Summary. Melatonin is an indoleamine with a wide spectrum of biological activities other than transmitting photoperiod information, including antioxidant, oncostatic, anti-aging and immunomodulatory properties. Although melatonin is synthesized mainly in the pineal gland, other tissues have the same capacity. In the present study, we examined whether two key enzymes in melatonin biosynthesis, arylalkylamine Nacetyltransferase (AANAT) and hydroxyindole-O- methyltransferase (HIOMT) and its receptor MT 1 are expressed in the two endocrine thyroid cells of the rat, follicular cells and C cells. Reverse transcriptase polymerase chain reaction analyses demonstrated that both AANAT and HIOMT mRNAs are expressed in the rat thyroid C-cells, and MT 1 expression has been detected in C cells and follicular cells. Immuno- fluorescence revealed that AANAT protein is localized in C-cell cytoplasm, and MT 1 protein in both cell populations. These findings demonstrate that the rat thyroid expresses AANAT, HIOMT, and its receptor MT 1 , showing that C cells are the main melatonin- synthesizing sites in the thyroid. This local C-cell- secreted melatonin may protect follicular cells from the oxidative stress inherent to the thyroid gland, and could also have paracrine and autocrine functions. Key words: Melatonin, MT 1 , Thyroid, C cells, Follicular cells Introduction Melatonin, an important indoleamine secreted by the pineal gland during the night, is mainly implicated in circadian rhythm control of mammalian and other vertebrates. Besides playing an important role as a transmitter of photoperiodic information, this indoleamine has antioxidant (Martinez-Cruz et al., 2002; Reiter et al., 2005, 2009; Mogulkoc et al., 2006), anti- aging (Reiter et al., 2002; Tajes et al., 2009) antiproliferative, and, potentially, anticancerogenic activities, including suppressing effects on secretory and growth processes of the thyroid gland (Lewinski and Karbownik, 2002). Melatonin production is catalyzed by two well-characterized enzymatic reactions from tryptophan. First, serotonin is converted to N- acetylserotonin (NAS) by the enzyme arylalkylamine N- acetyltransferase (AANAT) (Voisin et al., 1984). NAS is subsequently methylated by hydroxyindole-O- methyltransferase (HIOMT) to form melatonin (Axelrod and Weissbach, 1960). Although the pineal gland is considered the main site of melatonin synthesis, many extrapineal tissues have been identified as melatonin synthesizers, such as retina (Mennenga et al., 1991; Iuvone et al., 2002; Tosini et al., 2007), Harderian gland (Djeridane et al., 1998), gut (Raikhlin et al., 1975; Konturek et al., 2007), ovary (Itoh et al., 1997, 1999; Nakamura et al., 2003), immune system (Guerrero and Reiter, 2002; Carrillo-Vico et al., 2004, 2005; Naranjo et al., 2007), skin (Slominski et al., 2002; Fischer et al., 2008), and testes (Tijmes et al., 1996). Moreover, melatonin has been found in the rat thyroid gland (Kvetnoy, 1999). Considering the great production of melatonin in many organs and its wide spectrum of biological Melatonin-synthesizing enzymes and melatonin receptor in rat thyroid cells Rocío García-Marín 1 , Manuel de Miguel 1 , José Mª Fernández-Santos 1 , Antonio Carrillo-Vico 2,3 , José Carmelo Utrilla 1 , Jesús Morillo-Bernal 1 , Eduardo Díaz-Parrado 1 , Ismael Rodríguez-Prieto 2,3 , Juan Miguel Guerrero 2,3 and Inés Martín-Lacave 1 1 Department of Normal and Pathological Cytology and Histology, School of Medicine, 2 Institute of Biomedicine of Seville (IBiS), University Hospital Virgen del Rocío/CSIC and 3 Department of Biochemistry and Molecular Biology, School of Medicine, University of Seville, Spain Histol Histopathol (2012) 27: 1429-1438 Offprint requests to: Dra. Inés Martín-Lacave, Department of Normal and Pathological Cytology and Histology, School of Medicine, University of Seville, Av. Sánchez-Pizjuán s/n. 41009-Seville. Spain. e-mail: ilacave@us.es DOI: 10.14670/HH-27.1429 http://www.hh.um.es Histology and Histopathology Cellular and Molecular Biology