ABSTRACT
Melatonin was found in the earliest life forms and is present in all organisms studied to date, ranging from bacteria to humans (Conti et al. 2002, Tan et al. 2003). In mammals and humans, melatonin acts as a sleep regulator and was reported to have a role in sleep initiation (Shochat et al. 1997, Zisapel 2007, Pandi-Perumal et al. 2008). Melatonin has been described to have other roles: it can act as a dopamine release inhibitor from the hypothalamus and retina (Zisapel 2001), it can be involved in the aging process (Reiter et al. 1998), and it can regulate blood pressure (Cavallo et al. 2004, Grossman et al. 2006) and immune response (Carrillo-Vico et al. 2006) among other roles. Melatonin is phylogenetically conserved across vertebrates and acts as a time-keeping signal but also as a direct output of the circadian clock, playing an important role in the synchronization of the circadian system (Cassone 1998). Melatonin was found to be produced mainly by the pineal gland and retina but also by many other organs and tissues including the gastrointestinal tract (Bubenik and Pang 1997), skin (Slominski et al. 2005), lymphocytes (Carrillo-Vico et al. 2004) and bone marrow (Conti et al. 2000), suggesting that melatonin is involved in a range of physiological processes. The daily cycle of illumination is the main environmental factor infl uencing melatonin production and hence, it is described as a “zeitgeber” or the biological time keeping hormone which entrains circadian (daily)
1Department of Physiology, Faculty of Biology, University of Murcia, Campus of Espinardo30100. Murcia (Spain). Email: lmvera@um.es 2Institute of Aquaculture, University of Stirling-FK9 4LA. Stirling (UK). Email: hm7@stir.ac.uk *Corresponding author
and circannual (seasonal) rhythms in vertebrates (Menaker et al. 1997, Falcón et al. 2006, Pandi-Perumal et al. 2006). In all the species studied so far, the synthesis of this hormone by the pineal organ takes place during the dark phase of the daily light/dark (LD) cycle, providing an internal signal of night time to the organisms, and thus timing and controlling several biological rhythms (Arendt 1995, Zachmann et al. 1992a). Daily rhythms in fi sh include locomotor activity, rest, food intake, vertical migration and shoaling, skin pigmentation, osmoregulation and metabolism, whereas seasonal processes include growth, reproduction and smoltifi cation for migrating salmonids (Falcón et al. 2007, Migaud et al. 2010). However, the profi le of the melatonin rhythm varies among vertebrate species and three different variants have been described (Fig. 1). The A-type is characterised by a delay of melatonin rise after the start of the dark phase, showing the
Figure 1. Schematic representation of the different melatonin profi les recorded in vertebrates. Examples of species which express such pattern of plasma melatonin for each profi le are listed. Horizontal black bar denotes subjective dark period (Adapted from Reiter 1988, Falcón et al. 2010b).
