Influence of growth hormone on cerebral cortical RNA metabolism in immature hypothyroid rats

Expression of thyroid hormone receptor isoforms in the oligodendrocyte lineage

Thyroid hormone (T3) regulates brain development and function and in particular ensures normal myelination. Animal models and in vitro systems have been employed to demonstrate the effects of T3, which acts via nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression, such as myelin basic protein (MBP), in a hormone-dependent or -independent fashion. Two distinct genes, TR alpha and TR beta, encode several receptor isoforms with specific functions. This overview summarizes current knowledge on the cellular expression and the role of these isoforms and also examines the action of T3 on oligodendrocyte lineage cell types at defined developmental stages. Re-expression of TRs and also that of other transcription factors in oligodendrocytes may constitute some of the metabolic changes required for succesfull remyelination in the adult central nervous system after demyelinating lesions.

Enhancement of seminiferous tubular growth and spermatogenesis in testes of rats recovering from early hypothyroidism: a quantitative study

Testicular weight and DNA content were markedly reduced (63 and 69%) in weanling Long-Evans rat pups rendered hypothyroid from birth by administration of propylthiouracil (PTU), a reversible goitrogen. These growth deficits worsened to > 80% by continuing hypothyroidism beyond weaning, to days 50 and 90. Recovery of thyroid function, brought about by discontinuing PTU at weaning, resulted in a paradoxical stimulation of testis growth, amounting to increased weight (40%), DNA content (60%) and size by 90 days, compared to age-matched controls. In the 25-day or older hypothyroid rats, testicular structure was immature and spermatogenesis markedly delayed, as evident by closed lumen and significantly reduced diameter of seminiferous tubules (38%), thickness of germinal layer (70%), and number of primary spermatocytes (86%), compared to control. Hypothyroidism did not alter the number of tubules per testis cross section. In the 90-day recovery rats, numbers of seminiferous tubules were unchanged but tubular diameter was significantly (20%) larger than in controls and spermatogenesis appeared very active as indicated by significantly increased germinal layer thickness (22%) and total number and density of primary spermatocytes (55% and 40%). The results show that although postnatal hypothyroidism is deleterious for testicular growth and spermatogenesis, recovery from this condition leads to enhanced seminiferous tubular growth and spermatogenesis.

Effects of hormone therapy on the central cholinergic neurotransmission of the Snell dwarf mouse

Effects of growth and thyroid hormone therapy on central cholinergic neurotransmission has been followed in the cholinergic-deficient Snell dwarf mouse. Growth hormone and thyroxine can reverse the neurotransmission impairment even in adulthood. Furthermore, it appears that in the dw/dw mouse, hormone deficiency becomes determinant only after the critical period of neurogenesis, in a late postnatal developmental stage. At first sight, these observations show that the disturbed cholinergic neurotransmission of the mutant might be linked to the pituitary and thyroxine deficiency that is characteristic of this mouse. Hormone therapy selectively stimulates cholinergic activity in cholinergic-deficient structures, where it reactivates presynaptic markers. The differential responsiveness of the neuroanatomical and neurochemical targets could be linked to different regulatory hormone effects or to timing in respect to sensitivity to hormones during development.

Epidermal growth factor and bovine growth hormone stimulate differentiation and myelination of brain cell aggregates in culture

Bovine growth hormone (bGH) and epidermal growth factor (EGF) increased the activity of ornithine decarboxylase (ODC) in brain cell aggregates cultured in a serum-free chemically defined medium. ODC is considered as a marker of cell growth and differentiation. The effect of bGH and EGF on myelination was investigated by measuring two myelin markers, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and myelin basic protein (MBP). EGF treatment at days 2 and 5 caused a dose-dependent increase of both myelin markers at culture day 12. This increase could still be observed at culture day 19, indicating a prolonged action of EGF. The continual presence of bGH in the culture medium produced a large accumulation of MBP at day 19. This effect was dose-dependent and required the presence of triiodothyronine (T3). In contrast, the effect of bGH on CNP activity did not require the presence of T3. This is the first report showing a direct effect of bGH on CNS myelination in vitro and of EGF on both MBP accumulation and ODC activity.

Serotonin, 5-hydroxyindoleacetic acid and substance P content of discrete brain nuclei in rats made hypo- or hyperthyroid in the neonatal period: effect of growth hormone treatment

In order to determine the role of growth hormone (GH) in the therapeutic effect of thyroxine (T4), we measured the content of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and substance P in discrete brain nuclei of neonatally-induced hypothyroid rats and in neonatally-induced hypothyroid rats subsequently maintained on bovine growth hormone (b-GH) injections. Substance P was measured by radioimmunoassay whilst 5-HT and 5-HIAA levels were determined by HPLC with electrochemical detection. In neonatal hypothyroid rats, substance P concentration increased in 5 out of 11 brain nuclei dissected while 5-HT and 5-HIAA level increased in 7 out of 19 brain nuclei selected. Although b-GH-replacement therapy abolished the hypothyroid-induced accumulation of 5-HT and 5-HIAA in brain nuclei with exception of the substantia nigra zona reticulata, it did not influence the substance P accumulation. This suggests that the abnormal brain development observed during hypothyroidism may, in part, result from absence of growth hormone. We also observed that neonatal hyperthyroidism induced very little modification of 5-HT, 5-HIAA and substance P concentrations in discrete nuclei of the rat brain.

Complete recovery of growth deficits after reversal of PTU-induced postnatal hypothyroidism in the female rat: a model for catch-up growth

Newborn rats of both sexes were treated from birth with the anti-thyroid goitrogen, n-propylthiouracil (PTU) given in the drinking water of the litter (0.1% w/v). One group received the treatment for 25 days, another for 50 days, and a third group for 120 days. The experimental rats showed growth retardation as well as all other classical signs of developmental arrest or delays induced by postnatal hypothyroidism. In order to assess the ability of the hypothyroid animals to recover spontaneously from the retarded state, at days 25, 50 and 120 postnatal the PTU water was replaced with tap water. In each case, within 5-7 days after PTU withdrawal the animals began to show marked compensatory growth accompanied by many signs of behavioral and physiological recovery. In general, the male rats showed higher compensatory growth rates as compared to the females, enabling them to attain significantly higher body weights. However, when growth recovery was followed for up to 6 months it was found that the male rats were unable to attain complete catch-up growth, regardless of the age at which recovery began, while the females of all age groups were able to achieve this goal. In view of the severity of PTU-induced growth retardation, these results suggest significant plasticity of growth processes in the rat, especially in the female. It is suggested that male and female rats recovering from prolonged PTU-induced growth retardation offer a good model system for the study of biochemical, anatomical and physiological aspects of growth recovery and catch-up growth at both the cellular and organismic levels, particularly in relation to the effects of thyroid, growth hormone, and other growth-promoting factors.

Influences of both thyroid and bovine growth hormones on substance P, thyrotropin-releasing hormone, serotonin and 5-hydroxyindoleacetic acid contents in the lumbar spinal cord of developing rats

Neonatal hypothyroidism was induced by injection of 131I on the first living day whilst neonatal hyperthyroidism was induced by daily administration of high doses of thyroxine (T4). Following decapitation, segments of the lumbar spinal cord were microdissected by a punch technique. We measured serotonin and 5-hydroxyindoleacetic acid (5-HIAA) contents by high performance liquid chromatography and both substance P and thyrotropin-releasing hormone (TRH) levels by radioimmunoassay. We demonstrated that: (1) neonatal hyperthyroidism decreased substance P and TRH levels in the dorsal and ventral horns respectively, without modifying serotonin and 5-HIAA contents; (2) neonatal hypothyroidism increased the concentration of substance P in dorsal horn, of TRH in ventral horn (confirming our previous work), of serotonin in ventral horn, and of 5-HIAA in both ventral and dorsal horns; (3) T4-replacement therapy abolishes hypothyroid effects on substance P, TRH, and 5-HIAA, but not on 5-HT accumulation; and (4) bovine growth hormone-replacement therapy has no therapeutic action on the hypothyroid-induced accumulation of substance P, TRH, serotonin and 5-HIAA.

Postnatal action of growth and thyroid hormones on the retarded cerebral myelinogenesis of Snell dwarf mice (dw)

Snell dwarf mice (dw) showed a lower CNPase activity (59% of the normal controls) only in the cerebrum among different parts of the CNS, and a strikingly reduced level of spontaneous locomotion activity with an indistinct diurnal periodicity in a 24-h record at 40 days of age. Daily administration of bGH and T4 to the dwarfs during the first 40 days of postnatal life restored CNPase activity to the level of the normal controls, and was accompanied by normalization of the pattern of spontaneous locomotion activity. Daily administration of bGH alone also restored CNPase activity and spontaneous locomotion, but to a lesser extent. The daily administration of thyroid stimulating hormone (TSH) alone, however, failed to restore CNPase activity, in spite of the fact that the thyroid glands of the TSH-treated dwarfs were indistinguishable from the normal controls in organization and appearance. These results indicate that the restoration of both the retarded myelinogenesis and abnormal behavior of the Snell dwarf mice might essentially depend upon GH levels and the synergistic effects of T4.

Effects of bovine growth hormone on the retarded cerebral development induced by neonatal hydrocortisone intoxication

In comparison with normal controls, hydrocortisone-intoxicated rats (HC rats) had smaller cerebra, lowered 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) activity, and greatly reduced learning ability. The reduction in cerebral weight and DNA content was considered to be caused by a decrease in the number of proliferating glial cells, because the usual postnatal elevation of thymidine kinase (TK) activity was found to be suppressed in the cerebra from the HC rats. Electron microscopic observation of the pituitary gland revealed that the 5-day-old HC rat contained growth hormone (GH) secretory cells which were fully packed with GH granules, suggesting a disorder in the system which releases GH. In an attempt to promote cerebral development in the HC rats, we administered bovine GH (bGH) to some of the HC rats daily from the day of birth until weaning (HC + bGH rats). In the HC + bGH rats, the cerebral DNA was restored to normal levels and a concomitant increase in TK and CNPase activity was noted. Furthermore, in the brightness discrimination test, whereas the HC + bGH rats attained the learning ability of the normal controls after only 10 sessions, the HC rats were unable to reach an equivalent level even after 25 sessions.

Ethanol induced changes of in vitro protein synthesis during the development and maturation of brain tissue

Decreased protein synthesis in a cell free system of brain has been reported for male adult rats following chronic ethanol ingestion. To assess the developmental and maturational changes occurring in the neonatal brain, the effects of pre- and postnatal maternal ingestion of ethanol were determined. For these studies young female rats were given a 10% w/v ethanol/water solution for varying periods after impregnation and seven days post pregnancy. Data showed that maternal ethanol ingestion produced a large deficit in the in vitro incorporation of (14C) leucine into the hot TCA extractable residue of ribosomes of neonatal brain. Maximum inhibition was obtained when ethanol was given postnatally. To determine the molecular sites of ethanol's action, ribosomes and pH 5 enzymes from the adult and neonatal brain were examined. Data showed that the highest activity was obtained with control neonatal brain enzymes and ribosomal fractions.

Severe hypothyroidism and the maturation of the rat cerebral cortex

In order to study the influence of hypothyroidism on the maturation of the visual cortex, rats were thyroidectomized (T) surgically at 10 days of age and injected with 131I within 2 days after the operation. These T rats, and age-paired controls (C), were killed at 20, 25, 30, 40 and 80 days of life. The groups of T rats were subdivided into 'severely' hypothyroid and hypothyroid animals, on the basis, respectively, of complete or incomplete stasis of body growth and depletion of pituitary growth hormone stores. The pyramidal cells of the visual coretx of 'severely' hypothyroid rats were studied separately from those of hypothyroid animals. It was found that the number of spines along the apical shaft of such cells was markedly and equally reduced in all of the T rats, as compared to C rats, irrespective of the degrees of hypothyroidism attained, and stopped increasing in number by 30 days of age. From this age onwards, the distribution of the spines could no longer be fitted by a mathematical model31 which, however, adequately described for normal rats the developmental changes which take place with increasing age in the distribution of spines along the shaft of pyramidal cells from the visual cortex. This lack of fit was only found for the 'severely' hypothyroid animals; data from the other hypothyroid animals could still be fitted by the model. Such data show that the above-mentioned model31 may be a useful tool for the study of the intensity of the effects of hypothyroidism on the maturation of cortical neurons.

The effect of phenylthiocarbamide (PTC) on mouse brain development

PTC--when dispensed during the whole fetal development--causes a disturbance of brain development in new born mice. This disturbance in manifested by a reduction of the cell number, a reduced protein content and a reduced activity of acetylcholinesterase in the brain.

Effects of neonatal thyroidectomy on thymidine metabolism and deoxyribonucleic acid synthesis in the developing rat cerebellum

Thyroid deficiency at birth caused a temporal decrease of the DNA content in rat cerebellum during the periods from 7 to 21 days of life as compared with the normal. At 2 h following the subcutaneous injection of [3H]-thymidine, the specific radioactivity of cellular dTTP in thyroid deficient rats was apparently increased on the 14th and 21st days, although the endogenous pool sizes of dTTP did not significantly differ between the normal and thyroidectomized rats throughout the entire ages studied. The ratio of incorporation of thymidine into cerebellar DNA was initially determined by the specific radioactivity and then expressed as the ratio of [3H]-DNA to [3H]-dTTP, the relative specific radioactivity of DNA. The age peak of incorporation occurred at 7 days in the normal; on the other hand, the specific activity of DNA in thyroid deficiency attained a maximum at 14 days and the relative specific activity appeared to be prolonged up to 14 days. The corresponding changes were observed in the formation of dTTP, suggesting that a transient retardation of cerebellar DNA synthesis taking place by neonatal thyroidectomy may be in part attributable to the variance of phosphorylation of thymidine. Indeed, thymidine kinase activity that regulated salvage pathway for DNA synthesis displayed a parallel variation in thyroid deficiency to characteristic age dependence of DNA synthesis.

Effect of neonatal thyroidectomy on nucleic acids and protein contents of the developing rat brain

(1) Thyroidectomy at birth caused a temporal reduction of the DNA content only in the cerebellum during the periods from 7 to 21 days after birth, but not in the cerebrum and brain stem. (2) The ratio of RNA to DNA in cerebrum from the thyroidectomized rats was underwent a marked decrease from 14 to 56 days as compared with those of the normal controls. On the other hand, the effect of thyroid deficiency on the ratios of cerebellum and brain stem as a whole was very small. (3) The ratios of protein to DNA generally declined in the three brain regions during the periods from 14 to 56 days as a result of throid deprivation.