Effects of thyroxine on postnatal cell acquisition in the rat brain

Neuropsychiatric aspects of pediatric thyrotoxicosis

The neurobehavioural and neuropsychiatric changes associated with thyrotoxicosis are multiple and varied. This association is well recognised although the true incidence of neuropsychiatric symptoms in thyrotoxicosis is not known. Review of available literature suggests that frank psychiatric symptoms in thyrotoxicosis may be in the order of 10%. In pediatric thyrotoxicosis the neuropsychiatric symptoms may be pronounced and may antedate the medical diagnosis by six months to one year. One of the classic presentation is deterioration in school performance. Frequently noticed cognitive and behavioural abnormalities in pediatric thyrotoxicosis are hyperactivity, irritability or anxious dysphoria, and problems of attention. Successful treatment of thyrotoxicosis usually leads to resolution of the major mental disturbances associated with it and delayed treatment possibly results in enduring neuropsychiatric problems. Awareness of neuropsychiatric symptomatology can help in early detection and appropriate management of children with thyrotoxicosis.

Growth factors, glia and gliomas

The abilities of growth factors to cause normal cells to express the properties associated with transformed cells is discussed in specific reference to the oligodendrocyte-type-2 astrocyte (O-2A) progenitor cell. In the O-2A lineage, it has been possible to use growth factors and other defined molecules to induce or promote in normal cells all of the main properties of tumor cells, these being continued cell division in the absence of differentiation, more subtle modulations of self-renewal probabilities, promotion of cell migration and inhibition of programmed cell death. In addition to our studies on primary cells, our application to the growth of human tumor specimens of techniques utilized to study primary glial progenitor cells has allowed us to isolate a human glioblastoma multiforme (GBM)-derived population that expresses many properties otherwise uniquely expressed by oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. Hu-O-2A/Gb1 (for Human O-2A lineage Glioblastoma number 1) cells responded to similar mitogens and differentiation modulators as rodent O-2A progenitors, and generated cells with features of precursor cells, oligodendrocytes and astrocytes. Moreover, 1H-NMR analysis of amino acid composition demonstrated a striking conservation of types and quantities of free amino acids between the human tumour cells and the rodent primary cells. Hu-O-2A/Gb1 cells represent the first human glioma-derived population for which unambiguous lineage assignment has been possible. Our results thus demonstrate that the human O-2A lineage can contribute to one of the most malignant of glial tumours. Our analyses further indicate that at least two distinct glial lineages can generate glioblastomas. In addition, the highly diagnostic 1H-NMR spectrum expressed by Hu-O-2A/Gb1 cells raises the possibility of eventual non-invasive identification of tumors of this lineage.

A novel role for thyroid hormone, glucocorticoids and retinoic acid in timing oligodendrocyte development

The timing of oligodendrocyte differentiation is thought to depend on an intrinsic clock in oligodendrocyte precursor cells that counts time or cell divisions and limits precursor cell proliferation. We show here that this clock mechanism can be separated into a counting component and an effector component that stops cell proliferation: whereas the counting mechanism is driven by mitogens that activate cell-surface receptors, the effector mechanism depends on hydrophobic signals that activate intracellular receptors, such as thyroid hormones, glucocorticoids and retinoic acid. When purified oligodendrocyte precursor cells are cultured at clonal density in serum-free medium in the presence of mitogens but in the absence of these hydrophobic signals, the cells divide indefinitely and do not differentiate into postmitotic oligodendrocytes. In the absence of mitogens, the precursor cells stop dividing and differentiate prematurely into oligodendrocytes even in the absence of these hydrophobic signals, indicating that these signals are not required for differentiation. The levels of these signals in vivo may normally regulate the timing of oligodendrocyte differentiation, as the maximum number of precursor cell divisions in culture depends on the concentration of such signals and injections of thyroid hormone into newborn rats accelerates oligodendrocyte development. As thyroid hormone, glucocorticoids and retinoic acid have been shown to promote the differentiation of many types of vertebrate cells, it is possible that they help coordinate the timing of differentiation by signalling clocks in precursor cells throughout a developing animal.

Effects of altered thyroid states and undernutrition on the calbindin-D28K (calcium-binding protein) content of the hippocampal formation in the developing rat

A quantitative study of calbindin-D28K (calcium-binding protein) was carried out on the developing hippocampal formation in normal, hypothyroid, hyperthyroid, and underfed rats. In normal animals, the calbindin-D28K content increased after birth in agreement with the distribution of the protein previously reported by immunocytochemistry. Calbindin-D28K was strikingly spared, compared to the other proteins, from the effects of hypothyroidism. On the contrary, the calbindin-D28K:protein ratio was transiently reduced by hyperthyroidism. Corrective doses of thyroxine to hypothyroid rats increased the calbindin-D28K content whatever the period of the hormonal treatment, but they also had a marked effect on the hippocampal weight and the protein content, especially when the hormone was given on days 2-3. With this latter replacement therapy schedule, the calbindin-D28K:protein ratio dropped from the high value of the hypothyroid animal to normal. Taken together, the results obtained in hypothyroidism, hyperthyroidism and replacement therapy are consistent with a pronounced action of thyroid hormone on hippocampal structures other than those containing calbindin-D28K. Undernutrition, which, like hypo- or hyperthyroidism, also markedly impairs hippocampal growth, affected the calbindin-D28K content per hippocampus but not the calbindin-D28K:protein ratio. This emphasizes the unique influence of thyroid hormone on brain development. The relative preservation of calbindin-D28K in the hippocampal formation of animals lacking thyroid hormone suggests that calbindin-D28K function may be crucial in this brain region.

Developing rat cerebellum--II. Effects of abnormal thyroid states and undernutrition on hyaluronic acid

The early postnatal pattern of hyaluronic acid (HA) deposition in rat cerebellum is affected by thyroid deficiency, thyroxine treatment and undernutrition. The modification of HA ontogenesis apparently reflects the smaller number of cells formed in undernourished rats, or alterations of cell maturation (accelerated in thyroxine-treated and slowed down in thyroid-deficient rats). The developmentally regulated loss of tissue water is also affected in the three conditions; this can be correlated with the roughly simultaneous disappearance of extracellular, but not of total, HA.

Effects of mild hyperthyroidism on levels of amino acids in the developing Lurcher cerebellum

This study examines the question of whether intrinsically defective mutant Lurcher Purkinje cells, which degenerate during postnatal weeks two to five, followed by later loss of granule cells are competent to respond to neonatal hyperthyroidism, which is known to cause premature differentiation of Purkinje cells and an acceleration of the peak of proliferation in granule cells in normal rodent cerebellum. Both total amounts and concentrations (per mg wet weight) of Tau, Glu, Asp and GABA were assayed as markers of cell function in Lurcher and wild-type mice made very mildly hyperthyroid by feeding nursing dams high-thyroxine food. Tau, which is present in relatively high concentrations in Purkinje cells, was affected by hyperthyroid treatment in the Lurcher in a manner that is most consistent with an acceleration of the degenerative process in Purkinje cells. The acidic amino acids Glu and Asp show later changes and response to hormone which seem to be a reaction to the Purkinje cell pattern, probably in the granule cells. We conclude that the Lurcher cerebellum is particularly sensitive to thyroid hormone, and that it responds to low levels of hyperthyroidism in a distinct way.

Role of thyroid hormone and nerve growth factor in the development of choline acetyltransferase and other cell-specific marker enzymes in the basal forebrain of the rat

The effects of treatment with L-thyroxine (subcutaneously 0.3 microgram/g body weight daily from birth, i.e., day 1) and 2.5S nerve growth factor (NGF; intraventricularly 2 micrograms on 1, 3, 5, 7, and 9 postnatal days), separately and together, were studied on the biochemical development of different cell types in the basal forebrain of 10-day-old rats. The development of cholinergic, gamma-aminobutyric acid-ergic (GABAergic), and glutamatergic neurons was monitored respectively in terms of choline acetyltransferase (ChAT), glutamate decarboxylase (GAD), and glutaminase activities, whereas glutamine synthetase (GS) and 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNPase) activities were used to judge the maturation of astroglial and oligodendroglial cells. Treatment with either thyroid hormone or NGF from birth significantly increased the expression of ChAT activity in the basal forebrain of neonatal rats. When both agents were administered to the same animal, in agreement with our earlier in vitro findings, the stimulation in ChAT activity was much greater than the sum of the individual effects. In hypothyroid rats, significant effects of NGF at the low doses used were not detectable, although the increase of ChAT activity induced by thyroxine was potentiated by NGF in these animals. Under the present experimental conditions neither thyroxine nor NGF treatment had an appreciable effect on the activities of glutaminase, GS, and lactate dehydrogenase. However, the administration of thyroxine markedly increased CNPase activity in normal rats, whereas in hypothyroid rats the effect on both CNPase and GAD was also significant. Similar elevations in CNPase and GAD activities were not observed after NGF treatment, suggesting that the effect of NGF was specific to the cholinergic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone modulates the expression of a neurofilament antigen in the cerebellar cortex: premature induction and overexpression by basket cells in hyperthyroidism and a critical period for the correction of hypothyroidism

Neurofilament expression by basket cells of the cerebellar cortex is suppressed in hypothyroidism. By using a monoclonal antibody (mabN210) that selectively recognizes an epitope associated with the 210-kDa neurofilament subunit, we have explored the relationship between thyroid hormone levels and basket cell maturation. In animals rendered hypothyroid by inclusion of propylthiouracil in the maternal drinking water from embryo age E17, there is a complete absence of mabN210 immunoreactivity in the basket cell axons, while the other immunoreactive axons in the cerebellar cortex, primarily Purkinje cell axons and mossy fibers, are apparently unaffected. This deficit can be corrected by treatment with thyroid hormone but there seems to be a critical period for full recovery, for animals treated from birth recover normally whereas there is a gradual diminution in the efficacy of treatment the later it begins. Thyroid hormone therapy begun after postnatal day 30 (P30) leads only to very minor recovery. By contrast, animals on a hyperthyroid regime show premature mabN210-antigen induction in the basket cells and supranormal levels of expression at P25, despite the severe reduction in the number of basket cell somata. This suggests either abnormal compensatory sprouting of axon collaterals by the remaining basket cells or the occurrence, during normal cerebellar corticogenesis, of competition between basket cell axons for a limited number of Purkinje cell targets followed by the elimination of the excess collaterals.

Fetal brain development in response to iodine deficiency in a primate model (Callithrix jacchus jacchus)

The common cotton-eared marmoset (Callithrix jacchus jacchus) has been used for the first time as a primate model to study the effects of dietary iodine deficiency on fetal brain development. Paired male and female marmosets were fed a low-iodine diet of maize, peas, meat meal, Torula yeast, maize oil and added vitamins, minerals and amino acids for 6 months before mating. Offspring from first and second pregnancies were compared with offspring from control marmosets fed the same diet but supplemented with iodine. Severe iodine deficiency in the fetus at birth was evident by reduced plasma thyroxine levels, increased plasma thyroid stimulating hormone levels, increased thyroid weight and reduced thyroid iodine content. Thyroid histology revealed hyperplasia, hypertrophy and absence of colloid material in the follicles. Iodine deficiency caused a reduction in the weight of the fetal brain and in particular the cerebellum. Brain cell number was reduced in the cerebellum and brainstem but cell size was reduced in the cerebral hemispheres. Histology of the brain revealed morphological changes in the cerebellum and cerebral hemispheres. In the-cerebellum there was: an increase in the thickness of the external germinal layer indicative of impaired cell acquisition; a decrease in total area; a decrease in molecular layer area; and an increase in Purkinje cell (Pc) linear density due to a reduction in the length of the Pc line. The decrease in molecular layer area and increase in Pc linear density imply diminished ascending and lateral extension of Pc dendrites. Changes in the cerebral hemispheres consisted of an increase in the density of neuronal cell bodies in the granular band and a decrease in synaptic counts in the layer between the pia mater and supragranular band of the visual cortex. Offspring from second pregnancies compared to those from first pregnancies were more severely affected and associated with lower plasma levels of maternal and fetal thyroxine. These findings indicate the importance of maternal and fetal thyroid function in relation to fetal brain development in the primate.

L-triiodothyronine stimulates growth by means of an autocrine factor in a cultured growth-hormone-producing cell line

L-Triiodothyronine (T3) stimulates DNA synthesis and replication of cultured GC cells, a T3-responsive growth hormone (GH)-secreting cell line. To determine whether T3 stimulates secretion of an autocrine growth factor, we compared the growth-promoting activity of medium conditioned by T3-stimulated and T3-depleted cells to that of unconditioned medium. Addition of polyclonal rabbit anti-T3 serum to T3-containing media decreased cellular T3 content by 50-70%. In unconditioned medium, anti-T3 serum decreased T3-induced cell growth and GH production by 40-70%. In conditioned medium, anti-T3 serum also effected a 45-70% decrease in induction of GH secretion but did not attenuate the growth-promoting activity. Growth-promoting activity was not detected in medium conditioned by T3-depleted cells. Thus, conditioned medium from T3-containing GC cell cultures contains growth-promoting activity that is independent of T3. Further, the induction of GC cel growth by T3 may occur, at least in part, by induction of an autocrine growth factor.

Effects of morphine on DNA synthesis in neonatal rat brain

Several observations have led to the hypothesis that endogenous opioids may modulate the growth and development of the brain. In the present study, we have examined the effect of morphine on the incorporation of [3H]thymidine into the DNA of neonatal rat brains in vivo and in vitro. We have found that morphine, when administered to one-day-old rats, inhibited [3H]thymidine incorporation into brain DNA in a long-lasting, naloxone-reversible manner. Morphine inhibited DNA synthesis in animals one and 4 days of age but not in older animals. This effect was tissue-specific, and did not appear to be due simply to respiratory depression or decreased availability of precursor to the brain. Naloxone, when administered acutely, or naltrexone, chronically, had no effect on [3H]thymidine incorporation, indicating that endogenous opioids do not tonically depress DNA synthesis. When neonatal brain tissue was incubated with morphine in vitro. [3H]thymidine incorporation values were not different from controls. These data indicate that the effect of morphine on DNA synthesis in vivo may be an indirect one, rather than a direct action on proliferating cells.

Thyroid hormone and development of the rat hippocampus: cell acquisition in the dentate gyrus

A quantitative autoradiographic histological study was carried out to examine mechanisms underlying the reduction in the rates of growth and of cell acquisition, including that of granule cells, in the dentate gyrus of hypothyroid rats. Thyroid deficiency in early life had no effect on the replication of intrinsic cells present in the polymorph and granular layers. The pyknotic index was also normal in the "proliferative zone", polymorph layer and granule cell layer, indicating that thyroid hormone had no effect on the survival of replicating, migrating or maturing granule cells. By contrast, the arrival of migrating cells from the "proliferative zone" to the granular layer was severely retarded in thyroid deficiency. This deficit was rapidly restored after a physiological dose of thyroxine given to hypothyroid rats. The present findings are consistent with our previous proposal that the role of thyroid hormone in the formation and/or the maintenance of nerve cells is related to changes in either cell migration or maturation, rather than to alterations in the replication of germinal cells.

Selective suppression of neurofilament antigen expression in the hypothyroid rat cerebral cortex

As an integral component of the cytoskeleton neurofilaments play a central role in the establishment and maintenance of neuronal form. In particular, high neurofilament concentrations are characteristic of many classes of axons in the central nervous system. Isolated neurofilaments from rat brain consist of 3 distinct polypeptides with apparent molecular weights 210K, 160K and 68K. A murine monoclonal antibody, mabN210, has been produced which specifically recognizes an epitope associated with the high molecular weight subunit and this antibody has been used to explore the regulation of neurofilament expression during brain development. It has been shown that in the rat cerebellar cortex, the expression of mabN210-immunoreactivity in basket cell axons is severely suppressed in hypothyroidism while neurofilament antigen expression in other cerebellar axons seems not to require thyroid hormones. In view of the well-known cortical deficits in hypothyroidism, these studies have now been extended to include the developing rat cerebral cortex and selected cortical afferent and efferent axons. In hypothyroid rats there is a marked suppression of mabN210-immunoreactivity in the cerebral cortex and corpus callosum and, to a lesser extent, there is a reduction in staining in the internal capsule. By contrast, hypothyroidism did not reduce mabN210-immunoreactivity in the lateral olfactory tract or the stria medullaris. In rats, serum thyroid hormone starts to rise to adult levels on postnatal day 4. It appears that axons that have attained their mature distribution prior to the onset of thyroid hormone expression are not affected by hypothyroidism whereas mabN210-immunoreactivity is suppressed in those axonal tracts that reach a mature distribution after P4.

Effect of thyroid state on histamine H1 receptors in adult and developing rat brain

The effect of thyroid status on histamine H1 receptors in adult and developing rat brain was investigated using the (3H) mepyramine binding assay. Hypothyroidism induced by treatment with 6-n-propyl-2-thiouracil resulted in a 31% decrease in the density and total content of adult rat brain (3H) mepyramine binding sites and a significant retardation of the developmental increase in H1 receptor binding in neonates. At 30 days of age, when euthyroid rats reached binding levels of the adult, hypothyroid animals presented reductions of 22 and 39% in (3H) mepyramine bound per unit weight and per brain respectively. In contrast, hyperthyroidism induced by treatment with L-thyroxine did not alter H1 receptor numbers in the adult rat brain but accelerated the developmental increase in (3H) mepyramine bound per unit weight that reached normal adult levels by 21 days of age. The results suggest that thyroid dysfunction during early life and adulthood may cause derangements of the histaminergic system in the brain.

Thyroid hormones reversibly suppress somatostatin secretion and immunoreactivity in cultured neocortical cells

Thyroid hormone effects on brain somatostatin-like immunoreactivity (SRIF-LI) were studied in an in vitro model system. Serum was removed from the nutrient culture medium of fetal day-18 rat cerebral cortex cells maintained in primary, long-term, dispersed monolayer culture. Chronic administration of either T3 or T4 in serum-free medium was associated with suppressed release of SRIF-LI into the culture medium (36-43 h accumulation), cell content of peptide and acute release in response to potassium-induced depolarization. Suppression was dose-dependent with an IC50 of less than 1 nM for T3. The most dramatic effects were observed for K+-induced release. Thirty-five to 50% suppression was typically observed with T3 at a near maximum dose (3 nM). Reverse T3 and diiodotyrosine were less potent and effective than T3. TRIAC and diiodothyronine also possessed significant suppressive activity. T3 suppression of release depended on duration of pretreatment. Administered for less than 16 h, T3 failed to significantly suppress K+-induced release, but significant suppression was observed for pretreatment periods of 16 h or longer. Indirect fluorescent immunohistochemical examination revealed a reduction in the number of cells positively stained for SRIF-LI in T3-treated dishes relative to controls. Upon removal of T3 and subsequent recovery in serum supplemented medium for 24 h, T3-treated and control cells exhibited similar levels of SRIF-LI release and cell content. T3-treated and control cells incorporated [3H] leucine into trichloracetic acid precipitable counts to similar extents. Dexamethasone and several sex steroids failed to modify the effects of T3 and did not independently influence SRIF-LI levels. Acute cycloheximide administration did not reverse T3 effects. The data indicate that primary brain cell cultures may be useful models to examine direct peripheral hormone actions on nervous tissue. Thyroid hormones suppress SRIF-LI levels in a dose, time and structure-dependent manner, which appears to be reversible. The findings are consistent with a possible integration of peripheral hormone and brain peptide physiology.

Cellular location of cytosolic triiodothyronine binding protein in primary cultures of fetal rat brain

The evolution of a cytosolic triiodothyronine (T3) binding protein was studied in primary cultures of fetal rat brain. These cultures exhibited neuronal characteristics during the first week. T3 binding activity in cell supernatants increased during this period from 39 +/- 7 (mean +/- SD) to 159 +/- 24 fmoles T3/culture flask. A similar increase was observed in the soluble proteins. After day 8, neuronal death occurred and glial cells multiplied and differentiated. On day 11 an 86% drop in the binding activity was observed (24 +/- 7 fmoles T3/culture flask); the pool of soluble proteins remained stable. Scatchard analysis revealed two types of binding site in both 7- and 14-day cultured cell cytosols. Binding affinities were similar in both cytosols (KA1 approximately 1.5 X 10(9) M-1, KA2 approximately 1 X 10(8) M-1); in contrast, the number of sites was 4-fold smaller in 14-day cytosols. In subcultures mostly composed of glial cells, almost the same affinities were measured, but the numbers of both types of sites were 20 times smaller than in 7-day cells. These results show that in cell cultures from embryonic rat telencephalon, cytosolic T3 binding protein is mainly located in the neurons.

Perinatal age and sex variations of the triiodothyronine nuclear receptors in the chick pectoralis major muscle

The ontogenesis of the nuclear triiodothyronine receptors was determined in the pectoralis muscle of male and female chicken at 18 days in ovo and 0, 3, 6, 14 and 35 days ex ovo. Our results show the presence of putative T3 nuclear receptors with equilibrium dissociation constant values (Kd approximately 5.50 X 10(-10) M) in good agreement with these reported in other tissues. The T3 receptor numbers decrease from 18 days incubation to 6-day-old animals, then increase until 35 days of age. Compared to the already reported levels of thyroid hormone in plasma and tissues, the results seem to correspond to a down-regulation of the muscle T3 receptors. The nuclear binding capacity of T3 was higher in females than in males, which could be related to the known effects of various sex steroid hormones on the T4 to T3 tissue conversion.

Effects of thyroxine on methionine adenosyltransferase activity in rat cerebral cortex and cerebellum during postnatal development

Methionine adenosyltransferase (MAT) activity was evaluated in cerebral cortex and cerebellum in controls and in rats treated with thyroxine. In controls the enzyme showed a different pattern in cerebral cortex and cerebellum during neonatal and late suckling periods. Hyperthyroid rats showed a significant increase of the enzyme in cerebral cortex only at the 2nd day of the neonatal period; in cerebellum the developmental pattern of MAT in neonatal period was anticipated temporally by 2-4 days. During the late suckling period thyroxine treatment produced in cerebellum a significant decrease in MAT activity at the 15th day after birth. From these data, we propose that hyperthyroidism may cause precocious induction of MAT both in cerebral cortex and in cerebellum and that the increased availability of S-adenosyl-L-methionine during the neonatal period could be related to its utilization also in polyamine biosynthesis.

Effects of ethanol on postnatal cell acquisition in the rat cerebellum

Cell generation and cell survival were investigated in the cerebellum of young rats exposed to 10% v/v ethanol in drinking water throughout gestation and lactation. At 12 days postpartum, cell cycle parameters in the external granular layer showed no significant change from control values, and the cell acquisition rate was unaffected. However, the external granular layer appeared thicker in ethanol-treated than in control 12-day-old animals, and a significant increase in cell death in the internal granular layer was observed. It is suggested that the effect of ethanol on the developing cerebellum may involve reduction of granule cell number consequent to increased granule cell death, and possibly retarded migration from the proliferating precursor cell pool.

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.

Early barbiturate treatment eliminates peak serum thyroxine levels in neonatal mice and produces ultrastructural damage in the brains of adults

Serum thyroxine levels peak sharply at the end of the second postnatal week in mice. Treating neonatal mice with barbiturates (PhB) completely eliminates this marked thyroxine (T4) peak without significantly affecting nonpeak levels at other ages. PhB treatment at this same time also establishes or induces long-lasting degenerative processes that continue to result in ultrastructural neural deficits in adults, long after treatment has been discontinued. Similar neural deficits have been observed in animals which were deficient in T4 during this same period of development.

Effects of haloperidol on cell proliferation in the early postnatal rat brain

Haloperidol, a widely used neuroleptic, produced a significant depression of the rate of [3H]thymidine incorporation into the DNA of 11-day-old rat brain. The reduction of in-vivo DNA synthesis rate was detectable by 4 h after subcutaneous injection of a single dose of haloperidol (20 mg/kg) and through the period 10-24 h after drug treatment the rate was less than 50% of that of controls in the forebrain. [3H]Thymidine incorporation returned to control values by 32 h. The effect on the cerebellum was similar but less pronounced. The depression was dose-dependent and a half-maximal effect was produced with haloperidol doses of 5-10 mg/kg. Parallel histological studies on treated rats suggested prolongation of the DNA synthesis phase of the cell cycle in the forebrain subependymal layer, associated with an increase in turnover time of about 15%.

Effects of altered thyroid states on myelinogenesis

Myelinogenesis was studied in controls and in rats treated since birth with Methimazole (hypothyroid) or thyroxine (hyperthyroid). The amount of myelin in forebrain and its protein composition were determined between 13 and 40 days of age, the period of most rapid myelin accumulation. Hypothyroid rats had reduced on both and brain weights relative to controls and the yield of myelin was reduced on both a per brain and a per milligram brain protein basis. Developmental changes in the protein composition of isolated myelin followed the pattern of control animals (the percentage of total myelin protein present as proteolipid protein, large basic protein, and small basic protein increased, as did the ratio of proteolipid/large basic protein) but were delayed temporally by 1-2 days. Hyperthyroid rats also had reduced body and brain weights. At 13 days myelin accumulation was greater than that of controls, corresponding to an earlier initiation of myelination. At later ages myelin yield was reduced on a per brain basis but not on a per milligram brain protein basis. The developmental pattern of myelin protein composition was accelerated temporally by 1-2 days. Myelination in optic nerve, assayed by proteolipid protein content, also was slightly delayed in hypothyroid animals and somewhat accelerated in hyperthyroid animals. The relative synthesis of myelin proteins (determined as incorporation of intracranially injected [(3)H]glycine into myelin protein relative to incorporation into whole brain protein), as well as distribution of radioactivity among individual myelin proteins, was determined. The results supported the conclusion of the myelin protein accumulation study; hypothyroidism retards the developmental program for myelinogenesis, whereas in the hyperthyroid state myelin synthesis is initiated earlier but is also terminated earlier.

Effect of chlorpromazine on cell proliferation in the developing rat brain. A combined biochemical and morphological study

Chlorpromazine, a widely used drug in current clinical practice, produced a severe reduction of the rate of [3H]thymidine incorporation into brain DNA of 11-day-old rats. The depression of in vivo synthesis rate was detectable by 6 h after chlorpromazine administration (50 mg/kg, s.c.) and the rate was less than 40% and 60% of controls during period 14-30 h in forebrain and 6-30 h in cerebellum respectively. The depression was dose-dependent and half maximal effect was produced with about 10 mg/kg chlorpromazine. The drug caused some retardation in the rate of conversion of [3H]thymidine to [3H]thymine nucleotides in the brain, but the severe depression in DNA labelling was also evident after correcting the values on the basis of [3H]thymine nucleotides concentration. Mitotic activity was significantly reduced in the cerebellar external granular layer. Increased numbers of cell degenerations, shown by Feulgen cytophotometry to be postmitotic, were seen in both layers 12 and 32 h after chlorpromazine. Analysis of cell cycle parameters showed no significant changes. However, the labelling index in subependymal cells was reduced, indicating an increase in turnover time of about 40%. The results are consistent with an action of chlorpromazine on cell proliferation, either by direct effects on the generation and survival of cells, or via its major pharmacological actions on neurotransmitter balance. These effects are potentially of functional and clinical significance.

Effects of thyroid state on brain development: beta-adrenergic receptors and 5'-nucleotidase activity

The effect of thyroid status on beta-adrenergic receptor binding and 5'-nucleotidase activity was studied in the forebrain and the cerebellum of the rat during the first 5 postnatal weeks. The developmental increase in beta-adrenergic receptor binding was significantly depressed in thyroid deficiency in both the forebrain and the cerebellum. The effect was more pronounced in the cerebellum, where at day 35 the concentration and the total number of beta-adrenergic receptor sites were reduced by 35% and 50% respectively. In contrast, hyperthyroidism had no significant effect on the development of beta-adrenergic receptors in the brain. On the other hand, hyperthyroidism led to a sustained increase in the forebrain in the activity of 5'-nucleotidase, an enzyme which is also associated with plasma membranes and has been proposed to play some role in neurotransmission. In thyroid deficiency the enzyme activity was markedly depressed. The effect was significant from day 12 in the cerebellum and from day 21 in the forebrain, the maximal depression, at day 21, being 55% and 45% respectively. In comparison with these plasma membrane markers, the accretion of membranous proteins was less affected: although this was retarded in hypothyroidism and advanced in hyperthyroidism there was no residual effect at 35 days except those attributable to changes in organ size. The results indicated, therefore, that the biochemical specialization of cells, as reflected in certain plasma membrane constituents, are chatacteristically influenced in the developing brain by thyroid disorders.

Thyroid deficiency and cell death in the rat cerebellum during development

The [3H]-DNA content of the cerebellum remained constant during a period from 4 h to 8--9 days after the administration of [3H]-thymidine both to normal rats at 6 and 12 days of age and to thyroid deficient rats at 6 days. On the other hand, when [3H]-thymidine was given to 12-day-old thyroid deficient rats, a progressive decrease in the cerebellar content of [3H]-DNA was observed during the first 4 days after the injection. These findings, together with previous results showing a markedly elevated pyknotic index in the internal granular layer of the 12-day-old hypothyroid cerebellum, are consistent with there being an increased death of newly formed and differentiating granule cells in thyroid deficiency towards the end of the second week of postnatal life.