Deficit of galanin-like immunostaining in the median eminence of adult hypothyroid rats

Thyroid hormone and remyelination in adult central nervous system: a lesson from an inflammatory-demyelinating disease

Re-myelination in the adult CNS has been demonstrated in different experimental models of demyelinating diseases. However, there is no clear evidence that re-myelination is effective in multiple sclerosis (MS), the most diffuse demyelinating disease. Moreover, chronic disabilities in MS are believed to be due to remyelination failure and consequent neuron damage and degeneration. Due to the presence of numerous oligodendrocyte precursors inside demyelination plaques, reasons for remyelination failure are unknown. In this paper, we reviewed data from embryonic development and in vitro studies supporting the primary role of thyroid hormone in oligodendrocyte maturation. We also reviewed personal data on the possibility of promoting myelination in chronic experimental allergic encephalomyelitis (EAE), a widely used experimental model of MS, by recruiting progenitors and channeling them into oligodendroglial lineage through the administration of thyroid hormone.

Expression of peptides and other neurochemical markers in hypothalamus and olfactory bulb of mice devoid of all known thyroid hormone receptors

We have investigated with histochemical techniques the expression of peptides and other neurochemical markers in the hypothalamus and olfactory bulb of male mice, in which the genes encoding the alpha and beta thyroid hormone receptors (TRalpha1, TRbeta1 and TRbeta2) have been deleted. Thyrotropin-releasing hormone messenger RNA levels were increased in the hypothalamic paraventricular nucleus and in the medullary raphe nuclei of mutant mice lacking the thyroid hormone receptors alpha1 and beta (alpha1(-/-)beta(-/-)), as compared to wild-type mice. In contrast, galanin messenger RNA levels were lower in the hypothalamic paraventricular nucleus of mutant animals, as was galanin-like immunoreactivity in the internal layer of the median eminence. Substance P messenger RNA levels were unchanged in the medullary raphe nuclei. Thyrotropin-releasing hormone receptor messenger RNA levels were increased in motoneurons, unchanged in the subiculum, and lower in the amygdala of mutant animals. Galanin messenger RNA levels were unchanged in the hypothalamic dorsomedial and arcuate nuclei of the thyroid hormone receptor alpha1(-/-)beta(-/-) mice, as was the immunocytochemistry for oxytocin and for vasopressin in the hypothalamic paraventricular nucleus. A reduction in tyrosine hydroxylase messenger RNA levels was found in the arcuate nucleus of mutant mice. In the olfactory bulb, immunohistochemistry for calbindin and for tyrosine hydroxylase revealed a reduction in the intensity of labeling of nerve processes in the glomerular layer of thyroid hormone receptor alpha1(-/-)beta(-/-) mice. The tyrosine hydroxylase messenger RNA levels were also slightly reduced. In contrast, the levels of galanin and neuropeptide Y messenger RNA in this region were unchanged in thyroid hormone receptor alpha1(-/-)beta(-/-) mice as compared to wild-type mice. Together these studies reveal many regional and neurochemically selective alterations in neuronal phenotype of mice devoid of all known thyroid hormone receptors.

Thyroid hormone-induced plasticity in the adult rat brain

It is well known that thyroid hormone plays a crucial role in the development and maturation of the nervous system. However, little is known about the role of thyroid hormone in the adult brain. In this short review we have dwelt on this point, with regard to the role of thyroid hormone on neuropeptide gene expression regulation in the paraventricular nucleus of the hypothalamus and in extrahypothalamic brain areas, on neurotrophin and neurotrophin receptor expression in the hippocampus and basal forebrain in basal conditions, and after neurotoxic challenges. Effects of hypothyroidism are discussed in view of a possible role of thyroid status in brain aging quality.

Regulation of anterior pituitary galanin gene expression by thyroid hormone

Thyroid hormone is required for basal and estrogen-induced expression of anterior pituitary galanin. Steady-state anterior pituitary galanin mRNA levels decreased 6-fold in hypothyroid rats after 3 weeks of treatment. Similarly, hypothyroidism resulted in a 2.6-fold decrease in estrogen induction of galanin gene expression. The effect of thyroid hormone on anterior pituitary galanin gene expression appears to be exerted, at least in part, at the pituitary itself. Transient expression assays in GH3 cells suggest the involvement of transcriptional mechanisms in the regulation of galanin gene expression by thyroid hormone. A region between -41 and -132 bp upstream of the transcriptional start site confers thyroid hormone responsiveness to the galanin gene. Gel-mobility shift assays show specific binding of 'SPI-like' proteins in GH3 nuclear extracts to this region of the galanin gene. This binding was greatly enhanced by thyroid hormone.

NGF content and expression in the rat pituitary gland and regulation by thyroid hormone

The involvement of nerve growth factor (NGF) in neuroendocrine regulation is supported by several lines of evidence. In this paper, we investigated the NGF content and expression in the pituitary gland and other endocrine organs during dysendocrine states (thyroidectomized, adrenalectomized and gonadectomized male rats). We found an increase of NGF-IR in the pituitary gland and testis of hypothyroid rats whereas no differences were found in the adrenal gland and blood. Also, NGF mRNA expression had increased in the anterior pituitary of hypothyroid rats whereas it had not changed after adrenalectomy and gonadectomy. Moreover, other neurotrophins and neurotrophin high-affinity receptors were unchanged in the anterior pituitary of hypothyroid rats. These data indicate that pituitary NGF is selectively modulated by thyroid status of the animal, further supporting a close link between NGF and thyroid hormone.

The rat galanin-gene promoter: response to members of the nuclear hormone receptor family, phorbol ester and forskolin

We have cloned a rat genomic DNA fragment of approximately 12.5 kb. Nine kb of the cloned fragment lie in the 5'-flanking region of the gene and contain the promoter elements, while the remaining 3.5 kb contain the first four complete exons, the first three introns, and part of the fourth intron of the rat galanin gene. We have partially analysed some of the elements within the proximal sequence of this promoter which may influence the transcriptional regulation of the rat galanin gene. The rat galanin-gene promoter contains many regions which share homology with both the human and the bovine galanin genes and certain cis-elements appear to be conserved among the three species. In an attempt to test whether some of these elements are functional in the rat gene, transient transfection studies were carried out in selected cell lines. Estrogen, thyroid hormone and retinoic acid all showed a minimal degree of promoter stimulation when the rat galanin-gene promoter was co-transfected with the appropriate hormone receptors in Neuro 2A cells, while co-transfection of the nuclear orphan receptor ELP1 was able to stimulate transcription of a galanin promoter-driven reporter-gene construct (-374 bp) by 35-fold. The galanin promoter mediated a 3-4-fold induction in response to forskolin or TPA. Deletion of a 5-bp element at -50 bp from the start of transcription was able to greatly reduce the forskolin response but not the TPA response. These results point to several elements that may be targets of transcription factors linked to extracellular stimuli.

Thyroid hormone regulates NGF content and p75LNGFR expression in the basal forebrain of adult rats

Several lines of data from human and animal studies have suggested a role of thyroid hormone in the regulation of cholinergic neurons in the adult brain. In this study we have investigated the content of nerve growth factor (NGF) and the expression of NGF low affinity receptor (p75(LNGFR)) in the basal forebrain of adult hypothyroid rats. We describe an increase of both NGF and p75(LNGFR) expression in the basal forebrain of adult hypothyroid rats. The administration of colchicine up-regulates p75(LNGFR) expression in both hypo- and control rats, whereas it fails to down-regulate choline acetyl transferase mRNA expression during hypothyroidism. These data offer a possible neurobiological explanation to cognitive defects observed during adult hypothyroidism in humans.

Expression of enkephalin and dynorphin precursor mRNAs in brain areas of hypo-and hyperthyroid rat: effect of kainic acid injection

An abnormal thyroid status induces morphological and neurochemical modifications in the adult brain. In this study we have analyzed the expression of enkephalin (ENK) and dynorphin (DYN) precursor mRNAs by means of in situ hybridization in the brain of hypothyroid and hyperthyroid rats. The influence of thyroid hormones on kainic acid (KA)-induced expression of ENK and DYN mRNAs in the granule cells of the dentate gyrus was also studied. Our results can be summarized as follows: (1) hypothyroidism induces an up-regulation of ENK mRNA in the granule cells of the dentate gyrus and layers V/VI of the cingulate cortex and of DYN mRNA in the granule cells of the dentate gyrus; (2) the up-regulation of ENK mRNA expression in the granule cells induced by KA is not modified by altered thyroid status; (3) in contrast, the KA injection fails to up-regulate DYN precursor mRNA expression in the granule cells of the rostral dentate gyrus of the hypothyroid rats; (4) injection of KA in hyperthyroid rats increases the expression of DYN mRNA in the granule dentate gyrus more than in euthyroid rats. The present results suggest that thyroid hormones exert an inhibitory control of expression of ENK and DYN mRNAs in selected brain areas. This effect could be directly mediated though the thyroid hormone nuclear receptor or could be secondary to changes in glutamatergic transmission in the dentate gyrus, as suggested by the profound alteration of the KA-induced expression of DYN mRNA in the dentate gyrus of hypo-and hyperthyroid rats.

Regulation of VIP mRNA expression by thyroid hormone in different brain areas of adult rat

A role of thyroid hormone in the regulation of neuropeptide synthesis has been demonstrated in different tissues. In this paper we investigated the vasoactive intestinal peptide (VIP) mRNA expression by means of in situ hybridization in several brain areas of hypo- and hyperthyroid adult rats. Neither hypo- nor hyperthyroidism modified the VIP mRNA levels in the thalamus and in the hypothalamic suprachiasmatic nucleus. In contrast, in the anterior cingulate and frontoparietal motor cortex of hypothyroid rats there was a marked increase in the signal for VIP mRNA per cell, but the number of VIP expressing neurons did not change. These data indicate that also central VIP synthesis can be influenced by the levels of circulating thyroid hormone, but that this effect is confined to specific areas and cell populations of the brain.

Neuroendocrine regulation of thyrotropin-releasing hormone (TRH) in the tuberoinfundibular system

[...] It is now required to list each part needed for mucous excretion. They are two ducts in the brain substance, then a thin portion of membrane shaped as the infundibulum, then the gland that receives the tip of this infundibulum and the ducts that drive the mucus (pituita) from this gland to the palate and nares. [...] and I said that one (duct) [...] from the middle of the common cavity (third ventricle) descends [...] into the brain substance, and the end of this duct is [...] the sinus of the gland where the brain mucus is collected [...].