Effect of the Pineal Hormone Melatonin on Teleost Fish Phagocyte Innate Immune Responses After in vitro Treatment ALBERTO CUESTA, ALEJANDRO RODRI ´ GUEZ, M a VICTORIA CALDERO ´ N, JOSE ´ MESEGUER, AND M a A ´ NGELES ESTEBAN Ã Department of Cell Biology, Fish Innate Immune System Group, University of Murcia, Murcia, Spain ABSTRACT Although neuroendocrine-immune system interaction has been shown in teleost fish, no study has evaluated the role of melatonin (Mel) on fish immune response even considering that it is affected by the photoperiod. Gilthead seabream (Sparus aurata L.) and sea bass (Dicentrarchus labrax L.) head-kidney leucocytes were incubated with Mel (0-control-, 20 pM–400 mM) and leucocyte viability and main innate cellular immune parameters were evaluated. Overall, seabream and sea bass head-kidney leucocytes incubated with low (similar to physiological) doses of Mel unchanged the innate immune response, whereas very high (pharmacological) dosages did. Phagocytosis was not affected by any Mel treatment while the peroxidase activity was significantly inhibited with the highest Mel concentration. In contrast, the sea bass respiratory burst activity was increased in a dose-dependent manner with 400 nM Mel or higher. Further studies are needed to clarify whether there are interactions between the fish pineal gland, and its hormone Mel, and the fish immune system. J. Exp. Zool. 307A:509–515, 2007. r 2007 Wiley-Liss, Inc. How to cite this article: Cuesta A, Rodrı ´guez A, Caldero ´n MV, Meseguer J, Esteban MA. 2007. Effect of the pineal hormone melatonin on teleost fish phagocyte innate immune responses after in vitro treatment. J. Exp. Zool. 307A:509–515. Endocrine-immune system interactions have been widely demonstrated in mammals, whereas in fish these relationships are still under investi- gation. Melatonin (Mel) hormone is secreted by the pineal gland and retina in vertebrates follow- ing a daily circadian rhythm with the peak of production during the dark phase of the photo- period (see Falco ´n et al., 2007). These light-dark rhythm influence many behavioural processes in fish, such as locomotor activity, sedation, skin pigmentation, oxygen consumption, osmoregula- tion, thermoregulation, food intake, shoaling behaviour and, recently, immunity (Esteban et al., 2006; Falco ´n et al., 2007) in which Mel might be involved. Regarding the potential role of Mel on the immune system, in mammals and birds, in vivo studies have demonstrated that Mel stimulates both innate and acquired immunity, whereas in vitro treatment has provided contra- dictory results (Maestroni, ’93; Poon et al., ’98; Guerrero and Reiter, 2002). Thus, in vivo Mel enhances the antibody titres, antibody-dependent cellular cytotoxicity and natural killer-cell activity, cytokine release, lymphocyte proliferation, anti- gen presentation and the number of granulocyte- macrophage colony forming units in the bone marrow and restores immunosuppression (Del Gobbo et al., ’89; Maestroni, ’93, ’95; Persengiev and Kyurkchiev, ’93; Morrey et al., ’94; Moore and Siopes, 2002, 2003; Arias et al., 2003). In contrast, although some authors have found decreased immune responses in vitro, others described immune cell activation using physiological or even pharmacological doses of Mel (Lewinski et al., ’89; Morrey et al., ’94; Poon et al., ’94, ’98; Arzt et al., ’98; Kriegsfeld et al., 2001; Arias et al., 2003; Silva et al., 2004). Whatever the effects, the mechanism Published online 9 July 2007 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jez.404 Received 26 March 2007; Revised 8 June 2007; Accepted 9 June 2007 Grant sponsor: Spanish Ministry of Education and Science; Grant number: AGL2005-07454-C02-01/ACU; Grant sponsor: Fundacio´n CajaMurcia. Ã Correspondence to: M.A. Esteban, Department of Cell Biology, Faculty of Biology, University of Murcia, 30100 Murcia, Spain. E-mail: aesteban@um.es r 2007 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 307A:509–515 (2007)