**3.4. Ghrelin**

62 Thyroid Hormone

**3.3. TRH** 

basal activities [109].

skin and thyroid tissues [112-119].

secretion as an antagonist to TSH [122].

thyroid tissues [101] as well as in cultures of human thyrocites [102]. Besides its effects on thyroid function, suppression of thyroid follicular cell growth has been demonstrated by the

These evidences of the expression of SS in thyroid, more precisely limited to the C cells, and the actions in the follicular cells have led us to the speculation that this peptide could elicit

Thyrotropin-releasing hormone (TRH) is a tripeptide (pGlu-His-ProNH2) that was originally described to be synthesized in the mammalian hypothalamus, released into the hypothalamic–pituitary portal system and has the capability of inducing the release of thyroid-stimulating hormone (TSH) from the anterior pituitary [104, 105]. TRH binds to specific seven transmembrane domain, Gq/11-protein-coupled receptors of which, two subtypes have been cloned and characterized so far: TRHR1 and TRH-R2 [106, 107]. The two receptor subtypes exhibit similar affinities to TRH but different tissue distribution [108] and

Besides stimulation of TSH secretion at the pituitary, main role of TRH as key factor in the hypothalamic-pituitary-thyroid axis, numerous morphological and pharmacological studies have described TRH as neurotransmitter or neuromodulator in the CNS affecting behaviour, temperature regulation, food intake, and nociception [110, 111]. Furthermore, as suggested by the wide tissue-distribution of TRH and TRH receptors, TRH is implicated in many physiological and pathological processes of prostate, pancreas, testis, adrenal gland, heart,

In the case of thyroid, it has been well established that C cells express TRH at both mRNA and protein levels (see Figure 7) [36]. Furthermore, our research group have described that C-cell cultures express TRH-R1 and TRH-R2, whereas only TRH-R2 subtype is expressed by PC-Cl3 rat thyrocytes [60]. It has also been reported that TRH inhibits the TSH-induced increase of cAMP and subsequent release of thyroid hormones by the dog thyroid gland [120, 121], suggesting that TRH could participate in the regulation of thyroid hormone

In accordance to this point, Rausell et al. in 1999 [122] found that the levels of TRH and TRH-like peptides in the thyroid were strongly influenced by thyroid status. Furthermore, Rausell et al. demonstrated that TRH exerted a direct effect on thyroid hormone release in vitro [123] and administration of TRH to hyperthyroid patients with very low levels of TSH

Although the effects of TRH on thyroid hormone secretion could, at least in part, be due to hypothalamic TRH, the results so far published in the literature and reviewed above would open the possibility of a paracrine regulation of follicular cell activity through C-cell

resulted in decreased levels of circulating thyroid hormones [124].

released TRH as a putative additional mechanism for thyroid function.

inhibition of both TSH and IGF-1 proliferative stimulation [103].

local effects on thyroid hormone release acting locally in a paracrine fashion.

Ghrelin, is a 28 amino-acid acylated-peptide with powerful GH-releasing, orexigenic and adipogenic functions that, at hypothalamic level, regulates appetite, food-intake and energy metabolism in mammals [125, 126]. Since its initial description in 1999 [125], a variety of new functions for ghrelin have been characterized in the literature. Thus, apart from its GHreleasing and orexigenic effects, ghrelin has been reported to influence sleep and behaviour [127, 128], and the pituitary-gonadal axis at both peripheral [129] and central levels [130, 131].

The enteroendocrine cells of the stomach mucosa are the main source of circulating ghrelin [132]. Ghrelin synthesis and plasma levels rise and fall in relation to food intake, increasing with fasting and decreasing after eating [133, 134].Besides its gastric secretion, ghrelin is also expressed in many other tissues and organs [38, 135, 136] and despite the fact that molecular mechanisms have not yet been characterized, recent investigations have implicated ghrelin in many pathological conditions of heart, bone, liver, kidney and, specifically thyroid tissues [137].

The first observation regarding the presence of ghrelin in thyroid tissue was that by Kanamoto et al. in 2001 [138], describing the production of ghrelin in human medullary thyroid carcinoma tissue. Also, Gnanapavan et al. in 2002 [139] showed a very low expression of ghrelin at mRNA level and suggested to be carried-out by a very minor thyroid cell-population in normal human thyroid tissue. Those findings were concordant to those from Volante et al. [140] who failed to detect ghrelin-immunopositive cells in normal human adult thyroid gland. This last fact, being probably due to the very scarce presence of C cells in the human adult thyroid gland as compared to rats, where their percentage, in relation to follicular cells, ranges from 4.5 to 10.4% [18]. These observations were supported by the study by Raghay et al. in 2006 [38] in which ghrelin was described in the thyroid gland to be synthesized only by C cells.

Moreover, the ghrelin functional receptor has been demonstrated to be expressed in the neighbouring follicular cells [141], supporting, one more time the idea, increasingly found in the literature, that C cells would modulate thyroid function [38, 60, 63, 64], in this case, in a paracrine fashion, via ghrelin. Addressing this last point, up to date there are evidences

that would help to elucidate some aspects of this putative new mechanism. Thus, Volante et al. showed differences in ghrelin content in foetal and pathologic thyroid as compared to normal adult glands, [140]. Moreover, Park et al. [141] demonstrated that ghrelin, via GHSreceptor and calcium signalling, enhanced the TSH-induced proliferation of FRLT5 rat follicular cells, suggesting the thyroid function as a target for ghrelin, via GHS-receptor and protein kinase C, one of the key signal-pathways for thyroid follicular-cell function and thyroid hormone synthesis [142].

Paracrine Regulation of Thyroid-Hormone Synthesis by C Cells 65

gastrointestinal tract and adrenal gland, thus suggesting that the widespread expression of

Besides as nervous system neurotransmitters , CART peptides have been implicated in the regulation of feeding and body weight, drug reward and reinforcement and other processes [156]. They also have neuroprotective properties and promote the survival and differentiation of neurons in vitro [157]. Moreover, there is some evidence of its role as a

While the importance of CART peptides is clear, little is known about the cellular mechanisms by which CART exert their effects. No receptor for CART peptides has yet been identified, but some cellular effects have been observed, such as the induction of c-Fos activity in brain areas that are related to feeding and energy expenditure [159] or the induction of phosphorylation of

Due to important CART roles in the regulation of food intake and energy balance, where the thyroid plays a relevant function, several studies have been focused in the effects of CART in the regulation of Hypothalamus-Pituitary-Thyroid axis (HPT) and whether the thyroid status could regulate the expression of CART in the hypothalamus. CART-IR neurons in the paraventricular hypothalamic nucleus are reported to co-express thyrotropin releasing hormone (TRH) in rats, this neuronal populations co-containing TRH and CART project their axons to the median eminence, suggesting that CART peptides may have an important role in the regulation of thyroid-stimulating hormone (TSH) in the anterior pituitary [161]. In 2002, López et al. [162] demonstrated the existence of functional interrelations of the HPT axis with CART peptides. In other studies, the importance of the CART signaling system in the regulation of the HPT axis is suggested by the potent stimulatory effect of CART on the TRH gene expression of hypophysiotropic neurons [163]. Intracerebroventricular administration of CART increases TRH mRNA in hypophysiotropic neurons of fasting animals, and CART increases TRH content and release of hypothalamic cells in vitro [163]. These results together with those from Wierup et al. [155] described above provide the basis

for future studies of the role played by C-cell secreted CART on thyroid function.

Serotonin or 5-hydroxytryptamine (5-HT) is a biogenic amine synthesized by serotonergic neurons of the CNS, pineal gland and enterochromaffin cells of the gastrointestinal tract of humans and other mammals. Serotonin was isolated for the first time by Rapport et al. in 1948 [164] as a vasoconstrictor plasma agent. In fact, platelet serotonin is released to blood clots contributing to the haemostasis regulation. Serotonin synthesized by enterochromaffin cells is mainly involved in intestinal motility, whereas that synthesized in CNS acts as a neurotransmitter implicated in the regulation of mood, appetite, sleep, memory and learning. Besides, serotonin has antidepressant actions and regulates behaviour, cardiovascular function, muscle contraction, endocrine activity and body temperature [165]. Serotonin is derived from the essential amino acid L-tryptophan. The biosynthetic pathway of serotonin has two enzymatic steps: the first is catalyzed by the enzyme tryptophan-

CART reveals a role for CART as modulator of neurohormonal functions.

ERK in AtT20 cells [160] which activates the MAP kinase pathways.

modulator of anxiety and stress response [158].

**3.6. Serotonin** 

In regard to the implication of ghrelin in thyroid hormone synthesis, after the demonstration of the ghrelin receptor in human thyroid tissue [143], Kluge et al. [144] described a ghrelinmediated decrease in TSH levels and an increase of serum T4, probably due to a ghrelin direct stimulatory action on the thyroid gland. These results has been supported by a study from our research group which have demonstrated that, effectively, ghrelin has a direct effect on the three tissue-specific genes involved in thyroid hormone synthesis: thryroperoxidase (TPO), Na+/I symporter (NIS) and thyroglobulin [144, 145]. This direct effect on follicular-cell activity could be responsible for the effects observed at the hypothalamus-pituitary-thyroid axis [144, 145].
