Effect of thyroid deficiency on cell acquistion in the postnatal rat brain: a quantitative histological study

Methods for Inferring Cell Cycle Parameters Using Thymidine Analogues

Tritiated thymidine autoradiography, 5-bromo-2'-deoxyuridine (BrdU) 5-chloro-2'-deoxyuridine (CldU), 5-iodo-2'-deoxyuridine (IdU), and 5-ethynyl-2'-deoxyiridine (EdU) labeling have been used for identifying the fraction of cells undergoing the S-phase of the cell cycle and to follow the fate of these cells during the embryonic, perinatal, and adult life in several species of vertebrate. In this current review, I will discuss the dosage and times of exposition to the aforementioned thymidine analogues to label most of the cells undergoing the S-phase of the cell cycle. I will also show how to infer, in an asynchronous cell population, the duration of the G₁, S, and G₂ phases, as well as the growth fraction and the span of the whole cell cycle on the base of some labeling schemes involving a single administration, continuous nucleotide analogue delivery, and double labeling with two thymidine analogues. In this context, the choice of the optimal dose of BrdU, CldU, IdU, and EdU to label S-phase cells is a pivotal aspect to produce neither cytotoxic effects nor alter cell cycle progression. I hope that the information presented in this review can be of use as a reference for researchers involved in the genesis of tissues and organs.

Cell cycle analysis in the rat external granular layer evaluated by several bromodeoxyuridine immunoperoxidase staining protocols

An important step in bromodeoxyuridine (BrdU) immunohistochemistry is the production of single-stranded DNA to make the incorporated BrdU accessible to the antibodies. This paper examines the effect of distinct DNA denaturation pretreatments (DNase I, sodium citrate buffer, endonuclease Eco RI and exonuclease III, and HCl hydrolysis) on detection of BrdU. We found that all the methods used in the partial denaturation of DNA combined good nuclear immunostaining with acceptable tissue integrity. We also observed that these immunohistochemical protocols revealed a spatial pattern in the distribution of DNA-synthesizing cells within the cerebellar external granular layer (EGL) of 10-day-old rats, allowing us to estimate the fraction of S-phase cells. Our results indicate that detection of BrdU-stained cells is affected by the distinct histological procedures used in such detection. Additionally, as the duration and phases of the cell cycle in EGL neuroblasts are estimated in accordance with BrdU detection, an effect on this detection can render the measurement of cell cycle inaccurate. The present work shows that DNase I and citrate buffer, at appropriate conditions, may be good alternatives for acid denaturation. However, they are less sensitive than autoradiographic techniques that use ³H-thymidine administration. Finally, current data reveal that short survival times after a single BrdU exposure do not seem to affect the cell cycle progression of the EGL neuroblasts.

Formation of excess sublobules in the cerebellum of hypothyroid rats

Cerebellar folia may increase in number in hypothyroid rats (Lauder et al., 1974; Hasebe et al., 2008a). In this study, we aimed to confirm the formation of an excess sublobule and to determine whether excess sublobules are consistently formed in conserved positions in hypothyroid rats. Instead of the foliation pattern partitioned by cerebellar fissures, we employed the bifurcation pattern of the internal granular layer for investigation of complexity of the cerebellar cortex in hypothyroid rats. The basic foliation pattern of the cerebellum was intact in hypothyroid rats, but lobules III to IX frequently showed an increase in the number of sublobules. The excess sublobules were mainly found in the folia and along the shallow region of the fissure. In other words, the excess sublobules were not located in random locations but rather in specific locations. The area in the internal granular layer of lobules V to IX was significantly larger than that in control rats. From the increased area of the internal granular layer it may be inferred that internal granular cells increase in number than those in normal rats. In our study, regions within the cerebellum that show an excess of sublobules correlate with regions that show an intermediate to late-forming internal granular layer (Altman, 1969). Our observations fit with the view that excess sublobules are formed by the external granular layer showing prolonged cell proliferation and hypothyroidism predominantly has an adverse impact on the intermediate to late phases in development of the internal granular layer.

Current and Potential Rodent Screens and Tests for Thyroid Toxicants

This article reviews current rodent screens and tests to detect thyroid toxicants. Many points of disruption for thyroid toxicants are outlined and include: (a) changes in serum hormone level; (b) thyroperoxidase inhibitors; (c) the perchlorate discharge test; (d) inhibitors of iodide uptake; (e) effects on iodothyronine deiodinases; (f) effects on thyroid hormone action; and (g) role of binding proteins (e.g., rodent transthyretin). The major thyroid endpoints currently utilized in existing in vivo assay protocols of the Organization for Economic Cooperation and Development (OECD), Japanese researchers, and U.S. Environmental Protection Agency (EPA) include thyroid gland weight, histopathology, circulating thyroid hormone measurements, and circulating thyroid-stimulating hormone (TSH). These endpoints can be added into the existing in vivo assays for reproduction, development, and neurodevelopment that are outlined in this chapter. Strategic endpoints for possible addition to existing protocols to detect effects on developmental and adult thyroid endpoints are discussed. Many of these endpoints for detecting thyroid system disruption require development and additional research before they can be considered in existing assays. Examples of these endpoints under development include computer-assisted morphometry of the brain and evaluation of treatment-related changes in gene expression, thyrotropin-releasing hormone (TRH) and TSH challenge tests, and tests to evaluate thyroid hormone (TH)-dependent developmental events, especially in the rodent brain (e.g., measures of cerebellar and cortical proliferation, differentiation, migration, apoptosis, planimetric measures and gene expression, and oligodendrocyte differentiation). Finally, TH-responsive genes and proteins as well as enzyme activities are being explored. Existing in vitro tests are also reviewed, for example, thyroid hormone (TH) metabolism, receptor binding, and receptor activation assays, and their restrictions are described. The in vivo assays are currently the most appropriate for understanding the potential effects of a thyroid toxicant on the thyroid system. The benefits and potential limitations of the current in vivo assays are listed, and a discussion of the rodent thyroid system in the context of human health is touched upon. Finally, the importance of understanding the relationship between timing of exposure, duration of dose, and time of acquisition of the endpoints in interpreting the results of the in vivo assays is emphasized.

Methylmercury inhibition of type II 5'-deiodinase activity resulting in a decrease in growth hormone production in GH3 cells

We have recently reported that methylmercury (MeHg) inhibits type II iodothyronine 5'-deiodinase (D2) activity in mouse neuroblastoma NB41A3 cells. In the present study, we determined the biological significance of D2 inhibition by MeHg. GH3 rat pituitary tumor cells were treated with MeHg and D2 activity and production of a thyroid hormone-responsive gene product, growth hormone (GH) were analyzed. MeHg inhibited D2 activity and decreased thyroxine (T4)-induced, but not 3,3',5-triiodothyronine (T3)-induced GH secretion in GH3 cells. Our results suggest that MeHg inhibition of D2 activity might be involved in the inhibition of GH production in GH3 cells. Thus, D2 inhibition could be a novel mechanism involved in MeHg toxicity.

Environmental perchlorate: why it matters

The only known mechanism of toxicity for perchlorate is interference with iodide uptake at the sodium-iodide symporter (NIS). The NIS translocates iodide across basolateral membranes to the thyroid gland so it can be used to form thyroid hormones (TH). NIS is also expressed in the mammary gland during lactation, so that iodide can be transferred from a mother to her child. Without adequate iodide, an infant cannot produce sufficient TH to meet its developmental needs. Effects expected from perchlorate are those that would be seen in conditions of hypothyroidism or hypothyroxinemia. The probability of a permanent adverse effect is greatest during early life, as successful neurodevelopment is TH-dependent. Study of perchlorate risk is complicated by a number of factors including thyroid status of the mother during gestation, thyroid status of the fetus, maternal and infant iodine intake, and exposure of each to other TH-disrupting chemicals. Perhaps the greatest standing issue, and the issue most relevant to the field of analytical chemistry, is the simple fact that human exposure has not been quantified. This review will summarize perchlorate's potential to adversely affect neurodevelopment. Whether current environmental exposures to perchlorate contribute to neuro-impairment is unknown. Risks posed by perchlorate must be considered in conjunction with iodine intake.

Review of animal models for autism: implication of thyroid hormone

Autism is a behaviorally defined disorder associated with characteristic impairments in social interactions and communication, as well as restricted and repetitive behaviors and interest. Its prevalence was once thought to be 2/10,000, but recently several large autism prevalence reviews revealed that the rate of occurrence was roughly 30/10,000. While it has been considered a developmental disorder, little is certain about its etiology. Neuroanatomical studies at the histological level in the brains of autistic patients provide many arguments in the etiology of autism. Results from postmortem and imaging studies have implicated many major structures of the brain including the limbic system, cerebellum, corpus callosum, basal ganglia and brainstem. There is no single biological or clinical marker for autism. While several promising candidate genes have been presented, the critical loci are yet unknown. Environmental influences such as rubella virus, valproic acid, and thalidomide exposure during pregnancy are also considered important, as concordance in monozygotic twins is less than 100% and the phenotypic expression of the disorder varies widely. It is thus hypothesized that non-genetic mechanisms contribute to the onset of autistic syndrome. In light of these ambiguities, hope is held that an animal model of autism may help elucidate matters. In this article, we overview most of the currently available animal models for autism, and propose the rat with mild and transient neonatal hypothyroidism as a novel model for autism.

Methylmercury inhibits type II 5'-deiodinase activity in NB41A3 neuroblastoma cells

Methylmercury (MeHg) is a well-known neurotoxicant and prenatal exposure to MeHg results in severe brain damage. Since MeHg has a high affinity for thiol groups, we sought to determine whether MeHg inhibited type II iodothyronine deiodinase (D2) activity, by which prohormone thyroxine (T4) is converted to active thyroid hormone, 3,5,3'-triiodothyronine (T3) in the brain, using NB41A3 mouse neuroblastoma cells. In MeHg-treated cells, D2 activity was inhibited in a dose- and time-dependent manner; relatively low concentrations of MeHg (30 nM) inhibited D2. Kinetic analysis using a double reciplocal plot of D2 activity revealed competitive inhibition by MeHg. DTT protected D2 from MeHg when cells were incubated with both MeHg and DTT or when MeHg was added to the assay buffer containing DTT and cell sonicates from untreated cells. Removal of MeHg from culture medium did not recover D2 activity. These results demonstrate that MeHg inhibited D2 activity in NB41A3 cells and the selenocysteine in the catalytic subunit of D2 may be involved in the inhibitory action of MeHg. Further our results suggest that T3 deficiency due to D2 inhibition in the brain may be involved in the neurotoxicity of MeHg.

Regulation of cerebellar neuronal migration and neurite outgrowth by thyroxine and 3,3',5'-triiodothyronine

The timing of granule cell migration in the developing cerebellum is regulated by thyroid hormone. Granule cell migration depends on the recognition of extracellular neuronal guidance molecule(s), such as laminin, and this, in turn, requires cell surface adhesion molecules (integrins) that are anchored on the cell membrane by the actin cytoskeleton. While many of the actions of thyroid hormone, specifically 3,5,3'-triiodothyronine (T3), are mediated by regulated gene expression, both thyroxine (T4) and 3,3',5'-triiodothyronine (rT3) also exert direct, positive control of the quantity of polymerized actin in cultured astrocytes without affecting gene expression. T4-dependent actin polymerization has been shown to (i) participate in the immobilization of laminin to the cell surface, (ii) help deposit laminin in the molecular layer of the developing cerebellum, and (iii) anchor integrin(s) that recognize laminin present in the extracellular matrix. In this study, we show that both T4 and rT3, but not T3, directly regulate the F-actin content of elongating neurites of cerebellar neurons. T4 and rT3 also promoted extensive granule cell migration from cerebellar explants, as well as, dense cell clustering and extensive neuronal process formation when granule cells were grown on a laminin-coated surface. Both granule cell migration and neuronal process outgrowth were markedly attenuated by the addition of integrin-blocking antibodies or binding peptides, by the absence of thyroid hormone or the presence of T3. These data suggest that the T4-dependent actin polymerization in developing neurons is necessary for these migrating cells to recognize the laminin guidance molecule, thereby providing a novel molecular mechanism for the profound influence of thyroid hormone on brain development that is independent of regulated gene expression.

Effects of excess thyroid hormone on cell death, cell proliferation, and new neuron incorporation in the adult zebra finch telencephalon

Widespread telencephalic neuronal replacement occurs throughout life in birds. We explored the potential relationship between thyroxine (T4) and cell turnover in the adult male zebra finch. We found that many cells in the zebra finch brain, including long-projection neurons in the high vocal center (HVC), stained positively with an antibody to thyroid hormone receptors (TR). Labeling was generally weak in the ventricular zone (VZ) that gives rise to new neurons but some proliferative VZ cells and/or their progeny, identified by [3H]-thymidine labeling, co-labeled with anti-TR antibody. Acute T4 treatment dramatically increased the number of pyknotic and TUNEL-positive cells in HVC and other telencephalic regions. In contrast, degenerating cells were never observed in the archistriatum or sub-telencephalic regions, suggesting that excess T4 augments cell death selectively in regions that show naturally occurring neuronal turnover. VZ mitotic activity was not altered shortly after acute T4 treatment at a dosage that stimulated cell death, although [3H]-labeling intensity per cell was slightly reduced. Moreover, the incorporation rates for neurons formed shortly before or after acute hormone treatment were no different from control values. Chronic T4 treatment resulted in a reduction in the total number of HVC neurons. Thus, hyperthyroidism augmented neuronal death, which was not compensated for by neuronal replacement. Collectively, these results indicate that excess T4 affects adult neuronal turnover in birds, and raises the possibility that thyroxine plays an important role in the postnatal development of the avian brain and vocal behavior.

Thyroid hormone regulates the extracellular organization of laminin on astrocytes

Astrocytes produce laminin, a key extracellular matrix guidance molecule in the developing brain. Laminin is bound to transmembrane receptors on the surface of astrocytes known as integrins, which are, in turn, bound to the microfilament meshwork inside the astrocyte. Previous studies have shown that T4 regulates the pattern of integrin distribution in astrocytes by modulating the organization of the microfilaments. In this study, the effect of thyroid hormone on the secretion and topology of laminin in astrocytes was examined. Linear arrays of secreted laminin were observed on the surface of the T4-treated astrocytes within 10 h after seeding the cells onto poly-D-lysine-coated coverslips and became an organized meshwork by 24 h. In contrast, little if any laminin was identified on the surface of either hormone-deficient or T3-treated cells until 36 h after seeding and then was restricted to punctate deposits. Secretion of laminin into the medium by hormone-deficient and T3-treated cells was significantly greater than that by T4-treated cells. Conversely, deposition of laminin into the extracellular matrix was significantly greater in T4-treated cells than in hormone-deficient and T3-treated cells. Thyroid hormone had no effect on the production of laminin by astrocytes. These data show that T4 regulates the extracellular deposition and organization of laminin on the surface of astrocytes and provide a mechanism by which this morphogenic hormone can influence neuronal migration and axonal projection in the developing brain.

Thyroid hormone regulates the expression of laminin in the developing rat cerebellum

In the rat cerebellum, migration of neurons from the external granular layer to the internal granular layer occurs postnatally and is dependent upon the presence of thyroid hormone. In hypothyroidism, many neurons fail to complete their migration and die. Key guidance signals to these migrating neurons are provided by laminin, an extracellular matrix protein that is fixed to the surface of astrocytes. Expression of laminin in the brain is developmentally timed to coincide with neuronal growth spurts. In this study, we examined the role of thyroid hormone on the expression and distribution of laminin in the rat cerebellum. We show that laminin content steadily increased 2- to 3-fold from birth to maximal levels on postnatal day 8-10 then steadily decreased to a plateau by postnatal day 12 in the euthyroid cerebellum. Immunoreactive laminin appeared in the molecular layer of the euthyroid cerebellum by postnatal day 4, reached maximal intensity by postnatal day 8-10, and was gone by postnatal day 14. In contrast, laminin content in the hypothyroid cerebellum remained unchanged from birth until postnatal day 10 and then increased to maximal levels over the next two days; maximal levels were approximately 35% less than those levels in the euthyroid cerebellum. Laminin staining did not appear in the molecular layer of the hypothyroid rat cerebellum until postnatal day 10, reached maximal intensity by postnatal day 15 and disappeared by postnatal day 18, despite the continued presence granular neurons in the external granular layer. These data indicate that the disruption of the timing of the appearance and regional distribution of laminin in the absence of thyroid hormone may play a major role in the profound derangement of neuronal migration observed in the cretinous brain.

Neurotrophins promote the survival and development of neurons in the cerebellum of hypothyroid rats in vivo

The development of cerebellar cortex is strongly impaired by thyroid hormone (T3) deficiency, leading to altered migration, differentiation, synaptogenesis, and survival of neurons. To determine whether alteration in the expression of neurotrophins and/or their receptors may contribute to these impairments, we first analyzed their expression using a sensitive RNAse protection assay and in situ hybridization; second, we administered the deficient neurotrophins to hypothyroid animals. We found that early hypothyroidism disrupted the developmental pattern of expression of the four neurotrophins, leading to relatively higher levels of NGF and neurotrophin 4/5 mRNAs and to a severe deficit in NT-3 and brain-derived neurotrophic factor (BDNF) mRNA expression, without alteration in the levels of the full-length tyrosine kinase (trk) B and trkC receptor mRNAs. Grafting of P3 hypothyroid rats with cell lines expressing high levels of neurotrophin 3 (NT-3) or BDNF prevented hypothyroidism-induced cell death in neurons of the internal granule cell layer at P15. In addition, we found that NT-3, but not BDNF, induced the differentiation and/or migration of neurons in the external granule cell layer, stimulated the elaboration of the dendritic tree by Purkinje cells, and promoted the formation of the mature pattern of synaptic afferents to Purkinje cell somas. Thus, our results indicate that both granule and Purkinje neurons require appropriate levels of NT-3 for normal development in vivo and suggest that T3 may regulate the levels of neurotrophins to promote the development of cerebellum.

Thyroid hormone promotes BCL-2 expression and prevents apoptosis of early differentiating cerebellar granule neurons

Programmed cell death is a basic cellular process that has aroused much interest in recent years. Like immune cells, cultures of cerebellar granule neurons are very homogeneous and provide a unique opportunity for quantifying by flow cytometry one form of programmed cell death in the CNS, the apoptosis, and for studying its regulation by neurotrophic factors. We found that thyroid hormone promoted postmitotic survival by preventing the apoptosis of newly formed and early differentiated granule neurons in a dose-dependent manner. This regulation could be through the protein bcl-2, which is known to prevent cell death. This protein was present at all stages of granule neuron differentiation and appeared to be developmentally regulated. It was underexpressed in apoptotic granule neurons. The protein content of the cerebellum in hypothyroid rats was drastically reduced. In contrast, thyroid hormone caused a marked dose-dependent increase in the amounts of this protein in granule neuron cultures. The possibility that thyroid hormone may be directly or indirectly required to promote cell survival is discussed, in terms of the hormone control of the local delivery of neurotrophins, such as NGF and NT-3, as well as the expression of their low affinity receptors, gp75. We suggest that thyroid hormone has a permissive action on the developing CNS.

Quantitative [3H]thymidine autoradiography of neurogenesis in the olfactory epithelium of developing normal, hypothyroid and hypothyroid-rehabilitated rats

We recently reported that postnatal hypothyroidism results in marked reduction in surface area and cell number in the rat olfactory epithelium (OE) and recovery from this condition is accompanied by compensatory growth and restitution of these parameters. To explore the correlative changes in olfactory neurogenesis, i.e. mitotic activity of basal cells (BCs) and migration and survival of developing olfactory receptor neurons (ORNs), hypothyroid rats at postnatal (P) days of P10, P25 and P75 were injected with [3H]thymidine and OE was examined by quantitative autoradiography to determine the density of labeled nuclei at the BC and ORN zones at days 1, 5 and 15 post-injection. These data were compared with those of age-matched controls as well as young adult rats allowed to recover from hypothyroidism at the end of the suckling period (P25). Hypothyroidism was induced by administration of propylthiouracil (PTU) from birth in the drinking water (1 g/l) for 10, 25 and 90 days; recovery was induced by withdrawal of PTU at P25. The results indicated that the densities of labeled nuclei in the BC and ORN zones were not significantly altered in the suckling hypothyroid rats. In the P75 hypothyroid rats density of labeled BC nuclei was unaffected 1 day after injection but was significantly (36%) more than controls 5 days after injection; the density of neuronal nuclei in the ORN zone of P75 injected rats was markedly and significantly reduced (56% and 37% at 5- and 15-days post-injection). Data indicate that mitotic activity of BCs and their migration into the ORN zone is not affected in the hypothyroid infant rats but migration and/or survival of developing ORNs are markedly reduced in the postweaning growing rats made hypothyroid from birth. In rats allowed to recover from hypothyroidism at P25 and injected with labeled thymidine at P75, the density of labeled BC nuclei were significantly increased (48% and 43% at 1- and 5-days post-injection) compared to normal rats suggesting elevated levels of neurogenesis; density of ORN nuclei, however, were the same as controls. The results indicate critical regulatory influences of thyroid hormones on olfactory neurogenesis in the rat olfactory receptor sheet, in particular during the postweaning period.

Autocrine regulation of proliferation of cerebellar granule neurons by nerve growth factor

Premigratory cerebellar granule neurons, which highly express nerve growth factor (NGF), low (gp75NGFR) and high (gp140trkA) affinity NGF receptors, were used as a physiological model to investigate the effects of NGF on neuronal replication. Studies in vivo and on cultures showed that NGF stimulates DNA synthesis, mitotic activity and related cell acquisition by initiating the entry of cells into the S phase and regulating their time in the G1 and S phases. The NGF-induced effects were blocked in vivo and in vitro by both monoclonal anti-blocked in vivo and in vitro by both monoclonal anti-NGF and anti-gp75NGFR antibodies. These results clearly demonstrate that NGF is essential for the crucial first step of cerebellar ontogenesis and support the idea that low affinity receptors are involved in the biological response, possibly by interacting with gp140trkA. By comparison with a number of well known mitogens, the high affinity form could be the main transducer of the mitogenic signal pathway. The early developing cerebellum appears therefore to be the first autocrine (and/or paracrine) model of NGF action on neurogenesis in the CNS.

Developmental plasticity of the rat olfactory receptor sheet as shown by complete recovery of surface area and cell number from extensive early hypothyroid growth retardation

To assess the effects of early thyroid deficiency, and recovery from this condition on growth and development of olfactory epithelium (OE), male Sprague-Dawley rat pups were rendered hypothyroid by addition of propylthiouracil (PTU) to their drinking water from birth. At weaning some rats continued to receive PTU while others ere allowed to recover by withdrawal of PTU. Body weights and plasma thyroxine levels were determined in all groups. At the ages of 25, 50 and 90 days, the OE of these hypothyroid and 'recovery' rats were compared with age-matched controls for surface area, epithelial thickness, density and total number of olfactory receptor neurons, basal cells and supporting cells, using morphometric and cell counting methods. Normal rats showed marked and highly significant increases in the OE surface area and olfactory neuron number (2.6- and 2.3-folds) during the post-weaning period. In the hypothyroid rats, body growth and thyroxine levels were severely suppressed. The OE in the 25-day-old hypothyroid rats showed more than 40% reduction in surface area and cell number, compared to controls, but mean epithelial thickness and surface density of cells were unchanged. In the post-weaning hypothyroid rats, the expansion of surface area was severely retarded, and increase in cell number ceased entirely. In rats allowed to recover by PTU withdrawal, by 90 days of age, body weight and size had markedly increased but had not caught up completely; however, thyroxine levels were restored to normal and the surface area and cell number in the OE had increased in a compensatory manner, completely restoring the deficiencies in OE growth, including surface area, numbers of receptor neurons, basal cells and supporting cells. The results indicate marked growth plasticity of OE in the post-weaning rats. This pronounced ability to recover from early growth retardation contrasts with that seen in central neural structures, and indicates the great potential of OE for use as a model neural system for the study of recovery from early damage and growth retardation.

Thyroid hormone receptor/c-erbA: control of commitment and differentiation in the neuronal/chromaffin progenitor line PC12

The c-erbA proto-oncogenes encode nuclear receptors for thyroid hormone (T3), a hormone intimately involved in mammalian brain maturation. To study thyroid hormone receptor (TR) action on neuronal cells in vitro, we expressed the chicken c-erbA/TR alpha-1 as well as its oncogenic variant v-erbA in the adrenal medulla progenitor cell line PC12. In the absence of T3, exogenous TR alpha-1 inhibits NGF-induced neuronal differentiation and represses neuron-specific gene expression. In contrast, TR alpha-1 allows normal differentiation and neuronal gene expression to occur in the presence of T3. Finally, TR alpha-1-expressing cells become NGF-responsive for proliferation when T3 is absent, but NGF-dependent for survival in presence of T3. A similar differentiation induction by NGF plus T3 was observed in a central nervous system-derived neuronal cell line (E 18) expressing exogenous TR alpha-1. Together with the finding that TR alpha-1 constitutively blocked dexamethasone-induced differentiation of PC12 cells into the chromaffin pathway, these results suggest that TR alpha-1 plays an important role in regulating commitment and maturation of neuronal progenitors. In contrast, the v-erbA oncogene, a mutated, oncogenic version of TR alpha-1, partially but constitutively inhibited NGF-induced neuronal differentiation of PC12 cells and potentiated dexamethasone-induced chromaffin differentiation, giving rise to an aberrant "interlineage" cell phenotype.

Thyroidal influence on the cell surface GM1 of granule cells: its significance in cell migration during rat brain development

1. No difference was observed in the in vitro growing ability of granule cells isolated from hypothyroid or normal rat brain. When granule cells were taken from hypothyroid rat brain and grown in normal culture medium containing 10% fetal calf serum, they behaved similarly to the granule cells obtained from normal rat brain. 2. In both cases there were progressive losses of in vitro growing ability of the granule cells with the age of the animal and it became impossible to grow them when derived from 21 days or older animals. 3. A marked decrease in cell surface GM1 was observed when the cells were maintained under thyroid hormone-deficient conditions in culture. 4. Anti-GM1 antibody was found to inhibit significantly the migration of granule cells along the astrocyte fibers. 5. These results indicate that GM1 has an important role in thyroid hormone-dependent postnatal brain maturation in rat.

Neuron addition in the postmetamorphic frog

Neuron number among somatic motoneurons, sensory neurons, and sympathetic postganglionic neurons that innervate the hindlimb was correlated with body length in the bullfrog, Rana catesbeiana. Two to three times more dorsal root and sympathetic ganglion neurons are found in the largest than the smallest specimens. Hindlimb motoneurons show a 20% increase in number, but this increase is restricted to the caudal third of the motor pool. Within this region, 60% more motoneurons are found among the largest frogs. Cell division does not appear to be the mechanism of neuron addition. Instead, we propose that a pool of undifferentiated neurons mature to maintain functional capabilities as the animal increases in size.

Effects of hypothyroidism upon the granular layer of the dentate gyrus in male and female adult rats: a morphometric study

The effects of hypothyroidism upon the structure of the central nervous system of adult rats are poorly understood in spite of evidence that the mature brain is vulnerable to this condition. Existing developmental studies show that the morphological changes induced by thyroid hormone deficiency are related to alterations in neurogenesis. We studied the granular layer of the dentate gyrus under different experimental conditions of hypothyroidism, because in rodents the neurogenesis of the granule cells continues during adulthood. The following groups of rats were analysed: 1) control; 2) hypothyroid from day 0 until day 180 (hypothyroid group); 3) hypothyroid until day 30 and henceforth maintained euthyroid (recovery group); and 4) hypothyroid since day 30 (adult hypothyroid group). Groups of 6 male rats and 6 female rats were analysed separately. The volume of the dentate gyrus granular layer and the numerical density of its neurons were evaluated, so we were able to estimate the total number of granule cells. Because in the experimental groups the volume of the granular layer and the numerical density of its neurons were reduced, the total number of granule cells was decreased. In the hypothyroid and recovery groups the alterations were identical and more striking than in the adult hypothyroid groups. The total number of granule cells displayed sexual differences in all groups studied except in the hypothyroid groups. The present results support the view that thyroid hormone deficiency interferes with the process of cell acquisition by reducing neuronal proliferation and that it also leads to increased cell death. These events underlie the irreversible morphological changes observed in the brain of hypothyroid rats, either during development or at maturity. The referred structural alterations are probably related to the functional deficits observed in this condition.

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

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

Postnatal development of the wild-type and weaver cerebellum after embryonic administration of propylthiouracil (PTU)

In this study we used propylthiouracil (PTU), a thyroid hormone-inhibiting compound, to render +/+, wv/+, and wv/wv embryos hypothyroid in order to test if the appearance of external granule layer (EGL) cell death in the weaver cerebellum is affected by alteration of granule cell development. At birth, the number of EGL cells in the PTU-treated cerebellum was reduced, compared to control animals, by 50%. Also, the amount of cell death was reduced in the PTU-treated wv/wv cerebellum. As adults, no differences were seen between PTU-treated and untreated mutant or normal cerebella. If the hypothyroid treatment that results in a 50% decrease in EGL cell number is due to an extension of cell cycling time, then the expression of the weaver phenotype of cell death likely follows granule cell exit from the cell cycle.

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.

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

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

Long-lasting effects of naltrexone, an opioid receptor antagonist, on cell proliferation in developing rat forebrain

Several studies have demonstrated in the past that endogenous opioid peptides and opioid receptors may be involved as mediators of brain tissue growth and function in the neonate. Applying histological and autoradiographic methods, we have examined the effect of the mu-receptor-specific antagonist, naltrexone, on the proliferation of the 4-12-week-old rat forebrain subependymal layer. We found that naltrexone, when given daily throughout the weaning period, evoked a long-lasting increase of the mitotic rate and the [3H]thymidine labelling index. This effect was most significant about 8-10 weeks after ending the naltrexone treatment. Although a direct influence of naltrexone on long-term subependymal cell proliferation cannot be excluded, we are discussing evidence of an indirect effect via suppression of noradrenergic activity in the forebrain.

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.

Correlated ultrastructural damage between cerebellum cells after early anticonvulsant treatment in mice

The anticonvulsants phenobarbital and diphenylhydantoin administered early in life to mice resulted in significant and long-lasting ultrastructural damage, including abnormalities of mitochondria, myelin sheaths, and lamellar inclusion bodies inside identified cells throughout the cortical layers of the cerebellum in treated vs control mice. The magnitude, distribution and duration of damage was age and treatment specific. No differences were detected in density of parallel fiber processes nor in synapse density within the molecular layer. Neuron profiles containing damaged organelles were not homogeneously distributed but made up only a small fraction of the total cell population examined. In our experiments, there was an overall within-animal correlation explaining 45% of the magnitude of damage in different cerebellar regions, but between synaptically connected cells, specifically mossy fiber axon varicosities and granule cell dendrite profiles, the subset population ratio of damaged-to-total mitochondria was highly significantly correlated (70-90%; P less than 0.001). We hypothesized that some correlated transneuronal degeneration and death in the central nervous system may have a transynaptically regulated component that first appears as correlated damage between synaptically connected cells, perhaps regardless of the degree of toxicity. The orderly cytoarchitecture and cell connections of the cerebellar cortex can be used to study these patterns of degeneration.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

Neonatal vasopressin antagonist treatment facilitates adult copulatory behavior in female rats and increases hypothalamic vasopressin content

This study confirms that i.c.v. administration of Arg-vasopressin (AVP, 20 ng) inhibits copulatory behavior in female rats (lordosis response, LR) and demonstrates that this inhibitory effect is blocked by the vasopressin antagonist (3-mercapto-3-methylbutyryl-Tyr-[Me])arginine vasopressin (dPTyr(Me)AVP, 20 ng, i.c.v.). The effects of neonatal AVP antagonist treatment on adult female copulatory behavior, hypothalamic content of AVP and AVP-like immunoreactive (AVP-ir) neurons were examined. dPTyr(Me)AVP was given to female rats 1 microgram/animal/day s.c. from day 1 through to day 7 (day 0 = day of birth). The females were ovariectomized as adults and sexual receptivity activated by submaximal doses of estradiol plus progesterone. The LR was significantly facilitated in the neonatally dPTyr(Me)AVP treated females who also showed a higher content and an increased number of AVP-ir neurons in the suprachiasmatic nucleus compared to saline controls. Functional, biochemical and immunocytochemical evidence is provided that neonatal exposure to an AVP antagonist induces persistent changes in central vasopressinergic neuronal mechanisms.

Thyroxine injections do not cause premature induction of thymidine kinase in sg/sg mice

Severe hyperthyroidism from the time of birth causes a premature induction and termination of thymidine kinase activity in the cerebella of wild-type mice. This leads to elevated enzyme levels at postnatal days 5 and 6, with significantly lower levels by postnatal day 7 (which is actually the time of peak activity in normal animals). In this study, neonatal hyperthyroidism does not have significant effects on postnatal day 5, 6, or 7 enzyme levels in the neurological mutant staggerer. This is consistent with the hypothesis that thyroid hormone exerts its effects via the Purkinje cells, which are reduced in number and grossly stunted in the mutant.

Survival and development of embryonic and postnatal cerebellum transplanted into adult rat hosts: effect of growth as explants in culture prior to transplantation

The survival and organization of embryonic and postnatal cerebellum transplanted into intracephalic cavities in adult rat hosts was studied before and after growth as explants in tissue culture. It is well known that embryonic cerebellum survives transplantation and develops a typical cerebellar cytoarchitecture. This study demonstrates that early postnatal cerebellum survives transplantation poorly if at all using the intracephalic transplant paradigm. Postnatal cerebellum is capable of extensive growth and organization in explant culture; however, prior growth in explant culture did not enhance survival in the transplant situation. On the other hand, embryonic cerebellum grown in explant culture for as long as two weeks, to an age comparable to the postnatal cerebellar transplants, survives transplantation and develops similar cytoarchitectural features to directly transplanted embryonic cerebellum.

The cellular neurobiology of neuronal development: the cerebellar granule cell

Cerebellar granule cells in vivo and in vitro have been widely used in the study of the cellular neurobiology of neuronal development. We have described the basic neuroanatomical data on the granule cell in the developing and mature cerebellum. The importance of the cytoskeleton in determining the morphology of the granule cell and in process outgrowth and cell migration has been described. Extensive information is now available on the composition of the granule cell cytoskeleton. Cell surface glycoproteins are thought to be involved in the control of cell adhesion and cellular interactions during development. A number of surface molecules belonging to either the N-CAM or the Ng-CAM groups of glycoproteins have been studied in detail in the cerebellum. The role of these proteins in cell adhesion and in granule cell-astroglial interactions during granule cell migration has been reviewed. The survival and differentiation of neurones is controlled by soluble trophic factors. Several factors have been described which act as trophic factors for granule cells in vitro and may do the same in vivo. The numerous studies that have been carried out on the cerebellar granule cell have allowed us to describe certain aspects of the cellular neurobiology of this class of neurones as an example with general significance for the understanding of neuronal differentiation and function.

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.

Disappearance of Rohon-Beard neurons from the spinal cord of larval Xenopus laevis

Rohon-Beard neurons are primary sensory cells located in the spinal cord of embryonic lower vertebrates. The kinetics of their normal, gradual, but complete disappearance in Xenopus tadpoles has been followed. Levels of acid phosphatase activity, a common histochemical correlate of cell death, were assayed and found to increase at the time of onset of disappearance of Rohon-Beard cells. Ultrastructural examination revealed the presence of numerous secondary lysosomes, swelling of endoplasmic reticulum and mitochondria, and a decrease in nuclear density. The disappearance of Rohon-Beard neurons may be attributed to autophagic cell death involving lysosomal acid hydrolases. This process begins only a few days after the maturation of voltage- and neurotransmitter-dependent membrane conductances and the electrical uncoupling of these neurons. The loss of Rohon-Beard neurons in embryos whose development was arrested by crowding was appropriate for the developmental stage of the animals rather than their chronological age.

Neuronal maturation in the normal and hypothyroid rat cerebellar cortex studied with monoclonal antibody MIT-23

By immunocytochemistry we have studied the expression of the mitochondria-associated polypeptide MIT-23 during the postnatal development of the normal and hypothyroid rat cerebellar cortex, in afferent fibers, and also in neurons of the cerebellar nuclei. The glial cells are never immunoreactive. In all neurons of the cerebellar cortex, MIT-23 expression always occurs after the final mitosis and migration are complete, and persists throughout adult life. Almost all MIT-23 expression begins postnatally. A few Purkinje cells are already immunoreactive at birth and the rest begin expression during the following two days. Immunoreactive Golgi and granule cells are found from postnatal day 4 (P4), basket cells from P10, and stellate cells from P16. Purkinje cells from different anteroposterior regions of the vermis express different levels of MIT-23 with higher staining intensities in lobules I to IV. These differences appear early in development and are retained in the adult. MIT-23 expression in the hypothyroid cerebellar cortex differs from that in control animals only in minor ways. However, sections immunoperoxidase-stained with anti-MIT-23 antibody reveal that, in addition to previously reported alterations in cerebellar development due to a shortage of thyroid hormones, Purkinje cell axonal development is slowed down in the hypothyroid condition, and occasional Purkinje cells in normal and especially in hypothyroid animals have their somata and or dendrites in ectopic locations. Analysis of these cells reveals a preferential direction of dendritic trunk growth in the direction of the molecular layer. Furthermore, secondary branching of ectopic dendrites is confined exclusively to the developing molecular layer, as in normal Purkinje cells, thus suggesting that neither the mature nor immature granule cell environment is sufficient to sustain normal dendritic development.

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.

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.

Effect of removal of glutamine and addition of dexamethasone on the activities of glutamine synthetase, ornithine decarboxylase and lactate dehydrogenase in primary cultures of forebrain and cerebellar astrocytes

The regulation of glutamine synthetase (GS) and ornithine decarboxylase (ODC) was studied in primary cultures of two types of astrocytes derived from either newborn forebrain or 8-day-old cerebellum of the rat. In the 14-day-old cultures the specific activities of both these enzymes were about twice as great in forebrain astrocytes as in cerebellar astrocytes. Treatment with dexamethasone or removal of glutamine from the culture medium caused a marked increase in the specific activity of GS. The glutamine-mediated relative increase in GS activity was similar in both types of astrocytes. Removal of glutamine caused a transient reduction in ODC activity in the forebrain astrocytes, while in cerebellar astrocytes the activity remained markedly decreased throughout the period of glutamine deprivation. The severe reduction in ODC activity had relatively little effect on the cell numbers of protein content of the astrocyte cultures. The increase in GS activities, involving protein synthesis de novo, caused by removal of glutamine and by addition of dexamethasone, were additive and therefore probably mediated by different mechanisms. The induction of GS after glutamine removal was blocked by cycloheximide but not by alpha-amanitin, suggesting regulation at the post-transcriptional level. In contrast, the dexamethasone-mediated induction of GS appeared to be regulated at the transcriptional level, as it was markedly reduced by alpha-amanitin. None of these conditions had any effect on lactate dehydrogenase activity. Treatment with alpha-amanitin resulted in a complete suppression of the activity of ODC (a protein with a very short half life), in both the control and dexamethasone treated cultures. However, this enzyme activity was reduced only partially in astrocytes cultured in glutamine deficient medium, suggesting that under these experimental conditions the mRNA may be markedly stabilized in astroglial cells.

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

Stellate cell development was studied in the bullfrog cerebellum during spontaneous and thyroxine-induced metamorphosis using the Golgi-Kopsch method and electron microscopy. Cells that possessed axosomatic synapses and resembled stellate cells were present even in the incipient molecular layer of the cerebellum in the premetamorphic tadpole. These cells may have originated from the early, transient wave of external granule cells that have been reported in the cerebellum of premetamorphic tadpoles in the first 6 months of development, and may constitute the variant population of stellate cells that are present later during development or the degenerating cells that have been observed during metamorphosis as scattered dying cells in the molecular layer. Typical stellate cells, whose development resembled the genesis and differentiation of stellate cells in birds and mammals, were initially observed at the outer border of the molecular layer that is adjacent to the external granular layer during the onset of metamorphosis. These stellate cells were bipolar with processes extending in a plane perpendicular to elongating parallel fibers, and with progressive development, became multipolar with dendrites oriented in various directions with respect to the pia. Stellate cell axons innervate the dendrites and somata of Purkinje cells and other stellate cells, and can be categorized into two types: (1) axons with extensive branching near the soma of origin, and (2) long axons with few branches that occasionally terminate in the Purkinje cell layer. Atypical neurons that did not resemble typical stellate cells were also present in the molecular layer; these might be classified as a stellate cell variant. The generation and differentiation of stellate cells can be induced 1 to 2 years prematurely by administering thyroid hormone to premetamorphic tadpoles. Like most events of cerebellar histogenesis in the frog, stellate cell development also appears to be largely a thyroid-dependent phenomenon.

Con A-binding glycoproteins in the developing cerebellum of control and hypothyroid rats

Concanavalin A (Con A)-binding glycoproteins were studied during the postnatal development of the cerebellum of control and hypothyroid rats. Only 4 glycoprotein bands have a transient behavior in control animals. They progressively increase until the 13th day and markedly decline between the 15th and the 18th postnatal day. In the cerebellum of hypothyroid rats, the level of these compounds is greatly reduced and the previous decrease observed in controls is not found again. This defect of Con A-binding glycoproteins mainly localized on the plasma membrane of parallel fibers might be related to the reduced synaptogenesis observed in the molecular layer of hypothyroid rats between parallel fibers and Purkinje cell dendritic spines.

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.

Observations on cell growth and regulation of glutamine synthetase by dexamethasone in primary cultures of forebrain and cerebellar astrocytes

Cell growth, development of glutamine synthetase and its regulation by glucocorticoids, were studied in primary cultures of two types of astrocytes derived from rat brain, one from newborn forebrain and another from either newborn or 8-day-old cerebellum. Cell number per dish increased linearly following an initial decrease due to removal of non-astrocytic cells, and after the second week reached a stationary phase in both types of cultures at more or less the same time, although the cell number in cerebellar cultures was about 35% lower than in forebrain cultures of the same age. At all ages, irrespective of cell density, the cerebellar astrocytes were larger in size than the forebrain astroglial cells. The developmental curves for glutamine synthetase activity were similar in vitro and in vivo; however, the increase in enzyme activity in vitro was significantly greater than in vivo and this difference was more marked in forebrain than in cerebellar cultures. Throughout the period studied the specific activity of glutamine synthetase was significantly higher in forebrain astrocytes than in cerebellar astroglial cells. Treatment with dexamethasone caused a marked increase in the specific activity of glutamine synthetase. However, in agreement with our previous in vivo findings, the steroid induction in forebrain astrocytes was significantly less than that in cerebellar astrocytes. In culture, both types of astrocytes remain responsive to the hormone for longer than in vivo. The differences in the biochemical properties of the forebrain and cerebellar astrocytes seem to be intrinsic, and not related to the cell density or to the purity of the cultures.

Effect of thyroid hormone and serum on the development of Na+, K+-adenosine triphosphatase and associated ion fluxes in cultures from rat brain

The effect of culture conditions, serum supplementation or chemically defined medium and the influence of thyroid hormone were studied on the development of the Na+, K+-adenosine triphosphatase (Na+,K+-ATPase) and on the intracellular content of K+ and Na+ ions in cultures which either were greatly enriched in a neuronal cell type, the cerebellar granule cells, or contained a mixed population of cells (brain reaggregates). Foetal rat brain reaggregates displayed lower Na+,K+-ATPase activity when cultured in chemically defined medium than in the presence of serum. Supplementation of the serum-free medium with thyroid hormone resulted in a rise in the Na+,K+-ATPase activity and [3H]ouabain binding to levels similar to those found in the cultures grown in the serum-containing medium. Thyroid hormone had no significant effect on the Mg2+-ATPase activity and on the intracellular content of Na+ and K+ ions. In the granule cell-enriched cerebellar surface cultures the Na+,K+-ATPase activity was lower when the cells were grown in chemically defined medium compared with the serum-containing medium, and the intracellular Na+ to K+ ratio was higher. Thyroid hormone had no effect on the Na+,K+-ATPase activity, [3H]ouabain binding or Mg2+-ATPase activity. The hormone also failed to influence ATPase activities in cerebellar astrocytes maintained in chemically defined medium. Although thyroid hormone had no effect on the Na+,K+-ATPase activity of cultured cerebellar granule cells, treatment with the hormone resulted in a decrease in the ratio of intracellular Na+ to K+ ion content. The effect of the hormone on the Na+,K+-pump activity in live cells was therefore tested by estimating ouabain-sensitive 86Rb uptake. This was regulated as in other cell types, by the rate of Na+ entry: the Na+-ionophore monensin trebled the rate of 86Rb uptake, which was also increased (+30-100%) by 10% foetal calf serum, the maximal response being obtained by about 20 min exposure to serum. The effect was completely blocked by the Na+/H+ exchange inhibitor amiloride. The factor(s) in the serum responsible for the regulation of the Na+,K+-pump were, however, not the thyroid hormones, which failed to affect 86Rb uptake. On the basis of comparing thyroid hormone effects on the different cultures studied it was concluded that not every type of neural cell is target of the hormone action during development.