Teratogenic mechanisms of dysthyroidism in the central nervous system

G.J. Boer, M.G.P. Feenstra, M. Mimiran, D. F. Swaab and F. Van Haaren Progress in Brain Research, Vol. 7 3 0 1988 Elsevier Science Publishers B.V. (Biomedical Division)

71

CHAPTER 6

Teratogenic mechanisms of dysthyroidism in the central nervous system Andrea Vaccari Department of Neurosciences. Chair of Medical Toxicology. Via Porcell4. 09124 Cagliari. Italy

Introduction The regulatory influence of hormones on the development of the Central Nervous System (CNS) is manifest both pre- and postnatally. Consequently, alterations in hormone homeostasis may affect a number of biochemical and morphologic events involved in functional and behavioral maturation (see Timiras, 1972). Thyroid hormones, thyroxine (T4) and triiodothyronine (T3)are well-known growth-promoting factors for the CNS, their effects being at a peak when the cell ceases to divide and begins to differentiate. In humans a decrease in or lack of thyroid influences, whatever the cause may be, in late fetal or early postnatal periods, when the brain is more plastic and its growth more accelerated, may result in developmental abnormalities such as stunted body growth and skeletal maldevelopment, spastic displegia, ataxic gait, abnormal hearing, strabismus and, most importantly, in various levels of mental deficiency (cretinism) (Whybrow and Ferrell, 1974; Jubiz, 1979). One must recall that the placental transport of thyroid hormones is minimal (Fisher et al., 1977; Varma et al., 1978), which, from the beginning of the second trimester of gestation onwards, makes the brain development of the human fetus dependent on the activity of its own thyroid gland. Causes of thyroid deficiency may be: (1) congenital agenesis or maldevelopment of the fetal or

neonatal gland, (2) impaired mechanisms for making or secreting thyroid hormones, or (3) resistance to thyroid hormones perhaps due to a decreased number of their specific binding sites (Sawin, 1985). Thus, thyroid deficiency in humans, during the sensitive period of brain development (from about the 19th gestational week until the 2nd or 3rd year after birth) may represent a strong teratogen the effects of which can be reversed or prevented only when adequate doses of thyroid hormones and iodine (Vanderpas et al., 1986) are administered as soon as the diagnosis is made postnatally. Although one cannot precisely state whether the deadline for starting the therapy lies at 3, 6 or 9 weeks after birth (Glorieux et al., 1983; New England Congenital Hypothyroidism Collaborative, 1984), thyroid hormone supplementation should preferentially be initiated during the first 30 days of life in order to prevent irreversible brain damage (Wolter et al., 1980), and continued up to the 4th year of life (Klein, 1986). In other words, the critical period for brain development seems to extend well beyond the very first weeks after birth, although clinical evidence shows that hypothyroidism starting after the 2nd year of life allows the achievement of a relatively good level of mental performance. Of course it is to be expected that if an adequate treatment of neonatal thyroid hypofunction leads to a return of the normal situation, this is obtained by repairing structural

12

abnormalities and by a normalization of the CNS developmental schedule, which would be highly retarded otherwise (Brasel and Boyd, 1975). Thyroid hyperfunction is a rare pediatric pathology which may lead to psychomotoric and behavioral perturbances (Grave’s disease) or represent a true teratogenic factor as it occurs in neonatal thyrotoxicosis, when underweight, skull malformations and, most frequently, mental retardation may occur (Hollingsworth et al., 1980). During the last 15 years a great number of experimental studies have dealt with morphological (Legrand, 1984), biochemical (Vaccari, 1983; Puymirat, 1985) and molecular (Nunez, 1985) mechanisms which probably do underlie the dysthyroidism-related abnormal development of the CNS, and may eventually lead to cretinismassociated neurological and mental abnormalities. This effort has been almost entirely based upon experimental models of congenital or acquired adult hypothyroidism, the rat being the most widely used animal (DOhler et al., 1979). The popularity of rodents in the ‘market’ of experimental dysthyroidism is justified by a wellstated similarity in the general scheme of neurodevelopmental events in humans and several mammalian species. The newborn rat ‘may be compared with a human fetus in the second trimester of pregnancy, and the newborn human baby to a 6-10-day-old rat’ (Fisher et al., 1977; Legrand, 1984). Of course, similarity of human and animal brain development does not mean that information obtained from experimentally provoked dysthyroidism, when the causative agent is well-known, may entirely reflect spontaneously occurring thyroid disease in humans, where pathogenesis may be complex and of unknown origin. In this survey I shall briefly describe the mechanisms of action of thyroid hormones in the brain, their effects on the establishment of neuronal networks, and dysthyroidism-associated abnormalities of central monoaminergic neurotransmission as an index for altered number of interneuronal interactions.

Molecular mechanisms of dysthyroidism-related

CNS teratogenesis

Teratogenic influences of early thyroid insufficiency may result from impaired DNA transcription or depressed synthesis of active proteins in the immature brain. Since thyroid hormones do normally have a regulatory (inductive, repressive) role in the expression of the genetic program of nerve cells, protein perturbance may result in a retarded differentiation and proliferation, and in a defective establishment of the neuronal network in the immature brain. Protein depression is just the last of several events where T, (3,5,3’-triiodothyronine), the active 5’ -deiodination product of T4 (3,5,3’,5’-tetraiodothyronine), after having been taken up by the neuropile and nerve endings (Dratman et al., 1982), binds to nuclear, chromatin-associated receptor proteins (Schwartz and Oppenheimer, 1978a). The T,-nuclear receptor complex may then trigger the production of messenger RNAs to be translated to specific proteins (see Menezes-Ferreira and Torresani, 1983), thus influencing the thyroid-dependent gene expression in specific neuronal cells of different brain regions. Thyroid hormone-binding proteins have been also identified in the cytosol or plasma membranes and cytoplasmic reticulum of brain cells (Dillman etal., 1974; Geel, 1977), where they would regulate the retention and supply of T, to the various cell compartments, primarily nuclei and mitochondria (Dozin-Van Roye and De Nayer, 1978; Menezes-Ferreira and Torresani, 1983). The discovery that reverse T3(rT3)(3,3’,5’triiodothyronine), the alternative extrathyroidal 5-deiodination metabolite of T4, also binds to nuclear receptors where it can competitively inhibit all of the T3 binding (Latham et al., 1976) is of great interest. Since the levels of rT, may be high in fetal circulation (and are more than 10-fold the T, levels, as is the case in the human amniotic fluid before 30 weeks; Chopra et al., 1978), rT, might conceivably result in a significant occu-

73

pation of T, receptors, thus influencing fetal thyroid hormone-regulated functions. Furthermore, rT, would inhibit the conversion of T4 to T,. Information is lacking about hypothyroidismassociated increases in the levels of rT, which would hinder T, from triggering its action in humans. Certainly the antithyroid drug propylthiouracyl (PTU), which is widely used to induce experimental hypothyroidism in rats, is a potent producer of rT, (Jubiz, 1979). The presence of thyroid hormone receptors in the CNS has stimulated investigators to study whether there are receptor alterations in the developing hypothyroid brain (Fig. 1). At first it was stated that the density of nuclear T, receptors is at a peak either in cerebral hemispheres (Valcana and Timiras, 1978)or in the whole brain and cerebellum (Schwartz and Oppenheimer, 1978b) of rats during the first 2 days of postnatal life, and that it declines to adult levels by the 2nd week, thus correlating with the critical dependence of the brain on thyroid hormones. Hypothyroidism is known to affect cerebellar development markedly, in spite of the low T, binding capacity in this region (Rue1 et al., 1985), and it has been shown to increase the density of nuclear T, receptors in the brain consistently, as early as

Bmax0'13

1

6

9

13

33

60 -180

days of age

Fig. 1. Postnatal development of triiodothyronine (T,) binding to nuclear receptors in rat cerebral hemispheres (0), and effects of neonatal radiothyroidectomy on T, binding (open column = euthyroids; solid column = hypothyroids). * P < 0.05 vs. euthyroids. Results are from Valcana and Timiras (1978).

13 days after neonatal radiothyroidectomy (Fig. l), compared to euthyroid rats (Valcana and Timiras, 1978). Neonatal hyperthyroidism does not affect binding characteristics of T, (Timiras, 1979). Therefore the developing hypothyroid brain would react to a smaller availability of the T, ligand, with an adaptive, greater binding capacity for thyroid hormones. Similarly, in synaptic transmission, excess of neurotransmitters decreases, and a deficit increases the number of pertinent receptors. In contrast, researchers have also observed a decreased binding of T, due to a lower than normal affinity for the hormone of nuclear receptors in human lymphocytes or fibroblasts from a hypothyroid subject suffering from familial peripheral tissue resistance to thyroid hormones (Bernal et al., 1978).Beyond all these considerations and the need for additional studies aimed at ascertaining whether alterations in T, binding capacity are long-lasting (even lifetime), little doubt exists that thyroid hormones in health and disease have a regulatory role on their nuclear receptors (Valcana and Timiras, 1978). Morphogenetic alterations in dysthyroidism The stuctural consequences of altered thyroid states during the neonatal age have been thoroughly considered in an excellent review (Legrand, 1984). Here I will only summarize some of most relevant findings. The most important maturational steps in the CNS are (a) cell proliferation, which occurs prenatally in the rat cortex, starts from birth and continues up to approximately the 15th day of life in the cerebellum, and continues after birth in cortical glial cells, the hippocampus and olfactory bulbs (Pate1 et al., 1976); and (b) neuronal differentiation, a process that is mostly postnatal in the rat, begins with a change in cell shape, followed by neurite outgrowth, leading to the establishment of interneuronal contacts (synaptogenesis). T4 administration has been shown to increase the synaptic density and the number and size of nerve processes in the cerebellum of neonatal rats

14 cell acquisition (DNA/ brain area) of controls

%

5

10

20

30

40

age days

Fig. 2. Effects of propylthiouracyl (PTU>provoked hypothyroidism on postnatal cell acquisition (cell number) in the forebrain (O), cerebellum (0),olfactory bulbs (A) and hippocampus ( A ) of newborn rats. Results are expressed as the percent content of DNA ( p g P/brain region) with respect to euthyroid counterparts. The horizontal bars indicate the period when the differences between hypothyroid and euthyroid rats werz significant. Results are from Patel et al. (1976) and RabiC et al. (1979).

(Legrand, 1979). Conversely, thyroid hormone deficiency does dramatically affect the neuronal cell differentiation and to a smaller extent the region-specific cell acquisition (Fig. 2) (Patel et al., 1976; Legrand, 1984; Nunez, 1984). This statement is based upon a number of experimental studies where attention was focused on the rat cortex and cerebellum. The latter region proved to be particularly useful for studying dysthyroidismrelated alterations (Lauder, 1977), thanks to its simple and homogeneous structure and its almost entirely postnatal development. A number of observations are noteworthy. First of all, the density of cortical axonal terminals in layer IV and the dendritic branching of pyramidal neurons are decreased to the point that a deficit in dendroaxonal processes may lead to a 50% decrease in synaptic numbers in the neonatal hypothyroid brain (Eayrs, 1971). Disturbances in synaptogenesis have also been demonstrated in rat visual and auditory cortex, and in the caudate nucleus (see Legrand, 1984), a region where the postnatal density of neurons is lower, compared to

euthyroid counterparts. There is also a marked reduction in the size and density of hippocampal cells (RabiC et al, 1979). Secondly, the migration of cerebellar, external granular cells towards the molecular or the inner granular layer is delayed, and there is a compensatory prolongation of proliferative activity, as compared to the euthyroid cerebellum. The final result is a normal total number of cells in the hypothyroid cerebellum (Patel et al., 1976), while similar disturbances in the cell migration from the germinal ventricular layer in the hippocampus and olfactory bulbs do not coincide with compensatory mechanisms. The number of Purkinje cells is not affected in hypothyroidism, though their histological maturation is delayed and the density and branching of their dendritic spines is markedly diminished (Rebibe and Legrand, 1972). Thirdly, there is increased death of newly formed and differentiatingcells in the internal granular layer, perhaps a consequence of a lower availability of synaptic sites in the hypothyroid cerebellum (RabiC et al., 1977; see Rami et al., 1986). As a matter of fact, the most relevant effect of neonatal hypothyroidism seems to be a deficient synaptogenesis along with alterations in the qualitative distribution and specification of available synapses (Nunez, 1984), which might help in explaining the electrical, neurotransmitter and behavioral alterations which are associated with both experimental and spontaneous hypothyroidisms. Finally, the myelination process is retarded and myelin content is decreased in the hypothyroid brain, which is partially the result of a delayed formation and maturation of oligodendrocytes (Legrand, 1980). Conversely,in neonatal hyperthyroidism, both oligodendrocytes and myelin accumulate more precociously than in the euthyroid cerebellum. Organization of cytoplasmic structures in dysthyroidism Changes in cell shape and neurite outgrowth, i.e. dendritic branching, axonal sprouting and synapse formation, thus all those differentiation

75

processes which are under thyroid control, strictly depend on the massive assembly of cell microtubules and neurofilaments (Yamada et al., 1970; Nunez, 1985). Microtubules are linear, polymeric structures composed of tubulin and microtubuleassociated proteins (MAPs and TAU) which work as assembly-promotingfactors of the microtubule itself. Neurofilaments also have a protein matrix, including actin and several actin-binding proteins (ABP). All groups of proteins share the build-up of the nerve cell structure, axons and dendrites included. The finding that the efficiency of microtubule assembly depends on the stage of brain development is of great relevance to the understanding of dysthyroidism effects at this level (Lemon et al., 1980). Thus, the rate of tubulin polymerization steeply increases from the fetal period up to peak levels at approximately 1 month after birth in rat and mouse brain supernatants. Furthermore, prenatally caused hypothyroidism proved to slow down the rate of assembly during the first 15 days of life, though this effect was transient in the rat. Replacement therapy with thyroid hormones brought the rates of microtubule assembly back to normal (see Nunez, 1985). Finally, the composition of MAPS changes with age and is thyroid-dependent. More precisely, among the three major types of MAPs, the HMW, (350 kDa), the HMW, (300 kDa) and TAU (65 kDa), the TAU fraction is heterogeneous and displays neonatal- and adult-specific components which are probably regulated by different genes (Nunez, 1984). The TAU, protein is almost missing in microtubules from hypothyroid brains (Nunez, 1985). If one assumes that different kinds of MAPS underlie the expression of specific types of microtubules which, in their turn, trigger the differentiation of specific neuronal cells, it is easy to suspect the importance of any dysthyrodism-associated impairment of the microtubule machinery.

Effects of dysthyroidism on central neurotransmission Thyroid deficiency during brain development leads to a dramatic reduction in the number and specificity of synaptic junctions (Legrand, 1984). Therefore, it is no wonder that both pre- and postsynaptically-located neurotransmitter functions in neonatal dysthyroidism are extensively impaired (see Vaccari, 1983; Puymirat, 1985). Such alterations involve levels and turnover of neurotransmitters, their release and uptake processes, the activities of pertinent synthesizing and catabolic enzymes, and their specific receptors. Although consistent evidence shows that disruptions of biogenic amine homeostasis may be of importance to neuropsychiatric disorders (Barchas et al., 1977), neurotransmitter alterations in dysthyroidism often display conflicting trends. This makes it difficult to correlate them with neurobehavioral aberrations. There may be several reasons for such inconsistency. (1) The ageand brain region-specific sensitivity of mammalians to dysthyroid procedures, and the time schedule of brain development in different species. (2) The experimental model chosen to provoke hypothyroidism. Thus, surgical thyroidectomy cannot be performed neatly soon after birth in rats, and in any case it involves parathyroid ablation. Radiothyroidectomyat birth can only be accomplished by using huge amounts of 13'1, about one hundred times (on a body weight basis) the quantity applied in the treatment of human hyperthyroidism. Therefore the involvement of extrathyroidal tissues in the response to radioactivity cannot be excluded. The sulphydryl goitrogens methirnazole (MMI) and propylthiouracyl (PTU) have membrane-perturbing effects (Biassoni and Vaccari, 1985) which may superimpose on those of the thyroid deprivation they provoke (Fig. 3). (3) The time-lag between the last administration of antithyroid drugs and the death of the animal may be important in order to avoid any direct influence of MMI or PTU on neurotransmitter parameters studied. (4) The

76 X

of controls

I

..

...

.

-. ... ... ... ... ...

I

m

L

-5.1

5-HT,

5-HT2

D2

IMIR

5-HTuptske

Fig. 3. Effects of methimazole (MMI>provoked hypothyroidism (solid column) and in vitro influences of 1* M MMI (open column) or PTU (dotted column) on 5-HT,, 5-HT,, D, and imipramine binding sites of rat brain membranes, and on the synaptosomal uptake of 5-HT. Rats were given MMI, 5-20 m@g/day S.C. form birth to day 30 of age. * P < 0.05; ** P < 0.02; *** P < 0.005 or 0.001 vs. controls. Results are from Vaccari and Timiras (1981). Vaccari etal. (1983b), Vaccari (1985) and Biassoni and Vaccari (1985).

effects of malnutrition, which are often associated with both experimental hypo- (Strupp and Levitsky, 1983) and hyperthyroidism (PascualLeone et al., 1985), may partially mask dysthyroidism-specific alterations. ( 5 ) Last but not least, thyroid disease is known to influence corticosteroid metabolism, hypo- and hyperthyroidism being associated with decreased or increased catabolism of corticosteroids in humans, respectively (Pittman, 1971; Linquette et al., 1976). Furthermore, there are adrenal changes in genetically hypothyroid mice (Shire and Beamer, 1984). Inasmuch as corticosteroids may influence the developing neurotransmission and act as teratogen agents by themselves (see Meyer, 1985; Biegon et al., 1985), the effects of adrenal dysfunction may become confused with those of dysthyroidisn. In the final analysis, the fact that similar experimental schedules lead to contrasting results may just indicate that it is normal to have physiological fluctuations within a certain range. It is always

difficult, indeed, to determine when an alteration of neurotransmitter homeostasis is statistically significant, in order to provoke neurobehavioral impairment. Presynaptic alterations

At the time that these considerations were reviewed for the first time (Vaccari, 1983), the overall picture of dysthyroidism-associatedpresynaptic alterations in neurotransmitter pathways was rather confusing. Now, in spite of a degree of uncertainty, mostly related to the status of monoamine levels (see Savard et al., 1983), little doubt exists that both neonatal and adult hypothyroidism depress synthesis and turnover rates of 5-HT, thus suggesting a serotonergic hypoactivity in the CNS (Vaccari, 1982). In addition, the existence of a depression of the dopaminergic system is supported by several studies showing decreased steady-state levels of dopamine (DA) in the whole brain, the diencephalon,or in striatal, hypothalamic and brain stem tissues of newborn hypothyroid rats (Rastogi et al., 1976; Puymirat, 1985; Leret and Fraile, 1986), along with a consistent decrease of hypothalamic DOPAC content (Puymirat, 1985). According to the available literature, a depression of norepinephrine (NE) pathways in the neonatal or adult hypothyroid brain also seems likely to occur. Finally, some evidence indicates that neonatal and adult hyperthyroidisms increase the synthesis and turnover rates of 5-HT and catecholamines (see Vaccari, 1983; Puymirat, 1985). Neurotransmitter-linked enzyme activities have been shown to respond differently to thyroid manipulations. Thus, tyrosine hydroxylase and monoamine oxidase (MAO) activities may be either increased or decreased in the hypothyroid brain (Rastogi et al., 1976; Vaccari et al., 1977 and 1983a) depending on the experimental variables previously discussed in the text. In summary, it is sufficiently proven that neonatal and, to a lesser extent, adult hypothyroidism depress the monoaminergic tone, while the opposite occurs in hyperthyroidism.

TABLE 1

Receptor alterations in the dysthyroid CNS Receptor a

Dysthyroidism

Brain region

Age of onset of dysthyroidism

Effectb

References

Dopamine (Dz)

Hypo-

Striatum

Newborn

Decrease (c)

Newborn Weaned Adult Adult

No change Increase Decrease No change

Adult Newborn Adult

Increase (d) Decrease No change

Vaccari and Timiras (1981); Kalaria and Prince (1985) Del Cerro et al. (1986) Overstreet et al. (1984) Kalaria and Prince (1986) Crocker and Overstreet ( 1984) Crocker et al. (1986) Vaccari and Timiras (1981) Atterwill (1981)

Increase No change Increase No change No change No change

Vaccari et Vaccari et Vaccari et Vaccari et Vaccari et Vaccari et

Striaturn

(Tx) (d) (c) (c)

Serotonin 5-HT1

HYPO-

Brain

5-HT2

HYPO-

Brain

5-HTI 5-HTZ

HyperHyper-

Brain Brain

Newborn Newborn Newborn Newborn Newborn Newborn

Imipramine

HYPO-

Cortex Striatum Cortex, striatum

Newborn Newborn Adult

Decrease (c) No change (c) No change (c)

Vaccari (1985) Vaccari (1985) Vaccari (1985)

Adrenergic Alpha,

HYPO-

Cortex

Adult

Decrease (c)

Hyper-

Cortex Spinal cord Cortex Cortex Forebrain, cerebellum Cortex, striatum, hypothalamus Lymphocytes (human) Cortex, hypothalamus Striatum Forebrain, cerebellum Spinal cord Cortex Striatum, hypothalamus

Adult Newborn Adult Adult Newborn Adult

Increase No change Decrease (c) No change Decrease (c) Decrease (c)

Adult Newborn Newborn Newborn Newborn Adult Adult

Increase No change Increase Increase Increase Decrease Increase

Gross et al. (1981); Kalaria and Prince (1986) Gross et al. (1981) Lau et al. (1985) Gross and Schiimann (1981) Gross and Schiimann (1981) Smith et al. (1980) Gross et al. (1980b); Atterwill et al. (1984) Fantozzi et al. (1983) Atterwill et al. (1984) Atterwill et al. (1984) Smith et al. (1980) Lau et al. (1985) Schmidt and Schultz (1985) Atterwill et al. (1984)

Newborn Adult Newborn Adult

Decrease (c) Decrease (c) Increase No change

CodolA and CodolA and CodolA and CodolA and

Newborn

No change (c)

Newborn Adult

Increase (c) No change (c)

Patel et al. (1980); Kalaria et al. (1981) Patel et al. (1980) Kastrup and Christensen (1984); Kalaria and Prince (1986)

Alpha, Beta

HypoHyperHYPO-

Hyper-

Histamine ( H I )

Cholinergic (Muscarinic)

Hypo-

Brain

Hyper-

Brain

HYPO-

Forebrain, cortex, striatum Cerebellum Brain, Cortex, striatum

(c) (HI) (c) (HI)

al. (1983b) al. (1983b) al. (1983b) al. (1983b) al. (1983b) al. (1983b)

Garcia Garcia Garcia Garcia

(1985) (1985) (1985) (1985)

78

TABLE 1 (continued) Receptor"

GABA

Dysthyroidism

Brain region

Age of onset of dysthyroidism

Effectb

References

Hyper-

Forebrain Cerebellum Brain

Newborn Newborn Adult

No change Decrease No change

Patel et al. (1980) Patel et al. (1980) Kastrup and Christensen ( 1984)

HYPO-

Cerebellum Forebrain, striatum

Newborn Newborn

Decrease (c) No change (c)

Cerebellum, striatum Cortex Striatum Forebrain Cerebellum

Newborn Adult Adult Newborn Newborn

Increase (Tx) Decrease (c) No change (c) No change Increase

Patel et al. (1980) Patel et al. (1980); Kalaria and Prince (1985) Del Cerro et al. (1986) Kalaria and Prince (1986) Kalaria and Prince (1986) Patel et al. (1980) Patel et al. (1980)

HyperGlutamate

HYPO-

Striatum

Newborn

Decrease (c)

Kalaria and Prince (1985)

Benzodiazepine

HypoHyper-

Cortex Cortex

Adult Adult Adult

Increase (Tx) Decrease Increase

Medina et al. (1984) Medina et al. (1984) Gavish et al. (1986)

a The ligands used were: [3H]spiperone (D2; 5-HT2); [3H]5-HT(5-HT,); [3H]WB4101or [3H]prazosin (alpha,); [3H]clonidine (alpha,); [3H]dihydroalprenolo1 (beta); [3H]GABA or ['H]muscimol (GABA); [3H]glutamate (glutamate); [3H]QNB (muscarinic); [3H]mepyramine (histamine); [3H]flunitrazepam (benzodiazepine); [3H]imipramine (imipramine/S-HT uptake). The methods used to induce hypothyroidism in rats are indicated in brackets: (c) = methimazole or propylthiouracyl; (Tx) = surgical ablation; (r) = I 3 ' I ; (d) = iodine-deficient diet; (HI) = high-iodine diet.

Of particular relevance to the central effects of dysthyroidism is the possibility that iodothyronine hormones enter into a neurotransmitter pathway, serving as substrates for biogenic amine-forming enzymes (Dratman et al., 1984). A neurotransmitter-precursor role for thyroid hormones is strongly suggested by the structural similarity of T, and tyrosine (Dratman, 1974). Receptors and their functions in dysthyroidism Since thyroid hormones regulate the synthesis or degradation of specific proteins at the transcriptional or translational level, every receptor protein in central neurotransmission may be a putative target for thyroid influences in health and disease. Such a primary, direct effect of dysthyroidism may be either amplified or counteracted by subsequent receptor alterations (increased or decreased density of receptors, modifi-

cations in their afkity for ligands) secondary to the synaptic availability of pertinent transmitters (Reisine, 1981). Of course, the mere description of any receptor alteration is not very informative, unless they are correlated with modifications of receptor-mediated functions. The study of receptor functions is usually based upon the behavioral or biochemical effects of receptor-specific agonists or antagonists. Correlating receptors to functional aberrations is a difficult task. This is particularly true in dysthyroidism studies, where there is already disagreement about the behavioral effects intrinsic to hormone dysfunction (see Table 2), and where the use of receptor-specific drugs to assess receptor functions may introduce an additional variable in the system. Thus, all of us who are familiar with thyroid studies know that neonatal and adult hyperthyroid rats are hyperactive and hyperreactive,

19

TABLE 2 Putative receptor functions in dysthyroidism ~

Receptor system

Dysthyroidism"

Age of onset of dysthyroidism

Effectb

HYPO-

Newborn

Spontaneous locomotion

Decrease (c)

Newborn Newborn

Spontaneous locomotion Spontaneous locomotion

Increase (c) No change (c, d )

Newborn

Learning ability

Decrease (c, d, Tx)

Newborn Adult

Learning ability Spontaneous locomotion

No change (c) Decrease (c, Tx)

Adult Newborn Newborn (mice) Adult

Learning ability Spontaneous locomotion Spontaneous locomotion Spontaneous locomotion

Decrease (c) Increase No change Increase

Adult

Learning ability

No change

Newborn Adult Adult

5-HT syndrome 5-HT syndrome 5-HT syndrome

Decrease (c) Decrease (Tx) Increase

Adult (human)

L-Tryptophan effect

Increase

HYPO-

Newborn Adult Adult

Apomorphine behavior Apomorphine behavior Haloperidol catalepsy

Increase (d) Increase (r) Decrease (c. r)

Hyper-

Newborn Newborn Newborn (mice) Adult Adult Adult Adult

Apomorphine behavior Apomorphine behavior Amphetamine behavior Apomorphine behavior Apomorphine behavior Apomorphine behavior Haloperidol catalepsy

Decrease Increase Increase Decrease No change Increase Increase

Adult

Dopamine behavior

Increase

Adult

Dopamine behavior

No change

Newborn Adult Adult Adult Newborn Adult

Clonidine activity Clonidine activity Clonidine activity NE release Clonidine activity Clonidine activity

Increase Decrease Increase No change Increase Increase

Hyper-

Adult Adult Adult

cAMP accumulation Firing of Purkinje cells cAMP accumulation

Decrease (c) Decrease (c) Decrease

Heal et al. (1984) Attenvill et al. (1984) Heal et al. (1983) Gross et al. (1980a) Heal et al. (1984) Strijmbom et al. (1977); Attenvill et al. (1984) Gross et al. (1980a) Manvaha and Prasad (1981) Schmidt and Schultz (1985)

Hyper-

Adult

Anticonvulsant thresholds

No change

Attenvill and Nutt (1983)

References

~~~

Intrinsic

Hyper-

Serotonin

HYPOHyper-

Dopamine

Adrenergic Alpha,

HYPO-

HyperBeta

Benzodiazepine

HYPO-

Rastogi et al. (1976); Tamasy et al. (1986) Schalock et al. (1977) Overstreet et al. (1984); Comer and Norton (1985) Schalock et al. (1977): . ,. Hendrich et al. (1984); Overstreet et al.

( 1984)

Tamasy et al. (1986) Ito et al. (1977); Fundarb et al. (1985)

Fundarb et al. (1985) Rastogi et al. (1981) Forster et al. (1981) Ito et al. (1977); KulcsAr et al.

(1980)

Fundarb et al. (1985)

Vaccari (1982) Heal et al. (1983) Attenvill (1981); Brochet et al.

(1985)

Coppen et al. (1972) Overstreet et al. (1984) Crocker et al. (1986) Crocker and Overstreet (1984); Crocker et al. (1986) Attenvill (1981) Rastogi et al. (1981) Forster et al. (1981) Attenvill (1981) Str6mbom et al. (1977) Heal and Attenvill (1982) Attenvill (1981); Crocker and Overstreet (1984) Engstr6m et al. (1974); Attenvill (1981); Heal and Attenvill (1982)

Str6mbom et al. (1977)

(c) (c) (Tx) (c)

a The methods used to induce hypothyroidism in rats are indicated in brackets: (c) = methimazole or propylthiouracyl; (Tx) = surgical ablation; (r) = I3'I; (d) = iodine-deficient diet. Apomorphine behavior = stereotypy, spontaneous locomotion etc.; amphetamine behavior = hyperactivity; 5-HT syndrome = shaking behavior, side-to-side head movements, stereotypy; clonidine activity = sedation or hyperactivity, depending on the dose.

80

they bite, and that hypothyroids are calm and can be easily manipulated. Nonetheless, spontaneous activity and reactivity in hypothyroid rats have been described as either decreased, or increased, or fluctuating in different studies (see Table 2). Avoidance learning, curiosity and 'memory', however, appear to be consistently decreased in experimental hypothyroidism, thus mimicking some aspects of the human cretinoid syndrome. The up-to-date status of dysthyroidismassociated receptor alterations is summarized in Table 1. The density of dopaminergic D,-type receptors is consistently decreased in striatal membranes from newborn and adult rats given goitrogenic drugs (Vaccari and Timiras, 1981; Kalaria and Prince, 1985, 1986), whereas an iodine-deficient diet given to weaned pups (i.e. late in development) (Overstreet et al., 1984) or I3'I to adult rats (Crocker et al., 1986) increases the number of D, receptors. The hyperthyroidism-associated D,-receptor deficit we have found (Vaccari and Timiras, 1981)might either be due to T, toxicity, or represent the adaptive response towards an increased dopaminergic tone. As a matter of fact, the DA system is overactive in neonatal and adult hyperthyroids, as may be inferred from a number of studies showing enhanced behavioral responses to the stimulation of DA neurons as obtained with directly or indirectly acting DA-agonists such as apomorphine, LDOPA, DA itself, or amphetamine (Table2). However, no precise statement can be made concerning the status of DA transmission in hypothyroidism (Table 2). For this purpose it will be important to characterize the complex behavioral components of DA stimulation, e.g. locomotor activity, stereotypy, catalepsy, etc., more thoroughly. Such behaviours might be triggered by different types of DA receptors, or comediated by additional transmitters, and may respond differently to thyroid dysfunctions. Hypothyroid newborn and adult rats undergo a clear-cut serotonergic hypoactivity (Vaccari, 1982; Heal et al., 1983), while the opposite occurs in adult hyperthyroid rats and mice (Attend,

score for 5-HT syndrome

n

2ot

15

30 45

60

90

120

150 min.

Fig. 4. Time-course of the serotonergic behavioral syndrome (tremors, wet dog shakes, side-to-side head movements etc.) induced by an i.p. injection of 220 mg/kg L-5-hydroxytryptophan to 32-34-day-old, euthyroid (open column) and MMIhypothyroid (solid column) rats. Results are from Vaccari (1982).

1981; Brochet et al., 1985) as assessed by the behavioral syndrome which is believed to reflect activation of central 5-HT neurons, after treatment of dysthyroid animals with 5-HT precursors (Fig. 4). It is interesting that the antidepressant activity of the 5-HT precursor L-tryptophan or of imipramine (a marker for the 5-HT reuptake system) is potentiated in human hyperthyroids (Coppen et al., 1972). The maximum number of both 5-HT,- and 5-HT2-type receptors has been found to increase in the brain of MMI-treated newborn rats, and to be unchanged in T,- treated hyperthyroids (Vaccari et al., 1983b). The greater density of 5-HT receptors might be an additional adaptive reaction to the hypothyroidism-related depression of the 5-HT pathways. It is generally agreed that neonatal and adult hypothyroidisms decrease the number of a1-, qand p-adrenoceptors in cortical, striatal, hypothalamic and brain membranes (Table 1). The effects of hyperthyroidism are less obvious. Moreover, it is noteworthy that the number of membrane 8-adrenoceptors of lymphocytes from human hypothyroids is increased (Fantozzi et al., 1983).

81

The dysthyroidism-provoked alterations of adrenoceptors do not fit well with the impairments of functions they probably mediate. For example, clonidine has been widely used as a ligand or as an agonist of a,-receptors. The density of clonidine binding sites decreases or remains the same in cortical membranes of adult hypoand hyperthyroid rats, respectively (Gross and Schtlmann, 1981). In contrast, clonidine-induced sedation, an index for stimulation of presynaptic a,-receptors which regulate the feedback inhibition of NE release (Westfall, 1984), is enhanced in both hypo- and hyperthyroid newborn rats (Heal et al., 1984), and diminished in adult hypothyroids (Atterwill et al., 1984). An explanation for these discrepancies might be that receptor binding assays almost exclusively reveal postsynaptic a,-adrenoceptors (U’Prichard et al., 1980), which might not react to dysthyroid insults in the same way as presynaptically located a,autoreceptors (Heal et al., 1984). In addition, agerelated differences in the ratio of pre- and postsynaptic a,-receptor numbers may be an important variable between studies (Dausse et al., 1982). Thus, one might cautiously conclude that in the CNS of hypothyroid newborn rats only, or in hyperthyroidism, there is a greater %-mediated inhibition of NE release than in euthyroidism. As far as /?-adrenoceptorsare concerned, their hypothyroidism-associated decreased density in cortical, striatal and cerebellar membranes (Smith et al., 1980; Gross et al., 1980b; Atterwill et al., 1984) agrees well with the decrease of NE- or isoprenaline-stimulated CAMP accumulation in cortical slices of hypothyroid rats (Gross et al., 1980a) and with the depressed firing responses of Purkinje neurons to NE (Marwaha and Prasad, 198l), as indices of /?-mediatedcentral adrenergic activity. A receptor-to-function correlation is less easily stated in hyperthyroidism, due to inconsistency of the available results (see Tables 1 and 2). A similar conclusion can be drawn for benzodiazepine receptors : in fact, hyperthyroidism has been found to either increase or decrease the density of flunitrazepam binding sites in cortical

membranes of adult rats (Medina et al., 1984; Gavish et al., 1986). Furthermore, hyperthyroidism does not affect the flurazepam-induced increase or the FG 7 142-provoked decrease in pentetrazol seizure thresholds, as an index of benzodiazepine receptor function (Atterwill and Nutt, 1983). Cerebellar GABAergic and cholinergic muscarinic receptors seem to be sensitive targets for dysthyroidism-related influences as well (Table 1). Finally, the density of histamine HItype receptors and of imipramine binding sites decreases in the brain and cortex of newborn and adult hypothyroid rats (Vaccari, 1985; Codola and Garcia, 1985). Conclusions

Experimentally provoked thyroid dysfunctions during the fetal and early postnatal period in mammalians induce a morphologic and biochemical cascade of events underlying abnormal brain development, and ending with behavioral aberrations which are in some aspects similar to the neuropsychiatric disturbances of human dysthyroidism. A lack or deficiency of thyroid hormones results in an abnormal regulation of the synthesis or degradation of specific proteins such as nuclear or cytosol T, receptors, microtubules and neurofilaments, enzymes and receptors involved in neurotransmitter pathways. Alterations in the kinetics of microtubule assembly may affect the build-up of the nerve cytostructure and lead to a deficient synaptogenesis. Abnormalities in the synaptic network would then, as expected, result in a widespread malfunction of the neurotransmitter machinery. The receptor alterations that are primarily due to protein abnormalities might be further complicated by compensatory adaptations of their number or affinity, a consequence of dysthyroidism-associatedexcessiveor deficient availability of pertinent transmitters. Consistent with the hypothesis of a monoamine imbalance in the origin of mental disorders, a central serotonergic, noradrenergic and, prob-

82

ably, dopaminergic hypofunction in neonatal hypothyroidism would underlie behavioral teratogenesis. Such conclusions are necessarily inferred from animal studies; a promising approach to a better understanding of the mechanisms underlying central teratogenic consequences of human dysthyroidism will be based on the use of blood platelets as a model of peripheral neurons. References Attenvill, C.K. (1981) Effects of acute and chronic triiodothyronine (T,) administration to rats on central 5-HT and dopamine-mediated behavioral responses and related brain biochemistry. Neuropharmacology, 20: 131-144. Attenvill, C. K. and Nutt, D. J. (1983) Thyroid hormones do not alter brain benzodiazepine receptor function in-vivo.J. Pharm. Pharmacol., 35: 767-768. Atterwill, C.K., Bunn, S. J., Atkinson, D. J., Smith, S. L. and Heal, D. J. (1984) Effects of thyroid status on presynaptic alpha,-adrenoceptor function and beta-adrenoceptor binding in the rat brain. J. Neural Transm., 59: 43-55. Barchas, J. D., Elliott, G. R. and Berger, P. A. (1977) Biogenic amine hypotheses of schizophrenia. In J. D. Barchas, P. A. Berger, R.D. Ciaranello and G.R. Elliott (Eds.), Psychopharmacology, Oxford University Press, New York, pp. 100-120. Bernal, J., Refetoff, S . and De Groot, L.J. (1978) Abnormalities of triiodothyronine binding to lymphocyte and fibroblast nuclei from a patient with peripheral tissue resistance to thyroid hormone action. J. Clin. Endmrinol. Metab., 47: 1266-1272. Biassoni, R. and Vaccari, A. (1985) Selective effects of thiol reagents on the binding sites for imipramine and neurotransmitter amines in the rat brain. Br. J. Pharmacol., 85: 447-456. Biegon, A., Rainbow, T. C. and McEwen, B. S. (1985) Corticosterone modulation of neurotransmitter receptors in rat hippocampus: a quantitative autoradiographic study. Brain Res., 332: 309-314. Brasel, J.A. and Boyd, B. (1975) Influence of thyroid hormone on fetal brain growth and development. In D.A. Fisher and G.B. Burrow (Eds.), Perinatal Thyroid Physiology and Dbease, Raven Press, New York, pp. 59-71. Brochet, D., Martin, P., Soubrie, P. and Simon, P. (1985) Effects of triiodothyronine on the S-hydroxytryptophaninduced head twitch and its potentiation by antidepressants in mice. Eur. J. Pharmacol., 112: 41 1-414. Chopra, I. J., Solomon, D. H., Chopra, U., Wu, S.Y., Fisher, D. A. and Nakamura, H. (1978) Pathways of metabolism of thyroid hormones. Rec. Progr. Horm. Res., 34: 521-567.

CodolA, R. and Garcia, A. (1985) Effect of thyroid state on histamine H, receptors in adult and developing rat brain. Biochem. Pharmacol., 34: 4131-4136. Comer, C.P. and Norton, S. (1985) Behavioral consequences of perinatal hypothyroidism in postnatal and adult rats. Phannacol. Biochem. Behav., 22: 605-61 1. Coppen, A., Whybrow, P.C., Noguera, R., M a g s , R. and Prange, A. J. (1972) The comparative antidepressant value of L-tryptophan and imipramine with and without attempted potentiation by triiodothyronine. Arch. Gen. Psychiatry, 26: 234-24 1. Crocker, A. D. and Overstreet, D. H. (1984) Modification of the behavioural effect of haloperidol and of dopamine receptor regulation by altered thyroid status. PsychopharmaC O ~ O ~ V82: , 102-106. Crocker, A.D., Overstreet, D.H. and Crocker, J.M. (1986) Hypothyroidism leads to increased dopamine receptor sensitivity and concentration. Pharmacol. Biochem. Behav., 24: 1593-1597. Dausse, J. P., Le Quan-Bui, K. H., Gallagher, D. W. and Agajanian, G.K. (1982) Alpha,- and alpha,-adrenoceptors in rat cerebral cortex: effects of neonatal treatment with 6-hydroxydopamine. Eur. J. Pharmacol. 78: 15-20. Del Cerro, M. R., Somoza, G., Segovia, S.and Guillambn, A. (1986) Effects of neonatal thyroidectomy on neurotransmitter receptors in several regions of the rat brain. ZRCS Med. Sci., 14: 92-93. Dillman, W., Surks, M.I. and Oppenheimer, J.H. (1974) Quantitative aspects of iodothyronine binding by cytosol proteins of rat liver and kidney. Endocrinology, 95: 492-498. Dbhler, K. D.,Wong, C.C. and Von Zur Miihlen, A. (1979) The rat as a model for the study of drug effects on thyroid function: consideration of methodological problems. Pharmac. Ther., 5: 305-318. Dozin-Van Roye, B. and De Nayer, P. (1978) Triiodothyronine binding to brain cytosol receptors during maturation. FEBS Lett., 96: 152-154. Dratman, M.B. (1974) On the mechanism of action of thyroxin, an amino acid analog of tyrosine. J. Theor. Biol., 46: 255-270. Dratman, M.B., Crutchfield, F.L. and Gordon, J.T. (1984) Thyroid hormones and adrenergic neurotransmitters. In E. Usdin, A. Carlsson, A. Dahlstrom and J. Engel (Eds.), Catecholamines: Neurophannacology and Central Nervous System-Theoretical Aspects, A.R. Liss, New York, pp. 425-439. Dratman, M.B., Futaesaka, Y.,Crutchfield, F.L., Berman, N., Payne, B., Sar, M. and Stumpf, W.E. (1982) Iodine125-labeled triiodothyronine in rat brain: evidence for localization in discrete neural systems. Science, 215: 309-312. Eayrs, J.T. (1971) Thyroid and developing brain: anatomical and behavioral effects. In M. Hamburgh and E. J. Barring-

83

’

ton (Eds.), Hormones in Development, Appleton Century Crofts, New York, pp. 345-355. Engstram, G. Svensson, T.H. and Waldeck, B. (1974) Thyroxine and brain catecholamines: increased transmitter synthesis and increased receptor sensitivity. Brain Res., 77: 471-483. Fantozzi, R., Brunelleschi, S., Cuomo, S., Defeo, L., Maggi, M., Mannelli, M., Serio, M. and Ledda, F. (1983) Changes in the density of human lymphocyte beta adrenergic receptors in hypothyroidism. Acta Pharmacol. Toxicol., 53: Suppl. 1, 120. Fisher, D.A., Dussault, J.H., Sack, J.H. and Chopra, I.J. ( 1977) Ontogenesis of hypothalamic-pituitary-thyroid function and metabolism in man, sheep and rat. Rec. Progr. Horm. Res., 33: 59-1 16. Forster, M. J., Nagy, Z.M. and Murphy, J.M.(1981) Potentiation of amphetamine-induced hyperactivity in the adult mouse following neonatal thyroxine administration. Bull. Psychonomic SOC.,18: 337-339. Fundarb, A., Molinengo, L. and Cassone, M. C. (1985) The transition from a fixed ratio to a fixed interval schedule of reinforcement in hypo and hyperthyroid rats. Pharmacol. Res. Commun., 17: 463-470. Gavish, M., Weizman, A., Okun, F. and Youdim, M.B. (1986) Modulatory effects of thyroxine treatment on central and peripheral benzodiazepine receptors in the rat. J. Neurochem., 47: 1106-1 110. Geel, S. E. (1977) Development-related changes of triiodothyronine binding to brain cytosol receptors. Nature, 269: 428-430. Glorieux, J., Dussault, J.H., Letarte, J., Guyda, H. and Morissette, J. (1983) Preliminary results on the mental development of hypothyroid infants detected by the Quebec screening program. J. Pediarr., 102: 19-22. Gross, G., Brodde, 0.E. and Schiimann, H. J. (1980a) Effects of thyroid hormone deficiency on pre- and postsynaptic noradrenergic mechanisms in the rat cerebral cortex. Arch. Int. Pharmacodyn. Ther., 244: 219-230. Gross, G., Brodde, 0.E. and Schiimann, H. J. (1980b) Decreased number of beta-adrenoceptors in cerebral cortex of hypothyroid rats. Eur. J. Pharmacol., 61: 191-194. Gross, G. and Schiimann, H.J. (1981) Reduced number of alpha,-adrenoceptors in cortical brain membranes of hypothyroid rats. J. Pharm. Pharmacol., 33: 552-554. Gross, G., Brodde, O.E. and Schiimann, H.J. (1981) Regulation of alpha,-adrenoceptors in the cerebral cortex of the rat by thyroid hormones. Naunyn-Schmied. Arch. PharmaCOI. 316: 45-50. Heal, D.J. and Atterwill, C.K. (1982) Repeated administration of triiodothyronine (T,) to rats enhances nigrostriatal and mesolimbic dopaminergic behavioural responses. Neuropharmacology,21: 159-162. Heal, D.J., OShaughnessy, K.M., Smith, S.L. and Nutt, D. J. (1983) Hypophysectomy alters both 5- hydroxytryp-

tamine- and alpha,-adrenoceptor-mediated behavioural changes in the rat. Eur. J. Pharmacol., 89: 167-171. Heal, D. J., Smith, S. L. and Atterwill, C. K. (1984) Effects of thyroid status on clonidine-induced hypoactivity responses in the developing rat. J. Neural Transm., 60: 295-302. Hendrich, C.E., Jackson, W. J. and Porterfield, S.P. (1984) Behavioral testing of progenies of Tx (hypothyroid) and growth hormone-treated Tx rats: an animal model for mental retardation. Neuroendocrinology, 38: 429-437. Hollingsworth, D.R., Mabry, C.C. and Reid, M.C. (1980) Congenital Grave’s disease. In C. La Cauza and A.W. Root (Eds.), Problems in Pediatric Endocrinology, Academic Press, New York, P. 169. Ito, J.M., Valcana, T. and Timiras, P. S. (1977) Effects of hypo- and hyperthyroidism on regional monoamine metabolism in the adult rat brain. Neuroendocrinology, 24: 55-64. Jubiz, W. (1979) Endocrinology - A Logical Approach for Clinicians, McGraw-Hill, New York, pp. 45-79. Kalaria, R.N., Kotas, A.M., Prince, A. K. and Reynolds, R. (1981) Neuronal development in discrete areas of rat brain during neonatal thyroid deficiency. Br. J. Pharmacol., 77: 103P. Kalaria, R.N. and Prince, A.K. (1985) Effect of thyroid deficiency on the development of cholinergic, GABA, dopaminergic and glutamate neuron markers and DNA concentrations in the rat corpus striatum. Int. J. Dev. Neurosci., 3: 655-666. Kalaria, R. N. and Prince, A. K. (1986) Decreased neurotransmitter receptor binding in striatum and cortex from adult hypothyroid rats. Brain Res., 364: 268-274. Kastrup, J. and Christensen, N. J. (1984) Lack of effects of thyroid hormones on muscarine cholinergic receptors in rat brain and heart. Scand. J. Clin. Lab. Invest., 44: 33-38. Klein, R. (1986) Screening for congenital hypothyroidism. Lancet, 8503: 403. Kulcsir, A., Kulcsir-Gergely, J. and Kulcsir, L. (1980) A hyperthyreosis model experiment in rats. Arzneim.-Forsch., 30: 44-47: Latham, K. R., Ring, J. C. and Baxter, J. D. (1976) Solubilized nuclear ‘receptors’ for thyroid hormones. Physical characteristics and binding properties, evidence for multiple forms. J. Biol. Chem., 251: 7388-7397. Lau, C., Pylypiw, A. and Ross, L. L. (1985) Development of serotonergic and adrenergic receptors in the rat spinal cord: effects of neonatal chemical lesions and hyperthyroidism. Dev. Brain Res., 19: 57-66. Lauder, J. (1977) The effects of early hypo- and hyperthyroidism on the development of rat cerebellar cortex. 111. Kinetics of cell proliferation in the external granular layer. Brain Res., 126: 31-51. Legrand, J. (1979) Morphogenetic actions of thyroid hormones. Trends Neurosci., 2: 234-236.

84 Legrand, J. (1980) Effects of thyroid hormones on brain development, with particular emphasis on glial cells and myelination. In C. Di Benedetta, R. Balasz, G. Gombos and G. Porcellati (Eds.), MultidisciplinaryApproach to Brain Development, Elsevier, Amsterdam, pp. 279-292. Legrand, J. (1984) Effects of thyroid hormones on central nervous system development. In J. Yanay (Ed.), Neurobehavioral Teratology, Elsevier, Amsterdam, pp. 331-363. Lennon, A.M., Francon, J., Fellous, A. and Nunez, J. (1980) Rat, mouse and guinea pig brain development and microtubule assembly. J. Neurochem., 35: 804-813. Leret, M. L. and Fraile, A. (1986) Influence of thyroidectomy on brain catecholamines during the postnatal period. Comp. Biochem. Physiol., 83: 117-121. Linquette, M., Lefebvre, J., Racadot, A., Cappoen, J.P. and Fontaine-Delort, S. (1976) Taux de production et concentration plasmatique moyenne du cortisol dans I‘hyperthyroidie. Ann. Endocrinol. (Paris), 37: 331-345. Marwaha, J. and Prasad, K. N. (1981) Hypothyroidism elicits electrophysiological noradrenergic subsensitivity in rat cerebellum. Science, 214: 675-677. Medina, J.H., Tumilasci, 0. and De Robertis, E. (1984) Thyroid hormones regulate benzodiazepine receptors in rat cerebral cortex. IRCS Med. Sci., 12: 158-159. Menezes-Ferreira, M.M. and Torresani, J. (1983) Mtcanismes d’action des hormones thyroidiennes au niveau cellulaire. Ann. Endocrinol. (Paris), 44: 205-216. Meyer, J. S. (1985) Biochemical effects of corticosteroids on neural tissues. Physiol. Rev., 65: 946-1020. New England Congenital Hypothyroidism Collaborative ( 1984) Characteristics of infantile hypothyroidism discovered on neonatal screening. J. Pediatr., 104: 539-544. Nunez, J. (1984) Effects of thyroid hormones during brain differentiation. Mol. Cell. Endocrinol., 37: 125-132. Nunez, J. (1985) Microtubules and brain development: the effects of thyroid hormones. Neurochem. Int., 7: 959-968. Overstreeet, D. H., Crocker, A. D., Lawson, C. A., McIntosh, G. H. and Crocker, J. M. (1984) Alterations in the dopaminergic system and behaviour in rats reared on iodinedeficient diets. Pharmacol. Biochem. Behav., 21: 561-565. Pascual-Leone, A.M., Escrivil, F., Alvarez, C., Goya, L.and Rodriguez, C. (1985) Influence of hormones and undernutrition on brain development in newborn rats. Biol. Neonate, 48: 228-236. Patel, A. J., Rabit, A,, Lewis, P.D. and Balasz, R. (1976) Effects of thyroid deficiency on postnatal cell formation in the rat brain. A biochemical investigation. Brain Res., 104: 33-48. Patel, A.J., Smith, R.M., Kingsbury, A.E., Hunt, A. and Balbz, R. (1980) Effects of thyroid state on brain development: muscarinic acetylcholine and GABA receptors. Brain Res., 198: 389-402. Pittman, J.A. (1971) The Thyroid, Harper and Row, New York, pp. 644-655.

Puymirat, J. (1985) Effects of dysthyroidism on central catecholaminergic neurons. Neurochem. Int., 7: 969-977. Rabit, A., Favre, C., Clavel, M.C. and Legrand, J. (1977) Effect of thyroid dysfunction on the development of the rat cerebellum, with special reference to cell death within the internal granular layer. Brain Res., 120: 521-531. Rabit, A., Patel, A. J., Clavel, M.C. and Legrand, J. (1979) Effect of thyroid deficiency on the growth of the hippocampus in the rat. A combined biochemical and morphological study. Dev. Neurosci., 2: 183-194. Rami, A., Rabit, A. and Patel, A. J. (1986) Thyroid hormone and development of the rat hippocampus: cell acquisition in the dentate gyrus. Neuroscience, 19: 1207-1216. Rastogi, R.B., LaPierre, Y. and Singhal, R.L. (1976) Evidence for the role of brain biogenic amines in depressed motor activity seen in chemically thyroidectomized rats. J. Neurochem., 26: 443-449. Rastogi, R.B., Singhal, R.L. and LaPierre, Y.D. (1981) Effects of apomorphine on behavioural activity and brain catecholamine synthesis in normal and L-triiodothyronine-treated rats. J. Neural Transm., 50: 139-148. Rebikre, A. and Legrand, J. (1972) Donntes quantitatives sur la synaptogenkse dans le cervelet du rat normal et rendu hypothyrordien par le propylthiouracile. C. R. Acad. Sci. (Paris), 274: 3581-3584. Reisine, T. (1981) Adaptive changes in catecholamine receptors in the central nervous system. Neuroscience, 6: 1471-1 502. Ruel, J., Faure, R. and Dussault, J.H. (1985) Regional distribution of nuclear T, receptors in rat brain and evidence for preferential localization in neurons. J. Endocrinol. Invest., 8: 343-348. Savard, P., Mtrand, Y., Di Paolo, T. and Dupont, A. (1983) Effects of thyroid state on serotonin, 5-hydroxyindole acetic acid and substance P contents in discrete brain nuclei of adult rats. Neuroscience, 10: 1399-1404. Sawin, C.T. (1985) Hypothyroidism. Med. Clin. N . Am., 69: 989-1004. Schalock, R. L., Brown, N. J. and Smith, R.L. (1977) Neonatal hypothyroidism: behavioral, thyroid hormonal and neuroanatomical effects. Physiol. Behav., 19: 489-491. Schmidt, B.H. and Schultz, J.E. (1985) Chronic thyroxine treatment of rats down-regulates the noradrenergic cyclic AMP generating system in cerebral cortex. J. Pharmacol. Exp. Ther., 233: 466-472. Schwartz, H. L. and Oppenheimer, J. H. (1978a) Nuclear triiodothyronine receptor sites in brain: probable identity with hepatic receptors and regional distribution. Endocrinology, 103: 267-273. Schwartz, H.L. and Oppenheimer, J. H. (1978b) Ontogenesis of 3,5,3’-triiodothyronine receptors in neonatal rat brain: dissociation between receptor concentration and stimulation of oxygen consumption by 3,5,3’-triiodothyronine. Endocrinology, 103: 943-948.

85 Shire, J.G. and Beamer, W.G. (1984) Adrenal changes in genetically hypothyroid mice. J. Endocrinol., 102: 277-280. Smith, R.M., Patel, A.J., Kingsbury, A.E., Hunt, A. and Ballsz, R. (1980) Effects ofthyroid state on brain development: beta-adrenergic receptors and 5’-nucleotidase activity. Brain Res., 198: 375-387. Strombom, U., Svensson, T.H., Jackson, D.M. and Engstrbm, G. (1 977) Hyperthyroidism, specifically increased response to central NA-(alpha>receptor stimulation and generally increased monoamine turnover in brain. J. Neural Transm., 41: 73-92. Strupp, B. J. and Levitsky, D.A. (1983) Early brain insult and cognition: a comparison of malnutrition and hypothyroidism. Dev. Psychobiol., 16: 535-549. Tamasy, V., Meisami, E., Vallerga, A. and Timiras, P. S. (1986) Rehabilitation from neonatal hypothyroidism: spontaneous motor activity, exploratory behavior, avoidance learning and responses of pituitary-thyroid axis to stress in male rats. Psychoneuroendocrinologv, 11: 91-103. Timiras, P. S. (1972) Developmental Physiologv and Aging, , MacMillan Co., New York, pp. 129-165. Timiras, P. S. (1979) Distribution, development and function of thyroid hormone receptors in the brain. Proc. West. Pharmacol. SOC.,22: 371-374. U’Prichard, D. C., Reisine, T. D., Mason, S. T., Fibiger, H. C. and Yamamura, H. I. (1980) Modulation of rat brain alphaand beta-adrenergic receptor populations by lesion of the dorsal noradrenergic bundle. Brain Rex, 180: 143-154. Vaccari, A., Valcana, T. and Timiras, P. S. (1977) Effects of hypothyroidism on the enzymes for biogenic amines in the developing rat brain. Pharmacol. Res. Commun., 9: 763-780. Vaccari, A. and Timiras, P.S. (1981) Alterations in brain dopaminergic receptors in developing hypo- and hyperthyroid rats. Neurochem. Int., 3: 149-153. Vaccari, A. (1982) Decreased central serotonin function in hypothyroidism. Eur. J. Pharmacol., 82: 93-95. Vaccari, A,, Biassoni, R. and Timiras, P. S. (1983a) Selective effects of neonatal hypothyroidism on monoamine oxidase activities in the rat brain. J. Neurochem., 40: 1016-1025. Vaccari, A., Biassoni, R. and Timiras, P. S. (1983b) Effects of neonatal dysthyroidism on serotonin type 1 and type 2 receptors in rat brain. Eur. J. Pharmacol., 95: 53-63. Vaccari, A. (1983) Effects of dysthyroidism on central monoaminergic neurotransmission. In M. Schlumpf and W. Lichtensteiger (Eds.), Drugs and Hormones in Brain Development, Karger, Basel, pp. 78-90. Vaccari, A. (1985) Effects of neonatal antithyroid treatment on brain 3H-imipramine binding sites. Br. J. Pharmacol., 84: 773-778. Valcana, T. and Timiras, P.S. (1978) Nuclear triiodothyronine receptors in the developing rat brain. Mol. Cell. Endocrinol., 11: 31-41.

Vanderpas, J. B., Rivera-Vanderpas, M.T., Bourdoux, P., Luvivila, K., Lagasse, R., Perlmutter-Cremer, N., Delange, F., Lanoie, L., Ermans, A.M. and Thilly, C.H. (1986) Reversibility of severe hypothyroidism with supplementary iodine in patients with endemic cretinism. N . Eng. J. Med., 315: 791-795. Varma, S. K., Murray, R. and Stanbury, J.B. (1978) Effects of maternal hypothyroidism and triiodothyronine on the fetus and newborn in rats. Endocrinology, 102: 24-30. Westfall, T.C. (1984) Evidence that noradrenergic transmitter release is regulated by presynaptic receptors. FASEB Proc., 43: 1352-1357. Whybrow, P. and Ferrell, R. (1974) Thyroid state and human behaviour : contributions from a clinical perspective. In A. J. Prange (Ed.), The Thyroid Axis, Drugs and Behaviour, Raven Press, New York, pp. 5-28. Wolter, R., Noel, P., De Cock, P., Craen, M., Emould, C.H., Malvaux, P., Verstraeten, F., Simons, J., Mertens, S., Van Broek, N. and Vanderschueren-Lodewyckx, M. (1980) Neuropsychological study in treated thyroid dysgenesis. Acia Pediatr. Scand., 277: 41-46. Yamada, K.M., Spooner, B. S. and Wessels, N.K. (1970) Axon growth: role of microfilaments and microtubules. Proc. Nail. Acad. Sci. USA. 66: 1206-1212.

Discussion R. Balazs: I would take issue with your proposal that all the effects of thyroid disorders are the result of a single influence followed by a cascade of alterations. A. Vaccari: Of course, simplifying the intimate mechanisms of any pathological event like dysthyroidism is a difficult task. There is no doubt that the very first single influence of thyroid disorders is an alteration in the synthesis or turnover of specific proteins. For example, Lemarchand-Bkraud et al. (1987) have shown that the modulation of pituitary nuclear T,-receptors in hypothyroid rats is impaired by the protein synthesis inhibitor cycloheximide. From then onwards, all the different steps in the cascade sequence that I have suggested may occur either at the same time, or follow each other, or may even be absent, depending on variables such as the stage of brain development and the severity of thyroid insults. I have focused attention on the putative role of established neurotransmitters in the etiogenesis of behavioral disorders. It will be also necessary to consider the role of trophic factors and neurotransmitter peptides in the origin of dysthyroidism-related central teratogenesis. A. J. Patel (comment): In your talk you proposed a direct relationship between modification in the composition of microtubule-associated protein reduction and the reduction in the number of synaptic junctions in hypothyroid animals. In this context, another possibility to be considered is the alter-

86

ations in specific cell surface molecules that are believed to be involved in cell-to-cell interaction and recognition' (see Edelman, 1984). A severe reduction in the ontogenesis of neural cell adhesion molecule (N-CAM)-like glycoprotein (D2) and changes in its molecular forms on the one hand, and in the number of synapses per nerve on the other has been observed in the cerebellum of young hypothyroid or hyperthyroid rats (Patel et al., 1985).

References Edelman, G. M. (1984) Modulation of cell adhesion during induction, histogenesis and perinatal development of the nervous system. Annu. Rev. Neurosci., 7: 339-377. Lemarchand-Bkraud, T., Von Overbeck, K. and Rognoni, J. B. (1987) The modulation by 3,5,3'-triiodothyronine (T,) of pituitary T, nuclear receptors in hypothyroid rats is inhibited by cycloheximide. Endocrinology, 121: 677-683. Patel, A. J., Hunt, A. and Meier, E. (1985) Effects of undernutrition and thyroid state on the ontogenetic changes of D, , D, and D, brain-specific proteins in rat cerebellum. J. Neurochem., 44: 1581-1587.