**3.5. CART**

Cocaine and amphetamine related transcript (CART) is an abundantly expressed and widely distributed neuropeptide that has been implicated in a number of physiologic processes. The existence of a CART peptide fragment was first reported by Spiess et al. in 1981 in extracts of ovine hypothalamus as a somatostatine-like polypeptide [146], but its functional significance was not explored further. Almost fifteen years later, Douglass et al. found the first evidence of the existence of mRNA encoding neuropeptide CART in the rat brain after psychostimulant drugs were administered [147].

CART gene is composed of 3 exons and 2 introns, with rats and mice having alternatively spliced variants resulting in the production of two peptides, the longest of which has 129 amino acids (lCART) while the shortest is composed by 116 amino acids (sCART) [148]. The C-terminal end of CART, consisting of 48 amino acid residues and three disulfide bonds, it is thought to constitute the biologically active part of the molecule, and several fragments, notably, rlCART 55-102 and rlCART 62-102, have been shown to be active *in vivo*. In humans, only the short form is expressed (116 amino acids).

Distribution of CART mRNA and peptides has been demonstrated by several techniques showing a wide distribution throughout the nervous system, including sensory processing areas, central autonomic control areas, pathways involved in drug reward and reinforcement (*nucleus accumbeus*, lateral hypothalamus and projections to dopaminergic ventral tegmental area neurons), areas controlling feeding and body weight (hypothalamic nuclei), and stress related brain regions [149-152]. Furthermore, CART expressing cells have been found in the anterior pituitary, adrenal gland, islets of Langerhans, myenteric plexus of small intestine and in the ovary [153, 154]. Wierup et al. [155] reported for the first time the existence of CART IR in C cells in the porcine thyroid, in addition to porcine pancreas, gastrointestinal tract and adrenal gland, thus suggesting that the widespread expression of CART reveals a role for CART as modulator of neurohormonal functions.

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 modulator of anxiety and stress response [158].

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

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.
