Excess neonatal thyroxine: effects on learning in infant and adolescent rats

Adult onset of type 3 deiodinase deficiency in mice alters brain gene expression and increases locomotor activity

Constitutive loss of the type 3 deiodinase (DIO3) causes abnormally increased levels of thyroid hormone action in the developing and adult brain, leading to an array of behavioral abnormalities. To determine to what extent those phenotypes derive from a lack of DIO3 in the adult brain, versus developmental consequences, we created a mouse model of conditional DIO3 inactivation. Mice carrying "floxed" Dio3 alleles and a tamoxifen-inducible cre transgene were injected with tamoxifen at two months of age. Compared to oil-injected controls, the brain tissue of these mice showed a 75-80% decrease in DIO3 activity and 85-95% Dio3 mRNA was expressed from recombinant alleles. Mice with adult DIO3 deficiency did not show significant differences in growth, serum thyroid hormone parameters or behaviors related to anxiety and depression. However, female mice exhibited elevated locomotor activity and increased marble-burying behavior. They also manifested relatively modest alterations in the expression of T3-dependent genes and genes related to hyperactivity in a sex- and region-specific manner. Upon thyroid hormone treatment, the expression response of T3-regulated genes was generally more pronounced in DIO3-deficient female mice than in female controls, while the opposite effect of altered genotype was noticed in males. The extent of the molecular and behavioral phenotypes of adult-onset DIO3 deficiency suggests that a substantial proportion of the neurological abnormalities caused by constitutive DIO3 deficiency has a developmental origin. However, our results show that DIO3 in the adult brain also influences behavior and sensitivity to thyroid hormone action in a sexually dimorphic fashion.

Individualized treatment to optimize eventual cognitive outcome in congenital hypothyroidism

BACKGROUND

In congenital hypothyroidism (CH), age-specific reference ranges (asRR) for fT₄ and thyrotropine (TSH) are usually used to signal over/under-treatment. We compared the consequences of individual fT₄ steady-state concentrations (SSC's) and asRR regarding over-treatment signaling and intelligence quotient at 11 y (IQ₁₁) and the effect of early over-treatment with high L-T₄ dosages on IQ₁₁.

METHODS

Sixty-one patients (27 severe, 34 mild CH) were psychologically tested at 1.8, 6, and 11 y. Development scores were related to over-treatment in the period 0-24 mo, relative to either individual fT₄SSC's or asRR. Three groups were formed, based on severity of over/under-treatment 0-5 mo (severe, mild, and no over/under-treatment).

RESULTS

FT₄ and TSH asRR missed 41-50% of the over-treatment episodes and consequently 22% of the over-treated patients, classified as such by fT₄SSC's. Severe over-treatment 0-5 mo led to lowered IQ₁₁'s and to a 5.5-fold higher risk of IQ₁₁ < 85 than other treatment regimes. Under-treatment had no effect on development scores. Initial L-T₄ dosages >10 µg/kg resulted in a 3.7-fold higher risk of over-treatment than lower dosages.

CONCLUSIONS

Data suggest that asRR, compared to fT₄SSC's, signal over-treatment insufficiently. Using fT₄SSC's and avoiding over-treatment may optimize cognitive outcome. Lowered IQ₁₁'s are usually a late complication of severe early over-treatment.

The effects of thyroid hormone level and action in developing brain: are these targets for the actions of polychlorinated biphenyls and dioxins?

Alterations in thyroid hormone level or responsivity to thyroid hormone have significant neurologic sequelae throughout the life cycle. During fetal and early neonatal periods, disorders of thyroid hormone may lead to the development of motor and cognitive disorders. During childhood and adult life, thyroid hormone is required for neuronal maintenance as well as normal metabolic function. Those with an underlying disorder of thyroid hormone homeostasis or mitochondrial function may be at greater risk for developing cognitive, motor, or metabolic dysfunction upon exposure to substances which alter thyroid hormone economy. Polychlorinated biphenyls (PCBs) and dioxins have been argued to interfere with thyroid hormone action and thus may affect the developing and mature brain. Animal models provide useful tools for studying the effects of thyroid hormone disorders and the effects of environmental endocrine disruptors. The congenitally hypothyroid, hyt/hyt, mouse exhibits abnormalities in both the cognitive and motor systems. In this mouse and other animal models of thyroid hormone disorders, delayed somatic and reflexive development are noted, as are permanent deficits in hearing and locomotor and adaptive motor behavior. This animal's behavioral abnormalities are predicated on anatomic abnormalities in the nervous system. In turn, these abnormalities are correlated with differences in neuronal structural proteins. In normal mice, the expression of mRNAs coding for these proteins occurs temporally with the onset of autonomous thyroid hormone production. The hyt/hyt mouse has a mutation in the thyroid stimulating hormone receptor (TSHr) gene which renders it incapable of transducing the TSH signal in the thyrocyte to produce thyroid hormone. Some behavioral and possibly some biochemical abnormalities in mice exposed to PCBs are similar to those seen in the hyt/hyt mouse. In addition to direct effects on brain development and neuronal maintenance, thyroid hormone is necessary for maintaining metabolic functioning through its influence on mitochondria. Because the brain is particularly sensitive to inadequate energy generation, disorders of thyroid hormone economy also indirectly impair brain functioning. Alterations in thyroid hormone level result in differing expression of mitochondrial genes. Mutations in these mitochondrial genes lead to well-recognized syndromes of encephalomyopathy, myopathy, and multisystem disorder. Hence, PCBs and dioxins, by possibly altering the thyroid hormone milieu, may alter the functioning of mitochondria in the generation of adenosine triphosphate (ATP). The use of animal models of thyroid hormone deficiency for behavioral, anatomic, histologic, and molecular comparison will help elucidate the mechanisms of action of these putative endocrine-disrupting compounds. The study of thyroid hormone disorders provides a template for relating thyroid hormone mediated effects on the brain to these compounds.

High and low dose initial thyroxine therapy for congenital hypothyroidism

OBJECTIVE

To assess factors influencing thyroxine (T4) levels 1 month after initiating replacement therapy for congenital primary hypothyroidism.

METHODOLOGY

A retrospective review of 41 children with congenital hypothyroidism who received either high or low dose initial T4 therapy. Thyroid scintiscan was performed, and T4 levels determined before starting treatment and after 1 month.

RESULTS

T4 levels at 1 month were correlated (r2 = 0.38, P < 0.001) with the pretreatment T4 level (r = 0.48), as well as with the T4 dose (r = 0.46). Suboptimal treated T4 levels (< 130 nmol/L) were seen with greater frequency in infants with thyroid agenesis (7/11) rather than ectopia (7/28, P < 0.03), despite receiving similar doses of thyroxine. Infants with suboptimal treated T4 levels had lower pretreatment T4 levels than those with optimal levels (21 +/- 7 vs 48 +/- 34 nmol/L, P < 0.02). Biochemical hyperthyroidism (T4 > 216 nmol/L) occurred in six patients; four of six had ectopia.

CONCLUSIONS

These data suggest that infants with little residual thyroid function should receive higher initial T4 doses than those with significant ectopia.

Correlations between radial-maze learning and structural variations of septum and hippocampus in rodents

Large, but non-pathological, individual differences in neuroanatomy of the brain exist in rodents, which have been shown to covary with behavioral traits. In the present review, we explore the relationship between variations in the extent of the intra- and infrapyramidal mossy fiber projection of the hippocampus and spatial and non-spatial learning capacities in mice and rats. Preliminary data concerning anatomical variation in the septo-hippocampal cholinergic system and its consequences for individual behavior are also presented. We conclude that the hippocampal intra- and infrapyramidal mossy fiber projection is intimately involved in the regulation of spatial, but not of non-spatial learning capabilities. Although lesion studies have shown that a well-functioning cholinergic system is a prerequisite for performance in spatial learning tasks, our preliminary data suggest that individual differences in the cholinergic system do not explain individual differences in learning.

Effect of thyroid hormone level on temperament in infants with congenital hypothyroidism detected by screening of neonates

To determine the effect of congenital hypothyroidism and its treatment on infant behavior, we assessed temperament in 50 six-month-old infants with congenital hypothyroidism detected by means of screening of neonates. Intelligence and temperament were also evaluated at 12, 18, 24, and 36 months. More of these children were classified as "difficult" than children in the nonhypothyroid standardization sample. Temperamental difficulty was associated with increased nervous system sensitivity, reflecting more intense responses and a lower threshold of response to external stimulation. Greater temperamental difficulty was found to persist until at least age 2 years of age and to be associated with higher circulating triiodothyronine and thyroxine levels between 1 and 3 months of age. Our results suggest that behavioral features should be considered, as well as circulating hormone levels, in determining the proper dose of thyroid hormone replacement in infants with congenital hypothyroidism.

Neonatal hyperthyroidism in the rat: thyroxine accelerates the development of unconditioned but not learned responses to tastes

The effects of neonatal hyperthyroidism induced by thyroxine injection on the development of unconditioned and learned behaviors mediated by the gustatory system of the rat were investigated. The development of unconditioned ingestive responses evoked by 10% sucrose and 0.1% HCl taste solutions was advanced several days by thyroxine treatment. However, the emergence of taste aversion learning involving 10% sucrose and LiCl injection was not advanced and may have been slightly delayed. Thus, the ontogenesis of unconditioned and learned behaviors mediated by the gustatory system was not influenced uniformly by neonatal thyroxine treatment.

Olfactory granule cell development in normal and hyperthyroid rats

Dendritic development was examined in olfactory bulbs of both normal 7-, 14-, 21- and 60-day-old rats and littermates treated on postnatal days 1-4 with 1 microgram/g body weight of L-thyroxine sodium. Tissue was processed via the Golgi-Cox technique and subjected to quantitative analyses of mitral and internal layer granule cell development. These populations of granule cells were selected because their pattern of late proliferation suggested potentially greater susceptibility to postnatal hormonal alterations. Although neonatal hyperthyroidism induces widespread acceleration of maturation, including precocious chemosensitivity, granule cell development was unaffected relative to littermate controls. Both normal and hyperthyroid groups exhibited an inverted U-shaped pattern of cellular development, with rapid dendritic dendritic growth and expansion occurring during the earliest ages tested, but with loss of processes and dendritic field size occurring after day 21.

Behavioral effects of neonatal thyroid hormones and differential postweaning rearing in rats

Male and female rats (T rats) given an injection of 75 microgram tri-iodothyronine (T3) and 25 microgram thyroxine (T4) on Day 3 after birth were raised under enriched (E) and impoverished (I) postweaning conditions. Observations of their open-field behavior and Days 63 and 112 revealed a higher rate of activity in T groups as compared to controls (C) as well as faster running times and more entries/min in Hebb-Williams maze testing on Days 74-86, indicating behavioral hyperactivity in T rats. Active-avoidance testing at age 91 days revealed a higher number of intertrial crossings and significantly faster unconditioned response and conditioned response latencies in T as compared to C groups. Enriched postweaning rearing altered the behavioral effects in T rats comparatively little, least in the open-field and more so for escape- and avoidance-latencies and for the number of errors in the Hebb-Williams maze.

Development of cholinergic inhibitory capacities in the hyperthyroid mouse

The ontogeny of behavioral arousal and inhibition, as measured by spontaneous locomotor activity, was compared in four experiments for controls and mice injected with thyroxine as neonates. Mice treated with thyroxine at 1-3 days of age had higher activity levels at 10-15 days of age than controls, suggesting potentiation of arousal systems by the hormone treatment. Although thyroxine-accelerated development had no reliable effect upon the age at which peak activity occurred, scopolamine injections increased activity as early as 15 days of age in thyroxine-treated mice, whereas saline-treated or unhandled controls did not show a similar increase until 16-17 days of age. The findings were interpreted as indicating both a potentiation of arousal and a compensatory acceleration of cholinergic inhibitory capacities as a result of the neonatal hyperthyroidism. In addition, the importance of the behaviorally suppressive effects of a novel injection experience in the neonatal mouse was demonstrated.