**1. Introduction**

Thyroid hormones (TH) have major well-known actions on the growth and development of the maturing tissues including mammalian brain via activation of specific nuclear receptors leading to gene expression and subsequent target protein synthesis. Deficiency of THs has serious issues on the development on all types of tissues including brain leading to severe thyroid disorders and as a result imposes overall metabolic malfunctioning of all system organs. Endemic goiter was probably first described with cretinism by Paracelsus (1493 -1541) and by other physicians of the Alps and Central Europe. However, the relationship between cretinism and involvement of thyroid gland was lacking over centuries. Thyroid gland was literally described by Wharton in 1656. Since then the progress of research on thyroid gland gained attention particularly for its most observed pleiotypic action in number of species from aquatic animals to humans. Developments of new scientific technologies and the progress in the area of molecular biology from time to time are continually changing our concepts of the regulation of the functions of THs at the subcellular level [1,2].

Immunocytochemical localization studies revealed that TH receptors (TR) in adult vertebrates are highly concentrated within choroids plexus, dentate gyrus, hippocampus, amygdaloid complex, pyriform cortex, granular layer of cerebellum, mammillary bodies and medial geniculate bodies. Although specific nuclear receptors for THs in adult brain have been identified, their functions are unclear about target gene expression. Imunohistochemical mapping further documented that locus coeruleus norepinephrine stimulates active conversion of L-tetraiodothyronine (L-T4) to L-triiodothyronine (L-T3). A morphologic linking between central thyronergic and noradrenergic systems has been established. This changes in TH ontogeny gradually started drawing attention that possible

© 2012 Sarkar, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

TH action in mature brain switches its role which may be different from its classical action mediated through nuclear receptors. As the brain approaches adulthood, nuclear levels of iodothyronines decline gradually reaching a plateau and maintain it, and the TH levels increase within nerve terminals of adult vertebrates [1]. In particular, it showed decrease in nuclear L-T3 receptor binding in adult brain compared to developing brain. These switching differences in TH ontogeny between developing and adult vertebrate brain has gradually interested investigators to search for new functional role and mechanism of action of TH. Nevertheless, the action of THs remained limitedly judged in mature mammalian central nervous system (CNS) [3,4].

"Quo Vadis?" Deciphering the Code of Nongenomic Action of Thyroid Hormones in Mature Mammalian Brain 5

**Figure 1.** Thyroid hormones and their deaminated and decarboxylated products of interest.

TH treatment [1,11,12].

in red blood cells, acetylcholinesterase in neuronal plasma membrane, inhibition of synaptosomal membrane Na+-K+-ATPase (NKA), rapid action of L-T3 on synaptosomal Ca2+ influx, identification of specific L-T3-binding sites in rat thymocyte membrane, synaptosomal membrane, depolarization of actin filaments in cultured astrocytes by TH, and changes in second messengers and their corresponding regulatory systems following

Selective uptakes of THs have also been documented within the nerve terminals. Intravenous administration of [125I]-L-T4 in rats followed by thaw mount autoradiography has described selective distribution of L-T4 in specific adult rat brain areas particularly within the nerve terminals. Within the nerve terminal this was concentrated as L-T3 [10]. Other reports about the transportation of TH in adult brain also indicated role of transthyretin as a major serum binding protein for TH required for its transportation in cerebrospinal fluids and ultimately enable crossing of TH of the blood brain barrier directing to the brain. A role of monocarboxylate anion transporter protein-8 (MCT-8) also has been found to play a major role in TH transportation across the plasma membrane [10]. Three important enzymes called monodeiodinase are involved in TH metabolism. These are 5'-deiodinase type I (D-I), 5'-deiodinase type II (D-II) and 5'-deiodinase type III (D-III). D-I and D-II catalyzes conversion of the L-T4 to L-T3. D-I is the major deiodinating enzyme in the peripheral tissues. In brain D-II is predominantly localized in glial cells, astrocytes, and

Recent research highlights about the nonconventional nongenomic action of THs and its metabolites. Adult mammalian CNS is of specific interest. Clinical observations specifically have shown that the adult-onset thyroid disorders lead to several neuropsychological diseases including but not limited to anxiety, depression, mood disorders etc. in humans. These complications can be improved with appropriate adjustment of circulatory THs [5-8]. However, the defined mechanism to explain this is inadequate. The involvement of TH nuclear receptors in ameliorating these neuropsychiatric dysfunctions in mature CNS is controversial. Current knowledge about the TH-responsive gene expression in adult mammalian CNS is largely unavailable except some few discrete reports with differential effects in certain brain areas. Indication of new rapid nongenomic effects of THs and its metabolites, within seconds to minutes, poses special significance.

The interest about the action of TH in brain originated because like the classical neurotransmitters, catecholamines, THs are also derived from the amino acid, tyrosine. Tyrosine is decarboxylased by specific aromatic amino acid decarboxylase to produce catecholamines. There are possibilities that THs can also undergo decarboxylation and form biogenic amine-like neuroactive compounds, such as thyronamines or iodothyronamines as hypothesized. However recent experiment challenges this initial hypothesis since aromatic amino acid decarboxylase failed to produce this and thus presence of TH specific decarboxylase is speculated [9]. For example, L-T4 and L-T3 can be decarboxylated to produce L-T4-amine and L-T3-amine respectively (Figure 1). L-T3-amine can further be deiodinated to form L-T2-amine and then further deiodination can generate L-T1-amine. Important deaminated metabolites of L-T4 and L-T3 are tetraiodothyroacetic acid (TETRAC) and triiodothyroacetic acid (TRIAC) respectively [9,10]. Thyronamines may have neurotransmitter-like actions. However, no evidence is present to-date to identify physiologic formation of thyronamines that describe their physiologic functions, except one new report which identified 3-iodo-thyronamine in adult brain including other tissue homogenates in sub-picomolar concentrations [10]. Few pharmacologic actions for these synthetically prepared iodothyronamines are known in other tissues. This theory of action of thyroid hormones could be like classical neurotransmission led to search for the nongenomic mechanism of action of THs.

Thus, besides the genomic concepts, a parallel idea of nongenomic of TH action was emerging with demonstration of direct plasma membrane-TH interaction and expression of some hormonal effects in a variety of cells. These studies include activation on Ca2+-ATPase "Quo Vadis?" Deciphering the Code of Nongenomic Action of Thyroid Hormones in Mature Mammalian Brain 5

4 Thyroid Hormone

nervous system (CNS) [3,4].

TH action in mature brain switches its role which may be different from its classical action mediated through nuclear receptors. As the brain approaches adulthood, nuclear levels of iodothyronines decline gradually reaching a plateau and maintain it, and the TH levels increase within nerve terminals of adult vertebrates [1]. In particular, it showed decrease in nuclear L-T3 receptor binding in adult brain compared to developing brain. These switching differences in TH ontogeny between developing and adult vertebrate brain has gradually interested investigators to search for new functional role and mechanism of action of TH. Nevertheless, the action of THs remained limitedly judged in mature mammalian central

Recent research highlights about the nonconventional nongenomic action of THs and its metabolites. Adult mammalian CNS is of specific interest. Clinical observations specifically have shown that the adult-onset thyroid disorders lead to several neuropsychological diseases including but not limited to anxiety, depression, mood disorders etc. in humans. These complications can be improved with appropriate adjustment of circulatory THs [5-8]. However, the defined mechanism to explain this is inadequate. The involvement of TH nuclear receptors in ameliorating these neuropsychiatric dysfunctions in mature CNS is controversial. Current knowledge about the TH-responsive gene expression in adult mammalian CNS is largely unavailable except some few discrete reports with differential effects in certain brain areas. Indication of new rapid nongenomic effects of THs and its

The interest about the action of TH in brain originated because like the classical neurotransmitters, catecholamines, THs are also derived from the amino acid, tyrosine. Tyrosine is decarboxylased by specific aromatic amino acid decarboxylase to produce catecholamines. There are possibilities that THs can also undergo decarboxylation and form biogenic amine-like neuroactive compounds, such as thyronamines or iodothyronamines as hypothesized. However recent experiment challenges this initial hypothesis since aromatic amino acid decarboxylase failed to produce this and thus presence of TH specific decarboxylase is speculated [9]. For example, L-T4 and L-T3 can be decarboxylated to produce L-T4-amine and L-T3-amine respectively (Figure 1). L-T3-amine can further be deiodinated to form L-T2-amine and then further deiodination can generate L-T1-amine. Important deaminated metabolites of L-T4 and L-T3 are tetraiodothyroacetic acid (TETRAC) and triiodothyroacetic acid (TRIAC) respectively [9,10]. Thyronamines may have neurotransmitter-like actions. However, no evidence is present to-date to identify physiologic formation of thyronamines that describe their physiologic functions, except one new report which identified 3-iodo-thyronamine in adult brain including other tissue homogenates in sub-picomolar concentrations [10]. Few pharmacologic actions for these synthetically prepared iodothyronamines are known in other tissues. This theory of action of thyroid hormones could be like classical neurotransmission led to search for the

Thus, besides the genomic concepts, a parallel idea of nongenomic of TH action was emerging with demonstration of direct plasma membrane-TH interaction and expression of some hormonal effects in a variety of cells. These studies include activation on Ca2+-ATPase

metabolites, within seconds to minutes, poses special significance.

nongenomic mechanism of action of THs.

**Figure 1.** Thyroid hormones and their deaminated and decarboxylated products of interest.

in red blood cells, acetylcholinesterase in neuronal plasma membrane, inhibition of synaptosomal membrane Na+-K+-ATPase (NKA), rapid action of L-T3 on synaptosomal Ca2+ influx, identification of specific L-T3-binding sites in rat thymocyte membrane, synaptosomal membrane, depolarization of actin filaments in cultured astrocytes by TH, and changes in second messengers and their corresponding regulatory systems following TH treatment [1,11,12].

Selective uptakes of THs have also been documented within the nerve terminals. Intravenous administration of [125I]-L-T4 in rats followed by thaw mount autoradiography has described selective distribution of L-T4 in specific adult rat brain areas particularly within the nerve terminals. Within the nerve terminal this was concentrated as L-T3 [10]. Other reports about the transportation of TH in adult brain also indicated role of transthyretin as a major serum binding protein for TH required for its transportation in cerebrospinal fluids and ultimately enable crossing of TH of the blood brain barrier directing to the brain. A role of monocarboxylate anion transporter protein-8 (MCT-8) also has been found to play a major role in TH transportation across the plasma membrane [10]. Three important enzymes called monodeiodinase are involved in TH metabolism. These are 5'-deiodinase type I (D-I), 5'-deiodinase type II (D-II) and 5'-deiodinase type III (D-III). D-I and D-II catalyzes conversion of the L-T4 to L-T3. D-I is the major deiodinating enzyme in the peripheral tissues. In brain D-II is predominantly localized in glial cells, astrocytes, and

in the tanycytes lining the lower part of the third ventricles. D-III catalyzes the conversion of L-T3 to L-T2. Concentration of L-T3 within the nervous system has been attributed to the brain D-II which has major functions in regulating the overall neuronal homeostasis for TH. Expression of D-II in nervous tissue is implicated in the neuronal uptake of the circulatory L-T4 and its conversion to L-T3 followed by its supply to the neuronal targets. Expression of D-II is an important protective mechanism against hypothyroidism. This prevalence of TH homeostasis is a preventive measure and thought to be neuroprotective [1,13-16].

"Quo Vadis?" Deciphering the Code of Nongenomic Action of Thyroid Hormones in Mature Mammalian Brain 7

Termination

T3-neuronal membrane protein interactions

Activation of Second Messenger systems

**Figure 2.** Hypothesis: Proposed nongenomic action of thyroid hormones in adult mammalian brain.

Physiologic responses

Regulation of protein phosphorylation

Regulation of protein kinases/phosphatases

Hypothesis 3

Hypothesis 2

Hypothesis 1

**Figure 3.** (a) A typical neuron. (b) Cartoon of a neuron showing synaptosome. (c) Scanning electron

Synaptic vesicles, and the other intracellular components (Figure 3). Synaptosomes can be considered as isolated nerve terminals. Synaptosomes are obtained after homogenization and fractionation of nerve tissue. The fractionation step involves several centrifugations steps to separate various organelles from the synaptosomes. Synaptosomes are formed from the phospholipid layer of the cell membrane and synaptic proteins such as receptors.

microscopic image of synaptosome.

Interest also materializes to explore further the nongenomic mechanism of action of THs in adult mammalian CNS. In this context TH-mediated signal transduction pathways are also being investigated. Particularly the regulation of the activation of the second messenger systems and subsequent protein phosphorylation are of much awareness. Understanding of the mechanism of action of TH in adult mammalian brain has key implications in the higher mental functions, learning and memory, and in the regulation of several neuropsychiatric disorders developed during adult-onset thyroid dysfunctions in humans.
