Effect of thyroid deficiency on the synaptic organization of the rat cerebellar cortex

The Role of Thyroid Hormone in the Regulation of Cerebellar Development

The proper organized expression of specific genes in time and space is responsible for the organogenesis of the central nervous system including the cerebellum. The epigenetic regulation of gene expression is tightly regulated by an intrinsic intracellular genetic program, local stimuli such as synaptic inputs and trophic factors, and peripheral stimuli from outside of the brain including hormones. Some hormone receptors are expressed in the cerebellum. Thyroid hormones (THs), among numerous circulating hormones, are well-known major regulators of cerebellar development. In both rodents and human, hypothyroidism during the postnatal developmental period results in abnormal morphogenesis or altered function. THs bind to the thyroid hormone receptors (TRs) in the nuclei and with the help of transcriptional cofactors regulate the transcription of target genes. Gene regulation by TR induces cell proliferation, migration, and differentiation, which are necessary for brain development and plasticity. Thus, the lack of TH action mediators may directly cause aberrant cerebellar development. Various kinds of animal models have been established in a bid to study the mechanism of TH action in the cerebellum. Interestingly, the phenotypes differ greatly depending on the models. Herein we summarize the actions of TH and TR particularly in the developing cerebellum.

Absence of Thyroid Hormone Induced Delayed Dendritic Arborization in Mouse Primary Hippocampal Neurons Through Insufficient Expression of Brain-Derived Neurotrophic Factor

Thyroid hormone (TH) plays important roles in the developing brain. TH deficiency in early life leads to severe developmental impairment in the hippocampus. However, the mechanisms of TH action in the developing hippocampus are still largely unknown. In this study, we generated 3,5,3'-tri-iodo-l-thyronine (T3)-free neuronal supplement, based on the composition of neuronal supplement 21 (NS21), to examine the effect of TH in the developing hippocampus using primary cultured neurons. Effects of TH on neurons were compared between cultures in this T3-free culture medium (-T3 group) and a medium in which T3 was added (+T3 group). Morphometric analysis and RT-qPCR were performed on 7, 10, and 14 days in vitro (DIV). On 10 DIV, a decreased dendrite arborization in -T3 group was observed. Such difference was not observed on 7 and 14 DIV. Brain-derived neurotrophic factor (Bdnf) mRNA levels also decreased significantly in -T3 group on 10 DIV. We then confirmed protein levels of phosphorylated neurotrophic tyrosine kinase type 2 (NTRK2, TRKB), which is a receptor for BDNF, on 10 DIV by immunocytochemistry and Western blot analysis. Phosphorylated NTRK2 levels significantly decreased in -T3 group compared to +T3 group on 10 DIV. Considering the role of BDNF on neurodevelopment, we examined its involvement by adding BDNF on 8 and 9 DIV. Addition of 10 ng/ml BDNF recovered the suppressed dendrite arborization induced by T3 deficiency on 10 DIV. We show that the lack of TH induces a developmental delay in primary hippocampal neurons, likely caused through a decreased Bdnf expression. Thus, BDNF may play a role in TH-regulated dendritogenesis.

Endocrine disorders and the cerebellum: from neurodevelopmental injury to late-onset ataxia

Hormonal disorders are a source of cerebellar ataxia in both children and adults. Normal development of the cerebellum is critically dependent on thyroid hormone, which crosses both the blood-brain barrier and the blood-cerebrospinal fluid barrier thanks to specific transporters, including monocarboxylate transporter 8 and the organic anion-transporting polypeptide 1C1. In particular, growth and dendritic arborization of Purkinje neurons, synaptogenesis, and myelination are dependent on thyroid hormone. Disturbances of thyroid hormone may also impact on cerebellar ataxias of other origin, decompensating or aggravating the pre-existing ataxia manifesting with motor ataxia, oculomotor ataxia, and/or Schmahmann syndrome. Parathyroid disorders are associated with a genuine cerebellar syndrome, but symptoms may be subtle. The main conditions combining diabetes and cerebellar ataxia are Friedreich ataxia, ataxia associated with anti-GAD antibodies, autoimmune polyglandular syndromes, aceruloplasminemia, and cerebellar ataxia associated with hypogonadism (especially Holmes ataxia/Boucher-Neuhäuser syndrome). The general workup of cerebellar disorders should include the evaluation of hormonal status, including thyroid-stimulating hormone and free thyroxine levels, and hormonal replacement should be considered depending on the laboratory results. Cerebellar deficits may be reversible in some cases.

Aberrant Cerebellar Development in Mice Lacking Dual Oxidase Maturation Factors

BACKGROUND

Thyroid hormone (TH) plays a key role in the developing brain, including the cerebellum. TH deficiency induces organizational changes of the cerebellum, causing cerebellar ataxia. However, the mechanisms causing these abnormalities are poorly understood. Various animal models have been used to study the mechanism. Lacking dual oxidase (DUOX) and its maturation factor (DUOXA) are major inducers of congenital hypothyroidism. Thus, this study examined the organizational changes of the cerebellum using knockout mice of the Duoxa gene (Duoxa-/-).

METHODS

The morphological, behavioral, and electrophysiological changes were analyzed in wild type (Wt) and Duoxa-deficient (Duoxa-/-) mice from postnatal day (P) 10 to P30. To detect the changes in the expression levels of presynaptic proteins, Western blot analysis was performed.

RESULTS

The proliferation and migration of granule cells was delayed after P15 in Duoxa-/- mice. However, these changes disappeared by P25. Although the cerebellar structure of Duoxa-/- mice was not significantly different from that of Wt mice at P25, motor coordination was impaired. It was also found that the amplitude of paired-pulse facilitation at parallel fiber-Purkinje cell synapses decreased in Duoxa-/- mice, particularly at P15. There were no differences between expression levels of presynaptic proteins regulating neurotransmitter release at P25.

CONCLUSIONS

These results indicate that the anatomical catch-up growth of the cerebellum did not normalize its function because of the disturbance of neuronal circuits by the combined effect of hypothyroidism and functional disruption of the DUOX/DUOXA complex.

An in vitro method to study the effects of thyroid hormone-disrupting chemicals on neuronal development

Thyroid hormones (THs) play critical roles for normal cerebellar development. It has been reported that several environmental chemicals may affect cerebellar development through TH system. One such example is the suppression of TH receptor (TR)-mediated transcription by polybrominated diphenyl ethers (PBDEs). To determine the effect of these chemicals on brain development, we established a primary culture system of rat cerebellar Purkinje cells. Using this system, as low as 10(-10)M TH induced Purkinje cell dendritic arborization and such effect was dose-dependent. We examined the effect of decabromodiphenyl ether (BDE209) using this system. Dendritic development of the Purkinje cells was suppressed by 10(-10)M BDE209, that was compatible to the result of the suppression of TR-mediated transcription by using reporter gene assay. These results suggest that TH plays a pivotal role in the development of the Purkinje cell dendrites. Together with in vitro assay system such as reporter gene assay and liquid chemiluminescent DNA-pull down assay, an in vitro protein-DNA binding assay, these assay systems provide us with precise information about environmental chemicals on brain development.

Cerebellum: links between development, developmental disorders and motor learning

The study of the links and interactions between development and motor learning has noticeable implications for the understanding and management of neurodevelopmental disorders. This is particularly relevant for the cerebellum which is critical for sensorimotor learning. The olivocerebellar pathway is a key pathway contributing to learning of motor skills. Its developmental maturation and remodeling are being unraveled. Advances in genetics have led to major improvements in our appraisal of the genes involved in cerebellar development, especially studies in mutant mice. Cerebellar neurogenesis is compartmentalized in relationship with neurotransmitter fate. The Engrailed-2 gene is a major actor of the specification of cerebellar cell types and late embryogenic morphogenesis. Math1, expressed by the rhombic lip, is required for the genesis of glutamatergic neurons. Mutants deficient for the transcription factor Ptf1a display a lack of Purkinje cells and gabaergic interneurons. Rora gene contributes to the developmental signaling between granule cells and Purkinje neurons. The expression profile of sonic hedgehog in postnatal stages determines the final size/shape of the cerebellum. Genes affecting the development impact upon the physiological properties of the cerebellar circuits. For instance, receptors are developmentally regulated and their action interferes directly with developmental processes. Another field of research which is expanding relates to very preterm neonates. They are at risk for cerebellar lesions, which may themselves impair the developmental events. Very preterm neonates often show sensori-motor deficits, highlighting another major link between impaired developments and learning deficiencies. Pathways playing a critical role in cerebellar development are likely to become therapeutical targets for several neurodevelopmental disorders.

Brain-derived neurotrophic factor (BDNF) ameliorates the suppression of thyroid hormone-induced granule cell neurite extension by hexabromocyclododecane (HBCD)

Thyroid hormone (TH) plays an essential role in growth and differentiation of the central nervous system. Deficiency of TH during perinatal period results in abnormal brain development known as cretinism in human. We recently reported that an environmental chemical 1,2,5,6,9,10-α-hexabromocyclododecane (HBCD) suppressed TH receptor (TR)-mediated transcription. To examine the effect of HBCD on cerebellar granule cells, we used purified rat cerebellar granule cells in reaggregate culture. Low dose HBCD (10(-10)M) significantly suppressed TH-induced neurite extension of granule cell aggregate. To clarify further the mechanisms of such suppression, we added brain-derived neurotrophic factor (BDNF) into culture medium, since BDNF plays a critical role in promoting granule cell development and is regulated by TH. BDNF completely rescued HBCD-induced suppression of granule cell neurite extension in the presence of T3. These results indicate that HBCD may disrupt TH-mediated brain development at least in part due to a disruption of the T3 stimulated increase in BDNF and BDNF may possess ability to ameliorate the effect of HBCD in granule cells.

Participation of NMDA-glutamatergic receptors in hippocampal neuronal damage caused by adult-onset hypothyroidism

We analyzed the participation of N-methyl-d-aspartate (NMDA) receptors in the neuronal damage caused by adult-onset hypothyroidism. Wistar rats were randomly assigned into four groups. The euthyroid group received tap water. The hypothyroid group received methimazole (60 mg/kg) in their drinking water to induce hypothyroidism. Two more groups of rats received the antithyroid treatment and were injected daily with the NMDA antagonist ketamine (15 mg/kg, sc) or MK-801 (0.5mg/kg, ip). Treatments were administered during 4 weeks. At the end of the respective treatments rats were deeply anaesthetized and perfused intracardially with 0.9% NaCl followed by 4% paraformaldehyde. The brains were removed from the skull, and coronal brain sections (7microm thick) were obtained. Neurons were counted in the CA1, CA2, CA3, and CA4 hippocampal regions differentiating between normal and atrophic cells by an experimenter blind to the treatment. The percentage of neuronal damage found in the MMI group was significantly greater in the hippocampal regions compared to the euthyroid group. In contrast, both NMDA antagonists were able to prevent the neuronal damage secondary to hypothyroidism in all hippocampal regions. Our results suggest that the neuronal damage caused in the hippocampus of adult-onset hypothyroid rats requires activation of NMDA channels.

Perinatal exposure to PTU delays switching from NR2B to NR2A subunits of the NMDA receptor in the rat cerebellum

Certain kinds of developmental neurotoxicants are considered to act by affecting the levels of thyroid hormones, which are essential for the brain development of both humans and experimental animals. Hypothyroidism experimentally induced in rats with propylthiouracil (PTU) offers a useful animal model for developmental neurotoxicity. The purpose of the present study was to clarify developmental alterations in gene expression caused by PTU in this model, with the focus on eight genes implicated in neural network formation or synaptic functions, such as the brain-derived neurotrophic factor (BDNF) and NMDA receptors 2A/2B. First, we measured the developmental profile of gene expression in vehicle-dosed rat cerebellum by quantitative RT-PCR and then examined the effects of PTU on mRNA levels on postnatal day (PND) 22, when most of the cerebellar structures in mature animals are already formed. PTU induced up-regulation of NR2B mRNA and down-regulation of NR2A and BDNF mRNAs in the cerebellum on PND 22, but there were no changes in the other genes (growth associated protein-43, L1, neuronal cell adhesion molecule, synaptophysin, post synaptic density-95). Examination of the effects of PTU on maturation of NMDAR subunits (NR2A/NR2B) demonstrated changes in relative expression on PND 14, but not on PND 4, with recovery after maturation. The profile of NMDAR subunits in vehicle-dosed rats showed a shift from NR2B to NR2A during development. These results suggest PTU can delay this switching from NR2B to NR2A subunits in the maturation of NMDA receptors.

Regulation of nuclear coactivator and corepressor expression in mouse cerebellum by thyroid hormone

Thyroid hormone (TH) has an important role in central nervous system development. TH action is mediated by a number of transcription factors including thyroid hormone receptors (TRs) in combination with a group of coregulators that can either activate (coactivators) or repress (corepressors) transcription in the presence of TH. The aims of this report were to determine if regulation of the corepressor Hairless (Hr) by TH was TR-isoform- mediated in neonatal cerebellum and to determine if other cerebellar corepressors (SMRT and NCoR) and coactivators (SRC family) are also regulated by TH. In order to study this we examined 14-day-old and adult knockout mice that lack expression of the TRbeta or TRalpha isoforms and measured mRNA expression in untreated, hypothyroid and TH-treated young mouse pups. TH-treated wild-type and TRbeta-deficient mice demonstrated upregulation of Hr by 22.8- +/- 8.6- and 11.8- +/- 3.6-fold respectively, which was not upregulated in TRalpha-deficient mice. In wild-type mice, TH treatment results in a reciprocal decrease (61%) in the coactivator SRC-1. These changes were not observed in adult mouse cerebellum. No effect was seen with NCoR and SRC-3 expression. SMRT was 3-fold increased in TH treatment of only wild-type mouse pups. We conclude that (1) TRalpha is the major TR regulating Hr expression in the cerebellum of young mouse pups; (2) TH upregulates Hr and SMRT and downregulates SRC-1; (3) NcoR and SRC-3 may not be regulated by TH in the cerebellum at the transcriptional level; and (4) autoregulation of TH action may be mediated through TH-dependent expression of the cofactors necessary for TH action in the cerebellum and may be developmentally specific.

Postnatal changes of steroid receptor coactivator-1 immunoreactivity in rat cerebellar cortex

Steroid receptor coactivator-1 (SRC-1) interacts with nuclear hormone receptors (NRs) to mediate their action in a ligand-dependent manner. Among such ligands, thyroid hormone (TH) is particularly crucial for brain development. The expression of many TH target genes is regulated by TH only for a limited critical period, although TH receptor (TR) expression is not greatly altered after such period. To alter TH sensitivity, other factors may be involved. We thus examined the changes in SRC-1 expression during postnatal development in the rat cerebellum by immunohistochemistry and Western blotting. Strong SRC-1 immunoreactivity (IR) was constantly seen in Purkinje cell from postnatal days (P) 2 to P30. SRC-1 IR was also constantly observed in the internal granule cell layer. However, it was negative in the external granule cell layer at P2 and P7, whereas a weak IR was detected in the premigratory zone at P15. SRC-1 IR was detected in the molecular layer after P15. These results indicate that although TR is almost ubiquitously expressed in the developing cerebellum, the TH sensitivity could vary in each subset of cells. By Western blotting, SRC-1 protein level was greatest at P15, at which time TH action may be obvious. Taken together, the differential expression of SRC-1 may be crucial in mediating TH action during cerebellar development.

Current perspectives on the role of thyroid hormone in growth and development of cerebellum

The thyroid hormone (TH) is essential for growth and development of brain, including the cerebellum. Deficiency of TH during the perinatal period results in abnormal cerebellar development, which is well documented in rodent animal models. TH exerts its major effect by binding to the nuclear TH receptor (TR), a ligand-regulated transcription factor. Although TR is highly expressed in many brain regions, including the cerebellum, TH-target genes that likely play critical roles in brain development have not yet been fully clarified. At present, however, expression of many cerebellar genes is known to be altered by perinatal hypothyroidism. Interestingly, after the critical period of TH action (first 2 weeks of postnatal life in rodent cerebellum), the activities of many genes that are altered by perinatal hypothyroidism return to the same levels as those of euthyroid animal despite morphological alterations. Several prominent candidate genes that may play key roles in TH-mediated cerebellar development are discussed in this review. On the other hand, TR-mediated transcription may be modulated by various substances. The nuclear hormone receptor superfamily contains more than 40 transcriptional factors and, most of these receptors are present in the brain. Possible interactions between TR and such transcription factors are also discussed. Further, several additional issues that need to be clarified are discussed. One such issue is the discrepancy of phenotypes among TR-knockout and perinatal hypothyroid mice. Recent studies have provided several important clues to address this issue. Another current area that needs attention is the effect of endocrine disruptors on brain development. Since the molecular structures of TH and several endocrine disrupting chemicals are similar, the effect of such chemicals on brain may be exerted at least in part through the TH system. Recent studies have shown the possible interaction between TR and such chemicals. Overall, this review provides current findings regarding molecular mechanisms on TH action in cerebellar development.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

Deletion of the thyroid hormone receptor alpha 1 prevents the structural alterations of the cerebellum induced by hypothyroidism

Thyroid hormone (T3) controls critical aspects of cerebellar development, such as migration of postmitotic granule cells and terminal differentiation of Purkinje cells. T3 acts through nuclear receptors (TR) of two types, TRalpha1 and TRbeta, that either repress or activate gene expression. We have analyzed the cerebellar structure of developing mice lacking the TRalpha1 isoform, which normally accounts for about 80% of T3 receptors in the cerebellum. Contrary to what was expected, granule cell migration and Purkinje cell differentiation were normal in the mutant mice. Even more striking was the fact that when neonatal hypothyroidism was induced, no alterations in cerebellar structure were observed in the mutant mice, whereas the wild-type mice showed delayed granule cell migration and arrested Purkinje cell growth. The results support the idea that repression by the TRalpha1 aporeceptor, and not the lack of thyroid hormone, is responsible for the hypothyroid phenotype. This conclusion was supported by experiments with the TRbeta-selective compound GC-1. Treatment of hypothyroid animals with T3, which binds to TRalpha1 and TRbeta, prevents any defect in cerebellar structure. In contrast, treatment with GC-1, which binds to TRbeta but not TRalpha1, partially corrects Purkinje cell differentiation but has no effect on granule cell migration. Our data indicate that thyroid hormone has a permissive effect on cerebellar granule cell migration through derepression by the TRalpha1 isoform.

Thyroid hormone action and brain development

Thyroid hormone (TH) plays a crucial role in brain development. Developing rodent cerebellum might be an excellent model for studying the molecular mechanisms of TH action in the brain because perinatal hypothyroidism greatly affects its ontogeny. Although the TH-regulated genes that play crucial roles in cerebellar development have not yet been fully characterized, recent studies have provided novel insights into TH action in brain development.

Developmentally regulated expression of alpha- and beta-calcitonin gene-related peptide mRNA and calcitonin gene-related peptide immunoreactivity in the rat inferior olive

Immunohistochemical methods have revealed the transient neonatal expression of calcitonin gene-related peptide (CGRP) in olivocerebellar compartments, and it has been hypothesized that this peptide plays a role in the development of olivocerebellar connectivity. Furthermore, the distribution of the CGRP binding sites in the cerebellar cortex also favors this hypothesis. In this study, the pattern of postnatal expression of alpha- and beta-CGRP mRNAs in the inferior olive (IO) complex was analyzed using in situ hybridization histochemistry with RNA probes complementary to specific sequences of alpha- and beta-CGRP mRNAs, and the results were compared with the pattern of CGRP immunoreactivity. High levels of alpha-CGRP mRNA expression were found in specific subnuclei of the IO complex, i.e., the medial part of the dorsal fold of the dorsal accessory olive, the beta nucleus, the dorsal cap, the caudal third of the medial accessory olive, and the rostral part of the dorso-medial cell column; in the same subnuclei beta-CGRP mRNA was detected. The olivary distribution of the two CGRP mRNA coincided with that of CGRP immunoreactivity. The expressions of alpha-CGRP mRNA and CGRP immunoreactivity were restricted to the first 2 postnatal weeks, the peak being reached at the end of the first week; beta-CGRP mRNA was transiently expressed in the same olivary compartments, but only from postnatal day 6 to 9. In general, the alpha-CGRP signal was also more intense than the beta-CGRP signal. The present findings indicate that the alpha- and beta-CGRP mRNA expression in the olivary complex is under developmental control and restricted to specific olivocerebellar compartments. The data provide a basis for the transient expression of a CGRP olivocerebellar compartment and further support the hypothesis of a role for CGRP in the complex postnatal cerebellar phenomena of connectivity reshaping and synapse stabilization.

Response of fast muscle innervation to hypothyroidism

The early period of motor innervation development is characterized by multiple innervation of muscle cells. This transitory state in rat extensor digitorum longus (edl) muscle is normally concluded at weaning when a 1:1 ratio between nerve endings and muscle cells is reached. Motor innervation of edl muscle in rats made hypothyroid after weaning was studied in three ways: electrophysiology (intracellular recordings of muscle postsynaptic potentials) was carried out to study neuromuscular transmission; silver impregnation of terminal axons to observe sprouting; force production in twitch and tetanus following direct muscle stimulation and nerve stimulation. A number of multiply innervated muscle cells was found in hypothyroid rats following two months of treatment. This finding seems to be related to the appearance of nodal sprouting in motor axons. No sign of denervated end-plates was found. Twitch and tetanus tension were smaller than in controls, but they were bigger when referred to unitary muscle mass. Time course of twitch, particularly half relaxation, was slowed in muscles of hypothyroid rats. These findings suggest that plastic processes occur in muscle innervation of rats made hypothyroid after weaning. Therefore, thyroid hormones play a role in stabilizing motor innervation not only during development, but also in adults.

Reorganization of mossy fiber synapses in male and female hypothyroid rats: a stereological study

Thyroid hormone deficiency has been reported to interfere with synaptogenesis, particularly in those regions of the brain where the neurons display a late and protracted histogenesis, although the extent of the synaptic alterations remains unknown. To provide detailed quantitative data on the effects of hypothyroidism upon synapses, a link of the hippocampal circuitry was selected: the contact between mossy fibers and dendritic excrescences of CA3 pyramidal cells (MF-CA3 synapses). Groups of six male and six female rats aged 30 and 180 days were analyzed separately after being treated as follows: (1) hypothyroid from day 0 until day 30 (30 day old hypothyroid group); (2) hypothyroid from day 0 until day 180 (180 day old hypothyroid group); (3) hypothyroid until day 30 and thenceforth maintained euthyroid (recovery group); and (4) and (5) 30 and 180 day old control groups, respectively. Timm staining, Golgi impregnation, and electron microscopy were employed to estimate the volume of the mossy fiber system, the number and size of mossy fiber boutons, and the number and related features of MF-CA3 synapses. The volume of the mossy fiber system and the number of synaptic boutons were reduced in all experimental groups. The total number of synapses was decreased in 30 day old hypothyroid rats, but did not differ among 180 day old animals. Postsynaptic densities were shorter in hypothyroid and recovery groups than in controls, although the reduction was not as marked in recovery rats as it was in hypothyroid animals. Structural alterations were noted in the pre- and postsynaptic compartments of MF-CA3 synapses of both 180 day old hypothyroid and recovery rats. These changes can be regarded as mechanisms of reorganization as they underlie the compensation for the hypothyroid-induced numerical reduction of synapses observed in 30 day old animals and enable a complete catch-up of their total number. However, synaptic reorganization was not fully achieved, as revealed by the reduction in the size of the synaptic sites in hypothyroid and recovery animals. Finally, we demonstrate that hypothyroidism did not interfere with the sex-related differences of MF-CA3 synapses described in normal rats.

Gene expression in the developing cerebellum during perinatal hypo- and hyperthyroidism

The intensity of p75NGFR receptor-like immunoreactivity and the mRNAs encoding p75NGFR, T alpha 1 alpha-tubulin, GAP-43 and the myelin proteins MBP and PLP were measured in the developing cerebellum to study the effects of perinatal thyroid hormone imbalance in rats. Results compared to age-matched controls provide in vivo evidence for differential gene regulation by thyroid hormone in the developing cerebellum. We found that p75NGFR immunoreactivity was strikingly elevated in hypothyroid rats, whereas p75NGFR mRNA content remained only twice as high as that of control levels on postnatal day 15 (P15). When p75NGFR immunoreactivity was still elevated in hypothyroid rats, Purkinje cells exhibited proximal axonal varicosities, axonal twisting and differences in axonal caliber. The mRNAs encoding proteins involved with neurite growth-promoting elements, T alpha 1 alpha-tubulin and GAP-43, were also increased in hypothyroidism, possibly reflecting a neuronal response to a deficiency in, or damage to, cerebellar neurons, or a general delay in their down regulation. Similar increases were not observed for the myelin specific genes. MBP and PLP mRNAs were first detected on P2 of hyperthyroid rats, and they increased with age. Hypo- or hyperthyroidism did not affect the initial onset of MBP and PLP expression, however, hyperthyroidism increased levels of PLP and MBP mRNAs between P2 and P10. By contrast, the most consistent decrease in MBP and PLP mRNAs in rats with thyroid hormone deficiency was observed only on P10. At later times (P15 and P30), the two mRNA levels were similar to controls in all groups. These results are consistent with a role for thyroid hormone in the earlier stages of cerebellar myelination. Hypothryoidism led to specific increases in T alpha 1 alpha-tubulin and GAP-43 mRNAs, and in the immunoreactivity and mRNA levels of p75NGFR receptor--all changes that may play a role in the observed abnormal neuronal outgrowth.

Postnatal development of a granule cell-enriched, neurone-specific glycoprotein, gp50, in normal and thyroid-deficient rats

Glycoprotein gp50 is a neurone-specific, granule cell-enriched glycoprotein that is also a major component of isolated synaptic membranes. Here, we describe the use of a monoclonal antibody, mab SM gp50, to study the postnatal development of gp50 in the brain of normal and thyroid-deficient rats. Radioimmunoassay, enzyme-linked immunosorbent assay, and Western blotting show that gp50 is not detectable in brain until postnatal day 4 (P4) in both forebrain and cerebellum. In forebrain, the rate of increase of gp50 levels is maximal between P12 and P20. It is somewhat later in cerebellum, where peak levels are attained between P30 and P35. Immunocytochemical studies show little detectable gp50-like immunoreactivity before P16, and the staining is still weak, relative to adult tissue, at P25. The intense staining of the granule cell layer characteristic of adult cerebellum predominantly appears after P25. Development of gp50 is severely retarded in the cerebellum of thyroid-deficient rats, particularly during the second and third postnatal weeks. However, by the fourth postnatal week, gp50 levels in normal and hypothyroid animals are comparable. The results indicate that significant alterations in the pattern of gp50 expression continue to occur at a late stage of cerebellar development. In particular, the increase in immunocytochemical staining of the granule cells after P25 is striking in that by this time most major events associated with cerebellar development are essentially complete.

Effect of triiodothyronine on the muscarinic receptors and acetylcholinesterase activity of developing rat brain

The effect of triiodothyronine was investigated on the developing muscarinic system in rat cerebral cortex. Triiodothyronine was administered from birth to day 30. In the synaptosomal membrane fraction of the cerebral cortex, acetylcholinesterase activity and the binding of the muscarinic ligand 3H-quinuclidinyl-benzylate increased significantly as compared to the control. An analysis of the binding curve based on Hill plots suggested a positive cooperativity of receptors of the membrane samples obtained from both hormone treated and control animals. The results obtained are in accordance with morphological, biochemical and clinical data on the thyroid hormone effect on central nervous system maturation.

Forebrain development in control and hypothyroid larvae of Triturus cristatus carnifex

Some cytological and developmental aspects of the forebrain gray matter were studied in the control and the hypothyroid larvae of Triturus cristatus carnifex. The mitotic cell percentages in the periventricular germinal layer, the fusiform shape of the recently divided neuronal cells and the transitory gray matter cleavage, are characteristic of the immature status of the central nervous system (CNS) in this species. The hypothyroidism obtained during two months of thiourea treatment slows down the development in all the forebrain areas of the experimental larvae. Nevertheless, the nucleus olfactorius anterior and the corpus striatum, which have a longer mitotic activity with respect to the other neuronal structures, are more deeply affected by thyroid atrophy. Thyroid activity and forebrain maturation are discussed on the basis of the literature and our data with regard to their influence on behavior and evolution.

Effect of thyroid deficiency on the development of glia in the hippocampal formation of the rat: an immunocytochemical study

The development of glia in the hippocampal formation of normal and hypothyroid rats was studied using immunocytochemical staining for either glial fibrillary acidic protein (GFAP) or vimentin. Light microscopy showed lower GFAP immunoreactivity in the radial glial processes of young hypothyroid rats compared to normal animals. These processes followed the known path of neuroblast migration toward the proliferative zone of the dentate gyrus until the end of the 1st postnatal week. Vimentin immunoreactivity showed that the glial processes were present and therefore immature at least with respect to their cytoskeletal composition. We propose that this early defect in the maturation of the radial glial fibers accounts for the final deficit in the granule cells of the dentate gyrus. Later in development, thyroid deficiency also reduced the density and number of GFAP-labeled astrocytes and the growth of their processes. This observation is in complete disagreement with the glial hypertrophy induced by thyroid deficiency in the cerebellum. The considerably increased histogenetic cell death observed in the cerebellum of young hypothyroid rats could in turn induce glial hypertrophy, whereas the hippocampal formation, where a normal low number of cell deaths is observed, is only subjected to the general depressive effect of thyroid deficiency on cell maturation.

Altered Purkinje cell maturation in rats exposed prenatally to ethanol. II. Synaptology

Pregnant rats of Sprague-Dawley strain were offered a protein- and vitamin-enriched nutritionally balanced liquid ethanol diet, providing 35% of calories as ethanol. Control animals were pair-fed identical liquid diet except that maltose-dextrins were substituted for ethanol. Pups of both ethanol-exposed and pair-fed groups were surrogate-fostered by mothers which received normal rat chow ad libitum. An average of eight pups of each experimental and control group was killed at 0-1, 5, 7, 10, 14, 21, 28, and 42 days. After fixation, their cerebella were bisected in the midvermal plane and one-half embedded in Araldite. Thin sections cut from lobules II/III were stained with uranyl acetate and lead citrate and the synaptology of the Purkinje neurons was studied with the electron microscope in both the experimental and control groups. Impaired climbing fiber maturation was seen in ethanol-exposed pups as evident from the presence of somatic spines--climbing fiber synapses at 14 days postnatally. In the 14-day-old control pups the climbing fiber maturation was complete or nearly complete as evident by their synapses on spines situated on the smooth dendritic branches and by the absence or paucity of climbing fiber--somatic spine synapse. In addition, a reduction in the number of parallel fibers was evident from the increase in Purkinje cell spines per varicosity in the ethanol-exposed pups. We conclude, therefore, a direct or indirect effect of ethanol occurs on climbing fiber maturation and Purkinje cell parallel fiber relationship.

Morphometric analysis of the cerebellar Purkinje cell in the developing normal and hypothyroid chick

A morphometric analysis of Purkinje cells in the developing cerebellar cortex of the chick was performed in normal animals and embryos made hypothyroid by one or two spaced injections of tetramethylthiourea. Profiles of 162 Purkinje cells, from Golgi-Cox treated sections were analysed. Soma area, perimeter and circularity index, cumulative length of the dendrites and number of dendritic bifurcations were studied. The results showed significant differences between control and hypothyroid animals. There were no important differences between birds rendered transiently hypothyroid with a single injection and those made chronically hypothyroid with dual injections. This confirms that the Purkinje cell is very dependent on thyroid hormone especially during the early phases of its morphogenesis. The development of the Purkinje cell was the most affected process of cerebellar cortex maturation in the thyroid-deficient chick. The dendritic arborization was particularly hypoplastic. Moreover, a dynamic balance appeared to exist between the development of the dendritic arborization and that of the perikaryon.

Granule cell development in the frog cerebellum during spontaneous and thyroxine-induced metamorphosis

Granule cell maturation in the cerebellum of bullfrog tadpoles was studied during both spontaneous and thyroxine-induced metamorphosis by using electron microscopy and Golgi-impregnated preparations. The production of cerebellar microneurons, a majority of which are granule cell precursors, was quantitatively compared during spontaneous and thyroxine-induced metamorphosis by using stereological methods and biochemical measurements of DNA. Granule cell migration and differentiation appeared morphologically similar during spontaneous and thyroxine-induced metamorphosis. In both instances, granule cells migrated tangentially along the pial surface, migrated into the internal granular layer, developed dendritic arbors, and formed synaptic contacts with the processes of Golgi cells and with mossy fibers. These events are similar to developmental processes that have been described in detail in other animals. Quantitative stereological measurements demonstrated similar overall patterns of change during spontaneous and thyroxine-induced metamorphosis. Most notably, increases in the volume of the external granule layer correlated with increases in the relative and total amounts of DNA. However, measurements of total DNA were consistently reduced during the period of accelerated change that occurs in thyroxine-induced metamorphosis, although external granular layer volume was greater in these tadpoles after 2 and 3 weeks of thyroxine treatment than in spontaneously metamorphosing tadpoles. While granule cell development in the frog is largely dependent on thyroid hormone, differences between thyroid-hormone-induced and spontaneously metamorphosing tadpoles suggest that normal patterns of cerebellar development are also dependent on events that occur in premetamorphic tadpoles in the absence of thyroid hormone.

Influence of thyroidectomy on brain catecholamines during the postnatal period

The influence of early thyroidectomy (Tx) on changes in dopamine (DA) and noradrenaline (NA) during the postnatal period (30, 45 and 60 days old) was studied in the diencephalon and the rest of the brain of male and female rats. Thyroidectomy interfered with the normal growth of the animals, decreased brain weight and markedly influenced the developmental pattern of both DA and NA in the diencephalon. When compared with control values, the DA concentration, in 45- and 60-day-old Tx male rats, was 29 and 43% lower, respectively, and 21 and 43% lower, respectively, in Tx females. Diencephalic NA levels in Tx rats were also lower than those observed in controls, 15% inferior in 45- and 60-day-old males; 27 and 22% lower, respectively, in females. Thyroidectomy does not significantly alter the level of either amine in the rest of the brain.

Effects of undernutrition and thyroid state on the ontogenetic changes of D1, D2, and D3 brain-specific proteins in rat cerebellum

Disturbances in metabolic balance brought about by alterations in thyroid state and undernutrition during early life had a marked effect on the concentrations of the brain-specific proteins, D1, D2, and D3 in the developing rat cerebellum. In normal rats, the concentrations of D1 and D3 increased and that of D2 decreased during the first 3 weeks after birth. In the hyperthyroid state a small but consistent advancement was observed in the developmental curves of these proteins. The hypothyroid state caused a marked retardation in the maturational pattern of D1 and D2 but not of D3. In undernutrition, at 6 days the concentrations of D1 and D3 proteins were higher than in controls, but thereafter the developmental increase was markedly delayed for D1 only. The concentration of D2 was normal at 6 days, but after the first week a marked retardation was observed in the maturational pattern of this protein in undernourished rats. In addition, the "anodic-immature" form of D2 predominated in 6-day-old controls, but this was gradually replaced by a "cathodic-mature" form which progressively became the dominant form of D2 in 35-day-old rat cerebellum. The developmental switch in terms of the two forms was also advanced in hyperthyroidism and retarded in thyroid deficiency and undernutrition. Furthermore, daily treatment of hypothyroid rats with physiological doses of thyroxine from birth restored the concentrations of D1 and D2 to normal, but that of D3 was increased above control levels, indicating differences between the proteins in their sensitivity to mechanisms of control by thyroid hormone. Also, the overall effects of undernutrition were markedly different from those of hypothyroidism.

Effects of neonatal antithyroid treatment on brain [3H]-imipramine binding sites

The action of the antithyroid, sulphydryl reagent methimazole (MMI) on the specific binding of [3H]-imipramine in the cerebral cortex and corpus striatum of immature and mature rats has been examined. Chronic administration of MMI through the first 30 days of life decreased the number of imipramine binding sites in cortical but not striatal membranes, as assessed 48 h after the last injection of goitrogen. A similar treatment did not affect the binding profile of [3H]-imipramine in mature rats. Acute administration of MMI to 30 day-old rats increased the number of imipramine binding sites shortly after the injection, an effect no longer evident 48 h later. MMI in vitro increased the binding of [3H]-imipramine. It is concluded that maturational impairment of the hypothyroid cortex, rather than any alteration of membrane bound thiol groups, was a major cause for the diminished binding of [3H]-imipramine in MMI-treated, immature rats.

Changes in the expression of a neuronal surface protein during development of cerebellar neurones in vivo and in culture

The expression of the neurone-specific D2 protein changes both quantitatively and qualitatively during development in vivo and in cultures of cerebellar nerve cells. The total D2 content per unit protein shows a two-fold increase in vivo from birth to postnatal day 6, after which it declines progressively to about 50% of the maximal value. This increase can be accounted for by an immature form of the protein anodic D2 being preferentially expressed at the early stages of cerebellar development. After postnatal day 9 this form gradually switches to a mature form cathodic D2. This switch can be mimicked by neuraminidase treatment, suggesting a developmental loss of sialic acid from the D2 protein. In freshly isolated cells the total D2 content per unit protein is only 30% of that in the corresponding intact tissue from 8-day-old cerebella, but it increases rapidly during the first 8 days of culture to levels similar to those of the equivalent age in vivo. The switch from anodic D2 to cathodic D2 also occurs at a faster rate in culture, probably reflecting the culture conditions that favour differentiation. The changes in the expression of D2 during development of cerebellar nerve cells in culture suggest that anodic D2 is preferentially expressed on nerve cells that are proliferating, migrating, or in the initial stages of differentiation, whereas cathodic D2 is associated with differentiated neurones. The transition between the two forms appears to occur during the formation of interneuronal contacts.

Alterations in the postnatal development of the cerebellar cortex due to zinc deficiency. II. Impaired maturation of Purkinje cells

Zinc deficiency during the first 3 postnatal weeks retarded the maturation of Purkinje cells. The dendrites of the Purkinje cells of 21-day-old zinc-deficient (ZD) rats were reduced in size and had fewer branches. Somatic processes were found in 24% of the Purkinje cells of ZD animals. Only 3% of the Purkinje cells of normal animals had somatic processes. A basal polysomal mass in the Purkinje cells of 21-day-old ZD rats indicated that zinc deficiency impaired the cytoplasmic maturation of Purkinje cells. The development of the glial envestment of the dendrites and the maturation of climbing fibers also were retarded. Pair-fed controls were studied to control for the effects of inanition in the ZD dams. In the pups of pair-fed dams, undernutrition slightly impaired the growth of the dendrites but produced few qualitative changes in the maturation of the soma and climbing fibers. Somatic processes were found on 10% of the Purkinje cells of pair-fed animals. Thus, the findings in the ZD animals were not only caused by the decreased maternal food consumption but by zinc deficiency. The retarded maturation of Purkinje cells was related to the altered metabolism of Purkinje cells and to effects secondary to decreased numbers of parallel fibers.

Purkinje cell maturation in the frog cerebellum during thyroxine-induced metamorphosis

Purkinje cell maturation during thyroxine-induced metamorphosis in premetamorphic bullfrog tadpoles was studied using electron microscopy and Golgi (silver-impregnated) preparations. Cerebella from tadpoles were examined following 1, 2, or 3 weeks of thyroxine treatment. Particular attention was paid to possible differences between the two populations of Purkinje cells previously described, i.e. (i) the smaller population located in the dorsal part of the cerebellum, where the Purkinje cells show dendritic arborization long before the appearance of the external granular layer, and (ii) the larger population located in the middle and ventral regions of the cerebellum, where the Purkinje cells begin to undergo maturation during metamorphosis when the external granular layer is established. Following thyroxine treatment, both populations of Purkinje cells showed rapid maturational change. In the mature (dorsal) group, dendritic growth resumed in the presence of an external granular layer increasing the complexity of their dendritic arbors. Moreover, climbing fiber synapses translocated from contacts on the soma to the thorns of growing dendrites, and somatic processes often disappeared. The immature (ventral) group showed dramatic differentiation of the perikaryon including polarization of cytoplasm with subsequent dendritic outgrowth and formation of somatic processes in the presence of climbing fibers. Stellate cell contacts appeared on the smooth portion of the soma of many Purkinje cells. Dendritic growth during thyroxine-induced metamorphosis was characterized by growth (elongation) with minimal branching, which is initially observed during spontaneous metamorphosis. Typically, these growing dendrites ended in growth cones, some with one or several filopodia. Developing Purkinje cell dendritic spines formed synapses with parallel fibers. The present study has provided an example of the dramatic nature of thyroxine's action in inducing the complex series of detailed maturational changes in the cerebellum 1-2 yr ahead of schedule. In addition, the results show that thyroxine-induced Purkinje cell maturation is more rapid and synchronous than that seen during spontaneous metamorphosis. It is concluded that Purkinje cell maturation during metamorphosis is largely dependent on thyroid hormone.

Triiodothyronine enhances the morphological maturation of dopaminergic neurons from fetal mouse hypothalamus cultured in serum-free medium

In dissociated hypothalamic cell cultures of 16-day mouse embryos, growing in chemically defined medium, the catecholaminergic neurons were identified by autoradiography after labelling with [3H]dopamine and by immunocytochemistry with an anti-tyrosine hydroxylase antibody. Using selective inhibitors of amine transport and radioenzymatic determination of amine levels in these cultures, we show that these neurons were mostly dopaminergic. The number of dopaminergic neurons identified by the two techniques increased between days 5 and 8 and decreased after 15 days in vitro. The same number of neurons were identified by autoradiography and by immunocytochemistry and consisted of fusiform and multipolar neurons. The proportion of both types remained steady until 15 days in vitro. Under these conditions, the addition of triiodothyronine (10(-9) M) at the initiation of the culture increased the size but not the number of dopaminergic neurons after 8 days in vitro. Furthermore, triiodothyronine significantly increased the dopaminergic neurite length and arborization. This morphological effect of triiodothyronine was associated with an increase of 35% in [3H]dopamine uptake. Our study shows that hypothalamic dopaminergic neurons are responsive to triiodothyronine which acts as a maintenance or trophic factor having an effect on neurite extension and arborization.

Cochlear synaptogenesis in the hypothyroid rat

The effects of congenital hypothyroidism on cochlear synaptogenesis were studied using transmission electron microscopy in 30 propylthiouracil-treated rat pups 1-35 days of age. No difference with controls was observed at the level of the inner hair cells which are known to mature essentially during the prenatal period. On the contrary, hypothyroidism resulted in numerous abnormalities in synaptogenesis at the level of the outer hair cells: abnormal persistence of numerous afferent dendrites and presynaptic specializations, incomplete development of efferent terminals and absence of formation of postsynaptic cisterns. It can be concluded that hypothyroidism results in severe retardation in the postnatal development of the innervation of outer hair cells.

Altered developmental changes of neuromuscular junction in hypo- and hyperthyroid rats

1. Effects of thyroid hormone on the development of neuromuscular junctions (n.m.j.s.) were investigated electrophysiologically in the diaphragms (sternal region) of normal, hypo- and hyperthyroid rats from the age of birth (day 0) to day 35.2. Hypothyroidism in new-born rats was induced either by daily administration of propylthiouracil to mothers or by subcutaneous injection of 150 muCi (131)I on day 1. Hyperthyroidism was induced by daily injection of thyroxine.3. In normal rats up to day 10, muscle fibres were innervated polyneuronally. By day 20, multiple innervation was eliminated and muscle fibres received only a single input. In hypothyroid rats elimination of polyneuronal innervation was retarded by 5-8 days, while in hyperthyroid rats the elimination was accelerated by 2-3 days.4. The frequency of miniature end-plate potentials (m.e.p.p.s) in normal rats increased from one per 40 sec on days 0-5 to 1/sec on days 25-35. The m.e.p.p. frequency in hypothyroid rats was 25-65% of that in normal rats of the same age. In hyperthyroid rats the m.e.p.p. frequency was normal up to day 18 but subnormal afterwards. The duration of m.e.p.p. measured on day 22-23 was slower in hypothyroid rats and faster in hyperthyroid rats, relative to m.e.p.ps in normal rats.5. The sensitivity to acetylcholine (ACh) at extrajunctional regions in normal rats was about 100 mV/nC at birth and declined to 1 mV/nC by day 26. In hypothyroid rats, the ACh sensitivity was as high as 30 mV/nC on day 26; in hyperthyroid rats, ACh sensitivity on day 26 was undetectable.6. With pairs of nerve stimuli (applied at a 50 msec interval), the second end-plate potential was facilitated until day 10 and depressed after day 16 in normal rats. This shift from facilitation to depression during development was not altered in either hypo-or hyperthyroid rats.7. It is concluded that the lack and excess of thyroid hormone retards and facilitates the development of n.m.j.s. respectively. Possible mechanisms for this altered development are discussed.

S-100 protein in the brain of hypothyroid adult rats: an immunochemical and immunocytochemical study

The levels and the distribution of S-100 protein were studied in the brain of hypothyroid adult rats. The concentration of the protein was significantly increased in the soluble fraction of hypothyroid rat brain as compared with controls (P less than 0.001). This finding probably reflects the increased number of astroglial cells, which has been widely reported in hypothyroid adult rats. The immunocytochemical ultrastructural distribution of the protein in the cerebellar cortex was not affected by thyroid deficiency. The protein was thus confined to the cytoplasm and the nucleus of the astroglial cells both in treated and untreated rats.

Effect of hypo- and hyperthyroidism on hexokinase in the developing cerebellum of the rat

Total hexokinase levels (units/g tissue) have been measured during postnatal development of the cerebellum in control, hypothyroid, and hyperthyroid rats. In addition. distribution of hexokinase in the developing cerebellum has been observed with an immunofluorescence method. Hypothyroidism delays the normally observed postnatal increase in total hexokinase activity, whereas hyperthyroidism accelerates the increase. In normal animals, hexokinase levels in maturing Purkinje cells pass through a transient increase, with maximal levels at approximately 8 days postnatally followed by rapid decline to relatively low levels by 12 days; hypothyroidism delays this transient increase and subsequent decline, but hyperthyroidism does not appear to affect markedly the timing of this phenomenon. Cerebellar glomeruli are relatively enriched in hexokinase content, as judged by their intense fluorescence. Hypothyroidism delays the development of intensely stained glomeruli. Hyperthyroidism did not appear to cause precocious increase in numbers of glomeruli but may have increased the rate at which the hexokinase was assimilated by newly formed glomeruli. The effects of hypo- and hyperthyroidism on total cerebellar hexokinase levels are interpreted in terms of the effect of thyroid hormone on the biochemical maturation of synaptic structures rich in hexokinase.

Correction of the biochemical effects of neonatal hypothyroidism by daily low doses of thyroxine. Comparative effects of hyperthyroidism and these corrections

The purpose of the present communication was to study the corrective effects of low daily thyroxine doses, on the cerebellum biochemical maturation in propylthiouracile (PTU)-treated rats during the early postnatal life. The corrected hypothyroid animals were compared to the normal, hypo- and hyperthyroid ones. The protein, RNA and DNA cerebellar contents were evaluated at 6, 10, 14, 18 and 35 days old animals. At all ages hypothyroidism and hyperthyroidism decreased cerebellar protein, RNA and DNA contents, except in 35-day-old hyperthyroid animals, where DNA content returned to normal level. In these two experimental groups, protein/DNA and RNA/DNA ratios were higher than those of controls at 10 days and lower at 35 days. In hypothyroid animals treated by corrective doses of T4, cerebellar protein, RNA and DNA contents and DNA concentration were not different from hyperthyroid animal values at all stages, while protein/DNA and RNA/DNA ratios were higher than those of hyperthyroid animals. Administration of physiological doses of T4 to hypothyroid animals led to the same effects as higher doses in normal animals. Thus, neonatal hypothyroidism seems to lower the sensitivity threshold of the cerebellum to thyroid hormone effects.

Separation of cell types from the developing cerebellum

The heterogenous population of perikarya (cells) obtained by dissociating cerebellar tissue of developing postnatal rats was separated by sedimentation at unit gravity. Peak fractions, defined by monitoring the distribution of different size classes with a particle analyzer, were enriched in ultrastructurally well-preserved and metabolically competent specific cell types. These fractions included the peak of rapidly sedimenting cells comprising large neurones, such as Purkinje cells, which accounted for about 50% of the cells (vs. 2% in the initial cell suspension) and for a much greater proportion of the total cell mass (over 80%). More slowly sedimenting peak fractions contained 2-5-fold enrichments in replicating cells (assessed in terms of [3H]thymidine incorporation into DNA or by the frequency of mitotic cells), astroglia-like cells and external and differentiating granule cells, respectively. Separated, replicating cells continued synthesizing DNA in vitro; the [3H]thymidine incorporation rate was about 5-fold greater in the fraction enriched in proliferating cells than in the total cell suspension. Besides their structural integrity, the viability of the cells was also indicated by the finding that the proportion of trypan blue-excluding cells in all fractions exceeded 80%. Moreover, protein synthesis, in terms of incorporation of labelled amino acids, continued in the separated cells at a linear rate for a relatively long time. The rate per cell was highest in the large neuronal fraction, and lowest in the astroglia-like fraction.

The effects of postnatal hyper- and hypothyroidism on the development of D-amino acid oxidase in rat cerebellum and brain stem

Treatment of rats with propylthiouracil for the first 30 days of postnatal life drastically retards the ontogenesis of D-amino acid oxidase in the brain stem and cerebellum. There is a marked terminal deficit of D-AAO in both the brain stem (--64%) and cerebellum (--67%) at 94 days (adults) despite the near euthyroid status at this age. If initiated early enough, thyroxine replacement therapy reverses the effects of PTU on the development of D-AAO. Hyperthyroidism significantly accelerates the development of D-AAO in both brain stem and cerebellum. Nonetheless, animals treated with thyroxine the first month of life display a net deficit of cerebellar D-AAO content in adulthood. The results are discussed in terms of the localization of D-AAO in cell types especially sensitive to thyroid hormone: (1) a cell type which is among the last to derive from the external germinal zone in the developing cerebellum, and which in the adult is located adjacent to the Purkinje cell soma; and (2) mossy fiber neurons and cerebellar glomeruli.