Thyroid hormone control of preganglionic innervation of the adrenal medulla and chromaffin cell development in the rat. An ultrastructural, morphometric and biochemical evaluation

Nicotinic induction of preproenkephalin and tyrosine hydroxylase gene expression in butyrate-differentiated rat PC12 cells: a model for adaptation to gut-derived environmental signals

Accelerated maturation of peripheral sympathoadrenal transmitter levels and function occurs at 7-10 postnatal days in the rat. This event is temporally disconnected from the timing of major changes in physiologic stimuli evident after the birthing process (i.e. temperature, oxygen, sound, light, etc.). Colonization of the gut, fermentation of carbohydrates, and production of short-chain fatty acids (e.g. butyrate) mirrors this postnatal time course. In this report, we examined the interaction between butyrate differentiation of rat pheochromocytoma cells and cholinergic-nicotinic induction of the neuropeptide (enkephalin) and catecholamine-related biosynthetic enzymes (tyrosine hydroxylase, dopamine beta-hydroxylase, phenylethanolamine N-methyltransferase). Our results show that butyrate induces both preproenkephalin and tyrosine hydroxylase mRNA through a proximal promoter region and that this regulatory step is time and dose dependent. Moreover, there is an additional interaction with cholinergic-nicotinic inducible mechanisms consistent with classically described transsynaptic cholinergic regulation of these genes. Dopamine beta-hydroxylase and phenylethanolamine N-methyltransferase promoters were not affected by butyrate treatment. We speculate that colonization of the human gut (along with the attendant fermentation of enteral carbohydrates to short-chain fatty acids) may represent a mechanism through which environmental signals affect postnatal maturation of sympathoadrenal transmitter systems.

Adrenal gland SNAP-25 expression is altered in thyroid hormone receptor knock-out mice

SNAP-25 is a protein in neurons and neuroendocrine cells, which is involved, together with syntaxin and VAMP, in neurotransmitter release and neurite outgrowth. Since the thyroid hormone receptors TR alpha and TR beta are essential for nervous system development, their possible role in regulating the expression of these vesicle trafficking proteins was examined by analysing SNAP-25 levels in TR alpha and TR beta knock-out mice. Immunoblotting and RT-PCR showed that SNAP-25 levels are increased in the adrenal gland, but not in cerebellum, in knock-out mice, while syntaxin-1 and VAMP-2 are unaffected in either tissue. Treatment of the pheochromocytoma-derived cell line PC12 with the thyroid hormone L-3,5,3'-triiodothyronine (T3) decreased SNAP-25 expression. Together, these data suggest that thyroid hormones exert a negative regulatory effect on SNAP-25 in adrenal medullary neuroendocrine cells.

Development of cholinergic neurons in rat brain regions: dose-dependent effects of propylthiouracil-induced hypothyroidism

The effects of hypothyroidism on development of cholinergic system in brain regions (prefrontal cortex and hippocampus) were evaluated by measuring choline acetyltransferase (ChAT) activity and hemicholinium-3 binding to the high-affinity choline transporter. Various degrees of thyroid deficiency were produced by perinatal exposure to propylthiouracil (PTU) in drinking water ranging from 5 ppm (mg/l) to 25 ppm beginning at gestational day 18 until postnatal day 21. ChAT, a marker for cholinergic nerve terminals, was reduced by PTU in a dose-dependent manner. Concomitant with the enzyme deficits, hemicholinium-3 binding was elevated, suggesting an increase in neuronal impulse activity. Although similar changes were seen in both brain regions examined, the magnitude and duration of these changes were more definitive in the prefrontal cortex. Nonetheless, these neurochemical alterations appeared to be recoverable when the rats returned to a euthyroid state, and no further changes were observed as the animals reached adulthood. In comparison, data reported in a succeeding article indicate that deficits in cognitive function were first seen in weanling hypothyroid rats, but that the behavioral impairments lasted well into adulthood when thyroid status and cholinergic parameters in the brain appeared to have recovered to normal. These results suggest that alterations of cholinergic system caused by perinatal hypothyroidism are associated with neurobehavioral deficits at weaning, and these developmental deviations may cause permanent impairment of cognitive function despite recovery from the hormonal imbalance at adult ages.

The influence of dexamethasone treatment of pregnant rats on the development of chromaffin tissue in their offspring during the fetal and neonatal period

The aim of these examinations was to determine the influence of dexamethasone (Dx)-treatment of gravid females, on day 16 of gestation on the development of medullary chromaffin tissue of their fetuses and neonatal offspring. In conducting these investigations we used stereological as well as spectrofluorimetric measurements, in 20-day-old fetuses and 1-, 3-, 5-, 7-, 9-, 11-, 13- and 14-day-old neonatal rats. Single Dx-treatment (1.5 mg/kg bw) of the dams led to a significant decrease in body and adrenal weight of their fetuses and neonatal offspring, and also reduction of the medullary volume and the number of chromaffin cells during the entire period examined as a result of decreased cell proliferation in the fetal and early neonatal period (till the 5th day of age). The proliferative activity of the chromaffin cells was evaluated through the mitotic index after applying the cytostatic vincristine-sulphate. During the second neonatal week the mitotic index showed significantly higher values in comparison with the corresponding controls, which indicates that there is regeneration and recovery of the adrenal gland medulla. Adrenaline content in the adrenal gland tissue of offspring of Dx-treated dams was significantly reduced only on the 1st neonatal day. Thus, the change in blood glucocorticoid level of pregnant females after a single Dx injection during the period critical for development of the hypothalamo-pituitary-adrenal system in fetuses affects the development and kinetics of medullar chromaffin cell division.

Changes in proenkephalin gene expression in the developing hamster

Proenkephalin (Penk) gene expression is high in the adult hamster adrenal medulla and it is comparable to that found in both the hamster and rat striatum. In addition, Penk gene expression in the hamster adrenal medulla is more typical of adult mammalian adrenals than the rat. Since the nature of Penk gene expression in the developing hamster adrenal is not known, it was examined and compared to that found in the striatum were adult levels in the adrenal and striatum are similar. The results show that Penk gene expression progressively increases in the developing hamster adrenal to peak on postnatal day 4. There is then a small decline to adult levels by postnatal day 12 when the morphology of the developing adrenal resembles the adult. Functional splanchnic nerve activity, as assessed by the ability of reserpine to induce increases in adrenal tyrosine hydroxylase mRNA, is not present until after postnatal day 4. Therefore, early increases in Penk gene expression are independent of splanchnic nerve activity. Adrenal EC peptides resulting from the developmental increases in Penk gene expression appear to be unprocessed and proenkephalin-like. This is based on the very low levels of free enkephalin (met-enkephalin) detected in the adrenals from both newborn and adult hamsters (1-5% of total EC peptide levels). In the developing hamster striatum, Penk gene expression remains low and unchanged until postnatal day 4 and increases six-fold by adulthood. Free enkephalin (met-enkephalin) levels remain high (between 36 and 88% of total EC peptide levels) in the developing and adult hamster striatum. Therefore the results show early increases in adrenal Penk gene expression in the developing hamster that are independent of splanchnic nerve activity and adult Penk gene expression which is high and dependent on splanchnic nerve activity. This differs from what is observed in the frequently studied rat. However, developmental changes in the hamster striatum are similar to those in the rat.

Thyroxine replacement after hypophysectomy alters the pattern of enkephalin localisation in the adrenal medulla of the fetal sheep

It has been suggested that a pituitary-derived or -dependent factor may suppress enkephalin peptide synthesis in the central noradrenaline- containing cells of the sheep adrenal medulla in late gestation. We have investigated the effect of thyroxine (T4) replacement after fetal hypophysectomy on the localisation of enkephalin peptides within the peripheral adrenaline- and central noradrenaline-containing regions of the adrenal medulla of the fetal sheep. Fetal hypophysectomy (HX) was performed in 12 fetal sheep at 105-108 days gestation (term = 145 +/- 3 days gestation). T4 (40 micrograms/kg/24h) (HX + T4) or saline (HX + Sal) were infused between 110 and 140 days gestation. Adrenal glands were collected from the HX + T4, HX + Sal groups and from a group of intact fetal sheep (n = 4) for immunohistochemistry using anti- met-enkephalin and an avidin-biotin staining system. In adrenals from intact fetal sheep, there was intense positive staining for met-enkephalin in the peripheral adrenaline-containing region and sparse staining for enkephalins in the central noradrenaline-containing region. In contrast, in the HX + Sal group, enkephalin staining was uniformly present throughout the peripheral and central regions of the adrenal medulla. In the HX + T4 group, however, the staining density of met-enkephalin was higher within the adrenaline cells of the peripheral rim of the medulla than in the central medullary region. We have demonstrated that T4 replacement after fetal hypophysectomy restores the normal ontogenetic pattern of localisation of enkephalin peptides within the fetal adrenal. We postulate that T4 acts either indirectly via neural mechanisms or directly at the noradrenaline-containing cells to mediate the suppression of enkephalin staining within the central noradrenaline-containing region of the adrenal medulla of the fetal sheep in late gestation.

The transcriptional regulation of the preproenkephalin gene

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Regulation of adrenomedullary preproenkephalin mRNA: effects of hypoglycemia during development

To further evaluate whether transsynaptic mechanisms account for stress-induced changes in adrenomedullary preproenkephalin mRNA (ppEnk mRNA), neonatal rats were made hypoglycemic at a time when synapses are non-functional (less than 10 days postnatal age). While ppEnk mRNA in medullae from adult rats increased as much as 60-fold in this paradigm (insulin 10 U/kg), ppEnk mRNA levels in the newborn increased only 1.6-fold (insulin 20 U/kg). To evaluate whether postsynaptic cholinergic pathways of the neonatal adrenal medulla were functional, we treated 5-day-old pups with cholinergic agonists (nicotine [1 mg/kg, s.c., q 12 h] + carbachol [1.7 mumol/kg, s.c., q 12 h x 4 days]). Combined cholinergic agonist treatment augmented enkephalin prohormone and peptide levels up to 3-fold (P less than 0.05). To determine whether the blunted response to hypoglycemia in the newborn resulted from a deficiency in functional transsynaptic activity, synapses were matured using thyroid hormone pretreatment (postnatal days 2 and 3) before hypoglycemic stress. Hypoglycemia now caused a 40-fold increase in adrenomedullary ppEnk mRNA levels only in the T3/insulin treated group. To exclude other secondary effects of hypoglycemia (eg. hormonal, or insulin treatment-dependent), intracellular glycopenia was produced in the presence of secondary hyperglycemia by injecting adult rats or pups with 2-deoxyglucose (500 mg/kg). Similar to the insulin-hypoglycemia group, a large increase in adrenomedullary ppEnk mRNA resulted in the adult but not in the 5-day-old neonatal adrenal medullae. We conclude that enkephalin biosynthesis, like co-stored catecholamines, is induced by a transsynaptic process.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of thyroid hormones but not their excess affects the maturation of olfactory receptor neurons: a quantitative morphologic study in the postnatal rat

To study quantitatively actions of thyroid hormones on maturation of olfactory receptor neurons (ORN), surface density and total number of receptor knobs (1 knob/ORN) were measured in 1 mu sections from septal olfactory epithelium of newborn, 12- and 25 day normal, hypo- and hyperthyroid rats. Hypothyroidism was induced by adding to drinking water n-propylthiouracil (0.1% w/v) from birth. Hyperthyroidism was induced by daily injection of pups with T4 (1-thyroxine, 0.3 microgram/g b.w., s.c.). Experimental pups showed all the signs of hypo- and hyperthyroidism. Between days 1-25, normal pups showed marked increase in surface area of septal olfactory epithelium (6x), total number (12x) and surface density (#/mm2, 2x) of mature ORNs. Thyroid deficient rats showed, by day 12, marked reductions in epithelial surface area and total number of mature ORNs; these and the surface density deficits became very pronounced by 25 day (30% area, 27% density, 47% # mature ORNs). Hyperthyroid rats, however, did not show an increase in any of these parameters over controls. Although total number of ORNs (mature and immature), as measured by number of nuclei, was also reduced in hypothyroid pups, surface density was not altered, indicating that maturation of ORNs, but not their local accretion is altered in thyroid deficiency. The results indicate that thyroid hormones are essential for normal proliferative expansion of olfactory epithelium and for maturation of ORNs postnatally. These actions of thyroid hormones are not increased or accelerated by excess T4 suggesting saturation of the hormone receptor system at the normal plasma level.

Prenatal exposure to nicotine impairs nervous system development at a dose which does not affect viability or growth

Prenatal exposure to high doses of nicotine (greater than 6 mg/kg/day) via maternal infusions has been shown to impair nervous system development and to decrease viability and growth. In the current study, we have examined the effects of infusing pregnant rats with 2 mg/kg of nicotine per day from gestational days 4 through 20. At this lower dose, there was neither interference with maternal weight gain nor any increase in resorption rate. Intrauterine and postnatal growth was maintained at normal or supranormal rates in the exposed offspring. Nevertheless, sufficient nicotine penetrated the fetal brain to cause persistent alterations in [3H]nicotine binding sites, abnormalities of cellular development [assessed by measurements of ornithine decarboxylase (ODC) activity and deoxyribonucleic acid (DNA)], and impairment of development of peripheral noradrenergic projections (assessed by kidney norepinephrine levels); in each case, the neural alterations were virtually equivalent to those obtained previously at the higher, growth-suppressant dosage. These findings indicate that growth impairment alone is insufficient to predict the adverse effects of nicotine on development.

Ontogeny of the opiate phenotype: an approach to defining transsynaptic mechanisms at the molecular level in the rat adrenal medulla

Transmitter phenotypic expressions is a dynamic cellular process governed by multiple interactions with the neuronal environment. During sympathoadrenal development the arrival of presynaptic nerve terminals at the adrenal chromaffin cell (in the immediate postnatal period), coincides with the acquisition and subsequent development of a variety of transmitter biosynthetic capacities. Data discussed herein supports the contention that synaptic connections serve a central role in triggering the ontological cascade. Disruption of the normal timing of innervation events is detrimental to subsequent function and results in permanent deficiencies in development. In addition, alteration of transmitter biosynthetic regulatory mechanisms appears to reside at the level of gene expression. In view of this, additional molecular approaches are necessary to further elucidate the fundamental basis of neuronal transmitter phenotypic plasticity. Our approach to this problem represents a logical extension of previous research in this area and ultimately, will involve characterizing transcription activator molecules important in transmitter gene expression at various ontological ages.