Thyroid hormone-processing systems

ONTOGENY OF THYROID HORMONE-PROCESSING SYSTEMS IN RAT BRAIN... 

Among vertebrate species, the neuro-endocrine-immime system is responsible for many complex, inter-related physiological processes including neuronal, homeostatic, reproductive and immune functions. There are four main types of hormone polypeptides, eicosanoids, steroids and thyroid hormones. Reflecting the inter-dependency of the neiiro-endocrine and immune systems, hormones, neuropeptides and other neiirotransmitters are known to be produced by some immune cells and play a role in the regulation of the immune system, while endocrine and nervous tissues express receptors for many substances produced by the immune system. The major focus of interest in endocrine disruption has... 

There is no such clear cut difTcrcnlialiun as metamorphosis in the mammal, but development is an extremely complex process and has been shown to depend upon the presence of adequate amounts of thyroid hormones. Deficient development, especially of the central nervous system, is marked in ehildren suffering from thyroid deficiency early in life, ansi this inadequacy cannot be overcome completely by medication commenced after the first few weeks. In the adult, thyroxine is important in the maintenance of energy turnover in most of the tissues of the body, such as the heart, skeletal muscle, liver, and kidney, Other physiological functions, most notably brain aclivity and reproduction, are also dependent upon thyroxine, although the metabolic rales of the tissues concerned in these functions do not seem to be altered. 

Several types of in vitro culture systems have evolved for the study of myelination. Cultures of cerebellar explants produced myelin-like processes, which can be stimulated to grow by thyroid hormone (13). However, the explant system produces only small amounts of tissue which may be sufficient for morphological and histochemical studies but are usually insufficient... 

Fifth, certain kallikreins, such as human kallikrein 6, are highly expressed in the central nervous system. It has previously been shown that hK6, and possibly some other kallikreins, are implicated in inflammatory reactions within the central nervous system that lead to demyelination. The association of hK6 and some other kallikreins with AD and multiple sclerosis points to the possibility that some of these enzymes may play important roles within the central nervous system. In addition, many of these enzymes have been found in endocrine tissues such as the islets of Langerhans, thyroid, pituitary, and others, pointing to the possibility that they may participate in prohormone or hormone processing. 

Several lines of evidence indicate that macromolecules of as yet unidentified chemical nature, produced by cancers and released into the systemic circulation, are responsible for the biochemical alterations in the liver and other host organs. In view of the diverse regulatory properties of the many different enzymes that increase or decrease towards their immature level (see Table III), a deficiency or excess in any given endocrine or dietary factor can clearly not explain the phenomenon. Nor has it been possible to implicate reductions in the efficacy of these factors. Subnormal concentration of the nuclear thyroid hormone receptor has been noted in the liver of tumor-bearing animals(24) however, since losses in the T3-inducible catalysts of the same liver occurred at much earlier stages of tumor-bearing,(24) the subnormal receptor concentration could clearly not be the cause of these losses but was probably another, and rather late, reflection of the process of biochemical undifferentiation. 

Other endocrine. systems are interrelated. Both corticosteroids and thyroid hormones may increa.se the requirement for pyridoxine and affect pyridoxal S-phosphate-dependent metabolic processe.s. Moreover, there appear to be associations between vitamin B(, and anterior pituitary hormones that seem to involve the hypothalamu.s. S-hydroxytiypta-mine. and dopamine. The latter two neurotransmitters are synthc.sizcd by metabolic procetises that require pyridoxal S-phosphate. 

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