Hormonal regulation of brain development. II. Effect of neonatal thyroidectomy on succinate dehydrogenase and other enzymes in developing cerebral cortex and cerebellum of the rat

Thyroid hormones: Possible roles in epilepsy pathology

Thyroid hormones (THs) L-thyroxine and L-triiodothyronine, primarily known as metabolism regulators, are tyrosine-derived hormones produced by the thyroid gland. They play an essential role in normal central nervous system development and physiological function. By binding to nuclear receptors and modulating gene expression, THs influence neuronal migration, differentiation, myelination, synaptogenesis and neurogenesis in developing and adult brains. Any uncorrected THs supply deficiency in early life may result in irreversible neurological and motor deficits. The development and function of GABAergic neurons as well as glutamatergic transmission are also affected by THs. Though the underlying molecular mechanisms still remain unknown, the effects of THs on inhibitory and excitatory neurons may affect brain seizure activity. The enduring predisposition of the brain to generate epileptic seizures leads to a complex chronic brain disorder known as epilepsy. Pathologically, epilepsy may be accompanied by mitochondrial dysfunction, oxidative stress and eventually dysregulation of excitatory glutamatergic and inhibitory GABAergic neurotransmission. Based on the latest evidence on the association between THs and epilepsy, we hypothesize that THs abnormalities may contribute to the pathogenesis of epilepsy. We also review gender differences and the presumed underlying mechanisms through which TH abnormalities may affect epilepsy here.

Protective effects of selenium to placental lead neurotoxicity in rat pups

The neurotoxic effects of lead are age-related phenomena, which in turn are dependent on nutrition status as well as the presence of other micronutrients. The present study was designed to study the protective effects of selenium on rat pups exposed to lead. The activities of different enzymes in cerebrum and cerebellum regions were estimated following maternal lead neurotoxicity through placenta. The parental animals were exposed to different treatments for 5 weeks prior to gestation and 3 weeks during gestation; that is, animals were exposed for a total period of 8 weeks and rat pups obtained from exposed parents were allowed to survive on their mothers' milk for a period of 1 month, after which they were sacrificed. Activities of succinic dehydrogenase (SDH), acetylcholine estrase (AChE), and Na(+)/K(+)-ATPase were studied in the cerebrum and cerebellum of the rat pups at 4 weeks of age. A significant reduction in the activity of all the three enzymes-SDH, AChE and Na(+)/K(+) -ATPase-was observed in the lead-treated group, except in the case of AChE, which increased in the case of cerebrum. However, changes in different enzyme activities were less pronounced in the animals that were treated simultaneously with selenium and lead. No pups were delivered to the mothers who were exposed to selenium alone for period of 8 weeks.

Revisiting thyroid hormones in schizophrenia

Thyroid hormones are crucial during development and in the adult brain. Of interest, fluctuations in the levels of thyroid hormones at various times during development and throughout life can impact on psychiatric disease manifestation and response to treatment. Here we review research on thyroid function assessment in schizophrenia, relating interrelations between the pituitary-thyroid axis and major neurosignaling systems involved in schizophrenia's pathophysiology. These include the serotonergic, dopaminergic, glutamatergic, and GABAergic networks, as well as myelination and inflammatory processes. The available evidence supports that thyroid hormones deregulation is a common feature in schizophrenia and that the implications of thyroid hormones homeostasis in the fine-tuning of crucial brain networks warrants further research.

Thyroid hormone and γ-aminobutyric acid (GABA) interactions in neuroendocrine systems

Thyroid hormones (THs) have critical roles in brain development and normal brain function in vertebrates. Clinical evidence suggests that some human nervous disorders involving GABA(gamma-aminobutyric acid)-ergic systems are related to thyroid dysfunction (i.e. hyperthyroidism or hypothyroidism). There is experimental evidence from in vivo and in vitro studies on rats and mice indicating that THs have effects on multiple components of the GABA system. These include effects on enzyme activities responsible for synthesis and degradation of GABA, levels of glutamate and GABA, GABA release and reuptake, and GABA(A) receptor expression and function. In developing brain, hypothyroidism generally decreases enzyme activities and GABA levels whereas in adult brain, hypothyroidism generally increases enzyme activities and GABA levels. Hyperthyroidism does not always have the opposite effect. In vitro studies on adult brain have shown that THs enhance GABA release and inhibit GABA-reuptake by rapid, extranuclear actions, suggesting that presence of THs in the synapse could prolong the action of GABA after release. There are conflicting results on effects of long term changes in TH levels on GABA reuptake. Increasing and decreasing circulating TH levels experimentally in vivo alter density of GABA(A) receptor-binding sites for GABA and benzodiazepines in brain, but results vary from study to study, which may reflect important regional differences in the brain. There is substantial evidence that THs also have an extranuclear effect to inhibit GABA-stimulated Cl(-) currents by a non-competitive mechanism in vitro. The thyroid gland exhibits GABA transport mechanisms as well as enzyme activities for GABA synthesis and degradation, all of which are sensitive to thyroidal state. In rats and humans, GABA inhibits thyroid stimulating hormone (TSH) release from the pituitary, possibly by action directly on the pituitary or on hypothalamic thyrotropin-releasing hormone neurons. In mice, GABA inhibits TSH-stimulated TH release from the thyroid gland. Taken together, these studies provide strong support for the hypothesis that there is reciprocal regulation of the thyroid and GABA systems in vertebrates.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Effects of L-thyroxine on brain monoamines during parr-smolt transformation of Atlantic salmon (Salmo salar L.)

During spring, seaward migrating juvenile Atlantic salmon (Salmo salar) undergo parr-smolt transformation (PST) which involves changes in physiology, including one or two peaks in plasma thyroxine (T4). To investigate if changes in plasma T4 influence neural function, we measured levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and also measured serotonin (5-hydroxytryptamine, 5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in brain regions of two groups of Atlantic salmon parr on an 8:16 h light/dark photoperiod. One group was treated with ambient T4 to simulate the natural smolt peak in plasma T4. T4 treatment depressed DOPAC levels as well as DOPAC/DA and 5-HIAA/5-HT ratios in the olfactory system but with no changes in the optic tectum. We conclude that during PST monoaminergic functions in specific brain regions of juvenile Atlantic salmon are affected by T4 treatment.

Glucose transport in developing rat brain: glucose transporter proteins, rate constants and cerebral glucose utilization

Developing rat brain undergoes a series of functional and anatomic changes which affect its rate of cerebral glucose utilization (CGU). These changes include increases in the levels of the glucose transporter proteins, GLUT1 and GLUT3, in the blood-brain barrier as well as in the neurons and glia. 55 kDa GLUT1 is concentrated in endothelial cells of the blood-brain barrier, whereas GLUT3 is the predominant neuronal transporter. 45 kDa GLUT1 is in non-vascular brain, probably glia. Studies of glucose utilization with the 2-14C-deoxyglucose method of Sokoloff et al., (1977), rely on glucose transport rate constants, k1 and k2, which have been determined in the adult rat brain. The determination of these constants directly in immature brain, in association with the measurement of GLUT1, GLUT3 and cerebral glucose utilization suggests that the observed increases in the rate constants for the transport of glucose into (ki) and out of (k2) brain correspond to the increases in 55 kDa GLUT1 in the blood-brain barrier. The maturational increases in cerebral glucose utilization, however, more closely relate to the pattern of expression of non-vascular GLUT1 (45 kDa), and more specifically GLUT3, suggesting that the cellular expression of the glucose transporter proteins is rate limiting for cerebral glucose utilization during early postnatal development in the rat.

Developmental expression of GLUT1 and GLUT3 glucose transporters in rat brain

Two glucose transport proteins, GLUT1 and GLUT3, have been detected in brain. GLUT1 is concentrated in the endothelial cells of the blood-brain barrier and may be present in neurons and glia; GLUT3 is probably the major neuronal glucose transporter. Of the few studies of glucose transport in the immature brain, none has quantified GLUT3. This study used membrane isolation and immunoblotting techniques to examine the developmental expression of GLUT1 and GLUT3 in four forebrain regions, cerebral microvessels, and choroid plexus, from rats 1-30 days postnatally as compared with adults. The GLUT1 level in whole brain samples was low for 14 days, doubled by 21 days, and doubled again to attain adult levels by 30 days; there was no regional variation. The GLUT3 level in these samples was low during the first postnatal week, increased steadily to adult levels by 21-30 days, and demonstrated regional specificity. The concentration of GLUT1 in microvessels increased steadily after the first postnatal week; the GLUT1 level in choroid plexus was high at birth, decreased at 1 week, and then returned to near fetal levels. GLUT3 was not found in microvessels or choroid plexus. This study indicates that both GLUT1 and GLUT3 are developmentally regulated in rat brain: GLUT1 appears to relate to the nutrient supply and overall growth of the brain, whereas GLUT3 more closely relates to functional activity and neuronal maturation.

Stimulation of acetylcholinesterase activity by triiodothyronine in the brain of Singi fish, Heteropneustes fossilis (Bloch)

Three consecutive days of injections of triiodothyronine (T3)(0.038, 0.075, 0.15 and 1.54 nmoles/g) significantly elevated the acetylcholinesterase (AchE) activity in the brain of Singi fish, Heteropneustes fossilis (Bloch). The higher doses of 0.075, 0.15 and 1.54 nmoles of T3/g induced a greater increase in enzyme activity than 0.038 nmoles/g. A T3 dose of 0.019 nmoles/g was found to be ineffective. The T3 action on AchE activity was blocked by cycloheximide. Thiourea treatment for 30 days decreased the AchE activity below the control level. This reduced level of the enzyme activity was brought back even above the control level by T3 injections. It is, therefore, suggested that thyroid hormone is involved in the sustenance of AchE activity in fish brain.

Influence of soluble environmental factors on the development of fetal brain acetylcholinesterase-positive neurons cultured in a chemically defined medium: comparison with the effects of L-triiodothyronine (L-T3)

In cerebral hemisphere neuronal cultures derived from 15-day-old rat embryos, the addition of L-triiodothyronine (L-T3) or nerve growth factor (NGF) enhanced the expression of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities in a dose-dependent manner. When cultures were supplemented with both agents at maximal effective concentrations, the stimulation in ChAT and AChE activities was significantly greater than the sum of the individual effects. Conversely, when the cultures were exposed to astrocyte conditioned medium grown in the presence or absence of L-T3 (CM + L-T3 or CM-L-T3). laminin and fibroblast growth factor (FGF), ChAT and AChE activities were not stimulated above those of control cultures when added alone or in combination with L-T3. Furthermore, L-T3, NGF, CMs, laminin and FGF did not affect AChE+ cell survival, but significantly increased neurite outgrowth and branching with NGF and L-T3 being the most powerful agents followed by CMs, laminin and FGF. Additionally, the simultaneous addition of L-T3 with either laminin or FGF in culture, caused an additive effect of L-T2 in the neurite density of AChE+ cells with both agents. This study shows that (1) thyroid hormones do not act through the regulation of soluble neurotrophic factors produced by astroglial cells, (2) thyroid hormones interact with the effect of NGF on ChAT and AChE activities, (3) the regulation of ChAT and AChE activities and the neurite outgrowth are independently regulated. and (4) the regulation of ChAT and AChE activities is very specific compared with that of neurite outgrowth.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors determining the differential tissue response of Na/K-ATPase to thyroid hormone in the neonatal rat

The concentration of ouabain binding sites (OBS), a measure of the abundance of sodium/potassium-dependent ATPase (Na/K-ATPase), was measured in cerebral cortex (CC), cerebellum (CBL), brown adipose tissue (BAT), and heart of neonatal rats with congenital hypothyroidism. In euthyroid rats, the concentration of OBS was 50- to 100-fold greater in the nervous tissue than in the nonnervous tissue. Congenital hypothyroidism resulted in a significant reduction in the number of OBS in all four tissues. Although in absolute terms (pmol/mg protein) the reduction was greater in the nervous tissues, in relative terms it was much greater in the nonnervous tissues. The restoration of OBS concentration was much more sensitive to T4 in CC, CBL, and BAT than in heart. In contrast, the response of OBS concentration to T3 in hypothyroid rats was greater in the heart, followed by the BAT and CBL, being minimal in CC. The sensitivity to T4 replacement correlated with the degree of the stimulation of the type II 5'deiodinase (5'D-II) by hypothyroidism, whereas the response to exogenous T3 correlated with the fraction of the tissue T3 that derives from plasma T3 and inversely with the plasma T3 concentration required to saturate 50% of the nuclear receptors as reported in previous studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical localization of thyroid hormone nuclear receptors in cultured acetylcholinesterase-positive neurons: a correlation between the presence of thyroid hormone nuclear receptors and L-tri-iodothyronine morphological effects

A monoclonal antibody against the rat liver L-tri-iodothyronine nuclear receptor and acetylcholinesterase cytochemistry were used for the localization of thyroid hormone nuclear receptors in acetylcholinesterase-positive cell nuclei in fetal rat cerebral hemisphere neuronal cultures. After 3 days in vitro, the ratio of acetylcholinesterase-positive cells that were immunoreactive for the thyroid hormone nuclear receptor to those not stained for this receptor (74-26%, respectively) remains unchanged despite an increase in the number of acetylcholinesterase-positive cells with time (from day 3 to day 21) in culture. Furthermore, the addition of 3 X 10(-8) L-tri-iodothyronine in culture did not modify this ratio or have an effect on the number of acetylcholinesterase-positive cells, but significantly increased the neurite density in those acetylcholinesterase-positive cells that were immunoreactive for the thyroid hormone receptor. Conversely, no difference in the neurite densities of those acetylcholinesterase-positive cells not stained for this receptor was observed when cultured in the presence or absence of thyroid hormone. In other experiments with the same fetal brain cultures, treatment of cultures for 8 days with L-tri-iodothyronine, beginning on culture day 20, demonstrated the presence of a critical period which occurs in vitro around day 20, since the stimulatory effect of L-tri-iodothyronine on immunoreactive acetylcholinesterase-positive cell neurite density is lost after 20 days in vitro. These results demonstrate, for the first time, the presence of L-tri-iodothyronine nuclear receptors in fetal rat acetylcholinesterase-positive neurons and the existence of a cellular heterogeneity in the distribution of the thyroid hormone receptor. The presence of these receptors in fetal brain acetylcholinesterase-positive neurons suggests that some effects of L-tri-iodothyronine on the maturation of a subpopulation of acetylcholinesterase-positive neurons may result from a direct effect of this hormone through an interaction with its specific nuclear receptors.

An interaction between thyroid hormone and nerve growth factor promotes the development of hippocampus, olfactory bulbs and cerebellum: a comparative biochemical study of normal and hypothyroid rats

The effects of treatment with L-thyroxine (T4;20 ng/g body weight, given subcutaneously on days 1, 3, 5, 7 and 9), 2.5 S nerve growth factor (NGF; 2 ng/mg brain weight, given intracerebroventricularly on days 1, 3, 5, 7 and 9), monoclonal anti-NGF (2 ng/mg wet weight, given intracerebroventricularly on days 1, 3, 5, 7 and 9), and monoclonal anti-NGF receptor (192 IgG; 2 ng/mg wet weight, injected daily from day 1 to day 9) antibodies, separately or together, were studied on the biochemical development of hippocampal formation, olfactory bulbs and cerebellum in 10-day-old and 15-day-old normal and hypothyroid rats. The results provide the following information: (1) CNS structures other than the basal forebrain are sensitive to NGF during early development. (2) Both normal and hypothyroid rats are more sensitive to NGF deprivation than NGF supplementation. (3) The effects of anti-NGF antibodies in normal rats are similar to those induced by anti-NGFr antibodies. (4) NGF alone had little or no effect, but interacts with T4 in promoting cell maturation, especially in hypothyroid rats. (5) Hypothyroid rats are more sensitive to T4 and to T4 plus NGF than are normal ones. (6) The synergistic action of both trophic factors, but not that of T4, tend to disappear at long term in hypothyroid rats. (7) The differential sensitivity of the brain areas to T4, NGF, or both trophic factors correlates with their cell acquisition rate, especially in hypothyroid rats. (8) T4 and NGF together act more markedly (but not exclusively) on the cholinergic structures in both normal and hypothyroid rats. (9) RNA appears to be very sensitive to NGF, especially in hypothyroid rats. In close correlation with preliminary morphological observations, the results clearly demonstrate that an interaction between T4 and NGF regulates the ontogeny of a number of neuronal structures in CNS independently of their neurotransmitter phenotype, but with a regional specificity. The possibilities of accounting for this interaction, in particular the major role of thyroxine, are discussed.

Postnatal changes in local cerebral blood flow measured by the quantitative autoradiographic [14C]iodoantipyrine technique in freely moving rats

The postnatal changes in local cerebral blood flow in freely moving rats were measured by means of the quantitative autoradiographic [14C]iodoantipyrine method. The animals were studied at 10, 14, 17, 21 and 35 days and at the adult stage. At 10 days after birth, rates of blood flow were very low and quite homogeneous in most cerebral structures except in a few posterior areas. From these relatively uniform levels, values of local cerebral blood flow rose notably to reach a peak at 17 days in all brain regions studied. Rates of blood flow decreased between 17 and 21 days after birth and then increased from weaning time to reach the known characteristic distribution of the adult rat. The postnatal evolution of local cerebral blood in the rat is in good agreement with previous studies in other species such as dog and humans that also show higher rates of cerebral blood flow and glucose utilization at immature stages. However, in the rat, local cerebral blood flow and local cerebral glucose utilization are not coupled over the whole postnatal period studied, since blood flow rates reach peak values at 17 days whereas glucose utilization remains still quite low at that stage. The high rate of cerebral blood flow in the 17-day-old rat may reflect the energetic and biosynthetic needs of the actively developing brain that are completed by the summation of glucose and ketone body utilization.

Influence of triiodothyronine (L-T3) on the morphological and biochemical development of fetal brain acetylcholinesterase-positive neurons cultured in a chemically defined medium

In cerebral hemisphere cultures initiated from 15-day-old rat embryos, the number of acetylcholinesterase-positive (AChE+) cells increased from 6.8 +/- 1.6 cells/well on day 3 to 112 +/- 16 cells/well on day 15. With time in culture, AChE+ cells increased both in size of the perikarya and neurite length. The addition of L-triiodothyronine (L-T3) at a concentration of 3 x 10(-8) M at the initiation of the culture had no effect on the number of AChE+ cells but significantly increased the size and neurite length of AChE+ neurons after 5 days in vitro. These morphological effects are associated with biochemical effects. L-T3 increased AChE activity in both a dose- and time-dependent manner (the stimulatory effect of L-T3 becomes significant between day 8 and day 15). Since a major part of AChE+ cells may be cholinergic neurons, we have also measured the effect of L-T3 on ChAT activity. L-T3 also increased ChAT activity in a dose and time dependent manner. Furthermore, treatment of cultures with L-T3 at different times in culture demonstrated the presence of a critical period which occurs in vitro around day 20, since the stimulatory effect of L-T3 on ChAT activity is lost after 20 days in vitro. Studies of the time necessary for L-T3 to increase both ChAT and AChE activities show that 2 days and 15 days, respectively, are required for L-T3 to significantly stimulate both enzyme activities. This in vitro analysis demonstrated the morphological effect of L-T3 on the size and the neurite length of AChE+ cells. These effects are associated with biochemical effects on ChAT and AChE activities. Thus, it appears that thyroid hormones regulate several steps of neuronal maturation.

Malic enzyme response to triiodothyronine in the brain of neonatal rats

Malic enzyme activity in the soluble fraction of the neonatal brain of hypothyroid rats was observed to be lowered as compared to that of the control animals (p less than 0.01). Administration of triiodothyronine to the neonates of control animals resulted in significant enhancement (p less than 0.001) in the activity of the Malic enzyme. Our studies show that brain malic enzyme which is involved in lipogenesis and hence in myelination responds to triiodothyronine in the early stage of life.

Effect of thyroid deficiency on the regional development of glutaminase, a glutamatergic neuron marker, in the rat brain

The effect of thyroid deficiency on the activity of phosphate-activated glutaminase (the marker for glutamatergic neurons) was studied in different parts of the rat brain at ages 5, 10, 15 and 25 days, and at day 130 following 102 days of rehabilitation. The brain regions investigated were the cerebral cortex, basal forebrain, hippocampus and cerebellum. During normal development, the activity of glutaminase increased relatively earlier in the cerebral cortex and hippocampus than in the cerebellum, while the absolute value reached a much higher level in the hippocampus than in other brain regions. In the basal forebrain, the developmental pattern of glutaminase was bimodal, and the rise in enzyme activity after 15 days coincided with the decrease in the cerebral cortex. These regional developmental changes in glutaminase activity correlated well with known information on the formation of glutamatergic cells and pathways in the brain. Neonatal thyroid deficiency had little effect on the developmental patterns of enzyme activity, the exception being a transient decrease in 10-day-old hypothyroid hippocampus. The present results, together with previous findings, indicate that the effect of thyroid hormone on neural maturation is cell-type specific and the glutamatergic neurons are not the main targets of thyroid hormone action.

Developmental pattern for phosphatidylserine decarboxylase in rat brain

In adult rats, a significant portion of brain ethanolamine glycerophospholipids are synthesized by a pathway involving phosphatidylserine decarboxylase, a mitochondrial enzyme. We have now examined whether this enzyme plays a particularly prominent role during development. Activities for both phosphatidylserine decarboxylase and succinate dehydrogenase (another mitochondrial enzyme) were determined in brain homogenates from rats 5 days of age to adulthood. Succinate dehydrogenase activity, expressed on a per unit brain protein basis, increased markedly during development. This pattern has been reported previously and is as expected from the postnatal increase in oxidative metabolism. In contrast, phosphatidylserine decarboxylase activity decreased 40% from 5 to 30 days of age. The apparent Km for brain phosphatidylserine decarboxylase was 85 microM in both young (8- and 20-day-old) and adult animals. Parallel studies in vivo were carried out to determine the contribution of the phosphatidylserine decarboxylase pathway, relative to pathways utilizing ethanolamine directly, to the synthesis of brain ethanolamine glycerophospholipids. Animals were injected intracranially with a mixture of L-[G-3H]serine and [2-14C]ethanolamine and incorporation into the base moieties of the phospholipids determined. The 3H/14C ratio of ethanolamine glycerophospholipids decreased about 50% during development. Our studies in vitro and in vivo both suggest that phosphatidylserine decarboxylase plays a significant role in the synthesis of brain ethanolamine glycerophospholipids at all ages, although it is relatively more prominent early in development.

Cholinergic neurotransmission in the central nervous system of the Snell dwarf mouse

An unequal decrease in cholinergic activity has been evidenced in discrete brain areas in the growth hormone, thyroid-stimulating hormone and prolactin deficient Snell dwarf mouse. The effect of the mutation's pituitary deficit on central cholinergic mechanisms appears to be selective: Normally high cholinergic activity areas such as striatum, olfactory tubercles, and hippocampus show strong alterations in this neurotransmitter system. Structures which appear earlier in ontogenesis are less affected, if at all. The lack of pituitary hormones seems to have effects on choline acetyltransferase activity and/or synthesis as well as on the development of high affinity (H.A.) cholinergic uptake mechanisms, both strongly defective in hippocampus and striatum. Therefore, a lower density of cholinergic terminals can be inferred. Furthermore, our observations are consistent with a close functional coupling of the choline H.A. transport and of subsequent choline acetylation. Acetylcholinesterase activity does not seem to be affected. Moreover, a compensatory effect at the postsynaptic level may have occurred due to developmental or functional plasticity for cholinergic responsiveness. In conclusion, the dwarf mouse seems to be a useful model for a better understanding of the influences of growth hormone and thyroid hormones on the development of central cholinergic mechanisms. It also provides the possibility to attempt a functional restoration of the deficient cholinergic neurotransmission and the behavioral disturbances which may be linked to them, by hormone replacement.

Comparison of iodothyronine 5'-deiodinase and other thyroid-hormone-dependent enzyme activities in the cerebral cortex of hypothyroid neonatal rat. Evidence for adaptation to hypothyroidism

Recent studies have shown that approximately 75% of the nuclear 3,5,3'-triiodothyronine (T(3)) present in adult rat cerebral cortex (Cx) derives from 5'-deiodination of thyroxine (T(4)) within this tissue. The activity of iodothyronine 5'-deiodinase (I 5'D), the enzyme catalyzing T(4) to T(3) conversion, increases rapidly after thyroidectomy, suggesting that this could be a compensatory response to hypothyroxinemia. To evaluate this possibility during the period of central nervous system maturation, we studied several thyroid hormone-responsive enzymes (aspartic transaminase [AT], succinic dehydrogenase [S.D.], and Na/K ATPase) in the Cx of 2-, 3-, and 4-wk-old rats. The rats were made congenitally hypothyroid by placing 1, 2, 5, and 20 mg methimazole (MMI) in 100 ml of the mothers' drinking water from day 16 of gestation throughout the nursing period and to the litters after weaning. In addition, serum thyroid hormones, I 5'D, and, in some experiments, in vivo T(4) to T(3) conversion in Cx were measured in the same pups. Serum T(4) concentrations varied from <1 to 40 ng/ml and were generally inversely related to maternal MMI dose. Serum T(3) was less affected by MMI than was T(4). At 2 wk, decreases in AT, S.D., and ATPase were present in the 20-mg-MMI group but not in the 5-mg-MMI pups despite low serum T(4) (<10 ng/ml) in the latter. At 3 and 4 wk, both 5- and 20-mg-MMI groups had significant reductions in these cortical enzymes despite a normal serum T(3) in the 5-mg-MMI rats. Cortical I 5'D activity was 10-fold the control value in 5- and 20-mg-MMI animals at 2 wk but increased only three- to fivefold at 3 and 4 wk. I 5'D correlated inversely with serum T(4) (r >/= 0.96) at all ages, but the less marked elevation of this enzyme in 3- and 4-wk-old pups was not accompanied by an increase in serum T(4). Serum T(3) increased or remained the same between 2 and 3 wk. These results suggested that the 10-fold increase in I 5'D at 2 wk protected the 5-mg-MMI group from tissue hypothyroidism, but that the three- to fivefold increase at 3 and 4 wk could not. Injection of approximately 250 ng T(4)/100 g body weight to 2-wk-old, 20-mg-MMI pups (one-sixth the normal T(4) production rate) led to both a 1.8-ng/g cortical tissue increment in cortical T(3) and a significant increase in AT at 24 h, compared with a 0.38-ng/g cortical tissue T(3) increment and no change in AT in euthyroid controls. The larger increment in T(3) of the 20-mg-MMI pups was due in great part to increased fractional T(4) to T(3) conversion. Although the latter resulted in greater serum T(3) concentrations, three-fourths of the newly formed T(3) in the cortex was generated in situ, and it was blocked by iopanoic acid as was the increase in AT. We conclude that 70-80% of the T(3) in the Cx of the neonatal rat is produced locally. Serum T(4) appears to serve both as a precursor for T(3) and as a critical signal for increases in cortical I 5'D. The increased I 5'D can result in normal or near-normal cerebrocortical T(3) concentrations despite marked reductions in serum T(4). This mechanism seems to be particularly effective around 2 wk of age when many thyroid-hormone-dependent maturational changes occur in the rat Cx.

Triodothyronine enhancement of neuronal differentiation in aggregating fetal rat brain cells cultured in a chemically defined medium

Triiodothyronine (30 nM) added to serum-free cultures of mechanically dissociated re-aggregating fetal (15-16 days gestation) rat brain cells greatly increased the enzymatic activity of choline acetyltransferase and acetylcholinesterase throughout the entire culture period (33 days), and markedly accelerated the developmental rise of glutamic acid decarboxylase specific activity. The enhancement of choline acetyltransferase and acetylcholinesterase specific activities in the presence of triiodothyronine was even more pronouned in cultures of telencephalic cells. If triiodothyronine treatment was restricted to the first 17 culture days, the level of choline acetyltransferase specific activity at day 33 was 84% of that in chronically treated cultures and 270% of that in cultures receiving triiodothyronine between days 17 and 33, indicating that relatively undifferentiated cells were more responsive to the hormone. Triiodothyronine had no apparent effect on the incorporation of [3H]thymidine at day 5 or on the total DNA content of cultures, suggesting that cellular differentiation, rather than proliferation was affected by the hormone. Our findings in vitro are in good agreement with many observations in vivo, suggesting that rotation-mediated aggregating cell cultures of fetal rat brain provide a useful model to study thyroid hormone action in the developing brain.

An electron microscopic study of the developing caudate nucleus in euthyroid and hypothyroid states

Thyroid hormone exerts a powerful influence on CNS growth and maturation. Hypothyroidism early in life has long been known to cause disturbances in innate behavior, motor performance, severe and frequently irreversible mental retardation. In this deficiency, depressed caudate neurogenesis, cell migration and neuropil development during the rapid period of CNS growth may contribute to the clinical picture of perceptual handicaps often seen in cretins. Light microscopic and Golgi studies of the developing caudate nucleus in thyroid deficiency have been carried out to help attain insights into the mechanisms whereby the extrapyramidal system regulates motor function. The ultrastructural study of caudate nuclear cytogenesis and synaptogenesis in normal and hypothyroid states provides more detailed information for further analysis of the problem. Hypothyroidism was induced from birth by adding prophylthiouracil to the food and drinking water of lactating dams. Linear development of the caudate nucleus of both normal and hypothyroid rats at ages 8, 14, 20, 30 and 42 days was studied by electron microscopy. Thyroid glands were examined by light microscopy to assess the normal and deficient states. Immature cells, primitive processes and synapses were the characteristic features of the 8-day-old normal caudate nucleus. Distinctively wide cisternae of the rough endoplasmic reticulum, loosley packed Golgi apparatus and chromatin clumps throughout the nuclei of the neurons were significant early morphologic variations. The dramatic cytoarchitectural maturation in the 14- and 20-day normal caudate neuropil points to the rapidity of developmental rate. After the growth spurt of the first three weeks a maturational plateau occurs which is characterized by well-formed neuronal cytoplasmic organelles, myelinated and non-myelinated axons, axon terminals, dendrites and their spines, and synapses. Thyroid deficiency causes a marked maturational delay of approximately 7 days in caudate neuronal proliferation, the elaboration of neuronal networks and the attainment of mature synaptic contents and membranes. This delay is evidenced by comparison of the structural similarities between 8-day-old normal and 14-day-old deficient rats; and additional comparisons between the 14-day-old normal and 20-day-old hypothyroid rats. A rapid "catch up" process in fine structural morphogenesis takes place in the period between days 14 and 30 in the deficient animals. Repression of thyroid function does not entirely prevent development of the caudate nucleus but allows a fairly extensive, though critically incomplete degree of maturation. This imperfection is manifested by a decrease in the number of synaptic contacts that persists even after the rapid "catch up" phenomenon of caudate synaptogenesis.

The effects of early hypo- and hyperthyroidism on the development of rat cerebellar cortex. III. Kinetics of cell proliferation in the external granular layer

The effects of early hypo- and hyperthyroidism on the rates of cell acquisition and proliferation have been studied in the external granular layer (EGL) of the developing rat cerebellar cortex at 10 days of age using quantitative autoradiographic methods. Both altered thyroid states reduce the rate of cell acquisition in the EGL, but appear to do so for different reasons. Hyperthyroidism shortens the average length of the cell cycle by decreasing the duration of the pre-DNA synthetic phase (G1), indicating that excess thyroxine may exert a direct effect on the EGL. This action involves the early onset of neuronal differentiation (cessation of proliferation)46 which presumably leads to the observed decrease in the rate of cell acquisition (increased doubling time). Such differentiating cells do not, however, leave the proliferative zone or the EGL prematurely, resulting in a reduced labeling index, mitotic index, and growth fraction as non-dividing cells dilute the proliferating cell population. Hypothyroidism, on the other hand, leads to no significant change in the length of the cell cycle or in the mitotic index, but causes a decreased labeling index and growth fraction, as well as a reduced rate of cell acquisition (increased doubling time). No significant change in the amount of cell death in the EGL could be found to explain this apparent discrepancy between the rate of cell proliferation (cell cycle length) and cell acqusiition. The answer to this puzzle appears to lie in the mitotic index, which is not affected to the same extent as the labeling index, although it is also slightly reduced. If cells were to remain longer in mitosis, this could result in a decreased labeling index and growth fraction but nearly normal mitotic index and cell cycle length (as measured using the % labeled mitoses method), since those cells dropping out of the cycling population would be counted as mitoses...

Aspects of biochemical differentiation in the central nervous system

A demonstration of cell-specific patterns of development in the immature CNS is provided by examples of characteristic, cell-specific time-courses of enzyme development in different classes of brain cells isolated in highly purified form by bulk-separation from the cerebral and cerebellar cortex of the growing rat. The enzymatic analysis was carried out at the level of the nerve and glial cell lysosomes and mitochondria, two subcellular organelles crucial to the economy of all cells. The findings reveal rather similar developmental patterns for the lysosomal hydrolase N-acetyl-beta-D-glucosaminidase in neurons and glial cells of the cerebral cortex as well as in two different cerebellar nerve cell types, the Purkinje and the granule cell. However, significant differences in the post-natal chronology of development of the mitochondrial enzyme alpha-glycerophosphate dehydrogenase were noted between cortical nerve and glial cells, the glial enzyme exhibiting 6-fold higher levels of activity than the neuronal one throughout the first month of postnatal life. The findings emphasize the feasibility as well as the necessity of studies aimed at the elucidation of the cell-specific aspects of the biochemistry of developing nerve and glial cells.