e. on administration of thyroid hormone (Balazs et al ., 1969; Trojanova and Mourek, 1973). Reductions in the different classes of lipids per cell have been demonstrated in both cerebrum and cerebellum in a number of studies. We see lowered contents of cerebrosides, cholesterol, phospholipids, sphingomyelin and gangli- osides. There is also a reduction in proteolipids in the cerebrum (Balazs et al , 1969; Faryna et al, 1972).

Early studies provided a clue to the biochemical basis of disturbed brain function in hypothyroidism when it was shown that oxygen consumption was as much as 40 % lower in hypothyroid animals (Barrnett, 1948). It has since been- confirmed that thyroidectomy lowers oxygen consumption in both developing and adult brain (Trojanova and Mourek, 1973). Cerebral oxygen consumption in euthyroid rats is low at birth and remains so in the early days of life. The adult level is reached at 45 days of life. If thyroid hormone, T 4 , is given to newborn rat pups it raises oxygen consumption. However after 45 days it has no effect (Fazekas et al, 1951). It has been postulated that T 4 affects protein biosynthesis and so only stimulates oxygen consumption in tissues with a high rate of protein turnover (Sokoloff, 1961). Using liver slices it has been shown that protein biosynthesis is low in hypothyroidism but can be stimulated by addition of T 4 (Du Toit, 1952). Thyroxine administered to normal animals in vivo also increased amino acid incorporation into protein in cell free liver homogenates (Sokoloff and Kaufman, 1961). Other studies have shown that amino acid incorporation into protein is rapid in cell free preparations of infant rat brain in comparison to adult brain (Gelber et al, 1964). However incorporation of amino acids into cerebellar tissue protein during the first three weeks of life has been shown to be slowed considerably by hypothyroidism (Dainat, 1974).

Mitochondria are essential for the incorporation of amino acids into microsomal proteins. Mitochondria from immature rat brains are several times more effective than adult brain tissue in this respect (Klee and Sokoloff, 1964). It seems that the mitochondria interact with T 4 to produce a substance that stimulates the biosynthesis of protein at the step involving the transfer of soluble RNA-bound amino acids to the microsomal protein (Sokoloff et al, 1963; Weiss and Sokoloff, 1963). The mitochondrial fraction can also incorporate amino acids into the proteolipid fraction of myelin. Once again this process occurs more readily in young brains than in mature brains (Klee and Sokoloff, 1965).

T 4 also accelerates the incorporation of inorganic 32 P into brain phospholipids. Conversely thyroid deficiency results in a marked decrease in the incorporation of 32 P into this lipid fraction (Myant, 1965). Thus thyroid deficiency inhibits phospholipid synthesis in the brain.

Many enzymes involved in brain metabolism are affected by hypothyroidism. Glutamate decarboxylase, a mitochondrial enzyme in the synaptosomal fraction, has a reduced activity per cell at 17-32 days of life. Lactate dehydrogenase, a supernatant enzyme, is similarly reduced in activity (Balazs et al, 1968). Succinic dehydrogenase activity has been shown to be suppressed in the cerebral cortex, and cholinesterase is also suppressed but to a lesser extent (Van Wynsberghe and Klitgaard, 1973). These enzyme changes do not occur, or are reversed, if thyroid hormone is administered by the tenth day of life.

Enzymes involved in fat synthesis are similarly affected. Fatty acid synthetase, which is involved in the formation of fatty acids such as palmitate from acetyl Co-A, malonyl Co-A and reduced nicotinamide adenine dinucleotide phosphate, is reduced in activity by about 45 % in the young hypothyroid rat (Volpe and Kishimoto, 1972). In control animals this enzyme is most active in the brains of foetuses and young animals as it is concerned with the early appearance and continuous turnover of phospholipids in cell membranes (Volpe et al , 1973). The activity of galactocerebroside sulfatransferase, which is important in the process of myelination, is also reduced in hypothyroidism; this time by about one-third (Walravens and Chase, 1969; Mantzos et al, 1973).


Hypothyroidism and behaviour

Hypothyroidism in the newborn delays the appearance of behavioural characteristics such as eye opening and acquisition of righting and placing reflexes (Van Wynsberghe and Klitgaard, 1973). More complex adaptive behavioural manifestations, for example maze learning and performance in an escape avoidance situation in aversion to an adverse stimulus (Eayrs, 1971) using an auditory conditioning stimulus, are also impaired. Most of these learning deficiencies may be reversed with thyroid replacement therapy even if it is given late. However some deficiencies persist unless replacement therapy is given before the tenth day of life. One such test where this is true is the Hebb-Williams closed field test (Dodge et al, 1975).


Hypothyroidism in primates

A few studies have been conducted on the effects of prenatal hypothyroidism in primates on development of the CNS. They have shown that hypothyroidism gives rise to considerable reductions in the non-chloride space, protein and non-protein solids, RNA, cholesterol, NeuNAC, sodium, potassium, ATPase and carbonic anhydrase in both the cerebrum and cerebellum, but no change in DNA content of either area of the brain was found (Kerr et al , 1972; Holt et al, 1973).


Hypothyroidism ( cretinism ) in children

Most studies of hypothyroidism or cretinism in children have used a population suspected of contracting the disorder in very early life or during the foetal period. Unfortunately the early diagnosis of hypothyroidism is often missed in these children because of their rather nondescript symptoms such as lethargy, constipation and feeding problems (Raid and Newas, 1971). Neonatal jaundice, enlarged tongue and umbilical hernias are also found in a large number of cases. If a case is suspected, confirming evidence may be obtained from observations on their osseous development. Bone age of cretins in early postnatal life often approximates osseous development of normal foetuses of 7-9 months of age (Caffey, 1950; Anderson, 1961).

Cretins can show a variety of characteristics, namely low IQ, spasticity, a shuffling gait, uncoordination, awkwardness, jerky movements and a coarse tremor. On the other hand, those with mild cretinism may not show these signs (Money, 1956).

If the abnormalities are to be reversed in the young child they must be recognized at an early age and treated with thyroid hormone. In one carefully conducted study it was shown that when treatment was initiated before the end of the third month of life almost all cretins could be improved to such an extent that their psychometric testing scores exceeded 90. By contrast, when treatment was not started until after three months many children had scores below 90 (Raid and Newas, 1971).

In another study it was shown that if replacement therapy began after one year of age there was little hope of raising the IQ over 90. On the other hand if treatment was begun before six months of age it was shown that as many as 45 % of all children in the study achieved an IQ in excess of 90. As one would expect, children given therapy between the ages of seven months and one year were apt to give results intermediate between the two extremes cited here (Smith et al , 1957).

If the hypothyroidism was contracted at a later age (after six months of age) as many as 41 % achieved IQ’s better than 90. Only a small percentage fell into the uneducable group with IQ’s of less than 50. If the hypothyroidism arose after 13 years the prognosis was even better. By this time the brain has developed and any neurological effects of myxodema exhibited are reversible.

In patients that are deprived of early treatment the prognosis for normal brain and neurological development is undoubtedly the poorest. However, even those cases caught immediately after birth and treated at that time show impaired development because of the damage sustained in utero (Wilkins, 1962). In early foetal life thyroxine crosses the placenta. Hence in the event of reduced maternal T 4 production or placental transport of the same the growth of the foetus could be compromised (Man et al ., 1963).

After the first trimester of pregnancy the foetus is able to make thyroid hormone. How important the foetal T 4 is in comparison to that arising in the mother is not clear (Hodges et al , 1955; French and Van Wyk, 1964). In the beagle we know that four times as much T 4 is transported from foetus to mother as vice versa (Beierwaltes and Mato vino vie, 1963). In this case most maternal T 4 is bound to protein and not available for placental transport. On the other hand, foetal T 4 binds less readily to protein and so tends to flow to the mother more easily (French and Van Wyk, 1964). However, the placenta does have the capacity to transport large doses of administered triiodothyronine (Raiti et al ., 1967). The administration of T 4 to mothers who have a history of giving birth to cretins seems to produce children with less severe cretinism which might be indicative of placental transportation (Bacon et al ., 1967) of administered T 4 . Unfortunately the maternal pituitary is unresponsive to foetal needs. Hence if the foetal thyroid secretes little T 4 the maternal thyroid does not respond by secreting more of the same (Dodge et al ., 1975).

Antithyroid agents like propylthiouracil affect both maternal and foetal thyroid glands. Similarly iodine deficiency in the maternal diet also impairs thyroid function in mother and offspring (Lotmar, 1933). Protein-bound iodine in cretins is as low as 0.2 mg/100 ml (Beierwaltes et al, 1959).

The neurological pathology in these children is much the same as that already described for experimental animals. We see a 50% reduction in total brain size. The cerebellum is smaller in size and has small gyri and prominent sulci. The cerebrum has ill-differentiated cortical layers and shows degenerative changes. The pyramidal tracts, basal ganglia and thalamus also appear to be reduced in size. The nerve cell population is greatly reduced and myelination seems somewhat impaired (Lotmar, 1933; Beierwaltes et al ., 1959; Adams and Rosman, 1971).


Hyperthyroidism

There is a danger in administering thyroid hormone to mothers with a history of giving birth to cretins in that hyperthyroidism can be induced (Raiti and Newas, 1971). In animals (Koldros, 1968) excessive doses of T 4 may lead to early maturation of certain developmental processes in the brain (Hamburgh, 1968), but the brain growth and body growth achieved by maturity are stunted (Nicholson and Altman, 1912a,b,c; Pelton and Bass, 1973). Both the cerebrum and cerebellum are characterized by having reduced cell numbers, neural processes and myelin. Synaptogenesis is accelerated early on but by 21 days of life we see a reduced synaptic complement (Gourdon et al., 1973). The effect of T 4 and T 3 administration in early life has a similar effect on behaviour. Whereas young animals have an accelerated behavioural and electroencephalographic maturation (Schapiro and Norman, 1967), at maturity their learning ability seems to be significantly impaired (Eayrs, 1964).


Other hormones

Corticosteroids and brain growth

Hormones other than thyroid hormone have a profound influence on brain growth and development. Cortisone given to young mice or rats during the first few days of life impairs somatic and brain growth. In 1965, Howard demonstrated a 35% decrease in forebrain growth in mice treated with cortisone during days 2—7 of life. He reported that during the time when the mice were receiving the treatment DNA production ceased altogether but RNA continued to be produced at a lower rate than normal. He also reported impaired cholesterol synthesis (Howard, 1965).

Other workers (Balazs, 1971; Cotterall et al, 1972) found similar results in the rat. Again administration of corticosteroid during the first four days of life adversely affected the accumulation of DNA in the cerebrum and cerebellum. So profound was the effect that the accumulation of DNA was reduced by 90 % in the cerebrum and 70% in the cerebellum. Following cessation of treatment on the fifth day of life, DNA accretion speeded up to normal rates but at day 35 the cerebrum and cerebellum still had 20 % and 30 % fewer cells respectively than control animals. It was further shown that the incorporation of 14 C thymidine into brain tissue was significantly impaired when the cortisol was given. Later on, when cortisol treatment was stopped, mitotic activity increased; perhaps in an attempt to compensate for any deficiency in cell number. In contrast to Howard’s experiment (Howard, 1965) this study showed that protein /DNA and RNA/DNA ratios were not altered by corticosteroid treatment.

We are still not clear as to how cortisol brings about this impediment to growth. The only clue we have comes from a study in which hydrocortisone was given to ten-day-old mice. This resulted in elevated levels of brain glucose. It was postulated that this might indicate a facilitation of brain glucose uptake or an impaired glucose utilization. Perhaps these high glucose concentrations affect growth in some way as yet poorly understood (Thurston and Pierce, 1969). A reduction in brain growth is also invoked by administration of oestradiol, de- hydroepiandrosterone and testosterone (Howard, 1965).

Rats given cortisol in early life show definite behavioural changes. For instance disturbed swimming has been reported (Schapiro et al ., 1970). However, other behaviour seems unaffected such as adaptive behaviour (Howard and Granoff, 1968). Children given corticosteroids early in life show definite behavioural effects (Dodge et al ., 1976).


Growth hormone

Growth hormone deficiency reduces the rate of cell division without altering the time at which the cells divide (Winick, 1976). However, growth hormone only seems to have an effect on growth at certain periods during the animal’s development. Mice genetically devoid of growth hormone show normal organ growth until 10 days of life; at which time the rate of cell division slows down in comparison to that characteristic of control mice and growth stunting occurs (Fig. 4.1). It would thus appear that early hyperplastic growth is independent of the effects of growth hormone (Winick and Grant, 1968).

Rats hypophysectomized at 21 days of age and fed ad libitum were shown to have reduced cerebral weight as well as DNA, RNA and protein contents at both 38 and 49 days of life. When growth hormone was administered during the experimental period of 21-49 days these defects were partially corrected. But cytoplasm to nucleus ratios remained low as did the RNA levels (Cheek and Graystone, 1969). Hypophysectomy at a later stage in a different experiment did not affect brain weights or protein contents but incorporation of phenylalanine was significantly reduced (Takahashi et al, 1970). This again could not be completely restored by growth hormone therapy. It has been postulated that the effect of growth hormone on DNA synthesis may be explained by the fact that it elevates DNA polymerase activity (Jasper and Brasel, 1973).


Figure 4.1 Growth of hypopituitary dwarf mice and their normal littermates. After 12 days of life the dwarf mice are shown to suffer from growth retardation. Before that time growth, as measured by body weight, was the same in the two groups (from Winick, 1976, with permission).


Insulin

Insulin given daily to rats hypophysectomized at 21 days of life also had a profound effect on brain growth. At 38 and 49 days of life brain RNA levels were found to be higher than in untreated rats but DNA levels were not affected (Cheek and Graystone, 1969). However, in a further study it was shown that insulin can inhibit growth. Here, when protamine zinc insulin was given to intact rats fed ad libitum from 26-38 days of age, there was an extra whole body weight gain but cerebral weights, water and DNA and RNA contents were all significantly reduced. It was hypothesized that the hypoglycemia induced by the insulin damaged or inhibited cerebral growth in some way (Graystone and Cheek, 1969).


Nerve growth factor (NGF)

Nerve growth factor was first demonstrated to exist in 1953 when it was shown that a heat labile non-dialyzable nucleoprotein in the microsomal fraction of neoplastic cells caused increased growth of the sympathetic nervous system of experimental animals (Levi-Montalcini and Hamburger, 1953). Winick and Greenberg later demonstrated its presence in the developing human foetus from 9-16 weeks of gestation (Winick and Greenberg, 1965a, b). Whether or not it has a role in the normal growth and development of the brain has yet to be discerned. However, anabolic metabolic processes are enhanced by NGF including RNA, protein and lipid synthesis (Levi-Montalcini and Angoletti, 1968). It causes increased incorporation of glucose into brain gangliosides (Graves et al ., 1969). It has also been shown to stimulate the production of axonal sprouts from injured monamine neurones (Bjerre et al , 1973).


Environment—hormones and behaviour

Behaviour in experimental animals and in children can be modified by environmental stimulation during the period of their brain development and to a lesser extent at a later time. Such effects are accompanied by changes in neurochemistry which are possibly mediated by changes in hormonal levels (Winick et al , 1975; My Lien et al ., 1977; Morgan and Winick, 1980).

When infant rats are exposed to the cold the corticosteroid content of their adrenal gland decreases in response to the cold. However, if such animals are regularly handled this depletion is accelerated. The earlier the rats are handled the greater the rate of corticosteroid loss. Handled animals are more responsive to the effects of ACTH leading to the secretion of larger quantities of corticosteroid than in the non-handled rats. Exposure to high steroid levels in early life seems to lead to altered adrenal function in the adult which in turn affects behaviour. Hence here is an example of early stimulation affecting the secretion of a hormone which in turn permanently alters behaviour, perhaps by inhibiting DNA synthesis (Levine et al ., 1958; Levine and Mullins, 1968).


Toxic substances

Foetal alcohol syndrome

In historical documents there are many references to the fact that alcoholic mothers often gave birth to infants with malformations (Warner and Rosett, 1975). In more recent times Lemoine and co-workers (1968) from France and Jones, Smith and co-workers (1973) of the United States have carefully characterized these defects termed “foetal alcohol syndrome” (FAS). The congenital defects usually exhibited by such children include growth retardation, small head size, mental and psychomotor retardation, craniofacial peculiarities and cardiovascular defects. Other defects include anomalies of the joints and genitalia, micrognathia, epicanthic folds, hypoplastic midfacial structures such as a broad nasal ridge, upturned nares, long upper lip, small palpebral fissures, abnormalities of the ears, drooping of the eyelids (ptosis) and crossed- eyedness (strabismus), wide mouth, narrow bifrontal diameter, cleft palate, visceral anomalies and small hemangiomas (Hanson et al ., 1976; Oulette et al ., 1977; Clarren and Smith, 1978). Figure 4.2 shows several children suffering from some of these defects. It seems that if a woman consumes more than 2 grams of alcohol per kilogram body weight per day during gestation then her offspring will suffer from the full FAS and will exhibit most of the above congenital malformations. However, nobody has as yet been able to determine exactly how much alcohol a woman can consume during pregnancy without causing deleterious effects to her unborn child. Further, whether binge drinking or a steady consumption has the more harmful effects is also an unsolved problem.

Infants exposed to alcohol in the prenatal period show growth failure at the time of exposure as well as postnatally. This tends to be disproportionate with body length being affected more than body weight. Many children die in the perinatal period and those that live tend to show profound neurological difficulties. This is often characterized by tremulousness, hyperactivity and irritability in the immediate postnatal period. To some extent these symptoms are due to alcohol withdrawal but the tremulousness often persists beyond the usual withdrawal period and often the children are left with a permanent fine motor dysfunction (Hurley, 1980). As we have seen in Chapter 2, brain growth is time dependent and hence the reduced brain growth in these children, where brain development is impaired in litero, is permanent.

As there are many similarities between offspring that have been nutritionally deprived in litero and offspring that have been exposed to alcohol in litero it has been postulated that malnutrition is the underlying cause of FAS (Chernoff, 1977; Randall, 1977; Randall et al ., 1977). This is supported by the fact that alcoholics all suffer from primary undernutrition. However, Jones hotly disputes this scenario and maintains that FAS and malnutrition are unrelated (Jones and Smith, 1976).


Figure 4.2 Facial features of boy with FAS at birth (top left) and at 6 months (top right). Girl with FAS at 16 months (bottom left) and 4 years (bottom right) (from Hanson et al., 1976, with permission).


Children suffering from FAS have permanent somatic and brain growth stunting as well as alterations in brain function secondary to this impaired growth. There are few long-term follow-up studies of children exposed to early malnutrition which makes comparisons difficult. However, it seems from the results of animal and human studies at our disposal that if a mother is given adequate calories but insufficient protein during gestation, the offspring show similar characteristics to FAS sufferers. Once again, we see long-term growth retardation, behavioural and intellectual abnormalities and neurochemical alterations (Hsueh et al , 1967; Kenney and Burton, 1975). Thus these observations fail to support the view that the retarded growth in FAS and undernutrition have a different etiology.

The specific nutrient deficiencies characteristic of alcoholics are vitamin A (Smith and Lindebaum, 1974), folic acid (Halstead et al , 1967), zinc (McBean et al , 1972) and magnesium (Hurley, 1980). All of these have teratogenic effects which are similar to the effects of prenatal alcohol exposure.

Vitamin A deficiency during pregnancy causes the same kind of defects seen in animal models of FAS. Characteristically we see ocular, cardiovascular, and urethral anomalies (Wilson et al , 1953). Folic acid deficiency at this time also causes the same kind of urogenital abnormalities (Gross et al, 1974).

Zinc deficiency during pregnancy leads to congenital anomalies similar to those seen in FAS. Such deficiencies severely retard cerebellar development in experimental animals (Fosmire et al, 1977). The same kind of cerebellar abnormalities have been described in rats exposed to alcohol during infancy (Bauer-Moffett and Altman, 1975) which supports the concept of nutrient deficiency being the cause of this impaired neural development rather than the effect of alcohol itself. Another way in which this effect on neural development could be mediated is via vitamin A, as zinc is intimately involved in retinol metabolism (Smith et al, 1973).

Finally, magnesium deficiency during pregnancy in the rat leads to decreased maternal weight gain and reduced food intake. If the deficiency occurs during the period 5-12 days of gestation there is a high neonatal mortality and degree of abnormal brain histology. Those offspring that survive have a high incidence of cleft lip, short tongue, hydrocephalus, micrognathia, agnathia, club feet, herniations and heart and lung teratogenital anomalies (Hurley et al, 1976).

All of these observations tend to link the effects of alcohol on foetal growth and development to malnutrition. However, it is possible that there is an alternative explanation. Alcohol could have a direct effect on the foetal tissues. It is readily transported by the placenta and there is a direct correlation between the levels of alcohol in the maternal and foetal circulations (Mann et al, 1975). Alternatively chronic alcoholism could affect maternal metabolism in some way which in turn could have a teratogenic effect (Hurley, 1980).


Cannabis and development

With the increasing popularity of the use of marijuana amongst young women of child-bearing age, many studies have been designed to investigate the possible effects of cannabis and its constituents on foetal growth and development (Bonnerjee et al., 1975; Joneja, 1976; Wright et al, 1976).

Marijuana and its derivatives have been shown to freely cross the placenta in experimental animals and depending on the dose level and method of administration have various teratogenic effects on the developing foetus (Abel, 1980). It would seem that low doses of cannabinoids have little discernible effect (Haley et al , 1973; Keplinger et al, 1974). Similarly chronic high doses given before and/or throughout gestation have little effect on the growing foetus. This is believed to be due to either altered maternal distribution (Mantilla-Plata et al.,

1975) , masking (Joneja, 1976), or acquisition of a physiological tolerance mechanism (Fried, 1977). Prenatal deaths and congenital abnormalities do, however, occur if high doses of cannabinoids are administered during discrete periods of gestation, Further, the foetus is more susceptible in early gestation (Pessoud and Ellington, 1967; Harbison and Mantilla-Plata, 1972) than in late (Gianutsos and Abbatiello, 1972). Low to moderate doses of cannabinoids given throughout gestation or during critical periods of development, e.g. organogenesis, result in subtle developmental changes but no gross prenatal effects. The usual outward manifestations of moderate exposure are a reduced physical growth and a delayed ontogenetic appearance of such physiological features as incisor eruption, eye opening and visual placing (Borgen et al, 1973; Fried,

1976) , hyperactivity (Borgen et al , 1973), hypoactivity (Fried, 1976) and inferior maze learning (Gianutsos and Abbatiello, 1972).

Certain developmental changes have also been observed in children exposed to cannabis in utero at moderate to heavy doses. Once again, the human foetus seems to be more susceptible in early pregnancy. Such children have an elevated auditory threshold which persists for 6 weeks postnatally (Fried, 1980). Others have reported distal limb teratology (Hecht et al, 1968; Carakushansky et al, 1969). However, in the latter studies, the women were suspected of being multiple drug users which makes it difficult to discern whether the marijuana or some other drug was responsible for the abnormalities.

Many of the developmental consequences associated with prenatal exposure to cannabinoids are shared by offspring from mothers which have been malnourished during gestation (Chow and Lee, 1964; Barnes, 1966; Smart and Dobbing, 1971; Whatson et al, 1978). Malnutrition also potentiates the effects of marijuana on rat pups. When rat dams are exposed to marijuana and fed a low-protein diet during gestation the developmental defects in the offspring are far greater than if dams are fed a high-protein diet at this time. Fried suggests that perhaps the protein content of the diet determines the ability of the dam to detoxify the drug (Fried, 1980). This observation could be of great practical importance in the human situation.

Smoking

It is well known that mothers who smoke during pregnancy are significantly more likely to give birth to premature and short-for-dates (SFD) babies (Miller et al , 1978). The effect is thought to be mostly due to hypoxia, but may also be due to reduced maternal supply of amino acids to the foetus. It used to be considered that smoking was not teratogenic, but more recent studies by Himmelberger et al (1978) have suggested that children of smokers are more likely to have all types of congenital abnormalities, including those of the central nervous system. Long-term studies of smokers’ children have shown deficiencies in growth, general intellectual ability, reading age and mathematical ability (Butler and Goldstein, 1973) and in emotional development. It has also been reported that neurological abnormalities, including cerebral dysfunction, abnormal electroencephalograms, impaired auditory tests, squints and hyperactivity, are more common in children of heavy smokers compared with those of non-smokers (Denson et al ., 1975).

Drugs

A number of drugs increase the requirement for specific nutrients and therefore if taken during pregnancy may divert these nutrients away from the foetus. This situation is unlikely to affect the foetus except in circumstances when the intake of the nutrient is already barely adequate. We have already seen that the foetal alcohol syndrome may be due to this mechanism. Anti-convulsant drugs, particularly dilantin and phenobarbitone, induce biochemical evidence of folate deficiency when the diet contains a barely adequate amount of the vitamin, possibly due to induction of drug-metabolizing enzymes (Labadarios et a/., 1978). There is evidence that pregnant women taking anti-convulsant drugs have a greater than normal likelihood of giving birth to a malformed baby. Experiments in rats on a folate-deficient diet resulted in the production of foetuses with neural tube defects and hydrocephalus (Labadarios, 1975).

Congenital malformations of the central nervous systems associated with hormone treatment were first reported by Gal et al (1967) in babies born to mothers who had hormonal pregnancy tests.

Lead

Lead is a neurotoxin and much of the debate about the possible dangers of contemporary lead burdens centres on the possibility that these may produce adverse effects on behaviour and intelligence. It is clearly not possible to debate this delicate issue here but it is a matter of potential concern to all those interested in brain development. Lead is trapped in the teeth and skeleton and it might therefore be considered that analysis of dentine and bone would give a better measure of exposure to lead, thus measuring blood levels. Bryce-Smith et al (1977) reported that stillbirth bones contained a higher concentration of lead than those of apparently normal neonates. It was significant that in those stillbirths with malformations of the central nervous system (hydrocephalus, spina bifida, etc.) there was an excess of lead and cadmium with respect to calcium.

Obvious malformations are apparently not the only neural damage associated with a high body burden of lead. More subtle defects may become apparent after birth in those exposed to lead in utero. This is a crucial question on which there is at present no information. It has, however, been reported (Bryce-Smith, 1979) that mentally retarded children tend to have higher blood lead levels than controls, and hyperactivity associated with high blood lead levels improves when the levels are reduced with penicillamine.

Neurologic manifestations of lead intoxication can be produced in suckling rats by feeding lead in the diet of the lactating mother (Pentschew and Garro, 1966). The concentration of lead in the brains of young rats first exposed to the mineral in the mother’s milk and then in the diet are dose-related and significantly higher at each dose level than in the mother (Table 4.1). This difference in brain lead concentration accounts for the fact that rats at 21-35 days of age showed neurological evidence of lead intoxication whereas their mothers did not. It was of interest that the brain lead concentration in these young animals remained high in spite of a fall in the blood lead concentration with increasing age. This suggests that blood lead concentration in young children may be a poor guide to the tissue concentrations. It is also of interest that the young rats showed evidence of abnormal behaviour including a degree of hyperactivity (Mykkanen, 1977).


Table 4.1 Effect of age on the concentration of lead in the brain of rats exposed to diets containing different concentrations of lead acetate (Mykkanen et al, 1979)


Concentration of lead acetate Aqe in the diet (a1100 q )

{days) 0 0.5 1.0 2.0


7

0.15

0.36

_

0.60

21

0.11

1.27

2.55

2.92

35

0.20

1.24

2.55

4.07

49

0.20

•1.22

2.23

3.01

Dam

0.22

0.49

0.91

1.69


124


Conclusions

Normal growth and development of the brain depends on the interaction and balance of a large number of factors both endogenous and exogenous. Some of the effects of hormonal deficiencies and excesses are similar to those resulting from malnutrition (Chapter 3) and it may be that the effects of the latter are normally mediated by hormones. In addition, the brain is particularly sensitive to potentially toxic substances, some of which (alcohol, cannabis and tobacco) may be considered ‘social poisons’, whilst others, such as lead, are environmental pollutants. Thus the milieu in which the brain grows must be protected from all these as well as being supplied with adequate nutrients and stimulation if it is to reach its true potential. Failure in respect of the factors reviewed in this chapter may have more dire consequences than protein-energy malnutrition.


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Dickerson JWT. and McGurk H. Brain And Behavioural Development. (1982) Blackie & Son Ltd., Glasgow.

Brain and Behavioural Development - 1982: 1 Neural Development | 2 Comparative Neural | 3 Malnutrition | 4 Hormones and Growth Factors | 5 Cortical Activity | 6 Functional Asymmetry | 7 Plasticity | 8 Sex Differences


Cite this page: Hill, M.A. (2019, October 16) Embryology Book - Brain and behavioural development 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Brain_and_behavioural_development_4

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